A tide-influenced delta complex in the upper Proterozoic Shaler Group, Victoria Island, Canada

Abstract

The Reynolds Point Formation is the second formation of the Shaler Group of Victoria Island. It has a maximum known thickness of about 1100 m. It comprises four members as follows: basal terrigenous clastics, a thick carbonate-dominated unit, a second terrigenous clastic member, and a mixed carbonate–clastic sequence. The lower clastic member is the subject of this report. Its greatest known thickness is about 215 m. The basal part consists of dark green and purple shaly mudstones interpreted as prodelta and distal delta slope deposits. These fine grained sediments were spread out westward across an extensive, strongly current-influenced shelf sea that contained abundant stromatolites. These stromatolites comprise the orange-weathering stromatolitic biostrome of the topmost unit of the underlying Glenelg Formation. The abrupt contact between the two formations and the strongly contrasted nature of the rock types involved suggest that the platform underwent rapid subsidence. Sandstones, siltstones, and minor mudstones of the overlying (main) part of the lower clastic member are interpreted as platform deposits of a prograding marine delta complex. These include deposits of tidal channels and associated tidal flats, commonly forming fining-upward sequences, beach deposits, and mudstones and siltstones that may be lagoonal or interdistributary bay deposits. These fine grained rocks contain casts of salt crystals. Subsidence of the delta platform is recorded by the overlying suite of deeper water grey shales and shaly carbonates that constitute the lower part of the carbonate member of the Reynolds Point Formation. Gradual filling and shallowing is indicated by upward transition from shaly beds to a thick, current-influenced stromatolitic biostrome. This pattern of pulsatory subsidence, followed by gradual sedimentary filling is shown by both the basal clastic member and the overlying beds.