Intertidal boulder pavements in the northeastern Gulf of Alaska and their geological significance

Sedimentary Geology, 88 (1994) 161-173 Elsevier Science B.V., Amsterdam

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Intertidal boulder pavements in the northeastern Gulf of Alaska and their geological significance C.H. Eyles Department of Geography, McMaster University, Hamilton, Ontario, L8S 4K1, Canada Received September 14, 1992; revised version accepted April 2, 1993

ABSTRACT Striated boulder pavements, consisting of planar concentrations of clasts having striated upper surfaces, are a common feature of glacigenic deposits but their origin is not well understood. Laterally extensive pavements are currently forming in the intertidal zone west of Icy Bay in the Gulf of Alaska. Pavements comprise "armoured" layers of interlocking boulders, one clast thick, that have been eroded from underlying outcrops of Late Cenozoic glaciomarine diamictites; they originate essentially as lag surfaces along a high energy, storm-dominated, mesotidal shoreline. Boulder pavements are either flat or show a "nucleated" plan form where successively smaller boulders have been accreted around a large core boulder. Nucleation imparts a hummocky surface topography to the pavements and suggests that some form of size sorting of clasts has occurred. Packing is promoted by repeated tamping of the clast lag by floating masses of glacier ice which become grounded across the intertidal zone at low tide. Repeated abrasion of the pavement surface by debris contained within ice blocks produces smooth, flattened clast upper surfaces and short, randomly oriented striations. Data from Icy Bay can be used to constrain the origin of laterally extensive boulder pavements exposed in Late Cenozoic glaciomarine sediments on Middleton Island, The significance of such pavements in the geologic record is that they form along erosional unconformities and may identify sequence boundaries.

I. Introduction S t r i a t e d b o u l d e r p a v e m e n t s , consisting o f laterally extensive p l a n a r c o n c e n t r a t i o n s o f clasts with s t r i a t e d u p p e r surfaces, a r e d e s c r i b e d f r o m a w i d e r a n g e o f glacigenic d e p o s i t i o n a l s e q u e n c e s ( D r e i m a n i s , 1976; K r u g e r , 1979; O j a k a n g a s a n d M a t s c h , 1981; G r a v e n o r a n d R o c h a - C a m p o s , 1983; G r a v e n o r a n d M o n t e i r o , 1983; V i s s e r a n d Hall, 1985; Eyles, 1988). H o w e v e r , p a v e m e n t s vary in t h e i r c h a r a c t e r i s t i c s , t h e i r p r e c i s e origin is p o o r l y u n d e r s t o o d a n d o f t e n difficult to d e t e r m i n e and, in t h e case o f p a v e m e n t s d e p o s i t e d b e l o w ice sheets, is c o n t r o v e r s i a l (e.g. Clark, 1991; M i c k e l s o n et al., 1992). Clast c o n c e n t r a t i o n s can d e v e l o p in g l a c i g e n i c s e d i m e n t s in m a n y ways, e i t h e r by selective d e p o s i t i o n d u r i n g t r a n s p o r t by w a t e r o r ice, o r by e r o s i o n o f s u r r o u n d i n g m a t e r i als r e s u l t i n g in t h e d e v e l o p m e n t o f a lag. F u r t h e r m o r e , such clast h o r i z o n s m a y b e s t r i a t e d in a

