The Ordovician Juniata Formation is a mapped bedrock unit in Pennsylvania , West Virginia , Virginia, Tennessee, and Maryland . It is a relative slope-former occurring between the two prominent ridge-forming sandstone units: the Tuscarora Formation and the Bald Eagle Formation in the Appalachian Mountains .
8-574: The Juniata is defined as a grayish-red to greenish-gray, thin- to thick-bedded siltstone , shale , and very fine to medium-grained crossbedded sandstone or subgraywacke and protoquartzite with interbedded conglomerate. The Juniata is a lateral equivalent of the Queenston Shale in western Pennsylvania . The depositional environment of the Juniata has always been interpreted as mostly terrestrial or shallow marine deposits resulting in
16-733: A molasse sequence produced by the Taconic orogeny . Very few fossils exist in the Juniata Formation, but different types of trace fossils such as tracks and burrows can commonly be found. Relative age dating of the Juniata places it in the Upper Ordovician period, being deposited between 488.3 and 443.7 (±10) million years ago. It rests conformably atop the Bald Eagle Formation in Pennsylvania and
24-401: A higher propensity for containing a significant clay fraction. Although often mistaken for a shale , siltstone lacks the laminations and fissility along horizontal lines which are typical of shale. Siltstones may contain concretions . Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding. Siltstone
32-798: Is an unusual rock, in which most of the silt grains are made of quartz . The origin of quartz silt has been a topic of much research and debate. Some quartz silt likely has its origin in fine-grained foliated metamorphic rock, while much marine silt is likely biogenic, but most quartz sediments come from granitic rocks in which quartz grains are much larger than quartz silt. Highly energetic processes are required to break these grains down to silt size. Among proposed mechanism are glacial grinding; weathering in cold, tectonically active mountain ranges; normal weathering, particularly in tropical regions; and formation in hot desert environments by salt weathering. Siltstones form in relatively quiet depositional environments where fine particles can settle out of
40-401: Is composed mostly of silt . It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility . Although its permeability and porosity is relatively low, siltstone is sometimes a tight gas reservoir rock, an unconventional reservoir for natural gas that requires hydraulic fracturing for economic gas production. Siltstone
48-599: The Martinsburg Formation in Maryland, and conformably below the Tuscarora Formation . The Juniata is a good source of road material, riprap and building stone. This article about a specific stratigraphic formation in Maryland is a stub . You can help Misplaced Pages by expanding it . Siltstone Siltstone , also known as aleurolite , is a clastic sedimentary rock that
56-557: The clay and silt fraction is composed of silt-sized particles. Silt is defined as grains 2–62 μm in diameter, or 4 to 8 on the Krumbein phi (φ) scale . An alternate definition is that siltstone is any sedimentary rock containing 50% or more of silt-sized particles. Siltstones can be distinguished from claystone in the field by chewing a small sample; claystone feels smooth while siltstone feels gritty. Siltstones differ significantly from sandstones due to their smaller pores and
64-419: Was prized in ancient Egypt for manufacturing statuary and cosmetic palettes . The siltstone quarried at Wadi Hammamat was a hard, fine-grained siltstone that resisted flaking and was almost ideal for such uses. There is not complete agreement on the definition of siltstone. One definition is that siltstone is mudrock ( clastic sedimentary rock containing at least 50% clay and silt) in which at least 2/3 of
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