Cleavage Development in Dolomite of the Elbrook Formation, Southwest Virginia

Cleavage development in dolomite of the Elbrook Formation, southwest Virginia

JANET SCIIV^l^ I'l'^F.R \ CAROL SIMPSON 1 department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061

ABSTRACT collapse of phyllosilicates and quartz to form clay seams (Beach, 1979, 1981; Gray, 1981; Marshak and Engelder, 1985). Argillaceous material Well-develo])ed cleavage in argillaceous Cambrian Elbrook For- has also been found to exert some control on cleavage formation, and a mation dolomite lias been analyzed petrographically and chemically to positive correlation between argillite content and cleavage intensity has evaluate the defoimation mechanisms responsible for cleavage forma- been observed such that well-cleaved limestones generally contain more tion and the lithologic controls on these mechanisms. Two main types than 10% clay (Nickelsen, 1972; Groshong, 1975; Gray, 1979; Marshak, of cleavage occur: moderately spaced cleavage defined by coarse, 1983; Tapp, 1983; Mitra and others, 1984). The effect of clays on the continuous, wavy domains, and narrowly to closely spaced cleavage behavior of dolomite has not been previously documented. defined by fine, discontinuous, anastomosing domains. This paper describes the microstructures and chemical changes asso- The two moist important deformation mechanisms during cleav- ciated with cleavage development in dolomite of the Cambrian Elbrook age formation were pressure solution and mechanical rotation. Evi- Formation near the base of the Pulaski thrust sheet. A model for cleavage dence for pressure solution includes off-set sedimentary structures, in impure dolomite is presented and is compared with existing models for grain dissolution, syn-cleavage fibrous veins, and accumulations of limestones. Observations are based on examination of polished thin sec- quartz, phyllosilicates, and pyrite in cleavage domains. Pressure solu- tions, using standard transmitted light microscopy and an ARL-SEMQ tion was enhanced by the presence of clay minerals. electron microprobe. All samples were cut perpendicular to bedding and A model for cleavage development in dolomite is presented in cleavage. which cleavage initiates by the dissolution of dolomite grains at dolomite-phyllosilicate grain boundaries. Rotation of phyUosilicate GEOLOGIC SETTING grains concomitant with dolomite removal resulted in a strong preferred orientation of layer silicates within cleavage domains. Finer The Pulaski thrust fault, one of four major southeast-dipping thrust cleavage domains coalesced to form coarser domains as the interven- faults in the southern Appalachian Valley and Ridge province, places ing lithon dolomite was dissolved. This model is comparable to those deformed Cambro-Ordovician sediments over deformed Cambrian to previously proposed for cleavage development in argillaceous lime- Mississippian sedimentary rocks of the Saltville thrust sheet (Fig. 1). Tim- stones and siltstones. ing of the initial emplacement of the Pulaski thrust sheet is poorly con- strained as the youngest rocks in the thrust sheet are Middle Ordovician INTRODUCTION (Schultz, 1983). Final emplacement occurred during the late Paleozoic (Alleganian) (Schultz, 1983). Near the study area, the Pulaski thrust sheet Well-develop* ;d spaced cleavage is commonly observed in lime- is 1,500 m thick (Schultz, 1983). Conodont alteration (Epstein and others, stones, but it is less commonly reported in dolomite. Experimental and 1977) and illite crystallinity (Schweitzer, 1984; Schultz, in press) indicate field data for deformed pure calcite and pure dolomite show that the two that the maximum temperature at the base of the thrust sheet was between minerals deform differently. Experimental studies indicate that calcite be- 200 and 350 °C. comes weaker and more ductile with increasing temperatures, whereas The Elbrook Formation occupies a structural position near the base dolomite increases in strength and deforms by brittle fracturing up to -400 of the Pulaski thrust sheet. In the lower part of the formation, thinly °C (Higgs and Handin, 1959; Turner and Weiss, 1963; Wenk and Shore, laminated dolomite and calcareous mudstones alternate with more mas- 1975). In a study of naturally deformed carbonates, it was found that for sive dolomites (Schultz, 1983). The dolomite is highly fractured and the same grain size, dolomite is stronger than limestone at shallow crustal veined in places (Cooper, 1939; Cooper and Haff, 1940), and lccally levels (Hugman and Friedman, 1979). Thus at low temperatures, a pure displays well-developed disjunctive, spaced cleavage, especially in the dolomite usually shows brittle behavior, whereas a limestone generally lowest, highly argillaceous horizons (Schultz, 1983). Cleavage is found in deforms by twinning and pressure solution to produce a spaced cleavage. Ramsay (1967) class IB, 1C, or 2 upright and inclined folds that trend Argillaceous dolomites in the central Valley and Ridge province, however, southwest-northeast (Fig. 2). It is axial planar to slightly convergent or are known to contain spaced cleavage (Helmstaedt and Greggs, 1980; defines southeast-dipping cleavage fans with planar to curved axial sur- Tapp, 1983), as aire similar lithologies in the Pulaski thrust sheet of faces (Fig. 2). Cleavage density is generally greatest in hinge regions, southwest Virginia. slightly less on overturned limbs, and lowest on upright limbs. The trace of Spaced cleavage and tectonic stylolites in impure limestones have been the cleavage is often continuous, with planar to anastomosing morphology, attributed to removal of calcite by solution processes and consequent and it is commonly refracted across lithologic contacts.

