Core-Based Evidence for Sandy Slump and Sandy Debris Flow Facies in the Pliocene and Pleistocene of the Gulf of Mexico: Implications for Submarine Fan Models

G. Shanmugam and Grant Zimbrick

Examination of nearly 3,500 feet of conventional core from Pliocene and Pleistocene deep-water reservoirs cored in 25 wells in 8 different areas covering the eastern, central and western Gulf of Mexico reveals that the reservoirs are predominantly composed of mass- transport deposits, mainly sandy slumps and sandy debris flows (60-100% of cored intervals). Bottom-current reworked sands are common (10-50%). Of importance to existing submarine fan models is that turbidites are extremely rare (<1 % of all cores). Sedimentary features indicative of slump and debris-flow origin include sand units with sharp upper contacts, slump folds, discordant, steeply dipping layers (up to 60°), glide planes, shear zones, brecciated clasts, rafted mudstone clasts, planar clast fabr c, inverse grading of clasts, and moderate-to- high matrix content (5-20 %).

These reservoirs have been interpreted by others to represent turbidite-dominated basin-floor fans and slope fans of the often used sequence stratigraphic model. However, our core data do not show a dominance of turbidites. Sandy debris flows exhibit a variety of log motifs (e.g., blocky, fining-up, and coarsening-up) due to changes in concentration of mudstone clasts, and a variety of internal seismic facies (e.g.. parallel-continuous, irregular-discontinuous, chaotic-discontinuous, and lateral pinch out) perhaps due to changes in stacking patterns of debris flows and slumps. Classic submarine-fan models, commonly advocated for these reservoirs, may not be appropriate. We propose a slump and debris-flow dominated slope model in which sea-floor topography and depositional freezing (i. ., plastic flows) control sand distribution and geometry. Contrary to popular belief, sandy debris flows can be thick, areally extensive, and excellent reservoirs.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California