EVIDENCE OF FRACTURE FLOW WITHIN THE WONEWOC FORMATION IN SOUTHCENTRAL WISCONSIN

The Cambrian-age Wonewoc Formation in southern Wisconsin is classically described as a homogeneous, well-sorted, well-rounded sandstone. It forms an important bedrock aquifer long conceptualized as dominated by porous-media flow. However, recent hydrogeologic studies of the Wonewoc Formation are altering our conceptual model of this aquifer by showing the existence of fractures that may influence groundwater flow.

Borehole flowmeter logs, geophysical logs and optical borehole images conducted through the Wonewoc Formation in Dane County, Wisconsin, provide evidence of important flow contributions from horizontal discontinuities interpreted to be bedding-plane fractures. These fracture flows are common in the Wonewoc Formation and sometimes can dominate the overall flow in a well or might provide less than 10% of the overall flow in other wells. We analyzed the occurrence of these bedding-plane fracture flows within the Wonewoc Formation to determine their regional extent and connectivity. Understanding whether these zones of bedding-plane fracture flow represent isolated fractures or regionally continuous features have important implications for properly characterizing this important regional aquifer. As part of an ongoing project to develop an updated groundwater flow model for Dane County, Wisconsin, efforts are underway to correlate these features between wells and determine their regional significance.

These insights will directly influence the conceptualization of the model layers for Dane County and provide a framework for evaluating potential bedding-plane fractures elsewhere within the Wonewoc Formation. The potential for significant flow contributions from fractures within homogeneous sandstone aquifers is important to consider when characterizing sandstone units in a wide variety of geologic settings.