Sedimentology and Depositional Sequences of the Jordan Formation (Upper Cambrian), Northern Mississippi Valley

Abstract

ABSTRACT The Jordan Formation, a typical cratonic quartz sandstone, has been the focus of study and contention for more than a century, especially because of its relation to the Cambrian-Ordovician boundary in the type area of Upper Cambrian strata in North America. The Jordan is approximately 30 m thick; the presence of trace fossils and scattered trilobite 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 nucleus, which supplied clastic sediments, and an extensive carbonate shoal, which developed on the continental margin and advanced into the craton interior during Late Cambrian time (the Great American Bank). We recognize three facies in the Jordan on the basis of sedimentary structures and grain size. The hummocky-stratified facies, typified by the (basal Jordan) Norwalk Member, is composed of very fine to fine, subangular, feldspathic quartz sandstone, with extensive planar to low-angle bedding. This facies is interpreted to represent episodic storm deposition below fair-weather wave base in an offshore setting. The trough 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 much scatter and local bi-modal patterns. This facies was deposited as subaqueou 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-stratified facies, present at the top of the Van Oser Member at a few scattered outcrops, is composed of coarse sandstone in cross-sets 1-2 m thick. This facies is interpreted as the shallowest and most energetic of all, whose deposition was dominated by currents of probable tidal origin. Because we recognize evidence for both storm and tidal processes in the Jordan, we favor a comparison with modern shelves in which combined processes are active. In general, the Jordan displays a coarsening-upward facies sequence, implying a progradational origin for the formation. Locally, part or all of a second coarsening-upward sequence is present, indicating that two progradational cycles took place. The upper contact of the formation is an unconformity, which variably truncates the progradational sequences. Above the unconformity, in overstep relationship to the truncated Jordan, lies the Lower Ordovician Prairie du Chien Group, a shoal-water carbonate unit with reworked quartz sand in its lower zone. The Prairie du Chien represents an extensive marine transgression that followed the sea-level lowstand at the end of Jordan deposition. Because of the scarcity of fossils in the upper Jordan, the age of the drawdown is not yet tightly const ained, but it is very near the Cambrian-Ordovician boundary.