Abstract This five-day field trip examines Pennsylvanian to Jurassic strata in the Paradox Basin on the Colorado Plateau in southeastern Utah. The trip will emphasize four major themes: 1) Permian-Triassic stratigraphy, 2) characteristics of non-marine depositional systems, 3) paleogeography, and 4) paleoclimate. The trip follows depositional facies in the Permian and Triassic section from proximal continental settings near the Ancestral Rocky Mountains in the Uncompahgre Highlands to distal marine settings within the Paradox Basin. Evolution of these depositional systems along the west coast of Pangea from the late Paleozoic to the early Mesozoic is a key indicator of the paleogeographic and paleoclimatic history of the region. Late Paleozoic red beds encountered during the trip provide additional, high-resolution paleoclimate information. The trip will highlight unconformities, and their origin as a result of sea-level change, regional tectonics, or salt diapirism. The Permian-Triassic unconformity, which is expressed worldwide, will be examined during the trip. In southeastern Utah, the Permian-Triassic unconformity influenced the diagenetic history of the White Rim Sandstone, resulting in formation of the Tar Sand Triangle—the largest tar sand deposit in the U.S. Rocks examined in detail during the trip include the Permian Cutler Group (informal lower Cutler beds, Halgaito Formation, Cedar Mesa Sandstone, Organ Rock Formation, White Rim Sandstone, and De Chelly Sandstone), the overlying Lower Triassic Moenkopi Formation, and locally the Upper Triassic Chinle Formation. The trip also passes through the underlying Pennsylvanian section and the overlying Jurassic section, including eolianites exposed in and around Canyonlands National Park. This field guide includes several sections. The introduction and following text sections present short discussions of the major rock units and topics to be presented on the field trip. The field trip section of the field guide begins with a brief list of significant geologic features encountered during the drive across the Rocky Mountains from Denver to Grand Junction. We will not stop at any of the sites in the list. The remainder of the field trip section provides brief descriptions of the individual field stops that will be visited.

Abstract Standing astride the Utah-Arizona border with bronzed shafts of sandstone, the buttes and pinnacles of Monument Valley rise above the high desert floor of the central Colorado Plateau. The valley is tucked into the apex of the southern end of the Monument Upwarp, a large asymmetrical anticline bounded on its southern and eastern margins by the Comb Ridge monocline (Fig. 1). As recently as 35 years ago, no paved roads served the remote region, today three U.S. highways (U.S. 160 running east-west, and U.S. 163 and U.S. 191 running north-south), provide access to the region. But the best way to see Monument Valley is via the back roads that provide access to some spectacular backcountry. Monument Valley is located within the Navajo Indian Reservation. Travel on paved and main dirt roads is generally unrestricted. However, travel in some areas is restricted and a permit is required for rock collecting or extensive geological study. Inquiries should be addressed Navajo Nation, Window Rock, Arizona 86515. Monument Valley and vicinity is covered by geologic maps of various scales 1:62,500 (Witkind and Thaden, 1963), 1:96,000 (Baker, 1936), 1:250,000 (U.S.G.S. Iseries, 345, 629, 744, 1003), and 1:500,000(Geological Maps of Utah and Arizona). Witkind and Thaden (1963) have provided the most comprehensive geological report of the region.

Regional geologic mapping using pictures from the first Earth Resources Technology Satellite (ERTS-1) has led to the recognition of two parallel northeast-trending systems of normal faults, each of which can be traced more than 100 km. Many eruptive centers appear to be localized along these fault systems or along their extensions. The faults are chiefly observed in Phanerozoic rocks and have minor displacement but are interpreted by us to reflect fault zones of major displacement in the crystalline Precambrian basement. The Bright Angel fault system extends as a continuous zone of normal faults from Cataract Creek on the southwest to the Echo Cliffs on the northeast. Beyond the Echo Cliffs, the system continues northeastward to the vicinity of Monument Valley as a more diffuse, discontinuous zone of normal faults. The Bright Angel fault, Vishnu fault, and Eminence Break graben are among the larger individual members of the total system. The Navajo mountain intrusive center lies along the discontinuous part of the system. Three major eruptive centers of the Mount Floyd volcanic field lie on the southwestern projection of the Bright Angel fault system. If the eruptive centers are included as part of the recognizable structural system, the Bright Angel system has a total known length of slightly more than 300 km. The Mesa Butte fault system, as now recognized, extends from Chino Valley on the southwest to Shadow Mountain on the northeast. Bill Williams Mountain, Sitgreaves Peak, and Kendrick Peak are principal silicic to intermediate eruptive centers of the San Francisco volcanic field that appear to be localized along the fault system. Red Mountain, Mesa Butte, and Shadow Mountain are prominent basaltic eruptive centers along the system; monchiquite diatremes at Tuba Butte and Wildcat Peak lie on the northeast projection of the fault system. The total distance from Chino Valley to Wildcat Peak is more than 200 km. Comparison of the Bright Angel and Mesa Butte fault systems with a residual aeromagnetic map of Arizona reveals a close correspondence between the positions of the observed relatively minor normal faults and the margins of a series of large northeast-trending magnetic anomalies. Perhaps the most noteworthy feature of the aeromagnetic map is a 400-km-long northeast-trending belt of large positive aeromagnetic anomalies that extends from the vicinity of Congress to the northern border of Arizona. The Mesa Butte fault system lies along the southeast margin of this anomaly belt. Another large positive anomaly, bounded on the southeast by the Bright Angel fault, corresponds in the Grand Canyon to a belt of Precambrian amphibolite and schist. Most of the large positive aeromagnetic anomalies along the Bright Angel and Mesa Butte fault systems may correspond to similar bodies of mafic metavolcanic rocks, which have been offset along two major and perhaps several minor faults of Precambrian age. The normal faults that displace the overlying Phanerozoic rocks have been formed by renewed movement along these ancient fault zones, in response to dilation of the crust from late Tertiary time to the present. The ancient fault zones inferred to be present along the Bright Angel and Mesa Butte fault systems may be related in origin to the Shylock and Chaparral fault zones in central Arizona described by Anderson (1967). Both the Shylock fault zone and the Chaparral fault have right-lateral transcurrent displacement. As shown by Anderson, the Shylock zone has a probable minimum horizontal displacement of 8 km. A large contrast in the magnetic properties of the rocks on opposite sides of the fault zone, indicated by the aeromagnetic map, suggests that the displacement may be several tens of kilometres or more. Comparably large right-lateral displacements may have occurred along the ancestral Bright Angel and Mesa Butte fault zones. The location of epicenters of recent earthquakes and reports of earthquakes by residents in the region indicate that the Bright Angel and Mesa Butte fault systems are currently active.