PALEOENVIRONMENTAL RECONSTRUCTION OF PYRITIC SHALE FORMATION

The Hamilton Group of Middle Devonian age represents epeiric sea deposits. The shales and carbonate rocks found within the Hamilton Group were deposited in a foreland basin associated with the Acadian Highlands. Within the Hamilton Group, as in other epeiric sea deposits, various types of shale can be found, in particular pyritic shales. This study looks at the characteristics of the pyritic shales through various techniques. Pyritic shales from the Hamilton Group were analyzed by looking at the occurrence of the brachiopod family Ambocoeliidae, through geochemical analysis of elemental abundance within sequential pyritic and non-pyritic shales, and by taking magnetic susceptibility measurements of sequential pyritic and non-pyritic shales. Various methods of analysis were employed in order to not only gain a better understanding of the depositional environment associated with pyrite formation but to also look for any correlations between the data that these methods produced. Both the geochemical and magnetic susceptibility data support the inference based on paleontological sampling that variation in the depositional environment for pyritic shales exists. Using functional morphology, abundance ratios of members of the Ambocoeliidae family suggest variation in substrate firmness among pyritic horizons. Spinous Ambocoeliidae increased in abundance in pyritic environments, but to varying degrees. Geochemical analysis provided a better understanding of the elemental constituents needed for the formation of these horizons as well as the concentration at which these elements occurred. The data collected implies that high total iron concentrations are not related to pyritic horizons. By testing the magnetic susceptibility of these shales, the prospect of using magnetic susceptibility data collected in the field to estimate iron abundance within shales was found to be unlikely. The broader implication of this study is that the environment of deposition for pyritic shales can be variable.