Stratigraphie Sequence of the Gile Mountain and Waits River Formations Near Royalton, Vermont

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Stratigraphie sequence of the Gile Mountain and Waits River Formations near Royalton, Vermont

GEORGE W. FISHER I jjepartment Qf Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 PAUL KARABINOS

ABSTRACT been controversial. This uncertainty has the Gile Mountain belt near Royalton, The stratigraphie sequence of the Gile given rise to numerous difficulties in inter- Vermont, show that the Gile Mountain Mountain and Waits River Formations, preting the regional structure of eastern Formation is younger than the Waits River two major Silurian-Devonian lithostrati- Vermont. Extensive sequences of compo- Formation, indicating that the belt is a graphic units in eastern Vermont, has long sitionally graded beds at 19 localities across syncline.

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Figure 2. Compositionally graded beds of micaceous quartzite rhythmically interbedded with garnet-mica schist in Gile Mountain Formation, 50 m east of contact with Waits River Formation in a Figure 1. Extent of Gile Mountain (Dgm) and Waits River pasture 2.8 km west of Royalton, Vermont (loc. A, Fig. 7). View is (Dwr) Formations in eastern Vermont, adapted from Doll and looking north; grading indicates tops to east, toward Gile

others (1961); rectangle indicates area shown in Figure 7, at Royal- Mountain Formation. S! is parallel to bedding; S2 is parallel to pen. ton, Vermont. Pen is 16 cm long.

Geological Society of America Bulletin, Part I, v. 91, p. 282-286, 7 figs., May 1980, Doc. no. 00506.

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INTRODUCTION vidual graded beds can be traced laterally (see Fig. 7), where a nearly continuous sec- for 30 m or more, and there is no sign of tion across the contact can be pieced to The stratigraphic sequence of the Gile channeling or erosional truncation of beds. gether from abundant discontinuous out- Mountain and Waits River Formations, The contact between the two formations crops in a pasture. Along the western edge two major Silurian-Devonian lithologic is commonly gradational over a zone 500 to of the pasture are outcrops typical of the units in eastern Vermont, has long been 100 m wide; it is best exposed at locality A Waits River Formation: brown-weathering controversial. Some investigators (for example, Em, 1963) have concluded that much of the Gile Mountain Formation is older than the Waits River Formation; others (for example, Dennis, 1956; Chang and others, 1965) have inferred the opposite stratigraphic sequence; and still others (for example, Murthy, 1957) have suggested that major parts of the two units are lateral facies equivalents, of approximately the same age. Most recent investigators have assumed that the Waits River Forma- tion is the older unit but have admitted that the question is still open. Because these two units underlie much of eastern Vermont (Fig. 1), the uncertainty about their relative age has contributed greatly to the difficulty of interpreting the regional structure of this complex area (Em, 1963; Rosenfeld, 1968; Woodland, 1977). We describe here some well-exposed sequences of graded beds near Royalton, Vermont, which demonstrate that the Gile Mountain Formation is younger than the Waits River Formation, at least in that area. Figure 3. Polished slab of interbedded micaceous quartzite (light gray) and garnet-mica schist (dark gray), showing sharp basal contact and gradational upper contact of mica- LITHOLOGY AND ceous quartzite layers. Slab is from top of outcrop shown in Figure 2. S, is parallel to bedding; S is parallel to scale. Scale is in millimetres. MINOR STRUCTURES 2

The Gile Mountain Formation consists predominantly of light-gray micaceous quartzite and quartz schist in beds 5 to 20 cm thick, rhythmically interlayered with fine-grained, dark-gray mica schists, and scattered beds of brown-weathering dolo- mitic marble. A few of the pelitic beds are aluminous enough to contain staurolite and kyanite. The Waits River Formation is characterized by beds of brown-weathering siliceous marble 10 to 30 cm thick, interbedded with black, rusty-weathering mica schist rich in sulfides and scattered beds of dark, micaceous quartzite. Many quartzite and quartz-schist beds in both formations are compositionally graded, from a quartz-rich base in sharp contact with the underlying pelite to a micaceous top that grades imperceptibly into the overlying pelite (Figs. 2, 3, 4, 5). The upper parts of some graded beds are delicately laminated, but it is very difficult to determine whether the laminae are relict Figure 4. Compositionally graded micaceous quartzite beds interlayered with subordi- sedimentary structures or metamorphic fea- nate mica schist in Gile Mountain Formation along White River, 1.2 km northeast of tures. In the extensive exposures in the bed Royalton, Vermont. View is looking north; sense of grading shows tops to west (left), of the White River near Royalton, indi- toward Gile Mountain Formation.

