Australian Drylands Depositional Environments: Local News, Globally Relevant

Wakelin-King, Gresley A.1

1Wakelin Associates, Melbourne, Australia

Landforms and sediments of continental drylands are relevant to the rock record and to sustainable land management. Globally, many strata were deposited in dryland settings, and modern landscapes are used as analogues for conceptual models of depositional processes. In the present day, ~40% of the world’s land is drylands, and it houses >30% of the people ; Australian drylands are ~80% of the main continent and contain communities, industries and biodiversity.  Understanding how drylands work is clearly desirable. Globally, drylands have highly variable rainfall, highly to extremely variable flow regimes, and a moisture deficit: attributes which govern biota life strategies, and create characteristic sedimentary deposits.

Previously, modern analogues focussed on northern hemisphere examples, typically from coarse-clastic, tectonically vigorous, high gradient catchments with flashy hydrology. Australia‘s context is different: low-relief, low-gradient, subtly expressed neotectonism, a blanket of regolith, and ephemeral rivers capable of big floods and sustained flows. Few Australian drylands sediments have been well-documented; other similar drylands (Thar desert, India; sub-Saharan Africa) may also be under-represented in (English-language) literature, or under-utilised as modern analogues.

This presentation journeys through some Australian continental sedimentary landscapes.

Mud-Aggregate Floodplain, Massive Mudrock

Mud aggregate floodplains are the modern analogue for massive mudrock. The current modern analogues (both located in Australia) are vertic soils transported as sand/ silt-sized bedload. Cooper Creek’s floodplain has black muds deposited/reworked in braid-like bars; Fowlers Creek is a cut-and-fill floodplain with vertically-accreted red muds.

Low-Angle Alluvial Fans

In Australia downstream-diverging fluvial networks are common, usually as low-angle alluvial fans. Channel systems range from coeval multithread to ~single channels sequentially moving across a depositional surface. Fine sediments transported by low-energy flows create broad low-gradient fans. Diverse topographic and climatic contexts lead to a range of sedimentary deposits, most of which are undocumented. Two examples are the mud-dominant Lodden Fan in semi-arid grassland, and the complex aeolian and fluvial sands in the Cooper Creek Fan (Strzelecki Desert).

Floodouts: a Fluvial Process

Floodouts are unchannelised river reaches (flow path is 100% floodplain). Declining discharge during development of Cainozoic aridity means that many Australian drylands rivers are underfit, or their flows no longer extend down the length of the network. Episodic and incomplete sediment transport promotes floodout formation, typically where flow emerges from lateral constriction and loses sediment transport capacity. Floodouts occur in rangefront plains, macrochannels, and valley confluences; they are valuable in land management. Some floodout bedding sequences could be identifiable in the rock record.

Flashy Flow Events, Froude Numbers, and Flat Bedding

Sedimentary fluvial rocks from a drylands setting may include planar-bedded sands. Comparison with the standard bedform diagram may lead to an interpretation of deposition during the high-energy (F = 1) flood peak of a “typical” flashy desert flow. However, bedforms in modern drylands rivers demonstrate more complex conditions including widespread subcritical (F <1) planar bedding, and low-energy gravel imbrication. Bedforms are governed by grain size and flow velocity, but also by flow depth and sediment composition; this leads to intriguing bedform combinations in rivers with rapid flood recession.


Biography

Dr. Wakelin-King is a geological geomorphologist, specialising in drylands rivers and sediments. Her professional practice aims at normalising geomorphology within sustainable landscape management. She researches fluvial processes in the Lake Eyre Basin and western NSW, and strongly advocates for fieldwork as a necessary component of remote-area landscape studies.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.