Cleavage in Some Sedimentary Rocks of the Central Valley and Ridge Province, Maryland

Home , Cleavage (geology), Fracture (geology)

P. A. GEISER Department of Geology, University of Connecticut, Storrs, Connecticut 06268

ABSTRACT Limited field evidence suggests that the preferred orientation shown by burrow cross sections may be controlled by paleocurrent The Cacapon Mountain anticline of the folded Appalachian directions, making the burrows a possible current indicator. Fi- Mountains contains rocks ranging in age from Early Silurian to nally, it is suggested that the fabric imposed by the burrowing or- Early Devonian. The Middle Silurian Bloomsburg Formation ap- ganisms may have localized and controlled the development of the proximates a thin, viscous plate embedded in a less viscous Sp cleavage. Key words: structural geology, finite strain, Silurian medium. The presence of scolithuslike burrows, useful as strain ichnology. markers, makes the Bloomsburg Formation an excellent unit for studying the folding of a sedimentary plate. INTRODUCTION Examination of the microscopic fabric of rock containing a so- This paper analyzes the deformation of some unmetamorphosed called "fracture cleavage" reveals the presence of discontinuous, sedimentary rocks in the folded Appalachian Mountains. Although platy zones containing tectonically oriented quartz and mica frag- numerous strain studies have been made in metamorphic terranes, ments. It is deduced that the shape and orientation of the quartz strain analysis has only infrequently been applied to sedimentary fragments is largely due to differential solution; however, there is rocks that have not undergone regional penetrative deformation some evidence of mechanical rotation of the fragments. As the (Nickelsen, 1966; Breddin, 1956; Plessman, 1966). "fracture cleavage" does not fit the classical definition (being lo- The Bloomsburg Formation in the Cacapon Mountain anticline cally penetrative), it has been designated Sp cleavage for the pur- is a thin (15-m) plate of siliceous siltstone, embedded in a thick se- poses of this paper. quence of incompetent sedimentary rocks. This is physically Finite strain analysis shows that the cleavage postdates analogous to a thin, viscous elastic plate embedded in a less viscous lithification, that it formed perpendicular to bedding and parallel to medium. The Bloomsburg Formation also contains natural strain the plane of the finite strain ellipsoid containing X! and X2, and that markers in the form of small vertical burrows, making it an excel- it is a plane of flattening. The Sp cleavage is interpreted as a lent unit to use in a deformation study. phenomenon marking a transition point in the material behavior of The Hancock region of the Cacapon Mountain anticline (Stose the rock. The transition point was reached after the rock under- and Swartz, 1912, p. 129-131; Swartz, 1923) is located im- went a maximum two-dimensional irreversible strain of e = 0.06. mediately west of Hancock, Maryland (Fig. 1). The anticline lies in This small strain is in contrast to the values of e = 0.30 that have the eastern half of the Valley and Ridge province of the central Ap- been suggested for slaty cleavage. During progressive deformation, palachians and west of the "tectonite" front (Fellows, 1943). the Sp cleavage became structurally passive, permitting it to be used Geologic mapping of the region was done by Stose and Swartz as a strain marker in the determination of strain about fold profiles (1912), and Swartz (1923, p. 152) did detailed stratigraphy. (ac surface). Rocks ranging in age from earliest Silurian to Early Devonian are

Geological Society of America Bulletin, v. 85, p. 1399-1412, 19 figs., September 1974

1399

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Study Samples containing burrows were collected at 17 localities (Fig. Area P*• 2). Where a fold is well exposed, three samples were collected from KWJH anco c k the same bed — a sample from each limb and one from the crest of the fold. At all other localities only one sample was collected. Each sample was cut in three perpendicular directions. The initial cut was perpendicular to the cleavage and approximately parallel to bedding (a in Fig. 3). The second cut was perpendicular to W. Va. both bedding and cleavags (b in Fig. 3), and the third cut was parallel to cleavage and perpendicular to bedding (c in Fig. 3).

