INTRODUCTION

This paper is a review of the salient features of the stratigraphy and the structural geology of rocks exposed in the Green Mountain-Sutton Mountain anticlinorium. Most of the paper is based on the work of many geologists, chief among whom are W. M. Cady, A. H. Chidester, J. B. Thompson, Jr., J. L. Rosenfeld, W. F. Brace, E-an Zen, J. W. Skehan, J. A. MacFadyen, J. G. Dennis, M. J. Rickard, and Jacques Beland. The writer has made relatively small contributions to the general understanding of the Green Mountain-Sutton Mountain anticlinorium in central Vermont (Osberg, 1952), southeastern Quebec (Osberg, 1965), and northwestern Massachusetts (Osberg et al., in press). Regional summaries on parts of the Green Mountain-Sutton Mountain anticlinorium have been published by Dresser nd Denis (1944), Cooke (1950), Cady, (1960, 1967), and Doll et al. (1961).

TECTONIC UNITS

The Green Mountain-Sutton Mountain anticlinorium extends from northwestern Massachusetts through Vermont and into southern Quebec (Fig. 1). The distribution of rock units exposed in the anticlinorium is complicated by sedimentary facies and the complex geometry of the anticlinorium and associated thrust faults. To facilitate description, the structural units are delineated in Figure 1.

The Green Mountain-Sutton Mountain anticlinorium consists of the axial anticline, the Lincoln-Enosburg Falls anticline on the west, and the Lowell Mountain-Stoke Mountain anticline on the east (Cady, 1960). The Pine Hill anticline in southern Vermont is analogous to the Lincoln-Enosburg Falls anticline. The Notre Dame anticline extends northeast from

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the Green Mountain-Sutton Mountain anticlinorium at Danville.

The Strafford-Willoughby arch on the east is generally parallel with the Green Mountain-Sutton Mountain anticlinorium.

The St. Albans-Hinesburg-Middlebury synclinorium (Cady, 1960) is west of the Green Mountain-Sutton Mountain anticlinorium. In southern Vermont the section in the Taconic klippe occupies the trough of that synclinorium.

Many thrusts interrupt the sequence west of the axial anticline; the more important ones are shown in Figures 1 and 2.

STRATIGRAPHY

Precambrian, Cambrian(?), Cambrian, Cambrian-Ordovician(?), and Ordovician rocks are exposed in the Green Mountain-Sutton Mountain anticlinorium. Their distribution is shown in Figure 2. Rocks of pre-Trentonian age in the west limb of the anticlinorium in Vermont are predominantly miogeosynclinal, but in northwestern Vermont and southern Quebec they intertongue

Fig. 1. Location of major structural features in Green Mountain-Sutton Mountain anticlinorium, Vermont and southern Quebec.

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Fig. 2. Generalized geologic map of Green Mountain-Sutton Mountain anticlinorium.

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with eugeosynclinal rocks. Equivalent rocks in the east limb of the anticlinorium are eugeosynclinal. Post-Trentonian rocks are exogeosynclinal in the west limb and eugeosynclinal in the east limb. Distinct unconformities are present beneath Cambrian(?) and middle Ordovician rocks.

Precambrian

Rocks assigned to the Precambrian are included in the Mount Holly Complex (Osberg, 1952; Whittle, 1894); they occupy the core of the axial anticline in southern Vermont (Doll et al., 1961; Fig. 2). Smaller exposures are present in the Pine Hill anticline, the Lincoln-Enosburg Falls anticline, and in the southern extension of the Strafford-Willoughby arch (Fig. 2).

The Mount Holly Complex consists of fine- to medium-grained biotite gneiss, quartzofeldspathic granulite, banded fine-grained mica schist and quartzofeldspathic gneiss, amphibolite, marble, calcsilicate gneiss, and quartzite. Relict minerals and textures indicate that the Mount Holly Complex underwent high-grade metamorphism before Cambrian(?) time, but that subsequently it has been affected by Paleozoic retrograde metamorphism.

Stratigraphy within the Mount Holly Complex is poorly understood. Only local sequences that cannot be adopted over larger areas have been established (Brace, 1953; Osberg, 1952; Thompson, 1964, oral commun.).

Cambrian(?)

Rock units of Cambrian(?) age include the Pinnacle Formation, White Brook Dolomite, Forestdale Marble, and much of the Pinney Hollow, Underhill, and Bonsecours Formations and Bennett Group. The distribution of the units is shown in Figure 2, and their stratigraphic positions are shown in Figure 3.

