Neoproterozoic stromatolites from Villa Mónica Formation (Sierras Bayas Group), Tandilia System: the oldest life record from Argentina

Abstract

The stromatolites from the Villa Mónica Formation, Tandilia System, Argentina, were initially reported as “small-scale anticlinal undulations” and “concave meniscated laminae”. Later on, these stromatolites have been studied on four scales: megascale, mesoscale, small scale and microscale. The megascale shows that the lower and the upper parts of the dolostone member are composed of 0.5-1.4 m thick domal biostromes and 0.1-0.5 m thick interbiostromal green shales. A few bioherms are present at the top of this unit. In contrast, in the middle part the stromatolites are absent at all. Domal biostromes are very conspicuous and they are reflecting the strong influence of palaeocurrents in their morphogenesis. The direction of elongation was perpendicular to the shoreline. Two types of elongated bioconstructions are distinguished: symmetrical and asymmetrical. The last one suggests their acute end points out to sea. The measured palaeocurrents suggest an N-S local shoreline direction, with open sea towards the East. In terms of small scale and microscale points of view, the following up-dated list of Villa Mónica Formation stromatolite groups and forms are as follow: Colonnella fm., Conophyton fm., Conophyton resotti, Cryptozoon fm., Gongylina fm., Gymnosolen fm., Inzeria fm., Jacutophyton fm., Jurusania cf. nisvensis, Katavia fm., Kotuikania fm., Kussiella fm., Minjaria fm., Parmites fm., Parmites cf. concrescens and Stratifera fm. Along the dolomitic unit of the Villa Mónica Formation, these stromatolites are strongly ordered. In mesoscale, two kind of patterns have been distinguished dependent of the vertical and lateral arrangement of the stromatolites inside the biostromes: i) when the stromatolites do not present any variation along the bed, these bioconstructions have been named as “monostromatolite layers”, and ii) on the other side, “stromatolites cycles” are defined when there are changes in stromatolite morphology in the same bed. In this sense, three “monostromatolite layers” and six “stromatolite cycles” have been identified in these dolostones. The former ones consist of A) flat laminated stratiform stromatolite, B) globoidal laminated bulbous stromatolite, and C) gently folded laminated stratiform stromatolites. On the other hand, “stromatolite cycles” consist of D) planar laminated stratiform stromatolites passing upwards to convex laminated digitated columnar stromatolites, E) gently folded laminated stratiform stromatolites grading upwards to convex laminated columnar stromatolites, F) wide conical laminated columnar stromatolites with smaller columnar branching stromatolites in their apex, G) wide conical laminated columnar stromatolites with convex laminated smaller cylindrical columns branching from their apex, H) convex laminated cylindrical unbranched, some of which suddenly stop their growth while others develop wide conical laminated falcate columns, and I) wavy laminated gently domed stratiform stromatolites grading upwards to very close spaced convex laminated unbranched columns, in a dendroid style. Basically, the stromatolite cycles resemble parasequences and high frequency cycles and they contain basic surfaces of the sequence stratigraphy framework. These stromatolites are the oldest life record from Argentina, whereas the body fossil Cloudina sp. has been recorded also from the Sierras Bayas Group, but from the younger Loma Negra Formation. Some Intrites sp. have been found it in the overlaying Cerro Negro Formation.