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Rocks of the Casco Bay Group preserve the clearest record of multiple folding. The earliest folds, designated F1 folds, are recumbent folds formed during the first deformation, D1. Structural features produced during D1 include minor recumbent parasitic folds, cleavages (schistosity, spaced cleavage), downward-facing beds (as determined by graded bedding), and, rarely, lineations, all of which have been folded by subsequent deformations. In most areas where they are seen, F1 folds are minor features and only rarely affect the map pattern of formations. Exceptions to this are seen in the Small Point and Boothbay Harbor areas. Figure 16 shows relatively small-scale but mappable recumbent folds of minor units in the Cape Elizabeth Formation at Small Point. Hinges of these folds are not seen in outcrop, but are indicated by the intricate distribution of the exposures of these units.
Folds in the Falmouth-Brunswick sequence and the Central Maine sequence
At the western edge of Merrymeeting Bay (Figure 5), in the Nehumkeag Pond Formation, the easternmost belt of the Onpr unit outlines the nose of a fold that closes to the southwest (Bath map sheet - pdf format). It is interpreted as an antiform(?) plunging gently to the southwest. The two western belts of rusty schist (Onpr) may represent either long extended antiformal(?) fold hinges, or simply lenses of similar lithology. Roadcuts along the northbound lane of I-95 in Topsham expose beautiful examples of strongly overturned antiforms (Figure 35) and synforms (Figure 36). These folds plunge gently to the north-northeast with axial planes dipping 35-40 degrees southeast.
The outcrop belt of the Falmouth-Brunswick sequence in the northwestern corner of the map is inferred to be a regional scale antiform (Figure 33 and Figure 37). The fold nose toward the eastern edge of the outcrop belt of the sequence would represent the core of this fold.
The Merepoint anticline (ma, Figure 33) is defined by the map pattern of the Merepoint Member and the stratigraphic sequence of the Cushing Formation. Based on the mapped distribution of rusty schist of the Merepoint Member, the anticline is inferred to plunge to the northeast.
Harpswell Sound syncline and Hen Cove anticline
The Hen Cove anticline exposes the formations of the East Harpswell Group, with the oldest in the core and younger units symmetrically disposed about the axial trace (Figure 17). Dips are away from the core of the anticline. On the limbs of the Hen Cove anticline, rocks of the Casco Bay Group lie in thrust-fault contact with the East Harpswell Group 1 (Figure 33 and Figure 37).
The map pattern of the units around the Harpswell Sound syncline closing to the northeast indicates that this structure plunges generally southwest. Axes of the minor folds on both limbs of the syncline are congruent and plunge predominantly to the south-southwest (Figures 39d and 39e). Offshore submarine topography just south of Ragged Island suggests that the Hen Cove anticline closes to the south at its southernmost known extent, its plunge there being to the south-southwest. The plunge then reverses to where it definitely plunges gently northeast between Yarmouth Island and the south end of Quahog Bay (Figure 17). It then reverses again to become southwest in the northern end of Quahog Bay. Plunges of minor folds in the axial zone of the Hen Cove anticline are consistent with this interpretation.
A careful analysis of the predominance of east-verging vs. west-verging parasitic folds on the different limbs of the Harpswell Sound and Hen Cove folds (Figure 39) reveals a correlation with position on the major F2 folds. The presence of some minor folds with opposite vergence suggests either
The minor folds within the Harpswell Sound syncline have a well-developed schistosity, dominated by biotite, parallel to their axial planes. Muscovite is roughly parallel to fold axial planes, but not as perfectly as the biotite. In the area of the Harpswell Sound syncline, metamorphic conditions were apparently conducive to recrystallization of both biotite and muscovite from an earlier schistosity that was parallel to the axial planes of F1 recumbent folds.
In contrast, the west limb of the Hen Cove anticline shows extensive parasitic folding of muscovite-dominated schistosity. The rotation sense of these folds is generally compatible with their position on the west limb of the anticline (Figure 39b). These minor folds of the S1 muscovite schistosity are interpreted to be F2 folds and are probably the same age as the upright folds of the Cape Elizabeth Formation around the Harpswell Sound syncline in which schistosity, dominated by biotite, is parallel to their axial planes. This agrees with observations of schistosity and minor folds of the Cape Elizabeth Formation in the Small Point area of Phippsburg (described below).
