Tectonic Control on Sedimentation – Wyoming
Andrew Nicholson
The Ericson Sandstone is a Campanian fluvio-estuarine sandstone and has been re-examined on and around the Rock Springs Uplift in SW Wyoming. The latter is a Laramide feature, usually believed to have been uplifted in latest Cretaceous and Paleogene times. ÊIt is proposed, based on Ericson thickness changes and the occurrence of multiple internal erosion surfaces, or internal unconformities, around the Rock Springs Uplift that the broader area was repeatedly active during the deposition of the Trail and Rusty members of Ericson Formation, possibly as old as 79my (early Middle Campanian). This is justified by gross interval isopach maps of the Trail and Rusty members that show a pronounced area of decreased accommodation (thinner sediment accumulation) across the crest of the uplift, the presence of unconformities at the base of Trail and top of Rusty members, and increased internal erosion surface density (# of surfaces per vertical measured section divided by the gross thickness) trends within the Trail and Canyon Creek members from flank to crest.
However, the unconformities have a much more widespread distribution than the uplift itself and are likely to be partly caused by significant basinward shifts of alluvial plain deposition, possibly forced by large-scale isostatic uplift of the Sevier fold-and-thrust belt at these times. ÊThe uplift activity during Ericson deposition was primarily in the form of reduced rates of subsidence around the uplift rather than positive landscale relief as there is no evidence of stratal onlap from flank to crest, only systematic thinning.