Conodont Color Alteration Index (CAI) Isograd Maps for Ordovician Rocks of the Michigan Basin

Repetski, John E.¹, Stig M. Bergström², Daniel O. Hayba³, Christopher S. Swezey³, and Joseph A. East^3
1 U.S. Geological Survey, Reston, VA
² The Ohio State University, Columbus, OH
³ U.S. Geological Survey, Reston, VA

Conodonts from more than 60 wells have been used to construct color alteration index (CAI) isograd maps for two stratigraphic intervals in the Michigan basin: (1) Lower and lower Middle Ordovician (Ibexian through middle Whiterockian), including the Prairie du Chien (PdC) Group and the “Foster” and “Bruggers” Formations; and (2) Upper Ordovician (Blackriveran to Edenian), including the Black River and Trenton Groups.

For the PdC/Foster/Bruggers interval, CAI values of 3 are seen only in Clare, Missaukee, and Ogemaw Counties, at depths in excess of 11,000 ft. CAI values of 2 to 2.5 are seen in the rest of the basin center, most at depths <11,000 ft. CAI values of 1.5 and 2 are seen elsewhere in southern Michigan at depths of about 4,000-6,000 ft.

In the Trenton-Black River interval, CAI values of up to 2.5 are observed only in the basin center, in Missaukee, Clare, and Gladwin Counties. The geographic field for CAI 2 values broadly covers the rest of the central part of the basin, with values of 1.5 on the basin flanks, again correlated with decreasing burial depths. All Trenton-Black River samples seen near the Albion-Scipio trend are in the CAI 1.5 field, consistent with the oil-bearing strata there.

For both stratigraphic intervals, isograd patterns based on CAI follow the approximately symmetrical pattern of overburden thickness for Paleozoic strata of the basin. However, burial temperatures indicated by CAI values and basin modeling require an overburden thicker than that present, suggesting post-Ordovician erosion and/or higher than normal geothermal gradient.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004