Bloomsburg Formation
Encyclopedia
The Silurian
Bloomsburg Formation (Sb) 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. Early American geologists theorized that these beds correlated with Old Red Sandstone
found in Scotland
.
, argillaceous siltstone
, shale
, and very fine to fine-grained sandstone
, arranged in fining-upward cycles. Although the Bloomsburg can sometimes be green to gray, its most dominant color is red and they are often called "The Bloomsburg Red Beds". This is significant in the Appalachian Basin since it marks the first large scale terrestrial collection of sediments in the rock record. Most of the sediments that were deposited before this time were marine, or a small scale terrestrial zone.
In New Jersey, the Bloomsburg Red Beds were previously called the "High Falls Shale" or "High Falls Formation".
of the Bloomsburg was mostly terrestrial or shallow marine deposits resulting in a molasse
sequence produced by the Taconic orogeny
. The red color is often evidence that the sediments were deposited in arid or oxidizing
conditions. The upper parts of the Bloomsburg are thought to be a marine transition zone.
fossils in the upper marine transitional zones.
period, being deposited between 417 to 410 (±5) million years ago. It rests conformably
a top the Clinton Group
and Shawangunk Formation
, and has a complex upper contact. In eastern Pennsylvania and New Jersey, the upper part of the Bloomsburg marks the Wallbridge Unconformity. As you move west, the Wills Creek Formation
lies conformably atop the Bloomsburg. Further west, the McKenzie Member
of the Mifflintown Formation interfingers with the Bloomsburg and eventually, the Bloomsburg pinches out into the Mifflintown.
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...
Bloomsburg Formation (Sb) is a mapped bedrock unit in 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...
, 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...
and Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
. It is named for the Town of Bloomsburg, Pennsylvania
Bloomsburg, Pennsylvania
Bloomsburg is a town in Columbia County, Pennsylvania, 40 miles southwest of Wilkes Barre along the Susquehanna River. In 1900, the population of Bloomsburg stood at 6,170; in 1910, 7,413; in 1940, 9,799, and in 1990, 12,439. The population was 14,855 at the 2010 census...
for which it was first described. The Bloomsburg marked the first occurrence of red sedimentary rocks in the Appalachian Basin. Early American geologists theorized that these beds correlated with Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
found in Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
.
Description
The Bloomsburg is defined as a grayish-red and greenish-gray claystoneClaystone
Claystone is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles ....
, argillaceous siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, 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...
, and very fine to fine-grained sandstone
Sandstone
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,...
, arranged in fining-upward cycles. Although the Bloomsburg can sometimes be green to gray, its most dominant color is red and they are often called "The Bloomsburg Red Beds". This is significant in the Appalachian Basin since it marks the first large scale terrestrial collection of sediments in the rock record. Most of the sediments that were deposited before this time were marine, or a small scale terrestrial zone.
In New Jersey, the Bloomsburg Red Beds were previously called the "High Falls Shale" or "High Falls Formation".
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 Bloomsburg was mostly terrestrial or shallow marine deposits 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 red color is often evidence that the sediments were deposited in arid or oxidizing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
conditions. The upper parts of the Bloomsburg are thought to be a marine transition zone.
Fossils
There are numerous fossils found in the Bloomsburg, mostly in the upper parts of the formation. There are trace fossils of early land plants and brachiopodBrachiopod
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...
fossils in the upper marine transitional zones.
Age
Relative age dating of the Bloomsburg places it in the Upper 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 417 to 410 (±5) million years ago. It rests conformably
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...
a top the 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...
and Shawangunk Formation
Shawangunk Formation
The Silurian Shawangunk Formation is a mapped bedrock unit in eastern Pennsylvania, New Jersey, and New York. It is named for the Shawangunk Ridge for which it is the dominant rock type. The division of the Shawangunk between the Tuscarora Formation and Clinton Group has not been conclusively...
, and has a complex upper contact. In eastern Pennsylvania and New Jersey, the upper part of the Bloomsburg marks the Wallbridge Unconformity. As you move west, the Wills Creek Formation
Wills Creek Formation
The Silurian Wills Creek Formation is a mapped bedrock unit in Pennsylvania, Maryland, Virginia, and West Virginia.-Description:The Wills Creek is defined as a moderately well bedded greenish-gray shale containing local limestone and sandstone zones, or more specifically as an olive to...
lies conformably atop the Bloomsburg. Further west, the McKenzie Member
McKenzie Formation
-Description:The McKenzie Formation is a Gray, thin-bedded shale and argillaceous limestone, with interbedded red sandstone and shale in its eastern extents in Maryland. The thickness ranges from 160 to 380 feet.-Age:...
of the Mifflintown Formation interfingers with the Bloomsburg and eventually, the Bloomsburg pinches out into the Mifflintown.