An aeolian component in Pleistocene and Holocene valley aggradation: evidence from Dicks Creek catchment, Yass, New South Wales

Abstract

Four late Quaternary depositional units are identified overlying sub-vertically dipping Ordovician bedrock in the upper reaches of the Dicks Creek catchment, near Yass in the Southern Highlands of New South Wales. The units are spatially discontinuous and separated from each other by erosional unconformities. They are found only on the lower slopes and in the valley floors, often exposed by recent gully erosion. The oldest unit (Unit 4) is a competent consolidated well-sorted fine to medium silt that unconformably overlies bedrock. It often forms the base to erosion gullies. Unit 3 is strongly dispersible and frequently has the characteristics of a debris flow. Unit 3 is particularly prone to sheet erosion and exhibits a high risk of dryland salinity development. Unit 2 is light to dark grey, poorly sorted, and often contains irregularly dispersed charcoal. Unit 2 is unconformably overlain by a predominantly pale yellow sand (Unit 1) that shows clear evidence of very recent deposition.

Physical and chemical characteristics of Units 2, 3, and 4 suggest an aeolian component. The silt size (4–8 on phi scale) fraction of Unit 4 is often >70% of the total mass, with grain sizes consistent with an origin as aeolian dust. Unit 3 is yellow brown in colour and often has the characteristics of a diamict with a major grain size component similar in size to Unit 4. Unit 2 is typically uniform in appearance and contains a predominantly kaolinite and illite clay mineralogy that contrasts strongly with a predominance of quartz in the underlying bedrock. A simple sediment budget indicates that the volume of Unit 2 could be accounted for by a combination of sheet and rill erosion within the catchment and additional aeolian deposition in the order of 4–8 t/km ² year.

Radiocarbon dates for charcoal recovered from Unit 2 indicate that some deposition was associated with cooler, drier conditions of the late Holocene ‘Little Ice Age’, approximately 200–600 years ago.