Late Quaternary sedimentation in St. George's Bay ...

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Late Quaternary sedimentation in St. George's Bay , southwest Newfoundland: acoustic stratigraphy and seabed deposits 1 J. SHAW AND D. L. FORBES Geological Survey of Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2 Received December I , 1989 Revision accepted March 20, 1990 Shallow seismic reflection data collected in St. George's Bay, southwest Newfoundland, reveal a complex pattern of subsurface topography and acoustic facies. Two basins in the inner bay are underlain by glacially overdeepened valleys that extend to depths in excess of 180m. Within the thick Quaternary sequence in the inner bay we recognize eight acoustic units. Units 1 (ice contact), 2 (subaq ueous outwash), and 3 (draped glaciomarine) record the presence and retreat of a major Late Wisconsinan icc margin. Unit 4 (postglacial mud) has resulted from reworking of glacio genic sediments in response to changes in relative sea level. Unit 5 (postglacial sand) is a shoreface deposit on the seaward front of the former moraine. Unit 6 (postglacial delta) was formed by fluvial reworking of glaciogenic sediments during the postglacial lowstand of relative sea level. Unit 7 (postglacial barrier- platform) comprises seaward-fining clinoform prisms that have prograded into the basins, and underlie grav::l ~each-ridge plains at Stephenville and Flat !sland. Unit 8 (postglacial spillover) results from entrainment of coarse sediment on the shallow sill and subsequent progradatiOn into the basins of the inner bay. Seabed sediment in the basins is mud where sampled. The submarine platforms associated with the barriers at Stephenville and Flat Island are largely sandy. The sill is covered by a gravel veneer, with irregular patches of sand that coalesce and thicken seawards. Extensive areas of gravel ripples testify to the continued mobility of much of the coarse sediment on the sill. A major ice front in the inner bay was present prior to 13.7 ka BP. The draped glaciomarine sediments, dated at 13.7- 11.2 ka BP in nearby Port au Port Bay, were deposited after withdrawal of the ice front to the vicinity of the present coast, during a read vance, and subsequently. During the postglacial lowstand of relative sea level much of the present sill area was emergent. Les donnees de sismique reflexion peu profonde obtenues dans Ia baie St. George, dans le sud-ouest de Terre-Neuve, devoilent une topographic de subsurface et des facies acoustiques complexes. Deux bassins dans Ia baie interieure sont formes par des vallees glaciaires surcreusees qui atteignent des profondeurs de plus de 180m. Dans I' epaisse sequence quaternaire qui occupe Ia baie interieur~ on reconnait huit unites acoustiques. Les unites I (au contact du glacier), 2 (fluvio-glaciaire subaquatique) et 3 (couverture glaciomarine) temoignent de Ia presenceet du retrait d'une marge glaciaire majeure au Wisconsinien tardif. L'unite 4 (boue postglaciaire) resulte du remaniement des sediments glaciogeniques provoque par les changements du niveau de mer relatif. L'unite 5 (sable postglaciaire) est un dep6t d'avant-plage sur le front d'une ancienne moraine du cote de Ia mer. L 'unite 6 (delta postglaciaire) a ete formee par le remaniement fluvial de sediments glaciogeniques durant une periode postglaciaire de stagnation du bas niveau de Ia mer relatif. L'unite 7 (plate-forme- barriere postglaciaire) comprend des prismes inclines a granulometrie decroissante vers le large qui ont prograde dans lcs bassins, et qui sont recouverts par des graviers de levees de pi age etales a Stephenville eta l'ile Flat. L'unite 8 (debordement postglaciaire) resulte de l'entrainement du sediment grassier sur le fond peu profond et de Ia progradation subsequente dans les bassins de Ia baie interieure. Le sediment du fond de mer dans les bassins est fait boue ou il y a cu echantillonnagc. Le sable dornine sur les plates-formes subaquatiques associees aux barrieres a Stephenville eta l'ile Flat. Le seuil est couven par une mince couche de gravier. avec des poches de sable irregulieres qui se fusionnent et gagnent en epaisseuren s'eloignant vers le large. De grandes surfaces couvertes de rides de gravicr temoignent de Ia mobilite continue de Ia majeure partie du sediment grossier sur le seuil. Un front glaciaire majeur dans Ia baie interieure existait il y a deja plus de 13,7 ka BP. Le dep6t des sediments de Ia couverture glaciomarine, date de 13,7- 11 ,2 ka BP, dans !a baie de Port au Pon situec a proximite, a ete forme apres le recut du front glaciaire aux positions de Ia cote actuelle, lors d'une reavancee, et subsequemment. Durant Ia periode postglaciaire de stagnation de bas niveau de mer relatif une partie imponante de Ia surface du seuil etait exondec. [Traduit par Ia revue] Can. J. Eanh Sci. 27,964- 983 (1990)

