Ocean Drilling Program Leg 178 (Antarctic Peninsula): sedimentology of glacially influenced continental margin topsets and foresets

Marine Geology 178 (2001) 135±156

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Ocean Drilling Program Leg 178 (Antarctic Peninsula): sedimentology of glacially in¯uenced continental margin topsets and foresets Nicholas Eyles a,*, James Daniels b, Lisa E. Osterman c, Nicole Januszczak a a

Environmental Earth Sciences, Department of Geology, University of Toronto, 1265 Military Trail, Scarborough, Ont., Canada M1C 1A4 b School of Earth Sciences, University of Melbourne, Parkville 3052, Vic., Australia c US Geological Survey, MS955 Reston, VA 20192, USA Received 11 January 2000; accepted 14 May 2001

Abstract Ocean Drilling Program Leg 178 (February±April 1998) drilled two sites (Sites 1097 and 1103) on the outer Antarctic Peninsula Paci®c continental shelf. Recovered strata are no older than late Miocene or early Pliocene (,4.6 Ma). Recovery at shallow depths in loosely consolidated and iceberg-turbated bouldery sediment was poor but improved with increasing depth and consolidation to allow description of lithofacies and biofacies and interpretation of depositional environment. Site 1097 lies on the outer shelf within Marguerite Trough which is a major outlet for ice expanding seaward from the Antarctic Peninsula and reached a maximum depth drilled of 436.6 m below the sea ¯oor (mbsf). Seismic stratigraphic data show ¯at-lying upper strata resting on strata that dip gently seaward. Uppermost strata, to a depth of 150 mbsf, were poorly recovered, but data suggest they consist of diamictites containing reworked and abraded marine microfauna. This interval is interpreted as having been deposited largely as till produced by subglacial cannibalization of marine sediments (deformation till) recording ice sheet expansion across the shelf. Underlying gently dipping strata show massive, strati®ed and graded diamictite facies with common bioturbation and slump stuctures that are interbedded with laminated and massive mudstones with dropstones. The succession contains a well-preserved in situ marine microfauna typical of open marine and proglacial marine environments. The lower gently dipping succession at Site 1097 is interpreted as a complex of sediment gravity ¯ows formed of poorly sorted glacial debris. Site 1103 was drilled in that part of the continental margin that shows uppermost ¯at-lying continental shelf topsets overlying steeper dipping slope foresets seaward of a structural mid-shelf high. Drilling reached a depth of 363 mbsf with good recovery in steeply dipping continental slope foreset strata. Foreset strata are dominated by massive and chaotically strati®ed diamictites interbedded with massive and graded sandstones and mudstones. The sedimentary record and seismic stratigraphy is consistent with deposition on a continental slope from debris ¯ows and turbidity currents released from a glacial source. Data from Sites 1097 and 1103 suggest the importance of aggradation of the Antarctic Peninsula continental shelf by till deposition and progradation of the slope by mass ¯ow. This may provide a model for the interpretation of Palaeozoic and Proterozoic glacial successions that accumulated on glacially in¯uenced continental margins. Published by Elsevier Science B.V. Keywords: Ocean Drilling Program; Antarctic Peninsula; Continental margin topsets and foresets; Diamictite; Deformation till; Debris ¯ow * Corresponding author. Tel.: 11-416-287-7231; fax: 11-416-287-7204. E-mail addresses: [email protected] (N. Eyles), [email protected] (J. Daniels), [email protected] (L.E. Osterman), [email protected] (N. Januszczak). 0025-3227/01/$ - see front matter Published by Elsevier Science B.V. PII: S 0025-322 7(01)00184-0

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1. Introduction Scouring of continental surfaces by large ice sheets releases large ¯uxes of sediment to marine basins. This results in accelerated growth of continental shelves and slopes (Piper et al., 1990; Boulton, 1990; Syvitski, 1991; Eyles et al., 1991; Gipp, 1993; Bart and Anderson, 1995; Clausen, 1998; Solheim et al., 1998; Steckler et al., 1999). In Antarctica, the Antarctic Peninsula Paci®c continental margin (APPcm) has experienced very rapid Late Cenozoic growth in response to repeated expansion of the Antarctic Ice Sheet to the shelf edge (Larter and Barker, 1989; Larter and Cunningham, 1993; Bart and Anderson, 1995; Barker et al., 1999; Anderson, 1999). In broad terms, the stratal geometry of the APPcm consists of uppermost large-scale `topset' strata, recording aggradation of the shelf, and underlying clinoform `foreset' strata recording progradation of the continental slope (Vanneste et al., 1994; Bart and Anderson, 1995; Vanneste and Larter, 1995). This structure closely resembles that identi®ed elsewhere around Antarctica (e.g. Bart and Anderson, 1995; Vanneste and Larter, 1995; Nitsche et al., 1995; ten Brink et al., 1995; Larter et al., 1995; De Santis et al., 1995; Brancolini et al., 1995). The same structure is evident on other glaciated margins in the northern hemisphere (Larsen et al., 1994; Clausen, 1998). There have been several attempts to understand the evolution of glaciated continental margins by modeling the interplay of tectonics, glacio-isostatic and eustatic sea level variation and sediment supply (e.g. Boulton, 1990; Gipp, 1993; ten Brink et al., 1995; Steckler et al., 1999). Such models are based on substantial seismic, age and sedimentary databases. In contrast, understanding and modeling of the evolution of the Antarctic continental margin is retarded, despite considerable seismic data, by a lack of information regarding subsurface stratigraphy, age and depositional processes (see discussions in ten Brink et al. (1995) and Bart and Anderson (1995)). Better geological sampling of the Antarctic margin is needed and this is the principal objective of the Antarctic Offshore Stratigraphy project (ANTOSTRAT) centered around the efforts of the Ocean Drilling Program and the Cape Roberts (Ross Sea) drilling program (see Barker and Cooper, 1998).

