Structural sequences and styles of subsidence in the Michigan basin

Abstract

Subsidence in the Michigan basin produced ∼5 km of sedimentation over a period of more than 200 m.y. during Paleozoic time. Utilizing well-log correlations and constrained by compaction corrections and estimates of paleobathymetry, we recognize four different styles of subsidence in the basin: trough-shaped, regional tilting, narrow basin-centered, and broad basin-centered. Subsidence began as a trough-shaped, northerly extension of the Illinois basin during Late Cambrian to Early Ordovician time. This was followed by narrow, basin-centered subsidence in Early to Middle Ordovician time. Basin-centered subsidence ceased for ∼30 m.y. during long-wavelength (>1000 km) eastward tilting in Middle to Late Ordovician time, a pattern incompatible with thermal-contraction subsidence models. Basin-centered subsidence resumed in Silurian time, but with a broader distribution. A second episode of narrow, basin-centered subsidence occurred in latest Silurian through Middle Devonian time and was replaced by broad, basin-centered subsidence at the end of Middle Devonian time. The geometry of Upper Devonian and younger Paleozoic deposits suggests another eastward-tilting event, but results remain inconclusive due to erosion of strata and uncertainties in their paleobathymetry. In addition to these subsidence patterns, two distinct unconformity styles are present: basin-wide and marginal erosion. There is no evidence for significant basin-centered unconformities as predicted by purely thermal mechanisms. A history of episodic subsidence reactivations is interpreted as the result of a stress-induced, crustal-weakening mechanism for the narrow, basin-centered subsidence, whereas broad basin-centered subsidence is interpreted as thermal contraction related to lower crustal attenuation during the narrow-subsidence episodes. Recently proposed dynamic topography related to initiation of Ordovician subduction provides a driving mechanism for long-wavelength eastward tilting. Together with a temporal correlation to Appalachian tectonism, these mechanisms provide a plate tectonic framework for the history of the Michigan basin.