DEEPWATER ATOKA FORMATION MUDROCK CHEMOPROVENANCE COMPARISONS FROM NORTHERN AND SOUTHERN OUACHITAS OF OKLAHOMA: IMPLICATIONS FOR RHEIC OCEAN CLOSURE

Middle Carboniferous Atoka Formation deepwater mudrocks from northern (Laurentia proximate) and southern (Gondwana proximate) Ouachitas are examined for provenance based on bulk geochemistry and heavy minerals. In addition, Atoka results are compared to Lower Carboniferous Stanley Group mudrock geochemistry (published from others). The Carboniferous stratigraphy records sedimentation in a very rapidly subsiding basin as the Rheic Ocean closed along the southern margin of Laurentia.

Chemical index of alteration (CIA) plotted against index of chemical variability (ICV) are similar for all Atoka with CIA ranging 80-85 and ICV ranging across expected felsic to mafic values. By contrast, Stanley CIA is lower, 60-75; ICV is similar to that of the Atoka. This result suggests Carboniferous mudrocks were derived from similar terranes with contrasting chemical weathering, or Atoka mudrocks are partially derived from recycling of Stanley mudrocks. Redox sensitive trace elements all indicate oxic bottom water conditions.

Trace elements, including Zr, Th, Hf, Sc, Y, Cr, Ni and V, and REE reveal similarities and differences among Atoka and between Atoka and Stanley in terms of provenance. Atoka northern samples are highly variable indicating mixed felsic and mafic, Stanley tuff affinity, and recycled sedimentary sources. Atoka southern samples show very little variation with mixed felsic and mafic sources. Although trace element plots for Atoka and Stanley overlap, Atoka mudrocks indicate a stronger mafic influence.

Heavy mineral separates investigated in order to understand mineral hosts for trace elements and REE. This is work in progress. Survey study of Atoka northern sample separates include in declining order of abundance: zircon, rutile, apatite, Fe- and FeTi-oxides, chromite, detrital barite, monazite and xenotime. A similar suite of heavy minerals are reported from Stanley.

Contradictory plate tectonic evolution scenarios are in the literature. Preliminary results here support the convergence of an island arc-continental crustal block (peri-Gondwana) terrane with closure of the Rheic Ocean along southern Laurentia. Atoka northern and Stanley samples suggest stronger recycle sedimentary component (Laurentia). Atoka southern samples suggest stronger influence of mafic source (peri-Gondwana).