Early Olenekian Microbialites in Chaohu Area, Anhui Province, East China

INTRODUCTION

The basal Triassic microbialites have been well documented from many parts of the world (Algeo et al., 2007; Baud et al., 2007, 2005; Riding and Liang, 2005; Della Porta et al., 2003; Lehrmann et al., 2003; Ezakil et al., 2001; Sano and Nakashima, 1997; Qian, 1995). Here, we present two kinds of microbialites from the lower Olenekian in the Lower Yangtze, South China.

MATERIALS AND METHODS

Located in Anhui Province in East China, Chaohu area in Lower Yangtze (Fig. 1) (Zuo et al., 2003; Feng et al., 1997) has been considered as a deepwater open marine basin bordering the carbonate slope during the Early Triassic (Li et al., 2007; Zhao et al., 2005; Zuo et al., 2003). The Lower Triassic in this area consists of the Yinkeng Formation (Induan and early Olenekian), Helongshan Formation and Nanlinghu Formation (Olenekian), and is mainly composed of carbonates such as micrite limestones and nodular limestones, as well as shales and calcareous marls. Some microbialites were found in Helongshan Formation and lower Nanlinghu Formation and were observed with light microscopes and Zeiss Ultra Plus Field Emission Scanning Electron Microscope (SEM).

Figure  1.  Location of the study area.

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RESULTS AND DISCUSSION

Two types of microbialites were found in the lower Olenekian in the study area, microbial mats and stromatolites. The microbial mats can be found in the nodular limestone in Helongshan Formation and are always associated with ammonites, bivalves and trace fossils. Cyanobacteria, such as Girvanella (Fig. 2-1), Sphaerocodium (Fig. 2-2), and Codiaceae are the dominant microfossils in the microbial mats. These microbes have been confirmed as the shallow photic zone microbial community (Flügel, 2004). So the nodular limestone of Helongshan Formation, in which microbial mats have been found, might be formed in shallow water. The stromatolite thin layers or bioherms are cropped in the yellowish thin-bedded calcareous mudstone layers in the upper Helongshan Formation and the lower Nanlinghu Formation.

Figure  2.  Images of Girvanella (1) and Sphaerocodium (2) in the microbial mat, cross thin section.

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The stromatolite bioherms are only a few centimeters thick and have semi-spherical dome on the top surface. The stromatolite columns are about 2 mm in diameter. The stromatolites are similar in morphology to the counterparts reported before, though not exactly the same (Figs. 3-1, 3-2). SEM observation shows that tubelike fila (2.5 μm in average diameter) are very popular in the stromatolites, especially in the sparitic parts (Fig. 3-3). Spherical, rodshaped and spiral microfossils can also be found in stromatolites (Figs. 3-4, 3-5). The nannofossils might be calcified cyanobacteria and other bacteria. Some of the light colour laminae in the stromatolites might be tube-like filament which have been calcified in situ. This microbial community has not been discribed in previous studies in Early Triassic microbialites (Wu et al., 2007). Such distinct microbial communities might indicate that the marine benthic microbial communities were not only widespread but also diverse during the Early Triassic.

Figure  3.  Images of stromatolites and the microbial fossils. (1) Top view of a stromatolite bioherm; (2) a polished slab of the stromatolite showing the column formed by the intercalating of different laminae; (3) the tubelike fila microfossils in stromatolite, SEM image; (4) spiral microfossils in the stromatolites, SEM image; (5) spherical microfossils in the stromatolites, SEM image.

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CONCLUSIONS

Two distinctive microbialites, microbial mats and stromatolites, were found in the lower Olenekian in the study area, showing that the marine benthic microbial communities were widespread and diverse during the Early Triassic. The depositional environment in Chaohu area during the early Olenekian might be much shallower than the previously estimated.