STRATIGRAPHY AND STRUCTURE OF THE CASTLETON AREA VERMONT I By PHILLIP FOWLER VERMONT GEOLOGICAL SURVEY CHARLES G. DOLL, State Geologist - Published by VERMONT DEVELOPMENT COMMISSION MONTPELIER, VERMONT BULLETIN NO. 2 1950 SECOND PRINTING 1958 CONTENTS PAGE Abstract 5 Introduction 6 General ......................... 6 Acknowledgments . . . . . . . . . . . . . . . . . . . 8 Previous work ...................... 9 Regional physiography .................. 10 Drainage ........................ 11 Valley sequence . . . . . . . . . . . . . . . . . . . . . 13 General ......................... 13 Mendon series ...................... 14 Lower Cambrian series .................. 17 Cheshire quartzite ................... 17 Dunham dolomite . . . . . . . . . . . . . . . . . . 18 Monkton quartzite ................... 20 Winooski dolomite ................... 20 Lpper Cambrian series .................. 21 Danby formation .................... 22 Clarendon Springs dolomite ............... 23 Lower Ordovician series .................. 24 Boardman formation .................. 24 Bascom formation ................... 27 Ordovician—Chazy group ................. 28 Burchards limestone . . . . . . . . . . . . . . . . . 28 Beldens formation ................... 29 Middlebury limestone .................. 30 Ordovician—Trenton group ................ 31 Orwell limestone .................... 31 Whipple marble .................... 32 Hortonville slate .................... 34 Taconic sequence ...................... 37 General ......................... 37 Lower Cambrian series .................. 38 Nassau formation ................... 38 Bomoseen grit ..................... 46 Mettawee slate .................... 47 Eddy Hill grit ..................... 49 PAGE Schodack formation .................. 50 Zion Hill quartzite ................... 53 Ordovician system .................... 55 Normanskill formation ................. 55 Pre-Ordovician unconformity . . . . . . . . . . . . . 57 Dikes ........................... 58 Structural geology ..................... 59 Major features ...................... 59 General setting .................... 59 Folds ......................... 61 Faults ..........................2 Taconic overthrust ................... Minor features ...................... Cleavage . . . . . . . . . . . . . . . . . . . . . . 69 Joints ......................... 71 Drag folds and flowage folds ............... 71 Time of deformation ................... 71 Metamorphism ....................... 72 Historical geology . . . . . . . . . . . . . . . . . . . . 74 Economic geology . . . . . . . . . . . . . . . . . . . . 78 References cited ...................... 80 Illustrations FIGURE 1. Geographic location of Castleton area . 7 2. Columnar sections of strata ................12 3. Major structural features of Castleton quadrangle and vicinity .................60 PLATE I. Correlation of Castleton area with ad- joining regions . Following page 57 II. Geological map of Castleton quadrangle .......In pocket III. Structure sections A-A' and B-B' ..........In pocket IV. Structure sections C-C' and D-D'..........In pocket STRATIGRAPHY AND STRUCTURE OF THE CASTLETON AREA, VERMONT PHILLIP FOWLER ABSTRACT During this investigation approximately two-thirds of the Castleton quadrangle was mapped geologically. The sedimentary rocks of the Castleton area belong to two stratigraphic sequences, contrasting in lithology and more or less parallel in time. The 10,000-foot Champlain Valley (Autochthonous, Valley) sequence includes Lower Cambrian or Pre-Cambrian clastics, Lower and Upper Cambrian dolomites and quartzites, Lower Ordovician and Chazyan marbles and dolomites, and Trenton marble, dolomite, and black slate. Although the widespread lithologic discordance between the Lower Cambrian Cheshire quartzite and the underlying Mendon series has been interpreted by some as an angular unconformity, it is here in- terpreted as due to facies changes within the Mendon series. Middle Cambrian is missing probably because of non-deposition. Erosional disconformities probably intervene between the Lower Ordovician and the Chazy and between the Chazy and the Black River-Trenton. Slight warping and subaerial erosion in middle Trenton time produced a significant unconformity. The 30004000-foot Taconic sequence crops out in the Taconic Range and the slate belt. The thick phyllite, quartzite, and grit that underlies the Taconic Range was formerly considered to be Upper Ordovician and was called the Berkshire schist. Because of its relations to the Cambrian slates of the slate belt and its similarity to the Nassau formation of New York it is here mapped as the Nassau formation. Above the probably late