Quaternary Sedimentary Processes and Budgets in Orphan Basin, Southwestern Labrador Sea

QUATERNARY RESEARCH 45, 160–175 (1996)ARTICLE NO. 0017

Quaternary Sedimentary Processes and Budgets in Orphan Basin,Southwestern Labrador Sea

RICHARD N. HISCOTT AND ALI E. AKSU

Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, A1B 3X5, Canada

Received June 1, 1995

as the recessional Trinity Moraine (Fig. 2). TransverseThe continental slope in Orphan Basin, northeast of Newfound- troughs characterize the east coast of Canada north of the

land, is underlain by several seaward-thinning debris-flow wedges Grand Banks (Piper, 1988), the Greenland margin (Mienertalternating with acoustically stratified, regionally extensive, et al., 1992), and the Norwegian Sea margin (Elverhøi etmainly hemipelagic sediments. d18O stratigraphy and volcanic ash al., 1995), and were occupied by ice tongues during glaciallayers in a 11.67-m core indicate that the uppermost debris-flow maxima.wedge formed during the last of several sea-level lowstands in

Aksu and Hiscott (1992) hypothesized that each compos-isotopic stages 2–4. Similarly, seismic reflection correlation ofite debris-flow wedge represents the culmination of a glacierdated levels at DSDP Site 111 with the Orphan Basin successionadvance. Their data show four to five such wedges, whichsuggests that two deeper debris-flow wedges were deposited duringpotentially correspond to advances of isotope stages 2–4, 6,oxygen isotopic stages 6 and 8. The oldest of the debris-flow depos-

its in at least three of the wedges formed well into the correspond-ing glacial cycle, after ice sheets had reached the edge of thecontinental shelf. Slower deposition by hemipelagic processes andice rafting formed the acoustically stratified units, including Hein-rich layers. The youngest three debris-flow wedges each have vol-umes of 1300–1650 km3. Approximately two-thirds of this mate-rial is attributed to glacial erosion of Mesozoic and Tertiary stratabeneath the Northeast Newfoundland Shelf. The remainder is be-lieved to have been derived by glacial erosion of older bedrock thatnow forms the island of Newfoundland. The observed sedimentvolumes and the inferred basal and upper ages of the debris-flowwedges imply an average glacial denudation rate of about 0.13mm/yr for this older bedrock, and an average of about 60 m ofglacial bedrock erosion since oxygen isotope stage 22. This denuda-tion rate is similar to estimates from the Barents Sea region offNorway. q 1996 University of Washington.

INTRODUCTION

Orphan Basin is situated north of the Grand Banks ofNewfoundland, landward of Orphan Knoll (Fig. 1). Thewestern slope and rise of the basin are smooth and undis-sected (Figs. 2 and 3) and are underlain by an alternation ofsheet-like acoustically stratified units and wedge-shapedunits formed of debris-flow deposits (Aksu and Hiscott,1992). A broad erosional trough cuts more than 100 m intothe Northeast Newfoundland Shelf and extends from the

FIG. 1. Map showing location of the study area (box) on the Canadianshelf edge toward the Bonavista Peninsula. This was calledcontinental margin. Note the serrated nature of the 200-m isobath northeast

the Trinity Trough by King and Fader (1992) and is referred of Newfoundland, reflecting deep erosion of the shelf. Hudson Strait andto in this paper as the Trinity Transverse Trough. King and Laurentian Channel were the sites of prominent ice streams during Pleisto-

cene glaciations (Hughes, 1987).Fader (1992) interpreted an arcuate feature within the trough

1600033-5894/96 $18.00Copyright q 1996 by the University of Washington.All rights of reproduction in any form reserved.

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161QUATERNARY SEDIMENTARY BUDGETS, LABRADOR SEA

FIG. 2. Map showing location of survey tracks and cores in Orphan Basin (see Fig. 1 for location). Dashed lines are profiles collected during threeMemorial University of Newfoundland (MUN) cruises. The Glomar Challenger seismic profile is shown in Figure 12, and the USNS Lynch 71B profilein Ruffman (1989). Open circles mark the locations of seven õ2.5-m-long gravity cores of Schafer et al. (1985); their core sites (79017-) 4 and 5 southof Orphan Knoll are labeled. Filled circles are MUN core locations; core sites (92045-) 7P and 9P through 13P are labeled. Bathymetry, in meters, isfrom Canadian Hydrographic Service (1990).

8, 10, and 12. In this paper, we report new core data and tia Research Foundation Corporation (NSRFC) 6-m-long,21-element, high-resolution Mark 5a hydrophone array. Ver-seismic ties to DSDP Site 111 on Orphan Knoll which sup-

port this suggested chronology and permit quantification of tical resolution of the Huntec DTS, 3.5 kHz, and NSRFCprofiles is about 10–20, 100, and 250–300 cm, respectively.the volumes of sediment carried through the Trinity Trans-

verse Trough during past glacier advances. In the upper few hundred milliseconds (msec) of sediment,we convert acoustic travel times to sediment thicknessesusing a velocity of 1500 m sec01, based on similar velocitiesDATA ACQUISITIONfor sediment depths of 0–300 m at Labrador Sea OceanDrilling Program (ODP) Site 647 (Shipboard ScientificThis study is based on Memorial University of Newfound-Party, 1987, p. 725), on acoustic velocity measurements ofland (MUN) seismic reflection profiles and cores collectedabout 1500 m sec01 throughout a recently acquired 35-m-during Canadian Scientific Ship (CSS) Hudson cruises 90-long core near MUN core 92045-11P, and on stacking veloc-007 and 92-045, and CSS Dawson cruise 91-029. Comple-ities of 1450 to 1550 m sec01 used by industry seismicmentary seismic data come from United States Navy Shipcontractors in the upper 1 sec of sediment penetration north(USNS) Lynch cruise 71B, Deep Sea Drilling Projectof the Grand Banks.(DSDP) Leg 12, and profiles collected by the Geological

