Late-glacial stratigraphy and history of the Gulf of St. Lawrence: Discussion

DISCUSSION

Late-glacial stratigraphy and history of the Gulf ofSt. Lawrence: Discussion1

Rudolph R. Stea

Discussion 482

Discussion

The cruise of the C.S.S. Hudson in 1990 was a memorableintroduction to marine geoscience and North Atlantic storms.Land- and sea-based geologists were thrown together to linkland and sea sedimentary records and get a better under-standing of the complex glaciation of Atlantic Canada.

Atlantic Canada is an amalgam of distinctive tectonic ter-ranes, where lithic markers can be used to reconstruct glacialflow patterns (e.g., Stea and Pe-Piper 1999). Provenanceanalysis can also help to establish correlations between ter-restrial and marine stratigraphic records. Once offshore icemargins and onshore flow lines are linked up, former Pleis-tocene glaciers can be modelled. Stea et al. (1998) usedthese methods to develop an ice advance and retreat historyof Maritime Canada. The Gulf of St. Lawrence region was acritical area for the reconstruction, and little data was avail-able. Josenhans and Lehman (1999) have made a significantadvance in the understanding of the marine record to com-plete the link with the terrestrial records in the Gulf of St.Lawrence.

While on the cruise, I logged piston cores and examinedthe lithologies of large (1t) IKU grab samples from the tillunits in the Gulf of St. Lawrence that were mapped usinghigh-resolution seismic data (Stea 1991). Much of that lithicdata was not presented in the summary paper by Josenhansand Lehman (1999), which is unfortunate because of its im-portant ramifications for ice reconstructions. The lithic dataand terrestrial radiocarbon dates from the surrounding landareas imply a much greater thickness and extent of late-glacial ice than in the model presented by Josenhans andLehman (1999). I will briefly review the pebble lithologicalresults, addressing the questions of ice-flow paths and icethickness, then discuss contradictions between theJosenhans–Lehman model of deglaciation of the Gulf of St.

Lawrence and the Stea et al. (1998) and Stea and Mott(1998) models.

Lithological data from IKU samples andglacial flow lines

Sample IKU 4 was taken in 80 m water depth near themouth of the Cape Breton trough (Fig. 1) and consists of asorted, gravelly sand. It is believed to have been reworkedduring lower relative sea levels from a till unit that blanketsmuch of the Magdalen Plateau (Josenhans and Zevenhuisen1993). Several dark anorthosite (labradorite), granulite, andgneiss pebbles suggest a Canadian Shield source, and thusan early Laurentide flow. Josenhans and Lehman’s (p. 1331)reference to “quartz pebbles” of shield origin from the low-ermost till may relate to these. Whitish anorthosite can befound in the Precambrian Blair River (Grenville) Inlier inCape Breton Island (Miller 1996).

Sample IKU 1 was taken from the surface of the upper tillunit (cf. Josenhans and Lehman 1999) in the Cape Bretontrough at 254 m water depth (Fig. 1). The pebble assemblage(average of 4 subsamples) consists of 70% red and greymudstones and sandstones, 12% metamorphic rocks, 3%limestone, 11% plutonic rocks, and 4% volcanic clasts. Mostof the sedimentary lithologies are common to the MagdalenBasin that surrounds the sample location; however, redbedsare most likely derived from the Pictou Group confined tothe Magdalen Plateau (Sanford and Grant 1990).

The key to the provenance of the upper till is the rela-tively undeformed plutonic rocks, characteristic of the Appa-lachian bedrock terranes (Williams 1978). The closestmatches of source rock terrane and sample assemblages arethe Blair River Inlier and Aspy Terrane of northern CapeBreton. The Blair River Inlier is the only major source ofsyenite and gabbro in the region (Fig. 1). A distinctive anti-perthitic syenite with sphene-rimmed ilmenite, found insample IKU 1, is confined to the Blair River Inlier in CapeBreton Island (Miller 1996). Amphibolitic gneisses withinthe Blair River Inlier may also be the source for the abun-dant quartz–amphibole fragments found in the IKU samples(Fig 1). The Cheticamp Pluton of southwestern Cape BretonIsland is a possible source region for the granodiorite peb-bles in both IKU samples. The cordierite-bearing gneissclast found in IKU 1 is probably derived from the Bras d’Or

