Triassic bacteria preserved in dolomitic lacustrine carbonates

BENAVENTE, CECILIA ANDREA; MANCUSO, ADRIANA CECILIA

Otro; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2019

The Santa Clara sub-basin remains a fairly unexploredregion of the Triassic Cuyana rift basin. The Santa Clara Arriba Formation isthe upper unit forming the Peñasco Group and represents a delta plain with developmentof mouth bars, prodelta, and offshore lacustrine setting. The offshore lacustrinedeposits present carbonates located at the top of the unit and system. Theobjective of this study is to describe the finding of extremely well-preservedbacteria in dolomitic lacustrine carbonates. Thin sections of the carbonatesshow two types of matrix: a micrite one and a dolomite one. Micritic carbonatesare characterized by wavy thin lamina 10 µm thick, interpreted as algaebiofilms. Dolomitic carbonates form homogeneous mosaic with anhedral crystalswith irregular sutures and cloudy centers. Within these mosaics, rounded opaquestructures of 5 µm in diameter are found. These structures are mostly observedas clusters or aggregates randomly distributed in the dolomite. This finding isinterpreted as preserved bacteria. Similar remains have been described fromlacustrine carbonates and interpreted as the results of encapsulation ofbacteria. Their presence is a strong indicator of primary dolomite and anoxicconditions in the system since modern bacteria involved in dolomiteprecipitation have sulfate reduction or methanogenic metabolism. Evidence presentedsuggests dolomite-rich carbonates of the Santa Clara Arriba Formationcorrespond to a meromictic model of dolomite genesis.