DESCRIPTION OF CLEAVAGE IN THE BLOOMSBURG FORMATION The Bloomsburg and lower Wills Creek Formations show a consistent cleavage as a conspicuous plane of parting, roughly perpendicular to the bedding (Fig. 4). This cleavage, frequently as promi- Figure 1. Location of study area. nent as bedding, has been mentioned only briefly in the literature on the central folded Appalachians. exposed in the anticline. The lower units consist of shale and Gair (1949, p. 30) described it as "appearing to be fracture fine-grained sandstone. The rocks become increasingly calcareous cleavage." Stevens (1959, p. 25—30) referred to "radial joints nor- upward, and the highest units are limestone. A generalized strati- mal to bedding," attributed to early tensional stresses. Cloos graphic column is given in Table 1. (1951, p. 156) described the structure simply as "cleavage normal The structures exposed in the western Maryland railroad cuts at to the bedding and fanning through a large angle." Roundtop and Woodmont have been described by Gair (1949), Both the fanning nature of the cleavage and its occasional sig- Cloos (1951), and Stevens (1959), but there has been no systematic moidal form have been described by Stevens (1959) and Cloos study of strain or use of strain markers. Because the rocks are in the i'1951). Cloos suggested that the cleavage formed early and possi- early stages of deformation, they retain an almost complete strain bly prior to folding. Gair also implied an early origin for the cleav- history. age, finding that the "fracture cleavage" in the Wills Creek induced Finite strain was measured in 20 samples from 17 different no preferred crystallographic orientation. The lack of preferred localities (Fig. 2). X-ray photographs were made of 15 samples to orientation suggested to Gair that the cleavage may have formed reveal internal structure. prior to consolidation. Field Methods. Mapping was done on parts of the two W2' A second type of cleavage found in the Bloomsburg Formation is quadrangles (U.S. Geological Survey, 1951a, 1951b), enlarged to a restricted to the shaly unirs and is characterized by a small angle, scale of 1:6,156. A total of 837 stations were made over the 32.7 between 5° and 45°, made with bedding (B-3 in Fig. 4b). This 2 km mapped, with three to four measurements made per station. In cleavage has been designated Sa. The so-called "fracture cleavage," an effort to utilize as much of the generally poor exposures as pos- approximately perpendicular to bedding, is designated Sp. The two sible, traverses were made close to one another with some lines cleavages never occur in the same bed. only 164 m apart. TABLE 1. GENERALIZE: STRATIGRAPHIC SECTION Sp CLEAVAGE AT HANCCCK, MARYLAND* Macroscopic Description Age Formation Thickness Llthology Or) The Sp cleavage is a set of prominent subparallel irregular surfaces (Figs. 4a, b), which are usually 1 to 2 cm apart. Examination Oriskany Group 65.5 Massive orthoquartzite; thin, discontinuous cherty layers at base of the bedding surfaces shows a faint lineation parallel to the Helderberg Formation 52.4 Thin- to massive-bedded limestone cleavage-bedding intersection designated Lx. with occasional shale interbeds or scattered chert layers One of the most consistent characteristics of the cleavage is its angle with bedding. The angles between cleavage and bedding Keyser Limestone 95.} Massive, nodular limestone at base; poles from 130 stations were measured on a stereographic net, and grades upward to thin-bedded limestone with considerable shale the sets were plotted on ?. frequency diagram (Fig. 5). Half the Tonoloway Limestone 132.0 Thin- to medium-bedded laminated angles fall between 82° and 90°; the remainder are scattered over IC limestone with platy appearance the intervals between 58° and 82°. Wills Creek Shale 147.0 Limestone interbedded with calcareous shale, sandstone,and mudstone The Sp cleavage ends abruptly at the base of the Bloomsburg Bloomsburg Formation 14.7 Massive red siltstone with a few Formation and gradually disappears upward as the siltstone be- yellow and red shale and sandstone interbeds comes progressively more calcareous and less burrowed.

The close correlation between burrowing and Sp cleavage is visi- McKenzie Formation 75.3 Dark calcareous shale and thin argillaceous limestone ble at a number of exposures in a fine-grained sandstone member of Rochester Shale 9.£5 Dark calcareous shale the lower Bloomsburg Formation. The cleavage, well developed in Keefer Sandstone 8.1 Fine- to medium-grain quartz sand- adjacent beds, is completely lacking in the unburrowed sandstone. stone, thin- to medium-bedded Figure 4 shows this relation clearly; S cleavage in burrowed silt- Dark shale with scattered interbeds p of thin- to medium-bedded fine stone ends at the edge of a i>ed (B-3 in Fig. 4b) consisting of inter- Rose Hill Formation 196.5 sandstone bedded shales and sandstone lenses.

Thin- to thick-bedded orthoquartzite Tuscarora Formation 124.5 with scattered interbeds of thin Microscopic Description shale Although cleavage is visible in all hand specimens, its develop- * After Swartz (1923Ì. ment varies considerably on the microscopic scale. The framework

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of the Bloomsburg siltstone consists of more than 95 percent quartz 42) and Ramsay (1967, p. 180) hypothesized mechanical rotation clasts. Three types of quartz grains occur: as a mechanism to produce dimensional orientation, although 1. Well rounded to subangular grains: These are generally the neither offered supporting data. The grains shown in Figure 7, as largest and have well-defined borders. They are typically found well as many other similar examples in the Bloomsburg, are felt to where hematite is almost absent. The grains show no preferred be strong evidence that mechanical rotation is an important process orientation or other indication of postdepositional alteration. They in early cleavage formation. are interpreted as being the original clasts. The cuspate grains are thus interpreted as forming by a combina- 2. Grains with highly irregular borders and embayments: These tion of mechanical and solutional processes. Clasts are broken up grains are also among the larger size fraction but include many by brittle fracture, the fragments are pulled apart, and synkine- which are less than 0.15 mm. Grains of this type occur only in areas matic layer silicates form parallel to the stretching direction (Fig. of hematite cement and may have reacted both chemically and 8). Their final cuspate form is the product of solution on faces ap- mechanically with the hematite. The highly irregular and diffuse proximately perpendicular to the maximum principal compressive borders suggest partial replacement of the quartz by hematite. Two stress (see Durney, 1972; Williams, 1972, p. 41). The dimensional observations suggest that these grains have also undergone intra- orientation of the cuspate grains is thus a product of both a granular deformation as a result of their reaction with the hematite: mechanical rotation and differential solution of the clast fragments. (1) almost all grains in the Bloomsburg Formation that have deformation lamellae (Tullis and others, 1973, p. 300) are cemented Alignment Zones and Patches by hematite; and (2) occasional grains appear to have been par- The alignment zones are subparallel strips in which almost all the tially shattered by the hematite. Observations similar to the second elongate grains are approximately parallel to one another and to one have been made by Smith (1948, p. 218-225) in his study of the trend of the zone (Figs. 6, 7). The degree of alignment varies ferruginous concentrations in sand and sandy clays. within a specimen as well as from thin section to thin section. The 3. Elongate, cuspate grains, generally less than 0.2 mm in alignment zones were found in all siltstones with cleavage. In gen- length, aligned parallel to one another and to secondary clay min- eral, the greater the number of zones per thin section and the erals. Although most grain boundaries are well defined, a few are greater the degree of alignment, the more prominent the cleavage is diffuse. This grain type most frequently occurs in narrow zones in in hand specimen. which almost all the grains are elongate and oriented with their longest dimension (a axis, dimensional) parallel to the zone border. Alignment Zone These characteristics have suggested :he term "alignment zone" for the structure (Figs. 6 and 7). In addition to the zones, irregular patches of preferentially oriented cuspate grains are also found. These areas have been termed "alignment patches." About 50 percent of the cuspate grains show undulatory extinction, but less than 5 percent of the grains show deformation lamellae. Williams (1972), working with low-grade metamorphic rocks, has described structures very similar to the alignment zones; he called them "A domains." He has also noted the presence of two types of quartz grains: detrital grains and "corroded" grains that show a strong preferred orientation and occur in mica-rich layers. These grains are identical to types 1 and 3 described above. Figure 7 shows three fractured clasts (grains 1, 2, and 3), which separated parallel to the stretching direction indicated in the figure. Grains 4, 5, and 6 show the characteristic cuspate form produced by solutional processes. Although the fractured clasts can be recon- Figure 3. Schematic diagram showing relative orientation of structures and sections of samples; a, parallel to bedding and perpendicular to S cleavage; b, perpen- structed, they show evidence of mechanical rotation—they have p dicular to bedding and Sp cleavage; c, parallel to Sp cleavage and perpendicular to bed- close but not identical optical orientation. Both Williams (1972, p. ding.

