Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine
Central Maine Sequence
The Central Maine sequence consists of flyschoid metamorphosed feldspathic wackes and pelitic and calcareous rocks ranging in age from Late Ordovician to Early Devonian. It occupies the Central Maine trough, extending northeasterly through Maine in a 100 km-wide belt between the Coastal Lithotectonic block on the southeast and the Bronson Hill-Lobster Mountain-Weeksboro/Lunksoos anticlinoria on the northwest (Figure 1). Within the Bath map sheet, rocks of this sequence crop out only in a small part of the northwestern corner (Figure 3), and are represented by the Torrey Hill, Richmond Corner, and Hutchins Corner Formations (Figure 2).
Torrey Hill Formation (SOth)*
The name "Torrey Hill Formation" comes from Torrey Hill near the center of the town of Freeport, west of the Bath map sheet, where typical outcrops of the formation are well exposed (Hussey, 1985). The formation consists of very rusty-weathering sulfidic quartz-muscovite-biotite-sillimanite-graphite ± garnet schist, with occasional thin micaceous quartzite interbeds.
This formation is included with the Central Maine sequence on the basis that the continuous, narrow map pattern in this map sheet and the Portland 1:100,00 map sheet (Berry and Hussey, 1998) to the west suggests conformity with the Richmond Corner Formation. To the east the Torrey Hill Formation lies in contact with different units of the Falmouth-Brunswick sequence. The eastern contact of the Torrey Hill Formation is shown as a normal contact on the Bath map sheet. The nature of the contact, however, is equivocal. Where the contact with the Nehumkeag Pond Formation is exposed in a roadcut, on the east side of the northbound lane of I-95, approximately 1.5 km west of the Bath map sheet, it appears conformable. Despite this local appearance, the contact of the Torrey Hill Formation with the Falmouth-Brunswick sequence is interpreted to be either an unconformity or a premetamorphic thrust fault because of the mapped low-angle truncation of units of the Falmouth-Brunswick sequence.
Richmond Corner Formation (SOrc)
The Richmond Corner Formation (Hussey, 1985) lies east of, and adjacent to, the Hutchins Corner Formation. Typically, the Richmond Corner Formation consists of moderately migmatized dark to medium brownish gray slightly schistose quartz-plagioclase-biotite ± garnet ± sillimanite gneiss, and granofels, with sporadic isolated beds 2 to 5 cm thick of coticule (medium reddish brown garnet-quartz-plagioclase-biotite granofels). Calc-silicate interbeds are essentially absent. Garnet occurs as small (½ mm) euhedra and irregular patches up to 1 cm. The migmatization, and the presence of sillimanite and garnet, together with the rarity of calc-silicate form the principal bases for distinguishing rocks of the Richmond Corner Formation from those of the Hutchins Corner Formation. The two formations are otherwise very similar, characterized in general by a salt-and-pepper appearance due to the contrast between the dark biotite and the light feldspar and quartz in equant ½ to 1 mm grains.
Outcrops along I-295 in Yarmouth, Cumberland, and particularly Falmouth, expose the contact zone between the Hutchins Corner and Richmond Corner Formations which appears to be gradational over a 2 to 4 meter interval. There is no evidence for a fault contact. For these reasons, the Richmond Corner Formation is assigned to the Central Maine sequence, conformably below the Hutchins Corner Formation (Figure 2).
Hutchins Corner Formation (SOhc)
The name "Hutchins Corner Formation" (Osberg, 1988) is applied to some of the rocks previously mapped as "Vassalboro Formation" by Osberg and others (1985). As noted by Osberg (1988) "the Vassalboro Formation includes rocks that can be correlated lithologically with the Sangerville and Waterville Formations, as well as rocks older than the Waterville Formation. As a consequence, the name Vassalboro should be abandoned. ...Rocks that lie beneath the Waterville Formation are called Hutchins Corner Formation (new name)."
Within the Bath map sheet, rocks of the Hutchins Corner Formation are medium to dark brownish gray 3 cm to >5 meter beds of quartz-plagioclase-biotite ± hornblende granofels and gneiss with sporadic 1 to 5 cm interbeds of greenish gray calc-silicate granofels. Rare pelitic interbeds contain sillimanite, indicating sillimanite or possibly sillimanite + K-feldspar grade metamorphism. Pegmatite and quartzo-feldspathic dikes and lit-par-lit stringers of simple mineralogy (quartz, albite, microcline, biotite, muscovite, and sporadically, schorl) are common.
* Letters in boldface type are used throughout the text to identify rock units shown on the Bath map sheet.
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