Episodic post?glacial sea?level rise and the sedimentary evolution of a tropical continental embayment (Cleveland Bay, Great Barrier Reef shelf, Australia)

Abstract

Cleveland Bay is a 400 km² landlocked tropical embayment located at 19° S and 146° 55´E The bay is protected from the dominant southeasterly tradewind by Cape Cleveland, but lies open to northerly and northeasterly weather and to the effects of occasional tropical cyclones. Water‐motion within the bay is dominated by the effects of refracted southeasterly‐generated waves (mostly 0.5–1.2 m high, 4–6 s period) and by semi‐diurnal tidal currents, which reach speeds of 15–30 cm/s during spring tides. Residual circulation within the bay is anticlockwise and results in preferential sediment accumulation on the eastern side. The bay contains three main Holocene stratigraphic units (A‐C) which rest on weathered Late Pleistocene clay. The Pleistocene land surface is planar, dips seawards at 0.8 m/km and is incised by a major complex of fluvial and tidal channels. Seismic unit C encompasses cross‐bedded or draped fill of the channel system. Seismic unit B, occurring laterally to C, comprises massive grey mud with mangrove roots, is up to 4 m thick, ranges in radiocarbon age from 8 (depth 15 m, outer bay) to 7 ka BP (depth 5 m, adjacent to the coastline) and accumulated at vertical rates up to 680 cm/ka. Unit A is the main Holocene bay‐fill, comprising transgressive beach sand at the base which passes up into bioturbated offshore muddy sand. The unit A prism is the offshore continuation of the coastal chenier plain and represents rapid transgression in the Early‐mid Holocene followed by coastal and bay progradation since 6.5 ka BP at horizontal rates of 0.5–1.0 km/ka and vertical rates up to 100 cm/ka. Surficial sediment distribution within the bay has been markedly affected by the dumping of harbour and shipping channel dredge spoil. The predumping sediment distribution comprised a coastal platform of beach sand and subtidal silty sand, passing offshore into bioturbated muddy sand with scattered shells; minor bioclastic detritus accumulates adjacent to fringing reefs on Magnetic Is. The Pleistocene land surface is exposed on the sea floor adjacent to the shipping channel in the western bay, and at depths > 20 m seawards of the shore‐connected Holocene sediment prism. The combined sedimentary evidence is consistent with pauses or slowdowns in the post‐glacial sea‐level rise at ‐28 m at 10 ka bp (shoreline S₃) and ‐10 m at 8 ka bp (shoreline S₂).