Abstract: Overview of Upper Cretaceous Marine Depositional Sequences in Alabama and Their Probable Correlation with Coeval Depositional Sequences in the Western Interior Seaway

David T. King, Jr.

Upper Cretaceous marine sedimentation in the inner Gulf coastal plain of Alabama spans the late Santonian to latest Maastrichtian (85-67 Ma) and encompasses 13 depositional sequences. The depositional sequences are separated by either type 1 (subaerial to submarine) or type 2 (mainly submarine) unconformities that delineate and, in some instances, subdivide formal stratigraphic map units in the region. In stratigraphic order, the outcropping and shallow-subsurface marine section in Alabama consists of the Eutaw, Mooreville-Blufftown, Demopolis-Cusseta, Ripley, and Prairie Bluff-Providence intervals. Each of these intervals encompasses two to four depositional sequences; each sequence is comprised of separate transgressive and highstand systems tracts. Both paralic (i.e., lagoonal, bar ier-island, and lower shoreface facies) and shelfal facies associations are present within transgressive and highstand systems tracts. Systems tracts are separated by (1) shelfal condensed zones comprised of chalky marl or shelly clay or (2) coeval paralic maximum-flooding surfaces delineated by sharp facies discontinuities. Sequence boundaries are delineated by conglomeratic transgressive surfaces, incised-valley fills, paleosols, karst paleotopography, and/or regional low-angle erosional truncation. The global pattern of third-order depositional sequences from 85 to 67 Ma is present with the Alabama marine section but is modified within the upper parts of both the Eutaw and Mooreville-Blufftown intervals and the entire Prairie Bluff-Providence interval. In those places, regional and lo al tectonic adjustments caused single global depositional sequences to be partitioned into two or three smaller local depositional sequences. Thus, 13 local depositional sequences comprise the Alabama Gulf coastal plain section vs. 8 in the current global synthesis. Six depositional sequences comprising the Eutaw and Mooreville-Blufftown intervals (85-80 Ma) are correlative with the middle and upper parts of the Western Interior Seaway (WIS) Niobrara depositional cycle. The overlying WIS Clagett depositional cycle correlates with four depositional sequences comprising the Demopolis-Cusseta interval plus the lower part of the Ripley interval (80-71 Ma). Four depositional sequences comprising the upper part of the Ripley interval plus the Prairie Bluff-Providence interval correlate with th WIS Bearpaw depositional cycle (71-67 Ma).

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994