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

Intrusive Rocks

There is a variety of metamorphosed and unmetamorphosed plutonic rocks in the Bath map sheet. The metamorphosed intrusive bodies are mostly dike-like or sill-like. These include the Lincoln Sill, Edgecomb Gneiss, Oak Island Gneiss, and small unnamed metadiorite and metagabbro plutons. Minor metamorphosed dikes and sills of mafic and ultramafic composition are sporadically distributed in the Cape Elizabeth and Cushing Formations.

Unmetamorphosed igneous rocks range from Silurian to Mesozoic age. Major granitic intrusions are the Silurian Spruce Head pluton and the Devonian Waldoboro pluton. Gabbro and diorite are present in the Raccoon pluton in the northeastern part of the map in the town of Cushing, and a gabbro pluton of unknown size underlies Monhegan Island (Figure 5; Lord, 1900). A belt five km wide in the Woolwich-Bath-Phippsburg area (Figure 5) is characterized by numerous highly elongate two-mica-garnet granite plutons. Some have weakly developed foliation, but most are massive. Some of these minor intrusions are of granodiorite to quartz diorite composition. Granitic pegmatite dikes, lenses, and irregular masses are common throughout areas where the metamorphic rocks are strongly migmatized. Mesozoic dikes of basalt and diabase are sparse but relatively well distributed in the area. Only one, the Christmas Cove dike, is shown on the map.

Raccoon pluton (Srgb, Srgbh)

Gabbro and diorite form an irregular igneous complex on the west side of the St. George River estuary near the northeastern corner of the Bath map sheet (Figure 3). Within the pluton, Sidle (1991) recognizes noritic gabbro, hornblende gabbro, and diorite. Noritic gabbro (Srgb) is dark olive to brownish green, containing plagioclase, augite (varying to ferroaugite), non-pleochroic green to colorless orthopyroxene, magnetite, and minor amounts of hornblende and biotite. Cumulate layers with assemblages plagioclase-orthopyroxene and plagioclase-orthopyroxene-augite occur in the noritic gabbro. Igneous lamination is weakly to moderately developed. Sidle (1991) reports that diorite and amphibole gabbro (Srgbh) are the predominant rock types of the pluton. No systematic descriptions or modes are given for these rocks.

South of the Raccoon pluton, in the towns of Cushing and Friendship on either side of Meduncook River estuary, Sidle (1991) maps foliated masses of diorite which he states "are gradational with northeastward-striking amphibolites and amphibolite schists." These amphibolites are included in the unnamed amphibolite unit (Obv) of the Benner Hill sequence on the Bath map sheet. Some of the coarser-grained metadiorites may represent feeder reservoirs for the pillow-structured amphibolites described above.

Spruce Head pluton (Ssg, Ssgb, Ssgd)

The southern third of the Spruce Head pluton is exposed in the northeastern corner of the Bath map sheet (Figure 3). An extensive investigation of its geochemistry and petrology was done by Ayuso and Arth (1997). According to them, it is a composite pluton consisting predominantly of granodiorite and granite with subordinate yet substantial volumes of gabbro, diorite, and tonalite. Ayuso and Arth (1997) identified four petrographic groups, each with its own trace element and rare earth element composition. They deduce at least three distinct pulses of magma, all in close temporal and spatial relationship. Most of the pluton in the Bath map sheet is shown generally as coarse-grained, massive to foliated biotite-muscovite granodiorite and granite and hornblende-biotite granodiorite (Ssg). The gabbro (Ssgb) and diorite to granodiorite (Ssgd) units of Ayuso and Arth are shown separately.

Figure 25

In the early 1900's the granite (Ssg) was quarried extensively in the Tenants Harbor area along the coast (Figure 25). Several quarries that were active at the time are described by Dale (1907). Chayes (1952) included specimens of Spruce Head granite in his modal analysis of New England calc-alkaline granites. The most recent age given for the Spruce Head pluton is a 421 ± 1.0 Ma concordant zircon age from hornblende granite (Tucker and others, 2001).

