Late Pleistocene sedimentation on the continental slope and rise off western Nova Scotia
STEPHEN A. SWI FT Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
ABSTRACT is typically much <50 m over 2 km (1:40). In slope progradation off LaHave Channel (infor- contrast, canyon relief east of 61°30'W is of the mal name proposed here for the shallow depres- Reflectivity patterns in echograms record- order of 200-1,000 m over 6 km (1:30-1:6) and sion between LaHave Bank and Emerald Baik), ed from relatively smooth continental slope extends —90 km from the shelf edge to the and Uchupi (1969) noted that the slope to the and rise off Nova Scotia can be grouped into 3,500-m isobath. east was eroded by canyons. More recent seis- four depositional associations. The shallowest Emery and Uchupi (1967) found evidence of mic and well data support their observations and association correspDnds to gravelly sand on the outer shelf that spills over the shelf edge 6
currents carried sediment (originating in deep 41° canyons to the east of 61°30'W) along the lower rise toward thie south and west. In the west, the primary depocenter was the slope and upper rise; in the east, the depocenter was on the lower rise. A glacial-sediment source on land and relict shelf morphology probably controlled sedimentation processes and, thus, the location of depocenters sea- ward of the shelf edge.
INTRODUCTION
The Nova Scotian continental slope between 65° and 61°30'W is among the smoothest in the North Atlantic. The steepest relief (maximum of 300 m over 2 km, 1:7) occurs in a narrow zone ~3 km wide between the shelf edge and the Figure 1. Bathymetry of Nova Scotian continental slope and rise from Shor (1984) modified 800-m isobath (Fig. 1). Seaward, sea-floor relief using RC 2408 data.
Geological Society of America Bulletin, v. 96, p. 832-841, 7 figs., July 1985.
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turbidity-current, channel-fan, and slump-slide features of late Pleistocene age between 180 and 1,500-m water depth (Box I in Fig. 2). Between 500 and 2,500 m directly east of 62°30'W (Box II in Fig. 2), shallow seismic structure and sedi- mentology of piston cores suggest that sedimen- tation was predominantly hemipelagic in the late Pleistocene (Hill, 1981; Piper and others, unpub. data). On the lower Nova Scotian continental rise (4,600-5,000 m), studies show that at the pres- ent time, southwest-flowing bottom currents and "abyssal storms" erode and deposit sediment (High Energy Benthic Boundary Layer Experi- ment, HEBBLE; Box III in Fig. 2; Hollister and McCave, 1984 and references therein). Mean flow at these depths is probably due to recircula- tion of the Gulf Stream within this basin and not to shallower thermohaline circulation of the Western Boundary Undercurrent (Hogg, 1983). Hollister and McCave (1984) attributed the in- tense abyssal storms to mesoscale variability of the Gulf Stream; Hogg (1983) suggested that the storms are related to instability of Gulf Stream recirculation. Pleistocene paleoceanography for this region is uncertain (Hill, 1981; Alam and others, 1983), and HEBBLE results cannot be confidently extrapolated to the late Pleistocene. Sedimentological studies on the rise have produced conflicting conclusions. Hollister (1967) and Hollister and Heezen (1972) sug- gested that reddish lutite interbedded with well- sorted silt laminations predominate in Pleisto- cene sediments of piston cores, and that reflectivity patterns of 12-kHz echograms paral- lel isobaths. They inferred that contour currents deposited both surface and subsurface sediment. Using core data, Zimmerman (1972) also con- cluded that southwest sediment transport oc- curred during the Pleistocene. In contrast, Emery and Uchupi (1972, Figure 2. Echo-sounding tracklines. Thickened lines and letters show locations of profiles p. 397-399) concluded that downslope proc- illustrated in Figures 4-7. Box I is detailed study area of Hill (1983 and 1984), Box II is study esses (turbidity currents, slumps, and slides) area of Piper and others (unpub. data), and Box III is HEBBLE study site (Hollister and emplaced most of the sediment on this rise. Oth- McCave, 1984). Tracklines are labeled with vessel abbreviation (All = Atlantis II, CH = Chain, ers argue that Pleistocene sediments on the rise KN = Knorr, RC = Robert Conrad, V = Vema) and cruise number. Cruises All 32, KN 31, and west of the Laurentian fan (insert in Fig. 2) are CH 70 used 12-kHz echo sounding; all others used 3.5 kHz. Isobaths from Figure 1. turbidites: Stanley and others (1972), on the basis of sediment texture and mineralogy; Piper (1975), using sedimentary structures; and Stow suggest that Tertiary shelf-edge topography be- instability and slumping along the heads and (1978 and 1979), using clay mineralogy, heavy- tween 65° and 61°30'W was filled and flanks of canyons. Piper (1975) suggested that mineral analyses, and sediment texture and smoothed during the Pliocene-Pleistocene, during low sea level, canyon heads received sed- structure. Piper and Slatt (1977) and Stow probably by slumps and turbidites, whereas iment from longshore drift. Piper and Slatt (1979 and 1981) suggested that south of Nova canyons formed on the slope to the east (Jansa (1977) and Stow (1978) proposed that ice- Scotia the reddish lutite and montmorillonite and Wade, 1975; Parsons, 1975; King and rafting from Laurentian Channel was also a sig- interpreted by Hollister and Heezen (1972) as Young, 1977; Uchupi and others, 1982). East of nificant sediment source. contourite was derived from ice transport across 61°30'W, Stanley and Silverberg (1969) and Detailed studies within the smooth slope re- Nova Scotia and from coastal-plain erosion fol- Stanley and others (1972) concluded that ero- gion west of 61°30'W found a striking change lowed by transport onto the rise by downslope sion of glacial-outwash debris exposed on Sable in sedimentation between 63° and 62°W. To the processes. and Emerald Banks during the Pleistocene led to west, Hill (1983 and 1984) mapped small Recent studies suggest that both cross-isobath