The Pinnacle Formation (Booth, 1950; Clark, 1934), 600-2,000 ft thick, is the basal unit of the Paleozoic sequence. Other names used for the unit are Hoosac and Dalton, and it has been considered to be part of the Camels Hump Group. In southern Vermont it crops out in a narrow belt adjacent to the Mount Holly Complex, but on the north it underlies large tracts in the core of the Lincoln-Enosburg Falls anticline (Fig. 2; Doll et al., 1961).

The Pinnacle Formation typically consists of light-gray albite-biotite schist and greenish-gray chlorite-mica schist. Relict grains of clastic quartz are abundant and pebble beds are locally common. Lenticular bodies of conglomerate are present in contact with the Mount Holly Complex.

A distinctive greenstone (Tibbit Hill Schist) crops out in northern Vermont and southern Quebec. It consists of chlorite schist, locally containing abundant knots of epidote. Relations in northern Vermont indicate that it lies stratigraphically within the Pinnacle Formation (Dennis, 1964).

The White Brook Dolomite (Clark, 1936) and the Forestdale Marble (Keith, 1932) are equivalent. These rocks form a unit in the west limb of the Green Mountain-Sutton Mountain anticlinorium too thin to be shown separately in Figure 2, and thus are included with the underlying Pinnacle Formation. The White Brook or Forestdale rocks are light-gray to white, somewhat sandy dolomite; they range in thickness from a few feet to 150 ft.

The Pinney Hollow (Perry, 1928), Underhill (Doll et al., 1961), and Bonsecours Formations (Osberg, 1965) are physically continuous. However, the relation between the Pinney Hollow and Underhill Formations is uncertain. Doll et al. (1961) show the Pinney Hollow to be directly above the Underhill in central Vermont. On the north they show the Pinney Hollow and the Underhill to be separated stratigraphically by black phyllite (Hazens Notch Formation), and farther north they show the Pinney Hollow to thin and disappear in the vicinity of Montpelier (Fig. 2). The writer's interpretation is that the disappearance of the Pinney Hollow Formation near Montpelier is structural, that the Hazen Notch Formation is the Ottauquechee Formation, and that th Underhill Formation as mapped in northern Vermont includes the Pinney Hollow-Underhill sequence of central Vermont.

The Pinney Hollow Formation crops out in the east limb of the axial anticline in southern Vermont (Doll et al., 1961), but in central Vermont it bridges the axial anticline, and in northern Vermont it is coextensive with the Underhill Formation in the west limb of the axial anticline. The Bonsecours Formation occupies the core of the axial anticline north of Knowlton, Quebec (Fig. 2). The Bennett Group (Dresser and Denis, 1944) in the Notre Dame Mountains of Quebec is in part lithologically similar to the Bonsecours and occupies

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Fig. 3. Correlation chart for Green Mountain-Sutton Mountain anticlinorium.

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the same stratigraphic position.

The Pinney Hollow, Underhill, and Bonsecours Formations and part of the Bennett Group are lithologically similar. They consist of light-greenish-gray chlorite-mica schist and phyllite. Quartz segregations are common and locally porphyroblasts of albite are abundant. Beds of schistose sandstone and quartzite, lenses of carbonate rock, and units of greenstone are locally prominent.

The thickness of these units in central and northern Vermont and in southern Quebec is difficult to estimate because of their schistose character and intricate folding. On the basis of their mapped distribution, thicknesses of a few thousand feet or more are reasonable.

Cambrian

Rocks of Cambrian age include the Cheshire Quartzite, Gilman Quartzite, Dunham Dolomite, Monkton Quartzite, Parker Slate, Winooski Dolomite, Rugg Brook Dolomite, Danby Formation, Saxe Brook Dolomite, Clarendon Springs Dolomite, Woods Corners Group, Sweetsburg Formation, Rosaire Group, and Ottauquechee Formation. In Figure 2 the carbonate units are shown by a single symbol, and the pelitic rocks of the Parker Slate, Woods Corners Group, Sweetsburg Formation, Rosaire Group, and Ottauquechec Formation are grouped and shown by another symbol. Stratigraphic relations are indicated in Figure 3.