The purpose of this paper is to present detailed lithofacies and biofacies data from the APPcm that helps constrain depositional processes involved in growth of the continental margin. A dominant seismic stratigraphic component of the continental shelf of the Antarctic Peninsula is acoustically chaotic and transparent seismic facies (Figs. 1 and 2). These have been interpreted as tills deposited under or at the margin of an ice sheet extending to the shelf edge (Pope and Anderson, 1992; Bart and Anderson, 1995; Canals et al., 2000). Ground truthing of the sedimentology and origin of these seismic facies has hitherto, been limited to short (,5 m) piston cores (Pope and Anderson, 1992; Pudsey et al., 1994). Collection of sedimentological data from acoustically chaotic transparent seismic facies was one of the principal objectives of Ocean Drilling Program Leg 178. 2. Ocean Drilling Program Leg 178 and methodology In early 1998, Leg 178 of the Ocean Drilling Program drilled nine sites across the APPcm from the continental rise to the mid-shelf (Fig. 1) and collected a total of 1806 m of core. One of the objectives was to identify and compare sedimentary records preserved in shelf topsets, slope foresets and hemipelagic sediment `drift' deposits found on the continental rise. Three sites (1095, 1096 and 1101) were drilled in deep water (3±4 km) on the continental rise where sedimentation has been dominated by distal ®ne-grained turbidity currents sourced from the adjacent continental slope (Rebesco et al., 1996). These sites record glacial/interglacial cyclicity back to 9 Ma, details of which are reported elsewhere (Barker et al., 1999). Shelf sites (1097, 1098, 1099, 1100, 1102, 1103; Fig. 1) were drilled in water depths between 400 and 500 m. Drilling was slow and dif®cult because of interruptions by icebergs, by limitations on drilling imposed by ship heave in heavy swells and large boulders on the iceberg-scoured sea ¯oor. This paper focuses on sedimentological descriptions of core recovered from two continental shelf sites 1097 and 1103 (Figs. 1 and 2). Overall recovery was poor (Fig. 3). Cores are dominated

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Fig. 1. Location of ODP Leg 178 drill sites along the Antarctic Peninsula Paci®c margin. This paper describes core recovered at Sites 1097 and 1103 (Rebesco et al, 1998; Barker et al., 1999). Location of seismic lines PD4 (Fig. 2A) and 195-152 (Fig. 2B) are also shown.

by diamictites which were described using formal descriptive lithofacies schemes (e.g. Eyles et al., 1983). Matrix ®nes from representative facies were systematically processed for marine microfossils (Osterman et al., 2001). In many cases where no core was recovered, sediment was retained by the

core catcher and was sampled for biofacies. In addition, physical sediment properties such as porosity, water content, density and P-wave velocities were also collected (Shipboard Scienti®c Party, 1999a,b); additional constraints on sediment origin are provided by seismic stratigraphic data.

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Fig. 2. Acoustic stratigraphy at Leg 178: (A) Site 1097 and (B) Site 1103 showing ¯at-lying topsets and underlying more steeply dipping foresets; reproduced from Bart and Anderson (1995) and Barker et al. (1998). See Fig. 1 for location of pro®les. Seismo-stratigraphic units S1 (topset) and S2, 3 (foresets) are from Barker et al. (1998).

3. Site 1097 Site 1097 is located on the outer continental shelf in 552 m water depth, some 14 km from the shelf edge within Marguerite Trough which crosses the continental margin (Fig. 1). Rotary drilling at Site 1097 reached a depth of 436.6 m below the sea ¯oor (mbsf) using a rotary±core±barrel (RCB) method. Recovery was poor in the upper 80 m where sediment is loosely consolidated but a total of 57 m was recovered from the lower part of the hole (14% recovery) where strata are more consolidated (Fig. 3). Single and multichannel acoustic stratigraphic pro®les oriented perpendicular to the shelf edge through Marguerite Trough show a wedge-shaped succession

of seismic re¯ectors. These dip gently seaward from a structural mid-shelf high some 30 km landward of Site 1097 (Fig. 2A). In contrast to the situation at Site 1103, a well-developed topset/foreset architecture cannot be identi®ed at Site 1097 (Fig. 2). Nonetheless, uppermost seismic re¯ectors are ¯at lying and rest on gently dipping re¯ectors that steepen seaward below the outer shelf at depth. 3.1. Age of drilled interval at Site 1097 Age dating of the ®rst 80 m of the recovered section at Site 1097 is not possible because of the predominance of diamictite facies that are barren or contain reworked microfossils (Fig. 3). In addition,

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Fig. 3. Summary ®gure for Sites 1097 and 1103 showing core recovery and dominant lithofacies types. Biostratigraphic age control is provided by diatoms and radiolarian assemblages. See text and Fig. 4 for de®nition of foram-iniferal biofacies.