Within the more competent Yarmouth Island Formation of the East Harpswell Group, the arch bend of the Hen Cove anticline is seen at three exposures. The most spectacular of these is a shoreline exposure on the eastern side of Yarmouth Island (Figure 40). Minor folds on the eastern limb at this locality are compatible with the position of the arch bend and are interpreted to be parasitic F2 folds (Figure 41). On the east limb of the Hen Cove anticline, the Bethel Point Formation is very thin, with essentially no parasitic folding.
Phippsburg and Cape Small synforms
The Phippsburg synform (ps, Figure 33) is mapped from the west side of Cape Small at the southern tip of Phippsburg (Figure 16) north to Doubling Point on the Kennebec River. The structure is complex due to minor faults and deflection that may be related to the intrusion of the Pitchpine Hill pluton. This fold is regarded as an F2 fold. Relics of earlier recumbent folds are outlined by detailed mapping of thin units in the Cape Small synform just east of Hermit Island, Small Point (Figure 16), but parasitic folds to these are infrequently seen in outcrop. In the southern end of the structure at Cape Small (Figure 16), parasitic folds are extremely common and are seen mostly as folds of muscovite schistosity and S1 fracture cleavage. Muscovite schistosity must therefore have been formed during D1 deformation. In the minor F2 folds at Cape Small, biotite locally forms a weakly developed S2 schistosity. This schistosity is essentially parallel to axial planes of the numerous parasitic folds of the Harpswell Sound syncline and Hen Cove anticline where muscovite as well as biotite is parallel to F2 axial planes. Hussey has interpreted these relations to indicate that the D2 deformation and the accompanying recrystallization of muscovite and biotite into axial plane parallelism has been much more intense in the Harpswell Sound syncline area than in the Cape Small area.
Robinhood Cove synform, Georgetown antiform, and Bay Point synform
Three closely related structures that merge southward to form a single synform on Georgetown Island are recognized on the basis of the map pattern of the coticule (Ocec), amphibolite (Ocea), and other associated minor units of the Cape Elizabeth Formation (Figure 13). The folds from west to east are the Bay Point synform (bps, Figure 33), the Georgetown antiform (ga, Figure 33), and the Robinhood Cove synform (rcs, Figure 33). The Bay Point synform is defined by the belt which includes scattered outcrops of coticule, amphibolite, and rusty schist (Ocep) which probably are equivalent collectively to the same units better exposed and separately mapped around the Robinhood Cove synform. The Georgetown antiform decreases in amplitude to the south, and the two bordering synforms merge into one at the south shore of Georgetown Island. These are all interpreted to be F2 folds. To the north of Georgetown Island the continuity of the Georgetown antiform is obscured by lack of suitable marker horizons. In the vicinity of Westport Island, to the north of Georgetown Island, the distribution of outcrops of the thin amphibolite and calc-silicate member (Ocea) within the Cape Elizabeth Formation suggests a general convergence to the south. This may indicate an antiform plunging south(?), but disruption by the Back River fault and lesser unnamed faults make this interpretation uncertain. The axial trace of the Robinhood Cove synform on Georgetown Island is deflected northeastward (Figure 33) and can be recognized along the northeastern shore of the Island at Lowe Point (Figure 5). Here the foliation in migmatized Cape Elizabeth rocks abruptly changes from strikes averaging N 30oE and dips to the southeast, to northwest strikes and dips to the southwest. The structure here postdates not only the gneissic foliation and schistosity, but also migmatite stringers in the Cape Elizabeth Formation. Figure 42 and Figure 43 are stereographic plots of axes and axial planes of minor F2 and possibly later folds associated with the Georgetown antiform and Robinhood Cove synform, respectively. These plots illustrate the predominance of plunges to the south. Also, there is a greater range of fold orientations in the Robinhood Cove synform as indicated by more scatter in Figure 43.