Of the nine piston cores collected on MUN cruises (Figs.Survey of Canada (Dartmouth, Nova Scotia). The MUN da-2 and 3), the longest are 92045-9P (10.66 m), -10P (10.68taset (Fig. 2) includes 3950 line-km of single channel reflec-m), and -11P (11.67 m). Subsamples were taken at 10-cmtion profiles collected using a 40 in.3 (655 cm3) air gun, 700intervals from core 92045-11P, then dried and weighed. For-line-km of Huntec DTS boomer profiles, and 2620 line-kmaminifera were separated from the sand fraction (ú63 mm)of high quality 3.5 kHz subbottom profiles. The air-gun databy floatation in carbon tetrachloride. The foraminifera werewere recorded using two separate hydrophone arrays: a Seis-

mic Engineering (SE) 30.5-m-long streamer and a Nova Sco- further screened at 150 mm, and about 200 specimens of

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162 HISCOTT AND AKSU

FIG. 3. Detailed bathymetry of the study area, compiled using MUN 12-kHz profiles (uncorrected meters, v Å 1463 m sec01) and data from CanadianHydrographic Service (1990). Dotted lines are MUN seismic profiles. Filled circles are core locations; 7P and 9P through 13P are piston cores (CSSHudson cruise 92045) referred to in the text. Solid bold lines are MUN seismic profiles shown in Figures 4, 5, and 6. The dashed bold line marks thelocation of an industry multichannel seismic line simplified and interpreted in Figure 16.

Neogloboquadrina pachyderma (sinistral) were hand-picked was determined by replicate leaching of a dolomitic shalepowder (rock standard SGR-1) which contains no calcitefrom the ú150 mm split for oxygen-isotope analysis follow-

ing methods of Aksu et al. (1989). (based on XRD analysis). The acid filtrate from each samplewas analyzed for calcium and magnesium by ICP-ES, andIn this paper, the weight percentage of rock fragments

and silicate grains in the 150- to 3000-mm fraction is referred calcite and dolomite abundances were calculated and cor-rected to account for only partial leaching of dolomite.to as the ‘‘Heinrich variable,’’ being conceptually like Hein-

rich’s (1988) percent ice-rafted debris that he calculated from XRD mineral abundances in bulk powders from seven car-bonate-rich layers were determined by multiplying peak heightsgrain counts of rock fragments, silicate grains, and foramini-

fera tests. The weight of ú150 mm foraminifera in each by peak widths at half-height and then peak intensity factors(Cook et al., 1975, p. 1005). The only minerals present insample was determined by adding the weight of the screened

carbon tetrachloride float to a secondary separation of fora- significant amounts are quartz, K-feldspar, plagioclase feldspar,calcite, and dolomite. With few exceptions, calcite and dolomiteminifera achieved using a hydrocell similar to that described

by van der Westhuizen et al. (1993). percentages, determined semi-quantitatively by XRD and quan-titatively by partial acid leach, show good agreement (Table 1).A second set of subsamples was taken at 10-cm intervals

from core 92045-11P for carbonate analysis, augmented by Six subsamples were taken from near-surface debris-flowdeposits in cores 92045-7P, -12P, and -13P (Fig. 3) in ordera small number of additional subsamples in carbonate-rich

intervals. These samples were powdered, and 1 g of powder to determine the provenance of microflora. Palynomorphswere separated from eucalyptus-spiked samples and exam-was digested in glacial acetic acid for six hours, buffered to

a pH of 5.0 by 1 M sodium acetate. This partial leach proce- ined for pollen, terrestrial spores, marine dinoflagellates,acritarchs, and algal spores by Dr. Peta Mudie, Geologicaldure dissolved all calcite and about 45% of the dolomite

present in the samples; the proportion of dolomite dissolved Survey of Canada.

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TABLE 1Mineral Percentages, Core 92045-11P

Sample depth (cm)

Mineral 150 305 460 600 710 800 940

A. XRD results

Calcite 24 11 22 10 17 11 25Dolomite 17 11 13 12 12 10 17K-feldspar 13 28 12 15 20 34 16Plagioclase 20 38 34 49 32 26 34Quartz 27 12 19 13 18 20 9

B. Acid leach results

Calcite 21.3 8.7 18.6 13.4 12.8 7.9 19.3Dolomite 2.1a 10.5 14.7 16.6 15.5 11.5 18.7

a Anomalous result, not plotted in Figure 11.

SEISMIC STRATIGRAPHY lenses have widths from õ1 to ú20 km and central thick-nesses of a few meters to ú50 m. In dip profiles (Fig. 6),

The slopes of Orphan Basin are blanketed by õ20 msec acoustically incoherent lenses vary in length from õ10 to(õ15 m) of acoustically stratified sediment in which reflec- ú60 km. They are thin at their upslope end, progressivelytions show considerable lateral continuity and relatively con- thicken in the downslope direction, and gradually pinch outstant spacing from the upper slope to the rise. Immediately near the lower slope area. MUN cores 92045-7P, -12P, andbelow is a wedge-shaped unit of stacked, mound-shaped -13P recovered debris-flow deposits, which are structurelessacoustically incoherent to transparent lenses that are inter- sandy muds with scattered pebbles.preted as debris-flow deposits (Aksu and Hiscott, 1992). On MUN air-gun profiles acquired with the NSRFCIndividual lenses exhibit either no coherent internal reflec- streamer, the upper 300–500 m of the slope and rise consisttions or a faint, discontinuous, and discordant reflection con- of alternating sheet-like units of acoustically stratified sedi-figuration, suggesting a disordered arrangement of weak re- ment and composite mound-shaped debris-flow deposits.