Introduction Because of its key position for interpreting the extent of the Laurentide Ice Sheet, southwest Newfoundland (Fig. 1) has attracted, from an early date, the attention of Quaternary geologists (e.g., Daly 1921; Coleman 1926; Flint 1940; MacCiintock and Twenhofel 1940). Although well-exposed deposits onshore have provided the basis for detailed reconstructions of late- and postglacial events around St. George's Bay (e.g., Brookes 1969, 1970, 1974, 1977a, 1977b, 1987; Grant 1987), information was needed from parts of the study area below present sea level to constrain interpretations of the late Quaternary geology. In addition, the presumed placer mineral potential of adjacPP..t Port Ill' Pur~>
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FIG . 1. Location of the study area, also showing track plots for cruises 69-048 (CNAV Sackville) and 89-008 (css Baffin), and the location of the seismic section illustrated in Fig. 13.

from an earlier set of seismic records reported by Shearer ( 1973) and data gathered in 1989 from css Baffin.

Environmental setting The bedrock geology of the region has been described by Riley (1962) and Williams (1985). Briefly , the south shore of the bay comprises sandstones, siltstones, conglomerates, limestones, and gypsum of the Mississippian Codroy Group . The north shore is composed of Middle and Lower Ordovician dolomites, limestones, and shales of the St. George and Table Head groups. The massif immediately east of Port Harmon (Fig. 2) is formed in Late Proterozoic anorthosite. The coast of St. George's Bay (Fig. 1) is characterized main! y by soft eroding cliffs of glacigenic sediments, typically about 30m high, fronted in places by narrow mixed sand and gravel beaches. West of the P0:t au Port isthmus (Fig. 2) , the coast of tile Pon au Port peniliSu;d is rocky, as is the short and precipitous coastal fringe of the Indian Head massif south of Stephenville. Extensive areas of coastal sediment accumulation

include the Flat Island barrier complex (Shaw and Forbes 1987), extending for 12 km along the south coast of St. George's Bay, the broad beach-ridge plain at Stephenville, the barrier at Stephenville Crossing, and the spit on the southeast coast of the Indian Head Range. The bathymetry of inner St. George's Bay is shown in Fig. 2 . A sill extends across the bay from just west of Bank Head to the Port au Port isthmus . In the middle of the bay, it is typically at a depth of 25m, but shallows to about 17 m in places. Landward of the sill are two basins. The northern basin is located offshore from the Stephenville beach-ridge plain and is relatively shallow , with a maximum depth of 57 m. This contrasts with the southern basin, where the maximum depth is 97 m. The sill slopes gently seaward, but on the landward side it terminates in an abrupt slope, typically 25- 30 m high, but up to 55 m high where it borders the deep southern basin . The Stephenville beach-ridge plain and the Flat Island barr1er rest 01. gently shelving submarine platforms, which are relatively steep below pronounced breaks in slope . Offshore from Flat Island, the

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FIG. 2. Bathymetry of inner St. George's Bay, core and sample locations, and the distribution of gas in Quaternary sediments . The presence of gas is a proxy indicator of relatively thick postglacial mud in basins.

break in slope associated with the platform rises from - 25m in the southwest, where the platform merges with the sill, to about -5 m at the distal end of the spit.

