Microbially induced sedimentary structures indicating climatological, Hydrological and depositional conditions within recent and pleistocene coastal facies zones (Southern Tunisia)

Abstract

Extensive tidal areas of the Recent coast of southern Tunisia are overgrown by microbial mats. Different mal types of which each are dominated by distinct and ,,veil adapted cyanobacterial species develop. Ecological rcsponse of the mat-forming microorganisms to climatological, hydrological and sedimentological factors produce characteristic sedimentary structures (= microbially induced sedimentary structures).A succession of Pleistocene rocks crops out near the lagoon El Bibane, southern Tunisia. The stratigraphic section comprises structures that we regard as fossil equivalents to those microbially induced structures we observe in the Recent coastal area. Preservation of the structures ix result of lithification of the microbial mats. This we conclude from fossil filaments of cyanobacteria visible within the rock matrix.The Recent microbially induced sedimentary structures indicate facies zones within the modern tidal environment. Comparison of the Recent structures with the fossil analogues recorded in the stratigraphic section aids to identify the same distinct facies zones within the Pleistocene coastal environment also.Erosion by water currents forms step-like cliffs, and the microbial mat is undermined and ripped off piece by piece. Shallows within the supratidal area are overgrown by copious microbial mats comprising structures like biolaminites and -varvites, as well as polygons of cracks. The features originate from effects triggered by seasonal variations of climate. Tufts and reticulate pattern of bulges indicate supernatant water films covering the mat surfaces. Morpholo,L,ically higher parts of the Recent tidal area are overgrown bv single-layered mats forming petecs, induced by microbial mat growth and evaporitive pumping.The study demonstrates that microbially induced sedimentary structures can be used to reconstruct small-scaled facies zones within coastal environments. They also include hints on palcoclimatological, hydrological and scdimentological conditions.

Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in theca.3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia

“…In response to a semi-arid climate, the mats cause ''polygonal oscillation cracks'' and ''gas domes'' ( Fig. 1c; Noffke et al, 2001a). Polygonal oscillation cracks are polygon-shaped patches of microbial mat separated from each other by a crack.…”

“…Using this set of criteria, more than 14 studies have systematically explored MISS from modern to ancient sites, comparing structures in equivalent environmental settings from the modern right back to the early Archean (e.g., Noffke 1999Noffke , 2000Noffke and Krumbein, 1999;Noffke et al, 2001aNoffke et al, , 2002Noffke et al, , 2003Noffke et al, , 2006aNoffke et al, , 2006b. This suite of studies has assembled a data set that enables the evolution of MISSprokaryota to be monitored throughout Earth's geological record.…”

A case study of microbial mat-related features in coastal epeiric sandstones from the Paleoproterozoic Pretoria Group (Transvaal Supergroup, Kaapvaal craton, South Africa); The effect of preservation (reflecting sequence stratigraphic models) on the relationship between mat features and inferred paleoenvironment

“…However, the pervasive large palimpsest ripple fields complemented by a rich variety of associated MRS observed at most extensive outcrops of the latter unit (Parizot et al, 2005;Bosch and Eriksson, 2008;Eriksson et al, 2010), contrast strongly with the near-absence of MRS in the Daspoort outcrops. As has been discussed in section 2 of this paper, for both modern and ancient mats (including Precambrian examples) growing on clastic substrates, a school of thought supports a bias in the occurrence of MRS in the shallow marine -tidal flat-supratidal setting-sabkha environmental continuum (e.g., Schieber, 1998;Gerdes et al, 2000;Noffke et al, 2001b;Schieber et al, 2007a;Gerdes, 2007;Bose and Chafetz, 2009;Noffke, 2010). The widespread occurrence of MRS in the Magaliesberg Formation supports the view that these mat-related features might be partially diagnostic of such coastal palaeoenvironmental settings, but the paucity of Daspoort MRS does not.…”

“…salt works and laboratory models; Schneider, 1995;Gerdes and Klenke, 2003). Gerdes et al (2000) and Noffke et al (2001b) provide further examples of the perceived shallow marine -coastal bias in clastic mat studies in Pleistocene-modern tidal flats, as do Parizot et al (2005) for Palaeoproterozoic equivalents where they related MRS to inferred water depths and wave heights. In the Precambrian record, where the absence of grazers enhanced preservation and abundance of MRS, most marginal shallow marine settings formed part of preserved epeiric seas (e.g., Eriksson et al, 2008); as an example, Schieber (1998) relates MRS in the c. 1450-850 Ma Belt Supergroup of North America to a palaeoenvironmental spectrum ranging from calm offshore -shallow nearshore -sea-marginal sandflats -shallow nearshore lagoons.…”

“…For modern mats growing on clastic substrates, the most commonly observed environments are, firstly, the shallow marine tidal-supratidal-sabkha continuum (Schieber et al, 2007a;Gerdes, 2007;Bose and Chafetz, 2009), and secondly, hypersaline lagoons (e.g., Gerdes and Krumbein, 1987;Noffke et al, 2001b;Noffke, 2010) or analogues thereof (viz. salt works and laboratory models; Schneider, 1995;Gerdes and Klenke, 2003).…”