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

Correlations and Ages of the Stratified Rocks

Ages of most stratified rocks in the Bath map sheet are only loosely constrained. No bedrock fossils are known from anywhere in the Bath map sheet. Even beyond the map sheet, fossils are altogether unknown from the Casco Bay and East Harpswell Groups or Falmouth-Brunswick sequence. Fossils are known from elsewhere in the central Maine and Fredericton sequences, though not in the particular formations exposed in the Bath map sheet. The Hart Neck Formation of the Benner Hill sequence has yielded Ordovician fossils in the Rockland area, about 13 km north of the Bath map sheet; rocks correlative with the Penobscot Formation of the Megunticook sequence contain Early Ordovician fossils in southern New Brunswick, about 200 km distant.

Radiometric ages have been determined for volcanic rocks of the Casco Bay Group (Cushing and Spring Point Formations), the Falmouth-Brunswick sequence (Nehumkeag Pond and Mount Ararat Formations), and Megunticook sequence (Penobscot Formation). Radiometric ages of many of the plutonic rocks provide a minimum possible age of the rocks they cut. Most sequences are cut by dated Devonian plutons, and some by Silurian plutons. Numerous 40Ar/39Ar hornblende ages (West and others, 1988, 1993) date post-metamorphic cooling which does not further constrain ages of the stratified rocks themselves.

The following sections discuss correlations of units within and beyond the map sheet based on similarity of rock type and similarity of unit sequences in order to constrain regional stratigraphic and structural models. Age equivalence is not necessarily implied.

Central Maine sequence

The Hutchins Corner Formation has been traced from the type area near Palermo, Maine, to the northwest corner of the Bath map sheet through regional mapping (Hussey, unpublished reconnaissance mapping; Newberg, 1984; Pankiwskyj, 1996; Osberg, 1988). The Torrey Hill Formation is correlated with the Beaver Ridge Formation in the Brooks-Palermo area (Pankiwskyj, 1996). Pankiwskyj, however, assigns the Beaver Ridge Formation to the Falmouth-Brunswick sequence, and infers a major thrust fault (Hackmatack Pond fault) between it and the Hutchins Corner Formation of the central Maine sequence. Further comparisons of stratigraphic relations, lithology, deformation styles, and radiometric ages are necessary to resolve the question of sequence assignment.

The Richmond Corner and Torrey Hill Formations were originally designated as members of the Cushing Formation (Hussey, 1985; Newberg, 1984). They were elevated to formation rank when rocks west of the Flying Point fault were separated from the Cushing to form the Falmouth - Brunswick sequence (Hussey, 1988, 1989). Reexamination of many of the easternmost exposures of biotite granofels originally included with the Hutchins Corner Formation revealed the presence of sparing garnet and sillimanite; consequently they were reassigned to the Richmond Corner Formation. Despite these differences of mineralogy, the two formations are otherwise similar in general appearance. This, along with the fact that the rocks of the Torrey Hill, Richmond Corner, and Hutchins Corner Formations appear to be a conformable sequence, warrants reassignment of these two formations to the Central Maine sequence rather than the Falmouth-Brunswick sequence. Since the Torrey Hill Formation lies adjacent to different units of the Nehumkeag Pond and Mount Ararat Formations, the lower (eastern) contact of the Torrey Hill Formation is either a pre-metamorphic folded thrust fault or an unconformity.

Falmouth-Brunswick sequence

The Falmouth-Brunswick sequence consists, in the Bath sheet, of the Nehumkeag Pond Formation and its various members, and the Mount Ararat Formation. The Mount Ararat Formation may correlate with the Marden Hill and Parmenter Cemetery Formations in the Palermo area on strike 60 km to the northeast (Pankiwskyj, 1996). Preliminary U-Pb ages on zircons from the Mount Ararat and Nehumkeag Pond Formations suggest a Middle Ordovician age for these formations. John Aleinikoff (written communication, 2002) obtained a 458 ± 3 Ma TIMS age for a 1-meter wide granitic gneiss sill cutting the Mount Ararat Formation at Little River on the east side of Wolf Neck, Freeport. He also analyzed zircons by SHRIMP from felsic interbeds of the Mount Ararat Formation at the same locality that were discordant, but suggest an age of about 450 Ma. SHRIMP analysis of zircons from the Nehumkeag Pond Formation in the Androscoggin River between Brunswick and Topsham (Figure 5) were also discordant, but suggest an age of about 460 Ma. A U-Pb monazite age of 316 ± 1 Ma from the same sample (Aleinikoff, personal communication, 1994) is probably a metamorphic age of some sort, and may even represent a mixed population of monazites of different ages. These preliminary ages are consistent with the 460 ± 2 Ma age reported for the Carrs Corner Formation (correlated with the Nehumkeag Pond) in Palermo, north of the Bath map sheet (Tucker and others, 2001).

The few radiometric ages available suggest that rocks of the Falmouth-Brunswick sequence may be somewhat younger (ca. 460 Ma) than rocks of the Casco Bay Group (ca. 470 Ma). This supports the interpretation that the Falmouth-Brunswick sequence is not part of the Cushing Formation. The Falmouth-Brunswick sequence may represent a basement to the Late Ordovician - Devonian rocks of the Central Maine sequence.

Casco Bay Group

Units of the Casco Bay Group correlate with rocks along strike to the north-northeast in the Liberty-Palermo area mapped by Pankiwskyj (1976, 1996) and West (2000). This correlation is based primarily on similarity of lithologic sequence.

U-Pb zircon dating of the Peaks Island Member of the Cushing from Danford Cove in South Portland (Hussey and Bothner, 1995, stop 2) gives eruption ages for the upper part of the member of 471 ± 3 Ma (J. N. Aleinikoff, in Hussey and others, 1993) and 473 ± 2 Ma (Robert D. Tucker, personal communication, 1998). Another age constraint on the Casco Bay Group is the 469 ± 3 Ma U-Pb zircon age for the Spring Point Formation in the Liberty area, north of the Bath map sheet (Tucker and others, 2001). The dates show that these volcanic rocks were erupted in Middle Ordovician time. Overlying parts of the Casco Bay Group may be Middle to Late Ordovician. Felsic and intermediate volcanic rocks of similar age are reported by Moench and Aleinikoff (2002) for the Ammonoosuc Volcanics in the Bronson Hill belt of New Hampshire.

The relationship of the Cape Elizabeth Formation to the Merrimack Group of southern Maine and southeastern New Hampshire has been a matter of uncertainty and confusion. Originally, Katz (1917) correlated the Eliot Formation of the Merrimack Group with the Cape Elizabeth Formation. Ongoing work by Hussey demonstrates that Katz (1917) included two lithically dissimilar rock units as part of the Cape Elizabeth Formation. That part of Katz's (1917) Cape Elizabeth outcrop belt extending from Saco northeastward toward Portland is now correlated with the Eliot Formation. These rocks have the same calcite and ankerite-rich modal composition as the Eliot Formation in the Eliot area at the southwestern tip of Maine, and are otherwise lithically identical. Also, rocks in Casco Bay that Katz (1917) mapped as the Mackworth Formation are lithically similar, and are thus correlated with the Eliot Formation (Hussey and others, 1993; Berry and Hussey, 1998). In addition, Katz (1917) mapped as part of the Cape Elizabeth Formation those heavier bedded but similar carbonate-rich rocks in the Two Lights State Park area in the town of Cape Elizabeth. He suggested that the rocks at the locality of High Head in Two Lights State Park are typical of the Cape Elizabeth Formation. Hussey and others (1993) recognize the similarity of those rocks to the Kittery Formation of the Merrimack Group, with which they are now correlated. In contrast, the other rocks that Katz (1917) mapped as part of the Cape Elizabeth Formation are the pelitic and psammitic rocks that lie in sequence above the metavolcanic rocks of the Cushing Formation and beneath the mafic metavolcanic rocks of the Spring Point Formation. These rocks are characterized by an aluminous composition in which pelitic mineral assemblages (with sillimanite, andalusite, staurolite, or garnet) are developed at the higher grades of metamorphism as in the Bath 1:100,000 map sheet. At similarly high grades of metamorphism in the Portland area, rocks equivalent to the Eliot Formation develop a calc-silicate paragenesis (with epidote, hornblende, or diopside).