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Figure 4a. S„ cleavage in Bloomsbur; Formation. Roundtop cut. Bed B-3 outlined.

Note that Sp cleavage does not penetrate bed B-3 containing Sa cleavage.

The zones are discontinuous, generally splaying out into the sur- The grains were divided into two size groups, those more than rounding rock. The larger zones are from 1 to 2 mm wide and from 0.15 mm long and those less than 0.15 mm long. Two plots of the 1 to 2 cm long. The spacing of the zones varies: some thin sections data were made: angle versus location (Fig. 9) and ale versus loca- 2 cm wide show no strongly a_igned zones at all, whereas others tion (LS, AZ, RS — Fig. 10). The rose diagrams of angle versus lo- have several within 1 cm of each other. cation and the graph of ak versus location show that (1) in most The degree of dimensional alignment of the grains is not as read- cases, the preferred orientation increases as the center of the zone is ily visible in all sections. The highest degree of alignment is seen in approached, (2), the ale ratio of all grains increases as the center of

sections parallel to bedding and perpendicular to Sp (section a, Fig. the zone is approached, and (3) the smaller grains show a better 3). A slightly weaker alignment, with less well defined zones, is seen degree of alignment than the larger ones. Although the larger grains

in sections perpendicular to both Sp and the bedding (section b, Fig. are strongly aligned only in :he zones, fine silt, glauconite, and seri- 3). No alignment is seen in sections cut in the plane of Sp (section c, cite are aligned parallel to the trend of the zones almost everywhere Fig. 3), indicating that the alignment zones are essentially tabular in they occur. Some of the aligned platy minerals are clearly the result shape. Since a axes of the cuspate grains within the zones are the of synkinematic crystallization (Fig 8), but mechanical rotation of same size in both sections a and d, the shape of the grains is that of detrital clays cannot be ruied out. The preferred orientation of an oblate ellipsoid. these constituents is best seen in the scattered clay patches. An alignment zone in sample C (Fig. 2) was studied in detail. Another characteristic of the alignment zones is the rapid in- Three traverses were made with a mechanical stage across the zone crease in the amount of fine-grained matrix from the sides of the shown in Figure 6. The locations of the traverses is indicated on the zones to the centers. This sudden change in the ratio of matrix to figure. The parameters measured were (1) angle between the a axis framework helps to make the zones and patches visible (Figs. 4, 5). of the grain and the trends of the zone (Fig. 9); (2) axial ratio, ale, The alignment patches are distinguished from the zones by both of the grain (Fig. 10); and (3) distance from the center of the zone. the irregular shape of the alignment area and the lack of orientation

Figure 5. Frequency diagram of number of readings (total, 127) versus angle between bedding and S„ cleavage. Data collected at 68 stations in Bloomsburg Forma- tion.

of the larger components. The patches are apparently irregular, but their exact shape could not be determined because the borders are diffuse and the alignment itself can only be seen at high magnifications. Extension fractures that lie at about 80° to the cleavage are found in some samples (Fig. 11). These fractures are filled with elongate quartz grains showing undulatory extinction. The new quartz is synkinematic: it grew on the ruptured detrital nuclei and progressively filled the fractures growing between the fragments. This behavior indicates that the clasts were already cemented at the time of fracture. Because the alignment zones transect (Fig. 12a) and thus postdate the extension fractures, the zones must have been active following lithification. Unfortunately, the evidence shown in Figure 11 does not demonstrate conclusively that the zones were initiated after lithification. The anastomosing morphology and the greater solutional effects in the centers of the alignment zones indi- cate that the zones grew with time; thus it is possible that initial solutional activity may have started prior to lithification.

