THE ALTAMONT FORMATION (MIDDLE PENNSYLVANIAN) AT THE I-170 OUTCROP IN ST. LOUIS, MISSOURI

Pennsylvanian strata spanning the Desmoinesian-Missourian boundary have been preserved in the St. Louis area due to a local downwarp called the St. Louis basin. The upper Desmoinesian (Middle Pennsylvanian) Altamont Formation is extensively exposed along both sides of the southbound lanes of I-170 near the St. Louis metropolitan airport. The Altamont Formation includes three members, including (ascending) the Amoret Limestone, Lake Neosho Shale, and Worland Limestone. In terms of Midcontinent cyclostratigraphy, the Amoret Limestone is the transgressive limestone of the Altamont cyclothem, the Lake Neosho Shale is the phosphatic core shale of the cyclothem, and the Worland Limestone is the regressive limestone. All three of these members have been reported at the I-170 exposure. A brightly-colored claystone paleosol that has been identified as the Bandera Shale is at the base of the exposure.

Our work shows that the shale underlying the Worland Limestone Member at I-170 actually includes two phosphatic shales separated by a paleosol, and therefore represents two depositional cycles. Conodonts in these units indicate the lower phosphatic zone correlates with the unnamed core shale of the Farlington cyclothem, and only the upper phosphatic shale is the true Lake Neosho Shale. The so-called Amoret at I-170 underlies both phosphatic shales, and therefore must be older than the Amoret at its type locality, correlating instead with the Farlington Limestone; there is no Amoret equivalent at the I-170 exposure. Since the Bandera Shale in its type area lies between the Farlington Limestone and the Lake Neosho Shale, the paleosol between the two phosphatic zones at I-170 must have formed at the time of Bandera Shale deposition. The more prominent paleosol beneath the Farlington-equivalent limestone at I-170 formed during an earlier depositional cycle. Data collected at this site are crucial for proper correlation of strata along the Midcontinent outcrop belt to the west with those in the Illinois basin to the east.