Sedimentary Facies, Sea Level History, and Paleogeography of the Upper Cretaceous (Maastrichtian) Providence Sand, Central and Eastern Coastal Plain of Alabama

Abstract

ABSTRACT The Maastrichtian Providence Sand crops out in an east-west trending belt in the inner Coastal Plain of Alabama. The Providence Sand (0 to 250 ft (76m) thick) rests disconformably on incised valleys (type-1 unconformity) at the top of the underlying Maastrichtian Ripley Formation. The Providence is topped by a high-relief stratigraphic break thought to be another type-1 unconformity; the Maastrichtian-Danian Clayton Formation overlies the Providence. In the study area, the Providence is comprised of six main facies. These Providence facies are: 1) cross-stratified Ophiomorpha-bearing fine to medium sands (barrier-island shoreface), 2) cross-stratified pebble- and clast-bearing medium sands (barrier-island tidal inlet), 3) sandy mottled clays and intercalated fine sands (back-barrier deposits), 4) carbonaceous silts (back-barrier marsh), 5) interlaminated sands and carbonaceous silts (flood tidal delta), and 6) interlaminated clayey sands and clayey silts (proximal back-barrier lagoon). The Providence facies are arranged in two genetic packages (probable parasequences) which developed during a single cycle of relative sea-level change. The lower genetic package, comprising the basal Perote Member of the Providence Sand and the laterally equivalent tongue of the Prairie Bluff Chalk (lower-shoreface and inner-shelf facies), is separated from the upper genetic package by a sharp lithologic break. At the break, barrier-island shoreface and tidal-inlet sands of the upper genetic package overlie various fine-grained facies of the lower genetic package. The break between the two genetic packages is a facies shift (discontinuity) delineating a probable parasequence boundary. This parasequence boundary developed at the time of maximum rate of shelf flooding and thus corresponds to a subsurface condensed section. A change in depositional strike from approximately northwest to nearly east-west at the parasequence boundary is indicated in depositional facies correlations. The barrier-island shoreface and barrier-island tidal-inlet facies possess the coarsest sands in the formation and therefore are the most permeable facies and best aquifers in the Providence Sand.