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Casco Bay Group
Rocks of the Casco Bay area were first mapped and named by Katz (1917) west of the Bath map sheet. He defined the Cape Elizabeth formation, Spring Point greenstone, Diamond Island slate, Scarboro phyllite, Spurwink limestone, Jewell phyllite, and Mackworth slate, which he referred to collectively as the Casco Bay Group. The units have since been renamed as formations, except for the modified Spurwink Metalimestone (Bodine, 1965; Hussey, 1985, 1988). The Mackworth Formation, which is not exposed in the Bath map sheet, is now correlated with the Eliot Formation of the Merrimack Group (Berry and Hussey, 1998) and is therefore no longer included with the Casco Bay Group. The Cushing Formation was mapped separately by Katz (1917) as the Cushing Granodiorite. Subsequent investigations by Bodine (1965) and Hussey (1971a, 1971b) revealed the fragmental volcanic nature of the formation, and Hussey (1971a, 1971b) included it as a stratigraphic unit at the base of the Casco Bay Group.
With the separation of the Mount Ararat and Nehumkeag Pond Formations from the original Casco Bay Group to form the separate Falmouth-Brunswick sequence, Hussey (1988) referred to the remaining parts of the Casco Bay Group as the "Saco-Harpswell sequence," thus introducing some confusion. In order to undo this confusion we now abandon the name "Saco-Harpswell sequence" and retain simply "Casco Bay Group" for the same rocks. The Casco Bay Group now includes Katz's original stratigraphic units (except for the Mackworth Formation which is correlated with the Eliot Formation of the Merrimack sequence), and the Cushing Formation, but not rocks of the Falmouth-Brunswick sequence.
Cushing Formation (Ocp, Oca, Ocm, Ocw)
The Cushing Formation consists predominantly of metamorphosed volcanic rocks of felsic to intermediate composition that lie below the Cape Elizabeth Formation. The Cushing Formation is subdivided into the Peaks Island, Merepoint, and Wilson Cove Members. Units of the East Harpswell Group that lie structurally below the Cape Elizabeth Formation in Harpswell (Figure 3 and Figure 5) were formerly included as members of the Cushing Formation (Hussey, 1985). Recent radiometric age determinations, discussed below, show conclusively that these volcanic rocks are distinctly younger than, and therefore cannot correlate with, the Cushing Formation. The contact between the Casco Bay Group and the East Harpswell Group, nowhere exposed, is inferred to be either an unconformity or a pre-metamorphic thrust fault.
Peaks Island Member (Ocp)
The Peaks Island Member of the Cushing Formation (Hussey, 1985, 1988) underlies a portion of the Bath map sheet in the general area of Middle Bay, Harpswell (Figure 5). To the southwest, this belt continues through Casco Bay to the town of Cape Elizabeth. This is the part of the Cushing Formation originally interpreted by Katz (1917) to be a deformed granodiorite pluton. The principal lithology is plagioclase-quartz-biotite-muscovite ± microcline granofels and gneiss. Textures and structures suggesting that these rocks are volcanic rather than intrusive in origin include
Composition of these metavolcanic rocks based on the metamorphic mineralogy, ranges from rhyolite to dacite with rhyodacite predominating.
Amphibolite (Oca), representing a basaltic to andesitic protolith, occurs at or near the top of the Peaks Island Member as a narrow (150 m-wide) lens extending from the north end of Middle Bay 10 km northeast to the Topsham shore of Merrymeeting Bay (Bath map sheet - pdf format; Figure 5). Minor rock types include muscovite-biotite-quartz-plagioclase schist (Figure 9), and calc-silicate granofels or gneiss in lenses and concretions.
Part of the Peaks Island Member is overlain conformably by and locally interfingers with the Wilson Cove Member of the Cushing Formation. In other places, it is overlain unconformably(?) by the Cape Elizabeth Formation (discussed below).
Merepoint Member (Ocm)
The Merepoint Member of the Cushing Formation is a rusty-weathering muscovite-biotite-quartz-plagioclase gneiss (Hussey, 1985, 1988). Its mineralogy is very similar to that of the Bethel Point Formation of the East Harpswell Group with which it was originally correlated (Hussey, 1971b). However, because of significant age differences (about 472 Ma for Merepoint vs. about 445 Ma for Bethel Point), the correlation is no longer tenable. Within the Bath map sheet, the Merepoint Member is exposed on Merepoint Neck, Brunswick (Figure 5), as a thin (about 100 meters) lens within gneisses of the Peaks Island Member.
