Shelf Sand Body Distribution and Transgressive Systems Tract Development on a Low Energy Shelf: An Example from the Alabama Continental Shelf

Louis R. Bartek, Robert W. Wellner, Tonja L. Young

Approximately 2,125 km of high-resolution (<1 m) Geopulse seismic reflection data were collected within a 900 km² grid that extends from ~5 km offshore of the Alabama coast to the upper continental slope. The Alabama shelf is characterized by a low sediment supply, relative to the Louisiana and Texas shelves, moderate to light storm activity, and a microtidal hydrodynamic regime. Therefore, for our purpose of analyzing process driven facies distributions within a sequence stratigraphic context, this area is considered a typical low energy, low sediment supply "end-member" type setting.

Preliminary analysis of the seismic data reveals that high amplitude, high frequency, and laterally continuous, seafloor parallel seismic reflections lie above the late Wisconsin (oxygen isotope stage 2) sequence boundary and transgressive surface. Assuming a sediment velocity of 1,550 m/s, this facies ranges from 1 to 5 m in thickness. Other investigators have documented that this facies is characterized by a sandy lithofacies. Between water depths of ~30-40 m there is an abrupt transition to a high amplitude, low-frequency, reverbatory facies immediately below the water bottom. This facies is confined to a narrow east-west oriented belt across nearly the entire Alabama shelf. The reverbatory nature of this facies indicates the presence of a relatively hard water bottom within this b lt. Carbonate-cemented sandstone hardgrounds have been documented in this area and we believe that they are much more extensive than previously reported.

The transgressive systems tract offshore of Alabama overlies a highly irregular, often channelized surface that we interpret as the Late Wisconsin exposure surface (sequence boundary). A wide range of seismic facies have been identified beneath this surface, and in general, are similar to those observed on other margins (e.g. Texas Shelf, East China Sea). Statistical analysis of horizontal and vertical facies changes and their relationship to sequence stratigraphic surfaces are presently being conducted to determine the minimum amount of data necessary to identify the nature and orientation of reservoir sands and seals that developed under typical low energy "end-member" type depositional conditions.

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