barren sections for foraminifera, diatoms and radiolarians occur below this depth. Age constraints below 80 mbsf at Site 1097 are provided by diverse assemblages of diatoms. These are typical of the Fragilariopsis barroni and the upper part of Thalassiosira inura Zones (Fig. 3). Radiolarian assemblages include species typical of glaciated continental shelves (e.g. Spongotrochus glacialis, Spongopyle osculosa and Porodiscid spp.). Several species are consistent with an age in the Upsilon Zone (e.g. Helotholus vema, Prunopyle titan and Lampromitra coronata) (Shipboard Scienti®c Party, 1999a; Osterman et al., 2001). The radiolaria below 218±289 mbsf in the Tau Zone (Fig. 3) suggests an age range of 4.85±5.6 Ma. Regardless of this uncertainty regarding the age of the middle section at Site 1097, it can be concluded from the biofacies

data that recovered strata at Site 1097 are no older than Late Miocene to Early Pliocene (Fig. 3). What follows is a description of the principal lithofacies and biofacies recovered and an interpretation (Fig. 4) of depositional conditions. 3.2. Lithofacies 3.2.1. Massive diamictite (Dmm) The term diamictite refers to poorly sorted and lithi®ed admixtures of clasts (de®ned as larger than sand-sized) and matrix ®nes. Massive, matrixsupported diamictite (facies Dmm; Figs. 3, 5 and 6A) represents the most common sediment type identi®ed at Site 1097 comprising some 65% of the total cumulative length of core recovered. The maximum recovered interval through such facies is 3 m (Core 37;

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Fig. 4. Environmental interpretation of lithofacies and associated biofacies recovered from Sites 1097 and 1103.

Figs. 3 and 5A). Matrix grain size is highly variable and varies from a silty mud (10±20% sand, 50±75% silt, and 10±40% clay) to a sandy silty clay (up to 30% sand). Matrix color varies from dark gray (5Y 3/1) to green (5GY 3/1). Clast contents vary from clast-rich (.20% gravel) to poor (10±20%) including large boulders at least 50 cm in diameter. The frequent occurrence of freshly fractured pieces of the same lithology also denotes large boulders and may be typical of poorly recovered intervals. Clast lithologies include volcanic (basalt, volcaniclastics, andesite, and rhyolite) and plutonic igneous rocks (gabbro, granite, and diorite) all derived from the Antarctic Peninsula (e.g. Pope and Anderson, 1992; Elliot, 1995). The mineralogy of the sand and silt fraction is dominated by quartz (30±80%) and feldspar (5±20%) with minor lithic fragments, mica and hornblende. Trace amounts of tephra occur (Core 13) and manganese micronodules are also present (Cores 10 and 12). With a single exception (Core 44; see below), massive diamictite facies at Site 1097 contain reworked and abraded diatoms, foraminifers, and sponge spicules (see below). In Core 44, at a depth of 362 mbsf, massive diamictite with moderately

well-preserved marine microfossils and burrows (facies Dmm), is transitional upwards into bioturbated sandy mudstone (facies Fmd) containing dropstones (Figs. 3 and 5B). 3.2.2. Graded and strati®ed diamictite (Dmg, Dms) These facies show a distinct bedding and internal structure created by size sorting of clasts and account for 10% (about 5 m) of recovered sediments at Site 1097. Graded diamictite facies (facies Dmg; Figs. 5 and 6B) show either an upward decrease in clast size (normal grading) or increase (inverse grading). Core 27 shows a normally graded diamictite bed up to 40 cm thick capped by bioturbated muds containing dropstones, well-preserved sponge spicules and in situ marine microfossils (Fig. 5A). The graded diamictite bed rests with a sharp erosive contact on the ®negrained top of the underlying bed and contains a clast of altered volcanic tephra. Strati®ed diamictite facies in Core 25 (facies Dms; Fig. 5A) are de®ned by a crude bedding imparted by variation in matrix texture and clast content. These facies are interbedded with thin (,10 cm thick)

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mudstones (facies Fl; Fig. 5A) containing an in situ marine microfauna and ¯ora. 3.2.3. Mudstone (Fm, Fl) Mudstone facies account for 25% of recovered sediment at Site 1097. Facies are either massive or weakly laminated (Fig. 6C) and range in texture from silty clay (e.g. Core 36; 10% sand, 40±50% silt, and 30±50% clay) to clayey silt (e.g. Core 34; 5% sand, 25±69% silt and 30±75% clay; Fig. 5A; Shipboard Scienti®c Party, 1999a). Mudstones are bioturbated and contain dropstones and shell fragments. In Cores 34±36, the content of ice-rafted clasts in several thin intervals is suf®ciently high (.10%) to warrant description as diamictite (Figs. 5A and 6D). Core 34 shows slump structures (Fig. 5A). 3.3. Biofacies at Site 1097 At Site 1097, radiolarians, diatoms and foraminifers are all present but relative abundance and degree of preservation varies from sample to sample (Shipboard Scienti®c Party, 1999a). Three qualitative biofacies (A±C) can be de®ned for benthic foraminiferas at Site 1097 and these assist in identi®cation of depositional environment for lithofacies described in core. The distribution of biofacies types is shown downhole in Fig. 3. Biofacies A consists of poorly preserved, reworked assemblages of benthic foraminifers. Typical samples contain less than 12 robust foraminifer specimens (normally the benthic foraminifers Globocassidulina subglobosa and Cassidulinoides parkerianus), which are commonly yellowcolored, broken or ®lled with sediment indicating post-mortem transport (Fig. 7). Worn, dark brown and gray-colored fragments of biogenic matter such as Inoceramus (Bivalvia) prisms, mollusc shell fragments and echinoderm spines are also common. This biofacies is characteristic of massive diamictite lithofacies. Biofacies B contains a more abundant and diverse assemblages of foraminifers. The dominant benthic foraminifers (Globocassidulina subglobosa and Cassidulinoides parkerianus) are the same as in Biofacies A but the difference is their better preservation. Biofacies B occurs in strati®ed and graded diamictites that are interbedded with bioturbated mudstones (Fig. 5).

