Geology of the Starksboro Area, Vermont 1

Geology of the Starksboro Area, Vermont

Joseph A. DiPietro Department of Geology University of Vermont Burlington, Vermont 05405

LOCATION AND PREVIOUS MAPPING

The field area is centered in the Lewis Creek Western Sequence watershed in the township of Starksboro, Vermont (P1.1. L-4). It encompasses the northeastern part of the Pinnacle Formation. The Pinnacle Formation is Bristol 7.5' quadrangle and the northwestern part of divided into lower, middle and upper members. All the Mt. Ellen 7.5' quadrangle. three members are variable from north to south and each consists of two or more map units. The facies Cady and others (1962) mapped the Mt. Ellen variations and distinguishing features of each member quadrangle at a scale of 1:62,500. Cady mapped the are summarized in Figure 3. Most of the rock in the eastern part of the Bristol quadrangle for the Geologic Pinnacle Formation consists of schistose sandstone or Map of Vermont (Doll and others, 1961). Four semischist, which I refer to as metagreywacke. formations and two members were recognized: the Late Precambrian Pinnacle and Underhill Formations; the Fairfield pond phyllite and Forestdale Dolomite Members Lower Pinnacle Member of the tlnderhill Formation; and the Lower Cambrian Cheshire and Dunham Formations. They interpreted the CZpl - Undifferentiated quartz-sericite mete- Pinnacle Formation as the core of the Lincoln anticline greywacke and phyllite (Type bc. pi.i, C-s and 1-7). with younger rocks of the Underhill Formation in The lower member consists predominantly of massive, depositional contact on both limbs. The Underhill medium grained, schistose, shiny light grey, brown Formation on the east limb was also considered to be a weathered, quartz-sericite-chlorite-albite meta- finer grained stratigraphic equivalent of the Pinnacle greywacke with blue quartz pebbles and brown weathered Formation. disseminated carbonate. The rock is quartz-sericite rich with albite generally making up less than 10 Tauvers (1982, 1982b) mapped the Lincoln area percent. Siotite is rare. Phyllite, schistose to the south of the Starksboro region. He and I quartzite and mica schist are present locally. This (Dipietro, 1982) developed a stratigraphy in the unit becomes thicker to the north. The lower part pinnacle Formation and mapped thrust faults in the contains wide intervals of fine grained, grey or green, Underhill Formation on the east limb of the Lincoln fissile, sericite-chborite-quartz-epidote phyllite and anticline (p1.2). The Fairfield Pond and Forestdale schist which become more abundant north of the field Members on the west limb of the Lincoln anticlirie could area. The lower member overlies the basal Pinnacle not be correlated with the Underhill Formation. As a conglomerate in the Lincoln area (Fig.l). consequence, the upper phyllitic part of the Fairfield Pond Formation was mapped as a separate formation. The CZpll - Laminated magnetite schist (Type bc. Forestdale Dolomite and overlying metagreywacke were P1.1, E-a). This unit is a fine grained, greenish- mapped as units in the upper member of the Pinnacle grey, sericite-chborite-quartz-epidote schist charac- Formation. terized by abundant white quartz laminations and secondary magnetite. It is similar in appearance to STRATIGRAPHY the laminated schist (CZu1) except that it lacks biotite, contains abundant magnetite and, in general, is compositionally less variable. Contacts with the The formations in the field area are separated surrounding quartz-sericite metagreywacke (CZpI) are into a western and an eastern sequence. The western not exposed. sequence consists of the Pinnacle, Fairfield Pond, Cheshire and Dunham Formations. The eastern sequence consists entirely of the Underhill Formation and is Middle Pinnacle Member separate:3 from the western sequence by the Underhill and Jerusalea thrust faults. The Late Precambrian CZpm - Undifferentiated guartz-albite-serjcjte pinnacle and Underhill Formations are rift clastic metagreywacke and phyllite (Type bc. P1.1, D-4 and 0- deposits that overlie, with profound unconformity, the 6). The Middle Pinnacle Member is a very massive, Precambrian-Y North American basement (Rankin, 1976; resistant, granular, medium and coarse grained, quartz- Doll and others, 1961). The Cheshire and Dunham albite-sericite-chlorite-biotite metagreywacke With Formations constitute the lowest part of the Cambrian - albite pebbles, local conglomerates, graded beds and Ordovician shelf sequence in Vermont. Figure 1 shows intraclasts. South of Lewis Creek it is medium-to-dark the correlation of the western sequence with Doll and grey with biotite, but becomes more schistose light-to- others (1961) and with Tauvers (1982). A detailed medium grey north of Lewis Creek. Here, blue quartz is stratigraphic column of the western sequence is shown more common and biotite is sparse. Mica schist, in Figure 2. magnetite-rich layers, lower member-type metagreywacke (CZpl) and sai.dy laminations similar to those The rocks are metamorphosed to the biotite in the laminated schist (cZul), are present locelly. grade. Cady and others (1962) mapped the garnet Interlayered sericite-chlorite-quartz-epidote phyllite isograd along the eastern boundary of the area. All and schist are not common but become more abundant map units strike a few degrees West of north and dip north of Lewis Creek. The lower contact is gradational east. Graded beds indicate that the west limb of the over an interval of 10 to 30m and is well-exposed at Lincoln anticline is overturned. locations E-5 and L-7 (P1.1). 1 CZpm- - Graded bed unit (Type bc. P1.1, c-s type phyllite and metagreywacke. The dolomictic layers and F-5). T-iis unit consists of beds (<30cm) grading are typically 5 to 15cm thick and rarely greater than from pebbles to fine sand that are overlain, with sharp 30cm. Layers of Forestdale-type rock are much thicker, contact, by thin (<10cm) layers of phyllite. These locally up to four meters. Dolomite weathers tan-to- tutbidite sequences are interlayered with lower and dark brown and forms deep reentrants between the middle member-type metagreywacke (Czpl, CZpm) as well resistant phyllitic layers. In some areas, dolomite is as discontinuous phyllite. hidden beneath phyllite and unless the rock is broken, it can be overlooked. The dolomictic layers become CZpmc - Congbo'neratic unit (Type bc. P1.1. G-4 less abundant and pinch Out into dark green fissile and N-7 1 . Toe cungbomeratic unit consists of discon- phyllite (CZpuc) north of Lewis Creek. tinuous lenses of pebble and cobble conglomerate up to lOm thick. Graded as well as poorly-sorted beds are Czpufm - Massive Forestdale Dolomite (Type bc. present. Clasts are subrounded to rounded and are R-6). Three meter thick layers of white to buff, predominantly matrix supported. In observed order of brown weathering, sandy dolomite alternate with four to abundance, the clasts are