Grupo Mina Verdún

POIRÉ, DANIEL G.

Geología del Uruguay - Tomo I: Predevónico

Universidad de la República

Lugar: Montevideo; Año: 2014; p. 233 - 251

The Mina Verdún Group was erected by Poiré et al. (2003b) to include a sedimentary succession exposed in the homonymous mine, which is made up (from base to top):Don Mario Formation(40 m, metapelites), La Toma Formation (15-20 m, marls), El Calabozo Formation (170m, grey stromatoliticlimestones) and Gibraltar Formation (60 m, dolomitic marls and pelites, pinklimestones and massive brown, green, grey and black dolostones). Thickness of the succession at its stratotype reaches 285 m. An important feature of the thick (up to 170 m) limestones of El Calabozo Formation is the occurrence of a stromatolite assemblage composed almost exclusively by Conophyton fm. A minimum age for the succession is provided by the unconformably overlying Las Ventanas Formation, which on the basis of acritarch biostratigraphy is assigned to the lower Ediacaran (635-575 Ma, Blanco y Gaucher (2005). Later on, Gaucher et al. (2004b, 2007) assign the succession exposed in the Burgueño Quarry, 40 km to the south of Mina Verdún, to the MVG, but included the Cerro de las Víboras Formation as a basal unit of the Mina Verdún Group (1433 Ma in age, Gaucher et al., 2011). In this sence, the marine volcanosedimentarysuccession exposed in the Paso del Molino and Burgueño Quarry area shows 275 m in thickness, which is conform from base to top by the following formations:Cerro de lasVíboras, Don Mario, La Toma, El Calabozo and Gibraltar. At the stratotype in Mina Verdún and in Burgueño Quarry, conglomerates overly with erosional unconformity the MVG. These conglomerates, assigned to Las Ventanas Formation (Blanco & Gaucher, 2005), include a level of diamictites and breccias at the contact to the Mina Verdún Group, which has been recognized as a palaeokarst surface. The association of palaeokarst surfaces developed on top of carbonate successions and overlain by glacigenicdiamictites is well known for a number of Neoproterozoic successions. Dropstones occur in laminated mudstones, associated with diamictites. However, no cap carbonates occur on top of the diamictites.The Mina Verdún Group is a marine succession occurring in the Nico Pérez Terrane. Limestones of ElCalabozo Formation contain an almost monospecific assemblage of stromatolites, dominated by Conophyton (Poiré et al., 2003a, 2003b, 2005; Gaucher et al., 2004b, 2006, 2007a). They consist of unbranching, subcylindrical columnar stromatolites with typical conical laminae. Their profile is angulate to geniculate, and their cross section circular to oblong (Poiré et al., 2005). Column height reaches 70 cm, and diameter 25 cm. Rare occurrences of digitated columnar stromatolites were reported from the Mina Verdún Group by Poiré et al. (2005). Later on, brunched digitatedstromatolites have been reported in dolostones of Gibraltar Formation (Poiré et al., 2006). Carbon isotopic data presented by Gaucher et al. (2005, 2006, 2007) show low-amplitude δ13C oscillations and a plateau around 2 ? PDB, thus consistent with a late Mesoproterozoic to Tonian age, which confirm the 1433 Ma obtained from primary zircons from Cerro de lasVíboras Formation tuffs (Gaucher et al., 2011).