The Champlain Thrust and Related Features Near MiddleburyVermont

97 Trip B-4 THE CHAMPLA IN THRUST AND RELATED FEATURES NEAR MIDDLEBURY, VERMONT by Peter J Coney, Robert E Powell, Marilyn E Tennyson, Brewster Baldwin Department of Geology, Middlebury College M i d d l e b u r y , Vermont This field trip will review preliminary results from i n v e s t i ­ gations of the Champlain thrust, Middlebury synclinorium, and Green Mountain a n t i c l i n o r i u m near the latitude of Middlebury by senior geology majors at Middlebury College During the past seven years 20 senior theses have been completed; seven of these provide a n e a r ­ ly continuous geologic map of the Champlain thrust belt between Vergennes and Route 125 w e s t of Middlebury at a scale of 500 feet to the inch Other theses have included regional and local gravity studies, geologic mapping of critical localities in the Middlebury s y n c l i n o r i u m and Green Mountain anticlinorium, petrologic studies of greenstone and ultramafic bodies east of the anticlinorium, sedimcntological studies of lower Paleozoic rocks, and mapping and petrologic studies of Mesozoic igneous rocks These efforts have built upon earlier studies in w e s t - c e n t r a l Vermont by Cadv (1945) , Welby (1961), Osberg (1952), and the unfortunately unpublished work of Crosby (1963) Although the field trip will concentrate on the Champlain thrust w e s t of Middlebury (Figure 1), the regional tec­ tonic setting of the thrust is briefly discussed here as bac k gr o u n d for participants TECTONIC SETTING At the latitude of Middlebury four distinct tectonic p r o v i n ­ ces are from w e s t to east: the Adirondack massif, the Lake C h a m ­ plain lowland, the Middlebury s y n c l i n o r i u m bounded on its w e s t side by the Champlain thrust belt, and the Green Mountain anticlinorium The tectonic significance of the Champlain thrust must be sought in the nature of these provinces and their boundaries In the Lake Champlain lowland (Welby, 1961) a relatively u n ­ d ef ormed 5,000 foot sequence of Upper Cambrian through Middle O r d o ­ vician clastic and mainly carbonate shelf assemblage rocks rest with pr o found unconformity on a crystalline Precambrian b a semen t (Figure 2) The Precambrian is extensively exoosed in the A d i r o n ­ dack massif The boundary between the Adirondacks and the lowland is a complex of fault blocks, down-fa ulted to the east, and s t r u c ­ tural relief on the Precambrian b a s e m e n t is at least 5,000 feet To the east of the Champlain lowland an Eocambrian to Middle Or d ov ic i an clastic and mainly carbonate shelf assemblage nearly 10,000 feet thick presumably rests on Precambrian basement (Figure 98 Vergennes ROUTE MAP FOR FIELD TRIP B-4 NEIG C slat* €-0 € 1972 carbonate quartzite c Champlain thruet o Orwell s St Q e o r g e thrust th ru st field trip route Middlebury milts FIG 99 Iben/iHe Sheie Stony black P oint Sh ale: calcareous shale th thin limes to n e s Hortonville Slate - H a t * state F o r m a t is* G len s F a lls /./mestone: thm -bedded c r e n u l a t e d d a rk G t e n j Fa i t s & L i me sten t VSO' QrweH is.y x e b l o e - ^ r a y )imes>»ne rujfJ) ilTrrT^jTo^Tf-; lijht-yray textured limestone- ca leite L im esto n e: Voice* *■ f p J C rem e Ponrt Ls d olom itic tJeaiheriny ,dark re ms th in -b ed d e d incom petent, blue *9 r a y iim esten t Pjty Tjint F*» ^ TSaidens fm : t a n do) 4mJ Subite merbie 'Bridfr* V o lo 'to n g VSo' a scorn Form at t o n lim e s to n e t d o l o s t o n e , * Cutting J* I Dolostone Vvo' H VJ hi te hall T>o) n C ut tin y I / D o lo sto n e : l i m e s t o n e a n d d o l o s t o n e ; c h e r t y in u p p e r p a r t ; t e n d stone unth b r e c c i a * d Sc»)\fhet ( v e r m h tles) a f l -‘ I t I r ^ 1a«n tmear muKlm mrmmo /wiwt T / I I fu a rtn te S hel b u r n t Marble u*hite w » r b U y Cl arendon I m ass/re Ityht g r a y Springs gra y l i m e s t o n e , so m e do) os tone t- Q u a r t s u»i>K d o lo sto n e Dolostone: D a n b y F o rm a tio n : upper hi g e o d e t ( s Potsdam) d o lo sto n e j in^rbidx p a r t ( v j a i l i n g jo r4 M em ber) m ostly d o lo sto n e W«'nooski Dolostone: p i n k , Cream, a n d < )ra y ; b e d s < / ' , jOmtly , ' onk ton red Quartzite: C h e sh ire I C om upper H bedded ui/th scattered ram s Quartzite: 3So' lohiie m a s due yuartsite tf^uencc of strata near Middlebury, Vt G pi a»Ustdnr > _ D o lo sto n e : doloston e F g u erifte Quarts i l e , siliceous m u d s t o n e , and doUstsne D unham ,O0bt lam in ated led f r o m C a x l y , 1*1 ^ 5”; K a y a n d C a d y , M H - C a d y and Zent I b U tib y , I H I 100 2) This sequence is