DECIPHERING THE ORDOVICIAN TECTONIC HISTORY OF A PORTION OF THE LIBERTY-ORRINGTON BELT IN SOUTH-CENTRAL MAINE

Ordovician rocks of the Liberty-Orrington belt in south-central Maine have been repeatedly deformed, metamorphosed and intruded by igneous bodies during the Silurian-Devonian Acadian Orogeny. While much research has been focused on Acadian tectonism in the region, very little is known about the Ordovician history of these rocks. This work is focused on unraveling the Ordovician history of a portion of the Liberty-Ordovician belt (Razorville 7.5’ quad.) through a combination of bedrock mapping, metamorphic stratigraphy, and geochemical analyses of selected metaigneous rocks.

Rocks in the study area have been previously assigned to the Casco Bay Group (Cushing and Cape Elizabeth Formations) and more recent studies in adjacent areas have subdivided the Cushing Fm. into several different units. Because original relative age relationships have been obscured by Acadian tectonism, the following stratigraphic descriptions proceed from southeast to northwest across the strike of the belt. The metasedimentary Cape Elizabeth Fm. is composed primarily of interlayered feldspathic mica schists and granofels. Immediately to the northwest of the Cape Elizabeth Fm. is a thin but distinctive rusty weathering garnet-grunerite gneiss (Wilson Cove Mbr. of the Cushing Fm.), which is interpreted to represent a metamorphosed iron rich horizon (rock contains up to 40 wt.% iron). The next major unit within what had previously been mapped as the Cushing Fm. is the Sheepscot Pond Gneiss. This unit, previously unrecognized in the study area, is characterized by felsic gneisses with thin interlayers of amphibolite, and is interpreted to represent highly deformed and metamorphosed volcanic rocks and/or intrusives. Northwest of the Sheepscot Pond Gneiss is a thick heterogeneous assemblage of lithologies that likely correlates with the Nehumkeag Pond Fm. of the Falmouth-Brunswick sequence. The unit is dominated by quartz-plagioclase-biotite gneiss with subordinate amounts of amphibolite, rusty schist/gneiss and calc-silicate/marble. This unit likely represents an assemblage of metamorphosed volcanogenic sediment and volcanic rock.

Contact relationships between the above units are obscured by poor exposure, extreme deformation and metamorphism. Geochemical analyses of metaigneous rocks in the study area are in progress and should provide insight on the tectonic setting.