Anomalous Thermal Maturity of Michigan Basin: New Hypothesis

Karen Rose Cercone, Henry N. Pollack

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, or heat flow. Likewise, higher heat flow from the basement in the past, no matter what the timing, is unsatisfactory as an explanation of the elevated maturities. Evidence from fission-track dating, shale compaction, and basin geometry suggests that a significant amount of Carboniferous overburden has been removed from this region by early Mesozoic erosion. If the missing strata included thermally resistive carbonaceous mudstones and coals, the high geothermal gradients which characterize these lithologies would have elevated the temperature at the base of the Carboniferous section. This "thermal blanket effect would have affected underlying pre-Carboniferous strata as well, enhancing the process of organic maturation throughout the basin late Paleozoic and early Mesozoic time. Models of organic maturation which include as little as 500 m of eroded Carboniferous fluvial-deltaic mudstones, carbonaceous mudstones, and coals can explain the anomalous maturity of most strata in the Michigan basin without postulating any increase in ancient heat flow.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.