Ferruginous microstromatolites related to Middle Jurassic condensed sequences and hardgrounds (Bucegi Mountains, Southern Carpathians, Romania)

Frutexites encrustations on rugose corals (Middle Devonian, southern Morocco): complex growth of microbial microstromatolites

“…The calcitic laminae, in turn, are typically rather homogeneous in appearance and consist of microspar to small, granular spar crystals, but without the fibrous crystal bundles observed in other studies of Frutexites shrubs (e.g., Myrow and Coniglio 1991;Böhm and Brachert 1993), nor have filamentous or rod-shaped structures (cf. Chafetz et al 1998;Lazȃr et al 2012) been observed under SEM in the calcitic and hematitic parts of the Frutexites shrubs (Fig. 4a).…”

“…Woods and Baud 2008), nor encrusted the roofs of sub-seafloor cavities (cf. Reolid and Molina 2010;Lazȃr et al 2012). This reflects probably a deficiency of firm, stable substrates, which were apparently required for Frutexites columns to grow, and the available bioclasts constituted the only favorable sites for Frutexites nucleation.…”

“…In the first model, the formation of the hematitic parts of Frutexites is considered a direct result of the activity of iron-oxidizing microbes (cf. Mamet and Préat 2006;Lazȃr et al 2012). Although microbes cannot dominate the iron-oxidation processes at circum-neutral pH in oxidized seawater, it is now well documented that oxidation of iron can be readily mediated by neutrophilic microbes under very low oxygen conditions (Straub et al 1996;Benz et al 1998;Chaudhuri et al 2001).…”

“…Instead, it has been proposed that Frutexites formed as a result of the activity of iron bacteria (Playford et al 1976;Chafetz et al 1998), but the role of various microbes and the exact mechanism of iron precipitation are still poorly understood. While some studies carried out in past two decades shed some light on this issue (Mamet et al 1997;Mamet and Préat 2006;Lazȃr et al 2012), none of the models provides a plausible explanation for the internal structure of Frutexites and, more particularly, for the intimate association between the iron-rich and calcitic laminae.…”

“…Generally, Frutexites seem to have preferred low-energy environments, characterized by low sedimentation rates and often limited availability of oxygen; they are consequently commonly found on the bottoms of deep, dysaerobic basins (Playford et al 1976;Tsien 1979;Böhm and Brachert 1993;Reitner et al 1995;Mamet and Préat 2006;Woods and Baud 2008;Lazȃr et al 2012), not uncommonly in association with hardground surfaces (Mišík and Aubrecht 2004;Reolid and Nieto 2010). Where present in shallow-water settings, in turn, Frutexites are typically limited to cryptic environments, such as such as small cavities and sheet cracks (Hofmann 1969;Horodyski 1975;Walter and Awramik 1979;Myrow and Coniglio 1991).…”