Coarsening-Upward Shelf Sequences in Aphebian Wishart Quartzite, Knob Lake Area, Quebec and Newfoundland: ABSTRACT

Abstract

The Wishart Quartzite of the Aphebian Labrador Trough contains coarsening-upward sequences about 10 to 20 m thick consisting of three successive subfacies: (A) basal centimeter-scale beds of ripple cross-laminated, very fine to fine sandstone separated by shaly partings, (B) decimeter-scale beds of fine to medium sandstone with thin lamination and hummocky cross-bedding, and (C) medium to coarse sandstone with herringbone trough cross-bedding. The top and bottom contacts of the sequences are sharp, the transition from A to B is completely gradational, and the transition from B to C is abrupt but still gradational. Scattered slabs of syndepositionally cemented sandstone cap most of the sequences. The upsection changes in primary sedimentary structures and mean grain size b th indicate a temporal progression from distal, low-energy to proximal, high-energy shelf deposition. The sandstone slabs are interpreted as lag gravels deposited during erosional hiatuses. Individual coarsening-upward sequences could have formed either by coastal progradation coupled with erosional truncation of the tops of the sequences (e.g., ravinement) or by aggradation on the offshore shelf. An offshore shelf origin is preferred for three reasons: (1) no sedimentary structures requiring subaerial exposure are recognized in any of the sequences, (2) very similar coarsening-upward sequences in Cretaceous strata of the western United States have been interpreted as offshore shelf deposits on firm stratigraphic grounds, and (3) although no known Holocene offshore shelf deposit is entir ly analogous, all of the sedimentary structures in the Wishart sequences have been observed in cores from one or more Holocene shelf environments. End_of_Article - Last_Page 785------------