Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine
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
- there are no reasonably close lithic correlations between the Cushing Formation on the one hand and the Mount Ararat and Nehumkeag Pond Formations on the other hand;
- the Mount Ararat and Nehumkeag Pond Formations are separated from the Cushing Formation by the Flying Point fault, one of the principal segments of the Norumbega fault system (Swanson, 1995; Hussey, 1988, 1989); and
- the sequences on either side of the Flying Point fault have been subjected to significantly different thermal histories (West and others, 1993).
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)
The Nehumkeag Pond Formation was named by Newberg (1984) for exposures near Nehumkeag Pond in the town of Pittston, north of the Bath map sheet. The dominant lithology is gray plagioclase-quartz-biotite ± garnet gneiss and granofels of probable metavolcanic origin (Figure 4). Along with this are thin- to medium-layered plagioclase-quartz-biotite granofels and gneiss with thin partings of biotite-rich gneissose schist of volcanogenic origin, and zones of quartz-plagioclase-biotite- muscovite-sillimanite ± garnet gneiss or schist interpreted to be metamorphosed interlayered aluminous wackes.
Minor units mappable at the 1:62,500 scale or larger include
- impure marble,
- rusty-weathering, sulfidic muscovite-biotite-quartz ± graphite schist, and
- dark gray hornblende amphibolite with minor associated calc-silicate gneiss.
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.
Introduction Central Maine sequence Falmouth-Brunswick sequence Casco Bay Group East Harpswell Group Fredericton sequence Megunticook sequence Benner Hill sequence Sequence uncertain Correlations Intrusives Structure Metamorphism Timing Minerals Acknowledgements References
Last updated on February 1, 2008