Late Cenozoic sea-level chan the onset of g tion: impact on c o n t u t a l progr ation off eastern Canada David J. W. Piper Atlantic Geoscience Centre, Geological Survey of Canada, Bedford Institute of Oceanography, PO Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2

and William R. Normark US Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

Received 20 November 1988; revised 14 June 1989

Late Cenozoic sedimentation from four varied sites on the continental slopes off southeastern Canada has been analysed using high-resolution airgun multichannel seismic profiles, supplemented with some single channel data. Biostratigraphic ties are available to exploratory wells at three of the sites. Uniform, slow accumulation of hemipelagic sediments was locally terminated by the late Miocene sea-level lowering, which is also reflected in changes in foraminiferan faunas on the continental shelf. Data are very limited for the early Pliocene but suggest a return to slow hemipelagic sedimentation. At the beginning of the late Pliocene, there was a change in sedimentation style marked by a several-fold increase in accumulation rates and cutting of slope valleys. This late Pliocene cutting of slope valleys corresponds to the onset of late Cenozoic growth of the Laurentian Fan and the initiation of turbidite sedimentation on the Sohm Abyssal Plain. Although it corresponds to a time of sea-level lowering, the contrast with the late Miocene Iowstand indicates that there must also have been a change in sediment delivery to the coastline, perhaps as a result of increased rainfall or development of valley glaciers. High sedimentation rates continued into the early Pleistocene, but the extent of slope dissection by gullies increased. Gully-cutting episodes alternated with sediment-draping episodes. Throughout the southeastern Canadian continental margin, there was a change in sedimentation style in the middle Pleistocene that resulted from extensive ice sheets crossing the continental shelf and delivering coarse sediment directly to the continental slope.

Keywords: sea level changes; sedimentation styles; contintental slope progradation; eastern Canada

Introduction

The eastern Canadian continental shelf lies on a passive continental margin that rifted in the Triassic (Grant et al., 1986). It was emergent for much of the late Tertiary (King and Maclean, 1976; King and Fader, 1986) and was crossed by late Pleistocene ice sheets. Parts of the adjacent continental slope have been eroded throughout much of the Pleistocene (Piper and Normark, 1982a), probably as a result of canyon incision, whereas elsewhere there has been gradual slope progradation through the late Cenozoic (Piper et al., 1987). A regional biostratigraphic framework is provided by eight deep-water exploratory wells (that are no longer confidential) in water depths of more than 700 m and further well control on the outer shelf (Figure 1). The area thus provides a good opportunity to examine the evolution of a continental margin during the sea-level changes and climatic cooling of the later Cenozoic.

High resolution multichannel airgun profiles have been collected from selected areas of the continental slope as part of a programme to investigate the constraints to exploratory drilling posed by potential slope instability. The most detailed data sets were acquired in the vicinity of the 1929 Grand Banks earthquakeepicentre (Piper and Normark, 1982b) on the St Pierre Slope off St Pierre Bank and in the vicinity

0264-8172/89/040336-12 $03.00 ©1989 Butterworth & Co. (Publishers) Ltd

of the first deep-water drilling on the Scotian Slope, the Acadia K-62 well just west of Verrill Canyon. Subsequently, additional single-channel airgun profiles were acquired in Flemish Pass because of the drilling activity in that area. The shelf break at the top of the Scotian and St Pierre Slopes lies between 80 and 400 m depth at the edge of the irregular, glaciated continental shelf. The slope is crossed by submarine canyons that debouch onto the continental rise and the Sohm Abyssal Plain at water depths of around 5500 m.

Flemish Pass lies in a graben that developed during Cretaceous rifting of the Labrador Sea (Grant et al., 1986). Maximum water depths in the pass are about 1100 m, and the slope on the west side of the pass rises steeply to about the 400 m isobath and then more gently to the shelf break at the edge of the Grand Banks in about 100 m of water. There is an intensification of southward-flowing bottom currents in the restricted waters of the pass (Pereira et al., 1985).

