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首页> 外文期刊>Journal of South American earth sciences >Pleistocene molluscan assemblage in the southern Coastal Plain of Rio Grande do Sul, Brazil: Implications in the evolution of a Barrier-Lagoon System
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Pleistocene molluscan assemblage in the southern Coastal Plain of Rio Grande do Sul, Brazil: Implications in the evolution of a Barrier-Lagoon System

机译：巴西南里奥格兰德州沿海平原的更新世软体动物组合：对屏障-泻湖系统演化的启示

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The Barrier-Lagoon System III is the better preserved Pleistocene depositional system in the southern Coastal Plain of the state of Rio Grande do Sul, Brazil. The origin of this unit is correlated to the sea-level highstand of the last interglacial stage, but so far very little is known about the evolution and stratigraphy of this unit due to the scarcity of outcrops. A fossil assemblage consisting of thousands of bivalve and gastropod shells were recovered from drilling holes and SPT (Standard Penetration Test) surveys performed at depths up to 17 m on that unit. The characterization of these fossils in addition to the analysis of the sediments and SPT data allowed to interpret the depositional setting of the fossil-bearing sediment as backbarrier deposits representing a retro-gradational sequence characterized by lagoon bottom and margin facies superposed by sandy aeolian facies, covered by loess deposits. The fossil assemblage found in the lagoon bottom and margin deposits is dominated by bivalve and gastropod molluscs, mostly characteristic of open marine, shallow (= 30 m) environment, but including some Erodona mactroides and Anomalocardia brasiliana, bivalves typical of lagoon environments. The presence of species that do not currently live in the region suggests the influence of warmer coastal waters during the last interglacial stage. The good degree of preservation of a large part of the molluscan assemblage, with no signs of abrasion, bioerosion or incrustation, indicates short residence time at the sediment-water interface after death and disarticulation, followed by rapid burial below the Taphonomically Active Zone (TAZ). The presence of marine species together with species from low energy mixohaline environments indicates the transport of sediments and marine fauna to the backbarrier, which would have occurred during the transgressive phase of System III evolution. This process could have been related to increased storminess during the last interglacial, which promoted the transportation of large amounts of sediments and shells from the shoreface to the backbarrier through overwash and opening of ephemeral inlet channels.
机译：屏障-泻湖系统III是巴西南里奥格兰德州南部沿海平原保存最完好的更新世沉积系统。该单元的起源与最后一个冰间期的海平面高位有关，但由于露头稀少，迄今为止对该单元的演化和地层了解甚少。从钻孔中回收了由数千个双壳类和腹足动物壳组成的化石组合，并在该装置上进行了深度达17 m的SPT（标准渗透试验）调查。除了对沉积物和SPT数据进行分析外，这些化石的特征还有助于将含化石的沉积物解释为后壁沉积物，代表了反泻层序，其特征是泻湖底部和边缘相被砂质风积相叠加，黄土沉积物覆盖。在泻湖底部和边缘沉积物中发现的化石组合主要由双壳类和腹足类软体动物主导，主要表现在开放的海洋浅层（<= 30 m）环境中，但包括一些Erodona mactroides和Brasiana Anomalocardia brasiliana，这是泻湖环境中典型的双壳类动物。当前不在该地区生活的物种的存在表明，在最后一个冰期间，温暖的沿海水域产生了影响。软体动物群大部分的良好保存程度，没有磨损，生物侵蚀或结垢的迹象，表明死亡和分离后在沉积物-水界面的停留时间很短，随后在Taphonomically Active Area（TAZ）下快速埋葬）。海洋物种以及来自低能混合盐碱环境的物种的存在表明沉积物和海洋动物向后垒的运输，这是在系统III进化的过渡阶段发生的。此过程可能与上一次冰间期的暴风雨增加有关，这通过大量冲洗和临时进口通道的开放促进了大量沉积物和贝壳从岸面到后壁的运输。

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