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Abstract
A massive occurrence of microbial carbonates, including abundant sponge remains, within the Devonian Elbingerode Reef Complex was likely deposited in a former cavity of the fore-reef slope during the early Frasnian. It is suggested that the formation of microbial carbonate was to a large part favored by the activity of heterotrophic, i.e., sulfate-reducing bacteria, in analogy to Quaternary coral reef microbialites. The Elbingerode Reef Complex is an example of an oceanic or Darwinian barrier reef system. In modern barrier reef settings, microbialite formation is commonly further facilitated by weathering products from the central volcanic islands. The Devonian microbialites of the Elbingerode Reef Complex occur in the form of reticulate and laminated frameworks. Reticulate framework is rich in hexactinellid glass sponges, the tissue decay of which led to the formation of abundant micrite as well as peloidal and stromatactis textures. Supposed calcimicrobes such as Angusticellularia (formerly Angulocellularia) and Frutexites, also known from cryptic habitats, were part of the microbial association. The microbial degradation of sponge tissue likely also contributed to the laminated framework accretion as evidenced by the occurrence of remains of so-called “keratose” demosponges. Further typical textures in the microbialite of the Elbingerode Reef Complex include zebra limestone, i.e., the more or less regular intercalation of microbial carbonate and cement. Elevated concentrations of magnesium in the microbialite as compared to the surrounding metazoan (stromatoporoid-coral) reef limestone suggests that the microbialite of the Elbingerode Reef Complex was initially rich in high-magnesium calcite, which would be yet another parallel to modern, cryptic coral reef microbial carbonates. Deposition and accretion of the microbialite largely occurred in oxygenated seawater with suboxic episodes as indicated by the trace element (REE + Y) data.
Highlights
Biotic carbonate formation in carbonate factories is either biologically controlled as in tropical and cool water reefs or biologically induced as in mud-mounds and microbialitesHandling Editor: Jörn Peckmann.28359 Bremen, Germany 4 Göttinger Zentrum Geowissenschaften, UniversitätGöttingen, Goldschmidtstraße 3, 37077 Göttingen, Germany (Schlager 2000; James and Wood 2010). Riding and Virgone (2020) have further discussed the fact that combinations of these themes (“hybrid carbonates”) are widespread both in fossil and modern depositional systems
The following major findings were made during the revisit of the conspicuous late Devonian (Frasnian) microbialite occurrence in Rübeland, Elbingerode Reef Complex, Harz Mountains, Germany
The microbialite of the Elbingerode Reef Complex is composed of large amounts of fine carbonate and consists of a reticulate and a laminated framework
Summary
Biotic carbonate formation in carbonate factories is either biologically controlled as in tropical and cool water reefs or biologically induced as in mud-mounds and microbialitesHandling Editor: Jörn Peckmann.28359 Bremen, Germany 4 Göttinger Zentrum Geowissenschaften, UniversitätGöttingen, Goldschmidtstraße 3, 37077 Göttingen, Germany (Schlager 2000; James and Wood 2010). Riding and Virgone (2020) have further discussed the fact that combinations of these themes (“hybrid carbonates”) are widespread both in fossil and modern depositional systems. 28359 Bremen, Germany 4 Göttinger Zentrum Geowissenschaften, Universität. Goldschmidtstraße 3, 37077 Göttingen, Germany (Schlager 2000; James and Wood 2010). Microbialites forming reef mounds and within metazoan reef systems have been common during earth history (Riding 1991a; Webb 1996; Riding and Virgone 2020; and references therein). Recent studies have shown that microbialites may occur in great abundance in cryptic habitats of modern, postglacial high-energy tropical coral reefs (Reitner 1993; Montaggioni and Camoin 1993). Several studies have shown that the presence of heterotrophic, sulfate-reducing bacteria is crucial for the formation of microbial carbonate within coral reef cavities, i.e., in cryptic reefal habitats (Heindel et al 2010, 2012; Gischler et al 2017), rather than
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