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

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 (from 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 outbound of Laurentia in an evolving Iapetus/Rheic Ocean basin and then 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. Detailed studies in this area focus on unraveling both the Ordovician pre-accretionary history of the protolith rocks, and the superimposed deformational and metamorphic events associated with their Acadian accretion.