The Cheshire Quartzite (Emerson, 1892) and the Gilman Quartzite (Clark, 1934) are somewhat similar in lithology and both are Early Cambrian (Clark, 1936; Seely, 1910; Walcott, 1888). They are found in the west limb of the Green Mountain-Sutton Mountain anticlinorium, and are the oldest stratigraphic units containing fossils.(FOOTNOTE 3)

The Cheshire Quartzite crops out in southern and west-central Vermont. The upper 300-500 ft consists of thick-bedded white quartzite; the lower 500-700 ft consists of thinner beds of gray and white mottled quartzite. The Gilman Quartzite crop out in northwestern Vermontand southern Quebec; it is lithologically similar to the lower part of the Cheshire Quartzite. The Gilman Quartzite is possibly 1,000 ft thick in Vermont, but thins to a featheredge in the vicinity of Danville, Quebec (Osberg, 1965; Fig. 2).

The Dunham Dolomite (Clark, 1934) is exposed in the west limb of the anticlinorium from southern Vermont to the vicinity of Danville, Quebec. It consists mainly of buff sandy dolomite in beds 6-10 in. thick; locally the upper part is more thinly bedded (Cady, 1945). The thickness in Vermont is about 1,500 ft (Cady, 1945), but decreases markedly in southern Quebec to a few tens of feet (Eakins, 1964; Osberg, 1965).

The Monkton Quartzite (Keith, 1932) crops out in the west limb of the Green Mountain-Sutton Mountain anticlinorium and in the St. Albans-Hinesburg-Middlebury synclinorium. It consists of interbedded reddish quartzite, variegated red and white quartzite, minor phyllite, and buff dolomite. It is approximately 1,000 ft thick in west-central Vermont (Cady, 1945) and thins eastward, disappearing west of Starksboro, Vermont (Fig. 2). Fossils contained in these rocks indicate an Early Cambrian age (Kindle and Tasch, 1948; Perkins, 1908).

The Parker Slate (Keith, 1932) is exposed in northwestern Vermont. Its continuation into southern Quebec is represented by the Oak Hill Slate. The Parker consists of gray to black slate, sparse beds of sandy dolomite, and local bioherms (Shaw, 1958); maximum thickness is 1,000 ft. Fossils indicative of Early Cambrian age have been found in this unit (Shaw, 1958; Walcott, 1886, 1891), and suggest that it is the correlative of the Monkton Quartzite (Shaw, 1962).

The Winooski Dolomite (Cady, 1945; Hitchcock et al., 1861) crops out in the west limb of the axial anticline in southern Vermont and in the St. Albans-Hinesburg-Middlebury synclinorium in northern Vermont. It consists of white to slightly pinkish dolomite in beds 6-12 in. thick. Locally siliceous partings are present along bedding planes. The Winooski Dolomite is about 1,000 ft thick in west-central Vermont but thins and disappears in northwestern Vermont (Cady, 1945; Shaw, 1958). No fossils have been collected from the unit.

The Rugg Brook Dolomite (Howell, 1939; Schuchert, 1933) has been recognized in northwestern

FOOTNOTE 3. Fossils reported in quartzite in northwestern Massachusetts have been interpreted to date rocks stratigraphically beneath the Cheshire, but the stratigraphic relations of the rocks are open to more than one interpretation (see papers by Walcott, 1888; Whittle, 1894; Keith, 1932; Prindle and Knopf, 1932; and Herz, 1961).

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western Vermont (Doll et al., 1961). The Scottsmore Dolomite occupies a similar stratigraphic position in Quebec. The Rugg Brook is buff to salmon, sandy dolomite and conglomerate, and ranges in thickness from a few tens of feet to as much as 300 ft. No fossils have been collected from the Rugg Brook.

The Danby Formation (Cady, 1945; Keith, 1932) is exposed in the west limb of the axial anticline in southern Vermont and in the St. Albans-Hinesburg-Middlebury synclinorium in northern Vermont. It consists of beds, 8-15 in. thick, of white, commonly cross-bedded quartzite separated by 12-15-ft intervals of buff to light-gray dolomite in beds 8-12 in. thick. It is about 300 ft thick in central Vermont; in northwestern Vermont it apparently intertongues with the Woods Corners Group (Cady, 1945; Shaw, 1958). Fossils in the Danby indicate a Late Cambrian age (Cady, 1945; Rodgers, 1937).

The Saxe Brook Dolomite (Howell, 1939; Shaw, 1958) is found in the St. Albans-Hinesburg-Middlebury synclinorium in northwestern Vermont. It consists of light-gray massive dolomite, sandy dolomite, and dolomitic sandstone, and is approximately 700 ft thick (Shaw, 1958). It is apparently the lateral equivalent of the Dunham Dolomite, Parker Slate, Winooski Dolomite, and Danby Formation. The Saxe Brook Dolomite is unfossiliferous.

