IC14. Geology of the Baraboo District, Wisconsin: A description and ...

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fine crenulation longrain- Figure 7. Block diagrams illustrating the suggested mode of origin of the longrain, phyllitic cleavage (S1), and the late main-phase crenulations and cleavage (SIL) in phyllitic layers of the Baraboo Quartz- ite. The diagrams show the situation on the south limb of the syncline. The three principal sections of the finite strain ellipsoid resulting from deformation of an original reference sphere are shown in each diagram. Where flattening alone was involved (a) a phyllitic cleavage without any longrain would be developed. Where the ellipsoid was triaxial, however (i.e., where there was some shortening parallel to the hinge line of the Baraboo syncline), the phyllo- silicates would tend to have been aligned parallel to the longest axis of the finite strain ellipsoid (b). More extreme shortening parallel to the hinge line would have led to crenulation of the S1 surface about axes parallel to the longrain (c), resulting in the formation of the SIL crenulation cleavage. Note: (i) The use of the strain ellipsoid here as an indicator of the deformation does not assume any mechanical significance for the cleavage surfaces as in Figure 6. (ii) It is not possible at present to be certain whether or not significant rotational strain was involved in the formation of S1. If S1 did not form in the principal section of the finite strain ellipsoid normal to the maximum shortening axis, it would have rotated towards this section (see text). The longrain and crenulations would have developed in the same way as that outlined above even if S1 did not come to lie normal to the maximum shortening axis. They would be parallel to the longest axis of the elliptical section of the finite strain ellipsoid in which S1 lay.
Genesis of the Baraboo Syncline (Dalziel The Baraboo syncline developed when a thick pile of Precambrian sediments was subjected to regional tectonic stress in a mobile belt under conditions that favored ductile behavior. The sedimentary pile now exposed was aniso- tropic and inhomogeneous, consisting of arenaceous material with pronounced stratification (and cross stratification), containing numerous thin argillac- eous partings, and overlain by other lithic units. Hence no matter how homogeneous the regional tectonic stress field may have been, the pattern of stress and of resultant strain within the deforming sediments must have been complex. Moreover, it must have been constantly changing as the fold developed. Reinterpretation of Riley's data (1967) on the orientation of quartz deformation lamellae throughout the Baraboo Quartzite in the light of experimental studies (Carter and Friedman, 1965; see Fig. 8) shows that the lamellae were formed by a stress system in whichcl and c3 (respectively the greatest and least compressive principal stresses) lay in a vertical plane striking north- northwest--east-southeast. At a number of different localities where bands of quartz-filled tension gashes form distinct conjugate bands, the attitude and shear sense indicate that the gashes resulted from a stress system in which Cl and 6 (intermediate compressive principal stress) lay in the same plane (Fig. 8, Balziel, l969a). These interpretations are not at variance. Recent studies indicate that deformation lamellae characteristically reflect the stress field early in the fold history (Dieterich and Carter, 1969). Shear along the tension gash bands at Baraboo has deformed the (Sll) cleavage in the quartzite (Fig. 54; Dalziel, 1969a), which is locally marked by deformed quartz grains. Hence the tension gashes probably reflect the stress system at a later stage in the progressive development of the syncline than do the deformation lamellae. The available evidence, therefore, indicates a regional stress system consistent with the over-all geometry of the Baraboo syncline. The plane normal to the hinge-line of the syncline, the hinge lines of the minor folds, and the strike of the quartzite, slaty, phyllitic, and secondary phase cleavages was the plane early in the fold history and later the r1/r2 plane. The orientation of Cl within this plane was variable (Fig. 81, probably in time as well as in space. The dominant joints in the quartzite (striking north-northwest--south-southeast and vertical, P1. V) may have formed as extension fractures normal to r3 at approximately the same stage in the fold history as the tension gashes. Some are quartz-filled. No other joint sets consistent enough to warrant dynamic interpretation were distinguished. In attempting to explain the process of folding in rocks in response to tectonic stress, geologists have tended to invoke relatively simple mechanisms that can account for recognizable fold shapes and associated small-scale structures such as slickensides and axial surface cleavage. However, it should be borne in mind that just as the various mesoscopic structures in the Baraboo Quartzite represent only recognizable stages in the progressive strain of the lithic units in which they occur, so the various mechanisms that can be called upon to explain them represent only the readily distinguishable components of a much more complex deformation. These components must be integrated in
Page 2 and 3: GEOLOGY OF THE BAMBOO DISTRICT, WIS
Page 4 and 5: ILLUSTRATIONS Cover. Stanley A. Tyl
Page 6 and 7: Figure 43. 44. 45. 46. 47. 48. Map
Page 8 and 9: FOREWORD For more than half a centu
Page 10 and 11: (Dalziel and Dott) Regional Geologi
Page 12 and 13: The most comprehensive account of t
Page 14 and 15: the obvious terminal moraine loopin
Page 16 and 17: TABLE 2: Rubidium-strontium and pot
Page 18 and 19: Rhyolite. Much of the rhyolite is f
Page 20 and 21: grains in the formation have very t
Page 22 and 23: crustal parent rocks early in earth
Page 24 and 25: Main Phase Structures. Mesoscopic s
Page 26 and 27: 'arious - bubt f ul TABLE 3 (contd.
