An Early Jurassic deep water stromatolitic bioherm: a possible methane seep in the Los Molles Fm (Neuquén Basin, Argentina)

An assemblage of microbial stromatolites and worm tubes makes up a limestone bioherm, 15 m thick and 60 m in diameter, enclosed in marls and dark shales of the Early Jurassic Los Molles Formation (Neuquén Basin, Argentina). The Los Molles Formation is a basinal unit consisting primarily of shales, silts and sandstone turbidites deposited after the first general marine transgression in the basin, below storm wave base, in relatively deep-water, poorly oxygenated conditions. The bioherm is composed of stacked stromatolites, 1–4 m in diameter and 1–2 m thick. They initially grew horizontally (tabular stromatolites), or vertically (columnar stromatolites), and later became domal, equidimensional to asymmetric structures. They often display vertical and overturned flanks. Each stromatolite consists of micritic laminae alternating with early botryoidal cements, which make up the bulk of the structure. Calcareous worm tubes are embedded in peloidal micrite associated with the stromatolites. The growth of the assemblage was uninterrupted by terrigenous input. The diagenetic sequence indicates evolution from marine phreatic to burial environments. Isotopic values for micrite, worm tubes, early botryoidal cements, and spar cements yield very negative δ¹³C values (down to −33) and negative δ¹⁶O values (down to −11). These depleted values indicate precipitation in very anomalous conditions in the Jurassic ocean, under the influence of methane-rich fluids. These characteristics suggest that the stromatolitic bioherm formed in relation to methane seeps in the seafloor of the Neuquén Basin. It is suggested that the structure grew within the photic zone, although a chemosynthetic component cannot be excluded. Minimum depositional depth has been estimated at 50 m, and could be up to 100 m. The vent fluids would have migrated through shallowly buried early rift faults, which bound underlying depocenters infilled with continental clastic and volcanic rocks. The origin of these fluids could be related to magmatic activity which resulted in accumulation of the syn-rift volcanics. Alternatively, the structure could indicate seepage related to the presence of hydrocarbon accumulations in the syn-rift succession during the Jurassic, the importance of which is being gradually acknowledged in the petroleum system of the Neuquén Basin.