SEQUENCE STRATIGRAPHY INTERPRETATION OF LOWER AND UPPER BOUNDARIES OF THE CAMBRIAN ST. LAWRENCE FORMATION IN SOUTHWESTERN WISCONSIN

Depositional environment and sequence stratigraphic interpretation of Wisconsin sedimentary deposits have been the focus of active research since Sloss, (1963). Wisconsin geology has become economically important in recent years due to increasing demand for high quality fracking sand by energy companies. This study focuses on the sequence stratigraphic interpretation of the Cambrian St. Lawrence Formation. The St. Lawrence Formation is fine to very fine sandy dolomite and dolomitic siltstone, which outcrops in southwest Wisconsin. It is subdivided into Lodi and Black Earth Members. It is underlain by the Lone Rock Formation and overlain by the Jordan Formations. New stratigraphic sections, petrology and mineralogy data were collected and interpreted.

The lower contact with the Lone Rock formation is characterized by an upward loss of glauconite, a gradual increase of dolomite, the appearance of stromatolites, a high concentration of flat pebbles and a decrease in sand content. We interpret this contact to represent a transgressive surface, which separates the underlying Lone Rock highstand systems tract from the overlying St. Lawrence transgressive system tract. The upper contact, with the Jordan Formation, can be identified by an abrupt upward increase in sandstone, cross bedding, bioturbation and a decrease in dolomite from underlying St. Lawrence. We interpret this contact to be an unconformity, and therefore a sequence boundary. This sequence boundary separates the St. Lawrence transgressive systems tract and the Jordan highstand systems tract.

Wisconsin was geographically situated in the tropics during the Cambrian and a shallow epeiric sea dominates deposition during this period. We conclude that St. Lawrence Formation deposited in deeper waters than either the Lone Rock or the Jordan Formations. The former was deposited by prograding shoreline during regression of the epeiric sea caused by a drop in sea level. This was followed by rising sea level during St. Lawrence time, which reduced the influx of clastic sediments and allowed carbonate sediment to accumulate on the shelf. Finally, sea level fell again and the Jordan sandstone was deposited as wedges of near shore marine sand during this second regression of the Sea. These formations are part of the Sauk sub-transgressions and sub-regression cycles.