LATE CRETACEOUS-PALEOCENE SYNOROGENIC SEDIMENTATION AND KINEMATIC HISTORY OF THE SEVIER THRUST BELT, NORTHEAST UTAH AND SOUTHWEST WYOMING

Integration of cross-cutting structural relationships, overlapping sed imentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distr ibution and timing of Late Cretaceous through Paleocene thrust faultin g in the northeast Utah-southwest Wyoming part of the Sevier thrust be lt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-vergin g events. Provenance data delimit a sequential restoration of a region al cross section. The principal thrust systems in northeast Utah and s outhwest Wyoming are the Willard, Ogden, Crawford, Absaroka, and Hogsb ack thrusts. The Willard is the westernmost, structurally highest, and oldest of the thrusts; it carries a unique section of thick Proterozo ic sedimentary rocks, as well as Paleozoic rocks, and was folded durin g displacement on younger thrust systems. The next youngest thrust sys tem is the Ogden, which comprises several basement-rooted imbricate th rusts that together form a large antiformal stack with a structural cu lmination in the Wasatch Range. Growth of the Wasatch culmination took place during Coniacian through Paleocene time, contemporaneous with s equential displacement on the frontal Crawford, Absaroka, and Hogsback thrusts. Total shortening in this part of the thrust belt was approxi mately 100 km, and structural relief of approximately 25 km developed in the area of the Wasatch culmination. During Coniacian through Paleo cene time, thrusting followed an overall eastward progression that was interrupted by local out-of-sequence and hinterlandward-verging event s. Several episodes of synchronous displacement on two or more thrusts can be demonstrated. Shortening occurred in three main episodes. The first episode (approximately 89-84 Ma) involved approximately 33 km of shortening on the Crawford thrust and its footwall imbricates. Approx imately 19 km of structural relief on the basement-cover contact devel oped in the area of the Wasatch culmination. The second episode (appro ximately 84-62 Ma, with a break between approximately 75-69 Ma) involv ed approximately 30 km of shortening, mainly on the Absaroka thrust, a nd development of an additional approximately 6 km of structural relie f in the culmination. The third episode of shortening (approximately 5 6-50 Ma) took place on the Hogsback thrust, involved approximately 21 km of horizontal shortening, and produced no significant increase in s tructural relief in the culmination. Long-term rates of shortening ran ged between 3.0 mm/yr and 6.6 mm/yr. These three episodes of shortenin g produced three large accumulations of synorogenic conglomerate, tota ling approximately 3 km in thickness. The Henefer Formation and Echo C anyon and Weber Canyon Conglomerates were deposited during Crawford th rusting. The Evanston Formation was deposited during and after Absarok a thrusting, and the lower conglomeratic part of the Wasatch Formation was deposited during and after Hogsback thrusting. Most of the sedime nt in these synorogenic units, however, was derived from repeated upli ft of the Willard thrust sheet and from the eastern flank of the Wasat ch 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. P eriods 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 o f 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 de formation in close proximity to the tip of the Crawford thrust. Compar ison 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 subsidenc e and sediment supply were the growth and erosion of the Wasatch culmi nation. Growth of the duplex beneath the culmination may have been a m eans of maintaining critical taper in the thrust wedge.