Role of fracture localization in arch formation, Arches National Park, Utah KENNETH M. CRUIKSHANK* 1 Rock Fracture Project and Bailey Willis Geomechanics Laboratory, Department of Geological ATILLA AYDIN J and Environmental Sciences, Stanford University, Stanford, California 94305 ABSTRACT ever, because these processes are not re- themselves participate in arch formation. In stricted to the sites of arches in rock fins. many sites we are able to demonstrate that Spectacular rock fins on the flanks of Salt There must be some factor that locally en- the reason for fracture localization is shearing Valley anticline in southeast Utah are formed hances the effects of erosion within a rather along preexisting discontinuities, together with by erosion along zones of joints. Within a rock small part of a rock fin to produce an arch. the interaction between adjacent sheared fin, arches form where intense fracturing is lo- How erosion is localized within a rock fin to discontinuities. calized. Fracture localization is controlled by form an arch is enigmatic. In the case of nat- shear displacement along existing horizontal or ural bridges (for example, Natural Bridge Geological Setting vertical discontinuities. Horizontal discontinu- National Monument, Southeast Utah), a ities may be shale layers, shale lenses, or bed- river or stream is assumed to be the agent Arches National Park is centered on the ding planes, whereas vertical discontinuities providing localized erosion. Arches within salt-cored Salt Valley anticline (Fig. lb), are usually preexisting joint segments. The Arches National Park, however, are not as- which represents the northwest extent of the roof and overall shape of an arch is controlled sociated with fluvial activity. Paradox basin salt diapirs. Rocks of Jurassic by existing shale layers, interfaces between In this paper we address mechanisms that and Cretaceous age are exposed on the flanks sandstones of different properties, or second- locally fragment rock, making it more sus- of the anticline, which dip up to about 15°. ary fractures due to shear on vertical joints. ceptible to erosion. Where such a fracture- The majority of arches are within the Jurassic Joints that bound rock fins are related to the damaged zone exists within a rock fin, the Entrada Sandstone. formation of the diapir-cored Salt Valley anti- effects of erosion are locally enhanced and an The Entrada Sandstone is underlain by the cline. Shear displacement along existing dis- arch will probably form. Local enhancement Navajo Sandstone (Fig. lc), a massive eolian continuities, which localizes intense fracturing, of erosion by fracture concentration probably sandstone with a thickness of 41-91 m (Dyer, is probably related to the growth of Salt Valley accounts for the majority of arches within 1983). The lowest member of the Entrada anticline and its subsequent collapse due to dis- Arches National Park. We do not imply, Sandstone, the Dewey Bridge Member, con- solution of the anticlines salt core. however, that all arches within the park sists of interbedded fine-grained-silty sand- are controlled by a single mechanism. It stone and siltstone and rests unconformably INTRODUCTION should be remembered that the term on the Navajo. The Dewey Bridge Member "arch" has been applied to a geomorphic ranges in thickness from ~6 to >30 m. The Arches are structures formed by perfora- feature without regard to the mechanisms overlying Slickrock Member is a dark red, tion of rock walls. Arches National Park in that shaped it, which makes it difficult massive, fine-grained sandstone, whereas southeast Utah has >700 arches—with spans when applying a mechanism to the local- the upper Moab Member is a light-colored, ranging from 1 to 93 m and heights up to 34 ization of an "arch." We have examined clean, fine- to medium-grained sandstone. m—within an area of 29,695 ha (Stevens and ail major named arches and many smaller The Slickrock Member ranges from 60 to 160 McCarrick, 1988). Native American inhabit- arches and rock shelters. At all sites we m in thickness, whereas the Moab is —20-40 ants of the region believed that arches were see evidence for the fracture localization m thick. The Navajo, Slickrock, and Moab built by the Great Sky Father, whereas some that we describe in this paper. Other fac- sandstones are more resistant to weathering early settlers believed that the arches were tors may have assisted in locally acceler- than the Dewey Bridge. The Slickrock Mem- handcrafted by prehistoric native Americans ating the effects of erosion, such as under- ber is the major cliff-forming unit within the (Barnes, 1978). Currently, localized erosion cutting of cliffs or channeled runoff from park (Lohman, 1975). The Entrada Sand- of rock fins by wind and water is considered buttes. Each arch is unique, but in this pa- stone is overlain by the