Late Wisconsinan events Whereas Flint (1940) and MacCiintock and Twenhofel (1940) thought that the St. George's Bay region (Fig. I) had been glaciated by ice from Labrador, Brookes ( 1970) showed that a Newfoundland-centred ice cap existed during the Late Wisconsinan maximum. The concept of an independent ice centre in Newfoundland represented a return to earlier views first enunciated by Bell ( 1884) and later supported by Tyrrell (cited in Fairchild 1918). Publications by Grant (1977a, 1977 b, 1987), Grant and King 1984, Brookes (1969, 1970, 1974, 1977a, 1977b, 1987) , and Dyke and Prest (1987) showed that during the Late Wisconsinan, the last and most limited of three successive glaciations of western Newfoundland (Grant 1977 b), St. George's Bay was occupied by a piedmont glacier, confined by the nunataks of the Anguille Mountains and the Lewis Hills (Fig. 1). However, the position of the ice limit in the

bay has remained uncertain . Shearer (1 973) reported an "end moraine feature" about 37 km west of Port au Port peninsula and suggested that this might represent the limit, but Grant (1980) argued that this moraine is outside the limit of Late Wisconsinan glaciation. Quaternary sediments exposed in the coastal areas around the bay record the presence and retreat of Late Wisconsinan ice and changes in relative sea level . The oldest Quaternary unit on land is the St. George's River Drift of MacClintock and Twenhofel (1940). This is a compact lodgement till, 1- 5m thick, of Late Wisconsinan age (Brookes 1974). Marine submergence was registered when the ice had retreated to the approximate position of the present coastline. The limit of submergence is 29-44 m above present sea level, and radiocarbon dates on samples from the marine sediments range from 13.7 to 12.6 ka BP (Brookes 1969, 1974, 1977 b; Grant 1987). Marine silty clays are overlain by delta bottomset beds (silty clay, sandy silt) , which are in tum overlain by delta foreset and topset beds (gravel and sand). This suite of predominantly coarse-grained deposits, which prograded into the sea only a short distance from the ice front,

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ILLUSTRATED SEISMIC S ECTION \IIIII I LLUSTRATED SI DESCAN SONAR RECORD

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FIG. 3. Track chart of MY Navic11la cruise 88-018 (E) and the locations of seismic sections and a sidescan sonar record used as illustrations.

was termed the Bay St. George Delta by MacC!intock and Twenhofel ( 1940). Ice-contact sediments overlie the marine deposits in places , forming the Robinson's Head moraine, which along the south coast was formed by a read vance of the ice front. At Stephenville, where the moraine records a temporary halt in a general recession, it was dated by Brookes (1977a) at 12.6 ka BP. He inferred that it was contemporaneous with a relative sea level of +28m. However, evidence by Grant (1987) suggests that, just west of Stephenville, the moraine might predate 13.1 ka BP, although several dates from elsewhere in the region are consistent with the accepted age. Relative sea levels Brookes et al. (1985) showed that relative sea level, which was at +44 mat 13.7 ka, dropped below present datum ca. 10 ka BP, reach~d a minimum 0f -11 to -14m ca. 6 ka BP, and subsequently rose. While additional dates presented by Shaw and Forbes ( 1987) were in agreement with the later part of this curve, recent evidence from several sources suggests that a modified curve is required . Grant (1 987) showed that at Romaines,

0.7 km west of the mouth of Romaines Brook (Fig. 2) , freshwater conditions existed close to present sea level at 11.5 and 12.7 ka BP. He inferred that relative sea level fell below the present level just after 13 ka BP. This is in conflict with Brookes' conclusion ( 1977 a) that relative sea level was at + 28 m at 12.6 ka BP. Grant ( 1987) also suggested that relative sea level subsequently fell below the postglacial minimum of Brookes et al. ( 1985). Forbes and Shaw (1989) describe seismostratigraphic evidence from Port au Port Bay and St. George's Bay showing deltas graded to a level of approximately 25m below present sea level. This is suggestive of a postglacial (early Holocene) minimum at this level, rather than the -II to - 14 m argued on the basis of previous evidence . Methods The 1988 N avicula survey (Forbes and Shaw 1989) yielded 313 line kilometres of acoustic data (Fig. 3) . Bathymetry was obtamed with a 30 kH:t. Etac echo sounder, and sidescan sonar imagery was collected using a 100kHz Klein towfish. Shallow seismic data were obtained with a Datasonics bubble-pulser system and also with an ORE Geopulse receiver, NSRFC

