University of New Hampshire University of New Hampshire Scholars' Repository

New England Intercollegiate Geological NEIGC Trips Excursion Collection

1-1-1988

The Skitchewaug in the Mascoma Area, West-Central New Hampshire

Thompson, James B. Jr.

Follow this and additional works at: https://scholars.unh.edu/neigc_trips

Recommended Citation Thompson, James B. Jr., "The Skitchewaug Nappe in the Mascoma Area, West-Central New Hampshire" (1988). NEIGC Trips. 445. https://scholars.unh.edu/neigc_trips/445

This Text is brought to you for free and open access by the New England Intercollegiate Geological Excursion Collection at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in NEIGC Trips by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. C-2

THE SKITCHEWAUG NAPPE IN THE MASCOMA AREA, WEST-CENTRAL NEW HAMPSHIRE

James B. Thompson, Jr. Department of Earth and Planetary Sciences Harvard University, Cambridge, MA 02138

INTRODUCTION

The highway cuts on Interstate 89 provide an excellent opportunity to see some of the more interesting features of the of the Bronson Hill anticlinorium in the area between the Mascoma and Croydon domes. The highway crosses at the structural depression between the domes. This depression displays a segment of the Skitchewaug nappe, and a floored outlier of the Bethlehem Gneiss of the Mount Clough Pluton.

The area has been of interest since the early work of C. H. Hitchcock (1877,1890,1908,1912). Later work by C. A. Chapman (1939,1942,1952); by F. C. Kruger and Daniel Linehan (1941); and by J. B. Lyons (1955) laid the ground work for the present interpretation (Thompson, 1954,1956; Thompson et al.„ 1968; Robinson et al., 1979). The discovery of fossils at Skitchewaug Mountain, Vermont (Boucot et al., 1958; Boucot and Thompson, 1958,1963) and later in the vicinity of the Croydon and Mascoma domes provided some of the key information leading to the identification of the Skitchewaug nappe as a major structural feature of the Bronson Hill anticlinorium. The emplacement of this nappe represents an early stage of the tectonic history of the region, antedating the rise of the domes.

The stratigraphic sequence shown in figure 1 is essentially that worked out by Marland P. Billings (1937) in the Littleton-Moosilauke area, farther north, but on-strike with figure 1. An early Devonian age for the Littleton Formation is established on the basis of abundant fossils near Littleton, NH (Boucot and Arndt, 1960; Boucot and Rumble, 1980). The Silurian age of the Fitch Formation is based on fossils at the Fitch farm and vicinity, also near Littleton (Harris et al., 1983). The dating of the Clough Formation as early Silurian, however, is based mainly on fossils immediately south of figure 1, at Hetty Brook and Beaver Brook in the Croydon Mountain northwest of Newport, NH (Boucot and Thompson, 1963). These localities, unfortunately, are inaccessible to an NEIGC group, and we must therefore be content with the less-well-preserved material in the area of figure 1.

The only rocks to be seen on this trip that are older than the fossiliferous Clough, are assigned to the Partridge (Quimby?) Formation. It is of probable Ordovician age, but may include some younger strata. As pointed out by Hepburn et al. (1984, p. 143, see also Trip A-2 this volume) the pre-Littleton rocks in the Skitchewaug nappe represent a more southeasterly lithofacies than their correlatives mantling the domes. The rocks in the nappe stratigraphically beneath the fossiliferous Clough quartzites, include a variety of pelites and conglomerates shown on earlier maps as part of the Clough. These (and also some of the pelites previously mapped as Partridge or Littleton) are now regarded as correlative with parts of the Rangeley and Perry Mountain Formations of Hatch et al. (1983).