Can. J. Earth Sci.38: 479–482 (2001) © 2001 NRC Canada

479

DOI: 10.1139/cjes-38-3-479

Received March 22, 2000. Accepted June 7, 2000. Publishedon the NRC Research Press Web site on March 14, 2001.Paper handled by Associate Editor R. Gilbert.

R.R. Stea.Nova Scotia Department of Natural Resources,P.O. Box 698, Halifax, NS B3J 2T9, Canada. (e-mail:[email protected]).

1Paper by H. Josenhans and S. Lehman. 1999. CanadianJournal of Earth Sciences,36: 1327-1345.

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Terrane, characterized by metamorphic suites with sillimaniteand cordierite (Raeside and Barr 1990).

I differ from Josenhans and Lehman (1999) in postulatinga cross Magdalen Shelf and Cape Breton flow of ice to feedthe Cape Breton trough ice stream, whereas they show anisolated ice tongue that just skirts the western coast of CapeBreton and the eastern coast of Prince Edward Island. Thetransport of erratics from the distinctive bedrock terranes ofCape Breton can be best explained by an ice stream in theCape Breton trough confluent with ice crossing the 300 mhighland plateaus of southwestern Cape Breton Island and alocal ice cap in the 500 m northern Cape Breton highlands(Fig 1). Grant (1994) mapped westward- and northwestward-trending striae across the plateau regions of southwesternCape Breton. According to our model, the ice stream in theCape Breton trough was first generated from the Scotian IceDivide over mainland Nova Scotia and offshore south ofCape Breton Island (Scotian Phase), which produced north-ward-trending ice-flow features across all of northern main-land Nova Scotia (Stea et al. 1992, 1998). The Scotian Phasemargin off mainland Nova Scotia was dated between 15–17ka (King 1996). During the ensuing Chignecto Phase (ca 13ka), the Cape Breton trough ice stream formed the upper till.Sometime after 13 ka, ice flow across the southwest high-land plateau was cut off, leaving a radially spreading localice mass on Cape Breton.

Ice-extent reconstructions

Josenhans and Lehman have made a strong case for theage of the readvance of the upper till unit at 13.2 ka, basedon the positioning of core 90-028-10 at the till–tongue mar-gin. This ice advance can be correlated with the ChignectoPhase (ca. 13 ka; Fig. 1) of Stea et al. (1998) in spite ofslightly older dates, because the Chignecto Phase age wasestablished using terrestrial wood (accelerator mass spec-trometry, AMS) radiocarbon dates. The age of the middle tillis less well constrained as the till tongue margin was notcored or dated. It wasn’t clear to me, for example, where themiddle till overlies ice-proximal sediments of the lower till(ibid. p. 1337). Proximal glaciomarine sediment on the topof the Laurentian Moraine, which represents the terminus ofthe lower till (King and Fader 1986), was cored and dated at13.1 ka (G.B.J. Fader, personal communication, 1997; Steaet al. 1998).

The inferred age of the Laurentian Moraine is less than 14ka, making it equivalent in age to the middle till.

Josenhans and Lehman (1999, Fig. 14) concluded thatmost of the central Magdalen Plateau coastal areas, includ-ing southeastern New Brunswick and northern Nova Scotia,were ice free from 13.5 ka onward. I have a different viewbased on till provenance data discussed earlier and radiocar-bon dating of the adjacent land areas. Calving bay-ice retreatin the channel areas adjacent to the Magdalen Plateau can betraced to western Prince Edward Island just before 12.4 ka(Prest 1970). Eastern P.E.I. was ice covered much longer, atleast until 10 ka, inferred by radiocarbon ages on basal lakesediments and bogs (Anderson 1985), and shells from off-shore sediment cores (Kranck 1972). The persistence of iceafter 10 ka in northern Cape Breton is verified by AMS dat-ing of wood from the base of lakes, a similar story to the

Gaspé, where ice remained until 10 ka (Richard et al. 1997).Having quoted Richard et al. in their paper, it is unclearwhat evidence Josenhans and Lehman used to interpret ice-free conditions in eastern New Brunswick and the Gaspé at13.2 ka. What could possibly drive the Baie des Chaleursand Cape Breton trough ice streams, unless it was ice in theadjacent land areas?

