The Rochester Shale of western New York was formally subdivided into two members, the Lewiston and Burleigh Hill, by Brett (1983a, b). The lower, or Lewiston Member, was named for exposures along the east side of the Niagara River Gorge about a mile south of the village of Lewiston, Niagara County, NY. Here and elsewhere, the member consists of calcareous shale and fossiliferous, muddy limestone. The upper, or Burleigh Hill Member as recognized for a large road cut exposure along Burleigh Hill Road at the Niagara Escarpment near Thorold, Ontario, Canada. This interval consists of about 10 meters (30 feet) of barren to sparsely fossiliferous calcareous to dolomitic shale with thin dolomitic calcisiltite beds that increase in thickness and frequency upward. This member has very few fossiliferous limestone beds and bears a sparse, low diversity fauna, with small, thin-shelled brachiopods, Amphistrophia, Coolinia, as well as the trilobites Dalmanites limulurus and Trimerus delphinocephalus, but generally lacking bryozoans and echinoderms.
Brett (1983a, b) further divided the Lewiston Member into five informal submembers, designated Lewiston A-E (Figures 2 and 5). The Lewiston A represents approximately 1 meter thick transitional facies of muddy limestone and very calcareous shale at the base of the formation which is characterized by a fauna of atrypids, Eoplectodonta and large Leptaena and some unusual echinoderms; it is less rich in bryozoans than overlying mudstones. Locally, small mounds of micritic limestone and leaf-like fistuliporoid bryozoans protrude upward from the Irondequoit Limestone into the Lewiston A submember. Lewiston B consists of lenses of medium dark gray, muddy, bryozoan-rich limestone (packstone) and interbedded sparsely fossiliferous, calcareous mudstone; in most localities the B is about 5 meters thick and displays a middle portion that is more sparsely fossiliferous mudstone. Upper and lower portions tend to yield specimens of the rhombiferan cystoid Caryocrinites and the coronoid Stephanocrinus angulatus. The transition of the Lewiston B into overlying C beds is abrupt but typically marked by a distinctive thin interval (typically < 10 cm) with exceptionally preserved crinoids, cystoids and other fossils. Because this interval typically shows articulated specimens of the tiny disparid crinoid Homocrinus parvus, it has been informally termed the Homocrinus beds (Figure 3). Lewiston C is a distinctive sparsely fossiliferous interval characterized by an absence of lenticular bryozoan limestone and the presence of abundant thin, tabular, laminated calcisiltites. This portion of the Lewiston closely resembles the upper Rochester or Burleigh Hill in lithology and fossil content. The D submember rather closely mirrors Lewiston B with lenses of bryozoans, mainly Chilotrypa and bedding planes covered with the brachiopod Striispirifer niagarensis. Caryocrinites is locally abundant again at this level (Figures 1 and 2). Lewiston E corresponds with the "bryozoa beds" of Grabau (1901), a package of about 1-2 meters of stacked bryozoan-rich limestones with abundant ramose as well as fenestrate bryozoans and thin calcareous shale and a diverse assemblage of brachiopods, echinoderms, and trilobites. The upper contact of the Lewiston E beds with the overlying basal Burleigh Hill Member is typically very sharp; the highest bryozoan-rich limestone is abruptly overlain by barren dark gray shale. The five divisions of the Lewiston Member have been recognized approximately from Brockport, NY westward to near Jordanville, Ontario and the position of the E beds or "bryozoa beds" can be correlated still farther east and west.
The Rochester Shale is of middle Silurian age, belonging to the early Sheinwoodian Stage of the Wenlock Series -both terms derived from the classic Silurian in the Welch Borderland area of Great Britain. It is, in part, equivalent to the rather similar Coalbrookdale Shale of England, while the underlying Irondequoit Limestone is equivalent to a rather similar Buildwas or Woolhope limestone in Britain. These correlations are based upon the occurrence of conodonts, tiny tooth-like elements of small, eel-like proto-vertebrate animals. These tiny (< 1 mm) apatite (calcium phosphate) elements can be extracted from limestones and limey mudstones like the Rochester Shale, using weak acids such as muriatic acid to dissolve the calcium carbonate and release the conodonts. Conodonts of the Kockellella ranuliformis and Ozarkodina sagitta rhenana zones are present in the Rochester Shale with the boundary of the two zones occurring approximately at the upper part of the Lewiston Member (Cramer et al., 2005). These conodont species also occur in the type area of the Silurian in England and permit age assignment. The base of the Sheinwoodian is presently dated at approximately 431 to 433 million years old based upon U/Pb radiometric dating of zircon crystals obtained from volcanic ash layers close to the boundary with the underlying Telychian Stage of the Llandovery Series (Melchin, 2012). The Rochester deposition may record approximately a million years of geologic time.