Monhegan pluton and other gabbro and diorite bodies (Sgb)

According to the mapping of Lord (1900), gabbroic rocks occupy Monhegan Island, adjacent Manana Island, and Duck Rocks. He mapped three phases:

olivine norite,
gabbro-diorite, and
hornblende gabbro.

Olivine norite occupies the southern two-thirds of Monhegan Island, and gabbro diorite the northern third, with hornblende gabbro confined to a small oval zone within the gabbro-diorite near the north tip of the island.

Lord (1900) describes the olivine norite as coarse-grained, feldspathic ("resembling, in part, at least, the olivine bearing anorthosyte of the Saguenay District, Canada"), purplish gray to steel gray in color, and consisting of plagioclase, olivine, hypersthene, magnetite, diallage, and hornblende. Minor variants are characterized as troctolite, olivine gabbro, and gabbro "so intimately associated that no distinctive lines could be drawn between them in the field." The olivine norite and associated phases pass gradationally into gabbro-diorite and hornblende gabbro of the northern end of the island. These phases are finer grained and variegated in color due, as Lord (1900) notes, to irregular inclusions of quartz-bearing, light gray diorite.

Randomly-oriented mafic dikes of basalt or diabase from a few millimeters to one meter in thickness cut both major phases of the Monhegan pluton. Mineralogically they range in composition from hypersthene gabbro to gabbro diorite, quite similar to the major phases of the pluton. Younger than the mafic dikes are a variety of felsic dikes of relatively uniform strike (N 50 to 60 degrees W) in the northern half of the island. Some of these felsic dikes can be mapped across the entire width of the island. They are rare in the olivine norite. They range from a few centimeters to a few meters in thickness and display a wide variety of textures from aplitic to pegmatitic. Principal minerals present are albite, quartz, muscovite, orthoclase, garnet, apatite, and magnetite.

Other bodies of gabbro and diorite, some with textures of commingled magmas, are mapped in the Port Clyde area (Guidotti, 1979) and on the St. Georges Islands (Eden and Pavlik, 1996).

Lord noted the similarity of the gabbroic rocks of Monhegan with occurences on Vinalhaven, the St. George Peninsula (gabbro associated with the Spruce Head pluton), and at Addison, Maine. These occurrences are now considered to be of Late Silurian age (Bradley and others, 1998), and it is likely that the Monhegan Gabbro and other mafic plutons of the Georges Islands and Port Clyde are of similar age.

Miscellaneous metamorphosed mafic to intermediate plutons (DSmdg)

Near the southern tip of Southport Island at Newagen (Figure 5) a small (0.9 x 0.5 km) pluton of metadiorite or metagabbro (DSmdg) intrudes highly migmatized Cape Elizabeth Formation. Marginal portions of the pluton are amphibolite with gneissic foliation or weak schistosity, and are heavily injected by irregularly textured garnetiferous granite. Spatial relations of the metagabbro and granite suggest contemporaneous injection and mixing of granitic and gabbroic magmas. Later pegmatites intrude these rocks irregularly. Foliation in the amphibolite is parallel to the gneissic foliation in the Cape Elizabeth host rock. Toward the interior of the pluton, relict ophitic texture is present. A similar but much smaller body of metadiorite or metagabbro is present near Pratts Island along the west shore of Southport Island, about 3 km north-northwest of Newagen (Figure 5).

At Fitch Point along the east shore of the Damariscotta River estuary (Figure 5) a roughly oval pluton approximately 0.8 km in diameter intrudes heavily injected biotite granofels of the Bucksport Formation. The rock is a metamorphosed gabbro with relict equigranular texture and weak tectonic foliation (DSmdg). Locally, the metagabbro has abundant almost monomineralic garnet xenoliths up to 4 cm long and irregular poikiloblastic masses. The xenoliths resemble coticule beds which are common in the Cape Elizabeth Formation elsewhere but not in the surrounding Bucksport Formation at this locality. Garnets of similar habit and distribution have been described from the Kinsman Quartz Monzonite by Clark and Lyons (1986). They interpret that these garnets formed by magmatic crystallization. The garnets in the metagabbros of the Bath map sheet may have had a similar magmatic origin.