Stea and Mott (1998) present stratigraphic evidence of areadvance of eastern Gulf of St. Lawrence ice during theYounger Dryas (10.5 ka), which dammed a large lake in thelowland areas of south western Cape Breton and northernNova Scotia (Fig. 1). Recent pipeline excavations in north-ern Nova Scotia revealed a widespread buried soil horizon,under till, with wood dated to 10.8 ka (R.J. Mott and R.McNeely, personal communications, 2000). These data im-ply that Gulf-based ice advanced southward into Nova Sco-tia to nearly the foot of the Cobequid Highlands, sometimebetween 10.8 and 10 ka (Fig. 1). The sedimentological re-sponse of the Younger Dryas in the Gulf is not well under-stood, but Syvitski (1992) and Josenhans and Zevenhuisen(1993) describe a late-glacial “paraglacial” deltaic unit os-tensibly derived from ice sheets on land that may relate tothe Younger Dryas. It is not clear whether this unit was sam-pled in the Josenhans and Lehman (1999) study, but a simi-lar pre-Holocene high-amplitude seismic unit (Yankee BankFormation) off the Scotian Shelf was dated to between 10and 10.7 ka (Stea et al. 1996). It is interesting to note thatLoring and Nota (1973) defined Younger Dryas ice activitybased on shells from glaciomarine sediment in the Gulf ofSt. Lawrence dated to 10.2 ka.

Relative sea level

The relative sea-level story of the Gulf of St. Lawrence iscrucial to ice reconstructions and vice versa (e.g., Quinlanand Beaumont 1981; Tushingham and Peltier 1994).Josenhans and Lehman interpret complete ice over the Gulfand environs prior to 14 ka. If the eastern Gulf of St. Law-rence was deglaciated by 13.5, then why aren’t raisedbeaches found along these coasts? Grant (1980) and Dykeand Prest (1987) proposed thin or no Late Wisconsinan (18–21 ka) ice in the Gulf, which would explain the lack of sub-mergence. Ice cover over the 500 m Cape Breton Highlandsand over the eastern Gulf regions would undoubtedly requiresubstantial isostatic depression, resulting in marine submer-gence after rapid ice retreat. Emergent shorelines in westernP.E.I. and northeastern New Brunswick attest to this. Glacialstudies in New Brunswick and Nova Scotia (Rampton et al.1984; Pronk et al. 1989; McClenaghan and DiLabio 1995;Stea and Pe-Piper 1999) point to a Late Wisconsinan ice capover the Magdalen Plateau (Escuminac Ice Centre) thatcrossed the 300 m Cobequid Highlands to the south. Theparadox (lots of ice, no submergence) is best explained bythe persistence of an ice cap in the southern Gulf of St. Law-rence after rebound had partially equilibrated the area (Steaet al. 1998; Fig. 1).

The authors claim a post-glacial, sea-level lowering of110 m, alluding to well-developed terraces not shown in thepaper. Asserting a shallow-water origin for core 90-028-011seems problematic, as it is largely dark mud (Josenhans etal. 1991) and much of the terrestrial material could be re-

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Fig. 1. Map of the southern Gulf of St. Lawrence region showing the inferred late-glacial ice margins (modified from Stea et al. 1998).



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worked. Loring and Nota (1973), however, describe-70 mdeep terrace on the Magdalen Plateau and mapped well-sorted sand and gravel to about 80 m depth, so it wouldseem that much of the plateau was exposed to wave-reworking. A correlative shoreline at a depth of-70m on theAtlantic coast of Nova Scotia was dated to 11.6 ka (Stea etal. 1994), but no evidence was found for any lower shore-lines on the inner Scotian Shelf.

