Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine
Benner Hill Sequence
The Benner Hill sequence (Osberg and Guidotti, 1974; Guidotti, 1979; Berry and others, 2000) occupies an area between the Waldoboro pluton and the Spruce Head pluton (Figure 3). It includes the Mosquito Harbor Formation, the Hart Neck Formation, and the Benner Hill Formation (Figure 2).
Two unnamed units in the Friendship area (Figure 5), one containing amphibolite, calc-silicate rocks, and marble, and the other containing rusty-weathering schist, have been previously assigned to the Benner Hill sequence (Hussey, 1971c; Newberg, 1979). Work in the Thomaston 7½' quadrangle to the north of the Bath map sheet by Berry and others (2000) suggests, however, that those two units might not belong to the Benner Hill sequence. Considering this interpretation, those units are presented below under the heading of "sequence uncertain."
Mosquito Harbor Formation (Omh)
The Mosquito Harbor Formation (Berry and others, 2000) is named for exposures on the northern shore of Mosquito Harbor, northeast of Port Clyde (Figure 5). The unit is characterized by light-colored feldspathic quartzite and coarse-grained gray mica schist interbedded on a ¼ to 1 cm scale (Guidotti, 1979). When weathered, the rock develops a pronounced "ribbed" surface due to the thin quartzite layers that stand out in relief. Figure 22 shows the character of this unit at Marshall Point in Port Clyde (Figure 5) where it is migmatitic and deformed. The unit commonly includes layers up to 30 cm thick of flaggy-bedded dark green amphibolite. A few thicker amphibolite layers are also present. On the Bath map sheet, gneisses immediately east of the Waldoboro pluton are assigned to this unit with uncertainty, although if the interpretation of Berry and others (2000) proves correct, that assignment may not be tenable.
The Mosquito Harbor Formation is overlain conformably by the Hart Neck Formation, with the contact gradational over a thickness of 50 meters (Guidotti, 1979).
Hart Neck Formation (Ohn)
The Hart Neck Formation (Berry and others, 2000) is named for shoreline exposures on Hart Neck, south of Tenants Harbor (Figure 5). It is characterized by Guidotti (1979) as a heterogeneous unit consisting of numerous gray-weathering, biotitic rock types including quartzite, granofels, quartzose metapelite, grits, and quartz-pebble metaconglomerate, with granofels, grits, and metapelite being most common. Garnet is rare. Bedding thickness is commonly 3 to 10 cm, but ranges from a few millimeters up to 3 meters thick. Some of the quartz metapelites are fissile with mm-scale light-colored quartz laminae, and are difficult to distinguish from those in the Mosquito Harbor Formation. Metapelites in the Hart Neck Formation are almost always closely associated with biotite granofels and grits, both of which are nearly absent in the Mosquito Harbor Formation. The quartz-pebble metaconglomerate, though present only in thin units, is one of the most distinctive rocks of the Hart Neck Formation. Pebbles are mostly quartzite and vein quartz ranging up to 10 cm and set in a biotitic quartzite matrix.
The contact of the Hart Neck Formation with the overlying Benner Hill Formation is fairly sharp and interpreted to be conformable (Guidotti, 1979; Berry and others, 2000).
Benner Hill Formation (Obh)
The Benner Hill Formation (Osberg and others, 1985) consists mostly of thinly interbedded rusty-weathering quartz-mica schist and quartzite. In most areas thin bedlets rich in pale garnet characterize the formation (Osberg and Guidotti, 1974). In a few places rocks of this lithology are interbedded with mafic metavolcanic rocks, some of which have relict structures resembling pillows. Rusty mica schist is interbedded with garnet-bearing rocks typical of the Benner Hill Formation on Southern Island at the mouth of Tenants Harbor (Figure 5; Guidotti, 1979). Black, graphite-rich schist occurs at the base of the formation in the Thomaston quadrangle (Berry and others, 2000).
In places, such as along the west shore of the St. George River estuary just south of the Spruce Head pluton, rocks with abundant garnet bedlets are on strike with rusty-weathering quartz-mica schists described informally as the Prison Farm lithology (Hussey, 1985). This map relationship suggests that the Prison Farm unit is a facies variant of the Benner Hill Formation (Osberg and Guidotti, 1974; Guidotti, 1979; Osberg and others, 1985). Berry and others (2000) offer as an alternative explanation that a major regional thrust fault separates the Benner Hill Formation to the east from the Prison Farm unit and other units to its west.
Rocks Exposed on the Georges Islands, St. George
Eusden and others (1996) and Eden and Pavlik (1996) have mapped non-rusty to slightly rusty interbedded micaceous metawacke and metapelite on the islands comprising the inner part of the Georges Islands group off Port Clyde (Figure 5). Eusden (2001, unpublished compilation map) now correlates rocks on the northern island (Caldwell Island) with the Hart Neck Formation, and rocks on adjacent islands to its south and east (Little Caldwell, Teal, and Hooper Islands) with the Mosquito Harbor Formation. At the top of the Mosquito Harbor Formation is a thin, yet persistent unit of amphibolite and garnet coticule. The formations project directly to the mainland units at Port Clyde, so they are correlated with reasonable confidence. Isolated exposures along the shores of a few southerly islands in the Georges Island group (Davis and Benner Islands) are tentatively assigned to the Mosquito Harbor Formation because of their somewhat similar lithology. But due to lack of offshore exposure, stratigraphic continuity with other units of the Benner Hill sequence cannot be established, making their stratigraphic position somewhat tentative.
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