Morphology and distribution of submerged palaeoshorelines: Insights from the North West Shelf of Australia

Palaeoshorelines and associated palaeo-coastal features are studied to reconstruct past sea level, climate, and depositional environments. Their identification typically depends on direct field observations and is therefore challenging in the marine environment, where the interpretation mostly relies on sparse geophysical data. This review presents, based on 118 published case studies, a summary of morphological evidence that can be used to identify submerged relict coastal features worldwide, using only geophysical data. Four coastal feature categories that can be used as palaeoshoreline indicators were identified: (1) beach ridges of wind and wave origin; (2) shoreface strata; (3) marine terraces; and (4) coral-reef terraces.

In light of this proposed classification, an area of ~200,000 km² was investigated along the Rowley Shelf (North West Shelf, Australia), a carbonate-dominated platform, based on the integration of high-resolution bathymetry (i.e., seismic-derived bathymetry, satellite-derived bathymetry, multibeam echosounder bathymetry, spot depth soundings) and 2D reflection seismic lines. Relict features were discriminated from modern bedforms using five criteria: (1) stratigraphic position; (2) emersion features; (3) similarity with modern and published analogues; (4) integration of modern ocean conditions; and (5) evidence of early cementation. In total, over 500 submerged relict coastal features were identified, making this review the most comprehensive catalogue published to date.

Relict features are concentrated over specific depths, referred to as modal sea-level depths (MSLDs), which correspond to depths where the relative sea level remained stable over long periods of time. Nine MSLDs are observed at 20, 35, 50, 60, 70, 80, 90, 105 and 125 m below sea level. Each MSLD is the result of the accumulation of coastal features through multiple glacial/ interglacial cycles. Most of the features may nevertheless be related to the last glacial sea-level fall and were likely formed between Marine Isotopes Stages (MIS) 5 and 2.

The analysis of the submerged coastal features indicates that the overall shelf morphology is controlled by the distribution of these features, and that, while in a carbonate province, their formation is related to wind, tide, fluvial and wave processes. The higher concentration of relict fluvial-influenced features at shallower depths and of relict tide-influenced features at greater depths suggest that fluvial runoffs were limited during glacial periods. This, in turn, supports the hypothesis of a prevalent dry climate during glacial periods and in contrast, of a humid climate during interglacial periods. Finally, the study reveals that most modern coral reefs of the Rowley Shelf are growing on top of relict coastal features and that seabed ridges previously interpreted as drowned coral reefs are, in fact, likely to be relict coastal features formed through clastic processes.

Results from this study will support the identification of submerged palaeoshorelines on continental shelves around the globe and highlight the influence of associated relict coastal features on shelf morphologies. Additionally, this study provides new insights on processes shaping carbonate provinces.