Microbially Induced Sedimentary Structures (MISS) And Their Paleoenvironmental Significance

The Ruyang and Luoyu Groups, consisting mainly of siliciclastic sediments, are well developed and wide spread in Lushan Area, Western Henan province, with complete successions of the Meso- and Neoproterozoic strata for the southern margin of North China Platform. Abundant microbially induced sedimentary structures (MISS) have been identified from these two groups. These bio-sedimentary structures can be grouped into the four genetic categories suggested by Schieber (2004) as follows. 1) Mat growth related features, includings nodular cluster or tufts, petee structures, polygonal reticular growth or irregular ridges; 2) Mat metabolism related features, including residual mat pockets, micro reticular mats, wavy crinkled laminae and microbially coated grain; 3) Mat destruction related features, including various sand cracks, such as spindle, reticular, bird-foot, vein and radiate shaped, Manchuriophycus, curled margins, chips, and mat smoothed ripple marks; and 4) Mat decaying and diagenesis related features, including gas dome, rainpit-like structures, sandy volcanoes, Astropolithon and irregular reticular pits. In general, larger sand cracks more often occur on sandstone or mudstone surfaces, especially at the transitional positions from sandstone to mudstone. The features resulted from mat metabolism and decay also conform the existence of abundant microbial mats.The present study also shows that MISS were preserved in peritidal siliciclastic environments, especially from the supratidal to the subtidal zones. According to the distribution of MISS in the study area, four morphological association zones can be recognized, in each of which has its distinctiveness in MISS associations related to the changes in depositional environments. The upper supratidal zone abounds small sand cracks and growth ridges formed by relatively thin microbial mats; the lower supratidal is rich in various MISS, especially the large and morphologically complex sand cracks formed by thick mats; the upper intertidal is characterized by mat-protected or multi-direction ripple marks and mat chips; while the lower intertidal to subtidal commonly lacks in situ MISS, but contains redeposited mat chips.The paper has also discussed the relationship between MISS formation and depositional environments, and argued that the MISS morphological associations are mainly controlled by environmental factors such as hydrodynamics, substrate exposure, humidity of deposit surface, deposition ratio and the redox state after preservation. These factors are well correlated to the palaeotopography and the water depth. It is suggested, therefore, that the MISS morphological associations can be used for paleogeographcal and paleoenvironmental reconstructions, especially for the Precambrian siliciclastic successions.