The acoustically stratified units are similar to the surficialflectors. In strike profiles (Figs. 4 and 5), the concavo-convex

FIG. 4. Seismic strike-profile in about 1500 m of water showing reflections that define the tops of regionally extensive stratified units denoted bycolor as Bl Å Blue, G Å Green, A Å Aqua, and acoustically incoherent lenses interpreted as debris-flow deposits (Aksu and Hiscott, 1992; cf. Hiscottand Aksu, 1994). Location shown in Figure 3; vertical exaggeration 165; 0.1 sec (two-way travel time) Ç75 m.

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FIG. 5. Seismic strike-profile in about 1800 m of water showing reflections that define the tops of regionally extensive stratified units denoted bycolor as Bl Å Blue, G Å Green, A Å Aqua, and acoustically incoherent lenses interpreted as debris-flow deposits (Aksu and Hiscott, 1992; cf. Hiscottand Aksu, 1994). Location in Figure 3; vertical exaggeration 165; 0.1 sec (two-way travel time) Ç75 m. The U-shaped feature in the middle of theprofile, along the Green reflection, may be a local leveed channel.

stratified unit in reflection amplitude, continuity, and cycle Schafer et al. (1985), and (iii) correlation to DSDP Site 111on Orphan Knoll (Fig. 2). All dates reported in this paperthickness. The debris-flow units thin progressively ba-

sinward until they reach a feather edge (Fig. 6), so that on are in radiocarbon years.the rise seismic profiles show only parallel, high-intensity

Core Analysisreflections.Four regionally traceable seismic reflections each separate Piston core 92045-11P (Figs. 2 and 3) was taken at 497

the base of a wedge-shaped unit of debris-flow deposits from 47.54* N, 467 42.00* W in 2927 m of water. The core pene-an underlying unit of acoustically stratified sediments. These trated the Blue reflector at a depth of about 6 m below thefour reflections have been named, from youngest to oldest, seafloor (mbsf), downslope of the limit of debrim-flow depo-Blue, Green, Aqua, and Brown (Figs. 4, 5, and 6). Reflections sition (Fig. 10); the Green reflector lies beyond the limit ofBlue, Green, and Aqua were traced throughout the entire grid core penetration at a depth of about 18 mbsf. The core con-of MUN seismic profiles (Figs. 2 and 3), whereas the Brown sists mainly of burrowed, slightly to moderately calcareousreflection could not be traced with certainty in all areas. and/or dolomitic silty mud (Fig. 11 column f) with widely

Three isopach maps indicate thicknesses between reflec- scattered pebbles of plutonic, metamorphic, and carbonatetion pairs: the Seafloor/Blue isopach (Fig. 7), the Blue/Green rocks. There are several sand-rich, burrowed intervals, mostisopach (Fig. 8), and the Green/Aqua isopach (Fig. 9). The of which have gradational tops, sharp bases, and high car-volumes of sediment calculated from these isopachs and a bonate contents (Fig. 11, columns b, c, and d). Calcite is1500 m sec01 acoustic velocity are, respectively, 1325, 1525, somewhat more abundant than dolomite in the carbonate-and 1650 km3. In each case, most of the volume consists rich layers. At the core top and base, foraminifera are abun-of debris-flow deposits, because the acoustically stratified dant (see below), but elsewhere most of the carbonate occurssediments between each set of reflections are nowhere more as detrital rock fragments of limestone and dolostone.than about 15–20 m thick (e.g., Fig. 6). The distal limits of The Heinrich variable shows six prominent and four minordebris-flow deposits seen on seismic profiles follow isopach

peaks, most of which occur within or near sandy carbonate-contours and occur where the total deposit thickness is be-

rich beds (Fig. 11, column e). At least six Heinrich eventstween 20 and 40 msec (Figs. 7–9). Distal tongues of the

have been recognized in other parts of the North AtlanticSeafloor/Blue isopach are located where the underlying

Ocean (Heinrich, 1988; Andrews and Tedesco, 1992;Blue/Green unit is thinnest (compare Figs. 7 and 8), demon-

Broecker et al., 1992; Bond et al., 1993; Bond and Lotti,strating that the debris-flow wedges smooth previous topog-

1995). These have been ascribed to pulses in deposition byraphy by filling in bathymetric lows.

ice rafting just before the end of atmospheric cooling cyclesCHRONOLOGY (Bond et al., 1993). The detrital particles of limestone and

dolostone, common in cores from the Labrador Sea, wereSeismic reflections are dated using three methods: (i) pis-ton–core analysis, (ii) correlation to a dated gravity core of derived from Ordovician and Silurian carbonate series of the

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FIG. 7. Isopach map from the seafloor to Blue reflector, in milliseconds (msec) of two-way travel time. Map border is the same as in Figure 3.Sediment package thicker than 100 msec (Ç75 m) is shown with stipple. Discontinuous dotted lines mark the tips of the most distal debris-flow lensesin this stratigraphic interval. Volume of this sediment package was calculated out of the 20-ms contour.

