40AR/39AR GEOCHRONOLOGY OF THE UPPER MISSISSIPPI VALLEY, MIDDLE AND UPPER ORDOVICIAN GALENA GROUP: SEDIMENT ACCUMULATION RATES AND BIOSTRATIGRAPHIC IMPLICATIONS FOR THE HISTORY OF AN EPEIRIC SEA

Carbonates that constitute the Ordovician Galena Group of the Upper Mississippi Valley (UMV) contain numerous K-bentonites that represent isochronous and correlatable stratigraphic markers. ⁴⁰Ar/³⁹Ar multigrain incremental heating as well as single crystal laser fusion experiments on sanidine phenocrysts from the Millbrig, Dygerts, and Rifle Hill K-bentonites yield weighted mean ages of 449.6 ± 1.0, 447.1 ± 0.9, and 444.0 ± 2.8 Ma*, respectively (±2ó analytical uncertainty). The less precise age of the Rifle Hill bed reflects small crystal sizes, detrital contamination, and difficulty in sampling this unit. The age of the Millbrig is further refined when combined with the single-crystal incremental heating age of 448.4 ± 0.9 Ma obtained by Min et al. (2001) on biotite from the same unit. The best ⁴⁰Ar/³⁹Ar age of the Millbrig is 448.9 ± 0.7 Ma (±2ó analytical uncertainty only). Thus, for the first time deposition of the Galena Group in the UMV is constrained by precise radioisotopic dating to have occurred between 448.9 ± 0.7 Ma and 444.0 ± 2.8 Ma, a time interval of 4.9 ± 2.9 my. In turn, these ages allow for new interpretations of the relative durations of omission surfaces and sequence boundaries, further understanding of the processes controlling hardground formation, and allow a recalibration of the conodont based biostratigraphic framework for this succession. At face value, these ages imply sedimentation rates of ~34 m/my in the lower Galena Group, but suggest a decrease in sediment accumulation to less than 12 m/my in the upper part of the succession. Confirmation of this decrease, however, requires improvement in the age for the Rifle Hill K-bentonite.

*All ages relative to 28.02 Ma Fish Canyon Sanidine.