Stratigraphic controls on AUTHORS Chad A. Underwood ϳ Geological vertical fracture patterns in Engineering Program, University of Wisconsin, Madison, Wisconsin, 53706-1692; current Silurian dolomite, northeastern address: Montgomery Watson Harza, One Science Court, Madison, Wisconsin, 53711; Wisconsin [email protected] Chad A. Underwood received a B.S. degree in Chad A. Underwood, Michele L. Cooke, J. A. Simo, geology from the University of Wisconsin at and Maureen A. Muldoon 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 ABSTRACT Watson Harza (MWH) in Madison, Wisconsin. Vertical opening-mode fractures are mapped on quarry walls to Michele L. Cooke ϳ Geosciences assess the stratigraphic controls on fracture patterns in the relatively Department, University of Massachusetts, undeformed Silurian dolomite of northeastern Wisconsin. Our two- Amherst, Massachusetts, 01003–5820; [email protected] stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons (e.g., organic partings or Michele L Cooke received a B.S.E. degree in cycle-bounding mud horizons)in terminating opening-mode frac- geological engineering from Princeton tures. First, the mechanical stratigraphy of the exposures is inter- University and an M.S. degree in civil engineering and a Ph.D. in geological and preted from the observed fracture pattern. Both visual inspection environmental sciences (with mechanical and a newly developed quantitative method are employed to iden- engineering minor) from Stanford University. tify effective mechanical interfaces. The two methods show similar Currently an assistant professor of results, confirming the validity of qualitative visual inspection. The geosciences at University of Massachusetts– second stage of our study stochastically predicts mechanical stratig- Amherst, her research applies structural raphy and subsequent fracture pattern from empirical relationships geology and rock fracture mechanics to between the observed sedimentary stratigraphy and the interpreted investigate deformation of active and ancient mechanical stratigraphy. For example, 63% of cycle-bounding mud structures. horizons within the inner-middle and middle shelf facies associa- J. A. Simo ϳ Geology and Geophysics tions serve as mechanical interfaces. These empirical percentages Department, University of Wisconsin, are input to a Monte Carlo analysis of 50 stochastic realizations of Madison, Wisconsin, 53706–1692; mechanical stratigraphy. Comparisons of the stochastically pre- [email protected] dicted and interpreted mechanical stratigraphy yield errors ranging Toni Simo received both an M.S. degree from 13 to 33%. This method yields far better results than assuming (1982) and a Ph.D. (1985) from the University that all stratigraphic horizons act as mechanical interfaces. The of Barcelona. He was a Fulbright Scholar methodology presented in this article demonstrates an improved before joining the Department of Geology and prediction of fracture pattern within relatively undeformed strata Geophysics, University of Wisconsin–Madison from both complete characterization of sedimentary stratigraphy in 1989. His primary interest is in carbonate and understanding mechanical controls on fracturing. 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 Copyright ᭧2003. The American Association of Petroleum Geologists. All rights reserved. fracturing and arsenic contamination. Manuscript received November 16, 2000; provisional acceptance September 21, 2001; revised manu- script received June 17, 2002; final acceptance July 29, 2002. AAPG Bulletin, v. 87, no. 1 (January 2003), pp. 121–142 121 INTRODUCTION Maureen A. Muldoon ϳ Geology Department, University of Wisconsin, Oshkosh, Wisconsin, 54901–8649; Fracture patterns exposed on both horizontal surfaces (e.g., Dyer, [email protected] 1988)and vertical outcrops (e.g., Gross et al., 1995; Bahat, 1999) Maureen Muldoon received her M.S. degree have been used to infer paleostress orientations and, thus, give evi- and Ph.D. from the University of Wisconsin– dence of the tectonic history of a region. Interpreting the geologic Madison. She worked at the Wisconsin history from vertical outcrop patterns of fractures requires consid- Geological and Natural History Survey for eration of both tectonics and stratigraphy, which can both produce seven years before joining the faculty at the variations in the fracture pattern (see Dholakia et al., 1988; Gross University of Wisconsin–Oshkosh. Her et al., 1995). Furthermore, determining the stratigraphic controls research interests include the