3D Facies Architecture of a Dryland Shadow Bar, Umbum Creek, Central Australia

Payenberg, Tobias¹, Nathan P. Gardiner¹, Carmen B.E. Krapf¹, Mark R. W. Reilly² (1) The University of Adelaide, Adelaide, Australia (2) University of Adelaide, South Australia, Australia

Modern depositional analogues that quantify the distribution of facies types create essential data sets that aid in the development of reservoir models for comparable ancient subsurface deposits. Umbum Creek flows into the western side of Lake Eyre, Central Australia. High flow regime bedding structures predominate in a catchment area that provides infrequent flooding events as the dominant fluvial transport mechanisms. In its lower reaches Umbum Creek occupies a relatively straight incised channel belt except for two incised meanders immediately downstream a basement fault. The meanders form convex bars (pointbars) that are characterised by a series of sandy chute channels with complex shadow bars that vary in sizes between 5x5m metres to up to 50x50m. The shadow bars are forming in response to a low velocity zone created in the lee of an obstacle, dominantly vegetation.

A high resolution 30x30 m GPR 3D grid was acquired with a Mala X3M system with 250 MHz antenna across one of the shadow bars to investigate their 3D internal facies architecture. In addition, several trenches aligned to the GPS survey lines were excavated to allow a direct comparison with the interpreted GPR data. The internal facies architecture is characterized by multiple erosional surfaces, several discontinuous 0.5 to 5 cm thick mud drapes and fining upward cross-bedded sandy gravel packages. Former scour pools, imaged as “smilies” in the GPR data set, are filled with fine-grained sand comprising abundant climbing ripples, sinusoidal ripples and convex upwards parallel stratification (CUPS), mantled by mudcracked silts indicating waning flood deposits.