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 (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 outboard 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.

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). The field site lies along the western limb of the Harpswell Sound syncline where rocks of the Cushing, Cape Elizabeth, and Spring Point formations have undergone low pressure amphibolite facies metamorphism (staurolite zone). The rocks exposed here provide an excellent opportunity to explore the transition between primarily felsic volcanism in the Cushing Formation, to sedimentation in the Cape Elizabeth Formation. At this transition lies the distinctive Wilson Cove Member of the Cushing Formation – a thin Fe- and sulfide-rich unit previously interpreted to represent a mixture of hydrothermal exhalative and sediment. Finally, above the metasedimentary Cape Elizabeth Formation, a transition back to volcanism is recorded in rocks of the Spring Point Formation.

Major and trace element geochemical studies of all these rocks will provide information on the nature of the protoliths, and 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.