Bedrock Geology of the Ordovician Casco Bay Group, Harpswell, Maine

Rocks of the Casco Bay Group are continuously exposed in a narrow northeast striking belt in southern coastal and south-central Maine for a distance of nearly 150 kilometers (just south of Portland to nearly Bangor). This... [ view full abstract ]

Rocks of the Casco Bay Group are continuously exposed in a narrow northeast striking belt in southern coastal and south-central Maine for a distance of nearly 150 kilometers (just south of Portland to nearly Bangor). This Middle to Late Ordovician (≈ 460 to 475 Ma) sequence of metasedimentary and metavolcanic rocks has been interpreted to have initially formed outboard of Laurentia in an evolving Iapetus/Rheic Ocean basin and was subsequently accreted during the Silurian-Devonian Acadian orogeny. This study is focused on a portion of the Harpswell peninsula in northern Casco Bay, where rocks of the lower portion of the Casco Bay Group (Cushing, Cape Elizabeth, and Spring Point formations) are spectacularly exposed. This study involves the detailed mapping of a 9 km² area just south of Harpswell Center that was previously mapped by Arthur Hussey in his classic study of the bedrock geology of the Orrs Island 7.5’ quadrangle (Hussey, 1971). Major and trace element geochemical studies of all these rocks will provide insight on the nature of the protoliths and provide insight into their depositional/tectonic environments in Ordovician time.  Additionally, structural and petrographic investigation will shed light on the superimposed Devonian-aged Acadian deformation and metamorphism.