Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Corpus Christi Geological Society and Coastal Bend Geophysical Society monthly luncheon, Corpus Christie, December 15, 2004
Sequence of the South Texas Oligocene: Understanding the Relationship Between Shale Tectonism and Lowstand Deposition
Ramon H. Trevino, L. Frank Brown, Robert G. Loucks, and Ursula Hammes
Our studies of the Oligocene (Frio) in the Corpus Christi Bay region highlight the interplay between cycles of relative sea level changes, resultant shifts in depositional loci, and mud / shale tectonism. We propose a model that can be useful to explorationists throughout the Gulf of Mexico Basin and in similar basins worldwide.
Growth-faulted subbasins along the South Texas coast exhibit complex patterns of deposition related to 3rd order (i.e., 1 to 3 million year) sea-level cycles. The relative sea level lowstands of these cycles caused basinward shifts in facies that manifested in lowstand systems tracts (LST) whose base is a sequence boundary. On the shelf, the LST comprised incised-valley fill deposits. Below the self, the LST comprised relatively deep-water, gravity flow deposits (basin-floor fans and slope fans) overlain by prograding deltaic complexes. The incised valleys extended to the shelf edge, and the fluvial systems within the incised valleys delivered their load of coarse-grained clastics to the shelf edge and slope. The relatively coarse-grained LST deposits overloaded the underlying, previously-deposited, distal muds on the slope. The loading mobilized the muds and resulted in syndepositional listric (i.e., growth) faults, which developed along the previous shelf edge and slope (landward). Basinward, the mobile muds formed mud or shales ridges, which eventually composed part of a new slope. The thickest section of the LST (i.e., with the exception of the incised valley fills) accumulated between the shale ridges and the growth faults forming intervening intraslope subbasins, which typify much of the Gulf of Mexico in general and the South Texas Oligocene in particular. The LST prograding deltaic complexes filed in the intraslope basins, forming a shallower water shelf. A subsequent rise in relative sea level flooded the shelf and associated incised valleys causing a landward shift in facies and resulting in the deposition of a transgressive system tract, TST (e.g., wave-dominated deltas, estuarine and barrier island depositional systems), and ultimately a marine condensed section (i.e. maximum flooding surface). With a decrease in the rate of relative sea level rise, highstand systems tract (HST) deltas prograded basinward over the maximum flooding surface (i.e., continental shelf).
The unconstrained TST and HST extend broadly across the shelf and are relatively easily correlated. Conversely, the key to correlating between the various intraslope subbasins is to recognize their distinct temporal nature. Each subbasin is younger than subbasins landward from it and older than those basinward. Even though each subbasin composes similar facies successions and therefore similar wireline-log facies, correlating similar facies in one subbasin to those of another crosses time lines, and is, therefore, incorrect. Furthermore, the incised valley fill is the only depositional system on the shelf that is temporally equivalent to its concomitant subbasinal lowstand systems (i.e., basin floor fan, slope fans, and prograding complex). Therefore, it is also incorrect to correlate TST and HST deltaic systems with LST deltaic systems even though the log facies may be very similar.