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Biofacies C is characterized by large numbers (up to 1000 specimens) of well-preserved foraminifera species and other biogenic material per sample. Biofacies A occurs at Site 1097 in diamictite and mudstone facies (e.g. Cores 35, 36; Fig. 5). Species are dominated by Globocassidulina subglobosa, Cassidulinoides parkerianus, Astrononion echolsi, Epistominella vitrea and Fursenkoina pauciloculata (Fig. 7). These species are typical of the present day outer Antarctic shelf where water depths are no greater than 500 m (Osterman et al., 2001). A very similar biofacies is reported from the Antarctic Dry Valleys (Ishman and Webb, 1988). Section 3.4 integrates lithofacies and biofacies data from Site 1097 and presents an environmental interpretation. 3.4. Environmental interpretation of lithofacies at Site 1097 3.4.1. Massive diamictite facies Massive diamictites at Site 1097 occur at several different stratigraphic levels. By themselves, such facies are not diagnostic of any one depositional environment since they can be produced in a wide range of settings. The key to interpretation is assessment of included biofacies types and interbedded sedimentary facies. Massive diamictite facies recovered within the uppermost 150 m of the hole are characterized by relatively low porosities interpreted as the result of subglacial shear consolidation (Shipboard Scienti®c Party, 1999a). These diamictite facies contain Biofacies A, which is distinguished by abraded and reworked foraminifer species. The absence of in situ marine microfossils and a lack of internal strati®cation, mudstone interbeds or bioturbation in such diamictites would appear to preclude a subaqueous marine origin by the deposition of suspended sediment and ice-rafted debris. In addition, the lack of associated facies such as graded facies does not favor a debris ¯ow origin. Given the ubiquitous presence of reworked and abraded marine microfossils (Fig. 4), massive diamictites at Site 1097 are most simply interpreted as having been deposited subglacially. In this regard, diamictites are most likely to have originated as `deformation till' (Boulton, 1996). These form where pre-existing

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Fig. 5. Graphic logs from representative cores at Site 1097 showing distribution of benthic foraminifer biofacies A±C.

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Fig. 5. (continued)

marine and glacial sediment is cannibalized and retransported subglacially as a so-called `deforming bed' (e.g. Osterman, 1984; Boulton, 1990; ten Brink et al., 1995; Clark et al., 1996; Licht et al, 1999; Shipp et al., 1999; Tulaczyk et al., 2000). Recovery was poor in the interval to 150 mbsf (Fig. 3) and this is most

likely the result of large boulders and the generally low degree of lithi®cation. As no recovery was made of any other lithofacies within the upper part of Site 1097, it seems reasonable to suggest that this interval consists entirely of poorly sorted subglacially deposited sediment. It is signi®cant that Canals et al. (2000)

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Fig. 6. Lithofacies from Site 1097. (A) Massive diamictite (facies Dmm): Site 1097A Core 10R, Section 1 interval 38±49 cm. (B) Graded diamictite (facies Dmg) showing overlying laminated marine mudstones (see Fig. 4A for graphic log): Site 1097A Core 27R, Section 1 interval 108±120 cm. (C) Weakly laminated, bioturbated marine mudstones separating graded diamictite beds (see Fig. 4B): Site 1097A Core 27R, Section 1 interval 87±102 cm. (D) Weakly laminated mudstone with ice-rafted clasts. This facies is transitional to diamictite (Fig. 4A): Site 1097A Core 34R, Section 1 interval 26±40 cm.

have recently inferred the presence of deformation till along the western margin of the Antarctic Peninsula continental shelf based on recognition of a subglacially streamlined (¯uted) sea¯oor topography. This is strong evidence that much of the modern sea¯oor is underlain by subglacially deposited sediment and supports our interpretation. Massive diamictite facies are also present below 150 mbsf at Site 1097 but differ from those facies at shallower depths by occurring within gently seaward dipping seismic re¯ectors and being interbedded with graded and strati®ed diamictites and mudstones containing Biofacies B and C. Other massive facies are interbedded with and in some cases transitional to mudstones with ice-rafted dropstones. These massive facies are interpreted in turn below in the context of associated facies and biofacies. 3.4.2. Graded and strati®ed diamictites Strati®ed diamictite lithofacies are interbedded with bioturbated mudstone and are characterized by moderately preserved and more abundant and diverse

assemblages of foraminifera (Biofacies B; e.g. Core 25). Graded diamictite facies (facies Dmg; Fig. 5) that contain Biofacies A, are diagnostic of turbulent downslope ¯ow of a poorly sorted sediment giving rise to sorting of different size fractions (e.g. Walker, 1992). Correspondingly, strati®ed and graded diamictite (facies Dms, Dmg) are con®dently interpreted as subaqueous sediment gravity ¯ows (debrites). Interbeds of mudstone probably record pauses between debris ¯ows allowing accumulation of ®ne sediment from suspension. Foraminifera in Biofacies B are typical of ice-proximal marine environments characterized by low salinity and high sedimentation rates (Shipboard Scienti®c Party, 1999a). Consequently, an ice-proximal glaciomarine depositional setting is suggested (Fig. 4). This is important contextual evidence for the origin of associated massive diamictite facies such as in Core 25. The presence of moderately preserved microfossils in those massive diamictite facies suggests that massive facies were deposited as debris ¯ows derived by mixing of marine and glacial sediment by mass ¯ow in the ice-proximal

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Fig. 7. Most common benthic foraminifera recovered at Site 1097. 1: Fursenkoina pauciloculata (Brady), 136 £ , 1097A-35R-1W, 100± 102 cm. 2: Nonionella auricula (Heron±Allen and Earland), 153 £ , 1097A-35R-1W, 100±102 cm. 3: Cassidulinoides parkerianus (Brady), 153 £ , 1097A-35R-1W, 100±102 cm. 4: Globocassidulina subglobosa (Brady), 102 £ , 1097A-35R-1W, 100±102 cm. 5: Epistominella vitrea (Parker), 221 £ , 1097A-35R-1W, 100±102 cm. 6: Ehrenbergina glabra (Heron±Allen and Earland), 101 £ , 1097A-35R-1W, 100±102 cm. 7: Nonionella iridea (Heron±Allen and Earland), 169 £ , 1097A-35R-1W, 100±102 cm.

