K-Ar dates of Authigenic Illite from the Mississippian Marshall Sandstone, Michigan Basin, USA

Abstract:

The Michigan basin is an approximately 5 km deep intracratonic basin that is centered over the southern peninsula of Michigan, USA. The basin overlies Proterozoic basement rock and is filled with dominantly Paleozoic sedimentary units. Evidence from diagenetic mineralization and thermal maturity of organic material indicate that past thermal conditions in the basin were anomalously high relative to expected maximum burial temperatures at the current geothermal gradient of 20° C/km. A number of hypotheses have been advanced to explain the anomalous temperature indicators including burial under now eroded sedimentary cover (Cercone and Pollack, 1991) and hydrothermal fluid flow related to the Mesoproterozoic rift that underlies the basin (Girard and Barnes,1995 and others). Here we present K/Ar ages of authigenic illite separated from the Early Mississippian Marshall Sandstone. K/Ar ages of authigenic illite provide temporal constrains on diagenetic process because illite generally forms at temperatures above 100° C, which is in agreement with the formation temperatures estimated for other authigenic components in the Marshall (Zacharias et al., 1993). Samples are from present burial depths of <500 m. Clay separates were analyzed using XRD and were shown to contain abundant illite, with little other clay mineral content and no K-feldspar. A mean age of 272 Ma was measured for three size fractions from a sample from the central basin. This sample had ages that were indistinguishable across three clay size fractions. A second sample from the southern margin of the basin showed a decrease in age with size fraction with 2-1µm illite giving an age of 313±3 Ma and a <0.5 µm illite sample giving an age of 294±3 Ma. This sample may have a small amount of coarse detrital illite that increases the age of the coarse size fraction. Based on the formation conditions interpreted for authigenic illite in the Marshall Sandstone, these dates may record a mid-Permian hydrothermal fluid flow event in the Michigan basin associated with the Alleghanian orogeny. Similar fluid flow has been recorded across the Alleghanian foreland basin.