n u m b e r o f d i f f e r e n t ways, t h r o u g h a b r a s i o n by o v e r r i d i n g glacial ice, ice bergs, ice-shelves o r s e a s o n a l ice. This p a p e r is c o n c e r n e d with b o u l d e r p a v e m e n t s t h a t d e v e l o p specifically on glacially influe n c e d c o n t i n e n t a l shelves a n d which o c c u r within g l a c i o m a r i n e successions. M o d e m i n t e r t i d a l stria t e d b o u l d e r p a v e m e n t s a r e d e s c r i b e d for t h e first t i m e f r o m two sites n e a r Icy Bay in t h e n o r t h e a s t e r n G u l f o f A l a s k a (Fig. 1) w h e r e t h e origin a n d d e p o s i t i o n a l setting o f t h e s e pavem e n t s is well c o n s t r a i n e d . B o u l d e r p a v e m e n t s r e p o r t e d h e r e a r e lag c o n c e n t r a t e s f o r m e d as clast-rich s u b s t r a t e s e d i m e n t s a r e e r o d e d a n d w i n n o w e d by m a r i n e processes. L a g b o u l d e r s a r e subject to s t r o n g tidal a n d s t o r m c u r r e n t s a n d r e p e a t e d a b r a s i o n by d e b r i s - r i c h g l a c i e r ice b e r g s g r o u n d i n g at low tide. S i m i l a r p a v e m e n t s can b e i d e n t i f i e d in t h e a n c i e n t r e c o r d within t h e L a t e C e n o z o i c Y a k a t a g a F o r m a t i o n o f A l a s k a (Eyles,

0037-0738/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSD1 0037-0738(93)E0062-K

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1988). The importance of these pavements is that they record episodes of relative sea-level lowering and, where correctly identified in ancient glaciomarine strata, indicate sequence boundaries representing sea-level low stands.

2. Geological setting The Gulf of Alaska lies across the complex collision zone between the Pacific and North American plates. Compressive forces generated between the subducting Pacific plate and the North American plate have resulted in the ex-

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treme uplift of the Chugach and St. Elias Mountains in the northeastern Gulf of Alaska (Fig. 1). Strong uplift is reflected in the deposition of over 5 km of glacially influenced marine strata (Yakataga Formation) during the past 6 Ma in the offshore Gulf of Alaska basin (Eyles et al., 1991). The study area, comprising the coastline between Yakataga R e e f and Icy Bay (Fig. 1), is in one of the most seismically active areas in North America and experiences frequent high-magnitude earthquakes (Plafker, 1986). There is widespread geologic evidence for strong vertical ground motions accompanying such earthquakes

~

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Fig. 1. Location maps. (A) Northeastern Gulf of Alaska region with outcrop of the Yakataga Formation shown (heavy stipple). (B) Detail of the coastal area between Yakataga Reef and Icy Bay. Well developed boulder pavements occur in the intertidal zone at Umbrella Reef and along the western shore of Icy Bay (locations marked with asterisks). Successive positions of the retreating margin of Guyot Glacier in Icy Bay over the past 100 years are shown.

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in the Gulf (Plafker, 1990); uplift rates of 40 m in 5000 years are documented for Middleton Island which lies 80 km offshore in the Gulf (Fig. 1; Plafker and Rubin, 1978) and similar rates have been suggested for the Yakataga coastline (Bruns and Schwab, 1983). The coastal plain between Yakataga Reef and Icy Bay forms a series of raised marine terraces up to 150 m above sea level, which are the result of recent episodic uplift caused by high-magnitude earthquakes (Plafker, 1986) and post-glacial glacio-isostatic recovery. Raised marine terraces on the coastal plain indicate repeated uplift of between 7.5 and 17 m, reflecting an earthquake frequency of about 1400 years (Plafker, 1986). The Yakataga district last experienced a major earthquake in 1899 and is currently in an aseismic condition (Nishenko and Jacob, 1990). This part of the Alaskan coastline was unaffected by the 1964 Good Friday earthquake (Jacob, 1987). Development of the modern intertidal pavements is constrained by this uplift history to within the past 100 years. As a result of repeated uplift events the coastal mountains surrounding the northeastern Gulf of Alaska expose Late Miocene to Pliocene rocks of the Yakataga Formation (Eyles et al., 1991). These strata consist dominantly of glaciomarine diamictites, marine sandstones and mudstones and record the initiation and development of Late Cenozoic glaciation in the Gulf of Alaska (Lagoe, 1983; Eyles et al., 1991; Lagoe et al., 1993). Coastal outcrops of the Yakataga Formation diamictites provide a source of boulders for well-developed boulder pavements at Umbrella Reef and Icy Bay (Fig. 1); these diamictites are not well lithified and are easily weathered. Eyles (1988) described boulder pavements from Pleistocene Yakataga strata exposed on Middleton Island (Fig. 1) and proposed a model for their development involving the formation of lag deposits during low sea-level stands and their subsequent abrasion by tidewater glaciers. This model was tentative and was unconstrained by modern analogs. The modern intertidal pavements described in this paper show similar and dissimilar characteristics to those exposed on Middleton Island. However, the origin of the modern pave-