The pre-Wisconsinan Plio-Pleistocene history of the eastern Canadian continental margin is poorly known. Gradstein and Agterberg (1982) demonstrate that in outer-shelf wells, the late Miocene is commonly absent or represented only by benthonic or reworked foraminifera, whereas earlier Miocene and early Pliocene units are characterized by a rich foraminiferal fauna including planktonic forms. The late Pliocene and Pleistocene are also characterized by benthonic

336 Marine and Petroleum Geology, 1989, Vol 6, November

Late Cenozoic sea-level changes, eastern Canada: D. J. W. Piper and W. R. Normark 62"00'W 61030 ' 61"00' 60°30 ' 60 = 58* 5 0 ° 4 5 °

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and reworked foraminifera. There is, therefore, local biostratigraphic evidence for the episodes of eustatic sea-level lowering in the late Miocene and late Pliocene- Quaternary proposed by Vail and Mitchum (1979) and Haq et al. (1987). On the continental shelf, there is a major unconformity between the Tertiary Banquereau Formation and Quaternary glacial sediments (King et al., 1974), but the age of this unconformity is not well constrained. Continental ice sheets crossed at least parts of the continental shelf during the Wisconsinan glaciation (King and Fader, 1986), and the preceeding isotopic stage 6 glaciation was even more extensive (Alam et al., 1983). On the basis of identification of till on seismic reflection profiles, and extrapolated ages, Mosher et al. (1989) suggest that the stage 6 glaciation was the first major glaciation to cross the Scotian Shelf. Ice rafting is known from the late Early Pliocene in the Labrador Sea (Srivastava et al., 1987), and the early Late Pliocene on the Fogo Seamounts southwest of the Grand Banks (Piper, 1975), but there is no evidence for local glacial ice on the Scotian Shelf and Grand Banks in the Pliocene or Early Pleistocene.

set to 27 to 248 Hz. Digital processing resulted in a deconvolved 12-fold stack.

Additional data from the Scotian Slope, and all data from Flemish Pass, consist of single-channel airgun (0.65 1) profiles collected on various cruises over the last five years. Position control on all surveys used an integrated Loran C and transite satellite navigation system and is estimated accurate to +150 m.

Our processed seismic reflection profiles yield useful data typically to 1 to 1.5 s sub-bottom. Depositional style changes progressively from relatively continuous planar reflectors at depth to a deeply dissected near-surface section. The erosional complexity of the upper continental slope in the vicinity of submarine canyons makes it difficult to identify and correlate regional unconformities as advocated by Mitchum et al. (1977). We have, therefore, identified key seismic reflectors of packets or reflectors of distinctive character that can be traced through much of a local study area and that mark horizons of significant change in depositional style.

Data

The principal data set used in this study is about 1000 km of high resolution multichannel seismic reflection profiles from the upper continental slope off southeastern Canada obtained in 1981. Seismic data were recorded digitally using a DFS IV recording system with a 400-m long, 24 channel streamer towed at 2 -5 m below the sea surface. The sound source was a 0.65 litre (40 cubic inch) air gun towed at a depth of 3 m. The shot interval was 15-18 m, sample interval 1 ms, record length 3 s, and the recording filters were

Deposit ional sequences

Centra l Sco t i an S l o p e

In the Verrill Canyon area of the central Scotian Slope, a grid of seismic profiles allows a detailed seismic stratigraphy to be defined that extends from a relatively smooth progradational slope in the west to the deep valleys of Verrill and Dawson Canyons in the east. The Acadia K-62 and Shubenacadie H-100 wells provide both lithologic and biostratigraphic control. Piper et al. (1987) identified and illustrated eight key reflectors within this area. Similar studies in areas further east

Marine and Petroleum Geology, 1989, Vol 6, November 337

Late Cenozoic sea-level changes, eastern Canada: D. J. W. Piper and W. R. Normark

C E N T R A L S C O T I A N SLOPE

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along the margin show that four late Cenozoic horizons mark particularly significant changes in depositional style. Mid-Cenozoic sediment on the central Scotian Slope consists of a thick sequence of smooth slope-parallel reflectors, represented by biogenic claystones in wells. This monotonous sequence is terminated in this area by the cutting of a broad shallow channel and aggradation of low levees shortly after the deposition of the 'light green' reflector of Piper et al. (1987). (In Figure 2a the 'light green' reflector is shown as E, the channel as 1.) A prominent interval of uniform sediment 0.1 to 0.2 s thick is draped over this earlier relief. The top of this draped packet is marked by the 'red' reflector of Piper et al. (1987), which is

shown as D in Figure 2a. Immediately above the draped packet there ~s locally a sequence of short, discontinuous reflectors that fills hollows in the 'red' surface and forms closely-spaced mounds and gullies above it (2 in Figure 2a). It is at this stratigraphical level on the slope that there is the first clear indication of widespread gullies that tend to persist and enlarge up-section. Alternating sand and mud occur in the wells.

