ABSTRACT The Willard-Paris-Meade thrust sheet is the oldest and westernmost sheet to develop in the Wyoming salient of the Sevier fold-and-thrust belt. This dominant thrust sheet was transported ~60–70 km eastward, but despite its importance to the overall shortening budget of the Cordilleran orogenic system, the timing for onset and cessation of major fault slip is poorly constrained. Synorogenic strata associated with emplacement of the sheet thicken westward, reaching a total thickness >2 km near the thrust front, and are divided into five intervals: (1) lower and (2) upper parts of the Aptian(?) to Albian Gannett Group (Ephraim and Bechler Formations); (3) Upper Albian Smiths, Thomas Fork, and Cokeville Formations and correlative Bear River Formation; (4) uppermost Albian to Cenomanian Sage Junction Formation and partly correlative Aspen Shale; and (5) Upper Cenomanian to Turonian Frontier Formation. Integrated sedimentologic, detrital zircon provenance, and subsidence analyses of these foreland basin strata record a protracted history of thrust sheet emplacement. Detrital zircon U-Pb age spectra, conglomerate clast types, and sandstone petrography record an unroofing sequence, with progressive erosion of Lower Mesozoic to Upper Paleozoic to Lower Paleozoic bedrock levels, starting in the more western Paris sheet and continuing across the Meade and Willard sheets. Synorogenic strata also contain young grains derived from the Cordilleran magmatic arc that bracket maximum depositional ages of: 115.2 ± 1.8 Ma and 116.2 ± 2.3 Ma in the upper Ephraim Formation to 107.9 ± 2.4 Ma in the overlying Bechler Formation, which were deposited during early thrust slip; 101.6 ± 0.7 Ma, 101.4 ± 0.6 Ma, and 98.8 ± 0.4 Ma in the Cokeville, Sage Junction, and Aspen Formations, respectively, which were deposited during increased subsidence and thrust slip rates; and 95.6 ± 1.4 Ma in the Frontier Formation, which was deposited during late slip, eastward thrust propagation, and early uplift of the Wasatch anticlinorium. The lower Ephraim Formation was deposited during the Early Cretaceous Cordilleran magmatic lull and lacks young zircon grains, such that timing for thrust initiation is not well constrained, ranging from ca. 130 to 120 Ma. Regional variations in stratigraphic thickness and sedimentation rates, combined with flexural modeling, indicate that a foredeep began developing in front of the Paris thrust and then migrated eastward as the Meade and Willard thrusts propagated and overrode their own synorogenic strata, with increasing rates of thrust loading and subsidence from ca. 105 Ma to 95 Ma.

Abstract In sedimetary basins not currently undergoing primary compaction (e.g., Rocky Mountain Basins), p-wave velocities noticeably vary with azimuth, yet the mechanism(s) controlling the anisotropy remain uncertain. Possible geologic causes for azimuthal anisotropy include but are not limited to sedimentary fabrics, steep bedding, changes in local in-situ or residual stress, and open or mineralized fractures. To test these hypotheses, P-wave velocity azimuths (Vfast) from a proprietary seismic survey of a NNW-trending Laramide Anticline on Casper Arch in central Wyoming were compared to image log data from the seismic coverage area and fracture orientations from nearby analog structures.

Quantification of fault-related illite neomineralization in clay gouge allows periods of fault activity to be directly dated, complementing indirect fault dating techniques such as dating synorogenic sedimentation. Detrital “contamination” of gouge is accounted for through the use of illite age analysis, where gouge samples are separated into at least three size fractions, and the proportions of detrital and authigenic illite are determined using illite polytypism (1M d = neoformed, 2M 1 = detrital). Size fractions are dated using the 40 Ar/ 39 Ar method, representing a significant improvement over earlier methods that relied on K-Ar dating. The percentages of detrital illite are then plotted against the age of individual size fractions, and the age of fault-related neoformed material (i.e., 0% detrital/100% neoformed illite) is extrapolated. The sampled faults and their ages are the Absaroka thrust (47 ± 9 Ma), the Darby thrust (46 ± 10 Ma), and the Bear thrust (50 ± 12 Ma). Altered host rock along the frontal Prospect thrust gives an age of 85 ± 12 Ma, indicating that the 46–50 Ma ages are not related to a regional fluid-flow event. These ages indicate that the faults in the Snake River–Hoback River Canyon section of the Wyoming thrust belt were active at the same time, indicating that a significant segment of the thrust belt (100 km 2 +) was active and therefore critically stressed in Eocene time.