In light of our present understanding, thus it appears that Katz (1917) mapped as "Cape Elizabeth" two contrasting and unrelated groups of rocks, the one carbonate-rich, and the other pelitic. This mapping of two units under one name and the designation of High Head at Two Lights State Park as a reference locality for the Cape Elizabeth Formation (now Kittery Formation), might seem to warrant abandoning the name. However, since the name "Cape Elizabeth" is so widely used and generally understood to represent the package of pelitic/psammitic rocks of the Casco Bay Group below the Spring Point Formation (Osberg and others, 1985, for example), it would be inappropriate to rename the formation. Because there are other localities within the town of Cape Elizabeth where these pelitic rocks are in stratigraphic order within the Casco Bay Group, the name Cape Elizabeth can appropriately be retained; only the type locality need be changed, but that is beyond the limits of the Bath map sheet.

Within the Bath map area, all rocks represented as Cape Elizabeth lie conformably beneath the Spring Point Formation and are pelitic to subpelitic.

East Harpswell Group

The Yarmouth Island Formation is exposed in the core of the Hen Cove anticline. It is structurally overlain, in turn, by the Bethel Point, Sebascodegan, and Cape Elizabeth Formations. Originally, Hussey (1971b) interpreted this to be a conformable stratigraphic sequence, with the Yarmouth Island oldest and the Cape Elizabeth youngest. Recently, Robert D. Tucker (personal communication, 1995) obtained a U-Pb age of 445 ± 2 Ma on intermediate to felsic metatuffs from the upper part of the Yarmouth Island Formation, indicating it is latest Ordovician to earliest Silurian in age. The Bethel Point and Sebascodegan Formations, still interpreted to lie conformably above the Yarmouth Island Formation, are also taken to be latest Ordovician to early Silurian. This means that the entire East Harpswell Group is younger than any of the rocks of the Casco Bay Group, including the Cape Elizabeth Formation that structurally overlies it. Consequently, the contact of the East Harpswell Group with the structurally overlying Casco Bay Group is now interpreted to be a major thrust fault (the Boothbay fault; discussed in Structural Geology).

The age of the Sebascodegan Formation now appears to be about the same as that of the Bucksport Formation. This age and the lithologic similarity of parts of the two formations suggests that they may be equivalent. The Hutchins Corner Formation of central Maine and the Berwick Formation of southern Maine and New Hampshire may also correlate. No correlatives for the Yarmouth Island or the Bethel Point Formations are known in the area.

Fredericton sequence

About 20 km north of the Bath map sheet, the Bucksport Formation is intruded by the Late Silurian North Union Granite Gneiss dated at 422 ± 2 Ma (Tucker and others, 2001), indicating a Silurian or older age for the Bucksport Formation. In that same area, the Bucksport Formation is interpreted to be conformable and interstratified with the Appleton Ridge Formation (Stewart, 1998; West, 2000; Tucker and others, 2001), which has been correlated with the fossiliferous Digdeguash Formation of southern New Brunswick that contains Early Silurian graptolites (Fyffe and Riva, 2001). These relationships indicate a Silurian to perhaps Late Ordovician age for the Bucksport Formation. The Bucksport Formation correlates with the Flume Ridge Formation of eastern Maine.

Benner Hill sequence

Rocks of this sequence have been mapped in the Bath map sheet around Port Clyde (Figure 5; Guidotti, 1979) and north of the Bath map sheet into the Thomaston area (Osberg and Guidotti, 1974). This belt contains the type localities for all three formations (Berry and others, 2000). In the Thomaston 7½' quadrangle, a siliceous coquinite near the top of the Hart Neck Formation has yielded highly deformed, yet identifiable, brachiopods. Boucot (Boucot and others, 1972; Boucot, 1973) confidently interprets the fauna to be of Caradocian (Late Ordovician) age, although Neuman (1973) presents a case for possibly an older Ordovician age.