Relief features developed just above the "red' (D) reflector are maintained through the stratigraphic section to a prominent, surface with considerable topographic relief, termed C in this paper, about 0.2 s above 'red' (D) (Figure 2a and Table 1). (This is a horizon between 'blue' and 'gold' in the scheme of Piper et al., 1987). The sequence above C tends to drape over small-scale irregularities, and is gently undulating with the development of levees adjacent to the larger valleys (3 in Figure 2a). The levee sections are characteristically wedge-shaped packets of relatively planar, crisp reflectors that thin or pinch out away from gully and larger scale erosional features. The 'yellow' horizon (B in Figure 2a) in places marks a change from levee aggradation to a more draped style of deposition. On dip lines, the 'yellow' (B) reflector corresponds to the base of stacked diamict sequences and proglacial sediment wedges on the uppermost slope (Mosher et al., 1989). Strata above the 'yellow' (B) reflector show rapid downslope thinning, and marked changes in thickness observed in strike lines are not associated with the major canyons.

Biostratigraphic control is available from the Acadia K-62 and Shubenacadie H-100 wells (Figure 1), which yield a consistent stratigraphy based on planktonic foraminifera (Gradstein, in Piper et al., 1987). Data from the Shubenacadie well indicate that the 'light green' reflector is of mid-Pliocene age (zones N19-N20). Data from Acadia indicate that the base of the late Pliocene N21 zone lies between 'red' and 'light green'; and the base of the Pleistocene at about the C reflector (Figure 5 of Piper et al. 1987). There is no biostratigraphic evidence for the age of the 'yellow' reflector, but extrapolation of glacial sedimentation rates based on radiocarbon dating suggests that it may mark the base of isotopic stage 6 (0.2 Ma) (Mosher et al., 1989). Biostratigraphic data indicate Miocene to early Pliocene sedimentation rates of the order of a few centimetres per thousand years; late Pliocene rates of about 35 cm per thousand years, and Quaternary rates of 15 to 30 cm per thousand years (Piper et al., 1987).

A summary of the seismic framework for this region provides the basis for interpretation of late Cenozoic

Table 1 Nomenclature of key seismic reflectors recognized in this study and their probable age

St Pierre Slope Central Scotian Slope Flemish Pass Logan Canyon

A (top of lower till) (above yellow) B (base of lower till) yellow B' B'

gold C (erosional surface) (blue-gold) C' C'

blue D (top of blanket) red D' D' E (base of sands) light green E'

dark green F' F'

Age

Stage 6

Basal Pleistocene

Late Pliocene Mid Pliocene ? Late Miocene

* Colour designations from Piper et al. (1987)

3 3 8 M a r i n e a n d P e t r o l e u m G e o l o g y , 1989 , Vo l 6, N o v e m b e r

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depositional history in other areas where there is less reflecting fluvial transport of sediment across an well control for either lithology or age. Miocene and emergent shelf (Piper et al., 1987). Most of the gullies early Pliocene strata on the central Scotian Slope cut at this time persisted through to the Quaternary, accumulated slowly and uniformly (probably under and some enlarged to become submarine canyons. conditions of relative high sea-level stand), and no Deposits with levee-like structure accumulated seismic expression of the late Miocene low stand of sea between t h e gullies. In the earliest Quaternary (at level has been recognized. This phase of sedimentation horizon C), there was renewed gulley incision, but was terminated with the cutting and aggradation of an above this horizon there was a regional decrease in 'ancestral valley' in mid-Pliocene time. This valley was sedimentation rate and prominent levee-like growth partly filled by draped early Late Pliocene strata. There was restricted to the major channels. This suggests that was then a Late Pliocene event characterized by the these major channels acted more efficiently as widespread cutting of shallow gullies, associated with a pathways for sediment transported across the marked increase in sedimentation rates, perhaps continental slope. The onset of shelf-crossing

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Marine and Petroleum Geology, 1989, Vol 6, November 339

glaciations in stage 6 led to a change in sedimentation pattern with the continued incision of major canyons, the deposition of till and other coarse ice-proximal sediments on the upper slope, and the accumulation of proglacial sediment wedges on the middle slope.

