THE CHARACTER AND AGE STRUCTURE OF VALLEY FILLS IN UPPER WOLUMLA CREEK CATCHMENT, SOUTH COAST, NEW-SOUTH-WALES, AUSTRALIA

Extensive valley fills at the base of the escarpment in upper Wolumla Creek, on the south coast of New South Wales, Australia, have formed f rom a combination of 'cut and fill' processes. The valley fills compri se series of alternating, horizontally bedded sand and mud units, refl ecting reworking of detritus from deeply weathered granites of the Beg a Batholith. Sand units are deposited as sand sheets or splays on floo dplain surfaces or in floodouts that form atop intact valley fill surf aces downstream of discontinuous gullies. Alternatively, sands are dep osited from bedload and form bars or part of the valley floor within c hannel fills. Organic-rich mud units are deposited from suspension in swamps or in seepage zones at the distal margin of floodouts. Within 5 km of the escarpment, valley deposits grade downstream from sand sheet and splay deposition in floodouts, to mud deposition in swamp and see page zones. Radiocarbon dates indicate that virtually the entire valle y fill of upper Wolumla Creek was excavated prior to 6000 years BP, Re mnant terraces are evident at valley margins. The valley subsequently filled between 6000 years sp and 1000 years sp producing valley fills around 12m deep, but no greater than 300m wide. Reincision into the va lley fill, on a scale smaller than the present incision phase, is indi cated at around 1000 years sp, following which the channel refilled. P ortion plans dated from 1865 refer to the study area as 'Wolumla Big F lat', and show large areas of swampy terrain, suggesting that the vall ey fill had re-established by this time. Within a few decades of Europ ean settlement the valley fill incised once more. Upper: Wolumla Creek now has a channel over 10m deep and 100m wide in places, draining a c atchment area of less than 20km(2). (C) 1998 John Wiley & Sons, Ltd.