A REEXAMINATION OF PENNSYLVANIAN SINK FILL DEPOSITS IN EAST-CENTRAL MISSOURI: ARE THEY PENNSYLVANIAN AND ARE THEY SINK FILLS?

Sandstone and mudstone deposits of east-central Missouri form important sources of silica sand and refractory clay. These have been interpreted as Pennsylvanian sink fill deposits, primarily because of the roughly circular shape of many of the clay pits, the presence of high temperature clay minerals, bedding which is sometimes chaotic, and stratigraphic position, as the first significant sandstone, above known Ordovician carbonate units. Recent mapping along the Missouri River between St. Louis and Jefferson City, however, suggests that these deposits, tentatively assigned to the Cheltenham Formation, may actually consist of multiple generations of sand, silt, and clay ranging in age from early Ordovician to early Mississippian. Deposition appears to have been by fluvial processes over an initial paleokarst surface. Specific evidence includes:

Fluvial origin

1) Most deposits are flat-lying with jumbled strata being restricted to lateral margins. 2) The regional distribution pattern of sandstone resembles that of an integrated drainage system and commonly forms a sheet. 3) Internal structures of sandstones indicate deposition by running water. 4) Sandstone bodies are often flanked by successions of interbedded claystone, siltstone, and thinner sandstones.

Ordovician to Mississippian age

1) Invariably, these deposits are in contact with underlying Ordovician strata; whereas, similar deposits are not found to overly nearby Mississippian units. 2) In places, these deposits are interbedded with carbonates of early Ordovician age. 3) Small remnants of Devonian and Mississippian-age strata locally overlie these deposits. 4) Cobbles and small boulders of fossiliferous Mississippian-age chert are found on the surface above these deposits throughout their mapped extent. 5) Pennsylvanian sandstones in the western and northern parts of the state are typically associated with coal and are often interbedded with fossiliferous limestones and shales. No fossils are known from these deposits and coal is uncommon.