A Probability Analysis of the Distribution of Lateral and Vertical Seismic Facies Sequences within Paleoshoreline Indicators of Transgressive Systems Tract, Mobile Bay Tidal Deltas

D. S. Lewis, Louis R. Bartek

Mobile Bay, Alabama, an ancient incised valley, has a complex tidal delta system that appears to have developed and then stabilized in its present position when Holocene sea level transgression slowed or reached a relative still stand. These types of systems can be used as paleoshoreline indicators. They also have potential for becoming hydrocarbon traps, depending upon preservation of sandy facies within organic-rich estuarine or marine muds.

High-resolution Geopulse seismic reflection data were collected over the late-glacial/Holocene tidal delta complex in order to characterize the seismic facies and preservation potential of sediments. Limited core data available in the area has been used to ground-truth seismic facies and aid in defining the Late Wisconsin sequence boundary and the early Holocene flooding surface. Seismic profiles were then categorized into seismic facies based upon reflector amplitude, continuity, frequency, and orientation. Three major facies identified are: 1) high amplitude, high frequency facies with steeply inclined reflectors that are present both landward and seaward of Morgan Peninsula and are interpreted as channel migration deposits; 2) moderate to low amplitude, low frequency facies with ho izontal internal reflectors, interpreted to be back-barrier estuarine muds; 3) high amplitude, high frequency facies with horizontal internal reflectors, seaward of the barrier island complex, that are interpreted as interbedded deltaic silt and clay. Statistical analyses (a combination of Factor and Binomial Markov Process Analyses) of horizontal and vertical facies transitions and their relationship to sequence stratigraphic surfaces on seismic sections are being conducted to identify the minimum data necessary to identify the nature and orientation of reservoir sands and seals that develop in this type of environment.

The presence of near-surface Pleistocene barrier island facies in cores from Dauphin Island, Alabama (adjacent to the tidal inlet) indicates a possible pre-existing control for the location of the tidal delta system. Seismic profiles indicate westward migration of the main tidal inlet across the modern location of Morgan Peninsula; inlet migration appears to have occurred as sea level transgression slowed. The lateral accretion identified in these profiles indicates a record of multiple Late Pleistocene rapid sea level rise and still stand events is preserved within Mobile Bay.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995