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3 4 0 M a r i n e and P e t r o l e u m G e o l o g y , 1989, Vol 6, N o v e m b e r

Late Cenozoic sea-level changes, E); channel floors are marked by short discontinuous acoustic reflections (e.g. 1 in line 3, Figure 4 and in line 7, Figure 5). This subdued channel relief is then largely filled by a packet of draped reflectors: the base of this packet is marked by local erosion on the channel walls (e.g. 2 in line 3, Figure 4 and in line 13, Figure 5) and the top of the packet (reflector D) is a widespread seismic marker. This packet also marks the top of a thick sequence of parallel-stratified reflectors beneath Eastern Valley of Laurentian Fan that is overlain by a thick sequence characterized by short discontinuous reflections (extending west from 3 in line 7, Figure 5). The packet capped by reflector D is overlain locally by low relief channels filled with short discontinuous reflectors (e.g. 4 in line 3, Figure 4 and in line 7, Figure 5) and low adjacent levees. Most of these channels are broad and fill residual depressions in the surface of reflector D. Elsewhere, reflector D is overlain by parallel reflectors interpreted as a prograded mud sequence (e.g. 5 in line 7, Figure 5).

By 0.1 s above D, deposition of mud becomes more widespread, and irregularities in the surface of D have become masked. Locally, gullies are developed (6 in line 3, Figure 4 and in line 7, Figure 5), marked by erosional cutouts and low levees: these are generally narrower than the valleys immediately above D. About 0.2 s above D is an erosional surface (reflector C) showing numerous narrow gullies (line 3, Figure 4; line 13, Figure 5). Strata above C are generally draped, but show local wedging (thought to reflect levee development) particularly on the west flank of Halibut Canyon and along the small valleys at the eastern edge of Laurentian Fan (7 in line 3, Figure 4). There is a substantial contrast in thickness between a rather condensed sequence adjacent to Eastern Valley of Laurentian Fan (western end of line 7, Figure 5) and the thick progradational sequence on the high immediately west of Halibut Canyon (eastern end of line 7, Figure 5). Channel-fill facies (short discontinuous reflections) are seen adjacent to Eastern Valley at the western end of line 13 (8 in Figure 5).

There is then another pronounced change in depositional style at reflector B (e.g. southeast end of line 3, Figure 4; line 13, figure 5), that is locally marked by considerable erosion (e.g. 9 in line 7, Figure 5).

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eastern Canada: D. J. W. Piper and W. R. Normark Levee-like aggradation appears to terminate at this reflector, which upslope marks the base of the deepest acoustically incoherent sediments on the uppermost slope and outer shelf. Strata immediately above B show marked variation in thickness: they are thickest on the upper slope and locally onlap residual levee accumulations on reflector B downslope (10 in line 3, Figure 4). Upslope they are acoustically incoherent, but downslope they become well stratified. Reflector A marks the top of these well stratified sediments; the overlying sequence is acoustically transparent except on the upper slope where it becomes acoustically incoherent. The thickness variations in the transparent layer indicate that it is not solely an artifact of processing. There is some levee growth and gully cutting in this sequence above reflector A. At the margin of the Eastern Valley of Laurentian Fan, a stratified-sediment sequence has prograded across the deeper sequence below reflector B characterized by short discontinuous reflectors on channel floors and intervening levees (8 at western end of line 13, Figure 5).

There is no biostratigraphic control for the sequence on St Pierre Slope. The seismic sequence can be correlated with that on the central Scotian Slope on the basis of geometrical character and overall stratigraphic thickness (Figures 2 and 6). Some of the correlatable features on the central Scotian Slope are illustrated in Figure 2. Others have been illustrated in Figures 3, 4 and 5 of Piper et al. (1987). The most striking correlation is that of D with the 'red' reflector on the central Scotian Slope. In both areas, the reflector occurs at the top of a packet of reflectors that fill underlying broad valleys (1 in Figure 2: the valley on the central Scotian Slope is more deeply incised). Reflector D is overlain by a horizon with small-scale irregularities (2 in Figure 2). Reflector C appears to correlate with a pronounced reflector between 'blue' and 'gold' on the central Scotian Slope, but shows more irregular relief on the St Pierre Slope (Figures 2 and 6). This correlation is based on relative stratigraphic position and the most widespread evidence for local erosion in inter-canyon areas between the readily correlable reflectors B and D. Reflector B marks a change in sedimentation pattern similar to that at

SEABED DIAMICT ON UPPER SLOPE,TRANSPARENT DRAPE THINS DOWNSLOPE

( ~ BASE OF TRANSPARENT SEQUENCE DIAMICTON UPPER SLOPE,THIN SEDIMENT ELSEWHERE, LOCALLY ABSENT.