Oak Island Gneiss (DSoi)

The Oak Island Gneiss crops out in a belt varying from 250 to 650 meters wide for a distance of 11 km in the towns of Wiscasset, Westport, Woolwich, and Georgetown (Figure 5). As described by Hussey (1992),

"The dominant lithology of this unit is a light pink to light gray moderately foliated granite gneiss cut by relatively abundant irregular stringers of pink pegmatite. Associated with this, particularly on Oak Island and the western half of Chewonki Neck (Bath map sheet), are concordant layers 6 cm to several meters thick of medium dark gray dioritic gneiss. Concordant stringers of strongly migmatized metapelite of the host Cape Elizabeth Formation are common within the granitic gneiss, and foliation of the granite gneiss is commonly defined by discontinuous thin clots of muscovite and sillimanite."

Edgecomb Gneiss (DSeg)

Figure 26

The Edgecomb Gneiss was named and defined by Hatheway (1969) from exposures in the Wiscasset quadrangle just north of the Bath map sheet. It is a concordant body along the contact between the Cape Elizabeth Formation and the Sebascodegan Formation. It occupies a belt 0.25 km wide extending southward into the Bath map sheet a distance of 4 km before pinching out. The Edgecomb Gneiss is a medium dark gray plagioclase-quartz-biotite-hornblende orthogneiss with conspicuous plagioclase augen up to 1 cm in length (Figure 26). The augen in places resemble deformed phenocrysts. Lacking direct age determinations, it is presumed to be of Silurian-Devonian age.

Two-mica granite (DSg) plutons, Woolwich-Phippsburg belt

Figure 27

Small granite plutons, ranging from 0.2 to 1.5 km in width and 1 to 6 km in length, are concentrated in a belt extending from Popham Beach in Phippsburg northward to East Brunswick, Woolwich, Bath and Wiscasset (Figure 27; Bath map sheet - pdf format). These plutons are generally elongate parallel to the regional structure, intrude primarily the Cape Elizabeth Formation, and are restricted to zones of migmatization where the Cape Elizabeth metapelites have been metamorphosed to sillimanite + K-feldspar grade. All of the plutons are composed of light gray, fine- to medium-grained biotite-muscovite ± garnet granite. Most of the granites are nonfoliated to weakly foliated, but a few are moderately well foliated. Contacts with the country rock are relatively sharp and not chilled. Schlieren and recognizable xenoliths of country rock are rare. Two plutons, Towesic Neck and Bath (Figure 27) stand out from the others in being irregularly textured (fine-grained to locally pegmatitic), moderately foliated, and slightly rusty weathering, and generally transitional to the surrounding migmatite. No radiometric ages are available for these plutons, but based on variabilities of foliation, they may show the same Silurian to Devonian age range as reported by Tucker and others (2001) for granitic plutons on strike to the northeast in the Liberty area.

Lincoln Sill (DSls)

The Lincoln Sill, named by Trefethen (1937) for its excellent exposures in Lincoln County, is present in the Bath map sheet in two separate areas. In the Boothbay Harbor area (Figure 5) it crops out in a thin concordant antiformally-folded belt varying from 50 to 150 meters in width. For the most part it lies within the limits of the Bucksport Formation. Where it terminates to the north, on the western limb of the Boothbay anticline northwest of Boothbay Harbor, the sill widens. From distribution of outcrops of the sill, the Cape Elizabeth Formation, and the Bucksport Formation, it would appear that the sill cuts across minor folds of the contact and is locally in contact with the two formations implying it postdates the inferred Boothbay thrust. The second belt of the sill is a narrow concordant lens near the Cape Elizabeth-Bucksport contact at the northern edge of the map. This is the southern end of the main body that extends northward to the Liberty area.