Geological Society of America Bulletin, v. 97, p. 778-786,6 figs., June 1986.

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Figure 1. (a) Location of study area in southern Appalachian Valley and Ridge province. Pulaski thrust sheet marked by stipples. Boxed area enlarged in b. (b) Geologic map of study area (modified from Schultz, 1983). Sample collection localities marked by stars. P.M.W. = Price Mountain Window; (c) Cross section X-Y (from Schultz, 1983). Cr = Rome Formation, €b = Broken Formation of Schultz (1983), Ce = Elbrook Formation, Cc = Conococheague Formation.

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Figure 2. Outcrop sketch and orientation data of deformed Elbrook Formation near Pulaski, Virginia, (a) Structure section showing folds; in northwest portion of outcrop, f = faults, medium lines = bedding, fine lines = cleavage, (b) Enlargement of boxed area in 2a showing overturned syncline with convergent spaced cleavage. Sample localities numbered, (c) Equal-area projection of orientation data from fold illustrated in 2b. Poles to bedding ( • ), poles to cleavage (A). Best-fit 7r-circle (dashed) and /?-axis (®) determined by inspection.

CLEAVAGE TERMINOLOGY narrow, close, moderate, or wide (Fig. 3a). Cleavage morphology is de- scribed in terms of small-scale (Fig. 3b) and large-scale (Fig. 3d) Spaced cleavage, the most common type in carbonates, can be further topography of cleavage domains as well as domain length (Fig. 3c] and classified as disjunctive or crenulation cleavage (Powell, 1979). In the width (Fig. 3e). Elbrook Formation, the cleavage is predominantly spaced disjunctive, with rare crenulation cleavage. Disjunctive cleavage consists of cleavage UNDEFORMED ELBROOK FORMATION domains and micro lithons. The domain is the plane or zone which has been altered by deformation and is generally defined by concentrations of The lithology of the lithons in cleaved Elbrook Formation is the same insoluble material, elongated mineral grains, and foliated phyllosilicates as that of the undeformed rock away from the cleaved regions. The unde- (Powell, 1979). The microlithon is the material between the cleavage formed lower Elbrook Formation is a thinly laminated (0.75-5.0 mm), domains. Cleavage s pacing in the Elbrook Formation is commonly several fine-grained (9-60 ¡xm) ferroan dolomite with interbedded calcareous millimetres, and thus the term "microlithon" is modified to "lithon" (com- mudstone. Dolomite grains are equant and untwinned. Inhomogencities pare Marshak, 1983). within laminations include relict pellets, intraclasts, and irregular fenestrae. Several cleavage classification schemes have been proposed, but none Randomly oriented, <5-/nm-sized illite and chlorite grains comprise 1 of these is entirely appropriate for the cleaved dolomites of the Elbrook to 30 wt% of the dolomite layers (Fig. 4a). Fine-grained quartz silt is Formation. The classification in Figure 3 is a modification of those pre- always present in trace amounts and may constitute as much as 10% of the sented by Alvarez and others (1978), Powell (1979), Borradaile and dolomite by volume (avg = l%-2%). Pyrite content is approximately 3% others (1982), and Marshak (1983) and is based on spacing and morphol- (range is l%-8%) by volume. Calcite, found in only a few samples, is < 1% ogy of cleavage domains. Cleavage domain spacing can be classified as by volume. In general, with an increase in clay content, there is a deciease

