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AAPG Bulletin, V.
Stratigraphic controls on vertical fracture patterns in Silurian dolomite, northeastern Wisconsin
1Geological Engineering Program, University of Wisconsin, Madison, Wisconsin, 53706-1692; current address: Montgomery Watson Harza, One Science Court, Madison, Wisconsin, 53711; email: [email protected]
2Geosciences Department, University of Massachusetts, Amherst, Massachusetts, 01003-5820; email: [email protected]
3Geology and Geophysics Department, University of Wisconsin, Madison, Wisconsin, 53706-1692 ; email: [email protected]
4Geology Department, University of Wisconsin, Oshkosh, Wisconsin, 54901-8649; email: [email protected]
Chad A. Underwood received a B.S. degree in geology from the University of Wisconsin at Eau Claire and an M.S. degree in geological engineering from the University of Wisconsin at Madison. He currently works as a geotechnical engineer for Montgomery Watson Harza (MWH) in Madison, Wisconsin.
Michele L Cooke received a B.S.E. degree in geological engineering from Princeton University and an M.S. degree in civil engineering and a Ph.D. in geological and environmental sciences (with mechanical engineering minor) from Stanford University. Currently an assistant professor of geosciences at University of Massachusetts-Amherst, her research applies structural geology and rock fracture mechanics to investigate deformation of active and ancient structures.
Toni Simo received both an M.S. degree (1982) and a Ph.D. (1985) from the University of Barcelona. He was a Fulbright Scholar before joining the Department of Geology and Geophysics, University of Wisconsin-Madison in 1989. His primary interest is in carbonate sedimentology, sequence stratigraphy of sedimentary basins, and environmental sedimentology. Recent work in these areas includes Paleozoic epeiric sea deposition and shelf margins in the United States, synsedimentary facies associated with contractional settings in Spain and Indonesia, and stratigraphic implications in rock fracturing and arsenic contamination.
Maureen Muldoon received her M.S. degree and Ph.D. from the University of Wisconsin-Madison. She worked at the Wisconsin Geological and Natural History Survey for seven years before joining the faculty at the University of Wisconsin-Oshkosh. Her research interests include the investigation of groundwater quality and flow in carbonate rocks, relationship between stratigraphy and hydraulic properties, land use impacts on groundwater quality, and delineation of wellhead protection zones in fractured rock.
Vertical opening-mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed Silurian dolomite of northeastern Wisconsin. Our two-stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons (e.g., organic partings or cycle-bounding mud horizons) in terminating opening-mode fractures. First, the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern. Both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces. The two methods show similar results, confirming the validity of qualitative visual inspection. The second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy. For example, 63% of cycle-bounding mud horizons within the inner-middle and middle shelf facies associations serve as mechanical interfaces. These empirical percentages are input to a Monte Carlo analysis of 50 stochastic realizations of mechanical stratigraphy. Comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33%. This method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces. The methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing.
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