The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania , West Virginia , and Maryland . It is named for the North branch of the Mahantango Creek in Perry and Juniata counties in Pennsylvania. It is a member of the Hamilton Group , along with the underlying the Marcellus Formation Shale. South of Tuscarora Mountain in south central Pennsylvania, the lower members of this unit were also mapped as the Montebello Formation . Details of the type section and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database.
9-550: The Mahantango Formation is a gray, brown, and olive siltstone and shale , characterized by coarsening-upward cycles. The Mahantango Formation is typically found conformably overlying the Marcellus Formation , and underlying the Tully Limestone (where present). The Mahantango has been divided into four members: The Sherman Ridge and Fisher Ridge are ridge-formers . In south-central Pennsylvania,
18-401: 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
27-438: Is a clastic sedimentary rock that 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
36-798: 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
45-539: The Marcellus Formation shale. Its upper contact is also conformable to the Trimmers Rock Formation and Harrell Shale. In 2012, Read and Erikson reported the formation as Givetian in Virginia. The Mahantango represents a terrestrial to marine transition zone that went through many transgressive-regression sequences. The fine-grained rocks represent a shallow sea environment and accounts for many of
54-603: The Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members. There are numerous marine fossils found in the Mahantango including brachiopods , crinoids , trilobites , bivalves , and bryozoans . Atactotoechus furcatus Relative age dating of the Mahantango places it in the Middle Devonian period, being deposited between 392 and 385 (±3) million years ago. It rests conformably atop
63-557: 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
72-615: The fossils. Coarser grained sediments represent near-shore environments, beaches, or possibly delta lobes. These environments were tide-dominated and often had violent storms. The Montebello Sandstone member is an example of a storm dominated rock unit. Brachiopod fossils are scattered in massive sandstone beds throughout the rock unit, while the Sherman Ridge member is more laminated with fossilized ripple marks often indicating tidal current directions. Loyalsburg Formation Siltstone Siltstone , also known as aleurolite ,
81-419: 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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