Thermal maturity of the Michigan Basin

Almost all Paleozoic strata in the Michigan Basin display elevated levels of organic maturity that cannot be explained by present-day burial depths, geothermal gradients, and heat flow. Likewise, higher heat flow from the basement in the past is unsatisfactory as an explanation of the elevated maturities, particularly for the younger sediments. Previous studies have concluded that a significant amount of Permo-Carboniferous overburden has been removed from this region by early Mesozoic regional uplift and erosion. If the missing overburden were sufficiently thick and thermally resistive, a "thermal blanket" effect would have caused elevated temperatures throughout the underlying stratigraphic section during late Paleozoic and early Mesozoic time. Models of organic maturation that take into account this "thermal blanket" effect as well as other variations in thermal conductivity attributable to lithologic differences can explain the anomalous maturity of most strata in the Michigan Basin without postulating any increase in ancient heat flow. An overburden thickness of 2,000 m with a gradient the same as that observed in the present-day Carboniferous section would provide an adequate explanation for the elevated maturities. Lesser thicknesses of overburden would require correspondingly higher geothermal gradients, a reasonable possibility if the missing overburden was a fluvio-deltaic sequence containing low-conductivity carbonaceous strata.