ABSTRACT: Sequence Stratigraphy Along an Unstable Progradational Continental Margin--Pliocene-Pleistocene Offshore Louisiana

J. A. Pacht, B. E. Bowen

Systems tracts in offshore Louisiana Pliocene-Pleistocene strata are developed along an unstable progradational continental margin and differ greatly from those described for stable progradational continental margins. Basin-floor fans are developed within intraslope basins. They are thin and limited in area, although they are commonly characterized by well-developed, clean, blocky sandstones. Lowstand slope-fan systems tracts are characterized by stacked channel-overbank facies, chaotic facies (formed largely by soft sediment deformation), and thin units that probably represent hemipelagic sedimentation. In contrast to slope fans developed along stable progradational margins, updip deltaic facies are present. Lithology is highly variable in this systems tract. Lowstand pr grading-wedge systems tracts are characterized by wave-dominated deltaic deposits. Reflectors exhibit divergence toward downthrown sides of growth faults. Unlike stable passive margins, progradational patterns are not observed due to fault growth. Sand-rich upward-coarsening profiles characterize this unit. The transgressive systems tract is commonly very thin and demonstrates little reservoir potential, with the exception of a thin, very well-sorted basal sandstone formed by reworking of the underlying prograding wedge deposits. Highstand systems tracts consist largely of fluvial-dominated deltaic deposits. Reflectors are generally concordant. Oblique progradational patterns, common along stable progradational margins, are rare. Upward-coarsening profiles are observed; however, sandston s generally show poor reservoir quality.

Although morphology and depositional style of systems tracts in Pliocene-Pleistocene Louisiana Gulf Coast strata may differ from those developed along stable progradational margins, they can be correlated throughout the basin and effectively used to predict location of reservoir and seal strata.

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