environment. The absence of well-preserved microfossils in graded diamictite facies in Core 27 also indicates direct remobilization of till by downslope mass ¯ow. 3.4.3. Massive diamictite, muddy sandstone and mudstone facies Massive diamictites in Cores 35, 36 and 44 are distinguished from massive facies at other stratigraphic levels at Site 1097 by being transitional to bioturbated muddy sandstones with ice-rafted clasts (facies Fmd; Fig. 5A and B). Indistinct trace fossils such as burrows with poorly de®ned walls are also present and likely indicate soft sediment at the time of burrowing. At Site 1097, these lithofacies contain Biofacies C characterized by well-preserved foraminifera and associated biogenic material such as mollusk shells, sponge spicules and echinoderm spines. Because of these characteristics, massive diamictites containing Biofacies C are interpreted as glaciomarine in origin produced largely as `rain-out'

facies produced by suspension settling of mud with coarser material deposited by ¯oating ice (Fig. 4). Variation in the ¯ux of IRD through time gives rise to transitions from mud with suf®cient numbers of clasts to be identi®ed as diamictite (facies Dmm), to mudstone with isolated ice-rafted clasts (facies Fmd; Fig. 5). Weakly bioturbated, massive and laminated mudstones with high IRD content suggest episodic in¯uxes of meltwater, suspended sediment and icebergs to Site 1097 during late Miocene/early Pliocene time (Shipboard Scienti®c Party, 1999a). Such deposition suggests a subpolar or temperate glacier regime and the transport of ®ne sediment as suspended mud plumes derived from subglacial meltwaters. Notably, the total biogenic component of the mud within these glaciomarine intervals is generally less than 10% (Shipboard Scienti®c Party, 1999a), which is much less than the biogenic component of modern sediments accumulating on the present day Antarctic Peninsula shelf. For example, many of the cores

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described by Pope and Anderson (1992) are capped with diatomaceous muds; the upper part of cores analyzed by Pudsey et al. (1994) contain 30±60% diatoms. The low values recorded in muds at Site 1097 could re¯ect climatic factors such as more temperate conditions and an in¯ux of ®ne-grained meltwater-derived terrigenous sediment or other environmental factors such as reduced salinity around an ice margin terminating on the shelf. 3.5. Depositional setting of Site 1097 A subglacial origin for diamictites in the upper part of the drilled interval is suggested by the ubiquitous presence of reworked marine fossils (Biofacies A). Recovery was very poor in this interval and as related above, it is likely that this re¯ects the poor consolidation and the dominance of bouldery sediment. If correct, then ¯at-lying seismic re¯ectors at Site 1097 are interpreted as subglacial in origin. The work of Canals et al. (2000) is important in demonstrating the widespread presence of relict subglacial landforms on the sea ¯oor of the Antarctic continental margin. Data from Site 1097, if correctly interpreted, appear to con®rm that the shelf is underlain by thick subglacial sediment deposited as deformation till. In contrast, massive diamictites at depth are interbedded with sediment gravity ¯ow facies indicating deposition of massive facies by debris ¯ow, but in addition, are also transitional to mudstones with icerafted dropstones indicating a rainout origin. All three biofacies types are present. Gently dipping seismic re¯ectors below 150 mbsf in all probability identify former paleoslope surfaces that record alternating glacial conditions, when debris was discharged downslope from an ice margin at the shelf edge, more restricted ice extent with open marine conditions characterized by mud deposition and ice-rafting. 4. Site 1103 Site 1103 was drilled in 493 m of water northwest of Anvers Island (Fig. 1) in an area where the continental margin shows well-developed foresets and topsets (Fig. 2). The total depth of the hole was 362.97 mbsf (Fig. 3). Recovery from the upper 247 m was minimal (2.3%) but improved in the lower 115 m where 36.3 m of sediment was recovered

(34% recovery). Seismic re¯ectors are ¯at lying in the upper topset part of the hole to a depth of 220 mbsf (S1; Fig. 2B) but show an increased seaward dip in an underlying foreset unit (S3, Fig. 2B) below a major seismic unconformity (Fig. 3). 4.1. Age of drilled interval at Site 1103 From the top of the hole to 220 mbsf, several diatom species (Actinocylus ingens, F. barroni, T. insigna, T. inura, T. oestrupii and T. torokina) indicate a late Pliocene/Pleistocene age (Fig. 3; Shipboard Scienti®c Party, 1999b). Samples from approximately 290 mbsf contain well-preserved late Miocene diatoms such as Denticulopsis spp., Nitzschia januaria, and Rouxia californica. These data indicate a similar age range for the drilled section at 1103 as drilled at Site 1097 (Fig. 3). 4.2. Lithofacies Three dominant lithofacies can be identi®ed at Site 1103 (Figs. 3 and 8). These are diamictites (massive and chaotically strati®ed facies) (50% of recovered core), sandstones (25% of recovered core) and mudstones (25% of recovered core). 4.2.1. Diamictite (Dmm, Dms) Diamictite facies are massive or chaotically strati®ed. Thick-bedded (20 cm to 4.2 m) massive, unstructured and matrix-supported facies (facies Dmm; Figs. 8 and 9A, B) alternate with medium- to thin-bedded (,30 cm) chaotically strati®ed facies (facies Dms; Figs. 8 and 9C, D) showing a distinct `¯ow-banding' (Visser, 1993) de®ned by stringers and streaks of deformed mudstone (Fig. 10). Diamictites vary from clast-rich (.20% gravel) to clast-poor (10±20%) with the latter showing a smaller overall clast size (,2 cm). Gradation between poorly sorted muddy siltstone with dispersed clasts (facies Fmd) and diamictite (Dmm) occurs in several cores (Cores 33, 37; Figs. 8A and B). The largest clast size observed is 10 cm and clast shape varies widely from angular to rounded. In general, clasts are supported by a poorly sorted muddy sand matrix (ranging from 60% sand, 30% silt and 10% clay to 30% sand, 50% silt and 20% clay) (Shipboard Scienti®c Party, 1999b). The sand fraction is compositionally immature showing more than 40% lithic

