Facies Stratigraphy of Upper Cretaceous Chalk-to-Clastic Transition Zone and Discontinuity-Bounded Genetic Packages Created by Rapid Sea Level Change on Campanian Shelf of Central and Eastern Alabama

David T. King, Jr., Michael C. Skotnicki

The intertonguing Mooreville Chalk and clastic Blufftown Formation (Campanian) contain a transition zone from chalk to clastic rocks in the outcrop area between Montgomery, in central Alabama, and the Chattahoochee Valley of Alabama's eastern border, a span of 150 km. A dip-section transect of the shelf facies shows the shoreward sequence: chalk (outer shelf), clayey marl (inner outer shelf), marly glauconitic siltstone (inner shelf), and bioturbated sand (lower shoreface). The inner-shelf facies contains evidence of punctuated mixing of clastics in the form of hummocky cross-bedded sand, turbidite-emplaced coquinoid sandstone beds, and imbricated Exogyra shell beds. The outer-shelf facies contains sandy shelf-bar deposits associated with regressive episodes. A regional c rrelation of shelf and shoreline facies shows distinct genetic packages marked by facies discontinuities. These discontinuities are the result of rapid sea level changes. We interpreted four transgressive-dominant genetic packages, ranging from 15 m to 50 m thick, each of which can be traced from the shoreline clastic region in the east to the outer-shelf area on the west. In the shoreline clastic region, truncated, transgressive barrier-island sequences rest on lagoonal facies. In the shelf area, the tops of some genetic packages are marked by increased frequency of storm and turbidite sand layers and the development of sandy shelf bars.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.