Intracratonic Peripheral Bulge Depozones and Their Importance in Understanding the Lithospheric Rheology and Evolution of Intracratonic Basins: an Example From the Michigan Basin

Resolving the geodynamic evolution of intracratonic basins is often hampered by the inability to constrain important parameters such as lithospheric rheology and the size of the basin-forming load. The purpose of this study is to show: 1) peripheral bulges surrounding the Michigan Basin can be detected through stratigraphic and facies analyses; and 2) estimates of the load size and the paleo-lithospheric rheology can be determined with high precision by examining the location and wavelength of the peripheral bulge. The Paleozoic Michigan Basin was modeled using a radial flexure program, in which the lithosphere is an infinite elastic plate deflected by the subsurface emplacement of a disc-shaped load. Various values and combinations of effective elastic thicknesses and load-radii were used to produce modeled profiles that matched the actual stratigraphic profile of the basin (other parameters were held constant). Effective elastic thicknesses of 25-45 km and load-radii of 35-150 km produced best-fit profiles; a range of values too large to be very useful. The deposits flanking the Michigan Basin provide important constraints on the lithospheric properties of the region during the Paleozoic. Assuming that the lithosphere behaves like an elastic sheet, the area surrounding an intracratonic basin should respond to subsidence in the adjacent basin by uplifting as low-amplitude, long-wavelength peripheral bulges. The Kankakee arch in northern Indiana and its Paleozoic manifestation, the Wabash Carbonate Platform, represent the peripheral bulge of the Michigan Basin. The Wabash Platform was a structural and topographic high that formed synchronously with the Michigan Basin in an area where the modeled profiles predict a peripheral bulge would be located. Marine carbonate units thin over this high, show stratigraphic time-condensation and have facies reflecting deposition in much shallower water compared to those deposits to the north and south. Isopach maps of specific rock units, such as the Devonian Traverse Formation, are identical to those predicted by flexural models that use an effective elastic thickness of 40 km and a circular load radius of 60 km. The effective elastic thickness value of 40 km agrees with the thicknesses proposed by other models and the 60 km load radius used in the model is very similar in dimension to a large positive gravity anomaly in the region. The dimensions of the peripheral bulge thus provide an important constraint on the lithosphere rheology and load size. The techniques employed here can be used to refine geodynamic models of the Michigan Basin and can be applied to other marine intracratonic basins.