Summary of Effects of Strain on Fabric Strain effects have been both mechanical and chemical. Present evidence suggests that solutional processes have been dominant; however, the relative proportions of each process are not now known. Mechanical effects have taken the form of fracturing and displacement of grains. There is evidence that there has been some bodily rotation of the grains, as well as displacement. There is little evidence of intragranular deformation. Solutional effects are strongest in the alignment zones and have Figure 6. Alignment zone in Bloomsburg siltstone. Section cut perpendicular to been primarily responsible for the shape and orientation of the cus- cleavage and parallel to bedding. (Section a, Fig. 3.) pate quartz grains. Recrystallization in the form of synkinematic glauconite, sericite, and quartz is pervasive. Brace (1961) has mentioned the possible use of rotated scolithus tube as a strain measure in an artificial example that he devised. DETERMINATION OF FINITE STRAIN Several attempts have been made to utilize vertical burrows in Type of Strain Measured strain studies. Only one (Brace, 1961, p. 1073) allowed calculation The elongation and shortening of the tube cross sections record of the principal strains. Plessman (1966) used burrows to deter- strain developed in the bedding plane. If bedding is a principal mine the percent of shortening, following procedures outlined by plane, two of the three strain axes can be measured. Cloos (1947, p. 856-867). Wise (I960, p. 63-66) used scolithus The whole-rock strain in the Bloomsburg Formation can be di- simply to elucidate strain history. vided into two components, strain in the alignment zones and Plessman (1966), using the U-shaped burrows in the graywackes strain outside the alignment zones. The strain recorded by the bur- of the Harz Mountains, determined percent of shortening by row cross sections represents the second component and is a small measuring the ellipticity of the present burrow cross sections. He value. Although the largest strain occurs within the alignment assumed that the initial cross section of the burrow, as seen on zones, the zones represent less than 5 percent of the present total bedding surfaces, was circular. He also noted that the vertical bur- rock volume. rows had been systematically "tilted" out of the perpendicular by folding, but no attempt was made to use this information as a Geologic Strain Markers measure of strain. An attempt has been made to measure finite strain by using Wise's (1960) study at Chickies Rock, Pennsylvania, on the rela- geologic strain markers. Such a marker should fulfill the following tion between cleavage and scolithus tubes, showed that the tubes lie criteria: (1) it should be abundant; (2) the mechanical properties of in the plane of the cleavage and suggested that this indicates that the marker should be similar to those of the host rock (for example, the cleavage was originally perpendicular to the bedding. Only calcareous oolites in limestone); and (3) the initial form of the

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Figure 7. Photomicrograph of alignment zone showing brittle failure and separa- Figure 8. Synkinematic quartz; growth elongation parallel to trend of S„ cleavage. tion of quartz clasts. Clasts 1, 2, and show separation. Grains 4, 5, and 6 show cus- Section perpendicular to Sp and approximately parallel to bedding. pate form. marker should be known. As will be shown, the physical properties a means of emphasizing the contrast in order to measure the shapes of the burrows fulfill these three criteria. of the cross sections. Etching the slabs in a concentrated solution of hot KOH was found to increase the contrast considerably. The DESCRIPTION OF VERTICAL BURROWS etched slabs were then photographed with contrast copy film and Burrows weather out on bedding and cleavage surfaces with printed on high-contrast paper. varying degrees of relief or as a faint discoloration. At some locali- The etched slabs, cut perpendicular to Sp cleavage, show three ties, relief on the bedding is as much as 1 cm (Fig. 13). The discol- types of burrows: individual (Fig. 14a), cluster (Fig. 14a), and mi- orations on cleavage surfaces are narrow bands, slightly darker gratory (Fig. 14b). than the surrounding rock, except in the more calcareous units of Cluster types consist of roughly circular to elliptical groupings of the Bloomsburg Formation, where they appear as yellow-green individual burrows. "Migratory" refers to a sequence of overlap- weathering, irregular stringers. The burrows are approximately ping burrow sections, created as the position of the burrow was perpendicular to bedding. moved along the bedding surface. Burrows as much as 50 cm long are found, in cross section, they Most specimens contair all three tube types, but no specimen may have a maximum apparent diameter of as much as 2 cm. was found consisting of only one or two; however, a few samples Etched slabs, however, showed these very large "burrows" to be from the more calcareous units of the Bloomsburg contain migra- clusters of smaller ones whose principal diameters are only 0.1 to tory tubes almost to the exclusion of other types (Fig. 13). 0.2 cm (Fig. 14). In thin sections cut approximately parallel to bedding, the burrows appear as circular to elliptical patches of quartz clasts that are Textural Relations of Burrows both more rounded and slightly larger than those outside. The Because there was insufficient contrast between the material framework in the burrows is also more open than that of the sur- forming the burrow and that of the matrix, it was necessary to find rounding sediment.

ORIENTATION OF LONG AXES OF QUARTZ GRAINS

TRAVERSE 1

^F3 18 a

•+ TREND OF ALIGNMENT ZONE

TOTAL NUMBER

OF G RAI N S 34 14 f 10

—3

24 24

TRAVERSE 3

Figure 9. Rose diagrams of orientation of long axes of quartz grains in alignment zones. (See Fig. 6 for locations of traverses.) Traverse 1: Grains less than 0.15 mm: a, left-hand border of alignment zone; h. alignment zone; c, right-hand border of alignment zone. Grains greater than 0.15 mm: d, left-hand border of alignment zone; e, alignment zone; f, right-hand border of alignment zone. Traverse 2: Grains less than 0.15 mm: a, left-hand border of alignment zone; b, alignment zone; c, burrow in alignment zone; d, right-hand border of alignment zone. Grains greater than 0.15 mm: e, left-hand border of alignment zone; f, alignment zone; g, right-hand border of alignment zone. Traverse 3: Grains less than 0.15 mm, locations same as traverse 1.

An outline of the textural relations and general properties of the There is little internal structure in this section other than a tendency surrounding rock is given in Table 2. of the clasts nearest the burrow border to parallel it. Migratory Burrows. Cross sections of the migratory tubes show Internal Fabric an arcuate array of the clasts across the tube. The borders of the Individual and Cluster Burrows. Thin sections perpendicular to migratory tube sections are distinct and regular. the axes of the burrows show no well-defined internal structure. A Sections cut parallel to the burrow show that although the bur- few burrows show a crude arcuation of clasts. Most of the burrow axis is generally straight, the borders are highly irregular and rows' cross sections show a weak alignment of the longs axes of the somewhat diffuse. There is also a tendency for the upper part of the clasts parallel to the sides of the burrow. burrow to be inclined away from the general burrow trend. An ar- Sections parallel to the axes of the burrows show them to be es- cuate array of the clasts also occurs in this section. The arcuation sentially straight, with regular sides and well-defined borders. is concave upward, and the morphology of the tubes in general is

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Traverse 1 Grains -=.75mm Grains ».75mm similar to that produced by Sabellariids (Kirtley and Tanner, 1968), suggesting that the burrows may have been constructed in a similar way. 3.0 . The migratory type of burrow suggests an organism that begins by excavating a burrow downward, perpendicular to a surface. After burrowing verticr.lly for some distance, the organism causes the upper part of the burrow to migrate laterally, and the final posi- a/c tion of the burrow makes a large acute angle with the surface. The trend of the migration and the direction of the long axis of the bur- 20 . . row cross sections suggsst environmental control. Tube migration tends to be perpendicular to the direction of the long axis of the tube section (Fig. 14b).

