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.

Triassic palaeogeography and fluvial dispersal across the northwest European Basins

“…The plane bedded and climbing ripple laminated sandstones represent the product of deposition from shallow, unconfined flows, and their arrangement into coarsening-upward cycles is suggestive of progradation, although variably sharp (with evidence of desiccation) or gradational basal contacts suggest that these progradational sand systems alternated between entering standing water bodies or subaerially exposed surfaces. These may be analogous in depositional process to the modern terminal splay complexes described from Lake Eyre (Lang et al 2004;Fisher et al 2008) and ancient systems described by Saez et al (2007).…”

Fluvial Systems in Desiccating Endorheic Basins

“…They are interpreted as the products of unconfined flow on the floodplain, occurring when the flow in the rivers was not contained within the channels . Similar unconfined flows at the distal ends of river systems are referred to as floodouts by Tooth (1999a, b) or terminal splays by Lang et al (2004). The high proportion of floodplain sandstone sheets in the distal areas suggests that unconfined flow was more frequent in these areas compared with the medial and proximal parts of the distributary systems.…”

Estratigrafia y sedimentologia del abanico terminal de la Formacion Candeleros (Grupo Neuquen), Cretácico Inferior, Cuenca Neuquina, provincias de Neuquen y Mendoza, Argentina.

“…were fed by three different mechanisms, shallow channels that lost their transport capacity under the inf luence of extremely low gradients, f low expansion phenomena related to topographic lows where the water eventually emerges and a spilling f low occurs (Tooth, 1999;Lang et al, 2004) and small distributary channels during episodes of high discharge from headwater systems that allowed for the extension of f lows in the f lood basin. Channelized episodes covered limited distances in the f loodplain because of the rapid loss of speed by friction that increased at the base of the channel, with increasing radial distance from the origin and laterally from the axis of the stream (Cuevas Gozalo and Martinius, 1993).…”