HIERARCHICAL SEDIMENTOLOGICAL ANALYSIS OF FLUVIAL SEDIMENTARY PATTERNS IN THE UPPER CRETACEOUS BILLMAN CREEK FORMATION, LIVINGSTON, MT

To be predictive, fluvial facies models must portray lithologic heterogeneity in 3D sedimentary patterns from spatially limited data. The sedimentary architecture of a fluvial system is documented from a well-exposed 2D road-cut in the Upper Cretaceous Billman Creek Formation to evaluate the accuracy of facies models predictions. To quantify architecture, five detailed measured sections totaling 63 m and 900 m² of outcrop photo maps were collected. Data was restored and quantified in a sedimentological hierarchy of sedimentary facies, event beds, sedimentary bodies, and sedimentation regions. These attributes were correlated to different scale formative processes operating across a hierarchy of boundary layers.

Seventeen facies ranging from coarse sandstone to claystone record the interplay between fluid shear, sediment settling velocity, and flow viscosity during deposition. An acceleration matrix captures non-uniform and unsteady flow creating variation in facies assemblages in five event bed types. Four sedimentary body types record time-averaged flow confinement. Channelform sedimentary bodies form from confined flows scaling to the bank-full discharge. A hierarchy of complex, composite, and elementary channelforms record avulsion, switching, and thalwag migration. Partial to unconfined flow creates wedgeforms, lobeforms, and drapes in flanking fluvial strata. Sedimentary body types determine facies assemblages, event beds, and erosional surface frequency. Collectively, these attributes define channel, channel flank, and floodplain sedimentation regions.

Six paleogeographic maps characterize the evolution of the fluvial system. An avulsion event disconformably places the channel region into the floodplain region. Calculated channel paleo-discharge rates decrease through time from 3.45 to 0.19 m³/s, corresponding to a change in sinuosity and stacking pattern within a single-channel course. This records decreasing longitudinal gradient and channel switching due to chute and neck cutoff events prior to channel avulsion. This hierarchical approach correctly links scalar flow processes to varying sedimentary responses. The result is a more robust prediction of fluvial system evolution and resultant 3D sedimentary architecture.