CONICET | Buscador de Institutos y Recursos Humanos

In order to better refine the paleoenvironmental analysis of the lacustrine succession in the Triassic CerroPuntudo Formation (Anisian) located in the Cuyana rift basin of Argentina, a half-meter thick limestone wasstudied. This carbonate layer contains two tuff and two silty clay laminae about 1 cm in thickness that allows forthe differentiation of three units. All three units are characterized by the presence of cyanoids (oncolites containingcyanobacteria and algae), silty carbonate and siliciclastic matrix, and charophyte remains, and are classifiedas algal wackestones. The cyanoids have concentric laminae and range in diameter from 0.5 to 8.0 cm. Theyhave irregular margins and multiple laminae in the cortex coating of one or more nuclei. In some cases the nucleiconsists of charophyte thalli and reproductive structures (gyrogonites). Transversal sections of the thalli measurebetween 0.2 and 0.6 mm wide with longitudinal sections of gyrogonites 600 μm long with an approximate diameterof 400 μm. Both structures are replaced by spar and can be seen in longitudinal and transverse sections.Between the laminae algal filaments of cyanophytes have been preserved as tubules of 10 to 20 μm long withmicritized external walls. The matrix is composed of carbonate and siliciclastic silt grains as well as cyanoidcoated grains. These limestone units are interpreted as deposits of a marginal lacustrine paleoenvironment thathas undergone diagenesis within subaqueous conditions, no subaerial exposure or pedogenic features are evident.The process that gave rise to the cyanoids involved the growth of microbial mats above the thalli of charophyteswhile they were in situ in the littoral zone of the Cerro Puntudo paleolake. The existence of moderate energy currentsand agitation may have promoted the formation and growth of the concentric cyanoids in this Triassicnearshore environment.