Palynofacies analysis and palynology of the Agua de la Mula Member (Agrio Formation) in a sequence stratigraphy framework, Lower Cretaceous, Neuquén Basin, Argentina

GULER, M.V., LAZO, D.G., PAZOS, P.J., BOREL, C.M., OTTONE, E.G., TYSON, R.V., CESARETTI, N.N., AGUIRRE URRETA, M.B.

CRETACEOUS RESEARCH (PRINT)

ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2013 vol. 41 p. 65 - 81

0195-6671

Variations in the palynological organic matter were documented throughout the Agua de la Mula Member (Late Hauterivian) of the Agrio Formation, at a combined section in the type area of the Agrio Formation; the base at Agrio del medio and the middle to top at Bajada del Agrio. A main organic rich interval was identified in the basal Agua de la Mula Member, dominated by marine-derived Amorphous Organic Matter (AOM), coinciding with the highest Total Organic Carbon (TOC) content, between 1.4 and 3.8%, suggesting dysoxic conditions. The rest of the Member is predominantly characterized by terrestrially-derived organic matter, mainly phytoclasts, with low TOC values, around 1% or lower, indicating predominantly oxic depositional settings. By integrating stacking pattern and shell beds analysis, four depositional sequences SQ1 to SQ4 were recognized. The organic-rich, finely laminated bituminous black shales of the Spitidiscus riccardii zone constitute the major Transgressive System Tract (TST1) related to a third-order cycle, which represent the most important flooding episode within the Neuquén Basin during the Late Hauterivian. Each TST of the other three depositional sequences (SQ2, SQ3 and SQ4) identified within the Crioceratites diamantensis and Paraspiticeras groeberi Zones are of lower hierarchy, probably fourth order. With the exception of the oxygen-controlled, basinal and outer ramp settings indicated for the TST1 and the lower TST2, respectively, the prevalence of well oxygenated, inner to middle ramp depositional environments, is suggested for the rest of the sedimentary succession. Thus, a shallowing-upward trend with improved oxygenation is recorded through the Agua de la Mula Member, reflected by decreasing TOC, AOM content and preservation state of the organic matter. Based on the whole rock fluorescence analysis of the two selected organic-rich intervals from the Spitidiscus riccardi and the lowermost Crioceratites diamantensis zones, the presence of hydrocarbons suggests a very early in situ generation.