Canadian Arctic (Aksu and Piper, 1987; Hiscott et al., 1989; content of stage 5 deposits and H7 to H9. Only during thewarmest phase of stage 5 did high surface-water productivityAndrews and Tedesco, 1992; Bond and Lotti, 1995). Hein-

rich layers 1 through 6 have ages of about 15,000, 21,000, and reduced terrigenous input conspire to permit depositionof a foraminifera-rich mud near the bottom of core 92045-27,000, 35,500, 50,000, and 66,000 yr (Andrews and Te-

desco, 1992; Bond et al., 1993). The numbering of Heinrich 11P (11.24–11.29 mbsf). Presumably, conditions at this timewere like those in the Holocene, when hemipelagic sedimen-events H1 to H6 in Figure 11 is constrained by ages and

ash positions published by Bond et al. (1993) and ages of tation under south-flowing bottom currents promoted deposi-tion of foraminifera-rich mud at the core top (0.0–0.16oxygen-isotope stages identified in core 92045-11P (Fig. 11,

columns a and g). We have assigned the labels H7, H8, and mbsf).From about 2.0 to 2.7 mbsf, the sediment in core 92045-H9 to deeper peaks in the Heinrich variable (Fig. 11, column

e), but none of these correlate with high detrital carbonate 11P consists of several sharp-based, thin beds that gradefrom fine or medium sand to mud. These are interpreted ascontents and their identification is provisional. Note that H1

to H6 all correlate with peaks in the ú63-mm fraction, turbidites. This turbidite unit can be correlated to cores92045-9P and -10P (Figs. 2 and 3).whereas the ú63-mm fraction shows rapid, high-amplitude

variation below H6 during interglacial stage 5, suggesting a Sand-sized volcanic ash concentrations occur at depths of0.30 and 7.20 mbsf. The ash consists of colorless, platy,different style of coarse-fraction delivery to the slope and

rise. The reduced content of detrital carbonate suggests a bubble-wall shards and to a lesser extent black pumice. Simi-lar tephra occurs throughout the North Atlantic Ocean, form-smaller contribution from ice rafting.

The Western Boundary Undercurrent presently sweeps the ing three distinct ash horizons within the past 500,000 yr.The upper two ash horizons, Ash 1 and Ash 2, have beenlower slope and rise of Orphan Basin. According to Piper et

al. (1994), this deep thermohaline current intensifies during dated at 9800–10,500 and 57,500 yr (Ruddiman and Mcin-tyre, 1984; Andrews and Tedesco, 1992). On the basis ofinterglaciations, which might account for the variable sand

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FIG. 8. Isopach map from Blue reflector to the Green reflector, in milliseconds (msec) of two-way travel time. Map border is the same as in Figure3. Sediment package thicker than 100 msec (Ç75 m) is shown with stipple. Discontinuous dotted lines mark the tips of the most distal debris-flow lensesin this stratigraphic interval. Volume of this sediment package was calculated out to the 20-ms contour.

the types and relative abundances of ash particles, the two Core 79017-5 encountered Ash 1 at a depth of 0.3 mbsf,levels of ash in core 92045-11P are correlated with Ashes a sharp-based silty and sandy bed at about 1.2–1.5 mbsf1 and 2 in the North Atlantic (Fig. 11, column a). (interpreted by Schafer et al. as a thick turbidite because

Core 92045-11P has an intact record from about 128,000 of its sharp base and vertically variable faunal content),yr (base of substage 5e) to the present. The Blue seismic and thin graded couplets of sand and mud from about 2.0reflector has an age of about 35,500 yr, near the middle of mbsf to the base of the core at 2.4 mbsf. This stratigraphyisotope stage 3, although a possible error of one cycle in is identical to that of core 92045-11P, with differences ofreflection matching between NSRFC and 3.5-kHz profiles, less than 10 cm in the depths to lithologic contacts. Theor in reflection tracing in 3.5-kHz records (e.g., Fig. 10) 30-cm-thick bed that Schafer et al. (1985) interpreted as apermits an age assignment of anywhere from about 30,000 turbidite has a high content of detrital carbonate, includingto 40,000 yr (depths of 5–7 mbsf). The average sedimenta- dolomite (Table 1), which has a provenance from the Ca-tion rate for stratified sediments in core 92045-11P from the nadian Arctic (Andrews and Tedesco, 1992; Piper et al.,base of stage 1 to the depth of the Blue reflector is about 1994), not from the upslope area. X-radiographs published16.5–17.5 cm/1000 yr. The Green reflector lies about 6 m by Schafer et al. show no tractional structures characteris-below the approximate base of stage 5 (Fig. 11, column a). tic of turbidites, and fluctuations in foraminiferal abun-If the rate of stage 6 accumulation was also 16.5–17.5 cm/ dances are as likely the result of climatic shifts (Bond et1000 yr, then the age of the Green reflector can be estimated al., 1993) as they are of sorting by a turbidity current. Weas about 160,000 yr. reinterpret this bed as Heinrich layer 1, a sharp-based

deposit resulting from vigorous ice rafting and icebergCorrelation to Dated Gravity Core melting. This bed, like other Heinrich layers in core

92045-11P and elsewhere (Bond and Lotti, 1995), is char-Cores 79017-4 and 79017-5 of Schafer et al. (1985) arelocated 40–60 km southeast of core 92045-11P (Fig. 2). acterized by a peak in detrital carbonate content that lags

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FIG. 9. Isopach map from the Green reflector to the Aqua reflector, in milliseconds (msec) of two-way travel time. Map border is the same as Figure3. Sediment package thicker than 100 ms (Ç75 m) is shown with stipple. Discontinuous dotted lines mark the tips of the most distal debris-flow lensesin this stratigraphic interval. Volume of this sediment package was calculated out to the 20-ms contour.

behind the increase in sand delivery by ice rafting (Fig. using USNS Lynch 71B and CSS Sackville 69041 seismic11, column f versus column b). profiles (Fig. 2). The greatest success was achieved with

