Abstract: Sequence Stratigraphy of Jurassic Strata in the Eastern Gulf Coastal Plain: Comparison to the Global Cycle Chart

Berry H. Tew, Ernest A. Mancini, Robert M. Mink

On the basis of regional stratigraphic, petrographic, and lithofacies relationships, three third-order, unconformity-bounded depositional sequences that resulted from changes in relative sea level and coastal onlap during the Middle and Upper Jurassic (Callovian, Oxfordian, and Kimmeridgian) can be recognized for the eastern Gulf Coastal Plain area of southwest Alabama. These sequences are designated the LZAGC-3.1 (Lower Zuni A Gulf Coast), LZAGC-4.1, and LZAGCA.2. LZA refers to the time correlative supercycle set as shown on the Exxon global cycle chart, GC indicates that the sequences recognized here are indigenous to the Gulf Coastal Plain, and the numeric designation provides a unique number for the Gulf Coastal Plain sequences within the supercycle time intervals on the Exxon cha t. The above sequences are correlated to the LZA-3.2 sequence (155.5 to 150.5 Ma), a composite of the LZA-4.1 through LZA-4.4 sequences (150.5 to 144 Ma), and the LZA-4.5 sequence (144 to 142 Ma) in the global (Exxon) system, respectively.

The LZAGC-3.1 sequence consists of a basal unconformity, transgressive systems tract (Wemer Formation and Louann Salt), condensed section (Pine Hill Member of the Louann, basal Norphlet Formation shale), and highstand systems tract deposits (Norphlet Formation continental strata). A type 2 sequence boundary separates the LZAGC-3.1 from the LZAGC-4.1 sequence. Shelf margin systems tract (Norphlet Formation marine sandstone), transgressive systems tract ("lower" Smackover Formation), condensed section ("middle" Smackover Formation), and highstand systems tract ("upper" Smackover Formation, Buckner Anhydrite Member of the Haynesville Formation, "middle" Haynesville Formation) strata comprise the LZAGCA.1 sequence. Strata of the "upper" Haynesville Formation represent the component system tracts of the LZAGC-4.2 sequence. A type 2 sequence boundary separates the LZAGCA.1 sequence from the LZAGCA.2 sequence.

In the southwest Alabama part of the eastern Gulf Coastal Plain, highstand systems tract strata of the Norphlet Formation (LZAGC-3.1 sequence) and Smackover Formation comprise the primary Jurassic hydrocarbon reservoirs. Hydrocarbon source rocks for these reservoirs are organic-rich Smackover Formation carbonate mudstones that are in the condensed section of the LZAGC-4.1 sequence.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994