ments is well constrained and they provide very important data regarding the depositional setting of the Middleton Island examples.

i?

iilii ~--

Fig. 2. Boulder pavements exposed along the western shore of Icy Bay. (A) Lateral view of elongate "hummock" on pavement. Note flatter pavement form in background and ice bergs (bergy bits) resting on pavement surface. (B) Oblique view of pavement "hummock" showing well flattened clast upper surfaces. Sand covers the pavement in between hummocks. (C) Lengthwise view of "hummock" showing more rounded "stoss" end pointing seaward (top of picture).

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3. Description of pavements M o d e r n striated boulder pavements were observed at two sites along the Gulf of Alaska coastline near Icy Bay and at Umbrella R e e f (Fig. 1). On the western shore of Icy Bay close to Carson Creek (Figs. 1, 2) some 600 m 2 of the intertidal zone consists of a one-clast-thick veneer of closely packed clasts lying on a diamictite b e d r o c k platform; the largest boulders in the p a v e m e n t form elongate " h u m m o c k s " oriented perpendicular to the shoreline (Fig. 3). At Umbrella Reef (Figs. 1, 3) extensive (0.05 km 2) boulder pavements show a variety of forms including unoriented h u m m o c k s and flat surfaces (Fig. 4). A t both localities pavements occur between bedrock outcrops in the intertidal zone, and in some cases extend alongshore for several hundred metres and up to 100 m in a shore normal direction (e.g. Fig. 3). They rest directly on poorly

c.H. EYL,t'~S lithified diamictite bedrock and form a smooth, a r m o u r e d surface which is either flat, or shows slight undulations and hummocks. Pavements resemble a cobbled roadway and a vehicle can be driven over their surface with ease. Intertidal bedrock platforms are flat to gently undulating and slope seaward at between 1° and 2 ° (Fig. 5). Pavements are commonly " p r o t e c t e d " from the o p e n sea by a rim of exposed b e d r o c k lying close to low water mark and are covered in a landward direction by mobile beach sands (Figs. 3, 5); they are best exposed in the central portion of the intertidal zone. A boulder beach occurs at high tide level at Umbrella R e e f (Fig. 5).

3.1. Clast shape and organisation Clasts forming the pavements vary in size from 0.1 m to 2 m diameter and generally have flattened u p p e r surfaces (Fig. 6); A-axis orientation

Fig. 3. Aerial view of pavements at Umbrella Reef; sea lies toward lower left, land toward upper right. Width of pavement (in a land to sea direction) is approximately 100 m; pavements are covered by sand in landward direction. Horizontally bedded fine-grained Yakataga Formation rocks outcrop in the central part of the photograph and at lower left. Note "nucleated" forms on pavements.

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to be short (less than 5 cm long) and form a criss-crossing pattern. Clast organisation within the pavements varies between being extremely well developed to poorly developed. Well developed pavements consist of tightly packed clasts tessellated with smaller pebbles so that no intraclast movement is possible (Fig. 6). Clasts within these pavements have rounded edges and flattened upper surfaces. Occasionally loose clasts may be observed on the top of the pavements. Poorly developed pavements are less well organised with many clasts poorly packed and loose and have irregular upper surfaces (Fig. 7). Individual pavements may pass from being well developed to poorly developed over a distance of a few metres.

