STRATIGRAPHIC CONTROLS ON FRACTURE PATTERNS WITHIN THE SILURIAN DOLOMITE OF NORTHEASTERN WISCONSIN

The accuracy of predictions of groundwater movement in fractured-carbonate aquifers is often limited by how well the fracture network has been characterized. While hydrogeologists are starting to recognize that sedimentological and stratigraphic models provide a method of incorporating geologic variability into models of fluid flow, less attention has been focused on the stratigraphic controls on fracture patterns. Fracture patterns in sedimentary rock can be evaluated in terms of mechanical stratigraphy. Stratigraphic horizons that resist fracture propagation act as mechanical interfaces that bound mechanical units since fractures abut these horizons. In this study, we integrate lithostratigraphic data with a detailed characterization of the mechanical stratigraphy of the Silurian dolomite in order to better predict the distribution of fractures in the subsurface.

Silurian stratigraphy in the Door Peninsula consists of predictable alternations of two characteristic facies associations: the Inner Shelf Facies Association deposited in supratidal to shallow tidal environments and the Middle Shelf Facies Association deposited in subtidal environments. The Inner-Middle Shelf Facies Association is transitional between these two end members.

Fracture maps of vertical quarry walls are used to assess the stratigraphic controls on fracture patterns. Fractures in the thinly-bedded Inner Shelf Facies Association are generally evenly distributed, densely spaced, and frequently abut against depositional cycle boundaries such as organic or mud horizons. Less commonly, fractures abut against horizons within a depositional cycle. The massive Middle Shelf Facies Association contains longer and more widely spaced fractures; fractures do not appear confined to distinct stratigraphic horizons because of the overall absence of weak interfaces.

Data obtained from fracture maps are used to develop empirical relationships between lithostratigraphy and mechanical stratigraphy. These relations allow us to predict mechanical stratigraphy and fracture patterns at depth where only lithostratigraphic data are available. This methodology is tested by comparing stochastically predicted mechanical stratigraphy to observed mechanical stratigraphy.