A Neoproterozoic paleokarstic surface in the upper Villa Mónica Formation, Tandilia System, Argentina: Preliminary constraints on its origin and diagenesis

Abstract

The Neoproterozoic sedimentary successions of the Tandilia Basin are constituted by a wide range of sedimentary rocks. We analyse the upper section of the Villa M´onica Formation constituted almost entirely by dolostones. C-isotope pathways and mudstones with dropstones, just below the dolostones, suggest this succession is a probable cap carbonate. Therefore, the model of its diagenetic evolution is consistent with trends of primary marine origin. The age of this unit is still under debate, but stromatolites, Sr isotope data and δ 13C curve suggest Cryogenian age. An uplift registered on top of the formation is related to period of intense erosion and weathering with generation of a karstic surface over the dolostones as a telodiagenetic surface. This important unconformity was situated, from palaeomagnetic studies, in 595 Ma and is characterized by dedolomitization, intense dissolution and precipitation if iron oxide minerals, chert and calcite. Siliceous cementation and replacement have probably occurred when pH dropped during sub-aerial exposure associated to sea-level fall of uncertain relation with glaciations. This surface constitutes the contact between Villa M´onica and Colombo formations, representing an extensive period of erosion, and called “Piedra Amarilla surface”. A few meters below, further evolution of the dissolution processes developed a paleokarstic subsurface recognized by presence of up to 3 m caverns filled by weathered dolomite. C-O isotope data in this level are typical of meteoric fluids interaction showing lower with respect to unaltered dolostone, δ 13C varying from -0.45 to -1.87 and δ 18O VPDB from -6.16 to -10.25 .