EVIDENCE IN THE ALTAMONT CYCLOTHEM (MIDDLE PENNSYLVANIAN) AT ST. LOUIS, MISSOURI FOR LATE DESMOINESIAN TECTONIC ACTIVITY ALONG THE NORTHEASTERN FLANK OF THE OZARK UPLIFT

The Upper Desmoinesian (Kasimovian global stage; Upper Carboniferous) Altamont cyclothem (mostly in the Altamont Formation) is exposed along I-170 in St. Louis, Missouri. The outcrop area is on the northeastern flank of the Ozark uplift where an anticlinal trend connects the uplift with the Mississippi River arch to the north, and separates the Midcontinent region on the west from the Illinois basin on the east. Transgressive deposits in the cyclothem include claystone-pebble diamictites containing scattered clasts of phylloid algal limestone up to boulder size and phosphate nodules that were reworked from underlying cyclothemic beds. We believe these diamictites were deposited primarily by submarine debris flows following reactivation of near-by uplifts. Candidate structures include Waterloo-Dupo anticline, Cap au Grès faulted flexure, and the NW-SE trending structures that dominate the Ozark uplift to the south.

The phosphatic Lake Neosho core shale of the Altamont cyclothem is truncated by an erosion surface below the base of the regressive Worland Limestone. Channels, some exceeding a meter in depth, were cut into underlying strata during that erosional episode. Because this erosion surface is not widespread, we infer it to be a local feature of tectonic origin rather than a product of eustasy. The lower part of the Worland is dominated by skeletal and intraclastic packstones, and includes scattered phylloid algal mounds, indicating continued anomalously shallow conditions. Overlying lime mudstones and skeletal wackestones indicate decreasing average energy levels, and suggest increasing depths of deposition. The uppermost part of the Worland, however, displays root molds and columnar ped structure, indicating eventual withdrawal of the sea as lowstand approached.

Late Desmoinesian tectonic activity has yet to be documented for the area between the Ozark uplift and Mississippi River arch. Anomalous lithologic and stratigraphic features in the Altamont cyclothem at I-170, however, are consistent with near-by uplift at that time. Such uplift may have affected patterns of circulation in the Late Paleozoic Midcontinent Sea, and may have influenced cyclothemic facies patterns at the transition from Midcontinent region to Illinois basin.