A POSSIBLE ROGUE WAVE DEPOSIT IN THE JOPPA MEMBER OF THE STE. GENEVIEVE FORMATION IN THE ILLINOIS BASIN

A roadcut on US-51 north of Anna, IL, exposes the Mississippian Period, Valmeyeran Series, Aux Vases Formation and underlying Joppa Member of the Ste. Genevieve Formation. The purple and green shale of the Joppa is interbedded with siliciclastic and carbonate lenses. One unusual 0.2 km long carbonate sheet within the shale has been interpreted by previous authors as a storm deposit. The base of the possible tempestite indicates apparent scouring and loading features into the shale. Ooids (some broken), fossil fragments (some coated grains), rare clasts and some siliciclastic laminations make up the sheet. Ooids and fossils occur unmixed, mixed together, and sometimes roughly graded, suggesting a depositional process involving high agitation. Cross-bedding is essentially unidirectional. The surface of the carbonate is flat with truncation of cross-bedding. The unit thins to the north and south. Regional studies suggest this is a restricted deposit, essentially limited within the roadcut area. Apparent scour and load structures are not evenly distributed along the outcrop but are concentrated near the center. Textural and structural features in this rock unit were compared with tempestites formed by hurricanes, tsunamis, bolides, as well as shoal migration and eolian processes. The Anna deposit does not appear to match units produced by any of these methods. It is suggested that this layer may have been the result of a previously undocumented sedimentary process, a rogue wave. Rogue waves (Freak Waves, Killer Waves, Draupnir Waves) are known to reach massive size and power at sea and have been documented striking shorelines on numerous occasions. They commonly form downwind from islands. The paleoenvironment at the time of deposition of the Anna feature may have been conducive for the generation of rogue waves. Research into the nature of rogue wave depositional processes is needed.