A New Facies Model for Terminal Splays in Dryland Fluvial-Lacustrine Basins

Simon C. Lang¹, Mark Reilly¹, John Fisher², Carmen Krapf¹, Tobias Payenberg¹, and Jochen Kassan^3
1 Australian School of Petroleum, University of Adelaide, Adelaide, Australia
² Royal Holloway University
³ Whistler Research, Brisbane, Australia

Terminal splays are key features of dryland fluvial-lacustrine basins, and occur where a river enters a playa lake, or simply drains onto a subaerial floodplain. These are being increasingly recognized within many continental basins (eg. North Sea, northern and central Africa, South Caspian Sea). A new facies model for terminal splays is presented based on field work around Lake Eyre, central Australia. A variety of terminal splays occur at the terminus of the Neales River, Umbum and Douglas Creeks. The overall setting is an arid, low-accommodation continental interior basin, the Lake Eyre basin (>1.3M km2), Australia.

The facies model begins with a sandy low-sinuosity fluvial channel belt that becomes a network of bifurcating, downstream-narrowing, 5-1m deep distributive avulsion channels (>1:50 <1:150 t/w ratio) filled by sandy, fining-upward, simple or compound bars. Desiccated mudplugs are common within the channels. Numerous narrow crevasse splay channels occur (<1:25 t/w ratio) until flow becomes unconfined, constructing a splay lobe or low relief triangular middle ground bar that comprise the bulk of the terminal splay complex. These overlie desiccated and vegetated and terrestrially-bioturbated floodplains and/or the playa lake floor. They are typically fining-upward sheetlike deposits, with small-scale 2D and 3D dunes, parallel and/or upward-convex parallel lamination, abundant climbing ripples (< 0.1-1m thick, ~100-1000m wide; >1:1000 t/w ratio). Aeolian reworking is pervasive, resulting in a well-sorted sweet spot at the distal part of the terminal splay complex. Saline playa lake muds, fine-grained floodplain facies, and abandoned channel mud-plugs form the main baffles and seals.