DEPOSITION OF THE UPPERMOST CAMBRIAN (CROIXAN) JORDAN SANDSTONE, AND THE NATURE OF THE CAMBRIAN-ORDOVICIAN BOUNDARY IN THE UPPER MISSISSIPPI VALLEY

The Jordan Sandstone, of Late Cambrian (Croixan) age, is well known as one of several cratonic sheet sandstones in the Upper Mississippi Valley that are part of the classic Paleozoic ''orthoquartzite-carbonate suite.'' Evidence that the Jordan is an entirely marine, regressive sequence, rather than transgressive as commonly assumed, provides new insight into the origin of cratonic sheet sandstones. The lower half of the Jordan Sandstone is composed of two intercalated lithofacies: (1) extensively burrowed, very fine grained sandstone interbedded with hummocky cross-stratified beds deposited on the offshore shelf and (2) fine-grained, burrowed, cross-stratified sandstone deposited on the lower shoreface during storms. The upper half of the Jordan consists of medium- to coarse-grained sandstone deposited on the upper shoreface by tidal and storm-generated currents. This lithofacies is characterized by abundant scour surfaces, tidal inlet fills, and very large angular intraclasts of sandstone derived from incised beach-rock on the foreshore. Lithofacies associations demonstrate that the Jordan Sandstone is a regressive sequence deposited as part of a shoreline system that prograded across the Hollandale embayment, a shallow depression on the cratonic shelf in the Upper Mississippi Valley region. Progradation was interrupted by transgressive episodes recorded as local tongues of offshore deposits that disconformably overlie shoreface deposits. This sedimentologic model contributes to the evaluation of depositional controls in Late Cambrian time, and to solving some longstanding enigmas associated with cratonic sheet sandstones. Contrary to the long-held view that the Cambrian-Ordovician boundary is conformable in this region, sedimentologic evidence indicates that a regional unconformity separates the Jordan Sandstone from the Early Ordovician Oneota Dolomite. Deep incision into the lowest Jordan lithofacies occurred near the Transcontinental Arch on the western margin of the embayment, and near the Wisconsin Arch on the eastern margin. This differential erosion marks the earliest known tectonic uplift of the basin's margins relative to the center.