Schafer et al. (1985) obtained two 14C accelerator mass CSS Sackville 69041 records, on which the 6a/6b seismicspectrometer (AMS) dates of foraminifera just above and contact was traced down the steep western flank of Orphanbelow Heinrich layer 1: 13,090 { 140 (Beta 9261) and Knoll and westward to the first available crossing with a12,770 { 140 14C yr B.P. (Beta 9262), respectively. These MUN seismic line (Fig. 2, about 2250 m water depth), wheredates suggest an age of about 13,000 yr for Heinrich layer the traced reflection is 20 msec (about 15 m) below the1 in cores 79017-5 and 92045-11P, at least a few hundred Aqua seismic reflection. Caution is required because seismicyears younger than dates reported for core material from profiles in Orphan Basin show a seaward converging reflec-elsewhere in the North Atlantic Ocean by Andrews and Ted- tion geometry with many offlap terminations near the terminiesco (1992) and Bond et al. (1993). Despite this minor dis- of debris-flow wedges, so that tracing of individual reflec-crepancy, the dates from nearby gravity core 79017-5 sup- tions can lead to several equally plausible solutions. There-port the chronology we present in this paper for core 92045- fore, the correlation between DSDP Site 111 and the MUN11P (Fig. 11). seismic grid is only approximate. Correlation errors of one

or two cycles at sedimentation rates of about 20 cm/1000DSDP Site 111 Stratigraphy yr could result in the mistaken conclusion that two reflectors

have the same age when in fact they differ in age by perhapsDSDP Site 111 penetrates the pelagic cap of Orphan Knoll20,000 to 30,000 yr.(Fig. 2). The boundary between DSDP shipboard seismic

Pseudoemiliania lacunosa occurs in Site 111 samples be-units 6a and 6b (Fig. 12) occurs at 100 msec depth (Laughtonlow 97 mbsf, but is absent in sediments above this depthet al., 1972; Ç75 mbsf). An attempt was made to trace this

seismic boundary from Site 111 to the MUN seismic grid (Laughton, et al., 1972, p. 125). Based on the last appearance

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FIG. 10. Two short segments of a continuous 3.5-kHz profile passing through core site 92045-11P, and located on Figure 3. (A) One of the mostdistal debris-flow lenses above the Blue reflector. Blue was transferred to this profile by measuring its depth (in milliseconds) on the corresponding air-gun profile (NSRFC streamer). (B) Reflection character at the core site, beyond the toe of all debris flows. The Green reflection is faint on the originalrecord, but is more clearly seen at the same depth on the corresponding air-gun profile.

datum of this taxon at 474,000 yr (Berggren et al., 1985), layers) containing far-traveled carbonate detritus from theCanadian Arctic. The remainder of this material is burrowedthe average sedimentation rate from 0–97 mbsf is 20.5 cm/

1000 yr. Linear interpolation using this sedimentation rate hemipelagic mud and minor mud turbidites.The Blue reflector, which lies just below the youngestand a seismic velocity of 1500 m sec01 in near-surface sedi-

ments gives an age of about 342,000 yr for the boundary debris-flow wedge, is apparently 30,000–40,000 yr youngerthan the beginning of the isotope stage 2–4 glacial episodebetween seismic units 6a and 6b of Laughton et al. (1972),

essentially identical to the age of the top of oxygen isotope (Figs. 11 and 13). This age difference may correspond tothe time required for ice-cap growth and for ice sheets tostage 10 (Imbrie et al., 1984).

In the region where the CSS Sackville 69041 line inter- extend across the continental shelf and deposit sufficientsediment to supply a series of debris flows. Even if allowancesects the MUN seismic grid (Fig. 2), the seismic unit 6a/6b

boundary (Ç342,000 yr) lies at a depth of 85 msec (Ç65 is made for the proposed error bar associated with the age ofthe Aqua reflector (Fig. 13), the oldest debris-flow depositsmbsf), near the feather edge of debris-flow deposits, giving

an average sedimentation rate of 19 cm/1000 yr, marginally associated with the isotope stage 8 advance appear to post-date growth of continental ice sheets by at least 10,000 yr.higher than the rate above the Blue reflector in core 92045-

11P. The Aqua reflector lies 15 m higher at 50 mbsf and The Green reflector is estimated to be about 25,000 yryounger than the base of stage 6. Uncertainty in the age oftherefore has an inferred age of [342,000 0 1000(1500 /

19)] yr Å 263,000 yr. The Brown reflector lies 10 msec the Brown reflector does not permit a unique placement inone part or another of the stage 10 glacial cycle. Neverthe-(Ç7.5 m) below the 342,000-yr surface, and therefore has

an inferred age of 381,000 yr. As noted above, these ages less, we provisionally use the model provided by the lag ofthe post-Blue debris flows behind onset of the stage 2–4may be in error by {30,000 yr.glacier advance to position the other reflectors late in theirrespective glacial stages in Figure 13. Better chronologicalDISCUSSIONcontrol provided by longer cores is needed to confirm this

Chronological Framework model.