quartz-plagioclase gnelss, ten centimeter layers of Forestdale-type rock at loc- plagioclase, plagioclase gneiss, blue quartz and ation R-6 (P1.1). The massive dolomite occurs only in polycrystalline white quartz. The plagioclase is this one isolated area and is gradational to the main largely altered to albite. The interlayered body of Forestdale Dolomite. Thick layers of sandy metagreywacke (Czpm) is medium grey at location N-7 dolomite are more common south of the field area (P1.1) and dark grey with intraclasts at location 0-4 (Tauvers, 1982). (p1.1). CZpuu - Chloritic metagreywacke and phyllite (Type bc. P1.1, B-2 and R-5). South of Lewis Creek, Upper Pinnacle Member the chloritic unit consists of highly variable, grey or green, chloritic, fine and medium grained mets- Forestdale-type phyllite and metagreywacke (Not greywacke and phyllite with Forestdale-type rock common a map unit. Type bc. P1.1, 0-6). Lustrous silver- near the base. Secondary magnetite is abundant in this green phyllite and inetagreywacke are present throughout area but biotite is absent. North of Lewis Creek, the the upper member south of Lewis Creek. Because these lower part changes facies to Clpub, CZpul and CZpuq. rocks occur below, within and above Forestdale The upper part consists predominantly of Forestdale- Dolomite, they are referred to as Forestdale-type type rock but is more coarse grained with layers rich phyllite or metagreywacke. The term "Forestdale-type in blue quartz pebbles. Carbonate layers (identical to rock" refers strictly to metagreywacke and phyllite, the Forestdale Dolomite), biotite and secondary not to dolomite. Forestdale-type rock is not a map magnetite are locally present. The lower contact of unit. Units which contain Forestdale-type rock are this unit is gradational. mapped by the presence or absence of a specific rock type. Forestdale-type rocks are lustrous silver-green, CZpub - Biotite bearing metagreywacke and fine-to-medium grained, quartz-sericite-chlorite- phyllite (Type bc. P1.1, E-3 and 1-3). The lower part albite-iron oxide phyllite and metagreywacke that of the biotite-bearing unit consists predominantly of weather dark (dull) green. All gradations from fissile massive, granular, dark grey, medium and coarse iicaceous phyllite to very sandy metagreywacke are grained, abbite-quartz-chlorite-sericite-biotite meta- present. Secondary magnetite is common. greywacke that resembles the Middle Pinnacle Member. The upper part is finer grained and consists of green- CZpuz - Coarse metaureywacke and phyllite (Type to-dark grey, chloritic phyllite and metagreywacke. bc. Pl.L, F-.J anj R-'). Tne distinctive rock in this Cross beds, graded beds, blue quartz clasts, albite unit is a coarse, green to light grey, quartz-albite- clasts, detrital carbonate and small (<3cm) sericite-chlorite metagreywacke with abundant blue intraclasts, are present locally. This unit becomes quartz clasts and a spaced (5mm) 51 schistosity. Albite thicker, darker grey and more massive to the north. and white quartz clasts are also common. Carbonate The lower contact, at location J-3(Pl.1), is sharp layers and flakes of muscovite are present locally. (<2m), and graded beds are common at the base. Siotite is absent. The coarse metagreywacke is more abundant (>70 percent) north of Lewis Creek where it is CZpul - Thinly layered unit (Type bc. P1.1, J- interlayered with predominantly dark green, fissile, 3). The thinly layered unit consists of Forestdale- chloritic phyllite. Medium and fine grained, dark type rock interlayered with thin (<2 cm) fine-to-medium green, chloritic ruetagreywacke are common south of grained, white, quartz-albite-rich, sandy layers that Lewis Creek. The coarse metagreywacke, in this area, are spaced less than 15cm apart. Secondary magnetite oecomes less abundant and pinches Out south of the is abundant in this rock and although present in the field area (Tauvers, 1982). The lower contact is chboritic unit (CZpuu) south of Lewis Creek, it is a gradational over an interval of 8 to 30m and is well- mappable unit only in the East Mountain area. Its exposed at locations L-5 and D-4 (P1.1). It becomes lithic identity is lost at location 1-3 (P1.1) where increasingly gradational to the north and may become continuous exposure shows that it changes facies into impossible to delineate north of the field area. the underlying and in part, stratigraphically equivalent biotite-bearing unit (CZpub). The thinly Clpuc - Fine metagreywacke and phyllite (Type layered unit is a transitional fades between the fine bc. P1.1. D-3 and Q-6). This Unit consists predomi- grai.ned chboritic unit (CZpuu) and the coarse grained nantly of lustrous green, chloritic. Forestdale-type biotite bearing unit (CZpub). The lower contact with ohyllite and metagreywacke. Dark green varieties are the biotite bearing unit is sharp (<2rn). also common. It is distinguished from the underlying and overlying units by the lack of both coarse CZpuq - Quartz-pebble metagreywacke (Type bc. etagreywacke and carbonate layers. Sandy P1.1, K-3 and D-2). The distinctive rock in this unit rsetagreywacke is common south of Lewis Creek whereas is a greenish, coarse to pebbly quartz-albite-chborite pnyllite is more common to the north. Biotite is absent metagreywacke. Blue quartz is extremely abundant and in this unit. the rock is locally a quartzite. Blue quartz is less abundant at location E-2 (p1.1) where this rock closely CZpuf - Forestdale Dolomite (Type bc. P1.1, J- resembles the coarse metagreywacke in CZpug. The 3 and 0-5). The Forestdale Dolomite consists of white quartz-pebble metagreywacke is interlayered and :o buff, sandy dolonite, interbayered with Forestdale- surrounded by greenish chioritic rock that is typical of the chboritic unit (CZpuu). 2 Summary and Stratigrapnic Trends of the Pinnacle Eastern Sequence P0 ra at ion Underhill Formation. The tJnderhiil Formation The Pinnacle Formation is divided into three structurally overlies and truncates the east limb of nembers which change facies along strike (Fig. 3). the Lincoln anticline. It consists of two fault These changes indicate that the lithic character of slices: the Jerusalem slice and the Underhill slice. each member is time transgressive and can occur at different stratigraphic intervals. For example, layers Underhill Slice of dolomite occur both above and below the stratigraphic horizon that is mapped as Forestdale Dolomite. Another significant aspect of the Pinnacle Formation is CZu - Undifferentiated quartz-sericite schist that identical stratigraphic sequences appear at and metagreywacke (Type bc. P1.1, C-14 and K-13). The different stratigraphic intervals. For example, the Underhill slice consists predominantly of fine grained, CZpm-Czpug-CZpuc sequence is nearly identical to the light grey, brown weathered, quartz-sericite-chlorite- Czpub-CZpuq-CZpuu sequence that occurs north of Lewis albite schist interlayered with thin (

