BEDROCK MAP AND GEOLOGY OF THE WEEKS MILLS QUADRANGLE, EAST-CENTRAL MAINE

The Weeks Mills quadrangle is located within the Central Maine basin along the boundary between the Central Maine sequence and the Falmouth-Brunswick sequence. The area is dominantly underlain by a sequence of metasedimentary and metavolcanic units trending northeasterly and dipping to the northwest. The structurally lowest, southeastern portion of the area is composed of a group of garnet-andalusite schists and grey schists with garnet coticule. A belt of mylonitic rocks, up to several hundred meters thick, separates these rocks from the next belt of rocks to the northwest. Feldspathic, biotite and muscovite-bearing schists and gneisses, thought to be correlative with the Cape Elizabeth Formation, are structurally above the mylonitic rocks. The Nehumkeag Pond Formation, composed of a belt of biotite and/or hornblende-bearing gneisses with mappable layers of amphibolite, is structurally upsection. The Beaver Ridge Formation, which is composed of rusty weathering schists, is the next unit to the northwest. The Hackmatack Pond fault forms the boundary between the Beaver Ridge Formation and the Hutchins Corner Formation to the northwest. The Hutchins Corner Formation consists of variably interlayered calc-silicate and biotite-quartz granofels. One outcrop of feldspathic schist and garnet amphibolite, correlative with the Sandy Pond Formation, is located along the Hackmatack Pond fault. Granodioritic rocks of the Three Mile Pond pluton are found in the northwest corner of the field area, and pegmatite is found pervasively throughout and forms a mappable unit along the Hackmatack Pond fault.

Two phases of folding are recognized. The first produced isoclinal folds with shallowly plunging, northeast-trending fold axes. These are overprinted by Z-folds with variably plunging, north-trending axes. Dextral shear indicators are prevalent throughout the area and are associated with formation of the mylonite which forms part of the high strain zone of the Norumbega fault.

Sillimanite is found sporadically in rocks northwest of the mylonite, while andalusite is abundant in rocks to the southeast, suggesting post-peak movement along the Norumbega fault. Schists in the Sandy Pond Formation contain an earlier staurolite + kyanite assemblage which is clearly overprinted by a later cordierite + sillimanite assemblage.