Stress history of folding and cleavage development, Baraboo syncline, Wisconsin
I.W.D. DALZIEL* Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 G. L. STIREWALT Department of Geology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514
ABSTRACT arch as inliers in the flat-lying lower Subsequent reinterpretation of his data by Paleozoic sedimentary rocks of the midcon- Christie and Raleigh (1959), and by other Analysis of quartz subfabric elements tinent region (Fig. 1). The largest and best workers, in the light of recent experimental was undertaken in specimens of Precam- known of these inliers occurs in Columbia evidence (Friedman, 1964; Carter and brian Baraboo Quartzite collected around and Sauk Counties, south-central Wiscon- Friedman, 1965; Dalziel, 1969; Dalziel and the doubly plunging, asymmetric Baraboo sin, where Precambrian rocks composed Dott, 1970), has resulted in general agree- syncline. Principal stress axes determined mainly of Baraboo Quartzite form an elon- ment that the quartz subfabric bears some from quartz deformation lamellae in speci- gate ring of hills known as the Baraboo genetic relationship to the fold. Riley's "a" mens from the hinge zone of the fold are cr1 Ranges. The quartzite is certainly more fabric axes lie in a plane approximately (greatest compressive principal stress) and than 1,200 m thick and rests stratigraphi- perpendicular to the hinge line of the fold,
Geological Society of America Bulletin, v. 86, p. 1671-1690, 17 figs., December 1975, Doc. no. 51206.
1671
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 R.4E. R.5E. R.6E. R.7E. R.8E.
T.12N. T.12N.
T.11N. T.11N.
T.10N
R.5E. R.6E. R.7E. Figure 1. Pre-Paleozoic outcrop map of the Baraboo district, Wisconsin (after Dalziel and Dott, 1970), showing specimen locations. Inset illustrates the regional setting of the Baraboo district.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 N. Range S. Range 1,000 m BARABOO SYNCLINE PCs PCdk B/».. Q and P. 1,000 ft "•"SEA LEVEL
1 km
N. Range S. Range
BARABOO SYNCLINE
=1 B' T + + t +granite+ + + + + + +
Figure 2. Geologic cross sections of the Baraboo syncline (after Dalziel and Dott, 1970). Slate, pCf = Freedom Formation, pCdk = Dake Quartzite, pCro = Rowley Creek Slate, P Section locations are shown in Figure 1. pCr = rhyolite, pCb = Baraboo Quartzite, pCs = Seeley Paleozoic sedimentary rocks, Q = Quaternary deposits.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT
mutually perpendicular thin sections were faces and appears to reflect migration of The other mesoscopic fabric elements examined from each of the specimens col- iron-rich solutions through permeable sed- listed in Table 2 are less widespread and lected for the present study. In the case of iments prior to their final lithification. occur only in the less competent phyllitic one specimen each from the North and The most prominent mesoscopic struc- layers from which insufficient data could be South Ranges and one each from the west- tural elements imposed on the Baraboo obtained for stress analysis. These meso- ern and eastern hinge zones of the fold, Quartzite are a cleavage and its intersection scopic fabric elements are important in con- three mutually perpendicular sections were with bedding. The cleavage in the quartzite sidering the significance of the stress pattern examined. Where two sections were cut, layers (S/) is at right angles to bedding, and interpreted from the quartz microfabric one was taken perpendicular to the hinge therefore it is nearly horizontal on the verti- analysis and will be discussed later. For the line of the fold and one parallel to the bed- cal north limb (Fig. 4A) and steeply south present it is sufficient to state that, although ding. The third sections, therefore, were cut dipping on the gently north dipping south field relationships clearly indicate that one parallel to the hinge line of the fold and limb (Fig. 4B). It is refracted in the phyllitic structure postdates another, there is no perpendicular to bedding. layers so as to dip northward at a moderate reason to suppose that the various meso- angle on both limbs (Figs. 4A, 4B), approx- scopic structures in the Baraboo Quartzite MESOSCOPIC STRUCTURES imately parallel to the axial surface of the are unrelated to the evolution of the fold. Bedding/cleavage intersections parallel Baraboo syncline. With the possible excep- Mesoscopic fabric elements of the the hinge line of the fold. Therefore, these tion of a crenulation cleavage in the Baraboo Quartzite are listed in Table 2 and linear structures plunge gently west at the Ableman's Gorge area at the western end of illustrated in Figures 3 and 4. Inherited east end of the syncline, gently east at the the north limb (Fig. 1), all the secondary structures consist of bedding (So), cross- west end, and are nearly horizontal else- structures in the phyllitic layers are geneti- bedding, and color banding. S0 is marked where. In the quartzite, the cleavage ap- cally related to the syncline. Hence, despite by prominent thick quartzite beds that are pears on the mesoscopic scale as a rather the curved axial surface trace of the syn- separated by thin layers of more argillace- penetrative system of braided closed cracks cline (Riley, 1947, Fig. 1; Dalziel and Dott, ous material — phyllite, quartz-phyllite, or (Figs. 4A, 4B). The cleavage in the phyllitic 1970, Pis. I, II), there is little field evidence phyllitic quartzite (Figs. 4A, 4B). The ir- layers (SJ is marked by a concentration of for subsequent refolding, and the meso- regular color banding occurs on joint sur- aligned phyllosilicates (Fig. 4C). scopic structures appear to mark events in the progressive straining of the sedimentary sequence. TABLE 1. SPECIMENS COLLECTED FOR PRESENT STUDY