3.2. Pavement relief

Fig. 4. Boulder pavements at Umbrella Reef. (A) Areas of well developed fiat pavement lying between boulder hummocks. (B) Irregular hummock forms and areas of fiat pavement (covered by tidal pools and thin sand veneer).

is random. Clast lithologies are highly variable; most are sedimentary rocks of the Yakataga Formation (diamictites, sandstones, conglomerates and shales) but occasional volcanic and granitic rock types also occur. Fine striations are visible on many fine-grained rocks especially on the seaward-pointing ends of clasts (Fig. 6); striae tend

Boulder pavements form either flat surfaces extending for several tens of metres (Figs. 4A, 6A), or "hummocky" surfaces with a relief of up to 1 m (Figs. 2, 4B). Flat pavements occur most extensively at Umbrella Reef where they separate irregularly spaced boulder hummocks (Fig. 4). Individual hummocks consist of several large boulders (up to 2 m diameter) in the centre, surrounded by smaller clasts (Fig. 8). A well developed lateral size grading is shown away from the centre of each hummock towards the margin (Fig. 8), resulting in side slopes of between 25 and 30°; the underlying bedrock surface may be either flat or gently undulating (Fig. 5). Most hummocks contain extremely well packed clasts and their upper surfaces are highly smoothed

Outcropof

Umbrella

Hoiocene glaciomarine sediment HWM A

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-

Boulder

Pavements

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":~'~'~:'~:'~?-~.i.:.~..:~.:.:..

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20 m I

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Fig. 5. Schematic cross-section of the intertidal zone at Umbrella Reef.

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(Fig. 8). Hummocky forms at Umbrella Reef are irregularly shaped whereas those comprising pavements at lcy Bay are elongate mounds ap-

Fig. 7. Poorly developed pavement; boulder lag lying on poorly lithified Yakataga Formation sediments at Umbrella Reef. Some boulders have been packed together and their surfaces flattened but overall this lag is irregular and poorly arganised. Largest boulder in foreground is approximately 80 cm diameter

proximately 5 m wide and 10 to 15 m long, oriented perpendicular to the shore. Oriented mounds resemble small drumlins with their blunt "stoss-sides" facing seaward (Fig. 2). The largest boulders in the pavement tend to be concentrated in the central and seaward portions of the mounds (Fig. 2). Depressions in the boulder pavements at Umbrella Reef and Icy Bay are covered either by small tidal pools in which red calcareous algae or seaweed grow, or by a thin cover of sand (Figs. 2B, 4). Algae and seaweed may attach to the lateral margins of clasts forming the pavement, although clast upper surfaces are always kept clear. A variety of marine organisms, including mussels, barnacles and gastropods were also observed in the tidal pools and on clast margins. 4. Pavement formation

Fig. 6. Detail of clasts within pavements. (A) Area of flat pavement at Umbrella Reef showing tightly packed clasts with flattened upper surfaces. (B) Closely packed clasts "tessellated" with smaller clasts and coarse sand. Clast surfaces are faceted; no preferred A-axis alignment can be seen. (C) Striations on the upper surface of a fine sandstone clast within the pavement at Icy Bay. Note variable direction and crossing of striae; striae are rarely more than a few centimetres long.

The intertidal boulder pavements described above closely resemble those reported from subArctic and sub-Antarctic coastal settings, in which pavement formation occurs as marine lag surfaces are abraded by the grounding of ice bergs or seasonal pack ice (e.g. Araya and Herve, 1972; Hansom, 1983, 1986). The restriction of boulder pavements to areas where clast-rich Yakataga Formation sediments crop out, suggests that a

INTERTIDAL B O U L D E R PAVEMENTS IN THE NORTHEASTERN G U L F OF ALASKA

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critical to pavement development; the pavements in the Gulf of Alaska all occur on slopes of less than 2°. The nature of marine erosion processes in the study area, their influence on the development and organisation of boulder lags and the role of grounding ice in the formation of pavements will be examined below.

