The Role of Solution in the Formation of Boudinage and Transverse Veins in Carbonate Rocks at Rheems, Pennsylvania

The role of solution in the formation of boudinage and transverse veins in carbonate rocks at Rheems, Pennsylvania

DECLAN G. DE PAOR \ CAROL SIMPSON I CHRISTOPHER M. BAILEY I KENNETH J. W. MCCAFFREY > Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 ERIC BEAM* I ROBERT J. W. GOWER I GULZAR AZIZ /

ABSTRACT INTRODUCTION trix (see also Ramsay, 1967). Thus the shapes are often used as field mapping guides in high- The interaction of pressure solution with Boudinage structure has been observed for grade metamorphic rocks to determine the rela- extensional strain is the most important factor more than a century (see Cloos, 1947, for a tive timing of metamorphism and deformation. influencing boudinage and vein structures in review of the earliest literature). Quantitative Pinch-and-swell structures suggest a low viscos- a mesoscopically folded sequence of subgreen- rheological studies by Goguel (1948), Ramberg ity contrast, implying that metamorphism oc- schist-grade carbonate rocks from the over- (1955), Schwerdtner (1970), Stromgard (1972), curred during or prior to deformation, whereas turned limb of the Lebanon Valley fold nappe Sanderson (1974), Smith (1975), and Ferguson preserved angular boudin shapes suggest a high at Rheems, Pennsylvania. The formation of and Lloyd (1984) have led to the generally ac- viscosity contrast, implying deformation prior to stylolites and slickolites in association with cepted qualitative correlation between boudin metamorphism. This approach, however, ig- boudins and veins indicates that extreme bed- morphology and rigidity with respect to the ma- nores pressure solution, an important process normal contraction is intimately connected with bed-parallel extension. A variety of boudin shapes in dolomitic beds, from rectilinear to pinch-and-swell and fish-mouth III Triassic Basin E3 Valley and Ridge structures, are formed by a combination of Paleozoic rocks cataclastic flow and heterogeneous pressure ED Allochthonous solution. Ordovician rocks • Lebanon Valley Nappe Many veins record catastrophic failure and m Cambrian clastic rocks collapse of wallrock prior to or during crys- Efl Crystalline Basement tallization of the vein-fill. At the earliest stage T- Thrust Fault of boudinage, veins in boudin necks are straight and perpendicular to bedding. With Normal Fault continued extension, they adopt a bow-tie form in cross section as a result of inward flow of the surrounding matrix and pressure solution of the fragment corners and vein margins. With continued stylolitization and separation of boudins, bow-tie veins are con- verted into transverse veins of extraordinary dimension. Failure to recognize the role of pressure solution in the formation of boudinage structure may result in inaccurate estimates of extension, erroneous assumptions regarding volume change, and incorrect interpretations of the changes in stress regime with time.

*Present address: Department of Geological Sci- Figure 1. Map of Lancaster area, Pennsylvania, showing the location of Rheems quarry and ences, University of Texas, Austin, Texas 78712. the general stratigraphy and structure.

Geological Society of America Bulletin, v. 103, p. 1552-1563, 9 figs., December 1991.

