Print this pageSeismic and Sequence Stratigraphy of Late Quaternary Lowstand and Transgressive Deposition, Mobile Incised Valley System: Implications for Quantitative Prediction of Lithofacies
CABOTE, BRIAN S. and LOUIS R. BARTEK
In 1994, The University of Alabama Seismic Stratigraphy Group collected approximately 1600 km of high resolution Geopulse seismic profiles on the Alabama continental shelf. The Alabama shelf in Northeastern Gulf of Mexico is an "end member" depositional system in terms of low sediment supply, microtidal range and moderate to low storm activity. The late Quaternary and Holocene depositional history in the lower portion of the Mobile incised valley system reveals that very different conditions existed in recent history. Stage 2 (late Wisconsinan) lowstand led to bifurcation of the Mobile-Tensaw River system and yielded two major distinct incisions, an eastern lobe and a western lobe. The major incisions have very different geometries and orientations to the paleo-shoreline. These factors combined with changes in the rate of relative sea level rise result in significantly different and complex seismic facies assemblages. This warrants application of stochastic distribution models to surmise lateral and vertical seismic facies associations within each system independently. The stochastic distributions of seismic facies do indeed vary significantly between the lobes; several hypotheses exist for the stratigraphy. One of which is the western lobe of the system developed in a mixed wave-tide dominated environment as indicated by the presence of estuarine seismic facies (parallel, moderate amplitude and frequency reflectors) overlain by well developed tidal inlet facies (sigmoidal, pod-like, very high frequency reflectors). In contrast, the eastern lobe of the system developed in a tidally dominated environment as indicated by chaotic, low frequency seismic facies suggesting reworking of shoreface facies during large tidal variations amplified by incised valley geometry and orientation.