IMPRESSIVE SCOURING ON BURIED PRECAMBRIAN BARABOO QUARTZITE, WI

In Spring 2000, blasting in Williams Quarry near Baraboo, WI, exposed a unique outcrop of scours and potholes/channels in the Precambrian Baraboo Quartzite. Documentation of this and subsequent exposures was critical because exposures were destroyed approximately every three to eight months. In the quarry, the Baraboo Quartzite is buried beneath ~ 7-10m of Cambrian Parfreys Glen Conglomerate, interpreted to have been a local boulder-cobble-pebble beach of the Sauk transgression.

The initial outcrop was along the high wall and contained a vertical and slightly concave scour ~ 7m high and 20m long. Two low areas in the high wall resembled channels. The top of much of the quartzite "rolled in" under the overlying conglomerate, suggesting the erosion surface was more extensive. Indeed, all subsequent blasting revealed more scours and potholes and/or channels. For example, the second blast revealed smaller scours but with a bulbous pothole ~ 3m across. In two other exposures, jutting irregularities in the high wall showed apparent channels going into the quartzite parallel to the high wall. In each subsequent exposure, the scours diminished in size and number. Retreat of the high wall has been about 150m.

The mode of origin of the scours is ambiguous. Initially, the exposures appeared to be of a fluvial origin, however the classic interpretation of the Baraboo area is that of a tropical coastal environment. Modern fluvial analogs in southeast Missouri as well as modern coastal analogs in Oregon and Washington both reveal numerous features like those in the quartzite. In both modern analogs, scouring appears to be controlled more by low joint density rather than by the lithology or hardness of the bedrock.

Small potholes in Baraboo Quartzite surface exposures have been observed in at least three different localities in the area, and explanations for their time of formation range from Precambrian to Pleistocene. The quarry exposures are the only known locally scoured surfaces buried under rock of known age, providing stratigraphic control on possible age interpretations of the erosion. If the features are fluvial, then scouring could have been Precambrian. If coastal, then Late Cambrian would be the logical time for the erosion.