The age constraints for the Blue and Aqua seismic reflec- Sediment Budgets and Sourcestors support the hypothesis of Aksu and Hiscott (1992) thatthe debris-flow wedges accumulated in front of shelf-edge Ice sheets that enter the sea terminate at a ‘‘grounding

line’’ (or just beyond the grounding line in the case of iceice margins during glacier advances and that the acousticallystratified units accumulated during glacier retreat, intergla- shelves) where massive tills, till tongues (King and Fader,

1986; King et al., 1991), and proximal subaqueous outwashcial episodes, and early glacial times (Fig. 13). Part of thestratified sediment consists of ice-rafted deposits (Heinrich are deposited. During the maximum extent of glaciers, the

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FIG. 11. Stratigraphy of core 92-045-11P (location in Figs. 2 and 3), showing visual description (column f), texture (columns b and f), occurrencesof carbonate-rich sediments (brick pattern in column f; circled F Å abundant foraminifera), outsized dropstones (diamonds in column f), the Heinrichvariable and North Atlantic Heinrich events H1 to H6 (column e). Oxygen isotopic stages (column g) are based on d18O data (column a). Calcite anddolomite (columns c and d) were determined on acid-leach filtrates, except near 1.5 mbsf (dashed curve segment, column d), where Mg leach data wereunreliable (Table 1). Bold dots in columns c and d give calcite and dolomite abundances from semi-quantitative XRD analyses; the shallowest of theseconstrains the dolomite curve near 1.5 mbsf. Bold numbers are ages (103 yr) from the following sources: column a, Ruddiman and McIntyre (1984);column e, Bond et al. (1993) and Bond and Lotti (1995); column g, Imbrie et al. (1984). The Blue reflector occurs at a depth of about 6 mbsf.

ice margin remains at or near the shelf edge, providing sediment supplied through the Trinity Transverse Trough duringthese glacial events, average annual sediment yields of 33ment directly to the shelf break and the uppermost slope

(Piper, 1988). Some of this sediment may have been pushed 1 106 (stage 8), 43.5 1 106 (stage 6), and 56.5 1 106 m3

(stages 4–2) can be calculated. Given the uncertainties inseaward from a temporary interglacial site of deposition incoastal fjords, larger embayments, and shelf basins (An- all calculations and in the seismic tie to DSDP Site 111, an

approximate average annual yield of (50 { 10) 1 106 m3drews, 1990, 1992; Elverhøi et al., 1995), but if each glacialevent both remobilizes and leaves behind a similar volume might be considered appropriate for all of the major glacier

advances through the Trinity Transverse Trough. The totalof sediment in these temporary repositories, then this sedi-ment need not be taken into account in the evaluation of volume carried through the trough since about 290,000 yr

ago is approximately 4500 km3.sediment budgets during individual glacier advances.If it is assumed that debris-flow deposition is terminated The sediment yield calculations do not distinguish be-

tween glacial erosion that took place before the ice sheetsby global postglacial sea-level rise (i.e., rapidly decreasingd18O values, Fig. 13), then liberal estimates of the time avail- reached the edge of the continental shelf and erosion while

the ice was at its maximum extent. All eroded material wouldable for deposition of the debris-flow wedges of isotopicstages 8, 6, and 4–2 are, respectively, 50,000, 35,000, and eventually be pushed seaward and transferred to the slope,

even if it was generated before the ice reached the shelf23,500 yr (Fig. 13), using 293,000 yr as the oldest probableage of the Aqua reflector, and ages of 160,000 and 35,500 edge. The significance of this distinction is that, although

debris flow may have been an active transport process onyr for the Green and Blue reflectors. Using isopach-basedvolumes as conservative estimates of the quantity of sedi- the slope for a cumulative 110,000 yr during isotopic stages

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FIG. 12. Part of Glomar Challenger seismic profile over DSDP Site 111, located in Figure 2 (Laughton, et al., 1972, p. 44). Correlation of theboundary between seismic units 6a and 6b and the MUN seismic grid is discussed in the text.

8, 6, and 4–2, glacial erosion that generated the debris couldhave occurred throughout these three glacial intervals(175,000 yr; Imbrie et al., 1984). As a working hypothesis,we assume an intermediate value of about 140,000 yr forthe total duration of glacial erosion over this period. Theaverage annual yield from direct glacial erosion throughouteach glacial period is therefore reduced to 40 1 106 m3.

Most ice that passed through Trinity Transverse Troughmust have come from the northern part of an ice sheet thatextended beyond the modern coast of Newfoundland andonto the adjacent shallow shelf (Piper et al., 1990, p. 526).The shelf area has been deeply incised both along TrinityTransverse Trough and in its ‘‘tributaries’’ beneath Concep-tion, Trinity, and Bonavista Bays, which are locally deeperthan 500 m (Fig. 14). This deep erosion can be attributed tothe high basal shear stresses expected at the base of icestreams issuing from the periphery of large ice sheets andconverging into valleys (Hughes, 1987). During Pleistoceneice maxima, with grounding lines near the edge of the mod-ern continental shelf, both the present coastal area and innershelf (underlain by Precambrian and Paleozoic bedrock) andthe outer continental shelf including Trinity TransverseTrough (underlain by Mesozoic and Tertiary sediments; Fig.14) would have been subjected to erosion. The shape of the FIG. 13. Summary of inferred correlation between Blue to Brown seis-

mic reflectors, stratified sediments (parallel lines), debris-flow depositsshelf, cut in this area by a deep transverse trough, attests to(mounds), and glacial cycles and their ages (SPECMAP time scale) giventhe power of this glacial erosion.by the stacked, smoothed d18O record tabulated by Imbrie et al. (1984).How much sediment might have been eroded as glaciersAbundant debris flows are believed to correspond to times when sea level

crossed what is now the continental shelf? A first-order esti- was at its lowest (d18O ú 0‰), so that the glacier termini reached the shelfmate comes from an examination of shelf bathymetry (Fig. edge. Stippled vertical lines are probable error bars on the estimated ages

of the Aqua and Brown reflectors.14). Assuming no isostatic adjustments, the approximate vol-

