Controls on Carbonate Factory Distribution in a Restricted Intraplatform Basin: Upper Jurassic Smackover, Southwest Alabama
This study documents the stratigraphy and depositional facies of the Upper Jurassic Smackover Formation in the Manila Basin, southwest Alabama, and examines the carbonate factories responsible for sediment production. Four wells with associated cores and log data were analyzed, alongside geochemical proxy concentrations measured via X-ray Fluorescence (XRF) and Laser Induced Breakdown Spectroscopy (LIBS). These proxies were grouped into redox, terrestrial, and nutrient to assess how oceanographic conditions influence carbonate sediment production and accumulation.The transgressive Lower Smackover is dominated by microbially produced lime mudstone, with minor microbialite and peloidal packstone. A restricted marine environment led to low oxygen conditions, with high nutrient levels due to internal waves or regional upwelling. Although the contribution of terrestrial nutrients was likely minimal, nutrients transported via fluvial systems accumulated over time within a geographically-restricted basin.The regressive Upper Smackover documents the progradation of a deltaic system and oolitic shoals into the basin. The distal slope of the prograding system is primarily composed of microbialite while the shallower, mid slope is dominated by peloidal packstone, recording the transition from a microbial carbonate factory at depth to a skeletal carbonate factory. The shallowest part of the system consists of oolitic grainstone and mature sandstone, recording high saturation in the photic zone. Effective wave agitation and current flushing of nutrients contributed to supersaturation and the abiotic precipitation of oolites in shallow, near-shore environments. Alternating wet and dry climates resulted in high variability, albeit with low averages, of both nutrients and terrestrial inputs. This study reveals a strong correlation between water chemistry and carbonate factory types. Microbial factories operated in dysoxic, nutrient-rich environments, while skeletal factories thrived in well-oxygenated environments with moderate nutrients. Abiotic factories were constrained to very shallow depths, where wave agitation and light penetration enhanced supersaturation. Comparison with the Conecuh Embayment indicates similar depositional conditions, though oxygen levels were lower, in the Manila Basin, likely due to a greater geographic restriction.Overall, this study enhances the understanding of how oceanographic conditions control sediment production distribution in restricted marine environments. These findings can be applied in analogous settings, and aid in hydrocarbon exploration.
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