The Clarendon Springs Dolomite (Keith, 1932) crops out in the west limb of the axial anticline in southern Vermont and in the St. Albans-Hinesburg-Middlebury synclinorium in west-central Vermont. It is light-gray, fine-grained, crystalline dolomite, approximately 200 ft thick. Contained fossils indicate a Late Cambrian age (Rodgers, 1937). The Gorge Formation (Keith, in Raymond, 1925) in northwestern Vermont is probably equivalent to the Clarendon Springs (Cady, 1945).

The Woods Corners Group (Shaw, 1958) is exposed in northwestern Vermont in the St. Albans-Hinesburg-Middlebury synclinorium. The units composing the group are predominantly gray to black micaceous slate that locally has thin interbeds of dark-gray slate and lighter quartzite. The slate sequence intertongues on the west with dolomite units; on the east it is possibly as much as 2,000 ft thick. An abundant fauna from the group indicates a Middle to Late Cambrian age (Cady, 1960; Shaw, 1958).

The Sweetsburg Formation (Clark, 1934) crops out in southern Quebec in the west limb of the Lincoln-Enosburg Falls anticline and in the east limb of the axial anticline. Locally it forms the core of the syncline between these two anticlinal tracts. It is the northward continuation of the Woods Corners Group. Lithologically, it is a dark-gray slate locally containing thin interbeds of white quartzite; in the vicinity of Danville it contains a distinctive gray limestone member. The thickness of the Sweetsburg is estimated to be 300--1,000 ft (Eakins, 1964; Osberg, 1965).

The Rosaire Group (Beland, 1957) crops out in the Notre Dame Mountains of Quebec. It is lithologically similar to the Sweetsburg Formation and occupies the same stratigraphic position. Fossils have not been found in the Rosaire slate, but on stratigraphic grounds it is interpreted to be Cambrian (Beland, 1957).

The Ottauquechee Formation (Perry, 1928) is the southward continuation of the Sweetsburg Formation in the east limb of the axial anticline in Vermont. It typically is gray mica schist containing numerous quartz seams and lenses. Thick beds of gray quartzite are common and greenstone units are present locally. The formation is estimated to be approximately 2,000 ft thick in central Vermont, but it thins to a few tens of feet in southern Vermont. Fossils have not been found in the Ottauquechee. However, a Cambrian age is suggested because it has been traced into the Sweetsburg Formation and thence into the fossiliferous Woods Corners Group (Osberg, 1956).

In north-central Vermont the Ottauquechee Formation is contiguous with the "Hazens Notch Formation" (Cady et al., 1963), which is exposed in the core of the axial anticline. The two units are somewhat similar lithologically and have been separated on the basis of a discontinuous greenstone body. Cady et al. (1963) have interpreted the stratigraphic position of the "Hazens Notch Formation" to be beneath the Ottauquechee Formation. According to their interpretation, the Hazens Notch intertongues with the Underhill Formation on the west, and perhaps also with part of the Pinnacle Formation. Alternatively, the Hazens Notch may in fact be Ottauquechee. The large pattern of outcrop (see Doll et al., 1961) may be the result of structural complexit that causes its anomalous position at the core of the axial anticline. The latter interpretation is favored by

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the writer, and the Hazens Notch is shown as the Ottauquechee Formation in Figure 2.

Cambrian-Ordovician(?)

Rocks of Cambrian-Ordovician(?) age include the Stowe Formation, Caldwell Group, and Armagh Group. The rocks are unfossiliferous but lie between the Sweetsburg-Ottauquechee (Cambrian) and units assigned to the Ordovician.

The Stowe Formation (Cady, 1956) crops out in the east limb of the axial anticline in Vermont. It is light-grayish-green chlorite-mica phyllite that locally contains distinctive units of greenstone. In extreme north-central Vermont, numerous beds of sandstone and conglomerate are interbedded with the phyllite (Cady et al., 1963). The Stowe is estimated to be from 1,500 to 10,000 ft thick in northern Vermont (Cady, 1956; Cady et al., 1963), but it thins to perhaps 500 ft in southern Vermont (Skehan, 1961).

