Models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences

Palaeogeography, Palaeoclimatology, Palaeoecology, 51 (1985): 15--84

15

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

MODELS OF GLACIOMARINE SEDIMENTATION AND THEIR APPLICATION TO THE INTERPRETATION OF ANCIENT GLACIAL SEQUENCES

C. H. EYLES, N. EYLES and A. D. MIALL

Glaciated Basin Research Group, Department of Geology, University of Toronto, Toronto, Ont. M5S 1B3 (Canada) (Received March 20, 1984; revised version accepted December 10, 1984)

ABSTRACT Eyles, C. H., Eyles, N. and Miall, A. D., 1985. Models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences. Palaeogeogr., Palaeoclimatol., Palaeoecol., 51: 15--84. This paper argues that glaciomarine environments can be regarded as special, glaciallyinfluenced types of continental margin environments (e.g. continental shelf, slope, rise and basin plain). Knowledge of the stratigraphic architecture and typical sedimentary sequences of non-glacial margins is becoming well-known but remains limited for those that have been glacially modified. The principal influences on sedimentation in these environments relate to the glacial sediment input (controlled by relief of basin margin, glacier thermal regime and ice flow dynamics) and depositional environments (influence of traction currents, substrate relief and proximity to nearby ice margins). Typical ranges of sedimentation rates can be established for glacially-influenced continental margin environments and these may provide a framework for ancient sequences. Starvation of sediment supply to glacially-influenced continental margins is common. The nature of sub ice shelf sedimentation, a model that has been applied to m a n y ancient glacial sequences is critically reviewed; the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences. Existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation. In addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict(ite)s frequently fingerprint sediment source and not mode of deposition. The importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences. These are the Early Proterozoic Gowganda Formation (~2.3 Ga) of northern Ontario, Canada; the Late Proterozoic Port Askaig Formation ( ~ 6 7 0 Ma) of Scotland and Ireland, and the Late Cenozoic Yakataga Formation ( ~ 2 0 Ma to recent) of the Gulf of Alaska. These examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings. Diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance]retreat cycles, because of the varied controls on diamict accumulation and dlamictite preservation in marine basins. 0031-0182/85/$03.30

© 1985 Elsevier Science Publishers B.V.

16 RATIONALE Late Cenozoic glaciation has had a profound influence on nearly all continental margins by virtue of glacially perturbed sea level fluctuations (Crowell, 1978; Beard et al., 1982), consequent migration of facies belts and changes in the width of continental shelves (Frazier, 1974), and variation in the quantity and type of continental sediment reaching the shelf edge (Shanmugam and Moiola, 1982; Weaver and Kuijpers, 1983). Even more significantly some 40% of the Earth's continental shelves have been directly glaciated. Continental margins older than the Late Cenozoic (e.g. that of the Gondwana Supercontinent) have an even greater glaciomarine component (Frakes and Crowell, 1975). Continental margin environments include four main groupings; continental shelf and adjacent terrestrial margin, slope (and rise) including associated canyons, and basin plain, each identified by distinct depositional processes, lithofacies assemblages and stratigraphic architecture. Whereas these four components and their sedimentary products are becoming well known for non-glaciated continental margins understanding of the nature of glaciallyinfluenced sedimentation remains limited. However, industry-generated subsurface exploration programs in ancient continental margin stratigraphies that contain a significant glacial component are creating a considerable demand for generalized facies models_ (e.g., Martin and Cooper, 1984). OBJECTIVES OF THIS PAPER This paper attempts a broad review of glaciomarine sedimentation on continental margins as described in the current literature. In part I, a number of important factors are identified that control the evolution of typical lithofacies sequences ranging from those that influence the characteristics of the glacial sediment source to those that operate in the depositional environment. Ice proximal and distal glaciomarine environments are recognized with typical lithofacies sequences and deposition rates identified. Generalized lithofacies profiles are presented in the hope that they will stimulate modification and more detailed attempts at large scale stratigraphic modelling of such complex environments. Finally, in part II, ancient glacial sequences in Canada, Alaska and Scotland are described from the Early Proterozoic (2.2 Ga), Late Proterozoic (670 Ma) and Plio-Pleistocene (2 Ma) to illustrate the application of this sedimentological approach. INTRODUCTION The establishment of lithofacies criteria to distinguish glacioterrestrial palaeogeographic settings from glaciomarine is of critical importance in determining past ice sheet configuration, the distribution of land and sea, the tectonic setting of ancient glacial sequences, in resolving questions of global

17 stratigraphy employing glacial strata as event markers (Chumakov, 1981) and in certain economic investigations. The need is becoming more critical as hydrocarbon and placer mineral exploration moves into ancient glaciated basins. As argued elsewhere (N. Eyles et al., 1983a) most glacial studies emphasize traditional criteria derived from the study of small samples (e.g. cores and hand specimens) employing features such as clast shape, mineralogy, surface grain textures, fauna, composition and geochemistry. Few ancient glaciated basins to date have been investigated on a broad regional scale using multivariate basin analysis techniques in which lithofacies sequences, stratigraphic association and geometry ("basin architecture") are identified. Such synthesis is common in industrial practice. For certain sedimentary basin types having considerable industrial significance (e.g. alluvial), broad classifications of basin type and typical architectures are available (e.g. Miall, 1981, 1984; Kingston et al., 1983). The basis for such classification is fundamentally the plate tectonic setting, which is a major control on the depositional systems that accumulate within the basin. One of the main aims of the glacial sedimentologist is therefore the establishment of a parallel classification of glaciated basins. At present this is a long way off because, with few exceptions (e.g. Nystuen, 1976, 1982; Eisbacher, 1981; Link and Gostin, 1981; Visser, 1982, 1983a), ancient glacial sediments have not been examined using modern basin analysis methods. This contrasts with most other sedimentary environments (e.g. fluvial sediments, carbonates, or submarine fans) for which assessment of petroleum potential led to very considerable stimulus for sedimentological research during the nineteen-sixties (N. Eyles and Miall, 1984). Whilst the stratigraphic evidence for the Earth's glacial record is well catalogued (Hambrey and Harland, 1981) it has not been adequately described employing sedimentological facies techniques. The Earth's extensive glacial heritage, for the most part selectively preserved in glaciomarine basins, is a rich field for sedimentological investigation. A n o t e on t e r m i n o l o g y

To the sedimentologist the glaciomarine literature with its breadth of descriptive and genetic terms used for poorly sorted admixtures of gravel sand and muds, begs rationalization in the manner completed for carbonate environments (i.e. Dunham, 1962; Embry and Klovan, 1971). Existing glacial terms derived from studies of modern and Quaternary sediments are so encumbered with genetic inferences that they cannot be used in new approaches to the study of glaciated basins. Most of the definitions of the varieties and subtypes of "till" are based on models of glacial processes derived from theoretical modelling rather than on the kind of meticulous observation and facies analysis that have been so successful in other areas of clastic sedimentology. Application of facies analysis methods to glacigenic sediments by the writers is showing that many earlier genetic interpretations cannot be supported, and radical reinterpretations are required (see part II of this paper).

18 In this paper we follow a methodology urged previously for the description and environmental interpretation of giacioterrestrial sediments (N. Eyles et al., 1983a). The term "diamict(ite)" as defined by Frakes (1978) is employed for any poorly sorted gravel--sand--mud admixture, and a number of diamict(ite) lithofacies can be recognized. These can be described objectively in the field or in core logging by reference to lithofacies codes. Analyses of sediment genesis are based on detailed examination and bed-by-bed logging of surface outcrop and subsurface core sections, the generation of vertical profile models and the interpretation of depositional systems based on careful stratigraphic reconstruction. In this approach the term "till" (and "tillite") is reserved solely for those diamicts deposited directly at the base of a glacier by subglacial and supraglacial aggregation of englacial debris without subsequent reworking (see N. Eyles et al., 1983a, 1984). I. CONTROLS ON GLACIOMARINEDEPOSITION Glacially-influenced basins have occurred in a range of tectonic settings, but amongst the most significant are those of divergent Atlantic-type which are dominated by subsidence tectonics and greater preservation potential (e.g. Kingston et al., 1983). Review of an extensive literature indicates for example the strong association of Proterozoic diamictite sequences, continental rifting and the development of divergent trailing-edge margins (Strong, 1979; Aalto, 1981; Eisbacher, 1981; Hiscott, 1981; Anderton, 1982; Armin and Mayer, 1983; Karlstrom et al., 1983; Fralick, 1985). Divergent margins are usually characterized by broad continental shelves and adjacent slope prisms but also include those with a high relief continental shelf with slopes steeper than that of the continental slope (e.g. Antarctica). Another significant category of glacial basins includes those located in intracratonic settings. Examples include m o s t of the Late Paleozoic Gondwana basins of South America, South Africa and Australia (Hambrey and Harland, 1981), and the late Cenozoic Irish and North Sea Basins. Few major glacial basins are located on convergent or transform plate margins, although this may reflect either poor preservation potential or lack of study. An exception is the Yakataga Formation, located within the modern accretionary arc complex of the Gulf of Alaska (see part II, below). The controls on glaciomarine sedimentation at the present day and during Quaternary glaciations have been investigated by many workers. The importance of glacier thermal regime (and therefore regional climate), oceanographic circulation, characteristics of water masses, wave energy, bathymetry, basin floor relief, proximity to glacier margin, type of glacier margin, water depths, retreat rates and ice berg production rates have been identified, among others (Andrews and Matsch, 1983; Powell, 1984). However, many of these glaciomarine controls are difficult to employ as aids to classification in pre-Quaternary investigations of glaciated basins because with the recent exception of PoweU (1984), simple lithofacies criteria are not available for

19 recognition of m a n y environmental controls and these are therefore difficult or impossible to identify in rock sequences, particularly in the absence of any included fauna. Thus there is a need for generalized sedimentation models which can be identified in ancient sequences using stratigraphic means such as vertical profile analysis of either drill core or outcrop. Review o f ' a substantial literature relating to m o d e r n and ancient glaciomarine environments suggests that the dominant factors governing the evolution of lithofacies sequences in glaciomarine basins are those relating to the glacial sediment input and secondly, and more important, those factors appertaining to the depositional environment. 1. Glacial sediment input The glacial sediment input delivered to marine areas depends primarily on: (a) the relief of the basin margin; (b) glacier thermal regime; and (c) ice flow dynamics and discharge. These factors influence the abundance of certain textural size classes and the location and rate of sediment deposition. a. Relief o f basin margin Glaciated areas of high mountain relief with exposed bedrock areas typicaUy produce coarse-grained bouldery sediments by weathering of exposed valleysides. These sediments are transported both englacially and on the glacier surface (high-level transport; Boulton, 1978) and are characterized by large grain size range and clast angularity (Fig.l). This c o m p o n e n t is essentially glacially transported talus (Boulton and Eyles, 1979). In contrast, glaciation of low relief areas (e.g. cratons) produces m o r e fine-grained sediments as a result of large-scale areal abrasion at an ice sheet base. Repeated cannibalism and reworking of older glacial sediments, in particular those trapped in large marginal lakes and shallow seas, is also very significant (Scott, 1976; Gillberg, 1977; Sladen and Wrigley, 1983). As a result, whereas depositional processes m a y be c o m m o n to all glaciated areas, the textural characteristics of deposited sediments can show marked differences in areas of dissimilar relief and rock type. This is not widely appreciated in the literature and casts considerable doubt on those studies that try to establish the depositional environment of diamict(ite)s by reference to textural data (see part 1.5). b. Glacier thermal regime The importance of thermal regime lies in its control on the production of basal meltwaters, which determines whether the glacier will d u m p large volumes of suspended sediment into the marine environment. The thermal regime of m a n y m o d e r n ice masses is not well known but complex thermal mosaics are known to exist across their bases (Paterson, 1981). For the sedimentologist the thermal condition at an ice base can be divided into thawed (and thawing) zones where basal ice slides over the

20

Fig. 1. a. "Clast-clusters" of former supraglacial and englacial debris in fine-grained glaciomarine diamict (Dmm) of the Yakataga Formation, Alaska (see also Figs. 13--16). Clasts lack the characteristic faceting and shaping typical of basal debris (e.g. Boulton, 1978). b. Supraglacial debris on a valley glacier in an area of high bedrock relief; ice rafting from valley glaciers that terminate in marine waters results in distinctive "clastclusters" in glaciomaxine sequences.

TRENCH

ABYSSAL PLAIN

ANTARCTICA: CONTINENTAL SHELF AND UPPER SLOPE

FIORD

GULF OFALASKA: CONTINENTAL SHELF

<5.26 Ma < 0 . 6 Ma

Arctic Ocean (Canadian) Pacific Ocean (Alaskan) < 0.6 Ma

modern

T


mud/diamict

<5Ma

Aleutian

"~lcm

mud/diamict

modern < 0.035 Ma 2 5 - 5 Ma

mud/sand

3.5m

0.5--I.14mm 40mm

2--7cm 2--5cm 4cm

mud/diamict mud/diamict mud/diamict

<0.08 Ma

outer shelf and upper slope outer shelf and upper slope inner shelf sediment-starved inner shelf adjacent to ice shelf sediment-starved inner shelf adjacent to ice shelf mud/diamict mud/diamict

0.6--2.55m 0.1--2.0m

modern modern

1000m 0.84--4.3m

mud

1000m

mud/diamict

modern modern

4410m

mu d/diamict

modern

Piper et al., (1973) Von Huene and Kulm (1973)

Clark eta/., (1980) Piper et al., (1973)

Orheim and Elverhoi (1981)

Barrett (1975)

Orheim and Elverhoi (1981) Elverhoi and Roaldset (1983) Barrett (1975)

Glasby (1978) Richards (1976) Elverhoi et al., (1983)

Hoskin and Burrell (1972)

Elverhoi et al., (1983)

Powell (1981)

Moloia and Carlson (1978) Plafker and Addicott (1976)

2--3m lm

mud/diamict mud/diamict/sands

%2Ma

35m

0.01 Ma

Molnia (1979) Molnia and Carlson (1978)

3750m

mud/diamict

REFERENCES

mud/diamict/sand

RATE/Ka

LITHOLOGY

modern 0.01 Ma

AGE

mud mud

F T

THERMAL REGIME OF GLACIAL SOURCE

silty-sand

Alaskan; proximal to fiord-head delta New Zealand; mid fiord Norway; mid-fiord Spitsbergen; outer fiord

Alaskan; proximat to tidewater ice margin Spitsbergen; proximal to tidewater ice margin

coastal embayments inner shelf proximal to tidewater ice margins outer shelf outer shelf

SETTING

Representative deposition rates from suspension and ice-rafting in different glaciomarine and glacially-influenced marine settings. T = thawed base glacier, F -- frozen. For comparison, deposition rates of lodgement till may locally reach 30 m/ka (N. Eyles and Menzies, 1983).

