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

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Brookes identifies two points of difference in his discussion of our re-appraisal of the Late Wisconsinan stratigraphy and chronology of southern St. George’s Bay, Newfoundland: (i) the applicability of our sedimentological model and(ii) the relation of sediments to late-glacial sea level. The latter point is the subject of a manuscript submitted for review to this journal, incorporating 91 radiocarbon dates from onshore and offshore marine records, 19 of which arereported for the first time. We, therefore, restrict our rebuttal to what we believe are the critical disagreements in field observations and interpretations that underpin the two points of difference and invite readers to visit the contentiouscoastal sections to compare published accounts; access to these sites is described in several field guides to southern St. George’s Bay (Liverman and Batterson 1995; Liverman et al. 2001). In our paper, we explicitly state that for 80% of the 39 km-long coastal exposure (our “non-ridge” sections; p. 866) our observations broadly confirm earlier interpretations by Brookes (1969, 1974, 1977) and MacClintock and Twenhofel (1940), with the one important exception that we found no evidence for glacial readvance following initial deglaciation and marine submergence of the present coast-line. In almost 100 logged sections through “non-ridge” ex-posures, we did not encounter an upper till or ice-contact gravels (Robinsons Head Drift), indicative of a glacial readvance; instead, we described laterally continuous, moderately to well-sorted, planar bedded and trough cross-bedded, fine gravel capping sections, which we interpreted as glaciofluvial outwash (p. 859; figs. 4, 5c). Consequently, ourdeglacial depositional model for a large part of the coast of southern St. George’s Bay is neither compatible with, nor applicable to, published sedimentary sequences. In the remaining 20% of coastal exposure (our “ridge” sections, p. 866), our mapping revealed complex sedimentarysequences of interbedded diamicton, gravel and mud above the basal till (p. 861; figs. 4, 5a,5e,5f), in contrast to the simple three-fold stratigraphy of basal till, glaciomarine delta and readvance till–gravels described by Brookes (1969, 1974, 1977) and MacClintock and Twenhofel (1940). We argued that these rapid lateral and vertical facies transitions resembled the depositional sequences observed on modern and ancient grounding-line fans. Inland these coast-normal ridges are commonly associated with eskers and meltwater channels and become dominated by purely glaciofluvial sediments. We concluded that “ridge” sections represent subaqueous fans deposited at the grounding line of a quasi-stable tidewater glacier. Nowhere in these “ridge” sections did we encounter sediments resembling delta foresets (Bay St. George Delta) as described by Brookes (1969, 1974, 1977) and MacClintockand Twenhofel (1940), and consequently, our depositional model does not accommodate their sedimentary sequences. Although we chose to present in a separate manuscript the implications of our revised deglacial stratigraphy and chronology for local relative sea-level history, it is incorrect for Brookesto say we ignored sea-level history in our paper. Specifically, we contested Brookes’ observations that marine limit is recorded by: (i) the upper limit of delta foresets in Robinsons Head ridge (44 m) — we interpreted these sediments to be subaqueous fan gravels (p. 863 and p. 867); and (ii) a terraceof possible marine origin at ~45 m above sea level (asl) between Fischells and Bank Head — we observed (as did Grant 1991) that these extensive terraces were capped by glaciofluvial gravels and are more reasonably interpreted as outwash terraces that once graded to deltas some distance offshore (p. 859 and p. 867). Brookes’ assertion that a regressional terrace at30 m asl near Stephenville dates 12.6 ka BP was challenged by our reinterpretation of the Kippens section, in which we argued that shell ages relate to a former sea-level position closer to 18 m asl (p. 864 and p. 867). We concluded that grounding-line fan deposition must have required waterdepths in excess of the published marine limit (45 m asl) and that the best minimum estimate for the age and elevation of the revised marine limit was provided by glaciomarine sediments dated 13.5 ka BP from 35 m asl, but that the corresponding sea-level position was likely tens of metres higher, because these shells were recovered from deep-water mud. Brookes states in reference to the Kippens section that “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, a later collection of shells from the Stephenville site yielded the same date”. We find this comment puzzling and to some extent misleading. The papers referred to largely discuss the proposed glacial Lake Howley in