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FIG. 4. Quaternary sediment thickness in St. George's Bay, based on data from cruises 69-048 , 88-018 (E), and 89-008. Al so shown are the axes of bedrock valleys and the approximate position of a maj.or late Quaternary ice margin indicated by the d ata.

streamer, and a multitip sparker sound source. The sparker system provided the best quality of data, although the bubblepulser system gave superior resolution of bedrock structure. Navigation was by radar (I 0 min fixes) and Loran-C (raw time differences logged every minute). Twenty-one grab samples and two gr~vity cores were recovered. Particle size analyses were performed on the grab samples. Figure 2 indicates the locations of grab samples and a core. Figure 3 shows the survey tracks and the positions of seismic sections and sidescan records used as illustrations. Interpretation of the Navicula data was hindered by the strong bubble pulse that obscured the upper few metres of sediment, the strong surface multiples that obscured the record in shallower areas, and a poor return signal in some places, in particular on the shallow, gravel-covered sill. Perhaps the most frustrating problem was the loss of signal because of shallow gas, which is present over significant areas of the bay (Fig. 2). The gas is restricted to postglacial mud in basins and obscured the acoustic stratigraphy in those areas, making it difficult to determine the depth to acoustic basement. Data gathered from CNA v Sackville in 1969 (Shearer 1973) were also examined. The ship's tracks within the study area are shown in Fig. 1. The seismic equipment used was a Bolt 600-B airgun (1 in. 3 chamber (1 in. = 25.4 mm)) operated at air pressures of 1000-1500 psi (1 psi= 6.895 kPa). This data set was of some use in determining the geometry and thickness of Quaternary sediments outside the inner bay, but internal acoustic structure could not be resolved. Consequently, to

determine more clearly the relationship between sediments in the study area and those in St. George' s Bay proper, and in particular to define the westerly extent of thick glacial diamict sequences observed below the sill, additional data were gathered during 1989 during the css Baffin survey. The seismic equipment carried by this vessel included a 40 in. 3 airgun and a Huntec Deep Tow System (Josenhans et at. 1989). A plotofthe vessel's track in the study area is shown in Fig. 1.

The bedrock surface Bedrock in the inner bay typically comprises gently folded conformable strata. The lack of identifiable bedding immediately southwest of the Indian Head Range (Fig. 2) raises the possibility that bedrock in that area may be an extension of the Proterozoic anorthosite onshore. However, a strong anomaly in the magnetic data is confined very close to the shore at Indian Head, suggesting that the Proterozoic bedrock is confined close to the headland. The basins in the inner bay are underlain by deep submarine bedrock valleys (Figs. 4, 5). The deep valley extending from the head of the bay toward the southwest (Figs. 4, 5) is a continuation of the St. George's River lowland. For convenience we refer to it as the St. George Valley. Maximum depth of bedrock surface is typically greater than 160m. At the head of the bay, just offshore from Stephenville Crossing (Fig . 5), a maximum depth of 195 m was recorded. The valley swings to the northwest off Bank Head, where it shallows to 125 m, and continues to the southwest, gradually deepening to 140 m northwest of Highlands (Fig. 4). There are several tributary

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FIG . 5. Topography on acoustic unit 1 in metres below sea level. ln the few areas where the unit is absent , this is a represention of the bedrock ~urface .

valleys in the upper bay (Fig. 5) . One of these lies offshore from ·· Flat Bay Brook, and another is located just to the north of the first and is separated from the valley off Stephenville by a col at about 90 m water depth. The second major submarine valley, the Stephenville Valley, lies offshore from the Stephenville beach-ridge plain and appears to extend headward beneath it (Figs . 4, 5). Maximum depth of bedrock surface in this area exceeds 180 m, but the valley shallows to about I 00 m in the middle of the bay and appears to continue to the southwest where, like the St. George Valley, it becomes a relatively shallow feature .