TABLE I

t~

22 substrate producing basal meltwaters that are integrated into a well developed drainage network, and frozen and freezing zones where no free meltwater is available and ice moves either by internal deformation over a frozen ice/substrate contact or by deformation of the substrate (e.g. Moran et al., 1980). Subglacial meltwaters flush out abrasion products and transport large volumes of traction and suspended load into the marine environment (e.g. Powell, 1981). This sediment transport mechanism is largely absent from frozen and freezing zones (Anderson et al., 1983a, 1984). As a result, deposition rates in marine environments around thawed/thawing ice margins are much higher than around cold zones (Table I). The importance of glacier thermal regime with regard to erosional and depositional zones across large grounded ice masses has been demonstrated by Sugden (1977) and Denton and Hughes (1981) and possible relationships with typical glacioterrestrial lithofacies sequences and regional stratigraphies by N. Eyles et al. (1983a, b). c. Ice flow dynamics Regional ice volumes and the rate of ice discharge into marine margins of an ice sheet dictate whether an ice margin is land-based or sea-going. High regional ice volumes promote isostatic downwarping and submergence. Very high discharges, in the form of ice streams into marine portions of ice sheets give rise to floating sea-going margins which may be large ice shelves or smaller ice tongues. These project seawards, in response to creep-thinning, from ice sheet margins grounded well below sealevel (Fig.2). So far as is known, ice shelves and tongues lose mass by basal melting (De Q. Robin, 1979; Kohnen, 1982; Lennon et al., 1982) and ice bergs released from their margins may have lost most if not all of their englacial debris (Orheim and Elverhoi, 1981) so t h a t ice shelves may not be significant contributors to continental margin sedimentation (see discussion below). Ice shelves are also inherently unstable and during times of decreasing mass balance or rising sea level may rapidly disintegrate (Hughes, 1975, 1982) and revert to a grounded tidewater ice margin that terminates as a steep ice wall in shallower water along the basin margin (Powell, 1981). Ice bergs released from these grounded tidewater margins carry more debris than those from shelves or tongues as the basal debris layer may be transported away intact (e.g. Drewry and Cooper, 1981; Domack and Anderson, 1983). Finally, with ice volumes that are still further reduced, ice margins terminate on land away from marine waters. Isolation of ice sheet margins from the marine environment may be aided by isostatic recovery and emergence. Continental ice margins are fringed by large lakes (that may be directly connected to the marine environment by seaways), till plains and outwash plains (sandar). Glacially produced sediments can normally only reach the marine environment by glaciofluvial transport processes and are thus stored within large and rapidly evolving braided stream, deltaic or fan depositional systems (Boothroyd, 1976; Galloway, 1977; Molnia, 1980; Miall, 1983b).

23

Fig.2. Glaciomarine environments along a continental margin (for typical deposition rates see Table I). Basin margin (w): 1 -- grounded terrestrial ice margin; 2 = grounded marine ice margin and subaqueous fans (Cheel and Rust, 1982; Fig.3); 3 = ice shelf grounded below sea level and subaqueous fan (Orheim and Elverhoi, 1981); 4 -~ glacier fed marine delta (Galloway, 1977; Carlson and Molnia, 1977); 5 = glaciated fiord (Hoskin and Burrell, 1972; Baker and Friedman, 1973; Nelson, 1981; Powell, 1981, 1983; Vlsser, 1982). Shelf (with active ice rafting) (~): 1 = shallow bank; carbonates, shell banks, palimpsest sediments (Vorren et al., 1982); 2 = deep bank; mud and diamict drape (Molnia and Carlson, 1978; Solheim and Kristofferson, 1984); 3 = ice berg scour and turbation (Vorren et al., 1982); 4 = sediment starved shelf (Orheim and Elverhoi, 1981; Anderson et al., 1983a; Table I); 5 = moraine systems left by retreating ice lobe grounded on shallow bank (Andersen, 1979; C.H. Eyles and N. Eyles, 1984a); 6 -- stratiform sequences of diamict, muds and channelized resedimented facies (e.g. Fig.14). Continental slope, rise and canyon systems (o): 1 = glacially-influenced submarine fan (Stow, 1981; Piper and Normark, 1982); 2 = upper slope contourites (Anderson et al., 1982); 3 = downslope resedimentation; debris flow -* turbidites (Nardin et al., 1979; Wright and Anderson, 1982); 4 = canyon fill and feeder to fans (e.g. Fig.5; Miall, 1985). Abyssal plain (o): 1 -- pelagic muds, ice-rafted debris, turbidites (Piper et al., 1973; D.L. Clark et al., 1980; Goldstein, 1983).

2. Depositional environment Given initial d i f f e r e n c e s in t h e v o l u m e a n d t y p e o f glacial s o u r c e s e d i m e n t s c o n t r i b u t e d to the m a r i n e e n v i r o n m e n t as a result o f t h e f a c t o r s identified above, o t h e r c o n t r o l s relating t o the d e p o s i t i o n a l e n v i r o n m e n t t h e n c o m e i n t o play. T h e m o s t significant is w h e t h e r t h e d e p o s i t i o n a l site is p r o x i m a l or distal t o a g r o u n d e d ice m a r g i n (or g r o u n d i n g line o f an ice shelf o r t o n g u e ) . O t h e r i m p o r t a n t influences include t h e relief o f t h e basin f l o o r a n d t h e i n t e n s i t y o f b a c k g r o u n d m a r i n e t r a c t i o n currents. Clearly, m a n y o t h e r factors are i m p o r t a n t , e.g. salinity, w a t e r d e p t h s , o c e a n o g r a p h i c circulation, etc., b u t c a n n o t be readily d i s c r i m i n a t e d in a n c i e n t glacial sequences. Classifications o f g l a c i o m a r i n e e n v i r o n m e n t s r e c o g n i z e a characteristic

24 proximal facies belt (Boltunov, 1970; Vanney and Dangeard, 1976; Boulton and Deynoux, 1981). Andrews and Matsch (1983) set geographic limits to the extent of this zone as being within 1 km of the icefront and recognized intermediate (1--100 km) and distal environments (>100 km) beyond. Distance alone, however, is not necessarily significant and any definition of environment should ideally obey Walther's Law and therefore be process orientated. Thus Boulton and Deynoux (1981, p. 398) defined the proximal glaciomarine environment as one affected by "strong bottom currents, generated by density instability due to mixing of different water masses". In addition, supplementary evidence for proximity to an ice margin should be forthcoming. Without evidence such as glacitectonic structures and till(ite)s deposited in the grounded glacier environment the definition of Boulton and Deynoux (1981) is not specific enough and could also portray deposition at large marine fans or deltas deposited by glacial meltstreams many tens of kilometers from the ice margin (e.g. Galloway, 1977). Away from ice proximal environments, non-glacial marine processes dictate the pattern of sediment accumulations and direct glacial influence may be restricted to the supply of fine-grained suspended sediment, ice rafted detritus and deformation and reworking by berg grounding (ice berg turbation). Sediment gravity flow and marine currents may be of considerable, even predominant, importance in generating the final depositional product. Because of the episodic high energy nature of the ice proximal environment the boundary with more distal environments is not sharp but may extend over a broad area. The outer limit of distal glaciomarine sedimentation is taken here as the farthest extent of ice rafted debris. This definition recognizes that detritus is also rafted by ice of various origins (references in Piper, 1976; Andrews and Matsch, 1983), not necessarily glacial. In some cases these can be discriminated from ice berg transport in the rock record (see Part 1.2.c). a. Proximal glaciomarine e n v i r o n m e n t

Sedimentary components identified within lithofacies sequences deposited in the ice proximal environment near a grounded ice margin or grounding line include fan gravels and sands, diamicts, muds and tills (Fig.3). Tills (sensu stricto; N. Eyles et al., 1983a, 1984) will occur at or near the boundaries of individual sequences and may be associated with erosional unconformities that show glacitectonic structures. Extreme caution is required to discriminate glacitectonic deformation from that resulting from density loading or downslope mass movement. Visser (1983a--c) and Visser et al. (1984) have discussed criteria for discriminating these using core and outcrop data with regard to Permian fiord-head glaciomarine sequences of the Dwyka Formation of the Kalahari Basin. At those ice margins with a large efflux of sediment-laden meltwater, icecontact subaqueous fans form at the opening of subglacial or englacial meltwater conduits (Fig.3). Coarse-grained gravels having various axial fabrics

25

® ®

Fig.3. Proximal glaciomarine sedimentation close to meltstream exits during retreat of a grounded ice margin. (After Nystuen, 1976; Powell, 1981; Cheel and Rust, 1982). The same model probably applies to sites near meltstream outlets at the grounding line(s) of large ice shelves. See text for discussion of each component. 1 = glacitectonized marine sediments; 2 = lensate, thin lodgement till units; 3 = coarse-grained stratified diamicts; 4 = ice berg zone muds, diamicts; 5 = coarse-grained proximal outwash; 6 = interchannel crossStratified sands with channel gravels; 7 = slump facies (debris flows, slides and turbidites). Fan sedimentation m a y be local and controlled by local basins or indentations in ice margin. Lithofacies produced by grounded ice (till and glaciteetonic structures) may be overlain by "ice berg zone" muds and diamicts (e.g. Boulton, 1981; Boulton et al., 1982; Elverhoi et al., 1983; Osterman and Andrews, 1983). Deformation results from melt of buried ice and ice berg turbation. Not shown are density underflows and suspended sediment plumes. Former supraglacial debris where present, is a distinctive sedimentary component (Fig.1).

accumulate at the fan apex (Rust and Romanelli, 1975) with cross-stratified sands being typical of the main fan body away from bifurcating channels (Banerjee and McDonald, 1975; Nystuen, 1976; PoweU, 1981). Within these steep sided channels, massive, horizontally stratified and inversely graded gravels and sands accumulate by sediment gravity flow as episodic pulses of dense sediment laden meltwater sweep down the fan (Cheel and Rust, 1982). Depositional oversteepening results in slumps; kettle structures and associated faulting result from the melt of buried ice (e.g. McDonald and Shilts, 1975; N. Eyles, 1977; Domack, 1983). Coarse-grained diamicts with a muddy sand matrix may accumulate across the fan in areas of episodic traction current activity, mud deposition and icerafting (Figs.3, 4). These contain ice-rafted "outsized" clasts and diamict pellets, abundant traction current structures and clasts rolled along as bedload; soft sediment deformation is ubiquitous (C.H. Eyles and Jopling,

26 1985). Diamict facies commonly have gradational contacts with sands and gravels (Figs.3, 4B) and form part of a lithofacies continuum with pebbly sands and poorly sorted gravels. Examples of coarse-grained stratified diamict lithofacies associated with a major push ridge formed by the advance of a Late Pleistocene tidewater glacier in the Irish Sea Basin are described by C. H. Eyles and N. Eyles (1984a). Coarse-grained stratified diamict lithofacies formed where intermittent traction current activity, ice-rafting and RAIN OUT liCE'RAFTING AND ~ PELAGICFALL-OUT)

(A)

TRACTION CURRENT ACTIVITY

DOMINANT PROCESSES 1. RAINOUT

/ 4

OIAMICT LITHOFAClES Predominantly massive(Dram) Planar geometries

RESEDIMENTATION

ASSOCIATED LITHOFAClES Massivemuds (Fro)

2.

RAINOUT+ RESEOIMENTATION Massiveand stratified diamicts with ~" J flow structures, abundant silt and clay clasts, rafts (Dram, Dins)

Massive muds (Fro), laminated and clays (turbidites) (FI, FId) silts

3.

RESEDIMENTATION

Laminated silts and clays (turbidites) (FI, FId), gradedand massivesands (Sg, Sin).

.I

Stratified and massivediamicts with flow structures, abundant silt and clay clasts, rafts, variable grading characteristics (Dram, Dins, Dmg). Fills and flattens irregular topography.

Traction beddedsands (Sr, St, Sp) graded sands (Sg) and deformedunits of silty sand (Sd)

4. RESEDIMENTATION+ TRACTION J ~ CURRENT ACTIVITY

Predominantly stratified diamicts with evidenceof resedimentation and traction current activity (winnowed units, silt and sand stringers, rippled sands, flow structures, variable grading) (Dins, Dcs)

5, TRACTIONCURRENTACTIVITY

Winnowed diamicts, predominantly stratified (Dcs, Dms) Channelized geometries

Traction beddedgravelsand sands (Gm, St, St, Sp)

6, TRACTIONCURRENTACTIVITY ,,,.,,.,,.,,,,,,.,1=~"~,1,~ + RAIN OUT

Massiveand stratified diamicts with silt and sand stringers, rippled sands, somewinnowed units (Dram, Dins, Dcs)

Traction beddedsandsand gravels (Sr, St, Sp, Gm). Some mud drapes (Fm)

27 1

(B)

Fm Dram

2

6 Dmm

FId Dmm Dram(,)

~,m(o), Sr ~. ,

~

~._._

Dmm(¢)

oms(q

Omm

Stoc~c)

~

Drns(cl

~~m(c} ~

~mr.(c)

ores(r) Fm

Drnm FId

Dram

Dram(r) °ms(r}

:m,Sr mm(c)

RAIN1 OUT

_ ~ Drnm ~ Dram{r)

/\ 5 • ":~:~l.",::

/

:~:.~".

