FACIES ANATOMY OF LATE CAMBRIAN (STEPTOEAN-SUNWAPTAN) SANDSTONES, TUNNEL CITY GROUP, UPPER MISSISSIPPI VALLEY

The Upper Mississippi Valley (UMV) is a classic area for Cambrian lithostratigraphy and biostratigraphy and the basis for a sedimentation model for cratonic marine settings. Testing of this model requires finer scale analysis of lithofacies than has been completed previously and an updated faunal analysis. To evaluate the Late Cambrian (Steptoean-Sunwaptan) depositional history, I examined 19 sections in Wisconsin and Minnesota. The Tunnel City Group (Lone Rock and Mazomanie formations) represents a nearshore environment dominated by clastic sedimentation and storm deposition during a time of exceptionally high sea level. However, cratonic, supermature quartz sandstones are difficult to interpret genetically. Intraformational and flat-pebble conglomerates, hummocky cross-stratification (HCS) with burrowed upper surfaces and fossil lags provide evidence for relatively brief, frequent periods of highly energetic conditions followed by quiescence. In particular, the Lone Rock Formation permits detailed examination of HCS. At least five lithofacies, grouped into shoreface and proximal offshore lithofacies associations, represent different subenvironments along the depositional transect. Further, thorough analysis of the sequence stratigraphy in conjunction with trilobite biostratigraphy within this overall sedimentological and stratigraphic framework will permit the Late Cambrian sandstones in the UMV to serve as a model for wave- and storm-dominated, sandy epicontinental seaways.