Paleogeography of Lower Mississippian Rocks of Michigan Basin

Abstract

Five Lower Mississippian formations (Antrim, Ellsworth, Bedford, Berea and Sunbury) of Michigan were studied to determine the Early Mississippian paleogeography and to develop an effective and efficient technique for studying thick shale sequences which have variable properties both laterally and vertically. Standardized color descriptions of cuttings from 443 wells throughout the Southern Peninsula of Michigan were the main source of the data, which were later analyzed and plotted by computer techniques. Other variables assessed were thickness, lithologic ratio, and texture. Interpretations based on both intraformational and interformational variations indicate that the factors controlling local deposition became established in the Antrim sea and persisted until the deposition of the Sunbury Shale. The post-Antrim sediments were derived from two source areas northeast and northwest of Michigan. The former produced the birdfoot Bedford-Berea delta, and the northwest source contributed a thick green shale which may represent the prodelta deposits of a delta farther west. The development of these Early Mississippian deltas may be the first indication of greater deltaic sequences in the Pennsylvanian System. The localization of the Bedford-Berea and the Ellsworth rock types on opposite sides of the basin was not effected by a structural or topographic barrier, but by an environment in which all sediments deposited were primarily or secondary black. The organic content of the black shale was preserved in a reducing shallow-water environment. The green shale, however, was deposited in a mildly reducing environment in which the organic matter was not preserved. The green color is due to the abundance of green clay minerals, glauconite, and ferrous ions.