Geomorphic and stratigraphic complexity: Holocene alluvial history of upper Wollombi Brook, Australia

Holocene and post-European settlement alluvial histories of three nested drainage basins were reconstructed from detailed litho- and chronostratigraphy of cut and fill terraces and floodplains in the upper Wollombi Brook catchment. Fernances Creek (13.8 km(2)) valley fill consisted of intercalated thin mud sheets deposited in ephemeral swamps and thick sand sheets deposited by discontinuous channels. Dairy Arm (39.8 km(2)) valley fill was more complex, with inset alluvial fills in the upper basin and overlapping vertically stacked fills in the lower basin. However, correlative litho-stratigraphic units were not found on all tributaries. Furthermore, basal radiocarbon dates on the last inset fill of four tributaries did not overlap, allowing for plus or minus twice the standard deviation of the reported ages. Wollombi Brook (341 km(2)) valley fill was also complex, with longitudinally discontinuous units, most of which were not found in the two tributaries. Upstream late Holocene channel incision was coeval with downstream chain of ponds because sediment generated by incision was stored in the intervening valley. Historical channel incision occurred between 1838 and 1867 on Fernances Creek at a locally steeper section of valley floor during the period of peak population and frequent floods immediately after a road crossing was constructed, but coincided with a catastrophic flood on Dairy Arm (June 1949) and on Wollombi Brook (1927). Lack of correlative litho- and chronostratigraphic units plus out-of-phase post-European incision indicate that stratigraphic complexity is a function of geomorphic complexity due to the operation of geomorphic thresholds and complex response.