BEDROCK GEOLOGY OF THE BROOKS EAST AND BROOKS WEST 7.5’ QUADRANGLES, SOUTH-CENTRAL MAINE

Recent 1:24,000 scale bedrock mapping of ~ 270 km² in the Brooks area, south-central Maine has extended units mapped to the south and revealed new details of relationships among several narrow northeast trending lithotectonic belts. From northwest to southeast, the belts include: (1) Central Maine basin, (2) Falmouth-Brunswick sequence, (3) Casco Bay Group, (4) Passagassawaukeag Gneiss, and (5) Fredericton belt.

In the study area, the Central Maine basin is represented by a monotonous sequence of interlayered greenschist facies sandstone and pelite of the latest Ordovician (?) - Silurian Vassalboro Group; stratigraphic uncertainty does not allow assignment to a particular formation. The Ordovician Falmouth-Brunswick sequence is represented by rusty weathering schist and granofels of the Beaver Ridge Fm. and dominantly felsic gneisses of the Nehumkeag Pond Fm., presumably with volcanic or volcanogenic protoliths. This belt abruptly terminates midway through the study area along either a northwest trending high angle fault or a fold closure.

The Ordovician Casco Bay Group is represented by the distinctive Fe-Mn rocks of the Wilson Cove member of the Cushing Fm., and the Cape Elizabeth and Scarboro formations. Two northeast trending high strain zones of the Norumbega fault system within the Casco Bay Group extend the length of both quadrangles (> 20 km). The Hill 806 mylonite zone, recognized by Pankiwskyj (1996), is up to 150 meters wide and is dominated by quartzofeldspathic mylonitic gneisses. The newly discovered Ray Corner mylonite zone is significantly wider (up to a km) and contains a range of cross-cutting fault rock types including mylonite, cataclastite, lithified fault breccia, and pseudotachylyte. Several rock units southeast of the Ray Corner zone are truncated by it on the map.

To the southeast, the Casco Bay Group is separated by an unnamed mylonite zone from variably sheared upper amphibolite facies schist, gneiss, and migmatite of the Ordovician or older Passagassawaukeag Gneiss. Farther southeast, a narrow zone of striped gneiss separates the Passagassawaukeag Gneiss from the Fredericton Belt (Bucksport Fm.). Mappable intrusive rock bodies are limited to the southeastern portion of the study area and include the northern end of the Mixer Pond gneiss and a portion of the Late Devonian Mt. Waldo pluton.