Megunticook sequence

Fossils have not been found in the Megunticook sequence in the Bath map sheet. Tremadocian (Early Ordovician) graptolites, however, are present in the Calais Formation on Cookson Island in the St. Stephen area, southern New Brunswick (Ruitenberg and Ludman, 1978). The Calais Formation is correlated with the Penobscot Formation by Ludman (1987) and Berry and Osberg (1989). Tucker and others (2001) report a Late Cambrian-Early Ordovician U-Pb zircon age of 503 ± 5 Ma for a metamorphosed fragmental volcanic rock at the base of the Penobscot Formation, north of the Bath map sheet in Lincolnville. On the basis of the fossil age, the U-Pb age, and correlation, the Penobscot Formation in the Bath map sheet is assigned a Late Cambrian-Early Ordovician age.

Sequence uncertain

Much of the Cross River Formation (OCcr) may correlate with parts of the Penobscot Formation on the basis of its rusty and pelitic character. Some lower parts of the Cross River in the Pemaquid anticline are only slighly rusty and similar to parts of the Megunticook Formation in Lincolnville. This would indicate a Cambrian to Early Ordovician age for the Cross River Formation. Its contact with the overlying Bucksport Formation would therefore be either an unconformity or a low-angle fault.

The unnamed amphibolite unit (Ouv) had been assigned previously to the Benner Hill sequence because of its position between sulfidic schist of the Prison Farm unit to its east and gneisses possibly equivalent to the Mosquito Harbor Formation to its west (Hussey, 1985). Also, the Benner Hill Formation contains amphibolites, some of which reportedly may preserve relict pillow structure (Guidotti, 1979). Arguments against assigning the Ouv unit to the Benner Hill sequence include the association of marble and calc-silicate rocks with the pillow lavas, which are not known in the Benner Hill Formation, the absence of the Hart Neck Formation between the amphibolite unit (Ouv) and the possible Mosquito Harbor rocks (Omh?), and the lack of repetition of the amphibolite unit to the east of the unnamed sulfidic schist. Furthermore, the thrust fault proposed by Berry and others (2000) if extrapolated southward, would separate the unnamed amphibolite unit from the Benner Hill sequence. Therefore, the assignment of this unit to the Benner Hill sequence is now considered uncertain. An alternative might be the Megunticook sequence, since thin units of pillow basalt and marble are known below sulfidic schists at the base of the Penobscot Formation (Berry and Osberg, 1989). Correlation is hampered by migmatization against the Waldoboro pluton, intense deformation, and lack of continuity to the northeast.

The unit of unnamed sulfidic schist (Ouss) at Friendship (Figure 5) traces northward into the Prison Farm unit of Osberg and Guidotti (1974). If it is a facies of the Benner Hill Formation (Osberg and Guidotti, 1974; Guidotti, 1979), then the unnamed schist is Ordovician. If it is not part of the Benner Hill sequence (Berry and others, 2000) then its age is unknown, but presumably Cambrian or Ordovician, since it appears to share the same Silurian metamorphism with the Benner Hill sequence (West and others, 1995).

Rusty metawackes and metapelites (Ouss) mapped on the outer Georges Islands have been correlated by Eusden and others (1996) with the Penobscot Formation of the St. Croix belt. Alternatively these rusty rocks may correlate with the unnamed sulfidic schist in Friendship, which maps northward into the Prison Farm unit in the Thomaston 7½' quadrangle.

The few small exposures on the outer Georges Islands tentatively assigned to the Mosquito Harbor Formation (Omh?) are in contact with unnamed sulfidic schist. If this sulfidic schist is part of the Penobscot Formation, the contact might represent the Benner Hill sequence resting in stratigraphic contact above the Megunticook sequence. Local relationships on the outer islands, however, suggest that the sulfidic schist is younger than the rocks labeled "Omh?" (King and Eusden, 1994). It is partly due to this apparent inconsistency that the assignment to the Mosquito Harbor Formation is questioned, and the unnamed sulfidic schist is not assigned to the Penobscot Formation.


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