Pedology and geomorphology of the Northern Rivers catchments of northeastern NSW, Australia

David Thomas Morand

This is an overview of the pedology and geomorphology of the Northern Rivers. Links between pedology and geomorphology (geopedology) are discussed. The study is based on the work and knowledge acquired during extensive soil landscape mapping of the region. Two case studies examine specific issues of pedology and geomorphology. These are (i) acid sodic soils and (ii) uplift and relief inversion. The case studies were selected because of their general interest and their lack of previous research. A description of the study area and environment highlights both the geodiversity and biodiversity of the area. Relationships between soils and the region’s climate, vegetation and land use are illustrated. Geology has received a more detailed description as it is the main basis for the diversity of soils and geomorphic features within the Northern Rivers. Fourteen physiographic regions have been identified, with the soils and geomorphology of each one described. Soil classification is an integral aspect of pedology and classifications used in the Northern Rivers have been reviewed. A study of the applicability of two international schemes - the World reference base for soil resources (WRB) and Soil Taxonomy - was also undertaken and presented as a peer-reviewed paper. The 13 Australian Soil Classification (ASC) orders that are present in the study area are each discussed in terms of morphology, distribution, characteristics, associated soils and important soil forming factors. Some prominent soil and geomorphic relationships are identified. Case study 1 examined sodic and acid sodic soils utilising laboratory data acquired during soil landscape mapping and limited X-ray diffraction (XRD) data. Results highlighted the complex relationship between sodicity and soil dispersion. The known problems of using exchangeable sodium percentage (ESP) as a predictor of potential soil instability were further demonstrated. Use of electrochemical stability index (ESI) instead of ESP showed little improvement, possibly due to the generally low salt content of soils in the region. Acid soils generally have high exchangeable aluminium contents. If sodic, any dispersion in such soils is thought to be offset by the high exchangeable aluminium content. Results showed that although exchangeable aluminium is not always associated with low dispersion it was commonly associated with low to moderate dispersion in acid sodic soils and rarely with high to very high dispersion. Also, exchangeable magnesium of varying levels is rarely associated with high dispersion and its influence on soil dispersion in the Northern Rivers is not clear. Despite high exchangeable aluminum contents many acid sodic soils in the Northern Rivers have developed severe erosion problems typical of highly dispersive soils. The instability of these soils may be due to a combination of factors: (i) mineralogy - smectite was present in all XRD results; (ii) highly erosive rainfall; (iii) soil texture - many soils have a significant sand component; (iv) some dispersion. Although this study emphasised samples with high or very high dispersion, most samples had at least low to moderate dispersion, prompting the need to consider what constitutes significant dispersion. Case study 2 presented apparent evidence of uplift and relief inversion. Relief inversion has been identified with some confidence, but confirming the existence of uplift needs a more detailed study than that provided in this overview. Isostatic adjustment has been suggested as a possible mechanism, but uplift associated with continental rifting is also considered. The field evidence is not conclusive and supports both processes. An important aspect of this study is the confirmation of the continued importance of field science for the ongoing development of the disciplines of pedology and geomorphology. Further research topics are suggested, including more detailed investigation and analyses.

Pedology and geomorphology of the Northern Rivers catchments of northeastern NSW, Australia

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Pedology and geomorphology of the Northern Rivers catchments of northeastern NSW, Australia

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