Occurrence of Burrows J ' . ' In the Bloomsburg Formation, burrows are absent in the fine, 1 0 I ' ' .'' '' ."-.' I ' ' ' texturally mature sands of the middle units and the interbedded shales and sands of the basal units. The burrows are most common LS AZ RS is m RS near the tops of the massive siltstone beds, and they increase in numbers toward the top of the formation. The burrows are also Traverse 2 Grains -.75 mm Grajns , j5m[n sporadically common in the massive calcareous siltstone of the 3.0 lower Wills Creek Formation. Burrows were found at all outcrops of the Bloomsburg Formation I visited. On the basis of the foregoing textural and geometric properties, it is concluded that the burrows fulfill the strain marker criteria.

a/c FINITE HOMOGENEOUS STRAIN FROM TUBE CROSS SECTIONS 2.0 No assumptions about initial shape of the tubes are necessary to • I • • permit strain measurement. Several workers have dealt with the - • « . i : - ' • • problem of strain determination from nonspherical objects that have a primary fabric (Cloos, 1947; Ramsay, 1967; Elliott, 1970), and they have developed methods for strain measurement. . • - . v ;..'.' I • " 1.01-.; • •• ,"'•'.'•'. '•. ; In this study, strain measurements from tube sections have LS AZ RS LS AZ RS been made using the methods developed by Elliott (1970, p. 2221-2236). Two conditions are necessary to do this: there must Figure 10. ale ratios of quartz grains in alignment zone. LS = left side of zone; AZ be finite homogeneous strain on the scale of the sample, and it must = alignment zone; RS = right side of zone. Locations of traverses shown in Figure 6. be possible to determine the shape of the undeformed distribution on the "shape factor grid" (Elliott, 1970, p. 2225). Briefly, the procedure for strain measurement is as follows: 1. Measurement of the angle a between the principal axes and an arbitrary line (Fig. 15). The line chosen is parallel to the trend of

Sp (Lx) measured in a plane perpendicular to Sp and approximately parallel to bedding (section a in Fig. 3). 2. Measurement of the principal (a) and least (c) diameters of the sections (Fig. 15). 3. Calculation of the shape e = In ale. 4. Plotting the points («, 2a) on polar coordinates. The position of the point is determined by the value of 2a and e measured for each cross section; thus, each point represents a single burrow cross section. An example of a measured sample with the burrow cross sections plotted on polar coordinates is shown in Figure 16. 5. The set of points is plotted on the strain grid by superimpos- ing the grid on the polar coordinates, the origin of the strain grid coinciding with the origin of the polar coordinates. 6. The strain is removed by returning the deformed distribution along the strain grid to its undeformed shape. Figure 17 shows the strain points contoured by the Mellis method and the generalized shapes of the distributions. The data are summarized in Table 3.

Analysis of the Strain Data The plots in Figure 17 show two generalized shapes, termed (after Elliott, 1970, Table 1) "heart" and "banana," respectively. The principal problem in analysis of the data is the deduction of Figure 11. Synkinematic quartz, showing growth elongation at about 80° to exten- the undeformed distributicn shape of the burrow cross sections. sion fracture. Section cut perpendicular to bedding and cleavage. (Section b, Fig. 3.) The following distributions are possible:

Figure 12a. Alignment zone transecting extension fracture in Bloomsburg siltstone. Growth elongation of quartz clasts at about 80° angle to trace of extension fracture. Section cut perpendicular to bedding and cleavage. (Section b, Fig. 3.)

Case I. All burrows have circular cross sections, and the un- strained distribution shape is a point. Case II. Burrows range from circular to elliptical and have a perfectly random orientation. The plot for this case is a circle with its center at the origin. Case III. All burrows have elliptical cross sections but lack preferred orientation of their a axes. This case gives a ring-shaped distribution with its center at the origin and lacking any maxima. Case IV. Burrows range from circular to elliptical in cross section, and a axes show a preferred orientation. This results in a symmetrical distribution, which approaches a line with increasing degree of preferred orientation. To complete the analysis, the significance of three aspects of the geometric properties of the shape factor grid must be understood: (1) Distribution shapes lying near the center of the grid are subject to little shape change for comparatively large strains (Elliott, 1970, p. 2228); consequently, the shapes shown in Figure 17 are those of the initial distribution; (2) the fluctuation of all the shapes plotted is large (about 90°), indicating low strain values; and (3) none of the four cases mentioned above can be transformed by strain into any of the others; thus, their shapes are unique. Applying this information to the data shown in Figure 17 makes Figure 13. Burrows weathering in relief on bedding surface. Note apparent align- it clear that the initial distribution is that of case IV, with a ten- ment of burrows parallel to bedding-cleavage intersection.