References

Anderson, T.W. 1985. Late-Quaternary pollen records from easternOntario, Quebec, and Atlantic Canada:In Pollen Records ofLate Quaternary North American Sediments,Edited by MBryant and R.G. Holloway. AASP Foundation, pp. 281–326.

Dredge, L., Mott, R.J., and Grant, D.R. 1992. Quaternary stratigra-phy, palaeoecology, and glacial geology, Iles de la Madelaine,Quebec. Canadian Journal of Earth Sciences,29: 1981–1996.

Dyke, A.S., and Prest, V.K. 1987. Late Wisconsinan and Holocenehistory of the Laurentide ice sheet:In The Laurentide Ice Sheet.Edited byR.J. Fulton, and J.T. Andrews. Géographie physiqueet Quaternaire,41: 237–264.

Grant, D.R. 1980. Quaternary sea-level change in Atlantic Canadaas an indication of crustal delevelling.In Earth rheology, isos-tasy and eustasy.Edited byN.A. Mörner. John Wiley and Sons,London, pp. 201–214.

Grant, D.R. 1994. Quaternary Geology, Cape Breton Island. Geo-logical Survey of Canada, Bulletin 482.

Josenhans, H., and Lehman, S. 1999. Late glacial stratigraphy andhistory of the Gulf of St. Lawrence, Canada. Canadian Journalof Earth Sciences,36: 1327–1345.

Josenhans, H., and Zevenhuisen, J. 1993. Quaternary sedimentmaps of the Gulf of St. Lawrence. Geological Survey of Canada,Open File Report 2700.

Josenhans, H.W., Johnston, L., Jarrett, K., Smith D., andZevenhuisen, J. 1991. Surficial geological investigations of theGulf of St. Lawrence. Cruise Report Hudson 90–028, Geologi-cal Survey of Canada, Open File Report 2394.

King, L.H. 1996. Late Wisconsinan ice retreat from the ScotianShelf. Geological Society of America Bulletin,108: 1056–1067.

King, L.H., and Fader, G.B. 1986. Wisconsinan glaciation of theContinental Shelf of Southeast Atlantic Canada. Geological Sur-vey of Canada, Bulletin 363.

Kranck, K. 1972. Geomorphological development and post-Pleistocene sea level changes, Northumberland Strait, MaritimeProvinces. Canadian Journal of Earth Sciences,9: 835–844.

Loring, D.H., and Nota, D.J.G. 1973. Morphology and sedimentsof the Gulf of St. Lawrence. Fisheries Research Board of Can-ada, Bulletin 182.

McClenaghan, M.B., and Dilabio, R.N.W. 1995. Till geochemistryand its implications for mineral exploration: southeastern CapeBreton Island, Nova Scotia, Canada. Quaternary International,20: 107–122.

Miller, B.V. 1996. Geology, geochronology, and tectonic signifi-cance of the Blair River Inlier, Northern Cape Breton Island,Nova Scotia, Ph.D. thesis, Dalhousie University, Halifax, N.S.

Mott, R.J., and Stea, R.R. 1994. Late-Glacial (Allerød/YoungerDryas) buried organic deposits, Nova Scotia, Canada. Quater-nary Science Reviews,12: 645–657.

Prest, V.K. 1970. Quaternary Geology of Canada.In Geology andeconomic minerals of Canada,.Edited byR.J.W. Douglas. Geo-logical Survey of Canada, Economic Geology Report 1,pp. 676–764.

Pronk, A.G., Bobrowsky, P.T., and Parkhill, M.A. 1989. An inter-pretation of the late Quaternary glacial flow indicators in theBaie des Chaleurs region, northern New Brunswick. Géographiephysique et Quaternaire,43: 179–190.