The Boothbay anticline is interpreted to be a major F2 structure, deforming the Bucksport and Cape Elizabeth Formations, Boothbay thrust, and the Lincoln Sill. It should be noted that the Lincoln Sill on both limbs of the Boothbay anticline lacks the parasitic folding that is so abundantly developed in the Bucksport Formation. The significance of this may be related to the time of intrusion versus the time of F2 folding, or to rheological differences of the two rock types during deformation.
East Boothbay recumbent folds
The amphibolite unit within the Cape Elizabeth Formation (Ocea) is exposed extensively in the area north and west of East Boothbay (Figure 13). The convoluted shape of the outcrop belt of this unit indicates that an early (F1) recumbent fold set has been refolded by later (F2) folds. Inferred axial traces for folds of both ages are shown on Figure 45b. The mapped portion of the amphibolite mostly represents the core of a recumbent fold that has been refolded by a south-plunging, map-scale F2 synform (Figures 45b and 45c). The early fold is inferred to close generally to the east, with a minor west-closing infold on the east limb of the F2 synform (Figure 45c). On the west limb of the F2 synform, positions of the F1 fold limbs are mapped tenuously due to their thinness, incomplete outcrop, and the extensive development of pegmatites. The inferred upper limb of the recumbent fold is mapped southward to where it is cut by a high-angle fault just east of Spruce Point, Boothbay Harbor (Figure 5). The western (lower) limb is cut by the fault farther north, near the middle of Figure 45b. This F1 fold probably reappears on Southport Island in the reclined synform just west of the Boothbay anticline (Figure 45a). The same amphibolite (Ocea) is also in the Linekin Neck area to the east, implying that a major west-closing F1 hinge, now eroded, was present at higher structural levels (labeled "F1?" in Figure 45c).
Folds in the Linekin Neck - South Bristol area
Several doubly-plunging macroscopic anticlines with intervening synclines have been mapped between East Boothbay and South Bristol (Figure 5). They are delimited on the basis of alternating outcrop belts of the Cape Elizabeth and Bucksport Formations (Hussey, unpublished mapping in the Boothbay 15' quadrangle). The Cape Elizabeth Formation crops out in synclines and the Bucksport in anticlines. On Figure 33, scale limitation permits only a generalized representation of these folds. The detailed character of the folds is not clear. In many areas, plunges of minor folds do not agree with mapped closing directions, and plunges of minor folds reverse frequently with no indication from map pattern of similar changes in direction of fold closures. This inconsistency of plunge direction may be due to the presence of both F1 and F2 parasitic folds. The easternmost of the macroscopic folds is a syncline first recognized by Kirk (1971). It extends southward to Outer Heron and nearby islands south of Linekin Neck, East Boothbay (Figure 13), as suggested by Kirk (1971) and as shown on the Bath map sheet, or it may close near South Bristol, in which case the outer island exposures of amphibolite may represent the upper limb of a major recumbent fold.
Pemaquid Harbor anticline
The Pemaquid Harbor anticline (pha, Figure 33) is defined by the eastern outcrop belt of the Cross River Formation on the east shore of Johns Bay in the town of Bristol (Figure 5). A crude antiformal structure is indicated by the general orientation of foliation, dipping steeply to the east on the east side, and dipping moderately to steeply to the west on the west side.
Port Clyde refolded anticline
Guidotti (1979) has mapped a large-scale, complex anticline in the Port Clyde-Tenants Harbor area (pcra, Figure 33). From his map pattern and structural data we infer that rocks of the Benner Hill sequence were folded into an anticline and then refolded antiformally. The earlier anticline is defined by the symmetry of units getting younger away from the Mosquito Harbor Formation both to the east and to the west. In the Hart Neck area Guidotti (1979) describes the later fold, the Hart Neck antiform (hna, Figure 33), as plunging about 35o to the northeast. Attitudes of bedding indicate that to the west the fold has been irregularly warped, perhaps due to the intrusion of granite and diorite plutons in the area. This younger fold (hna) is probably a different age from F2 folds west of the Waldoboro pluton.