1552

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Figure 3. a. Geometry of recumbent-reclined folding, view to the east (down-plunge). Stratigraphy youngs to the north. Beds in lower portion of middle bench are right-way- up. b. Small-scale folding in micritic limestone. and others, 1972; Root and MacLachlan, 1978). Nappe folds such as the 100-km-wide Lebanon that can affect boudin morphology (Mullenax northwest-overthrust Paleozoic nappe (Wise, Valley Nappe were formed in Cambro-Ordovi- and Gray, 1984) by altering the shape of bou- 1958, 1960; Donath and Parker, 1964; Freed- cian rocks southeast of the Hamburg Klippe dins so that they appear to have a similar ductil- man and others, 1964; Duchaine, 1978; Faill (Gray, 1954; Wise, 1958). In Rheems quarry, ity to their matrix. and Geyer, 1987); small-scale extensional and the Epler Formation (Geyer and others, 1958; The orientations of veins are often used to contractional structures are compatible with Faill and Geyer, 1987) of the Beekmantown deduce paleostress orientations, with the as- bulk north-northwest-directed noncoaxial de- Group is folded into a series of recumbent, sumption that the opening direction is perpen- formation (compare with Ramsay, 1967, p. 120; gently reclined, north-northwest-verging meso- dicular to, or at a high angle to, the vein walls. Sanderson, 1979; Ramsay and others, 1983). scopic folds that are interpreted as parasitic folds Where clear evidence for the opening direction On close scrutiny, however, many of the small- on the overturned limb of the partially eroded is preserved, as in crack-seal veins (Ramsay, scale structures are found to be problematical. Lebanon Valley Nappe (Fig. 2; Wise, 1958, 1980; Henderson and others, 1990), incremental Boudins with pinch-and-swell structure are 1960; Donath and Parker, 1964; Freedman and strains can be measured (Durney and Ramsay, formed by a combination of fracturing, pressure others, 1964; Duchaine, 1978; Faill and Geyer, 1973), and tectonic inferences made (Ramsay solution, and vein formation. Furthermore, 1987). Mesozoic normal faulting resulted in and Huber, 1983). Many veins, however, do not some veins—here called "transverse veins"—are half-grabens such as the Gettysburg sub-basin, contain fibrous minerals, and the opening direc- bedding-parallel on overturned parasitic fold whose southeastern margin lies —300 m to the tion therefore has to be assumed. Bedding- limbs and can be shown to have opened in an northwest of the quarry (Socolow, 1980). parallel veins are usually interpreted to have extension field that was parallel to the bedding The Epler Formation comprises meter-scale, opened perpendicular to bedding. We will show trace. Correct interpretation of these deforma- dark gray, dolomitic beds; thinner, medium- that some bedding-parallel veins have opening tion mechanisms and sequences is important for gray, carbon-rich, micritic limestone interbeds; directions parallel to bedding. Failure to recog- the quantification of regional strain and correla- rare thin calcareous mudstone layers; and scat- nize this variety of vein may cause serious errors tion of displacement patterns. tered chert nodules (Faill and Geyer, 1987). The in interpretation of the tectonic history of a lack of recrystallization of primary calcite or region. General Geological Setting phyllosilicates suggests that metamorphism did A detailed field study of the interaction of of Rheems Quarry not attain lowest greenschist facies in this area, extensional, contractional, and dilatational de- although biotite-grade peak metamorphic condi- formation mechanisms was undertaken in a car- The lower Ordovician carbonate rocks of the tions were reached during the Taconian Orog- bonate quarry at Rheems, Lancaster County, Beekmantown Group (Stose and Jonas, 1933) eny in the Ordovician Conestoga Formation 25 Pennsylvania (Fig. 1). Textbook examples of re- formed part of the Cambro-Ordovician carbon- km to the southeast (Valentino, 1990; Valentino cumbent folds, boudins, and veins are exposed ate bank on the southeast passive margin of and Faill, 1990). on the quarry walls (for example, Suppe, 1985, Laurentia (Rodgers, 1970). During the Middle Figs. 5-1, 9-24, and 9-54), and at first glance to Late Ordovician Taconic orogeny, closure of FOLDS AND FAULTS their interpretation seems relatively straightfor- the proto-Atlantic Ocean caused thrust sheets ward. The large asymmetrical folds of dolomitic containing basement and shelf carbonates to be A map of the distribution, orientation, and and limestone multilayers are parasitic to the detached and overthrust to the northwest to geometry of mesoscale structures is shown in overturned limb of a now eroded, north- form the Hamburg Klippe (Stose, 1946; Piatt Figure 2. An earlier structural map exists in the

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breccia zone ENTRANCE

LEGEND

/ bedding / overturned bedding / selected veins // faults (interred) strike slip U/D dip slip // fold traces lineations, SL = slickenlines f fold axes / boudin axes

\J synform, overturned

f\ antiform, overturned

transverse veins

Mapping by: D. G. DePaor, C. Simpson, C. Bailey, R. J. W. Gower, K. J. W. McCaffrey, G. Aziz, E. Beam

transverse veins

Figure 2. Detailed structural map of Rheems quarry. Double lines represent quarry walls.

