Depositional History and Genetic Sequence Stratigraphic Framework of the Middle Miocene Depositional Episode, South Louisiana

By

COMBELLAS-BIGOTT, RICARDO I., and GALLOWAY, W.

The University of Texas at Austin, Austin, TX

Two widespread, transgressive deposits associated with the faunal tops Amphistegina B (15.5 m.a.) and Textularia W (12.4 m.a.) defined the Middle Miocene depositional episode (MM depisode).

Sediment supply coupled with mobile salt, subsidence and sea-level changes produced a mosaic of linked structural and depositional systems. Three distinct structural provinces were identified: The western province characterized by continuous E-W orientated faults with minor salt spreading. The highly-subsided central detachment province bounded by distinct lateral faults, shallow salt bodies, and a thin MM depisode, produced by salt withdraw, structural rebound, and differential subsidence. Arcuate-seaward-dipping normal faults (Roho system) and counterregional faults formed intricate pathways for sediments to reach the basin floor in this province. The eastern province was very stable with minor salt withdraw.

Four regressive cycles separated by maximum flooding events divided the MM depisode: Cycle 1 (Amphistegina B-Cibicides opima), cycle 2 (Cibicides opima-Cristelaria I), cycle 3 (Cristelaria I-Bigenerina humblei), and cycle 4 (Bigenerina humblei-Textularia W). Cycles were modified by sea level fall.

Characteristic depositional systems in cycles 1 and 2 are fluvial dominated platform and shelf margin deltas fed by the central mixed-load Mississippi fluvial system in the western and central structural provinces. During cycle 2 collapse of the shelf margin produced a retrogradational delta fed apron, flanked by a structural-controlled gorge that fed a long-lived submarine fan. During cycle 3 main deltaic depocenter shifted toward the east fed by the eastern Mississippi fluvial system. Extensive shorezone-shelf system tracts occupied most of the western and central provinces through cycles 3 and 4. During cycle 4 generation of submarine canyons in the east started infilling the eastern portion of the submarine fan.