Depositional Dynamics of Cyclic Carbonates from the Interlake Group (Lower Silurian) of the Williston Basin

Abstract

A set offour transgressive-regressive cycles is recognizable in the Lower to Middle Interlake Group of central Manitoba. The initial Silurian transgression reached its maximum with development of Virgiana (brachiopod) beds in the Fisher Branch Formation. Three subsequent transgressive peaks are identified by coral-rich layers in the upper part of the overlying Inwood Formation, the whole of theAtikamegFormation, and the middle part of the Cedar Lake Formation. These are separated by strata with flat to domal stromatolitic structures or other very shallow water features occurring in the lower to middle parts of the Inwood Formation, the upper part of the Moose Lake Formation, and much of the East Arm Formation. Regressive phases are marked by thin, reddish-brown shale beds bearing abundant frosted quartz grains. These marker beds are best known from middle and lower levels in the East Arm Formation (v and u2 horizons) and the middle of the Inwood Formation (ul horizon). A similar marker bed separates the Interlake Group from the underlying Stonewall Formation of latest Ordovician or earliest Silurian age. The number and spacing of these cycles suggest a correlation with other carbonate cycles found in the East Iowa, Michigan, and Anticosti Basins. While a Bahamian bank model of mosaic-facies deposition explains some aspects of the shallow-water Manitoban cycles, modification of the Israelsky wedge concept (facies in phase) enhances the interpretation of a vast, dynamically interconnected sea stretching far beyond the confines of the Williston Basin. During the first 10 m.y. of the Silurian, the Williston Basin was not an isolated topographic depression. Available evidence suggests that the region was still one of the shallowest parts of the periodically, extensively flooded North American platform.