Coquina Beach-Ridge System Sedimentary Architecture and Evolution in Hamelin Pool, Shark Bay

Abstract

Shark Bay World Heritage Area displays a unique Holocene coarse bioclastic (coquina) beach-ridge system in the supratidal environment of Hamelin Pool and L'Haridon Bight hypersaline basins. In Hamelin Pool, E and W shores have different degrees of vulnerability to the usually NW approaching storms. Aerial imagery, GPR profiles, 14C dating and sedimentology were used to delineate a depositional and evolutionary model for beach-ridge deposit in Hamelin Pool E and W shores by assessing internal and external architecture, facies and ages. The onset of the beach-ridge system took place about 5000 14C years BP facilitated by abundant shell supply and falling sea levels, under episodes of high water levels induced by storms. Prograding seaward inclined GPR reflections are the prevailing architectural elements and result from swash-backwash processes in the beachface during elevated water level episodes. An important feature is a diachronous blanket generated by storm-surge events and eolian processes which occasionally develop incipient soils that cap older beach-ridge deposits. During the latest Holocene, marked environmental changes within Hamelin Pool are suggested by change in the depositional style of beach-ridge deposits (complex geometries of spit ridges start to prevail over the seaward inclined reflectors of prograding beachfaces); lower ridge elevations and the occurrence of an erosive period after 1450 14C years BP in the W shore. Although the ages obtained for the ridge sets mapped in both E and W shores of Hamelin Pool are roughly similar, they exhibit differences in sedimentary architecture and coastal morphology as a result of different degrees of vulnerability to storms, and variable reworking by tidal currents, waves and longshore currents which may also be energetic enough during fair-weather conditions to transport sediment in the nearshore environment. A depositional model is proposed where washovers and storm-surge ridges are deposited by overwash episodes at the peak of major storms while berm construction occurs during the waning phase by swash, backwash, overtopping and minor overwashing. This model can be used as an analogue for other coquina deposits in the stratigraphic record providing an example of high-resolution depositional architecture that is useful for reservoir characterization and prediction. Finally, comparison is made with the prograding, well dated system at Telegraph Station, SE Hamelin Pool.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015