Douglas Creek terminal splay, sited on the western shoreline of Lake Eyre North, central Australia, covers a surface area of approximately 4 km2 with a down‐system length of 2·5 km from the distributary channels terminus to the splay fringe. Two distributary channels feed two sediment lobes which have amalgamated to form the terminal splay. Three primary facies associations have been identified sub‐dividing the creek terminus into distributary channel, proximal and distal splay sections. Proximal splay sediments are characterized by erosionally based, relatively thick (> 100 mm), stacked sheets of coarse to medium sand which commonly display trough and planar cross‐bedding, whereas the distal splay is characterized by thin (generally < 50 mm) massive beds of very fine sand, silt and clay. The change in splay sedimentology is interpreted as reflecting the transition from bedload‐dominated deposition to suspended load‐dominated deposition from decelerating sheetfloods as they spread out from the channel onto the dry lake bed. A proximal to distal splay transition zone is also noted where deposits of both facies associations interfinger laterally and vertically. In scale, geometry and facies associations, the Douglas Creek terminal splay is very different to the often cited Neales terminal splay complex located 70 km to the north. It is suggested that these architectural differences reflect variations in discharge volume, input sediment distribution and the degree of vegetation cover. Understanding the variation in terminal splay architecture has very significant implications for the modelling of analogous subsurface petroleum systems, which at present relies on few modern‐day analogues.

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Sheetfloods are typically invoked as the mechanism responsible for the kilometre-scale transport of sand-sized sediment grains in shallow-gradient fluvial systems. This concept is based on the lateral extent of ancient thin, sheet sandstone deposits rather than on fluid dynamics, which has resulted in a loosely constrained model for sheetfloods. This study tested the conceptual mechanism by developing a depthaveraged, 2D computational fluid dynamics model. The model results compare well against observations from modern deposits at Lake Eyre to provide a quantitative, physically sound basis for sheetfloods that can be applied in ancient and modern settings to constrain otherwise qualitative interpretations.Fluvially sourced, thin, sheet sandstones and splays are frequently observed at the distal fringes of endorheic, shallow-gradient drainage basins in ancient (Stear Hinds et al. 2004) and modern settings (Parkash et al. 1983;Abdullatif 1989;Lang et al. 2004;Tooth 2005). Process interpretations suggest that subaerial, unconfined, sheetfloods were responsible for depositing sand-sized sediment over distances of hundreds to thousands of metres. The term sheetflood is used here in reference to subaerial, unconfined, turbulent flow events that undergo expansion, thinning and deceleration with increasing radial distance from source. This concept of unconfined, fluvially sourced sheetfloods is qualitatively based on observations and dimensional measurements of sheet sand deposits rather than on a firm understanding and appreciation of the fluid dynamics of sheetflood events. Using descriptional observations to interpret the physical processes of sheetflood events is an unsafe way to proceed (see Paola 2000), as the fundamental question 'what are the fluid dynamic characteristics of sheetfloods that are capable of transporting sediments over the distances interpreted in the literature?' remains unanswered. As a qualitative, descriptional concept, the relevance to both ancient and modern datasets is difficult to test and predictions regarding sedimentary deposits are very loosely constrained. This concept cannot easily be tested via laboratory modelling, as scaling of the thin sheetflood will result in laminar flow conditions. This paper compares the results of a 2D computer model for sheetfloods with observed data collected from the Douglas Creek Terminal Splay, central Australia, to provide a quantitative, fluid dynamics framework for fluvial sheet sandstone interpretation. In doing so we present a mathematical solution that generates wellconstrained predictions that can be applied to and tested on analogous ancient and modern deposits. Douglas Creek Terminal SplayThe terminal splay deposited from numerous sheetflood events at the terminus of the ephemeral Douglas Creek, Lake Eyre, central Australia ( Fig. 1) was selected as a modern example of a sheetflood deposit. Thirty-one trenches were dug on the 4 km 2 splay at various measured distances up to 2 km from source at the channel mouth. Facies were described and thickn...

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The aeolian regime of the 100 km wide, hyperarid Namib Desert has been sporadically punctuated by the deposition of fluvial sediments generated during periods of higher humidity either further inland or well within the desert from Late Oligocene to Late Holocene. Four new Late Cenozoic formations are described from the northern Skeleton Coast and compared with formations further south: the Klein Nadas, Nadas (gravels, sands), Vulture’s Nest (silts) and Uniab Boulder Formations. The Klein Nadas Formation is a trimodal mass-flow fan consisting of thousands of huge, remobilised, end-Carboniferous Dwyka glacial boulders, many &gt;3 m in length, set in an abundant, K-feldspar-rich and sandy matrix of fine gravel. Deluge rains over the smallest catchments deep within the northern Namib were the driving agent for the Klein Nadas Fan, the termination of which, with its contained boulders, rests on the coastal salt pans. These rains also resulted in catastrophic mass flows in several of the other northern Namib rivers. The Uniab Boulder Formation, being one, consists only of huge free-standing boulders. Gravelly fluvial deposition took place during global interglacial and glacial events. The Skeleton Coast Erg and other smaller dune trains blocked the rivers at times. The low-energy, thinly bedded silt deposits of the central and northern Namib are quite distinctive from the sands and gravels of older deposits. Their intermittent deposition is illustrated by bioturbation and pedogenesis of individual layers. Published offshore proxy climatological data (pollens, upwelling, wind, sea surface temperatures) point to expansion of the winter-rainfall regime of the southern Cape into southwestern Angola during strong glacial periods between the Upper Pleistocene and Holocene. In contrast to deposition initiated by short summer thunder storms, we contend that the silt successions are river-end accumulations within which each layer was deposited by runoff from comparatively gentle winter rains that lasted several days.