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and parallel-to-isobath processes may have been often useful presentations of these data, recogni- reflectors (hemipelagic association; profile E, active. Shor and others (1984) inferred both tion of major depositional systems in such maps Fig. 4) appear with a transition zone —10 km types of processes from magnetic grain align- is sometimes difficult because (1) interpretation wide. West of 62°15'W, distinct subbottom re- ments in lower rise cores. Damuth and others based on echocharacter alone is often ambigu- flections are rare. (1979 and 1981) observed turbidite channels, ous, (2) the nature of contacts are usually not Surface sediments on the outer Scotian shelf mass-transport deposits, and migrating sediment included, and (3) echocharacter produced by re- are glacial tills deposited during late Pleisto :ene waves in 3.5-kHz echograms from the lower lated sedimentary processes (for example, low sea levels and coarse sands and grj.vels rise. slump, slide, and turbidites of a mass sediment formed by submarine reworking of tills and failure) are often mapped separately. Another outwash during the Holocene transgression DATA difficulty, in this study at least, is that tracklines (King, 1970; Drapeau and King, 1972). Beyond may be too widely spaced to adequately map the shelf edge, silt and clay content of surface In order to cons.train interpretation of late spatial variation in echocharacter. sediment increases with depth of water until Pleistocene sedimenlation, the bathymetry, phys- For these reasons, echocharacter data in this mud becomes the most common facies at - 500 iography, and reflectivity patterns (echocharac- study are organized into four "echocharacter as- m (Stanley and others, 1972; Hill and Bowen, ter) along continuous echo-sounding lines have sociations." Within an association, a characteris- 1983; Hill and others, 1983). As the mud con- been compiled principally between 64°-60°W tic process or closely related processes produce tent of coarse shelf sediment increases seaward, and between the shelf break and abyssal plain sediment types and accumulation patterns—and acoustic velocity and bulk density decrease (Fig. 2). Approximately 4,000 km of 3.5-kHz thus, presumably, echocharacter—that are dis- (Hamilton and Bachman, 1982). As a result, the and single-channel seismic data were collected tinct from those in other associations. Associa- sea-floor acoustic impedance contrast decreases, in June-July, 1983, aboard the R/V Robert tions may include regions of erosion (for and penetration increases. The seaward transi- Conrad (cruise RC 2408) and supplemented example, the source region of a slide or slump) tion to subbottom "fuzziness" (prolonged, later- with -5,000 km of 12- and 3.5-kHz data (pulse as well as deposition. Echocharacter associations, ally incoherent echo) and subbottom reflectors length <5 msec) archived by Woods Hole are analagous to "fades associations" of Read- may be due to deeper penetration of the acoustic Oceanographic Institution (WHOI) and La- ing (1978, p. 5) and to "fades tracts" of Teichert. signal, to actual changes in the number and mont-Doherty Geological Observatory (1958). thickness of beds (for example, Tucholke, 1980; (LDGO). Using RC 2408 records, sea-floor Interpretation of echocharacter associations is Damuth, 1980), or to scattering from increasing physiography (sea-liloor channels, scarps, and based on (1) reflectivity pattern, (2) water depth, sea-floor roughness (Bryan and Markl, 1966). In sediment waves) w.is mapped. Bathymetry was (3) physiography, (4) detailed morphologic, any case, because of the masking effects of the contoured at 50-m intervals after correction for seismic, and sediment studies, and (5) contact.'; sea floor, little can be inferred about late Pleisto- speed of sound in water using Mathews (1939) between associations. Each association includes cene sedimentation along the shelf edge from the and surface water-mass locations (determined one or more echotypes defined by Damuth echocharacter and distribution of shelf-spillover by 15 expendable bathythermographs to 500 m, (1975, 1978, 1980); no new echotypes were association. by hourly sea-surface temperature measure- recognized. Because many echotypes and struc- ments, and by weekly sea-surface oceanographic tures seen in these data are described elsewhere Downslope Association analyses from the National Earth Satellite Ser- (Jacobi, 1976; Flood and others, 1979; Damuth, vice, NOAA). To enlarge coverage, bathymetric 1980; Embley, 1980,1982; and references in all In general, downslope association includes contours (100-m interval) of Shor (1984) were of these), not all echotypes are illustrated here. areas where subbottom reflectors are scarce or modified to agree with the new data (Fig. 1). The associations probably reflect sedimenta - absent and where the sea-floor return is pro- Published and unpublished descriptions of tion conditions during the late Pleistocene. No longed compared with the shelf sea-floor ixho. WHOI and LDGO cores were examined but outcrops of significantly older rocks, as found This association includes reflectivity paiterns not used extensively in this study (except as elsewhere off Nova Scotia (Stanley and others, which elsewhere have been mapped separately noted below), because the cores did not ade- 1972; King and Young, 1977), were noted ex- (Damuth, 1980). quately sample all reflectivity patterns in the re- cept for scarps and canyons indicated in Figure The predominant echotype from 1,000-m gion mapped. 