In the opinion of the writer the fossiliferous quartzites at the top of the Clough Formation correlate with those of the uppermost Perry Mountain Formation of Hatch et al. (1983). The Fitch Formation, commonly containing rusty-weathering schists and calc-silicates in its lower part, and non-rusty calc-silicates in its upper part, would then be correlative with the Smalls Falls and Madrid Formations, respectively. Both Hatch et al. and the writer agree that the lower units of their Silurian section are cut out by onlap northwestward toward the Bronson Hill anticlinorium, a positive area during Silurian sedimentation. The writer respectfully suggests, therefore, that the Blanchard Pond fossil locality, that has been used to date the Rangeley Formation, may in fact belong to the Perry Mountain rather than the Rangeley, and that the Rangeley is simply missing at Blanchard Pond. The faunas at Blanchard Pond are essentially the same as those at the Croydon Mountain, and the Skitchewaug Mountain localities farther south. The lower parts of the Clough and Perry Mountain Formations and all of the Rangeley, could thus be Llandovery or older, possibly including some Ordovician strata.

274 43° 30‘

Li m l 0 0.5 1.0 Miles BETHLEHEM GNEISS a PARTRIDGE Fm. KINSMAN qm. FITCH Fm.

DOME GNEISS AMMONOOSUC LITTLETON Fm. CLOUGH Fm. VOLCANICS Figure 1. Geologic map of the Mascoma Lake area, New Hampshire. Modified from Thompson et al. (1968) and Chapman (1939,1942,1952). Numbers show stop localities. Line A-A' is section line of Thompson et al (1968, Pl. 15-lb). 275 REFERENCES

Billings, MP., 1937, Regional metamorphism of the Littleton-Moosilauke area, New Hampshire: Geol. Soc. Amer. Bull., v. 48, p. 463-566. Boucot, A.J. and Arndt, Robert, 1960, Fossils of the Littleton Formation (Lower Devonian) of New Hampshire: U.S. Geol. Survey Prof. Paper 334-B, p. 41-51. , MacDonald, G.J.F., Milton, Charles, and Thompson, J.B., Jr., 1958, Metamorphosed Middle Paleozoic fossils from central Massachusetts, eastern Vermont, and western New Hampshire: Geol. Soc. Amer. Bull., v. 69, p. 855-870. , and Rumble, Douglas, HI, 1980, Regionally metamorphosed (high sillimanite zone, granulite facies), Early Devonian brachipods from the Littleton Formation of New Hampshire: Jour. Paleontol., v.54, no. 1, p. 188-195. , and Thompson, J.B., Jr., 1958, Late Lower Silurian fossils from sillimanite zone near Claremont, New Hampshire: Science, v. 128, 362-363. , and Thompson, J.B., Jr., 1963, Metamorphosed Silurian brachiopods from New Hampshire: Geol. Soc. Amer. Bull., v. 74, p. 1313-1334. Chapman, C.A., 1939, Geology of the Mascoma quadrangle, New Hampshire: Geol. Soc. Amer. Bull., v. 50, p. 127-180. , 1942, Intrusive domes of the Claremont-Newport area, New Hampshire: Geol. Soc. Amer. Bull., v. 53, p. 889-916. , 1952, Structure and petrology of the Sunapee quadrangle, New Hampshire: Geol. Soc. Amer. Bull., v.63, p. 381-425. , 1953, Problem of inverted zones of metamorphism in western New Hampshire: Illinois State Acad. Sci. Trans., v. 46, p. 115-123. Hadley, J.B., 1942, Stratigraphy, structure, and petrology of the Mount Cube quadrangle, New Hampshire: Geol. Soc. Amer. Bull., v.53, p. 113-176. Harris, A.G., Hatch, N.L., Jr., and Dutro, J.T., Jr., 1983, Late Silurian conodonts update the metamorphosed Fitch Formation: Amer. Jour. Sci.., v. 283, p. 722-738. Hatch, N.L., Jr., Moench, R.H., and Lyons, J.B., 1983, Silurian-Lower Devonian stratigraphy of eastern and south- central New Hampshire: Extension from western Maine: Amer. Jour. Sci., v. 283, p. 739-761. Hepburn, J.C., Trask, N.J., Rosenfeld, J.L., and Thompson, J.B., Jr., 1984, Bedrock geology of the Brattleboro quadrangle, Vermont-New Hampshire: Vermont Geol. Surv. Bull., 32,162 p. Hitchcock, C.H., 1877, The Geology of New Hampshire. Part n-Stratigraphical Geology: Concord, New Hampshire, 684 p. , 1890, Significance of oval granitoid areas in the lower Laurentian (abstract with discussion by G. H. Williams): Geol. Soc. Amer. Bull., v. 1, p. 557-558. , 1908, Geology of the Hanover, New Hampshire, quadrangle: Vermont State Geologist 5th Report, p. 139- 186. , 1912, The Strafford quadrangle: Vermont State Geologist 8th Report, p. 100-145. Kruger, F.C., and Linehan, Daniel, S.J., 1941, Seismic studies of floored intrusives in western New Hampshire: Geol. Soc. Amer. Bull., v.52, p. 633-648. Lyons, J.B., 1955, Geology of the Hanover quadrangle, New Hampshire-Vermont: Geol. Soc. Amer. Bull., v. 66, p. 105-145. Robinson, Peter, Thompson, J.B. Jr., and Rosenfeld, J.L., 1979, , gneiss domes, and regional metamorphism in western New Hampshire and central Massachusetts: in The Caledonides in the U.S.A: Excursions in the Northeast Appalachians, edited by J. W. Skehan, S.J. and P. H. Osberg, p. 93-174. Thompson, J.B., Jr., 1954, of the Skitchewaug Mountain area, Claremont quadrangle, Vermont- New Hampshire: New England Intercollegiate Geol. Conf., 46th Ann. Mtg. guidebook, Hanover, NH, p. 35-41. , 1956, Skitchewaug Nappe, a major recumbent in the area near Claremont, New Hampshire [abs.]: Geol. Soc. Amer. Bull., v. 67, p. 1826-1827. , Robinson, Peter, Clifford, T.N., and Trask, N.J., Jr., 1968, Nappes and gneiss domes in west-central New England: in Studies of Appalachian Geology: Northern and Maritime, edited by E-an Zen, W.S. White, J.B. Hadley, and J.B. Thompson, Jr., John Wiley & Sons, New York, p. 203-218.