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fragments. Shell fragments are common, including a complete barnacle valve in Core 28. A distinctive and common component of diamictites is irregular blebs and stringers of darkcolored mudstone and light-colored, angular clasts of mudstone (Fig. 9B). Mudstone clasts are distinct in containing a diverse, well-preserved diatom assemblage typical of continental shelves (e.g. Stephanophyxis grunowii and Thalassionema) but the surrounding diamictite matrix is barren apart from shell fragments (Shipboard Scienti®c Party, 1999b). Mudstone clasts (,2 cm diameter) (variably termed `intraclasts', `rip up clasts', `silt clasts' or `chips' in the literature; Pickering et al., 1989; Eyles et al., 1993) form up to 20% of the total clast population, resembling the `clast breccias' of Pickering et al. (1989).

`blebs' of siltstone and mudstone. Mudstones show transitions from massive to laminated facies with no systematic trend upcore (e.g. Core 31; Fig. 8A). Dropstones are common in massive and laminated mudstone facies (facies Fmd and Fld, respectively; Fig. 8A and B) and bioturbation is absent. Deformation structures, such as small-scale normal faults and dewatering structures occur throughout (Figs. 8A, B and 10). Core 37 shows pillow structures and associated downward-penetrating dykes at the base of medium-grained massive sandstone (Fig. 8B). Core 31 is composed of interbedded laminated and massive mudstone with dispersed clasts, and diamictite (Fig. 8A). In other cores (e.g. Core 33; Fig. 8A), gradations occur between diamictite facies, mudstone with dispersed clasts, and mudstone lacking any clasts.

4.2.2. Chaotically strati®ed, graded and massive sandstones (Sd, Sg, Sm) Beds of chaotically strati®ed, graded and massive muddy sandstone (Sd, Sg, Sm, respectively; Fig. 8A and B) are between 2 cm and 3 m thick (Cores 37, 38; Figs. 8A, B and 9E, F). All sandstone facies are poorly sorted (65% sand, 25% silt and 10% clay) and gray in color. Isolated lithic clasts and mud clasts are scattered throughout and shell fragments are common. Massive facies (Sm) show thin (,5 cm) graded conglomeratic intervals at their base. Graded facies (Sg) exhibit an upward reduction in texture (normal grading; Fig. 8A and B) but ®ne-grained bed tops were not recovered. Bed bases are sharp and erosional. Chaotically strati®ed facies (Sd) show extensive soft-sediment deformations and incomplete mixing of different textural populations (Fig. 9E and F). Core 27 shows parallel-laminated and graded sandstone beds showing extensive `convolute' soft sediment deformation (Fig. 8A). Beds of steeply dipping graded sandstone occur in Core 33 interbedded with mudstone and diamictite (Fig. 8A).

4.3. Biofacies

4.2.3. Mudstone (Fm, Fl) The dominant mudstone facies is massive (facies Fm; Fig. 9G) with a subordinate weakly laminated facies (facies Fl; Fig. 9H). Mudstones are very poorly sorted (5±20% sand, 35±70% silt and 10±60% clay). Laminations have no systematic structure but are de®ned by thin (,1 mm) streaks and sheared-out

Microfossils are rare at Site 1103 (Fig. 3). Biofacies allow ages to be identi®ed but offer few clues as to speci®c paleoenvironmental conditions. Most samples contain rare but reworked foraminiferal assemblages (Biofacies A; see above) (Fig. 8A and B). Mudstones in Core 33 contain an in situ microfossil assemblage (Biofacies C). 4.4. Environmental interpretation of lithofacies at Site 1103 Sedimentary facies recovered from foresets at Site 1103 are con®dently interpreted as sediment gravity ¯ow deposits. Massive sandstone facies (facies Sm) are identi®ed as the `disorganized muddy sands' of Pickering et al. (1989) and are deposited either by high density turbidity currents or very ¯uid sand± mud debris ¯ows. A muddy matrix creates suf®cient buoyancy to enable clasts to be freighted within the ¯ow and grading is absent except for thin so-called `coarse-tail' graded intervals at bed bases. Chaotically bedded and massive poorly sorted sandstone (facies Sd) record rapid dewatering of concentrated dispersions transported by sediment gravity ¯ows. Graded sandstone (facies Sg) represents the B division of turbidites (Pickering et al., 1989). Massive and irregularly laminated siltstones (facies Fm, Fl) also record rapid deposition from dense turbidity currents or muddy debris ¯ows; lamination

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Fig. 8. Graphic logs from representative cores at Site 1103 showing location of benthic foraminifer biofacies A±C. For lithofacies codes, see Fig. 4.

