Black River Group

Black River Group
Stratigraphic range: Middle Ordovician to Late Ordovician Whiterockian-Mohawkian ~460–450 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mudcracked limestone (Tyrone Limestone, Black River Group, Late Ordovician; Frankfort, Kentucky)
Type	Group
Sub-units	Coboconk Formation Deicke and Millbrig bentonite layers Gull River Formation Hatter Formation (PA) Isle La Motte Limestone (NY,VT) Linden Hall Limestone (PA) Pamelia Formation (NY) Pecatonica Formation (IN) Peery Limestone (VA,WV) Plattin Formation (IN) Selby Limestone (NY) Snyder Limestone (PA,VA,WV) Ward Cove Limestone (VA,WV) Wardell Formation (VA, WV) Witten Limestone (VA,WV) Shadow Lake Formation. New York Amsterdam Limestone Waterton Limestone Lowville Formation
Underlies	Lexington Limestone and Trenton Group
Overlies	St. Paul Group and Wells Creek Formation
Lithology
Primary	Limestone
Other	Dolomite, Mudstone
Location
Region	Indiana  Kentucky  Michigan  New York  Ohio  Pennsylvania  Tennessee  West Virginia  Ontario  Quebec
Country	United States  Canada

The Black River Group is a geologic group that covers three sedimentary basins in the Eastern and Midwestern United States. These include the Appalachian Basin, Illinois Basin and the Michigan Basin. It dates back to the Late Ordovician period. It is roughly equivalent to the Platteville Group in Illinois.^[1] In Kentucky and Tennessee it is also known as the High Bridge Group. In areas where this Geologic Unit thins it is also called the Black River Formation (undifferentiated). One example of this is over the Cincinnati Arch and Findley Arch.^[2] Large parts of the Black River have been dolomized (where the parent limestone CaCO₃ has been turned into dolomite CaMg(CO₃)_2.) This happed when there was interaction of hot saline brine and the limestone. This created hydrothermal dolomites that in some areas serve as petroleum reservoirs.^[3]