FILLON, RICHARD H., Texaco, Inc., New Orleans, LA; and PAUL N. LAWLESS, CNG, Inc., New Orleans, LA

Abstract: The Lower Miocene Sequence in the Northern Gulf: Prelude to Progradational Slope Salt-Basin Deposition and Diminishing Slope-Bypass Deposition in the Deep Basin

Locally represented by over 1000 m of sand-rich section, the hydrocarbon-rich second-order "Lower Miocene Sequence" was deposited during the first 10 MY of the Miocene. It has been subdivided in considerable detail based on planktonic foraminiferal and calcareous nannofossils biostratigraphy. The Lower Miocene Sequence encompasses six third-order sequences designated according to equivalent traditional benthic foraminiferal biochronozones as: Lentic Jeff; Siph Davisi; Marg "A"; Gyro "6"; Amph "B"; and Lower Chris "I" /Cib Op. It also includes several important genetically related fourth-order sequences such as "Rob'C'"; "Rob 'L'"; and "Cib Op". Most of the Lower Miocene sequence is comprised of progradational wedges of shelf (sandy) and upper slope (shaley) systems tracts that suggest considerable seaward advancement of the shelf-edge. Only the basal Lentic Jeff sequence contains significant sand-rich proximal submarine fan section. Long recognized as a reservoir-prone deposystem downlapping rapidly subsiding, thin, basinal Paleocene through Oligocene section, Lentic Jeff deposition straddled the Liebusella Regression which terminated the second-order Late Oligocene Anahuac Sequence. Significantly, the rapid shelf/slope progradation that began with Lentic Jeff deposition accompanied the rapid extrusion of a large allochthonous salt canopy and the creation of a regionally extensive detachment surface above the Paleogene. It can be argued that the mobile salt both buttressed the prograding slope and restricted sediment delivery to the deep Gulf basin. Prior to the Liebusella Regression, sediments that made it to the shelf-edge bypassed the slope, accumulating in the deep Gulf of Mexico. The Pleistocene Mississippi fan is a good analog for pre-Liebusella distal fan deposition. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana