Shales belonging to the Carboniferous flysch exposed in the Ouachita Mountains of west-central Arkansas and southeastern Oklahoma have undergone postdepositional alteration that has significantly affected their whole-rock chemistry. Alteration is particularly pronounced in the northeastern Ouachitas and along the Benton-Broken Bow uplift. Altered rocks have lost Ca, Mg, K, Na, Fe, and Si relative to the “conservative” elements Al, Ti, Zr, Cr, and Ni. Some evidence exists for slight loss of Al and Ti relative to Cr, Ni, and Zr. Pyrophyllite- and chloritoid-bearing samples are enriched in 18O relative to less altered samples, and this enrichment is not due to changes in the δ18O of quartz.
Mineralogical and textural evidence support a postdepositional alteration model rather than mixing of sediment from different sources, or sedimentary sorting, to account for the variations in whole-rock chemistry. Development of microscopic foliation-parallel zones enriched in phyllosilicates and in Ti and Al is related to slaty cleavage development and suggests that chemical alteration is coeval with deformation and very low grade metamorphism. Volume-loss calculations based on conservation of “immobile” elements, and on deformation of detrital mica grains, give minimum estimates of ≈30%–50% volume loss in the most altered shales.
The postdepositional changes recorded in whole-rock chemistry of these shales may be unique to the Ouachita flysch, but suggest a need for greater caution in the uncritical use of shale chemistry in studies of crustal evolution and provenance.