During the Middle to Late Ordovician Period, Laurentia or ancestral North America, collided along its southeastern side (essentially the modern east side) with the Taconic island arc, a series of volcanic islands. An accretionary wedge of sediments and volcanics off scraped from the Iapetus (Protoatlantic) was overthrust onto the edge of the continent (Ettensohn, 2008). The landmass was uplifted to Alpine heights and shed sediments onto Laurentia. By the middle Silurian the Taconic Mountains had largely been eroded to relatively low relief cutting back the supply of siliciclastic sediments and leading to deposition of "homegrown" limestone sediments in the basin. However, during the Wenlock Epoch a new collision was under way, with a narrow peninsula-like continental area, Avalonia, which now forms the parts of the Eastern Seaboard of Canada, impinging upon eastern Laurentia. Siliciclastic mud and silt comprising the Rochester Formation was shed from highlands produced during an early tectophase of the so-called Salinic Orogeny (Ettensohn and Brett, 1998; Ettensohn, 2008), which uplifted mountainous terrains to the east of present day New York State. This orogeny also produced gentle subsidence of the foreland basin that propagated to the northwest and permitted accumulation of a substantial thickness of muds, silts and sands. The fine-grained sediments that comprise the Rochester deposits graded to the east, near present day Utica, NY, with nearshore sands of the Herkimer Formation. Contemporaneous with the Rochester deposition, shallow water carbonate banks with minor patch reefs, somewhat like the modern Bahama Banks, existed in the midwestern regions of the Laurentian continent(Figure 1). Thus, Rochester muds interfingered with carbonate skeletal sands and silts to the northwest and locally shelly organisms, especially bryozoans and brachiopods, produced sufficient skeletal debris to form thin limestones.
Stratigraphy of the Rochester Shale
The Rochester Shale is a classic unit in North American stratigraphy, as the first formally designated formation. James Hall (1839) applied the name to a gray shale unit, about 100 feet thick, exposed below High Falls in the Gorge of the Genesee River at Rochester, NY. This designation effectively established the practice of naming stratigraphic units for well-defined type sections or reference localities, a practice that has been formalized in the North American Code of Stratigraphic Nomenclature. In this sense, the Rochester Shale, or Rochester Formation, is an icon of stratigraphy.
The Rochester Shale consists of medium gray, calcareous mudstone, which weathers shaly (marl of European usage) interbedded with lenticular to tabular, muddy limestones (packstones), as well as some clean, skeletal limestones with little mud (grainstones) and calcisiltites (beds composed of silt-sized calcitic grains). Much of the "shale" is barren or sparsely fossiliferous and in some localities much of the Rochester Formation is sparsely fossiliferous (Figure 2). The unit ranges in thickness from about 43 meters (140 feet) east of Rochester, NY to just 0.6 meters (2 feet) at Clappison Corner, Ontario prior to its pinchout to the northwest (Brett, 1983a, b). To the east of Syracuse, NY the Rochester Shale begins to interfinger with fine-grained sandstones and east of Utica, NY it undergoes a transition into the Herkimer Sandstone (Gillette, 1947), locally, a massive quartz sandstone (Zenger, 1971). Along the outcrop belt the Rochester Shale persists northwestward to near Hamilton, Ontario, despite strong thinning and in the subsurface, the unit is known in southern Ontario and northern Ohio (Figure 3); in southern Ohio and Kentucky its equivalent is mapped as an informal shaly member of the Bisher Formation (Brett and Ray, 2005). To the south it persists into Pennsylvania, Maryland, West Virginia and northern Virginia as a medium to dark gray shale unit. Thus, the Rochester Shale was a very widespread interval of mud and silt deposition over eastern Laurentia (ancestral North America).
The Rochester Shale sharply, but conformably (no major time gap) overlies a 2 to 4 meter (6.5 to 13 feet) thick interval of fossiliferous limestone, termed the Irondequoit Limestone (Figure 2), a dolomitic packstone to the west, crinoidal packstone and grainstone (i.e. with or without muddy matrix between the fossil grains) in western New York and muddy packstone near Rochester, NY. The Rochester Formation is overlain, throughout western New York, by the DeCew Formation, a fine-grained, silty to sandy, laminated dolostone, typified by spectacular soft sediment deformation and slumping (Brett et al., 1990; McLaughlin and Brett, 2006).
Stratigraphy of the Rochester Shale
The Rochester Shale is a classic unit in North American stratigraphy, as the first formally designated formation. James Hall (1839) applied the name to a gray shale unit, about 100 feet thick, exposed below High Falls in the Gorge of the Genesee River at Rochester, NY. This designation effectively established the practice of naming stratigraphic units for well-defined type sections or reference localities, a practice that has been formalized in the North American Code of Stratigraphic Nomenclature. In this sense, the Rochester Shale, or Rochester Formation, is an icon of stratigraphy.
The Rochester Shale consists of medium gray, calcareous mudstone, which weathers shaly (marl of European usage) interbedded with lenticular to tabular, muddy limestones (packstones), as well as some clean, skeletal limestones with little mud (grainstones) and calcisiltites (beds composed of silt-sized calcitic grains). Much of the "shale" is barren or sparsely fossiliferous and in some localities much of the Rochester Formation is sparsely fossiliferous (Figure 2). The unit ranges in thickness from about 43 meters (140 feet) east of Rochester, NY to just 0.6 meters (2 feet) at Clappison Corner, Ontario prior to its pinchout to the northwest (Brett, 1983a, b). To the east of Syracuse, NY the Rochester Shale begins to interfinger with fine-grained sandstones and east of Utica, NY it undergoes a transition into the Herkimer Sandstone (Gillette, 1947), locally, a massive quartz sandstone (Zenger, 1971). Along the outcrop belt the Rochester Shale persists northwestward to near Hamilton, Ontario, despite strong thinning and in the subsurface, the unit is known in southern Ontario and northern Ohio (Figure 3); in southern Ohio and Kentucky its equivalent is mapped as an informal shaly member of the Bisher Formation (Brett and Ray, 2005). To the south it persists into Pennsylvania, Maryland, West Virginia and northern Virginia as a medium to dark gray shale unit. Thus, the Rochester Shale was a very widespread interval of mud and silt deposition over eastern Laurentia (ancestral North America).
The Rochester Shale sharply, but conformably (no major time gap) overlies a 2 to 4 meter (6.5 to 13 feet) thick interval of fossiliferous limestone, termed the Irondequoit Limestone (Figure 2), a dolomitic packstone to the west, crinoidal packstone and grainstone (i.e. with or without muddy matrix between the fossil grains) in western New York and muddy packstone near Rochester, NY. The Rochester Formation is overlain, throughout western New York, by the DeCew Formation, a fine-grained, silty to sandy, laminated dolostone, typified by spectacular soft sediment deformation and slumping (Brett et al., 1990; McLaughlin and Brett, 2006).
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