MINERALOGY AND PETROLOGY OF THE LOWER PALEOCENE CLAYTON AND PORTERS CREEK FORMATIONS, SOUTHEASTERN MISSOURI, USA

The uppermost Cretaceous through lower Paleocene strata of the Mississippi embayment were sampled at a strip mine in Stoddard County, southeastern Missouri to evaluate the mineralogy and origin of the basal Paleocene Clayton and Porters Creek formations. Twenty-five samples were described in detail using petrographic methods and analyzed by X-ray diffraction. The basal excavated strata in the mine include the uppermost Cretaceous McNairy Sand and Owl Creek Formation. Overlying the Cretaceous strata with unconformity is the lowermost Paleocene Clayton Formation (CF), 1.6 m thick, which is interpreted by Campbell et al. (2007) as a megatsunami deposit associated with the Chicxulub impact event. The contact with the overlying Porters Creek Formation (PCF) is gradational; the total exposed thickness of PCF was not measured, but is estimated to be 25 m.

The CF is comprised of a bioclastic rudstone and floatstone with a glauconitic calcitic mud matrix containing dispersed quartz, biotite, feldspars, and glassy to clay-replaced microspheroids (impact glass?). Recrystallization of carbonate mud and extensive pyrite replacement are common. Grain size, carbonate mud, and fossil abundance decrease in the upper CF. The PCF is mudstone with carbonate fossils present in the lower 2 m but rare above and minor glauconite. The mud is composed almost entirely of clay minerals, cristobalite, quartz, heulandite, calcite, muscovite, biotite, feldspars, and pyrite. Heulandite and calcite are common in samples from the lower 2 m, but rare above and cristobalite is common to abundant in all samples analyzed above 2 m, except an iron-oxide concretion and the sample from the upper contact. Preliminary clay mineral X-ray diffraction data indicate a prominent increase in expandable clays above the K-T unconformity. No petrographic or field evidence for primary volcanic deposits is observed, contrary to previous studies of the PCF in the region (Allen, 1934; Simms, 1972). The increase in expandable clay minerals and presence of heulandite and cristobalite may instead be interpreted to be from alteration of fine-grained impact glass and other labile silicate materials. Oxidation, gypsum, and loss of calcite in the middle and upper PCF are interpreted to be from post-PCF weathering.