INVESTIGATION OF THE CONTACT BETWEEN THE TRAVERSE GROUP AND SQUAW BAY LIMESTONE IN THE SUBSURFACE OF THE MICHIGAN BASIN

The Traverse Group-Squaw Bay interval is a poorly understood set of units in the subsurface of the Michigan basin. The interval these cores span has been described extensively in outcrop, but subsurface investigation has been limited. Selected cores came from both the northern and southern portions of the Michigan basin. Both cores were provided by and examined at the Michigan Geology Repository for Research and Education (MGRRE). Identification of facies, lithologies, contacts and stacking patterns was performed for both cores. The Traverse Group units were deposited on a shallow carbonate ramp. The Squaw Bay Formation was deposited under deeper water conditions with a greater influx of siliciclastic sediment.

Lithologies included limestone, argillaceous limestone, and dolomite. Contacts between the Traverse Group and Squaw Bay Limestone were picked based on observed lithology changes and presence of pyritized surfaces. Four facies were defined from both cores: cross-bedded, peloidal packstones, skeletal mud-lean packstones to grainstones, skeletal wackestones, and argillaceous lime mudstones. A strong similarity in facies and facies sequence was noted between the cores. Identified facies were interpreted to have been deposited in lagoonal, shoal, and deep water cratonic basin environments. The general depositional environment was interpreted as a shallow marine low gradient carbonate shelf with lagoons protected by skeletal grain shoals and open marine waters at a tropical to subtropical latitude. Dolomitized intervals occurred below the finer grained Squaw Bay in both cores indicating it may have acted as a seal for upward migrating hydrothermal fluids that may have altered the uppermost Traverse. Both cores are capped by facies interpreted as deeper water cratonic basin deposits and one or more hardground surfaces interpreted as flooding surfaces occur at or near the base of these facies. These flooding surfaces occur at or near the contact between the Traverse Group and the overlying Squaw Bay Limestone, which has led to the interpretation that the fine grained Squaw Bay was deposited after a basin wide increase in relative sea level represented by the pyrite surfaces.