143-10 Detrital zircon analysis of sandstones within the great American carbonate bank (Lower Ordovician, U.S. midcontinent)

Abstract:

Sandstones within the upper Cambrian (Pabian) to Lower Ordovician great American carbonate bank (GACB) have received relatively little study. Their provenance and sedimentary characteristics can provide new insights into paleogeographic and tectonic reconstructions as well as cross-platform fluvial and aeolian transport processes that remain poorly understood.

In the Laurentian midcontinent the GACB succession is dominated by fine-grained carbonate strata that record the maximum transgression of the Sauk megasequence. Several variably thick quartz arenite beds occur locally within the Prairie Du Chien Group. Fourteen samples were collected from this interval in Iowa, Wisconsin, Illinois and Indiana outcrops and cores. Detrital zircons were separated and analyzed by LA-ICPMS at the University of Arizona Laserchron Center (n=241).

The results of the detrital zircon analysis are largely consistent with paleogeographic reconstructions for central Laurentia during formation of the GACB. Twelve of the samples, collected from the shallower facies of the Wisconsin Arch, are statistically indistinguishable and share a dominant 2700 Ma age peak, reflecting sourcing from the Superior province several hundred kilometers upwind to the paleo-east. These results are consistent with the textural and structural evidence for eolian transport. The sample from Denzer Wisconsin, which is less than 5 km from the from the exposed Baraboo Hills, has a population of 1700-1900 Ma zircons, which were likely recycled from the Mesoproterozoic Baraboo Formation. The southern Indiana sample, which was collected from strata near the platform margin, has a distinct provenance signature, with abundant 1000-1300 Ma zircons that were derived from the Grenville terrane more than 500 km to the northeast.  A previously published age set from near the platform margin in Missouri shares this Grenville zircon population which may represent lowstand cross-platform fluvial transport and redistribution by storm and longshore currents.

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