3. Holocene sediment on the Nova Scotian slope depth to the abyssal plain is an echo 50-100 Damuth and others (1979 and 1981) com- and rise is commonly <2 m thick, and average msec long with low coherence, only vague and piled an echocharacter map showing the distri- late Pleistocene accumulation rates are typically fuzzy subbottom reflectors, and no hyperbolae bution of reflectivity patterns for the continental 100-500 m/106 yr (Stanley and others, 1972; [profile B, Fig. 4; downslope channels, profile H, rise off western Nova Scotia. Their study does Piper, 1975; Stow, 1979; Hill, 1981). Thus, the Fig. 5; prolonged echotype (IIB) of Damuth, not include the RC 2408 data and does not deepest reflectors at —100 msec vertical inci- 1980]. The Sohm Abyssal Plain also returns extend north of 42"N (~3,000-m depth). Their dence two-way traveltime (90 m at 1,800 echoes of this type; distinct subbottom reflectors, analysis differs from mine in that they used m/sec) are probably no older than 9 x 105 yr commonly found on other abyssal plains and HEBBLE results in their interpretation and that but may be much younger. ascribed to turbidites (Damuth, 1975; Tucholke, they did not group reflectivity patterns into 1980), were not found. associations. Shelf-Spillover Association Prolonged echo may be due in part to oblique return from sea-floor roughness features and, in ECHOCHARACTER ASSOCIATIONS The flat outer shelf is characterized by a short part, due to the nature of interference and (<10 msec) echo with no subbottom returns backscattering returns from many thick, coarse- In continental slope and rise environments (profile A, Fig. 4; echotype IA of Damuth, grained beds (Bryan and Markl, 1966; Tu- elsewhere, echocharacter surveys have been suc- 1978). The echo lengthens basinward of the cholke, 1980). Damuth's (1980 and references cessfully used to interpret sedimentation proc- shelf edge, and a single subbottom reflector sim- therein) echocharacter studies show that piston esses active both ai. present and during the late ilar to echotype IC of Damuth (1978) typically cores from areas of prolonged echoes (IIB) con- Quaternary (Damuth, 1980 and references appears at —400 m. In deeper water east of tain much thicker and more abundant silt/sand therein). Although echocharacter maps are 62°15'W, several distinct, coherent subbottom beds than do cores from other echotypes. Thick
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Figure 3. Distribution of echocharacter associations. Heavy line separating associations is broken where boundary is uncertain. Channels (dots and arrows) and debris flows (circles) are downslope deposits that could be distinguished by echocharacter and physiography. Channels indicated by arrows have apparent widths less than ~1 km. Channel boundaries and arrows are dashed where uncertain; scarp boundaries are hatched. "Horseshoe" pattern indicates hemipelagic deposits with hyperbolic echocharacter (profile P, Fig. 7). Wavy patterns mark contourite-fan association with sediment waves divided into two wavelength classes; limits are dashed where known and unmarked where questionable. Isobaths from Figure 1.
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Regular, overlapping hyperbolae with no subbottom reflectors and with vertices either at the sea floor or at varying elevations above the sea floor (profile C, Fig. 4; echotypes IIID ,ind IIIC of Damuth, 1980, and Damuth and Hayes, 1977) were uncommon on the rise. However, Damuth and others (1979 and 1981) mapped large fields between 65°-64°W below 3,000 m where my data are sparse and of poor quality. East of 64°W, such hyperbolae were recorded from the floors of channels (for example, profile C) and from a large debris-flow deposit which fills a topographic channel near 42°N, 61"W. 3140 I- . •• Such hyperbolic echoes are characteristic of the upslope portions of slump and debris-flow de- 52 posits (Embley, 1980; Damuth and Embley, $ 3180 1981). 5 N 1920 On the slope and upper rise to the west of •8» 62°15'W, no evidence was found in the physiog- 0.5 KM 5 1940 raphy, reflection pattern, or seismics for large- St "J !_ scale late Pleistocene slumps. In particular, no O . Y >; 1960 evidence, such as documented by Embley £ (1980, 1982, and references therein) elsewhere in the Atlantic, could be found on the slope in 1980 this data set or that of Hill (1983 and 1984) for a slump >5 km wide. On the rise, semi- 2000 D transparent layers resembling debris-flow depos- its could not be mapped over distances greater than -20 km, with the exception of a large de- posit near 61°W, 42°N (Fig. 3). Damuth and others (1979 and 1981) mapped a large area of debris-flow deposits between 40°30'-41°30'N and 62°-63°30' W. However, the RC 2408 data in this region show channel physiography and widespread areas of prolonged echo with minor hyperbolae. This evidence suggests turb:dite deposits, although the occurrence of debris-ilow fill in channels may be underestimated due to the difficulty of distinguishing thick debris flows in areas of prolonged echo.