Wilson Cove Member (Ocw)
In the vicinity of Lookout Point on Harpswell Neck (Figure 5) the Wilson Cove Member fingers out southward into white plagioclase-quartz-muscovite-biotite schistose granofels of the Peaks Island Member as mapped in detail by Hussey (1971b). Northeast of Lookout Point, the Wilson Cove Member is a coherent unit, the outcrop belt of which parallels the shore of Wilson Cove and then swings northward to where it is cut off by the South Harpswell and Cape Elizabeth faults in Middle Bay. On the west side of the faults, the Wilson Cove Member crops out in a narrow belt extending from the Pennelville area of Brunswick (Figure 5) a distance of 4 kilometers to the north-northeast where it pinches out against the Cape Elizabeth Formation. This pinch-out may be either
The main outcrop belt of the Wilson Cove Member continues to the southwest mostly underwater, but emerges on Scrag Island and Birch Island at the very western edge of the map. It extends beyond the Bath sheet to be exposed on Little Iron Island, Black Rock, and Irony Island, names undoubtedly taken from the black, iron-sulfide-rich rocks of the unit.
Cape Elizabeth Formation (Oce, Ocer, Ocep, Ocea, Ocec)
The Cape Elizabeth Formation is the most widely distributed formation of the Casco Bay Group within the map area. Its metamorphic grade ranges from staurolite to sillimanite + K-feldspar in a Buchan-type metamorphic facies series. At the highest grade it is extensively migmatized.
Exposed along the eastern shore of Bailey and Orrs Islands and on Jaquish Island (just west of the Bath map sheet) is a 1- to 3-meter wide conformable lens of hornblende-tremolite-talc-biotite schist within the Cape Elizabeth Formation (Oce). This probably represents a metamorphosed ultramafic sill. It has been mined on a small scale. In his report on the first geological survey of the State of Maine, Jackson (1837) briefly mentions a small talc (talcum of his day) mining operation on Jaquish Island, and this is probably an extension of the Bailey Island lens.
Locally in the Harpswell Neck and Cundys Harbor areas of Harpswell (Figure 5), a belt of very rusty-weathering muscovite-quartz-biotite-staurolite or sillimanite schist (Ocer) crops out at the base of the Cape Elizabeth Formation.
Several thin belts of amphibolite (Ocea) and associated units are mapped within the Cape Elizabeth Formation. The amphibolites represent two or more separate stratigraphic horizons, though their relationships to each other are not well known. They are shown on Figure 13 and are described here from west to east. Just west of Bath, a two-km long and 200-m wide belt of amphibolite and associated slightly rusty-weathering marble and calc-silicate granofels is present within migmatized Cape Elizabeth rocks. This lens may be more extensive, but has not been traced further north or south along strike. In and around The Basin in Phippsburg, and extending around the northern part of the Phippsburg synform (Figure 13), is another similar association, dark gray amphibolite, white marble, and coarse-grained skarn-like calc-silicate granofels with diopside, grossularite, vesuvianite, and scapolite. A small prospect in this unit on the north shore of The Basin is a world-class specimen locality for the mineral grossularite (Thompson and others, 1991), popularly known as "cinnamon" garnet. This belt and the Bath belt may represent the same stratigraphic horizon within the Cape Elizabeth Formation.
On Georgetown Island, amphibolite without marble or calc-silicate rock crops out in a belt from the southern part of Robinhood Cove to Indian Point (Figure 13). The amphibolite exposed on Seguin Island, about 4 km south of the tip of Georgetown Island, is probably a continuation of this belt. It is associated with a sequence of thin units of rusty schist, marble, and calc-silicate granofels which are not shown separately on the Bath map sheet. Also associated with the amphibolite, and separately represented on the Bath sheet as Ocec, is a distinctive reddish-brown garnet-magnetite-grunerite(?) granofels, known as coticule (Figure 14). Some specimens of the garnet have a smooth, dense texture, resembling jasper. The coticule is characteristically interbedded with quartz-plagioclase-biotite granofels in beds one to 4 cm thick. The coticule apparently pinches out at Robinhood Cove, but appears again on the western side of the island, being particularly well exposed in the West Georgetown area and on Marr and Long Islands in the lower Kennebec River (Figure 13). A second belt of coticule (Ocec) occurs at the north end of Arrowsic Island, and probably represents the same stratigraphic horizon as the Georgetown belt.
A thin but conspicuous belt (Ocea) of amphibolite and closely associated calc-silicate granofels (in part, very coarse-grained) occupies a fault-broken and folded belt near the Back River in Woolwich, Wiscasset, and Westport (Figure 13). The amphibolite is 10 to 15 m wide, consisting of hornblende, plagioclase, quartz, and biotite. Separated from this by about 20 meters of normal quartzo-feldspathic Cape Elizabeth lithology is a thin calc-silicate granofels or skarn, characteristically with bright green diopside and reddish orange grossularite, the combination of which gives the rock a brightly colored aspect that was responsible for the useful, if not highly technical, field term "Christmas-tree rock."
