Joe Bartlett
Geological Evolution of Lessor’s Quarry
The stratigraphy at Lessor’s Quarry is dominated by alternating layers of bioclastic
limestone and mudstone. Two classes of fossils: bryozoans and crinoids are found throughout
the stratigraphic column. The two classes of fossils are found in many different layers and vary
greatly with respect to their condition ranging from fossil hash to well-preserved organisms. The
stratigraphic column is dominated by somewhat regularly alternating bands of material with
different fossil content and grain size. Two distinct bands of fossil hash are visible. The lower
boundaries of all the beds are well defined but four of the transitions between larger to smaller
grain size show a gradual transition. No strong pattern is evident through the height of the
column. The bedding patterns and composition in the stratigraphic column leads to the
formation of two hypotheses concerning the environment present during the deposition.
The sediments present in the quarry suggest that the area was on the continental slope
during the time of deposition. This is evident through the fine-grained sediments and the types
of fossils contained in the limestone. The lack of large variations in the size of the sediments
shows that a fairly consistent low energy environment was present with no influence from tidal
variations. The smoothness of the beds suggests that no strong currents were present during
deposition. The fossils found in the quarry are most similar to those present in the Neritic zone.
Storm surges or other high-energy events caused the removal of these organisms from the
Neritic zone and movement further down the continental slope where they would be deposited.
The stratigraphic column (Figure X) suggests a fairly stable environment punctuated by
several events of varying energy levels. The column clearly shows evidence of two high-energy
events creating a bioclastic limestone fossil hash. The large percentage of broken fossils within
these beds implies that a short but high-energy event occurred. The gradual decrease in
sediment size and fossil content after these events demonstrates that a large amount of
sediment was carried off the continental shelf and then settled out over time. Several other
moderately high-energy events were also presented in the column possibly indicating weaker
storms.
The variations from the low energy mudstone to the high-energy bioclastic limestone
demonstrate that the quarry was once located on the continental slope. The stratigraphy
showed no strong evidence of tidal or seasonal influences. The sediment deposition was
influenced by storms and other high-energy events. The stratigraphy present in Lessor’s Quarry
demonstrates a variable marine depositional environment shown through the fossil content and
bedding structures.