Sequence architecture during the Holocene transgression: an example from the Great Barrier Reef shelf, Australia

Abstract

The application of sequence stratigraphic analysis to post-glacial sediments requires the correct identification of sequence boundaries, flooding surfaces and systems tracts. The nature of coastal sediments deposited during sea-level cycles is complex and misinterpretation is easy, particularly in ancient sequences for which eustasy is inferred rather than demonstrated. A large core, seismic and radiocarbon database from Cleveland Bay, in tropical Queensland, allows detailed analysis of the systems tracts and parasequences developed during the late Holocene. Despite the large database, a unique sequence stratigraphic interpretation is not possible, because oscillations in the sea-level curve apparently occurred at a frequency higher than the resolving power of radiocarbon dating. Our best estimate is that the final stages of the Holocene sea-level rise comprise four parasequences, each deposited during episodic slowdowns in sea-level rise and/or a pulse of rapid sediment supply. Sea-level pauses probably occurred at 12–10 m below modern levels at 8.5 ka BP, followed by a fall to ca. −17 m at 8.1 ka BP, a very rapid transgression to ca. −10 m at 7.9 ka BP, a further transgression to ca. −5 m at 6.8 ka BP, and a final rise to the Holocene highstand at 1.65 m at 5.5 ka BP. The stratigraphic record of these sea-level rises is complex, occurs within the `modern' shore-connected sedimentary wedge, and comprises parasequences associated with shorelines at −11 m (P 11), −17 m (P 17), −10 m (P 10), −5 m (P 5) and +1.65 m (P +1.7). Each parasequence comprises, in ascending order, onlapping estuarine-shoreline sediment (including mangrove mud and beach sand), and progradational shallow-bay muddy sand. Across the bay, units up to and including the lower parts of the shallow-bay mud correspond to the transgressive systems tract (TST) of the last 18 ka glacial/interglacial cycle (the particular parasequence represented being dependent upon location, especially depth). Bay mud above this level corresponds to the post −5.5 ka BP highstand systems tract (HST), which comprises sand cheniers on the coastal plain and bioturbated muddy sand in the shallow bay. The junction between the TST and the HST lies within the bay mud, and corresponds approximately to the local flooding surface for the P +1.7 parasequence deposited at the +1.65 m shoreline. Offshore, terrigenous TST strata do not occur over most of the middle and outer shelf, which instead is veneered with a thin ‘mid-cycle shellbed’. The lower part of this shellbed is time-equivalent to the later part of the TST and the upper part is time-equivalent to the HST. Offshore, the maximum flooding horizon (MFH) and peak eustatic sea-level horizon (PESH) of the Late Pleistocene-Holocene sea-level cycle lie at some level within the mid-cycle shellbed. Nearshore, the MFH and PESH lie within the shore-connected sediment wedge at levels near the TST/HST boundary. However, at no locality do either the MFH and PESH necessarily correspond to a physical boundary within the stratigraphy.