276 ITINERARY

Exit 15 (Montcalm) on 1-89 between Lebanon and Grantham, New Hampshire, on 1-89 turn left at foot of exit ramp, pass beneath highway, then turn left again at T-intersection beyond southbound ramps. Proceed south 1.3 mi parallel and adjacent to 1-89 to turnaround and parking area. Excess vehicles will be stored here. If southbound on 1-89 turn right at foot of exit ramp, then left immediately at T-intersection. Proceed 1.3 mi south as above. Park and set trip mileage at zero.

Quartzite of the Clough including the fossiliferous upper calcareous zone in the hinge region of the Skitchewaug nappe (Boucot and Thompson, 1963, Locality 9, on strike with Outcrop 6, and Locality 7, with Outcrops 1 and 2). Most of this outcrop consists of massive quartzite and quartz-pebble conglomerate separated by thin highly foliated layers of muscovite-garnet schist In some places the quartzite beds appear to be boudined and the schist injected between quartzite masses. Overall the bedding dips gently to moderately northwest, whereas the dominant schistosity in the schist interbeds dips gently to steeply southeast, apparently parallel to axial planes of early (nappe stage) folds. Two tabular masses of biotite amphibolite dip moderately to steeply southeast. They are roughly parallel to the axial plane in the schist and appear to truncate bedding, so they are probably dikes. A strong mica lineation on schist foliation planes and a strong hornblende lineation on amphibolite foliation planes trends N30- 50W in reasonable conformity with features assigned to the stage in adjacent outcrops.