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Figure 14a. Individual and cluster burrows. Photograph of etched slab. Section Figure 14b. Migratory burrow cross sections. Etched slab cut perpendicular to perpendicular to cleavage and approximately parallel to bedding. (Section a, Fig. 3.) cleavage and parallel to bedding. (Section a, Fig. 3.)

dency to approach case III (samples M and P), and that strain is a current-controlled vectorial property of the paleoenvironment. very small. Thus, the distribution shapes and the elliptical form of Somewhat similar deductions were made by Berg (1973, p. 137) the burrows themselves are not the result of strain, but must reflect regarding the burrowing upper Devonian pelecypod Archanodon a primary environmental control. sp. Berg concluded that "the orientation of burrow curvature and That the burrows may have initially had elliptical cross sections hinge impressions can be a useful vectorial feature in paleoenviron- is not at all improbable. In their study of Sabellariid worms, Kirtley mental analysis." and Tanner (1968) described an organism similar in occurrence Even though the distribution shapes are virtually undeformed, it and behavior to the Bloomsburg fauna. In particular, the Sabel- was decided to determine the maximum possible strain each sample lariid produce clusters of burrows with elliptical cross sections that could show. To do this, the location of the initial circle point is have a pronounced preferred orientation. Unfortunately, Kirtley found by locating the intersection of the broad base of the distribu- and Tanner did not suggest a cause for the orientation. A possible tion and the line of symmetry (Fig. 16; see Elliott, 1970, p. 2225). mechanism is suggested by a few scattered measurements of current The amount of strain is determined for each case by returning the ripple crests in the Bloomsburg Formation in Maryland and Penn- init al circle to the origin. The line joining the initial circle point sylvania. (Unfortunately, current indicators are rare in this unit.) with the origin is the principal direction. Using the coordinate sys- These measurements show the crests to be consistently oriented at tem in Figure 16, calculation of two-dimensional finite strains gives about 90° to the general trend of burrow elongation, leading to the an average value of 6 percent and lies near the lower limit of mea- tentative hypothesis that the preferred orientation of the burrows is surement. The results are shown in Table 3.

TABLE 2. MICROSCOPIC FABRIC OF BURROWS IN BL00MSBURG FORMATION

Individual Cluster Migratory burrows burrows burrows

Type of siltstone in which burrow Low carbonate Low carbonate High carbonate

Weathered appearance Dark red Dark red Yellow-green

Principal diameter of burrow 0.1-0.3 cm 0.1-0.3 cm 0.3-0.6 cm

Type of material filling burrow Framework: 85 percent Same as Framework: 40 percent of total ; consists of individual of total; angular subrounded quartz silt quartz silt, 0.1 -1.75 1.00-2.25 mm diam mm diam, scattered Matrix cement: 15 per- Matrix: 60 percent of cent of total; consists total consists of of a 35 percent hematite microspar cement, 50 percent clay particles, 15 percent fine quartz and calcite grains

Type of material in which burrow occurs Framework: 80 percent Same as Framework: 75 percent of total; consists of individual of total consists of angular quartz silt angular quartz grains generally less than 0.60 ranging from 0.1 to mm diam. 1.75 mm diam. Matrix and cement: 20 per- Matrix and cement: 25 cent of total; consists of percent of total; Figure 15. Schematic diagram of etched surface cut perpendicular to Sp and parallel hematite and sericite with consists of quartz, to bedding, showing geometric elements of burrows measured for strain determina- small amounts of quartz. silt, and hematite Hematite and quartz are cement and detritus; tion. Trace of Sp on bedding = Lx. both cement and detrltal some areas silica grains cemented

+ 45 Note: Percentages visually estimated using diagrams of Terry and Chilingar (1955).

cause the cleavage contains the two stretching directions, it is a

plane of flattening containing \j and k2. Thus the Sp cleavage is a principal plane of the finite strain ellipsoid. Estimates of the value

of A.! and \2 have been based on reconstruction of disrupted clasts in the alignment zones. The maximum value measured in section a

(Fig. 3) was Xi = 1.5 and in section b, X2 = 1-3. This is a minimum value because in larger separations, solution and rotation of the fragments prevent measurements. The dimensional habit of the cuspate quartz grains in the alignment zones approximates that of an oblate ellipsoid with the plane of its circular section lying in the zones; this tends to reflect the shape and orientation of the finite strain ellipsoid deduced from stretching directions. Hara (1966, p. 132; Hara and others, 1968, p. 51-113) noted similar behavior in his studies of flexurally folded quartz layers in pelitic schist, in which the vector means of grain orientation parallels the direction of the principal finite strain axes. Mukhopadhyay (1973) further substantiated this property of

-45 quartz. In his study of deformed quartz grains, very similar to the cuspate grains of this work, Mukhopadhyay showed that both the Figure 16. Sample B of Figure 17, showing strain points and method of strain analysis. Contours show areas of 1, 3, and 5 circle overlap. dimensional orientation and the axial ratios of the dimensions ac- curately reflect finite strain. The plot of bedding cleavage angles in Figure 5 shows a uni- INITIAL ORIENTATION OF Sp CLEAVAGE AND modal distribution of the angles about 90°, with 28 percent of the ITS RELATION TO THE PRINCIPAL DIRECTION angles falling in the range 86° to 90° (measurement error, ±2°). This observation is of considerable importance because for irrota- X-ray and pétrographie studies of 11 slabs of the Bloomsburg tional strain, Harker's formula (see Ramsay, 1967, p. 233) shows

siltstone, cut perpendicular to both Sp cleavage and bedding (see that the amount of strain required to rotate a line into a principal Fig. 18a, b) show that the burrows parallel the Sp cleavage. None of plane becomes infinitely great as the plane is approached. This sug- the specimens showed any deviation. gests that the 90° angle between Sp cleavage and bedding is the ini-

The Sp cleavage is also parallel to and thus contains two stretch- tial angle, because there is no strain sufficiently large to rotate the ing directions perpendicular to each other; one parallel to the axes two planes (bedding and cleavage) into orthogonality. of the burrows resulted in the rupturing of the burrows perpendicu- Although in general one can rotate a given plane to any orienta- lar to their length (Figs. 18a, b, 19) and a second direction, best tion by any number of superposed strains, this case is felt to be

developed in the alignment zones, parallel to Lx (Figs. 7, 8). Be- highly unlikely in the rocks studied because (1) there is evidence for

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Mirror Trace Initial C i rcl e * Principal Direction

Figure 17. Results of strain measurement of burrow cross sections. Data plotted on polar coordinates and contoured by Mellis method with generalized form superposed. Contours show areas of 1,3, and 5 circle overlap.

t/-«. I

r i

I 1

Figure 18. Positive radiographs of slabs of Bloomsburg siltstone. Sections cut perpendicular to bedding and cleavage. (Section L>, Fig. 3.) Microboudinage (1) perpendicular to alignment zones (2); alignment zones parallel to burrows (3).