The Caldwell Group (MacKay, 1921) is the northward extension of the Stowe into Quebec. It is exposed in the east limb of the axial anticline from the International Boundary northward, and rocks here assigned to it are exposed in the west limb of the Lincoln-Enosburg Falls anticline from Granby northward (Fig. 1). The Caldwell Group consists of grayish-green and purple phyllite; gray, red, and green sandstone; conglomerate; and greenstone. The thickness is estimated to be between 500 and 1,000 ft.

The Armagh Group (Beland, 1957) is continuous with the Caldwell Group west of the Notre Dame Mountains. It is in the same stratigraphic position as the Caldwell and is similar in lithology.

Ordovician

Rocks of Ordovician age include limestone and dolomite herein called "undifferentiated limestone and dolomite," and the Hortonville (Ira) Slate, Stanbridge Slate, Beauceville Group, Cram Hill Formation, and Moretown Formation.

Undifferentiated limestone and dolomite of Beekmantownian and Chazyan ages crop out in the St. Albans-Hinesburg-Middlebury synclinorium (Cady, 1945). Similar rocks also are exposed in the continuation of the synclinorium in southern Quebec. The units are locally fossiliferous and have a total thickness of approximately 3,000 ft.

The Hortonville Slate (probably equal to the Ira as used by Keith, 1932; see Zen, 1964) crops out in the St. Albans-Hinesburg-Middle-bury synclinorium in west-central Vermont. It is gray slate that locally contains thin beds of quartzite and thick beds of gray limestone. Fossils from the slate are consistent with a medial Ordovician age (Kay, 1959).

The Stanbridge Slate (Clark, 1934) in southern Quebec is lithologically similar to the Hortonville and has a similar stratigraphic relation to the underlying rocks. It is thin-bedded, dark-gray slate with thin interbeds of quartzite, blue limestone, and lenses of limestone conglomerate. Diamictite (defined by Flint et al., 1960a, b) is present at the base at several localities, and mafic volcanic rocks have been observed within the formation. Clark (1934) has suggested a medial Ordovician age for the Stanbridge, but Riva (1966), on the basis of graptolites, indicates an early Ordovician age.

The Beauceville Group (MacKay, 1921) is exposed in the east limb of the axial anticline in southern Quebec. It consists predominantly of black, gray, and locally green slate, gray sandstone, and layers of diamictite. Greenstone forms a distinctive unit. Fossils collected near Magog and near St. Justine indicate a medial Ordovician age (Berry, 1962; Gorman, 1954).

The Cram Hill Formation (Currier and Jahns. 1941) crops out in the east limb of the axial anticline in Vermont. It is the southern extension of at least part of the Beauceville Group. Lithologically, the Cram Hill Formation consists of black phyllite, thin interbeds of sandstone, agglomerate, and greenstone. No fossils have been found in the rocks, but they are similar to the part of the Beauceville from which medial Ordovician fossils were collected.

The Moretown Formation (Cady, 1956) underlies the Cram Hill Formation in the east limb of the axial anticline in Vermont. It has not been distinguished separately in southern Quebec. It consists of grayish-green, thinly interbedded phyllite and quartzite, gray sandstone, and greenstone. Its thickness has been estimated to be 6,000-25,000 ft (Cady, 1956; Cady et al., 1963). Cady et al. (1963) interpret the Moretown Formation to be a partial facies equivalent of the Cram Hill, and thus suggest that it is of medial Ordovician age. Recently, Cady (written commun., 1967) has suggested that it may be partly early Ordovician.

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Unconformities

Several unconformities are exposed in the Green Mountain-Sutton Mountain anticlinorium. Major unconformities are present at the base of the Cambrian(?) section and beneath middle Ordovician rocks. Other unconformities may be present within the Middle Cambrian sequence and at the base of upper Ordovician rocks. The unconformity beneath Silurian rocks that crop out in outlying areas is exposed only in the vicinity of Magog, Quebec, in the Green Mountain-Sutton Mountain anticlinorium (St. Julien, 1965).

Precambrian rocks beneath the Cambrian(?) unconformity have recrystallized under different metamorphic conditions than affected the overlying Paleozoic rocks. In addition, on a regional scale the Precambrian rocks contain major and minor structural features that are truncated at the unconformity (Billings et al., 1952; Doll et al., 1961).

