The elevated organic maturity observed in shallowly buried units from the Michigan Basin implies that higher temperatures and thicker overburdens once existed in the basin. Evidence from sediment-accumulation rates, regional dips, and maturity of Pennsylvanian-age coals suggests that up to 1,000 m (3,280 ft) of sediment were removed by erosion prior to the Late Jurassic, when the basin became stable. Geothermal gradients during Paleozoic basin subsidence ranged from 35 degrees to 45 degrees C/km (19 degrees to 25 degrees F/1,000 ft), in contrast to the average present value of 25 degrees C/km (14 degrees F/1,000 ft). Depths to the top of the oil window ranged from 1,900 to 2,300 m (6,230 to 7,540 ft) during the Paleozoic. Post-Pennsylvanian erosional uplift and further thermal maturation of the basin have combined to raise the top of the oil window to its present level of 500 m (1,640 ft).--Modified journal abstract.

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