Brackish water algal reefs
“Brackish-water carbonates are far less studied than their marine or limnic counterparts.However, their association with few, specialized species enables the documentation offine-scale changes in the depositional environment. The Cenozoic Mainz Basin (Germany) was only sporadically connected to the North Sea and the Paratethys, expos-ing 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 Hydrobiainflata, 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 p recipitations. Locally, quiver-shaped structures of Trichoptera sp. protective casesoccur. 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. Ourr esults 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 concludethat 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.”
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