STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE POTTSVILLE FORMATION AT THE UNION CHAPEL TRACK SITE

Amphibian and reptile trackways from the Pennsylvanian-age Pottsville Formation at the Union Chapel Mine provide exceptional insight into ancient life and environments. The trackways come from the Cincosaurus beds, which constitute one of many fossiliferous intervals exposed in the mine. These intervals contains different fossil assemblages representing a spectrum of terrestrial to marine environments of deposition.

The Union Chapel Mine is located in the Black Warrior foreland basin of Alabama, which formed adjacent to the Appalachian-Ouachita juncture during assembly of the Pangaean supercontinent. The Pottsville Formation contains a thick succession of siliciclastic rocks that was deposited near the paleoequator, and the cyclic nature of Pottsville strata is interpreted to be the product of fourth-order glacial eustasy driven by the Gondwanan ice sheet. The Union Chapel section spans parts of two fourth-order parasequences and contains transgressive strata in the upper part of the Mary Lee coal zone and highstand deposits in the lower part of the Gillespy coal zone.

At the base of the Union Chapel section, the Mary Lee coal was mined as a source of high-quality fuel for electric power generation. The Cincosaurus beds were deposited on an estuarine mudflat that formed as the Mary Lee swamp was inundated by sediment-laden water. The Cincosaurus beds represent a dynamic environment in which amphibians, reptiles, and a variety of invertebrates ventured onto the mudflat at low tide. Deposition of the Cincosaurus beds ended with a minor drop of relative sea level and widespread soil development. This event was succeeded by a return to peat-swamp sedimentation, as represented by the overlying New Castle coal, which was also mined at Union Chapel. The roof shale of the New Castle coal contains standing fossil forests and represents a swamp that was prone to overbank flooding. Above the roof shale is a thin bed of nodular limestone containing brachiopods and bivalves, which records condensation during a major marine transgression. Above the limestone is a progradational succession of shale and sandstone that forms the base of the Gillespy coal zone, contains marine trace fossils, and was deposited in prodelta and delta-front environments during a fourth-order highstand event.