SEDIMENTOLOGY AND DEPOSITIONAL SEQUENCES OF THE JORDAN FORMATION (UPPER CAMBRIAN), NORTHERN MISSISSIPPI VALLEY

The Jordan Formation, a typical cratonic quartz sandstone, has been th e focus of study and contention for more than a century, especially be cause of its relation to the Cambrian-Ordovician boundary in the type area of Upper Cambrian strata in North America. The Jordan is approxim ately 30 m thick; the presence of trace fossils and scattered trilobit e and brachiopod shells indicates a marine environment of deposition. The formation was deposited in an epeiric sea at least tens of meters deep (the ''moat'') lying between the still-exposed continental nucleu s, which supplied elastic sediments, and an extensive carbonate shoal, which developed on the continental margin and advanced into the crato n interior during Late Cambrian time (the ''Great American Bank''). We recognize three facies in the Jordan on the basis of sedimentary stru ctures and grain size. The hummocky-stratified facies, typified by the (basal Jordan) Norwalk Member, is composed of very fine to fine, suba ngular, feldspathic quartz sandstone, with extensive planar to low-ang le bedding. This facies is interpreted to represent episodic storm dep osition below fair-weather wave base in an offshore setting. The troug h cross-stratified facies, typified by the (overlying) Van Oser Member , is composed of fine to medium, well-rounded, pure quartz sandstone, in pervasive high-angle trough cross-sets. Cross-bed orientation shows a regional southward (modern coordinates) transport direction, with m uch scatter and local bimodal patterns. This facies was deposited as s ubaqueous dune bed forms. It is interpreted to have formed nearer the shoreline and above fair-weather wave base, with dune movement induced largely by storm-enhanced marine currents. The large-scale cross-stra tified facies, present at the top of the Van Oser Member at a few scat tered outcrops, is composed of coarse sandstone in cross-sets 1-2 m th ick. This facies is interpreted as the shallowest and most energetic o f all, whose deposition was dominated by currents of probable tidal or igin. Because we recognize evidence for both storm and tidal processes in the Jordan, we favor a comparison with modern shelves in which com bined processes are active. In general, the Jordan displays a coarseni ng-upward facies sequence, implying a progradational origin for the fo rmation. Locally, part or all of a second coarsening-upward sequence i s present, indicating that two progradational cycles took place. The u pper contact of the formation is an unconformity, which variably trunc ates the progradational sequences. Above the unconformity, in overstep relationship to the truncated Jordan, lies the Lower Ordovician Prair ie du Chien Group, a shoal-water carbonate unit with reworked quartz s and in its lower zone. The Prairie du Chien rep resents an extensive m arine transgression that followed the sea-level low-stand at the end o f Jordan deposition. Because of the scarcity of fossils in the upper J ordan, the age of the drawdown is not yet tightly constrained, bat it is very near the Cambrian-Ordovician boundary.