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

East Harpswell Group

Rocks of this group were originally included as members of the Cushing Formation (Hussey, 1971b, 1985, 1988). However, radiometric ages (presented below) suggest that the lowest unit, the Yarmouth Island Formation, is Late Ordovician-Early Silurian, younger than any dated rocks of the Casco Bay Group. Lacking evidence of large scale overturning of the sequence, the newly recognized age relationships warrant separation of these rock units into a distinct lithostratigraphic sequence, the East Harpswell Group, as proposed here (Figure 2). The Yarmouth Island, Bethel Point, and Sebascodegan are now regarded as formations of this group.

The East Harpswell Group is exposed in a single north-northeast-trending anticline, the Hen Cove anticline (Figure 3 and Figure 17). The Yarmouth Island Formation is exposed in the core of the fold in the southern part of the belt, where the plunge is gently northward. It is overlain by the Bethel Point Formation, which wraps around the Yarmouth Island Formation in the south and forms the core of the fold to the north where the plunge is nearly horizontal. The Sebascodegan Formation is exposed mainly on the west and east flanks of the fold.


Yarmouth Island Formation (SOy, SOya)

The Yarmouth Island Formation consists dominantly of light gray metamorphosed felsic to intermediate volcanic rocks with lesser calc-silicate granofels or gneiss, amphibolite (some of which may be of sedimentary rather than igneous origin), and feldspathic volcanogenic metasedimentary rocks. The dominant lithology is light gray feebly thin-bedded to massive plagioclase-quartz-gedrite-garnet-cummingtonite gneiss and granofels, probably representing relatively magnesian volcanics of dacitic composition (Figure 18). Other rock types, occurring as thin beds or zones, include plagioclase-quartz-biotite-sillimanite-garnet granofels, locally with cordierite, poikiloblastic staurolite, and pseudomorphs of muscovite, margarite, hercynitic spinel, and chlorite after staurolite (Faiia, 1996). These aluminous granofelses probably represent post-eruptive submarine alteration and reworking of the dacitic fragmental volcanic rocks. Andalusite has been observed in a quartz vein cutting the sillimanite-bearing rocks at one locality. Units within the formation, mappable at 1:24,000 scale (Hussey, 1971b), include calc-silicate gneiss with marble interbeds, and amphibolite. Only one of the amphibolite members (SOya) is shown on the Bath sheet. Most amphibolite is intimately associated with or has interbeds of calc-silicate rock, and may represent metamorphosed calcareous sediments rather than mafic metavolcanics.

The Yarmouth Island Formation is restricted in outcrop to the core of the Hen Cove anticline at its southern end (Figure 17), and is structurally, and probably stratigraphically, the lowest unit of the East Harpswell Group.

Bethel Point Formation (SObp)

The Bethel Point Formation (Hussey, 1971b, 1985) has a simple and consistent lithology. It consists of very rusty-weathering sulfidic quartz-muscovite-biotite schist with rare quartz-plagioclase-biotite-muscovite granofels interbeds. East of Brunswick (Figure 17), the formation includes a 5-10 meter wide lens of marble that is not known elsewhere. Within the Bath map sheet the Bethel Point Formation crops out only in the core and limbs of the variably-plunging Hen Cove anticline, and separates the Yarmouth Island Formation from the Sebascodegan Formation. The contacts with these formations appear conformable.

Sebascodegan Formation (SOs, SOsa, SOsr, SOspi)

On the western limb of the Hen Cove anticline (Figure 17), the Sebascodegan Formation consists of thinly interbedded rocks of the following lithology. From most to least abundant they are:

  1. light gray plagioclase-quartz-biotite-granofels or gneiss, locally preserving volcanic breccia structure; biotite commonly forms lineated clots giving the rock a coarsely spotted appearance;
  2. medium greenish gray calc-silicate granofels with grossularite; meionite (Ca-scapolite) was noted in thin section from calc-silicate granofels exposed on Bombazine Island in New Meadows estuary just north of Sebascodegan Island (Figure 17);
  3. quartz-plagioclase-biotite-muscovite-sillimanite granofels; and
  4. plagioclase-quartz-biotite-cummingtonite granofels.

Interbedded throughout the exposures are 1- to 2-meter-thick sections of rusty weathering biotite-muscovite-quartz schist.

Minor mappable units shown on the Orrs Island 1:24,000-scale map sheet (Hussey, 1971b), not all of which are shown separately on the Bath map sheet, include rusty-weathering sulfidic quartz-plagioclase-biotite granofels, schist, and gneiss (SOsr), locally with chromian muscovite (formerly referred to as fuchsite); three types of amphibolite (SOsa), described below; calc-silicate granofels (SOspi); and feldspathic biotite-tourmaline quartzite. The most mineralogically variable amphibolite, informally referred to as the Dyer Cove type after Dyer Cove (Figure 17; Hussey, 1985) consists of hornblende, gedrite, cummingtonite, light-brown biotite (probably magnesium-rich), and plagioclase in the compositional range labradorite-bytownite. Some outcrops show thin interbeds of calc-silicate rock, occasionally with calcite. The Dyer Cove type amphibolite includes a 2-meter-thick bed of massive pure calcite marble (Figure 19). Its outcrop belt is well marked by a linear series of narrow, shallow quarries where the marble was quarried during the early and middle 19th century for industrial and agricultural purposes. Because of its calcium-rich composition, this amphibolite type is interpreted to be a metamorphosed impure carbonate. In the same belt, especially along strike to the north and on the east limb of the Hen Cove anticline at essentially the same stratigraphic horizon (Figure 17) is a second type of amphibolite, of much simpler mineralogy (plagioclase-hornblende-biotite-sphene), more typical of a metamorphosed volcanic or intrusive rock of mafic to intermediate composition. Another belt of amphibolite, extending from Pinkham Point, Harpswell, to Cedar Ledge, and a 20-meter-wide unit at Long Point, Harpswell (Figure 17), constitute a third distinctive type of amphibolite. Generally of simple mineralogy, with hornblende, andesine, biotite, and sphene, this amphibolite type is significantly coarser-grained (Figure 20). The third type of amphibolite may represent metamorphosed pre-tectonic diorite sills.

lens of calcite marble
Figure 19
coarse-grained amphibolite
Figure 20

The calc-silicate granofels unit, informally referred to here as the Pole Island member of the Sebascodegan Formation (SOspi), occurs as a 10-km-long lens on the west limb of the Hen Cove anticline, but does not crop out along the eastern limb (Figure 17; see also Hussey, 1971b). The typical rock type of this lens is light greenish gray quartz-plagioclase-hornblende-biotite-microcline-clinozoisite-diopside ± grossularite granofels. The feldspathic quartzite unit, likewise, is exposed only on the west limb of the Hen Cove anticline, forming a 50-meter-wide belt east of the Dyer Cove type amphibolite and west of the Pole Island member. On the Bath sheet, the quartzite unit is not shown separately, but is included in SOspi.


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