The Middle Tithonian Los Catutos Member (Vaca Muerta Formation, Neuqu6n Basin), is lithologically and geochemically similar to limestone-marl alternations from the Late Jurassic of the northern hemisphere. Both marls and limestones are pelbiomicrites and biopelmicrites principally composed of pellets, radiolaria, forams, ostracods, equinoids, spicules of sponges and gastropods, cemented by several generations of calcite cement. Smectite and interlayers are the major epiclastic components of the fraction below 2 Ixm, reflecting pedogenic processes developed on volcanogenic source rocks. More abundant kaolinite in some marls reflects stronger humid conditions in the source area and enhanced terrigenous supply. A1203 content is demonstrated to be a reliable indicator of clastic input. The same is not true for silica, often related to high biogenic productivity of siliceous organisms. Rocks show total organic carbon (TOC) contents up to 1.95% and constitute regular to good sources for hydrocarbons, although thermally immature. Rhythmites formed gently sloping mounds accumulated in a distal submarine ramp under low-energy and poorly oxygenated open-sea conditions. Sedimentation rates were high due to high productivity on the sea surface, and supply of terrigenous and carbonate sediments transported by suspension plumes originated in shallow, photic areas. 613C values correspond well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of 613C point to a productivity crisis or a mixture of water layers in a stratified sea with a periodicity of 400 ka, which could be a result of changes in the orbital eccentricity of the Earth. Light isotopic composition of O in bulk rocks is the result of diagenetic neomorphism and cement precipitation. Calculated palaeotemperatures from 61Sc are consistent with those derived from measured vitrinite reflectance (Ro%) and burial history reconstruction. Data indicate initial burial during the Tithonian extending up to the Lower Cretaceous, a short period of uplift (Intravalaginian tectonic phase), and renewed uplift during the Cenomanian followed by significant Late Cretaceous sedimentation and Pliocene thrusting.
The Lower Cretaceous Chachao Formation in the Malargue anticline area consists of wackestone, packstone, and minor gramstone and mudstone rich in benthic fauna which were deposited in a shallow carbonate ramp. These shell beds are composed of a lowdiversity molluscan fauna mostly dominated by oysters, other benthic pelecypods (Eryphila sp., Ptychomya sp., Pecten sp., Pinna sp., Trigonia sp., Cucullaea sp.), serpulids (Sarcinella sp., Parsimoniasp.), and occasional ammonites (Olcosthepanus curacoensis).Most skeletal accumulations range from 15 to 70 em in thickness. Shell beds exhibit a sharp, flat and commonly undulating base. In some cases the tops were bioturbated pnncipally by Thalassinoides isp. Some shell beds show a densely packed biofabric (bioclastic-supported) but most of them have dispersed or loosely packed biofabric (mud-supported). Bioclasts include articulated or disarticulated shells, fragmented valves and shell debris. The analysis of shell concentrations of the Chachao Formation allow to recognize four taphofacies according to their taphonomic attributes.Different kind of processes, such as low-energy storm currents and waves, were interpreted, whose deposits are arranged into a coarseningand thickening-upward and then fining-and thinning upward trends. According to their biostratinomic features shell beds of Chachao Formationhave been interpreted as parautochthonous to autochthonous skeletal concentrations, mainly of sedimentologic origin.Towards the end ofthe Valanginian, the drowning ofthe carbonate ramp took place, related to a relative sea level rise and environmental stress. This drowning event was recorded by the alternating thin-to medium bedded dark gray carbonate and black shales (Agrio Formation) of LateValangiman-Late Hauterivian age.
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