~ b~::-."-;~:

am,S~

6

RAIN OUT 5 & CURRENT REWORKING

Dms(rl 2

4

\ RAIN OUT & RESEDIMENTATION

3

~:J

FId Dins(r) FId Dmstr}

4

Dms(rl

:.,r, k Dins(r) ~Dcslc) Meltwater

currents Basin floor currents

Resedimentation

Ic.'..:..-?-.p

0osl,I

Dmmlr) ~ Ores[r)

a

Fig.4. A. Processes, typical diamict lithofacies and associated sediments in distal glaciomarine environments. Numbers refer to vertical profiles depicted in B. The term rain-out identifies ice-rafting and suspended sediment deposition. B. Vertical profiles illustrating A. Scale : various. Rain-out (No. 1) produces diamict sequences tens to hundreds of metres thick (see Figs.6, 11, 15). From C. H. Eyles and N. Eyles (1983a). See Fig.15 for lithofacies code. Letters in parentheses refer to evidence for resedimentation (r) and coeval traction current activity (c).

deposition of suspended fines, slumping and resedimentation have operated concurrently appear to be a common component of Late Pleistocene proximal glaciomarine and glaciolacustrine successions (e.g. fig.6 in Thompson, 1982; "Kalix till" of Lundquist, 1983 and fig.184 in Sj6rring, 1983). In the distal reaches of subaqueous outwash fans away from energetic

28 meltwater input rapid deposition of fine-grained suspended sediments predominates (Gazdzicki et al., 1982). It has been suggested that overflow currents, formed as density driven underflows lose momentum and become buoyant, shepherd away ice bergs and other floating ice masses. As a result a proximal mud belt develops ("ice berg zone muds"; PoweU, 1981; Osterman, 1982; Elverhoi et al., 1983; Osterman and Andrews, 1983). Muds contain a low volume of ice-rafted debris (IRD) but may show faint structure due to varying current activity and downslope resedimentation. Deposition rates of up to 4.41 m yr -1 are reported (Powell, 1981; Table I). Interaction of ebb and flood tidal currents with suspended sediments within interflow and overflow sediment plumes results in deposition of finely laminated (graded) sandy and silty mud lithofacies termed "cyclopels" by Mackiewicz (1983) and Mackiewicz et al. (1984) 1. Whether these models of proximal mud deposition are widely applicable remains to be seen as in many cases ice bergs are not driven clear of the icefront (e.g. Baker and Friedman, 1973, p. 491) and may take several years to clear inner fiord areas. Herman (1983, p. 356) refers to nearly 6000 ice bergs "jammed in a single W-Greenland fjord." Ice bergs that melt close to the ice margin may contribute large volumes of IRD to proximal muds resulting in diamict facies with fine-grained matrix. Supraglacial and englacial debris, derived from mountain sides in areas of high basin margin relief, and cohesive diamict pellets derived from the basal zones of the glacier are readily identified glacial sedimentary components of ice-rafted debris (Fig.lb; Ovenshine, 1970; Visser, 1983c). Downslope resedimentation by sediment gravity flow is an active process in ice proximal environments and gives rise to sharp erosive or conformable diamict bases, and sheet-like, lenticular or channelized geometries. Flow noses and brecciated diamict are common. A wide variety of penetrative structures may form as a result of high porewater pressures caused by sudden loading of substrate sediments. Several studies of Late Pleistocene glacial sediments have recently associated the ice proximal glaciomarine (and lacustrine) environment with the deposition of "subaquatic flow tills" (Evenson et al., 1977; Hicock et al., 1981). Typical models (e.g. Boltunov, 1970; Dreimanis, 1979; Hicock et al., 1981) show such units being released by the slumping of debris either on and/or below the ice margin. It has been argued that the conditions under which sediment gravity flows originate from accumulations of supraglacial debris and become submarine are circumscribed (PoweU, 1983) and perhaps of greater significance is post-depositional failure of rapidly deposited proglacial subaqueous sediments (Fig.3; Cheel and Rust, 1982; Prior et al., 1982; Visser and Kingsley, 1982; Visser, 1983a; McCabe et al., 1984) 2. However, 1 Possible seasonal controls on sedimentation in this environment have been identified by Domack (1984) from Late Pleistocene sequences. 2 Major instabilities are caused by glacitectonism and the consequent creation o f growing structural highs immediately beyond the ice-margin (e.g., Thomas and Summers, 1984; Lea, 1985; C.H. Eyles et al., 1985).

29 matrix-supported clast rich debris flow lithofacies also form by mixing of coarse and fine sediments during downslope movement in deltaic and fan environments (Larsen and Steel, 1978; Cohen, 1983; Postma, 1983; Visser, 1983c). Gazdzicki et al. (1982) describe a Pliocene example of sediment gravity flow deposition from the distal reaches of a subaqueous outwash fan merging with a basin plain from the South Shetland Islands. Deposition is dominated by slow sedimentation of mud from suspension interrupted by repeated incursions of pebbly sandstone and diamict by sediment gravity flow. The flows are defined by tabular or broadly lenticular geometries, laminated tops and sharp, sometimes erosive, bases. The beds range from debris flows through density-modified grain flows to turbidity currents and are mantled by pelagic muds with a well-defined Penitella biotope (Birkenmajer, 1982). These are buried by sediment gravity flows. Similar sequences are described by Visser (1983a--c) and Visser et al. (1984) from Late Paleozoic valley-head settings in the Karoo Basin of South Africa. Late Pleistecene examples are described by Domack (1983) and McCabe et al. (1984). Whereas post-depositional downslope resedimentation is an exceedingly common submarine process in areas of either moderate to high relief or high deposition rate, it is still the case that the identification of mass movement processes in Quaternary glacial sequences is automatically associated with deposition of "flow tills" directly from an ice margin. This reasoning is also applied, without any detailed lithofacies logging, to ancient sequences even where there is no compelling evidence diagnostic of the presence of a nearby ice margin (e.g. Port Askaig Formation; Howarth, 1971 and Upper Proterozoic Gaskiers Formation; Gravenor, 1980). Clearly downslope resedimentation processes are not uniquely associated with the ice proximal glaciomarine environment and on steep palaeoslopes and continental shelves of high relief are of major significance (Anderson et al., 1983b; see below). The identification of complex stratigraphic geometries generated by ice push, ice melt and localized fan deposition is a major criterion for recognition of proximal glaciomarine sedimentary environments. Major geomorphic features include morainal banks and push ridges, De Geer and crossvalley moraine ridges (Andrews and Matsch, 1983 and references therein). A growing body of data suggest that many Pleistocene morainal ridge complexes on continental shelves, showing the internal characteristics depicted in Fig.3, were formed either at the limit of glaciation or as a result of short lived glacier surges (McCabe et al., 1984; Solheim and Kristofferson, 1984; Thomas and Summers, 1984; Lea, 1985). Figure 3 depicts a proximal glaciomarine depositional system at a stationary or slowly retreating tidewater ice margin dominated by thawed basal zones. Glacier retreat results in a fining up sequence with the upper part of lithofacies sequences dominated by ice berg zone muds and diamicts with a fine-grained matrix (Hayward and French, 1980; Elverhoi et al., 1983). However, isostatic emergence or rapid sediment accumulation may bring the sequence up above wave base, producing a coarsening up beach or deltaic sequence (Andrews, 1978;

30 Boulton and Deynoux, 1981; Boulton et al., 1982). Where ice retreat is rapid, a stable subaqueous fan does not develop and an irregular blanket of gravels, sands and ice-rafted debris, fine and coarse-grained diamicts is deposited over either glacitectonized substrate surfaces, locally reworked lodgement till or bedrock (Powell, 1981). Edwards and Foyn (1981) employed the absence of subaqueous outwash lithofacies between "basal tillites" and overlying marine shales with dropstones to argue for the rapid retreat of a tidewater ice margin in the Upper Proterozoic Mortensnes Tillite of Finnmark. The absence of subglacial/proglacial meltwater facies on the Ross Sea continental shelf is similarly employed by Anderson et al. (1984) to argue for rapid decoupling of an ice sheet following a rise in sea-level. b. Ice shelf sedimentation The nature of sedimentation under ice shelves remains enigmatic. Observation of m o d e m depositional processes are few in number and sedimentological data from Antarctic sea floor areas that may have been deglaciated by ice shelves in the recent past are controversial (Barrett, 1975; Kellogg et al., 1979). However, despite our limited understanding of ice shelf sedimentation the ice shelf model has been widely employed in the interpretation of ancient sequences (Carey and Ahmad, 1961; Easterbrook, 1963; Reading and Walker, 1966; King, 1969; Aalto, 1971; King et al., 1972; Tucker and Reid, 1973; Spjeldnaes, 1973; Young, 1978; Williams and King, 1979; Visser, 1982; MiaU, 1983a), frequently with the assertion that the past may be the key to the present. A commonly recurring model of ice shelf sedimentation is based on the identification of a couplet of massive diamict(ite), overlain by faintly stratified or laminated diamict(ite) with current deposited interbeds, passing up into marine sediments. To several workers this records continuous melt-out of debris from the ice shelf base and sedimentation through the water column "without substantial disaggregation or sorting" ("waterlain till"; Dreimanis, 1979, p. 168; Gibbard, 1980), followed by ice shelf break-up and distal glaciomarine deposition of stratified diamicts. In an influential paper, Reading and Walker (1966) argued that such massive diamictites required "continuous uninterrupted sedimentation" which they considered could occur only below a closed ice shelf cover typical of the present day Antarctic. In contrast, stratified diamicts were argued to result where glaciers discharged directly in the open sea and deposition occurred by suspension fallout and ice-rafting from ice bergs. The argument that massive diamict(ites) require continuous sedimentation typical of sub ice shelf locations is oversimplified, as a wide literature demonstrates that such diamict lithofacies form in distal and proximal glaciomarine (and glaciolacustrine) environments dominated by suspension deposition and ice-rafting (part 1.2.c). Recent evidence from the Antarctic also shows that englacial debris is rapidly exhausted by basal melting and that sedimentation

31 occurs only in the area of the grounding line (Drewry and Cooper, 1981: "complete melt-out model"; Anderson et al., 1984, p. 312). Little debris is transported across the whole shelf, though there may be local _~ezing-on of brackish water and debris (Orheim and Elverhoi, 1981; S~gden and Clapperton, 1981). Drewry et al. (1980) present geophysical evidence for rapid accumulation of sediments in a belt tens of kilometers wide along the grounding line of the Filchner Ice Shelf. Gravenor et al. (1984) argue that substantial thicknesses of massive and laminated diamicts accumulate close to the grounding lines of ice shelves as a result of continued delivery of basal debris and subsequent "undermelting". Presumably a substantial component of such sediments will be muds deposited from suspended sediment plumes. Whilst detailed sedimentological characteristics and lithofacies sequences are unknown deposition close to ice shelf grounding lines in the vicinity of meltstream outlets is also likely to be associated with subaqueous fans and coarse-grained diamicts where downslope resedimentation and traction current activity is important (Fig.3; Drewry and Cooper, 1981; Orheim and Elverhoi, 1981; Miall, 1983a. Repeated deformation and reworking of the sediment wedge by grounding line migration is also suggested (De Q. Robin, 1979; Thomas, 1979; Drewry and Cooper, 1981). These data, if representative of sub ice shelf sedimentation, may indicate that deposition of thick, stratiform and regionally extensive massive diamict lithofacies is more typical of distal glaciomarine environments where abundant ice-rafted debris is supplied by ice bergs from calving tidewater ice margins and where suspended sediment is available (Stauffer and Peng, 1984). However, given the paucity of data appertaining to sedimentation below ice shelves this contrast requires further qualification. A major problem is over-reliance on modern ice shelf analogues in particular, the "complete melt-out model" from the Antarctic. Conditions prevailing under ice shelves formed on midlatitude and sub-polar continental margins during periods of enhanced glaciation may have been significantly different with regard to dominant thermal regime, englacial sediment distribution and volume and substrate relief. Another variant of the ice shelf model is that the massive diamict(ites) record deposition when the ice shelf was grounded. Thus Link and Gostin (1981) identified "subaqueous basal till" in the Late Precambrian Sturt Formation of South Australia solely on the basis of the massive structure of the diamictite despite absence of local and regional basal unconformities, glaciotectonic structures and preferred clast orientation. H. Williams and King (1979) also simply interpret massive diamictites of the Gaskiers Formation (Upper Proterozoic) in Newfoundland as grounded ice shelf deposits. Indeed, the assumption that massive diamict(ites) are necessarily of subglacial origin is as widely employed in the interpretation of m o d e m and ancient sequences (e.g. Bj~rlykke et al., 1976; Nystuen, 1976; Edwards, 1976, 1978, p. 424 and references therein; Hambrey, 1982; Gravenor and Rocha-Campos, 1983) as the assumption that they record "closed" deposition below floating ice shelves.