Faults and shear zones
Faults within the Bath map sheet, shown in Figure 33, include a major folded thrust (Boothbay thrust), major segments of the right-lateral Norumbega ductile shear system (Flying Point and South Harpswell faults), and several longitudinal high-angle brittle faults locally showing silicification and brecciation (Cape Elizabeth, Back River, The Basin, Phippsburg, St. George Estuary, and other unnamed faults). Several short transverse faults with east-northeast to northwest trends and offsets of generally less than 100 meters, are shown in Figure 33. In addition, minor brittle faults, too small to be shown at the scale of the Bath map sheet, are common throughout the map area.
Faults and deformational fabrics of the western third of the Bath map sheet were included in the Casco Bay shear-zone system defined by Swanson (1999a, 1999b). This zone widens from about 4 km in the Bath map area to about 15 km in the south end of Casco Bay in the Portland 1:100,000 map sheet (Berry and Hussey, 1998). Between the well-defined fault strands which show intense deformation, metamorphic rocks contain a pervasive but less severe dextral shear fabric in a belt ~35 km wide, extending eastward to the general area of Muscongus Bay (Figure 5) in the Bath map sheet (West, 1999, p. 171).
Our view differs somewhat from Swanson's in that we do not think that all these features are related to Late Paleozoic dextral deformation. Some named faults that he included in the Casco Bay shear-zone system are better described as younger brittle normal faults related to the Early to Middle Mesozoic rifting and breakup of Pangea. In addition, some have left-lateral horizontal offset. A more detailed evaluation of the movement sense and deformational style of many faults is better supported by geologic features in the Portland 1:100,000 map sheet to the west and is not pursued here.
Boothbay thrust. The most significant structural feature within the map area is a major folded thrust, here named the Boothbay thrust (BBT, Figure 33). The contact of the Cape Elizabeth Formation with the Bucksport and Sebascodegan Formations is inferred to be the Boothbay thrust surface (Figure 37; sections A-A' and B-B' on the Bath map sheet - pdf format). This thrust is interpreted from the widespread presence of older rocks (Casco Bay Group) above younger rocks (East Harpswell Group, Fredericton sequence). The proposed thrust is deformed by map-scale F2 folds. It is inferred to have been formed prior to high-grade metamorphism because metamorphic isograds are not displaced; rocks on the upper and lower plates show no difference in grade of metamorphism. This could explain why fault-related fabrics and structures have not been observed at contacts of the juxtaposed sequences.
Exposures of the Bucksport Formation in the central third of the map represent autochthonous rocks of the lower plate of the thrust. This essentially follows the representation on the Bedrock Geologic Map of Maine (Osberg and others, 1985). In the area of the Hen Cove anticline, the contact between the Sebascodegan and Cape Elizabeth Formations is interpreted to be a continuation of this thrust (Figure 17). By this interpretation the East Harpswell Group is exposed in a window through the upper plate. Within the Bath map sheet, all exposures of the Casco Bay Group are in the upper plate of the Boothbay thrust and are therefore allochthonous. Figure 37 schematically represents the inferred structural relations between the lower and upper plates of the fault in the northwestern part of the Bath map sheet.
Direction of vergence of this thrust is not clearly indicated by evidence within the map sheet; however, an eastward vergence is favored because it appears to require far less transport than does a westward vergence, considering the distribution of the major rock units of the Casco Bay Group just to the west. This may be consistent with the interpretation of Tucker and others (2001) who include an early east-directed thrust, although they prefer a west vergence for the major thrusting in the region northeast of the Bath map sheet.
Possible blind thrust. The original relationship between the East Harpswell and Casco Bay Groups is not exposed on the map. It is shown in cross section (Figure 37) by a heavy line beneath the Yarmouth Island Formation. Present knowledge of age relationships allows that this contact may be an unconformity. Alternatively, a folded thrust may separate these rocks.