Figure 4. Profiles (3.5 kHz) from each echocharacter association. A: shelf-spillover; B-D: Late Pleistocene sediments mapped hers as downslope; E: hemiipelagic; F: contourite-fan. Locations in Figure 2. Profiles B and C cross down-slope association are interpreted as being channel deposits on the rise. Arrows in profile C indicate "tails" of some hyperbolae. In profile deposits emplaced predominantly by near- E, note continuity of some reflectors beneath two topographic channels. bottom gravity flows: turbidity currents and mass-transport mechanisms. The most coir pel- ling reason is the distinct, continuous eastern sand units and gravel are present in piston cores Fig. 6) is not accompanied by any noticeable boundary near 62°W. This feature stretches from areas identified as downslope association change in fuzziness. This suggests that sufficient nearly 370 km from a depth of -500 m dow n to but are rare elsewhere (Hollister, 1967; Stanley energy penetrates subbottom in some regions of the abyssal plain (Fig. 3). Echocharacter and others, 1972; Stow, 1977; Tucholke, unpub. prolonged echo to return a reflector if a suffi- changes in 5 km or less across the boundary. data). cient subbottom impedance contrast exists. Neither hemipelagic nor contour-following sed- On the levees of rise channels and on the Large, irregular hyperbolae (profile D, Fig. 4; imentation can account for these characteristics. slope between 400-1,500 m, the prolonged echotype III A of Damuth, 1978) were found The association boundary on the slope some- echotype often includes one or two coherent only on the slope south of Baccaro Bank. Side times coincides with a sea-floor scarp and out- subbottom reflectors (north end of profile L, echoes and diffraction off rough sea floor proba- cropping reflectors, clearly indicating channel Fig. 6). The disappearance of the reflector along bly produce such hyperbolae (Damuth, 1975, cutting to the west (profiles G and H, Fig. 5). a trackline into prolonged echoes (not shown in 1980; Damuth and Hayes, 1977). Occasionally, reflectors in channel walls indi-
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W On two crossings, outcropping reflectors indi- 1740 cate erosion (profiles I and K, Fig. 5). Else- where, continuous units as well as eroded units overlie the boundary (profile J, Fig. 5). The thicknesses of continuous units are typically un- 1820 Downslope changed across the boundary or are thicker to the east than on the sloping boundary, suggest- ing that sediment was draped over pre-existing topography. 1900 Three additional lines of evidence support in- terpretation of downslope processes to the west of the boundary. First, Hill (1983 and 1984) 3220 r W 2 KM found that slumps, slides, and channel-fan sys- tems, with widths <2 km along isobaths, origi- nate at the base of a 150-m escarpment which parallels the 500-m isobath. In cores, sand and 52 silt layers with chaotic stratigraphy are common * 3300 within reddish-brown to brown mud, indicating that much slope sediment was emplaced by gravitational processes in the late Wisconsin (Hill, 1983 and 1984). Although none of the 3380 echo-sounding lines used in this study cross ^ 4660 Hill's area, the physiography is similar to that o where my coverage shows downslope associa- ,
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N can be traced on the slope for as much as 5C> km (Fig. 3; Piper and others, unpub. data), but most 1200 are considerably shorter. Round bottom chan- nels 5-10 km wide could be identified at 4,000-4,200 m by changes in physiography and by fading out of subbottom reflectors, but these channels could not be confidently traced to up- slope sources (Fig. 3). 3400 Sediment in this association is interpreted as being predominantly hemipelagic (that is, com- posed primarily of fine-grained terrigenous mud 3480 - transported seaward well above the sea f.oor, sensu Pierce, 1976), because reflector units are continuous over a wide range of topography and slope both in strike and dip lines. Such an origin 3560 is also suggested by gradational contact with spillover association near 500 m and continuity of the association from the slope onto the upper 5 rise (Fig. 3).
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Hummocky sea floor (profile P; Fig. 7) occurs type IIA, Damuth, 1980, and references there- DISCUSSION on a ridge between two arms of the debris flow in). The association is found south of the and returns hyperbolic echoes with sea-floor re- 4,300-m isobath and east of 62°30'W. Al- The principal finding of this study is the re- lief of 10-20 m. Subbottom reflectors are con- though the northern boundary is poorly sampled gion of downslope association centered seaward formable and do not show migration. The only and not well constrained, it appears gradational of a gap in the outer shelf banks with a sharp pair of tracklines which intersect on the struc- over 2-12 km. Some reflectors in the hemipe- eastern boundary that extends from the shelf tures shows no difference in wavelength, sug- lagic unit crop out south of the boundary (pro- edge to the abyssal plain. To account for this gesting no elongation. These features appear file Q, Fig. 7). The west boundary is sharp; the pattern and its relationship to shelf topography, similar to structures ascribed elsewhere to creep transition to no subbottom reflectors occurs over a model is proposed below, presuming that the (Hill and others, 1982). 