The stratigraphy and geochronology of emergent fossil reef deposits of Western Australia

Corals provide the most widely used sea-level archive. Many coral species survive only in shallow water, therefore fossil corals emergent or submergent relative to present reefs, along stable coastlines, suggest variations in past sea level. Along the coastal margin of Western Australia (WA) an extensive series of marine isotope stage (MIS) 5e reefs outcrop at 3 ± 1 m above sea level. \ud \ud The consistency of reef elevation along thousands of km of WA coastline demonstrates the tectonic stability of this trailing intraplate continental margin. There is also evidence of erosional terraces or incipient reef development at elevations above this +3 m sea-level benchmark. Some workers reasoned that the higher elevation of these marine units is an artefact of localized tectonism or warping. This rationale fails to address: 1) intertidal deposits at multiple elevations in close proximity; 2) the distinct geomorphological difference between the lower and upper marine units; and 3) the similarity in elevation between WA emergent marine deposits and those found on stable carbonate platforms of The Bahamas and Bermuda. \ud With accurate and precise dating of these emergent reef deposits it may be possible to characterise the nature of sea level during MIS 5e.\ud \ud Coral skeletons are constructed of chemically unstable aragonite. Older coral material is generally more diagenetically altered, often suffering the addition or loss of uranium or thorium, which leads to inaccurate U-series age calculations. Coral diagenesis is generally determined by changes in carbonate mineralogy, but a more subtle form of geochemical alteration may affect the uranium-thorium age of corals that appear mineralogically pristine.\ud \ud In an attempt to extract meaningful ages from corals that have undergone isotopic exchange, modeled alpha-recoil processes were used to calculate open-system ages. These open-system ages are based on the assumption that alpha-recoil mobilisation\ud is the only diagenetic process operating within the coral/reef unit. Independent age controls including: 1) the known duration of MIS 5e; 2) stratigraphic superposition; and 3) age equivalence within individual corals, were used to test the reliability and accuracy of uncorrected and open-system corrected coral ages. Despite claims to the contrary, this study found open-system corrections to fail the above prescribed age tests, such that open-system corrections did not reflect the corals true age.\ud \ud Investigations into the nature of MIS 5e sea levels were focused on two contrasting locations, Cape Cuvier, a high-energy coastal site, and Shark Bay, a low energy marine embayment. Both sites point to an extended period of widespread coral reef development at +2 to +4 m elevation with incipient reef and erosional terraces at between +6 and +10 m. The incipient nature of the upper coralgal rim and the fact that the lower terrace was not able to utilize the newly available accommodation space points to a brief but rapid sea level excursion to this new elevation. High-precision U-series\ud dating returned coral ages that were inconsistent with the stratigraphic interpretation of the site and other known MIS 5e sea level curves.\ud \ud In an attempt to fill in the temporal and spatial gaps left by coral dating, the Useries method was applied to crustose coralline algae (CCA). This study shows that living CCA capture a modern seawater equivalent 234U value of 147.02 ± 1.5 ‰, and initial\ud uranium concentrations of 0.2 ± 0.07 ppm. These initial chemistries allow for the examination of uranium and thorium isotopic evolution over geological timescales, however uranium uptake and detrital 232Th contamination limit the usage of this material in U-series dating.\ud \ud Although we are confident of our interpretation of the sea-level history in WA during MIS 5e, we offer the following considerations: 1) field observations and relationships remain the most reliable means of determining the succession of events in the case of MIS 5e; 2) despite increased measuring precision of “high tech” dating methods, there is not necessarily a concomitant increase in the accuracy of the ages; and, 3) open-system corrections are not a reliable tool for determining a coral’s true age.\u