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