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Placing the Lake Eyre Terminal Splay Facies Model in a Global and Geologic Context: Similarities and Differences between Five Terminal Splays around Lake Eyre, and a Comparison with Fluvial Fans and Deltas

Amos, Kathryn J.1; Krapf, Carmen B.1
1 Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia.

Sedimentary fans that form at the terminus of an ephemeral fluvial system on a playa surface are used as a modern analogue for ancient dryland fluvial-lacustrine sandstone reservoirs such as the North Sea Triassic and South Caspian Pliocene. These have been named terminal splay complexes (TSCs) in Lake Eyre, central Australia, for which a facies model has been established. However, this facies model is based on observations from a limited number of these sediment complexes. The usefulness of this facies model in a geologic context is dependant upon this model applying to other terminal splay complexes.

This study presents a comparison between five terminal splay complexes from around the margins of Lake Eyre, the worlds fifth largest playa and depocentre for a 1.14 million km2 dryland continental interior basin. Their sizes range from 0.1 km2 to 36 km2, with grain sizes ranging from clay to gravel. For these analogues to be usefully applied in geological models, expected variation in facies characteristics between differing settings (e.g. climate and slope) needs to be explored. This study presents an initial comparison between the TSCs of Lake Eyre and published descriptions of fluvial fans from different environments.

In addition to an understanding of TSC variability, understanding temporal variability in dryland systems is critical for usefully applying modern analogues to the geological record. Temporal variations on a short time-scale (tens to hundreds of years) include fluvial discharge and lake filling events during dry phases. These can result in wave and wind-tide reworking of sediments, producing characteristic wave and tidal sedimentary structures in a continental fluvially-dominated deposit. Due to the ephemeral nature of sedimentation in these systems and low slopes, discrete lobes will be active during small discharge events which may have differing sediment characteristics to each other, and the TSC will accrete laterally as it progrades. Over longer time-periods, prolonged lake-level highstands during wet phases may result in the deposition of lacustrine muds over the TSC, and the sub-aqueous deposition of fluvially sourced sediments may produce shallow lacustrine delta facies. These temporal variations in sedimentary processes will have major implications for understanding the internal architecture characteristics of TSCs. These are explored, and some schematic hypothetical successions are presented.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009