MULTIPLE EROSION SURFACES IN TRANSGRESSIVE AND HIGHSTAND PORTIONS OF THE ALTAMONT CYCLOTHEM (MIDDLE PENNSYLVANIAN) IN ST. LOUIS, MISSOURI

The upper part of the Bandera Formation and the Altamont Formation (and cyclothem) are well exposed along I-170 in St. Louis, Missouri. At the base of the exposure is a claystone paleosol (upper Bandera). This is overlain by maroon diamictite in the basal Altamont containing rounded clasts, up to boulder size, of claystone and limestone, plus thin lenses of coal and carbonaceous shale, transported phosphate nodules, and lenses of shelly material, all of which represent a complex history of erosion and deposition early in the transgressive phase of the Altamont depositional cycle. As sea level rose, ravinement marked by channeling occurred. Fissile shell-hash limestone and micritic limestone containing phylloid algae and angular clasts of claystone (Amoret Limestone Member of the Altamont Formation) were deposited above the ravinement surface, filling the channels. A second ravinement surface truncates these limestones and the lower ravinement surface. This erosion surface is overlain, in succession, by cross-bedded calcareous shale, dark claystone, and fissile black phosphatic shale, all composing the Lake Neosho Shale Member of the Altamont Formation. The presumably deep-water phosphatic shale is truncated by a third erosion surface that is overlain by bioclastic shale containing reworked phosphate nodules, and then the Worland Limestone Member of the Altamont. In places, the highest erosion surface truncates nearly all underlying Altamont strata, and the Worland Limestone lies only a few centimeters above the Bandera paleosol.

The contact between the Bandera and Altamont Formations is a transgressive surface. The transgressive portion of the overlying Altamont cyclothem contains two ravinement surfaces formed by wave and/or tidal action, and is therefore similar to many transgressive intervals in passive margin settings. A third erosion surface truncates the apparent high-stand deposit (Lake Neosho phosphatic shale). The latter surface is difficult to interpret in context of the well-accepted cyclothem depositional model. Nevertheless, identifying and properly interpreting such surfaces will contribute to understanding the origin of Pennsylvanian cyclothems.