SEDIMENTARY PROCESSES AND FACIES GEOMETRY OF A SANDY EPHEMERAL TERMINAL SPLAY COMPLEX, UMBUM CREEK, LAKE EYRE, CENTRAL AUSTRALIA

Mark R.W. Reilly¹, Simon C. Lang¹, Tobias H.D. Payenberg¹, and Jochen Kassan^1,2
¹ Australian School of Petroleum (ASP) and Australian Petroleum Cooperative Research
Centre (APCRC), University of Adelaide, Adelaide, SA 5005, Australia
² Whistler Research Pty Ltd, Whistler Court, Greenbank, QLD 4124, Australia

Understanding the complexities of sub-surface fluvial-lacustrine sandstone reservoirs can be improved with the application of analogue datasets from comparable modern settings and outcrops. The Quaternary Umbum Creek terminal splay complex, located on the western shore of Lake Eyre, Central Australia, provides a useful dataset for facies distribution and geometry.

Within the active fluvial system, the topographically higher frontal splay complex, comprising older coarse-grained fluvial sediments, dams floodwaters in the main channel until it reaches erosional spill points at the head of each distributary splay channel. The main fluvial channel then mobilises silt and poorly-sorted, very fine- to coarse-grained sands and reworks the older, coarser fraction at the head of each distributary, leaving behind a granule to cobble lag. At the end of the distributary channels high net to gross, very well-sorted, fine- to upper medium-grained sandy lobate splays are constructed. Beyond the frontal splay complex, fine-grained silts and mud are deposited from suspension as a sheet across the frontal splay complex. After the floodwater water dissipates and evaporates, aeolian processes dominate, resulting in very-fine to fine-grained sands accumulating between the splays.

Aerial photo interpretation, facies mapping and surveying has identified four individual lobate splays. The geometry of the splay lobes range from 150 - 300 m wide, up to 500m long, and 0.6 - 1.0 m thick. The accompanying splay distributary channels are 50 – 150 m wide, 280 - 570 m long and 0.6 – 1.0 m deep. Although this system morphologically resembles a fluvial influenced delta on the aerial photographs, the depositional processes indicate that the ephemeral river terminates onto the playa lake surface during floods, limiting the progradation within a shallow sheet of rapidly dissipating floodwater, resulting in a high net to gross sand sheet.

Figure 1. Umbum Creek, western Lake Eyre, Central Australia showing a sandy terminal splay complex dominated by a single main fluvial channel downstream of an incised meanderbelt. Note the pooling of water in the fluvial channels updip of the frontal splay complex which is topographically higher than the base of the main fluvial channel. The white sediment in the foreground is a thin veneer of salt extending into the playa lake.