Acoustic units and environmental interpretations Inner St. George's Bay contains a considerable thickness of Quaternary sediments, amounting to more than 180 m at the head of the St. George Valley, offshore frow Stephenville Crossing (Fig. 4). Elsewhere in this basiu, thickness is at least 60 m and exceeds 80 m in the two tributary valleys . The maximum thickness in the Stephenville Valley is in excess of 125 m, whereas values of only 30 m are typical of the area between the two valleys. There is a major depocentre in the

vicinity of the sill, running from just southwest of the isthmus (85 m maximum) to west of Bank Head (90 m maximum) . West of the sill, thickness declines seaward . We define acoustic units as intervals that are recognized on the basis of acoustic attributes, bedding, and unit geometry. Our environmental interpretation of the units is based on these characteristics, together with stratigraph ic position and lithology. Depths and thicknesses are calculated from the acoustic return times using an assumed compressional sound velocity of 1.5 km · s- 1 . We identify eight acoustic units in the Quaternary sediments of St. George's Bay and interpret them as follows: ( I) ice-contact sediments, (2) subaqueous outwash, (3) draped glaciomarine sediments , (4) postglacial mud, (5) postglacial sand, (6) postglacial fluvial delta, (7) postglacial barrier- platform, (8) postglacial spillover. The near-seabed outcrops of the~w: units are shown in Fig. 6.

Unit 1: ice-contact sediments This unit (Fig. 7) is characterized by a dark acoustic tone and an absence of coherent internal reflectors. It unconformably overlies bedrock and has an undulating or hummocky upper

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FIG. 6. Near-seabed outcrop distribution of the acoustic units. Acoustic unit 2 is confined to deep troughs and consequently does not outcrop on the seabed.

surface, except where eroded. Because of the difficulty in determining the base of the unit in some areas and of distinguishing between it and bedrock in others, we are reluctant to present a detailed isopach map, and instead illustrate (Fig. 5) the surface topography of the unit in metres below sea level. Where the unit is only a thin veneer, this is an approximation of the bedrock basement. This is especially true on knolls on the flanks of the St. George Valley and on some of the ridges south of it. On the plateau between the two main valleys, unit 1 occurs as an irregular sheet, averaging about 20 m in thickness (up to 30m in places) and loosely mimicking the relief of the underlying bedrock. In places the unit has a ridged or hummocky surface expression, with relief of 3- 4 m. Below the sill, which extends across the bay, unit 1 sediments, overlain in places by deposits of unit 5 (postglacial sand), form a bank up to 85 m thick. The bank thins seawards and contains several sequences, separated by prominent reflectors. The uppermost sequence, about 25m thick, appears to pass laterally into draped glaciomarine sediments of unit 3. Outcrops of unit 1 on the seabed are characterized by an

irregular surface, which gives strong returns on the echo sounder, and a veneer of coarse gravel and boulders, which we interpret as a lag due to winnowing. The most extensive outcrop is in a zone extending across the bay on the sill (Fig. 6). It is also exposed on the seabed in a zone extending 2 km out from the coast between the isthmus and Stephenville, on the south coast close to Bank Head, and in irregular patches on basement highs, chiefly around the distal end of Flat Island barrier. We interpret this unit as unstratified ice-contact sediments. Elsewhere, units with similar acoustic character have been cored, and the lithologies have indicated till (see, for example, Josenhans et al. 1986; King and Fader 1986). Some, at least, of this unit is an equivalent of the St. George's River Drift (MacClintock and Twenhofel 1940), because on a number of survey lines approaching the coast, unit 1 directly overlies bedrock, and at nearby coastal exposures the St. George's River Drift is the lowest Quaternary sediment. In several places, pockets of stratified sediment are noticeable within the unit and are perhaps analogous to those contained within the St. George's River Drift (MacC!intock and Twenhofell940; Brookes 1974).

