STRATIGRAPHY AND STRUCTURE OF A PORTION OF THE CASTLETON QUAI)RANGLE, VERMONT By E-AN ZEN VERMONT GEOLOGICAL SURVEY CHARLES C. DoLL, State Geologist Piibli shed by .VERMONT DEVELOPMENT DEPARTMENT MONTPELIER, VERMONT BUILETIN No. 25 1964 CONTENTS PAGE ABSTRACT ..........................5 iNTRODUCTION ....................... 6 ACKNOWLEDGMENTS .................... 10 STRATIGRAPHY ........................ 11 Introduction ........................ ii Taconic Sequence: Lower Cambrian Units .......... 11 I3iddie Knob Formation ................. 11 Bull Formation .................... 13 West Castleton Formation ............... IS Taconic Sequence: Post-Lower Cambrian Units ....... 20 General Remarks ..................... 20 1-latch Full Formation ................. 21 Ponitney Slate ..................... 23 Indian River Slate ................... 24 lXuvlet Formation ................... 25 'laconic Sequence: Discussion ............... 27 Stratigraphic Relations Within the Post-Lower Cambrian Units 27 Stratigraphic Relations Between the Lower Cambrian Succes- sion and Later Rock Units ............... 28 Synclinoriuin Sequence ...................It Precambrian: Mount Holly Series ............ 31 Lower Cambrian: Cheshire Quartzite ............31 Lower Cambrian: Winooski Dolostone .......... 32 Upper Cambrian: Undifferentiated Danby—Clarendon Springs Fonnations ..................... 33 Lower Orciovician: Shelburne Formation ......... 34 Lower Ordovician: Bascom Fonnation .......... 36 T4ower Ordovieian: Chipman Formation .......... 37 Middle Ordovieian: Ira Formation .............IS STRUCTURAL GEOLOGY ................... 44 Taconic Seqnenee .................... 44 Introduction ...................... 44 Structure West of the Gorhamtown—East Poultney Line - 45 Structure of the Bird Mountain Slice ........... 47 Structure in the Vail Brook—North Brook Area ...... 53 Structure of the Green Phyilite Area East of Ira l3rook . - 54 Synclinoriutn Sequence .................. 56 Pine Hill Reverse Fault 57 Pittsford—Chippenhook Anticline .............57 West Rutland Marble l3eIt ............... .58 Relation Among the Tectonic Units ............ 62 Relation Within the Taconic Sequence .......... 62 Relation Between Taconic and Synclinorium Sequences 63 Summary of Chronology of Diastrophic Events ....... 65 Relerenccs Cited ...................... 68 Illusirotious FIGURE I'AGE 1. Index map of the area .................. 8 2. Columnar section of the Taconic sequence ......... 10 3. Columnar section of the synclinoriurn sequence ....... 12 4. A possible interpretation of the facies relation between the Bull and West Castleton Fonnations in the map area ...... 19 5. Three possible structural interpretations of the area near East Poultney ....................... 48 6. Reconstruction of the pre-fra structures near Vest Rutland 61 P LA'r E 1. Geologic map of the southern and eastern portions of the Cast- I eton 1 S-minute quadrangle, \Tennoiit......... (iii pocket) 2. Structural cross sections of the Ctistletoti area .... (in pocket) STRATIGRAPHY AND STRUCTURE OF A PORTION OF THE CASTLETON QUA DRANGLE, VERMONT By E-AN ZEN ABSTRACT f3eclrock of the southern half and northeastern portion of the Castleton quadrangle in western Vermont, exclusive of the Coxe Mountain area, comprises two distinct sequences: the Taconic sequence, and the synclin- orium sequence. The Taconic sequence is predominantly argillite, but includes minor greywacke, conglomerate, quartzite, and limestone. Major units in this sequence are, from oldest to youngest, the Biddie Knob Formation, the Bull Formation, the West Castleton Formation, the Hatch Hill Formation, the Poultney Slate, the Indian River Slate, and the Pawlet Formation. These units range in age from Early Cambrian through Late Cambrian, Early Ordovician, into the Middle Ordovician; field mapping reveals a major unconformity within this sequence, below the Pawlet Formation. A second possible unconformity is between the West Cast- leton Formation and the Hatch i-till Formation. Rocks of the Taconic sequence have been thrust in from outside, presumably from the site of the present Green Mountain anticlinorium. Thrusting may have occurred as submarine gravity slides and was a mid- Trenton event. As a result of this diastrophism, a number of imbricate tectonic slices were produced; within the map area at least two slices are identified. These are the Giddings Brook (lower) slice and the .l3ird Mountain (upper) slice; the Spaulding 1 -lill-Middletown Springs area in the south-central portion of the Casticton quadrangle is the north cud of a window of the Giddings Brook slice, framed by the Bird Moun- tain slice. Within each slice, the rocks are recumbently and isoclinally folded as well as locally thrust-faulted; these