Shawangunk Formation
Encyclopedia
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
Shawangunk Formation is a mapped bedrock unit in eastern Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
, New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
, and New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
. It is named for the Shawangunk Ridge
Shawangunk Ridge
The Shawangunk Ridge , also known as the Shawangunk Mountains or The Gunks, is a ridge of bedrock in Ulster County, Sullivan County and Orange County in the state of New York, extending from the northernmost point of New Jersey to the Catskill Mountains.The ridgetop, which widens considerably at...
for which it is the dominant rock type. The division of the Shawangunk between the Tuscarora Formation
Tuscarora Formation
The Silurian Tuscarora Formation — also known as Tuscarora Sandstone or Tuscarora Quartzite — is a mapped bedrock unit in Pennsylvania, Maryland, West Virginia, and Virginia.-Description:...
and Clinton Group
Clinton Group
The Silurian Clinton Group is a mapped bedrock unit in New York, Pennsylvania, Maryland, and West Virginia. It contains the Rochester Shale, Rose Hill Formation and the ridge-forming Keefer Formation...
has not been conclusively determined. The shift of nomenclature currently has the divide between Hawk Mountain
Hawk Mountain
Hawk Mountain is a mountain ridge, part of the Blue Mountain Ridge in the Appalachian Mountain chain, located in central-eastern Pennsylvania near Reading and Allentown. The area includes 13,000 acres of protected private and public land, including the 2,600 acre Hawk Mountain Sanctuary.The River...
and Lehigh Gap.
Description
The Shawangunk is defined as a light to dark-gray, fine to very coarsed grained sandstoneSandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
and conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, containing a few shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
interbeds. There are also four members of this formation: Tammany, Lizard Creek, Minsi, and Weiders.
The Lizard Creek member is described at Lehigh Gap as having conglomerates, sandstone, calcerious sandstone, siltstone, and shale with a few minor red beds. The Minsi Member is described as a congloeritic quartize and the Weiders is described as both a conglomerate and quartzite. The Tammany and Lizard Creek members are equivalent to the Clinton Formation and the Minsi and Weiders members are equivalent to the Tuscarora Formation in the central and western part of Pennsylvania. The Tammany Member was not described at Lehigh Gap.
Depositional environment
The depositional environmentSedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...
of the Shawangunk has always been intrepreted as mostly terrestrial to a shallow marine deposit resulting in a molasse
Molasse
The term "molasse" refers to the sandstones, shales and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse is deposited in a foreland basin, especially on top of flysch, for example that left from the rising Alps, or erosion in the Himalaya...
sequence produced by the Taconic orogeny
Taconic orogeny
The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the United States...
. The numerous coarse-grained beds, especially in the lower part, suggest a relatively close orogenic source with very high to high energy fluvial systems depositing the sediments. Shale interbeds may indicate a slowing or shifting of these ancient river systems.
Fossils
Very few fossils exist in the Shawangunk; however, EurypteridEurypterid
Eurypterids are an extinct group of arthropods related to arachnids which include the largest known arthropods that ever lived. They are members of the extinct order Eurypterida ; which is the most diverse Paleozoic chelicerate order in terms of species...
s can be found in the middle portions of the formation roughly 420–750 feet from the base in some locations in New York State. Linguloid brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s were identified in the Lizard Creek Member.
The ichnofossil Arthrophycus was described in the Minsi Member in the area of Lehigh Gap. These were most likely made by polychaete worms
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
of the family Terebellida
Terebellida
Terebellida make up a suborder of the Polychaeta class, commonly referred to as "bristle worms". Together with the Sabellida, the Spionida and some enigmatic families of unclear taxonomic relationship , they make up the order Canalipalpata, one of the three main clades of polychaetes...
.
Age
Relative age dating of the Shawangunk places it in the Lower SilurianSilurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
period, being deposited between 440 to 417 (±10) million years ago. It rests unconformably
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
above the Martinsburg Formation
Martinsburg Formation
The Ordovician Martinsburg Formation is a mapped bedrock unit in Pennsylvania, New Jersey, Maryland, Virginia, and West Virginia. It is named for the town of Martinsburg, West Virginia for which it was first described...
and conformably below the Bloomsburg Formation
Bloomsburg Formation
The Silurian Bloomsburg Formation is a mapped bedrock unit in Pennsylvania, New Jersey, New York and Maryland. It is named for the Town of Bloomsburg, Pennsylvania for which it was first described. The Bloomsburg marked the first occurrence of red sedimentary rocks in the Appalachian Basin...
.
Economic Uses
The hardness of the quartzite and conglomerate layers make the Shawangunk an excellent building stone. Some of the boulders that have accumulated as colluviumColluvium
Colluvium is the name for loose bodies of sediment that have been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope, transported by gravity. The deposits that collect at the foot of a steep slope or cliff are also known by the same name. Colluvium often...
at the base of Blure Mountain are used for decrotive stone, building stone, and aggrigate.
See also
- Geology of PennsylvaniaGeology of PennsylvaniaThe Geology of Pennsylvania consists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economic advantages and geologic hazards and play an important role in shaping everyday life in the state...
- Geology of New York
- Geology of New JerseyGeology of New JerseyThe Geology of New Jersey consists of four distinct physiographic provinces. They are: the Atlantic Coastal Plain Province, the Piedmont Province, the Highlands Province, and the Ridge and Valley Province.-Coastal Plain:...
- Blue Mountain (Pennsylvania)Blue Mountain (Pennsylvania)Blue Mountain is a ridge that forms the eastern edge of the Appalachian mountain range in the U.S. state of Pennsylvania. It cuts across the eastern half of the state from New Jersey to Maryland, providing a distinct boundary between a number of Pennsylvania's geographical and cultural regions...
- Shawangunk RidgeShawangunk RidgeThe Shawangunk Ridge , also known as the Shawangunk Mountains or The Gunks, is a ridge of bedrock in Ulster County, Sullivan County and Orange County in the state of New York, extending from the northernmost point of New Jersey to the Catskill Mountains.The ridgetop, which widens considerably at...
- Lehigh Gap, PennsylvaniaLehigh Gap, PennsylvaniaThe Lehigh Gap, located in Lower Towamensing Township, Carbon County, Pennsylvania, is a water gap formed by the Lehigh River where it cuts through Blue Mountain in the state of Pennsylvania in the United States. The gap allows easy travel between Carbon County to the north and Lehigh and...