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Figure 4. a. Ladder veins and rectangular boudin cross sections. Veins are long and thin; they opened perpendicular to their current long dimension, that is, parallel to bedding trace, b. Sharp-sided vein clusters with thin slivers of dolomitic rock (arrowed) characteristic of the initial stage of boudin development. literature (Wise, 1958), but the quarry is ac- tively worked and features observable in its walls have changed considerably during the past tain thin carbonaceous layers that have the mor- chocolate tablet shaped. The line of maximum 30 years. phology of stylolites but with rounded teeth at extension in bedding is generally oriented The structure of Rheems quarry is dominated angles of <30° to bedding, and which are folded downdip, so that the bulk symmetry is ortho- by recumbent, slightly reclined, asymmetrical into centimeter-scale asymmetrical folds whose rhombic. Rare asymmetric, rotated boudin sec- fold pairs with axes plunging and axial planes vergence is consistent with that of the major tions were seen, but the majority are symmetric dipping at 16° toward 075° (see also Duchaine, folds (Fig. 3b). On the right-way-up, north- within their enveloping surfaces. Most boudins 1978). Competent dolomitic beds are sand- northwest-dipping limbs of the recumbent folds, occur in single dolomitic beds, but 1- to 2-m- wiched between incompetent limestone inter- the sense of asymmetry of folded stylolites is top thick groups of beds are boudinaged on a wave- layers (Fig. 3a) that thicken by up to 50% in the to the north-northwest but is top to the south- length of a few meters in several places, whereas hinges, where a poorly developed cleavage is southeast on overturned south-southeast-dipping internal to these boudins, the individual 10- to defined by carbon-rich stylolitic cleavage planes. limbs. This change in shear sense around major 30-cm-thick beds show smaller wavelength Cleavage on fold limbs is confined to the more fold hinges precludes shearing prior to folding boudinage. argillaceous calcareous units. A number of and implies that localized ductile deformation of Boudin trains have cross-section shapes that steeply dipping faults have strike-slip or oblique the stylolitic micritic limestone beds occurred vary from rectilinear (Fig. 4) to ovoid, gentle displacement of a few meters or less (Fig. 2). during fold amplification. There is no evidence pinch-and-swell and fish-mouth (Fig. 5; see also They offset fold axes and may be related to the for twinning, pressure solution, or grain-bound- Suppe, 1985, fig. 5-1). Traditionally, this would formation of Triassic basins. ary migration recrystallization of the micron- indicate a wide variation in relative ductilities Younging directions in the quarry were pre- sized grains in thin section. Grain-boundary (Ramsay, 1967); however, we can demonstrate viously deduced from regional stratigraphic and sliding is thus the most plausible deformation that at this locality all geometries are formed by structural constraints; however, we have also mechanism. different proportional contributions of the same found direct evidence in the form of graded beds brittle processes. Rectangular boudins show and rip-up clasts within a single packstone bed. BOUDINAGE AND PRESSURE least separation (Fig. 4a) and are bounded by These sedimentary structures and the locally SOLUTION thin veins or groups of veins. Narrow, angular developed cleavage-bedding relations confirm slivers of dolomitic rock occur along, and ap- that the mesoscopic folds in the quarry are at Boudins (Figs. 4, 5) are the predominant proximately parallel to, the margins of many 10- least second-order structures on the overturned structures in dolomitic beds on overturned fold to 50 -cm-thick veins, indicating a multiple limb of a now eroded nappe as proposed by limbs; they are not observed at all in fold hinge fracture origin (Fig. 4b). The brittle nature of Wise (1958, 1960). There is no evidence in regions or on right-way-up limbs. Where boudin initial boudin fragmentation is recorded by Rheems quarry for distinct fold phases related to necklines (terminology of Jones, 1959) can be razor-sharp, jagged, and splaying fractures that separate tectonic events. observed, they are noncylindrical structures ori- opened in the boudin necks and filled with Higher-order, hand-specimen-scale folds oc- ented subparallel to fold hinges but intersecting blocky calcite (Figs. 5a, 5b). Angular or cur in stwo 30- to 50-cm-thick beds of micritic each other at acute angles. Their three-dimen- domino-faulted shards of dolomitic rock are limestone. These distinctive pale gray beds con- sional geometry is wedge shaped rather than preserved in boudin necks (Fig. 5a). Many more

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Figure 5. Boudin morphologies, a. Gentle pinch-and-swell boudin. Note the concentration of brittle dolomitic fragments in an overall bow-tie shaped vein. b. Ovoid boudin with strongly curved wedged-shaped bounding veins. Note bow-tie veins in the neck regions, c. Stylolitic surface crossed by furrows that mark the trend of veins in the underlying dolomitic unit. Note acute angle of intersection of furrows (arrowed), d. Fish-mouth structure. The mouth has closed around the left-hand 25 cm of a transverse calcite vein. The top and bottom of the boudin contain carbonaceous striations. Bedding in surrounding micritic limestone is highlighted.

wedge- and sickle-shaped veins are present near beds, and also between limestone and vein cal- residue thickness occurs in boudin necks along tjie perimeter than in the center pf individual cite; there is no evidence for pressure solution the "scar" (terminology of Jones, 1959) be- boudins, facilitating the development of convex along the initial fracture contacts between do- tween limestone beds and the calcite that precip- bedding surfaces and concave lateral termina- lomitic beds and vein calcite. Dissolution pits itated in early formed veins (Fig. 6a). There are tions (Figs. 5a, 5b). The distribution of veips has (Fig. 5c) are often 5 to 10 cm in diameter, with some dolomitic beds where one side has re- therefore permitted thin slices of the rigid dolo- amplitudes that range from a few millimeters or mained planar but the opposite side has the mitic rock to rotate toward the boudin necks. In less to several centimeters; they have thickest characteristic undulations of a boudinaged mar- this way, the original brittle rectangular boudin residues in their centers and faint striae within gin. The one-sided boudin necks contain narrow shapes have been modified by continued fractur- the residue material on their flanks. The flanks calcite veins, and insoluble residues along the ing and veining of their perimeters into shapes of boudins are coated by asymmetrical, parbon- limestone/calcite contacts are generally more more usually ascribed to ductile deformation. rich stylolites ("slickolites" of Ramsay and than 3 mm thick. Most of the boudin shapes in Rheems quarry Huber, 1983), which commonly grade into caN The vergence of the asymmetrically sheared have been extensively modified by pressure solu- citic and carbonaceous striae (slickenlines) on stylolites in the micritic limestone does not tion. A 1- to 4-mm-thick, sutured to undulatory boudin crests (compare Arthaud and Mattauer, change across boudin necks, implying that the layer of carbonaceous and sulfitic insoluble resi- 1969). In some extreme cases, cataclastic flow shearing preceded boudin formation. These due occurs along the interfaces betwgen dolo- and pressure solution have combined to yield folded stylolites serve as flow markers and show mitic boudins and the surrounding limestone fisfi-mouth boudin shapes (Fig. 5d). Maximum that, in places, the micritic limestone flowed into