32 Whereas subglacial till(ite)s may be massive over short intervals of core or section they have diagnostic structures, clast fabric, basal and upper contacts and vertical profiles (e.g.N. Eyles et al., 1982, 1983a) that must be considered in genetic studies. Basal deposits formed below large marine ice sheet margins grounded in deep water are presumably similar to those below strictly terrestrial ice margins except that they should occur in disconformable association with marine sediments (e.g. Anderson et al., 1984). Till(ite)s cannot be identified on the basis of "massive structure" alone in the absence of detailed facies analysis (N. Eyles and Miall, 1984, p. 32). If we look to areas of likely Quaternary ice shelf sedimentation (e.g. Antarctic continental margin) for reconstructions of typical vertical profiles the data are controversial. Kellogg et al. (1979) argue, for example, that a thick submarine unit of massive diamict, with a fine-grained matrix containing reworked diatoms and foraminifera, and recovered by both DSDP drilling and piston-coring, is a "till" deposited below a more extensive grounded Ross Ice Shelf. Barrett (1975) had earlier argued that the unit resulted from suspension deposition a n d ice rafting in a distal glaciomarine setting. Resolution of this conflicting interpretation awaits more data though Anderson et al. (1984) show that the mineralogy of Ross Sea diamicts, lying above a marked unconformity, demonstrate systematic regional variation in response to primary dispersion by a grounded ice sheet (see below). The difficulties of applying an ice shelf model and criteria that may be useful in identifying sub ice shelf sedimentation have been discussed by Domack (1983) with regard to extensive Late Quaternary glaciomarine sequences exposed in the Puget Lowlands of Washington. In this area massive diamicts, produced by suspension deposition and ice rafting, were identified by Armstrong and Brown (1954). Debate has subsequently focussed on whether they were deposited on an open marine shelf or below an ice shelf (Wagner, 1959; Easterbrook, 1963; Shaw, 1972). This debate was finally resolved following identification of water depths from foraminiferal faunas within the diamicts together with a sea level history that precluded the freeboard necessary to accommodate an ice shelf. The modern view is that distal glaciomarine shelf sedimentation in the Puget Lowlands area followed the retreat of a grounded ice m~gin (Thorson, 1980; Armstrong, 1981; Domack, 1983). Working from long drill cores in the Dry Valley area of Antarctica, McKelvey (1982) was able to discriminate diamict deposition below closed or open ice covers by reference to included diatom floras. Unfortunately the nature of the ice cover and a sedimentological model were not developed further. Miall (1983a) describing drill-core from the Lower Proterozoic Gowganda Formation, recognized that ice shelf models were difficult to apply to the rock record but identified critical evidence for sediment gravity flow into deep water. These flows appear to originate from extremely local basement highs that may have been "pinning point" areas of local glacial debris production below an ice shelf (see part II.1).

33 In the Late Tertiary-Quaternary Yakataga Formation of Alaska convincing evidence for the former existence of ice shelves is given by a series of striated boulder lags outcropping on Middleton Island (see below; part II.3). These form extensive planar surfaces within a thick (1.25 km) succession of glaciomarine diamict lithofacies, formed by suspension deposition and ice-rafting and containing marine micro-organisms and unbroken molluscs. Boulders have striated upper surfaces showing a consistent direction of glacial abrasion. The absence of subglacial diamicts, glacitectonic deformations and subglacial and proglacial glaciofluvial sediments suggests the skimming of the substrate by an ice shelf periodically expanding to the continental shelf edge. Former ice shelves have been identified in other Quaternary glaciated terrains by reference to "ice shelf moraine ridges" formed at a consistent elevation around fiord margins where the ice shelf impinged on the valleyside (Sugden and Clapperton, 1981 and references therein). The likelihood of preservation and identification of such ridges in ancient sequences is remote. At this point in time, lack of understanding of the variability and complexity of sub ice-shelf sedimentation is a major constraint on the derivation of facies models. It is argued that the widespread recognition of sub ice-shelf sedimentation in the rock record has been overemphasized because of simple reliance on massive diamictite lithofacies as indicators of deposition below an ice shelf cover. This is not to say that ice shelves have not been a significant component of the palaeogeography of ancient continental margins (they comprise for example about 45% of the present Antarctic perimeter; Kristensen, 1983) but that their sedimentary record remains ambiguous. c. Distal g l a c i o m a r i n e e n v i r o n m e n t

The distal glaciomarine environment is dominated by non-glacial marine processes (Figs.2, 4). In mud belts where there is unrestricted supply of suspended sediment, fine-grained diamicts accumulate where ice-berg rafted debris melts-out (D. J. Miller, 1953; Armstrong and Brown, 1954; Ferrians, 1963; Bj~brlykke, 1967; Spjeldnaes, 1973; R.D. Miller, 1973; Plafker and Addicott, 1976; Armstrong, 1981; Boulton, 1981; Stauffer and Peng, 1984). Diamicts have a blanket-like geometry thickening in topographic lows and thinning over highs. Marine units can be very extensive over thousands of square kilometers and may show homogeneous lithology and lithic components as a result of widespread dispersal of debris by pelagic suspension and ice-rafting. Analysis of included marine microfauna has yielded important data as to likely water depths in which distal glaciomarine diamicts have accumulated. Massive lithofacies are generated even in comparatively shallow water (see part II.3) where little or no sorting occurs by traction currents or downslope resedimentation or where large volumes of suspended sediment are available (e.g. McCave, 1971). Stratified

34 and laminated lithofacies develop where the influence of currents and resedimentation is marked or where the relationship between suspended sediment and the volume of ice-rafted debris fluctuates through time. Deposition rates of distal fine-grained diamicts and m u d sequences are extremely variable and depend primarily on the predominant thermal regime of adjacent ice masses and proximity to suspended sediment plumes and ice berg tracks (Table I and part 1.3). In addition to dumping debris through the water column, ice bergs m a y plough and rework b o t t o m sediments (Thomas and Connell, 1985). Ice berg turbation has been identified as an important process d o w n to depths of 300 m on the Norwegian continental shelf where bathymetric highs exhibit a drape of "ice berg turbate" (Vorren et al., 1982). Relatively low clay contents of churned lag horizons were ascribed to resuspension during ploughing. Dreimanis (1979) identified a priori "berg till' deposition by lodgement and melt-out from grounded ice bergs, but Vorren et al. (1982) argued that such deposits are rather rare and only likely to occur where there is a sudden water level fall. This is frequently seen in modern ice-dammed proglacial lakes b u t is unlikely in the marine environment. On shallow shelf areas the free passage of icebergs may be prevented and marine sediments may show no evidence of floating ice (Andrews, 1978; Anderson et al., 1980). Many coastlines are characterized by seasonal or perennial pack ice which ploughs the sea floor and can be major sources of ice-rafted detritus as a result of overriding, freezing-on, aeolian and fluvial deposition and bank collapse over shore ice (Spjeldnaes, 1973; Dionne, 1974b; Dalland, 1977; and references in Andrews and Matsch, 1983). Lewis et al. (1982) found that modern ice-scour due to bottom-dragging pressure ridges of Arctic Ocean pack ice extend to 80 m depth b u t are most abundant in depths from 15 to 30 m. D.L. Clark et al. (1980) discriminated glacially-derived IRD from that rafted by pack ice by reference to cohesive diamict "pellets" (see also Ovenshine, 1970; Piper, 1976) and carbonate maxima. Goldstein (1983) however argues that such "pellets" also form on pack ice surfaces. Von Huene et al. (1973) maintained that the coarse sand fraction is the most reliable indicator of glacially ice-rafted debris. Ledbetter and Watkins (1978) discriminated ice-rafted debris from lag deposits b y the association of IRD with manganese micronodules. d. Influence o f traction currents Traction currents of various types can serve to winnow or supplement the fine-grained c o m p o n e n t of diamicts accumulating in the distal glaciomarine environment (Fig.4). A detailed review of such currents is b e y o n d the scope of this paper and only a brief synopsis is presented here. Shallow marine shelves with moderate to large tidal ranges are characterized by vigorous tide and wind-generated currents of up to 1 m/sec (H. D. Johnson, 1978}. In deeper water of the Antarctic upper continental slope and lower shelf

35 circumpolar bottom currents (contour currents) have recorded velocities of between 15 and 30 cm/sec (Gill, 1973). Density driven thermohaline currents, produced by the formation of sea-ice, move periodically down the continental slope of Antarctica with velocities of between 1 and 7 cm/sec (Anderson et al., 1982) and may reach depths of 5000 m (Kennett, 1982). Cold b o t t o m currents may be derived from an ice shelf and flow across the shallow marine shelf reaching velocities of up to 40 cm/sec near the shelf break (Orheim and Elverhoi, 1981). Mid-depth currents up to 57 cm/sec are known from the Arctic Ocean Basin with weak traction currents (<6 cm/sec) on the deep Abyssal Plain (D.L. Clark et al., 1980). Currents of between 45 and 60 cm/sec are needed t o entrain cohesive particles of coarse silt and smaller size, and most deeper water currents simply maintain silt and clay in suspension resulting in selective deposition of coarser IRD particles (e.g. Orheim and Elverhoi, 1981). As a result, documentation of IRD ("erratic abundance curves") in marine cores as an index of the intensity of ice-berg rafting may be oversimplified (Watkins et al., 1982). Clark et al. (1980) report from the Central Arctic Basin modification of the silt and clay size fraction in fine-grained diamicts by mid-depth currents acting to keep such material in suspension. Selective deposition (winnowing) generates stratified gravelly-muddy-sand diamicts ("residual paratills" of Anderson et al., 1982) which are widespread in shallow water portions of the Antarctic continental shelf (Anderson et al., 1980, 1984). These may be associated with gravelly lags (e.g. Jacobs et al., 1970; Grant, 1971; Fig.4). In areas of low current velocity, silts may be deposited from waning traction currents to produce diamicts with a silt-sized mode ("compound paratills" of Anderson et al., 1982; e.g. Holtedahl, 1959; Ruddiman and Bowles, 1976; Fig.4). These diamicts occur as widespread veneers on low relief areas of the Antarctic shelf and slope (Anderson et al., 1980). Variation in intensity of traction currents (e.g. contour currents) generates a laminated IRDpoor mud. Contour currents moving around the outer shelf edge can be displaced above the sea floor by more vigorous meridional bottom water flow which in turn is controlled by the quantity of cold dense saline water formed by freezing sea ice (Anderson et ah, 1982). The sedimentological significance of these contour currents in the Antarctic is that they deposit laminated mud sequences (contourites) in otherwise "sediment-starved" shelf areas (see part 2.e below; Anderson et al., 1979, 1982, 1983b). e. Influence o f substrate relief There is widespread documentation of resedimentation processes involving sediment gravity flow in many environments (Hiscott and Middleton, 1979; Tillman and Ali, 1982; Hurst and Surlyk, 1983). Detailed frameworks for describing and recognizing resedimented lithofacies are now available (Middleton and Hampton, 1976; Nardin et al., 1979; Lowe, 1979; Hiscott and Middleton, 1979; Hein and Walker, 1982; Nemec and Steel, 1984). The role of basin floor relief in promoting mass movements is critical but, even in areas of low regional slope (as low as 0.5°), fine-grained subaqueous

36 sediments are unstable if sedimentation rates are high (J.M. Coleman and Garrison, 1977; Prior and Coleman, 1979; T.C. Johnson, 1980; Prior et al., 1981, 1982; J.M. Coleman et al., 1983). The glacial environment is characterized by deleveling in response to ice sheet and sediment loading (Andrews, 1978; Newman et al., 1980; Boulton et al., 1982), sudden shock due to earthquakes, ice berg calving and m e l t of buried ice (Rust, 1977; Powell, 1981; Cheel and Rust, 1982) and pore-water pressure changes due to changing water levels and bursts of rapid deposition and gas migration (Bugge, 1983; Solheim and Kristofferson, 1984). As a result many glacial and continental margin glaciomarine environments are dominated by downslope resedimentation processes (Marlowe, 1968; Frakes and Crowell, 1969; Boulton, 1972; Carlson and Molnia, 1977; Piper and Slatt, 1977; Damuth, 1978; Aksu and Piper, 1979; Lawson, 1981; Stow, 1981; JSrgensen, 1982; Piper and Normark, 1982; C.H. Eyles and N. Eyles, 1983a; Miall, 1983a, 1985; Paul, 1983; Postma et al., 1983; Visser, 1983a; Gravenor et al., 1984; Visser et al., 1984; Solheim and Kristofferson, 1984; C.H. Eyles, 1985). The term resedimented, as used here, refers to a variety of post-depositional mass movement processes ranging from slides to true flows that redeposit subaqueous sediments. Slides describe movements of essentially rigid, internally undeformed and coherent diamict masses over discrete internal or basal shear surfaces (Nardin et al., 1979); sediment gravity flows move downslope with clasts supported by interstitial fluids, fine sediment, buoyancy and dispersive pressure (Middleton and Hampton, 1976). Between these t w o end members a continuum of processes exists and it is often very difficult or impossible to isolate the dominant support mechanism and mode of transport which are known to change both temporally and laterally during a single mass movement episode (Lowe, 1979; Visser, 1983a). Fine-grained mass flow deposits can be recognized by the variable presence of flow noses, flow banding and folding, creep structures, incorporated rafts of associated lithologies, basal grooving and flutings, and abundant silt and clay clasts (Nichols, 1960; Rattigan, 1967; Roberts et al., 1976; Visser et al., 1984; Fig.4). Abundant silt and clay clasts are probably produced by brecciation of fine-grained laminated lithofacies during slumping, creep, subaqueous dewatering or subaerial exposure (Dionne, 1974a; Brodzikowski and Van Loon, 1982). Lack of sorting, grading or stratification within resedimented diamict facies may indicate proximity to the flow source (c.f. "disorganized bed model", Walker, 1975). Increased internal sorting by dispersive pressures results in a downslope sequence of internal grading characteristics (e.g. Walker, 1979) and may ultimately result in the transformation of large and highly concentrated sediment gravity flows into turbidites (Banerjee, 1966; Morgenstern, 1967; Nardin et al., 1979; JSrgensen, 1982; Wright and Anderson, 1982; Miall, 1985), explaining the close association found in vertical profile, of resedimented diamicts and graded silty clay laminations (units C, D, E of the Bouma sequence).