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Narrowly Closely Moderately Widely Moderately spaced cleavage domains are, on average, 4 cm apart and Spaced Spaced Spaced Spaced range in width from 30 to 70 /urn. In outcrop, this type of cleavage appears continuous, but on a microscopic scale, it is often discontinuous (Fig. 4c). The boundaries between lithons and domains are sharp; along them, some grain boundaries are concave toward the domain (Fig. 4d). Within do- 5 cm 5 em 5 cm mains, elongate carbonate and quartz grains are draped by phyllosilicates in sharply bounded subdomains (Fig. 4b). Chlorites oriented with (001) Sutured Undulatory Non-Sutured planes at a high angle to cleavage are sometimes found between cleavage- parallel clay minerals (Fig. 4b). The morphology of moderately spaced cleavage domains ranges from planar to anastomosing but is predominantly wavy (Fig. 4e). The domains commonly bifurcate, and those of intermediate width coalesce to form coarse cleavage domains (Fig. 4f). Domain surfaces are undulatory or Continuous Discontinuous nonsutured. Bedding laminae, intraclasts, and early formed veins are often truncated or offset by the cleavage domains (Fig. 4g). Narrowly spaced (incipient) cleavage, commonly developed between moderately to widely spaced cleavage domains (Fig. 5a), is usually not visible on the outcrop scale. In thin section, the cleavage is defined by 5 cm 5 cm aligned clay minerals and appears as "hair-like" networks of fine (<10

Planar Wavy Anastomosing /xm), discontinuous (0.5 to 3.5 mm) domains (Figs. 5a, 5b). Narrowly spaced cleavage domains anastomose and coalesce to form thicker domains (Fig. 5c). "Horse-tail" terminations are common. Domain surfaces are generally nonsutured but become more dentate if calcite is present. Cleavage refraction is associated with changes in argillite abundance m such that the angle between cleavage and bedding trace decreases with 5 cm 5cm 'II increase in clay content (Fig. 5d). An increase in the proportion of clay is Intermediate Fine also accompanied by a decrease in domain spacing, a change in domain shape from planar to anastomosing, and a more pronounced offset of laminations. Fibrous veins are associated with the cleaved dolomite. The majority of these are syntaxial ferroan dolomite veins, although antitaxial quartz 100 mm veins and composite ferroan dolomite + quartz + ferroan calcite veins are also found. These fibrous veins, ranging from 0.2 to 5.0 mm thick, are typically bedding-parallel and at a high angle to cleavage; the straight to Figure 3. Terminology used to describe disjunctive cleavage do- slightly curved mineral fibers in the veins are elongate parallel to the mains (after Alvarez and others, 1978; Powell, 1979; Borradaile and cleavage domains (Fig. 5e). In a single thin section with abundant veins, others, 1982; Marshak, 1983). (a) Spacing, (b) surface morphol- cleavage may be off-set, may overprint, or may be a point of termination ogy, (c) length, (d) shape, (e) thickness. for the fibrous veins (Fig. 5f).

