The Neoproterozoic sedimentary record from Tandilia System, Argentina

Abstract

The Tandilia System is an orographic belt located in the Buenos Aires Province, Argentina, which a maximum length of 350 km orientated in a NW-SE direction. The hills are mainly composed by an igneous metamorphic basement and a Neoproterozoic and Early Palaeozoic sedimentary cover, which displays sub-horizontal bedding. Whereas the Neoproterozoic carbonate-siliciclastic sedimentary succession crops out in the north-western part, around Olavarrıía and Barker-San Manuel region, the Lower Palaeozoic rocks (quartz-arenites, kaolinite-rich mudstones, conglomerates and basal diamictites) occur in the western part of the range, especially towards the southeast (Balcarce-Mar del Plata area). These deposits overlie a Palaeoproterozoic crystalline basement (Buenos Aires Complex). From a lithostratigraphic standpoint the Neoproterozoic sedimentary successions comprise the Villa Mónica, Colombo, Cerro Largo, Las Águilas, Olavarrıía and Loma Negra Formations (Sierras Bayas Group). The Cerro Negro Formation overlies the Sierras Bayas Group with erosional unconformity. Finally, an Early Palaeozoic succession is represented by the Sierra del Volcán and Balcarce Formations. From the sequence stratigraphy scheme, six sedimentary sequences have been distinguished: i) four Neoproterozoic sequences (Sierras Bayas Group), ii) a latest Ediacaran-Cambrian sequence (Cerro Negro Formation) and iii) an Ordovician-Silurian sequence (Sierra del Volcán and Balcarce Formations). The sedimentary sequences of the Sierras Bayas Group and Cerro Negro Formation are divided by regional unconformities and are composed from bottom to top as follow: A) Villa Mónica Formation (70m thick) is made up of two sedimentary facies associations, (a) arkosic/quartz sandstones, and (b) dolostone, stromatolites dolostone and shales, which stromatolite assemblages suggest a Tonian-Cryogenian age. Evidences of a significantly higher diagenetic degree of alteration in this unit with respect to the overlying units, was interpreted in favour of a significant hiatus in between. The top of this unit is characterized by red to purple marls that suggest a period of exposure and weathering of the succession. B) Colombo Formation (8m thick), shales, chert breccias and diamictites, composed of massive mudstones bearing exotic clasts up to 2.5 m in diameter. C) Cerro Largo Formation (40m thick), finely bedded, varicoloured, glauconitic, hetherolithic facies and cross-bedded quartz-arenites. D) Olavarría Formation (35m thick), siltstones and claystones suggesting shallow-marine deposits. Particularly, in the Barker area, the middle part of the unit comprises red claystones with high iron content (9 m thick), which could be correlated with other late Neoproterozoic iron deposits in SW Gondwana. E) Loma Negra Formation (40m thick, bearing Cloudina sp.), reddish and black micritic limestones, deposited in an open marine ramp and lagoon. F) Cerro Negro Formation (400m thick) characterised by reddish and greenish, brown olive claystones and heterolithic facies, formed in intertidal flats. The lower part of the Cerro Negro Formation consists of reddish residual clayish deposits, micritic limestones and marls, and breccias with a phosphatic level. The basal, highly irregular, karst surface (“Barker Surface”) has been correlated on a regional scale and related to a marine regression that exposed the Loma Negra shelf. If this correlation is valid would suggest that initiation of the Cerro Negro deposition took place at around 545 Ma.