MARKED EROSION SURFACE,TERMINATES LEVEE GROWTH

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IRREGULAR SURFACE, LOCALLY EROSIONAL

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Figure 6 Schematic dip and strike sections summarizing key events recognized in seismic reflection profiles from the Late Cenozoic sequence on St Pierre Slope

M a r i n e and Pet ro leum Geology , 1989, Vol 6, N o v e m b e r 341

Late Cenozoic sea-level changes, eastern Canada: D. J. W. Piper and W. R. Normark

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'yellow' on the central Scotian Slope (Figure 2). The St Pierre Slope sequence can also be correlated

with the seismic stratigraphy of the Laurentian Fan (Piper and Normark, 1982a). Seismic-profile correlation down the St Pierre Slope to the eastern levee of Eastern Valley (Wilson and Piper, 1986) shows that reflectors D and E correspond approximately to horizon L on the Laurentian Fan. This correlation also indicates that horizon A of Piper and Normark (1982a) lies somewhere between reflectors D and C on St Pierre Slope. An outcrop of mudstone sampled from DSRV Alvin on the floor of Eastern Valley at about 43 ° 37' N close to the level of horizon A, contained a late Pliocene planktonic foraminiferal fauna (F. Gradstein, personal communication, 1987).

Flemish Pass

Cenozoic sediment has prograded on the slope between the Grand Banks and the floor of Flemish Pass (Figures 7 and 8). No regional high resolution multichannel data are available; our interpretations in this region are based on single-channel airgun (0.651) seismic reflection profiles. At sub-bottom depths greater than about 0.3 s (below reflector C in Figure 9), there is a sequence of slope-parallel reflectors. This sequence is overlain by a packet of sediments 0.2 s thick (up to reflector B') that show gentle undulations of internal reflectors and locally appear to be associated with slope valleys (Figure 10). At about 0.1 s sub-bottom on the continental slope, a prominent and irregular reflector

(B') marks the first occurrence of acoustically incoherent sediment on the upper slope, the downslope occurrence of many small valleys and gullies, and the accumulation of a very thick sequence (>0.25 ms) of generally unstratified sediments in Flemish Pass.

Biostratigraphic control is available from the Gabriel C-60 well in Flemish Pass (Figure 7), where a change from predominantly sandy to predominantly muddy sediment at 657 m sub-bottom corresponds to the boundary between Plio-Pleistocene foraminifera above and early Miocene fauna and flora below (Gradstein, 1981).

The Flemish Pass stratigraphy can be tentatively correlated with that on the Scotian Slope and St Pierre Slope. Our shallow seismic reflection profiles do not allow correlation of the Pliocene-Miocene unconformity under the floor of the Flemish Pass with events on the flanks of the pass (Figure 8). This unconformity might be local and related to intensification of bottom current erosion, associated with the early stages of deep-water drift deposits in the North Atlantic in the Late Miocene and Early Pliocene (Tucholke and Mountain, 1986). We have used overall thickness considerations to tentatively correlate the onset of valley cutting on the flanks (at C') with reflector C on the Scotian Slope and St Pierre Slope. A distinctive package of reflectors below C' is tentatively correlated with D (Figures 9 and 10). The base of the upper slope acoustically amorphous section (B') is correlated with B on the central Scotian Slope and St Pierre Slope.

342 Marine and Petroleum Geology, 1989, Vol 6, November

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East Scotian Slope The East Scotian Slope is highly dissected by submarine canyons, which make recognition of a late Cenozoic seismic stratigraphy difficult in most areas. Thus, for example, it is not possible to correlate the late Cenozoic sequence m the adjacent deep water Tantallon well (Figure 1).

