Some good Baraboo cartoons in this section.
Baraboo 500 million years ago (98k)
Map of rhyolites in the Baraboo region (300k)
a double plunging syncline
Eruptions of silicic volcanoes and granitic intrusion took place in central and southern Wisconsin during the Mid-Proterozoic. Quartz sandstones were deposited over a wide area ontop of the volcanic rocks after the volcanic activity ceased. Small sections of these rocks are preserved as quartzites because of later deformation. We see them today scattered from South Dakota through Wisconsin.
The time period of 1.773 bya to 1.739 bya in the mid-Proterozoic, shows extensive volcanic and plutonic activity in Wisconsin. Rhyolites and granites cover an area in south central Wisconsin from the Fox River valley west through Baraboo where there extent is unknown. There are granites and rhyolites in central and northern Wisconsin that comprise much smaller volumes, and probably are similar in origin to the rocks of southern Wisconsin. The southern rocks are broken into two chemically distinct suites. A majority of these rocks yield a date of around 1.76 bya which is interpreted to be the end of peak magmatism. The first is a peraluminous suite of rhyolitic ash flow tuffs that vary in texture, and an associated subset of related granites. The second suite is mainly metaluminous rhyolite, with some associated biotite and hornblende granites.
Map of rhyolites in the Baraboo region (300k)The terms peraluminous and metaluminous describe the relative ratios of aluminum oxide to sodium and potassium oxides. These ratios can give you an idea of what the original source for the rocks was. Metaluminous rhyolites crop out occasionally along a zone 10 to 20 km wide that runs diagonally from the Baraboo hills to the north east past Berlin Wisconsin. To the north of this zone lie mainly metaluminous granites, and to the south, peraluminous rhyolites. Lesser amounts of dioritic intrusions, tholeiitic basalt, and andesitic dikes occur over a wide area and cut the previously mentioned rhyolite and granites. These rocks are possibly the first anorogenic rocks in the Great Lakes region, and are possibly linked to extensional tectonism. The magmas themselves were probably caused by the heat from a mafic source, from which the more basic dikes came that intrude the area.
This region reached a semi-stable period soon after the volcanics stopped, even though occasional emplacement of granites was still occurring. The region was eroding down and was probably close to sea level. Sediments began to collect, first terrestrial dirts and braided stream sediments, then later shallow marine sands. These quartz sandstones, now seen as Quartzites after metamorphosis, are very mature. This means they had to have come from a distant source, and have gone through a lot of mechanical weathering to become so homogeneous. Large volumes of quartz sandstones had accumulated in many parts of the Great Lakes region. We see the remnants of a probable semicontinuous sandy plain at the edge of a continent as quartzites all over Wisconsin with names such as the Barron, McCaslin, Waterloo, Flambeau, Baraboo, as well as the Sioux quartzite as far west as the Minnesota, South Dakota border. Each locality of quartzite has its own specifics, but they all relate closely to the Baraboo range which is by far the most studied. Baraboo, like the other quartzites, is made up of 95% quartz grains, with very small amounts of clays and feldspars. Baraboo in particular is 1200 meters thick, and has conformable shales and dolomite stratigraphically on top preserved in a few small areas, unlike any of the other quartzites.
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Assessing the time of deposition and subsequent deformation in the Baraboo area has been a continuing problem. Constraining the timing of sedimentation is a tricky thing. Unless there is some direct indicator of age interstratafied with the sediment, like a volcanic layers, indirect methods are required to constrain the beginning and end. In the case of the Baraboo quartzite, there are no easy indicators that can be dated directly. Deposition of the protolith quartz sandstone began some time after the deposition and partial erosion of the rhyolites in the area. At this contact, beneath the sand stones in the Baraboo area, lies a partially preserved dirt layer. This dirt layer was pivotal in the interpretation of the Baraboo area. For a long time, it was unclear if the rhyolite under the Quartzites was deposited before the sands, or if it intruded at a later date due to poor exposures. The existence of this dirt layer indicates the sands came after an erosional unconformity, and detrital zircons at the base of the baraboo, has settled the argument for most people. |
Picture of the contact by L.G.Medaris
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The underlying rhyolite and the detrital zircons from the Baraboo quartzite date to 1.76 bya . This is acting as an oldest age limit of deposition. A date for deformation is much harder because of the lack of the proper minerology. There is an episode recorded in the rhyolite around 1.64 to 1.615 bya, but its uncertain which metamorphic event is being recorded. Some workers suspect the deformation may be as late as the emplacment of the Wolf River Batholith 1.5 bya, which is described in the next section.
The geologic beauty of the baraboo region is the exposure and relationships of the quartzite and paleozoic strata. You look close in this region, you will see quartzite in a double plunging syncline. The cambrian sandstone that lies ontop with an angular unconformity has at the base extreamly coarse conglomerate. Some bolders are meters in diameter. This tells of a beach that was pounded with hurricanes 500 million years ago when the continent was much farther to the south. As time went on, the sea deeped, and the sands piled up, eventually buring the quartzite. Here is an image of what the tops of the Baraboo mountians might have looked like on a calm day 500 million years ago. |