At the northern end of Outcrop I, the quartzite is overlain by a well-bedded zone of diopside- and grossular-bearing calc-silicate rocks, fine-grained quartzites, and sulfidic mica schists, assigned to the upper member of the Clough Formation. Some calc-silicate rocks contain obvious shelly fossils, commonly replaced by diopside or grossular. DO NOT HAMMER: Plenty of loose material at Outcrop 2. In this vicinity there are several subhorizontal early folds in bedding that trend N20-40E with southeast dipping axial surfaces. As will be seen at Outcrop 2, these are equated with the hinge direction of the Skitchewaug nappe. Also note some late folds in mica schist with steeply dipping axial surfaces.

Climb onto top of north end of Outcrop 1, observing weathered fossils en route, then traverse grass and dirt slope south to low point in fence, and climb into quarry which lies above and south- southeast.

Rocks exposed in the quarry include typical quartzite and mica schist of the Clough Formation, the fossiliferous, bedded quartzite and calc-silicate member of the Clough, dikes of amphibolite, and a small post-metamorphic diabase intrusion

As you face upward into the quarry you are looking west-northwest up the southeast-dipping axial surfaces of a series of minor recumbent folds related to the frontal region of the Skitchewaug nappe. The fold axes are subhorizontal and trend N20-45E approximately at right angles to your line of view. At the mouth of the quarry you are standing in the quartzite member, which is also exposed on the left- and right- hand sides. The entire top and back of the quarry is composed of the fossiliferous bedded quartzite and calc­ silicate member that lies in a more frontal position in the nappe. Unfortunately, most contacts between the lower quartzite and the upper quartzite and calc-silicate members are either covered by debris or exposed on badly altered surfaces. The amphibolite is clearly a parallel to the axial surfaces of the northeast- trending nappe stage folds. It is possible that the dikes post-date the folds, but we prefer to think that the fold axial surfaces where localized by the dikes, because such dikes, though present in the Fitch, are unknown in the overlying Littleton Formation, and are presumed to be of Silurian age. A moderate to strong northwest-trending mineral lineation, equated with the dome stage in adjacent outcrops, is evident throughout the quarry.

The next six outcrops involve a hike of about 5.2 km over rough terrain with a vertical relief of 200 m. The feasibility of this hike is contingent on weather conditions. If weather does not permit, return to the bicycle path and proceed south along it about 0.8 km where you can pass through a hole in the fence to the prominent quartzite ledges of Outcrop 9. If weather does permit, on the other hand, ascend ridge north of quarry (Outcrop 2) in a southwesterly direction to the 578 m summit The route passes over calc-silicate rocks and associated schists and granultes of the Fitch Formation, on into the fossil zone and the massive quartzites of the Clough, and thence into the rusty schists, amphibolite, and minor quartzites of the Partridge Formation. The summit (Outcrop 3), is amphibolite, possibly intrusive into the Partridge Formation.

277 OUTCROP 1 Bedding at Quartzite Contact Fossil Bed N8E 26E N41E 85SE Schist between Quartzites Biotite Lineation N26V 15SE Foliation N41E 34SE Bedding N49E 43Mv Mica Lineation N52W 24SE

Amphibolite Foliation N44E 62E Fold Axis in Quartzite Bed Late Chevron Folds Hornblende Lineation N41W 62SE N27E 2NE Axial Planes N32E 79SE Axis N49E 22NE

OUTCROP Bedding N37E 36E Biotite Lineation N55W 36SE

Nappe Stage Minor Fold N26E 11SW

Conglomerate Bedding N24E 31E ^ N20E 7NE N31E 8NE N45E 8NE Foliation N42E 34E Mica Lineation N551 Pebble Lineation N66 Nappe Stage Fold

Amphibolite in Quartzite Foliation N51E 44E OUTCROP Hornblende Lineation N52W 43SE Amphibolite Foliation N11E 16E Hornblende Lineation N57W 14SE Tight Fold N65W 21SE

