Clarence River lowland floodplain : a morphostratigraphic analysis of a complex Holocene floodplain
Abstract
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.