Figure 6. a. A 4-mm-thick film of insoluble residue marks the stylolitic contact between limestone (sliver on top of block) and vein calcite. Left and right borders of vein are razor-sharp contacts with dolomitic layer, and they show some bridge fragments but no evidence of solution, b. Extreme wedge-shaped vein almost entirely occupied by micritic limestone that has flowed in from the host rock. c. Narrow vein of calcite in dolomitic host rock. Note the elongate protrusion of micritic limestone into center of vein and the truncation of stylolites against the calcite. d. Flowage of micritic limestone unit (light gray) into boudin neck region (white). The limestone-calcite contact is a stylolite. Note the rounded bedding contact of the dolomitic blocks (dark gray) against limestone.

open cavities in boudin necks prior to precipita- FRACTURES, VEINS, AND BRIDGES overturned limbs of folds, a wide variety of vein tion of the vein calcite. In Figure 6b, almost all morphologies and mechanisms are in evidence of the gap has been filled with micrite and only a Fractures and veins are visually striking, (Fig. 4; see also Fig. 8 below). thin rim of blocky vein calcite coats the vein small-scale structures in the Rheems quarry. walls. Spacing between the sheared stylolites in Veins are filled with 2-mm to 20-cm crystals of Ladder Veins the boudin neck is increased relative to that out- calcite, with accessory fluorite, barite, quartz, side boudin necks (Fig. 6b). In extreme exam- and orthoclase (I. Allison, 1991, personal com- Most veins are grouped in parallel sets con- ples (for example, Fig. 6c), micrite has flowed mun.). A few calcite-filled, thin (for example, 10 fined to individual beds; they strike parallel and up to 25 cm into the boudin neck. Sometimes m long by 2 cm thick), longitudinal veins occur dip perpendicular to bedding and thus resemble the micrite layer's internal banding is truncated parallel to bedding planes on right-way-up fold the rungs of a ladder in cross section (Fig. 4a; by a bounding stylolite (Figs. 6c, 6d); however, limbs. They do not have stylolitic margins, and compare with Ramberg, 1961). These veins are the thin layer of residue on the stylolite and the their calcite-fiber growth patterns show that they generally less than 1.5 cm thick and, where they extremely large grain size of the blocky calcite opened perpendicular to their longest dimension extend across the entire bed, are rectangular in vein fill suggest that the micrite flowed at least in in the standard fashion (Durney and Ramsay, cross section and are truncated by stylolites at part into an open cavity. 1973). They are not discussed further. On the the bedding surfaces (Fig. 6a). Wedge-shaped

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Figure 7. Bridge morphologies, a. En echelon vein array with long, narrow, straight bridges, b. Curved bridge separating en echelon veins. Note slickenlines and cracks in bend area. c. Broken bridge. Formerly connected ends of bridge segments are now marked by stylolites. Note bend in one half of bridge, d. Broken bridge with gaps between fragments indicative of extension subperpendicular to vein array.

veins terminate within the bed. Ladder veins (Fig. 7a) and on some surfaces perpendicular to followed by bridge collapse (Fig. 7d) and frag- may bifurcate (Fig. 4b) and frequently enclose bedding. The axes of the former arrays are sub- mentation. The wider veins are riddled with an- slender screens of host rock, like islands in a perpendicular to bedding,and the zones follow gular dolomitic shards, and their large calcite braided channel. A statistical study of vein spac- boudin neck lines; the latter arrays are oriented crystals have markedly bent crystal cleavage ing showed a tendency for individual veins to subparallel to the mean fold axial direction and planes. form clusters. The clusters in turn are not ran- at a high angle to bedding traces. Some en domly distributed but form a statistically anti- echelon veins merge laterally into parallel-sided Bow-Tie and Transverse Veins clustered distribution; that is, the distance longitudinal veins that cut across entire beds and between clusters is more or less constant. This across bedding-parallel stylolites. Veins within As boudin blocks move apart, the gaps be- characteristic spacing between clusters deter- arrays are generally straight to slightly sigmoidal tween them initially become rectangular and mines boudin spacing. Fibrous calcite, which in shape. They have asymmetric tips and a low then square. Subsequent stretching results in a tracks the incremental opening direction, is overlap (terminology of Rothery, 1988), with significant shape change of the veins, which dis- found in some millimeter-wide veins and for the well-preserved bridges of host rocks between play first "bow-tie," then "transverse" morphol- first few millimeters of coating on wider fracture veins. Some bridges step in opposite directions ogies. This vein morphology has been illustrated planes. It confirms that these veins initially within the same vein array, implying extensional (for example, Hossain, 1979; Suppe, 1985, opened by bedding-parallel extension. rather than shear displacement. Many bridges p. 355), but detailed descriptions are rare are homogeneously folded on the hand-speci- (Holmquist, 1930; Wise, 1958; Jones, 1959). En echelon Vein Arrays and Bridges men scale (Fig. 7b) by sliding on multiple micro- Wise (1958) introduced the term "bow-tie fold" faults; that is, they deformed by cataclastic flow. for the deflection of limestone bedding into Calcite-filled en echelon extension vein arrays Progressive separation of boudin fragments evi- boudin neck regions. Jones (1959) called these are visible on many dolomitic bedding planes dently leads to fracturing of bridges (Fig. 7c), "scar folds" after Wegmann's (1932) "cicatrice,"

