Yet, of these collections, columns and feeding appendages were found in fewer than ten specimens and no complete specimens of these or any crinoids were ever found. Likewise, articulated trilobites were found to be very rare. In contrast, within a few thin beds in about 20 cm of the transitional B-C a large number of complete crinoids and cystoids were found (Figures 293, 296, 301 - 303). This was long ago recognized by the famous crinoid paleontologist Frank Springer (e.g. Springer, 1926) and his veteran collector, Frederick Braun, who discovered these beds in a small gully at Lockport. Careful tracing of these beds by the author eventually resulted in discovery of similarly well preserved fossils at the same level for over 100 km along the Niagara Escarpment. Subsequently, similar horizons of exceptionally preserved fossils were discovered in the Lewiston B and C-D transition between Middleport and the town of Gates, west of Rochester, again in a narrow zone not more than 2m thick. The C-interval is generally very sparsely fossiliferous or completely barren so the chances of finding obrution beds are low even if the sedimentological conditions favorable to forming these deposits were common.
Evidently, the best conditions for fossil preservation occurred in a narrow environmental window in which organisms of various types were relatively abundant and conditions for rapid burial with minimal disruption of seafloor communities could take place, probably near the deeper end of storm wave base, the greatest depth at which strong storm waves touched down on the seabed. This "sweet spot" lay in somewhat deeper water near the lower end of tolerance of many organisms including Caryocrinites and some crinoids. Thus, the well-preserved fossils reflect a biased sample of the spectrum of fossil communities. Only certain types of organisms are present in these well-preserved assemblages. For example, these settings were apparently too low energy, deep or muddy for some sensitive forms like Stephanocrinus and callocystitids and therefore the opportunity to see these organisms as perfectly preserved specimens was rarely attained. The constraints on the Rochester taphonomic window probably include downramp settings that were shallow enough to have normal oxygenation and to sustain abundant life and yet deep enough that the bottom was not too heavily torn up and disrupted by storm waves and where abundant sediment winnowed from upslope area was rapidly dropped out, probably as a backwash of flocculated muds. Evidence of some current alignment in elongate fossils, such as trilobites (see below), suggests that some of this sediment came in the form of dense, bottom flowing currents.
Paleoecology and Depositional Environments of the Rochester Shale
The depositional environment and paleoecology of the Rochester Shale has been previously discussed in some detail (Brett 1983, 1999; Taylor and Brett, 1996, 1999, Tetreault 1994) and will be briefly summarized here with an emphasis on new data from the Caleb Quarry that provides some important new insights.
During deposition of the Rochester Shale the depositional strike (or orientation of environmental or facies belts) in western New York and Ontario was nearly east-west and water depth increased gradually from north to south (Brett, 1983b; 1999; Figure 4). In the present (east-west) outcrop belt of the Niagara Escarpment, facies are very similar for considerable distances. Indeed every bed of the thin (~30 cm) Homocrinus beds transition (Figure 3), including centimeter-thick crinoid beds was found to be traceable between Lockport and St. Catharines, Ontario (Brett and Taylor, 1997). And yet the nearly continuous north-south outcrops along Niagara Gorge show that the Rochester Shale facies change gradually but rather rapidly to the south, such that the fossil rich lower and upper parts of the Lewiston Member thin while the Lewiston C division thickens and most of the interval becomes nearly barren, dark gray shale near Niagara Falls. This evidence shows that the ramp was relatively steep to the south and that the environments to the south were deeper, dysoxic (low oxygen restricted) and nearly lifeless zones. Had the erosion of the Niagara Escarpment moved it a few miles further south, the Rochester would be viewed as a nearly barren shale unit.
Evidently, the best conditions for fossil preservation occurred in a narrow environmental window in which organisms of various types were relatively abundant and conditions for rapid burial with minimal disruption of seafloor communities could take place, probably near the deeper end of storm wave base, the greatest depth at which strong storm waves touched down on the seabed. This "sweet spot" lay in somewhat deeper water near the lower end of tolerance of many organisms including Caryocrinites and some crinoids. Thus, the well-preserved fossils reflect a biased sample of the spectrum of fossil communities. Only certain types of organisms are present in these well-preserved assemblages. For example, these settings were apparently too low energy, deep or muddy for some sensitive forms like Stephanocrinus and callocystitids and therefore the opportunity to see these organisms as perfectly preserved specimens was rarely attained. The constraints on the Rochester taphonomic window probably include downramp settings that were shallow enough to have normal oxygenation and to sustain abundant life and yet deep enough that the bottom was not too heavily torn up and disrupted by storm waves and where abundant sediment winnowed from upslope area was rapidly dropped out, probably as a backwash of flocculated muds. Evidence of some current alignment in elongate fossils, such as trilobites (see below), suggests that some of this sediment came in the form of dense, bottom flowing currents.
Paleoecology and Depositional Environments of the Rochester Shale
The depositional environment and paleoecology of the Rochester Shale has been previously discussed in some detail (Brett 1983, 1999; Taylor and Brett, 1996, 1999, Tetreault 1994) and will be briefly summarized here with an emphasis on new data from the Caleb Quarry that provides some important new insights.
During deposition of the Rochester Shale the depositional strike (or orientation of environmental or facies belts) in western New York and Ontario was nearly east-west and water depth increased gradually from north to south (Brett, 1983b; 1999; Figure 4). In the present (east-west) outcrop belt of the Niagara Escarpment, facies are very similar for considerable distances. Indeed every bed of the thin (~30 cm) Homocrinus beds transition (Figure 3), including centimeter-thick crinoid beds was found to be traceable between Lockport and St. Catharines, Ontario (Brett and Taylor, 1997). And yet the nearly continuous north-south outcrops along Niagara Gorge show that the Rochester Shale facies change gradually but rather rapidly to the south, such that the fossil rich lower and upper parts of the Lewiston Member thin while the Lewiston C division thickens and most of the interval becomes nearly barren, dark gray shale near Niagara Falls. This evidence shows that the ramp was relatively steep to the south and that the environments to the south were deeper, dysoxic (low oxygen restricted) and nearly lifeless zones. Had the erosion of the Niagara Escarpment moved it a few miles further south, the Rochester would be viewed as a nearly barren shale unit.
page 9