BEDROCK FRACTURES IN SOUTHEASTERN WISCONSIN: PALEOSTRESS ESTIMATES AND RELATIONSHIPS TO THE WAUKESHA FAULT

 

RYMASZEWSKI, Jody A., FRIEDRICH, Jason L., and CZECK, Dyanna M.,

Department of Geosciences, University of Wisconsin Milwaukee, P.O. Box 413,

Milwaukee, WI 53201 jody@uwm.edu

 

INTRODUCTION

 

The Waukesha Fault is an enigmatic structure cutting through Silurian dolomite in southeastern Wisconsin.  It is a normal fault, oriented ~N40E, 60SE with an apparent offset of 10 m (Mikulic and Mikulic, 1977; Sverdrup et al., 1997). It is only known to outcrop at the Waukesha Stone and Lime Quarry.  Detailed gravity surveys in the region reveal the lateral and vertical extent of the fault (Sverdrup et al., 1997; Skalbeck et al., 2006). The fault trace extends NE to the town of Port Washington, Wisconsin at the shore of Lake Michigan and can be traced to depths of 700 to 800 m (Sverdrup et al., 1997), showing that it is a major feature of an otherwise undeformed region.

 

The goals of our study are to 1) estimate the paleostress orientations that formed the Waukesha Fault, 2) measure the orientations of nearby small-scale fractures, 3) estimate the paleostress orientations that formed the small-scale fractures, and 4) compare the paleostress orientations for all the structures.

 

METHODOLOGY

 

The one known exposure of the Waukesha Fault is currently not available for active study. Therefore, we conducted paleostress orientation estimates on the Waukesha Fault, assuming its geometry matched that predicted by Sverdrup et al., 1997.  We measured the orientations of 158 fractures in Silurian bedrock at the Lannon Stone Products quarry (located in Lannon, Wisconsin: ~11km NE of the known Waukesha Fault outcrop and ~3 km NW of the Waukesha Fault trace) during field seasons from 2004-2006. We also measured the orientations of 38 fractures in Devonian bedrock at the Harrington Beach State Park abandoned quarry and beach outcrops (located ~ 10km NE of Port Washington, and ~2.5 km NW of the Waukesha Fault trace). We used standard stereonet procedures to calculate the orientations of the paleostresses likely to have formed the Waukesha fault and the fractures at each of the field locations.

 

RESULTS

 

Based on the assumed geometry of the Waukesha Fault, s1, the maximum principal stress was oriented vertically, s2, the intermediate principal stress, was oriented (plunge/trend) 0/040  s3, the minimum principal stress, was oriented 0/130.

 

The Lannon quarry contains fractures with no discernable offset and faults with small-scale (maximum ~4 cm, mostly normal sense) offsets.  Some rare fractures have preserved ridge and groove lineations or plumose features, allowing classification as shear fractures or extensional fractures, respectively.  The fractures at the top of the quarry have a random distribution.  The small faults found at the low-mid levels of the quarry exhibit an approximate N32E/56SE orientation.  Most of the fractures with no apparent offset at mid and lower levels have the approximate orientation N30E/65S; a second group has approximate orientation of N45W/ steep-subvertical.  Cross-cutting relationships between the two sets are inconclusive.  Two prominent conjugate fracture sets formed at Harrington Beach State Park. The approximate orientations of these two sets are 1) N74E/subvertical and 2) N25W/ subvertical.  Unfortunately, the textures of most fracture surfaces are too weathered to determine whether the fractures are extensional or shear fractures. Thus, the paleostress estimates must be partly based on the relative geometries.

 

The small faults at the Lannon Quarry have estimated paleostress orientations: s1 = vertical; s2 = 0/040,  s3 = 0/130.  Most of the fractures at the Lannon Quarry have a similar orientation (but often with slightly steeper dips) to the small faults.  Therefore, we interpret that most of these fractures are small extensional or shear fractures with the same paleostress orientations as the small faults.  The second group of fractures at the Lannon Quarry could either be extensional or shear fractures, but lack of any offset supports the likelihood that they are extensional fractures.  If so, the paleostress orientations are s1 = vertical; s2 = 0/315,  s3 = 0/225.

 

The fractures at Harrington Beach could either be 2 distinct sets of extensional fractures or a conjugate set of shear fractures.  If they are extensional fractures, the paleostress orientations are 1) s1 = vertical; s2 = 0/074,  s3 = 0/164 and 2) s1 = vertical; s2 = 0/335,  s3 = 0/245.  If they are shear fractures, the paleostress orientations are s1 = 0/114 s2 = vertical,  s3 = 0/024.

 

CONCLUSIONS

 

While the paleostresses on all fractures could not be conclusively determined, the paleostress orientations inferred from most of the geometries of the small-scale features in the Silurian bedrock at Lannon are consistent with those estimated for the Waukesha Fault.  However, the paleostress orientations of the Devonian rocks at Harrington Beach are inconsistent with those estimated for the Waukesha Fault.  Therefore, it seems likely that 1) the timing of the Waukesha Fault may be bracketed by the deposition of the Silurian rocks at Lannon and the Devonian rocks at Harrington Beach, or 2) the deformation associated with the Waukesha Fault encompassed a broader region in the south.  Further study of fractures in the region is required to test these hypotheses.

 

REFERENCES

 

Mikulic, D.G., Mikulic, J.L.,1977. History of geologic work in the Silurian and Devonian of southeastern Wisconsin: Guidebook 41st annual tri-state field conference, A19-A27.

Skalbeck, J.D., Couch, J.N., Helgesen, R.S., Swosinski, D.S., 2006. Coupled modeling of gravity and aeromagnetic data to estimate subsurface basement topography in southeastern Wisconsin. Geoscience Wisconsin 17, 53-64.

Sverdrup, K.A., Kean, W.F., Herb, S., Brukardt, S.A., Friedel, R.J., 1997. Gravity signature of the Waukesha Fault, southeastern Wisconsin. Geoscience Wisconsin 16, 47-54.

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