Stratigraphy and chronology of receding barrier‐beach deposits on the northern Illawarra coast of New South Wales

Abstract

Abstract Four Quaternary units occupying a shallow bedrock depression have been delineated from outcrop exposures and auger drilling on Sandon Point Beach, North Bulli, 60 km south of Sydney, N.S.W. In ascending stratigraphic order these units are: a basal muddy sand (2.2+ m) of fluvial origin; a mottled mud (3.5 m) with common iron oxides, representing alluvial deposits with a superimposed soil profile developed during a period of lower sea level; a sandy mud (3.9 m) containing estuarine shell fossils, small tree stumps (including one mangrove) and abundant wood fragments; and, at the top, a pebble layer overlain by beach and dune sands, all partly indurated with iron oxides (2.3 m). 14C dating in situ tree stumps, shell material and wood fragments from the estuarine sandy mud unit between mean sea level and +1.45 m provided ages ranging from 7520 ±. 150 to 6350 ± 100 years B.P. The age of this unit is supported by a date on in situ Myrtaceae root material (7000 ± 150 years B.P.) from a similar study sandy mud unit at Thirroul, some 2 km north of the Bulli site. These dates indicate that sea level reached its present position between 7500 and 6500 years ago and thus record the final stages of the postglacial marine transgression on the southern coast of New South Wales.