Figure 8. a. Bow-tie vein morphology. Note splay of fractures at high-stress corners of dolomitic boudin. Top and bottom walls of vein are stylolitic. b. Subhorizontal transverse vein with hammer-head-shaped terminations against dolomitic boudins at left and right. Note two longitudinal veins to either side of transverse vein and bow-tie structure at right end of photograph, c. Nesting of boudins. Necks in one layer are adjacent to boudin fragments in adjacent layer. This arrangement allows a maximum shortening perpendicular to bedding of 33% in an ideal case of equi-dimensional beds and boudins, d. Field observations of boudin nesting show less than the theoretical limit of 33% shortening (= 0.6 stretch).

but we consider Wise's description more geo- that bow-tie veins are an amalgamation of mul- cm-thick boudinaged dolomitic bed, thereby metrically appropriate. tiple thin ladder veins. The other two margins of producing a characteristic hammer-head shape The bow-tie vein of Fig. 8a is approximately the bow-tie vein are concave toward the sur- when viewed in cross section (Fig. 8b). Compar- the same thickness as the boudinaged dolomitic rounding limestone beds and are highly irregu- ison of vein morphologies reveals that transverse bed (29 cm) at its margins, but it is only 14 cm lar; some are jagged, bent, or broken fracture veins are the end product of a progressive se- thick in its center. Calcite in the center of the surfaces with dolomitic debris, but others are quence that starts with multiple parallel ladder vein is blocky, and its crystal cleavage planes stylolitic against limestone. The thickness reduc- veins or en echelon vein arrays and includes show 10° to 15° of bending. Dolomitic rock tion in the vein center is in part a consequence of bow-tie veins as an intermediate step. Although fragments appear to be isolated in the calcite pressure solution of both vein calcite and adja- the bow-tie geometry is no longer obvious, vein-fill, but this may be a cut effect, because we cent limestone, and in part a consequence of comparison of the vein in Figure 8b with its were unable to find a fragment entirely sur- flow of the limestone beds into the vein region. neighbors suggests that it also opened in a direc- rounded by vein calcite in three dimensions. The The term "transverse vein" is used here to tion perpendicular to the boudin edges: in other lateral fracture surfaces of the dolomitic boudins describe the paradoxical width-greater-than- words, parallel to its current long dimension. As are coated with a thin layer of calcite fibers and length geometry of veins that occur between with the bow-tie veins, boudin edges are often are slightly concave toward the vein. At boudin many of the most widely separated boudin concave toward the transverse veins and razor corners, multiple fractures associated with nar- fragments of dolomitic beds (Fig. 8b). A typical sharp, whereas the bedding-parallel vein mar- row ladder veins are enlarged into wedge- transverse vein is <2 cm thick, bedding-parallel, gins are undulatory and stylolitic. Angular frag- shaped veins in which fractured bridges are extremely long (>100 cm), bounded on either ments of dolomitic rock occur throughout the preserved. Some wedge veins acquire a Devil's- side by stylolites, and terminates abruptly at transverse veins, indicating that the initial stages horn-type morphology (Fig. 8a), which suggests both ends against the fracture faces of a —30 of opening involved brittle fracture. Most bow-

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tie and transverse veins are approximately or- played by dissolution. The maximum boudin from individual boudin trains using Ferguson's thorhombic in symmetry, but some have differ- thickness T was assumed to represent the origi- method as opposed to equation 1 are significant;

ent geometries to their top and bottom stylolites. nal layer thickness T0, and the original layer a stretch of 1.22 is increased to 1.36; and a Some boudin edges are ¡bent to very different length LQ, was calculated by conservation of stretch of 1.46, to 2.01. These differences are degrees even though they started as opposite cross-sectional areas, accommodated by the thickening of limestone sides of the same fracture; in extreme cases, bedding in the hinges of folds relative to their limbs during the initial fiber-loading phase (Fig. transverse veins terminate in a fish-mouth bou- Lo=2Ab/T (2) din (Fig. 5d). These transverse veins are not iso- 9a), and the results suggest that hinge thickening

