The Geography of the Region about Devil's Lake and the Dalles of the Wisconsin. Wallace Walter Atwood
the shear zones parallel to the bedding planes, there is one distinct vertical shear zone (Plate VIII) three to four feet in width. It is exposed to a height of fully twenty-five feet. Along this zone the quartzite has been broken into angular fragments, and at places the crushing of the fragments has produced a "friction clay." Slipping along vertical zones would be no necessary part of folding, though it might accompany it. On the other hand, it might have preceded or followed the folding.
Schistose structure probably does not always denote shearing, at least not the shearing which results from folding. Extreme pressure is likely to develop schistosity in rock, the cleavage planes being at right angles to the direction of pressure. It is not always possible to say how far the schistosity of rock at any given point is the result of shear, and how far the result of pressure without shear.
Schistose structure which does not appear to have resulted from shear, at least not from the shear involved in folding, is well seen in the isolated quartzite mound about four miles southwest of Baraboo on the West Sauk road (f, Plate II). These quartzite schists are to be looked on as metamorphosed quartzite, just as quartzite is metamorphosed sandstone.
At the Upper narrows of the Baraboo also (b, Plate II), evidence of dynamic action is patent. Movement along bedding planes with attendant development of quartz schist has occurred here as at the lake (Plate IX). Besides the schistose belts, a wide zone of quartzite exposed in the bluffs at this locality has been crushed into angular fragments, and afterwards re-cemented by white quartz deposited from solution by percolating waters (Plate X). This quartzite is said to be brecciated. Within this zone there are spots where the fragments of quartzite are so well rounded as to simulate water-worn pebbles. Their forms appear to be the result of the wear of the fragments on one another during the movements which followed the crushing. Conglomerate originating in this way is friction conglomerate or Reibungsbreccia.
The crushing of the rock in this zone probably took place while the beds were being folded; but the brecciated quartzite formed by the re-cementation of the fragments has itself been fractured and broken in such a manner as to show that the formation has suffered at least one dynamic movement since the development of the breccia. That these movements were separated by a considerable interval of time is shown by the fact that the re-cementation of the fragmental products of the first movement preceded the second.
What has been said expresses the belief of geologists as to the origin of quartzite and quartz schists; but because of popular misconception on the point it may here be added that neither the changing of the sandstone into quartzite, nor the subsequent transformation of the quartzite to schist, was due primarily to heat. Heat was doubtless generated in the mechanical action involved in these changes, but it was subordinate in importance, as it was secondary in origin.
Igneous rock is associated with the quartzite at a few points. At g and h, Plate II there are considerable masses of porphyry, sustaining such relations to the quartzite as to indicate that they were intruded into the sedimentary beds after the deposition of the latter.
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. IX.
A mass of quartzite in situ, in the road through the Upper Narrows near Ableman's. The bedding, which is nearly vertical, is indicated by the shading, while the secondary cleavage approaches horizontality. See larger image
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. X.
Brecciated quartzite near Ablemans in the Upper Narrows. The darker parts are quartzite, the lighter parts the cementing quartz.
See larger image
III. RELATIONS OF THE SANDSTONE OF THE PLAIN TO THE QUARTZITE OF THE RIDGES.
The horizontal beds of Potsdam sandstone may be traced up to the bases of the quartzite ranges, where they may frequently be seen to abut against the tilted beds of quartzite. Not only this, but isolated patches of sandstone lie on the truncated edges of the dipping beds of quartzite well up on the slopes, and even on the crest of the ridge itself. In the former position they may be seen on the East bluff at Devil's lake, where horizontal beds of conglomerate and sandstone rest on the layers of quartzite which dip 14° to the north.
The stratigraphic relations of the two formations are shown in Fig. 5 which represents a diagrammatic section from A to B, Plate II. Plate XI is reproduced from a photograph taken in the Upper narrows of the Baraboo near Ablemans, and shows the relations as they appear in the field. The quartzite layers are here on edge, and on them rest the horizontal beds of sandstone and conglomerate. Similar stratigraphic relations are shown at many other places. This is the relationship of unconformity.
Such an unconformity as that between the sandstone and the quartzite of this region shows the following sequence of events: (1) the quartzite beds were folded and lifted above the sea in which the sand composing them was originally deposited; (2) a long period of erosion followed, during which the crests of the folds were worn off; (3) the land then sank, allowing the sea to again advance over the region; (4) while the sea was here, sand and gravel derived from the adjacent lands which remained unsubmerged, were deposited on its bottom. These sands became the Potsdam sandstone.
This sequence of events means that between the deposition of the quartzite and the sandstone, the older formation was disturbed and eroded. Either of these events would have produced an unconformity; the two make it more pronounced. That the disturbance of the older formation took place before the later sandstone was deposited is evident from the fact that the latter formation was not involved in the movements which disturbed the former.
Although the sandstone appears in patches on the quartzite ranges, it is primarily the formation of the surrounding plains, occupying the broad valley between the ranges, and the territory surrounding them. The quartzite, on the other hand, is the formation of the ridges, though it outcrops at a few points in the plain. (Compare Plates II and XXXVII.) The striking topographic contrasts between the plains and the ridges is thus seen to be closely related to the rock formations involved. It is the hard and resistant quartzite which forms the ridges, and the less resistant sandstone which forms the lowlands about them.
That quartzite underlies the sandstone of the plain is indicated by the occasional outcrops of the former rock on the plain, and from the fact that borings for deep wells have sometimes reached it where it is not exposed.
The sandstone of the plain and the quartzite of the ridges are not everywhere exposed. A deep but variable covering of loose material or mantle rock (drift) is found throughout the eastern part of the area, but it does not extend far west of Baraboo. This mantle rock is so thick and so irregularly disposed that it has given origin to small hills and ridges. These elevations are superimposed on the erosion topography of the underlying rock, showing that the drift came into the region after the sandstone, limestone, and quartzite had their present relations, and essentially their present topography. Further consideration will be given to the drift in a later part of this report.
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XI.
The northeast wall of the Upper Narrows, north of Ableman's, showing the horizontal Potsdam sandstone and conglomerate lying unconformably on the quartzite, the beds of which are vertical.
See larger image
PART II.
Конец