Brackish water algal reefs – facies analysis as a tool to identify palaeoenvironmental variations in Miocene deposits (Mainz‐Weisenau, Germany). (4th January 2021)

• Record Type: Journal Article
• Authors: Nohl, Theresa; Knoblach, Julia; Teichert, Sebastian
• Other Names: Ramkumar Mu guestEditor.; Nagarajan R. guestEditor.; Santosh M. guestEditor.
• Is Part Of: Geological journal. Volume 56:Number 5(2021)
• Page Start: 2760
• Page End: 2771
• Publication Date: 2021-01-04
• Subjects: algal reefs -- brackish deposits -- Cladophorites sp. -- facies development -- microbial calcite precipitation; Geology -- Periodicals; 551
• DOI: 10.1002/gj.4067
• Languages: English
• ISSNs: 0072-1050

Abstract

Abstract : 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. Abstract : This study investigates the record of small‐scale changes in the depositional environment of Miocene brackish‐water deposits from the Mainz Basin (Germany) by facies analysis. Set‐up, distribution of the reef facies, and reef debris indicate short‐time variations of temperature, salinity and water depth. The apparent caliche horizons actually represent thin beds of increase Cladophorites growth. The example demonstrates that these brackish deposits are a sensitive recorder of palaeoenvironmental change.