PETROGRAPHIC CHARACTERISTICS OF MAXIMUM-TRANSGRESSIVE AND REGRESSIVE DELTAIC SANDSTONES OF UPPER-PENNSYLVANIAN (VIRGILIAN) OREAD CYCLOTHEM, NE OKLAHOMA

Deltaic sandstones deposited during maximum-transgression and regression are expected to differ in composition and texture, because of different environmental conditions associated with sea-level changes. This hypothesis is tested by petrographic study of two sandstones from maximum-transgressive Heebner delta and two from regressive Elgin delta in NE Oklahoma. 300 point-counts of each sample document composition, grain size, sorting, and skewness of framework grains, porosity, type and amount of cement and matrix. The data are compared to display their similarities to interpret the controlling processes.

All sandstones are feldspathic arenite. The lower Heebner sandstone is rich in K-feldspar (Q₂₈F₆₈L₄), less sorted (moderately), finer (mean φ=2.8), less rounded (angular-subangular), more symmetrical (skewness 0.03) than the upper one (Q₄₇F₅₃L₀, moderately well sorted, φ=2.4, angular-subrounded, fine skewed 0.10). These characteristics conform with the stratigraphic stacking of the lower distal and upper proximal delta lobes, where typical upward coarsening trend and increased textural and compositional maturity are common during deltaic progradation due to increase in wave energy and closeness to the river mouth. The lower Elgin sandstone is finer (φ=3.9), better sorted (moderately well), better rounded (subangular-subrounded), near symmetrical (skewness -0.09) than the upper one (Q₃₆F₆₁L₃, fine sand φ=2.2, moderately sorted, angular to subangular, fine skewed 0.15). Field evidence indicates that the lower sandstone is wave dominated, whereas the upper river dominated, conforming to the overall progradational pattern. The differences in wave and river processes and energy regime are probably the likely causes of an upward-decreasing textural maturity between the two sandstones. Elgin sandstones are more texturally heterogeneous than Heebner sandstones, indicating more variable environmental conditions in regressive deltaic environment. Overall the deltaic sandstones of Oread Cyclothem are feldspar-rich, texturally submature. The petrographic characteristics suggest that the lithology and location of provenance (i.e. Ouachita thrustbelt to the south), transport distance, and deltaic sedimentary processes are the dominant controls, rather than sea-level changes.