rather intensely deformed into the southplunging, w e s t w a r d inclined, Middlebury s y n c l i n o r i u m (Cady, 1945) The two disparate early Paleozoic shelf assemblages of the Lake Champlain lowland and the Mi d dl eb u ry s y n c l i n o r i u m (Figure 2) are separated by the Champlain thrust belt The thrust belt is a s e r ­ ies of east-dipping, low-angle faults w hi c h can be traced from at least southwestern Vermont and the adjacent Mew York northward into Canada (Cady, 1969) The Green Mountain a n t i c l i n o r i u m (Cady, 1945) rises sharply just east of the Middlebury s y n c l i n o r i u m and is a w e s t - v e r g e n t , doubly plunging, complex a n t i c l i n o r i u m with an exposed core of Precambrian Mount Holly ba s ement (Figure 3) Structural relief on the Precambrian unconformity between the floor of the M i d d l e ­ bury s y n c l i n o r i u m and the crest of the a n t i c l i n o r i u m is at least miles, and probably as much as miles, in an e as t - w e s t distance of about miles (Powell, 1969; Tennyson, 1970) The boundary b e ­ tween the s y n c l i n o r i u m and the a n t i c l i n o r i u m at the surface is loc­ ally marked by e a s t - d i p p i n g thrusts, but is mainly a des ce nd i ng cascade of w e s t - v e r g e n t folds (Osberg, 1952; Tennyson, 1970) termed the Green Mo u ntain front The Green Mountain front is a major s t r a t i g r a o h i c as well as tectonic boundary ma rk i n g an abrupt facies change in Eocambrian to Ordovician rocks from the mainly carbonate m i o g e o c l i n a l shelf assemblage on the w e s t to " e u g e o c l i n a l " graywacke assemblage on the east The Taconic "klippe", also of the eastern eugeoclinal facies, now lies athwart the Mid dl eb u ry s y n c l i n o r i u m on top of shelf rocks of the same age to the south of Middlebury, but has been removed by erosion northward With the exception, presumably, of the Chester dome, no bona fide "Yankee" Precambrian cratonic b a s e m e n t is known east of the Green Mountain anticlinorium Signifi c an tl y a belt of s e r p e n t i n ­ ized dunites (Beyer, 1972) and meta-greenstones (Crocker, 1972; Doolan and others, in preparation) lie just east of the a n t i c l i ­ norium embedded in m e t a - g r a y w a c k e s of the early Paleozoic "eugeo­ clinal" assemblage These rocks certainly mark a most significant tectonic boundary It w o u l d appear, thus, the entire lower P a l e o ­ zoic North American continental margin assemblage is exposed in a belt now less than 50 miles wide Putting it another way, in the context of current plate tectonic theory, once one takes a single step east of the Green Mountain a n t i c l i n o r i u m ev e ry th i ng to the Bay of Maine is of suspect geo-political allegiance An u n c o nf o rm it y of late Middle Ord o vi c i a n age seen at one place or another in most of the region, including the Taconic "klippe", separates the apparently w e s t - d e r i v e d shelf and "eugeo­ clinal" assemblage from an apparently east-de rived detrital-shale assemblage The critical overlying rocks are the Hortonville Formation in the Middlebury synclinorium, the Pawlet Formation in the Taconic "klippe" (Zen, 1962), and the Moretown Formation east STRUCTURE AND GRAVITY 1UJL of the um S i l u r i a n - D e v o n i a n detrital rocks mask all prior tectonic relationships east of the Moretown Formation to the Or v ic ia n O l i v e r i a n vo lcanic arc along the V e r m o n t - N e w Hampshire border (Rodgers, 1970), the presumed source of the d e t r i t a l - s h a l e flood Very p reliminary studies by Sedgwick (1972) suggest the Moretown Formation has a distinctly different heavy mineral assemblage compared to all older w e s t - d e r i v e d shelf and "eugeoclinal" rocks sampled to date The Champla in and related thrusts and the folds in the M i d ­ dlebury s y n c l i n o r i u m involve rocks as young as late Middle O r d o v i ­ cian The east side of the Green Mountain anticli n or iu m is often argued to involve S i l u r i a n - D e v o n i a n rocks as well (Cady, 1968) If all these m a j o r structures have any genetic relationship to one another, as is generally assumed, much d eforma tion is as young as Acadian (Middle Devonian) at least The e mplace ment of the T a c o n ­ ic "klippe" is well documented as Taconic (Middle Ordovician) and numerous ductile folds and mi no r structures beneath and adjacent to the "klippe" in s y n c li n or i um rocks are correlated with this e m ­ placement (Crosby, 1963) All these "Taconic" structures, i n c l u d ­ ing the "klippe" (Crosby, 1963; Johnson, 1970) are redeformed by y ou n g e r more "brittle" deformation thought to be Acadian (Crosby, 1963) The Middlebury region, then, has suffered at least two deformations To what extent these phases were discrete events or a single continuum is yet to be THE CHAMPLA IN AND RELATED THRUSTS General S t a t e m e n t The Champlain and related faults form a belt of east-dipping, low-angle thrusts w hi c h separate the Middlebury s y n c l i n o r i u m on the east from the Lake Champlain lowland to the west The belt of thrusts brings up resistant rocks, such as Cambrian Monkton Formation quartzite, w hi c h have p r oduce d a line of hills and ridges Snake and Buck Mountains are the prominent ridges at the latitude of Middlebury The system of faults forms a tightly packed series of slices exposed in a belt seldom more than miles wide The Champlain thrust is the most easterly of the faults w hi le the other faults lie just w e s t of, and s t r u c t u r ­ ally below, the Champlain thrust For regional strati g r ap h ic d e ­ tail the reader is referred to Cady (1945) and Welby (1961) , and to Figure Champlain T h r u s t The Champlain thrust (Stop 3) enters the area at the north end of Buck Mountain and extends southward for at least 15 miles to Route 125 west of Middlebury The fault can be easily traced northward into Canada, but its fate south of Route 125 is still in question (Stop 5) Cady (1945) and the Centennial Geologic Map of Vermont (Doll and others, 1961) t e r m i n ­ ate it in the very poorly exposed south-plunging anticline just west of Cornwall several miles south of Route 125 Over almost the entire trace w e s t of Middlebury, Cambrian Monkton Formation is 103 thrust over highly de f ormed Ordovician carbonate and shale The thrust plane lies w i t h i n several hundred feet of the base of the Monkton Formation and only at the south end of Buck Mountain (Cady, 1945; Welby, 1961; Egan, 1968) does it bite lower into s e v ­ eral tens of feet of Dunham Dolostone The Monkton Formation on the upper plate generally dips gently eastward into the Middlebury s y n c l i n o r i u m forming a prominent dip slope on the east sides of Buck and Snake Mountains Near the fault trace, however, local i m ­ brications and folds are evident in Monkton Formation layers The fault plane is only rarely exoosed, but at several points on Buck and Snake Mountains dips between and 25 degrees eastward are seen The trace of the fault wanders in topograohy and has right en echelon offsets from the north end of Buck Mountain south to Snake Mountain South of the summit of Snake Mountain it trends southward then angles southeasterly until lost beneath glacial cover south of Route 125 The en echelon offsets result in a s e r ­ ies of salients and re-entrants Structure contouring on the fault surface (Westervelt, 1967), gravity studies and geologic m ap p i n g (Davidson, 1970) suggest the salients are shallow downwarps and the re-entrants are mainly up-warps Thus, the marked offsets are apparently primarily due to topographic expression of undulations in the fault plane rather than numerous cross-faults as shown on earlier maps (Welby, 1961; Doll and others, 1961) The southern "termination" near Route 125, where the trace angles southeastward, is marked by a southward structural plunge of the Monkton Formation before it and the fault trace are buried by glacial cover If the fault continues southward it must climb up-section to place higher units on the sole If the fault i n ­ deed terminates near Cornwall then the plunge of the Monkton is probably due to rapid decrease in dip separation into the a n t i ­ clinal core (Smith, 1972) Stratig r ap h ic separation on the fault reaches nearly 5,000 feet If the shallow dips of the fault surface are projected eastward beneath the Middlebury synclinorium, and reasonable r e ­ constructions of geometry are made on the upper and lower plate, the dip separation is well over 10,000 feet Depending on how far the Monkton Formation extended w e s t w a r d from its present e x ­ posures along the fault trace, the dip separation could be much more If the fault terminates w e s t of Cornwall this dip s e p a r a ­ tion must decrease rapidly to zero, but where last seen Monkton Formation m u s t lie over at least Bridport Formation giving a s t r a t i g r a p h i c separation of about 3,000 feet Related Thrusts West of the Champla in Th r u s t The lower plate of the Champlain thrust is made up of a series of thrust slices from north of Buck M o u n t a i n to its apparent termination near Route 125 South of Route 125 the lower thrusts angle to the southwest away from the last seen southeasterly trend of the Champlain thrust 1