2-5 km and is accompanied by the depth upper 50-100 m of sediment, which produces changes dscribed above (profiles I, J, and K; reflectivity patterns, is approximately late Pleis- Contourite-Fan Association Fig. 5). South and east boundaries are not well tocene everywhere and that temporal changes in surveyed. sedimentation, whether episodic or fluctuating Contourite-fan association is mapped by oc- Sea-floor sediment waves occur only within (periods less than 100,000 yr), are less impor- currence of subbottom reflectors which are lat- this association (profile Q, R, and S; Fig. 7). tant than spatial changes. These assumptions are erally discontinuous over distances >1-2 km Subbottom reflectors are either offset with consistent with Hill's (1983 and 1984) studies of (profile F, Fig. 4; Fig. 5 in Damuth, 1975; echo- depth, indicating upslope migration, or conform- Wisconsin stratigraphy on the slope, with lack
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of evidence for wide-spread, episodic mass- with existing geologic and oceanographic data. significant lower fan progradation. Howevei, re- failure events west of 62°W, and with the occur- (1) Multi-channel seismic lines (obtained from flector relations in the termination of the bulge rence of drape, as well as erosional, relationships Canadian Oil and Gas Lands Administration) show little evidence of progradation in the sec- along the eastern baundary of the down-slope show no faults and no changes in sediment struc- tion resolvable with 3.5-kHz echo sounding association. ture of the upper 2 km associated with the (profiles I, J, and K; Fig. 5). Thus, the to;x>g- boundary. (2) Surficial pockmarks, which indi- raphy was likely relict during the late Pleisto- Sedimentation Model cate weakening of surface sediment by release of cene. Reflectors cropping out to the west a: the pre-Pleistocene gas (Josenhans and others, association boundary indicate that erosion by During the late Pleistocene, grounded ice 1978), have not been found on the slope or near turbidity currents helped maintain the relief probably reached no farther than the inner Nova the shelf edge (Hill, 1983; Hill and others, 1983; Scotian shelf, and sea level on the outer shelf King, 1983). (3) Hill and Bowen (1983) sug- Geologic Significance was 100-120 m lower relative to present (King, gested that spatial changes in present shelf-edge 1970 and 1983). Under these conditions, La- sediment transport may be due to longitudinal Sharply defined variations in late Pleistocene Have Channel was the only substantial water- changes in spawning rates of warm core rings off sedimentation occurred along strike in the slope way (-60 km wide, 30-50 m deep) between the Gulf Stream. However, recent data show no and rise off Nova Scotia. Aggradation of the grounded ice and the continental slope; the sharp change in ring spawning (Joyce, 1984), slope-rise apron west of 62°15'W was con- Gully (inset Fig. 2) is significantly narrower and and the appearance during glacials of disconti- trolled by small-scale downslope movement more sinuous. Midshelf basins probably trapped nuities in rate and location of ring genesis seems without a central canyon-channel system. This is sediment transported from the glacier near the unlikely. unlike most fan models (for example, Stow, sea floor, while ice-rafting and melt-water As a result of even sediment dispersal within 1981) and unlike the slope by-passing/coa- plumes carried sediment southward through the LaHave Channel plume, deltas did not de- lescing rise-fan model (Piper, 1975) which ap- LaHave Channel. Wisconsin ice may have abut- velop on the slope, failure was small in scale, plies better to the east. ted the landward edge of outer banks off western and canyons and steep slopes similar to those The reason for lateral variability was varia- Nova Scotia (Stanley and Cok, 1968; Emery east of 61°W did not develop. Small slides, de- tion in rate, process, and grain-size of sediment and Uchupi, 1972; Schlee and Pratt, 1970; bris flows, and turbidites transported sediment supplied from the shelf. During low sea levels in Schlee, 1973). If so, ice streams probably fun- from the slope to the upper rise, forming a depo- the late Pleistocene, LaHave Channel influenced neled sediment through LaHave Channel as center termed the "slope-rise apron" (Stow and slope and rise accumulation patterns by funnel- near-bottom transport as well as surface plumes. others, 1984). The lower rise aggraded as a dis- ing fine-grained sediment (glacial) to the slope. Seaward of the outer banks, in contrast to tal province of the slope-rise apron in a manner Thus, outer shelf morphology played a promi- conditions seaward of LaHave Channel, erosion significantly different than that of the lower rise: nent role in slope-rise sedimentation. In the past, of the outer banks, ice-rafting from Northeast or elsewhere off Nova Scotia, that developed as a under similar conditions of sediment source and Laurentian Channels, and erosion of canyon result of slope by-passing and deep-sea fan con- sea level, laterally variable shelf morphology walls were probably the principal sediment struction (for example, Piper, 1975). Downslope might also have influenced deep-water sedimen- sources for the slope. The greatest supply was transport of sediment occurred in many channel tation patterns. Temporal change in shelf mor- localized at canyon heads (Piper, 1975; Stow, systems; no