THE DEVELOPMENT OF ECHELON VEIN ARRAYS IN THE McKIM LIMESTONE: EASTERN MONUMENT UPWARP, UTAH Solomon Seyum and David D. Pollard Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 email: [email protected] Abstract Keywords: The McKim Limestone unit is a well-exposed, 3m- Echelon veins, shear zones, pressure solution seams, thick stratum that stretches over much of Raplee McKim limestone, Raplee, Comb, anticline, monocline, anticline and across the antiformal hinge of Comb fold, fractures monocline as nearly continuous, kilometer-scale outcrops in some areas. Two sets of systematic, Introduction carbonate-filled, echelon veins and complementary Shear deformation of sedimentary strata is often echelon pressure solution seams are common and localized within relatively narrow zones. The varieties apparently present only within this unit. Each set of shear zones are placed into three general categories, maintains sub-parallel orientations across those parts brittle, brittle-ductile, and ductile, based on the inferred of the folds with modest limb dips. Left-stepping and gradient in displacement across the shear zone. The end right-stepping echelon vein arrays have mean trends of member of idealized brittle shear zones would be 097°and 140°, respectively. These two sets of marked by a sharp discontinuity where the approximately planar shear zones often cross-cut in a displacement gradient is zero elsewhere in the material conjugate geometry. They also are observed to abut one but infinite at the discontinuity. A shear zone where another or appear as only one set. The initial formation sharp discontinuities in displacement develop in of thin echelon veins suggests a brittle elastic conjunction with significant displacement gradients is deformation. However, some veins have greater considered a brittle-ductile shear zone. A ductile shear apertures to trace length ratios, and some have zone has a continuous gradient in displacement between sigmoidal shapes, suggesting ductile inelastic the zone boundaries. deformation. Two kinds of brittle-ductile shear zones are Current conceptual models of brittle-ductile shear observed in the Earth’s crust. One example is that of zones of this type infer principal strain and stress dragged and folded rocks adjacent to a fault. The orientations based on geometries and displacement second example is the development of echelon fractures indicators, such that the regional greatest compressive in a narrow zone that otherwise has deformed by stress bisects the acute angle between a pair of continuous and significant displacement gradients conjugate shear zones at the time of formation. (Ramsay, 1980). Limestone strata exposed across Previous studies have suggested that the shear strain Raplee anticline and Comb monocline of the eastern across a localized zone produces echelon extension Monument Upwarp, in southeastern Utah, display fractures, whose initial orientation within the zone is systematic arrays of echelon veins and pressure solution defined by the local principal strain direction. seams (Fig. 1) that are classified as the second example Other limestone and grainstone strata in this field of brittle-ductile shear zones. We suggest that area do not exhibit conjugate shear zones with echelon understanding their origin and development will vein arrays. A suggestion is that the specific contribute to the currently established tectonic history compositional structure of the McKim Limestone of the area that has, to date, been inferred from regional favored vein formation over jointing under the same jointing events and fold development. stress state. These observations open the door to Echelon veins found on the eastern Monument investigate the constitutive properties of the different Upwarp have been found exclusively within the thin, lithologies and to compare models of brittle-ductile 3m-thick, McKim Limestone unit. The McKim unit and shear zones based on a complete mechanics to those several other similar limestone units in the area act as based on kinematic relationships alone. The final cap rocks for trapping oil in source rocks below. objective of such an investigation would be to relate the Understanding the nature of deformation of this unit causative stress field to the regional tectonic history. may shed light on the structural integrity and permeability potential of limestone cap rocks in oil reservoirs. Stanford Rock Fracture Project Vol. 22, 2011 L-1 displacement, and strain fields can not be related to a causative, stress field using a purely kinematic approach. A few studies (e.g. Olson and Pollard, 1991) have used a complete mechanics approach to investigate echelon veins, but these are limited to elastic deformation. A comparison of shear zone