Stratigraphy and Paleoenvironments of the Rochester Shale in Western New York.
By Carlton E. Brett
Department of Geology
University of Cincinnati
Cincinnati, OH 45221-0013
All figure number references are to illustrations and photographs in The Silurian Experience.
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Introduction
Extraordinary fossil assemblages, sometimes called fossil Lagerstätten (Seilacher et al., 1985), are celebrated for the richness of detail they provide about ancient life. Despite their rarity, Lagerstätten (literally fossil mother lodes) yield significant insights into paleobiology and ancient organism communities. The fossil assemblages from the Rochester Shale, illustrated in the book, The Silurian Experience, well qualify for that elite category as they reveal exquisite snapshots of Silurian life.
The Rochester Shale was already, and justifiably, famous for its fossil fauna in the days of James Hall. In 1852 Hall described the prolific fossil assemblages of the Silurian in the Fourth District (western New York). He mentions collecting a bushel basket full of the nut-like thecas of the cystoid Caryocrinites ornatus during excavation for the locks of the Erie Canal at Lockport. Hall also acquired the extensive collections of Colonel Jewett of Lockport, which also contained many new species. Among the more than 300 species described in this great monograph about two thirds were from the Rochester Shale. Many other researchers have discussed various aspects of the Rochester fauna (Ringueberg, 1882, 1886, 1888a, b; Bassler, 1901; Gillette, 1947).
Throughout much of its range, the Rochester Shale carries an abundant and diverse fossil assemblage, typically dominated by brachiopods and locally, bryozoans and echinoderms. In the outcrop belt of western New York and southern Ontario, Canada the lower Rochester and especially the upper Lewiston Member, contains thin lenses and biostromes of ramose (twiggy) and fenestrate (lacy) bryozoans (Grabau, 1901; Bassler, 1901). The fauna of these beds is exceptionally diverse and more than 200 species have been reported including diverse bryozoans, brachiopods, mollusks, echinoderms, and trilobites as well as a few corals, conulariids, dendroid graptolites, worm tubes and other miscellaneous rare fossils.
Although moderately well known, the fossil assemblages of the Rochester Shale were less famous than other middle Silurian echinoderm and trilobite rich beds such as those of the Much Wenlock Limestone of the Dudley area of Great Britain and the highly diverse assemblages of Wenlock and Ludlow age from the island of Gotland, Sweden (see Hess et al., 1999). The extraordinary preservation of Rochester Shale fossils at some levels has been documented in previous studies (Brett, 1978a-c; Taylor and Brett, 1996; Brett and Taylor, 1997). However, the most extraordinary assemblages were only revealed through excavation of large surface areas of bedding planes at the shale pit opened in the mid-1990s by Brent and Rose Caleb, in making a pond near Middleport, NY. It is those assemblages that are highlighted in the book,
The Silurian Experience. These assemblages not only provide extraordinary glimpses of Silurian life but an exceptional opportunity to document fossil occurrence, paleoecology and depositional processes of the lower Rochester shale. With this book, the Rochester biota and its occurrence is better documented than has ever been possible previously. This chapter is intended to provide an overview of the stratigraphy, geological setting and depositional environments of the Rochester Shale in western New York, as well as the taphonomy and paleoecology of the rich fossil assemblages with an emphasis on new insights from the exposures in the Middleport area.
Geologic Setting and Climate of the Middle Silurian Wenlock Interval
The Rochester Formation and associated sediments accumulated in the southern Subtropics at about 25 to 30˚S in the Appalachian foreland basin (Cocks, 2001; Blakey, 2011) adjacent to carbonate platform environments to the west and northwest (in modern directions; Figure 1). Climates were relatively warm, typical of the Subtropics, and probably affected by easterly Trade Winds. In the mid-continent more arid environments existed and these conditions ultimately gave rise to deposition of evaporites such as gypsum and halite, especially in the Michigan Basin and later also in New York State. However, the climates in the foreland basin were probably somewhat more humid and rivers emanating from highlands to the southeast carried a load of eroded sediment into the adjacent basin. Tropical storms and hurricanes would have been typical of this area owing to high sea surface temperatures and their storm tracks took them across the northern end of the foreland basin in present day western New York.
Extraordinary fossil assemblages, sometimes called fossil Lagerstätten (Seilacher et al., 1985), are celebrated for the richness of detail they provide about ancient life. Despite their rarity, Lagerstätten (literally fossil mother lodes) yield significant insights into paleobiology and ancient organism communities. The fossil assemblages from the Rochester Shale, illustrated in the book, The Silurian Experience, well qualify for that elite category as they reveal exquisite snapshots of Silurian life.