37 Resedimented diamicts may have a higher clast content than in situ nonresedimented facies as a result of loss of fines following expulsion of porewaters during flow. Clast long axes frequently show preferential alignment or imbrication (Boulton, 1971; Walker, 1975; Nystuen, 1976; Gravenor, 1985). The Antarctic continental shelf provides a good illustration of distal glaciomarine deposition heavily influenced by downslope resedimentation (Kurtz and Anderson, 1979). The shelf is characterized by a rugged topography with canyon-like troughs having steep flanks (10 °) leading down to the abyssal plain (e.g. Fig.2). Shelf gradients exceed those of the continental slope. In a study of the Weddell Sea area, Wright and Anderson (1982) identified large slides on seismic traces on inter canyon slopes composed of "unsorted glacial source material" (distal glaciomarine diamicts and tills). Downslope mass movement by sliding generates mass flows and finally turbidites resulting in sorting and removal of finer sediments by turbulence. As a result, diamicts are rapidly (within 10 km) converted to well sorted sandy turbidites which are deposited on extensive abyssal plain submarine fans. Much of the Antarctic shelf area is "starved" of sediment (Anderson et ah, 1979; Elverhoi and Roaldset, 1983) because of minimal meltwater and suspended sediment production, a glacioisostatically induced slope toward the land mass (the proglacial depression of Walcott, 1970) and limited volumes of debris in ice bergs released from ice shelf fronts. Anderson et ah, (1983a, p. 388) state that "cross-shelf transport of suspended s e d i m e n t . . , is the exception rather than the rule in Antarctica". Consequently, deposition rates on the shelf are very low (Table I and part 1.3). For example, Domack (1982) shows that on the King George V shelf, adjacent to a heavily glaciated coastline, recent sediments are laminated siliceous oozes and contain little IRD. Significant sediment transport is limited to contour currents at the shelfbreak (Anderson et al., 1979). The importance of downslope resedimentation on the Antarctic shelf (e.g. 3 in Fig.2) is that it is the only process at the present day by which sediment is transported into canyon systems.

3. Deposition rates in glaciomarine settings and their bearing on preservation potential in glaciated basins Table I summarizes deposition rates in different glaciomarine settings as represented in the current literature. Such data have considerable bearing on questions of preservation potential and therefore the origins of regionally extensive diamict sequences in ancient glaciated basins. Table I shows that the highest sedimentation rates are associated with ice proximal locations in fiords and coastal embayments in areas of thawed glacier bases (e.g. Alaskan Pacific coast; 3750--4410 m/Ka). These rates decrease toward outer shelf areas where deposition rates of between 1 and 3 m/Ka are recorded (Gulf of Alaska). Similar values appear to obtain in mid and outer fiord settings though rates are higher where turbidites are active. In contrast, deposition rates on the Antarctic shelf clearly illustrate restriction of sediment

38 supply (see above); a setting in which significant carbonate accumulations occur in association with glacial sediments (Domack, 1985). Sediment starved shelf areas adjacent to heavily glacierized coastlines are also widely encountered outside Antarctica~ In glacially indented coastlines overdeepened fiord basins m a y act as sediment traps preventing the supply of glacial sediment to the outer shelf. These shelf areas are frequently sites of non-deposition, active current scour, extensive formation of glauconite and are dominated b y carbonate accumulations (coquinas, shell hashes, foraminiferal detritus, e.g., Bj~brlykke et al., 1978; Luternauer, 1982; Wilson, 1982; Bornhold and Yorath, 1984). The considerable influence o f glaciation on pelagic deposition rates in deep abyssal plains can be demonstrated if the Arctic and Pacific Oceans are compared. The Arctic Ocean is fringed b y a deeply frozen coastline and cold based glaciers; the deposition rate there is m u c h less than that obtaining on the Alaskan Abyssal Plain o f the Pacific Ocean where there is a substantial sediment efflux from the adjacent Gulf of Alaska coastline characterized b y highly active ice masses with abundant meltwater supply (e.g. Galloway, 1977). By w a y of comparison, rates of till accumulation b y lodgement below thawed based sliding glaciers are also shown (caption; Table I). In addition to identifying the importance o f glacier thermal regime Table I substantiates the often-seen c o m m e n t that thick and regionally extensive diamictite sequences are most likely to be of glaciomarine origin. However, whereas the highest rates o f accumulation are associated with thick wedges of glaciomarine sediments in ice proximal fiord settings these sequences are of low preservation potential. In m a n y cases accumulation m a y outstrip subsidence and as a result sediments m a y be eroded and reworked (e.g. Lamplugh, 1911; Elverhoi et al., 1980). The same remarks apply to glacioterrestrial facies such as tillites. O f greater significance for the rock record are distal glaciomarine diamict sequences that have accumulated on subsiding shelf areas on the margins of heavily-glaciated coastlines. Modern deposition rates vary from Alaskan-type shelves (up to 3 m/Ka) to sediment starved Antarctic shelf areas ( 1--7 cm/Ka) and this range m a y be applicable to ancient sequences. These rates are generally less than k n o w n subsidence rates, indicating a greater preservation potential compared with basin margin accumulations.

4. Strat~graphic architecture of glaciomarine basins An integral part of the investigation of any sedimentary basin is an analysis of the geometry and interrelationships of the c o m p o n e n t stratigraphic units. This analysis is a two-way process. Stratigraphic documentation and correlation is an essential part of basin description~ b u t the information it provides a b o u t stratigraphic architecture provides important clues about environments and processes that can, in turn, be used to predict architecture in areas of limited section data (MiaU, 1984). Case examples of the architecture of glacial basins are few in number (Beuf et al., 1971; Bj~brlykke et al., 1976; Nystuen, 1976, 1982; Deynoux, 1980; Eisbacher, 1981; De la Grand-

39 ville, 1982) but such data are in increasing demand for a variety of purposes particularly with regard to petroleum and gas occurrences in thick basin fills of Carboniferous--Permian age relating to the Gondwana Supercontinent (Harris, 1981 and references therein; De la Grandville, 1982; B.P.J. Williams and Wild, 1984; Martin and Cooper, 1984). The glacial marine environment includes four main environments, the basin margin shelf, slope (plus rise), canyon and basin plain (Fig.2). Each is characterized by a distinctive set of depositional processes and by a distinct architectural style. Both aspects are reasonably well known for non-glacial environments (e.g. processes: Reading, 1978; architecture: Drake and Burk, 1974; Payton, 1978), but the superimposition of the additional glacial control adds numerous factors that have yet to be evaluated. Continental shelves are underlain b y subhorizontal, blanket-like units (e.g. Josenhans, 1983), although this simple geometry m a y be complicated by basement tectonics or growth faults. Marine currents, storm scour or scour by grounded ice during low sea level, m a y reduce preserved thicknesses and generate widespread erosion surfaces (Elverhoi and Solheim, 1983; Turcotte and Kenyon, 1984; Solheim and Kristofferson, 1984). On some shelves little net sediment accumulation occurs, most detritus bypassing the shelf to be swept down canyons or spilled over the shelf break. Continental slopes and rises grow by lateral accretion, producing a clinoform stratigraphy. The geometry (dip, thickness) of the clinoform units and their relationship to the shelf depends on the sediment type and its rate of accumulation. The shelf and slope are cut by submarine canyons, which funnel sediment down to the continental rise and abyssal plain (Fig.2). Canyons m a y be enlarged to depths of up to 2 km by submarine mass wasting or become plugged by sediment depending on the rate of sediment supply. On unglaciated shelves this is governed largely by sea level (e.g. Normark, 1978; Shanmugam and Moiola, 1982), which controls the efficacy of marine shelf processes and the proximity of coastal sediment sources to the canyon head. Climatic changes during glaciation cause rapid changes in sea level, and m a y result in grounded ice extending far out on to the shelf to feed coarse debris directly to the outer shelf edge, or down canyons. The northern Gulf of Alaska (see Fig.13) and Gulf of St. Lawrence provide good examples of areas where glaciers reached the shelf edge and discharged directly into slope systems (Piper et al., 1973; Slatt and Piper, 1974; Stow, 1981; Carlson et al., 1982). At the m o u t h s of m a n y submarine canyons the major sediment load is typically deposited as submarine fans. Our understanding of non-glacial fan composition and architecture is undergoing a rapid evolution at this time, but few studies have investigated the particular effects of glacial sediment supply and rapid sea level change on fan sedimentation (Piper and Normark, 1982). The Antarctic continental margin offers one example of a glacially-influenced shelf/slope/plain system and is of interest because it illustrates a "sedim e n t starved" shelf as a result of the proglacial isostatic depression (see above) around a continental ice sheet. In this case sediment only reaches

40 canyon systems as a consequence of post-depositional downslope resedimenration of sediments in inter-canyon areas. Only at times of an enlarged ice sheet is there a direct sediment supply to the canyon system (Anderson et al., 1983a). Glacially-influenced abyssal plain sedimentation has not y e t been recognized in the rock record and a detailed facies model has y e t to be established. The presence of extensive planar bodies o f turbidite silty clays and massive muds with ice-rafted clastics has been emphasized b y D. L. Clark et al. (1980) and Goldstein (1983) in studies of the Arctic Ocean Basin. Several recent studies have examined problems of regional correlation and palaeogeographic evolution of glaciomarine deposition across Quaternary continental margins, and these studies offer guidelines to investigations o f the stratigraphic architecture of older basins. G. H. Miller et al. (1977) and Andrews (1978) have discussed typical sedimentary sequences that result from marine transgressions and regressions in response to glacio-isostatic and glacio-eustatic changes on Arctic coasts that result from Late Quaternary continental ice sheet formation. Maximum amplitude o f transgressive phases is about 400 m whereas m a x i m u m regressive phases were associated with sealevel lowering of 200 m resulting in possible glacially-induced sea-level fluctuations of up to 600 m. Andrews (1978) shows very clearly however, that the timing and amplitude of sea-level cycles is very different according to distance from major centres o f ice loading. Sea-level histories in areas near ice sheet centres are the mirror image of those o f peripheral shelf areas several hundred kilometers distance that are removed from direct ice loading. This is because areas near ice centres are dominated b y isostatic depression and recovery (e.g., submergence -* emergence) in contrast to distal locations dominated, during the same time period, b y eustatic changes (emergence -* submergence). B o u l t o n et al. (1982) demonstrated important data as to the regional geometry of glaciomarine units developed around the periphery of large Quaternary ice masses. In the Spitsbergen Archipelago repetitive lithofacies sequences of lodgement till overlain b y m u d and distal glaciomarine diamict and sands can be identified across a large (100,000 km 2) area and have systematic thickness variations with regard to centres of m a x i m u m ice loading. An increase in regional ice volume results in crustal depression, submergence, and deposition of muds and distal glaciomarine diamicts. This facies belt broadens in the direction of maximum ice loading (maximum isostatic depression) as ice margins retreat and post glacial flooding ensues. The thickness of these muds and diamicts is greatest close to areas of m a x i m u m crustal depression and thinnest in areas several hundred kilometers distant from the ice centres, where depression is not only minimal b u t m o r e rapid isostatic recovery results in truncation of muds and diamict sequences b y sub-littoral erosion, and deposition o f sand and gravel cap units. In m a n y studies of ancient glacial marine sediments the presence of stratigraphically distinct intervals of diamictite sedimentation has been interpreted in terms o f distinct phases of glaciation (e.g. advance--retreat; Spencer, 1971,

41 1975; Gravenor, 1980; Young, 1981). As noted above, however, basin evolution is strongly influenced by isostasy and by distance from the ice centre. Many other influences acting independently can control diamictite distribution. Those formed by ice rafting depend on iceberg drift tracks, controlled by tides and currents which in turn influence the occurrence of mud deposition. Diamictites formed by sediment gravity flows require an appropriate palaeoslope as well as a glacial sediment supply. All these process controls are affected by changes in tectonic regime and by eustatic sea level change unconnected with changes in ice margin positions. Therefore although the presence of the kinds of facies described here can be interpreted in terms of glacial controls, their absence does not necessarily mean ice retreat or reduction in ice volume. As an example of these complexities, an ice tongue terminating on an outwash plain just above sea-level may float if a tectonic subsidence event of a few metres ensues (such adjustments are common in tectonically active areas such as Alaska). This change in ice margin environment, without ariy change in climate, would have profound effect on the type of sediment supply and depositional process in the adjacent sea. For example, it would initiate iceberg drifting, with the introduction of coarse, unsorted debris into the marine environment. Different parts of a sedimentary basin may develop quite different stratigraphies, reflecting local variations in palaeogeographic evolution, and none of the successions may contain complete or accurate records of climate change. The examples of ancient glacial sequences described below can be used to illustrate the difficulties of climatic interpretation from multiple diamict sequences. 5. Discrimination o f glacioterrestrial and glaciomarine diamict(ite)s by compositional characteristics -- a review

Frakes (1975) was able to discriminate distal glaciomarine diamicts from glacioterrestrial tills by reference to trace metal geochemistry. High latitude glaciomarine sediments are deficient with respect to crustal abundance, in Fe, A1, Ni, Ti and Cr and contain excess Cu and V in contrast to glacioterrestrial tills that have surplus Fe (see discussion in Naidu, 1975). Pevear and Thorson (1978) argued that glaciomarine sediments were identified from glacioterrestrial sediments by the presence of Na. Application of geochemical approaches to the genetic interpretation of ancient sequences has been both successful (Frakes and Crowell, 1975; Sumartojo and Gostin, 1976) and ambiguous (Howarth, 1971; Jackson and Van de Graaff, 1981). A large literature describes provenance studies in diamict(ites) (e.g. Shilts, 1976; Nystuen and Saether, 1979) whilst mineralogy and geochemistry have been used to infer plate tectonic setting (Bj~rlykke and Englund, 1979), plate motions and changing climate in source regions (Chapuis, 1974; Nesbitt and Young, 1982). Anderson et al. (1984) show how the mineralogy of diamicts across the Ross Sea continental shelf can be traced back to source areas allowing ice flow lines to be reconstructed. This reconstruction corroborates the interpretation of diamicts as basal tills and not glaciomarine diamicts.

