Composition of modern stream sand in a humid climate derived from a low-grade metamorphic and sedimentary foreland fore-thrust belt of North Georgia

Abstract

ABSTRACT Conditions of tectonics and drainage in north-central Georgia provide an excellent natural laboratory to study the composition of stream sand derived from a low-grade metamorphic and sedimentary foreland fold-thurst belt under humid climatic conditions. The streams head in a high-relief phyllite, schist, and metasandstone terrain and flow westward across a bounding fault into lower relief Paleozoic shales, cherty carbonates, and quartz arenites. The relative abundance of the rock types in the drainage basin of each sample site was determined and a source rock was calculated as the ratio of the area underlain by metamorphic rock to the total area of the drainage basin. Low-grade metamorphic rocks supply subequal amounts of monocrystalline quartz, polycrystalline quartz. and foliated quartz-mica rock fragments to stream sediment, whereas sand from the sedimentary terrain consists primarily of monocrystalline quartz, a surprisingly large number of pelitic rock fragments, and chert. Mixing of sediment from both rock types is recognizable on the quartz provenance diagram of Young (1976) by a high instability index (unstable polycrystalline quartz/total polycrystalline quartz) and a low polycrystallinity index (polycrystalline/total quartz). This diagram is especially sensitive to the presence of sandstones as rocks, which further decreases the polycrystallinity index. The ratio of metamorphic rock fragments to total rock frag ents also is directly related to the rock ratio. Modern sand derived from the foreland fold-thurst belt of north Georgia can be distinguished from Neogene subduction complex-derived sand and collision orogen-derived sand on QFL, LmLsLv, and QpLsmLvm provenance diagrams by containing less total feldspar, more total quartz, and no volcanic rock fragments. The concepts developed from the recent sand are applied to the Mississippian Parkwood Formation, Alabama, and the Cretaceous Muddy Formation, Montana. The results suggest that both sandstone units were derived from foreland fold-thrust belts but that the Parkwood area contained a greater proportion of low-grade metamorphic rocks than the area of the Muddy.