Cuenca De Michigan

Structural sequences and styles of subsidence in the Michigan basin

Paul D. Howell*

Department of Geological Sciences, University of Kentucky, 101 Slone Research Building, Lexington, Kentucky 40506-0053

Ben A. van der Pluijm Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063

ABSTRACT Subsidence in the Michigan basinproduced ~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 ofthe 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. Basincentered subsidence resumed inSilurian 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 strataand 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 mechanismfor the narrow, basin-centered subsidence, whereas broad basin-centered subsidence is interpreted as thermal contraction related to
*E-mail: phowell@pop.uky.edu.

lower crustal attenuation during the narrowsubsidence episodes. Recently proposed dynamic topography related to initiation of Ordovician subduction provides a driving mechanism for long-wavelength eastward tilting. Together with atemporal correlation to Appalachian tectonism, these mechanisms provide a plate tectonic framework for the history of the Michigan basin. INTRODUCTION The Michigan basin is a nearly circular, intracratonic basin 400 km in diameter and 5 km deep with only minor structural disruption (Fig. 1), yet it remains without a definitive origin (Leighton, 1996). The geometry of the Michigan basin has led tonumerous proposals for basin subsidence mechanisms, including thermal contraction following development of an isolated “hot spot” (Sleep, 1971; Nunn and Sleep, 1984; Houseman and England, 1986), metamorphic phase changes in the crust (Haxby et al., 1976; Middleton, 1980; Ahern and Dikeou, 1989; Hamdani et al., 1991), lithospheric stretching (McKenzie, 1978; Klein and Hsui, 1987), free thermalconvection (Nunn, 1994), and intraplate stress mechanisms (DeRito et al., 1983; Lambeck, 1983; Howell and van der Pluijm, 1990). Contention among these proposals centers mainly on irregular basin subsidence rates (Sleep and Sloss, 1978; Quinlan, 1987; Howell and van der Pluijm, 1990). Decompacted basement subsidence curves for three representative wells from the western, central, and eastern portions ofthe basin document the temporal and areal distribution of subsidence variability (Fig. 2). Sequence A displays rapid subsidence in the western and central basin wells, but is barely present on the eastern margin. Sequence B has a pronounced subsidence reactivation in the basin center, but is completely absent along the eastern margin due to nondeposition and erosion. Subsidence

rates...