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Unit 2: subaqueous outwash This unit (Figs. 8, 9) is characterized by a light tone and closely spaced, coherent, subhorizontal reflectors. In places these can be traced over relatively large distances; elsewhere they are irregular and discontinuous. The unit occurs as a ponded trough fill, onlapping the basin walls. It overlies either unit 1 (ice-contact facies) or bedrock and is invariably overlain by unit 3 (ice-distal facies) . The major occurrence is in the Stephenville Valley, where the un it is more than 60 m thick in places and forms an elongate body, confined between steep valley walls. The surface of the valley fill slopes from about -70 m just off Port Harmon, and just southeast of the isthmus , to about - 100 m in the deepest parts of the Stephenville Valley. Gas and surface multiples nbscure the recnrd of this facies in the St. George Valley. The unit is at:-o pr esent 10 valleys close to the end of Flat Island barrier. Here its upper surface is incised by valleys up to 15m deep and drops from - 40 m close to the shore to - 70 m. There are no seabed outcrops of unit 2.

We propose that unit 2 was formed by ice-proximal deposition of subaqueous outwash. Evidence for proximity to ice is found in the northern basin, where an interfingering wedge of unit 1 sediments (Fig. 9) was deposited by a minor read vance of an ice front oriented approx imately parallel to the present coast in that area. In the absence of seabed outcrop or core evidence, we speculate that these proglacial stratified sediments comprise interbedded sand and mud, possibly with pockets of homogenous mud or ice-rafted debris (Powell and Molni a 1989).

Unit 3: draped glaciomarine sediments This unit has a generally light tone and closely spaced , coherent, parallel reflectors of moderate to strong intensity, which can be traced laterally over distances of severn! kilometres. It is generally highly confonnable with the topvg~:1phy or llle underlying unit, forming a draped sheet (Fig. 7) that is typically 20 m thick but up to 35 m thick in places at the head of the bay. At one location in the northern basin draped sediments of unit 3

CAN. J. EARTH SCI. VOL. 27 , 1990

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15. Sidescan sonar record from the sill region, showing gravel ripples of various wavelengths.

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I973; Hunter et al. 1982; Forbes and Boyd I987, 1989), the ripples in St. George's Bay tend to occur in quasi-parallel bands or ribbo'"1S o f Yarying width align("d roughly normal to ripple crests . Patches of sea floor between ribbons may consist of relatively immobile gravel or may contain bedforms too small to be resolved on the sidescan records. In some cases, stringers of sand cover the area between ribbons. · Part of a sidescan sonar record showing gravel ripples on the St. George's Bay sill is reproduced in Fig. 15. The location of this area is indicated in Fig. 3. The appearance of the ripples as interpreted from the sidescan records is similar to that found at sites where ripple morphology has been observed directly (Forbes and Boyd 1989). Wave action is the dominant factor in their formation. We therefore interpret the gravel ripples on the St. George's Bay sill as wave-formed features. Their wavelength X., scaled to the nearbed orbital diameter d0 of the wave motion , is given by X. = k.d0 , where kr !S 1 is an empirical constant (Forbes and Boyd 1987). Ripple wavelength varies from less than I m to at least 3 m, and bedforms of very different wavelength are abruptly juxtaposed in many places (Fig. 15). This presumably reflects differences in grain size manifested through differing entrainment thresholds, such that ripples in coarser gravel are active only at higher shear stresses or orbital velocities (therefore preserve longer wavelengths) than ripples formed in finer sediments. In some cases, secondary ripple crests are observed in the troughs of the longer-wavelength features. This may reflect differing mobility of various gravel fractions at a given site. · A rough appreciation of sediment mobility in ripple patches on the sill can be gained from an analysis of threshold wave conditions for gravel entrainment. Following Forbes and Boyd ( 1987), the critical near-bed orbital velocity in metres per second can be computed as [I )

