The red clay mantle in the Wagga Wagga region, New South Wales: evaluation of an aeolian dust deposit (Yarabee Parna) using methods of soil landscape mapping

Abstract

The aeolian dust deposits (parna) in the Wagga Wagga region have formed the most common soil material of the region, ‘red clay’, which has a distribution closely related to landforms. On high plains it occurs across >90% of the total area, forming a nearly continuous mantle. On rises and low hills with average slope <10%, the red clay covers most crests and side-slopes, but does not occur in drainage depressions. On hills with average slope steeper than 30%, the red clay contains a significant amount (up to >50%) of gravels and exists only at lower and foot-slopes. On hills with average slope gradient of 10–30%, the distribution and gravel contents of the red clay vary between the above two situations. In floodplains the red clay is absent.

These distribution patterns are consistent with the aeolian origin of the red clay and may be explained mainly by landform and pedogenic processes occurring since the aeolian dust deposition, including: (1) erosion and deposition, which are more dominant in hills and f loodplains; (2) disturbance by soil creep and bio-turbation; (3) other pedogenic processes which may cause colour changes, and loss or accumulation of colloids and soluble components; and (4) development of soil structures. These post-deposition processes explain the presence or absence of the ‘red clay’ in various landforms and its variations in field texture from light clay to light-medium clay, colour (hue) from 2.5YR to 5YR, structure from weak to moderate, pH from 5.0 to 8.0, and thickness from 30 to >80 cm. Although this relatively uniform ‘red clay’ forms the subsoil, different soil types occur mainly due to the more variable topsoils.