Late-glacial stratigraphy and history of the Gulf ofSt. Lawrence: Reply1
H.W. Josenhans Josenhans
We appreciate the discussion by Rudolph Stea, because itbrings to light new data of relevance to the late-glacial re-construction of Atlantic Canada and presents lithologic datathat was beyond the focus of our paper, which was a re-gional assessment based on seismic reflection data andcores. Lithologic analysis and till provenance interpretationswere indeed one of the original cruise goals, and dedicatedshipboard analysis was undertaken by regional experts: R.Stea, L. Dredge, and R. Mott. Lithologic interpretationscompleted at sea were valuable in a local sense, but initialassessment of the limited samples available did not appear tolink directly with the seismostratigraphic framework thatforms the backbone of this paper. For example the IKU4sample mentioned by Stea was obtained on the easternMagdalen plateau from a seismic unit that could not be dif-ferentiated from the oldest or youngest of the till units.Without solid stratigraphic context, the lithology could rep-resent a mixture of multiple glacial deposits concentrated bytransgression of the bank top or attributed to transport to thesite by ice rafting. Sample IKU 1, as mentioned by Stea, wasa large volume sample representative of the upper till. Theinterpretations offered by Stea for this stratigraphicallywell-constrained unit are very helpful. Unfortunately, theyrepresent the only large volume sample of known strati-graphic setting obtained on the cruise.
Stea’s interpretations of IKU 4 allow us to bridge a shal-low water blind zone where marine seismostratigraphic datadoes not exist and land geologists cannot go. This blind zoneis sometimes laterally extensive due to removal of section bytrangression–regression, making onshore–offshore correla-tions difficult without extensive sample control. Stea’s inter-pretation of the upper till being derived from Cape Bretonand perhaps the Scotian ice divide is reasonable.
We acknowledge that our ice margin reconstructions(Josenhans and Lehman 1999, fig. 14) are only reliable off-shore, where seismostratigraphic control exists. We do notdisagree with the landward extension of these marine-basedice margins and accept the arguments proposed by Stea. Theevidence derived from the recent pipeline excavation inNova Scotia is significant and could be published as newwork that builds on the stratigraphy and regional mappingpresented in our paper.
Regarding the glacial dynamics within the marine areas,particularly with respect to isostatic loading and impact onrelative sea level, we disagree with Stea. The clearly recog-
nized till tongues that define the ice margins in our paperrepresent the location where the ice sheet becomes buoyantand therefore no longer contributes to isostatic depression.The high-water content and low consolidation of the upperglacial till–ice contact deposit (Josenhans et al. 1991, GSCOpen File 2394) is thought to reflect low basal shear, whichwould permit a very low-gradient ice stream, with little orno isostatic effect, to cover significant areas of the Gulf. Re-cent seismic reflection surveys conducted off north westernCape Breton along the eastern flanks of the Cape Bretonchannel reveal evidence of late ice lobes extending from thehighlands into the marine areas. We suggest that the lack ofraised shorelines is best explained by late ice emanatingfrom land that removed the raised marine deposits or pre-vented their formation. Alternatively, if raised sea levels didexist, they may have washed against late ice, thus preventingtheir development in the stratigraphic record.
The 110 m terrace mentioned in our paper is awell-mapped geomorphic feature that we believe is best ex-plained as a former sea level still low stand. We acknowl-edge that we have no satisfactory age control on this featureat this time, indeed this is the focus of our current research.However, well-constrained sea-level investigations near for-mer ice margins on Canada’s west coast (Josenhans etal.1997, Fedje and Josenhans 2000) indicate significantchanges in elevation over short lateral distances, and we cau-tion Stea in attempting to correlate shorelines from the Gulfof St Lawrence all the way to the Scotian Shelf.
References
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.W., Fedje, D.W., Pientitz, R., and Southon, J.R. 1997.Early Humans and rapidly changing Holocene sea levels in theQueen Charlotte Island – Hecate Strait, British Columbia Can-ada. Science,277: 71–74.
Josenhans, H.W., Johnston, L., Jarrett, K., Smith, D., andZevenhuizen, J. 1991. Surficial Geological Investigations of theGulf of St.Lawrence, Cruise report: Hudson 90–028. GeologicalSurvey of Canada, Open File 2394.
Fedje, D.W., and Josenhans, H.W. 2000. Drowned forests and ar-cheology on the continental shelf of British Columbia, Canada.Geology,28: 99–102.
Can. J. Earth Sci.38: 483 (2001) © 2001 NRC Canada
483
DOI: 10.1139/cjes-38-3-483
Received August 11, 2000. Accepted November 21, 2000. Published on the NRC Research Press Web site on March 14, 2001.Paper handled by Associate Editor R. Gilbert.
H.W. Josenhans. Bedford Institute of Oceanography, Geological Survey of Canada (Atlantic), Box 1006, Dartmouth, NS B2Y 4A2,Canada (e-mail: [email protected]).
1Discussion by R. Stea. This issue. Canadian Journal of Earth Sciences,38: 479–482.
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