Mancini, Ernest A.¹, Juan Carlos Llinas², William C. Parcell³ 
(1) University of Alabama, Tuscaloosa, AL 
(2) Center for Sedimentary Basin Studies and Department of Geological Sciences, University of Alabama, Tuscaloosa, AL
(3) Wichita State University, Wichita, KS

ABSTRACT: Upper Jurassic Shallow Water Thrombolites from the Northeastern Gulf of Mexico

Upper Jurassic coral-microbial, thrombolite-coral, and sponge-microbial reefal buildups have been described from outcrops in carbonate shelf and inner to middle ramp settings from France, Portugal and Spain. Pure thrombolite reefal buildups, however, have only been observed in deeper-water (slope or distal ramp settings) in these surface exposures. The development of pure thrombolite buildups in deeper water is attributed to abnormal marine conditions of depleted oxygen contents or fluctuating nutrient supplies and less light penetration which serve to exclude typical reef-building metazoans and the conditions of lower energy and reduced sedimentation. In the northeastern Gulf of Mexico, pure thrombolite reefal buildups of up to 45 m in thickness have been described. These buildups were associated with updip Paleozoic paleohighs located along the margins of the Conecuh and Manila embayments. They developed on the inner portion of a carbonate ramp. The construction of pure thrombolite buildups in this shallow water setting is attributed to abnormal marine conditions of restricted ocean circulation and freshwater runoff from the adjacent Appalachian Highlands. Such a setting resulted in a lower energy regime, low background sedimentation, and fluctuating salinity conditions and nutrient supply. This setting also had the potential to produce low oxygen conditions below the sediment-water interface. These abnormal marine conditions would serve to exclude typical reef-building metazoans. To the west, along the updip margin of the Mississippi Interior Salt Basin, coral-microbial reefal buildups were common. Open ocean circulation was evident in the Late Jurassic in the Mississippi Interior Salt Basin for the construction of coral-microbial buildups.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.