CONICET | Buscador de Institutos y Recursos Humanos

Dedolomitization is the process of replacement of dolomite by calcite which mostly occurs undermeteoric water influence. The shallow marine deposits of the Villa Mónica Formation (VMF) (~1150-720Ma) constitute here is a good example of where to study this process. VMF is the older unit of thesedimentary basin infill in Tandilia System (Buenos Aires, Argentina), and it is formed by a lower section(22 m) mainly composed of quartz-arkosic sandstones, and an upper mainly dolomitic section (36-52m) with thin interbedded green claystone levels and ending with red claystone and purple marl. Thecontact between the top of VMF and the base of the Colombo Formation is an erosive surface. Previousstudies indicate that the VMF was affected by deep burial diagenesis followed by telogenesis. However,the expression of the weathering processes over the dolostones is poorly known. Thus, the proposal ofthis work is to recognize and characterize features related to weathering processes in the last metersof the upper section of the VMF.With the aim of determining the weathering features imprinted in rocks, sedimentary sections fromquarry outcrops drill cores from subsoil sections located in the Sierras Bayas-Olavarría area weredescribed. In addition, 35 samples were selected for petrographic, X-ray diffractometry (XRD), Scanningelectron microscopy (SEM-EDS) and stable isotope analysis of C and O.The upper contact of the VMF presents deep weathering overimposed on dolomites, claystones andmarls modifying the petrologic characteristics of the last 3 to 5 meters of the unit. The main features ofweathering at mesoscale are the presence of terra-rossa discontinued levels (up to 50-60 cm thick),silicified dolomite levels, friable dolomite and marly levels staining with iron oxides. At microscale, Feoxidesare common and appear as inter- and intragranular cements; siliceous cements are mainlycomposed of chert filling secondary voids and as dolomite replacement; and calcite cements appearobliterating latest spaces of secondary porosity. XRD results obtained from both unaltered dolomite andweathered dolomite samples were compared in order to determine the main differences betweenprimary and secondary components. Unaltered dolomite is composed almost entirely by dolomite(stoichiometric), while weathered dolomite shows dolomite, calcite, quartz, hematite and illite in differentproportions according to the sample. SEM-EDS analysis was performed to detect evidence retainedand/or non-retained rhombic morphologies, replacements and cementations with high resolution, andto generate geochemical mappings. C and O isotopes stables results were obtained from samples ofweathered dolostones showing values influenced by meteoric fluids.Mesoscale observed variations on top of the VMF represented by friability, changes in coloration,silicification, oxides distribution together with mineralogical changes, replacements and isotopicvariations allow to affirm that the predominant processes affecting the top of the unit was thededolomitization that can be assumed as related to intense physicochemical changes under subaerialexposure.Considering that dolostones of VMF were interpreted as a post-Sturtian cap-dolostone (~ 720-710 Ma),here we propose that widespread pervasive dedolomitization may represent a post-glaciation intervalduring which the dolostone succession were exposed to weathering.

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