Upper Jurassic and Lower Cretaceous Thrombolite and Coral-Thrombolite Buildup Development in Shelf to Ramp Settings in the Northern Gulf of Mexico

Ernest A. Mancini¹, William C. Parcell², and Juan Carlos Llinás^1
1 University of Alabama, Tuscaloosa, AL
² Wichita State University, Wichita, KS

Although microbial buildups are found throughout the geologic record, thrombolites are particularly abundant in the Late Jurassic and Early Cretaceous in the northern Gulf of Mexico. These thrombolite buildups occurred in shallow to deeper water shelf to ramp settings creating topographic relief. Development of thrombolite and thrombolite-coral buildups shows correspondence to times of rising sea level. These thrombolites were formed by eurytopic organisms, which were not restricted by water depth, salinity, temperature, light penetration, oxygen content or nutrient supply. These opportunistic organisms required a hard substrate for nucleation and zero to low background sedimentation rates for initial development. Continued growth was dependent upon lower energy settings, low to moderate sedimentation rates, and eurytopic environmental conditions. Thrombolite buildups were predominated by calcimicrobes and encrusters. With a continued reduction in the rate of sea-level rise and resulting paleoenvironmental stabilization, corals colonized the area of thrombolite development resulting in a reduction in thrombolite growth. Microbial crusts were present but constituted a smaller percentage of the overall assemblage. The demise of the thrombolite and coral-thrombolite buildups corresponds with changes in paleoenvironmental conditions associated with increased energy and sediment supply during an overall regression of the sea.

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