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: ABSTRACT

Abstract

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 Chattahooche Valley of Alabama's eastern border, a span of 150 km. A dipsection 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 correlation of shelf and shoreline facies shows distinct genetic packages marked by facies discontinuities. These discontinuities are the result of rapid sea level changes. The authors 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 turbiditemore » sand layers and the development of sandy shelf bars.« less