investigation of on opening-mode fracture patterns helps us better predict the sub- groundwater quality and flow in carbonate surface flow paths of fluids such as ground water (e.g., Muldoon rocks, relationship between stratigraphy and and Bradbury, 1998)and hydrocarbons (e.g., Nelson, 1982).Un- hydraulic properties, land use impacts on derstanding the development of fracture networks within aquifers groundwater quality, and delineation of and reservoirs can improve fluid-flow prediction by constraining wellhead protection zones in fractured rock. some of the uncertainty in fracture characteristics, such as fracture length, aperture, and connectivity. In relatively undeformed sedimentary rocks, the density and height of opening-mode fractures (joints)are typically controlled by stratigraphy rather than faulting or folding (e.g., Becker and Gross, 1996; Hanks et al., 1997). Under these conditions, fracture density typically depends on the material properties and bed thick- ness of stratigraphic units (e.g., Huang and Angelier, 1989), because fractures commonly terminate at specific stratigraphic horizons (e.g., Gross et al., 1995). However, the pattern of fracturing is di- rectly controlled by mechanical stratigraphy, which does not nec- essarily correspond to the sedimentary stratigraphy (e.g., Corbett et al., 1987; Gross et al., 1995; Hanks et al., 1997). A mechanical unit represents one or more stratigraphic units that fracture inde- pendently of other units (Figure 1). Fractures typically span the thickness of the mechanical unit and commonly abut the bounding stratigraphic horizons. Such stratigraphic horizons along which many fractures abut are termed “mechanical interfaces” (Figure 1) (Gross et al., 1995). Consequently, the stratigraphic features that comprise the me- chanical stratigraphy of a sequence are those that control fracture initiation and termination in rock strata (e.g., Gross, 1993). Vertical opening-mode fractures (joints)commonly initiate from flaws somewhere within a mechanical unit and terminate at mechanical interfaces (Gross, 1993). In layered formations consisting of inter- bedded brittle/ductile rocks, fractures typically initiate in the brittle layer and terminate at the contact with ductile layers (e.g., Cook and Erdogan, 1972; Erdogan and Biricikoglu, 1973; Helgeson and Aydin, 1991; Rijken and Cooke, 2001). Within relatively homo- geneous layered formations, fractures may terminate at mechanical interfaces because of interface slip (e.g., Teufel and Clark, 1984; Renshaw and Pollard, 1995)and/or local debonding along inter- faces that are weak in tension (Cooke and Underwood, 2001). Whereas interface properties control fracture termination, me- chanical unit thickness, or spacing of mechanical interfaces, controls fracture density. Predicting fracture density therefore requires 122 Vertical Fracture Patterns in Silurian Dolomite (A) Figure 1. (A) Stratigraphic controls on fracture patterns. Fractures develop within me- Mechanical Unit chanical units and abut me- (may contain one or more chanical interfaces (modified stratigraphic horizons) from Gross et al., 1995). In- creased fracture density directly Mechanical Interface correlates with increased frac- at a Stratigraphic Horizon ture porosity. Both effective po- rosity and effective permeability are highly dependent on the connectivity of the fracture net- work; therefore, increased frac- ture density does not necessar- ily lead to increased Fracture permeability. (B) Termination of vertical fractures can redirect the vertical component of flow Exposed Vertical Fractures (B) Fluid to horizontal features, such as Flow dissolutionally enlarged bedding planes. Laterally extensive hori- Mechanical zontal high-permeability fea- Unit tures are continuous up to 14 km (9 mi) based on hydrogeo- logic studies of the Silurian do- lomite (Muldoon et al., 2001). Mechanical Interface Fracture knowledge of the distribution of mechanical interfaces. tigraphy, within a relatively undeformed setting, to test Field investigations indicate that in many relatively predictions of fracture pattern from stratigraphic in- thinly bedded rock types, the product of fracture den- formation. We interpret the mechanical stratigraphy of sity and bed thickness is approximately 1.0 (Price, a section of the shallow dipping (Ͻ2Њ)Silurian dolo- 1966; Hobbs, 1967; McQuillan, 1973; Huang and An- mite of northeastern Wisconsin (Figure 2)from careful gelier, 1989; Narr and Suppe, 1991; Gross, 1993; fracture mapping along two quarry exposures. This ar- Gross