Tectonics of the Baraboo Region

ES 767 Global Tectonics
Renee Wawczak
May 2, 2014 

Wisconsin Map
Image: Wisconsin Geologic Survey, http://wisconsingeologicalsurvey.org/bdrk.htm

Introduction
Origins and Formation of the Syncline
Rhyolite
Igneous Intrusives
Conclusions
References

Introduction
The Baraboo region is a unique geologic setting with various tectonic features that is found in the Sauk and Columbia counties of south central section of Wisconsin.  The most prominent and well known feature is the Baraboo Syncline, which is comprised of a North and South Range.   The Baraboo Range is approximately 25 miles long and 10 miles wide at its maximum width, and is composed of Precambrian rhyolites, quartzite of the Baraboo Formation, and various igneous intrusives (Dalziel and Dott, 1970).  Several chemical varieties of rhyolite are present, most predominantly in the North Baraboo Range, and the rhyolite is overlaid by the younger Baraboo Quartzite.  Together, these stratigraphic sequences make up the Baraboo Syncline.


Image: Precambrian stratigraphic sequence for the Baraboo region.  Adapted from Attig and Clayton, 1990, http://www.koubadrilling.com/well-drilling/docs/sauk-county-geological-report.pdf

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Origins and Formation of the Syncline
The Sauk and Columbia counties of south-central Wisconsin exhibit diverse structural geology.  The most obvious and well known structure is the Baraboo Syncline.  Approximately 1.7 billion years ago, sands rich in hematite along with clay were deposited in a shallow marine environment, creating massive formations of sandstone (Attig and Dott, 2004).  Between 1.65 and 1.45 billion years ago, folding and metamorphism of the rocks in the Baraboo region occurred, creating what is now known regionally as the Baraboo Syncline.  The hematite rich sandstone metamorphosed into the characteristically pinkish-red Baraboo Quartzite, while the interspersed clay layers that metamorphosed into slaty phyllitic layers within the quartzite.  The metamorphosed quartz sandstone is homogenous; overall it is composed of more than 80 percent quartz.  The formation is approximately 4,000 feet thick, and subdivided into three units based on observable stratigraphic features in each layer (Attig and Clayton, 1990).  These visible features, including well persevered ripple marks, cross-bedding, and cleavage, assist in determining the degree of metamorphism.  Based on the well preserved stratigraphic features found throughout the formation, it is believed that the Baraboo Syncline experienced a low grade of metamorphism (Dalziel and Dott, 1970).

The popular hypothesis for the folding and metamorphism is that they are a result of the collision of two continental plates approximately 1.65 billion to 1.45 billion years ago (Attig and Dott, 2004).  Due to the unique structure of the syncline, it is theorized that two separate convergences actually occurred (one pushing in a north-south direction, the other in an east-west direction).  The result of the two tectonic episodes resulted in the present day asymmetrical double-plunging syncline.  The asymmetrical nature of the syncline results in the reference to the North and South Ranges of the formation.  The North Range has experienced extremely steep folding; in fact, it is vertical and even overturned in several locations.  The South Range exhibits a gentle dip and creates broad outcroppings.  Visible differences in outcrop widths at one time gave the impression that one side of the syncline is thicker than the other; however, true thickness calculations reveal that both ranges are the same thickness.  It should be noted that the macroscopic syncline is not the only feature to be created as a result of the compressional forces.  Local anticlines, faulting, tension gashes, and boudinage have all been identified in the Baraboo Range.

Asymmetrical Syncline
Image: Diagram illustrating the asymmetrical shape of the Baraboo Syncline, http://digicoll.library.wisc.edu/cgi-bin/WI/WI-idx?type=HTML&rgn=div2&byte=1667058238

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Rhyolite
At 1.76 billion years old, rhyolite is the oldest rock type that outcrops locally (Attig and Dott, 2004).  Rhyolite outcrops can be found at the outer edges of both the North and South Ranges of the Baraboo Syncline, with the most extensive of the outcrops occurring on the north slopes of the North Range.  That the rhyolite occurs only on the outermost rims of the Baraboo Range provides strong evidence that it predates the other rock types known to the area.  The rhyolite found in the Baraboo region is slightly metamorphosed, ranges in texture from coarse to fine grained, and ranges in color from felsic to nearly black.

The rhyolite found in the North Range is red in color and exhibits feldspar phenocrysts (Attig and Clayton, 1990), and can be mistaken for the nearby Baraboo Quartzite.  Lapilli, small versions of volcanic bombs, have been identified in this formation which suggests a violent past eruption at this site.  This rhyolite has been categorized as an ignimbrite, a volcanic pyroclastic rock.  The South Range exhibits similar rhyolitic features, as well as a collection of large masses of igneous material containing at least 75 percent volcanic bombs, known as agglomerates.

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Igneous Intrusives
Two igneous intrusive bodies are evident in the Baraboo region, although these types of formations are not common to the Sauk and Columbia counties.  These intrusions are evidence of separate tectonic forces that existed outside the formation of the Baraboo Range.  The first igneous body is known as the Baxter Hollow Granite.  This body is a fine-grained granite and is estimated to be approximately 1.75 billion years old (Attig and Dott, 2004).  This is the only igneous rock that is known to have intruded into the Baraboo Quartzite (Attig and Clayton, 1990).  The second igneous body is known as the Denzer Diorite, which exhibits a medium-grained texture with red and gray quartz inclusions. Mineral alteration has been identified, indicating that the diorite has undergone minor chemical modifications.  
The diorite has been compared chemically to a subalkaline dike that intrudes exposed rhyolite approximately 50 km northeast of Columbia County (Attig and Clayton, 1990).  However, this body has no known direct contact with the Baraboo Syncline; therefore, the age of this intrusion had not been positively determined.

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Conclusions
The Baraboo area of south-central Wisconsin hosts numerous tectonic features, including a regional syncline, evidence of regional low-grade metamorphism, igneous structures and intrusions, and a variety of locally occurring small scale structures.  The Baraboo Syncline, which is composed primarily of quartzite, exhibits an asymmetrical structure which suggests that the region has been subjected to more than one episode of continental convergence.  Three types of igneous rocks have also been recognized in this region.  Rhyolite has been identified on either side of the Baraboo Syncline, and has been interpreted to be the oldest rock in the area.  Two igneous bodies, the Baxter Hollow Granite and the Denzer Diorite, are also present in the Baraboo region.  It has been noted that the Baxter Hollow Granite intruded the Baraboo Quartzite; however, there are no known contacts between the Baraboo Quartzite and the Denzer Diorite.  While many tectonic studies have been completed in this area, additional information is needed to definitively identify the structural forces that created the asymmetrical syncline, as well as positively identify and categorize the igneous intrusives.  This additional information would aid in completing the tectonic timeline for these localized features.

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References:
Geology of Baraboo District, Wisconsin, Dalziel, I.W.D., Dott, R.H., Geological and Natural History Survey, Madison, Wisconsin, 1970.
Available online at: http://wisconsingeologicalsurvey.org/pdfs/IC14.pdf 

“Geology of Sauk County, Wisconsin,” Attig, John W., Clayton, Lee, Wisconsin Geological and Natural History Survey, 1990. 
Available online at:
http://www.koubadrilling.com/well-drilling/docs/sauk-county-geological-report.pdf 

Roadside Geology of Wisconsin, Attig, John W., Dott, R.H., Mountain Press Publishing Company, Missoula Montana, 2004.