42 Many workers describe the development of clast fabrics in tills and in sediment gravity flows (Mark, 1973; Walker, 1975, 1979; Lawson, 1979; J. D. Collinson and Thompson, 1982). The distinct " r a n d o m " clast fabric held to be typical of glaciomarine (or glaciolacustrine) diamicts formed by ice-rafting and suspension deposition is probably an oversimplification (e.g. see review by Nystuen, 1976 and Domack and Lawson, 1985). Domack (1983) for example, argued that substrate hardness (in effect grain-size) determines whether ice rafted clasts assume an orientation with a/b planes horizontal or at a high angle. A particular problem is that clast fabrics are very difficult to identify in both cores and ancient deformed sequences, though pebble fabrics can be sometimes determined by X-ray analysis of cores (e.g. Domack, 1982). Emphasis on clast shape (e.g. distinct glacial "flatArons") to identify till(ite)s is misplaced unless it can be shown that such clasts have a direct glacially-induced orientation and are associated with other criteria identifying direct glacial deposition at an ice base. Similar remarks apply to the use of supposedly distinct "glacial" grain shapes and surface textures (Folk, 1975; Bull et al., 1980; Bull, 1981). Because these shapes and textures are frequently preserved during later transport and redeposition events, their use is limited in establishing a final depositional process though they can aid in interpreting a glacial origin for the debris (e.g. Miall, 1985, illustrated faceted clasts in debris flows within the Early Proterozoic Gowganda Formation). Studies of the micro-morphology of "tills" (i.e. either grain orientation or matrix fabric) was popular some time ago (e.g. Sitler, 1968; Evenson, 1971). An urgent need is to establish micro-morphological types for different diamicts by combined studies of sedimentology and fabric (e.g. Nystuen, 1976) similar to that for other marine sediments that have received detailed attention either because of geological engineering problems or with regard to hydrocarbon reservoir characteristics (e.g. Curtis et al., 1980; Brand and Brenner, 1981; Bull, 1981; Quigley, 1983). The use of palaeomagnetic remanence characteristics to discriminate between lodgement till (magnetic particles dispersed by subglacial shear) and diamicts deposited through a water column (unhindered orientation of magnetic particles) offers good possibilities as a genetic tool in Late Cenozoic sediments (N. Eyles et al., 1983c) but like the use of anisotropy of magnetic susceptibility to measure grain fabrics awaits further testing. A major problem in ancient sequences is that original palaeomagnetic characteristics are frequently overprinted by diagenetic and metamorphic remagnetization (Stupavsky et al., 1982). Geotechnical characteristics of both onshore and offshore Late Cenozoic sequences in many cases have provided rigorous criteria for genetic discrimination (see Fannin et al., 1979; Ardus, 1980; Sladen and Wrigley, 1983, for reviews and references). Analysis of biotic assemblages (where present) has been particularly useful in zoning distal glaciomarine diamict/mud sequences with respect to distance from the nearest ice margin (e.g. Nelson, 1981; Cooke and Hays, 1982; Osterman, 1982 and references therein; Mode et al., 1983), in estimating

43 age and water depths (Anderson, 1975b; J.D. Collinson, 1978; Domack, 1983; Lagoe, 1983; Kaye, 1984), in identifying displaced shallow water fauna that have either been resedimented into deep water by sediment gravity flow or picked up by an ice margin (Martin and Wilczewski, 1970; Kellogg et al., 1979; Visser, 1983a) and other environmental data (Knoll et al., 1981; Vidal, 1981). The presence of in situ biota in glaciomarine diamicts can perhaps b e overemphasized as a criterion for discrimination from glacioterrestrial sediments because several workers have shown that brackish glaciomarine waters may be inimicable to biota (Lord, 1979). In addition, Anderson (1975a) showed that biota are only selectively preserved in some areas of the Weddell Sea because of a very shallow (500 m) carbonate compensation depth (CCD). Therefore, absence of a marine biota in diamicts of questionable origin does not preclude glaciomarine deposition. It is commonly assumed that distal glaciomarine diamicts deposited in areas where current activity and resedimentation have not been important are characterized by a uniformly unsorted particle-size distribution similar to that of continental tills, reflecting similar glacial sources and lack of sorting (Warnke and Richter, 1970; Barrett, 1975). This assumption can be questioned because the texture of continental tills is strongly variable according to bedrock type and substrate relief (see above). Glacially ice-rafted debris may also retain the considerable textural diversity that is characteristic of englacial debris in the ice base (e.g. Lawson, 1981), which is mixed and eliminated by lodgement till deposition (N. Eyles and Menzies, 1983) and which thus goes unrecognized by studies using grain-size characteristics of the final till body. D. L. Clark et al. (1980, p. 39) have independently argued that non-sorted textures are not necessarily indicative of unaltered ice-rafted glacial sediment in the Arctic Ocean. In general, the use of criteria such as texture, fabric, compositional immaturity of sand fraction (e.g. Table 3 in Slatt and Eyles, 1981), clast shape, clast frequency, microfauna and geochemistry to establish diamict genesis is circumscribed given that these data predominantly reflect source characteristics rather than mode of sediment deposition. For example, Kellogg et al. (1979) argued that fine-grained diamicts from the Ross Sea Antarctica, interpreted as distal glaciomarine by Barrett (1975), were deposited as tills below a grounded ice shelf on the basis of grain-size similarities to Ohio tills, massive character, presence of abraded clasts and reworked diatoms and foraminifera. These characteristics however, reflect sediment source and are insufficient to interpret details of deposition. As a general comment, realistic interpretation of diamict genesis cannot be made until data pertaining to vertical and lateral lithofacies relationships and sequence context together with sediment body geometry are available from detailed sedimentological logging. Most glacial depositio~ual models are still based on highly generalized descriptions of sediment stratigraphies. In many cases suitable material (e.g. long drill core) has become available (e.g. DSDP, geotechnical and industrial investigations) but a detailed facies approach remains to be adopted.

44 IT. APPLICATION TO ANCIENT GLACIAL SEQUENCES

Having reviewed models of glaciomarine deposition i n a range of environmental settings it is now appropriate to describe three ancient glacial sequences ranging in age from Early Proterozoic to Pleistocene. Detailed accounts are given elsewhere and the object here is to illustrate the application of a sedimentological approach to the study of ancient glacial sequences and to identify characteristic features of glaciomarine sequences.

1. Early Proterozoic Gowganda Formation, Northern Ontario, Canada; distal glaciomarine sedimentation close to a divergent continental margin Between about 2.4 and 2.2 Ga there was a major phase of glaciation across northern North America. The Gowganda Formation, according to Young (1970, 1973) is one of several giaciogenic units deposited around an ice centre located in the central Canadian Shield, and is probably the best known and best exposed of these Early Proterozoic units. It occurs throughout the Huronian Basin of northern Ontario, an area which, according to a recent review of palaeomagnetic data (Irving and McGlynn, 1981) probably lay at a high latitude at this time. The formation has a maximum thickness of about 1 km in most areas and contains numerous bouldery diamictite beds and argillite units. These have long been interpreted as glacigenic in origin (A. P. Coleman, 1907) but, until recently, were regarded as lodgement tills, interbedded with lacustrine varved i

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Fig.5. Distribution of Gowganda Formation. Letters refer to principal locations described in text: A = Bruce Mines-Ironbridge (Frarey, 1977; Miall, 1985); B = Elliot Lake (Miall, 1985); C = Whitefish Falls (Young, 1981); D = Cobalt (Legun, 1981; Donaldson and Munro, 1 9 8 2 ) ; E = Rat Mountain (Miall, 1983a); F = Kenogami (Long and Leslie, 1985).

Fig.6. Lithofacies and stratigmphic architecture of the lower part of the Gowganda Formation, as reconstructed from drill holes near Elliot Lake, Ontario. Lithofacies code as in Fig.15.

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46 argillites (Lindsey, 1969). An area of glacial marine sedimentation was recognized only in the southern part of the basin (Fig.5; area C; Lindsey, 1971; Young, 1981), close to a possible continental suture (Sims et al., 1980). Recent work has focussed on careful lithofacies analysis, and it has been recognized that most of the formation is subaqueous in origin (Legun, 1981; Miall, 1983a, 1985; Long and Leslie, 1985). Some exposures of continental facies may occur in the Cobalt area (Fig.5; area D), where mapping shows a highly irregular contact between the Gowganda and underlying Archean rocks, indicating that initial deposits of the Gowganda Formation infilled a residual erosional relief (Legun, 1981; Donaldson and Munro, 1982). Donaldson and Munro (1982) identified regolith zones at the Archean-Gowganda contact, which they suggested are the product of subaerial freeze--thaw and downslope creep. Some of the typical lithofacies of the Gowganda are illustrated in Figs.6 and 7. They include a variety of sediment gravity flows, ranging from disorganized to graded-stratified bouldery debris flows (Fig.7A), massive sandy fluidized flows (Fig.7E) and Bouma-type turbidites (Fig.7D). The latter are relatively uncommon. They typically occur as isolated, thin units interbedded with argillites. The argillites contain thin streaks and lenses of silt and sand, diamict clots and dropstones (Fig.7G). Lindsey (1969) suggested that, in parts of the basin these beds were "varvites" implying a lacustrine environment and a seasonal depositional control. However, recent re-examination by Miall (1983a, 1985) and Long and Leslie (1985) has concluded that none of the beds are true rhythmic varves. Most are thin "distal" turbidites; others contain small scale ripple marks and other evidence of reworking by traction currents. Another important lithofacies is massive to faintly bedded diamictite consisting of sparse to abundant clasts, up to boulder in size, randomly interspersed in a sand--silt--mud matrix (Fig.7C). Some of the thinner units are interpreted as disorganized debris flows (cf. Walker, 1975), but intervals tens of metres thick were probably deposited as ice-rafted debris and pelagic muds. Where bedding is present contortions are common, suggesting slumping. Rare interbedded sandy sediment gravity flow units commonly are folded and disrupted as a result of loading on a saturated substrate (Fig.7F). Apart from the features described from the Cobalt area, there is no convincing evidence of subaerial sedimentation. Facies analysis has not led to the identification of any lodgement till, and there is no undisputed evidence of any basal striated pavements. The long "fingers" of Gowganda extending northward across Archean basement (Fig.5, areas E and F) have been referred to as palaeovalleys, but there is no evidence that they represent contemporaneous valleys or fiords. For example, they contain no zone of coarse debris along their margins, that would indicate proximity to a coastline (Miall, 1983a; Long and Leslie, 1985). Long and Leslie (1985) concluded: "a deepwater lacustrine environment (for the Gowganda Formation) is more likely than a marine setting as no unequivocal marine influence has been established

47

Fig.7. a - - c . F o r e x p l a n a t i o n see p. 49.

48

Fig.7. d--f. For e x p l a n a t i o n see p. 49.

49

Fig.7. Lithofacies variations in the Gowganda Formation, near Elliot Lake, Ontario. A. Typical debris flow (Drag), showing inverse and normal grading and clast imbrication. Scale is 1 m long. B. Ice-faceted clasts in a debris flow. C. Typical IRD diamictite (Dram). Scale is 1.5 m long. D. Bouma-type sandy turbidite (Sg). E. Dish structures in a fluidized flow. F. Sand unit containing rip-up clasts, deformed by loading into soft matrix of a mud-rich IRD diamictite. G. rhythmites with dropstones (Fld). At base is a sandstone bed showing a flow nose.

in the Huronian Supergroup to date". However, Miall (1983a, 1985, and this paper) argues that a marine basin is more probable on several grounds; lakes o c c u p y only a b o u t 1% o f the present earth's surface (J. D. Collinson, 1978) and are thus relatively unimportant in a geological sense. Lakes of the scale of the Huronian basin (400 × 500 kin) are rare (proglacial lakes of this size on continental interiors are short-lived), and the thickness of the Huronian Supergroup (~ 10 km) is probably thicker than any k n o w n lacustrine deposit. In addition, the Huronian rocks thicken southward toward a probable continental suture (Sims et al., 1980; Zolnai et al., 1984) and the overall architecture of the basin suggests a divergent continental margin, opening to the south toward a palaeo-ocean. The weight of evidence now indicates that the Gowganda Formation is virtually or entirely glaciomarine in origin, but we are a long w a y from establishing a comprehensive depositional systems model for the Gowganda basin. Miall (1983a) identified a massive diamictite association passing downcurrent into a cyclic sandy association in the R a t Mountain area (Fig.5, E), and suggested that the sediments there were formed as subaqueous outwash, possibly on a submarine fan. Channelized diamictites in the Kenogami area (Fig.5, F ) were also attributed to submarine fan deposition by Long and Leslie (1985) In the Elliot Lake area (Fig.5, B) an architectural model erected from drillhole data has been interpreted in terms of a southward prograding