CHEMICAL CHANGES in dolomite grain size, quartz silt content, and lamination thickness but an increase in pyrite concentration. To determine the nature of chemical changes associated with cleavage development, changes in element and oxide distribution were docu- CLEAVAGE MORPHOLOGY mented from the lithon into the cleavage domain. Ka X-ray back-scatter electron analyses show a qualitative enrichment in Si, Al, Fe, K, and Mg in Cleavage domains contain higher concentrations of clay, opaque cleavage domains, whereas Ca is depleted. Electron microprobe traverses minerals, and quartz silt than do the lithons. Clay minerals in domains are across lithons show that CaO and MgO have the highest abundance and generally aligned parallel to domain boundaries (Fig. 4b). Dolomite grains that concentrations of SiC>2, FeO, and AI2O3 are variable (Fig. 6). In the

within a domain are elongate parallel to cleavage due to dissolution on cleavage domains, CaO content is depleted and Si02, A1203, FeO and faces perpendicular to the direction of maximum shortening. In contrast, K2O are enriched with respect to the lithon (Fig. 6). The Mg content is dolomite grains in the lithon do not show a grain shape change, even very variable but is generally higher in the domains than in the lithons. near the domain boundary (Fig. 4b). Spacing of domains ranges from 5 cm (widely spaced) to 0.01 mm DISCUSSION (narrowly spaced) (Fig. 3a). The majority of the cleavage, however, can be classified as either moderately spaced or narrowly spaced. As spacings get Cleavage development in the Elbrook Formation dolomite involved wider, domain thickness and length increase, as does the volume of clay, solution processes and mechanical rotation as the major deformation quartz, and pyrite in cleavage domains. mechanisms. Petrographic and chemical evidence for pressure solution-

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Figure 4. SEM and transmitted-light photomicrographs, Elbrook Formation, (a) Randomly oriented, fine-grained chlorite and illite in undeformed dolomite in cleavage lithon. (b) Sharp contacts between lithon (L) and domain (D) (heavy dashed line) and between microdomains (light dashed line). Dolomite grains along L-D boundary show no preferred shape orientation. Phyllosilicates drape elongate grains of carbonate (C) in domain. Note chlorite books (arrowed) with (001) planes at a high angle to domain boundaries, (c) Discontinuous cleavage domain with branching terminations (arrowed), (d) Enlargement of portion of (b) to show concave edges (large arrow) suggesting dissolution. Some of the phyllosilicates are bent (small arrows), (e) Wavy cleavage domain with undulatory topography. (0 Coalescence of two intermediate domains (ID) to form a single coarse domain (CD), (g) Offset and truncation of intraclast and disruption of argillaceous dolomite lamination across wavy, moderately spaced cleavage. Scale bars for (a) 10 ¿im, (b) 20 jum, (c) (e) (g) 2 mm, (d) 4 ¿urn, and (f) 0.5 mm.

redeposition processes associated with disjunctive cleavage development in the cleavage domain. Local re-precipitation of the dissolved material is has been found in most samples. The presence of offset layers, veins and indicated by the presence of fibrous ferroan dolomite veins oriented per- intraclasts, and concave grain boundaries suggests that dissolution oc- pendicular to the cleavage planes. Individual fibers within these veins are curred in cleavage domains (Nickelsen, 1972; Geiser, 1974, 1975; Gray, parallel to the cleavage trace and are thought to have grown in this 1981). Preferential concentration of Si, Al, K, and Fe in the domains orientation during cleavage development. Concomitant cleavage and vein coincides with a high concentration of relatively insoluble illite, chlorite, development is also suggested by the complex overprinting relatio:3ships quartz, and pyrite. A depletion of Ca, and to a lesser extent Mg, in the among veins and cleavage domains (Fig. 5f). domains indicates that ferroan dolomite was the more soluble mineral. Within the cleavage domains, phyllosilicates are not only concen- The variable Mg trends may be the result of variations in chlorite content trated with respect to the lithons, but also show a strong preferred shape

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Figure 4. (Continued).

