Clarence River lowland floodplain : a morphostratigraphic analysis of a complex Holocene floodplain
Creators: name: Huq, Nasser Ejazul
Publisher: name: The Australian National University
Year: 1995

description: The Clarence Holocene fluvio-estuarine basin is, in fact, a double basin (inner and outer) separated by a bedrock barrier which acts as a "double" protector for the inner basin against marine influences. The Clarence fluvio-estuarine basin was inundated by the Holocene sea-level transgression about 8,000 years B.P. and fonned mud basin with tidal deltas at the entrance of the outer basin along the present coast and also, at the northern bedrock passage of the Inner basin. The inner basin for most of the Holocene time, was fluvial dominant with resulting brackish water environment for most of the basin. The outer basin was marine dominant for most of the Holocene time. The Holocene Clarence delta prograded through the inner basin in a protruding manner similar to elongated deltas of the Gippsland Lakes of southeast Australia. By about 2,500 years B.P. the bifurcating main channel within the inner basin was established, that closed-off most of the basin from tidal activity, and fluvial sedimentation became overwhelmingly dominant ever since. The estuarine deposits of inner basin is dominantly muds with tidal-delta sandy sediments confined to the northern-entrance area of the basin, whereas the outer basin has much more marine sands which are subsequently being buried by the fluvial sediments. The Clarence River within the inner basin (the study area) has been laterally stable in its present fluvial regime, that is since the Holocene establishment of the main channel in its present position about 2,500 years B.P. But unlike typical floodplains of the laterally stable channels, the Clarence floodplain is complex, having subsidiary channel-splay complexes nested within the broader floodplain, which cover about 40% by area of the Clarence floodplain. Geomorphic elements within these floodplainchannel zones, such as, channels, levees, plains, swamps etc., and their sediments are deposited by channel and overbank processes within these floodplain-channel zones rather than the main Clarence River. Geomorphic units and their sediments on the broader floodplain, outside these floodplain-channel zones, are formed primarily by the overbank vertical sedimentation of the main channel. The sediments of the Clarence floodplain include mainly : (i) medium sand, (ii) fine sand, (ill) very fine sand, (lv) sandy mud and (v) mud. Medium sands, which are mostly moderately-well sorted, mainly occur as channel deposits both in the main channel and in floodplain-channel zones. Moderately sorted fine to very fine sands generally occur closer to the channels, in dominantly levees, whereas sandy muds and muds in plains and swampy areas. The estuarine deposits are dominantly mud. The fluvial sediment body of the Clarence inner basin conformably overlies the estuarine deposits in areas of the floodplain covered by the plains and swamps that are located outside the floodplain channel zones. However, in floodplain-channel zones, fluvial deposits incise into the estuarine deposits generally up to 3 or 4 metres, resulting in relatively large sandy sediment tracts sitting within the estuarine muds. The Clarence fluvio-estuarine basin as a whole (both inner and outer basin together) contains the bmad elements, such as bay-head delta, tidal delta, mud basin etc. of typical barrier estuaries described in literature, but the inner basin with its distinct boundary conditions, departs from the typical element ensemble of barrier estuaries and hence, has distinctive facies assemblage.