Michael J. O’Leary1 and Paul J. Hearty2
1 Dept. Environmental & Geographical Sciences, Manchester Metropolitan University, Manchester, UK (email@example.com)
2 Dept. of Environmental Studies, University of North Carolina, Wilmington, NC
A detailed morphostratigraphic investigation of raised marine terraces at Cape Cuvier, Western Australia, reveals two geomorphically distinct units. A lower, well-developed accretionary reef terrace between +3 and +5.5 m (“+” denotes above mean low-water springs) represents an extended interval of stable sea level. An upper erosional terrace and incipient coralgal rim at +8.5 to +10.5 m represents a brief sea-level stillstand at this higher elevation. These geomorphologically distinct features suggest the lower and upper terraces developed during discrete sea-level fluctuations during MIS 5e. In an attempt to better define the timing of emplacement of each marine unit, 37 coral samples collected along vertical and lateral reef growth axes of both terraces were analysed with MC-ICPMS U-series dating. All coral samples exhibited elevated initial δ234U, suggesting that pervasive uptake of 234U-enriched uranium and 230Th thorium had occurred. However, despite these limitations of absolute dating, the site provides a means of testing the degree of reliability of both the conventional and modelled U-series ages though geomorphic and morphostratigraphic relationships.
At the seaward edge of the lower terrace, a 3 m high erosional scarp was stratigraphically logged and a total of 16 coral (Acropora humulis) framestone units were drilled for U-series analysis. While all coral exhibited elevated initial d234U values, 10 corals selected for their minimal detrital thorium contamination (232Th >1ppb), exhibited stratigraphically uniform ages of 130.3 ± 3 ka. These ages are considered to be too old in the context of the laterally prograding fringing reef growth model, considering the seaward position of the measured section – lateral progradation should produce age isochrons parallel to the reef front, such that progressively older corals are exposed on the reef surface landward of the reef crest. Applying the Thompson et al., (2003) model corrections to the conventional U-series measurements yielded equally uniform 116.5 ± 3 ka ages, which coincide with the end of MIS 5e. Considering the extent of Holocene marine erosion, it is highly unlikely that the current position of the scarp represents the original seaward edge of the fringing reef and therefore the ages are considered to be too young. Finally when coral ages are plotted on a on a 234U/238U-230Th/238U activity ratio diagram, they form a tight linear array, which intersects the closed system evolution curve at ~124 ka. Based on quantifiable erosion rates on limestone coasts, we are able to calculate an original terrace width of ~60 metres, with 35 m of the original terrace remaining. The current position of the scarp (based on a laterally accreting reef model) should represent growth interval approaching the middle of MIS 5e. We therefore conclude that the linear regression age model correction at ~124 ka, comes closest to representing true U-series coral ages. Thus, through the combined use of reef morphostratigraphy, conventional and modelled U-series ages, a comprehensive understanding of the timing and duration of sea-level highstand events can be gained.