The Rochester Shale was already, and justifiably, famous for its fossil fauna in the days of James Hall. In 1852 Hall described the prolific fossil assemblages of the Silurian in the Fourth District (western New York). He mentions collecting a bushel basket full of the nut-like thecas of the cystoid Caryocrinites ornatus during excavation for the locks of the Erie Canal at Lockport. Hall also acquired the extensive collections of Colonel Jewett of Lockport, which also contained many new species. Among the more than 300 species described in this great monograph about two thirds were from the Rochester Shale. Many other researchers have discussed various aspects of the Rochester fauna (Ringueberg, 1882, 1886, 1888a, b; Bassler, 1901; Gillette, 1947).
Throughout much of its range, the Rochester Shale carries an abundant and diverse fossil assemblage, typically dominated by brachiopods and locally, bryozoans and echinoderms. In the outcrop belt of western New York and southern Ontario, Canada the lower Rochester and especially the upper Lewiston Member, contains thin lenses and biostromes of ramose (twiggy) and fenestrate (lacy) bryozoans (Grabau, 1901; Bassler, 1901). The fauna of these beds is exceptionally diverse and more than 200 species have been reported including diverse bryozoans, brachiopods, mollusks, echinoderms, and trilobites as well as a few corals, conulariids, dendroid graptolites, worm tubes and other miscellaneous rare fossils.
Although moderately well known, the fossil assemblages of the Rochester Shale were less famous than other middle Silurian echinoderm and trilobite rich beds such as those of the Much Wenlock Limestone of the Dudley area of Great Britain and the highly diverse assemblages of Wenlock and Ludlow age from the island of Gotland, Sweden (see Hess et al., 1999). The extraordinary preservation of Rochester Shale fossils at some levels has been documented in previous studies (Brett, 1978a-c; Taylor and Brett, 1996; Brett and Taylor, 1997). However, the most extraordinary assemblages were only revealed through excavation of large surface areas of bedding planes at the shale pit opened in the mid-1990s by Brent and Rose Caleb, in making a pond near Middleport, NY. It is those assemblages that are highlighted in the book,
The Silurian Experience. These assemblages not only provide extraordinary glimpses of Silurian life but an exceptional opportunity to document fossil occurrence, paleoecology and depositional processes of the lower Rochester shale. With this book, the Rochester biota and its occurrence is better documented than has ever been possible previously. This chapter is intended to provide an overview of the stratigraphy, geological setting and depositional environments of the Rochester Shale in western New York, as well as the taphonomy and paleoecology of the rich fossil assemblages with an emphasis on new insights from the exposures in the Middleport area.
Geologic Setting and Climate of the Middle Silurian Wenlock Interval
The Rochester Formation and associated sediments accumulated in the southern Subtropics at about 25 to 30˚S in the Appalachian foreland basin (Cocks, 2001; Blakey, 2011) adjacent to carbonate platform environments to the west and northwest (in modern directions; Figure 1). Climates were relatively warm, typical of the Subtropics, and probably affected by easterly Trade Winds. In the mid-continent more arid environments existed and these conditions ultimately gave rise to deposition of evaporites such as gypsum and halite, especially in the Michigan Basin and later also in New York State. However, the climates in the foreland basin were probably somewhat more humid and rivers emanating from highlands to the southeast carried a load of eroded sediment into the adjacent basin. Tropical storms and hurricanes would have been typical of this area owing to high sea surface temperatures and their storm tracks took them across the northern end of the foreland basin in present day western New York.
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Please feel free to download this chapter from our book for reading at your leisure. When referencing any information from this chapter please use the following citation:
Brett, C.E., 2015, Stratigraphy and Paleoenvironments of the Rochester Shale of Western New York, pp. 1-33. In Chinnici, P., Smith, K., and Lask, M., eds.,
The Silurian Experience, Second Edition, Primitive Worlds, Rochester, New York, 2015, pp. 36-68. ISBN - 10 0-692-40170-9; ISBN - 13 978-0-692-40170-5
Brett, C.E., 2015, Stratigraphy and Paleoenvironments of the Rochester Shale of Western New York, pp. 1-33. In Chinnici, P., Smith, K., and Lask, M., eds.,
The Silurian Experience, Second Edition, Primitive Worlds, Rochester, New York, 2015, pp. 36-68. ISBN - 10 0-692-40170-9; ISBN - 13 978-0-692-40170-5
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