Coastal-Plain Incised-Valley Systems Characterized by Multiple Sequences: Examples from the Alabama Coast

Diana M. Duran, Robin Mattheus, Larry Greene, and Antonio B. Rodriguez
University of Alabama, Tuscaloosa, AL

Incised-valleys are generally categorized into piedmont and coastal-plain systems. Piedmont incised-valley systems are believed to be characterized by multiple cycles of incision and deposition, reoccupying previous valleys. Coastal-plain incised-valley systems are thought to exist through only one glacial-eustatic cycle, exhibiting a single depositional fill sequence. To test the validity of using incised-valley facies architecture as an indicator of incised-valley type, the Fish, Magnolia, Bayou LaBatre, and Fowl Incised-Valley Systems are examined via high-resolution seismic, lithologic, and radiocarbon data. These fluvial systems have small drainage basins, no greater than 388 km2, that do not extend beyond the coastal plain.

Regionally, two main surfaces are identified. The lower surface (1) is characterized by high-relief and downcuts to a maximum of 25 meters. The upper surface (2) is also characterized by high-relief, but only downcuts to a maximum of 13 meters. Both surfaces are rooted, indurated, and oxidized indicating sub-aerial exposure. Chronologic data indicate that surface 1 corresponds to oxygen isotope stage 6, and surface 2 correlates with oxygen isotope stage 2. These surfaces are also interpreted as sequence boundaries. Terraces are present along the flanks of each incised-valley system. These observations illustrate that coastal plain incised-valleys have gone through multiple stages of incision and deposition, and have reoccupied their valleys through multiple glacial-eustatic cycles. Results suggest that coastal plain and piedmont systems cannot be differentiated on fill-facies architecture alone.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005