Jurassic Tidwell to be the process responsible for the forma- per we describe a common theme for the Member of the Morrison Formation. The tion of arches (Barnes, 1978; Stevens and arches at Arches National Park. Tidwell Member consists of thin-bedded red McCarrick, 1988). Arch formation cannot be Previous workers ascribed the dense net- sandstone and shale with local concentra- due solely to weathering and erosion, how- work of fractures associated with arches to tions of chert (Doelling, 1988). the presence of an arch, rather than as a fac- The majority of arches are composed tor that controlled the location and formation of Entrada Sandstone, although arches are * Present address: M. King Hubbert Geome- chanics Laboratory, Earth and Atmospheric Sci- of an arch. The arch-forming fractures are present in the overlying Morrison and un- ences Department, 1397 CIVL, Purdue Univer- concentrated along preexisting discontinui- derlying Navajo and Wingate Formations. sity, West Lafayette, Indiana 47907-1397. ties—such as shale lenses and joints—that Within the Entrada Sandstone, most of the Geological Society of America Bulletin, v. 106, p. 879-891, 11 figs., July 1994. 879 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/7/879/3381965/i0016-7606-106-7-879.pdf by guest on 25 September 2021 uranium Yeliow mines , t Cat P r Dry Mesa g Window PARK Tower of Babel Arches Natl, g Park" 880 Geological Society of America Bulletin, July 1994 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/7/879/3381965/i0016-7606-106-7-879.pdf by guest on 25 September 2021 ROLE OF FRACTURES IN ARCH FORMATION Figure 1A. Simplified geologic map of Arches National Park and vicinity showing the location of the m^jor named regions within the park (Doelling, 1985). The major structure of interest is the asymmetric Salt Valley anticline. Dakota Sandstone arches are in the Slickrock Member (Ober- lander, 1977; Stevens and McCarrick, 1988). Moab Wember Joints in Entrada Sandstone appear to be related to the Salt Valley structure and do not Slickrock Member reflect a regional pattern (Kelley and Clinton, Dewey Bridge Member 1960; Doelling, 1988). These joints are ap- proximately parallel to the axis of Salt Valley, Navajo Sandstone and near Fiery Furnace on the northeast side of Salt Valley, joints change orientation to Kayenta Formation become parallel to the salt-cored Cache Val- Wing ale Sandstone ley anticline. On the northeast flank of Salt Valley, the intensity of jointing diminishes Chinle formation away from the valley. There are also many normal faults, with the down-dropped side toward the center of the anticline. On the southwest flank of the anticline, normal faults are older than the joints (Doelling, 1985; Dyer, 1988; Cruikshank and others, 1991); the normal faults have nucleated along zones Cutler Group of deformation bands (Aydin and Johnson, 1978; Zhao and Johnson, 1992). On the north- east flank of the anticline, joints predate the faulting, because many of the normal faults in the Devils Garden area formed along preex- isting joint surfaces (Cruikshank, 1993). Ë Paradox Formation Thus, the jointing at Arches National Park— ~ 100m fin-bounding joint zones, and arch-localizing joint zones—are related to the Salt Valley Figure 1C. Stratigraphie section for Arches National Park. The Entrada Sandstone is -140 m anticline (Doelling, 1985; Dyer, 1988). The thick (Phillips, 1989). development of the Salt Valley anticline is discussed in detail by Doelling (1985, 1988). face markings, such as hackle and plumose joints are composed of numerous subparallel Theories of Arch Formation structures (Hodgson, 1961; Pollard and Ay- joint segments that are confined to a narrow din, 1988) on the walls of the rock fins, indi- zone. This pattern can be seen in both verti- The first requirement to form an arch is the cate that these walls are joint faces. Linear cal and horizontal exposures (Hodgson, presence of a rock wall or fin that is strong traces seen from the air (Fig. 2) are zones of 1961). These zones of joints form a weak enough to support an arch structure; rock fins joints (Hodgson, 1961; Dyer, 1983; Cruik- zone that weathering exploits, leaving behind are abundant in Arches National Park. Sur- shank and others, 1991); that is, the traces of majestic rock fins separated by narrow can- Yellow Cat SW Moab anticline NE Kings Bottom Sait Valley graben A' A syncline Courthouse Salt wash syncfine syncline I I Figure IB. Most of the arches are in the Jurassic Entrada Sandstone, which is exposed on both flanks of the anticline and forms spectacular dip-slope exposures. Geological Society of America Bulletin, July 1994 881 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/7/879/3381965/i0016-7606-106-7-879.pdf by guest on 25 September 2021 CRUIKSHANK AND AYDIN Figure 2.4a) Aerial view of fin-bounding zones of joints at Klondike Bluffs.