The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta

Abstract

ABSTRACTHolocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea‐level rise within a deeply incised bedrock‐confined valley system. The present day Hawkesbury River is interpreted as a wave‐dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine‐dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood‐tidal delta. Ocean wave energy is partially dissipated by this flood‐tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river‐dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone.The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole‐core X‐ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C‐14 dates reveal four stages of incised‐valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse‐grained fluvial sand and silt was laid down under falling‐to‐lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine‐grained fluvial sand and silt, muddy bayhead delta and muddy central‐basin deposits developed as the incised valley was flooded during eustatic sea‐level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central‐basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth‐bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth‐bar, interdistributary‐bay and bayhead‐delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead‐delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead‐delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised‐valley systems.