Upper Cretaceous (Cenomanian to Maastrichtian) Stratigraphic Breaks in Alabama Coastal Plain: Criteria for Recognition and Relationship to Eustatic Sea Level Change: ABSTRACT

Abstract

In Alabama, the Upper Cretaceous section (450 m thick) rests on pre-Mesozoic basement and consists mainly of clastic shoreline and nearshore facies and hemipelagic shelf facies. The section contains several distinctive stratigraphic breaks, most of which are ascribed to third-order eustatic sea level drops on the current world sea level curve. However, a few distinctive breaks in the section are not related to eustatic sea level changes and cannot be attributed to changes in shelf subsidence rate. The eustatic-related stratigraphic breaks are delineated on correlations by erosional relief up to 25 m, lateral continuity (in dip section) over 200 km, and biostratigraphic discontinuity. On outcrop, a sharp facies discontinuity and a conglomeratic zone bearing teeth, bones, and phosphatic clasts mark these breaks. The most notable breaks correlate biostratigraphically with eustatic sea level drops at 90 Ma (125-m drop at the Tuscaloosa-Eutaw Formations contact), 80 Ma (65-m drop at the Blufftown-Mooreville and Cusseta-Demopolis lithosome contact), and 68 Ma (95-m drop at the Ripley Formation and Prairie Bluff-Providence lithosome contact). Other eustatic breaks indicated only by outcrop-scale evidence occur within the Ripley Formation (71 Ma, a 55-m drop) at the Cusseta-Demopolis and Ripley Formations contact (75 ma, a 50-m drop), and atmore » the Eutaw Formation and Blufftown-Mooreville contact (84 Ma, a 45-m drop). In addition, marine facies discontinuities (traceable over 200 km in dip section) are correlated with all minor (< 45 m) eustatic sea level drops. Some facies discontinuities which cannot be matched with coeval eustatic sea level change are uniquely characterized by sandy shelf-bar development and are attributable only to a briefly elevated clastic sedimentation rate.« less