HOLOCENE VALLEY AGGRADATION AND GULLY EROSION IN HEADWATER CATCHMENTS, SOUTH-EASTERN HIGHLANDS OF AUSTRALIA

Late Quaternary stratigraphy of a 50 km2 catchment on the south-easter n highlands of Australia reveals processes and history of denudation, and helps resolve a long-standing debate about factors controlling epi sodic valley aggradation and degradation during Holocene times. Valley sedimentation occurred when swampy vegetation fully colonized valley floors and obliterated all channels, promoting aggradation for periods of several thousand years, with most incoming sediment being trapped in swampy meadows. Much of the sediment was reworked from late Pleisto cene alluvial fan and valley fill deposits, and primary hillslope eros ion was minor during the Holocene. Differing sedimentation patterns be tween the Late Pleistocene, Holocene and Post-European settlement peri ods reflect regional changes in sediment supply and transport capacity as a result of major environmental change. Within the Holocene, howev er, valley fill stratigraphy is controlled by massive, episodic gully erosion terminating aggradation. Gully initiation appears to be contro lled more by thresholds of incision into vegetated valley floors than by changes to sediment supply. Whether the thresholds are exceeded bec ause of climatic change, autonomous change or extreme events cannot ye t be determined. Overall, the Holocene history represents continuing c omplex response to events of the Late Pleistocene, and does not suppor t the K-cycle concept, which has strongly influenced late Quaternary g eomorphology in Australia.