Year

2013

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Within Australia, as in many areas of the world, population is commonly concentrated in coastal areas. While some of these population centres are located on large deltas many coastal population centres are built around smaller river systems and barrier estuaries where increasing land-use pressures are initiating development on small-scale low-lying deltaic areas. As such, the management of these areas is of prime interest to government bodies and to the broader community. To achieve this it is necessary to have an understanding of their geological evolution and associated depositional processes in order to better forecast the implications of management decisions and the potential impacts of climate change.

The Holocene evolution of the Macquarie Rivulet and Mullet/Hooka Creek bay-head deltas, Lake Illawarra, New South Wales, is discussed in this thesis. Both deltas are forming at the interface between the rivers and creeks draining the largest and second largest sub-catchments with areas of 96.35 km2 and 75.2 km2, respectively. These waterways originate on the Illawarra Escarpment prior to falling steeply and flowing across relatively narrow coastal plains. Due to the orographic effect and the shape of the escarpment rainfall events are typically compartmentalised. Ten soil associations occur within the catchment, with 60% of them classified as being extremely erodible. Since European settlement large tracks of land were cleared for agricultural purposes and subsequently replaced with urban/industrial development. This study has documented how the regional characteristics have played a role in shaping the deltas’ evolution.

The information presented in this thesis is based on the sedimentological analysis of 147 cores and drill holes across the two study sites. The sedimentological analyses included core logging and detailed visual assessment, grain size analysis, X-ray Diffraction (XRD) and a variety of geochronological analyses. The geochronologies of the deltas were established through amino acid racemisation (AAR), radiocarbon (14C), accelerated mass spectrometry (AMS) and lead 210 (210Pb) techniques.

The study has shown that the key processes controlling the evolution of small scale bayhead deltas prograding into wave dominated barrier estuaries differs from the 14 key delta evolutionary processes as identified by Coleman and Wright (1975), Coleman (1976) and Coleman (1980). The observed differences in evolutionary processes reflect a combination of factors including the:

* negligible influence of marine processes, particularly large waves and tides;

* reduced size of catchment and associated sediment yields;

* tectonic stability within the receiving basin.

The spatial resolution of previous studies into the evolution of these smaller deltas is limited. Typically, these studies suggest the evolution of bay-head deltas is simplistic and consists of prodelta muds overlain by deltaic sands and floodplain sediments. The high spatial resolution of this study has illustrated that this simplistic view does not adequately represent the complex interactions between autocyclic, allocyclic and anthropogenic processes throughout the Holocene. Specifically the study has identified and described ten main facies/facies associations, three minor facies and two imported units within the 147 cores analysed. Furthermore, the increased spatial resolution of this study suggests the observed facies relationships at the highest level are independent of scale, however, at the more focussed level the scale differences between the mega/continental scale deltas and the smaller bay-head deltas become evident.

Previous wave-dominated barrier estuary evolutionary models have lacked the detail to adequately reflect the evolution of the fluvial/bay-head deltas. This lack of representation reflects the limited data available to inform the development of these models. Consequently, the high resolution nature of this study has enhanced the understanding of the evolution of bay-head deltas enabling refinements to be made to these previously published estuary models. The data has also allowed the development of a bay-head delta facies and evolution model applicable for use within any barrier estuary or lake.

The study has illustrated that an integrated approach to mapping, based on historical parish maps and aerial photographs within a GIS framework, can effectively track morphological changes since European settlement in Australia. This analysis has shown that Macquarie Rivulet and Mullet/Hooka Creek deltas have increased in area significantly, despite periods of negative growth. Both the positive and negative increases in area and associated morphological changes have occurred in response to a combination of anthropogenic catchment modifications and natural factors. Some of the key factors contributing to the observed changes include land clearing, urban/commercial development, flooding, wind-waves, bank stability, regional geology and erodible soils.

Predicting the future evolution of deltas under an altered climate scenario is challenging. This research has suggested the two deltas studied are highly vulnerable with large portions of the deltas likely to be inundated by 2050. This inundation will probably result in the deltas regressing and/or avulsing into new depocentres. Increased rainfall intensity and flooding, as predicted, may result in significant changes to the deltas’ morphologies and sedimentation patterns. Predicted wind patterns, are likely to have minimal effect on the deltas’ evolution. Future development within the deltas’ catchments is likely to increase the vulnerability of the deltas. Significantly, this research has shown that current land use planning practices do not necessarily consider the whole of catchment implications, particularly the end deltaic regions. To effectively manage these systems into the future these issues will need to be addressed.

FoR codes (2008)

040310 Sedimentology, 040311 Stratigraphy (incl. Biostratigraphy and Sequence Stratigraphy), 040399 Geology not elsewhere classified, 040601 Geomorphology and Regolith and Landscape Evolution, 040606 Quaternary Environments, 040607 Surface Processes

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.