A lithologic study of the Stony Mountain and Stonewall formations in southern Manitoba

Abstract

The outcrop section of the Upper Ordovician Stony Mountain formation and the Lower Silurian (?) Stonewall formation shows a mid-cratonic carbonate succession, into which terrigenous sediment was introduced in widely varying amounts. The bioclastic limestones of the lowest member were formed in an open sea, in which the sedentary fauna was scattered over the bottom and into which there were periodic introductions of abundant, fine terrigenous sediment. The overlying carbonate unit was deposited in slightly deeper waters and received a more continuous supply of this sediment. The third member was deposited in a locally semi-evaporitic, inter-shoal area which gradually changed to more open sea conditions, allowing the growth of biostromal coralline reefs.,, The unit containing this sediment is herein named the Williams member of the Stony Mountain formation. The overlying Stonewall formation was entirely formed in a reef environment... The change from solitary coralline colonies to full, though small, biohermal coralline reefs is due to the first appearance of encrusting, layered calcareous algae. The terrigenous sediment was supplied from three sources. One was a group of islands of Precambrian rock located just to the north, one the landmass to the southeast that also supplied the Maquoketa and Queenston formations, and one, a landmass to the east which produced the Lower Silurian (?) sandstones of the James Bay subsurface.