DECIPHERING THE CONTACT BETWEEN SEDIMENTARY UNITS AND PLUTONIC ROCKS BY DETAILED BEDROCK GEOLOGIC MAPPING OF MATINICUS ISLAND, MAINE

Bedrock contact relationships mapped on the Maine Bedrock Geologic Map (Osberg, et al., 1985) are problematic. Specifically, older Ordovician sedimentary rocks are mapped in direct contact with younger Devonian granitoids on Matinicus Island south of Penobscot Bay. The extent of the grainitoid pluton in the Penobscot Bay area suggest that heat associated with this pluton should have been sufficient to contact metamorphose the older sediments. Additionally, since the coarse grained granites must have intruded the sediments at depth the sediments should have experienced low to moderate burial metamorphism. In either of these scenarios, the mapped quartzose and argillaceous sedimentary units should have been metamorphosed to quartzites and schists. The presence of metamorphic rocks in parts of the island is suggested by high levels of Arsenic in some well water and evidence found on the Maine Bedrock Geologic Map (Osberg, et al., 1985). If the sediments are un-metamorphosed, as mapped, then the mapped intrusive contact must instead be a fault contact or the relative ages of the rocks may need a more detailed investigation. This research will specifically address these issues through detailed mapping of the lithologies and contacts present on Matinicus Island in August, 2011. The results of this investigation will be two-fold: 1) the results will be compiled on a 1:24,000-scale 7.5’ Matinicus Quadrangle Bedrock Geologic Map where none currently exists, and; 2) this research will clarify the nature of the contact present on the Island. The results of this investigation will also ascertain the source of Arsenic reported in island residents’ groundwater.