Lastly, a belt of amphibolite (Ocea) within the Cape Elizabeth Formation crops out in a convoluted complex of F1 and F2 folds in the East Boothbay area and some of the islands south of Boothbay Harbor (Figure 13). It occupies a position near the Boothbay thrust at the structural base of the Cape Elizabeth Formation. This amphibolite is of simple mineralogy (plagioclase-hornblende-biotite).
The Cape Elizabeth - Cushing Contact
Careful examination of the Bath map sheet from Middle Bay (just west of Harpswell Neck) northeast to Merrymeeting Bay reveals the pinch-out of different minor units of the Cushing Formation against the base of the overlying Cape Elizabeth Formation. Alternative explanations for the relations shown include:
Alternative 1 is favored because of simplicity, the lack of other indications of major faulting, and the presence of grit zones locally at the base of the Cape Elizabeth Formation in the Portland area (Hussey, 1988).
Spring Point Formation (Ospb, Ospf)
The Spring Point Formation, named for Spring Point, South Portland, near the entrance to Portland Harbor (west of the Bath map sheet) conformably overlies the Cape Elizabeth Formation (Figure 2). The mafic member (Ospb) consists of thin-bedded hornblende-garnet-plagioclase amphibolite (Figure 15), hornblende-cummingtonite amphibolite, and a distinctive garnet-hornblende-quartz-plagioclase band 3 m thick. In Harpswell Sound a felsic member (Ospf) of thin bedded light gray quartz-plagioclase-biotite-muscovite schistose granofels is mapped at the upper part of the formation. The amphibolites represent metamorphosed basaltic to intermediate volcanic rocks, probably pyroclastic in origin in view of the conspicuous bedding present. The quartzo-feldspathic upper member represents either felsic pyroclastic waterlaid volcanics or slightly reworked volcanogenic sediments.
Rocks assigned to the Spring Point Formation are also exposed in a convoluted, narrow belt in the middle of the Cape Small synform and on the east flank of the Phippsburg synform (Figure 16). On the Bath map sheet, those belts are included in "Upper Casco Bay Group, undifferentiated" (Ocbu) because they are so thin.
Diamond Island Formation (Odi)
The Diamond Island Formation, named for Great Diamond and Little Diamond Islands in Casco Bay, is a thin unit conformably overlying the Spring Point Formation (Figure 2). Within the Bath map sheet, the Diamond Island Formation crops out in the Cape Small and Phippsburg synforms (Figure 16), included on the Bath map sheet in Ocbu. There, a distinctive black, rusty-weathering quartz-graphite-muscovite phyllite occurs in association with, and presumably above, the amphibolite and calc-silicate gneiss tentatively correlated with the Spring Point Formation.
In the Harpswell Sound area (Figure 5) the Diamond Island Formation has not been observed within the Bath map sheet. It may be present but not exposed between outcrops of the Spring Point and Scarboro Formations; or it may be absent because of stratigraphic pinch-out, tectonic thinning, or faulting. It is present on Barnes Island approximately 2 km west of the map area (Hussey, 1971b).
Scarboro Formation (Osc)
The Scarboro Formation, named for the town of Scarboro (now Scarborough), is exposed on the shores of Harpswell Sound at the very western edge of the Bath map sheet. The following description is based on exposures at the northwestern end of Bailey Island (Figure 5). There, the Scarboro Formation is rusty-weathering fine-grained muscovite-biotite-garnet-quartz ± staurolite ± andalusite schist. Andalusite typically occurs as large poikiloblasts enclosing all other phases. In nearby exposures just west of the Bath map sheet along Merriconeag Sound, the Scarboro Formation includes non-rusty schist of the same mineralogy, and some graphitic dark-matrix schist with biotite, chlorite, and minor graphite, chloritoid, and staurolite.
Farther east, in the Small Point area, pelitic schist stratigraphically above the Diamond Island Formation is correlated with the Scarboro Formation (Figure 16). As with the Spring Point and Diamond Island Formations, this occurrence is included in the "Upper Casco Bay Group undifferentiated" unit (Ocbu) on the Bath map sheet.
Spurwink Metalimestone (Osk) and Jewell Formation (Oj)
At the very western edge of the map sheet, the two units above the Scarboro Formation are exposed. The Spurwink Metalimestone (Hussey, 1985, 1988) is a fine-grained meta- limestone thinly interbedded with phyllite. The Jewell Formation is lithologically identical to the Scarboro Formation, and is distinguished only by stratigraphic position above the thin Spurwink unit. These two formations, named for the Spurwink River in Scarborough and Cape Elizabeth and for Jewell Island in Casco Bay, are more widely exposed west of the Bath map sheet.
Last updated on February 1, 2008
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