ABSTRACT

Detailed petrographic studies were conducted to determine the compositional variations and the influences of tectonics on the deposition of the Pennsylvanian Atokan sediments in Arkansas. The rocks have a composition of Q_82-90F_3-10R_5-19 and are classified as sublitharenites. Quartz is dominated by monocrystalline types (82-71%) with minor amounts of polycrystalline variety (3-7%). Feldspars constitute a minor proportion (K-feldspar 2-5%; plagioclase feldspar 0-4%); and lithics (5-12%) are composed of metamorphic and sedimentary fragments. Paleocurrent data and high percentage of quartz suggest that the sediments were primarily derived from the northern craton. However, metamorphic and sedimentary rock fragments imply the Appalachians as a secondary source area. Point counting via the Gazzi-Dickinson technique indicates that all rocks are of a recycled orogen while the sandstone composition shows cratonic derivation. The high quartz content, and a dearth of feldspars indicate that warm and humid climate and low relief extensively altered the primary mineralogic signatures in the source area.

Compositional patterns of the Late Paleozoic sandstones with known tectonic origins suggest that the Dickinsonian diagrams are, in this case, insensitive to the true regional tectono-sedimentary scenario. Compositional variations and paleocurrent data suggest that the lithic and feldspathic Stanley sandstones were derived from the uplifted passive margin/craton which lay to the south of the Ouachita basin, while the more quartzose sandstones of the Jackfork and the Atoka were derived from the northern craton. In spite of deposition in a foreland basin adjacent to a thrust belt, the Stanley sandstone compositions plot on the continental block provenance, the Jackfork sandstones cluster around apex Q indicating a cratonic block provenance, and the Atoka sandstones with northern cratonic derivation plot on the recycled orogen field.