Indurated sand horizon influences present day coastal geomorphology of nearshore Northern Moreton Bay, South-East Queensland, Australia

Sea-level rise will lead to substantial changes to coastal geomorphology over the coming century and it is imperative to understand the implications. This includes the underlying stratigraphic influences on seabed morphology and the historical context with which they have formed. On the densely populated coastline of Eastern Australia, coastal erosion is a significant concern for residents and stakeholders. In South East Queensland, and particularly the coastal zone surrounding Bribie Island spit in Northern Moreton Bay, the accelerated erosion of the spit and discovery of indurated sand horizons in nearshore regions both above and below the seabed create a convergence of the past influencing the present.

Indurated sand horizons are predominantly considered to be the relict B horizon of the pedogenic processes that formed a podosol soil profile. Whilst not ubiquitous under present sea level, their presence presents a unique opportunity to study an accessible palaeosol unaltered by further pedogenesis and carbon input (as opposed to terrestrial indurated sand formations). This allows for an analysis of a time in Northern Moreton Bay during lower sea levels and how these horizons affect present day morphology. Data acquisition consisted of high and low frequency acoustics, coupled with core samples for geological analysis.

Our results show the indurated sands buried under 1-2 m of marine sands sloping downwards to the east. This suggests the present-day seabed follows the contours of the sub-surface indurated sand. High-resolution bathymetry of exposed indurated sand outcrops near Bribie Island spit indicate a dune-like shape suggesting a formation from coastal sand dunes into active terrestrial soil during lower sea levels. The dune troughs having accumulated greater mineral and organic material than the peaks, which can be attributed to the former surviving inundation from rising sea levels and the latter having undergone a weaker pedogenesis and subsequently erosion. Exposed indurated sand outcrops with a vertical face or ‘scour step’ are elevated to the surrounding marine sand seabed. Similar elevated structures were found to be a barrier to onshore sediment transport from offshore deposits and limiting beach replenishment whilst also offering protection from dampening long period waves and large storm swells. Core samples taken through the indurated layer from behind the spit to the shipping channel offshore showed elevated levels of aluminium and iron compared to surrounding marine sands, and consistent with podosol soil formation.

The techniques used here suggest that historical terrestrial geomorphology has determined the shape, mineralogy and strength of indurated sand layers. As these indurated sand layers were submerged and further modified by present day sea level, they may play an important role in coastal geomorphology and protection as sea levels rise further in the coming century.