'Nucleated' Boulder Pavements

4.1. Marine erosion processes

0

I _ _ l

A

lm

A'

i

Fig. 8. "Nucleated" boulder pavements. Upper: sketch showing plan view and cross-section of a nucleated boulder bummock on the pavement at Umbrella Reef. Note progressive size grading of clasts away from centre of hummock. Lower: photograph of nucleated hummock at Umbrella Reef and surrounding fiat pavement.

supply of boulder-sized material in the intertidal zone is critical to their formation. Pavements have not developed on fine-grained Yakataga Formation sediments exposed in the intertidal zone (Fig. 3). This condition of a local boulder source has also been identified as necessary for the formation of pavements in James Bay (Martini, 1981), the sub-Antarctic (Hansom, 1983) and Iceland (Hansom, 1986). Hansom (1983, 1986) argues that a substrate slope of less than 3° is also

Erosion of Yakataga Formation diamictites to produce a boulder lag is readily accomplished in the intertidal zone by wave and tidal processes. Tidal range in this part of the Gulf of Alaska is approximately 3 m and the coast is considered to be mesotidal; tidal currents cause localised erosion in the intertidal zone and probably enhance the erosional effects of storm waves. Storm waves are particularly powerful along the Gulf of Alaska coastline and have resulted in high rates of coastal erosion and retreat. The Gulf of Alaska has one of the highest winter storm frequencies in the Northern Hemisphere and experiences storms with energy levels comparable to Caribbean hurricanes (Nummedal and Stephen, 1978). The theoretical 100 year storm wave for the Gulf of Alaska has a height of over 30 m (Anderson and Molnia, 1989). The mean annual wave power in the study area is about 2 4 × 103 W S -1, identifying it as one of the highest wave energy environments in the world (Nummedal and Stephen, 1978). Along the coastline between Icy Bay and Yakataga Reef prevailing winds are from the east and southeast. Wave power calculations show a dominantly westward wave power vector with corresponding westward net sediment transport directions. The erosional effects of such powerful storm waves are apparent around the entrance to Icy Bay. Between 1922 and 1988 the eastern side of the mouth of Icy Bay receded more than 2 km (an average of 30 m yr-~); the western margin retreated by almost 5 km between 1922 and 1976 (an average of 92.5 m yr-1; Anderson and Molnia, 1989). High erosion rates are the result of a combination of high wave energies and poorly consolidated sediments. Computations of longshore sediment transport for the region indicate

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an annual average gross sediment transport of about 2 x 106 m 3 (Nummedal and Stephen, 1978). Winnowing and transport of fine-grain sizes from the heterogeneous Yakataga Formation diamictites exposed at Icy Bay and Umbrella R e e f leaves a lag of clasts resting on a low gradient, wave-cut bedrock platform. Simple marine lags of this type are widespread in the intertidal zone surrounding Middleton Island (Fig. 9), which is also underlain by poorly lithified Yakataga Formation sediments (Eyles and Lagoe, 1990). Bedrock exposed around Middleton Island is frequently eroded into shore-normal, elongate hummocks similar to the boulder pavement hummocks observed at Icy Bay (Figs. 2, 9). These erosional forms may be a product of strong wave and tidal currents in the intertidal zone.

4.2. Organisation of the boulder lags Simple marine boulder lags developed through erosion of heterogeneous sediment in the inter-

(!.H. EYLES

tidal zone must undergo subsequent size sorting, packing and abrasion to produce the organised pavements occurring at Icy Bay and Umbrella Reef. Size sorting of boulders into the hummocky forms observed on the pavements may, in part, be the result of "nucleation" of progressively smaller clasts around a single large boulder eroded out of the substrate. Large clasts act as obstacles to strong currents and are known to shield smaller particles and prevent motion even at velocities higher than that required for their movement (Schumm and Stevens, 1973). This process could eventually lead to progressively smaller clasts being stabilised on the lag surface away from a central large "core" boulder. The hummocky relief observed on intertidal pavements developed in the sub-Antarctic and sub-Arctic has been attributed to the "wallowing" action of grounded ice blocks (Araya and Herve, 1972; Dionne, 1978; Martini, 1981; Hansom and Kirk, 1989). The in-situ rotation of grounded ice blocks by tide or wave processes is thought to