In the vicinity of Logan Canyon, a systematic seismic stratigraphy can be distinguished (Clifford, 1986) from our high-resolution seismic reflection profiles (Figure 11). This section can be tied to the outer shelf Triumph P-50 well (Gradstein and Agterberg, 1982) through an industry, low-resolution seismic profile SAS 110. In this well, the later Miocene and undivided late Pliocene to Pleistocene have predominantly benthic foraminiferal assemblages, whereas the rest of the Miocene and the early Pliocene have richer assemblages with more planktonic species. The early Pliocene marker can be traced downslope to the high resolution, multichannel strike profile (Figure 11), and immediately underlies a reflector marking a prominent slope valley cutting event (E'). Higher in the section, the reflector at the top of a draping unit (D') and an irregular reflector marking the beginning of pronounced levee growth (C') are correlated with reflectors D and C respectively on St Pierre Slope (Table 1). The erosional features near reflector C on the St Pierre Slope (Figure 6) are analogous to those seen at the same stratigraphic level near the SAS 110 crossover in Figure 11. Levee growth above C' is marked by the mounded seismic character of sediment in the intervalley ridges. Reflector B' marks the top of the well stratified section on the upper slope (western end of profile in Figure 11), and is overlain by a •thick, generally acoustically incoherent section.

Regional and global correlations

On the central Scotian Slope, Piper et al. (1987) recognized a pronounced change in type of sediment and a five-fold increase in accumulation rate from the Miocene and early Pliocene to the late Pliocene and Quaternary (Figure 12). This was interpreted as the result of an increase in the rate of fluvial transport of sediment across the continental shelf. This change might have resulted from either a generally lower sea level or climatic change. This increased rate of fluvial sediment supply was accompanied by cutting of slope canyons and gullies. The larger erosional features have persisted throughout the late Plioc~ne and Quaternary, but many of the gullies appearephemeral: Gullied horizons alternate with draped horizons, suggesting the influence of fluctuating sea level.

An increase in fluvial transport of sediment across the continental shelves occurred about the mid Pliocene. It was associated with the cutting of several submarine canyons on the continental slope, in contrast" to the earlier cutting of a single broad shallow valley. This event must have been related to lowering of sea level, and can probably be correlated with the 3.7 cycle of Haq et al. (1987).

Although the pronounced late Miocene lowering of sea level (base of cycle 3.4 of Haq et al., 1987) had no recognizable effect on the seismic stratigraphic character of the central Scotian Slope (Piper et al., 1987), there is evidence of late Miocene erosion both in the Logan Canyon area and in Flemish Pass, although in the latter case it may be associated with bottom-current erosion. On the St Pierre Slope, the high resolution multichannel data do not penetrate sufficiently deep to show such an event. The lack of

Marine and Petroleum Geology, 1989, Vol 6, November 343

Late Cenozoic sea-level changes, eastern Canada." D. J. W. Piper and W. Ft. Normark

400 -

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3 4 4 M a r i n e a n d P e t r o l e u m G e o l o g y , 1 9 8 9 , V o l 6, N o v e m b e r

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M a r i n e and Petro leum Geology, 1989, Vol 6, N o v e m b e r 345

Late Cenozoic sea- leve l changes , eas tern Canada:

well control in the uppermost sections of these areas means that there is no clear indication of the style of Early Pliocene deposition except on the central Scotian Slope.

This contrast in the effects of the Late Miocene and the mid Pliocene lowstands of sea level suggests that sea level in itself may not account for the marked changes of sedimentation style in the mid Pliocene. The climatic cooling observed at this time in higher latitudes (Srivastava et al., 1987) may have been accompanied by an increased sediment discharge to the continental margin off southeastern Canada, either because of increased rainfall, or possibly local valley glaciation.

The Late Pliocene phase of canyon and gully cutting required continued supply of clastic sediment to the shelf edge in order to achieve the observed high rates of sedimentation and to provide a mechanism (turbidity currents) for canyon erosion. This time thus marks the first phase of cutting of the 'Quaternary unconformity' on the continental shelf.

The Late Pliocene canyon cutting can probably be correlated with the abrupt change in the style of deposition on the Laurentian Fan, marked by the widespread horizon L (Uchupi and Austin, 1979). This reflector is of Late Pliocene age or older, based on the samples recovered by DSRV Alvin, and appears to form the base of the later Cenozoic fan. The base of the turbidite section in DSDP Site 382, at the margin of the Sohm Abyssal Plain, is also dated at the base of the Late Pliocene (Tucholke et al., 1979).