Norumbega fault system
Flying Point fault. The Flying Point fault (FPF, Figure 33) trends northeastward through the Town of Brunswick, and separates the Falmouth-Brunswick sequence from the Casco Bay Group. This fault preserves evidence of both deep-seated ductile dextral shear deformation (Swanson, 1992, 1999a, 1999b) and later west-side-up normal-fault movement (West and others, 1993; West, 1999). Within the Bath map sheet, the only exposures that may represent the trace of the fault occur at the northern tip of Pleasant Point in Topsham (Figure 5). Here, protomylonitized feldspathic gneiss of the Peaks Island Member of the Cushing Formation is juxtaposed against sheared migmatized gneiss assigned to the Nehumkeag Pond Formation of the Falmouth-Brunswick sequence. Quartz veins in the migmatites and quartz in the pegmatite stringers in the fault zone are finely granulated to a translucent sugary texture. North of the map area, the ductile faulting aspects of the Flying Point fault appear to trace into the Sandhill Corner fault of Pankiwskyj (1996), one of the dextral mylonitic segments of the Norumbega fault system.
The idea that the Flying Point fault has undergone later normal movement, probably in Mesozoic time, comes from 40Ar/39Ar studies of the cooling history of biotite, muscovite, and K-feldspar reported by West (1999) and West and others (1993). West (1999, p. 174) states,
"A significant discordance in 40Ar/39Ar ages suggests that in early Mesozoic time a large thermal contrast existed in the rocks now juxtaposed across the Flying Point fault. Final juxtaposition and contemporaneous cooling between rocks currently juxtaposed across the Flying Point fault did not occur until after Triassic time."
West and others (1993) interpreted this time-temperature discontinuity to reflect significant (~4 km), post-Paleozoic, east-side-down displacement along the Flying Point fault.
South Harpswell fault. The South Harpswell fault (SHF, Figure 33) is another segment of dextral ductile high strain shear deformation of the Norumbega fault system (the Casco Bay shear-zone system of Swanson, 1999a). Features of ductile dextral shear movement in the area of Lookout Point on Harpswell Neck (Figure 5) have been elegantly described and interpreted by Swanson (1995, 1999a). This high strain zone is parallel to the western shore of Harpswell Neck, and near the head of Middle Bay (Figure 5) it appears to be cut out by the Cape Elizabeth fault.
Displacement across the Norumbega fault system is uncertain because the faults are parallel to strike and do not offset identifiable strain markers. Based mainly on the geometry of shear structures, Swanson (1999b) suggests that right-lateral movement integrated across the Casco Bay shear-zone system may be in the 100 to 150 km range. In eastern Maine, Chunzeng (2001) demonstrated pluton offsets of 25 km across one strand, the Kellyland shear zone. When added to other faults and shear zones of the Norumbega system, Ludman and others (1999) estimate total dextral displacement of 120 to 150 km in eastern Maine.
Cape Elizabeth fault. A northeast- trending fault (CEF, Figure 33) is inferred to pass between Birch Island and Harpswell Neck in the westernmost part of the Bath map sheet. This fault is responsible for a significant (~6 km) left-lateral horizontal offset of the contact between the Wilson Cove Member of the Cushing Formation and the Cape Elizabeth Formation. In addition, the sillimanite/staurolite-out isograd, which is closely mapped on the west shore of Harpswell Neck (Hussey, 1971b), is offset left-laterally through Middle Bay an unknown distance (outcrops of pelitic rocks in the Middle Bay area are too sparse to closely locate the isograd west of the fault). This fault is on strike with the Cape Elizabeth fault that is well exposed on the southwest shore of Casco Bay in Cape Elizabeth (Hussey, 1989). There, the fault is marked by extensive silicification and local brecciation. Swanson (1999a) included the Cape Elizabeth fault as a part of his Casco Bay shear-zone system. We interpret it to be a younger brittle fault that may correlate with the late, brittle deformation associated with the Flying Point fault. The left-lateral offset of the Wilson Cove-Cape Elizabeth contact may be due in part to dip-slip fault movement (down to the east) along the shared limb of an intermediate-scale sinistral fold set, in which case the actual displacement could be relatively minor.
