INSIDE THE JORDAN SANDSTONE; CONTINUOUS EXPOSURE OF SEDIMENTARY STRUCTURES IN AN UNDERGROUND MINE NEAR BAY CITY, WISCONSIN

In 2015-2016 Fairmount Santrol provided access to their underground mine located near Bay City, Wisconsin. The purpose of this project is to study the excellent exposures of the mine “ribs” (walls); to reveal the three-dimensional geometry, cross-bedding orientation patterns, and sedimentary features within the upper Jordan Sandstone. Compared with surface exposures, the subsurface exposures in the mine are much more extensive and nearly continuous. In addition, the spatial layout of the mine allows three-dimensional description of this unit.

This mine uses room and pillar construction, but with narrow elongate rooms (drifts) and larger square- or rectangular-shaped pillars. Intersections make “T’s” and form an alternating left-right pattern along primaries throughout the mine. Typical lengths of ribs (wall surfaces) are 20 to 25 m, the height of the “back” (roof) is generally 6.5 to 8.0 m, and the drifts (i.e., the ‘streets’) are roughly 6 to 7 m wide. Three-way intersections increase the stability of the mine.

We chose to focus on an area of roughly 150 by 400 meters in the eastern part of the mine. The typical stratigraphy consists of a lower unit of medium- to thickly-bedded, medium-coarse quartzose sandstone with variable cross-bed orientations; a middle unit of alternating clay and medium-coarse quartzose sandstone layers; and an upper unit composed of medium-coarse, cross-bedded quartzose sandstone that dips generally northeastward, with maximum dips of 20 to 25°. This sand wave thickens eastward to 4 m.

Large soft-sediment deformation features in the mixed clay-sand unit occur where the overlying sand wave thickness increases. Other sedimentary features include abundant intraclasts, burrows and bioturbation structures, load casts and flame structures, and several small-scale faults. Liesengangen banding is common. The cross bedded sands display strong diagenetic iron-oxide coatings.

Laminae from the thicker clay bed (beneath the sand wave), spaced at fairly regular intervals, extend southwestward partway up onto cross-bed foresets. This implies that the clay bed is contemporaneous with the sand wave and is present in slightly deeper water northeast of the sand wave. The repetitive spacing of the laminae are consistent with tidal processes.