Dual Preservation of Tectonic and Climatic Signals in Fluvial Outcrops of the Raton Basin (Colorado–New Mexico, USA)

The Raton basin (Colorado–New Mexico, USA) hosts a 4.5-km-thick succession of marine to continental strata deposited during regression of the Western Interior Seaway and the onset of Laramide intraforeland deformation. Outcrops of the Upper Cretaceous (Maastrichtian) to Paleocene Raton and Poison Canyon Formations display systematic up-section changes in fluvial architecture and sediment composition that may be used to interpret tectonic and paleoclimatic conditions, without a need for auxiliary geochemical data. The formations record sinuous channel networks that formed in vastly different environments: Larger, laterally isolated channels of the Raton Formation formed in ever-wet conditions and experienced steady discharge, whereas smaller, laterally amalgamated fluvial channels of the overlying Poison Canyon Formation formed in increasingly drier environments and experienced variable discharge. This transition is accompanied by an increase in in-channel grain size and angularity, a decrease in sorting, and a transition from quartzose sandstone to feldspathic-lithic sandstone. We interpret that as Laramide uplift progressed, distally sourced rivers of the Raton Formation were overrun by higher-gradient fluvial fans of the Poison Canyon Formation. Sandstone character and composition record a transition to local basement sources. Simultaneously, channel architecture records a transition from steady fluvial discharge to flashy discharge. Altogether, the up-section changes in fluvial architecture and sandstone composition to record Laramide uplift of the adjacent Sangre de Cristo-Culebra Range in the wake of increasing climatic variability during the early Cenozoic.