Fig, 9. Boulder lag lying on Yakataga Formation sediments in the intertidal zone of Middleton Island (see Fig. 1 for location). Clast lags are not organised into pavement forms on Middleton Island as the island is not affected by seasonal ice or floating glacier ice.

INTERTIDAL BOULDER PAVEMENTS IN THE NORTHEASTERN GULF OF ALASKA

displace larger clasts outwards, producing a raised "rim" or hummocky margin. This may be a contributory process to the formation and size sorting of boulder hummocks on the Alaskan intertidal pavements (see below). Enhanced packing of clasts on a boulder lag surface may also result from wave action. Tightly packed, "fitting" boulders are described from beaches in New Zealand where long-continued small movements of the clasts are induced by wave action (Hills, 1970). Schumm and Stevens (1973) report flume experiments simulating flow in a stream with coarse bed material and conclude that a tightly packed armoured bed may develop as a result of fine sediment migrating down between the coarsest particles. Packing is further enhanced by the in-situ vibration of clasts caused by combined lift and drag forces exerted on the clasts during high velocity flows. Such vibration also causes significant abrasion and rounding of clasts in place (Schumm and Stevens, 1973). Wave action could have similar effects on coarse-grained lag surfaces in the intertidal zone of the Gulf of Alaska, producing tightly packed pavement forms. However, modern intertidal boulder lags on Middleton Island show no great degree of packing or organisation into pavements (Fig. 9), yet are subjected to considerable wave energies. This suggests that wave action alone cannot account for the sorting and packing characteristics of the boulder pavements at Icy Bay and Umbrella Reef. The only significant difference between the mainland coast and Middleton Island is that the intertidal zone surrounding the island has not been affected by seasonal or floating ice since it emerged from the sea about 5000 years ago (Plafker, 1990); it is probable that the grounding of floating ice masses is critical to the formation of intertidal boulder pavements in this region.

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The climatic regime of the coastal plain, however, is remarkably temperate with average annual temperatures of about 6°C. In winter, sea ice only forms at the heads of fiords and does not develop along the open coastline (Powell and Molnia, 1989); the coast is never closed to winter navigation because of ice. During the first decade of this century, however, glaciers were much more extensive along the southern flanks of the coastal mountains. Guyot Glacier completely filled Icy Bay at this time and has since retreated over 40 km up-fiord (Fig. 1). The four separate glaciers presently at the head of Icy Bay calve large volumes of ice into the bay, but ice bergs are rarely seen on the open coast. Extremely rapid calving of ice fronts in Icy Bay has occasionally given rise to dense masses of drifting glacial ice bergs on the open coast; large masses of glacial ice from

4. 3. The role of grounding ice The coastal mountains flanking the northeastern Gulf of Alaska are extensively glaciated as a result of their elevation, high precipitation values and tectonic setting (Anderson and Molnia, 1989).

Fig. 10. Ice bergs (bergy bits) on the western shore of Icy Bay. (A) Stranded ice bergs lying on the beach adjacent to the boulder pavement at Icy Bay. T h e s e bergs were washed onshore during a s u m m e r storm. (B) Debris-rich ice berg resting on pavement at Icy Bay.