Pre-Cambrian Nassau formation lie Lower Cambrian slate, lime- stone, quartzite and grit separated by an erosional disconformity from Lower(?) and Middle Ordovician slate, grit, chert, and quartzite. The rocks of the Valley sequence exhibit broad, deep folds and one small break thmst. The Taconie sequence is a comparatively thin sheet super- ficially crumpled into isoclinal folds. The chief problem in this area is to account for the anomalous juxtaposition of the Cambro-Ordovician argillaceous Taconic sequence and the Cambro-Ordovician carbonate Valley sequence. The principal objective of this investigation was to ascertain the validity of Keith's concept of the Taconic overthrust, which is assumed to be a huge strip thrust that carried the Taconic sequence from the east westward to overlie the Valley sequence. Evidence for the fault in this area consists of lithologic discordance above and below the fault trace, large-scale structural discordance between the two sequences, the contrast between contemporaneous sequences of two dissimilar facies, the presence of klippen and fensters, and the exist- ence of the late Pre-Cambrian Nassau formation lying above Ordovician rocks of the Valley sequence. The Taconic overthrust is believed to be proved. The rocks of the Taconic sequence may have been laid down in the vicinity of the modem Connecticut Valley, and if so the minimun net slip on the Taconic overthrust must be about 50 miles. The folding, faulting, jointing, cleavage, and metamorphism of the rocks of both sequences took place probably near the close of the Ordo- vician period. The earlier stages of the Taconic orogeny produced a flow cleavage oriented parallel to the axial planes of the folds. During the later stages of the orogenv the first cleavage was folded by forces applied in a direction slightly different from those that produced the first cleavage. These later forces developed an axial plane shear cleavage. The northwestern third of the Castleton quadrangle was not investi- gated in detail, and data on that part of the quadrangle were obtained for the geological map from previous publications. INTRODUCTION General The Castleton area (Fig. 1) covers about 130 square miles in west- central Vermont and eastern New York. During this investigation all the rocks of the Castleton quadrangle that lie south of the Castleton River were mapped. In the northern part of the quadrangle only the rocks lying east of the Taconic Range were mapped. The remainder of the Castleton quadrangle has been mapped geologically in recent years by Kaiser (1945) and Cady (1945) . * *Referenc are given at end of the paper. 6 74 73 ° 72° 45° ---- ---.- .45° I I4 •St.Albans •1 (_I I - xz-I_-I L () -J Monkti • c: 44 0 Q) ( Mddlbury 440 ? V I Brendon N.H. .j.Rutland e ) Qj Bald'ICo ________2 l L 43 I N.Y. Bennington N.H. _lAlbany Becreft Mtn. c_Il ç lMass. 42° (fJ. 42° 73° 72° N.Y ______________ 0 10 20 30 40 __ Poughkeeosie MILES ' I i. Castleton Quadrangle 2. Saratoga District 3. Capital District 4. Taconic Quadrangle 740 FIGURE 1. GEOGRAPHIC LOCATION OF CASTLETON AREA AND OTHER PLACES MENTIONED IN THE TEXT. 7 Much of the Castleton area is occupied by a small part of the Tacoruc Range, which lies in east-central New York and adjacent parts of Vermont and Massachusetts. Nearly forty years ago Arthur Keith concluded that the Cambro-Ordovician rocks of the Taconic Range in Vermont are a klippe or thrust outlier composed of an argillaceous facies that had been thrust westward over a carbonate facies of similar age. Although this concept has found general acceptance among geologists working in the area, the roots of the overthrust have not been found. Moreover, modern detailed stratigraphic and structural studies have not been made in much of the Taconic region. The Castleton area was chosen in order to ascertain the validity of the concept of a large Taconic overthrust. Originally this study was confined to the rocks of the Taconic sequence, but it became evident that a clear picture of the relation between the Taconic sequence and the rocks of the marble belt bordering the Taconic Range on the east required remapping of the part of the marble belt that lies in the Castleton quadrangle. This investigation, therefore, has carried the mapping of the Champlain and Vermont Valleys (Cady, 1945) to the southern border of the Castleton quadrangle. Folding, faulting, and metamorphism of the rocks have made accurate strati- graphic work difficult. Identifiable fossils are rare, and the thickness of several formations is obscure. The area was mapped by the pace-and-compass method aided by Paulin altimeter and aerial photographs. Data were plotted on photo- static copies