THE WHEN, WHAT, AND HOW OF GULLY EROSION AND SEDIMENT DEPOSITION ON THE TABLELANDS OF NEW SOUTH WALES, AUSTRALIA
Gully erosion and its relationship to PEM have been extensively studied on the Tablelands of southeastern Australia. We test the long-held geomorphic interpretation that gully incision was initiated in water-saturated swampy meadow environments along pre-European valley bottoms. We measure concentrations of meteoric 10Be and the OSL of bulk sediment samples from two PEM profiles derived from initial erosion in Birchams Creek.
Bulk sediment OSL data collected from two upstream reference profiles that could each potentially serve as the source material for PEM –swampy meadow sediments and weathered regolith derived from sandstone bedrock – show two possible depth-sources from both the swampy meadow profile (39-87 cm and 102-147 cm) and the regolith profile (9-18 cm and 63-99 cm).
Homogeneous concentrations of meteoric 10Be measured from PEM deposits (8.4-9.4 x 108 atoms/g) show that PEM was well mixed during fluvial transport before deposition. Meteoric 10Be results show that PEM could only be derived from shallow erosion (~12-15 cm) of the swampy meadow source profile and not the deep incision indicated by bulk sediment OSL data. PEM could be derived from the regolith source up to depths of 81 cm, which agrees with potential depths suggested by bulk OSL analysis.
We interpret these data to suggest that incision leading to gully erosion throughout the Tablelands region did not begin in swampy meadows. European-induced reductions of vegetation on valley bottoms in the late 1800s to early 1900s was an attempt to improve the land for livestock grazing – a change in land-use that ultimately led to removal of PEM sediment from its source and the incision of erosional gullies, many of which are still active.