University of New Hampshire University of New Hampshire Scholars' Repository New England Intercollegiate Geological NEIGC Trips Excursion Collection 1-1-1972 Excursions at the North End of the Taconic Allochthon and the Middlebury Synclinorium, West-Central Vermont, with Emphasis on the Stucture of the Sundbury Nappe and Associated Parautochthonous Elements Voight, Barry Follow this and additional works at: https://scholars.unh.edu/neigc_trips Recommended Citation Voight, Barry, "Excursions at the North End of the Taconic Allochthon and the Middlebury Synclinorium, West-Central Vermont, with Emphasis on the Stucture of the Sundbury Nappe and Associated Parautochthonous Elements" (1972). NEIGC Trips. 162. https://scholars.unh.edu/neigc_trips/162 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]. Trip B-3 EXCURSIONS AT THE NORTH END OF THE TACONIC ALLOCHTHON AND THE MIDDLEBURY SYNCLINORIUM, WEST-CENTRAL VERMONT, WITH EMPHASIS ON THE STRUCTURE OF THE SUDBURY NAPPE AND ASSOCIATED PARAUTOCHTHONOUS ELEMENTS by Barry Voight, Department of Geosciences The Pennsylvania State University University Park, Pa., 16802 SUMMARY This excursion is designed to provide insight into the Paleozoic displacement and strain patterns at the juncture of the Taconic Allochthon and Middlebury Synclinorium. The nature of structural contacts will be examined in detail, and sequences of fault, fold, foliation and lineation evolution will be examined. Field trip stops will include locations within the allochthon and tne synclinorium in order to place their mutual boundary relation­ ships into proper perspective. Assembly point: front of the office of the Vermont Struc­ tural Slate Company, near junction Routes 4 and 22A south of Poult­ ney River in Fair Haven, Vermont. 8:39 A.M. sharp, Saturday, Oct­ ober 14 . Quadrangle maps: Sudbury(1946), Bomoseen(1944), Thorn Hill(1946), and Middlebury(1963) 7 1/2' topographic sheets. Most stops will be in the Sudbury quadrangle. The two standard published references for the area of the excursions are: Cady, W. M., 1945, Stratigraphy and structure of west-central Vermont: Geol. Soc. America Bull., v. 46, p. 515-558. Zen, E-an, 1961, Stratigraphy and structure at the north end of the Taconic Range in west-central Vermont: Geol. Soc. America Bull., v. 72, p. 293-338. Excursion participants may wish to review these references in ad­ vance of the field conference. Also to be recommended is the more recent review of Taconic geology by Zen (1967). Begin Excursion: Enter Fair Haven Village and turn west on Route 4^ Stop, approximately 2.5 miles at William Miller Chapel south of Route 4. Locality 1: Taconic thrust; Structural Window at William Miller Chapel - An area of intensely-deformed Ordovician Beldens t 50 CO o •H P • r < Ci co >-3 c • rH CO P m o X tL p§ •H o> x: CO c\ VT\ o c 03 NJ P 0 ) p Cm 03 pH cd jC c p a; in o •H ow; i—i o 03 o 03 P t il! •H &H On VO O C CD [S3 P CD -P <H cd CD bfl £ cd cr; o •H c o o cd Eh CD -P co co o £ o cd CO c o •rA -p c CD CO CO CO c o o •H rH cd u CD c CD o C\i • CD £ be •rH CL, 52 limestone and dolostone (Chipman formation, Cady and Zen, 1960) overlain in nearly horizontal contact by the early Cambrian Bomo­ seen graywacke member of the Bull formation (Figure 1; cf. Zen, 1961, Figure 4, p. 1.3; Plate 1). This is a structural window (Figures 1,3) in which the autochthonous carbonate is exposed through an overthrust fault sequence. The superjacpnt allochtho­ nous sequence within the general vicinity is normal, showing that at least here no recumbent fold (Figure 2, B-B') exists in the Taconic sequence (Zen, 1961, p. 319; Plate 1). The structural con­ tacts are locally exposed and can be examined in detail; there is no outcrop to the South. The Bomoseen graywacke (Zen, 1961, p. 301-302) is typical­ ly a hard, olive-grey, coarsely-cleaved rock, weathering to white or to pale brick red. Quartz and feldspar grains, typically 1 mm, are common; white mica occurs in alignment with cleavage surfaces. A common assemblage is muscovite-chlorite-albite-microcline-sti1- pnomelane-quartz. Zen has presented evidence suggesting that the Bomoseen graywacke is a lithofacies that becomes progressively older and thicker to the west. It is the oldest unit in the Tac­ onic sequence exposed west of Glen Lake. The Beldens member of the Chipman formation is typically a white marble limestone with local interbeds of orange- to buff- weathered dolostone. Return to Fair Haven and take the Scotch Hill road north to West Castleton. Interlude: The first quarrying of slate in Rutland County was done by Col. Alanson Allen