ABSTRACT: Late Wisconsinan stratigraphy and chronology of southern St. George's Bay, Newfoundland: a re-appraisal: Discussion.

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A sedimentological model has been presented by Bell et al.(2001) to explain the stratigraphy and chronology of glacigenic sediments at localities in southern St. George’s Bay, west Newfoundland. These authors claim that this model succeeds earlier interpretations of the sediments at these localities made by MacClintock and Twenhofel (1940) and Brookes (1969, 1974, 1977a), and, by implication, find fault with the history of late-glacial relative sea level for these localities and the wider area presented in Brookes (1974, 1977a) and, with some revision, in Forbes et al. (1993). This note draws attention to two points of difference I have with Bell et al.(2001) at the local and regional scales, namely the applicability of the adopted sedimentological model and the relation ofthe sediments concerned to late-glacial sea level. MacClintock and Twenhofel (1940) presented an inventory and interpretation of glacial and associated sediments exposed in tens of kilometres of coastal cliffs around St. George’s Bay (well portrayed in Bell et al. 2001, fig. 4). They divided the sediments into (i) a compact basal till, low in the cliffs, overlain in places by kame gravels named St. George’s River Drift, (ii) an overlying marine sequence, comprising lower muds and higher deltaic sands and gravels, in places underlying a delta surface named Bay St. George Delta,and (iii) in a few places, an upper unit of till and kame gravel, with morainic topography named Robinsons Head Drift.The sequence was interpreted to have resulted froman ice advance beyond the present coast of the bay, followed by ice retreat and local stagnation at the present coast, then by marine submergence, and, finally, an ice readvance in several lobes across the present coastline into the sea and sediments previously deposited in it. Brookes (1969, 1974, 1977a) followed and elaborated on this scheme, supplementing it with radiocarbon age control (courtesy of the Geological Survey of Canada), and relating glacial history to sea-level change. Thus, the St. George’s River Drift, derived from the island of Newfoundland, predated marine submergence, which, from radiocarbon dates on shells in glaciomarine stony muds at the base of the marine sequence, began at 13.5 14 C ka BP all around St. George’s Bay (standard deviations on the dates are 110-320 years). Marine limit was established and directly dated on the Port au Port Peninsula, beyond the complicating effect of the later Robinsons Head Drift readvance, at 45 m above sea level (asl) and 13.5 ka. In the Stephenville area, the Robinsons Head (Drift) Moraine (named by Brookes 1977a) abuts a regressional marine terrace at 30 m asl, and marine shells in sediments related to that terrace were dated at 12.6 ka BP, thus dating the Robinsons Head (Drift) readvance (locally only stillstand) and the 30 m sea level. Bell et al. (2001) concentrated their attention on one locality,near Highlands, and more briefly discussed similar settingsat Robinsons Head and Bank Head, farther northeast (their fig. 1 for locations). At Highlands, Brookes (1969, 1974) recognized a moraine ridge of Robinsons Head Drift, cut by the coastal cliff to expose till over Bay St. George Deltasediments, at the base of which, nearby one radiocarbon date of 13.5 ka BP, was obtained on glaciomarine stony clay. I agreed with MacClintock and Twenhofel’s (1940) conclusion that here, as at other headland sections along this shore, ice had readvanced below sea level, over a prior delta.The regional landform sediment associations at this and similar localities are well shown by Bell et al. (2001, fig. 2). Bell et al. (2001) based their reinterpretation of the Highlands section on what they recognized as sedimentary similarities, from the base of the marine sequence to the top of the ridge, to a model of deposition of a fan at the mouth of a glacial conduit at an ice-sheet grounding line (Powell 1990, 1991; Bell et al., fig. 9). The same grounding-line fan model was used by Liverman and Bell (1996) and Bell et al. (2001) regarding the sedimentary sequence at Robinsons Head, northeast of Highlands. Since the object of a reexamination of these sections was to test the validity of previous interpretattion(s), it seems insufficient merely to point to similarities with a model for grounding-line fans without considering the environment postulated previously. Are there diagnostic differences? Could the readvance hypothesis incorporate, if only partially, the grounding-line fan hypothesis? I previously concluded (Brookes 1974, 1977a) that ridged terrain at the coast of St. George’s Bay was produced by the Robinsons Head ice readvance at 12.6 ka BP, when sea level had fallen from a marine limit at 45 m to a temporary stand at 30 m. The grounding-line fan interpretation of Bell et al. (2001), however, leaves relative sea level at Highlands, at the time of deposition of the entire sediment package, at an unstated level somewhere above the package, while the question of sea-level change during accumulation of the sediments does not arise. Since earlier interpretations made general and then specific reference to the contemporary sea-level position, a reinterpretation should explain why sea level is ignored as a factor. Regarding sea levels elsewhere around this bay, 25-30 km northeast along the coast from this