USING TRACE FOSSILS TO INTERPRET THE HYDROCLIMATOLOGICAL SETTINGS OF DRYLAND ALLUVIAL-LACUSTRINE SYSTEMS: EXAMPLES FROM THE NEALES FAN AREA, LAKE EYRE, SIMPSON DESERT, AUSTRALIA

Such organism-media interactions and resultant sedimentary fabrics as those found in dryland alluvial-lacustrine systems in the modern Lake Eyre system in the Simpson Desert, Australia, have unique hydroclimatological signatures compared to those found in much wetter systems. The Neales Fan, Neales River, and Umbum Creek, the area of study, is a terminal splay complex on the western margin of Lake Eyre North, where fluvial systems enter the lacustrine basin. Organisms living in the Simpson Desert have adapted to a range of semiarid to hyperarid climates in which water is the limiting factor of biodiversity and its distribution. Organisms must burrow to escape extremes in temperature and avoid moisture loss. This behavior must be coupled with feeding and reproductive strategies in order for them to continue to survive in such hostile settings. The Lake Eyre is excellent as an analog for ancient hydroclimatological systems formed in dryland settings.

Our preliminary work suggests that the greatest abundance and diversity of traces found in the area of Neales Fan, Neales River, and Umbum Creek area of Lake Eyre is produced by terrestrial organisms during periods when the lakeplain and the proximal lake bed are dry and have physicochemical characteristics dominated by subaerial conditions. The diversity and abundance of tracemaking organisms decreases rapidly from the lakeplain to the proximal lakebed where no apparent living organisms have been found. Aquatic organisms are present only after floods reach this area; the diversity and abundance is controlled by the amount of freshwater that reaches the lake, the majority of which do not have enough water to fill the proximal portion of the lake. Snails and shield shrimp dominate, along with predaceous diving beetles and dipteran larvae; in some instances small fish are present. The traces produced by these organisms are mostly horizontal and minimally penetrative into the channel bottom or lakebed. The majority of these traces appear to be produced when the water level has fallen close to or at the sediment-water-air interface. The majority of vertically penetrative and sediment reworking traces, therefore, reflect subaerial conditions.