Stratigraphy of the Altamont Formation (Middle Pennsylvanian) in St. Louis, Missouri
The Altamont Formation, consisting of Amoret Limestone, Lake Neosho Shale, and Worland Limestone Members (ascending), is well exposed over a distance of about 200 meters along I-170 in St. Louis, Missouri, allowing detailed stratigraphic study. These units were deposited during the transgressive-regressive Altamont depositional cycle, with the black phosphatic Lake Neosho Shale representing maximum transgression (highstand). Such depositional cycles, attributed to Gondwanan glacio-eustatic fluctuations, produced the distinctive Pennsylvanian cyclothems of the Midcontinent region. In Iowa, only a single depositional cycle is apparent in the Altamont, but in other places including St. Louis the Altamont includes evidence for deposition during at least two separate episodes of high sea level, each separated by shallowing.
The Amoret Limestone Member in St. Louis is dominantly shale containing limestone lenses and pebble to bouldersized clasts of both limestone and shale. Most limestones are dense and micritic, but a persistent zone of shaly shellhash limestone is present in the upper part of the member. Channel structures, erosional truncation of beds, and the lithic clasts show high-energy conditions at various times during Amoret deposition. In places along I-170 the Lake Neosho was partially to completely eroded prior to Worland Limestone deposition, showing recurrence of erosional conditions near the time of sea-level highstand.
Regional differences and local variations in lithologic succession of the sort revealed at the I-170 outcrop document the potential for variability within and among Midcontinent Pennsylvanian cyclothems. Models for cyclothem deposition should include mechanisms for such regional and local variations.
AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009