lated in occurrence; similar but narrower veins where XAb is the cumulative area of boudins as may contribute somewhat less than 50% extra to occur between boudinaged 2- to 3-cm-thick do- above (compare with Voight, 1987). This meth- the stretch in the direction of the fold axial trace. lomitic beds of the Conestoga Formation in the od of calculation increased stretch values by 20% Lengths measured perpendicular to the axial H. R. Miller quarry near Millersville, southwest on average. Even greater increases are possible if trace are shortened by the flexural slip process; of Lancaster, Pennsylvania, where they have the maximum boudin thickness observed in the therefore, there is no reason to invoke overall been folded around late meter-scale folds. deformed state, T, is less than its original value area change, especially since neither veins nor To- There is, however, no microtextural evi- stylolites are well developed in these regions. STRAIN ANALYSIS dence to suggest homogeneous ductile stretching of dolomitic layers prior to boudinage, and there DISCUSSION Strain from Boudinage is ample evidence that solution actually played a dominant role in shaping neck regions; therefore The formation of boudins, and the infilling of A combination of the methods outlined in equation 1 probably gives the best stretch cavities between them, most likely occurred dur- Ramsay (1967), Ramsay and Huber (1983), estimate. ing the fold amplification stage of the Lebanon and Voight (1987) was used to determine the Shortening perpendicular to layering was es- Valley Nappe. Vergence of asymmetrically total stretch of the boudinaged dolomitic layers timated by comparison of neighboring boudin folded stylolites in the micritic limestone beds and their matrix. The veins between boudin geometries with theoretical stacking patterns. changes around the major fold hinges, but it necks serve as excellent minimum displacement Shortening by 33% (that is, stretching to 0.66 of remains constant across boudin necks, indicating markers. Photographs of views down the boudin original thickness) is achievable by nesting of that boudinage occurred after commencement axes were taken, and boudin shapes were meas- boudins in neighboring layers as illustrated in of fold amplification. ured in the field. Boudin shapes were further Figure 8c. Field observations (Fig. 8d) suggest The initial stretching of the competent dolo- analyzed using enlarged photographs placed on that this extent of shortening has not been fully mitic beds occurred on clusters of ladder veins a computer digitizing tablet. For each boudin attained, and so we must place a limit of 0.66 on that have a statistically anticlustered distribution train, we measured Lbl, Tbl> Lb2, Tb2, Lb3, Tb3, the stretch perpendicular to bedding. Multiply- (Fig. 9a). This grouping is in part a cut effect etc., the lengths and maximum thicknesses of ing this value by the minimum estimate of through en echelon vein arrays that are parallel individual boudins, and LvI, Tvi, LV2, TV2, Lv3, bedding-parallel stretch, 1.60, gives a product of to boudin neck lines. Rothery (1988), following Tv3, etc., the lengths and minimum thicknesses 1.06, which represents the change in cross- on from Shainin (1950), Ramsay (1967), and of veins between boudins. Using a digitizing sectional area. Although this figure is not precise Beach (1975), distinguished en echelon vein sets tablet, we calculated £Ab, the cumulative area (second decimal places are conventionally formed as shear arrays from those formed as of boudins, and £AV, the cumulative vein area. quoted in strain analysis simply to avoid accu- extension arrays based on the vein-vein zone These values were used to find upper and lower mulation of rounding errors), it suggests that the angle and amount of vein overlap, among other bounds of L0 = 2Lb, the original length of the processes of pressure solution and veining are parameters. The en echelon vein arrays in stretched layer, and L = 2Lb+2Lv, its final approximately balanced and that the overall Rheems quarry have the geometry of Rothery's length (following Hossain, 1979, measurement cross-sectional areas of fold limbs are conserved. (1988) extension arrays. Many of these veins of L included half-vein lengths at either end of Our longitudinal strain estimates yield have bridges that curve in both directions rela- the boudin train). The stretch, S, of a rigid frag- stretches of 1.60 ± 0.40. This does not necessar- tive to the overall stepping direction of the array, mented object or layer, is calculated from the ily give the total strain history of the rock, how- a geometry that is also consistent with extension formula ever. Elongate objects and layers facing the arrays (Pollard and others, 1982; Nicholson and S = L/Lo (1) principal compression direction tend to be con- Pollard, 1985; Rothery, 1988). Other indica- tracted or to rotate rigidly in their ductile matrix tions of an extensional origin for the veins in- clude the occurrence of bridge fragments on the (compare with Ramsay, 1967, p. 52). A min- during early stages of deformation. Boudinage sides of widely opened veins, and fibrous calcite imum value was obtained by ignoring thickness begins when they enter the extension field of the rinds on boudin end faces. and area changes, on the assumption that the strain ellipsoid, and then only after matrix strain boudins were absolutely rigid and that all reduc- has allowed fiber-loading stress to build up on As the overturned fold limbs continued to tions in thickness in the neck regions were due to the object-matrix interface (Goguel, 1948; de stretch, the clusters of veins accommodated the pressure solution. This gave an average stretch of Sitter, 1964, fig. 215; Lloyd and Ferguson, stretch of dolomitic beds. The intervening slivers I.60 over the exposed length of a typical bou- 1981; Ferguson and Lloyd, 1984). Despite the and slices of rock in the vein clusters would have dinaged fold limb. Local stretches in areas of incorporation of Hossain's (1979) modification, acted like pillars to hold apart the limestone intense veining were much higher, reaching the stretch estimates obtained using equation 1 beds on either side. One fracture in each cluster values of S = 30 where trains of transverse veins generally represent only part of the rock's strain eventually became the dominant fracture, or were concentrated. Some layers, however, were history. neck vein, and it opened parallel to the extended as little as 28%. Ferguson (1981) devised a method of sequen- maximum extension direction which lay in or A larger value of stretch was obtained by as- tial restoration of boudinage which yields an es- close to the bedding plane (Fig. 9b). The remain- suming that thickness changes were due entirely timate of the strain suffered by the matrix during ing veins in the cluster were carried passively to particulate flow, with no significant role fiber loading. Differences between stretch results with the dolomitic boudin at this stage. The very

