A FAULTED SECTION OF EOLIAN ST. PETER SANDSTONE, NEAR BRODHEAD, WISCONSIN

In 2007, a straightening of State Highway 104, approximately 4.5 miles north of Brodhead, Wisconsin, produced an impressive roadcut (>75 feet deep and nearly 700 feet long) through the St Peter Sandstone. As with other outcrops of this unit in south-central Wisconsin, the roadcut features large-scale cross-bedding, moderate- and high-angle cross-beds, prominent orange, rust, and pink color staining, and well sorted, well rounded, frosted, spherical grains, consistent with an eolian origin for this sandstone.

In the spring of 2012 we: 1) compiled photomosaics of the lower portion of the east and west walls in the southern half of this roadcut, 2) recorded strike and dip measurements, 3) measured the positions and attitudes of numerous high-angle faults, and the apparent displacements along these faults, 4) made observations of sedimentary features, and 5) collected samples for study in the laboratory. Cross-beds suggest a range of dip directions, with inferred principal wind directions from the northeast, east, and southeast.

Apparent displacements across the faults were readily determined by measuring the offsets of specific, brightly-colored, sub-horizontal, diagenetic bands within the sandstone. These offsets ranged from less than 1 centimeter to greater than 1.5 meters. Some faults extend vertically upward more than 6 m, but seem to disappear before reaching the top of the roadcut. Most faults are high-angle, normal faults, but some smaller reverse faults are also present.

Precipitation of iron-oxide compounds near the water table produced diagenetic bands that are offset by, and therefore must have formed prior to, faulting. The multiple bands observed in the profile are consistent with changing water tables. Faulting may have occurred shortly after the diagenetic bands formed, possibly while sand was still being deposited above.

A significant block (approximately 9-m wide) of the sandstone has dropped below the level of the roadway on the east wall. Possible origins for the faulting include sinkhole collapse beneath the sandstone, sand deposition over snowdrifts followed by melting, or movement due to instability within the sedimentary deposit itself. Each of these hypotheses has implications that are not easily resolved, so the cause of the faulting remains a mystery.