Two distinctive lithologic varieties comprise the Lincoln Sill in the study area. Where the sill is wide, interior parts preserve the original igneous mineralogy and texture. In these places the rock is a coarse-grained porphyritic mafic syenite with orthoclase phenocrysts in a groundmass of orthoclase, augite, hornblende, biotite, and minor hypersthene. The orthoclase phenocrysts retain euhedral compositional zoning and delicate microperthitic structure. Where the body is thin, the dominant rock type is a megacrystic biotite-hornblende-orthoclase schist. The megacrysts of purplish gray orthoclase are oriented with the two greater dimensions parallel to schistosity, yet retain delicate euhedral compositional zoning (Elders, 1969). Pankiwskyj (1976) and Tucker and others (2001) have applied the igneous rock name shonkinite (or metashonkinite where metamorphosed) to the distinctive rock that comprises the sill.

The age of the Lincoln Sill is latest Silurian to earliest Devonian, based on a 418 ± 1 Ma U-Pb zircon age from the nonfoliated phase of the sill in the Liberty area, north of the Bath map sheet (Tucker and others, 2001). Knight and Gaudette (1991) reported somewhat younger results from Rb-Sr whole rock and mineral analyses and U-Pb zircon analyses scattered in the 360 to 390 Ma range.

Quartz diorite at Georgetown (Dqd)

Figure 28

Two small bodies of quartz diorite have been mapped on the north end of Georgetown Island (Figure 5 and Figure 27). The western body is approximately 1 km long and 0.1 km wide, elongate in a N 40^o E direction which is at an angle of about 35 degrees to the regional trend of foliation of the metamorphic rocks, but nearly parallel to, and south of, the trace of the Back River fault. The eastern body is irregular but more nearly equant. Both are strongly foliated (Figure 28), generally parallel to their contacts. This is thought to be an igneous foliation because it is parallel to the pluton contact which locally is discordant to the structural trend of the metamorphic rocks. The rock is medium gray with essential plagioclase and quartz; varietal biotite and hornblende, and accessory apatite, sphene, zircon, and opaques. Zircon from the eastern body gives a Middle Devonian U-Pb age of 376 ± 3 Ma (J. Aleinikoff, written communication, 2002).

Waldoboro pluton (Dwga, Dwgp, Dwgn, Dwgl)

The Waldoboro pluton (Waldoboro Pluton Complex of Sidle, 1991, 1992; Sidle and Barton, 1992; Barton and Sidle, 1994) is a large irregular pluton centered around Muscongus Bay in the towns of Friendship, Bristol, and Bremen (Figure 5), and extending north of the Bath map sheet to Waldoboro. It is intimately mixed with migmatized metapelitic rocks tentatively assigned to the Cape Elizabeth Formation and lit-par-lit injected biotite and calc-silicate rocks assigned to the Bucksport Formation. Sidle (1991) maps several granitic phases in the pluton. The principal phase is garnet-bearing two-mica foliated granite variable to granodiorite (included on the Bath map sheet under the symbol Dwgn). Other varieties include:

fine to medium-grained, weakly foliated garnet-bearing leucogranite (Dwgl); and
very light gray, white, or pale pink moderately foliated garnet-bearing aplitic granite (Dwga).

A U-Pb zircon age of 368 ± 2 Ma is reported by Tucker and others (2001) for muscovite-biotite granite of the Waldoboro pluton from a quarry by Route 1 east of the center of Waldoboro. This phase of the pluton is not exposed in the Bath map sheet (Sidle, 1991).

