Abstract: Mississippi's Miocene in the Gulf Coast Context--A Stratigraphic Revision

Ervin G. Otvos

The Mississippi-Alabama Miocene forms a transitional zone between the thick Louisiana Miocene siliciclastics and the thinner carbonate-dominated units of northwest Florida. With recent recognition of marine cycles in Mississippi's coastal Miocene, this stratigraphic revision replaces currently used Neogene formation names. The old terms (Catahoula, Hattiesburg, Pascagoula, and Graham Ferry formations) had been applied to inadequately delineated terrestrial-paralic sequences.

Globigerinoides trilobus (N5-23), G. primordius (N4-5) and Globorotalia kugleri (N4) age ranges suggest that the Oligocene-Miocene epoch boundary, placed by various sources between 23.7 and 25.5 Ma, cuts across the Heterostegina zone in Mississippi. The age of the upper Heterostegina zone is

lower Miocene. A following hiatus is marked by absence of zones N4b-8 landward of newly recognized lower Miocene (N4-6) outer neritic deposits (new Hancock formation). Middle Miocene deposits in a number of wells directly overlie the upper Eocene Ocala facies. A nondepositional-erosional interval is also known in Alabama. Inner and middle-outer neritic Miocene deposits reached their greatest landward extent during the early Serravallian transgression (TM 2.4; zones N9-10, c. 15-14 Ma) that deposited the lower Pensacola formation interval. Its locally gas-rich Amos and younger members and evidence for a deep trench, cut obliquely across the Miocene shelf, are recognizable both in and off Alabama and Mississippi.

The early Tortonian transgression (TM 2.6; zone N16, c. 11-10 Ma) extended a deep neritic embayment into mainland Mississippi. Ancestral Mississippi delta lobes formed the bay's south shore. Regressions created brackish and terrestrial deposits that predominate in upper Miocene and lower Pliocene sequences. Late Miocene paralic units ("Pascagoula" and "coarse clastics") represent the middle Pensacola. Four Miocene intervals contain carbonates. A sharp unconformity between Pliocene paralics and the overlying Citronelle alluvium is considered the Pensacola Formation's upper boundary.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994