CGRG Bibliography of Canadian Geomorphology
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Author : Aitken, A.E.; and Bell, T.
Date : 2000.
Title : Temporal evolution of Holocene nearshore sedimentary environments, southeastern Axel Heiberg Island, Nunavut Territory.
Publication : GeoCanada 2000. Calgary, Alberta. May 29-June 2, 2000.
Issue : Abstract
Page(s) :
Abstract
The southeastern coastline of Axel Heiberg Island is characterized by the presence of narrow fjords separated by broad forelands (cf. Bednarski, 1998). Mapping of the surficial geology of this region has provided information on the variable nature of nearshore sedimentary environments, recorded in the lithostratigraphy and biostratigraphy of terrestrial and marine sediments, during postglacial emergence of the coastline. Holocene marine limit is demarcated by deltas and washing limits in diamicton ca. 125-140 m asl. Organic materials recovered from the marine deltas have yielded inconsistent AMS 14 C ages >10 ka BP (i.e. 12.4 ka BP, 49 ka BP) suggesting potential contamination from early Tertiary organic deposits cropping out upstream of the sampling sites. An AMS 14 C date of 9.6 ka BP derived from organic material recovered from sandy glaciomarine rhythmites at the head of Mokka Fiord provides the only chronological control of Holocene marine limit in the field area. At elevations below the Holocene marine limit coarse-grained glaciofluvial sediments forming terraces are graded to marine limit deltas that record Holocene marine regression. The terraces consist of coarsening-upward sequences of massive to trough cross-bedded sand and gravel overlying laminated or massive fine-grained glaciomarine sediments. An AMS 14 C date of 8.9 ka BP was obtained from peat buried beneath glaciofluvial gravel near the head of Mokka Fiord. The coarse-grained glaciofluvial sediments occur in the upper reaches of stream valleys and record the fluvial incision of the landscape accompanying glacio-isostatic emergence and marine regression. Fine-grained glaciomarine sediments record deposition in nearshore sedimentary environments. Proximal glaciomarine rhythmites are preserved within terraces along stream valley slopes. Molluscan faunas recovered from these sediments consist of Portlandia arctica, Thyasira spp. and Delectopecten greenlandicus. 14 C-dates for these molluscan faunas range from 7-9 ka BP. Distal glaciomarine massive muds, up to 3 m in thickness, mantle the landscape in the lower reaches of stream valleys. Molluscan faunas recovered from these sediments are characterized by the presence of Astarte borealis, Hiatella arctica and Mya truncata. 14 C-dates for these molluscan faunas range from 4-8 ka BP. Comparison with modern high arctic fjord mollusc assemblages suggest that the Portlandia-Thyasira-Delectopecten assemblage inhabited water depths ranging from 30-100 m, while the Astarte-Hiatella-Mya assemblage inhabited water depths ranging from 5-50 m (Thompson et al., 1986; Syvitski et al., 1989; Aitken & Fournier, 1993; Aitken & Gilbert, 1996). Of particular interest in this study is a marine offlap sequence, 2-6 m asl, exposed in a stream bank along the southern coast of the island. Five lithostratigraphic units are recognized in this sequence: 1) fine-grained fossiliferous diamicton; 2) massive marine muds; 3) pervasively bioturbated, laminated silty sand; 4) thinly bedded medium and fine sand; 5) gravel lag with shell hash. 14 C dating of mollusc shells indicate that Units 2-4 were deposited ca. 7-4 ka BP. The marine molluscan fauna recovered from Unit 2 consists only of Hiatella arctica, Mya truncata and Mya pseudoarenaria. The extralimital North Atlantic mollusc Mya pseudoarenaria is recorded for the first time in the eastern Canadian Arctic north of Baffin Island. Similar records of this taxa from early Holocene marine sediments on southeastern Ellesmere Island confirm this range extension (John England, pers. comm.). Units 3 and 4 contain a more diverse, shallow water (5-50 m depth) molluscan fauna dominated by Astarte borealis, Hiatella arctica, Mya truncata, Serripes groenlandicus, Clinocardium ciliatum and gastropods. The lithostratigraphy is interpreted to record the transition from a low energy, shallow (lagoonal?) nearshore environment to a high energy beachface environment accompanying marine regression. Temporal variation in the nature and rate of sedimentation in nearshore environments is correlated positively with variations in summer ablation recorded from the Agassiz Ice Cap on Ellesmere Island (Aitken & Bell, 1998; Koerner & Fisher, 1990; Fisher et al., 1995). Deglaciation of fiord-head environments (ca. 11-8 ka BP) is recorded by the rapid deposition of coarse-grained glaciofluvial and proximal glaciomarine rhythmites associated with large meltwater discharges into nearshore environments. Post-glacial sedimentation is dominated by suspension settling of fine-grained sediments and sedimentation rates are low, reflecting an interval (ca. 8 ka to present) of markedly reduced meltwater discharges into nearshore environments. Mollusc diversity increases throughout the Holocene. Models proposed by Syvitski et al. (1989) and Gordillo & Aitken (submitted) relate these changes in mollusc diversity to temporal variations in meltwater inflow and the supply of clastic sediments and labile organic matter to marine nearshore environments accompanying deglaciation.
Bibliography of Canadian Geomorphology