Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine

Sequence Uncertain

For several rock units, especially in the eastern part of the Bath map sheet, it is not clear to which stratigraphic sequence they belong due to structural isolation, insufficient exposure, the presence of intervening plutons, or unknown age. As more is learned about these rocks, it may be possible to assign them to one of the sequences already described, or they may need to be assigned to new sequences. The following units are described generally from west to east.

Cross River Formation (OCcr, OCcrg)

The Cross River Formation is named from exposures in and around the tidal basin of the Cross River in Boothbay (Figure 5; Hussey, 1985). In this area, the core of the Boothbay anticline, the main part of the formation (OCcr) is an extremely migmatized rusty-weathering gneiss. Locally where least migmatized, it consists of very sulfidic muscovite-graphite-sillimanite-quartz schist. Where most migmatized, the original metamorphic character is obliterated and the rock is a rusty-weathering quartz-plagioclase-biotite-muscovite gneiss with megacrysts of feldspar scattered throughout (Figure 23). The migmatite includes rafts of restitic quartzite, amphibolite, and metamorphosed calc-silicate concretions, all randomly oriented. In the Cross River area (Figure 5) a thin granofels unit (OCcrg) is mapped at the top of the formation. This unit consists of fine-grained dark gray quartz-plagioclase-biotite-garnet granofels with salt-and-pepper texture. Interbedded with this, particularly near the top, adjacent to outcrops of the Bucksport Formation, are zones of medium-grained amphibolite with large irregular porphyroblasts of garnet. The contact between the Bucksport and Cross River Formations is exposed in a small outcrop along the northwest shore of Cross River in the town of Boothbay (Figure 5). There, regrettably, the contact relation of the two formations is obscured by a pegmatite sill one meter thick. Conformability that is suggested by parallelism of bedding on either side of the contact thus cannot be confirmed. There is no evidence of shearing or faulting at this locality.

The Cross River Formation (OCcr) is exposed in a second belt to the east, in the core of the Pemaquid Harbor anticline. In this belt, the formation includes the same rusty-weathering migmatite as in the Cross River area, but also contains non-rusty, less migmatized quartz-plagioclase-biotite ± sillimanite gneiss. The granofels unit (OCcrg) has not been identified there. In both areas, the Cross River Formation crops out in the cores of antiforms and is structurally overlain by the Bucksport Formation (cross-sections A-A' and B-B' on the Bath map sheet - pdf format).

Unnamed amphibolite unit (Ouv)

Amphibolite with associated calc-silicate rocks and impure marble is mapped in a belt east of the Waldoboro pluton in Friendship (Figure 5). Locally, the amphibolite beds show well-preserved relict pillow structures with vesicular texture (Figure 24) indicating they are metavolcanic rocks. In most places, however, the pillows are strongly stretched by deformation. This unit is between gneisses possibly equivalent to the Mosquito Harbor Formation to its west, and unnamed sulfidic schists to its east (Hussey, 1971c; Newberg, 1979). The amphibolite unit is not repeated to the east as would be expected if the sulfidic schist unit were in the core of a map-scale fold.

Unnamed sulfidic schist (Ouss)

Several areas of unnamed sulfidic schist (Ouss) are mapped east of the Waldoboro pluton. One of these areas is along the shore in Friendship, east of the unnamed amphibolite unit (Ouv) (Hussey, 1971c). This unit projects northeastward into the Thomaston 7½' quadrangle (north of the Bath map sheet) where it was mapped informally as the Prison Farm unit by Guidotti (Maine Geological Survey, unpublished report, 1973; Osberg and Guidotti, 1974). The most prominent rock type is rusty-weathering muscovite-biotite-quartz schist with abundant completely to partially pseudomorphed andalusite and staurolite porphyroblasts. Minor quartzite and quartz-biotite schist are present. Bedding, where discernable, is on a 3 to 10 cm scale. In Friendship, the rusty schist contains abundant sheets of amphibolite ranging from ½ to 3 meters thick, which may represent metamorphosed mafic sills.

Another area of sulfidic schist, in the outer part of the Georges Islands (Burnt, Little Burnt, Allen, Benner, and Davis islands; Figure 5) has been referred to informally as the Allen Island Formation (King, 1994). It consists of rusty-weathering micaceous and feldspathic quartzite and medium-grained to coarse-grained muscovite-biotite-sillimanite schist. Bedding thickness varies from 1 cm to 1 m. Oval lenses of calc-silicate rock are fairly common (Eusden, written communication, 2001).

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