(Type bc. P1.1, E-2 and S-3) The argillaceous CZug - Greenstone (Type bc. P1.1, M-12). This member consists of grey, quartz-feldspar-sericite- rock is a massive spotted green, white and black, chlorite argillite with thin phyllitic partings, epidote- chborite-quartz-amphibol e-bioti te- albite green- rippled beds, trace fossils and local quartzite beds. stone. Locally it is less massive, epidote-quartz rich It also contains potassium feldspar which is not and interlayered with the biotite-rich unit (CZub) present in pre-Chashire Formation rocks. The lower The abundance of quartz suggests that the mafic contact appears gradational but is disrupted by the volcanic material was mixed with clastic sediment. ttinesburg thrust.

Ccm - Massive quartzite member Jerusalem Slice

(Type bc. P1.1. C-2) The massive member CZu1 - Quartz laminated schist (Type bc. P1.1, consists predominantly of massive white-to-pink 1-10, N-7 and N-b). The laminated schist consists of quartzite and is generally structureless. The lower grey or green, fine grained, sericite-chborite-quartz- contact is interlayered over an interval of ten meters epidote-albite schist with abundant, very continuous, and is well-exposed at location B-2 (P1.1). white laminations that define Sb. The laminations consist predominantly of recrystallized quartz and are Dunham Formation - Cd (Type bc. P1.1, .3-2). metamorphic in origin. A few are sandy and may The lower part of the Dunham Formation is exposed and represent original bedding. Quartz veins are abundant consists of pink or brown weathering, sandy dolomite and commonly impregnate the rock. The laminated schist wttn thin phyllitic partings and local trace fossils. contains biotite and imbricated pods and layers of The lower contact with the Cheshire Formation is sharp Middle Pinnacle Member metagreywacke (CZpm) along the (<3m) and is exposed at location J-2 (P1.1). Jerusalem slice on the east limb of the Linco1 anticline (eg. 91.2, E). Ob

Correlation of the Eastern Sequence with the Formation. Rippled beds and vertical burrows in the Western Sequence overlying argillaceous Cheshire (Cca), suggest near sea level deposition. The Fairfield Pond Formation The Underhill slice and Lower Pinnacle Member therefore, is interpreted as a shallow water, slow contain identical sequences of metagreywacke. The subsiding basin deposit. The phyllitic Cheshire (Ccp) major difference between the two rock units is that the may represent a shalbowing upward sequence to the metagreywacke in the Lower Pinnacle Member occurs in argillaceous Cheshire. The phyllitic Cheshire also layers of lOm or more whereas layers of metagreywacke marks, for the first time, the influx of sediment rich in the Underhill slice are commonly less than one meter in potassium feldspar. This compositional change and separated by wide intervals of quartz-sericite suggests a major shift in source area. The upper schist. The Underhill slice is also somewhat more Cheshire Formation (Ccm) and Dunham Formation mark the variable with metabasite and albite-biotjte rich beginning of the stable continental shelf that existed stetagreywacke. These rock types are not found in the until the Ordovician Taconian orogeny. Lower Pinnacle Member. The abundance of similar rock types however, suggests the Underhill slice is a finer Structural complications make the eastern grained facies equivalent of the Lower Pinnacle Member sequence somewhat more difficult to interpret. The (Fig.l). Stratigraphic equivalence cannot be proven lithic similarity to the Lower Pinnacle Member, because lithic units in the Pinnacle Formation are time suggests a similar, but more distal, source area for trangressive and cyclic. the Underhill slice. Metabasite in the Underhill slice suggests local volcanic or volcaniclastic source areas The Jerusalem slice is closely associated with not found in the Lower Pinnacle Member. The Jerusalem the Middle Pinnacle Member. Laminated rock, similar to slice may represent a distal intrabasinal mud or the quartz laminated schiat (CZu1), occurs locally in possibly, a facies peripheral to the main depocenter of the Middle Pinnacle Member and both rock units are the Middle Pinnacle Member submarine fan. truncated by the Underhill slice at location H-b (P1.1). The major difference between the two is that the laminated schist is much finer grained. Because of STRUCTURAL GEOLOGY their close association, the laminated schist is interpreted as a finer grained facies equivalent of the Two major deformational events are recognized Middle Pinnacle Member (Fig.l). The Jerusalem slice in the Starksboro area. The first (Dl) is associated may represent a stratigraphic facies of the Middle with the dominant regional foliation (Sl), the Pinnacle Member that is similar in origin to the thinly development and westward overturning of the Lincoln layered unit (CZpul) and chloritic metagreywacke anticline (Fl), biotite grade metamorphism (Ml), and (CZpuu) facies of the biotite bearing unit (CZpub) in late-Fl, syn-Ml, westward displacement of the the Upper Pinnacle Member. Underhill, Jerusalem and Hlnesburg thrusts. Radiometric dates indicate a Taconian age for Dl Environment of Deposition (Harper, 1968; Lanphere and others, 1983). The second deformational event (D2) is associated with open to overturned, upright folds (F2), axial plane crenulation Tauvers (1982) interprets the basal Pinnacle cleavage (S2), chlorite grade metamorphism (42), and conglomerate as an alluvial-fan deposit and interprets large-scale warping of the Lincoln anticline. Minor the overlying Pinnacle Formation rocks as submarine deformational events include poor ly-deve loped open or fan, mass flow or turbidite sequences. The presence of kink folds (F3), east-west fracturing and dike graded beds, phyllitic intraclasts, resedimented intrusion. carbonate deposits, and massive conglomeratic sand lay.rs, support a submarine fan origin for the Pinnacle Formation in the Starksboro area (Walker, 1978). Fold Events

