The primary predators of the day were represented not by fish as in modern marine communities, but by swimming nautiloid cephalopods, distant relatives of today's Nautilus with conical shells up to a meter or more long. At times of relatively lowered sea level these bryozoan thicket communities migrated southward into the study area of present day Niagara County.
The dense bryozoan patches faded southward into deeper water, where local outlying colonies provided substrates for the relatively long stemmed cystoid Caryocrinites ornatus, that occurred in clusters around bryozoans, and a few crinoids, but the primary organisms were the brachiopods Striispirifer and a few thin-shelled strophomenids, which formed local clusters and pavements on the muddy seafloor. Locally, brushy dendroid graptolites formed filter-feeding colonies somewhat analogous to the fenestrate bryozoans in shallower water. Trilobites, including Arctinurus, Calymene, Dalmanites and Trimerus, were relatively abundant and sometimes aggregated in groupings of several dozen individuals. These animals, lacking jaws, were probably soft-prey carnivores and scavengers adapted to furrowing into the muddy seafloor for worms and organic detritus. Their clusters may record mating aggregations, although they were in part very locally transported and oriented by basin-flowing gradient currents. During the relative deepening recorded by the Lewiston C submember, which occupies a majority of the Caleb Quarry section, the bryozoan thickets migrated northward and were replaced by these less diverse assemblages; however, the bryozoan dominated communities returned to this area tens of thousands of years later during a shallowing period recorded in the Lewiston D and E submembers.
Still further basinward, bottom waters became less well oxygenated and the unstable soft seafloor supported only sparse benthic life; epifaunal attached organisms became very rare. The primary remaining organisms were a few small flat shelled brachiopods, rare long stemmed crinoids in rare local patches, the trilobites Dalmanites and small Trimerus and rare ophiuroids. Deepest parts of the Rochester basin were seemingly nearly barren of life with the exception of soft-bodied animals, which produced the small branching feeding burrows of Chondrites. These deepest parts of the section are represented by the majority of the Lewiston C submember and again in the upper Rochester or Burleigh Hill Member. However, nearly all of the Rochester is barren dark gray, silty calcareous shale in the south end of Niagara Gorge, which provides a window into the deeper parts of the basin.
It is not entirely clear why these deeper areas remained so barren, but this pattern seems typical of the middle Silurian shales in eastern North America, including the nearly age-equivalent Massie Shale and the somewhat younger Waldron Shale in the North American mid-continent. Presumably a combination of low oxygen
(though not anoxia), sediment instability and turbidity made these deeper areas largely uninhabitable.
Muds and silts of the Rochester Shale were ultimately derived from erosion of uplifted "Salinic" mountain belts in the tectonically active or orogenic areas, caused by ongoing collisions of the eastern margin of Laurentia to the east. The sediments were locally deposited as pulses associated with storm-created gradient currents mainly flowing from the north-northeast. At times, plumes and/or bottom flows of flocculated muds blanketed offshore areas disrupting, locally transporting, and exterminating seafloor communities and preserving their skeletal remains intact. Thin, shell rich limestones were formed by a combination of storm processing and interludes of sediment starvation, perhaps associated with minor episodes of sea level rise in relatively shallow water.
The dense bryozoan patches faded southward into deeper water, where local outlying colonies provided substrates for the relatively long stemmed cystoid Caryocrinites ornatus, that occurred in clusters around bryozoans, and a few crinoids, but the primary organisms were the brachiopods Striispirifer and a few thin-shelled strophomenids, which formed local clusters and pavements on the muddy seafloor. Locally, brushy dendroid graptolites formed filter-feeding colonies somewhat analogous to the fenestrate bryozoans in shallower water. Trilobites, including Arctinurus, Calymene, Dalmanites and Trimerus, were relatively abundant and sometimes aggregated in groupings of several dozen individuals. These animals, lacking jaws, were probably soft-prey carnivores and scavengers adapted to furrowing into the muddy seafloor for worms and organic detritus. Their clusters may record mating aggregations, although they were in part very locally transported and oriented by basin-flowing gradient currents. During the relative deepening recorded by the Lewiston C submember, which occupies a majority of the Caleb Quarry section, the bryozoan thickets migrated northward and were replaced by these less diverse assemblages; however, the bryozoan dominated communities returned to this area tens of thousands of years later during a shallowing period recorded in the Lewiston D and E submembers.
Still further basinward, bottom waters became less well oxygenated and the unstable soft seafloor supported only sparse benthic life; epifaunal attached organisms became very rare. The primary remaining organisms were a few small flat shelled brachiopods, rare long stemmed crinoids in rare local patches, the trilobites Dalmanites and small Trimerus and rare ophiuroids. Deepest parts of the Rochester basin were seemingly nearly barren of life with the exception of soft-bodied animals, which produced the small branching feeding burrows of Chondrites. These deepest parts of the section are represented by the majority of the Lewiston C submember and again in the upper Rochester or Burleigh Hill Member. However, nearly all of the Rochester is barren dark gray, silty calcareous shale in the south end of Niagara Gorge, which provides a window into the deeper parts of the basin.
It is not entirely clear why these deeper areas remained so barren, but this pattern seems typical of the middle Silurian shales in eastern North America, including the nearly age-equivalent Massie Shale and the somewhat younger Waldron Shale in the North American mid-continent. Presumably a combination of low oxygen
(though not anoxia), sediment instability and turbidity made these deeper areas largely uninhabitable.
Muds and silts of the Rochester Shale were ultimately derived from erosion of uplifted "Salinic" mountain belts in the tectonically active or orogenic areas, caused by ongoing collisions of the eastern margin of Laurentia to the east. The sediments were locally deposited as pulses associated with storm-created gradient currents mainly flowing from the north-northeast. At times, plumes and/or bottom flows of flocculated muds blanketed offshore areas disrupting, locally transporting, and exterminating seafloor communities and preserving their skeletal remains intact. Thin, shell rich limestones were formed by a combination of storm processing and interludes of sediment starvation, perhaps associated with minor episodes of sea level rise in relatively shallow water.
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