Estromatolitos de aguas profundas en la Formación Los Molles (Neuquén, Argentina): ？chimeneas de metano en el fondo marino jurásico?

a stromatolitic bioherm embedded in deep-water shales and marls of the los molles formation was found in the sw of the neuquén basin (argentina). the neuquén basin is located in west central argentina and eastern chile, between 36-40o ls. it was formed in the late triassic by continental extension and evolved as a backarc basin from the middle jurassic. it was infilled by an upper triassic to early tertiary succession of marine and continental deposits included in different sedimentary cycles, formed in response to relative sea level variations and tectonic pulses. the los molles formation was deposited in response to the first marine transgression in the neuquén basin in the early jurassic. it is made up of dark shales, silts and sandstone turbidites. it overlies shallow marine clastic and carbonate deposits of pliensbachian age (chachil and sierra de chacaico formations), and it is overlain by tidal clastic deposits of bajocian age (lajas formation). a series of late triassic synrift depocentres were shallowly buried under these units in the studied area, which also unconformably overly paleozoic granitic or metamorphic basement (fig. 1). the described stromatolitic bioherm is located to the sw of the locality of zapala, in southern neuquén, in the area of the sierra de chacaico (fig. 1). it is 15 m thick and 60 m in diameter, and made up by individual stromatolites averaging 2 meters across and 1.5 m high (fig. 2). they show variable geometries, including tabular and columnar shapes in the lower, micrite-rich, part of the buildup, and domed shapes in the upper, cement-dominated, part. these individual stromatolites form a discrete bioherm, wrapped as a whole by an outer coating of laminations which locally shows very steep to vertical margins (fig. 2b). the individual stromatolites are made up of encased laminated calcitic layers, locally oversteepened, vertical or even overturned (fig. 3a). the microscropic structure shows the bulk of the lamination consisting o