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| Logan Formation | |
|---|---|
| Name | Logan Formation |
| Period | Silurian |
| Type | Formation |
| Lithology | Shale, limestone, siltstone |
| Namedfor | Logan County |
| Namedby | William H. Easton |
| Region | Ohio, Kentucky, Indiana, West Virginia, Pennsylvania, Tennessee |
| Country | United States |
Logan Formation
The Logan Formation is a Silurian stratigraphic unit recognized across parts of the Midwestern and Appalachian United States. It is significant for its fossil assemblages, stratigraphic position within the Niagaran Series and Salina Group correlations, and its role in regional stratigraphy used by state geological surveys and petroleum exploration. The unit has been studied by paleontologists, stratigraphers, and economic geologists from institutions such as the United States Geological Survey and several state geological surveys.
The Logan Formation records deposition during the middle to late Silurian and forms part of a succession that includes units correlated with the Niagara Escarpment carbonate facies and the overlying evaporitic and clastic sequences tied to the Michigan Basin and the Appalachian Basin. Regional syntheses by the United States Geological Survey and state agencies in Ohio, Indiana, Kentucky, Pennsylvania, and West Virginia place the Logan as a key marker used in correlation with the Medina Group and the Waverly Group. Historic mapping by the Ohio Geological Survey and modern subsurface frameworks inform petroleum and groundwater studies involving the Logan.
Lithologically, the Logan Formation typically comprises dark gray to black shale interbedded with thin limestones, calcareous siltstones, and local sandstone horizons. These lithologies reflect siliciclastic input and episodic carbonate production evident in cores and outcrops studied by researchers from Ohio State University and the Purdue University Department of Earth, Atmospheric, and Planetary Sciences. Stratigraphically, the Logan rests above older Silurian carbonates that correlate with the Lockport Group in areas and below units that grade into evaporites associated with the Salina Group. Biostratigraphic and chemostratigraphic studies referencing work from the Smithsonian Institution and the American Association of Petroleum Geologists have refined boundary definitions.
The Logan Formation is assigned to the Wenlock to Ludlow epochs of the Silurian Period and is constrained by conodont biostratigraphy, graptolite occurrences, and tie-ins to global Silurian stage schemes promulgated by the International Commission on Stratigraphy. Fossil assemblages include diverse brachiopods, bryozoans, crinoids, trilobites, and mollusks documented in monographs and field guides from the Paleontological Society and the National Museum of Natural History. Notable paleontologists such as Charles D. Walcott-era workers and later researchers at Yale University and the University of Cincinnati contributed collections that underpin regional taxonomic work. Graptolite zonation and conodont indices correlate Logan strata with units in the United Kingdom and Scandinavia where Silurian sequences are well-exposed.
Interpretations based on sedimentology and ichnology indicate deposition in a distal shallow-marine to offshore shelf setting influenced by relative sea-level changes linked to Silurian eustatic events documented in global syntheses by the International Union of Geological Sciences. Organic-rich shales and thin carbonates suggest intervals of reduced oxygenation and episodic carbonate-platform development analogous to environments described for the Michigan Basin and the Cincinnati Arch. Paleoecological reconstructions using assemblage data published by researchers affiliated with Harvard University and the University of Wisconsin–Madison depict communities structured around suspension feeders, with episodic benthic colonization after transgressive-regressive cycles comparable to those inferred for contemporaneous units in Ontario and New York (state).
The Logan Formation is mapped in subsurface and outcrop across portions of the Midwest and Appalachian regions, with prominent exposures in roadcuts, stream valleys, and quarries documented by the Ohio Department of Natural Resources and the Indiana Geological and Water Survey. Correlative horizons are traced into western Pennsylvania and eastern Kentucky, and subsurface logs from the Energy Information Administration and industry wells provide regional cross sections. Notable exposures that have been the subject of regional field trips include outcrops near the Cuyahoga Valley National Park area, roadside cliffs along state highways, and classic sections described in guides produced by the Geological Society of America.
Although not a major reservoir unit, the Logan Formation is important for regional hydrogeology, shale-hosted mineral occurrences, and as a marker in petroleum and natural gas exploration conducted by companies registered with the Securities and Exchange Commission. The formation's organic-rich shales have been evaluated for hydrocarbon source potential in basin modeling studies by the U.S. Energy Information Administration and industry consortia. Aggregate quarrying of interbedded limestone lenses and the role of Logan strata in influencing groundwater flow have been documented in environmental assessments prepared for the Environmental Protection Agency and state agencies.
The Logan Formation was first described and named in 19th-century regional surveys by geologists working for state geological surveys; later formalization of the name and boundaries occurred through publications by stratigraphers associated with the United States Geological Survey and academic institutions. Historical figures in early American stratigraphy, contemporaneous with the era of Josiah Whitney and later workers, contributed to lithostratigraphic frameworks that incorporated the Logan. Subsequent revisions and correlation efforts by professional organizations such as the Geological Society of America and the Paleontological Society refined its definition and place within Silurian chronostratigraphy.
Category:Geologic formations of the United States Category:Silurian System of North America