There are two distinct but very closely related Fl Folds. The Lincoln anticline is a doubly end member rock types in the pre-Cheshire Formation plunging, west vergent, Fl fold, that encompasses the rocks: 1) albite-biotite rock (CZpm, CZpub, CZua, CZub) entire western sequence (P1.1). The west limb is and 2) quartz-sericite rock with local carbonate (all sheared by the Hinesburg thrust. The east limb is other rock units). The albite-biotite rock types are truncated by thrusts in the Underhill Formation. The commonly massive and coarse grained. They may Underhill thrust does not follow the plunge of the fold represent a proximal facies of the quartz-sericite (P1.2). This suggests the Lincoln anticline was doubly rock. This possibility is supported by the abundance plunging prior to late-Fl thrusting. Older rock units of albite in the underlying Mount Holly Complex in the core of F2 synclines on the west limb, indicate (Tauvers, 1982) and the association of albite-biotite that the Lincoln anticline was overturned prior to F2. rock with metabasite in the Underhill Formation. The dominant planar element in the field area Large-scale stratigraphic trends are present in is the penetrative Sl foliation, which strikes gener- the Pinnacle Formation. The lower member becomes ally north and dips 50 to 90 degrees east (Fig.4b). thicker (>500m) and finer grained to the north. The The morphology of Sl in pre-Cheshire Formation rocks, stratigraphic trend from the Lower to Middle Pinnacle varies from fissile to schistose to spaced as the Member, is coarsening upward (Fig.2). This sequence is percentage of mica decreases. Sl is expressed as thin interpreted as a large prograding submarine fan. phyllitic partings in the Cheshire and Dunhan Formations but is non-pervasive or absent in the The stratigraphic trend of the Upper Pinnacle massive Cheshire quartzlte (Ccm). Minor shear zones Member varies from north to sout)i. It fines upward are commonly developed parallel to SI. (eg. P1.1. P-5). south of Lewis Creek, but contains the intervening Forestdale Dolomite (Fig.2). North of Lewis Creek, it Mineral lineations of mica or quartz are fines upward to a sharp contact (at the base of the present on most Sl surfaces. They rake between coarse grained biotite bearing unit, Czpub) and then, 60 and 90 degrees to Sl and plunge east-southeast (Fig. an almost identical, but much thinner, fining upward 4d). This orientation is parallel to slickensides sequence repeats itself (Fig.2). This may represent along the Underhill and Jerusalem thrust faults, which abandonment or migration of the lower-middle member suggests that the mineral lineations define the re- submarine fan lobe and the interfingering of a second gional transport direction of the rock. submarine fan. Small scale Fl folds are rare and observed only Active riftiny ceased by the end of Upper in the Pinnacle and Underhill Formations. They are Pinnacle Member time. The fine grained Fairfield Pond tight to isoclinal, doubly plunging folds, with Formation represents an abrupt change from the highly thickened hinge areas and amplitudes generally less variable coarse grained deposits of the Pinnacle than one meter (Fig.4c). Sl is axial planar to these folds which do not affect the map pattern. 4 $1 and its Relationship to the Lincoln relict igneous mineral was found in the metabasite. Anticl.ine. Except for minor Fl fold hinges, Si is This is a red-brown hornblende that occurs in the parallel (or at an imperceptibly small angle) to greenstone (CZub and CZug). bedding on both limbs of the Lincoln anticline. The clearest exanpies are the thin (0cm) sand layers in Syn-Si Mineralogy. Much of the detrital the thinly layered unit (CZpul). The sand layers are feldspar and quartz were recrystallized during the Oriented parallel to Si and, although boudined and development of the penetrative SI schistosity. The pulled apart, are not folded by Fl. Notwithstanding recrystallized grains are fine sand sized and not as the parallelism of bedding to Si across the Lincoln strongly undulose or as highly sutured as the relict anticline, cross section c-c' (P1.1) indicates that the grains. Other syn-Sl minerals include sericite, li.-nbs of the fold are at an angle to the axial surface. chlorite, biotite, epidote, sphene, magnetite, The cross section is constrained by field data that carbonate, hornblende, actinolite and ilmenite. indicates bedding decreases gradually from near vertical in the Cheshire and Dunham Formations to about Syn-Si epidote, actinolite and hornblende in 70 degrees east in the Middle Pinnacle Member on the the metabasite are commonly zoned. Zoned hornblende west limb of the Lincoln anticline. The dip of beds on occurs in the amphibolite-bearing unit (Clue) and in the east limb however, changes abruptly to between the biotite rich unit (Club). Zoning is patchy but 40 and 60 degrees east. Displacement on the Hinesburg early Si cores are generally light greenish-blue and thrust is minor in this area (P1.1, cross section C- late-Si rims deep blue-green. Blue-green hornblende C'). The Lincoln anticline therefore, is structurally also occurs as unzoned grains. Syn-Si actinolite connected to and part of, the relatively undeformed occurs in greenstone (CZug and Club) where it Cambrian - Ordovician shelf sequence to the west. The pseudomorphs relict hornblende. Individual grains parallelism of bedding and Si on both limbs of the locally show patchy zoning. In general, cores are Lincoln anticline is due to the early development and light aqua-green and rims are light pale green. The persistence of a bedding plane foliation in the former actinolite is rare. Pinnacle Formation. Thus, Si is interpreted as a foliation that developed parallel to bedding rather Post-S1-Pre-$2 Mineralogy • Biotite, blue-green than axial planar to the Lincoln anticline. It is hornblende, chlorite, muscovite and spessartine occur possible however, that an axial plane foliation is as randomly oriented, euhedral grains that truncate Si. developed in the hinge area of the major fold. Such a The edges of grains commonly deflect Si, indicating foliation would represent a S1.5 surface. that growth occurred late to post-Sl. Late to post-Sl mineral growth was not found west of the Pinnacle P2 Folds. F2 folds are conspicuous throughout Formation. the field area. They range in amplitude from less than one centimeter to 200m or more. Major F2 folds Syn-62 mineralogy. M2 mineral growth is (amplitude > 5Dm) are responsible for variations in both generally confined to S2 cleavage surfaces and commonly the strike and map width of each rock unit (P1.1). involves only quartz, sericite, chlorite and carbonate. They are tight, upright, doubly plunging folds with Sphene, epidote, iron oxide and tourmaline are rotated steep overturned west limbs and narrow fold hinges into S2, become stretched and granulated and then (Fig. 5). Major F2 folds warp and tighten the Lincoln emerge without recrystallization. anticline so that the Lower Pinnacle Member (CZp1) is exposed on the east limb rather than in the core (P1.1, Nylonitic Uecrystallization. A well-developed cross section c-c'). mylonitic foliation is present along the Underhill thrust. The foliation is defined by laminae of fine Minor F2 folds (amplitude <50m) are not grained, recrystallized quartz, albite, biotite, prevalent except near the hinge area of major folds. chlorite, sericite, epidote and zoned hornblende. The They are tight and similar in geometry to the major mylonite foliation truncates Sl but is folded by F2. folds with open folds developed in the limb areas. F2 This indicates that displacement occurred late-Si is disharmonic in the laminated schist. Fold axes during biotite grade, Ml metamorphism. Biotite locall diverge oy as such as 90 degrees in a single outcrop. grows across the mylonite foliation. This indicates Mi F2 is well-developed in all rock units except the persisted until the late stages and locally outlasted Cneshire and Dunham Formations west of the Hinesburg the thrust event. The thrust, therefore, does not thrust. Much of the F2 shortening in these formations truncate Ml metamorphic zones. was apparently taken up by 02 sneer along the Hinesburg thrust. Hinesburg Thrust Zone The S2 axial plane cleavage is generally restricted to the hinge areas of major folds and tight -nmor folds. It commonly occurs as a spaced The Hinesburg thrust was mapped by Keith (1932) crenulation or pressure solution cleavage that locally to explain the stratigraphic discontinuity that extends develops into a slip cleavage. It is rare in the from Starkaboro northward to the Winooski River Valley. Middle Pinnacle Member and occurs locally as a fracture Both Keith (1932) and Cady (1945) postulated that it cleavage in the Upper Pinnacle Member and the continues to the south but found no evidence for argillaceous Cheshire member (Cca). 52 is well- faults. Evidence, presented here, suggests that the developed in pl-iyllltic rock, especially the Fairfield J-Iinesburg thrust Continues south of Starksboro village Pona Formation where it commonly forms a dominant as a shear zone with minor displacement. This zone was foliation. mapped by Tauvers (1982) who reports slickensides, transposed worm burrows and stratigraphic repetition in the lower part of the Cheshire Formation, in the Metamorphism Lincoln area.

