Sediment accumulation and Holocene - OzCoasts

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Sediment accumulation and Holocene evolution of the Fitzroy River lower floodplain 2004). The geochemical analyses also show evidence of a change in sediment source at around 60 cm depth, switching from dominantly the Bowen and Surat Basin soils to a more local source in the New England Fold Belt (Douglas et al in prep). This switch in source continues until 40 cm depth. This period of major change in sediment, from 60 to 40 cm depth, occurred at around the time of major land clearing as part of the Brigalow Development Scheme, which started in the early 1960s (Kuhnen, 2004). The carbon and nitrogen isotopes also show a dramatic change at 20 cm depth that likely records the 1991 flood, which engulfed both Crescent and Frogmore Lagoons (Figure 2; Kuhnen, 2004). 52
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. 53
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Sediment accumulation and Holocene - OzCoasts

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