EVOLUTION OF ALLUVIAL FANS DRAINING FROM THE COLORADO FRONT RANGE DURING THE LARAMIDE OROGENY

As relief developed on the Front Range during the Laramide orogeny, large river systems carried sedimentary debris into the subsiding Denver Basin. In certain areas and during certain times long-lived streams occupied incised canyons in the mountain front and lobes of coarse sand–rich sediments were deposited at the foot of the mountains. These strata are interpreted as having accumulated in alluvial fans and in fluvial distributary fans.

The alluvial fans are characterized by debris-flow textures that are best developed in the southern part of the Denver Basin near Colorado Springs. Distributary fans, (one covering an area in excess of 500 square miles), lacking debris-flow textures and dominated by fluvial cross-bedded strata, are present in the central and northern portions of the basin west and southwest of Denver. Subsurface studies based on extensive well-control reveal sandstone distribution patterns that are interpreted as resulting from the dispersal of sediment by the focused rivers debouching into the basin from long-vanished canyons. The distribution of fans is best documented in the D1 sequence of synorogenic strata, ranging in age from 68 to 63 million years. Studies suggest that major river systems changed their loci of emergence over the span of time represented by the D1 sequence. Provenance studies suggest this change is due to regional evolution of catchment areas, a process that might involve large-scale stream piracy.

Integrated studies at the Denver Museum of Nature & Science suggest that local climate changes possibly driven by tectonically-driven orographic effects may have influenced the precipitation patterns responsible for the deposition of the fan detritus.