Figure 29

One of the most conspicuous phases (Dwgp) of the Waldoboro pluton, named the South Pond Porphyry by Newberg (1979), is a coarsely porphyritic foliated biotite-muscovite granite with megacrysts of K-feldspar (Figure 29) up to 11 cm long (Sidle, 1991). Along its eastern margin, the South Pond Porphyry is strongly sheared. Megacrysts are elongate and have serrated margins. The matrix is characterized by protomylonitic to mylonitic fabric. On the east, the porphyry is in contact with gray sillimanite-bearing gneiss which may in part be equivalent to the Mosquito Harbor Formation of Guidotti (1979; Berry and others 2000), and in part may represent extremely sheared porphyry. This suggests that the eastern edge of the porphyry represents a major shear zone, perhaps a continuation of the Sennebec Pond fault. On the west the South Pond Porphyry abuts different granitic phases of the Waldoboro pluton. Sidle (1991) interprets the map pattern in the Friendship 7½' quadrangle as suggesting that the South Pond Porphyry cuts the main phase (Dwgn) of the Waldoboro pluton, although he notes that field relations elsewhere argue for a predominantly gradational contact between the two. The aplite-granite phase (Dwga) appears to cut the South Pond Porphyry in the Friendship - Long Island area.

Granitic pegmatites

Granitic pegmatites (Dp) are very abundant in the Bath map sheet, particularly where the host rock has been metamorphosed to sillimanite or sillimanite + K-feldspar grade. In the Falmouth-Brunswick sequence, two temporally distinct suites of pegmatites are present. The older suite consists of relatively small but abundant pegmatites that occur as neosomes in migmatites. Brookins (Brookins and Hussey, 1978) obtained an Rb-Sr whole rock age of 367-377 Ma (recalculated here using ⁸⁷Rb decay constant of 1.42 x 10^-11/yr), suggesting a Middle to Late Devonian age for this older suite. North of the Bath map sheet, Tucker and others (2001) report U-Pb zircon and monazite ages of small pegmatite bodies intruding the Falmouth-Brunswick sequence near Gardiner of 367 ± 1 Ma, and at Greely Corner, Palermo, of 371 ± 1 Ma, within the same age range.

Tomascak and Francis (1995) identify a younger suite of pegmatites in the Topsham - Brunswick area (Figure 5) that are almost universally associated with small concordant lenses of two-mica granite. Figure 30 shows one of the larger pegmatites exposed in the quarry of the Consolidated Feldspar Company in Georgetown. The younger pegmatites in the Topsham area give U-Pb ages on monazites ranging from 268 to 275 Ma (Tomascak and others, 1996), thus suggesting a Permian age for intrusion of both pegmatite and granite. These pegmatite and granite bodies are larger than the migmatite neosomes and form a nearly continuous narrow zone within the Mount Ararat and Nehumkeag Pond Formations from Brunswick northward through Topsham into Bowdoinham (Figure 5). ⁴⁰Ar/³⁹Ar hornblende cooling ages of 266 and 270 Ma from this vicinity (West and others, 1993) indicate cooling soon after intrusion. Quarries and mineral production from the larger pegmatites within the map area have been described in detail by Cameron and others (1954) and Shainin (1948).

