Pennsylvanian Pewamo Formation and Haybridge strata of central Michigan: The youngest rocks of the Michigan Basin?
Kathleen C. Benison, "Pennsylvanian Pewamo Formation and Haybridge strata of central Michigan: The youngest rocks of the Michigan Basin?", Paleozoic Stratigraphy and Resources of the Michigan Basin, G. Michael Grammer, William B. Harrison, III, David A. Barnes
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Pennsylvanian red beds are the youngest known rocks in the Michigan Basin. Two new formation-level units, the Pewamo Formation and the Haybridge strata, have recently been described. The Pewamo Formation, composed of Pennsylvanian red sandstones and minor laminated mudstones, is known from outcrops, abandoned quarries, and one core in Ionia County. The Haybridge unit is located in the shallow subsurface and in coal mine tailing piles in Shiawassee County. It consists of red sandstone, red mudstone, coal, and gray mudstone, all hosting Pennsylvanian macroscopic plant fossils. Neither the Pewamo nor the Haybridge rocks have any demonstrated relationship to red core cuttings reported as Jurassic from the central Lower Peninsula of Michigan. No firm evidence exists for Jurassic, or any other post-Pennsylvanian rocks in the Michigan Basin. The red core cuttings may be glacial sediments with reworked palynomorphs from rocks transported from elsewhere. A shallow coring project, followed by detailed sedimentologic, petrographic, mineralogic, and paleontologic studies, is necessary to: (1) refine the vertical and lateral stratigraphy of the Pennsylvanian rocks in Michigan; (2) solve the "Jurassic red bed problem," and (3) understand the late Pennsylvanian–Pleistocene history of the Michigan Basin.