A second major unconformity lies beneath the Hortonville (Ira) Slate and the Stanbridge Slate west of the Green Mountain-Sutton Mountain anticlinorium in Vermont. In west-central Vermont the Hortonville (Ira) truncates rock units ranging in age from early Ordovician to Precambrian (Osberg, in prep.; Thompson, 1959). The unconformity may diminish northward and be absent beneath the Stanbridge Slate. Riordon (1954, 1957; St. Julien, 1961) reports an unconformity in the east limb of the axial anticline beneath the Beauceville Group in southern Quebec. However, Cady (1956) suggests that the unconformity is not present in the east limb of the axial anticline in central and southern Vermont.

A local unconformity has been suggested by Shaw (1958) at the base of the Rugg Brook Dolomite in northwestern Vermont. In southern Quebec the extension of this unconformity is marked by limestone conglomerate and sandstone. The unconformity extends from northwestern Vermont into Quebec at least as far north as Danville (Fig. 2).

St. Julien (1961) reports an unconformity beneath an upper Ordovician section in southern Quebec.

Facies Relations and Correlation

The axial trace of the Green Mountain-Sutton Mountain anticlinorium cuts at a low angle across the zone of intertonguing between a predominantly miogeosynclinal sequence on the west and a eugeosynclinal sequence on the east. The anticlinorium in Vermont, where Precambrian rocks form the core, has miogeosynclinal rocks in the west flank and eugeosynclinal rocks in the east flank.

The Cambrian section exposed in the west limb of the anticlinorium has been traced into northwestern Vermont (Cady, 1945; Shaw, 1958) where the Monkton Quartzite, Winooski Dolomite, Danby Formation, and Clarendon Springs Dolomite intertongue with the Parker Slate and Woods Corners Group. In southern Quebec the equivalent section is represented by the Sweetsburg Formation (Oak Hill Slate included for convenience), Dunham Dolomite, and Gilman Quartzite. Both the Dunham Dolomite and Gilman Quartzite are thinner than in Vermont; the Bonsecours Formation thickens as the Gilman thins. The sequence Bonsecours-Gilman-Dunham-Sweetsburg in the west limb of the anticlinorium has been traced through the fold depression at Danville (Fig. 2) and into the east limb (Osberg, 1965). The Gilman Quartzi e and Dunham Dolomite thin and disappear in the east limb of the anticlinorium in the vicinity of the St. Francis River, and, consequently, the Cambrian sequence in the east flank is represented mainly by the Bonsecours and the Sweetsburg Formations. These formations have been traced southward into the Underhill and Ottauquechee Formations of Vermont.

A correlation chart is shown in Figure 3.

STRUCTURAL GEOLOGY

The Green Mountain-Sutton Mountain anticlinorium has complex structural geometry. Ancestral folds predate the formation of the anticlinorium, and its present form is the result of the imprints of several deformations.

Ancestral Geanticlines

The stratigraphic interval that is absent beneath the middle Ordovician unconformity defines the general position of an ancestral geanticline. This ancestral geanticline may have developed partly by block faulting (Thompson, 1959). In west-central Vermont, the Hortonville (Ira) Formation and the associated Middlebury Limestone lie directly on limestone of Beekmantownian age (Cady, 1945; Doll et al., 1961); on the east, in the west flank of the Pine Hill anticline, they lie on marble of Beekmantownian age. In the west limb of the axial

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anticline, the Hortonville (Ira) Formation lies on Precambrian rocks, and, in the east limb, the stratigraphic equivalents (Moretown? and Cram Hill) lie on rocks of Cambrian-Ordovician(?) age. Thus, at this latitude an ancestral geanticline was present approximately in the same position as the Green Mountain-Sutton Mountain anticlinorium.

On the north, near Starksboro, Vermont (Fig. 2), previously unreported Hortonville (Ira) rocks were found to overlie quartzite of Early Cambrian age in the west limb of the Lincoln-Enosburg Falls anticline. At the same latitude, the axial anticline contains rocks of Cambrian(?) and Late Cambrian ages. In the east limb of the axial anticline, the equivalents of the Hortonville overlie a Cambrian-Ordovician(?) sequence, as they do farther south. The ancestral geanticline at this latitude was slightly west of the present position of the axial anticline.