50 continental slope, cut by a submarine channel (Fig.6; Miall, 1985). Coarse bouldery debris flow diamictites occur throughout the basin and there is a problem explaining them unless there were multiple sediment sources, because it is unlikely that debris flows carrying such coarse detritus could travel for m o r e than a few tens of kilometers from the basin margin. Miall (1983a) suggested deposition beneath an ice shelf that was stabilized by ice rises or "pinning points" which might form local debris sources. As discussed elsewhere in this paper, ice shelves probably do not deposit regionally extensive diamict sequences because of rapid exhaustion of basal debris. An alternative model is that most o f the debris was carried into the basin by ice rafting, with some of the resulting debris remobilized as sediment gravity flows by depositional oversteepening or earthquake shock on local submarine slopes. Basement tectonism is known to have controlled sedimentation of the basal Huronian sediments (Roscoe, 1969; Fralick, 1 9 8 5 ) a n d m a y have continued to maintain local basin relief throughout deposition of the Huronian Supergroup. Interbedded argillites record intervals of reduced IRD supply because of reductions in iceberg frequency. This could reflect a decrease in the number of tidewater calving points as a result of climatic amelioration, but local variations in iceberg density may simply reflect the control of drift tracks by marine circulation and wind patterns, as in the m o d e r n Baffin Bay (fig.2 in Andrews and Matsch, 1983). 2. Late Proterozoic Port A s k a ~ Formation, Scotland, distal glaciomarine sedimentation on a subsiding intra-cratonic shelf The Dalradian Supergroup (810--570 Ma) contains a prominent multiple diamictite sequence up to 850 m thick that can be traced over 700 km along strike from western Ireland to northeast Scotland (Fig.8). The Port Askaig Formation (c. 670 Ma) overlies the dolomitized Islay Limestone and is overlain by the stromatolitic subtidal to supratidal Bonahaven Dolomite (Figs.9, 10). The base of the Formation is comformable over its entire 700 km exposure (Max, 1981). The glacial formation marks a major change in the character of Dalradian sedimentation (Anderton, 1982). Underlying units are regionally extensive sequences of sands, muds and carbonates deposited on a slowly subsiding intracratonic shelf (Fig.9). Overlying formations (sands and turbidites) are shelf and basinal facies deposited in deeper fault-bounded basins with lateral thickness variations as a result of syndepositional faulting. Increasing tectonism culminated in eruption of tholeiitic lavas, rifting of the Laurentian and Baltic plates and opening o f the Iapetus Ocean at about 600 Ma (Fig.9). The Port Askaig Formation has attracted m u c h attention as a result of the association of diamictites with dolomites and palaeomagnetic data that indicated low depositional palaeolatitudes. Recent data however, suggests that the dolomites are detrital, reworked from the Islay Limestone, and that the

51

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PoA~'k'i~~ Mull of Oa Garbh

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Fig.8. Distribution of Dalradian Supergroup. Triangles indicate diamictite outcrops of Port Askaig Formation (Argyll Group). Place names: Cleggan (C), Glencolumbkille (Gc), Fanad (F), Mull of Oa (M), Port Askaig (PA), and Garvellach Islands (G).

palaeomagnetic characteristics are the result of metamorphic remagnetization in L o w e r Ordovician time (Stupavsky et al., 1982; Fairchild, 1983; C. H. Eyles and N. Eyles, 1983b) leaving t h e question of depositional palaeolatitude open. Forty-seven diamictite horizons are identified in total (Spencer, 1971), with diamictite units 1--38 exposed on the Garvellach Islands (Fig.10) and units 13--47 on the island of Islay at Port Askaig (Fig.8). A detailed sedimentological log through nearly 500 m of section exposed on Garbh Eileach (Fig.8) is shown in F i g . l l . Individual units have a planar geometry, and in some cases can be identified along strike in both Ireland and Scotland. Erosional basal contacts and lenticular geometries typical of glacioterrestrial till(ite)s are absent as are boulder pavements, and grooved and striated basal

52 GROUPS

FORMATIONS

TECTONIC ENVIRONMENT

CHRONOLOGY (?) Cambrian

Southern Highland

Tayvallich Volcanics

Easdale Slates Argyll

Glaciomarine Sedimentation c.670 Ma

opening of lapetus major volcanism

c.600

c.625

Vendian start of syndepositional faulting

stable subsiding shelf

Appin

Ma

_ _ _

~

faulting produces deep basins

Jura Quartzite Port Askaig = Islay limestone

_ _

f _ ?

c.650

Riphean

Grampian >700 limestones and dolomites

predominantly siltstone

predominantly sandstones

predominantly mudstone

Fig.9. Stratigraphic and tectonic setting of Port Askaig Formation within the context of faulting and subsidence accompanying the opening of Iapetus (from Anderton, 1982). The four stratigraphic groups comprise the Dalradian Supergroup with a total cumulative thickness of c. 25 kin.

surfaces, despite excellent bedding plane exposure throughout the regional outcrop (Max, 1981; Spencer, 1981, this volume). The lowermost diamictites (1--13) have a dolomitic silt matrix and contain less than 1% of extrabasinal clasts (Fig.10). Clasts and matrix are reworked from the underlying Islay Limestone. Granite clasts increase upwards in the remaining units and the matrix becomes quartzofeldspathic. The diamictite units are predominantly massive or stratified with discontinuous sand interbeds, have sharp basal contacts and irregular upper contacts characterized by conglomeratic lags, "sandstone downfold structures" (Spencer, 1971) and polygonal structures and irregular wedges (Figs.ll, 12A). The formation has been interpreted as representing 17 major glaciations with deposition of diamicts by in situ melt-out of basal debris (melt-out till) from the base of a grounded ice sheet close to sea level (Spencer, 1971). Discontinuous sand interbeds with abrupt lateral terminations within certain stratified diamictites were considered to be the result of sub- and englacial stream activity as the ice sheet decayed in situ. The absence of glacially eroded surfaces and subglacial structures has been explained by assuming that the ice sheets had frozen bases (Spencer, this volume). Additional support for the origin of diamictites as "melt-out tills" cited by Spencer (1971) are large dolomite rafts in the "Great Breccia" (diamictite 13; Figs.10, 11, 12B). This unit contains large (up to 320 X 64 X 45 m) dolo-

z:

IONAHAVEN

DOLOMITE

Gc

M

PA

-200m

-0

G

and

some sst

and

39-44

iamictites

45 47

diamict ites

1-12

Diamictites

GREAT BRECCIA

)ISRUPTED BEDS

14-18

t9-32

Diamlctites

t h i c k sst

and

33- 38

)iamictites

hin

S h e l f sst

b

20

rare exotics

% EXOTIC CLASTS

/o

~ig.10. a. Stratigraphic section approximately parallel to depositiona! strike, showing variations in thickness of Port Askaig Formation ~etween Cleggan (C), and Garvellach Islands (G); see Fig.8 for other locations. Romain numerals at left refer to members identified ~y Spencer (1971). Diamictites 1--12 constitute member I. b. Percentage exotic clasts (Rapakivi granite and gneiss) in diamictites. ~'rom C.H. Eyles and N. Eyles (1983a).

9"

"

54 Sr Sh

Sm

St

r.,.

Sm

St~,Sh Sr,$h

S, Sh,Sd S~

FI Dms

Sh,Sg St,Sh 3e

SI 18

Dmm

Dram Dins

Oml FI

Sh Dms

Sh,Sd Sm Sh,Sd

Sh.S~ 37

14-17

Omm

DEs

Sp,Sh Sh,sd

DISRUPTED BEDS 12e-,om)

FI St Dmm

LOWER DOLOMITE (~l-10m)

Sm $'.Sp

Dms

St.Sd 13

Sh. Sp

GREAT BRECCIA (~m)

36

Dram Sd

12

Dmm TI

$1

Din, Pmm 10

.

-,

Dcl Dram 35

~'nm

Sh.Sp st,Sp Sd

9 Ores

sh, Sp DEs

34 Dms Sr,Sh p ~ r l y expos,~l I

O~m

Sr,St Sh

S~

Dmm

Sh,Sr Sd Om,,Sd

s~,sh

0

N

Dram. Sd Dms Sp,Sh

Sd

Dmm

~:>

Sh.$r Dms

Sh

~ .~.

55

mite blocks and chaotic masses of siltstone and diamictite and the analogy was drawn with the glacially-quarried chalk rafts of East Anglia associated with Pleistocene glaciation. Wedge structures and polygons on the surface of diamictite and siltstone units were interpreted as permafrost in origin and have been employed to define terminations of major glacial episodes (Spencer, 1971). Diamictites are separated by a variety of shallow marine sediments including planar and trough cross-stratified, rippled and horizontally bedded sandstones, some with dewatering structures. Massive and laminated siltstones reported as "varvites", contain dropstones and dropped sandy diamict "clots". At one location (below diamictite 36; Fig.ll) dropstones occur within planar cross-stratified sands. Detrital dolomite sandstones and siltstones associated with diamictites 1--18 show similar characteristics. Many lines of evidence now indicate a distal glaciomarine origin for the diamictites. Diamictite units occur in conformable sequence context with marine sediments and exhibit a planar regional geometry strikingly similar to that of the glaciomarine Yakataga Formation, Alaska (Figs.12C, 13). Substrate structures, contacts, bedforms and associated sediment type and geometries characteristic of deposition by grounded glacier ice (e.g. Deynoux and Trompette, 1976, 1981; Sugden, 1977; Moran et al., 1980; Davis and Mallett, 1981) are conspicuously absent over 700 km of outcrop. The argument by Spencer (1971) that cold-based glaciers and ice sheets do not erode underlying substrates is regarded as incorrect. Examples where modern glaciers have retreated to leave, untouched, a preglacial surface are extremely rare and limited to small areas close to ice margins that either advanced over rigid snow patches or developed in situ by the merging of extensive snow fields. The argument, more fundamentally, omits to examine how such a frozen ice mass would be able to erode and transport debris. It is concluded that the diamictites are not the product of sedimentation in the grounded ice environment. As related elsewhere in this paper, massive fine-grained diamict lithofacies accumulate in distal glaciomarine areas by the processes of deposition of suspended fines and ice-rafting. Traction current activity generates a wide range of interbeds from discontinuous sandy stringers to conglomeratic lags and results in textural sorting and stratification within the diamict (Fig.12D). Sandstone stringers are frequently depressed by ice-rafted clasts (Fig.12E). Irregular pockets and lenticular masses of sandstone with diamictites (e.g. diamictites 19--20, Figs.ll, 12F) are typical of traction current deposited interbeds that have been disrupted by loading (Mills, 1983 and references Fig.11. Vertical profiles (drawn from detailed field logging at a scale of 1:25) of diamictite units 1--38 (540 m) exposed on Garbh Eileach in the Garvellach Islands (see Fig.8). R o m a n numerals refer to members identified in Fig.10. See Fig.15 for lithofacies code. Numbering of diamictite units follows that of Spencer (1971). Note change of scale between diamictite units 12 and 14.

56

F i g . 1 2 . a--c. F o r e x p l a n a t i o n see p . 59.

57

Fig.12. d--f. For explanation see p. 59.

58

Fig.12. g--h. For explanation see p. 59.

59

therein). Evidence for loading of sands and conglomerates into low-strength diamict is very c o m m o n in the Port Askaig Formation (diamictites 19--22, 26, 30, 31/2, Figs.ll, 12G). Irregular polygonal structures, sandstone downfolds, irregular dykes and wedges may relate to syndepositional soft sedm e n t deformation due to subaqueously density-induced loading (Butrym et al., 1964; Anketell et al., 1970; Mills, 1983) possibly aided by earthquake activity during early rifting of the basin (e.g. Waller, 1968; Hesse and Reading, 1978; Mills, 1983; N. Eyles and B. Clark, in press). Seismically induced liquefaction structures are abundant in the overlying Jura Quartzite (Anderton, 1982) resulting from the increasing tectonic instability of the area (Fig.9). Sharp and conformable basal contacts between diamictites and sandstone interbeds record abrupt changes of energy regimes and sediment supply. This may result from punctuated glacio-isostatic or tectonic subsidence and eustatic sea level changes. The upper surfaces of several diamictite units are regionally extensive planar erosion surfaces overlain by conglomeratic lags which are the product of reworking of diamict in shallow water. In Scotland, the diamictite units occur in sequence with shallow marine shelf sandstones; to the SW in Ireland they occur within thicker mudstone sequences either indicating deposition in deeper water or greater volumes of suspended sediment (Max, 1981). Spencer (1971, 1975, this volume) ruled out a glaciomarine origin for the diamict(ites) because of the shallow water depths indicated by intervening sandstone units and subaerial exposure indicated by periglacial structures. As Bj~brlykke (1969) pointed out however, this argument ignores submergence due to isostatic or tectonic causes. Diamictite 14 (the Disrupted Beds; Spencer, 1971; Figs.10, 11, 12H) shows many structures indicative of repeated downslope mass movement. Conformable beds of siltstone, dolomite, dolomitic conglomerate and diamictites with scattered ice-rafted clasts show faults, folds, pull-apart and boudin structures that indicate multiple episodes of downslope creep (Fig.12H). In addition, the underlying "Great Breccia" shows several characteristics of carbonate slope deposition where downslope resedimentation by rockfall, slide and sediment gravity flow occur (Hurst and Surlyk, 1983 and references therein). These characteristics include folding of large rafts in the Fig.12. Lithofacies variation in the Port Askaig F o r m at i o n on the Garvellach Islands, Scotland. A. Polygonal networks of wedges on the surface of diamictite 35 on east coast of Eileach an Naoimh. B. Large dolomite raft in Great Breccia, arrowed figure for scale. West coast o f Eileach an Naohnh. C. Planar geometry of diamictites and shelf sands (units 12--22, see Fig.11) exposed on A'Chuli (foreground) and Eileach an Naoimh (see Fig.8). Compare with Fig.13A. D. Stratified diamictite of unit 26 (Dins); Eileach an Naoimh. Base, below hammer, rests with a sharp, conformable contact on rippled and planar crossstratified sandstones. E. Dms of diamictite 38 on Garbh Eileach. Dropstones penetrate sandy interbeds deposited by traction currents. F. Deformed sandstone bed (Sd; at hammer head) loaded into diamictite (units 19--22; see Fig.11). G. Loaded surface of diamictite 22 Garbh Eileach. H. 18 m section of Disrupted Beds, Garbh Eileach; note boudins and pull-apart structures.

