Depositional Facies and Eustatic Effects in Upper Cretaceous (Maastrichtian) Ripley Formation, Central and Eastern Alabama: ABSTRACT

Abstract

ABSTRACT In central and eastern Alabama, the Maastrichtian Ripley Formation (40 to 123 m thick) is comprised of six depositional facies: (1) barrier-island shoreface/tidal-inlet fill sands; (2) back-barrier lagoon/marsh siltstones; (3) storm-generated lagoonal sand beds; (4) back-barrier tidal-flat/lagoonal claystones; (5) lower shoreface sands; and (6) inner-shelf deposits. The Ripley is divided into two genetic packages of facies; the genetic packages are bounded by stratigraphic breaks of eustatic origin. The basal Ripley break is correlated with the end-Campanian (c. 74 Ma) eustatic sea-level drop, and the middle Ripley break (separating the two genetic packages) marks the mid-Maastrichtian (c. 71 Ma) sea-level drop. The basal and middle Ripley breaks are low-relief erosional surfaces distinguished by facies discontinuities and thin conglomeratic lag deposits. The break at the top of the Ripley has a maximum erosional relief of 67 m in the outcrop area studied (83 m in the shallow subsurface) and is mantled by a conglomeratic lag bed up to 80 cm thick. The break at the top of the Ripley represents a Late Maastrichtian (c. 68 Ma) sea-level fall estimated to have been nearly 95 m.