Quaternary Geologic History of Lady Franklin Bank, Southeastern Baffin Shelf, N.W.T.
Praeg, Daniel B.
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The Lady Franklin Sand and Gravel is an informal term for a compositionally sand and/or gravel surficial sediment unit of the southeastern Baffin Shelf, delineated on the basis of grab samples and acoustic data by Praeg and MacLean (in preparation). The unit tends to occur above a bathymetric depth of about 200 m below which there is often surficial till of the Baffin Shelf Drift, and above which there is either (1) thin sand and gravel over thick tills, (2) thin sand and gravel over bedrock, or (3) an intermediate thickness cover for which it is uncertain whether there is thin sand and gravel over thin till, or solely thick sand and gravel. The Lady Franklin Bank area is an example of the latter case. In this area, the sand and gravel is overlain or grades into the silty/clayey sediments of the restricted Monumental Basin. Twenty-five randomly distributed surficial van Veen grab samples are available from the unit in the Lady Franklin Bank area. These had undergone grain size analysis prior to inception of the thesis, and were subsequently submitted to detailed lithologic and textural analysis of three selected grain size intervals: gravel (-50 to -30), coarse sand -20 to 00), and fine sand, (20-30). Grain size distributions indicate assemblages dominantly bimodal between gravel and fine sand, with silt/clay mode of variable importance. Grain size parameters across the study area indicate a general coarsening with increasing depths, except for two coarse grained medium depth samples off Loks Land. Hydraulic interpretation indicates currents of ~20 to 35 cm/s are required to mobilize the fine sand modes, values in the range of existing oceanographic currents. Lithology of the three selected intervals of the sediments are dominated by crystalline siliclastic material and limestone, with minor uncertain brown siltstones and very minor quartz sandstones, largely reflecting the underlying Pre-Cambrian granites and gneisses and Ordovician limestones of the Lady Franklin Bank/Monumental Basin bedrock. Lithologic distributions of all three selected intervals are non-random, and similar, showing a relation to the bedrock adjacent to bathymetrically shallow areas, (< 150 m) and no relation to the bedrock over deeper areas. Textural study of the three grain intervals delineates the presence of a distinct grain surface history comprised of an older, rounded, low relief surface, broken by a younger, angular high relief surface, which has been subsequently slightly modified by rounding. Entirely young surface grains are most common, followed by grain exhibiting both the old and new surfaces and entirely old surface grains. Scanning Electron Microprobe study of -20 to 00 quartz grains also recognizes the surface history, although it is confused by silica precipitation/solution features. Environmental discrimination indicates that the old surface is of the aeolian, subaqueous, and possibly source material environments, while the new surface is of the glacial environment. The new surface has been modified by rounding and surface forms of the subaqueous environment. Roundness distributions for the new surface are non-random, and show a noticeable increase in roundness modification with increasing depth. The evidence indicates a probable relative sea level low of 100 to < 150 m to produce the old surface, followed by ice advance (glaciation) to produce the new surface, and the observed lithologic distribution. The resulting till was modified and is probably till being modified by oceanographic bottom currents, which have decreasing strength to the west (decreasing depths), thus producing the existing grain size and roundness distributions. Keywords: Pages: 112 Supervisors: Ron Boyd / Martin Gibling