Transformation of high-relief canyon topography by an ancient rock avalanche, Hop Valley, Zion National Park, Utah, USA

Zion National Park preserves a rich geological record of Holocene landslide-dammed canyons in its deeply incised topography, with 11 hypothesized valley-blocking deposits within the park boundaries. Despite consistent stratigraphic, tectonic, and climatic settings, the occurrence of and subsequent landscape response to these natural dams varies. As such, the region provides a unique natural laboratory for quantifying the effect and evolution of landslide dams in high-relief canyon topography. Here we present a detailed study of a rock avalanche deposited at the mouth of Hop Valley in Zion National Park, describing its age, size, emplacement conditions, impact on local geomorphology and sedimentology, as well as the subsequent usage of the valley by native Ancestral Puebloans. Topographic reconstructions indicate the original deposit was ~75 million m3 and 1.5 km long with a maximum thickness of 180 m. New ages from cosmogenic 10Be surface exposure dating indicate a single-event failure at 6.7 ± 0.7 ka. The rock avalanche impounded ~55 million m3 of sediment, transforming Hop Valley from a relatively narrow gorge to a broad flat-floored canyon. Stratigraphic sections of accumulated upvalley sediments, calculated sedimentation rates (averaging 8.2 ± 0.8 m/ky), and paleoclimate records suggest the deposit primarily dammed sediment, rather than water, to produce an extensive alluvial plain. This detailed case-history analysis, together with our review of other Holocene landslide dams in Zion National Park, helps clarify the legacy of valley-blocking mass movements in steep canyon topography.

Evaluating the Traffic Capacity of the Zion–Mt. Carmel Tunnel in Zion National Park

Constructed in 1930 and recently designated as a National Historic Civil Engineering Landmark, the 1.1 mile long Zion–Mt. Carmel Tunnel has served Zion National Park in Utah well for several decades. With the passage of time, however, vehicles have become larger and this has necessitated the use of one-way operation to allow large vehicles to pass through the narrow tunnel. In recent years the number of visits to National Parks in the Colorado Plateau region has greatly increased. For example, visits to Zion National Park increased by 69% from 2010 to 2017. Accompanying the increase in visitor numbers has been an increase in traffic volume. As traffic volume has grown, two questions have become more obvious: What is the highway capacity of the Zion–Mt. Carmel Tunnel? And, how soon will the tunnel reach capacity? This paper covers a unique traffic engineering/highway capacity problem and describes: (a) the current method of operating the tunnel (both two-way and one-way operation); (b) data collection and analysis; (c) how one-way operation degrades tunnel capacity; (d) calculation of tunnel capacity and waiting times; (e) how soon the tunnel will reach capacity; and (f) alternatives for addressing the capacity problem.

Upper Triassic lithostratigraphy, depositional systems, and vertebrate paleontology across southern Utah

The Chinle Formation and the lower part of the overlying Wingate Sandstone and Moenave Formation were deposited in fluvial, lacustrine, paludal, and eolian environments during the Norian and Rhaetian stages of the Late Triassic (~230 to 201.3 Ma), during which time the climate shifted from subtropical to increasingly arid. In southern Utah, the Shinarump Member was largely confined to pre-Chinle paleovalleys and usually overprinted by mottled strata. From southeastern to southwestern Utah, the lower members of the Chinle Formation (Cameron Member and correlative Monitor Butte Member) thicken dramatically whereas the upper members of the Chinle Formation (the Moss Back, Petrified Forest, Owl Rock, and Church Rock Members) become erosionally truncated; south of Moab, the Kane Springs beds are laterally correlative with the Owl Rock Member and uppermost Petrified Forest Member. Prior to the erosional truncation of the upper members, the Chinle Formation was probably thickest in a southeast to northwest trend between Petrified Forest National Park and the Zion National Park, and thinned to the northeast due to the lower Chinle Formation lensing out against the flanks of the Ancestral Rocky Mountains, where the thickness of the Chinle is largely controlled by syndepositional salt tectonism. The Gartra and Stanaker Members of the Ankareh Formation are poorly understood Chinle Formation correlatives north of the San Rafael Swell. Osteichthyan fish, metoposaurid temnospondyls, phytosaurids, and crocodylomorphs are known throughout the Chinle Formation, although most remains are fragmentary. In the Cameron and Monitor Butte Members, metoposaurids are abundant and non-pseudopalatine phytosaurs are known, as is excellent material of the paracrocodylomorph Poposaurus; fragmentary specimens of the aetosaurs Calyptosuchus, Desmatosuchus, and indeterminate paratypothoracisins were probably also recovered from these beds. Osteichthyans, pseudopalatine phytosaurs, and the aetosaur Typothorax are especially abundant in the Kane Springs beds and Church Rock Member of Lisbon Valley, and Typothorax is also known from the Petrified Forest Member in Capitol Reef National Park. Procolophonids, doswelliids, and dinosaurs are known but extremely rare in the Chinle Formation of Utah. Body fossils and tracks of osteichthyans, therapsids, crocodylomorphs, and theropods are well known from the lowermost Wingate Sandstone and Moenave Formation, especially from the St. George Dinosaur Discovery Site at Johnson Farm.

Upper Triassic lithostratigraphy, depositional systems, and vertebrate paleontology across southern Utah

The Chinle Formation and the lower part of the overlying Wingate Sandstone and Moenave Formation were deposited in fluvial, lacustrine, paludal, and eolian environments during the Norian and Rhaetian stages of the Late Triassic (~230 to 201.3 Ma), during which time the climate shifted from subtropical to increasingly arid. In southern Utah, the Shinarump Member was largely confined to pre-Chinle paleovalleys and usually overprinted by mottled strata. From southeastern to southwestern Utah, the lower members of the Chinle Formation (Cameron Member and correlative Monitor Butte Member) thicken dramatically whereas the upper members of the Chinle Formation (the Moss Back, Petrified Forest, Owl Rock, and Church Rock Members) become erosionally truncated; south of Moab, the Kane Springs beds are laterally correlative with the Owl Rock Member and uppermost Petrified Forest Member. Prior to the erosional truncation of the upper members, the Chinle Formation was probably thickest in a southeast to northwest trend between Petrified Forest National Park and the Zion National Park, and thinned to the northeast due to the lower Chinle Formation lensing out against the flanks of the Ancestral Rocky Mountains, where the thickness of the Chinle is largely controlled by syndepositional salt tectonism. The Gartra and Stanaker Members of the Ankareh Formation are poorly understood Chinle Formation correlatives north of the San Rafael Swell. Osteichthyan fish, metoposaurid temnospondyls, phytosaurids, and crocodylomorphs are known throughout the Chinle Formation, although most remains are fragmentary. In the Cameron and Monitor Butte Members, metoposaurids are abundant and non-pseudopalatine phytosaurs are known, as is excellent material of the paracrocodylomorph Poposaurus; fragmentary specimens of the aetosaurs Calyptosuchus, Desmatosuchus, and indeterminate paratypothoracisins were probably also recovered from these beds. Osteichthyans, pseudopalatine phytosaurs, and the aetosaur Typothorax are especially abundant in the Kane Springs beds and Church Rock Member of Lisbon Valley, and Typothorax is also known from the Petrified Forest Member in Capitol Reef National Park. Procolophonids, doswelliids, and dinosaurs are known but extremely rare in the Chinle Formation of Utah. Body fossils and tracks of osteichthyans, therapsids, crocodylomorphs, and theropods are well known from the lowermost Wingate Sandstone and Moenave Formation, especially from the St. George Dinosaur Discovery Site at Johnson Farm.