Brackish water algal reefs – facies analysis as a tool to identify palaeoenvironmental variations in Miocene deposits (Mainz‐Weisenau, Germany),Geological Journal

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Brackish water algal reefs – facies analysis as a tool to identify palaeoenvironmental variations in Miocene deposits (Mainz‐Weisenau, Germany)
Geological Journal ( IF 1.4 ) Pub Date : 2021-01-04 , DOI: 10.1002/gj.4067
Theresa Nohl ₁ , Julia Knoblach ₁ , Sebastian Teichert ₁

Affiliation

Brackish‐water carbonates are far less studied than their marine or limnic counterparts. However, their association with few, specialized species enables the documentation of fine‐scale changes in the depositional environment. The Cenozoic Mainz Basin (Germany) was only sporadically connected to the North Sea and the Paratethys, exposing several transitions from marine to fresh water influence. Focusing on one outcrop of the Rüssingen Formation of Mainz‐Weisenau (Aquitanian, Miocene), we present a detailed analysis of the faunal and sedimentological responses to changing salinities and water depth, including algal reef growth and facies development. The deposits include allochthonous limestones surrounding an autochthonous reef complex and several smaller reef patches. The allochthonous facies is dominated by the gastropod Hydrobia inflata, and the reef facies is mainly made up by the green alga Cladophorites sp. The algal thalli are overgrown by cryptocrystalline, organic precipitations, and laminated, chemical precipitations. Locally, quiver‐shaped structures of Trichoptera sp. protective cases occur. The depositional setting was a shallow, low energy, and brackish environment supersaturated by carbonate. We could not confirm a general trend of reducing salinities as reported for the Rüssingen Formation. Our results question previously reported episodic desiccation events, because apparent caliche horizons actually represent thin beds of increased Cladophorites growth. Set‐up, distribution of the reef facies, and reef debris indicate short‐time variations of temperature, salinity and water depth. We conclude that these variations are based on the geographic position at the edge of an algal reef barrier, separating the Mainz Basin from the Rhine Rift Valley.

中文翻译：

苦咸水藻礁–相分析作为识别中新世古环境变化的工具（德国美因茨威瑟瑙）

咸淡水碳酸盐的研究远远少于海洋或利姆酸盐。但是，它们与少数专业物种的联系使得可以记录沉积环境中的小尺度变化。新生代美因茨盆地（德国）仅与北海和Paratethys零星相连，暴露了从海洋到淡水影响的几次过渡。我们着眼于美因茨-魏瑟瑙（阿基坦，中新世）鲁辛根组的一个露头，详细分析了动物和沉积学对盐度和水深变化的反应，包括藻类的生长和相的发育。沉积物包括围绕本地珊瑚礁复合体的外生石灰岩和几个较小的礁石斑块。腹足动物以异质相为主珊瑚礁（Hiabia inflata）和珊瑚礁相主要由绿藻Cladophorites sp。组成。藻类藻类被隐晶，有机沉淀和层状化学沉淀所覆盖。本地的Trichoptera sp。的箭形结构。发生保护性情况。沉积环境是被碳酸盐过饱和的浅层，低能，微咸的环境。我们无法确定鲁辛根组所报道的降低盐度的总体趋势。我们的结果质疑先前报道的间歇性干燥事件，因为明显的钙化层位实际上代表着增加的枝状辉石的薄层生长。礁岩相的设置，分布和礁石碎片表明温度，盐度和水深的短期变化。我们得出的结论是，这些变化是基于将美因茨盆地与莱茵河裂谷分隔开的藻礁屏障边缘的地理位置。

更新日期：2021-01-04

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