DEEPWATER ATOKA SANDSTONE AND MUDROCK PROVENANCE FROM THE SOUTHERN OUACHITAS OF OKLAHOMA: IMPLICATIONS FOR RHEIC OCEAN CLOSURE

Atoka Formation in the Boktukola syncline is the southernmost and youngest section of the very rapidly subsiding Carboniferous Ouachita trough. Physical sedimentology, sandstone petrography, and mudrock geochemistry studies provide insights to the final stages (pre-inversion) of Rheic Ocean closure along the southern margin of Laurentia.

Seven repetitive retrogradational stratigraphic successions are recognized through 1400 m thick section. Each succession consists thicker Ta-b sandstones transitioning upward to thinner Ta-c sandstones, and upward decreasing sand: mud. In addition, channel-fill successions are limited to the lower successions. Very few paleocurrent indicators are available, but they indicate a north-northwest transport direction. Atoka is considered to be representative of middle submarine fan deposition.

Sandstone major framework composition varies from subarkose to sublitharenite to quartz arenite, with restoration of altered framework grains. Lithic grains include metamorphic and volcanic with lesser sedimentary rocks. Heavy mineral fraction is dominated by zircon (variety of shapes, angularity and color including distinctive volcanic elongate bipyramidal grains) with some samples including prominent magnetite component, and also include tourmaline, rutile and garnet (ugrandite group). Sandstone framework suggests a mineralogically mature to submature sand grains derived from recycled orogen provenance, as has been reported by others.

Bulk-rock geochemistry of mudrocks is considered for provenance; although work is in progress, preliminary results support mixed mafic and felsic source. Trace elements and REE show little variation and indicate admixtures of mafic and felsic sources. NASC normalized REE plots demonstrate 1.5 to 2.0 enrichment, and Eu anomaly is very slightly negative. Plots of REE vs Al₂O₃ and SiO₂ imply that LREE are associated with clays either by absorbtion or ultrafine heavy minerals. REE trends are consistent with mafic and felsic sources.

Different, contradictory plate tectonic evolution scenarios are presented by other workers. Preliminary results of this study support the convergence of a combined island arc and continental crustal block (peri-Gondwana) terrane with closure of the Rheic Ocean along southern Laurentia.