Knowledge of Utah Thrust System Pushes Forward

By Grant C. Willis

Over the past few decades, study of the geometry, tectonic forces, and dating of thrust systems produced some of the most exciting advances in geology. Utah has been at the center of some of this work. This is a brief overview of the development and evolution of the Utah thrust system, synthesizing the work of many geologists. The western or Cordilleran thrust system extends from Mexico to Alaska, and formed mostly in the Middle Jurassic to early Tertiary (170 to 40 million years ago). It formed as dense oceanic crust beneath the Pacific Ocean (Farallon plate) converged with, and slid beneath the more buoyant continental crust of the North American plate during a mountainbuilding episode called the Sevier orogeny (the Sevier River area of central Utah is the namesake of this event). The Utah part of the Cordilleran thrust system is called the Sevier thrust system.

Though the basic geometry and age of the Sevier thrust system in Utah have been known for more than 50 years, knowledge of the timing, method, and sequence of emplacement of individual thrust sheets has advanced slowly.

Advances

Probably the biggest advance in Utah thrust system studies has come through improved dating and correlation methods. In an active thrust system, coarse alluvium is shed from rapidly eroding mountains formed by the thrusted rock. In some cases, soon after deposition, the advancing thrust plates override, fold, and fault these “synorogenic deposits.” To unravel thrust history, it is essential to accurately date these rocks.

In Utah, geologists scoured many miles of outcrops searching for datable materials. They collected and identified pollen, spores, and volcanic ash, then correlated the samples with well-dated strata elsewhere. They also mapped the conglomerates, unconformities, and cross-cutting relationships, and matched conglomerate clasts with the formations from which they were derived. Analyzing this data with new tools and thrust models has significantly refined our knowledge of timing and the sequence of events that formed the Utah thrust system.

Sevier Thrust System

The Sevier thrust system is a typical thrust system consisting of, from west to east, a thrust belt, a foredeep basin, a forebulge, and a back-bulge basin. The thrust belt is the wedge of stacked thrust plates. In Utah, single plates are up to 50,000 feet thick and, when thrusted into thick stacks or culminations, may have formed mountains similar in magnitude to the modern Andes Mountains of South America. The tremendous load of the stacked plates depressed the crust under and in front of the thrust belt (visualize forcing down the end of a raft floating on water by loading it with rock) forming a “foredeep” basin into which thousands of feet of coarse synorogenic sediment was shed. Foredeep-basin deposits in Utah commonly exceed 10,000 feet. Farther east, the land bowed upward, a counter-response to the depressed foredeep basin, forming a forebulge, a relatively high area with minor or no deposition. At times, the Utah forebulge was an area of erosion. Still farther east, a second, much shallower basin formed, the back-bulge basin.

The Farallon plate, subducting beneath the continental crust in the approximate position of modern central California, was the driving force behind the Sevier thrust system. The collision produced deformation that started in the west and migrated eastward. Thus, each of these four parts of the thrust system migrated eastward over time. Back-bulge basin deposits provide the earliest evidence of thrusting in Utah.

Middle Jurassic Back-bulge Basin

During the Middle to early Late Jurassic epochs, most of Utah was a broad, shallow back-bulge basin. The basin was covered by a shallow sea, tidal flats, sabkhas (flat evaporating pans), and coastal sand dunes (Twin Creek and Pruess Formations in northern Utah; Twin Creek, Arapien, and Twist Gulch Formations in central Utah; Carmel, Entrada, Curtis, and Summerville Formations in east-central and southern Utah), and later, by broad, low-elevation river floodplains (Stump and Morrison Formations in northern Utah, Morrison Formation in central and southern Utah, among others).

Late Jurassic to Early Cretaceous Forebulge High

By the Late Jurassic epoch, the backbulge basin had migrated east of Utah, and Utah was mostly a forebulge high. Modest erosion across this broad, gentle uplift produced an unconformity beveled across the Jurassic strata. The forebulge gradually migrated east of Utah during the Early Cretaceous. As the bulge subsided, sporadic deposition produced the late Early Cretaceous Cedar Mountain Formation (Kelvin Formation in northern Utah), a discontinuous unit noted for many minor internal unconformities and ancient soil horizons. This unit is also the most important producer of early Cretaceous dinosaurs in North America.

Early Cretaceous Thrust Faulting

Thrust faulting began in northwestern Utah in the latest Jurassic or earliest Cretaceous. Sparse evidence is found in Emigration Canyon near Salt Lake City, where boulder conglomerate strata near the base of the Kelvin Formation were derived from the westernmost and oldest thrust sheet. Additional evidence is preserved in synorogenic conglomerate beds in southern Idaho and western Wyoming.

Late Cretaceous Thrust Faulting

Thrust faulting reached its zenith in Utah during the Late Cretaceous when most of the major thrust plates were emplaced. By this time, most of the forebulge high had migrated east of Utah. Many plates were pushed eastward 25 to 30 miles, and in some cases, more than 50 miles. Drill holes have penetrated up to five stacked plates at single locations. Thrusted rock was folded, faulted, overturned, brecciated, and metamorphosed to a low grade as it was pushed eastward, forming large mountains and creating the spectacular tilted and complexly folded formations now exposed in many of the ranges of northern, central, and southwestern Utah (for example: Devils Slide in Weber Canyon, the complexly folded rocks in Parleys Canyon, and the great block of overturned strata that forms Mount Nebo).

The Late Cretaceous was also the time of peak oil and gas generation in the thrust belt. For example, Cretaceous organic-rich rocks buried by thrust sheets near the Wyoming border generated the oil and gas that migrated into reservoirs in the thrust-created folds in the Coalville area. A few of these folds became some of the best oil and gas fields in Utah (for example: the Pineview and Anshutz Ranch fields).

As the “thrust front” migrated eastward, it abandoned one thrust fault as the “wedge” of thrusted rock became too thick, and “stepped” forward to a new fault. Thrust faults to the rear “locked” into place or experienced only minor renewed movement. In general, thrust plates in the eastern part of the Sevier belt didn’t move as far as western plates. Likewise, the eastern plates were thinner and deformed into folds of smaller amplitude between wider spaced thrust faults than the thick western plates.

Because the Late Cretaceous was the time of peak thrusting, it was also the time of peak synorogenic sedimentation in wedge-top basins on the thrust plates and in the foredeep basin in front of the thrust belt. These deposits include the thick conglomerate beds along Interstate Highways 80 and 84 (Echo Canyon, Weber Canyon, and Evanston Conglomerates), near U.S. Highway 6 in Spanish Fork Canyon (Indianola and Price River Formation), in the mountains near Cedar City (Iron Springs Formation), and at several other places in Utah.

In general, synorogenic conglomerate beds grade eastward into fluvial sandstone and shale, coastal-plain deposits, and deltaic deposits comprising the extensive coal-bearing deposits of Utah (parts of the Frontier Formation of northern Utah, the Blackhawk Formation of central Utah, and the Straight Cliffs Formation of southern Utah, among others). These in turn grade eastward into fine sand, mud, and clay shallow-marine deposits (parts of the Mancos Shale of central and southern Utah, and most of the Frontier Formation and the Hilliard Shale north of the Uinta Mountains).

Late-Phase Thrusting

Thrust faulting continued into middle to late Eocene time. In the northern Sevier thrust belt, the late-phase faults are mostly in western Wyoming. In many areas near the front edge of the thrust belt in central and southern Utah, duplicated strata are present only within individual formations, making the deformation more difficult to recognize and map in the field. In addition, the frontal thrust faults are typically covered by younger undeformed deposits, and extend well east of the easternmost major thrust faults that break the surface. The shortening in this frontal zone is taken up by folds that decrease in amplitude to the east. The Sanpete-Sevier Valley anticline (the white hills along I-70 near Salina) and the Virgin anticline (the tilted rock east of I-15 near St. George) are two examples of large thrust-cored folds in the frontal part of the thrust belt.

The End of Thrusting

The most recent evidence of thrust faulting is about 50 million years old in northern Utah, and about 40 million years old in central and southern Utah. However, the end of thrust faulting is not clearly defined in the rock record because compression declined gradually as the rate of convergence between oceanic and continental crust decreased. As the compressional forces declined, the Cordilleran thrust system (including the Sevier thrust belt) was left unsupported. Many of the original thrust faults “relaxed” and slid backwards (to the west). In general, this backsliding was not extensive, but it was enough to complicate the evidence that geologists have had to unravel.

I THOUGHT THAT WAS THE LARAMIDE OROGENY!

The Sevier orogeny is often confused with the Laramide orogeny, even by geologists, because they overlap in time and location. The Laramide orogeny developed in the Late Cretaceous and continued into the Oligocene epoch, mostly synchronous with late stages of the Sevier orogeny. Laramide structures were produced in central and eastern Utah, western Colorado, and most of Wyoming — thus, some overlap eastern Sevier thrust belt structures. Classic Laramide structures in Utah include the Uinta Mountains uplift, the San Rafael Swell, and the Circle Cliffs (Waterpocket Fold). Some structures, such as the Uinta Mountains, were affected by both events. The two orogenies were produced by the same crustal shortening event, collision of the Farallon and North American plates, but they are distinguished by style of deformation. The Sevier orogeny defines a more western event that took advantage of weak bedding planes in thick Paleozoic and Mesozoic sedimentary rock. Shortening in basement metamorphic and igneous rocks was transferred tens of miles eastward along the weak shale and evaporite layers, producing “thin-skinned” thrust faulting that, in its eastern part, only involved sedimentary strata. In contrast, the Laramide orogeny produced “basement- cored” uplifts because thin sedimentary rock in those areas did not easily “decouple” from the basement rock.

This paper is extracted from: Willis, G.C., 1999, The Utah Thrust System – An Overview, in Spangler, L.W., and Allen, C.J., editors, Geology of northern Utah and vicinity: Utah Geological Association Publication 27, p. 1-9. Sources used to prepare this paper are listed in that publication.

Survey Notes, v. 32 no. 1, January 2000