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The Falmouth-Brunswick sequence in the Bath map sheet (Figure 3) consists of the Mount Ararat and Nehumkeag Pond Formations (Figure 2). They were first mapped as members of the Cushing Formation (Hussey, 1985) because of their generally similar metavolcanic composition. However, subsequent studies have shown that
Because of these differences, the Nehumkeag Pond and Mount Ararat Formations are interpreted to constitute a separate sequence, perhaps of a different structural level. The type of boundary with the Central Maine sequence is not clearly known. It may be a thrust-fault boundary (Pankiwskyj, 1996) or it may be an unconformity as suggested by Osberg (1988; Tucker and others, 2001) from studies north of the Bath sheet.
The rocks of the Falmouth-Brunswick sequence are moderately migmatized, and have been metamorphosed to sillimanite or sillimanite + K-feldspar grade.
Nehumkeag Pond Formation (Onp, Onpr)
Minor units mappable at the 1:62,500 scale or larger include
These commonly occur together as discrete units, but for reasons of scale are lumped together on the Bath map sheet as Onpr. Other minor rock types form thin lenses in the formation.
Mount Ararat Formation (Oma)
The name for this formation is taken from the exposures on and around Mount Ararat, a low hill in Topsham, Maine. (See Figure 5 for localities in the Bath sheet mentioned in the text.) The name "Mount Ararat Member" was originally applied by Hussey (1985) when these rocks were first mapped and interpreted to be part of the Cushing Formation. The Mount Ararat Formation typically consists of thin alternations (1 to 10 cm) of amphibolite and felsic granofels or gneiss (Figure 6). Proportions of the two rock types vary considerably; in some areas amphibolite dominates while in others felsic granofels dominates. The amphibolite consists of plagioclase, quartz, hornblende, biotite, minor sphene, and opaque minerals. The felsic granofels or gneiss consists of plagioclase, quartz, and biotite. The alternations of felsic and mafic layers are interpreted to have been beds of felsic and mafic volcanic ash prior to metamorphism. Thin layers of calc-silicate granofels occur sporadically with the amphibolite.
Distribution of outcrops of the Mount Ararat Formation relative to the Nehumkeag Pond Formation suggests a facies interfingering or structural interleaving of the two.
Last updated on February 1, 2008
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