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Sedimentary Geology and Sequence Stratigraphy in Continental to Shallow-Marine Deposits |
1 Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, 1° Piso, 1428 Ciudad de Buenos Aires, Argentinarscasso{at}gl.fcen.uba.ar
2 M&P Systems, Miró 525, 1428 Ciudad de Buenos Aires, Argentina
The Middle Tithonian Los Catutos Member (Vaca Muerta Formation, Neuquén 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 µm, 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.
13C 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 13C 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 18C 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.
