Lithological and drainage network determinants of the character of drowned, embayed coastlines

The perception that coastal character reflects lithology via Lithology's control on embayment size is tested by an examination of embayment sizes along a tectonically stable coastline to which ancient river systems drain (the New South Wales coast). This coastline is comprised of four distinct geological provinces exhibiting a wide range of lithological and structural variation and is taken to be representative of physiographic settings controlling coastline configuration on a wide range of continental-margin types. Our analysis shows that embayment size (L, the length of the embayment from headland to headland at sea level) is related to the size of the river draining to the embayment (catchment area, A) such that L = 0.60(root (A) over bar)(0.82) (r(2) = 0.68), rather than to coastal lithology, contrary to earlier assumptions. Modal non-dimensional embayment sizes (L/root (A) over bar) and variances vary little between the geological provinces; if anything, the modal non-dimensional embayment size is larger in provinces with greater lithological resistance. Two patterns discernible between the various provinces are the greater variability of embayment sizes and the greater abundance of ''under-sized'' embayments in provinces exhibiting strong coast-parallel structures. These structures are thought to facilitate the ''opening-up'' of large catchments inland. The fundamental control of catchment area on embayment length reflects both the long-term (Cenozoic) development of drainage networks in the bedrock fluvial domain and their subsequent drowning in the Late Quaternary (post-glacial) marine transgression. Shoreline configuration therefore reflects the interplay of Cenozoic fluvial influences, Quaternary sea-level fluctuations, and Late Quaternary sediment supply and accommodation volumes. High sea levels along embayed bedrock coasts result in a more crenulate and compartmentalized coast with smaller embayments because the sea level penetrates up into the lower-order drainage network; conversely, longer embayments and more open, less compartmentalized coastlines are associated with the lower sea levels resulting from eustatic sea-level fall and/or uplift of the land. Late Quaternary sedimentation governs the extent to which the paleochannel drainage network is buried, whereby the densities of paleochannels, interfluves (and hence headlands) along the coastline are reduced. The effect of this deposition (coastal progradation) therefore corresponds to sea-level regression or to tectonic uplift of the coast and should tend to enhance littoral sediment transport in contemporary coastal processes.