Medina Group

Consists of: Thorold Sandstone; Grimsby Formation-sandstone, shale; Power Glen and Cabot Head Shales; Whirlpool Sandstone.
State New York
Name Medina Group
Geologic age Lower Silurian
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone
Sedimentary > Clastic > Mudstone > Shale
Comments 800 ft. (250 m). Unit descriptions from USGS Lexicon website (ref. NY046) and reference NY018: The Medina Group in NY is revised to include (ascending) the Whirlpool Sandstone, Power Glen Shale, Devils Hole Sandstone (new name), Grimsby Formation, Thorold Sandstone, Cambria Shale (new name), and Kodak Sandstone. The interval containing the Thorold, Cambria, and Kodak has previously been assigned to the overlying Clinton Group. The term Medina replaces the Albion Group used previously in NY. The Medina consists of 80 to 115 ft of white, green, red, barren to moderately fossiliferous sandstone, siltstone, and shale. Unconformably overlies the Ordovician Queenston Shale and unconformably underlies the Clinton Group. The upper contact of the Medina was formerly placed by Rickard (1975) at the top of the Grimsby Formation; it is here revised as the top of the Kodak Sandstone. The Medina is of Early Silurian (Llandoverian) age. [This report uses the Early and Late Silurian time scale of Harland and others (1982).] (Brett and others, 1995). Revised the Thorold Sandstone of the Medina Group in western NY by removing the upper part of the unit and creating a new formation, the Cambria Shale. The revised Thorold consists of two laterally correlative facies: (1) to the east, between Rochester and Lockport, NY, unit is mottled pink to red, cross-bedded, channel sandstone with trace fossils, (2) in Niagara region and westward, it is a light-gray to white, massive, argillaceous, pelletal sandstone. Unit is typically interbedded with thin, green, silty, shales. Thickness is 4.5 to 10.0 feet. Overlies the Grimsby Formation with a sharp but nearly conformably contact, and conformably underlies the Cambria Shale. Between Niagara Falls, NY, and Hamilton, Ont, CAN, a low-angle unconformity occurs at the top of the unit and the Neagha Shale of the Clinton overlies the Thorold. The Thorold is of Early Silurian (Llandoverian) age (Brett and others, 1995). The Grimsby Formation of the Medina Group is revised to include the uppermost beds formerly (Rickard, 1975) included in the Power Glen Shale. The lowest beds of the revised Grimsby are named the Artpark Phosphate Bed. The Grimsby consists of interbedded red and green sandstone, siltstone, and shale, and tends to be more argillaceous toward the base. The matrix of the Artpark Phosphate Bed and the basal 5 to 10 ft of the Grimsby is typically intensely burrowed greenish-gray and maroon shale. Fossils are common in the basal beds and include pelecypods, cephalopods, ostracodes, bryozoans, and brachiopods. The remainder of the Grimsby consists of red and white mottled, fine- to medium-grained sandstone and conglomerate interbedded with shale. The ratio of sandstone to shale increases upward to the contact with the overlying Thorold Sandstone. Thickness is 56 to 72 ft. Overlies the Devils Hole Sandstone with a sharp contact, underlies the Thorold Sandstone with a sharp contact. Unit can be traced from Rochester, NY, to Hamilton, Ont, CAN. The Grimsby is of Early Silurian age, probably early Llandoverian based on correlation with the Brassfield Limestone of the Midcontinent (Brett and others, 1995). The Power Glen Shale of the Medina Group is revised to include only the lower part of the Power Glen Shale of Rickard (1975). The upper part is now considered the Devils Hole Sandstone (new) and the basal beds of the Grimsby Formation. The Power Glen consists of sparsely fossiliferous dark-gray to greenish-gray marine shale with light-gray to bluish-gray, hummocky cross-stratified to bioturbated, quartzose sandstone interbeds; also contains interbeds of dolomite, calcareous siltstone, and sandy limestone. Well exposed along Niagara Street railroad cut, Lockport, Niagara Co., NY. Thickness ranges from 20.0 to 27.5 feet. Gradationally overlies the Whirlpool Sandstone and underlies the Devils Hole Sandstone with a sharp contact. The Power Glen is of Early Silurian (Llandoverian) age (Brett and others, 1995). The Whirlpool Sandstone of the Medina Group in western NY consists of light-gray to white, medium- to coarse-grained, trough cross-bedded, quartzose sandstone with thin, dark-gray to greenish-gray shale clasts and interbeds. Unit contains floral microfossils. Thickness is 18 to 28 feet. Well exposed on Niagara Street at the railroad overpass, Lockport, Niagara Co., NY. Unit is lowest formation in Medina Group. Unconformably overlies the Upper Ordovician Queenston Shale and underlies the Power Glen Shale (contact is gradational over a few feet of interbedded sandstone and shale). Unit is mappable as far west as Hamilton, Ont, CAN, where in grades into carbonate of the Manitoulin Dolomite. To the east, near Medina, NY, the overlying Power Glen Shale pinches out, and the Whirlpool underlies the Devils Hole Sandstone (new). The Whirlpool is of Early Silurian age, probably early Llandoverian, based on correlation with the Manitoulin Dolomite (Brett and others, 1995).
References
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Counties Monroe - Niagara - Orleans