MODERN ANALOGUES FOR DRYLAND SANDY FLUVIAL-LACUSTRINE DELTAS AND TERMINAL SPLAY RESERVOIRS

Abstract

Ephemeral sandy fluvial-lacustrine deltas and terminal splays associated with dryland depositional environments are important reservoirs in many basins around the world, in both pericratonic and intracratonic settings (Triassic of Algeria; Triassic of the North Sea; and Pliocene of the Caspian Sea). Research on modern depositional analogues from dryland basins offers insights into these types of reservoirs. Australia’s modern Lake Eyre Basin, an arid to hyper-arid, low-accommodation intracratonic basin in central Australia, provides an ideal natural laboratory.

This paper highlights field observations of modern, sand-prone, reservoir analogues from the Neales River and Umbum Creek, on the western fringe of Lake Eyre, including unique aerial observations of sedimentation from a rare flood event in an ephemeral fluvial system. These rivers flow irregularly in a dryland setting, but are prone to flash flooding and highly variable discharge that moves large volumes of sediment over a few hours or days. Although there are variations in sediment type and discharge, similarities exist with the key reservoir elements common to most modern and ancient dryland fluvial-lacustrine systems.

Distinctive elements include fluvial point bar and associated overbank deposits, distributive avulsion channels and down-dip terminal splays, either on the floodplain or onto the playa lake fringe. The terminal splays are formed, where there is not a pre-existing standing body of water, during rapidly decelerating flows with high-flow regime, transitional to low-flow regime conditions. Typical structures include parallel lamination, convex-upward parallel lamination, climbing ripples and small-scale 2D and 3D dunes. Flow interference with in-channel and floodplain vegetation is an important sediment-trapping mechanism with reservoir quality implications. Aeolian deflation is also significant as it causes the removal of fine-grained sediments during dry periods. The main controls on sediment preservation include the overall low-accommodation setting and rare major lake-filling events controlled by flooding out-of-phase with flows down the western rivers. Depositional products are either high-net-to gross fluvial- terminal splay sheet sands or lower net-to-gross fluvial- terminal splay-lacustrine delta sand sheets or stringers.