Figure 6 summarizes mineral growth relative to The Hinesburg thrust in the Starksboro area, is 31 and S2 cleavage development for both elastic rock placed along the base of the argillaceous member of the and metabasite. Cheshire Formation (Cca) . Fault zone evidence is well- exposed in the area north of Starksboro village. Here Detrital Mineralogy. Quartz and albite are the phyllitic Cheshire member (Ccp) is absent. The preseit as detrital grains throughout the field area. Fairfield Pond Formation (cZf) thins and locally The aloite is altered from original Ca-plagioclase. pinches out. The argillaceous Cheshire member (Cca) Individual grains vary in size from fine sand to thins and pinches out north of the field area. coboles. Larger grains are strongly undulose with Quartzose rocks in the argillaceous Cheshire (Cca) sutured grain boundaries that are surrounded by fine locally contain a strong intersecting (phacoidal) recrystallizel subgralns. Detrital K-feldspar and cleavage. The fault appears parallel to the regional oiotlte are restricted to the Cheshire Formation. One 31 foliation. 5 The trace of the Hinesburg thrust at locations to the mineral lirieation. In other examples, a spaced 5-3 and H-2(?1.1) is marked by iter1ayered phyllite mylonite foliation truncates intrafolial folds of Si and argillite. Fault zone features are absent but a (P1.2, Fig.7). The rock is not strongly foliated and a fault zone is suggested by the truncation of the pervasive mineral lineation is not developed. phyllitic member (Ccp) and the thickness variations in the Fairfield Pond Formatton(CZf). Local shear may Well-Developed Stage. have extended to the Fairfield pond Formation - Pinnacle Formation contact in areas such as J-3(21.1), where the Fairfield Pond Formation is thin. Mylonite at this stage is strongly foliated, non-schistose and well lineated. The earlier 51 One significant aspect of the argillaceous foliation occurs as discontinuous lenses and stringers. Cheshire member, along the trace of the Hinesburg The intersecting (phacoidal) foliation is well- thrust, is the common occurrence of phyllitic partings developed only on sections perpendicular to the mineral up to 10cm thick. This is believed to represent a lineation. The foliation on sections parallel to the fault zone pressure solution cleavage. A pressure lineation is planar and compositionally layered solution cleavage is also suggested by transposed trace compared to the incipient stage. fossils in the phyllitic Cheshire member that lack a strong mineral lineation. Syn-Di quartz veins attest Pervasive Stage. to the selective solution of quartz.

The Hinesburg thrust developed from the sheared Mylonite at this stage is strongly foliated and overturned limb of the Lincoln anticline (P1.1). This very fine grained. Si is obliterated, but a few albite relationship, and the parallelism of the thrust to the porphyroclasts (<400 microns) are present. A pervasive 'regional Sl foliation, suggests displacement occurred mineral lineation is defined by elongate quartz-rich late-Fl during Ml metamorphism. Displacement on the rods or layers. Mica wraps around the quartz-rich Hlnesburg thrust is between .5 and 2km in the field layers (or rods) so that sections parallel to the area but increases northward to at least 10km in the mineral lineation show strong compositional layering Hinesburg area. and sections perpendicular to the mineral lineation show a well-developed phacoidal foliation (P1.2, Fig.8). Individual quartz and albite grains are Underhull Thrust Zone generally less than 100 microns and elongate parallel to the mineral lineation. They are undulose but are The Underhill thrust is the best defined and not highly sutured. most significant thrust in the area. The Jerusalem slice (CZul) is considered a large lithic sliver that The relative sense of shear obtained from formed during emplacement of the Underhill slice. intrafoiial folds and deformed mineral grains, is predominantly east over west. Intrafolial fold axes, Mylonite foliation and lithic slivers of slickensides and quartz rods plunge down dip and trend metabasite are well-exposed along the Underhill thrust roughly S 66 E. between locations J-lO and N-12 (P1.1). The fault trace is poorly exposed outside this area but was Quartz Mylonite. The quartz mylonite is crussed at locations 1-10, G-lO and C-9(Pl.1). It is characterized by a pervasive mylonite foliation with placel east of all known exposures of laminated schist slickensides and quartz rods (P1.2, Fig.9). It is the (Clul) at location I-lO(P1.1) and separates thick most common mylonitic rock at the fault contact and is soluences of metagreywacke (CZp1) from finer grained always in knife-sharp contact with country rock. sc.ilst and netagreywacke, of similar cousposition (CZu), at locations G-lO and C-9(Pl.1). The contact is In thin section, the quartz mylonite consists interlayered at locations 1-10 and G-10 but is abrupt of greater than 95 percent quartz with sericite and et location C-9. lesser amounts of iron oxide. Quartz varies in grain size from very coarse (>1.5mm) to very fine Mylonitic Foliation. Mylonite along the (<50 microns). All grains are undulose. Larger grains Underhill thrust is a syn-M1, incipient to pervasive are strongly undulose, contain deformation lamellae and foliation that truncates the earlier Si schistosity sutured grain boundaries. Individual grains are equant (P1.2). Two compositional types are recognized: 1) on sections perpendicular to the mineral lineation mylonitic country rock, which represents sheared (P1.2, Fig.10) but strongly elongate on sections metagreywacke, phyllite, schist and metabasite; and 2) parallel to the lineation (P1.2, Fig.11). The larger quartz mylonite, which represents sheared quartz veins. grains are surrounded by shear bands of fine They occur interlayered in sequences that range from a recrystallized quartz that define the mylonite few centimeters to five meters thick. Mylonite is foliation. present 250m east of the fault trace and makes up more than 90 percent of the outcrop within 20m of the thrust The lack of feldspar, abundance of quartz and at location C(P1.2). The mylonite foliation appears the occurrence in very thin layers with knife-sharp parallel to the regional Si foliation of the sur- contacts, suggests that the quartzite represents rounding country rock. sheared quartz veins.