formation with jointing in adjacent rock layers suggests that lithology plays a key role in governing the difference in failure mechanisms that is apparently manifest in strength and ductility contrasts. Geographic and Geologic Settings The study area is located in the far, southeastern corner of the state of Utah, between the towns of Bluff Figure 1. Echelon veins within conjugate shear zone and Mexican Hat. Comb monocline forms the 130 km arrays. Image of the top surface of the McKim boundary of the eastern Monument Upwarp, a Limestone on the antiformal axis of Comb prominent topographic high of the Colorado Plateau monocline. Left-stepping echelon veins (left of rock tectonic province (Fig. 2). The San Juan River, a major hammer) often trend east-west. Right-stepping tributary to the Colorado River, bisects the two main veins (right of hammer) often trend northwest- southeast. folds; having incised deep valleys that showcase the red hues of the jointed stratigraphy. The river marks the northern border of the Navajo Nation. Identification of these previously undocumented Comb monocline and Raplee anticline are adjacent, echelon vein and pressure solution seam arrays adds north-south trending, arcuate, reservoir-scale folds new information about the tectonic history of the dipping away from each other as much as 40° west eastern Monument Upwarp. They are investigated at (Raplee) and 60° east (Comb). Raplee anticline is a Raplee anticline and Comb monocline to make double-plunging fold that dwarfs in size next to Comb interpretations of the evolving tectonic stress state. monocline with a respectable 14 km fold axis length. Analyses of their geometries, mechanical interactions, The spatial relationship of the two folds and their vein fill textures, spatial and structural distributions, vertical geometries are illustrated in the geologic map and the textural features of the host rock composition and cross-section in Fig. 3. The age and lithology of are used to provide parameters to model deformation. strata exposed across Comb monocline range from Most previous models of echelon vein shear zones are Pennsylvanian marine sedimentary rocks to Jurassic limited to analyzing deformation as homogeneous terrestrial sedimentary rocks. Raplee anticline reveals simple shearing or variable simple shearing ina pre- Pennsylvanian- to Permian-age marine strata (Fig. 4). defined zone. Because the equations of motion are not invoked and no constitutive laws are prescribed, the Figure 2. Geologic map of southeastern Utah displaying the geographic locations of Monument Upwarp, Raplee Ridge and Comb Ridge. Modified from map by Bump and Davis (2003); (Mynatt et al., 2009). A white dashed line traces the length of Comb Ridge. Raplee Ridge is outlined with a rectangular box. Stanford Rock Fracture Project Vol. 22, 2011 L-2 Figure 3. a) Geologic map (modified from USGS map, Bull. 11, Plate 1, O’Sullivan et al. (1964)). The stratigraphic column to the right highlights the PPr unit (the portion of the Rico Formation exposed across the folds whose the uppermost stratum is the McKim Limestone) and the Pch unit (Halgaito Tongue that rests on top of the McKim in the study area). Green outlined regions indicate the extent of the McKim Limestone, so far visited, that exhibit echelon vein and pressure solution seam arrays. Red outlined regions are the extent of the McKim visited that do not display these structures. To the far left of the map is an incomplete red-outlined region that is partially outside of the map limits. The red arrow at the top right corner points in the direction of a McKim roadside outcrop visited 2 km outside of the map limits that does not have echelon vein or solution seam arrays. b) Geologic cross section from the linear transect displayed on the map. Both folds are interpreted as forming in response to Moore, 1931). Since there are no exposures of thrust slip along deep-seated thrust faults during an east-west faults associated with these folds, this interpretation is tectonic compressional event, likely due to the based on their similarity to other major folds found Cretaceous to Eocene Laramide orogeny (Gregory and elsewhere on the Colorado Plateau (Kelley, 1955; Stanford Rock Fracture Project Vol. 22, 2011 L-3 and/or thrust faulting processes (Mynatt et al., 2009). Fig. 5 is a regional geologic structure map of systematic joint distribution that illustrates this method of determining fracture-fold age relationships. Abutment relationships of joint sets at the outcrop scale was used to determine the relative ages of jointing (Fig. 6). Abutting joints are suggested to be younger than the joints they terminate against. Mynatt et al. (2009) identified three main, systematic joint sets across Raplee