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Petrology of Some Pennsylvanian Sedimentary Rocks
John C. Ferm
The middle Allegheny Kittanning Formation near Brookville, Pennsylvania is composed mainly of detrital rocks--sandstone, siltstone, and claystone, with minor volumes of coal, limestone, and ironstone. Detrital constituents are quartz (approximately 25%), micaceous metamorphic and sedimentary rock fragments, coarse micas and clays (about 70%), and miscellaneous silicates totalling about 5%. The average grain size is medium silt.
Sandstones and coarse siltstones can be classified into four descriptive-genetic groups based on quartz content and color relative to grain size and on primary structure and fossil content. Alluvial channel sandstone and coarse siltstone are characterized by relatively low quartz content, intermediate relative color, well-developed tabular-lenticular stratification, and a "large stem" flora. In contrast, beach sandstone and siltstone are relatively quartz rich and, excepting for possible worm burrows, are without distinctive fossils. Coarse grained lacustrine deposits have properties intermediate between those of channels and beaches. Lagoonal sandstone and siltstone are distinctive because of their relatively dark color and low quartz content. They also la k stratification and contain a marine fauna.
Varieties of finer grained sediments are distinguished by relative color and by structure and fossil content. Fine grained alluvial sediments are relatively light colored, generally contain roots or upright stems and often lack stratification. Offshore lacustrine, brackish, and marine deposits are differentiated mainly by fossil content.
Both fine and coarse grained sediments of similar genetic type occur in stratigraphic groups of considerable areal extent and reflect two times of marine transgression interrupted by an episode of fluvial-lacustrine deposition.
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