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Bureau Seminar, February 15, 2013

Oceanic and Depositional Models for
Leonardian Mudrocks, Permian Basin, Texas

Link to streaming video: available 02.15.2013 at 8:55am

Dr. H. Seay Nance
Bureau of Economic Geology
Jackson School of Geosciences
Univ. of Texas at Austin

Rapid scanning X-ray fluorescence (XRF) analyses enable interpretation of mineralogical profiles in mudrock successions that are otherwise difficult to characterize. Integrated with isotope and total organic carbon (TOC) analyses these data provide clues to the depositional and ocean-chemical history of a basin and facilitate well-log calibration. Geochemical analyses were performed on three Leonardian-age mudrock-dominated cores from the Permian Basin. For two cores from the Bone Spring Formation (Delaware Basin) and one core from the Lower Leonard interval (Midland Basin) a portable XRF scanner was used to generate the analytical suite of 28 elements. TOC, δ13CTOC, and δ15N data were collected from two cores, one from each basin. Based on relationships interpreted between mineralogy (by x-ray diffraction) and elemental abundances (by XRF) six mudrock facies have been defined that reflect modeled proportions of carbonate (mainly calcite), clay minerals (mainly illite), and quartz. These facies characterizations provide a means to interpret mineral compositional profiles for these successions otherwise characterized by elemental variations that are less familiar to geologists.

The stratigraphic successions in both basins include carbonate- and siliciclastic-dominated facies. TOC ranges up to 6.4% with slightly higher values occurring in the Midland Basin. TOC content positively co-varies with Mo and clay mineral content in most samples. OC richness favors slow sedimentation rates (thus, its coincidence with elevated clay-mineral content) in combination with sufficiently reducing conditions (signified by elevated Mo concentrations). δ13CTOC (-29.4to -24.9‰), and δ15N (8.9 to 17.5‰) vary systematically with overall mineralogical variations that probably reflect sea-level changes. Lower δ13CTOC and elevated δ15N values coincide with dominance by siliciclastic facies and mark sea-level lowstands. Elevated δ15N values probably record extensive denitrification attending nutrient recycling during periods of reduced oceanic circulation and deep-sea anoxia. Carbonate-dominated facies mark sea-level highstands accompanied by increased δ13CTOC and lower 15N values, the latter which suggest somewhat reduced nutrient recycling.

The Bone Spring cores contain equivalent intervals enabling facies evaluation across a 10-km west-to-east dip transit. Updip core contained a greater proportion of carbonate facies sourced from the Central Basin Platform. Downdip core contained more quartz-dominated facies signifying closer proximity to north-south-aligned axes of siliciclastic turbidite systems sourced from uplifted areas north of the basin.

TOC richness records a balance of OM productivity, slow sedimentation, and chemical potential for OC preservation. TOC does not imply maximum reducing or productivity conditions. Comparison between cores documents that TOC content was highest during sea-level lowstands (more siliciclastics) in Delaware Basin, whereas TOC accumulation was highest during sea-level highstands (more carbonates) in Midland Basin. Discrepancies between basins are explained by increased reducing intensity (elevated Mo) in Delaware Basin when exchange with the open ocean to the west is reduced over shallow sills during sea-level lowstand; in geographically more restricted Midland Basin, however, lowstand circulation may have been too low for optimum OM productivity although preservation potential was high.



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