interior Newfoundland. Whilst we have nowish to revisit the arguments in detail, we must address Brookes’ contention that his objections are accepted in later work. The Kippens section is relevant in that a proposed outlet of Lake Howley requires that the Stephenville area be ice-free prior to 12.2 ka. If the Kippens section does indeed provide evidence for a glacial readvance at 12.6 ka covering the entire Stephenville area, this would prevent such an outlet forming. Discussions by Brookes (1995, 1997) and responses from Batterson et al. (1995) and Batterson (1997) largely debate various aspects of the proposed Lake Howley, and to suggest that the challenge of Brookes (1995, 1997) has been sustained by Batterson (1997) is incorrect. Batterson (1997) discusses the Kippens section in more detail, provides two new radiocarbon dates from the area and, far from accepting Brookes’ objections, concludes that this new work casts further doubt on the dating of the Robinsons Head readvance. In our paper, we discuss the Kippens section, expanding on the abstract of Batterson (1997). Brookes suggests that a second collection of shells from the site yielding a similar age strengthens his case. Nowhere in our paper, or in previous work have wequestioned the validity of the 12.6 ka date; our argument, however, rests on the sedimentology of the section itself, and the relationship of this 12.6 ka date to the Robinsons Head readvance. We refer readers to the original paper where the field evidence and our interpretation are presented at length, rather than repeat them here. In his discussion of our paper, Brookes goes to somelength to discount the significance of our date of 13 680 ± 90 BP (Beta 120124) from the upper diamicton at Highlands. He does this by suggesting that the shells may be reworked from older sediments by readvance, citing as supporting evidence observations from two sites elsewhere in the region. We do not question that shells may be transported as paired valves, but note that the presence of fossils in apparent growth position is only one line of evidence suggesting that these shells do indeed date the readvance manifest in the upper diamicton. This section and the interpretation of the age of the readvance have been discussed in more detail in Sheppard (2000) and Sheppard et al. (2000). Brookes suggests that “it is certainly not special pleading to argue that valves…could remain paired while being ploughed from lower and older sediments by an ice advance into the sea”. Such shells however would normally be from the sediments deposited immediately prior to readvance, not those deposited nearly 1000 years before. Although circumstances can be envisaged in which older shells might be reworked into the readvance sediments, whilst not incorporating any younger material, perhaps it makes more sense to accept the sedimentological evidence provided rather than risk being accused of special pleading. Brookes asks “Could the readvance hypothesis incorporate, if only partially, the grounding-line fan hypothesis?” In our paper, we explicitly state that the occurrence of coarsening upward sequences and rare subglacial till in the grounding- line fan sediments of Highlands, for example, likely represent atemporary advance of the grounding line during overall deglaciation of St. George’s Bay (p. 867). We also point out that this advance, dated 13.7 ka, need not be a response to climatic forcing, as proposed for the Robinsons Head readvance, because other second-order controls such as relative waterdepth may influence tidewater termini fluctuations. Brookes concludes his discussion as follows “In the context of sedimentological inventory and explanatory models, however, such models often drive the interpretation of field evidence: ‘seeing is believing’ as the saying goes”. It is not clear that this comment is aimed directly at our work; if it is, it levels the accusation that our field observations are coloured by a preconceived expectation or model. We believe that the reader who examines our paper, and further has the opportunity to examine the field evidence will recognize that our model isderived from extensive field observations and the difficulty that was apparent in reconciling them with published work. As such we were driven to find other, and perhaps more plausible, interpretations of the field evidence.Geology is not an exact science, and the approach of multiple working hypotheses, whilst desirable, cannot always yield a unique answer. The data collected rarely provide an opportunity to falsify a hypothesis completely. Brookes perhaps interprets our work as a claim that his interpretations are falsified or rejected. In contrast, we view our work as presenting an alternative to the previously published interpretations of the St. George’s Bay stratigraphy, and one that we believe fits the field observations well.

Bibliography of Canadian Geomorphology