Mylonitic Country Rock. Three stages are Regional Extent. The mapped lateral extent of recognized in the progressive development of mylonite the Underhill thrust is 21km (Tauvers, 1982, this foliation in country rock: 1) incipient, 2) well- report). It probably extends northward along the developed and 3) pervasive. Mylonites that show a Pinnacle - Underhill Formation contact on the Geologic continuous grain size reductionS from country rock Map of Vermont (Doll and others. 1961). This trend toward the center of the shear zone, contain all separates the predominantly clastic Pinnacle Formation gradations between these stages. Those in knife-sharp to the west from finer grained, metabasite-bearing rock contact with country rock contain a well-developed or east of the thrust. To the south, the Underhill thrust pervasive mylonite foliation. In this discussion, a may continue along the Hoosac - Underhill Formation "section" refers to a cut perpendicular to the dominant contact. Stanley and Ratcliffe (1983) correlate the foliation in the rock. A section is either Underhill thrust with the Hoosac Summit - Middlefield perpendicular or parallel to the mineral lineation. thrust zone along the eastern border of the Berkshire massif in western Massachusetts. Large horizontal displacement (>15km) is suggested by: 1) the presence incipient Stile. of large lithic slivers (including metabasite ani the Jerusalem slice), 2) tectonic kltppen 3km west of the Rocks at the incipient stage of mylonitization fault trace and 3) the absence of volcanic rock in the are distinguished by a strong intersecting (phacoidal) Pinnacle Formation. foliation in sections both perpendicular and parallel Major £1 Folds in the tinderhill Slice. Quartz laminations in the laminated schist are Compositional layers in the Underhill slice are closely associated with quartz veins. Both are parallel to 51. The Underhill slice is underlain by a extremely abundant and are seen in outcrop and in thin major thrust fault and is not part of the Lincoln section to grade directly into each other (P1.2, anticline. tletagreywacke in the Underhill slice shows a Fig.13). The transition from a quartz lamination to a greater deyree of deformation than similar metagrey- quartz vein (grain size lmin) is directly related to wacke in the Lower Pinnacle Member. The micaceous the percentage of mica at grain boundaries. The grain foliation is well-developed with through-going size of quartz in a lamination is Less than 100 microns anastotnosing bands that separate distinct lenses of where interstitial mica is abundant but becomes recrystallized quartz and albite. This contrasts with considerably larger (forming quartz veins) where mica the Lower Pinnacle Member in which the micaceous is absent. All gradations may exist in a single foliation is generally expressed as interstitial lamination depending on the distribution of mica. The flakes. The percentage of recrystallized grains quartz veins truncate rather than deflect the relative to detrital grains is also greater in the surrounding micaceous foliation. This suggests that Underhill slice. These relationships suggest that the quartz growth occurred late to post-Sl (i.e. late Ml). Underhill slice is tightly folded relative to the Pinnacle Formation. It is possible that the Underhill Significantly, a sliver of laminated schist slice represent's a recumbent fold with a sheared lower along the Underhill thrust (P1.2, B) grades directly limb (i.e. the Underhill thrust). The dominant into a quartz mylonite. It was previously argued that foliation in this formation, therefore, may be the 51.5 the quartz mylonite represents sheared quartz veins. axial plane foliation into which the Sl bedding The close association of quartz laminations to both foliation has been rotated. The S1.5 foliation becomes quartz veins and quartz mylonite, suggests that the pervasive at the base of the recumbent fold where it quartz laminations themselves represent sheared quartz forms the mylonite foliation in the Underhill thrust veins. This interpretation is supported by the zone. Dl deformation apparently progressed from east presence, in quartz veins, of fine grained shear bands to west so that the Underhill slice was already tightly that cut across the larger strongly undulose grains folded and the axial plane Sl.5 foliation was well- (P1.2, Fig.13). developed, prior to the development of a S1.5 foliation in the Pinnacle Formation. This geometry, of recumbent The presence of sheared quartz veins suggests folds floored by thrusts overlying less deformed rock, that silica-rich fluids migrated into the fault zone is similar to Structures reported in the Taconic during active shear. Fluid migration is also suggested allochthons ( Zen, 1961, 1972). Thus, the Underhill in mylonitic metagreywacke by the presence of thrust may represent the root zone for the (now eroded) overgrowths and fibrous infillings along brecciated northern extension of the syn-metamorphic high Taconics albite porphyroclasts (White, 1976). The knife-sharp (Stanley and Ratcliffe, 1983). contacts of the laminated schist and quartz mylonite with country rock, may also be indicative of fluid Jerusalem Thrust Zone migration. Thus, the laminated schist is interpreted as an originally fine grained rock that has been tectonically modified by distributed shear along The Jerusalem slice consists of undifferen- raylonitic zones represented by the crystalline quartz tiated quartz laminated schist (CZul; Fig.12, P1.2). laminations. The sheared fabric of the quartz mylonite It is exposed as a belt on the east limb of the Lincoln relative to the recrystallized fabric of the laminated anticline and as erosional outliers at locations 14-7 schist, suggests that displacement continued along the and -lO(Pl.l). Field and petrographic evidence Underhill thrust after distributed shear in the suggest that shear related to emplacement of the Jerusalem slice had ceased. Jerusalem slice was not restricted to