corners (Fig. 9b), which were further rounded by rotation of dolomitic rock fragments into the neck regions. The sources for the minerals precipitated in the low pressure necks between boudins ("parti- tions," Read, 1934) are generally thought to be in the surrounding less competent units (however, see de Sitter, 1964). The predominance of calcite in the Rheems quarry veins suggests that the limestone acted as the main source rock, an assumption supported by the ubiquitous stylolites in the quarry. First recognized by Sorby (1853) as surfaces of dissolution, stylolites occur along all bedding planes between dolomitic and limestone beds, and were probably initiated on local heterogeneities of bedding surfaces during sediment compaction (see review by Durney, 1978). In addition, pressure solution was extremely important in changing the boudin shapes. The thickest insoluble residues occur against boudin necks and wedge-shaped veins (Fig. 6a), and asymmetrical carbon-rich stylolites and slickolites occur on boudin flanks, which suggests that dissolution was concurrent with, and an integral part of, the boudinage process. Our strain measurements support this assumption and indicate that the amount of veining and pressure solution was approximately balanced, with little or no change in cross-sectional area of the overturned fold limbs. c There are several published calculations of the amount of material removed in solution from dolomitic boudin rounded boudin corners naturally deformed rocks (for example, Ramsay and Wood, 1973; Wright and Piatt, 1982; Beutner and Charles, 1985). Most of these anal- yses were on slates or shales in which estimates of volume loss were permitted by the presence of strain markers such as reduction spots, grapto- lites, etc. Accurate estimation of volume loss 10 cm was not attempted for the unfossiliferous Epler Formation rocks in Rheems quarry because of Figure 9. Model for the development of transverse veins in overturned limbs of the Lebanon the lack of suitable strain markers. For meaning- Valley Nappe folds, a. Layer-parallel extension of overturned beds produces statistically anti- ful volume loss calculations, it would be neces- clustered distribution of en echelon fractures. Hinge thickening in limestone beds contributes sary to obtain original unaltered material of the less than 50% extra stretch, b. One fracture becomes the dominant neck vein. The remaining same initial composition as the now stylolitic fractures are carried passively with the boudin. Bow-tie veins form as wall-rock limestone layers, and these we were unable to obtain. flows into boudin neck. Wedge fractures open and allow boudin ends to become concave. Pressure solution between limestone and vein fill commences. Dolomitic boudin shapes are Several of the dolomitic beds have one planar modified by rotation of rock slivers at corners and by stylolites along dolomite-limestone side and one undulatory, boudinaged margin interfaces, c. Continued extension and separation of boudins, with pressure solution between with clustered veins in the boudin necks. Coe limestone and calcite vein, result in transverse vein morphology. Bridges of dolomitic rock (1959) described one-sided boudinage from the collapse and may help to hold transverse vein open. Variscan fold belt of southern Ireland, but attributed the structure to competence differences between the layers on either side of the boudinaged layer. He also observed truncated veins large size (> 10 cm) of many of the calcite crys- which we interpret to be collapsed bridges. A in neck regions which he considered the result of tals in wide neck veins, the dearth of crack-seal similar conclusion was reached by Jones (1959) out-of-section shear displacement along bedding textures except as narrow rinds on some boudin who suggested that open neck cavities between surfaces. Comparison of his illustrations with the end faces, and the significant flow of micritic boudins of hornblende gneiss from the Vernon structures we have mapped, however, strongly limestone into adjacent boudin necks, all attest area, British Columbia, were implosively filled suggests that pressure solution rather than dis- to the sustained presence of open, fluid-filled with wall-rock fragments. Flow of the limestone crete offset may have been responsible for vein cavities. In addition, many of the transverse beds into boudin neck regions caused wedge- truncation. Mullenax and Gray (1984) de- veins contain angular fragments of dolomite shaped fractures to open up near the boudin scribed the occurrence of what they termed