Specimen Geographic location Position Potential MICROSCOPIC STRUCTURES number (structural domain) in fold structural significance Introduction WR 1, 2, 3 West Range Part of complex To test stress pattern (western closure). western closure in main closures EN 1, 2, 3, 4 East Range Eastern closure To test stress patterns A complete discussion of the microscopic (eastern closure) 1n main closures structures in the Baraboo Quartzite has AG 1, 2, 3 Ableman's Gorge Vertical north To test homogeneity of (northwestern domain) limb stress pattern across been presented by Riley (1947, p. 180 m of section in 458—461). The descriptive information north range in an area from which stress ori- presented here is intended only to supple- entations can be de- rived from tension-gash ment Riley's descriptions by enlarging upon bands points essential to the present study. WBQ 1, 2 West Baraboo quarry Vertical north To test local homo- (northeastern domain) limb geneity of stress pat- tern and possible Recrystallization variation along north 1 imb MM 1, 2 Man Mound Vertical north To test local homo- Grains that are completely polygonized (northeastern domain) limb geneity of stress pat- tern and possible occur only patchily, and no major recrystal- variation along north lization has erased or altered microstruc- limb tures in the quartz. Polygonized quartz LNE 1, 2, 3,* 4 Lower Narrows of Baraboo Vertical north To test homogeneity River, east side limb through 600-m section marking sites of incipient recrystallization (northeastern domain) on north limb, also possible variation occurs at the margins of some grains, but along limb most grains have strongly sutured bound- DL 1, 2,* 3, 4* Devil's Lake Gently north To test homogeneity at aries. Relict seimentary grain margins out- (south limb) dipping south limb one locality on south limb lined by a thin film of impurities also are DL(B) 1, 2, 3, 4 Devil's Lake Gently north To determine variation preserved. (south limb) dipping south limb 1n stress pattern through a thick quartzite bed Deformation Lamellae To test homogeneity LRQ 1, 2, 3, 4* La Rue quarry Gently north (south limb) dipping south limb along south limb, also in more phyl- litic zone of As noted by Riley (1947), deformation quartzite lamellae are well developed in specimens of RSF 1, 2, 3,* 4* South of Rock Springs Hinge zones of inter- To determine variation relatively pure quartzite (Fig. 5), and (west-central domain) mediate scale fold in stress pattern (Ableman's syncline) around an intermediate- specimens that contain large amounts of scale fold phyllosilicate reveal few lamellae. Lamellae RC 1,* 2* South of Devil's Lake Base of section on To determine variation (south limb) south limb in stress pattern commonly cut across detrital quartz grain through section on boundaries and pass into quartz over- south limb growths but are always confined to indi- * Yielded Insufficient data for dynamic interpretation. vidual grains. Hence, lamellae development
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 NNW SSE
M«'«16
ru^e
g ' (quartzite cleavage)
c.
Figure 3. Mesoscopic structures associated with the S0 i>2 (crenulation cleavage) Baraboo syncline (after Dalziel and Dott, 1970). A. South limb Diagrammatic cross section of the Baraboo syncline showing attitude and rela- tionships of cleavages on the north and south limbs of the Slickensides with chatter marks fold. B. Block diagram il- facing up dip (south ) lustrating the relations of main phase mesoscopic struc- tures on the south limb of the syncline.
Quartzite
Phyllite
Longrain and fine crenulation
Trace of S-|E (slaty cleavage)
Longrain - • Fine crenulation
"S-iL Crenulation cleavage
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT
fractures are marked by slightly meander- ing closed cracks that contain phyllosilicate and sometimes polygonized quartz (Fig. 5).
QUARTZ SUBFABRIC ANALYSIS
Introduction
Lamellae of the crystallographic orienta- tion commonly observed in naturally de- formed quartz were first produced experi- mentally by Heard (1962), and more exten- sive laboratory studies of "natural" lamel- lae have recently been described (Heard and Carter, 1968; Christie and others, 1968; Ave Lallemant and Carter, 1971). Thus, there is not a large amount of experimental data on deformation lamellae in quartz that was unavailable either to Riley (1947) dur- ing his initial study or to Christie and Raleigh (1959) for their reinterpretation of his data. Figure 6 illustrates the variation in the angle between quartz c axes and normals to lamellae in the Baraboo Quartzite and indi- cates that lamellae in the quartzite are of the subbasal I type as specified by Ave Lal- lemant and Carter (1971). Subbasal I lamel- lae show a strong concentration of lamellae poles between 10° and 30° to [0001] and may have a weak submaximum of poles be- tween 75° and 85° to [0001]. Experimental studies (Heard and Carter, 1968; Carter and Raleigh, 1969; Carter, 1971) and fab- ric studies of naturally deformed rocks Figure 4. Photographs of mesoscopic struc- tures in the Baraboo Quartzite. A. Nearly verti- (Hansen and Borg, 1962; Carter and Friedman, 1965; Burger and Thompson, cal bedding (S0), northerly dipping cleavage in phyllite (S,), and subhorizontal cleavage in 1970; Friedman and Stearns, 1971) indi- quartzite (S,') on the north limb of the Baraboo cate that data from subbasal I lamellae can syncline at Van Hise Rock. Length of pen is 12.5 be used in paleostress interpretations. The cm. Note strong refraction of cleavage from phyl- "acute angle," Ci-C2 and "lamellae- litic layer into quartzite. The beds face to the arrow" methods for paleostress interpreta- right (south). B. Gently north-dipping bedding (S0), north-dipping cleavage in phyllite (St), and nearly tion all are well defined in Carter and vertical cleavage in quartzite (S,') on the south limb of the Baraboo syncline at Devil's Lake. Scale length is 15 cm. Cross bedding is visible in the quartzite above the phyllitic layer. C. Photomicrograph Friedman (1965). For the Baraboo study, all three methods were utilized to obtain in- showing concentration of aligned phyllosilicates