Assessment of the Relationship between Coastal Geometry and Depositional Environment along the Australian Coastline; Developing Models for Application to Ancient Systems

Nanson, Rachel ¹; Vakarelov, Boyan ¹; Ainsworth, Bruce ¹; O'Reilley, Daniel ¹; Midtkandal, Ivar ^1
1 School of Petroleum, The University of Adelaide, Adelaide, SA, Australia.

Coastal depositional systems can be broadly defined as deltaic, estuarine or non-fluviatile and can be classified by wave, tide or fluvial process domination and influence. Coastal geometries (local accommodation space, shelf morphology and degree of embayment) and processes (sediment supply rates and calibre, fluvial discharge and the nature of sediment supply) modify the influence of waves, tides and river flows, the resulting deposystem types and, in turn, the nature and geometries of the depositional elements they contain. Detailed topographic, bathymetric, tide, wave and river data are available for Australian coastal regions and allow geometric and process controls on coastal sedimentary systems to be investigated.

The morphologies of coast-normal transects (shelf profiles) influence wave / tide energy ratios at the shoreline. Wide shelves generally amplify tidal strength and dampen wave action through frictional losses across the shelf, resulting in tide-dominated coastal systems. In contrast, narrow shelves generally have a more subdued effect on wave energy and do not amplify tidal ranges, resulting in wave-dominated coastal systems. A preliminary investigation of shelf profiles sampled from non-fluviatile Australian coastlines indicates a shelf profile - system type trend: the modal distance to shelf edge for wave-dominated strandplains (40km) is less than one quarter the distance for mudflats (185km). However, many systems deviate from this trend and indicate the influence of other geometric and process factors.

Embayed coastlines have the potential to attenuate wave energy and amplify tidal energy, resulting in different process domination at the shoreline for any given shelf slope. A model for coastline embayment to shelf edge relationship to predict system types is under development; the preliminary shelf accommodation measure, which incorporates both distance to shelf edge and coastal embayment, demonstrates strandplain shelf accommodation less than one sixth of that for mudflat shorelines.

The applicability of modern coastal morphology to system type relationships to ancient systems is currently being tested.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009