section, what I continue to maintain is the top of the Bay St. George Delta stands at 45m asl. It is not overlain by morainic or grounding-line sediments, nor is it anywhere surmounted by a ridge. For nearly 10 km >30 m of cross-bedded sands underlie a transversely level surface rising gently inland, graded to a sea level at 45 m (Brookes 1974, Map 15-1973; Liverman and Bell 1996). Since Bell et al. (2001) appear not to disagree with this interpretation, the presence of the delta has sea-level implications which should extend to the Highlands locality.Across St. George’s Bay, at Abrahams Cove, on the south shore of Port au Port Peninsula, marine limit is marked also at 45 m by a limit of wave-washing of till over bedrock. Exposed by gravel extraction, marine shells from joints in the bedrock near the marine limit were dated at 13.5 ka BP, the same age as shells from the base of the marine sequence here (Brookes 1974). A beach-ridged, regressional terrace at 30 m was also obvious, but has since succumbed to gravel extraction. At Robinsons Head, closer to Highlands, marine shells in stratified muds at 35 m asl were dated at 13.5 ka BP, while sediments uncertainly interpreted as deltaic were recognized to 44 m asl, overlain by ice-marginal (morainic) sediments (Brookes 1974). The interpretation of the sediments above the dated unit was refuted by Liverman and Bell (1996) and Bell et al. (2001), but the indication nevertheless is that sea level had been at >35 m at 13.5 ka. At Robinsons Head, and Bank Head to the northeast, ridges of rolling topography, plainly interpretable as moraine crests,slope steeply inland to water-filled depressions, in what canbe seen as ice-contact faces and kettle depressions (well displayed in Fig. 3 of Bell et al. 2001). It seems reasonable from the above evidence of (i) ridged landforms and sediments that have yet to be shown to be incompatible with ice readvance into the sea, (ii) the age and height of the 45 m marine limit at Abrahams Cove and the broad delta surface northeast of Robinsons Head, and (iii) the 30 m regressional terrace abutting the ridged terrain (at least at Stephenville and at Highlands), to challenge the postulated subaqueous origin of the entire sediment package at Highlands and at Robinsons Head, proposed by Bell et al. (2001), regardless of any perceived similarity to the Powell model. These authors may yet go on to challenge the sea-level evidence I have summarized here, but at present it stands. They rightly state that my interpretation of the 12.6 ka age of shells near Stephenville as dating a sea level at 30 m, contemporaneous with the Robinsons Head moraine (Brookes 1977a), has generated some debate (p. 853). Mention is not made, however, of the fact that, concerning this debate, not only has the challenge of Brookes (1995, 1997) to Batterson et al. (1993) been sustained (Batterson 1997, 1995), a later collection of shells from the Stephenville site yielded thesame age. What remains unexplained is why the 30 m relative sea-level plane around St. George’s Bay appears to be undeformed isostatically. As further evidence in favour of a subaqueous origin of the sediments at Highlands, Bell et al. (2001) mentioned a radiocarbon date on paired valves of Hiatella arctica of 13.7 ka BP at 20 m asl in a diamicton occupying the upper part of the section and extending close to the cliff-top. This is statistically the same age as shells from the bottom of the marine section nearby. While paired valves may strengthen an argument in the making, it is certainly not special pleadingto argue that valves, particularly of robust Hiatella arctica, could remain paired while being ploughed from lower and older sediments by an ice readvance into the sea, as originally proposed by MacClintock and Twenhofel (1940). I have seen large, paired valves of Mya arenaria remaining in growth position in late-glacial deltaic sands, after these had slumped about 30 m vertically and flowed about 300 m horizontally on liquified mud at a site beside Bonne Bay, 170 km north-northeast of Highlands. Furthermore, from low coastal cliffs on Cabot Strait, 60 km south-southwest of Highlands, I reported pods of reddish mud enclosed in sandy-gravelly diamicton beneath morainic topography and containing delicate marine shell fragments that were dated to 13.8 ka. The occurrence was simply interpreted as produced by a readvance of ice, which ploughed up shell-bearing muds and incorporated them into a moraine (Brookes 1975, 1977b). In the surrounding area there are no sea-level indicators, such as terraces, to indicate where sea level may have been during deposition of the muds or the moraine. While the context is not exactly the same as at Highlands, it does warn of a possible alternative explanation for shells in a diamict to that of Bell et al. (2001). The record of late-glacial sediments at several sites around St. George’s Bay has been greatly refined by Bell et al. (2001), and none too soon, since this is the first re-appraisal published in a refereed journal since my work of thirty years ago. In the context of sedimentological inventory and explanatory models, however, such models often drive the interpretation of field evidence: believing is seeing, as the saying goes. Many (perhaps too many) will argue this is the purpose of modelling, whereas the converse is at least desirable, if not preferable. Alternatives, even in the absence of models, should be left open until falsified.

Bibliography of Canadian Geomorphology