IMPACT OF ASTRONOMICALLY-DRIVEN CLIMATIC AND OCEANOGRAPHIC CYCLICITY ON MICROBIAL CARBONATE SEDIMENT PRODUCTION AND FACIES DISTRIBUTION IN A SEMI-RESTRICTED BASIN: UPPER JURASSIC SMACKOVER FORMATION, ALABAMA

The Upper Jurassic Smackover Formation in the semi-restricted to restricted Conecuh Embayment of southwestern Alabama consists of three distinct carbonate factory types (microbial, skeletal, and abiotic) that provide a genetic record of sediment production and deposition in a mixed carbonate-evaporite-siliciclastic system. Sedimentary and geochemical proxy data from analysis of 29 cores indicate that the lower, transgressive interval of the Smackover Formation in the Conecuh Embayment consists of microbially dominated sediment that accumulated in an oxygen-depleted and nutrient-enriched environment, possibly associated with regional upwelling and propagation of internal waves. The upper, regressive Smackover Formation consists of microbial and oolitic facies in the more restricted part of the embayment and a cyclic repetition of two distinct associations of depositional and geochemical facies variants in the less restricted part of the basin. Cyclicity in the regressive Smackover likely records pulses of riverine water from bay-head deltas controlled by dry-wet climate cycles and supports the inference of high-frequency climatic cyclicity in the equatorial Late Jurassic. Time-series analysis of terrestrial influx proxies (titanium and aluminum) from a 90 m section of slabbed core in southwest Alabama demonstrates the occurrence of precession-scale cyclicity, supporting and extending previous observations on monsoonal conditions during the Middle and Late Jurassic. The climatic cycles of the Smackover likely resulted in a pronounced alternation of intense weathering and river discharge with a dry climate and enhanced evaporation. The periodic sediment influx and freshwater discharge into embayed, restricted environments played a major role in the Smackover's lateral and vertical geochemical and sedimentary heterogeneity and in the deposition of slope-centered microbial carbonates.

Extension of results from this study should improve understanding of the influence of sea-water controls on microbial carbonate sediment production and accumulation, thereby enhancing the predictability of facies distribution in analogous settings.