FLOOD DEPOSITS OF THE SAND‐BED EPHEMERAL STREAMS OF CENTRAL AUSTRALIA

Abstract

ABSTRACTThis study investigates the deposits of sand‐bed ephemeral streams within an area of 250,000 km2 in the western Lake Eyre basin, central Australia, being initiated after the record floods of February–March 1967.Large‐scale ripples were the most common bed forms preserved in the majority of channels, covering 30–40% of the depositional area. Other bed forms included longitudinal, transverse and linguoid bars (20%), upper‐regime plane beds (<5%), small‐scale ripples (25–30%), and flute marks (<1%). Major bed forms could be related to flood stage: plane beds (upper regime), large‐scale ripples, and longitudinal bars usually were generated by high‐stage flow, and transverse and linguoid bars by waning and low‐stage flow.Trough cross‐stratification was produced by migrating small‐scale and large‐scale ripples, large‐scale sets comprising about 60% of the deposits. Tabular cross‐stratification (25% of deposits) resulted from the downstream and lateral growth of channel bars. Flat‐bedding with parting lineation (<5%) was dominant only along the middle reach of the Finke, the major stream in the western Lake Eyre basin. Disturbed bedding was almost totally absent. Bed forms, in order of increasing overall mean grain size (?) of deposits, are plane beds (upper regime), small‐scale ripples, transverse and linguoid bars, and large‐scale ripples. Sorting (σI) and skewness (SkI) are dependent on mean grain size (Mz).Lower‐regime flow prevailed for the majority of streams, upper‐regime flow being dominant only in the middle reach of the Finke. Using estimated Froude numbers and Darcy‐Weisbach resistance coefficients, the greatest mean velocities of flow have been calculated at about 0.8–3.7 m/sec.