60

(b)

::~

"

e ~ o

1

i ¸

•

o

GuLF

OF

ALASKA

~

Fig.13. A. 1000 m section of glaciomarine diamict, muds and shelf sands of the Yakataga Formation exposed on Kulthieth Mountain, Yakataga District, Gulf of Alaska. B. Northern Gulf of Alaska continental margin and location of Middleton Island (see Fig.14). Broad submarine valleys (SV) connect the intensely glacierized margin to the Aleutian Trench (Piper et al., 1973). The basal part of the Middleton Island sequence (see Figs.15, 16A) may give a clue to sedimentation in these valleys.

61 direction of the inferred basin depocentre, subaqueous reworking of coarse dolomitic debris in the form of conglomerates (some of these may also record sediment gravity flow) and a distinct wedge-shaped regional geometry (Fig.10) as a result of syndepositional tectonics. The Port Askaig diamictites illustrate very clearly the importance of detailed genetic studies of glacial rocks to tectonic and palaeogeographic reconstructions. The palaeogeography of the North Atlantic region during Port Askaig time is not well known but is thought to be one of sagging intra-cratonic basins to the NW of an incipient Iapetus (Anderton, 1982) 1. The petrology of the formations below the diamictites indicates a sediment source to the NW whereas that of the diamictites indicates a southern glacial source area for the Port Askaig Formation. Consequently, Anderton (1980) ruled out the existence of an extensive Iapetus Ocean to the SE of the Port Askaig area at this time given the inability of ice sheets to cross large water bodies. A glaciomarine origin for the diamictite sequences helps resolve this palaeogeographic reconstruction. No structures associated with glacier traction are available to indicate either ice sheet movement or its direction. As related above episodes of folding ("ice push folds" of Spencer, 1975) are more likely related to tectonically triggered downslope movement (Figs.9, 11, 12) 2. Given the importance of tectonic subsidence on Dalradian sedimentation (Fig.9; Harris et al., 1978) the deposition of repeated distal glaciomarine diamicts and shelf sands is likely to have been controlled only in part by direct glacial influence. To argue that individual diamictite units of the Port Askaig Formation each records a separate regional ice advance is questionable and illustrates the difficulties of interpreting glacial advance/ retreat cycles from multiple diamict(ite) sequences where a number of controls, other than simple ice fluctuations, influence evolution of stratigraphic sequences. Simple attempts to identify "Milankovitch-type" climatic fluctuations directly from such sequences (e.g. Harland, 1981; Hambrey, 1983) are especially problematical. 3. Late Cenozoic Yakataga Formation, Alaska; distal glaciomarine sedimentation on a convergent plate margin Ghcial sedimentation began in the Gulf of Alaska Tertiary Basin (Fig.13) about 20 Ma in response to uplift along the Pacific Border Ranges (Plafker, 1981). The tectonic setting of the Yakataga Formation is that of an accre1 The formation coincides with a dramatic increase in tectonic activity marking a change from predominantly shallow marine shelf environments to fault-bounded deeper water basins (Fig. 9). It seems likely, on the basis o f petrographic data alone (Anderton, 1982), that the formation records glacially-influenced marine sedimentation on a subsiding shallow marine shelf adjacent to a southern glaciated landmass perhaps marginal itself to an incipient Iapetus Ocean; an analogous tectonic setting is provided by the example of the North Sea Basin relative to the Atlantic Ocean (Harris et al., 1978).

62 tionary arc complex in a convergent plate margin setting (Bruns and Schwab, 1983; Bruns, 1983). The formation has an aggregate outcrop thickness of nearly 5 km and is argued to be the largest and most complete sedimentary record of Late Cenozoic glaciation in the world (Plafker and Addicott, 1976). It has been the subject of regional mapping for hydrocarbon exploration (D. J. Miller, 1971) but despite good exposures around the Gulf coast no detailed descriptions are available apart from Armentrout (1983) and Lagoe (1983). The upper part of the formation, dated by palaeomagnetism and nannoplankton at about 2 Ma, is exposed on Middleton Island, a small island (8 X 2 km) close to the southern edge of the continental shelf and the adjacent Aleutian Trench axis (Figs.13, 14). Over 1.25 km of formation thickness is exposed on cliff sections and wave cut platforms uplifted during the 1964 earthquake (Prescott and Lisowski, 1977). The following is only a brief description of this offshore exposure and a comprehensive treatment of the Y akataga Formation is presented elsewhere. Figure 14 shows a planar geometry of outcropping diamicts, sands, muds and laminated silts that dip to the W at between 25 and 28 °. The base of the exposed succession is a thick package of channelized diamicts and graded gravels (Figs.15, 16A). Diamicts generally have a silty sand matrix with a clast content that varies from < 1 0 / m 2 (clasts greater than 1 cm diameter) to 263/m ~. Clast clusters, with clasts up to 5 m in diameter are common throughout; many clusters are composed of angular debris of similar lithology. Unbroken molluscs are locally common and marine microfossils are abundant (Plafker and Addicott, 1976). Stratified diamicts show textural banding, pebble bands and discontinuous sandy stringers and pockets which are frequently affected by soft sediment deformation. Sand dykes, stringers and irregular wedges emanate from sand interbeds and show very similar characteristics to those seen in the Dalradian of Scotland (see above). Stratification is often contained within broad shallow channels. Basal contacts of the diamicts are conformable with either massive and laminated muds or other diamicts having different matrix textures or clast content. Upper contacts are either transitional into massive muds, m u d d y sands with burrow structures (Diplocraterion?) or sharply overlain by channelized laminated muds with dropstones or stratified diamicts with the contact marked by concentrations of clasts or shell fragments. In places the contact between conformable diamict lithofacies is loaded. Of particular importance are extensive boulder pavements (Figs.14, 16) and shell bands (coquinas; Fig.16) that occur repeatedly through the succession (Fig.15). The former are usually composed of single large clasts (up to 3 m in diameter) aligned as a discrete horizon underlain and overlain by massive diamict that may contain large pockets of loaded silty sands. Individual clasts have a planar upper surface showing a consistent striation direction from the N (Fig.13, 16). Few clasts show the characteristic "bullet" shape that identifies transport in the basal traction zone of grounded glacier ice (Boulton, 1978; Fig.16). Individual pavements can be traced along the whole length of the island (Fig.14).

Fig.14. Middleton Island. Note planar outcrop and geometry of glaciomarine units of the Yakataga F o r m a t i o n exposed on wave cut platforms. Units dip to NW at 25--28 °. BP ffi outcrop of typical boulder pavement (e.g. Figs.16C, D). Q = outcrop of typical coquina (e.g. Fig.16B). National Oceanographic Administration photographs.

o~ co

64

OC

OB OA

Ot

02 rrl 0

,oot L I T H O F A C I E S CODES

B.P B.P B.P

DIAMICT

=

D

Dm -

=

m a t r i x supported

Oc -

=

clast supported

D - m

=

rues=ire

D - s

=

stratified

O-

=

graded

SANDS

=

S

Sm

=

massive

Sh

=

horizontal lamination

Sr

=

ripples

St

=

trough cross bedding

Sp

=

planar cross bedding

Sg

=

graded

Sd

=

with soft sediment

g

SYMBOLS FOR LOGS

DIAMICT m a t r i x supported stratified with sand interbeds clast supported SAND

ripples trough cross bedding

M U D massive laminated with dropstones

Q

deformation structures SSA-SdWU~aA~

coquina band

B.P. boulder pavement

F I N E - G R A I N E D (MUD) = F CONTACTS

FI

=

laminated

Fm

=

massive

erosioMI

F-d

=

with dropstone=

conformable toeded

65

Coquinas similarly form extensive surfaces within the succession and comprise large bivalves, gastropods, brachiopods, worm tubes and barnacles in both living and reworked positions. The beds have a matrix of either calcareous sand or m u d and overlie gravel lags (Fig.16B). Ice-rafted clasts are particularly abundant within the coquinas b u t overlying muds are clast poor. The base of the succession on Middleton Island consists o f multiple units of graded diamicts, gravels and sands which o c c u p y channels up to 150 m wide. Various grading characteristics can be identified and occasional outsized clasts are present. In a classic paper D. J. Miller (1953) demonstrated that the diamicts have been deposited on a shallow marine shelf by combinations of suspension deposition, ice-rafting and highly variable traction currents. He proposed the term "Yakatagite", to describe the diamicts. Plafker and Addicott (1976) after reviewing the available micro- and macrofaunal data concluded that deposition occurred in water depths of less than 100 m at a rate of about 1 m/Ka (Table I). Allison (1978) argued from analysis of molluscan assemblages that deposition occurred in water depths up to 136 m. Distinctive clusters of angular clasts of similar lithology can be identified as former valleyside debris released from floating ice (see Fig.l). The boulder pavements that sit within undisturbed diamict cannot be explained as the p r o d u c t of lodgement processes beneath glacier ice as they lack distinctive "basal" shaping and are n o t associated with deformed diamict. A possible key to their origin is the observation of Molnia and Carlson (1978) of lag boulder surfaces on shallow water bank areas in the present Gulf of Alaska. Boulder surfaces lying on a low relief submarine bank may have been abraded by ice shelves that are known to have repeatedly reached the edge of the continental shelf (Plafker, 1981; see above). The coquinas record depositional hiatuses and periods o f increased shelf circulation when higher energy regimes prevented suspension deposition but ice rafting continued. These horizons m a y thus represent lowered sea levels or episodic storms and lowering of wave base (Bloos, 1982; Kaye, 1984). Extensive loading o f diamict upper and lower surfaces and the many types of sand injection features indicate low strength substrates and episodically high pore water pressures. The region is tectonicaUy hyperactive and seismic shock may initiate such deformation processes (Waller, 1968; Page, 1975). Basin fills of laminated silts and clays that overlie diamicts (e.g. Fig.15) are o f turbidite origin. The graded and channelized gravel and diamict sequence at the base of the exposed section on Middleton Island is interpreted as the infill of submarine channels in which sediment gravity flow processes have predominated Fig.15. Vertical profile (drawn from detailed field logging) through 1.25 km (corrected for dip) of glaciomarine diamicts on Middleton Island. Lithofacies code and symbols from N. Eyles et al. (1983a).

66

~i~ii!i~i~i~i

~ii~

iiiiiii!ii!~! il !

~

~

~

i.... ii

Fig. 16. a--b. For explanation see p. 67.

~

~ ~ ~

~

~

~

iI i ~

i i ~I~ i il i

67

ii~iiiiii~ !i~ il i i~

Fig.16. Typical lithofacies within the Yakataga Formation, Middleton Island, Alaska. A. ChanneUzed sediment gravity flows at base of sequence. B. Coquina band on east coast. C. Boulder pavement. D. Faceted and striated upper surfaces of boulders within a boulder pavement.

68 (C. H. Eyles, 1985). The sequence may be representative of sedimentation in the broad channels that cross the Gulf of Alaska continental shelf from large outlet glacier and delta sources on the adjacent coastal margin (Fig.13; Ness and Kulm, 1973; Molnia, 1981; Carlson et al., 1982). The Late Cenozoic Yakataga Formation is of especial interest to the interpretation of ancient continental margins because of its thickness, extent and age; similar depositional processes continue in the area at the present day. The basin provides a good model of distal glaciomarine sedimentation and is the only example known to us of an accessible glaciomarine sequence formed in a convergent plate margin setting. FUTURE PROSPECTS Much new work is now being published on continental margin geology using seismic, sonar and drill-core data to investigate m o d e m shelves and slopes (e.g. Stanley and Moore, 1983). If glaciomarine environments can be categorized simply as special, glaciaUy-influenced types of continental margin environments (e.g. continental shelf, slope and rise and associated canyons, and basin plain) an exciting possibility seems to be emerging that this wealth of marine geological and geophysical information may form a firm basis for facies model studies and numerous analogues for ancient sequences. Consequently the sedimentological models for glaciomarine sedimentation presented in this paper are to be regarded as a first step in this process and invite further modification as knowledge of continental margin geology expands. The stratigraphic architecture of glaciated continental margins is clearly a fertile field for research. Much useful data are starting to accrue from regional seismic investigations but it remains to be seen whether the rather coarse resolving power of the seismic method can provide precise data about the complex stratigraphy of the late Cenozoic. A combination of deep-tow shallow-penetration seismic techniques, side-scan sonar and coring seems likely to provide the best results. ACKNOWLEDGEMENTS The Glaciated Basin Research Group at the University of Toronto is supported by grants from the Natural Science and Engineering Research Council of Canada, Mobil Oil, Geological Society of America, American Association of Petroleum Geologists, Canada Works and the University of Toronto. We wish to thank J. T. Andrews, M. Deynoux and P. Fralick for their critical comments on earlier drafts of the manuscript. W. A. "Buck" Braun and L. "Curt" Curtin of the Federal Aviation Administration in Anchorage, Alaska, and Lachlan McLachlan of Cullipool, Scotland, contributed valued logistical assistance. Field discussions with B. Clark, B. Kaye, P. Fralick and A. Waheed were very helpful. We are particularly indebted to

69

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