The lower Lewiston Member (A and B submembers) records continued upward deepening although probably at a lower rate. The extent of deepening is uncertain, but probably ranges from near normal wave base to deep storm wave base or a few tens of meters. Maximum water depth was attained near the boundary of units B and C of the Lewiston Member and we interpret the B-C boundary as a maximum flooding zone. Lewiston C, like the upper Rochester Burleigh Hill Member, is interpreted as representing highstand conditions. That is, maximum depth was attained near the base of the unit and that minor gradual shallowing took place through the unit. A further implication is that the rate of siliciclastic sedimentation was gradually increasing. Here depths were generally too great and conditions too turbid and muddy to support abundant shelly organisms so that skeletal limestones are essentially absent. However, at some levels, remains are found of small thin shells (e.g. Amphistrophia) and mobile animals including trilobites such as Dalmanites and Trimerus and, rarely Arctinurus, as well as soft-bodied worms, which left trace fossils, especially in silty deposits and very likely formed important food for predators including the trilobites. Beds of carbonate and/or siliciclastic silt are abundant within this interval. These beds show clear evidence of deposition by deep storm gradient currents with slight interference from waves. Such currents scoured the muddy seafloor removing loose muds and depositing slightly graded, hummocky laminated silt transported out of shallower areas to the north.
Near Whirlpool Bridge in Niagara Falls, NY, exposures of Rochester Shale in the upper C-D units show thicker (up to 40 cm), channelized and somewhat deformed siltstones/calcisiltites that closely resemble the DeCew Formation at the top of the Rochester (Brett, 1983). These units are both interpreted as rapidly deposited, coarse sediment that was poured into the basin during a time of falling sea level, analogous to the falling stage conditions. No hint of this silty package occurs in the Lewiston D submember at Middleport, or for that matter, at the north end of the Niagara Gorge. Presumably it was confined to local submarine channel fills along the southward trending slope or ramp into deeper parts of the Rochester basin. Regardless, this evidence strongly suggests that the Lewiston D submember represents a falling stage deposit - i.e. deposited during a regression forced by an actual and probably eustatic sea level fall. The Lewiston E, "bryozoa beds" limestone package was formerly interpreted as the maximum shallowing. However, reconsideration of this situation suggests that it may actually record the very initial transgression following a shallowing. As noted above, during transgressions terrigenous sediments tend to become trapped or sequestered in coastal areas leading to offshore starvation of siliciclastic sediments and promoting growth of carbonate-forming organisms. The sharp upper contact of the Lewiston E is interpreted as a maximum flooding surface associated with rapid rate of rise. It is analogous to the top of the Irondequoit limestone.
Interestingly, patterns very similar to those in the Rochester Shale are found in Ancestral Europe or Baltica, for example, the Jaani Formation, of essentially the same age seen in cliff sections along the island of Saaremaa, in Estonia. The basal shaly Mustjala Member, resembling lower Lewiston Member, is followed by bryozoan and crinoid rich limestones in the Ninase Member, as in similar aged Lewiston E strata, and these are, in turn, overlain by gray shales and calcisiltites of the Paramaja Member (Männik and Nestor, 2014) that very closely resemble the Burleigh Hill Member of the Rochester. This suggests that even the smaller scale cycles of the Rochester may have been produced by eustatic sea level changes.
Community Succession: The succession of changing environments and biofacies (communities) in the middle Lewiston Member is particularly well displayed at Middleport (Figure 5). By combining data and observations from nearby creek exposures with the excavations at the Caleb Quarry we can reconstruct a remarkably symmetrical transition of fossil assemblages from relatively shallow to deeper environments and then back again.
Exceptionally weathered exposures in the lower Rochester Lewiston B occur along banks of Jeddo Creek in Middleport. These were studied in detail by the writer both by sieving the weathered mudstone in the creek to collect all resistant fossils, and by collecting slabs from the banks; this sampling revealed an exceptionally rich fauna in several bryozoan-rich beds, separated by sparsely fossiliferous shale (Brett, 1978a).
Near Whirlpool Bridge in Niagara Falls, NY, exposures of Rochester Shale in the upper C-D units show thicker (up to 40 cm), channelized and somewhat deformed siltstones/calcisiltites that closely resemble the DeCew Formation at the top of the Rochester (Brett, 1983). These units are both interpreted as rapidly deposited, coarse sediment that was poured into the basin during a time of falling sea level, analogous to the falling stage conditions. No hint of this silty package occurs in the Lewiston D submember at Middleport, or for that matter, at the north end of the Niagara Gorge. Presumably it was confined to local submarine channel fills along the southward trending slope or ramp into deeper parts of the Rochester basin. Regardless, this evidence strongly suggests that the Lewiston D submember represents a falling stage deposit - i.e. deposited during a regression forced by an actual and probably eustatic sea level fall. The Lewiston E, "bryozoa beds" limestone package was formerly interpreted as the maximum shallowing. However, reconsideration of this situation suggests that it may actually record the very initial transgression following a shallowing. As noted above, during transgressions terrigenous sediments tend to become trapped or sequestered in coastal areas leading to offshore starvation of siliciclastic sediments and promoting growth of carbonate-forming organisms. The sharp upper contact of the Lewiston E is interpreted as a maximum flooding surface associated with rapid rate of rise. It is analogous to the top of the Irondequoit limestone.
Interestingly, patterns very similar to those in the Rochester Shale are found in Ancestral Europe or Baltica, for example, the Jaani Formation, of essentially the same age seen in cliff sections along the island of Saaremaa, in Estonia. The basal shaly Mustjala Member, resembling lower Lewiston Member, is followed by bryozoan and crinoid rich limestones in the Ninase Member, as in similar aged Lewiston E strata, and these are, in turn, overlain by gray shales and calcisiltites of the Paramaja Member (Männik and Nestor, 2014) that very closely resemble the Burleigh Hill Member of the Rochester. This suggests that even the smaller scale cycles of the Rochester may have been produced by eustatic sea level changes.
Community Succession: The succession of changing environments and biofacies (communities) in the middle Lewiston Member is particularly well displayed at Middleport (Figure 5). By combining data and observations from nearby creek exposures with the excavations at the Caleb Quarry we can reconstruct a remarkably symmetrical transition of fossil assemblages from relatively shallow to deeper environments and then back again.
Exceptionally weathered exposures in the lower Rochester Lewiston B occur along banks of Jeddo Creek in Middleport. These were studied in detail by the writer both by sieving the weathered mudstone in the creek to collect all resistant fossils, and by collecting slabs from the banks; this sampling revealed an exceptionally rich fauna in several bryozoan-rich beds, separated by sparsely fossiliferous shale (Brett, 1978a).
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