Abstract

The nature and effects of the Laramide orogeny in southwestern Wyoming leads one to consider the criteria used to identify and date such phenomena. Recognition of an orogeny on the basis of evidence preserved in the rock record involves analysis of regional stratigraphic patterns, local lithologic detail, and contact relationships between successive lithologic units. Age assignments of the Laramide and associated lithic units require dating the event by some means such as radiometrics. Conventional paleontological tools are widely used in the region; palynology and mammalian paleontology are also used in this analysis. This paper reviews, and in some cases modifies, the stratigraphic nomenclature currently applied to units of later Cretaceous and early Tertiary age.

The name "Laramide" is generally given to deformational and associated depositional events of lengthy duration extending from mid-Cretaceous to early (or even mid to upper) Eocene time. Deformation migrated from west to east through time. However, it is concluded here that the name would be better applied to the climactic event when the entire Rocky Mountain region was raised above sea level, ending recurrent cycles of marine transgression and regression. In latest Cretaceous (late Maastrichtian) and early Paleocene (Puercan) time, broad areas were elevated above base level accompanied by an episode of regional erosion. Deposition resumed on a topography of low relief cut into deformed and truncated strata, i.e., a surface of unconformity. This surface and the associated depositional hiatus mark the climactic phase of the Laramide in southwestern Wyoming.