Ut

=

[5(D31) 117]/2

where D is the grain size in metres and T the wave period in seconds. The orbital diameter, d0 , is obtained from the relation d0 = u1TITr , and the associated wave height, H, from linear wave theory asH == d0 sinh(kh), where k = 27TIL (Lis the wavelength) and h is the depth. Two samples ( 17 and I8) were obtained from ripple patches on the sill in water depths of 24 and 26 m , respectively. Computations of threshold wave conditions for the dominant 2 mm mode m sample 17 give combinations ranging from H = 2m(d0 = 0.6m) for a 7 s wave period toH = I .2m (d0 = 0.9 m) at a wave period of IO s. The combinations obtained for the dominant 29 mm gravel mode in sample 18 range from H = 4 .5 m (d0 = 2.3 m) at a 9 s wave period to H = 3.8 m (d0 = 2.9 m) at T = II s. Although wave data against which to compare these results are extremely limited, observations reported by Ploeg (197I) in the central Gulf of St. Lawrence indicated a seasonal mean peak wave period of 5.9 s, with median significant wave height of 1.37 m , and a I % exceedance peak period of I0. 8 s with a wave height of 4.57 m. The latter condition is more than adequate to satisfy the entrainment condition for 29 mm gravel in 26 m water depth, where it would produce ripples with a wavelength of approximately 3 m (assuming X. = d0 ). These computations demonstrate that gravels covering a wide size spectrum, forming ripples at a wide range of scales, are mobilized frequently under present-day conditions over much

of the St. George's Bay sill. Entrainment frequency and transport rates would have been higher with reduced water depths earlier in the Holocene. Discussion The thick sequence of ice-contact sediments in the St. George's Bay sill, which blocks two overdeepened bedrock valleys, is evidence of the sustained presence of a margin of the Late Wisconsinan Newfoundland ice sheet. The lateral transition into draped sediments of unit 3, and the presence of iceberg furrowing, shows that the ice front terminated in relatively deep water. The moraine (Fig. 4) lies obliquely across the inner bay, extending northeast from about 10 km west of Bank Head, towards the Port au Port isthmus. From west of Bank Head it appears to extend to the southwest, reaching the coast at the northern end of the Anguille Range. We remain uncertain of the position of the equivalent ice margin north of St. George's Bay. A detailed seismic survey within Port au Port Bay (Forbes and Shaw I989) revealed no evidence of a terminal position . If a lobe of ice did extend across the shallow bay, as Grant (1987) has suggested , we speculate that it may have been relatively thin compared with the ice in St. George's Bay, and consequently might have fom1ed a Jess welldefined moraine. A moraine comprising up to 100 m of icecontact sediment was observed on seismic records at the mouth of the Bay of Islands during cruise 89-008, in the position of the arcuate body illustrated by Grant (1987). The moraine, with numerous internal reflectors, is perched on the seaward lip of a deep bedrock basin. Combined with the evidence from St. George's Bay, this suggests that on the west coast of Newfoundland the accumulation of thick deposits associated with Late Wisconsinan ice is localized to those settings where strong ice streams debouched to the coast between nunataks . Radiocarbon dates on overlying marine sediments (see below) suggest that the ice-marginal deposit on the St. George's Bay sill is Late Wisconsinan in age and predates 13.7 ka BP. However, this does not resolve the controversy concern ing the extent of Late Wisconsinan ice in the region. It may have extended farther into the bay, but confirmation of this awaits coring of the locally complex sequences of till and glaciomarine mud in the outer bay and beyond. Relative sea level registered a postglacial maximum by ca. 13 .7 ka BP (Brookes 1974). Ice recession to the approximate position of the present coastline (B rookes I 974) took the form of a calving ice front (as suggested by Brookes 1969; Grant 1987). By ca. 13.5 ka BP, open water ocnicc.! expertise was provided by Donald Locke and Darrell Beaver. We also acknowledge Captain Lewis and the crew of css Baffin

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