that mark cleavage in phyllite (St) at Devil's Lake. Compositional layering is bedding (S 0). Length of bar scale is 1.6 mm. (Fracture along S, was induced formation on the stress history of the by slabbing of the specimen for the photograph.) quartzite. The crc2 method sometimes yielded ambiguous results both in the pres- postdates the overgrowths, and the lamellae undulatory extinction or incipient kinking ent Baraboo study and in that of Riley are not inherited from a presedimentation oriented at high angles to the c axes of (1947). source rock. As Riley (1947) also noted, quartz grains also occur. These structures quartz grains containing two sets of lamel- were termed "deformation bands" by Riley Description and Interpretation lae at nearly right angles are sometimes ob- (1947). served, but lamellae usually are developed A total of 34 specimens were collected in single sets. Curvature of lamellae appears Microfractures from 11 localities around the Baraboo syn- related to the development of undulatory cline (Fig. 1; Table 1). The orientations ofc extinction bands or incipient kink bands. Microfractures are common in the quartz- axes, deformation lamellae, and microfrac- ite, with some continuous across several tures were measured in 75 to 100 grains in Undulatory Extinction and Kink Bands grains and others confined to individual each of two perpendicular thin sections grains. Both types of microfractures have from 30 oriented specimens and in each of Incipient kink bands and undulatory ex- the same statistical attitudes, but several three mutually perpendicular sections from tinction zones are commonly subparallel to sets occur so that the orientation of micro- 4 oriented specimens. Strong internal con- the c axes of quartz grains in the Baraboo fractures is less homogeneous than that re- sistency of microfabric data was commonly Quartzite since natural lamellae are largely corded by Riley (1947). It appears likely observed between individual specimens of the subbasal type (Fig. 5). Riley (1947) that he concentrated on the coarser scale from the same locality and also in speci- described similar undulatory extinction microfractures that are the microscopic mens collected from different positions in bands in his study. Less frequent bands of manifestation of S/. These coarser micro- the same bed.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 FOLDING AND CLEAVAGE DEVELOPMENT, BARABOO SYNCLINE, WISCONSIN 1677
also may be suggested by preliminary studies of incipient intragranular kinks in quartz at this locality (Stirewalt, 1974). The lamellae-arrow technique yields a unique position for cr3 (Fig. 7F) that is about 90° to the position of o^ as inferred from the lamellae themselves (Fig. 7D). Peripheral concentration of the arrows at a single posi- tion indicates that cr3 and cr2 are not equal. Poles to microfractures (Fig. 7G) show a strong "cut effect," a feature that is com- mon to many of the microfracture diagrams and that makes most of our microfracture data difficult to use in the interpretation of paleostress orientations. Quartz subfabric in specimen WR-1 from the western closure of the syncline is shown in Figure 8. C axes in all grains again show a diffuse nature (Fig. 8B), and c axes in grains with lamellae show a slight tendency toward a preferred orientation (Fig. 8C). Lamellae poles form two distinct non- Figure 5. Microscopic structural elements in Baraboo Quartzite from Devil's Lake. (Thin section is peripheral spreading maxima that position cut perpendicular to S/ and parallel to S0.) Well-developed subbasal deformation lamellae occur cri rather precisely (Fig. 8D). Data on crc2 nearly at right angles to zones of undulatory extinction (incipient kinking) in the strained quartz grain (Fig. 8E) agree with the position of o~l ob- at the left side of the photomicrograph. The narrow zone of fine phyllosilicate trending diagonally tained from lamellae poles and diverge across the right side of the photomicrograph is the microscopic manifestation of S/. Length of bar from the position of cr3 indicated by the scale is 0.5 mm. lamellae-arrow method (Fig. 8F). Note that Figure 7 illustrates quartz subfabric data evidence that subbasal I lamellae most in this case lamellae poles tend to form a from a specimen collected from the west commonly form at angles <45° to o^, this girdle about
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT
the reliability of Riley's data from other along the vertical north-northwest- axes: localities. Previously, the inhomogeneity of south-southeast plane perpendicular to the 1. The cr3 axis lies off the nearly vertical Riley's quartz subfabric data, the fact that hinge line as also noted for Riley's data by north-northwest-south-southeast girdle he had relied mainly on a single thin section Christie and Raleigh (1959) and by Carter (Fig. 10A) to the east in the eastern closure per specimen, and the marked difference and Friedman (1965). Fewer cr3 axes de- (Fig. 10E) and to the west in the western between the paleostress pattern interpret- termined in our study have moderate or low closure (Fig. 10G). In both closures cr3is es- able from his results and those obtained in plunges than do those based on Riley's re- sentially normal to bedding and close to other studies of folded layered rocks led sults, while a few more newly determined lying in the quartzite cleavage, whereas IT, most workers to avoid attaching too much cr3 axes fall off the north-northwest- and (t2 lie almost in the bedding. The orien- significance to interpretations based on his south-southeast girdle. Together, cr, and tation of Over the entire fold, cr3 is dominantly amination of Figures 10 through 12 reveals 3. On the south limb (Fig. 10D), a1 is es- nearly vertical (Fig. 10A) but does spread the following facts about deduced stress sentially parallel to the hinge line of the 20 AG-1,2,3 (676 SETS OF LAMELLAE) WBQ-1,2 (169 SETS OF LAMELLAE) LNE-1,2,4 (439 SETS OF LAMELLAE) 5> 15 o 5 io D UOJ E 5 0 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 C AXIS A i LAMELLAE C AXIS A 1 LAMELLAE C AXIS A 1 LAMELLAE 20 20 20 r MM-1,2 (350 SETS OF LAMELLAE) |DL(B)1,2,3.4 DL-1,3 (988 SETS OF LAMELLAE) LRQ-1,2 (82 SETS OF LAMELLAE) 5 15 >- 16 o 5 1° 10 D UOJ 5 ff 5 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 60 90 C AXIS A i LAMELLAE C AXIS Al LAMELLAE C AXIS A 1 LAMELLAE WR-1.2,3 (675 SETS OF LAMELLAE) RSF-1,2 (192 SETS OF LAMELLAE) 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 C AXIS A 1 LAMELLAE C AXIS A 1 LAMELLAE C AXIS A 1 LAMELLAE Figure 6. Histograms illustrating variations in the angle between quartz c axes and poles to deformation lamellae for specimens collected in this study. Specimen locations are shown in Figure 1. