40Ar / 39Ar geochronology and provenance of detrital K-feldspars, Ordovician, Upper Mississippi Valley

Abstract

The Upper Ordovician strata of the Upper Mississippi Valley contain numerous thin, shale layers. Eighteen of these shale layers were previously thought to be altered volcanic ash beds, known as K-bentonites. Radioisotopic geochronology of six of the multiple shale-bed-partings throughout the Upper Ordovician indicate a non-volcanic origin and provide clues for clastic sediment provenance and dispersal, and depositional environments on epeiric seas. The shale-bed-partings occur throughout the Tippicanoe I supersequence; all but one are within carbonate units. One hundred thirty-six 40Ar / 39Ar Single Crystal Laser Fusion analyses of K-feldspar derived from these beds yield apparent ages that range from 754 Ma to 3.1 Ga. The age distributions of the K-feldspars show distinctive populations when the data are plotted according to their stratigraphic position. Samples from the Glenwood Shale and Platteville Group show an age distribution clustering around 850 Ma and 1.0 Ga respectively. Shale samples from four beds within the Galena Group show an age distribution expanding from 754 Ma to 3.1 Ga. The Glenwood and Platteville clusters correspond with the ages of nearby basement rock and Cambrian–Ordovician sandstones, while the shale-bed-partings of the Galena are interpreted to have been sourced from a broader region. These differences suggest that the Glenwood and Platteville samples were sourced from local rocks, whereas the Galena shale samples correspond with flooding of the local region area and connection to a new and complex source region. These shale-bed-partings can be interpreted in two ways: as pulses of terreginous material entering the system and overwhelming carbonate deposition, or as background sediment, accumulating during pauses in carbonate deposition. While the older Glenwood and Platteville samples can be interpreted in either way, the samples from the Galena are better interpreted as condensed background sediment.