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Controls on Facies Distributions in Dryland Rivers; A Tale of Two Catchments, a Fault and Transitions from Single Channel to Anabranching Planforms


The intracratonic dryland Lake Eyre basin is a widely used analogue for ancient dryland successions. However, controls on fluvial planform morphology and lithofacies are poorly understood, limiting applicability of the Lake Eyre basin as a modern analogue for the subsurface. Rivers to the west of Lake Eyre are ephemeral and often exhibit repeated downstream variation between single and anabranching channel planform geometries. We present a detailed investigation of the relationship between grain-size distributions, depositional elements and planform geometry of two rivers to the west of Lake Eyre. This study aims to improve understanding of fundamental controls on modern dryland fluvial deposits and apply these findings to improved understanding of deposit heterogeneity and reservoir quality characteristics. Initial investigation focused on a reach of the Neales River in which river pattern changes from single channel to anabranching. Field observations were surprising; the coarsest bed material was found distal to the main channel in the single channel reach. Largest clasts are transported in shallow channels not observable in satellite imagery. Depositional elements with distinct lithofacies characteristics are interpreted to be deposited during flow events of differing magnitude. Channel dimensions and lithofacies vary with channel pattern. To test our explanation of deposit variability, a second investigation was conducted on the Peake Creek, which traverses the same Geology and displays a similar change in planform geometry. Observations are comparable to those from the Neales River, with distinct lithofacies and grain size distributions in different depositional elements, organized across the floodplain according to their controlling discharge event magnitude. We present a comparison of depositional element maps, surveyed topographic transects and quantitative analysis of grain size from these two rivers, and a model to explain the primary controls on planform geometry and deposit facies variability and relationships. Key findings are that reservoir properties and deposit architecture will vary with river pattern, which in these rivers changes often. Primary controls on river pattern are valley slope and floodplain width. The classic trends of decreasing grain-size downstream and with distance from the channel across the floodplain are not applicable. Facies distribution is controlled by planform morphology and discharge variability.