LITHOSTRATIGRAPHIC CONTROLS ON NATURAL GAMMA INTENSITY AND PREFERENTIAL GROUNDWATER FLOW THROUGH THE TUNNEL CITY GROUP SANDSTONES, SOUTHERN WISCONSIN

An outcrop analog study of the Tunnel City Group sandstones in southern Wisconsin supports the utility of lithostratigraphic information in hydrostratigraphic studies of sandstone aquifers. Recent work has demonstrated that the Tunnel City Group (Upper Cambrian) has a highly correlatable natural gamma signal. This characteristic signal has been used in association with borehole slug testing and groundwater flow modeling to support the lateral continuity of preferential groundwater flow through the Tunnel City Group sandstones. Outcrop analog studies in southwestern Wisconsin show that the characteristic natural gamma signal is closely associated with storm-generated sequences of deposits in the Lone Rock Formation of the Tunnel City Group. The sequences are laterally continuous on the scale of an outcrop and typically consist of flat pebble intraclast conglomerate with a glauconite-rich matrix; glauconitic and feldspathic subquartzose sandstone with horizontal-planar, low-angle, or high-angle laminae; and feldspathic subquartzose sandstone with dolomite-filled burrows. Bedding plane fracture densities are highest at cycle boundaries and within the laminated subquartzose sandstone lithofacies. The sequences show upward increases in natural gamma radiation, which positively correlate with concentrations of potassium feldspar. The outcrop studies support the hypothesis that laterally continuous bedding plane fractures influence groundwater flow through the sandstones of the Tunnel City Group and they provide an explanation for the highly correlatable nature of the characteristic natural gamma signal.