Irene Gomez-Perez¹

(1) CASP, Cambridge, United Kingdom

ABSTRACT: A Deep Water Stromatolitic Bioherm : Methane Seeps in the Jurassic Sea Floor of the Neuquén Basin (Los Molles Formation, SW Neuquén, Argentina)

A community of stromatolites and tube worms makes up a 15m thick and 60m in diameter limestone bioherm, embedded in marls and dark shales of the Los Molles Formation (Neuquén Basin, Argentina). The Los Molles Formation is a basinal unit dominated by shales, silts and some sandstone turbidites, deposited after the first marine transgression in the basin, in relatively deep-water conditions. The limestone bioherm is made up by stacked-up pillow, domal, and columnar stromatolites, 2-4m across and 1-2m thick. Each stromatolite consists of algal-bacterial laminae, largely replaced and cemented by botryoidal-fibrous cements. Tube-like calcifying worms are embedded in peloidal micrite at the cores or flanks of the stromatolites. Minimum depositional depth has been estimated in at least 50 m, below the storm wave base, in the limit of the photic zone. The cement sequence indicates a marine phreatic to burial diagenesis. Isotopic analysis for sedimentary and diagenetic components yield very negative |Ä13C values (down to -33) and negative |Ä16O values (down to -11). These depleted values indicate deposition under the influence of methane. Based on this and other characteristics, the bioherm is interpreted as formed in relation to seeps of methane-rich fluids in the Jurassic sea floor. Migration of the fluids presumably happened through shallowly buried syn-rift faults. The fluids origin may be related to magmatic activity, which resulted in the emplacement and extrusion of volcanic rocks during syn-rift stages. The structure could also indicate the generation of hydrocarbons during the Jurassic.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado