Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indicators of an applied end load stress

Soft-sediment deformational structures associated with paleoseismicity (e.g., planar clastic dikes) exist within Upper Cretaceous Mesaverde Group strata in the Laramide Elk Basin anticline, northern Bighorn Basin (Wyoming, USA). Retrodeformation of the Elk Basin anticline to a horizontal Mesaverde Group position indicates that all basement offset is removed and that clastic dikes exhibit a dominant northeast trend. The trend of clastic dikes corresponds to the interpreted northeast-southwest direction of early Laramide layer-parallel shortening, suggesting that the development of clastic dikes recorded initiation of basement deformation and Laramide tectonism. To determine the timing of clastic dike development, we present zircon U-Pb geochronology from the stratigraphically lowest sand-source bed generating upwardly injected clastic dikes and a volcanic bentonite bed (Ardmore bentonite) above the stratigraphic interval containing clastic dikes. Weighted mean ages bracket clastic dike development between 82.4 and 78.0 Ma. Our results imply initiation of basement deformation ∼8–15 m.y. prior than other estimates in the Bighorn Basin. Therefore, we interpret the development of clastic dikes in the Elk Basin anticline to represent an initial phase of Laramide tectonism associated with an applied end load stress transmitted from the southwestern North American plate margin in response to the collision of the conjugate Shatsky Rise oceanic plateau ca. 90–85 Ma. Results demonstrate how sedimentary responses in the foreland can be used to understand tectonic processes at plate boundaries and provide spatial-temporal parameters for models of Laramide deformation.

Origin of a Clastic Dike at St. Ludger, Quebec: An Alternative Hypothesis

Structures visible in a recent photograph of a clastic dike protruding downward from 'till' into sand at St. Ludger, Quebec suggest that it may not be till as previously reported, but part of a layer of stony lake clay deposited beneath floating ice. Flame structures and faults indicate that the underlying sand changed from a plastic to an elastic state. Probably a layer of stony lake clay was drawn down into the underlying sand by reduced pore pressure developed in part of a closed system. This occurred when adjacent sand in the system was dilated by the shear stress resulting when floating glacier ice grounded. Glacial thrusting may have provided further intrusive impulse after the initial downward deflection.

A Pleistocene Clastic Dike, Upper Chaudière Valley, Québec

At St. Ludger, Upper Chaudière Valley, in southern Québec, glacio-lacustrine sands of the Gayhurst formation which are older than 20 000 years B.P., are cut by a vertical till dike, 40 cm in width and more than 200 cm in height. The infilling materials are similar to those of the overlying Lennoxville till. The trend of the dike is at right angles to the major direction of flow of the last glacier. It is believed that infilling from above occurred when the Laurentides ice sheet overrode and fractured the underlying sands during the Wisconsin stage.