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

Megunticook Sequence

The Megunticook sequence (Osberg and Guidotti, 1974) represents the southwestward extension of the St. Croix Belt of eastern Maine (Ludman, 1987). In the Bath map sheet, this sequence is represented by the Penobscot Formation (Figure 2) exposed on islands at the east edge of the map (Figure 3). Rocks of uncertain sequence which might belong to the Megunticook sequence include the sulfidic and nonsulfidic migmatites of the Cross River Formation, sulfidic schist of the Friendship area, and rusty migmatites exposed on the outer islands of the Georges Islands archipelago (Eusden and others, 1996). Until stratigraphic relationships are better understood, those rocks are listed below under "sequence uncertain."

Penobscot Formation (OCp)

Rocks exposed on Metinic Island, toward the eastern edge of the map sheet (Figure 5), are assigned by Guidotti (1979) to the Penobscot Formation (formerly his Owls Head unit) of the Megunticook sequence. According to Guidotti (1979), "The rocks there are virtually identical with the strata at Owls Head, "just north of the Bath map sheet. Guidotti (Maine Geological Survey, unpublished report, 1973) describes the rocks at Owls Head as "Mainly thick-bedded, dark biotite quartzites, schists, and granulites plus massive meta-graywacke or andesite tuffs. Most outcrops have rusty weathering surfaces. Also considerable bedded, buff-weathering quartzite and some metamorphosed mafic volcanics...Some rusty weathering pelites and minor limestones." The informal Owls Head unit has since been correlated with the Penobscot Formation (Osberg and others, 1985; Berry and Osberg, 1989; Berry and others, 2000).

Two Bush Island at the northeast corner of the Bath map sheet (Figure 5), is underlain by rusty-weathering metamorphic rocks that Guidotti (1979) suggests may belong to the Penobscot Formation (his Owls Head unit), based on reconnaissance from a boat. Rusty-weathering metamorphic rocks on Mosquito Island (Figure 5) were originally assigned by Guidotti (1979) to the Benner Hill sequence, but he now assigns them (Berry and others, 2000) to the Penobscot Formation of the Megunticook sequence. The structural relationship of Mosquito Island to smaller islands to the southwest, tentatively assigned to the Benner Hill sequence on the Bath map sheet, is not firmly established.


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