In southern Quebec, the Stanbridge Slate (equivalent to the Hortonville) may be a partial facies equivalent of carbonate rocks of early Ordovician age on the south. In the St. Albans-Hinesburg-Middlebury synclinorium, the Stanbridge overlies early Ordovician rocks (Eakins, 1964); in the west limb of the Lincoln-Enosburg Falls anticline in southernmost Quebec, it lies on Late Cambrian phyllite (Eakins, 1964); and, in the vicinity of the St. Francis River, it overlies Cambrian-Ordovician(?) rocks. The Beauceville Group in the east limb of the axial anticline has a similar stratigraphic position. Unconformities at its base and above it in the vicinity of Magog have been described by St. Julien (1961). Apparently, the stratigraphic break defined by the middle Ordovician unconformity incre ses on the southeast (Cady, written commun., 1967), thus suggesting that the ancestral geanticline swings to a northeasterly direction in southern Quebec. The position of the ancestral geanticline is shown in Figure 4.

A second geanticline developed in post-medial Ordovician to late Silurian time, but, because of the sparse distribution of Silurian rocks in Vermont and southern Quebec, its position cannot be ascertained. Thin bodies of conglomerate and orthoquartzite crop out locally east of the Green Mountain-Sutton Mountain anticlinorium in both Vermont and southern Quebec. Silurian rocks are not exposed in the west limb of the anticlinorium, but, on the west in New York, a clastic wedge of Silurian redbeds and pelite thins westward from a source on the east, presumably in the vicinity of the ancestral geanticline.

The emplacement of the Taconic klippe was related to the development of the second geanticline (Potter, 1967; Zen, 1967). Taconic rocks slid westward from the geanticline, and some of the thrust faults in the St. Albans-Hinesburg-Middlebury synclinorium may have developed contemporaneously with the emplacement of the Taconic rocks. However, the age of the thrusts is not definite; Cady (1960) suggests that they are Acadian.

St. Julien (1965) has mapped two folds east of the axial anticline in southern Quebec that are overprinted by folds of Acadian age. The older folds, exposed only in pre--medial Ordovician rocks, are truncated at an unconformity by Silurian-Devonian rocks. Both the older section and the Silurian-Devonian section are deformed by younger folds.

Recumbent folds with axes that diverge from the general trend of the St. Albans-Hinesburg-Middlebury synclinorium have been described by Crosby (1963) in west-central Vermont. Zen (1967) reports folds of the same type in Taconic rocks. These structural features have been interpreted to be associated with the development of the Taconic rocks.

East-trending minor folds have been reported in rocks exposed in the Green Mountain-suton Mountain anticlinorium (Albee, 1957; Cady et al., 1963; Chidester, 1953; Osberg, 1965). They predate all other minor structural elements, but their relation to the ancestral geanticline is unknown.

Development of Green Mountain-Sutton Mountain Anticlinorium

The present Green Mountain-Sutton Mountain anticlinorium was developed in Devonian time as a result of at least three episodes of deformation. Each episode is indicated by distinctive structural features, but whether the episodes were essentially continuous or were separated in time is unknown.

Folds with north trends have developed in Devonian(?) rocks in northwestern Massachusetts (Hatch et al., 1968); where they have not been deformed by younger structural features, they are mainly isoclinal and vertical. In pre-Devonian units, however, these folds over-print

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older folds and thus cause a complex and obscure structural geometry.

The Lincoln-Enosburg Falls anticline and the Pine Hill anticline may be in part manifestations of this deformation (Fig. 4). In southern Quebec the Lincoln-Enosburg Falls anticline is deformed by possibly two younger structural features.

NE-trending slip cleavage and minor folds deform the older Acadian structural features. In southern Vermont and adjacent Massachusetts, this later deformation is represented by slip cleavage and minor folds (Hatch et al., 1968). In central Vermont, minor folds with similar trends plunge northeastward in the east limb of the younger axial anticline and southwestward in its west limb (Osberg, 1952). The age of the folds in central Vermont cannot be demonstrated in the field. In northern Vermont, Dennis (1961) has observed both cleavage and folds with northeast trends in Devonian rocks. In southern Quebec, larger folds, in addition to minor structural elements, are similar in style and trend to the NE-trending folds

Fig. 4. Age relations among structural features of Green Mountain-Sutton Mountain anticlinorium.

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in Vermont (Osberg, 1965; Rickard, 1965). Although the age of these folds in Quebec has not been ascertained, their proximity to those in northern Vermont suggests that they also are Devonian.

The axial anticline is the latest development in the construction of the Green Mountain-Sutton Mountain anticlinorium. It is an open fold for which the surface of deformation is schistosity. Older folds are folded into the axial anticline so that classic relations between stratigraphy and structure are eliminated (Osberg, 1965). A slip cleavage that cuts all other structural features is parallel with the axial surface of the axial anticline.