Brett (1983a, b) further divided the Lewiston Member into five informal submembers, designated Lewiston A-E (Figures 2 and 5). The Lewiston A represents approximately 1 meter thick transitional facies of muddy limestone and very calcareous shale at the base of the formation which is characterized by a fauna of atrypids, Eoplectodonta and large Leptaena and some unusual echinoderms; it is less rich in bryozoans than overlying mudstones. Locally, small mounds of micritic limestone and leaf-like fistuliporoid bryozoans protrude upward from the Irondequoit Limestone into the Lewiston A submember. Lewiston B consists of lenses of medium dark gray, muddy, bryozoan-rich limestone (packstone) and interbedded sparsely fossiliferous, calcareous mudstone; in most localities the B is about 5 meters thick and displays a middle portion that is more sparsely fossiliferous mudstone. Upper and lower portions tend to yield specimens of the rhombiferan cystoid Caryocrinites and the coronoid Stephanocrinus angulatus. The transition of the Lewiston B into overlying C beds is abrupt but typically marked by a distinctive thin interval (typically < 10 cm) with exceptionally preserved crinoids, cystoids and other fossils. Because this interval typically shows articulated specimens of the tiny disparid crinoid Homocrinus parvus, it has been informally termed the Homocrinus beds (Figure 3). Lewiston C is a distinctive sparsely fossiliferous interval characterized by an absence of lenticular bryozoan limestone and the presence of abundant thin, tabular, laminated calcisiltites. This portion of the Lewiston closely resembles the upper Rochester or Burleigh Hill in lithology and fossil content. The D submember rather closely mirrors Lewiston B with lenses of bryozoans, mainly Chilotrypa and bedding planes covered with the brachiopod Striispirifer niagarensis. Caryocrinites is locally abundant again at this level (Figures 1 and 2). Lewiston E corresponds with the "bryozoa beds" of Grabau (1901), a package of about 1-2 meters of stacked bryozoan-rich limestones with abundant ramose as well as fenestrate bryozoans and thin calcareous shale and a diverse assemblage of brachiopods, echinoderms, and trilobites. The upper contact of the Lewiston E beds with the overlying basal Burleigh Hill Member is typically very sharp; the highest bryozoan-rich limestone is abruptly overlain by barren dark gray shale. The five divisions of the Lewiston Member have been recognized approximately from Brockport, NY westward to near Jordanville, Ontario and the position of the E beds or "bryozoa beds" can be correlated still farther east and west.
The Rochester Shale is of middle Silurian age, belonging to the early Sheinwoodian Stage of the Wenlock Series -both terms derived from the classic Silurian in the Welch Borderland area of Great Britain. It is, in part, equivalent to the rather similar Coalbrookdale Shale of England, while the underlying Irondequoit Limestone is equivalent to a rather similar Buildwas or Woolhope limestone in Britain. These correlations are based upon the occurrence of conodonts, tiny tooth-like elements of small, eel-like proto-vertebrate animals. These tiny (< 1 mm) apatite (calcium phosphate) elements can be extracted from limestones and limey mudstones like the Rochester Shale, using weak acids such as muriatic acid to dissolve the calcium carbonate and release the conodonts. Conodonts of the Kockellella ranuliformis and Ozarkodina sagitta rhenana zones are present in the Rochester Shale with the boundary of the two zones occurring approximately at the upper part of the Lewiston Member (Cramer et al., 2005). These conodont species also occur in the type area of the Silurian in England and permit age assignment. The base of the Sheinwoodian is presently dated at approximately 431 to 433 million years old based upon U/Pb radiometric dating of zircon crystals obtained from volcanic ash layers close to the boundary with the underlying Telychian Stage of the Llandovery Series (Melchin, 2012). The Rochester deposition may record approximately a million years of geologic time.
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