The amount of slope dissection increases up section from the Late Pliocene to the Early Pleistocene. This is manifested in different ways in different sections. On the central Scotian Slope and in the Logan Canyon area, the Early Pleistocene section is marked by deep gully erosion with intervening levee growth. In Flemish Pass, evenly stratified Late Pliocene sediments are followed by the cutting of broad, shallow valleys in early Quaternary time. On St Pierre slope, the Early Pleistocene section (including reflector C) is marked by widespread erosional irregularities that probably represent shallow gullies. This universal increase in slope dissection might have resulted from either longer periods of sea level lowstand, or a greater proportion of sandy sediment being delivered to the shelf break.

The Haq et al. (1987) sea level curves (Figure 12) do not portray the numerous glacio-eustatic sea level variations during their cycles 3.7 to 3.9 (and which were most pronounced in cycle 3.9). These variations in sea level are reflected in the multiple erosion events seen, for example, near reflector C on the St Pierre Slope (Figure 6) and above this level in highly dissected areas such as Logan Canyon (Figure 11).

In all the areas studied, there is a major change in sedimentation style in the Middle Pleistocene. There is a termination of pronounced levee growth and a widespread erosional event. On the upper slope, thick acoustically incoherent sediments were deposited above reflector B: these sediments thin rapidly downslope. On the central Scotian Slope (Mosher et al., 1989) and on the St Pierre Slope, there are two major acoustically incoherent units, which Mosher et al. (1989) demonstrate can be correlated with the early Wisconsinan (stage 4) and late Illinoian (stage 6) glaciations. These acoustically incoherent sediments comprise till on the outer shelf and uppermost slope, passing downslope into slumped diamicts and coarse

D. J. W. Piper and W. R. Normark

sub-glacial outwash (Piper, 1988). On the Laurentian Fan, Piper and Normark (1982a)

identified a phase of major erosion and channel reorganization (that they termed 'erosion 3') that terminated a prolonged period of levee growth. By analogy with the sequence on the continental slope, this event is correlated with reflector B at the beginning of isotopic stage 6.

In the Labrador Sea, to the north of Flemish Pass, Myers and Piper (1988) have shown that the onset of rapid sedimentation on the continental slope, accompanied by canyon cutting, began as late as the Middle Pleistocene. The greater depth and more rapid subsidence of the Labrador Shelf may account for this difference in effect of sea-level changes. The Middle Pleistocene event on the Labrador Shelf probably represents the first shelf-crossing glaciation in that region and can be correlated with the similar event in Flemish Pass and on the Scotian Shelf.

Conclusions

On the continental slope off southeastern Canada, uniform, slow accumulation of hemipelagic sediments was locally terminated by the Late Miocene sea-level lowering (base of cycle 3.4 of Haq et al., 1987), which is also reflected in changes in foraminiferan faunas on the continental shelf (Gradstein and Agterberg, 1982).

Data is very limited for the Early Pliocene, but where available on the central Scotian Slope, indicates a return to slow hemipelagic sedimentation.

At the beginning of the Late Pliocene, there was a marked change in sedimentation style: accumulation rates increased several fold and slope valleys were cut. This Late Pliocene cutting of slope valleys corresponds to the onset of late Cenozoic growth of the Laurentian Fan and the initiation of turbidite sedimentation on the Sohm Abyssal Plain.

High sedimentation rates continued into the Early Pleistocene, but the amount of slope dissection by gullies increased. Episodes of gully cutting alternated with episodes of sediment draping.

Throughout the southeast Canadian continental margin, there is a marked change in sedimentation style in the Middle Pleistocene that resulted from extensive ice sheets crossing the continental shelf and delivering coarse sediment directly to the continental slope. On the continental slope, this event is marked by considerable erosion; and there was a major reorganization of channels on the Laurentian Fan.

Acknowledgements

We thank N. J. Rushton and J. Stewart, then of Geomarine Associates, for their help in acquiring the seismic data. Felix Gradstein and Graham Williams provided biostratigraphic information; Laughie Meagher discussed local and Bernie MacLean regional seismic correlations. We thank Vince Clifford, Edna Wilson and Roy Sparkes for assistance. This work was funded by the Federal Panel on Energy Research and Development, Task 6.3. The manuscript was reviewed by Bernie MacLean, Mike Lewis and Steve Lewis. Geological Survey of Canada contribution number 37388.

346 Marine and Petroleum Geology, 1989, Vol 6, November

Late Cenozoic sea-level changes, eastern Canada: D. J. W. Piper and W. R. Normark References

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