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FIG. 14. Map of shelf and bay morphology, showing geographic names, and proposed ice divide from which glacially eroded sediment convergedon the Trinity Transverse Trough. Bathymetric contours are plotted at 100, 150, 200, 300, 400, 1000, and 2000 m. Bathymetry is not shown in detail inNotre Dame Bay or south of the Avalon Peninsula. The dotted line crossing the shelf from NNW to SSE is the landward limit of Mesozoic and Tertiarysedimentary strata on the shelf (Grant and McAlpine, 1990). The string of / signs encloses the shelf area, deeper than 150 m, that is believed to havebeen excavated by glaciers converging on Trinity Transverse Trough.

ume of sediment needed to fill the modern erosional mor- industry multichannel seismic lines in Orphan Basin showseveral wedge-shaped, poorly reflective or internallyphology between the northern Grand Banks and Cape Freels

(outlined by / signs in Fig. 14) to the same level as the mounded seismic units bounded by more continuous reflec-tion sets below the Brown reflector (Fig. 16). In explorationpresent-day northwestern edge of the Grand Banks (present

water depth about 150 m) is 4800 km3. Following each well Blue H28 (Figs. 2 and 3), the upper 550 mbsf consistsof unconsolidated sandy and gravelly deposits which weglacial-erosional phase, the deepened continental shelf

would have undergone isostatic uplift, making this volume infer to be glacial in origin. A prominent reflection at aboutthis depth, when traced beneath the lower slope, underliesestimate too small by a factor of about 1.75. Assuming an

average density of 2.0 g cm03 for the eroded glacio-marine about eight of the wedge-shaped units. We therefore feelconfident that at least 8 and perhaps 10 shelf-crossing glaciersediments, and a mantle density of 3.35 g cm03 (Anderson,

1989, p. 360), the adjusted volume that can be accommo- advances eroded the Trinity Transverse Trough. This is morethan the number of ice advances that, according to Piperdated by the present-day erosional relief on the Northeast

Newfoundland Shelf is about 8500 km3. et al. (1994), completely covered the Grand Banks, but isconsistent with their proposal for local or regional ice sheetOne might argue that the volume of sediment on the slope

and rise (4500 km3 in isotopic stages 2–4, 6, and 8) can be advance during all glacial isotopic stages starting at stage22. The Northeast Newfoundland Shelf is much narroweraccounted for simply by glacially induced shelf erosion (as

much as 8500 km3 available from shelf depressions). How- than the Grand Banks, so that grounded ice may havereached the shelf edge in Orphan Basin even as forests cov-ever, this is unlikely because continental glaciations in east-

ern Canada began well before stage 8, at least as early as ered parts of the Grand Banks, as suggested by Piper et al.(1994).stage 22 (Piper et al., 1994). Sedimentation rates in Orphan

Basin have been high since at least the middle Pliocene If each major advance contributed about 1500 km3 of sedi-ment, then eight such events would have contributed about(Fig. 15), suggesting protracted glacial erosion. In addition,

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deposits of the Seafloor/Blue unit in cores 92045-7P, -12P, and-13P. Most of the palynomorphs are of pre-Quaternary age andare strongly compressed, indicating recycling from formerlydeeply buried sediments. P. J. Mudie (personal communication,1995) gives semi-quantitative assessment of floral abundancesas follows: 11% Late Quaternary to Holocene tundra pollen andthe fresh-water alga Pediastrum, 37% Tertiary warm-temperatetree pollen, 22% Tertiary (mostly pre-Pliocene) marine dino-flagellates, 26% Mesozoic pollen and spores, and 4% Paleozoicacritarchs and terrestrial spores. By comparison, tills from innerBonavista Bay contain only Cambro–Ordovician acritarchs(Cumming et al., 1992, their facies 5). These data indicatethat a significant amount of the sediment transported onto thecontinental slope and rise by debris flows was eroded fromMesozoic and Tertiary sedimentary units on the shelf. Accuratequantification of the proportion of sediment contributed by this

FIG. 15. Age-depth plot and average sediment accumulation rates near source is beyond the scope of this paper and would requireexploration well Blue H28 (Figs. 2 and 3). Stratigraphy of Blue H28 from

knowledge of microfloral abundances in all potential sourceGradstein and Williams (1986) and Aksu and Hiscott (1992).materials, including Paleozoic bedrock. However, crude volumecalculations outlined above suggest that about one-third of thesediment in the debris flows came from erosion of Precambrian

12,000 km3, or 1.5 times the volume that can be accounted forand Paleozoic bedrock, and the rest from erosion of younger

by shelf erosion. The remainder may have come from erosionsedimentary deposits on the shelf. If correct, then during various

of the adjacent land mass, i.e., the Precambrian and Paleozoicglacial stages the Precambrian and Paleozoic bedrock must have

bedrock of central and eastern Newfoundland. The result wouldcontributed sediment at a rate of about 13 1 106 m3/yr.

be a mixture, in debris-flow deposits, of material from a varietyBedrock Denudation Ratesof sources, including Mesozoic and Tertiary strata that currently

underlie the outer shelf (Fig. 14). The accuracy of this predic- The area of bedrock subjected to subglacial erosion canbe approximated using inferred ice-flow directions and ice-tion is shown by microflora content of near-surface debris-flow

FIG. 16. Tracing of the essential elements of an industry multichannel seismic line (courtesy of Esso Resources Canada Limited) located in Figure3 and showing deeper features than the nearby MUN seismic profile reproduced in Figure 6. Solid lines are prominent reflections, the shallower of whichcoincide with Blue, Green, Aqua, and Brown reflections of Figure 6. Dashed lines are weaker, yet widely traceable, reflections. Single-barbed arrowsshow reflection terminations by onlap, downlap, and truncation. At least eight cycles of presumed stratified to mounded or acoustically transparentdeposits occur above the approximate deepest level of inferred Pleistocene sandy and gravelly deposits penetrated at nearby exploration well Blue H28(Fig. 3).