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Icy Bay were washed ashore at Yakataga Reef following the 1964 Good Friday earthquake. Floating glacier ice is normally contained within Icy Bay as a result of local longshore currents which produce eastward net transport along the bayshore past Claybluff Point (Fig. 1; Nummedal and Stephen, 1978). The shore to the north of Claybluff Point is invariably covered with stranded ice bergs, most of which contain abundant debris (Fig. 10B). Tabular bergs up to 6 m diameter (bergy bits--herein refered to as ice bergs) were observed on the boulder pavements exposed along the western shores of Icy Bay following a summer storm (Fig. 10A). Although ice bergs are rarely transported alongshore as far as Umbrella Reef under present conditions, this would be more common in the recent past when Guyot Glacier filled Icy Bay. The repeated grounding and tide- and wave-

induced movement of ice bergs on a boulder lag surface is probably responsible for the development of tightly packed boulder pavements with flattened and striated upper surfaces (e.g. Figs. 6, 11). Grounding ice bergs push clasts together and pound them into the poorly lithified bedrock substrate; abrasion occurs as debris-rich ice blocks and loose clasts are dragged across the boulder surfaces (Fig. 11A). The short, criss-crossing striae observed on clast upper surfaces at Icy Bay and Umbrella Reef (Fig. 6C) are typical of those resulting from abrasion by grounding ice bergs or ice floes as they are moved across the substrate by wave and tidal action (Dionne, 1973, 1985; Hansom, 1983, 1986). The seaward ends of boulders are particularly prone to abrasion by shoreward-moving ice blocks. Boulder upper surfaces are also kept free of algae and weed as a result of frequent abrasion by grounding ice bergs or loose

STRIATED BOULDER PAVEMENT FORMATION IN MARINE SETTINGS B. Ancient subtidal(?) pavements: Middleton Island

A. Modem intertidal pavements: Umbrella Reef and Icy Bay (i) Development of armoured boulder lag surface •

~

~

(i) Development of armoured boulder lag surface waveandtidal

waveandtidal

erosion

(subtidal)

~

Yakataga Formation

~

Yakataga Formation

(ii) Packing and abrasion by grounding ice sheet

(ii) Packing and abrasion by grounding ice bergs

Tid ran!

p=V=mUllL=U~,a~¢ (iii) Final pavement form

(iii) Final pavement form ~ c r o s s i n g

striae

i

Fig. 11. Contrasting models of striated boulder pavement formation in marine settings.

I N T E R T I D A L B O U L D E R P A V E M E N T S IN T H E N O R T H E A S T E R N G U L F O F ALASKA

boulders (Fig. llA). Size sorting of "nucleated" forms may also result from the outward pushing of large clasts as stranded bergs rotate and wallow on the boulder lag surface (Araya and Herve, 1972; Dionne, 1978; Martini, 1981; Hansom and Kirk, 1989). Similar boulder pavements to those described here are described from low energy subarctic intertidal, lacustrine and fluvial environments. These subarctic pavements form as clasts are tightly packed by the rocking of ice cakes on a clast-rich substrate and are subsequently abraded by drift ice (Dionne, 1976, 1978, 1979; Martini, 1981; Gilbert et al., 1984). In summary, the striated boulder pavements at Icy Bay and Umbrella Reef probably formed as a result of the combined effects of marine erosion and the grounding of floating ice bergs. Wave processes are particularly important in eroding underlying clast-rich bedrock to release boulders directly to the shore zone and may contribute towards the size sorting of clasts into "nucleated" forms (Fig. llA). Grounding glacial ice bergs are responsible for the packing and stabilisation of boulders and for flattening, polishing and striating clast upper surfaces. 5. Discussion

The intertidal boulder pavements described in this paper are well constrained in terms of their depositional setting and the processes that led to their formation. Striated boulder pavements preserved in the rock record, however, are often difficult to interpret and can lead to controversial palaeoenvironmental reconstructions (e.g. Clark, 1991; Mickelson et al., 1992). Eyles (1988) described a series of ancient striated boulder pavements contained within the thick glaciomarine diamictite stratigraphy of the Early Pleistocene Yakataga Formation (approximately 2 Ma) exposed on Middleton Island, Alaska. These pavements form extensive bedding plane surfaces within fossiliferous glaciomarine diamictites and show abruptly truncated and striated clast upper surfaces with parallel striae; each pavement in the succession showed a consistent north-south striation direction (Fig. llB). Pavements are associated with shallow water