Page 28 and 29: (S ) Strain-slip or 2 crenulation c
Page 30 and 31: It will be noted that the term "axi
Page 32 and 33: Figure 5. Photomicrographs showing
Page 34 and 35: Structural Geometry (Dalziel) Gener
Page 36 and 37: At the few localities where it coul
Page 40 and 41: time and space before the strain is
Page 42 and 43: (dashed lhne Dalzid* ldd data) Figu
Page 44 and 45: the result of a minor amount of sho
Page 46 and 47: PALEOZOI C GEOLOGY (Dott ) Stratigr
Page 48 and 49: latter name has been restricted by
Page 50 and 51: TABLF: 6. MAJOR SILICATE MINERALS O
Page 52 and 53: TABLE 8: SIZE DISTRIBUTION DATA FOR
Page 54 and 55: The Oneota Dolomite, basal unit of
Page 56 and 57: Cross Laminae: Trough Axes: number
Page 58 and 59: At least three transgressive "cycle
Page 60 and 61: sedimentologist's equations. At Bar
Page 62 and 63: 1. Grinding and chipping between th
Page 64 and 65: wave lengths. Thus it is improbable
Page 66 and 67: truncation surfaces separating cros
Page 68 and 69: processes or "environments" by the
Page 70 and 71: experiments in 1941, a sequence of
Page 72 and 73: GLACIAL GEOLQGY (Black) Introductio
Page 74 and 75: of pits and hollows. Knob and swafe
Page 76 and 77: fronts on prominent outwash aprons
Page 78 and 79: and Paleozoic rocks. Such pebbles a
Page 80 and 81: Windrow Formation of supposed Creta
Page 82 and 83: Kilbourn I Prairie du Sac Figure 17
Page 84 and 85: also have had their direction of fl
Page 86 and 87: TABU 11. SUMMARY OF NATURAL PLANT C
Page 88 and 89:
Needless to say, white man's agricu
Page 90 and 91:
Black, Robert F., 1964, Potholes an
Page 92 and 93:
Fahnestock, R. K. , 1963, Morpholog
Page 94 and 95:
Leith, C.K., 1913, Structural geolo
Page 96 and 97:
Salisbury, R.D., 1895, Preglacial g
Page 98:
Wells, J.W., 1963, Coral growth and
Page 101 and 102:
East end of Baraboo Ranges at eleve
Page 103 and 104:
Cross the road - but watch out for
Page 105 and 106:
49.3 Ochsner Park on left. Note wal
Page 107 and 108:
Figure 21. Geologic map of Upper Na
Page 109 and 110:
on the north bank of the river acro
Page 111 and 112:
Van Hise (1896), Steidtman (1910),
Page 113 and 114:
Large. Old Quartzite Quarry behind
Page 115 and 116:
can be traced in the woods. It is o
Page 117 and 118:
A fault may be present in the gap,
Page 119:
solution from the quartzite into Ca
Page 122 and 123:
Remarkably penetrative cleavage (Sl
Page 124 and 125:
Junction with South Shore Road Turn
Page 126 and 127:
Junction U.S. 12 Turn left, crossin
Page 128 and 129:
Figure 29. Structures in the phylli
Page 130 and 131:
Beneath the main phase fold shown i
Page 132 and 133:
Figure 31. Geologic map of the La R
Page 134 and 135:
Figure 33. Sketches of structural r
Page 136 and 137:
edding surfaces are invariably slic
Page 138 and 139:
At the mouth of Hemlock Draw there
Page 140 and 141:
Return directly downhill (south) th
Page 142 and 143:
uses). Pine Hollow exposes excellen
Page 144 and 145:
Figure 37. Geologic map of the Denz
Page 146 and 147:
67.5 Tunnel City in cliff at left w
Page 148 and 149:
0 10 20 feet Figure 40. Sketch of e
Page 150 and 151:
Cambrian conglomerate and sandstone
Page 152 and 153:
SUPPLEMENTARY STOP B -- Upper Narro
Page 154 and 155:
147 SUPPLEMENTARY STOP C -- Flagsto
Page 156 and 157:
Orientation Data for Crorr Strrta P
Page 158 and 159:
SUPPLEMENTARY STOP E -- Diamond Hil
Page 160 and 161:
of STOP 6) because its southerly ve
Page 162 and 163:
Figure 49. Structural relations in
Page 164 and 165:
SSE NNW Figure 51. Secondary phase
Page 166 and 167:
Features to be Seen: Compositional
Page 168 and 169:
Figure 54. Tension gashes and quart
Page 170 and 171:
Remarks : This is an excellent plac
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