Geology of the Moscow Landing Section, Tombigbee River, Western Alabama, with Focus on Ichnologic Aspects of the Lower Paleocene Clayton Formation

Abstract

Sedimentologic and ichnologic studies were performed on units of the Moscow Landing section, a ~1.5-km exposure of Upper Cretaceous and Lower Paleocene strata along the west bank of the Tombigbee River, Sumter County, Alabama. Goals of the study were (1) to generate a detailed geologic map of the entire Moscow Landing exposure, (2) to characterize and interpret large burrow systems expressed in an alternating marl/marly limestone sequence in the middle portion of the Clayton Formation, and (3) to characterize the sedimentology and ichnology of a thin chalk bed (referred to as the “Clayton chalk”) in the upper part of the Clayton Formation. This study produced the first detailed map of the Moscow Landing section, which should benefit future visitors to the site. The map, comprising nine partly overlapping segments or strips, depicts the distribution of stratigraphic units (upper part of the Prairie Bluff Chalk, discontinuous Clayton sand bodies, the Clayton Formation, lower part of the Porters Creek Formation, and select marker beds therein) and structural features (faults and bedding attitudes). Large, highly irregular, branching burrow systems hosted in Clayton Formation marls are attributed to decapod crustaceans and assigned to the ichnotaxa Spongeliomorpha iberica and Thalassinoides paradoxicus. Both of these forms are indicative of the firmground Glossifungites ichnofacies. Consistent with previous sequence stratigraphic interpretations, firmground conditions developed in response to erosional exhumation of semiconsolidated marls at parasequence-bounding marine flooding surfaces, which are marked by limestone interbeds. The Clayton chalk is characterized by a Zoophycos ichnofacies assemblage of traces. Compared to chalk-dominated units of the Cretaceous Selma Group in the eastern Gulf coastal plain, the Clayton chalk is most similar to the Demopolis Chalk with respect to sediment texture, composition, and ichnofabric. This suggests that the Clayton chalk was deposited in a relatively quiet, outer-shelf setting. Based on biostratigraphic constraints, the transgression responsible for Clayton chalk deposition occurred ~62-63 Myr ago and is in part coeval with the development of the Cannonball Sea in the U.S. Western Interior. The transition from Clayton chalk to clay-dominated Porters Creek deposits reflects relatively abrupt changes in sea-level, paleogeography, and depositional environments along the eastern flank of the Mississippi embayment.