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 sys tems drain (the New South Wales coast). This coastline is comprised of four distinct geological provinces exhibiting a wide range of litholo gical and structural variation and is taken to be representative of ph ysiographic settings controlling coastline configuration on a wide ran ge of continental-margin types. Our analysis shows that embayment size (L, the length of the embayment from headland to headland at sea leve l) is related to the size of the river draining to the embayment (catc hment area, A) such that L = 0.60(root (A) over bar)(0.82) (r(2) = 0.6 8), rather than to coastal lithology, contrary to earlier assumptions. Modal non-dimensional embayment sizes (L/root (A) over bar) and varia nces vary little between the geological provinces; if anything, the mo dal 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 grea ter abundance of ''under-sized'' embayments in provinces exhibiting st rong coast-parallel structures. These structures are thought to facili tate the ''opening-up'' of large catchments inland. The fundamental co ntrol of catchment area on embayment length reflects both the long-ter m (Cenozoic) development of drainage networks in the bedrock fluvial d omain and their subsequent drowning in the Late Quaternary (post-glaci al) marine transgression. Shoreline configuration therefore reflects t he interplay of Cenozoic fluvial influences, Quaternary sea-level fluc tuations, and Late Quaternary sediment supply and accommodation volume s. High sea levels along embayed bedrock coasts result in a more crenu late and compartmentalized coast with smaller embayments because the s ea level penetrates up into the lower-order drainage network; converse ly, 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 go verns the extent to which the paleochannel drainage network is buried, whereby the densities of paleochannels, interfluves (and hence headla nds) along the coastline are reduced. The effect of this deposition (c oastal progradation) therefore corresponds to sea-level regression or to tectonic uplift of the coast and should tend to enhance littoral se diment transport in contemporary coastal processes.