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, <j2 in the plane of the bedding and cr3 (least cally upon poorly exposed silicic volcanic although there is no consistent relationship compressive principal stress) normal to rocks. Precambrian metasedimentary rock between these a axes and bedding or cleav- bedding. In the closures, crj is essentially units overlying the Baraboo Quartzite have age (Dalziel and Dott, 1970, Fig. 8b). The a perpendicular to the hinge line. On the been recorded in mining records (Weidman, axes are clearly axes of least compressive limbs of the fold, cr, is mostly parallel to the 1904; Leith, 1935; Schmidt, 1951; Dalziel principal stress (erg), as we shall discuss hinge line of the fold and cr3 is nearly verti- and Dott, 1970) but have not been posi- later. Hence, it seemed worthwhile to con- cal. At the eastern end of the northern limb tively identified in outcrop. duct a detailed study of the quartz subfabric where the fold is tightest, cr3 is again verti- The entire Precambrian succession has in the Baraboo Quartzite and utilize the in- cal, but o~i is horizontal north-south and been folded into a complex doubly plunging formation from recent experimental evi- nearly perpendicular to vertical east- asymmetric syncline with an axial surface dence. Also, most studies of the stress his- striking bedding and to the hinge line. striking approximately east-northeast and tory of natural folds have been undertaken Gliding flow features in minerals of most dipping steeply north-northwest. The on open, high-level structures lacking folded layered rocks have reflected early northern limb is essentially vertical, and the well-developed cleavage (Hansen and Borg, layer-parallel compression (cr,) oriented southern limb dips gently northward (Fig. 1962; Carter and Friedman, 1965; Scott perpendicular to the hinge line of the fold. 2). Radiometric dates indicate that the se- and others, 1965; Friedman and Stearns, Prior to our new data from the Baraboo quence is of late Precambrian age and was 1971), and the Baraboo syncline provided syncline only macrofractures, tension gash deformed and metamorphosed during an an opportunity to undertake a paleostress bands, and (in one case) twin lamellae in orogenic event 1,200 to 1,500 m.y. ago study on a fold with well-developed cleav- age. calcite have shown a1 parallel to hinge lines (Dott and Dalziel, 1972). of folds or perpendicular to bedding. The The work of C. R. Van Hise, C. K. Leith, two latter stress configurations are theoreti- W. J. Mead, and other geologists of the METHODS OF INVESTIGATION cally characteristic of stresses associated "Wisconsin School" in the latter part of the with a later stage in the history of folding. nineteenth and early part of the twentieth Specimens were collected (Fig. 1) from Hence, the detrital quartz grains on the centuries made the structures in the de- the Baraboo syncline for the present mi- limbs of the Baraboo syncline apparently formed Baraboo metasedimentary rocks crofabric study to provide information contain deformation lamellae produced known to structural geologists throughout from positions around the fold considered mainly by later stage stresses rather than the world. An account of the history of to be critical with regard to the stress his- earlier in the fold history, when cr1 was geologic research in the Baraboo district is tory of the fold (Friedman and Sowers, parallel to bedding and perpendicular to the given by Dalziel and Dott (1970). The pres- 1970) and to test the reproducibility of the hinge line. The closely spaced cracks form- ent study combines microscopic analysis of results at any locality, particularly with re- ing the cleavage in the quartzite possibly the Baraboo Quartzite by Stirewalt with the gard to the position of specimens within a were controlled in orientation by relaxation field structural study of the mesoscopic fab- single bed of quartzite and their proximity following the early layer-parallel compres- ric by Dalziel. to the phyllitic layers (Table 1). Not all of sion. Key words: structural geology, folds, the specimens collected contained sufficient cleavage, structural analysis, petrofabrics, OBJECTIVES quartz grains with deformation lamellae for Precambrian. analysis. Those specimens with few lamel- The microscopic fabric of the Baraboo lae were rich in phyllosilicate. INTRODUCTION Quartzite was studied by Riley (1947), who From most of his specimens Riley (1947) concluded that it appeared to bear no rela- cut only a single thin section. In a few cases Precambrian basement rocks are locally tionship to either the mesoscopic or the he did have the control of a second section exposed along the axis of the Wisconsin macroscopic fabric. Riley's interpretation perpendicular to the first, thereby appar- was based upon the early "fabric" ap- ently justifying the use of single sections be- proach to the understanding of deforma- * Also of Department of Geological Sciences, Colum- cause the same fabric axes were obtained bia University, New York, New York 10027. tion lamellae (Ingerson and Tuttle, 1945). from each of the two sections. At least two 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.