Schist Foliation N19E 32E Biotite Lineation N43W 29SE Fold Axi N88E 21E

Late Chevron Folds Axial Plane N57E 56SE Schist near Axial Plane Amphibolite Axis N55E 3SW Foliation N65E 38SE Axial Plane Foliation N24E 27SE Biotite Lineation N26W 15SE Hornblende Lineation N80W 26SE

Figure 2. Field sketches (by Peter Robinson, 1979) of outcrops at Stop 1. Outcrop 1. Clough Quartzite including upper, fossiliferous, quartzite and calc-silicate member in frontal region of Skitchewaug nappe, cut by dikes of amphibolite. Dominant folds and axial plane foliation seem to be , but there are also lineations. Length of outcrop approximately 45 meters. View into quarry in Clough Quartzite, frontal region of Skitchewaug nappe. View is looking WNW approximately parallel to the transport direction of the nappe. Quartzite member in foreground to right and left (stippled) is succeeded, up along the axial surfaces, by the fossiliferous quartzite and calc­ silicate member. Both members are cut by amphibolite dikes approximately parallel to nappe stage axial surfaces. There is a small plug of post-metamorphic diabase at base of wall in center of quarry. Width of view in foreground is about 45 meters. Back wall of quarry is about 30 meters. Partridge Formation in core of Skitchewaug nappe with folds. Length of outcrop approximately 30 meters.

278 Proceed SW from the 578 m summit across lower ground, crossing the Clough, and ascending to the 589 m summit in calc-silicate rocks (note concretions) and associated rocks of the Fitch Formation. (Outcrop 4). The Fitch Formation also contains amphibolites, possibly intrusive, to the west and south, along crest of ridge. From the 589 m summit proceed SSW along high ground, along or near the Clough- Fitch contact, to NW base of a 590 m knob, and ascend SE to its summit in quartzites of the Clough (Outcrop 5). The quartzite here has pelitic partings containing almandine garnet to 1 cm. From here we proceed SSW, along or just W of ridge crest, following Clough-Fitch contact in search of fossils. At about 2.7 km from the start of hike is old town road crossing ridge. (This is the old road from East Plainfield to North Grantham). Knoll just north of road at crest (Outcrop 6) is fossil locality 7 of Boucot and Thompson (1963). Much of the best material, however, was removed by them. The terrain, then freshly logged, is now densely overgrown. From here we follow the old road E over quartzite with minor schistose partings. At about 0.3 km E of crest, drop south to E-flowing gully. Outcrop 7 (locality 6 of Boucot and Thompson) is on north slope of gully. The Clough-Fitch contact here tops east. We will then visit exposures of Bethlehem gneisses on south slope of 513 m hill NW of Leavitt Pond, and then cut N around W ridge of hill to road,and follow it E to N end of Leavitt Pond. LUNCH!

After lunch we will proceed NNE about 0.4 km in search of Outcrop 8, a fossil locality at Clough- Fitch contact (tops again east), near western margin of Bethlehem Gneiss. From here the route continues north and northeast over scattered quartzite outcrops of the Clough Formation 1.3 km to bicycle path.

Pass through hole in fence and climb to top of outcrop away from highway face. Quartzite of the Clough Formation on upper (right-side-up) limb of Skitchewaug nappe. Massive to weakly bedded quartzite, muscovite schist, and minor conglomerate. Bedding strikes N30-40E and dips 30-45 SE. Mica lineation and apparent pebble elongation lineation trends N60-70W.

Large exposures to east across 1-89 and on the mountain above are Bethlehem Gneiss of the Mount Clough pluton that lies structurally above. Minor amounts of sillimanite-mica schist of the Littleton Formation, granulites, and calc-silicate rocks of the Fitch Formation, and quartzites occur of the Clough below overhangs along the west face of the mountain. The dominant mineral lineation in all these rocks has the same N60W trend and is believed to have formed during the dome stage of deformation in the compressed zone between the Mascoma and Croydon gneiss domes.

