--> --> Sedimentology, chemofacies, and stratigraphic architecture of the Lower Cretaceous Burro Canyon Formation, Nine-Mile Canyon, Colorado

AAPG Rocky Mountain Section Meeting

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Sedimentology, chemofacies, and stratigraphic architecture of the Lower Cretaceous Burro Canyon Formation, Nine-Mile Canyon, Colorado

Abstract

The fluvial sedimentology and architectural elements of the Burro Canyon Formation vary both laterally and stratigraphically within the Piceance Basin of northwestern Colorado. Analysis of a well-exposed outcrop along Nine-Mile Canyon, near Whitewater, Colorado, provides further insight into the stratigraphic variability and heterogeneity of the fluvial deposits. Additionally, comparison to other previous studies of outcrop locations of the Burro Canyon Formation in the surrounding areas (e.g. Escalante and Rattlesnake Canyons) further defines the spatial and lateral distribution of the depositional characteristics. In order to address the stratigraphic variability of the lithofacies and chemofacies of the sequence, a detailed measured section, outcrop spectral gamma-ray profile, measured x-ray fluorescence profile, and thin-section analysis are used to identify the elemental abundance, mineralogy, and lithology of the sequence. The genetically related lithofacies are grouped into architectural elements that form semi-amalgamated to amalgamated channel complexes. The dominant indicator elements (proxies) are determined and related to environmental conditions. Using machine-learning techniques, chemofacies are defined based on the elemental abundances of the sequence. The roughly 80 foot-thick measured section predominantly consists of lithologies ranging from fine-grained to coarse-grained sandstones and greenish mudstones. Sedimentary structures primarily include trough crossbedding, planar bedding, and channel scour. At least two distinct fluvial systems are characterized within this measured section: a lower interval characterized by a low-sinuosity high net-to-gross braided fluvial system and an upper interval characterized by a more sinuous low net-to-gross fluvial system. The architectural elements defined by this outcrop study are then compared to other nearby localities that have been previously measured to interpret the depositional characteristics at Nine-Mile Canyon and further constrain the lateral variability of the Burro Canyon Formation.