of Fair Haven in 1839 on Scotch Hill, about a mile north of Fair Haven village. The first quarry was worked for 8 years, using the products for hearths, headstones for cemeteries, school slates and flagging for walks, before any roofing slate was manufactured. It was one year more, in 1848, before the first roof was covered with Vermont slate. This was done by Col. Allen under the following conditions. He was to wait for one year for his pay, and if, in the meantime the roof should break down from the weight of the slate, he was to receive no pay, but should pay all damages. The farmer was disappointed and the roof is good today (Smith and Rann's, History of Rutland County, 1866). The barn still stands on the farm of Stanley Kruml, about a mile south of Fair Haven on Rt. 22-A; the roof is in excellent condition. Locality 2 : Scotch Hill Syncline, West Castleton - This is private property. We are permitted to be here by the courtesy of the owners. Sampling of rock specimens is not permitted; LEAVE ALL HAMMERS IN THE VEHICLES ! Please cooperate — thank you. 53 - s a -, ' %%% ~ ^ ~ ^ %%%%% N' v ♦ * c % % " - . — — rr;::_;% .........h \ % « x A. > •*„ s* *» Nr -** 1 \ ; • * * i . 5 * • «Jill. * • * j J - _ i» - ? • * m 1 t J *■ * J ^* Jft * ii • a X ! * W * * # N\li.l*< APci_ 04 Figure 3. Plane-table outcrop map of structural window, William Miller Chapel - *40/ _ #*• 2 o A ^ ,- -f- ^ — 5* Exposed cross-section of the Scotch Hill syncline: the east limb is nearly vertical at this locality (Figure 4). The west limb dips east at a shallow angle. This is the West Castle­ ton formation (Zen, 1961, p. 304), within which early Cambrian fos­ sils have been found by several workers, in ledges near this local­ ity. The West Castleton formation ranges from a dark-grey, hard, poorly-cleaved sandy or cherty slate that weathers white or pale-red to a jet-black, fissile, graphitic and pyritic slate that contains many paper-thin white sandy laminae and commonly also black cherty nodules, and when weathered displays much alum bloom. Locally in­ terbedded in the fine black slate are beds of buff-to yellow-weath­ ering black dolostone or dolomitic quartzite, a few inches thick, some of which, however, become massive, siliceous, and heavily bed­ ded in the harder black slate. The varieties of black slate do not form mappable units but grade into each other along strike (Zen, 1961, p. 304-305). The rock becomes a phyllite to the east. Immediately to both the east and west of this fold are quarry belts of green and purple slates (Mettawee member, Bull for­ mation) . The simplicity of the Scotch Hill structure and the rela­ tion to the underlying Bull formation is consistent with the rela­ tive ages of the two units as proposed by Zen (1961, p. 317). The flexural flow fold displays a host of well developed minor struc­ tural elements, e.g. cleavage in several rock types, "refraction" of cleavage, slip and flow phenomena in the dolostones; calcite- filled fractures; and pressure-shadows associated with porphyro­ blasts. Cleavage is axially-planar in thicker slate beds. The dating of the cleavage is an important but unclosed question; Zen (1967, Appendix 7, p. 101) has suggested a post-Ordo­ vician age of the regional metamorphism based both on regional re­ lations and on radiometric dating. Zen noted that the regional metamorphic grade increases steadily eastward, from the non-meta- morphosed rocks of the Champlain Valley, to almandine-kyanite grades in eastern Vermont. No multiple-metamorphic effects were observed in the Paleozoic sequence; rocks as young as Devonian are involved and the metamorphic episode was presumed to postdate this. Radiometric dates in the range 350-390 m.g., from the southern Ta­ conic region and from north-central Vermont (Camels Hump area) are cited by Zen in support of this view. Yet Harper (1968) has cited radiometric dates from slate belt minerals in the range 445-460 m.g., which dates would be compatible with Ordovician (Taconic) de­ formation. This problem remains unresolved, and demands attention; a correct interpretation of regional structural relationships with­ in the "Taconic" and "synclinorium" areas hinge upon its resolution. Drive eastward along the Cedar Mountain Road to the(dead) end. Quarry on the left at the end. Locality 3 : Cedar Mountain Syncline; slate quarry This, the first major structure west of Lake Bomoseen, is specta­ cularly exposed in the abandoned quarry cut into Cedar Mountain (Figure 4). Fold is overturned to the west, nearly recumbent, and plunges south; axial plane cleavage dips gently eastward.