Geochemistry and petrology of a Middle Tithonian limestone-marl rhythmite in the Neuquén Basin, Argentina: depositional and burial history

SCASSO, R.A.; ALONSO, M.S.; LANÉS, S.; VILLAR, H.J.; LAFFITTE, G.

The Neuquén Basin, Argentina: A case study in sequence stratigraphy and basin dynamics

The Geological Society (London)

Lugar: Bath, BA1, Reino Unido; Año: 2005; p. 207 - 230

The Middle Tithonian Los Catutos Member (Vaca Muerta Formation, Neuquen 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 microns, 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. Al2O3 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. d13C 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 d13C 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 d18C are consistent with those derived from measured vitrinite reflectance (Ro%) and burial history reconstruction. Data indicate initial burial during theTithonian extending up to the Lower Cretaceous, a short period of uplift (Intravalaginiantectonic phase), and renewed uplift during the Cenomanian followed by significant LateCretaceous sedimentation and Pliocene thrusting.