Descripción: The lower section of Canadon Asfalto Formation (Callovian-Oxfordian) is located at Las Chacritas, Cerro Condor area, on the middle course of Chubut River. It is represented by lacustrine carbonate faciès with interbedded volcanic layers accumulated in the rift system of the Somuncura-Canadon Asfalto Basin. Eight microfacies (MF), defining the lake environments, and three biohermal faciès (MFB) corresponding to an extensive microbialitic belt overlying eulittoral sediments have been recognized: The microfacies are: MF1, mudstone with microbial lamination, related to supralittoral shallow ponds; MF2, biohermal intraclastic floatstone linked to storm episodes; culminating with evaporitic levels; MF3, mudstone with evidence of dessication formed in stagnant water; MF4, microbial-peloidal mudstone/wackestone with evidence of sporadic wave currents, in a eulittoral environment; MF5, intraclastic grainstone/rudstone with pedogenic alterations showing evidence of wave action in a eulittoral area; MF6, oncoidal floatstone interpreted as an hiperconcentrated flow; MF7: Bio-intra-siliciclastic packstone/wackestone accumulated in infralittoral conditions and MF8, silicified bulbous stromatolites related to the outlet of fluvial channels. The biohermal faciès are: MFB9, mudstone with stromatolites and wackestone levels; MFB10, planar stromatolites and MFB11, hemispherical stromatolites with chalk crust. Behind the biohermal belt, palustrine limestones locally show layers with episodic storm floods. The microfacial and facial distribution represents expansion and contraction cycles of a closed lake, controlled mainly by the precipitations. ©1999 Asociacián Gcológica Argentina.

Descripción: Microbial oncoids have been found in the Oxfordian limestones of the La Manga Formation in the La Vaina section at Potimalal River, Mendoza province. The oncoids ocurr either in packstone or floatstone-rudstone or are scattered in the wackestones. They are mostly elliptical, ameboidal and subordinately spherical in shape. Different types of oncoids were recognized, according to their features of envelopes: (1) micritic laminations, (2) grumose laminations, and (3) organism-bearing laminations. Molluscs and echinoid fragments, peloids, and intraclasts acted as oncoid nuclei. Laminae follow the shape of nuclei, especially in the inner zone of the cortices. However, in the outer zones the laminae contain encrusting organisms dominated by nubeculariids and serpulids. Some oncoids, especially the ameboidal or elliptical forms, are characterized by multiple nuclei, represented by small oncoids. The oncoids are associated with bivalves, echinoderms, forams, and serpulids. The fauna is indicative of calm, shallow conditions and the excellent preservation of echinoderms suggests minimal transport prior to burial. Growth histories are in evidence. The oncoids grew in a shallow, low energy, slightly to moderate agitated subtidal normal sea water environment. The limited rolling growth oncoids was probably accompanished by intermittent currents that reoriented the oncoids parallel to stratification. The discontinuous organisms-bearing laminations reflect periods of non-agitation and litification, which facilitated the growth of encrusting organisms on static oncoid particles during a period of low sedimentation rate. Deposition of the oncoid-bearing limestones took place during shallowing of the carbonate interval and associated with emersion, subaerial exposition and paleokarst as consequence of sea level fluctuations.