Aminostratigraphy and sea-level history of the Pleistocene Bridgewater Formation, Mount Gambier region, southern Australia

Abstract

A geochronological framework based on amino acid racemisation (AAR) and constrained by previously reported optically stimulated luminescence (OSL) ages is presented for the evolution and paleosea-level record of the Pleistocene Bridgewater Formation of the Mount Gambier region, of southern Australia. Within the study area, the Bridgewater Formation is represented by late early Pleistocene [Marine Isotope Stage (MIS) 23 at 933 ka] to Holocene barrier shoreline successions deposited during sea-level highstands. Regional monotonic uplift (0.13 mm yr^–1) and pervasive calcrete development during the Pleistocene have preserved the sequence of calcarenite (mixed quartz-skeletal carbonate sand) shoreline complexes from denudation. AAR analyses confirm that the barriers generally increase in age landwards and correlate with sea-level highstands associated with interglacials as defined by the marine oxygen isotope record. AAR analyses on the benthic foraminifer Elphidium crispum have proved more reliable than the whole-rock method in extending the age range of AAR dating of these relict shoreline successions. Paleosea-levels from the coastal plain are as follows: MIS 7, –9 ± 2 m; MIS 9, 4 ± 1 m; and a minimum sea-level of 2 ± 2 m is derived for MIS 11. Paleosea-level could not be determined for MIS 15, 19 or 23 as diagnostic sea-level indicators were not identified within these sedimentary successions. Dismal Range, dated at 933 ± 145 ka (MIS 23), represents a correlative feature to the East Naracoorte Range but is some 25 km seaward of the Kanawinka Fault compared with the same barrier at Naracoorte. Mingbool Range (788 ± 18 ka) is of similar age to the West Naracoorte Range (MIS 19) and formed as an arcuate shoreline complex that became attached to the higher relief of the area represented by the Mount Burr Volcanic Province. The higher topographical relief resulted from crustal doming of the Oligo-Miocene Gambier Limestone caused by the intrusion of magma associated with the volcanic province. The AAR age of 788 ± 118 ka for Mingbool Range indicates that the Mount Burr volcanics predate the deposition of this shoreline complex.

为了研究澳大利亚南部甘比尔山地区更新世布里奇沃特组的演变和古海平面，我们提出了一个地质年代学框架。 该框架基于氨基酸外消旋（AAR）并受限于以前报道的光释光（OSL）年龄。在研究区域之内，布里奇沃特组是早更新世晚期[在933万年时海洋同位素阶段（MIS）23]至全新世屏障岸线地层，沉积于海平面高水位的期间。更新世期间的区域性单隆起（0.13毫米/年）和钙质结砾岩普遍发育，保存了灰岩海岸线复合体序列（混合石英 - 碳酸骨骼砂）以免遭剥蚀。 AAR分析确认，岩屛年龄通常在向陆地方向上增加，并与海平面高水位正相关，后者与由海洋氧同位素记录所确定的间冰期有关。底栖有孔虫Elphidium crispum的AAR分析证明，在对这些残余海岸地层进行AAR年代确定上，它比全岩法更可靠。沿海平原的古海平面列述如下：MIS7，-9±2米; MIS9,4±1米;从MIS11得出至少为2±2米的海平面。我们无法确定MIS15、MIS19或MIS23的古海平面，因为在这些沉积序列内没有鉴别出海平面的标示物。地索马里山脉，年代为933±145ka（MIS23），显示与东纳拉库特山脉相对应的特征，但位于卡纳文卡断层向海方向约25公里处。名博奥尔山脉（788±18ka）在年代上与西纳拉库特山脉（MIS19）相似，形成弧形海岸线复合体，接附伯尔山火山区的较高地形。与火山区相关的岩浆侵入导致的更新世-中新世Gambier灰岩的地壳隆起造成较高的地形。名博奥尔山脉的AAR年龄为788±118ka，表明伯尔山火山活动发生在这种海岸线复合体的沉积作用之前。

KEY WORDS:

• amino acid racemisation
• Bridgewater Formation
• interglacial sea-levels
• Elphidium crispum
• Pleistocene
• coastal barriers

ACKNOWLEDGEMENTS

We thank Professor Patrick De Deckker and an anonymous reviewer for their thoughtful reviews, which have enhanced this paper. We also thank Deirdre Ryan, Bob Dalgarno and Don McLennon for field assistance. Research funding was provided by GeoQuEST Research Centre, University of Wollongong. Ken Hurst is thanked for allowing us access to his property at Lake Hawdon South and Laurie Hein for permission to sample within Myora Forest.

SUPPLEMENTAL DATA

Appendix: Details of sample sites.