Datapages, Inc.Print this page

ABSTRACT: Marl-Limestone Bedding and Sea Level Change, Upper Cretaceous Alabama Coastal Plain

David T. King, Jr.

Primary Upper Cretaceous marl-limestone sequences in Alabama are superficially similar to sequences interpreted as orbitally forced (Milankovitch) cycles with apparent 100,000 yr periodicity. However, key petrographic and stratigraphic observations cast doubt on a Milankovitch interpretation of the Alabama sequences. These observations include (1) varying thickness from less than 1 m to several meters within marl-limestone sequences, (2) fining upward among the coarse clastic grain sizes within each marl-limestone sequence, and (3) switching the primary mode of biochemical deposition between the marl and limestone beds (e.g., the limestone bed in each marl-limestone sequence contains abundant benthic algal grains (over 50%), whereas the marl contains only pelagic nannofos ils). Periodicity is not supported by observed thickness variations and the other petrographic and stratigraphic observations previously noted are inconsistent with cyclic (Milankovitch) productivity/dilution genesis of bedding. In Alabama, marl-limestone sequences and correlative coeval prograding clastic barrier-shoreline facies comprise the upper 20 m of a highstand systems tract within the lower Campanian Mooreville-Demopolis depositional sequence. A sequence-bounding (type 2) stratigraphic break, which has been biostratigraphically correlated with the global sea level drop at 80 Ma occurs at the top of the 20 m marl-limestone interval. Tectonic effects related to the early stages of the Laramide orogeny (about 80 Ma) affected short-term relative sea level thus producing the marl-lim stone sequences. In this hypothesis, the marl-limestone sequences reflect either short-term buildup and relaxation of intraplate stresses or short-term eustatic changes that resulted from slight variations in Farallon spreading rates.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990