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 FOLDING AND CLEAVAGE DEVELOPMENT, BARABOO SYNCLINE, WISCONSIN 1677 syncline. Where cr3 can be uniquely deter- mined (mainly from Riley's data), it is nearly vertical and at a high angle to bed- ding. There is, however, more of a tendency for a3 to spread along a north- northwest-south-southeast girdle than on the north limb. In a number of cases, cr3 could not be uniquely determined. This has usually been interpreted in the literature to indicate that cr2 and cr3 were of approxi- mately equal value when the lamellae formed. It could also mean that cr2 and cr3 changed orientation within the o-2-cr3 plane during the time lamellae were forming, an attractive explanation for lamellae on the limb of a fold when the o-2-o-3 plane is per- pendicular to the hinge line, as here. We believe the conclusions of Riley (1947) that the microscopic structures in the quartz grains of the Baraboo Quartzite reflect deformation that postdates the major syncline needs to be re-examined for two reasons. First, there is little indepen- dent evidence of refolding at Baraboo. De- spite the curvature of the axial surface trace of the syncline (Fig. 1), the geometry and field relations of nearly all the structures in the Baraboo Quartzite indicate that these structures formed at various stages during the progressive straining of the sedimentary succession as the syncline developed (Dal- ziel and Dott, 1970, p. 16-26). Second, in light of the data now available from numerous other folds, it appears unlikely that the quartz grains in the Baraboo Quartzite would not have deformed by glid- ing flow during the development of the syncline. Little recrystallization has oc- curred to obliterate intracrystalline micro- structures, hence the effects of any gliding flow influencing the quartz grains likely is preserved. The patterns of principal stress axes reflected by the deformation lamellae in specimens from the eastern and western closures of the Baraboo syncline are com- parable to those obtained by Carter and Friedman (1965) and Burger (1972) from the hinge zones of the Dry Creek Ridge an- ticline in Montana and also by Scott and others (1965) from the hinge zone of a small fold in the Tuscarora Sandstone. Namely, cr3 is normal to the layering, with cr, and cr2 lying in the layering and (Tj per- pendicular to the hinge line of the fold. The pattern clearly could reflect the stress re- gime in that part of a fold hinge zone where Figure 7. Quartz subfabric in three mutually perpendicular thin sections from the Ableman's the maximum principal compressive stress Gorge locality (specimen AG-2). Diagrams are equal-area, lower hemisphere projections. Numbers 1, was parallel to the layering and perpendicu- 2, and 3 indicate paleostress axes cr„ Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT workers. The most consistent feature is that least compressive principal stress lies in the plane perpendicular to the hinge of the fold and is mostly nearly vertical (Fig. 10A). Hence cr3 axes are parallel to bedding on the steep northern limb and cut across the gently north dipping southern limb of the fold. Theoretical and model studies of the stress history of folding (Chappie, 1968, 1969, 1970; Sherwin and Chappie, 1968; Dieterich and Carter, 1969; Dieterich, 1970) indicate that least compressive prin- cipal stress should approach parallelism with the folded layering at a stage in the shortening history after initiation of fold- ing. Thus, the steep cr3 axes on the north limb at Baraboo could be related to the formation of the syncline if the lamellae formed at a later stage in the shortening his- tory than the initiation of the folding. The steep cr3 axes on the southern limb that cut across the gently north dipping bedding may reflect the asymmetry of the fold. At the eastern end of the north limb (that is, the eastern part of the northeastern do- main) o-j is oriented normal to the nearly vertical layering and is similar in orienta- tion to The orientation of crl parallel to the hinge line of the fold that predominates on the limbs of the Baraboo syncline has been re- ported from the hinge zones of other natural folds, where it has been described as a stress orientation that occurred after fold- ing was initiated (Hancock, 1964; De Sit- ter, 1964; Stearns, 1968; Burger and Thompson, 1970; Friedman and Stearns, 1971). However, only in the case reported by Burger and Thompson (1970) was this indicate paleostress axes au Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 FOLDING AND CLEAVAGE DEVELOPMENT, BARABOO SYNCLINE, WISCONSIN 1677 based on plastic flow features in minerals (twin lamellae in carbonate grains) as well as on macrofractures and tension-gash bands. To our knowledge no quartz defor- mation lamellae data reflecting such an orientation for cr, have been obtained from any other fold. From the foregoing consideration of in- dividual domains, it appears to us that the orientations of principal stress axes from different locations around the fold are too systematic to be fortuitous. Moreover, from theoretical and model studies of the stress history of folding, as