dominant channel-fan system devel- phology (the present Nova Scotian shelf mor- 1978). oped on the rise. phology was largely determined by fluvial proc- Based on lithology of slope sediments (Hill, To the east of 62°15'W, hemipelagic mud esses during a mid-Tertiary low sea level, King 1983 and 1984) and distribution of echocharac- accumulated on smooth slope. Downslope; and others, 1974) might also have produced ter associations, the dominant seaward sediment transport in narrow channels and thin, broad stratigraphic change in patterns of accumula tion, flux through LaHave Channel is inferred to be a slump-slide features (Piper and others, unpub. lithology, and seismic facies on the slope and sea-surface plume analogous to plumes off mod- data) were less common here than to the west. rise. ern glaciers (Edwards, 1978; Pfirman, 1984). Farther east, hemipelagic accumulation was re- Near-bottom transport probably carried to the stricted to canyon divides (Stanley and Silver- CONCLUSIONS upper slope lesser ¿.mounts of coarse debris re- berg, 1969). Turbidity currents transported leased from icebergs grounded along the north- sediment down canyons to fans on the lower Late Pleistocene sedimentation varied along ern channel edge. rise. Below 4,500 m, channel overbank flow the slope and rise off western Nova Scotia as a Most fine-grained sediment in the LaHave may have deposited load as sediment waves result of a glacial source on the shelf and spatial Channel plume probably settled out upslope of (Normark and others, 1980). Alternatively, ab- variation in outer shelf morphology. Seaward the 1,200-m isoba:h. Because failure of fine- yssal currents may have redistributed sediment sediment transport in deep water differed across grained sediment on a slope is more likely in to the southwest either by deflecting overbank a sharp boundary near 62°15'W extending; 370 faster-accumulating sediments (Morgenstern, spillage or by resuspension and transport. Win- km from the upper slope to the abyssal plain. 1967; Coleman and others, 1983), the boundary nowing may have occurred but probably was Fine-grained glacial debris accumulated rela- between downslope and hemipelagic associa- insignificant, because rise cores from this region tively rapidly on the slope seaward of LaHave tions at 500-1,000 m and 62°15'W probably contain substantial amounts of mud (Piper, Channel. Small-scale sediment failure created a corresponds to the southeast limit of significant 1975; Stow, 1977, 1978). slope-rise apron that filled rather than carved sediment transport in surface and intermediate The southwestward bulge of contours near topographic relief. Turbidity currents, slides, and waters. Other explanations are not consistent 62°30'W appears to mark the southwest limit of debris flows distributed sediment onto the lower
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rise through many small channels that lacked a Edwards. M. B„ 1978, Glacial environments, in Reading, H. G., ed„ Sedimen- tion: Marine Geology, v. 37, p. 1-18. tary environments and facies: New York, Elsevier, p. 416-438. Parsons. M. G., 1975, The geology of the Laurentian Fan and the Scotia rise, in major slope or shelf source. On smooth slope Embley, R. W., 1980, The role of mass transport in the distribution and Yorath, C. J.. Parker, E. R., and Glass, D. J., eds.. Canada's continental character of deep-ocean sediments with special reference to the North margins and offshore petroleum exploration: Canadian Society of east of the boundary, failure rate of hemipelagic Atlantic: Marine Geology, v. 38. p. 23- 50. Petroleum Geologists Memoir 4. p. 155 167. sediment was significantly less. Well-stratified 1982, Anatomy of some Atlantic margin sediment slides and some Pfirman. S. L., 1984, Modern sedimentation in the Northern Barents Sea: Input, comments on ages and mechanisms, in Saxov, S., and Nieuwenhuis, dispersal, and deposition of suspended sediments from glacial mcltwater sediment accumulated down to 4,300-m depth J. K., eds.. Marine slides and other mass movements: New York, [Ph.D. thesis]: Woods Hole, Massachusetts, Massachusetts Institute of Plenum Press, p. 189-213. Technology/Woods Hole Oceanographic Institute, 360 p. with minor sliding and channeling. Farther east, Embley, R. W., and Langseth, M. G., 1977, Sedimentation processes on the Pierce, J. W., 1976, Suspended sediment transport at the shelfbreak and over continental rise of northeastern South America: Marine Geology, v. 25, the outer margin, in Stanley, D. J., and Swift, D.J.P., eds., Marine outer banks reduced the rate of supply to the p. 279 297. sediment transport and environmental management: New York, John slope of fine glacial outwash and ice-rafted de- Emery, K. O., and Uchupi, E., 1967, Structure of continental margin off Wiley and Sons, p. 437- 458. Atlantic coast of United States: American Association of Petroleum Piper, D.J.W., 1975, Late Quaternary deep water sedimentation off Nova bris. Turbidity currents transported sediment Geologists Bulletin, v. 51, p. 223-234. Scotia and western Grand Banks, in Yorath, C. J., Parker, E. R.. and eroded from outer banks down canyon-channel 1972. Western North Atlantic Ocean: Topography, rocks, structure, Glass. D. J., eds., Canada's continental margins and offshore petroleum water, life, and sediments: American Association of Petroleum exploration: Canadian Society of Petroleum Geologists Memoir 4. systems to the lower