Stratigraphy of Smackover Formation (Upper Jurassic), Southwest Alabama: Eustatic and Tectonic Influences on Depositional Patterns

Mary L. Barrett

The modeled eustatic sea level rise during Smackover deposition is recorded in rocks along the Wiggins Arch-Conecuh Ridge areas of the Manila embayment as a "large-scale" transgressive-regressive carbonate package. Transgression of Smackover carbonates in the Conecuh Ridge area was associated with the end of pronounced fault-block movement characteristic of earlier Norphlet deposits. Underlying Louann and Norphlet strata created a ramplike surface of transgression in the Wiggins arch area, whereas the Louann pinch-out and confinement of Norphlet coarse clastics between basement blocks in the far eastern Manila embayment resulted in a rapidly changing paleotopographic surface. The subdued paleotopographic (ramp) setting was dominated by lateral progradation of sedimentary nvironments; in contrast, carbonates to the east over basement features responded to the same eustatic changes by vertical upbuilding (aggradation) of facies-a "tectonic dictator" existed that controlled depositional relief and localized environments through time.

Direct evidence for meter-scale relative sea level oscillations is from multiple exposure surfaces within upper Smackover grainstones. These diagenetic caps (up to four zones recognized in the Chunchula field area) at the top of coarsening-upward sequences are characterized by chemically coated grains, gravitational vadose cements, multiple dissolution-reprecipitation features, and localized radial-fibrous cements. Stacked sedimentary packages indicate that repeated relative sea level rises again initiated carbonate deposition after exposure. Correlation of exposure caps in four wells in Chunchula field reflect internal time lines generally parallel with the Norphlet-Smackover contact, and indicate lateral equivalence of grainstones with updip anhydrite.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.