well as from studies of the mesoscopic and microscopic fabrics of other natural folds, principal stresses with the orientations observed are likely to have existed at some stage in the formation of the fold. We do not believe that a subse- quent deformation unrelated to the de- velopment of the Baraboo syncline is re- quired to explain the heterogeneous nature of the quartz subfabric of the Baraboo Quartzite. The only other structures in the Baraboo Quartzite that are suitable for dynamic in- terpretation, tension gashes and brittle frac- tures, also reflect stress patterns genetically related to the syncline. Tension-gash bands are common and in a few limited subareas show consistent patterns that indicate that cr3 lay parallel to the hinge line of the fold and a vertical <~r,-a2 plane was oriented north-northwest—south-southeast (Figs. 15, 16). Poles to randomly measured joints from the north and south limbs of the syn- cline show maxima parallel to the hinge line of the fold (Figs. 17A, 17B). Submaxima corresponding to macroscopic manifesta- tions of Sj' are oriented nearly vertical with an east-west to northeast-southwest strike on the south limb and near horizontal on the north limb. Hence the majority of the joints are classical "ac" joints normal to the hinge line of the syncline. Many of the joints are filled with quartz and are appar- ently extension fractures. Absence of slickensides on the filled ac joints suggests that the joints formed after the Baraboo syncline had "stabilized" as a fold. Thus, they likely reflect the regional orientation of cr3 and the ara2 plane. The tension-gash bands reflect the same orienta- tion of cr3 parallel to the hinge line of the fold. They formed after S / was initiated be- cause the latter is deflected across the gash bands, the more so where continued slip along the bands is indicated by sigmoidal Narrows of the Baraboo River (specimen LNE-2). Diagrams are equal-area, lower hemisphere pro- gashes (Dalziel and Dott, 1970, Fig. 54b). jections. Numbers 1, 2, and 3 indicate paleostress axes Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT DISCUSSION Stress History of Folding The relative simplicity of stress systems deduced from gliding-flow features in natural folds has been explained in two ways. It has been suggested, on the basis of two-dimensional analyses, that layer- parallel compression perpendicular to the hinge line early in the fold history attains a higher value than compression in any other orientation later in fold history (Dietrich and Carter, 1969; Burger and Thompson, 1970) and also that layer-parallel compres- sion perpendicular to the fold hinge is ac- companied by a greater amount of finite shortening strain than compression in any other direction (Friedman and Stearns, 1971). As shown in the preceding section, the stress-axis orientations determined in this study and those derived from Riley (1947) can be interpreted to bear a genetic rela- tionship to the syncline, even though this re- lationship is more complex than for other natural folds that have been studied. The stress pattern obtained from the closures of the Baraboo syncline, namely, cr, parallel to the layering and perpendicular to the hinge line with cr3 normal to the layering, com- pare favorably with patterns obtained from similar positions in other folds and with patterns predicted in theoretical and model studies of bent beams. The unusual orienta- tions obtained from the flanks of the Baraboo syncline must be explained in terms of both spatial and temporal stress variations through the fold history. Although the 54 specimens analyzed were collected from a variety of positions with respect to bedding surfaces and phylli- tic layers, the orientation of stress axes ob- tained from any one locality is remarkably consistent (Figs. 10 through 12). This is also true of specimens collected through 300 to 400 m of quartzite in the northwest- ern domain. In the northeastern domain, the al axes in the lower stratigraphic levels of the syncline are more nearly perpendicu- lar to the hinge line of the fold than those at higher stratigraphic levels — exactly the opposite of what would be expected for a synclinal fold if the entire thickness of quartzite had acted as a single buckling "plate." Therefore, the available data ap- pear to rule out the possibility that the stress pattern obtained from the limbs of the syncline is dependent solely upon posi- tion with respect to "neutral surfaces" dur- ing the formation of the fold. It appears and 3 indicate the orientations of o"i, cr2, and cr3 determined from single specimens. Circled numbers more likely that the stress pattern obtained refer to our specimens, numbered dots to Riley's data. General orientations of S0 (solid line) and S,' reflects the stage of fold development dur- (dashed line) are shown for the north and south limbs, and the eastern and western closures are based ing which the quartz subfabric was formed. on all the mesoscopic fabric data available (Dalziel and Dott, 1970, PI. V). Structural domains are shown in Figure 13. A. Data from the whole syncline. Note the position of the a-c plane of the fold. Orientation of cr parallel to layering and 3 C. Data from the northeastern domain. D. Data from the south limb. The 2-3 plane is from our data. perpendicular to the hinge line (as on the E. Data from the eastern closure. F. Data from the west-central domain. G. Data from the western northern limb of the fold), o^ normal to closure. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 EN 1,3,4 EXPLANATION Stereoplots Map Present study Riley's (1947)data Prepaleozoic outcrop (T, axis ® -1 Axial surface trace of anticline 0*2 axis @ .2 —J J- Axial surface trace of syncline Specimen No. AG 1,2,3 etc R3 etc Figure 11. Stereoplots (equal-area, lower hemisphere) showing orientation of paleostress based on data from a number of individual specimens at the locality. axes deduced from microfabric data around the Baraboo syncline. Larger stereoplots show axes Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 PRESENT STUDY Avg. angle 85" Avg. angle 80" Avg. angle 83" Avg angle 23" a, A1 s0 0 10 20 30 40 50 60 J70 80 9 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 7J0 80 90 10 20 30 40 50 60 70 80 90 8 8 8 r 8 „ 7 Avg. angle 79° Avg. angle 37° Avg. angle 69° Avg. angle 74° 2 6 a3AlS0 - u "" 0 1 90 .L90 190 Avg. angle 76° Avg. angle 20 Avg. angle 27 Avg. angle 67° CE Avg. angle 86° Avg. angle 15 Avg. angle 30 Avg. angle 89 OC -S D 2 Mis „ z 8 8 8 8 < 7 C Avg. angle 85° Avg. angle 53° Avg. angle 80° Avg. angle 74c O 6 o Ai fD <7-, A 1 S, • ^ Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 No. of determinations No. of determinations -> to U> -U • 15- D! w 5- — U1 L o10 o oo T 1 1 1 1—1 1 1 1 1 1 1 1 1 — : CO _ 1- D0) co <9. r o L o m 1 1 1 1 1 1 1 1 ! &> > » » i. H t § F c 00 1—I—I—I—I—I—I 1 1 1 1 1 1 1 1 1 ® I > a, h- =® — - (f> ® KJ oc L o10 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT Figure 13. Structural domains around the Baraboo syncline within which the orientations of principal stress axes determined in this study are homogeneous. layering (as in the northeastern domain), strong gliding flow within the quartz grains dicular to the hinge line of the fold is not and ai parallel to the hinge line (as is com- as it did in other natural folds. Gliding-flow strongly reflected by deformation lamellae mon on both limbs) may all be expected at features do reflect the cumulative strain his- in the quartzite remains unsolved. Lamellae a stage in the shortening history of a tory of a rock, and some of the deformation in other natural folds studied may reflect sedimentary pile after folding has been lamellae at Baraboo may have been pro- mainly the early layer-parallel compression initiated. duced during earlier stages in the shorten- perpendicular to the hinge line of the fold Deformation lamellae at Baraboo occur ing history of the fold. This explanation because later stage stresses resulted in brit- in detrital quartz grains that have not re- may hold for grains that contain more than tle failure (M. Friedman, 1973, written crystallized, yet the lamellae on the limbs of one set of lamellae. However, such grains commun.). the fold appear to reflect a stage in the are relatively uncommon, and the stereo- shortening history after initiation of fold- plots strongly suggest that most of the de- Cleavage Development ing. Thus, if the early layer-parallel com- formation lamellae now observed in the pression perpendicular to the hinge line quartz grains were not induced by early This study was initiated in part to pro- predicted by theory did occur, it took place layer-parallel compressive stresses. It is pos- vide new information on the mechanical without extensive plastic deformation of sible that pore fluid and matrix material significance of fracture-type cleavage. The the quartz grains. The intragranular struc- may have provided some type of cushioning most consistent and impressive thing about tures of all natural folds previously studied effect between the quartz grains early in the the cleavage in the Baraboo Quartzite is reflect early compression parallel to layer- deformation history of the Baraboo syn- that in the thick quartzite layers the cleav- ing and perpendicular to the hinge line. Pre- cline. There is abundant matrix material, age is always perpendicular to bedding. It is liminary investigations of the magnetic chiefly pyrophyllite, and evidence of post- hard for us to conceive that any stress sys- anistropy of the Baraboo Quartzite also in- depositional migration of fluids through the tem other than the early layer-parallel com- dicate that layer-parallel shortening oper- sediments prior to their lithification is pres- pression indicated by theory and by the ated during the formation of the syncline ent (Dalziel and Dott, 1970). However, it studies on other natural folds could have (R. F. Kligfield, 1973, personal commun.). appears doubtful that there could have been been responsible for such a consistent rela- It is difficult to explain why early com- enough cushioning effect to eliminate most tionship between S,' and bedding on both pression perpendicular to the hinge line of of the quartz-quartz grain contact, and the limbs of the asymmetric syncline. Also, the the Baraboo syncline did not result in problem of why early compression perpen- suggestion of a relationship between early Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 FOLDING AND CLEAVAGE DEVELOPMENT, BARABOO SYNCLINE, WISCONSIN 1677 layer-parallel compression and the initia- Lower Narrows tion of S/ is supported by preliminary magnetic anisotropy studies that indicate that maximum strain of finite shortening was normal to this cleavage (R. F. Kligfield, 1973, personal commun.). S i consists of a braided system of closed cracks, and only toward the margins of these layers, where there is more argilla- ceous matrix, is significant grain alignment observed. This cleavage grades into a slaty type that refracts into the phyllitic layers. Price and Hancock (1972) have suggested that hydraulic fracturing may have been re- sponsible for the formation of fracture cleavage in Pembrokeshire and Aragón. This cleavage is apparently comparable in character and in orientation with respect to fold geometry to S / at Baraboo. At all three localities, the cleavage in the competent beds is perpendicular to bedding and paral- Strike and dip of overturned bedding lel to the fold axes, and at all three localities, it has the appearance of extension fractures while being refracted into a slaty km mi type of cleavage in the less competent inter- beds. The narrow concentrations of phyl- losilicate material along S/ at Baraboo (Fig. 5) might support the suggestion that the cleavage is somehow related to dewatering. However, the theory of Price and Hancock (1972) seems to offer no satisfactory