Using fluvial stratigraphic architecture to isolate the role of avulsion processes in alluvial-basin filling

Alluvial architecture, the spatial distribution and character of sediments in alluvial stratigraphy, is an important record of past climate, tectonic, and eustatic conditions, and helps us understand terrestrial landscape processes over long timescales. A combination of sedimentary basin dynamics and terrestrial landscape processes control alluvial architecture, particularly accommodation-creation, sediment supply, and river avulsion (the rapid relocation of a channel to a new spot on its floodplain), but deconvolving these processes from alluvial architecture is difficult. In this dissertation, I use field studies of ancient deposits and numerical modeling to identify characteristic signatures of avulsion processes in fluvial stratigraphy, and I propose novel metrics of alluvial architecture that give insight into paleo-avulsion pattern, relative sedimentation and accommodation-creation rates, and the grain size of paleo-sediment supply. Part of this dissertation specifically addresses how avulsion-generated channel deposits and different avulsion patterns can be identified in the stratigraphic record and over what spatiotemporal scales. To this end, I analyzed published literature and mapped channel sand bodies in the Williams Fork Formation (Cretaceous, Colorado) to show that avulsions generate characteristic multistory sand bodies with irregular bounding surfaces, vertical story stacking, and stories that correlate with distinct floodplain horizons. Results from an object-based model of basin filling suggest that in mud-dominated deposits, different avulsion patterns generate sand bodies with unique number of stories per sand body. Statistical analysis of outcrop-scale sand body distributions in the same lower Williams Fork deposits is also consistent with a random paleo-avulsion pattern, and I use object-based modeling to understand the sensitivity of statistical analyses for detecting avulsion patterns in a given dataset. In addition, this dissertation uses novel alluvial architecture measurements to deconvolve avulsion and basin-scale controls on sand-dominated fluvial deposits, which are traditionally iv interpreted as generated during times when the space to deposit sediment (accommodationcreation rates) is low relative to the sediment supply, causing the deposits to be poorly preserved, or “reworked”. Here I use fluvial bar geometry and lithofacies as a proxy for deposit reworking, and I use this method to show that some sand-dominated successions (such as the Castlegate Sandstone (Cretaceous, UT) have well-preserved bars and no evidence for rapid reworking. Finally, I use a new method to explore relative changes in sandiness of paleo-sediment supply using the grain size distribution of paleo-bed-material and paleo-slackwater deposits in the Blackhawk and Castlegate formations. Results suggest that a decrease in the supply of finegrained sediment may be driving the increase in deposit sandiness at the Blackhawk-Castlegate boundary, rather than an increase in reworking. Overall, these novel approaches to alluvial architecture analysis show that paleo-avulsion patterns can be uniquely interpreted from both mudand sand-dominated deposits, and we can estimate the relative influence of avulsion, accommodation-limited reworking, and sedimentsupply grain size for given patterns in alluvial architecture.