Unravelling hidden glacial effects in the Cryogenian marine depositional settings of the Tandilia Basin, Argentina.

• A drastic shift in environmental conditions is recorded in Villa Mónica Fm. • Basal section represents a shoreface-offshore transition system. • Upper section evidence stromatolite reef in a tidal carbonate platform. • Phosphorous input controlled kilometer-scale biostromes and bioherms development. • Distal glacimarine and interglacial evidence remains hidden in the complete unit. The Cryogenian Period (720–635 Ma) is wide known by the occurrence of two global glaciations, Sturtian and Marinoan, responsible of dramatic climate and sea level changes as well as seawater oxygenation events. Constrained 87Sr/86Sr ratios from 0.7064 to 0.7071 and δ13C excursion are consistent with Cryogenian cap carbonates as documented in the upper Villa Mónica Formation. This unit constitutes the basal sedimentary cover of the Río de La Plata Craton in Central Argentina which begins its deposition in a shallow marine siliciclastic shelf, turning to a carbonate succession with exceptionally well-preserved stromatolites. The detailed facies analysis added to stromatolite morphologies included in paleoenvironmental models of deposition remain unknown. The later are considered necessary to establish the stratigraphic framework and the regional correlation of the unit across the basin as a piece for the Cryogenian assembly. The entire Villa Mónica Formation in the Tandilia Basin was deposited on a shelf with sedimentary facies variations observed from the shallow margins to the offshore transition zone. The transition from the lower to the upper sections implies the conquer of microbial communities to paleogeographically controlled chemocline fluctuations where the stromatolites evidence the development from a post-glaciation phosphorus-rich ocean. This drastic shift in paleoenvironments represented on the two distinctive marker sections was influenced by changes in climatic and oxygenation conditions. In this context, presumed glacial influence is predictable in a partially ice-covered Cryogenian Earth surface. [ABSTRACT FROM AUTHOR]