expla- nation for the regular orientation of this cleavage with respect to bedding, the most striking feature of the cleavage in the quartzite at Baraboo and in Pembrokeshire and Aragón. Given the nature of S,' at Baraboo, it appears most likely to us to have developed as an extension fracture upon relaxation of early compression paral- lel to the bedding and perpendicular to the hinge line of the syncline. This would ex- plain not only the appearance of the cleavage as a system of closed cracks, but also its re- lationship to the slaty type of cleavage in the phyllitic interbeds. SUMMARY AND CONCLUSIONS Detailed study of quartz subfabric (c axes, deformation lamellae, and microfrac- tures) in the Baraboo Quartzite has demon- strated the reproducibility of stress in- terpretations based on the data presented by Riley (1947). Results are reproducible even though the stress pattern is heterogeneous on the scale of the whole Baraboo syncline and differs markedly from paleostress orientations obtained from other natural folds. The orientation of the stress axes is not affected by proximity of the specimens studied to bedding planes or phyllitic horizons in the formation. Figure 14. Variation in the orientation of cr, at the eastern end of the north limb of the Baraboo On the scale of the entire syncline, cr3 is syncline. Numbers preceded by "R" refer to Riley's localities. A. Detail of specimen localities across mostly nearly vertical and always lies in the this segment of the north limb. B. Stereoplot (equal-area, lower hemisphere projection) showing the plane perpendicular to the hinge line of the systematic variation in orientation of o", at these localities. fold. Analysis of the results from different Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT Southeast A N Figure 15. Stereoplots (equal-area, lower hemisphere projection) illustrating principal stress axes deduced from tension-gash bands in the subareas shown in black. Numerous gash bands measured in areas shown cross-hatched revealed random orientations and slip vectors on the scale of this study. parts of the fold reveals a closer relation- can also be used to deduce the local stress the limbs, however, seems to reflect a stress ship between the stress axes deduced from system at the time the gashes and joints pattern characteristic of a stage in the short- the quartz subfabric and fold geometry: (1) formed. The nonslickensided extension ening history later than initiation of fold- In the hinge zones, o^ and cr2 lie in the bed- fractures show cr3 parallel to the hinge line ing that is reflected only by macrofractures, ding with o-! perpendicular to the hinge of the fold. The tension-gash bands, in tension-gash bands, and (rarely) by calcite line. cr3 is essentially normal to the bedding. areas where they are homogeneous, also in- twin lamellae in other folds studied. We are (2) On the fold limbs, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 FOLDING AND CLEAVAGE DEVELOPMENT, BARABOO SYNCLINE, WISCONSIN 1677 tion of S/ perpendicular to bedding was sure; possibly tension-gash development. controlled by the early compression that (2) Fold amplification with rotation of cri was parallel to bedding and perpendicular away from parallelism with the layering to the hinge line and that this cleavage in everywhere except in the hinge zone; the quartzite is an extension fracture de- cementation of matrix; cleavage refraction; veloped as a relaxation feature after the some lamellae; possibly tension-gash de- stress system shifted during fold develop- velopment. (3) Continued plastic deforma- ment. Hydraulic fracturing may have been tion of quartz grains; further amplification partially responsible for opening the cleav- of fold; extensional modification of cleav- age but could not have controlled its orien- age in quartzite. (4) Mesoscopic fracturing tation. to produce north-northwest-south-south- Contrary to the conclusion of Riley east vertical extension fractures. (1947) that the quartz subfabric in the Baraboo Quartzite resulted from a stress ACKNOWLEDGMENTS system later than, and unrelated to, de- velopment of the syncline, we believe that This study was supported by National the stress system reflected by the quartz Science Foundation Grants GA 12926 and subfabric is related to development of the GA 28279. Earlier field work by Dalziel Figure 16. Summary stereoplot (equal-area, syncline. We envisage the development of and R. H. Dott, Jr., was carried out under lower hemisphere projection) illustrating orienta- the Baraboo syncline and related structures the auspices of the Wisconsin Geological tions of stress axes deduced from the tension- as follows: (1) Early layer-parallel compres- and Natural History Survey. gash data presented in Figure 15. Numbers 1, 2, sion resulting in shortening normal to the We are grateful to the following for and 3 indicate the orientations of a-u cr2, and cr3 hinge line of the fold, pressure solution and numerous profitable discussions: R. H. deduced from the individual subareas of Figure elastic strain in the quartzite layers, and de- Dott, Jr., R. E. Bischke, K. F. Palmer, and 15. At two other locations, only the strike of the velopment of incipient slaty cleavage in the C. H. Scholz. Constructive criticism of the o~f fj2 plane could be determined. phyllitic layers; build-up of pore fluid pres- manuscript was offered by N. L. Carter, W. M. Chappie, M. J. de Wit, T. Engelder, M. Friedman, W. D. Means, and R. S. Stan- ley. We are particularly appreciative of the many helpful comments made by M. Friedman. The general computer program modified by Stirewalt for plotting, rotation, and contouring of universal stage data was kindly provided by K. J. Rosengren (Aus- tralian National University) and H. Helm- staedt (Queen's University). REFERENCES CITED Ave Lallemant, H. G., and Carter, N. L., 1971, Pressure dependence of quartz deformation lamellae orientations: Am. Jour. Sci., v. 270, p. 