Abstract

To test the influence that carboniferous graywacke sandstones of the Ouachita Mountains and Black Warrior basin represent deposition in a linked dispersal system analogous in tectonic setting to the modern Ganges delta-Bengal fan system, sandstones from the two areas were compared petrographically. Two point counts, a standard QFL count and a special count of lithic grains, made for each of 24 selected samples, half from the Ouachitas and half from the Black Warrior basin, reveal that sandstones from both areas are rich in quartz and metasedimentary lithic grains, but poor in feldspar and volcanic lithic grains. QFL plots show that Ouachita rocks have quartz percentages consistently higher than coeval Black Warrior basin sandstones because of transport-related attrition of lithic grains and/or contributions of craton-derived, quartz-rich detritus to the Ouachita sandstones. However, Ouachita and Black Warrior basin samples cluster tightly together on triangular plots of polycrystalline quartz grain (Q p ), aphanitic sedimentary/metasedimentary grains (L s ), and volcanic/metavolcanic grains (L v ). Moreover, the lithic populations of the two sample suites are indistinguishable in detail. Eight lithic grain types, mostly metamorphic, can be recognized in point-counting, although care must be taken to discriminate between true matrix and pseudomatrix consisting of deformed, fine-grained lithic fragments. We conclude that graywackes of the Ouachita Mountains and the Black Warrior basin had a common sedimentary/metasedimentary provenance, to the exclusion of significant igneous sources. This conclusion is compatible with the continental-collision model by which a sedimentary/metasedimentary terrane uplifted along a suture belt supplied sediment dispersed longitudinally through alluvial and deltaic systems to depositional sites in a remnant ocean basin. When plotted on QFL, Q p L v L s and other diagrams, Ouachita-Black Warrior sandstones and more arkosic sandstones from known arc-related settings form two distinct fields that reflect basic differences in provenance. The data thus appear to underscore the importance of collisional settings for the development of thick lithic-rich graywacke sequences of non-volcanic derivation.

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