TABLE 3. SUMMARY OF FINITE STRAIN DATA CALCULATED FROM BURROW CROSS SECTIONS

Location* No. of points Natural strain e

A 50 0.07 B 52 0.03 C 45 0.04 D 51 0.12 E 47 Ö.04 F 37 0.03 G 42 0.10 H 43 0.04 I 51 0.02 J 39 0.07 K 52 0.02 L 49 0.05 M 49 0.01 N 65 0.09 0 40 0.09 P 52 0.07 Q 63 0.06

* See Figure 2 for locations and Figure 16 for diagrams.

only two deformational events, flattening perpendicular to cleavage and finite amplitude folding, (2) many of the 90° bedding-cleavage angles are found in nonfolded rock, and (3) the maximum strain recorded by the burrows is only e = 0.06; had there been addi- tional strain, it seems unlikely that there would be no record of it in the fabric. Consequently, it is concluded that the 90° angle between cleavage and bedding is the initial angle. Because the burrows are

always parallel to the Sp cleavage, the same arguments can be applied to them, leading to the conclusion that they also formed at a 90° angle to bedding.

The presence of Sp cleavage in rocks without measurable folds strongly suggests that the cleavage formed prior to finite amplitude folding. In their study of a minor fold of a quartzite layer in the Tuscarora Formation, Scott and others (1965, p. 729-746) in- ferred the presence of an initial "trigger" fold with compressive stress trajectories parallel to the bedding. Given the appropriate stress symmetry, such a trajectory would, in a pure shear phase of deformation, generate structures reflecting a plane of flattening perpendicular to bedding. Similar trajectories have been modeled in studies of folding using finite element analysis. Parrish (1973, p. 318—334), using nonlinear flow laws, and Dieterich and Carter (1969, p. 129—154), using a linear model, have generated models of principal compressive stress parallel to layering prior to the development of large-scale finite amplitude folding. In both studies, once finite amplitude folding has begun, the orientation and magnitude of o^ changes with respect to bedding. Thus, any material plane initially perpendicular to bedding becomes subject to angular shear strain and is rotated from its origi- Figure 19. Bloomsburg siltstone showing weathered microboudinage (1) with ex- nal position with respect to bedding. E.ehavior of this type is sug- tension parallel to length of burrows (2). gested by Scott and others (1965) in their conclusion that quartz deformation lamellae, initially forming as an "active" tectonic element when o"i was approximately parallel to bedding, were pas- SUMMARY sively rotated into their present position as the layer rotated in the The cleavage in the Bloomsburg Formation is a discontinuous stress field. planar structure containing dimensionally oriented quartz grains

With these ideas in mind, the cases in which the Sp cleavage- and mica. The zones containing the dimensionally oriented quartz bedding angle is no longer 90° can be understood simply as the ro- and mica have been termed alignment zones. These zones are simi-

tation of a material plane, which occurs once the Sp cleavage is no lar to the A zones of Williams (1972), but they differ in that the longer parallel to a principal plane of stress and is subject to angu- presence or absence of the zones is not related to structural position

lar shear strain. In effect, during progressive deformation, the Sp in a fold. The zones predate finite amplitude folding and appear to cleavage becomes structurally passive and acts as a strain marker. be homogeneously distributed throughout the study area.

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Strain within the alignment zones has been recorded by brittle Dieterich, J. H., and Carter, N. L., 1969, Stress history of folding: Am. failure, separation, rotation, and solution of quartz clasts. Analysis Jour. Sei., v. 267, p. 129-154. of strain features both within the alignment zones and in the sur- Durney, D. W., 1972, Solution-transfer, an important geological deforma- rounding rock leads to the conclusion that the cleavage is a plane of tion mechanism: Nature, v. 235, p. 315-316. flattening formed parallel to a principal plane of the finite strain el- Elliott, D., 1970, Determ nation of finite strain and initial shape from deformed elliptical objects: Geol. Soc. America Bull., v. 81, p. lipsoid. The shape of the quartz grains within the zones is the result 2221-2236. of differential solution due to anisotropic stress, and it approxi- Fellows, R. E., 1943, Xecrystallization and flowage in Appalachian mates an oblate ellipsoid conforming to the initial orientation and quartzite: Geol. Soc. America Bull., v. 54, p. 1399-1432. shape of the finite strain ellipsoid. Gair, J. E., 1949, Some effects of deformation in the central Appalachians Although it cannot be conclusively shown that the Sp cleavage [Ph.D. dissert.]: Bait more, Johns Hopkins Univ., 97 p. was not initiated prior to consolidation, petrographic study does Geiser, P. A., 1970, Deformation of the Bloomsburg Formation, Cacapon