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Fig. 8. (continued)

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Fig. 9. Lithofacies at Site 1103. (A) Massive diamictite (facies Dmm) (Fig. 8): Site 1103A Core 27R, Section 2 interval 9±23 cm. (B) Massive diamictite facies with numerous light-colored silt clasts containing diatoms (Fig. 8): Site 1103A Core 34R, Section 4 interval 43±53 cm. (C) Chaotically strati®ed diamictite (facies Dms) (Fig. 8): Site 1103A Core 28R, Section 2 interval 83±92 cm. (D) Chaotically strati®ed diamictite (Fig. 8): Site 1103A Core 28R Section 2 interval 24±41 cm. (E, F) Chaotically bedded and massive sandstone (facies Sm, Sd) (Fig. 8): Site 1103A Core 38, Section 1 interval 53±67 cm; Site 1103A Core 38R, Section 1 interval 40±45 cm. (G) Massive mudstone facies (Fm) (Fig. 8): Site 1103A, Core 31R, Section 1 interval 137±146 cm. H. Weakly laminated mudstone facies (facies Fl) (Fig. 8): Site 1103A Core 31R, Section 1 interval 95±106 cm.

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151

Fig. 10. Thin-section photomicrographs of diamictite and mudstone facies at Site 1103. (A) Deformed, and (B) laminated mudstone (facies ¯d): Site 1103A; Core 35R (Fig. 8B). (C) Microfaults in deformed laminated mudstone (facies ¯d): Site 1103A Core 36R (Fig. 8B). (D) Massive diamictite: Site 1103A Core 34R with numerous dark-colored mudstone blebs (Fig. 8B).

is probably generated when the ¯ow progressively `freezes' from the bed base to the top during deposition. Widespread soft-sediment deformation structures attest to rapid deposition and local ¯uidization generated by rapid dewatering. Additional deformation structures, such as faults, result from post-depositional downslope creep. The common presence of sandstone and siltstone facies interpreted as sediment gravity ¯ows at Site 1103 provides critical contextual evidence for the origin of the massive diamictites with which such facies are associated. Diamictites are interbedded with poorly sorted massive and graded sandstones and siltstones (Fig. 8A and B) deposited from turbidity currents. Massive diamictites (facies Dmm; up to 4.2 m thick) are, consequently, identi®ed as debris ¯ows (debrites). Repeated ¯ow is recorded by the interbedding of massive facies with beds of crudely strati®ed diamictite with mudstone stringers (facies Dms; Figs. 8A, B and 9C, D). Muds were probably

deposited on bed tops between ¯ow events and were subsequently reworked and incorporated in later ¯ows (e.g. Eyles, 1990; Visser, 1993). 4.5. Depositional setting and environmental history of Site 1103 No information is available form the upper topset part of the succession drilled at Site 1103. However, in view of results obtained at Site 1097 and the work of Canals et al. (2000) reviewed above, we feel con®dent that topsets are composed primarily of subglacially deposited tillite. Lithofacies types recovered from the lower stratigraphic interval at Site 1103 within seaward dipping strata are relatively unambiguous indicators of sedimentary processes and overall environment. On the basis of the presence of sediment gravity ¯ow facies and the dip of seismic re¯ectors (Fig. 2B), it is suggested that the lower, foreset part of the stratigraphy at Site 1103 is a record of deposition

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on an earlier continental slope. A nearby source of glacially produced debris (till?) is suggested by the presence of large numbers of clasts of diatom-bearing marine sediment and the presence of a mixed and abraded biofacies assemblage (Biofacies A) in diamictites. Associated poorly sorted sandstone and mudstone facies were produced by downslope sorting of poorly sorted debris ¯ows in response to the onset of turbulence (e.g. Wright and Anderson, 1982; Walker, 1992). Such processes explain the upward transitions from clast-rich to clast-poor diamictite, and from diamictite to pebbly mudstone to mudstone (e.g. Core 33; Fig. 8A). In general, lithofacies at Site 1103 do not likely record protracted downslope ¯ow because extended downslope transport results in the generation of better-sorted graded conglomerates, sandstones and siltstone turbidites (e.g. Wright and Anderson, 1982; Pickering et al., 1989; Walker, 1992; Eyles et al., 1993). Because of the very poor overall textural sorting shown by sandstones at Site 1103, close proximity to a glacial source is suggested and an upper slope setting can be inferred. It must be emphasized that seaward-dipping re¯ectors in seismostratigraphic unit S3 at Site 1103 (Fig. 2) may have experienced some post-depositional tilting as a result of tectonic subsidence of the outer shelf. It is also possible that they retain their original depositional position within an upper continental slope foreset stratigraphy. 5. Discussion Suf®cient information was derived from Sites 1097 and 1103 to add to current understanding of depositional processes on the glaciated continental margin of the Antarctica Peninsula. The broader implications of the results are brie¯y identi®ed here. The drilled intervals at Sites 1097 and 1103 are no older than the late Miocene or early Pliocene (c. 4.6 million years; Fig. 3) and indicate ice reached the shelf edge. Bart and Anderson (1995) argued for a middle Miocene (after 15 Ma) onset of glaciation along the Antarctic Peninsula (possibly related to uplift of the Graham Land plateau; Elliot, 1995). More recently, Dingle and Lavelle (1998) have suggested glaciation began around 10 Ma. Nevertheless, all workers are agreed that the earliest

