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SUBSIDENCE CONTROLLED STRATIGRAPHIC SEQUENCES AND THE ORIGIN OF SHELF SAND RIDGES, WINNIPEG GROUP (MIDDLE ORDOVICIAN) MANITOBA,SASKATCHEWAN, NORTH DAKOTA

ABSTRACT In southern Manitoba and eastern North Dakota, three large (kilometers in length and width) elongate east-west trending sand bodies of probable storm and tidal-current origin have been deposited on mostly muddy continental shelf deposits of the Icebox Formation of the Middle Ordovician (Caradoc) Winnipeg Group in the Williston Basin. Several smaller stratigraphically equivalent sand bodies of similar origin are observed in western Manitoba and southern Saskatchewan. These sand bodies all occur in the second of three subsidence controlled depositional sequences in the Winnipeg Group. The largest of the sand bodies, the Carman sand in southern Manitoba, is approximately 30 m thick, 250 km long, and 45 km wide and was identified by using well log correlation and sample description. This sand body consists of at least five amalgamated sand ridges which accreted in a N to NNW direction. Individual sand ridges comprise a lower unit of bioturbated sandy siltstones (gradational with the underlying muddy shelf), grading upward into fine-grained, less silty sandstone of probable storm-current origin, and an upper unit of medium to coarse-grained, well-sorted, clean sandstone at the irregular top of the ridge. This clean sandstone was probably deposited by strong periodic or regular currents generated in a tidal regime. About 150 km to the south of the Carman sand in eastern North Dakota, two smaller elongate sand bodies in the Icebox Formation are of similar storm and tidal origin. These two ridges may be outer shelf equivalents of the Carman sand. The sand bodies in southern Saskatchewan are cross-bedded and burrowed and up to 5.0 m thick, 5.0 km long, and 2.0 km wide. The three Winnipeg Group depositional sequences (W1-W3), which are loosely constrained by conodont biostratigraphy, are the result of episodes of basin margin subsidence and sediment input. Early Williston Basin thermally controlled subsidence and subsequent basin margin erosion were the principal sediment supply mechanisms. Shelf ridge development in the Icebox Formation was related to a lower than usual basin subsidence rate for the Middle Caradoc eastern Williston Basin margin during transgression. This slow subsidence resulted in the development of shoaling sand ridges to a critical depth for the deposition of tidal sediments.