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"stylo-boudins" in hummocky cross-bedded standing of the bow-tie and flow folds that form layers, and between limestone and calcite in limestone layers within calcareous mudrock in between boudin necks but have not addressed boudin necks, the Martinsburg Formation of southwest Vir- the role of solution and reprecipitation in boudin (2) an obvious connection between veining ginia. They showed that the gentle pinch-and- formation. Ramberg (1955) and Griggs and and pressure solution adjacent to calcite veins in swell morphology of some boudins was slightly Handin (1960) have demonstrated the tendency the neck regions of boudins and the subsequent modified by pressure solution seams within the for competent dolomitic units at 300 °C and 5 final shape of the boudins themselves, and boudins in neck regions, but there was no indi- kb under experimental strain rates to fracture (3) curved bridge structures in the overlap cation of significant pressure solution along the into angular boudins in extension while the sur- regions of en echelon veins that help to accom- interface between the boudinaged layer and its rounding limestone flowed into the intervening modate the extension. matrix. They suggested that hummocky cross- gaps. There are few available constraints on the All of the different boudin and vein morphol- bedding was necessary for subsequent localiza- deformation conditions prevalent during bou- ogies in Rheems quarry were formed entirely in tion of pressure solution. We, however, have dinage of the dolomitic beds on the lower limb the brittle field by a combination of cataclastic found no evidence for hummocky cross-bedding of the Lebanon Valley Nappe. Bent, large (> 10- flow, pressure solution, and veining. Because in Epler Formation beds that contain one-sided cm diam.) calcite crystals in the veins are not pressure solution has concentrated along litho- boudins. We recognize that one-sided boudinage recrystallized, nor are phyllosilicates in impure logical boundaries, its contribution to the total structure is compatible with an origin by me- limestone beds. Folded bridges of dolomitic strain could easily be overlooked, the most strik- chanical necking alone, as suggested by Coe rock were deformed entirely by cataclastic flow, ing features of these rocks being the recumbent (1959), if competence contrasts are extremely and the rounding of boudin corners to produce folds, boudins, and calcite-filled veins. Failure to high across the straight side of the stiff layer, and pinch-and-swell and fish-mouth structures oc- recognize the contribution of pressure solution that hummocky cross-beclding may indeed play curred by a combination of cataclasis and pres- to boudin shape could lead to a substantial over- a role in the localization of boudin necks. It is sure solution. These features suggest that under estimate of the amount of extension involved in also possible, however, to produce one-sided subgreenschist-grade conditions, although the the deformation, and to erroneous assumptions boudins by stretch accompanied by pressure so- micritic limestone could flow plastically, the do- regarding volume change at all metamorphic lution only along the undulatory side of the lomite could not. grades. competent bed. There is little available informa- There are examples in the literature of struc- There is an important implication for the tion on the exact amounts of clay, carbon, or tures that are geometrically similar to the trans- study of regional tectonic patterns. It is relatively other insoluble material needed for pressure so- verse veins in the Rheems and Millersville easy to recognize transverse veins in low-grade lution to commence in dolomitic rocks, but the quarries, but all of them involve high-tempera- rocks, especially when they terminate in noncarbonate content may need to be 10% or ture deformation, and none of them involves hammer-head-shaped areas at boudin edges. At more (Schweitzer and Simpson, 1986). There- pressure solution as a deformation mechanism. higher grade, however, such veins may easily fore, variations in the degree of pressure solution Holmquist (1930, 1931) described quartz-filled become isolated due to wide separation of bou- could reflect a difference in initial composition ladder veins between skarn rock boudins sur- din blocks. Polyphase deformation may further across the bed. rounded by leptite from Persholmen, Sweden. obscure the transverse nature of veining, leading Flow of limestone into boudin necks, together He suggested three developmental stages— unsuspecting field geologists to assume a stan- with pressure solution between the limestone extension and fracturing of the skarn rock, flow dard, bedding-perpendicular opening mecha- and the calcite vein-fill, formed the bow-tie vein of leptite into neck regions, and filling of quartz nism. Such isolated transverse veins may appear geometries. These veins preserve only indirect veins, but he did not consider pressure solution to require a separate phase of extension perpen- evidence for their origin in clusters of fractures: as contributing to the final boudin or vein dicular to their long dimensions in the strain spikes or wedge fractures on bedding-parallel shapes. Jones (1959) described boudins of history of the rock, whereas they actually contacts, the narrow slivers and broken bridges hornblende gneiss separated by transverse peg- formed by extension parallel to their final long of dolomitic rock parallel to their sides, and the matite veins in a quartz-biotite gneiss host rock dimension, like elongate mineralized pressure angular dolomitic shards scattered around their from the Vernon area, British Columbia. His shadows or dynamically recrystallized tails that margins. With further separation of the dolo- structures are geometrically similar to the trans- are often associated with porphyroblasts. When mitic boudins, pressure solution along the inter- verse veins described here, but he attributed veins are elongated in the same direction as face between the surrounding limestone and the them to ductile flow at high temperature. Our boudinage, it is essential to check for evidence of calcite vein-fill reduced vein thicknesses to 2 cm finding that pressure solution is an important transverse versus longitudinal morphology be- or less and transformed their geometry from contributing mechanism during transverse vein fore drawing conclusions about the causal stress bow-tie to transverse form (Fig. 9c). Although formation in low-grade rocks leads us to suggest regime. The critical evidence is the development boudin fragments have been seen as far apart as that similar solution transfer mechanisms may of stylolitic, as opposed to fibrous, vein margins. 150 cm, the upper and lower bounding lime- also be involved in the higher grade structures. stone beds never meet except at stylolitic high ACKNOWLEDGMENTS points. Fragments of failed dolomitic rock pillars CONCLUSIONS and undulating stylolite surfaces may have Thanks to Roger Faill, Iain Allison, and Don served to hold transverse veins open and pro- The significant aspects of the geometries of Wise for discussions, Pennsylvania Geological duce their irregular shape. Note that no exten- the boudin, stylolite, and vein associations that Survey for financial support, and the Rheems sion of the veins themselves is invoked in this we observe in Rheems quarry are quarry manager for granting access. The manu- explanation of transverse vein morphology. (1) thickly residue-coated stylolitic surfaces script was improved by the reviews of Ed Experimental studies have aided our under- between limestone and boudinaged dolomite Beutner and Steve Laubach.

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Primed in U.S.A.

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