rise where both overbank Geologists Memoir 17, 532 p. p. 195 204. Flood, R. D., Hollister, C. D., and Lonsdale, P., 1979, Disruption of the Feni Pipe/, D.J.W., and Slatt, R. M., 1977, Late Quaternary clay-mineral distribu- spillage and contour-following currents depos- Drift by debris flows from Rockall Bank: Marine Geology, v. 32, tion on the eastern continental margin of Canada: Geological Society of ited sediment in a lower rise fan. p. 311-334. America Bulletin, v. 88. p. 267-272. Flood, R„ Kent, D. V., and Shor. A. N„ 1985, The magnetic fabric of surficial Reading, H. G„ 1978, Facies, in Reading, H. G., ed., Sedimentary environments deep-sea sediments in the HEBBLE area (Nova Scotian continental and facies: New York, Elsevier. rise): Marine Geology (in press). Schlee, J„ 1973, Atlantic continental shelf and slope of the United States ACKNOWLEDGMENTS Hamilton, E. L., and Bachman, R.T., 1982, Sound velocity and related proper- Sediment texture of the northeastern part: U.S. Geological Survey Pro- ties of marine sediments: Acoustical Society of America Journal, v. 72, fessional Paper 529-L. p. 1891 1904. Schlee, J., and Pratt, R. M., 1970, Atlantic continental shelf and slope of the Hill, P. R„ 1981, Detailed morphology and late Quaternary sediments of the United States—Gravels of the northeastern part: U.S. Geological Sur- A. Shor and B. Tucholke kindly provided Nova Scotian slope, south of Halifax [Ph.D. thesis): Halifax. Nova vey Professional Paper 529-H. echo-sounding records, navigation for LDGO Scotia, Dalhousie University, 331 p. Shor. A. N., 1984, Bathymetry, in Shor, A. N., and Uchupi, E„ eds.. Ocean 1983. Detailed morphology of a small area on the Nova Scotian Margin Drilting Program Regional Data Synthesis Series Atlas 2. East- lines, and core descriptions. A. Shor provided continental slope: Marine Geology, v. 53, p. 55-76. ern Northern American continental margin and adjacent ocean floor, 1984, Sedimentary facies of the Nova Scotian upper and middle contin- 39° to 46°N and 54° to 64°W: Woods Hole, Massachusetts. Marine valuable assistance in correcting and compiling ental slope, offshore eastern Canada: Sedimentology, v. 31, p. 293 309. Science International, Sheet 1. bathymetry. I thank D. Piper for providing an Hill, P. R., and Bowen, A. J., 1983, Modern sediment dynamics at the shelf- Shor, A., and Lonsdale, P., 1981, HEBBLE site characterization: Downslopc slope boundary off Nova Scotia, in Stanley, D. J., and Moore, G. T„ processes on the Nova Scotian lower continental rise [abs.J: EOS unpublished manuscript. Reviews by E. Uchupi, eds., The shelfbreak: Critical interface on continental margins: Society (American Geophysical Union Transactions), v. 62, p. 892. of Economic Paleontologists and Mineralogists Special Publication 33. Shor. A. N.. Kent. D. V.. and Flood. R. D„ 1984. Contourite and turbidite?: R. Flood, J. Milliman, D. Aubrey, and C. Ebin- p. 265 276. Magnetic fabric of fine-grained Quaternary sediments. Nova Scotian ger significantly improved the manuscript. Dis- Hill, P. R„ Moran, K. M., and Blasco. S. M., 1982, Creep deformation of slope continental rise, in Stow, D.A.V., and Piper, D.J.W., eds.. Deep sea fine sediments in the Canadian Beaufort Sea: Geo-Marine Letters, v, 2. grained sediment transport: Geological Society of London Spccial cussions with S. Pfirman and H. Josenhans were p. 163 170. Publication. helpful. R. Davies drafted the'figures. Funding Hill. P. R„ Piper. D.J.W., and Normark. W. R„ 1983, Pices IV submersible Stanley. D. J., and Cok, A. E„ 1968, Sediment transport by ice on the Nova dives on the Scotian slope at 63°W: Current Research, Part A, Geologi- Scotian shelf: Ocean Sciences & Engineering of the Atlantic Shell. was provided by National Science Foundation cal Survey of Canada Paper 83-1 A, p. 65- 69. Marine Technology Society Transactions, Philadelphia, p. 109 125. Hogg, N. G., 1983, A note on the deep circulation of the western North Stanley. D. J., and Silverberg. N., 1969. Recent slumping on the continental Contract OCE-8118014. Atlantic: Its nature and causes: Deep-sea Research, v, 30, p. 945- 961. slope off Sable Island Bank, southeast Canada: Earth and Planetary Hollister. C. D„ 1967. Sediment distribution and deep circulation in the west- Science Letters, v. 6, p. 123- 133. ern North Atlantic [Ph.D. thesis]: New York. Columbia University, Stanley, D. J.. Swift. D.J.P.. Silverberg, N„ James. N. P., and Sutton, R. G.. REFERENCES CITED 471 p. 1972, Late Quaternary progradation and sand spillover on the outer Hollister. C. D„ and Heezen. B. C„ 1972, Geologic effects of ocean bottom continental margin off Nova Scotia, southeastern Canada: Smithsonian Alam. M.. Piper. D.J.W.. and Cooke, H.B.S., 1983. Late Quaternary stratig- currents: Western North Atlantic, in Gordon, A. L., ed.. Studies in Contributions to Earth Science, v. 8. 88 p. raphy and paleo-oceanography of the Grand Banks continental margin, physicat oceanography. Volume II: New York, Gordon and Breach, Stow. D.A.V.. 1977, Late Quaternary stratigraphy and sedimentation on the eastern Canada: Boreas, v. 12. p. 253-261. p. 37 66. Nova Scotian outer continental margin [Ph.D. thesis): Halifax, Nova Bryan, G. M„ and Markl, R. G.. 1966, Microtopography of the Blake-Bahama Hollister, C. D., and McCave, I. N., 1984. Sedimentation under deep-sea Scotia. Dathousie University. region: Columbia University/Lamont Geological Observatory Techni- storms: Nature, v. 309, p. 220-225. 