Late Quaternary Evolution of Twofold Bay, Southern New South Wales.

The data collected for this report are the results of a detailed analysis of the texture and composition of onshore and offshore sediments within Twofold Bay, regional geomorphic mapping of the onshore Quaternary geology and a seismic survey (Uniboom) of the adjacent continental shelf between Merimbula Bay and Disaster Bay (alongshore distance of c.50km) and radiocarbon dating of selected barrier and estuarine deposits in Twofold Bay are presented. Seismic and geomorphic mapping data indicate that the bulk of the Quaternary sediments in the region are contained within barrier, estuarine and nearshore deposits at or adjacent to the present coast. A number of prograded and stationary barrier types impounding coastal lagoons and barrier estuaries are encountered on the open coast and within Twofold Bay. The Whale Beach barrier at the entrance to the Towamba River in Twofold Bay is an unusual example of a prograded barrier greatly modified by the erosive influences of floods and tidal currents and is not readily classified into any of the common barrier types encountered in Eastern Australia. Available radiocarbon dates indicate that these barrier and estuarine deposits are mid to late Holocene in age. Radiocarbon dates reported here for the Boydtown barrier, a prograded foredune ridge plain on the western shores of Twofold Bay, indicate a similar age but reveal a marked increase in the rate of barrier progradation from c.3000 yrs. B.P. to the present. A detailed examination of the texture and composition of a variety of onshore and offshore deposits in Twofold Bay and beach deposits on adjacent open coast confirmed the presence of a number of distinct sediment types. Although there are significant textural variations between the types, they contain similar mineral assemblages- predominantly quartz with variable amounts of feldspar (K~feldspar dominant) and lithics and heavy mineral assemblages with a mixture of ultrastable (zircon, rutile, tourmaline) and metastable (andalusite, kyanite, garnet, pyroxene, amphibole, epidote) mineral species. A multivariate statistical analysis of the light and heavy mineral assemblages indicates a localised increase in the proportions of feldspar and pyroxene-hornblende in barrier and nearshore sediments adjacent to the Towamba River entrance in Twofold Bay. A marked increase in the proportions of these minerals are also noted in the Boydtown barrier deposits from c.3000 yrs. B.P. to the present. A model of late Quaternary marine and terrestrial sedimentation for the region in general, and Twofold Bay in particular, is presented. Main features of the model include:
1. Inundation of the narrow coastal valleys in the foothills of the Southern Highlands in the latter stages of the postglacial marine transgression (PMT) to create a highly embayed open ocean coastline and Twofold Bay.
2. Significant marine sedimentation in coastal embayments following stabilisation of sea level at its present position c.6500 years ago with the majority of barrier and estuarine deposits formed in the early stages of the sea level stillstand (6500 to 3000 years ago) comprised of sediments derived from the adjacent continent during the PMT.
3. A trend towards coastal recession in the latter stages of the stillstand (3000 years ago to present day) as the offshore supply of sand to the coast dwindles. Notable exceptions include barrier deposits on both the open ocean coast (Wonboyn Beach - Disaster Bay) and more protected marine environments within Twofold Bay (Boydtown Beach, Whale Beach, Fisheries Beach). In these areas, renewed coastline progradation is initiated by the supply of quartzose sediment from offshore via littoral drift to the north (Wonboyn Beach), the supply of feldspathic sediments from the Towamba River (Boydtown Beach, Whale Beach) and the supply of biogenic material (shell fragments) from a localised nearshore source (Fisheries Beach).

本报告所用数据包含：对双湾（Twofold Bay）境内陆与近海沉积物的结构与组分开展的详细分析结果、沿岸第四纪地质（Quaternary geology）的区域地貌制图成果、对梅林布拉湾与灾难湾之间（沿岸距离约50公里）邻近大陆架实施的单道地震（Uniboom）测量数据，以及双湾内选定障壁岛（barrier）与河口沉积的放射性碳测年（radiocarbon dating）结果。
地震与地貌制图数据显示，该区域绝大多数第四纪沉积物赋存于现今海岸带或其邻近区域的障壁岛、河口与近岸沉积体中。在开阔海岸及双湾内部，可见多类进积型障壁岛（prograded barrier）与稳定型障壁岛，它们围限了沿海潟湖与障壁型河口。位于双湾托万巴河河口的鲸滩（Whale Beach）障壁岛是一个特殊案例：该进积型障壁岛受洪水与潮流的侵蚀作用发生了显著改造，难以归入澳大利亚东部常见的任何一类障壁岛类型。
现有放射性碳测年结果表明，这些障壁岛与河口沉积形成于全新世（Holocene）中晚期。针对双湾西侧的博伊敦障壁岛（一处进积型前滨脊岭平原）所测得的放射性碳年代显示其形成年代相近，但同时揭示出自约距今3000年至今，障壁岛的进积速率出现了显著提升。
对双湾内各类沿岸与近海沉积，以及邻近开阔海岸的海滩沉积开展的结构与组分详细分析，证实了多种独特沉积类型的存在。尽管各类沉积的结构存在显著差异，但它们的矿物组合特征相似：以石英为主，伴生含量不等的长石（feldspar，以钾长石K-feldspar为主）、岩屑（lithics），以及由超稳定矿物（ultrastable mineral，包括锆石zircon、金红石rutile、电气石tourmaline）与亚稳定矿物（metastable mineral，包括红柱石andalusite、蓝晶石kyanite、石榴子石garnet、辉石pyroxene、角闪石amphibole、绿帘石epidote）混合组成的重矿物组合。
对轻矿物组合（light mineral assemblages）与重矿物组合的多变量统计分析（multivariate statistical analysis）显示，在双湾托万巴河河口邻近的障壁岛与近岸沉积物中，长石与辉石-角闪石的占比出现了局部升高。同样，在博伊敦障壁岛沉积中，自约距今3000年至今，这类矿物的占比也出现了显著提升。
本文还提出了该区域（尤其是双湾）的晚第四纪海洋与陆地沉积模式。该模式的主要特征包括：
1. 在冰后海侵（postglacial marine transgression, PMT）的后期阶段，南高地山麓地带的狭窄沿海山谷被海水淹没，形成了高度曲折的开阔洋海岸线与双湾。
2. 约6500年前海平面稳定至现今位置后，沿海海湾内出现了显著的海洋沉积作用。在海平面稳定初期（6500至3000年前）形成的绝大多数障壁岛与河口沉积，其物质来源于冰后海侵期间邻近大陆的沉积物。
3. 自海平面稳定阶段后期（3000年前至今），由于向岸搬运的近海砂源逐渐枯竭，区域整体呈现海岸后退趋势。但存在显著例外：开阔洋海岸（万博恩海滩（Wonboyn Beach）——灾难湾（Disaster Bay））以及双湾内受保护的海洋环境（博伊敦海滩（Boydtown Beach）、鲸滩、渔业海滩（Fisheries Beach））的障壁岛沉积。在这些区域，海岸重新进积的动力来自：通过北向沿岸漂移（littoral drift）从近海搬运而来的石英质沉积物（万博恩海滩）、托万巴河供给的长石质沉积物（博伊敦海滩、鲸滩），以及局部近岸源区产生的生物成因物质（biogenic material，贝壳碎屑，渔业海滩）。