glaciations were restricted in extent to the Peninsula. A subsequent stage of larger-scale glaciation is supposed to have commenced at the Miocene/Pliocene boundary (c. 5 Ma) recorded by enhanced delivery of sediment to the shelf edge and accelerated slope progradation (Bart and Anderson, 1995). The lowermost seismic stratigraphic unit (S3) at Site 1103 may record this event (Fig. 2) because lithofacies recovered from this interval are consistent with slope progradation by mass ¯ow resulting in deposition of a distinct foreset section. Comparable successions of mass ¯ow facies have been recovered by piston coring of the modern Antarctic continental slope (Anderson et al., 1979). They are also widely reported from many other Pleistocene glacially in¯uenced continental slopes (e.g. Hill, 1984; Vorren et al., 1989; Boulton, 1990; Stoker et al., 1991; Aksu and Hiscott, 1992; King et al., 1998). Sedimentological and biofacies data from the upper part of the succession at Site 1097 (0±150 mbsf) suggests the importance of large-scale subglacial reworking of pre-existing glacial and marine sediment during episodes of ice sheet expansion to the shelf edge. If correct, this interpretation con®rms that the Antarctic Ice Sheet crossed the entire APPcm during the early Pliocene as proposed by Bart and Anderson (1995). In this regard, ten Brink et al. (1995) suggested that large amounts of marine sediment were retransported subglacially from inner shelf basins to the outer shelf and upper slope and their model is supported by data at Site 1097. Several workers have commented that ice expansion across the shelf likely reworked large volumes of marine sediment (e.g. Pudsey et al., 1994; Bart and Anderson, 1995) and there has been much discussion of subglacial depositional mechanisms (Alley et al., 1989; Vanneste and Larter, 1995). The occurrence of subglacially formed ¯ute ®elds on the APPcm shelf (Pudsey et al., 1994; Canals et al., 2000) in our view provides strong evidence of `soft-bed' conditions below the last ice sheet involving the reworking of preexisting sediment. Subglacial bedforms such as ¯utes and drumlins are now commonly attributed to a deforming subglacial bed (e.g. Boulton and Hindmarsh, 1987; Boulton, 1996; Boyce and Eyles, 1991; Shipp et al., 1999). Previous work suggested that the APPcm shelf aggrades principally during interglacials (Larter

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and Barker, 1989; Larter and Cunningham, 1993) but seismic data reveal the volumetric importance of massive seismic facies that can now be identi®ed as tillite. This indicates that shelf aggradation occurs during glaciation primarily in response to accommodation of thick till. Conversely, interglacial deposition rates on the shelf are likely to be very low given the lack of meltwater-derived sediment. Accommodation of thick till requires tectonic subsidence and the situation evident along the Antarctic margin can be contrasted with that along other passive margins where the lack of subsidence prohibits accumulation of thick successions of glacial deposits (e.g. eastern Canadian continental margin; Eyles, 1993, p. 190). Clearly, quantitative modeling of the relationship between subsidence and sediment supply along the lines of that completed by ten Brink et al. (1995) is needed. The lower sediment gravity ¯ow dominated part of the succession at Site 1097 below 150 mbsf was deposited in an ice proximal outer shelf or slope setting where ice discharged poorly sorted debris downslope. This succession may comprise a foreset stratigraphy similar to that seen at Site 1103 (Fig. 3). However, seismic data for the site do not show the clear division into topset and foreset components as exhibited at Site 1103 (Fig. 2). The predominance of sediment gravity ¯ow facies could simply re¯ect local depositional conditions in a shelf-crossing trough (Fig. 1). Finally, the broad-scale stratigraphic architecture seen at Site 1103 comprising a well-de®ned topset and underlying foreset stratigraphy (Fig. 2B), is seen on other glaciated continental margins. It underlies much of the Antarctic margin studied elsewhere (e.g. Alley et al., 1989; Hambrey et al., 1991; Anderson and Bartek, 1992; Alonso et al., 1992; Cooper et al., 1995), the southeast Greenland margin (Clausen, 1998), the northwestern Norwegian margin (Saettem et al., 1992) and the eastern Canadian continental margin (Hiscott and Aksu, 1994). In addition, comparable shelf and slope successions are reported from other glacially in¯uenced continental margin stratigraphic records from the Pleistocene and pre-Pleistocene (Visser, 1983; Miall, 1985; Eisbacher, 1985; Young and Gostin, 1991; Eyles, 1990, 1993; Eyles and Lagoe, 1990; Eyles et al., 1991; Eyles and Lagoe, 1998; McB Martin, 1999).

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Taken together, this may suggest a commonality of depositional processes and resulting stratigraphic architectures along glacially in¯uenced continental margins. 6. Conclusions 1. Seismic data indicate that the glacially in¯uenced APPcm is underlain by seaward dipping foresets and ¯at-lying topsets. Description of biofacies and sedimentary facies recovered by the Ocean Drilling Program (Leg 178) at two sites (1097 and 1103) suggests that topsets are composed primarily of subglacially deposited facies (till) recording aggradation of the shelf during episodes of continental ice sheet expansion to the shelf edge. An important process has been cannibalization of pre-existing marine sediment and deposition of deformation till resulting in diamictite facies with abraded and mixed biofacies assemblages. 2. Foreset strata are dominated by mass ¯ow facies indicative of deposition on an unstable continental slope close to a source of poorly sorted glacial debris. Foreset strata record progradation of the slope and the creation of a successively wider continental shelf. 3. This overall architectural structure of mass ¯ow dominated foresets and topsets dominated by subglacial sediment is seen on other glacially in¯uenced continental margins implying a commonality of depositional processes. This may provide a model for interpretation of other ancient glacial successions in the pre-Pleistocene.

Acknowledgements We thank the Master and crew of the JOIDES Resolution for their skill in working under dif®cult conditions, our shipboard colleagues for assistance and Leg 178 co-chiefs Peter Barker and Angelo Camerlenghi for stimulating discussions. Eyles' participation in Leg 178 was supported by the Natural Science and Engineering Research Council of Canada (NSERC); Januszczak acknowledges support from an NSERC Postgraduate Scholarship.

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We are particularly grateful to John Anderson, Geoff Norris and Andrew Miall for constructive comments on earlier drafts of this paper that was formally reviewed by Alan Cooper and Kathy Licht.

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