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Lockport Group

Lockport Group - consists of: Guelph, Oak Orchard, Eramosa, and Goat Island Dolostones; Gasport Limestone-local bioherms.
StateNew York
NameLockport Group
Geologic ageUpper Silurian
Original map labelSl
Comments150-200 ft. (45-60 m). Unit descriptions from USGS Lexicon website (ref. NY046) and reference NY018: The Guelph Dolomite of the Lockport Group is revised to include strata at its base that was formerly assigned as the upper part of the Oak Orchard Dolomite of the Lockport Group or the Oak Orchard Member of the Lockport Formation by Zenger (1965). The Oak Orchard is abandoned in this report. This revision allows for uniformity in nomenclature and stratigraphy for this interval with Ontario, CAN. Consists of medium-gray to dark-gray, light-gray to tan weathering, laminated, fine-grained, commonly oolitic dolomite. Formation is divided into three informal units. Unit A consists of very light gray, fine-grained, stromatolitic dolomite and is marked by a distinctive biostromal bed. Unit B is a 4 to 6 foot thick interval of contorted, dark-gray, laminated, oolitic dolomite with black shale partings, and includes a middle zone of uniform, medium bluish-gray, oolitic dolomite with thin, dark shaly partings and beds with dolomicrite intraclasts. Unit C forms a transition zone between the Guelph and the overlying Vernon Shale and is similar to unit B, but contains vugs and interbedded zones of fine-grained, olive-gray to greenish-gray, argillaceous dolomite and thin, black or dark-gray, shaly partings. In Ontario, CAN the Guelph is between 200 and 300 feet thick, however, only a thin wedge of the formation extends into the Niagara Co. region of western NY where it is 33 to 36 feet thick in the subsurface. Overlies the Eramosa Dolomite of the Lockport Group and conformably underlies the Vernon Shale of the Salina Group. The Guelph-Vernon contact is placed at the base of the first black shale bed that is greater than 1 inch thick (this horizon may no be equivalent in all sections). The Guelph is of Late Silurian (late Ludlovian) age based on conodonts. [Report uses Silurian time scale of Harland and others (1982).] (Brett and others, 1995). Proposed abandonment of the Oak Orchard because unit is similar to Eramosa Dolomite and allows uniformity of nomenclature and stratigraphy with interval in Ontario, CAN (Brett and others, 1995). The Eramosa Dolomite of the Lockport Group is revised to include lower interval formerly called the Oak Orchard Dolomite of the Lockport Group or the Oak Orchard Member of the Lockport Formation by Zenger (1965) and an upper interval formerly assigned to the Guelph Dolomite. The Oak Orchard is proposed for abandonment herein. The revised Eramosa consists of massive, pale brownish-weathering, vuggy, commonly biostromal dolomite with intervals of sparsely fossiliferous, medium-bedded, flaggy-weathering, brownish-gray, bituminous dolomite and stromatolite bioherms. Formation can be divided into six informal units. Unit A is a 7 to 8 foot thick, massive, biostromal dolomite characterized by thickets of ramose tabulate coral and abundant white chert nodules. Unit B is a 13 to 20 foot thick, fine-grained, sparsely fossiliferous and bituminous dolomite that weathers medium-bedded to flaggy. Unit C consists of tabulate coral biostromes (similar to unit A) and masses of stromatolites; unit is massive or thick-bedded, brownish-weathering, saccroidal dolomite with large vugs. Unit D is a 16.8 foot thick flaggy-weathering, dark brownish-gray, nonfossiliferous, saccroidal dolomite with a middle massive interval that contains coral. Unit E is a 1- to 2-foot-thick marker bed of light-gray, laminar, stromatolitic dolomite. Unit F consists of 7.4 ft of medium-grained, olive-gray dolomite that locally contains scattered oolites and corals (similar to unit D). Thickness of the Eramosa is 38 to 50 ft. Well exposed in the Niagara River Gorge on old sewage treatment plant access road in Niagara Falls, Niagara Co., NY. Unconformably overlies the Goat Island Dolomite and underlies the Guelph Dolomite, both of the Lockport Group. The Eramosa is of Late Silurian (Ludlovian) age based on conodonts (Brett and others, 1995). The Goat Island Dolomite of the Lockport Group in western NY is revised by the formal naming of three members, (ascending) the Niagara Falls, Ancaster, and Vinemount Members. The Niagara Falls Member is a gray to buff, biohermal grainstone, the Ancaster Member is a buff, thin-bedded, fine-grained, chert-rich dolomite, and the Vinemount Member is a light-gray to black, thin-bedded, shaly dolomite with some chert. Thickness of the Goat Island is 26 to 56 feet and averages 42 feet. Conformably and unconformably overlies the Gasport Dolomite and unconformably underlies the Eramosa Dolomite. East of Brockport, Monroe Co., NY, the Goat Island appears to correlate with the upper part of the Penfield Formation. The Goat Island is probably of Early and Late Silurian (latest Wenlockian and earliest Ludlovian) age based on conodonts (Brett and others, 1995). Revised the Gasport to the Gasport Dolomite of the Lockport Group and divided unit into two members, the Gothic Hill and Pekin Members. Gasport consists of a lower light pinkish-gray, crinoidal- and brachiopod-rich dolograinstone to dolopackstone (Gothic Hill Member) and an upper dark olive-gray, argillaceous, fine- to medium-grained dolomicrite that grades laterally into bioherms and flanking dolorudites (Pekin Member). Thickness is 19.5 to 37.2 ft. Unconformably overlies the DeCew Dolomite of the Clinton Group and unconformably underlies the Goat Island Dolomite of the Lockport Group. The Gasport is of Early Silurian (Wenlockian) age based on conodonts (Brett and others, 1995).
Primary rock typedolostone (dolomite)
Secondary rock typelimestone
Other rock types
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone
Incidental
Sedimentary > Carbonate > Limestone
Map references
NYS Museum, NYS Geological Survey, NYS Museum Technology Center, 1999, 1:250,000 Bedrock geology of NYS, data is distributed in ARC/INFOr EXPORT format (with ".e00" extension) in 5 seperate files based on printed map sheets, http://www.nysm.nysed.gov/gis.html.
Unit references
D. W. Fisher; Y. W. Isachsen, L. V. Rickard, 1970, Geologic Map of New York State, consisting of 5 sheets: Niagara, Finger Lakes, Hudson-Mohawk, Adirondack, and Lower Hudson, New York State Museum and Science Service, Map and Chart Series No. 15, scale 1:250000.
Brett, C.E., Tepper, D.H., Goodman, W.M., LoDuca, S.T. and Eckert, Bea-Yeh, 1995, Revised stratigraphy and correlations of the Niagaran Provincial Series (Medina, Clinton, and Lockport Groups) in the type area of western New York: U.S. Geological Survey Bulletin, 2086, 66 p., Prepared in cooperation with the U.S. Environmental Protection Agency and the University of Rochester, Department of Earth and Environmental Sciences.
USGS Geologic Names Lexicon (GEOLEX)

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