ABSTRACT: Sequence Stratigraphy of the Silurian Carbonates and Evaporites in the Michigan Basin

HOWELL, PAUL, and A. W. LIEBOLD, III, University of Michigan, Ann Arbor, MI

New correlations from several hundred wells and many cores in the Michigan basin require substantial revision of the sequence stratigraphy for the carbonates and evaporites of the Niagara and lower Salina groups. Four composite sequences are delineated: Niagaran, A-1 Unit, A-2 Unit, and the B Unit. Numerous Type 1 unconformities on the carbonate platform related to evaporative drawdown events necessitate a composite sequence framework.

The Niagaran composite sequence contains two sequences. The lower sequence consists of three parasequence sets: aggradational ("White Niagaran" ramp), transgressive ("Gray Niagaran"), and progradational ("Brown Niagaran"). The Brown Niagaran includes a distinct shelf margin and pinnacle reef growth. The Brown parasequence set is interrupted by one drawdown unconformity, represented by a subaerial exposure surface that extends below the shelf margin into the inter-pinnacle reef areas.

A second drawdown event ended the Niagaran and initiated the A-1 Unit as a thick (140 m) lowstand evaporite wedge in the basin. Flooding renewed carbonate deposition across the basin and platform region. Several distinct drawdown events within the A-1 carbonate are responsible for shallow water facies intercalated within deeper water lithologies. The A-1 composite sequence terminated from drawdown resulting in a second thick lowstand evaporite

wedge (A-2 halite). Reflooding initiated A-2 carbonate aggradational sequences, separated by several drawdown unconformities. The B Unit is a composite of carbonate/evaporite sequences, each consisting of an aggradational carbonate parasequence and a lowstand evaporite wedge. These sequences mark the final filling of the basinal depression created during Niagaran carbonate deposition. Composite sequences provide a powerful framework for describing and interpreting complex carbonate and evaporite relations.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)