Joe Bartlett
Michala Peabody
9/3/02
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. These fossils are indicative of varying energy environments during the
deposition period. The stratigraphic column contains evidence of two layers of fossil hash
suggesting a high-energy environment. The remainder of the column consists of more subtle
variations between grain size and fossil content suggesting less drastic energy fluctuations.
The lower boundaries of all the beds are well defined but four of the transitions between high
and low energy show evidence of gradual settling occurring during deposition. No strong
pattern is evident through the height of the column therefore no recurring seasonal deposition is
apparent. The bedding 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 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 highenergy events caused the removal of these organisms from the Neritic zone and movement
further down the continental slope.
The stratigraphic column 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 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 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.