Quinlan, G., and Beaumont, C. 1981. A comparison of observedand theoretical relative sea levels in Atlantic Canada. CanadianJournal of Earth Sciences,18: 1146–1163.

Raeside, R.P., and Barr, S.M. 1990. Geology and tectonic develop-ment of the Bras d’Or suspect terrane, Cape Breton Island, NovaScotia. Canadian Journal of Earth Sciences,27: 1371–1381.

Rampton, V.N., Gauthier, R.C., Thibault, J., and Seaman, A.A.1984. Quaternary geology of New Brunswick. Geological Sur-vey of Canada, Memoir 416.

Richard, P.J.H., Veillette, J.J., Larouche, A.C., Hétu, B., Gray, J.T.,and Gangloff, P. 1997. Chronologie de la Déglaciation enGaspésie: Nouvelles Données et implications, Géographie phy-sique et Quaternaire,51: 163–184.

Sanford, B.V., and Grant, A.C. 1990. Bedrock geological mappingand basin studies in the Gulf of St. Lawrence.In Current re-search, part B. Geological Survey of Canada, Paper 90–1B,pp. 33–42.

Stea, R.R. 1991. Clastic dispersal in the Laurentian Channel offCape Breton Island and mainland Nova Scotia: test of a glacialdispersal model. Dalhousie University, Centre for Marine Geol-ogy, Technical Report No. 13.

Stea., R.R., and Mott, R.J. 1989. Deglaciation environments andevidence for glaciers of Younger Dryas age in Nova Scotia, Can-ada. Boreas,18: 169–187.

Stea R.R., and Mott, R.J. 1998. Deglaciation of Nova Scotia: stra-tigraphy and chronology of lake sediment cores and buried or-ganic sections. Géographie physique et Quaternaire,41: 279–290.

Stea, R.R., and Pe-Piper, G. 1999. Using whole-rock geochemistryto locate the source of igneous erratics from drumlins on the At-lantic coast of Nova Scotia. Boreas,28: 308–325.

Stea, R.R., Mott, R.J., Belknap, D.F., and Radtke, U. 1992. ThePre-Late Wisconsinan Chronology of Nova Scotia, Canada.InThe Last Interglaciation/Glaciation Transition in North America.Edited by P.U. Clark and P.D. Lea. Geological Society of Amer-ica, Special Paper 270, pp. 185–206.

Stea, R.R., Boyd, R., Fader, G.B.J., Courtney, R.C., Scott, D.B.,and Pecore, S. 1994. Morphology and seismic stratigraphy ofthe inner continental shelf off Nova Scotia, Canada: evidencefor a –65 m lowstand between 11,650 and 11,25014C yr B. P.Marine Geology,117: 135–154.

Stea, R.R., Boyd, R., Costello, O., Fader, G.B.J., and Scott, D.B.1996. Deglaciation of the inner Scotian Shelf, Nova Scotia: cor-relation of terrestrial and marine glacial events.In Late Quater-nary palaeoceanography of the North Atlantic margins,Editedby J.T. Andrews, H.H. Bergsten, and A.E. Jennings. GeologicalSociety, Special Publication 111, pp. 77–101.

Stea, R.R., Piper, D.J.W., Fader, G.B.J., and Boyd, R. 1998.Wisconsinan glacial and sea-level history of Maritime Canada: acorrelation of land and sea events. Geological Society of Amer-ica Bulletin, 110: 821–845.

Syvitski, J.P.M. 1992. Marine geology of the Baie Des Chaleurs.Géographie physique et Quaternaire,46: 331–348.

Tushingham, A.M., and Peltier, W.R. 1993. Ice 3G: A new globalmodel of Late Pleistocene deglaciation based upon geophysicalpredictions of post-glacial relative sea-level change. Journal ofGeophysical Research,96: 4497–4523.

Williams, H. 1978. Tectonic lithofacies map of the AppalachianOrogen. Memorial University of Newfoundland, St. John’s,Nfld., Map No. 1, scale 1 : 1 000 000.



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Late-glacial stratigraphy and history of the Gulf of St. Lawrence: Discussion