Look northwest down 1-89. First two outcrops are Partridge Formation in core of Skitchewaug nappe, the third outcrop is quartzite of the Clough ^Outcrop 1) in the anticlinal hinge region. Pass back through hole in fence onto bicycle path. Proceed northwest on path to Outcrop 10.

Partridge Formation in core of Skitchewaug nappe. Rusty-weathering pyrrhotite-mica- garnet-fibrolite schist with subordinate biotite, amphibolite, and fine-grained felsic gneiss. All large folds with moderately inclined axial surfaces in this outcrop, as well as in the large outcrop across 1-89 to the east, appear to trend N50-80W parallel to the dominant mineral lineation and are believed to have formed during the dome stage of deformation. At the extreme south end of the outcrop is an example of late chevron-type folds with steeply inclined axial surfaces.

Proceed northwest along bicycle path past Outcrop 1 to cars.

STOP 2. Littleton Formation. Staurolite-sillimanite mica schist. Bedding obscure. Foliation strikes N40E, dips 40SE. Dome stage mineral lineation trends N40W. Fibrolitic sillimanite is best seen with hand lens on upper surface of outcrop.

Mileage

0.0 Reboard vehicles. Proceed northwest

1.0 Littleton Formation, left

1.1 Montcalm underpass. Take northbound ramp onto 1-89

279 Montcalm entrance ramp, northbound. Stop right.

Littleton Formation (staurolite zone) with coarse staurolite. Mesozoic lamprophyre dike. (Brief stop -10 minutes). Proceed north on 1-89.

Littleton Formation, left

Littleton Formation, right Garnet zone

Littleton Formation, right. Slaty phyllite, biotite zone

Take Exit 16 - Purmont. Cross trace of Clough Quartzite (Hardy Hill), upper limb of Cornish nappe.

Turn right then right again to end (3.1 mi) of road parallel to 1-89. Walk to outcrop SE.

Phyllite of Littleton Formation. Metamorphic grade has fallen off markedly since Stop 3. There are rare, tiny garnets and no staurolite. Return to cars and drive to northbound ramp (3.3 mi) and onto 1-89.

Lebanon City Line. Roadcuts over next 1.5 mi are in rusty, carbonaceous phyllites of Partridge Formation. Rust is from weathering of pyrrhotite.

Take Exit 17, turn right (East) on US-4.

Turn right on Stony Brook Road.

Large outcrops right and smaller one left Turn cars and park.

. The more easterly cuts are in rust schists and horblendic rocks of the Orfordville Formation (Chapman, 1939; Hadley, 1942), here regarded as correlative with the Partridge. The rocks immediately west are part of the Hardy Hill Quartzite of Chapman (1939), now regarded as possibly correlative with the Clough. The Hardy Hill includes quartzites, phyllites, and quartz-pebble conglomerates, some with a phyllitic matrix. Cross-bedding tops west toward phyllites of the Littleton Formation exposed in the more westerly cuts. Return by cars to US -4.

Turn right on US-4.

Cross Mascoma River.

Turn left (north) on Ruddsboro Road.

Turnout just past bridge, park cars.

Follow Lebanon-Hanover townline ESE about 0.2 km to clearings. ENE 0.3 km across clearings, are exposures of gray schists of the basal Littleton. Immediately E in old sugarbush are calc-silicates and associated rocks of the Fitch Formation. The fossil zone in the upper part of the Clough Formation is exposed farther east, across the old town road in the woods at ±400 m elevation. Pelmatozoan columnals were found here by the writer and G J. Wasserburg in 1962. Later material collected by A. J. Boucot in 1966 yielded a highly deformed cup coral and a possible Stricklandia. The fossil zone can be traced farther north into and Hanover and south, across the northeast comer of Lebanon, into Enfield.

If time permits, we can ascend to the nearby open ledges of Clough quartzites with excellent views. Bears have been haunting here this year.

TRIP