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impure marble interbedded with dark-gray suggest that the Gile Mountain Formation Royalton. The evidence at locality A (Fig. to black rusty-weathering garnet-mica is younger than the Waits River Formation 7), a pasture straddling the contact between schist containing scattered beds of dark- and that the Gile Mountain Formation oc- the Waits River and Gile Mountain Forma- gray micaceous quartzite, locally graded. cupies the core of a major syncline with an tions, is especially convincing. Nearly 100 Farther east is a transitional zone about 100 axial trace passing about 1.2 km west of graded beds — most in the Gile Mountain m thick in which the schist becomes pro- gressively lighter gray and loses its rusty- weathering character, while the psammitic beds become more abundant, richer in quartz, and more distinctly graded. At the southeast corner of the pasture, light to medium-gray mica schist is rhythmically interbedded with 5- to 10-cm micaceous quartzite layers typical of the Gile Moun- tain Formation. There is no sign of tectonic disruption of the stratigraphic sequence at any point in the section. As discussed by Woodland (1977), tectonic structures are the same in both formations: pervasive schistosity (Si) that parallels bedding (So) except at the noses of early minor folds (Fig. 6); closely spaced crenulation cleavage of variable intensity

(S2) which commonly cuts S0 and Sj at a high angle (Fig. 6) and is approximately parallel to the axial planes of a second set of folds; and a more widely spaced crenulation cleavage or kink banding (S3) which locally folds S2. Most of the F, folds (to which S! is Figure 5. Detail of graded beds in outcrop shown in Figure 4; scale is 17 cm long, axial) are markedly asymmetrical, with one limb ten or more times as long as the other; consequently, the facing directions indicated by graded beds in most outcrops are generally consistent, with only scattered re- versals corresponding to the short limbs of

F; folds. Most of the F2 folds (to which S2 is axial) are relatively open and have limbs of subequal length. Despite the complexities introduced by these two periods of folding, the river-polished outcrops along the White River near Royalton are large enough to permit determination of the three-dimen- sional form of minor folds, so that true stratigraphic facing directions can be inferred from graded bedding.

ORIENTATION OF GRADED BEDDING

The direction of stratigraphic tops as inferred from graded beds at 19 localities near Royalton are plotted in Figure 7. Each sym- bol represents the overall facing direction as Figure 6. Minor F, fold in interbedded micaceous quartzite and mica schist of Gile inferred from all the beds in a large outcrop Mountain Formation in outcrop along White River, 1.25 km northeast of Royalton, Ver-

or in a group of small outcrops. Many sym- mont, looking north parallel to fold axis. Fold is defined by bedding (S0), has Si parallel to bols are based on observations of dozens of its axial plane, and has S2 about 90° to axial plane. Right-handed sense of fold and sense of beds; 14 are based on observations of more graded bedding on long limbs of fold show that upright major F! syncline lies to west. Scale than 10 beds. Most of the data plotted is 17 cm long.

Formation, but some in the Waits River Formation — show tops toward the Gile Mountain Formation (Figs. 2, 3,). Only three beds, presumably on the short limbs of minor F, folds, show tops in the reverse direction. At only one locality (B in Fig. 7) does the inferred facing direction appear to conflict with a synclincal interpretation of the Gile Mountain belt. The western part of the outcrop displays only a few graded beds, all topping to the west, whereas the eastern part displays many beds, nearly all facing east, which give an eastward-facing sense to the outcrop as a whole. These east-facing beds are affected by a minor F, fold with a left-handed sense viewed parallel to its north-plunging axis, suggesting that these beds are on the western limb of a larger Fi synform. By contrast, the many nearby outcrops, which are dominated by west-facing beds (Figs. 4, 5), consistently contain

right-handed Ft folds (Fig. 6), suggesting that they lie on the eastern limb of a major F, synform. Because outcrops with west- facing beds and right-handed Fj folds greatly predominate in this area (Fig. 7), it seems likely that locality B is on the short limb of a local F, fold and that the regional F, synclinal axis lies to the west, near the center of the Gile Mountain belt, as shown in Figure 7. At one locality (C in Fig. 7) minor Fi folds are so numerous that no overall facing direction could be determined. However, the graded beds show that the downward- closing minor folds are all synclines, and the upward-closing folds are all anticlines, confirming that the regional F! syncline is a normal upright structure.