orientation that is not observed in the lithons. The most commonly proposed mechanism for grain-shape fabric development in micaceous rocks is that of mechanical rotation ("March model"; see reviews by Tullis, 1976; Means, 1977; Oertel, 1983), which may or may not be accompanied by grain-boundary sliding and/or new grain growth (Williams, 1972; Etheridge and others, 1974; Knipe and White, 1977; Gray, 1981; Knipe, 1981). With the March model, variation in the concentration of marker phyllosilicates from area to area in the rock should not alter the intensity of preferred orientation development if the strain is homogeneously distrib- uted and is due to mechanical rotation alone (Oertel, 1983). In addition, mechanical rotation of elongate grains without attendant recrystallization or dissolution processes in the surrounding matrix material would tend to produce numerous bent or kinked grains (Tullis, 1976). In the dolomite cleavage domains, bent phyllosilicates are observed (Fig. 4d), and clays sometimes wrap around larger quartz and dolomite gTains (Fig. 5b). These phenomena have not, however, been observed in the intervening lithons, which suggests that mechanical rotation occurred only in the cleavage Effect of Clays on Geavage Development domains. Oriented growth of layer silicates in cleavage domains may be an In nature, pure dolomite typically deforms at low temperatures by important contributor to fabric development (Etheridge and others, 1974; fracturing and brecciation (Stearns, 1968; Beach, 1982). In dolomite with Knipe and White, 1977; Williams, 1977; Knipe, 1979, 1981; Weber, a minimum argillite content of -10%, however, pressure solution was 1981; Mitra and others, 1984). Unambiguous evidence for neocrystalliza- found to be more important. In cleaved argillaceous dolomite, it has been tion/recrystallization, however, was not found in the Elbrook Formation observed that cleavage density, the amount of carbonate dissolution, and dolomite. These rocks were deformed in the temperature range 200-350 argillite content all show a positive correlation. Cleavage domains are °C (Schweitzer, 1984; Schultz, in press), temperatures too low for signifi- sometimes developed on the edges of relict pellets and intraclasts that have cant neocrystallization/recrystallization to have occurred. higher argillite contents than their adjacent matrix, and between lamina- Several authors have suggested that grain interference during me- tions of contrasting argillite content. Nickelsen (1972) reported a compa- chanical rotation can be reduced by dissolution of the intervening mineral rable cleavage initiation on worm burrows and mud cracks. phases (Nickelsen, 1972; Williams, 1972; Tullis, 1976; Gray, 1981; Mar- Qualitative chemical analyses of clay mineral species in the Elbrook shak and Engelder, 1985). A strong preferred shape alignment of phyllosil- Formation dolomites have shown no detectable change in relative propor- icates results, in a manner analogous to the "house of cards" collapse tions of illite and chlorite from lithon to domain. These results suggest that model of Gray (1981). We suggest that this model is the most likely to clay mineralogy alone does not directly affect the process of dolomite explain the observed phyllosilicate shape orientation in the cleavage do- dissolution. A similar conclusion was reached by Marshak and Engelder mains of the Elbrook Formation dolomite. (1985) in their discussion of the pressure solution effects of clays in lime-

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Figure 5. SEM and transmitted-light photomicrographs of narrowly spaced cleavage and fibrous veins, (a) Fine, narrowly spaced cleavage (vertical) forms hair-like network in the lithon (L) of intermediate, moderately spaced cleavage (MC). (b) Fine cleavage network with phyllosilicates (arrowed) wrapping around equant dolomite grains, (c) Fine, anastomosing, narrowly spaced cleavage domains coalesced to form a thick, planar domain, (d) Bifurcation of planar cleavage domain (arrowed) as it crosses from silty dolomite (pale) to lamina of h igher argillite content (dark). Note lower angle with bedding for the narrower, finer domains. Scale bars for (a) and (c), 2 mm; for (b), 10 jum; for (d), 0.5 nm.

stones and sandsto nes. They suggested that the role of the clay minerals Marshak, 1983). Based on theoretical studies, diffusion is thought to be the was to enhance removal of dissolved ions by providing an interconnected rate-determining step in pressure solution (Fisher and Elliott, 1974; Fisher, pathway for free-fluid movement. Other workers have suggested that the 1978). In natural systems, it has been found that the development of presence of clay minerals increases grain boundary diffusion through a cement in sandstones and decreased permeability in limestones, both of rock which in tun increases the rate of dissolution and, therefore, en- which inhibit grain boundary diffusion, result in pressure solution climin- hances pressure solution (Weyl, 1959; McEwen, 1978; Beach, 1982; ishing or ceasing altogether (Williams, 1977).

