Proceedings of the 2011 Stanford Rock Fracture Project FIELD TRIP GUIDEBOOK Outcrop-scale Deformation in Folded Sedimentary Strata, Eastern Monument Upwarp, UT by Solomon Seyum and David D. Pollard June 9, 2011 Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-1 Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-2 Preface Folds are one of the most common and inspirational geologic structures, occurring in most rock types, displayed at scales ranging from thin-section to mountain range, making ornate patterns in outcrop, serving as the backbone of spectacular topographic features, and are associated with a wide spectrum of tectonic regimes. A question we wish to address here is, what deformation mechanisms operate within sedimentary strata at the thin section and outcrop scales during kilometer-scale folding as elastic bending gives way to the inelastic accumulation of large curvature and finite strains? This one day field trip to Raplee and Comb Ridges of the eastern Monument Upwarp in Utah is an introduction to three types of systematic deformation structures recorded within the folded marine and terrestrial sedimentary strata that make up these topographic highs. Systematic joints control the landscape; shaping physical geographic features such as the mesas and rock monuments, and providing pathways for surface drainage as well as highways. Arrays of echelon veins and pressure solution seams are confined to relatively thin and extensively well-exposed limestone layers that span the folds, and act as cap rocks for oil. Deformation bands are structures observed exclusively at outcrops of massive cross-bedded sandstones that dominate the stratigraphy at, and east of, Comb monocline. Objectives for the day include differentiating between systematic and nonsystematic outcrop-scale structures, deducing their relative ages, and interpreting the possible mechanisms for their formation based on observations of relative displacement indicators and structural position. With this information it might be possible to relate the different structures to different stages in the evolution of the folds and to regional tectonic stress states. Field and optical petrographic observations and measurements, and detailed mapping of these systematic structures at select outcrops serve to identify the mechanisms of deformation and relative age relationships, as well as their spatial variations with respect to structural position on the folds. Interpretations of the evolving tectonic stress state during folding will be made from geometric and kinematic data of deformation structures, appropriate material properties gleaned from the literature, and boundary conditions inferred from the geologic and tectonic setting. This information will motivate and constrain mechanical models of the structures using finite element methods. The underlying objective is to reconcile the occurrence, for example, of joints and shear zones in adjacent strata that apparently formed at the same time and under the same stress state. We suggest that this investigation will lead to a better understanding of the constitutive properties and strengths of these sedimentary rocks during folding. We have completed reconnaissance geological mapping, which brought to our attention conjugate shear zones with sigmoidal veins in the McKim Limestone on both Raplee anticline and Comb monocline. We anticipate that two aspects of this investigation may generate two transformative concepts. Unlike most, if not all, previous studies of conjugate shear zones with sigmoidal veins that rely on kinematic relationships alone, we propose models based on the fundamental conservation laws of mass and momentum formulated into the governing equations of motion. And secondly, the juxtaposition of apparently brittle (jointing) and ductile (shear zone) mechanisms in adjacent outcrops offers the opportunity to deduce how the constitutive properties of the respective formations varied at the time of folding. In general, the interplay of field mapping and petrographic analysis with continuum mechanical modeling has the potential to offer new insights and original concepts about the folding process. Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-3 The San Juan River is approximately 50m wide in both images Top left photo: Comb monocline. Viewing direction is south-southeast looking at the antiformal hinge, geographically known as Lime Ridge. Standing on the McKim Limestone unit overlooking the San Juan River, with Comb Ridge in the background characterized by “hogbacks”. Bottom right photo: Raplee anticline. Viewing direction is south-southeast looking at the forelimb. Standing on a butte of the Halgaito Tongue Formation near the length-midpoint of the fold overlooking the San Juan River. Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-4 Field Trip Road Map & Contents Stop 1: Introduction and Geological Setting - Mexican Hat Oil Field View of Raplee anticline. Tectonic and depositional setting. History of oil production. Stop 2: Jointing and Regional Stress History - Mexican Hat Rock Overview of jointing at Raplee anticline. Halgaito Tongue stratigraphic unit. Stop 3: Jointing: McKim Limestone - Mexican Hat syncline / Halgaito anticline Comparison of jointing in the McKim Limestone and in the overlying Halgaito Tongue. Stop 4: Echelon Veins: McKim Limestone - Northern plunge of Raplee anticline Joints in the Halgaito Tongue. Echelon veins and solution seams in the McKim Limestone. Stop 5: Deformation Bands: Navajo Sandstone - Abandoned road at Comb Ridge Deformation bands in the Navajo (cross-bedded) Sandstone. Overlook of Comb monocline. Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-5 Stop 1: Introduction and Geological Setting Bluff, UT -to- Stop 1 “Mexican Hat Oil Field” driving distance: 39.1 km (1 km well-groomed dirt road) driving duration: ~32 min Stop 1 Contents 1.1 Geographic Setting and Fold Geometries • Comb monocline • Raplee anticline 1.2 Regional Tectonics • Laramide Orogeny • Thrust fault-induced folding 1.3 Stratigraphy 1.4 Story of Petroleum Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-6 Figure 1. A geologic map of southeastern Utah displaying the geographic locations of Monument Upwarp, Raplee Ridge, and Comb Ridge. (Mynatt et al., 2009) (modified from map by Bump and Davis (2003)). The white dashed line traces the length of Comb Ridge. Raplee Ridge is outlined with a dashed rectangular box. 1.1 Geographic Setting and Fold Geometries Raplee anticline and Comb monocline are located on the Monument Upwarp in southeastern Utah, near the middle of the Colorado Plateau tectonic province and northwest of the Four Corners (Fig. 1 inset). The San Juan River bisects the two folds and marks the northern border of the Navajo Nation. Comb and Raplee are asymmetric, arcuate folds with approximately north-south trending axes. The spatial relationship of the two folds and their geometries are illustrated in the geologic map and cross-section in Fig. 2a. The steep limb of Comb monocline dips as much as 60° eastward and forms the 130 km eastern boundary of the upwarp. Comb Ridge refers to the triangular peaks resembling an upturned “comb”, located near the synformal hinge of Comb monocline. The massive, cross-bedded Navajo Sandstone prominently marks this topographic high. The antiformal hinge is about 2.4 km west of Comb Ridge, forming a local high known as Lime Ridge. The age and lithology of strata exposed across Comb monocline range from Pennsylvanian marine sedimentary rocks to Jurassic terrestrial sedimentary rocks. West of Lime Ridge, the tops of layers slope about 05° up to Raplee Ridge. The ridge marks the antiformal hinge of the 14 km-long Raplee anticline with its steeper limb dipping as much as 40° westward toward the Mexican Hat Syncline and the town of Mexican Hat (Fig. 2b). This anticline reveals Pennsylvanian- to Permian-age marine sedimentary rocks. The sedimentary rocks are well-exposed in this high desert environment (Jentgen, 1977; Ziony, 1966), and vertical profiles of these two folds are revealed by the deep incision of the San Juan River and steep drainage channels (Fig. 3). The combination of good exposure and deep incision in the last several Ma (Wolkowinsky and Granger, 2004) provide an exceptional opportunity to map outcrop-scale deformation structures and the varying kilometer-scale shapes of the folds over a stratigraphic interval ranging from Pennsylvanian to Jurassic. Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-7 Figure 2. a) Geologic map of Raplee anticline and Comb monocline (modified from USGS map, Bull. 11, Plate 1 O’Sullivan et al. (1964)). The yellow stars mark the sites to be visited on this field trip. The red boxes in the stratigraphic column allude to the units of interest. The McKim Limestone marking the top of PPr. Pch is the Halgaito Tongue. JTRn is the Navajo Sandstone. b) Cross section showing the structure of Raplee and Comb folds. Stanford Rock Fracture Project Vol. 22, 2011 Field Trip Guide-8 Figure 3. Shaded relief map of Airborne Laser Swath Mapping (ALSM) data showing the spatial distribution of the five bedding-plane surfaces (geologic units) exhumed within the fold. From stratigraphically highest to lowest, they are the McKim, Goodrich, Shafer, Mendenhall, and Unnamed surfaces. Locations of structural sections are noted in the geologic map. Structural sections have no vertical exaggeration. Arrows in the stratigraphic column note the stratigraphic