QUANTIFYING TECTONIC AND CLIMATIC CONTROLS ON ALLUVIAL ARCHITECTURE: WASATCH FORMATION OF WESTERN COLORADO

While regional tectonism has previously been postulated to explain a conspicuous increase in sand deposition within the Wasatch Formation of Western Colorado, the influence of the climatic aberration associated with the Paleocene/ Eocene Thermal Maximum (PETM) may be another important control of sedimentation patterns. This study seeks to quantify the role of tectonics, allowing us to determine the influence of climate on sedimentation.

Previous workers have postulated that the increase in sand at the Paleocene/Eocene boundary is attributed to an increase in sediment availability. This hypothesis relates uplift of the Uncompaghre Mountains to unroofing of the mud-rich Cretaceous strata, exposing Jurassic eolian sandstones as source material. However, ongoing petrographic analysis indicates that Wasatch sandstones are lithic arenites, inconsistent with a quartz arenite eolian source. While tectonic forcing constrains subsidence patterns, the sharp increase in sand cannot be attributed to tectonic unroofing exposing a new sediment source. Nonetheless, tectonic changes in the fluvial gradient may have provided more efficient sediment transport. However, increased fluvial gradients are not consistent with observed highly aggradational channel fills that indicate oversupplied conditions.

Pollen data constrains the Paleocene/Eocene boundary to this sand rich interval. Changes in the proportion of lateral accretion, vertical accretion, and paleosol development have been observed though this sequence. The transition between the underlying Atwell Gulch Member mudstones and the Molina Member sandstones represents a shift from vertical to lateral accretion, while the overlying Shire Member is characterized by paleosol development. These shifts represent changes in fluvial architecture related to variations in river discharge. The overlap of gradual tectonic activity and a short term climate aberration may have created optimal conditions allowing for the increased sediment transport observed in the Molina Member.

While tectonics may determine basin geometry and subsidence, changes in climate provides a mechanism to transport a higher sand volume into the basin. By further constraining the role of tectonics, we hope to quantify the affect climate change has on fluvial architecture.