early folds are tight and are believed to he concomitant with thrusting. They have been refolded during the main phase of the development of the Middlehury synelino- rium. The synelinorium sequence consists of rocks of the same age range as the l'aconic sequence. By contrast, however, it is largely of clean carbonate and orthoquarizite. Apart from the basal Lower Cambrian Menclon Series, the only major exceptional rock type is the youngest unit, the mid-Ordovician Ira Formation of black phyllite with its basal limestone, the Whipple Marble. The Ira Fonnation marks a permanent break in the early Paleozoic sedinientary pattern of this region and it no doubt records orogenie activities in the source area to the east; these activities may be directly associated in time and space with Taconie thrusting. Despite local complexities, the structure of the synelinorium sequence is essentially simple and within the map area constitutes the west-facing east liml) of the south-plunging Midcllehury synclinorium. The 'anti- dine" of marble at West Rutland is reinterpreted as a homoclinal se- quence of pre-Middle Ordovieian rocks. The true nature of the structure has been obscured by the masking Inc Formation and by axial eulmina- tions and depressions within the pre-Ira structures. Within the synelinorium sequence, there are a number of north-south trending high angle normal and reverse faults, of which the Pine Hill reverse fault is one. The inception of these faults cannot be accurately dated but may have been in mid-Trenton time, an hypothesis which Ilts all (lata and explains certain peculiar rock types of that age in the area. If proven, then in mid-Orclovician time the area of the east limb of the Middlehury synclinorium may have been one of graben and horst, in addition to one where the major structures of the synelinorium began to develop, and into which slices of the Taconic thrust moved. The Taconic sequence now occurs in the center of the Middlebury synchinorium and rests above the right-side up Ira Formation, with major inversion of the sequence near the contact. The delineation of the contact between the two sequences is not easy, partly because of the similarity of rocks on the two sides of the dislocation and partly perhaps because no sharp surface of contact ever did exist, clue to the particular manner of emplacement of the allochtbone. The contact, how- ever, probably lies within the black phyllite rather than at either of its boundaries. Local outcrops of Ordovician marbles near this contact, in the town of Ira, are interpreted as slivers dragged along the sole of the thrust slices, rather than as indigenous anticlines. INTRODUCTION This report describes the bedrock geology of an area covering most of the southern portion of the 15-minute Castleton quadrangle in western \Termont, as well as a narrow strip extending northward from West Rut- land to Florence, in the northeastern part of the Castleton qUadrangle or the eastern half of the 7 1/2-minute Proctor quadrangle. The area is approximately hounded, to the north, by the low, east-west ridge north of the Castleton River; to the east, by Pine Hill and its southward continuation, called by T. N. Dale (1594) the ''ridge between the Taconic and Green Mountain Ranges''; to the west, by the 7 1/2-minute Thorn 1-lill quadrangle (Vermont-New York; southeastern quarter of the 15- minute Whitehall quadrangle); and to the south, by the 15-minute Pawlet quadrangle (Fig. 1). Geologically, the area is underlain by two distinct and contrasting stratigraphic and structural units. These are the "Taconic sequence" and the 'Valley sequence" of authors (see, for example, Kaiser, 1945; Fowler, 1950; Billings, Rodgers, and Thompson, 1952). There are strong topographic expressions of this geologic junction. A line of valleys and swamps follows Whipple 1-[ollow south from Florence to West Rutland. This line is continued by the valley of Clarendon River southward to Chippenhook, and leaves the Castleton quadrangle at the north end of Tinmouth Channel, an extensive swamp. To the west of this line rise the steep and often rugged hills of the Taconic Range, which is the eastern limit of the Taconic sequence. East of the line of swamps is relatively open country, underlain by rocks of the 'Valley sequence." The name, "Valley sequence," applies to a section of Lower Paleozoic sedimentary rocks, ranging from Early Cambrian to Middle Ordovician in age, and consisting largely of orthoquartzite and carbonate. The only significant argillaceous and quartzofeldspathic rocks in this sequence are the basal Cambrian (?) Mcndon Series (Brace, 1953; Fowler, 1950; Osberg, 1959) and the Middle Ordovician Ira Formation (Keith. 1932; Zen, 1961). The sequence overlies the Precambrian gneisses of the