Controls on Deposition of the St. Peter Sandstone (Middle-Late Ordovician), Michigan Basin

NADON, GREGORY C., A. SIMO, C. W. BYERS, and R. H. DOTT, JR., University of Wisconsin, Madison, WI

The St. Peter Sandstone (Middle-Late Ordovician) of the Michigan basin represents an approximately 10-m.y. interval of clastic deposition in an otherwise carbonate-dominated Ordovician succession. This interval, up to 320 m thick, also coincides with a change in basin

configuration from the nearly circular depocenter of the underlying Shakopee Formation to an east-west elongated trough situated west of Saginaw Bay. Interpretation of well logs and core from throughout the basin indicates that the clastics are composed of 20-25 sequences of well-cemented quartz sandstone, each 3-30 m thick, deposited in upper shoreface to tidal-flat environments. The sequences are interbedded with heavily bioturbated, shaly, lower shoreface sandstones (1-14 m thick) and, in the central and southeastern parts of the basin, with carbonate shales, stromatolites, and oolitic grainstones (2-39 m thick). The eastern and southeastern margins of the basin contain the thickest carbonate accumulations.

Hydrocarbon fields are located over structural highs formed by reactivation of basement structures. Detailed comparison of well logs within fields shows that sedimentary cycles thin over the structures as a result of the local reduction in the formation of accommodation space by syndepositional movements on the faults. The presence of thick carbonates along the southeastern margin of the basin is a result of the combination of distance from the clastic source and the episodic formation of accommodation space by syndepositional normal faulting along the basin margin.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)