show that the Sp cleavage was active after lithification. Strain Mountain Anticline, Hancock, Md. [Ph.D. dissert.]: Baltimore, Johns analysis indicating that the cleavage is a principal plane of the finite Hopkins Univ., 128 p. strain ellipsoid and the lack of measurable strain outside the zones Hara, I., 1966, Dimensional fabric of quartz in a concentric fold: Japanese Jour. Geology and Geography, v. 37, nos. 2-4, p. 123-139. support a postlithification origin. Thus the S cleavage is not be- p Hara, I., Uchibayashi, S., Yokota, Y., Umemura, H., and Oda, M., 1968, lieved to be a soft-sediment or Maxwell- (1962) type structure. Geometry and internal structures of flexural folds. I. Folding of a On the basis of the sequence of tectonic structures, the Sp cleav- single competent layer enclosed in a thick incompetent layer: age is interpreted as a transition phenomenon between brittle be- Hiroshima Univ. Jour. Sei., ser. C, v. 6, no. 1, p. 51-113. havior (extension fractures) and continuous nonrecoverable strain Kirtley, D. W., and Tanner, W. F., 1968, Sabellariid worms: Builders of a (finite amplitude folding). major reef type: Jour. Sed. Petrology, v. 38, no. 1, p. 73-78. The very small two-dimensional strain (maximum e = 0.06) that Mukhopadhyay, D., 1973 , Strain measurements from deformed quartz occurred outside the alignment zones is in contrast to the higher grains in the slaty rocks from the Ardennes and the northern Eifel: Tectonophysics, v. It, p. 279-296. values suggested by studies of the development of slaty cleavage Maxwell, J. C., 1962, Origin of slaty and fracture cleavage in the Delaware (e = 0.30; see Ramsay, 1967, p. 180). On the basis of this analysis, Watergap area, New Jersey and Pennsylvania in Engel, A.E.J., James, the elliptical form of the burrow cross sections is interpreted as a H. L., and Leonard, B. F., eds., Petrologic studies (Buddington vol- primary vectorial property produced by currents rather than by de- ume): New York, Geol. Soc. America, p. 281-311. formation. Thus, their distribution shapes reflect current directions Nickelsen, R. P., 1966, Fossil distortion and penetrative rock deformation rather than deformation. in the Appalachian Plateau, Pa.: Jour. Geology, v. 74, p. 924-931. The restriction of cleavage to units containing vertical burrows Parrish, D. K., 1973, A nonlinear finite element fold model: Am. Jour. Sei., suggests a causal relation between the two; primary fabrics induced v. 273, no. 4, p. 318- 334. by burrowing organisms may have served to initiate and localize Plessman, W., 1966, Diagenetische und kompressive Verformung in der Oberkreide des Harz-Nordrandes sowie im Flysch von San Remo: the alignment zones. Neues Jahrb. Geologie u Paläontologie Monatsh., v. 8, p. 480-493. The angular relation between Sp cleavage and bedding, the paral- Ramsay, J. G., 1967, Folding and fracturing of rocks: New York, lelism of burrows and alignment zones, and the deduction that the McGraw-Hill Book Co., 568 p. alignment zones were a principal plane of the finite strain ellipsoid Scott, W. H., Hansen, E., and Twiss, R. J., 1965, Stress analysis of quartz all lead to the conclusion that the cleavage and burrows formed at a deformation lamellae in a minor fold: Am. Jour. Sei., v. 263, p. 90° angle to bedding. This condition allows the cleavage itself to be 729-746. used as a strain marker, and, with appropriate calculations, it al- Smith, L. L., 1948, Hollow ferruginous concretions in South Carolina: lows the determination of finite strain due to folding about the fold Jour. Geology, v. 56, p. 218—255. profile (Geiser, 1970). Stevens, G. R., 1959, Nature and distribution of S planes in Maryland and southern Pennsylvania [Ph.D. dissert.]: Baltimore, Johns Hopkins ACKNOWLEDGMENTS Univ., 101 p. Stose, G. W., and Swartz, G. K., 1912, Paw Paw-Hancock folio: U.S. Geol. I thank Ernest Cloos and David Elliott for assistance with this Survey, folio 179 (field ed.), 176 p. work, and E. Hansen and W. Scott for their many valuable sugges- Swartz, C. K., 1923, Stratigraphic and paleontologic relations of the tions in both field and lab. Silurian strata of Maryland: Baltimore, Maryland Geol. Survey, Financial support was provided by the Maryland Geological Silurian vol., 778 p. Survey and Johns Hopkins University, and by a Geological Society Terry, R. D., and Chilingar, G. V., 1955, Summary of article by M. S. of America Penrose grant and the Society of Sigma Xi. Shretsov: Jour. Sed. Petrology, v. 25, p. 229—234. Tullis, J., Christie, J. M., and Griggs, T. D., 1973, Microstructure and pre- J. Gevirtz critically read parts of this manuscript and made many ferred orientations of experimentally deformed quartzites: Geol. Soc. valuable editorial suggestions. America Bull., v. 84, p. 297-314. REFERENCES CITED United States Geological Survey, 1951a, Md.-Pa.-W. Va., 7Vi' series, topographic map AMS 5363 III NE: Series V833. Berg, T. M., 1973, Pelecypod burrows in the basal sandstone member of the Williams, P. F., 1972, Development of metamorphic layering and cleavage Catskill Formation, northeastern Pennsylvania: Geol. Soc. America, in low-grade metamorphic rocks at Mermaqui, Australia: Am. Jour. Abs. with Programs (Northeastern Sec.), v. 5, no. 2, p. 137. Sei., v. 272, p. 1—47. Brace, W. F., 1961, Mohr construction in the analysis of large geologic Wise, D. U., 1960, Deformation at Chickies Rock, Pa., in Pennsylvania strain: Geol. Soc. America Bull., v. 72, p. 1059-1080. Geologists Guidebook 25th Ann. Field Conf., Lancaster, Pa. 1960: Breddin, H., 1956, Die tektonische Deformation der Fossilien im Rhein- Lancaster, Pa., Franklin and Marshall College, Dept. Geology ischen Schiefergebirge: Deutsch Geol. Gesell. Zeitschr., v. 196, p. 227-305. Cloos, E., 1947, Oolite deformation in the South Mountain fold, Mary- land: Geol. Soc. America Bull., v. 58, p. 843-918. 1951, Stratigraphy and structural geology of Washington County, Maryland, in Physical features of Washington County: Baltimore, MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 13, 1973 Maryland Dept. Geol., Mines and Water Resources, p. 124—161. REVISED MANUSCRIPT RECEIVED JANUARY 22, 1974

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