the laminated schist - Middle Pinnacle Member contact but was The room needed to accommodate the observed distributed throughout the laminated schist within the quartz veins and laminations may have been created as Jerusalem slice itself. the thrusts overrode the cross-synclinal trough of the doubly plunging Lincoln anticline (P1.2). The trough The erosional outliers occur on topographic was filled by the Jerusalem slice and was accompanied highs in knifesharp contact with the structurally by silica-rich fluid intrusion. underlying Middle Pinnacle Member. There is no change in texture or gradation in either rock unit as the contact is approached. The contact is parallel or at a Structural Evolution very low angle to Si and is folded by F2. The outliers do not plunge into the ground either to the north or The major structural events are summarized in south, change facies with the Pinnacle Formation or Figure 14. DL deformation began during the Taconian reappear as a belt on the west limb of the Lincoln orogeny with the onset of metamorphism, initial flexure antici me. of the Lincoln anticline and the incipient development of a bedding plane foliation (Si) in the Pinnacle Tectonic imbrication and raylonitization is Formation (Fig.14a). With continued deformation the evident along the belt on the east limb of the Lincoln Lincoln anticline became upright and doubly plunging. anticline. The laminated schist, in this area, The bedding foliation was accentuated on the limbs of contains discontinuous pods and layers of Middle the Lincoln anticline but may have been destroyed in Pinnacle Member metagreywacke which commonly contain a the hinge area during the development of an axial plane weil-developel phacoidal foliation. foliation (51.5, Fig.14b). Late-Fl faulting occurred along the Underhill thrust perhaps along the sheared Thin sections of laminated schist, taken lower limb of a recumbent fold. During westward anywhere in the Jerusalem slice, show the same general displacement, the Underhill thrust incorporated lithic fabric (P1.2, Fig.13). There is strong metamorphic slivers of metabasite and the Jerusalem thrust was differentiation of quartz-rich and mica-rich layers; formed. The Lincoln anticline became west vergerit at grain size in well-developed quartz laminations is less this time, broadly folding the overlying thrust slices than 100 microns and mica-rich layers commonly contain (Fig.14c). Shear along the overturned limb resulted in tnaes and stringers of quartz or epidote. These formation of the Hinesburg thrust. Ml metamorphism characteristics are typical of rnylonite along the ceased during the final emplacement of the Underhill Underhill thrust but are rare in fine grained country slice. There may have been a time gap before the rock in either the Underhill or Pinnacle Formations. initiation of 02 but this deformational event is also believed to be Taconian in age. It is responsible for large scale warping of the Lincoln anticline which produced the present configuration (Fig.14d, compare with cross section C-C' (P1.1)). 7 Harper, C.T. • 1968, Isotopic ages from the Appalachians ACKNOWLEDGMENTS and their tectonic significance: Can. J. Earth Sci., v.5, p.50-59. Dr. Rolfe Stanley read several early drafts of Keith, A., 1932, Stratigraphy and structure of this report. His supervision, comments, criticisms and northwestern Vermont: Wash. Acad. Sci. .7. • v.22, discussions have helped immensely. I would also like p.357-379, 393-406. to thank Ragan Cary who drafted the plates and figures. Lanphere, M.A. • Laird, J. and Albee, A.L., 1983. Partial funding for the 1982 field season was provided Interpretation of 40Ar/39Ar ages of by a grant from Sigma Xi. This report is based on a polymetamorphic mafic and pelitic schist in liasters Thesis (DiPietro. 1993) and was published with northern Vermont: Geol. Soc. Ant. Abstr. w/Progs., the support of the U.S. Department of Energy, Grant v.15, no.3, p. 147 . •DE-FG02-81WM 46642. However, any opinions, findings, Rankin, D.W., 1976, Appalachian salients and recesses: conclusions, or recommendations expressed herein are Late Precambrian continental breakup and the those of the author and do not necessarily reflect the opening of the lapetus Ocean, J. Geophy. Keg., views of D)E. v.81, no.32, p.5605-5619. Stanley. R.S. and Ratcliffe, N.M., 1983, Tectonic synthesis of the Taconic Orogeny in western New England: in press, Geol. Soc. MI. Bull. Tauvers, P.R., 1982. Basement-cover relationships in REFERENCES CITED the Lincoln area Vermont [M.S. thesis]: Burlington, Vt., Univ. of Vermont, 177 p. Cady. W.M. • 1945, Stratigraphy and structure of west- Tauvers, P.R., 1982b, Bedrock geology of Lincoln area, central Vermont: Geol. Soc. Am. Bull., v.56, Vermont, Spec. Bull. no.2. Vermont Geol. Survey. p.515-587. Walker, R.G., 1978, Deep-water sandstone facies and Cady, W.M., Albee, A.L. and Murphy, J.F., 1962, Lincoln ancient submarine fans: models for exploration for Mountain quadrangle: U.S.G.S. Geol. Quad. Map GO- stratigraphic traps, Ama. Ass. Petrol. Geol. 164, 1:62,500. Bull., v.62, p.932-966. DiPietro, J.A., 1982, The geology in Starksboro-South White, S.H. 1976, The effects of strain on the Starksboro, Vermont: a progress report, The Green microstructures, fabrics and deformation Mount. Geol., v.9, no.1, p.6. mechanisms in quartzites: Phil. Trans. R. Soc. DiPietro. J.A., 1983, Contact relations in the Late Lond. A., v.283, p. 69-86 . precambrian Pinnacle and Underhi-li Formations, Zen, E-an, 1961, Stratigraphy and structure at the Starksboro, Vermont [M.S. thesis]: Burlington, north end of the Taconic Range in west-central Vt., Univ. of Vermont, 131 p. Vermont: Geol. Soc. Mt. Bull., v.72. p.293-338. Doll, C.G. • Cady, W.M., Thompson, J.B. and Billings, Zen, E-an, 1972, Some revisions in the interpretation M.P., 1961, Centennial Geologic Map of Vermont: of the Taconic Allochthon, west-central Vermont: Vermont Geol. Surv., 1:250,000. Geol. Soc. Am. Bull., v.83. p.2573-2587.