218-235. Burger, H. R., 1972, Stress analysis of the Dry Creek Ridge anticline, Montana: Prelimi- nary report: Montana Geol. Soc., 21st Ann. Field Conf., p. 129-134. Burger, H. R., Ill, and Thompson, M. D., 1970, Fracture analysis of the Carmichael Peak anticline, Madison County, Montana: Geol. Soc. America Bull., v. 81, p. 1831-1836. Carter, N. L., 1971, Static deformation of silica and silicates: Jour. Geophys. Research, v. 76, p. 5514-5540. Carter, N. L., and Friedman, M., 1965, Dynamic analysis of deformed quartz and calcite from the Dry Creek Ridge anticline, Mon- tana: Am. Jour. Sci., v. 263, p. 747-785. Carter, N. L., and Raleigh, C. B., 1969, Principal stress directions from plastic flow in crys- Figure 17. Poles to randomly measured joint surfaces around the Baraboo syncline (after Dalziel tals: Geol. Soc. America Bull., v. 80, p. and Dott, 1970, PI. IV). Stereoplots are equal-area, lower hemisphere projections. A. Poles to 200 1231-1264. joint surfaces measured on the north limb. Contours = 0.5, 1.5, 3, 4.5, 6, and 9 percent per 1 percent Chappie, W. M., 1968, A mathematical theory of area. B. Poles to 450 joint surfaces measured on the south limb. Contours = 0.1, 0.5, 1, 3, 4, and 6 finite-amplitude rock-folding: Geol. Soc. percent per 1 percent area. C. Poles to 43 joint surfaces measured in the eastern closure. Contours = 2, America Bull., v. 79, p. 47-68. 7,14, and 20 percent per 1 percent area. D. Poles to 18 joint surfaces measured in the western closure. 1969, Fold shape and rheology: The folding Contours = 5 and 15 percent per 1 percent area. of an isolated viscous-plastic layer: Tec- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT tonophysics, v. 7, p. 97-116. Earth Sci., v. 7, p. 477-497. Baraboo Quartzite: Jour. Geology, v. 55, p. Chappie, W. M., 1970, The finite-amplitude in- Friedman, M., and Stearns, D. W., 1971, Rela- 453-475. stability in the folding of layered rocks: tions between stresses inferred from calcite Schmidt, R. G., 1951, The subsurface geology of Canadian Jour. Earth Sei., v. 7, p. 457-466. twin lamellae and macrofractures, Teton Freedom Township in the Baraboo iron- Christie, J. M., and Raleigh, C. B., 1959, The anticline, Montana: Geol. Soc. America bearing district of Wisconsin [M.S. thesis]: origin of deformation lamellae in quartz: Bull., v. 82, p. 3151-3162. Madison, Univ. Wisconsin. Am. Jour. Sei., v. 257, p. 385-407. Hancock, P. L., 1964, The relations between Scott, W. H., Hansen, E. C., and Twiss, R. J., Christie, J. M., Tullis, J. A., and Blacic, J. D., folds and late-formed joints in south Pem- 1965, Stress analysis of quartz deformation 1968, The nonrational nature of deforma- brokeshire: Geol. Mag., v. 101, p. lamellae in a minor fold: Am. Jour. Sci., v. tion in quartz (abs.): Am. Geophys. Union 174-184. 263, p. 729-746. Trans., v. 49, p. 314. Hansen, E. C., and Borg, I. Y., 1962, The Sherwin, J., and Chappie, W. M., 1968, Dalziel, I.W.D., 1969, Kinematic and dynamic dynamic significance of deformation lamel- Wavelengths of single layer folds: A com- analysis of the Baraboo Quartzite, Wiscon- lae in quartz of a calcite-cemented sand- parison between theory and observation: sin, from tension gash bands [abs.]: Am. stone: Am. Jour. Sci., v. 260, p. 321-336. Am. Jour. Sci., v. 266, p. 167-179. Geophys. Union Trans., v. 50, p. 323. Heard, H. C., 1962, The effect of large changes Stearns, D. W., 1968, Certain aspects of fractures Dalziel, I.W.D., and Dott, R. H., Jr., 1970, in strain rate in the experimental deforma- in naturally deformed rocks, in Riecher, Geology of the Baraboo district, Wisconsin: tion of rocks [Ph.D. thesis]: Lbs Angeles, R. E., ed., Rock mechanics seminar: Bed- Wisconsin Geol. and Nat. History Survey Univ. California, Los Angeles, 202 p. ford, Mass., U.S. Air Force Cambridge Re- Inf. Circ. 14, 164 p. Heard, H. D., and Carter, N. L., 1968, Experi- search Labs., p. 97-118. De Sitter, L. U., 1964, Structural geology: New mentally induced "natural" intragranular Stirewalt, G. L., 1974, Dynamic analysis of York, McGraw-Hill Book Co., 551 p. flow in quartz and quartzite: Am. Jour. Sci., natural intragranular kinks in quartz from Dieterich, J. H., 1970, Computer experiments on v. 266, p. 1-42. the Baraboo Quartzite, Wisconsin — Com- mechanics of finite amplitude folds: Cana- Ingerson, E., and Tuttle, O. F., 1945, Relations parison with results from deformation dian Jour. Earth Sei., v. 7, p. 467-476. of lamellae and crystallography of quartz lamellae: Geol. Soc. America Abs. with Dieterich, J. H., and Carter, N. L., 1969, Stress and fabric diagrams in some deformed Programs, v. 6, p. 548. history of folding: Am. Jour. Sei., v. 267, p. rocks: Am. Geophys. Union Trans., v. 26, Weidman, S., 1904, The Baraboo iron-bearing 129-155. p. 95-105. district of Wisconsin: Wisconsin Geol. and Dott, R. H., Jr., and Dalziel, I.W.D., 1972, Age Leith, A., 1935, The Precambrian of the Lake Nat. History Survey Bull. 13, 190 p. and correlation of the Precambrian Superior region, the Baraboo district, and Baraboo Quartzite of Wisconsin: Jour. other isolated areas in the Upper Missis- Geology, v. 80, p. 552-568. sippi Valley, in Kansas Geol. Soc. Friedman, M., 1964, Petrofabric techniques for Guidebook, 9th Ann. Field Conf.: p. the determination of principal stress direc- 320-332. MANUSCRIPT RECEIVED BY THE SOCIETY tions in rocks, in Judd, W. R., ed., State of Price, N. J., and Hancock, P. L., 1972, Develop- FEBRUARY 7, 1974 stress in the Earth's crust: New York, ment of fracture cleavage and kindred REVISED MANUSCRIPT RECEIVED MAY 16, 1975 Elsevier, 732 p. structures: Internat. Geol. Cong., 24th, MANUSCRIPT ACCEPTED MAY 23, 1975 Friedman, M., and Sowers, G. M., 1970, Pet- Montreal 1972, sec. 3, p. 584-592. CONTRIBUTION No. 2072, LAMONT-DOHERTY rofabrics: A critical review: Canadian Jour. Riley, N. A., 1947, Structural petrology of the GEOLOGICAL OBSERVATORY Printed in U.S.A. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021