Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming

Abstract

Integration of cross-cutting structural relationships, overlapping sedimentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of Late Cretaceous through Paleocene thrust faulting in the northeast Utah-southwest Wyoming part of the Sevier thrust belt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-verging events. Provenance data delimit a sequential restoration of a regional cross section. The principal thrust systems in northeast Utah and southwest Wyoming are the Willard, Ogden, Crawford, Absaroka, and Hogsback thrusts. The Willard is the westernmost, structurally highest, and oldest of the thrusts; it carries a unique section of thick Proterozoic sedimentary rocks, as well as Paleozoic rocks, and was folded during displacement on younger thrust systems. The next youngest thrust system is the Ogden, which comprises several basement-rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the Wasatch Range. Growth of the Wasatch culmination took place during Coniacian through Paleocene time, contemporaneous with sequential displacement on the frontal Crawford, Absaroka, and Hogsback thrusts. Total shortening in this part of the thrust belt was ∼100 km, and structural relief of ∼25 km developed in the area of the Wasatch culmination. During Coniacian through Paleocene time, thrusting followed an overall eastward progression that was interrupted by local out-of-sequence and hinterlandward-verging events. Several episodes of synchronous displacement on two or more thrusts can be demonstrated. Shortening occurred in three main episodes. The first episode (∼89-84 Ma) involved ∼33 km of shortening on the Crawford thrust and its footwall imbricates. Approximately 19 km of structural relief on the basement-cover contact developed in the area of the Wasatch culmination. The second episode (∼84-62 Ma, with a break between ∼75-69 Ma) involved ∼30 km of shortening, mainly on the Absaroka thrust, and development of an additional ∼6 km of structural relief in the culmination. The third episode of shortening (∼56-50 Ma) took place on the Hogsback thrust, involved ∼21 km of horizontal shortening, and produced no significant increase in structural relief in the culmination. Long-term rates of shortening ranged between 3.0 mm/yr and 6.6 mm/yr. These three episodes of shortening produced three large accumulations of synorogenic conglomerate, totaling ∼3 km in thickness. The Henefer Formation and Echo Canyon and Weber Canyon Conglomerates were deposited during Crawford thrusting. The Evanston Formation was deposited during and after Absaroka thrusting, and the lower conglomeratic part of the Wasatch Formation was deposited during and after Hogsback thrusting. Most of the sediment in these synorogenic units, however, was derived from repeated uplift of the Willard thrust sheet and from the eastern flank of the Wasatch culmination in the rear part of the thrust belt. Only local, minor accumulations were derived from the frontal ramp anticlines. Sediment accumulation and structural deformation were generally out-of-phase. Periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin. Periods of structural inactivity were marked by accumulation of aerially widespread, braided-river conglomerate on top of the thrust belt. One exception to this pattern is the Henefer-Echo Canyon-Weber Canyon conglomerate deposit, which contains evidence of progressive deformation in close proximity to the tip of the Crawford thrust. Comparison of the sequential restoration with the Late Cretaceous subsidence history and isopach patterns in the distal foreland basin of western Wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the Wasatch culmination. Growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge.