AN EARLY HOLOCENE RECORD OF RISING SEA-LEVEL ALONG A BATHYMETRICALLY COMPLEX COAST IN SOUTHWESTERN AUSTRALIA

The Holocene sedimentary sequence in the Bouvard Reefs area, southwest ern Australia is located in a transitional zone between two contrastin g coastal sectors. It contains a complex stratigraphic sequence record ing a dramatic coastal change associated with a rising, then falling, Holocene sea level. With a rising sea level, during the early Holocene , sedimentation and progradation of seagrass bank, beach, and beachrid ges were abruptly terminated, and succeeded by the development of a re trograding barrier dune with its associated barred lagoon. The reason for the change in sedimentation style (i.e., sedimentation processes a nd patterns) relates to the inundation of the offshore limestone ridge . At 8000 C-14 yrs B.P., sea level coincided with, or was below, the t op of an offshore limestone ridge, and a cuspate foreland accumulated leeward of the rocky reefs. Continuing sea-level rise inundated the re efs to sufficient water depth (i.e., more than 2 m) such that they cea sed to shelter the coast, which became more exposed, and a barrier dun e developed. The barrier dune then retrograded over the cuspate forela nd plain. The dramatic change in sedimentation style within 500 years indicates that marked changes in coastal sedimentation can also in a r elatively short time. This paper also illustrates the contrasting sedi mentary style that may be superimposed and juxtaposed as sedimentation continues with rising sea levels along a bathymetrically complex coas t, and provides insight into coastal response where a sea rises into a bathymetrically complex area.