The trace of the axial anticline is shown in Figure 4. It extends from the Massachusetts-Vermont boundary across the ancestral geanticline in southern Vermont, and into northern Vermont and southern Quebec at least as far as Danville. The Strafford-Willoughby arch in eastern Vermont and the St. Albans-Hinesburg-Middlebury synclinorium also may have been formed during this deformation.

The age of this latest deformation is certainly post-Early Devonian and pre-Cretaceous. Monteregian and White Mountain stocks cut these structural features. Relations giving a closer date are equivocal. The Ordovician date given by Rickard (1965) from muscovite aligned in slip cleavage related to the axial anticline is questionable, because it is inconsistent with the folding of the Silurian and Devonian in the vicinity of Magog (Fig. 2).

Although the Notre Dame anticline generally is considered to be the same age as the axial anticline, and its divergent trend is considered to be caused by a salient in the fold system, it is tempting to speculate that it belongs to the older Devonian NE-trending generation of folds. Thus, the intersection of the Notre Dame anticline by the axial anticline and the Lincoln-Enosburg Falls anticline at Danville would have produced the culmination in that area (Fig. 2). On the southwest, near Actonvale, two domal structures (W. A. Gorman, written commun., 1967) exposing black slate and gray limestone beneath slate and sandstone of the Caldwell Group are in line with the Notre Dame anticline. The domal structures are interpreted to be intersections of late folds related to the development o the axial anticline and the older Notre Dame anticline. More work is required in the Actonvale-Danville region to ascertain definitive relations.

SUMMARY

The Green Mountain-Sutton Mountain anticlinorium cuts at a low angle across a facies boundary between a predominantly miogeosynclinal sequence on the west and a predominantly eugeosynclinal sequence on the east. Although the two facies are dissimilar in stratigraphy, approximate stratigraphic equivalence between them can be ascertained by tracing the stratal units through the facies boundary in northwestern Vermont and southern Quebec. Distinct unconformities have been identified at the base of the Cambrian(?) sequence and at the base of the Trentonian section. Lesser unconformities are suggested within the Upper Cambrian section in northwestern Vermont and southern Quebec, and at the base of the upper Ordovician in southern Quebec.

The position of the Green Mountain-Sutton Mountain anticlinorium approximately coincides with the position of the ancestral geanticlines of medial Ordovician and post-medial Ordovician to Silurian ages. The present anticlinorium is probably Acadian. Its development involved at least three geometrically different constructional configurations, the latter two each being overprinted on its predecessor. It is not known whether the deformation was essentially continuous or periodic.

COMPARISON OF GREEN MOUNTAIN--SUTTON MOUNTAIN ANTICLINORIUM WITH LONG RANGE, NEWFOUNDLAND

Preliminary observations in the Long Range in Newfoundland suggest that it has many similarities to the Green Mountain-Sutton Mountain anticlinorium in Vermont and southern Quebec. Therefore, the Green Mountain-Sutton Mountain anticlinorium might be, to a first approximation, a good "blueprint" to use in working out the geology of the Long Range.

The gross features of the two areas are similar. The Long Range, like the Green Mountain-Sutton Mountain anticlinorium, is anticlinal and overturned, and perhaps partly overthrust, toward the west. Both fold systems have a carbonate-quartzite sequence of Cambrian and Ordovician age in the west flank and a succession of schist, partly interrupted by a Carboniferous graben in the Long Range, in the east flank.

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Klippen of Ordovician to Silurian age are associated with both anticlinoriums.

In addition to the general similarity of the two fold systems, certain stratigraphic details are remarkably parallel. The Lower Cambrian sequence exposed in the Long Range near Bonne Bay is similar, if not identical, to the Lower Cambrian section in the west limb of the Green Mountain-Sutton Mountain anticlinorium. The succession at Table Head west of the Long Range, although different in detail, is similar to the Ordovician section exposed in the St. Albans-Hinesburg-Middlebury synclinorium in west-central Vermont. Both sections contain unconformities beneath limestone of medial Ordovician age. The graywacke and greenstone in the Maiden Point Group north of the Long Range are similar lithologically to the Pinnacle Formation in southern Quebec. Although a comparison of the schist sequ nces in the east flanks of the two structural features cannot be made because of insufficient information, the schist exposed farther east at Gander Lake and west of Grand Falls is lithologically and stratigraphically similar to the Moretown-Cram Hill sequence in the east limb of the Green Mountain-Sutton Mountain anticlinorium.