DISCUSSION AND REGIONAL SIGNIFICANCE

Because of the gradational nature of the contact between the Waits River and Gile Mountain Formations, the consistent direction of tops indicated by graded bedding in both units, and the absence of any indication of tectonic disruption near the contact, we assume that the two formations are in normal stratigraphic contact. Our observations of graded beds suggest a stratigraphic sequence opposite to that inferred Figure 7. Stratigraphie sense inferred from graded beds near Royalton, Vermont, in by Ern (1963, p. 70—71) from observations Gile Mountain (Dgm) and Waits River (Dwr) Formations, together with orientation of of minor folds and bedding-cleavage rela-

bedding (S0) and schistosity (S, and S2). A, B, and C denote localities referred to in text. For tions, locally combined with graded bed- location of area shown, see Figure 1. ding. However, Ern did not show his

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graded-bedding data on his map, so we 1122) suggested that the Royalton synform REFERENCES CITED cannot compare our data with his. Fur- "probably represents the downward facing thermore, although Em recognized the nose of a recumbent nappe" with a root Chang, P. H., Ern, E. H., and Thompson, J. B., multiple deformation of these rocks, he did zone far to the east. Our stratigraphic data Jr., 1965, Bedrock geology of the Wood- stock quadrangle, Vermont: Vermont not differentiate between F: and F2 folds on indicate that the structure is a syncline with Geological Survey Bulletin 29, 65 p. his map, and he appears to have used folds a west-dipping axial surface, as originally Dennis, J. G., 1956, The geology of the Lyndon- of both generations in working out the di- suggested by Dennis (1956, p. 36). At pres- ville area, Vermont: Vermont Geological rection of stratigraphic tops. Second- ent, it is not clear how this fold geometry Survey Bulletin 8, 98 p. generation folds and bedding-cleavage in- fits the regional structure, but at least two Doll, C. G., and others, 1961, Centennial geological map of Vermont: Vermont Geological tersections are much more numerous than interpretations are possible: (1) as the result Survey, scale 1:250,000. first-generation structures, and many give a of easterly directed recumbent folding, as Ern, E. H., 1963, Bedrock geology of the Ran- misleading indication of the facing direc- suggested by Hepburn (1975) for the Pros- dolph quadrangle, Vermont: Vermont tion, owing to the effects of the early fold- pect Hill fold in southeastern Vermont, or Geological Survey Bulletin 21, 96 p. ing. In multiply folded rocks such as these, (2) as the result of westerly directed flow in Hepburn, J. C., 1975, Tectonic and metamorphic chronology of the Devonian and Silurian primary sedimentary structures are the which the flow lines near the Waits rocks in the Guilford Dome area, southeast- most reliable indicators of facing direction. River-Gile Mountain contact pass strati- ern Vermont: U.S. Geological Survey Pro- In view of the consistency of our observa- graphically downward from the Gile fessional Paper 888, p. 33-49. tions of the sense of graded beds, we con- Mountain Formation into the Waits River Murthy, V. R., 1957, The bed rock geology of the East Barre area, Vermont: Vermont clude that the Gile Mountain Formation is Formation, as suggested by Rosenfeld Geological Survey Bulletin 10, 121 p. younger than the Waits River Formation in (1968, Fig. 11) for part of the "Ascutney Rosenfeld, J. L., 1968, Garnet rotations due to the Royalton area. sigmoid," the northern continuation of the major Paleozoic deformations in south- Woodland (1977) concluded that the Hepburn's Prospect Hill fold. east Vermont, in Zen, E-an and others, eds., Studies of Appalachian geology: Northern Gile Mountain belt at Royalton occupies and maritime: New York, Wiley-Intersci- the core of a north-plunging synform clos- ence, p. 185-202. ing about 8 km south of Royalton (Fig. 1). AC KNOWLEDGMENTS Woodland, B. G., 1977, Structural analysis of the Because S, roughly parallels the axial trace Silurian-Devonian rocks of the Royalton of the structure and S2 trends across the belt area, Vermont: Geological Society of We thank E. H. Em, J. B. Lyons, J. L. American Bulletin, v. 88, p. 1111-1123. at a high angle, he inferred that the synform Rosenfeld, and B. G. Woodland for helpful formed during the early folding. Our obser- comments on a preliminary version of this MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 26, 1979 vations also support this interpretation (Fig. paper. The work discussed here was sup- REVISED MANUSCRIPT RECEIVED SEPTEMBER 28, 7). In his interpretive cross section and dis- ported by National Science Foundation 1979 cussion, however, Woodland (1977, p. Grants EAR-7612278 and EAR-7812968. MANUSCRIPT ACCEPTED OCTOBER 8, 1979

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