Sedimentology, chemofacies, and stratigraphic architecture of the Lower Cretaceous Burro Canyon Formation, Ninemile Hill, Unaweep Canyon, Colorado

Abstract

Well-exposed outcrops of the Lower Cretaceous Burro Canyon Formation at Ninemile Hill and surrounding areas in western Colorado, provide insight into the depositional characteristics and stratigraphic variability of these fluvial deposits. Comparison of the fluvial heterogeneity to other outcrop studies of the Burro Canyon Formation further defines the spatial and lateral heterogeneity of the fluvial deposits. The sedimentology, chemofacies, and stratigraphic architecture are addressed through a detailed 73-ft (22.3-m) measured section with gamma-ray and x-ray fluorescence profiles and thin-section petrography. Burro Canyon Formation lithofacies consist of ripple-bedded fine-grained sandstone, green mudrock, cross-stratified and planar-bedded medium- to coarse-grained sandstone, slightly conglomeritic cross-stratified medium- to coarse-grained sandstone, and massive-bedded sandstone. Dominant indicator elements (lithologic and depositional environment proxies) are grouped into chemofacies using k-means and hierarchical clustering, identifying carbonate-rich facies, clay-rich facies, and sand-rich facies in outcrop. Genetically related lithofacies define architectural elements that stack to form an amalgamated channel complex that is overlain by a non-amalgamated channel complex. The lower interval is characterized by low-sinuosity to braided, higher net-to-gross ratio fluvial deposits and the upper interval consists of lower net-to-gross ratio floodplain deposits. Lower Burro Canyon deposition was by low-sinuosity to braided-fluvial systems within incised valleys, whereas the upper Burro Canyon was deposited within a floodplain-dominated environment.