CORRELATION CHART Western Sequence Western Sequence Eastern Sequence Doll et at 1961 Tauvers 1982 This Report Reçion:l Storksboro South of North of Lincoln Area Stratigraphy Area Lewis Creek Lewis Creek Dunham Dunham Du nham Formation Formation Formation Cd 6cm an Cheshee Cheshire 6Cm Cheshire \\\\ Formation Coo Formation Formation Cco £co L.... o CP \Hinesburglhiust Fairfield £zp Fairfield CZf czt Fairfield fçpi Em.. Pond Fm Pond £Zpuu CZpug CZpct £Zpuu IMember :ZP!LJCZ tSJb ,, \ = IForestdot CZfd jJ €Zpuf CZpuf in Fn 03 jMember % 2 5 - CZpuC CZpuc CZpct CZpu9 CZpu Ld Pinnacle - CZpm U. a- Formation CZpbQ CZpm CZut 5 j ______- _l CZpcm ______£Zpt ______\CZu ______- CZpbc Mount Hotly Mount Hotly Ymha ______CZII CZug Complan Comex I CZub Ymhg ______FP ______1_VI

Figure 1. Stratigraphy and correlation chart for the Starksboro area. Half arrows indicate the direction of relative displacement of either the hanging wall or foot wall along faults.

8 ••

GE FORMATION Lewis SOUTH Creek NORTH Dunham Fm

JX Ccm Cheshire Of Ca Formation ------

Fairfield 2500 Pond Formation - - -

ICZPUI

£Zpufn

z O- .-e- 4) • 1 -0- --- - . Pinnacle

:r Formation 4) - Mcb , CL b G CZpng

—J -e- . €ZpI I

Not Exposed 1 PRECAMBRIAN Y ', L £Zpll EXPLANATION 'Mount Holly Complex: b - biotiie 6 - grodedbed 1 - disseminated 'ExpsedinUncoln Vermont, carbonate -e - blue quartz ciost . - 1 0 meters 4) - olbite clost

Figure 2. Detailed stratigraphy of the western sequence. Half arrow on the foot wall indicates displacement on inferred Late Precambrian normal fault.

9 DISTINGUISHING SOUTH NORTH Lewis FEATURES Creek ssive metagreywacke Blue quartz-rich layers UPPER Phyllite with lbite and chlorite-rich Phyllite and metagreywacke and metagreywacke Biotite bearing PINNACLE carbonate layers Local carbonate Carbonate layers Chlorite-rich Graded beds Blue quartz layers MEMBER Green Chlorite-rich Highly variable Intraclasts Variable lithic types Greeif Blue quartz clasts Variable lithic Albite clasts types Gradational (8-30m) ssiVe metagreywacke MIDDLE Massive metagreywacke Albite-hiotite bearing Massive metagreywacke Albite-rich Lacks carbonate Albite-Biotite bearing PINNACLE Biotite bearing Blue quartz clasts Lacks carbonate Lacks carbonate AJ.bite clasts Light to dark grey MEMBER Albite clasts ight to dark grey Conglomeratic Schistose C Med. to dark grey Granular Graded beds Conglomeratic Conglotneratic Graded beds Intraclasts Gradational (10-30in) Massive metagreywacke LOWER Quartz-sericite rich Disseminated carbonate Massive metagreywacke PINNACLE Lacks biotite and phyllite Albite not abundant Quartz-sericite rich MEMBER Blue quartz clasts Disseminated carbonate Light grey Lacks biotite Schistose Albite not abundant Weathers brown Light grey Weathers brown Thick MOUNT phyllitic interraltervals HOLLY

COMPLEX

Figure 3. Summary of south-to-north facies changes and distinguishing features in the Pinnacle Formation. Half arrow symbol is same as figure 2. S

n941 n 36

UJAM, 101

D JL ED so-SI LI n25 n 68

Figure 4. Lower hemisphere equal area projections of Dl -structural data. a) 36 poles to bedding (SO')-. Contour interval is 3 and 8 percent per 1 percent area; b) 941 poles to Si. Contour interval is 0.1, 2.0, and 9.0 percent per 1 percent area; C) 25 So-Si intersection lineations. Contour interval is 4 and 12 percent per 1 percent area; d) 63 mineral lineations. Contour interval is 2 and 9 percent per 1 percent area.

11 n=135 n= ir

L F2 n 124 n =34 =

Figure 5. Lower hemisphere equal area projections of D2 structural data, a) 135 poles of S2. Contour interval is 0.7, 6.0, and 18.0 percent per 1 percent area; b) 127 polesto F2 axial surfaces. Contour interval is 0.8, 6.0, and 17.0 percent per 1 percent area; c) 34 Sl-S2 intersection lineations. Contour interval is 3 and 15 percent per 1 percent area; d) 124 F2 fold hinges. Contour interval is 0.8, 4.0, and 14.0 percent per 1 percent area.

12 aIL

a) MINERAL PRE-Si I SYN-SI I SYNSM I POST-SI sec Quartz — I Albite — I Biotite — Chi. Sericite Epidote sec Carb I Hem/urn I I Mag. I I sec see Tour. — I Sphene I Spess. I J Pyr. I I Sec sec K-spar.

b ) MINERAL PRE-S1 f SYN-Si I SYN-SM I POST-S1 SYN -S21 Posi-S Aihite Epidote I R. Horn. — I E. Horn. L. Horn. E. Act. L.act. I Chi. I - Biotite I nag. [- I Sphene I Quartz Sericite Carb. I I sec

Figure 6. Relative time - mineral growth chart for a) clastic rock and b) metabasite. Sm represents the mylonitic foliation. Sec indicates secondary mineralization.

13 LJ

b) S1

c) SI Fill S•5 S15 S01181 Fill

\ F2 s15 c\F1II S15

Figure 14. Progressive development of the Lincoln anticline. a) Initial fiexure of the Lincoln anticline and the development of a foliation (Si) parallel to bedding (So) in the Pinnacle Formation. b) Development of an axial plane foliation (S1.5) in the hinge area of the Lincoln anticline (Fl). c) Truncation of the Lincoln anticline by the Underhill fold-thrust nappe. The bedding foliation (SO and Si) in the Underhill slice has been rotated into patallelism with the axial plane foliation (Sl.5) of the nappe (Fl). d) Large-scale warping (F2) of the Lincoln anticline.