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Seismic
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.