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foraminiferal biofacies indicating formation in water depths of less than 50 m (Eyles and Lagoe, 1990). The boulder pavements are interpreted as marine lag surfaces which formed by wave and tidal erosion in relatively shallow water depths; similar lags are found on shallow water shoals and banks on the modern Gulf of Alaska continental shelf (e.g. Tarr Bank, Fig. 1; Molnia and Carlson, 1980). These marine lags were subsequently abraded and striated by a grounding ice sheet that expanded offshore at times of lowered sea level (Fig. llB; Eyles, 1988). The planar form of the Middleton Island pavements exposed in stratigraphic section, and the parallel and consistent striation direction preserved on clast surfaces suggest abrasion by an extensive and unidirectional ice mass rather than randomly moving ice bergs or ice floes (Fig. llB). The excellent state of preservation of striae on the clast surfaces also suggests rapid burial of the pavements following formation. These characteristics may differentiate pavements formed in glacially influenced subtidal environments in offshore areas of sedimentary basins from those formed in intertidal settings along basin margins. Marine boulder pavements displaying short, crossing striae may also be used as evidence for the former presence of grounding ice bergs. Although the effects of floating ice are well known on modern cold-climate shallow marine shelves (e.g. Barnes, 1987; Lewis and Woodworth-Lynas, 1990), identification of ice berg scours in sedimentary successions is problematical and few are reported in the literature (e.g. Eyles and Clark, 1988; Woodworth-Lynas and Guigne, 1990). In contrast, striated boulder pavements are well represented in glaciomarine strata and may provide the only means of identifying ice berg and ice floe activity in the rock record. Furthermore, the striated boulder pavements described in this paper are significant features of temperate glaciated coastlines; as such, they may be considered as elements of the proglacial depositional system rather than as uniquely periglacial (cold climate) features. Striated boulder pavements have been described from many ancient glacigenic successions and are often used as evidence for terrestrial

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subglacial deposition (Dreimanis, 1976; Gravenor and Rochas-Campos, 1983; Fairchild and Hambrey, 1984; Visser and Hall, 1985). However, it is now increasingly recognised that striated boulder pavements are commonly preserved in glaciomarine successions and record significant erosional episodes resulting from lowered sea levels (Theron and Blignault, 1975; Ojakangas and Matsch, 1981; Eyles, 1988; Matsch and Ojakangas, 1991). Such boulder pavements can be considered as significant erosional unconformities and therefore identify "bounding surfaces" for the subdivision of glacigenic successions into allostratigraphic units (NACSN, 1983). This form of stratigraphic subdivision, based on identification of features recording significant sea-level variation, is critical both for the reconstruction of glacial palaeoenvironmental change and for the discrimination of isostatic and eustatic sea-level fluctuations. Thorough description and interpretation of boulder pavements from many different environmental settings is necessary for accurate assessment of the erosional significance of such features. Acknowledgements This work was supported by an NSERC University Research Fellowship and Operating Grant. I would like to thank Don and Lahoma Leischmann for their hospitality at Cape Yakataga and Will Jones for his skillful operation of our helicopter. Nick Eyles gave valuable field assistance and greatly improved the manuscript. The helpful reviews of Jim Hansom and an anonymous reviewer are also gratefully acknowledged. Mike Doughty kindly drafted the figures. This paper is a contribution to IGCP 260, Earth's Glacial Record. References Anderson, J.B. and Molnia, B.F., 1989. Glacial-Marine Sedimentation. American Geophysical Union, Washington, D.C., 127 pp. Araya, R. and Herve, F., 1972. Patterned gravel beaches in the South Shetland Islands. In: R.J. Adie (Editor), Antarctic Geology and Geophysics. I.U.G.S., Oslo, Universitetsforlaget, pp. t11-114.

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