40AR/39AR GEOCHRONOLOGY AND PROVENANCE OF DETRITAL K-FELDSPARS, ORDOVICIAN, UPPER MISSISSIPPI VALLEY

The Middle and Upper Ordovician strata of the Upper Mississippi Valley (UMV) contain numerous thin shale layers. Some such shale layers have been attributed to altered volcanic ash beds, known as K-bentonites. Geochronologic and XRD work on three of these clay-rich deposits shows that these deposits are not volcanic in origin and provide crucial information of source areas in the Ordovician.

The clay-rich beds studied are part of the St Peter Sandstone, Glenwood Shale, and Platteville Group. The St Peter Sandstone is a cross-bedded, well-sorted, fine- to medium-grained eolian sandstone overlain by massive bioturbated marine sandstone. The sample analyzed is approximately 1 meter above the base of the formation in Cannon Falls, MN. The Glenwood Shale, Hanover, WI, is thin and consists of poorly sorted, fine- to coarse-grained sandstone, green shale, and mud-cracked dolomicrite deposited in a peritidal to subtidal environment. The Platteville Gr. consists of subtidal carbonates, containing variable amounts of shale and skeletal grains. A shale between the Mifflin and Grand Detour members from Monroe, WI, was analyzed. XRD analysis of the clays present in each of the three beds does not show the illite/montmorillonite signature typical of altered volcanic ash beds, suggesting a non volcanic source.

Using the ⁴⁰Ar/³⁹Ar Single Crystal Laser Fusion (SCLF) method, experiments were conducted on more than 50 grains of K-feldspar. The ⁴⁰Ar/³⁹Ar ages for these K-feldspars range from 1.0 to 2.4 Ga with most grains falling around 2.3 Ga in the St Peter, from 800 to 960 Ma for the Glenwood with the majority of grains closer to 800 Ma, and from 0.97 and 2.0 Ga with most grains clustering at 1.0 Ga in the Platteville.

These results indicate a detrital Precambrian source for this K-feldspar, but variable provenance. Potential source regions include the Marquette Range for the St Peter deposit, the Wolf River Batholith of central Wisconsin for the Glenwood, and Mid-continent rift volcanics for the Platteville deposit. It is currently being tested whether the variance in source rock is regionally or stratigraphically controlled.