The Bluestone Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the Bur ...

. It is the youngest unit of the Upper Mississippian-age

Mauch Chunk Group The Mauch Chunk Group is a geologic group in West Virginia. It preserves fossils dating back to the Carboniferous period. See also * List of fossiliferous stratigraphic units in West Virginia This article contains a list of fossil-bearing s ...

. A pronounced

unconformity An unconformity is a buried erosional or non-depositional 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 o ...

separates the upper boundary of the Bluestone Formation from

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) b ...

of the overlying Pennsylvanian-age Pocahontas Formation.

Fossils

Two formally named subunits of the Bluestone Formation, the Bramwell and Pride Shale Members, are quite fossiliferous. Plant fossils allow the top of the Bluestone Formation to be correlated with the Namurian A-Namurian B boundary, the European equivalent of the Mississippian-Pennsylvanian boundary.

Conodonts Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

belonging to the '' Adetognathus unicornis'' or lower '' Rhachistognathus muricatus'' conodont zones have been found in the Bramwell Member. This would make the Bluestone Formation the youngest occurrence of Mississippian-type conodonts in eastern North America. A diverse

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

fauna was present in both the Bluestone Formation and the older Hinton Formation. Brachiopods were also fairly diverse, though not as dominant in the ecosystem as bivalves. Among the most abundant fossils were fossilized tubes of '' Microconchus hintonensis'', a

microconchid The order Microconchida is a group of small, spirally-coiled, encrusting fossil "worm" tubes from the class Tentaculita found from the Upper Ordovician to the Middle Jurassic (Bathonian) around the world. They have lamellar calcitic shells, ...

. The Pride Shale Member has produced '' Tanypterichthys pridensis'', an endemic species of deep-bodied

palaeonisciform The Palaeonisciformes (Palaeoniscida) are an extinct order of early ray-finned fishes ( Actinopterygii). Palaeonisciformes ''sensu lato'' first appeared in the fossil record in the Late Silurian and last appeared in the Late Cretaceous. The na ...

fish.

Deposition

The Pride Shale Member of the Bluestone Formation preserves

interbedded In geology, interbedding occurs when beds (layers of rock) of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimen ...

variations in sediment coarseness. This variation is cyclical at multiple scales, with cycles from smaller than a millimeter to up to several meters in thickness. The smallest-scale cycles likely represent sediments settling out during daily

tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...

. The largest-scale cycles are tied to changes in the tides influenced by the 18.6-year lunar nodal cycle. Cycles are also observed at half-year, monthly, and half-month scales. These correspond to seasonal changes, perigee/apogee tidal variation, and neap/spring tidal variation, respectively. Together, the different cycles reconstruct a

deltaic A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...

environment near the equator which was heavily influenced by tides and a monsoonal climate. Modern equivalents would be the deltas of the

Amazon Amazon most often refers to: * Amazons, a tribe of female warriors in Greek mythology * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon River, in South America * Amazon (company), an American multinational technology c ...

Ganges The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian states." is ...

Brahmaputra The Brahmaputra is a trans-boundary river which flows through Tibet, northeast India, and Bangladesh. It is also known as the Yarlung Tsangpo in Tibetan, the Siang/Dihang River in Arunachali, Luit in Assamese, and Jamuna River in Bangla. It ...

, and

Fly Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwing ...

rivers. During glacial periods, the monsoonal climate would have mellowed out into a less seasonal tropical climate in equatorial regions.

References

Carboniferous West Virginia Carboniferous geology of Virginia Carboniferous southern paleotropical deposits Serpukhovian {{Carboniferous-stub