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).

本报告所采用的数据涵盖多项成果：对双湾（Twofold Bay）沿岸及近海沉积物的结构与组成开展详细分析的结果、陆上第四纪地质（Quaternary geology）区域地貌填图数据、对梅林布拉湾（Merimbula Bay）与灾难湾（Disaster Bay）之间沿岸距离约50km的相邻大陆架实施的单道地震震源（Uniboom）勘探数据，以及双湾内选定的海岸障壁（barrier）与河口沉积（estuarine deposits）的放射性碳测年（radiocarbon dating）结果。
地震与地貌填图数据显示，该区域绝大多数第四纪沉积物赋存于现今海岸及邻近区域的海岸障壁（barrier）、河口与近岸沉积体中。在开阔海岸及双湾范围内，可见多种进积型与稳定型海岸障壁（barrier），它们围限了沿岸泻湖与障壁式河口。
位于双湾汤巴姆河（Towamba River）河口的鲸滩（Whale Beach）障壁（barrier），是一类受洪水与潮流侵蚀作用强烈改造的进积型障壁（barrier）的罕见实例，难以归入澳大利亚东部常见的任何一类障壁（barrier）类型。
现有放射性碳测年（radiocarbon dating）结果表明，这些障壁（barrier）与河口沉积（estuarine deposits）形成于全新世（Holocene）中晚期。本次报告中针对双湾西岸博伊德镇（Boydtown）障壁（barrier）——一处进积型前沙丘脊平原——的放射性碳测年（radiocarbon dating）结果显示其形成年代与之相近，但揭示出自约3000年前（B.P.，即Before Present）至今，障壁（barrier）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，以及相邻开阔海岸的海滩沉积开展的详细结构与组成分析，证实了多种独特沉积类型的存在。尽管各类沉积的结构特征存在显著差异，但它们的矿物组合特征相似：以石英（quartz）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（zircon, rutile, tourmaline）与亚稳定矿物（andalusite, kyanite, garnet, pyroxene, amphibole, epidote）共同组成的重矿物组合（heavy mineral assemblages）。
对轻矿物组合（light mineral assemblages）与重矿物组合（heavy mineral assemblages）开展的多变量统计分析（multivariate statistical analysis）显示，双湾内汤巴姆河（Towamba River）河口邻近的障壁（barrier）与近岸沉积中，长石（feldspar）与辉石-角闪石（pyroxene-hornblende）的占比出现了局部升高。自约3000年前（B.P.）至今，博伊德镇（Boydtown）障壁（barrier）沉积中上述矿物的占比也出现了显著提升。
本研究构建了该区域（尤其是双湾（Twofold Bay））晚第四纪海相与陆相沉积模式，该模式的主要特征如下：
1. 冰后期海侵（Postglacial Marine Transgression，简称PMT）的晚期阶段，南部高地（Southern Highlands）山麓地带的狭窄沿岸河谷被海水淹没，形成了多海湾的开阔海岸与双湾（Twofold Bay）。
2. 约6500年前海平面稳定在现今位置后，沿岸海湾内出现了显著的海相沉积；在海平面稳定期（sea level stillstand）的早期阶段（6500至3000年前）形成的绝大多数障壁（barrier）与河口沉积（estuarine deposits），其物质来源于冰后期海侵（Postglacial Marine Transgression）期间相邻大陆的沉积物。
3. 在海平面稳定期（sea level stillstand）的晚期阶段（3000年前至今），由于向岸输送的近海砂源逐渐减少，海岸整体呈现侵蚀后退趋势。但存在显著例外：开阔海岸（灾难湾（Disaster Bay）的温博伊恩海滩（Wonboyn Beach））以及双湾（Twofold Bay）内受保护程度更高的海域（博伊德镇海滩（Boydtown Beach）、鲸滩（Whale Beach）、渔业海滩（Fisheries Beach））的障壁（barrier）沉积。
在这些区域，海岸线重新进积的驱动力分别为：温博伊恩海滩（Wonboyn Beach）通过沿岸输沙（littoral drift）向北输送的近海石英质沉积物（quartzose sediment）补给、博伊德镇海滩（Boydtown Beach）与鲸滩（Whale Beach）由汤巴姆河（Towamba River）提供的长英质沉积物（feldspathic sediments）补给，以及渔业海滩（Fisheries Beach）由局部近岸源区供给的生物成因物质（biogenic material，贝壳碎屑）补给。