Eastern Section Meeting

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Revising the Basement Map of the Lower Peninsula; New Constraints from Cores and Cuttings

Abstract

The basement of the Lower Peninsula of Michigan is poorly understood with the last basement map generated in the 1970s. The Michigan Geological Repository for Research and Education (MGRRE) core repository holds nine basement cores and thirty-five wells with cuttings that intersect the Michigan Basin-Basement contact. Most of these samples were not used in the 1970's version of the basement map. The majority of the wells are from southern and southeastern Michigan. Initial investigation of the collection of available cores has shown a complex set of lithologies preserved in the basement of the Michigan Basin in Branch and St. Joseph counties. Four closely spaced wells alternate between porphyritic granite and biotite-rich gneisses. The porphyritic granites exhibit weathering profiles at their contact with the Mt. Simon Sandstone including mineralogical alteration and fissure development. A deep stratigraphic test drilled in Gratiot Co. cored through fine-grained turbiditic sediments interpreted to being part of the mid-continent rift System. The southeastern Lower Peninsula (Arenac, Huron and St. Clair counties) wells with core exhibit a diverse assemblage of metasediments, granites, and granite gneisses. Previous basement maps, suggest that the basement of southern Michigan consists of multiple terranes – the Grenville province in the southeast, the Penokean province in the north, the Yavapai province in the northwest and the Mazatzal province in the southeast. Unfortunately a paucity of geochronological data and samples makes it difficult to map out the boundaries of these terranes in detail. The eastern arm of the mid-continent rift cross-cuts the majority of the Precambrian basement terranes before being truncated by the Grenville province. New integration of geophysical data and the existing rock sample material can shed light on the complexity of Michigan's basement terranes and perhaps allow for more detailed delineation of the complex basement under the Michigan basin.