Doctor of Philosophy
School of Earth and Environmental Sciences
Blakemore, Amy, Middle Pleistocene to Holocene sea-level changes and coastal evolution on the Mount Gambier coastal plain, southern Australia, Doctor of Philosophy thesis, School of Earth and Environmental Sciences, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4243
Reconstruction of palaeo-sea levels provides a framework for determining Quaternary ice volumes and assisting in modelling predictions of future sea-level change. Commonly, records of palaeo-sea level derived from past shorelines only extend back to the last interglacial, marine isotope stage (MIS) 5e, due to erosion of older coastal successions by successive sealevel changes on tectonically stable coastlines. On the Mount Gambier coastal plain, southern Australia, palaeo-shorelines of the Bridgewater Formation, a succession of aeolianite barriers, are preserved by the gentle uplift of the region and the strongly indurated calcrete horizons which blanket the regional landscape, and provide a sea-level archive extending back to at least MIS 17 (680 - 710 ka). The southern continental margin of Australia is within the far-field of former Quaternary ice sheets. Palaeo-sea level records from this region are significant as they are more likely to reflect ice-equivalent sea level with minor but spatially variable hydroisostatic signals for different coastal sectors.
This thesis provides a geochronological framework for the deposition of interglacial barrier shorelines on the Mount Gambier coastal plain using amino acid racemisation (AAR), radiocarbon, and optically stimulated luminescence (OSL) dating. Depositional environments of barrier successions were reconstructed through stratigraphical analysis and assessments of facies architecture. Fixed and relational palaeo-sea level indicators were identified in the form of back barrier lagoonal and beach facies.
Petrological analysis of bioclastic sediments identified that the carbonate content of the barriers ranges from 66% to 99%. The percentage of quartz increases in inland barriers reflecting greater dissolution of carbonates in older successions. Thin section analysis identified an increase in sparite cements within the calcarenite with age, while XRD analysis highlighted a decrease in proportions of aragonite and an increase of low-Mg calcite. Scanning electron microscope (SEM) analysis of foraminifera indicated that test surface textures reflect transportation and diagenetic environments. Findings reveal a correlation between diagenesis and proposed age of the barriers and aid in constraining the evolution of the coastal plain.
This thesis reveals that the barrier shoreline complexes were formed during successive interglacial sea-level highstands and correlate with the shoreline barriers on the Robe coastal plain, 100 km northwest of the study area. Several barriers are found to be composite features, with facies deposited during separate interglacials. For example, Compton Range, 23 km landward of the present coastline, has a core of MIS 13 subaqueous sediment and overlying MIS 11 aeolian sands. Stacked aeolianites within Robe Range, at the present coastline, were deposited during successive interstadial highstands (MIS 5c, MIS 5a) of the last interglacial. Geochronological analysis also identified several episodes of reworking of older sediments into younger successions during sea-level regression and subsequent transgressional cycles further highlighting the complexity of coastal evolutionary processes within the region. Using a predefined uplift rate for this region, the combination of stratigraphical, petrographical and geochronological analyses allowed for the construction of a palaeo-sea level record of interglacial highstands over the past 680 ka.
This thesis identifies the first recorded example of subaqueously deposited late Pleistocene interstadial coastal strata above present sea level in Australia, represented by a flint conglomerate beach facies with interstratified shells. This deposit is constrained to MIS 5c in age and an uplift-corrected minimum palaeo-sea level of -14 ± 2 m is derived. OSL analysis confirms that MacDonnell Range, located 7 km from the present coastline, is MIS 5e (124 ± 10 ka) in age and a correlative of the Woakwine Range which formed during the sea-level highstand of the last interglacial maximum. Palaeo-sea level is also derived for MIS 7 and MIS 9 as 1.5 m and -3.5 m respectively, while minimum sea-level is derived from barrier shorelines deposited during MIS 11, MIS 13 and MIS 17. The sediment volume of individual barriers, predicted using Shuttle Radar Topography Mission (SRTM) data, corresponds with proposed interglacial intensity. The largest barriers on the coastal plain were formed during the more intense interglacials of MIS 5e and MIS 11 whereby warmer temperatures are proposed to have resulted in increased carbonate productivity on the Bonney Shelf, increasing the volume of available sediment for onshore transportation and barrier shoreline construction. AAR analyses revealed that individual foraminiferal tests provide a more accurate indicator of barrier age than AAR whole-rock analyses which are based on the averaging the D/L value of several thousand bioclastic carbonate grains. Analyses of single foraminifer tests allows for the identification and immediate rejection of contaminated, reworked or significantly diagenetically altered grains and provides numeric ages of greater accuracy and precision.
The results of this thesis are generally consistent with palaeo-sea level estimates from other tectonically uplifted coastal successions from the far-field of Quaternary ice sheets and oxygen isotope records. The palaeo-sea level record and coastal evolutionary processes derived from this research will provide an important dataset for modelling Quaternary ice volumes and interglacial climate dynamics.