Back River fault. The Back River fault (BRF, Figure 33 and Figure 13) is a late left-lateral fault, traced from just east of Wiscasset southwestward along the linear course of Back River between Wiscasset and Westport, through Hockomock Bay and onto Arrowsic Island in the vicinity of Mill Island (Figure 5; Hussey, 1992). It is delineated on the basis of
The Basin fault. The Basin fault (TBF, Figure 33) extends from the Sebasco area of Phippsburg north-northeastward through The Basin to just south of Pitchpine Hill (Figure 5). This fault is delineated on the basis of truncation of many minor units within the Cape Elizabeth Formation, offset of Ocea, truncation of several minor granite bodies in the area (DSg), and several minor silicified zones with drusy quartz vugs and occasional agate. Deflection of schistosity close to the fault trace suggests left-lateral movement. The fact that the fault is so nearly on strike with the trace of the Back River fault suggests that The Basin fault might be an extension of the Back River fault. However, the outcrop belt of Ocea in the north end of Phippsburg, and the two bodies of granite that lie between the ends of the two faults, show no evidence of offset. Movement along The Basin fault postdates consolidation and cooling of the three minor granite plutons which are offset in the vicinity of The Basin in Phippsburg. It is a late brittle fault as indicated by the brecciated silicified zones along its trace.
Phippsburg fault. The Phippsburg fault (PF, Figure 33) is inferred along, and to be the cause of, the very marked lineament that parallels Maine Route 209 between Phippsburg village and the north end of Small Point Harbor (Figure 5). The actual fault zone is not exposed, and the offset of several minor units within the Ocbu unit suggests only minor movement along the fault.
St. George Estuary fault. The estuary of the St. George River between Cushing and St. George (Figure 5) follows the trend of an inferred fault, here referred to as the St. George Estuary fault (SGEF, Figure 33). The existence of this fault is suggested by the linear trend of the estuary, by offset of formational contacts, by the marked difference in migmatization, and by the significant difference in structural trends on either side of the fault. On the northwest side bedding and schistosity strike uniformly north-northeast, parallel to the fault trace, and the rocks are not significantly migmatized. On the southeast side, in contrast, structural trends are very irregular and the rocks are strongly migmatized and extensively injected by granite. Mapped contacts between formations on the Port Clyde Peninsula generally trend westerly toward the fault. Because there are no exposures of the fault zone itself, the nature of faulting, whether ductile or brittle, dip- or strike-slip, east- or west-dipping, cannot be ascertained, nor can an estimate of amount of offset be made.
Miscellaneous minor faults. Several faults of relatively short mappable extent and minor offset are shown on the Bath map sheet. Many of these are late faults (probably Mesozoic) with an east-northeasterly trend, parallel to the principal east-northeast regional joint set, and have gouge, fault breccia, or slickensides along the fault planes where they can be observed along the shore or in roadcuts. Numerous minor indentations of shorelines are probably the result of wave erosion of similar incoherent material (gouge?) along minor late faults where they are exposed along the shore.
Lineations in the metamorphic rocks of the Bath map sheet include
In the Harpswell Sound syncline and Hen Cove anticline, mineral lineations, S-surface lineations, bedding mullions, and quartz-vein rodding have similar orientations (Figure 49). They plunge gently to moderately toward S 15-35o W or N 15-35o E, essentially parallel to axes of parasitic folds in this area. Crenulation axes (Figure 49b) plunge gently to moderately to the northeast, representing oblique but mostly vertical shortening in the plane of schistosity.
Similarly, mineral, mullion, and quartz-vein rodding lineations in the Boothbay anticline (Figure 50b) and in the South Bristol-Linekin Neck fold belt (Figure 50c) show relatively little scatter and are essentially parallel to the axes of parasitic folds in each area. Steep lineations are rare.
In contrast, lineations of all types in the area of the Georgetown antiform and Robinhood Cove synform (Figure 50a), show a moderate scatter of plunge angles with steep to vertical plunges common. Directions of plunges of these lineations show less scatter about a nearly north or south direction than the axes of parasitic folds (Figure 42 and Figure 43).
Last updated on February 1, 2008
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