Biohermal thrombolites of the crater lake Rincón de Parangeo in Central México

This work describes the distribution, macro and microstructures of modern microbialites deposited in the marginal facies of a Quaternary paleo-lake at Rincon de Parangueo, within the central trans-mexican volcanic belt. This paleo-lake formed after the volcanic eruption and maar formation that provided suitable conditions at its margins for substrate colonization and microbial proliferation. Vesicular thrombolites, grouped into domical and irregular clusters, were mapped, measured and analyzed. These microbialites are characterized by a 2 cm-thick stromatolitic crust and a thrombolitic matrix that accreted by carbonate precipitation. The stromatolitic crusts represent the last accretion stages at intertidal conditions before the complete evaporation of the paleo-lake. The asymmetrical distribution and preservation of these microbial deposits suggest that the rapid subsidence and tectonic deformation began before the lake completely dried out. The maar at Rincon de Parangeo represents not only an important record of recent geological processes in dynamic interaction with microbial processes under increasingly extreme alkaline conditions (pH between 10 and 10.6), but also a possible analogue for some Precambrian sequences. It provides an example of a rapidly transforming environment where macro and micro geobiological interactions directly influence biological succession.