Bajo de Veliz stromatolites revisited

Abstract

The discussion about the true biogenicity of Ca-rich deposits has been extensive, and in the past, abiogenic structures have been referred tostromatolites when their origin was linked to inorganic precipitation of carbonate. Later on, different types of carbonate precipitation have beendescribed including: mineralization (inorganic); and biogenic precipitation of carbonates, namely: biologically controlled, biologically induced andbiologically influenced mineralization. The Bajo de Véliz subbasin, located on the east margin of the Paganzo Basin, in the NE of San Luis province,presents the Bajo de Véliz Formation, referred to as Permian. The unit is a siliciclastic succession, well known by its extraordinary fossiliferouscontent and includes carbonate levels. The carbonates have been previously described as stromatolites based on microfacies analysis with theidentification of bryophyte and cyanobacteria as producers. Nevertheless, new polished sections and thin section microfacies analysis under lowmagnification binocular microscope along with SEM - EDS analysis shows no trace of biogenicity at a microstructure level. New data shows that themicrofabric is formed by a combination of Ca-rich parallel and subparallel veins (spar), included in a Si-rich matrix with Fe content. Thesemicrostructures were previously identified as bryophyte remains but no feature other than morphological resemblance was found. This points to agenesis linked to a probable carbonate rich seep that caused Ca and Si mineralization in the form of complex dendrites. Further cathodoluminescenceanalysis can confirm the timing and different phases of mineralization and if there was incremental growth.1