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Figure 5. (Continued). (e) Syn- taxial ferroan dolomite veins with fibers aligned parallel to cleavage trace (vertical). (0 Fibrous dolomite veins are crosscut by (CC), are off-set along (OS), terminate against (T), and overprint (OP) intermediate to thick cleavage domains in a small area. Scale bars for (e), 2 mm; for (f), 0.5 mm.

Another explanation for increased pressure solution with increase in clay content is that the clay-mineral grains act as stress concentrators (Wanless, 1979). A change in clay content would cause a change in the Theological properties of the carbonate and produce an inhomogeneous stress distribution. Both the initiation and propagation of pressure solution seams may thus be explained by stress concentrations at points of contrasting rheology. In the anticrack model of Fletcher and Pollard (1981), the solution seams initiate at a point in the rock, and their lengthwise propagation away from that point then occurs in both directions. It is suggested that the points of initiation for cleavage domains (anticracks) in the El- brook Formation dolomite are the newly aligned phyllosilicate minerals. Figure 6. Major The observed positive correlation between domain length and width, the oxide trends across horse-tail domain terminations, and veins perpendicular to cleavage are all closely spaced cleav- predicted by the anticrack model (Fletcher and Pollard, 1981). age domains (D) and lithons (L). Analyses MODEL FOR CLEAVAGE DEVELOPMENT at 50-/nm intervals. IN ELBROOK DOLOMITE

The first stage of cleavage development involved dissolution of dolomite at dolomite-phyllosilicate grain contacts and rotation of the phyllosilicates to produce narrowly spaced disjunctive cleavage domains. Phyllosilicate grains acted first as stress concentrators to localize the pressure solution seams, and secondly to increase grain-boundary diffusion and, hence, the extent of dissolution. The cleavage domains, defined by phyllosilicates with their (001) planes oriented perpendicular to the instan- taneous shortening direction, then propagated longitudinally as anticracks. As cleavage formation progressed, pressure solution continued to be the dominant deformation mechanism. Widening of cleavage domains occurred by rotation of the phyllosilicates upon dissolution of dolomite, Distonce, Microns accompanied by passive accumulation of quartz and pyrite. Changes in

