Two karst events bounding drastic changes in the Neoproterozoic Tandilia Basin history, Argentina: paleogeographic relevance

The sedimentary infill of the Tandilia Basin is characterised by at least four different stages of deposition during the Neoproterozoic (< 1160 to ~ 550 Ma), two of them recognisable in the Sierras Bayas Group and two in the La Providencia Group. The Villa Mónica Formation, representing the first stage of basin infill (< 1160 to < 720 Ma), consists of a lower siliciclastic section that passes transitionally into an upper dolostone section. This dolostone platform, rich in standard Cryogenian stromatolites, was interpreted as deposited under typical cold, suboxic to oxic seawater conditions. A detailed analysis of the dolostone section allowed us to distinguish two karst events imprinted in the formation. The hypogene karst genesis is interpreted as hydrothermal with the development of Mn-oxide dykes associated with intense silicification and brecciation of the host dolostones. The younger epigene karst, which developed over the post uplifted unit, is related to intense weathering and subaerial exposure with the generation of an irregular surface in its upper contact. This unconformity, known as the Piedra Amarilla Surface, is represented regionally in the Tandilia System. It may have implied a post-uplift period of erosion, dissolution and non-deposition that predates the accumulation of the ~ 400-m-thick overlying succession (the Sierras Bayas and La Providencia groups). Accordingly, the Piedra Amarilla Surface may represent a large spatio-temporal interval bounding two contrasting histories in the basin infill, which would justify separating the Villa Mónica Formation from the Sierras Bayas Group. This karst-related surface may be considered a fingerprint of the transition from the break-up of Rodinia to the configuration of southwestern Gondwana.