Figure 30

Figure 31

Large pegmatites (greater than 25 meters thick, and up to a few hundred meters long) occupy a belt extending from Brunswick northeastward through Bowdoinham and are restricted to the Falmouth-Brunswick sequence. These large pegmatities commonly show zonal mineral development (Cameron and others, 1954; Shainin, 1948). The zones inward from the contact with the country rock are the border zone, wall zone, intermediate zone, and core. Border zones in contact with wall rock are thin, discontinuous, and finer grained than interior parts. According to Shainin (1948), border zones of pegmatites in the Topsham area generally vary from one inch to one foot in thickness, have sharp contacts with the wall rock, and consist of fine-grained aggregate of quartz, plagioclase, perthite, biotite, muscovite, and garnet. Wall zones make up the bulk of most pegmatites and consist of graphic granite, coarse crystals of quartz, plagioclase, perthite, and biotite. The biotite in wall zones commonly forms large bladed crystals up to 25 cm long and 8 cm wide. The longest observed were 20 feet long, eight feet wide, and three inches thick and apparently formed along fractures (Shainin, 1948, p. 17). Intermediate zones, where present, consist of quartz, muscovite, and perthite with subordinate biotite and garnet. Perthite crystals commonly attain dimensions of two to ten feet. Core zones are generally centrally located, small, disconnected segments separated by wall or intermediate zone material, and consist of coarse perthite and/or quartz. Cavities are rare in the pegmatites of the area, but Palache (1934) described one from the Fisher Quarry in Topsham from which crystals of cleavelandite, topaz, herderite (more correctly hydroxyl-herderite), beryl, tourmaline, cassiterite, columbite, microlite, and apatite were recovered. Figure 31 shows the discoverer, Benjamin Burbank, standing in the cavity in 1934.

The pegmatites within the Casco Bay Group commonly show a variation in mineralogy that is closely dependent on the composition of the host rock. In aluminous phases of the Cape Elizabeth Formation, pegmatites have abundant fibrolitic sillimanite. These pegmatites generally have pink K-feldspar, and if black tourmaline is present, small bluish acicular grains of dumortierite (an aluminum borosilicate) are sparingly present also. Albite is commonly milky-white with a slight greenish cast indicating a minor degree of deuteric alteration to saussurite. Where pegmatites are present in dominantly quartz-plagioclase-biotite-muscovite phases of the Cape Elizabeth Formation, and in the Bucksport Formation, sillimanite is absent and the K-feldspar is generally white or pale buff. Dumortierite is generally not present. The local dependence of mineralogy of these pegmatites on the composition of the host rock suggests very local derivation by the process of partial melting of the host rock that has led to migmatization. This relation suggests the possibility that the neosomes and larger pegmatite lenses may be temporally closely related. These pegmatites show abundant evidence of having been intruded during deformation. Some preserve a crude foliation, and thin lenses are commonly folded semiconcordantly with the country rock.

Light gray, non-foliated pegmatites of simple mineralogy (quartz, perthitic microcline, albite, biotite, and minor muscovite and black tourmaline) occur throughout the Bucksport Formation. They are generally discordant irregular stringers ranging from 10 cm to 30 m in width. They are especially well exposed in coastal outcrops at the end of Linekin Neck in the town of Boothbay, and at Pemaquid Point in Bristol (Figure 5). At Pemaquid Point some of the thinner pegmatites are folded nearly concordantly with bedding of the Bucksport Formation.

Christmas Cove dike

Figure 32

The Christmas Cove dike (Figure 32) is the largest and most extensive of the mafic dikes mapped in the Bath map sheet. It is known to extend from Falmouth in the Portland area, to the south end of Barter Island just off Port Clyde (Figure 5), a distance of 70 km. On the Bath map sheet it is represented by a red line as several discontinuous but essentially on-strike segments from Harpswell Neck in a N 80^oE direction to Barter Island. It varies in dip from vertical to 45 degrees and in thickness from 15 to 35 meters. Columnar jointing is well developed and was the means of recognizing the dike on Barter Island during an overflight of the Muscongus Bay area. The rock is a fine-grained non-porphyritic diabase with diabasic texture. Minerals present, based on thin section examination, include plagioclase (labradorite), titaniferous augite, pigeonite, and magnetite.

McHone (McHone and others, 1995; McHone, 1996) correlates this with the Higganum dike of Connecticut and Massachusetts and interprets it to be part of a large feeder system for Jurassic-age flood basalts associated with rifting that accompanied the opening of the central Atlantic Ocean. 40Ar/39Ar analyses of whole rocks from the Christmas Cove dike produced discordant age spectra ranging in age from 196 to 205 Ma, indicating a Late Triassic to Early Jurassic time of emplacement (West and McHone, 1997).

Last updated on February 1, 2008

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