1978, Regional review of the Nova Scotian outer margin geology: cal Report No. 8, CU-8-66-NObsr 85077, 26 p. Jacobi, R. D., 1976, Sediment slides on the northwestern continental margin of Maritime Sediments, v. 14. p. 17-32. Coleman, J. M„ Prior, D. B„ Lindsay. J. F., 1983, Deltaic influences on Africa: Marine Geology, v. 22. p. 157-173. 1979, Distinguishing between fine-grained turbidites and contourites shelf-edge instability processes, in Stanley, D. J., and Moore, G. T„ eds.. Jansa, L. F„ and Wade, J. A., 1975, Geology of the continental margin off on the deep-water margin off Nova Scotia: Sedimentology. v. 26, The shelfbreak: Critical interface on continental margins: Society of Nova Scotia and Newfoundland, in van der Linden, W.J.M.. and p. 371 387. Economic Paleontologists and Mineralogists Special Publication 33. Wade, J.A., eds., Offshore geology of eastern Canada: Geological Sur- 1981, Laurentian Fan: Morphology, sediments, processes, and growth p. 121- 137. vey of Canada Paper 74-30. v. 2, p. 51 -105. pattern: American Association of Petroleum Geologists Bulletin, v. 65, Damuth, J. E.. 1975. Echocharacter of the western equatorial Atlantic floor Josenhans, H. W., King, L. H., and Fader, G. B., 1978, A side-scan sonar p. 375- 393. and its relationship to the dispersal and distribution of terrigenous sedi- mosaic of pockmarks on the Scotian shelf: Canadian Journal of Earth Stow, D.A.V., Howell. D. G., and Nelson, C. H„ 1984, Sedimentary, tectonic, ments: Marine Geology, v. 18. p. 17-45. Science, v. 15, p. 831-840. and sea-level controls on submarine fan and slope-apron turbidite sys- 1978, Echocharacter of the Norwegian-Greenland Sea: Relationship to Joyce, T. M„ 1984, Velocity and hydrographic structure of a Gulf Stream tems: Geo-Marine Letters, v. 3, p. 57 64. Quaternary sedimentation: Marine Geology, v. 28, p. 1-36. warm core ring: Journal of Physical Oceanography, v. 14, p. 936 947. Teichert, C., 1958, Concepts of facies: American Association of Petroleum 1979, Migrating sediment waves created by turbidity currents in the King, L. H., 1970, Surficial geology of the Halifax-Sable Island map area: Geologists Bulletin, v. 42, p. 2718-2744. northern South China Basin: Geology, v. 7, p. 520-523. Marine Science Branch, Department of Energy, Mines, and Resources. Tucholke. B. E„ 1980. Acoustic environment of the Hatteras and Nares Abyssal 1980, Use of high frequency (3.5-12 kHz) echograms in the study of Ottawa, Paper 1, 16 p. Plains, western Atlantic Ocean, determined from velocities and physical near-bottom sedimentation processes in the deep-sea: A review: Marine 1983. Aspects of regional surficial geology related to site investigation properties of sediment cores: Acoustical Society of America Journal, Geology, v. 38. p. 51- 75. requirements—Eastern Canadian shelf, in Ardus, D. A., ed.. Offshore v. 68, p. 1376 1390. Damuth. J. E.. and Embley. R. W„ 1981, Mass-transport processes on Amazon site investigation: London, Graham and Trotman, p. 37 102. Uchupi. E.. 1969, Marine geology of the continental margin off Nova Scotia, Cone: Western equatorial Atlantic: American Association of Petroleum King, L. H., and Young. 1. F., 1977, Paleocontinental slopes of east coast Canada: New York Academy of Science Transactions, Scries II, v. 31. Geologists Bulletin, v. 65. p. 629-643. geosyncline (Canadian Atlantic margin): Canadian Journal of Earth p. 56 65. Damuth, J. E., and Hayes, D. E.. 1977, Echocharacter of the east Brazilian Science, v. 14. p. 2553 2564. Uchupi. F... Ellis, J. P., Austin. J. A., Jr.. Keller, G. H.. and Ballard. R. D„ continental margin and its relationship to sedimentary processes: Marine King. L. H., MacLean. B.. and Fader. G. B., 1974. Unconformities on the 1982. Mesozoic-Cenozoic regressions and the development of the mar- Geology, v. 24, p. 73-95. Scotian shelf: Canadian Journal of Earth Science, v. 11, p. 89 100. gins off northeastern North America, in Scrutton, R. A., and Talwani. Damuth, J. E., Tucholke, B. E.. and Coffin, M. F„ 1979, Bottom processes on Mathews, D. J., 1939, Tables of the velocity of sound in pure water and M.. eds.. The ocean floor: New York, John Wiley and Sons. p. 80 95. the Nova Scotian continental rise revealed by 3.5 kHz echocharacter seawater (second edition): London. The Hydroeraphic Department. Zimmerman, H. B.. 1972. Sediments of the New England continental rise: [abs.]: EOS (American Geophysical Union Transactions), v. 60. p. 855. Admiralty. Geological Society of America Bulletin, v. 83, p. 3709 3724. Damuth. J. E., Tucholke. B. E.. and Shor, A„ 1981, Bathymetry and near- Morgenstern, N. R., 1967, Submarine slumping and the initiation of turbidity bottom sedimentation processes of the Nova Scotian continental rise currents, in Richards. A. F„ cd., Marine geotechnique: Urbana, Illinois, MANUSCRIPT RECEIVED BY THE SOCIETY JULY 5, 1984 [abs.]: EOS (American Geophysical Union Transactions), v. 62. p. 892. University of Illinois Press, p. 189-220. REVISED MANUSCRIPT RF.CF.IVED DECEMBER 13. 1984 Drapeau. G.. and King. L. H., 1972, Surficial geology of the Yarmouth-Browns Normark. W. R.. Hess, G. R., Stow. D.A.V.. and Bowen. A. J„ 1980. Sediment MANUSCRIPT ACCEPTED DFCEMBF.R 17. 1984 Bank map area: Geological Survey of Canada Paper 72-24, 6 p. waves on the Monterey Fan levee: A preliminary physical interpreta- Wc ons HOLE OCEANOGRAPHIC INSTITUTION CONTRIBUTION NO 5741
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