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elemental distribution during this stage of cleavage development include 1975, Slaty cleavage and the dewatering hypothesis, an examination of some critical evidence: Geolo^, v. 3, p. 717-720. (1) concentration of K, Al, Si, and Fe within cleavage domains; (2) re- Gray, D. R., 1979, Microstructure of crenulation cleavages: An indicator of cleavage origin: American Journal of Science, moval of Ca from the domains; and (3) incorporation of Mg from the v. 279, p. 97-128. 1981, Compound tectonic fabrics in singly folded rocks from southwest Virginia, U.S.A.: Tectonophysiis, v. 78, dissolving dolomite into chlorite, resulting in a variable distribution of Mg p. 229-248. Gregg, W. J., 1985, Microscopic deformation mechanisms associated with mica film formation in cleaved pssmmictic between cleavage domains and lithons. rocks: Journal of Structural Geology, v. 7, p. 45-56. Groshong, R. H., Jr., 1975, "Slip" cleavage caused by pressure solution in a buckle fold: Geology, v. 3, p. 411-^-13. The model presented here is similar to those proposed for the defor- Helmstaedt, H., and Greggs, R. G., 1980, Stylolitic cleavage and cleavage refraction in lower Paleozoic carbonate rocks of mation of argillaceous limestones by Gray (1981) and Marshak and En- the Great Valley, Maryland: Tectonophysics, v. 66, p. 99-114. Higgs, D. V., and Handin, J., 1959, Experimental deformation of dolomite single crystals: Geological Society of America gelder (1985) and for argillaceous siltstones by Gregg (1985). The major Bulletin, v. 70, p. 245-278. Hugman, R.H.H., III, and Friedman, M., 1979, Effects of texture and composition on mechanical behavior of experimen- difference between cleavage formation in argillaceous limestones and in tally deformed carbonate rocks: American Association of Petroleum Geologists Bulletin, v. 63, p. 1478-1489. the Elbrook Formation dolomite is that calcite grains commonly change Knipe, R. J., 1979, Chemical changes during slaty cleavage development: Bulletin of Mineralogy, v. 102, p. 206-209. 1981, The interaction of deformation and metamorphism in slates: Tectonophysics, v. 78, p. 249-272. their shape by twinning to form a grain-shape fabric (Tapp, 1983), where- Knipe, R. J., and White, S. H., 1977, Microstructural variation of an axial plane cleavage around a fold—A H.V.E.M. study: Tectonophysics, v. 39, p. 355-380. as dolomite grains do not twin at the anchimetamorphic deformation Marshak, S., 1983, Aspects of deformation in carbonate rocks of fold-thrust belts of central Italy and eastern New York conditions of the Pulaski thrust sheet. state [Ph.D. thesis]: New York, Columbia University, 223 p. Marshak, S., and Engelder, T., 1985, Development of cleavage in limestones of a fold-thrust belt in eastern New York: Journal of Structural Geology, v. 7, p. 345-359. McEwen, T. J., 1978, Diffusional mass transfer processes in pitted pebble conglomerate: Contributions to Maeralogy ACKNOWLEDGMENTS and Petrology, v. 67, p. 405-415. Means, W. D., 1977, Experimental contributions to the study of foliations in rocks: A review of research siice I960: Tectonophysics, v. 39, p. 329-354. The initial impetus for this work was provided by D. R. Gray. Mitra, G., Yonkee, W. A., and Gentry, D. J., 1984, Solution cleavage and its relationship to major structures in the Idaho-Utah-Wyoming thrust belt: Geology, v. 12, p. 354-358. Financial support from the Appalachian Basin Industrial Association, Nickelsen, R. P., 1972, Attributes of rock cleavage in some mudstones and limestones of the Valley and Ridge province, Sigma Xi, and the Department of Geological Sciences at Virginia Tech is Pennsylvania: Pennsylvania Academy of Science, Proceedings, v. 46, p. 107-112. Oertel, G., 1983, The relationship of strain and preferred orientation of phyllosOicate grains in rocks—A review: gratefully acknowledged. Assistance with analyses was given by C. Price Tectonophysics, v. 100, p. 413-447. Powell, C. McA., 1979, A morphological classification of rock cleavage: Tectonophysics, v. 58, p. 21-34. and T. N. Solberg. The manuscript was considerably improved by the Ramsay, J. G., 1967, Folding and fracturing of rocks: New York, McGraw-Hill Book Co., 568 p. Schultz, A. P., 1983, Broken formations of the Pulaski thrust sheet near Pulaski, Virginia [PhD. thesis]: Blicksburg, comments of R. H. Groshong, S. Marshak, G. Oertel, D. D. Pollard, and Virginia, Virginia Polytechnic Institute and State University, 82 p. J. B. Tapp. 1986, Broken formations of the Pulaski thrust sheet near Pulaski, Virginia: Virginia Polytechnic Institute and State University, Lowry Volume, Memoir Series No. 3 (in press). Schweitzer, J., 1984, Development of cleavage in dolomite, Pulaski thrust sheet, southwest Virginia [M.:>. thesis]: Blacksburg, Virginia, Virginia Polytechnic Institute and State University, 111 p. Stearns, D. W., 1968, Certain aspects of fracture in naturally deformed rocks, in Reicker, R. F., ed., N.S.F. Advanced REFERENCES CITED Science Seminar in Rock Mechanics: Bedford, Massachusetts, Air Force Cambridge Research Labcrato y Special Report, p. 97-118. Alvarez, W., Eagetder, T., and Geiser, P. A., 1978, Classification of solution cleavage in pelagic limestones: Geology, v. 6, Tapp, J. B., 1983, Relationships of rock cleavage fabric to incremental and accumulated strain in a portion of the Blue p. 263-266. Ridge, Virginia [Pb.D. thesis]: Norman, Oklahoma, University of Oklahoma. Beach, A., 1979, Pressure soljtion as a metamorphic process in deformed terrigenous sedimentary rock: Lithos, v. 12, Tullis, T. 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MANUSCRIPT ACCEPTED JANUARY 17,1986

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