Sediment accumulation and Holocene - OzCoasts

Sediment accumulation and Holocene evolution of the Fitzroy River lower floodplain
Conclusions
This study of the estuarine basin of the Fitzroy River has provided new insights
into the Holocene evolution of the estuarine reach of the river and the significant
spatial and temporal changes that have occurred in rates of sediment mass
accumulation. Our findings provide a broader understanding of sedimentary
process in the various depositional environments of the estuary, new insights
into the impacts on the estuary of changes in catchment land use and provide an
essential geological context for studies of modern biophysical processes. The
main findings of this study are:
1. The stratigraphy of the Fitzroy River floodplain reveals a
transgressive/highstand sequence typical of a stable Holocene tide-dominated
estuarine basin.
2. Since the mid Holocene, 16 000 megatonnes of sediment has been trapped
and stored in the floodplain below Rockhampton. This is equivalent to 1992
kilotonnes yr -1 of bulk sediment and 1636 kilotonnes yr -1 of fine sediment,
approximately 23% and 36% of the modern average annual discharge past
Rockhampton (Kelly and Wong, 1996; Dougall et al., 2005). However, the
majority of the accommodation space in the floodplain is now filled in; therefore,
there is relatively little modern sediment accumulating in this area (1 mm yr -1 ).
3. The present focus of sedimentation is the river mouth and the tidal creeks in
southern Keppel Bay. The highest rates of modern sediment accumulation are in
the mangrove lined tidal creeks, with sedimentation rates up to 3.3 cm yr -1 .
These data indicate that approximately 1706 kilotonnes yr -1 of sediment is
presently accumulating in this environment. This is approximately 19% (Kelly and
Wong, 1996; 8800 kilotonnes yr -1 ) to 30% (Dougall et al., 2005; 4575 kilotonnes
yr -1 ) of the Fitzroy River’s average annual suspended sediment load.
4. The freshwater lagoons receive sediment during flood events and provide a
detailed sedimentary record of major historical hydrological events. Variations in
sediment physical properties, chemistry and organic matter composition appear
to reflect major changes in land use of the Fitzroy River catchment. Significant
changes occurred in sediments that accumulated in Crescent Lagoon around the
1960’s that appear to have coincided with a phase of large-scale land clearing
associated with the Brigalow Development Scheme.
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