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thickness reconstructions (Grant, 1989, pp. 404, 414, and REFERENCES428). This area is approximately 6 1 104 km2 (2.2 1 104

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Aksu, A. E., and Piper, D. J. W. (1987). Late Quaternary sedimentation inFig. 14). A uniform glacial denudation rate of 0.13 mm/yrBaffin Bay. Canadian Journal of Earth Sciences 24, 1833–1846.over this area could supply the slope and rise annually with

Aksu, A. E., De Vernal, A., and Mudie, P. J. (1989). High resolution13 1 106 m3 of sediment, assuming an average sedimentforaminifer, palynologic and stable isotopic records of Upper Pleistoceneporosity of 40% (e.g., Hegarty et al., 1988) and zero bedrocksediments from the Labrador Sea: Paleoclimatic and paleoceanographic

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College Station, TX.eastern Newfoundland during stages 8, 6, and 4–2 lastingAnderson, D. L. (1989). ‘‘Theory of the Earth’’. Blackwell, Oxford.a total of 175,000 yr, is about 23 m. One major implication

is that the cumulative erosion of 8 to 10 glacial events since Andrews, J. T. (1990). Fiord to deep sea sediment transfers along thenorth-eastern Canadian continental margin: Models and data. Geographiestage 22 is probably at least 60 m in this area, assumingphysique et Quaternaire 44, 55–70.that all glacial events had similar erosive effect. This value

Andrews, J. T. (1992). Sedimentary geofluxes, global. Encyclopedia ofexceeds the few tens of meters that Sugden (1976) believedEarth System Science 4, 93–101.could be attributed to Quaternary glacial erosion of northern

Andrews, J. T., and Tedesco, K. (1992). Detrital carbonate-rich sediments,hemisphere Precambrian shields and is somewhat less thannorthwestern Labrador Sea: Implications for ice-sheet dynamics and ice-

the 120 to 200 m of average continental glacial denudation berg rafting (Heinrich) events in the North Atlantic. Geology 20, 1087–postulated by Bell and Laine (1985) to account for Quater- 1090.nary and upper Pliocene marine sediment aprons around Bell, M., and Laine, E. P. (1985). Erosion of the Laurentide region of NorthNorth America. America by glacial and glaciofluvial processes. Quaternary Research 23,

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base and calculations, it does allow a stronger case to be on millenial time scales during the last glaciation. Science 267, 1005–made that denudation rates beneath the perimeters of conti- 1010.nental ice sheets are in the range 0.1–0.4 mm/yr during Bond, G., Broecker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel,glacial times. J., and Bonani, G. (1993). Correlations between climate records from

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Broecker, W. S., Bond, G., Klas, M., Clark, E., and McManus, J. (1992).ACKNOWLEDGMENTS Origin of the northern Atlantic’s Heinrich events. Climate Dynamics 6,

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Canadian Hydrographic Service (1990). ‘‘Map 802-A, Newfoundland ShelfThis research was supported by Natural Sciences and Engineering Re-Bathymetry, 1:1,000,000 scale.’’ Department of Fisheries and Oceans,search Council of Canada grants to Hiscott (OGP0006088) and AksuGovernment of Canada, Ottawa.(OGP0042760). Special thanks are extended to the Geological Survey of

Cook, H. E., Johnson, P. D., Matti, J. C., and Zemmels, I. (1975). MethodsCanada, Bedford Institute of Oceanography, for providing air-gun, 3.5-kHz,of sample preparation and X-ray diffraction data analysis, X-ray mineral-and long coring systems for the MUN cruises, and previously collectedogy laboratory, Deep Sea Drilling Project, University of California, River-seismic profiles; and to the Department of Fisheries and Oceans for theside. In ‘‘Initial Reports of the Deep Sea Drilling Project’’ (D. E. Hayes,research-vessel time. We thank the officers, crew, and scientific personnelL. A. Frakes, and A. G. Kaneps, Eds.), Vol. 28, pp. 999–1007. U.S.of CSS Hudson cruises 90-007 and 92-045, and CSS Dawson cruise 91-Government Printing Office, Washington, DC.029, for their assistance in data collection. Special thanks are extended to

Helen Gillespie (MUN), Jane Mills (MUN), Jeannine Benoit (high school Cumming, E. H., Aksu, A. E., and Mudie, P. J. (1992). Late Quaternarysummer student), and Chris Finch (Newfoundland Department of Natural glacial and sedimentary history of Bonavista Bay, northeast Newfound-Resources chemist) for their assistance in laboratory analyses. We also land. Canadian Journal of Earth Sciences 29, 222–235.thank Dr. Peta J. Mudie (Geological Survey of Canada) for palynological Eidvin, T., Jansen, E., and Riis, F. (1993). Chronology of Tertiary fananalysis of debris-flow samples. Esso Resources Canada Limited generously deposits off the western Barents Sea: Implications for uplift and erosionprovided us with copies of a number of multichannel seismic lines in Orphan history of the Barents shelf. Marine Geology 112, 109–131.Basin. An early draft of this manuscript was significantly improved based

Elverhøi, A., Svendsen, J. I., Solheim, A., Andersen, E. S., Milliman, J.,on comments of David Liverman (Newfoundland Department of NaturalMangerud, J., and Hooke, R. LeB. (1995). Late Quaternary sedimentResources), John Andrews (INSTAAR, Boulder, CO), and G. B. J. Faderyield from the high arctic Svalbard area. Journal of Geology 103, 1–17.(Geological Survey of Canada, Bedford Institute of Oceanography). Con-

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Quaternary Sedimentary Processes and Budgets in Orphan Basin, Southwestern Labrador Sea

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