ABSTRACT

Ninety-three electric logs were used to analyze deep-seated sand bodies in two fields in the Hackberry embayment of southeastern Texas and the Houma embayment of southern Louisiana. The geometry and occurrence of these sand bodies can be related to the regional structural and sea level history of the study areas. The Hackberry embayment originated by shelfedge retreat after a major drop in sea level. Large amounts of sediment were then supplied by the Frio delta systems that built out to the shelf margin. The Houma embayment originated during a slight increase in the rate of sea level rise, while the Miocene deltaic systems built out over the continental shelf, triggering mass movement on the continental slope.

Both areas are characterized by large-scale synsedimentary faulting and salt tectonics. Large arcuate growth faults form the updip boundary of each embayment. Displacement across faults is from 900 to 2000 feet (300-610 m). In both embayments, the sand bodies are overlain by thick shale masses which contain similar "flysch-type" faunas. The sands have a blocky appearance on the SP curves of electric logs, are slightly more than 100 feet (30 m) thick, and can be correlated only for a few miles. The structures show typical rollover, or reverse drag, into the main growth faults. In the Hackberry embayment, it can be shown that some intervals on the expanded downthrown side of the fault contain additional section, while marker beds below and above continue across the fault. This expansion developed during a time interval represented by 400 feet (120 m) of section and are interpreted as erosional and depositional events across the growth fault.

Comparison with present-day large-scale rotational slumps on the continental slope off the Mississippi Delta shows that the overall characteristics of the Tertiary sands are very similar to such modern slump masses; therefore, these sands are interpreted as having originated in a similar fashion. Thus, it appears that these analogous sand bodies accumulated as a result of comparable processes, even though the associated changes in sea level in the two areas differed considerably.