IMPACT OF SEA-WATER CHEMISTRY ON CARBONATE SEDIMENT PRODUCTION AND STRATIGRAPHIC ARCHITECTURE OF PROGRADATIONAL MICROBIAL-OOLITIC CARBONATE MARGINS

Carbonate factories are characterized by specific depth ranges where carbonate sediment is generated from either biotic or abiotic processes and varies in composition and production rates. The type of carbonate factory is controlled by the oceanographic setting and impacts the facies architecture through a complex interaction with several space- and time-dependent variables.

Two analogous sedimentary systems are compared herein, the Lower Triassic Yelang and Daye Formations of the Nanpanjiang Basin in south China and the Upper Jurassic Smackover Formation of the Eastern U.S. Gulf Coast, both consisting of prograding carbonate shelves with microbialite-dominated slopes and oolitic margins. The Lower Triassic example displays a complete and continuous exposure through interior, margin, and slope depositional environments, providing exceptional information on the facies architecture. The Upper Jurassic example is an exclusively subcropping unit that has been documented through a wealth of log, core, and seismic data.

Integrated sedimentary and geochemical proxy data indicate that carbonate sediment production and accumulation in microbial-oolitic carbonate systems occur in semi-restricted basins and are controlled by changes in ocean circulation and variations in the chemical and physical stratification of the water column. The spatial and temporal distribution of carbonate factories in both examples indicates a strong oceanographic control on the location and type of carbonate sediment production and ultimately on the distribution of depositional facies. Microbial carbonates in slope environments form in dysoxic and mesotrophic to eutrophic conditions, whereas skeletal and abiotic production prevails in more oxygenated and nutrient-depleted environments with moderate terrestrial input.

Numerical modeling indicates that microbialite production on the slope adds to the volume and the stability of the slope through in-situ growth of stabilized material and is responsible for enhanced progradation of the shelf. Increased oolite production rates do not impact the large-scale architecture of the shelf because the systems are accommodation limited and any excessive production is either shed basinward or accumulated locally to form islands.