CLINTON-TYPE DEPOSITS IN THE MICHIGAN BASIN: A PRELIMINARY DISCUSSION OF THE SIGNIFICANCE OF LOWER SILURIAN HEMATITIC, PHOSPHATIC GRAINSTONES IN WESTERN MICHIGAN

Newly examined core samples from the western Michigan Basin in Oceana County exhibit an unusual lithology between the Cabot Head Shale (upper Cataract Group, Lower Silurian) and the Burnt Bluff Group (lower Middle Silurian), consisting of skeletal grainstones with significant amounts of hematitic oncoids and phosphatic grains. Previous literature describes the presence of a similar horizon from cuttings and core samples in the central to southwestern basin – however, the previous literature places this lithology lower in the section at the base of the Manitoulin Dolomite (lower Cataract Group).

These samples from Oceana County, likely reflect a condensed interval between the argillaceous mudstones of the Cabot Head and the argillaceous dolomites of the “Manistique-like facies” of the Burnt Bluff Group. The bounding units were deposited under deeper water conditions, likely near storm wave base. The upper Cabot Head Shale consists of red and green argillaceous mudstones and grades into the skeletal grainstones with an increasing content of phosphatic grains and oncoids up section. The overlying contact of the condensed interval is sharply planar and reflects a change to skeletal, burrow-mottled, argillaceous wackestones and mudstones. In a sequence stratigraphic framework, this interval likely represents a maximum flooding surface.

Two hypotheses are currently being considered for the stratigraphic placement and correlation of these unusual skeletal grainstones: that they are correlative with the Neda “Iron” Formation of Wisconsin or with the Wilhelmi Member of the Manitoulin Dolomite of Illinois.