Resumo

GOMEZ-PEREZ, Irene. Deep water stromatolites from the Los Molles Formation (Neuquén, Argentina): jurassic sea-floor cold seeps?. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.2, pp.1-14. ISSN 1853-6360.

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 of thin micritic laminae or peloids cemented by early fibrous and botryoidal cements growing centripetally. Locally associated clusters of tube-like worm communities are embedded in peloidal micritic sediment. Other biota includes ammonites and very rare ostracods, gastropods and bioclasts. The depositional environment deduced for this build-up is a low energy marine setting, bellow the storm wave base, and in the limit or slightly above the photic zone, in poorly oxygenated waters. This environment is deduced from the lack of phototrophic organisms, no sedimentary structures indicating current action, preservation of lamination, lack of bioturbation, and thriving opportunistic faunas, characteristic of oxygen-depleted environments in associated facies. Depositional depths for shaly-silty facies of the Los Molles Formation in the upper, regressive part of the section, where these facies interfinger laterally with tidal sandstones of the Lajas Formation, has been estimated in base to clinoform reliefs in at least 40 meters. Minimal depositional depth for the stromatolitic bioherm is estimated to be about 50 m, and it was likely between 50-100 m. Carbonate deposits analogous to the one described in this work have been found in deep water environments associated to cold seeps in subduction zones or oceanic ridges. These structures share many of their characteristics with the described buildup, as are being an anomalous carbonate accumulation in a relatively deep siliciclastic basin, the scale, the low diversity-high abundance faunas, and depositional facies as tube worms and bacterially induced lamination. What makes this example unique is the presence of stromatolites, which have not been described in other analogous localities. Differences between this example and hydrothermal vent deposits are the lack of associated ore deposits and mineralization, and the absence of chemosynthetic bivalves, non light-dependant faunas common in some hydrothermal vents and cold seeps. These communities form in association to sea-floor vents or seeps which reach the surface through faults or porous beds. In a similar context, this buildup is interpreted as associated to a sea floor vent in the Jurassic sea-floor of the Neuquén Basin. Given the location of the bioherm over the margin of one of the shallowly buried syn-rift depocentres, migration of the fluids could have followed its bounding faults. This depocentre is infilled by continental, volcanic, and pyroclastic rocks, and the origin of the fluids could be related to the magmatic and hydrothermal activity associated with the emplacement of these deposits during rift stages, shortly before deposition of the Los Molles Formation. On the other hand, some examples are known from deepwater communities formed in relation to cold seeps which origin is related to subsurface hydrocarbon accumulations (Gulf of Mexico, Greenland). The importance of hydrocarbon accumulations in the syn-rift succession of the Neuquén Basin is being gradually acknowledged, and the deposition of this bioherm could indicate the presence of subsurface hydrocarbons during the Jurassic in the Neuquén Basin. Future isotopic analysis will help to evaluate this model and decide on the origin of this unique structure.

Palavras-chave : Bioherm; Stromatolites; Calcareous tube worms; Deep water; Siliciclastic basin; Cold seeps; Rift faulting; Jurassic.

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