The East Traverse Mountains are an E-W trending mountain range dividing Utah and Salt Lake valleys in northern Utah. Geologically perplexing, the nature of the East Traverse Mountains has been under investigation for 140 years. Previously, the mountain range was proposed to be a dismembered but still coherent down-faulted block that experienced 4 km of post-thrusting extension within the Charleston-Nebo thrust sheet. However, new insight on the origin of the East Traverse Mountains indicate that it is a mega landslide, roughly ~100 km3 in size, which catastrophically slid from the upper reaches of the Little-Cottonwood stock to its present-day location. The primary evidence for this landslide includes two unusual dike swarms whose roots are in the Wasatch Range and whose upper reaches are now in the East Traverse Mountains, 16 km to the SW. A swarm of pebble dikes, indicative of porphyry mineralization is found at the center of the East Traverse Mountains and contain pebbles of Little-Cottonwood stock as well as two other intrusions found at the center of a mineralized zone. These granitic clasts have phyllic alteration, contain molybdenite grains and are sourced from a subeconomic molybdenum-copper porphyry deposit located 16 km to the NE. The other dike swarm occurs on the SE corner of the range near Alpine, Utah, which contains various andesitic and phaneritic dikes of intermediate-felsic compositions (56-69 wt.% SiO2) with localized marble on their southern margin. These dikes range in U-Pb ages from 36-29 Ma. Moreover, other evidence includes brecciation of the entire mountain range as well as along the slide path of this landslide. Breccia, as well as pseudotachylyte and cataclasite have been discovered that formed in the rapid transportation of the 1-2 km thick detached block. Devitrified pseudotachylyte veins range in thickness from 1 cm to 1 m and are present in the roof zone of the pluton. Sixteen kilometers of sliding caused 70-80% of the Oquirrh Group rocks of the East Traverse Mountains to be fractured to less than 1-inch diameter clasts in breccias and broken formations, as documented by 16 years of mining. U-bearing opal replaced significant areas of brecciated volcanic rocks when hot water seeped into highly-fractured, argillically altered rock. U-Pb ages of 6.1 ± 0.9 Ma from these opalite areas could provide a minimum age for the emplacement of the mountain block. Underlying the East Traverse Mountains slide block is a layer of fallout tuff deposited in the Jordan River Narrows member with 40Ar/39Ar ages of 6.62 ± 0.07 Ma which provides a maximum age of emplacement. Therefore, we propose that the East Traverse Mountains mega-landslide occurred between 6.1 ± 0.9 Ma and 6.62 ± 0.07 Ma. Our interpretation for the East Traverse Mountains mega-landslide model builds upon previous research and data, with the addition of these recent findings. This new interpretation is crucial for understanding the potential for large normal fault systems to create significant landslide hazards.

The East Traverse Mountains are an E-W trending mountain range dividing Utah and Salt Lake valleys in northern Utah. Geologically perplexing, the nature of the East Traverse Mountains has been under investigation for 140 years. Previously, the mountain range was proposed to be a dismembered but still coherent down-faulted block that experienced 4 km of post-thrusting extension within the Charleston-Nebo thrust sheet. However, new insight on the origin of the East Traverse Mountains indicate that it is a mega landslide, roughly ~100 km3 in size, which catastrophically slid from the upper reaches of the Little-Cottonwood stock to its present-day location. The primary evidence for this landslide includes two unusual dike swarms whose roots are in the Wasatch Range and whose upper reaches are now in the East Traverse Mountains, 16 km to the SW. A swarm of pebble dikes, indicative of porphyry mineralization is found at the center of the East Traverse Mountains and contain pebbles of Little-Cottonwood stock as well as two other intrusions found at the center of a mineralized zone. These granitic clasts have phyllic alteration, contain molybdenite grains and are sourced from a subeconomic molybdenum-copper porphyry deposit located 16 km to the NE. The other dike swarm occurs on the SE corner of the range near Alpine, Utah, which contains various andesitic and phaneritic dikes of intermediate-felsic compositions (56-69 wt.% SiO2) with localized marble on their southern margin. These dikes range in U-Pb ages from 36-29 Ma. Moreover, other evidence includes brecciation of the entire mountain range as well as along the slide path of this landslide. Breccia, as well as pseudotachylyte and cataclasite have been discovered that formed in the rapid transportation of the 1-2 km thick detached block. Devitrified pseudotachylyte veins range in thickness from 1 cm to 1 m and are present in the roof zone of the pluton. Sixteen kilometers of sliding caused 70-80% of the Oquirrh Group rocks of the East Traverse Mountains to be fractured to less than 1-inch diameter clasts in breccias and broken formations, as documented by 16 years of mining. U-bearing opal replaced significant areas of brecciated volcanic rocks when hot water seeped into highly-fractured, argillically altered rock. U-Pb ages of 6.1 ± 0.9 Ma from these opalite areas could provide a minimum age for the emplacement of the mountain block. Underlying the East Traverse Mountains slide block is a layer of fallout tuff deposited in the Jordan River Narrows member with 40Ar/39Ar ages of 6.62 ± 0.07 Ma which provides a maximum age of emplacement. Therefore, we propose that the East Traverse Mountains mega-landslide occurred between 6.1 ± 0.9 Ma and 6.62 ± 0.07 Ma. Our interpretation for the East Traverse Mountains mega-landslide model builds upon previous research and data, with the addition of these recent findings. This new interpretation is crucial for understanding the potential for large normal fault systems to create significant landslide hazards.

The Early Pennsylvanian (Bashkirian/Morrowan) Bridal Veil Limestone of north-central Utah was deposited in the eastern portion of the rapidly subsiding Oquirrh basin. The 420 meter-thick Bridal Veil Limestone displays distinct cyclicity formed by stacked, meter to decameter scale high-frequency sequences and their constituent parasequences. Though no one ideal cycle may be defined for the Bridal Veil Limestone, each high-frequency sequence and parasequence contains a general shallowing upward trend that ranges from anaerobic to dysaerobic mudstone at the base to skeletal wackestone to mud-dominated packstone, capped by heterozoan grain-rich carbonates or siliciclastic tidalites. Cycles bounded by exposure surfaces, indicated by micro-brecciation, rhizoliths, laminated calcite or silica crusts, rip-up clasts, centimeter-scale teepee structures, and/or pronounced erosional relief are termed high-frequency sequences. Those bounded by marine flooding surfaces are defined as parasequences. Thusly defined, the Bridal Veil Limestone is divided into 25 high-frequency sequences designated BVL-1 through BVL-25. Overall, two distinct sets of high-frequency sequences may be observed in the Bridal Veil Limestone. Sequences comprising the lower half of the formation (BVL-1 through BVL-12) are thicker, muddier, and less sand-prone than sequences in the upper half of the formation (BVL-13 through BVL-25), indicating an overall change in oxygenation, depositional texture, and accommodation upward in the section. Tracing of key beds and surfaces between the Thorpe Hills, Lake Mountain, and the Wasatch Range (spanning a distance greater than 50 miles) reveals that deposition was remarkably uniform across the southeastern part of the Oquirrh basin which we herein designate the Bridal Veil sub-basin and distinct from coeval formations in the southern Oquirrh basin, Ely basin, and Wyoming shelf. Mudstone and wackestone textures comprise a large portion of the formation by volume. Grain-rich carbonates are almost exclusively heterozoan in composition, indicating that the sub-basin was subphotic to aphotic through Early Pennsylvanian time.