The Middle Ordovician St. Peter Sandstone and Glenwood Formation (Ancell Group) represent a significant target for gas exploration at the base of the Tippecanoe sequence in the Michigan basin. Core and well log data show that the St. Peter-Glenwood interval contains numerous carbonate units that provide the basis for both regional correlation and subdivision of the section into at least 20 high-frequency sequences. The temporal resolution afforded by these sequences allows a detailed analysis of sediment partitioning as the basin evolved. The spatial distribution of the basal sequences illustrates the pronounced east-to-west onlap of the Wisconsin arch. An abrupt increase in sequence thickness upsection indicates that a major episode of basin-centered subsidence began during middle St. Peter deposition and continued through the deposition of the Glenwood Formation. The upper sequences show a significant beveling of the Glenwood Formation and the top of the St. Peter Sandstone in the north, south, and southeast areas of the basin prior to deposition of the overlying Black River carbonates. Although eustatic sea level changes were undoubtedly operating at several scales, the facies distribution of this mixed clastic/carbonate system also documents significant changes of local and regional tectonics.

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