ROBERTS, H. H., Coastal Studies Institute, Baton Rouge, LA, and J. SYDOW and A. H. BOUMA, Louisiana State University, Baton Rouge, LA
ABSTRACT: Seismic Stratigraphy, Sedimentology, and Reservoir Potential of a Late Pleistocene Shelf-Edge Delta
Evaluation of a 92-m continuous boring through a shelf-edge delta sequence, in Main Pass lease area, Block 303 (MP303), and 4500 line-km of high-resolution seismic profiles on the outer Mississippi-Alabama shelf have led to a better understanding of the internal architecture of fluvio-deltaic systems during falling to low sea level periods. The data set is the best available from any outer shelf setting, and has resulted in the most detailed documentation of a shelf-edge delta. Although only one boring is used, its high-quality data form a useful calibration point for the seismic grid. The resulting stratigraphic mapping supplies valuable knowledge about the lithologic character, stratigraphic position, distribution, and compartmentalization of reservoir facies in an outer shelf setti g.
The clinoforms of the landward-thinning delta wedge have prograded onto a shelf-wide, high-amplitude reflector. The deltaic clinoform wedge appears to be sand rich. Steep clinoforms at the boring site correspond to the top half of the deltaic sequence, consisting of silty to clean, upper delta front fine sands. Mapping of steep clinoforms indicate that delta sands are volumetrically the most significant reservoir facies on the outer shelf. The delta reservoir is compartmentalized into clinoform sets, interpreted as individual delta lobes. The lobes are separated by bounding surfaces interpreted as the result of lobe-abandonment periods. From data associated with boring, these surfaces appear to be composed of muddy facies. Within a clinoform set the best developed
reservoir facies are most likely associated with the steep (up to 5 degrees) clinoforms near the progradational center axes of diverging downlap, such as found at the boring site. A progradational center is interpreted as the depositional locus associated with an advancing feeder distributary. Each individual delta lobe may be comprised of one or more progradational centers related to distributary bifurcation. Stacking of clinoform sets indicate over-progradations from the northeast to southwest.
Paleobathymetric indicators suggest shoaling of at least 100 m within the ~10 m thick shelf-to-delta transition at the base of the delta sequence, and is evidence for a "forced" regression. Successively lower elevations of clinoform topsets from ~40 m on the middle shelf to -118 m of the distal-most lobes, provide further support that the majority of the fluvio-deltaic system prograded during an overall falling sea level to glacial maximum (18,000 y.B.P.) lowstand. The majority of the shelf-edge delta therefore belongs to the falling stage systems tract. Deposition during turnaround of sea level appears to be represented in the distal most lobes by a small volume of clinoforms, which prograded with a strong aggradational component. The final clinoforms are assigned to the lowstand sys ems tract.
The broad evacuation created by fluvial scour (~15 km wide, 20-25 m deep) is a northeast to southwest trending feature, which is filled with mainly medium to coarse sands and gravel, and minor estuarine sand silts at the boring site. The clean fluvial sands form the best quality reservoir facies. The combined fluvial and estuarine fill in this broad evacuation feature equates to approximately 9% of the volume of underlying delta clinoform material. Delta reservoir facies have been removed where overridden by widespread fluvial scour on the middle shelf. Fluvial reworking during sea level lowering thus results in poor preservation potential of preceding highstand-delta deposits on the middle and inner shelf. This reworking provides remobilized sand-rich sediment for shelf-edge delta co struction.
The fluvial deposits in the boring are capped by the bayline flooding surface, which is in turn overlain by estuarine deposits, the transgressive ravinement surface, and finally, submarine shoal and thin sheet deposits. The estuarine and shoal deposits form the transgressive systems tract. Submarine shoals are sandy, but contain abundant shell material. Shoals are of the poorest quality, and volumetrically least important, reservoir facies.
AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.