The origin and stratigraphic significance of the Quaternary Waterloo Rock of the Botany Basin of south-east Australia

ABSTRACT

The Waterloo Rock is an informally defined, variably indurated layer widely encountered in the Quaternary deposits of the Botany Basin of south-east Australia. The unit has been identified as a regionally significant stratigraphic marker, as playing an important role in the hydrogeological behaviour of the Botany Sands Aquifer and as a distinctive component of the soils of the Tuggerah Soil Landscape of the Botany Lowlands. Yet no systematic account exists of its nature, origin and stratigraphic significance. The Waterloo Rock may develop in at least two distinct contexts: as a result of the precipitation of cementing agents in the B horizons of soils and as the product of deposition at or close to the groundwater table. Its origin is thus more complex than suggested by many commentators and the unit should not be employed as a simple indicator of the location of the palaeo-water table or the presence of a palaeosol. Nor may the deposits of the Waterloo Rock be correlated stratigraphically, controverting those who have promoted the idea of a single, basin-wide episode of Waterloo Rock formation and who have sought to use the Waterloo Rock as a stratigraphic marker of the time gap separating the deposition of Units 3 and 4 within the Botany Basin sequence. Instead, the deposits provide no more than a pseudo-stratigraphy superimposed on a pre-existing sedimentary sequence. These findings have implications for understanding groundwater behaviour in the Botany Basin. Rather than the widely accepted picture of a single unit of Waterloo Rock, the Botany Sands Aquifer is likely to possess multiple layers of Waterloo Rock of varied but probably limited lateral extent. These may complicate groundwater movement and groundwater chemistry in an aquifer long regarded as homogeneous and isotropic at the regional scale. The term Waterloo Rock has been applied to deposits that have different origins, that occur in different stratigraphic contexts and that probably formed at different times. It would be helpful if the expression were replaced by a simple descriptive term with no genetic or stratigraphic connotations: sandrock and indurated sand are possibilities that already exist in the literature.

KEYWORDS:

• Waterloo Rock
• Australia
• Botany Basin
• Quaternary
• stratigraphy
• pedogenesis
• groundwater
• duricrust

Acknowledgements

We are grateful to ACCIONA Infrastructure Australia and Artefact Heritage Services for permission to publish some of the data employed in this paper.

Disclosure statement

No potential conflict of interest was reported by the authors.