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.