KING, DAVID T., JR., Auburn University, Auburn, AL

ABSTRACT: Eustatic and Tectonic Effects in Sequence Stratigraphy of the Paralic-Shelfal Marine Section, Upper Cretaceous, Alabama

In Alabama, sequence stratigraphic facies relations within the late Santonian to latest Maastrichtian (85-67 Ma) marine section have been strongly influenced by second-, third-, and fourth-order sea level changes related both to global eustasy and regional tectonics. Through detailed lithostratigraphic and biochronostratigraphic correlations, one can delineate the dictating mechanism (eustasy versus tectonics). In the outcrop region, the marine section is 450 m thick and is comprised of paralic and shelfal facies associations that occur within all formal stratigraphic units and, in turn, make up the 13 genetic packages (i.e., stratigraphic sequences) of the total marine section. Second-order eustatic oscillations of 3.5-5.0-m.y. duration dictated the paleogeographic (depositional) str ke and the characteristics of lateral and vertical facies relations along paleogeographic strike. Third-order sea level oscillations (0.5-3.5 m.y.) are related to either eustasy or regional tectonics. Oscillations driven by eustasy produced stratigraphic breaks with appreciable relief, significant regional paleogeographic reorganizations, increased lateral and vertical facies entropy,

and notable faunal changes. Stratigraphy of tectonically induced third-order oscillations generally lacks most or all of these features. Fourth-order sea level oscillations (less than 0.5 m.y.) are related most closely to minor regional tectonic adjustments coeval with the onset of episodic, Farallon-North American plate interactions. The episodic (rather than periodic) nature of fourth-order shelfal parasequences in the area, their fining-upward textural signature, characteristic transitional benthic-to-pelagic fossil record, and stratigraphic correlations with thin, prograding clastic tongues (paralic facies) collectively demonstrate a non-Milankovitch origin of the shelfal parasequences.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.