GEOCHEMISTRY OF ATOKA SANDSTONE AND MUDROCK FROM THE SOUTHERN OUACHITAS OF OKLAHOMA: IMPLICATION FOR PROVENANCE

Atoka Formation consists of sandstone and mudstone deposited in the rapidly subsiding Carboniferous Ouachita trough, formed during the final stages of Rheic Ocean closure. Mudrock geochemistry, identification and geochemical analysis of heavy minerals, and mineralogical study of mudrock and sandstone are key for the provenance analysis of the Atoka Formation to evaluate the possible plate tectonic configuration between southern Laurentia and Gondwana during the closure of the Rheic Ocean.

From preliminary results, geochemistry of mudrocks from Boktukola syncline suggests both mafic and felsic igneous provenance. Plots of REE vs Al₂O₃ and SiO₂ imply that LREE are associated with clays either by absorption or ultrafine heavy minerals. REE trends are consistent with both mafic and felsic sources. Environment of deposition is changing from oxic to dysoxic to suboxic toward younger strata indicated by transition element ratios [V/(V+Ni) vs Ni/Co, V/Cr vs Ni/Co, and Mo vs Ni/Co]. Decreasing pattern of CIA is observed towards younger strata (from CIA 51.15 to 83.26). Cross-plot of CIA vs. ICV also indicates mixing of mafic and felsic sources. Plot of Zr/Sc vs Th/Sc shows stratigraphically Zr/Sc is increasing more rapidly than Th/Sc, which indicates zircon enrichment towards younger strata. Plot of Cr/V vs Y/Ni indicates possible ultramafic source. These results will be further evaluated using trace element analysis of zircon, rutile, magnetite and garnet detrital grains.

Sandstone framework composition varies from subarkose to sublitharenite to quartz arenite, with altered framework grain restoration. Lithic grains include metamorphic, volcanic, and lesser sedimentary lithic. Heavy mineral fraction includes zircon (variety of shapes, angularity and color) as dominant grains, with significant amounts of magnetite in some sandstones, and minor tourmaline, rutile, and garnet (ugrandite group grains). Sandstone provenance indicates a predominantly felsic igneous source with lesser contributions from metamorphic and recycled sedimentary sources. By contrast, mudrocks indicate mixed felsic and mafic igneous sources with lesser contribution from recycled sedimentary sources. Such apparent provenance differences are explained by proximity to source and/or susceptibility to chemical weathering.