Middle Jurassic marine anoxia and related forced regression in the north of the Neuquén Basin, Mendoza province, Argentina

PALMA, R.M.; MARTÍN CHIVELET, J; KIETZMANN, D.A.; LÓPEZ GÓMEZ, J.,

Zaragoza

Congreso; 28 Meeting of sedimentologists; 2011

During the Callovian-Oxfordian times, in the Neuquén Basin, a typical retro-arc foreland basin, a widespread carbonate ramp deposition resulted in a vertical and lateral mosaic of shallow-water facies, adjacent to deepwater, which are known as La Manga Formation. This unit is part of the Lotena Mesosequence developed from middle Callovian to late Oxfordian-Kimmeridgian times. Different depositional settings have been defined in the ramp. Coral reef and oolitic-skeletal shoal were developed in inner ramp and upper middle ramp areas. The distal mid-ramp area was characterized by the interbedded of fine limestones and calcareous tempestites. Well bedded wackestones- packstones and marls are found in the outer ramp. Basin deposits of Los Blancos area crops out continuously as a regular hemipelagic alternace of dark to black rhythmic mudstones, black shales and marly limestones succession, with a thickness ranging up to 35 m. This section records a major anoxia episode in the basin that preclude a rapid forced regression that later derived in the deposition of largest evaporates sequences.This succession is rhythmically-bedded and are composed mostly of the variably neomorphosed (“grained”) micritic and pressure-welded (stylolaminite) argillaceous bands. With the exception of bioclastic material, mainly calcified radiolarian and spicules of sponge, an impoverished macrofauna is limited to largely inarticulate filamentous bivalves and ammonites. Biostratigraphic result from ammonite (Perisphinctes (?Arisphinctes) sp.) indicates that deposition of the La Manga Formation in the area was initiated in the middle Oxfordian (Riccardi, pers. com.).Samples from Los Blancos section show a content of organic carbon with TOC ranging from 0.88 to 4.33% (2.86% on average) whereas the maximum for HI values is only 2 mg HC/g TOC. An increase in organic matter is noted in the mostly black-shales interbedded where Perisphinctes (?Arisphinctes) of the Bifircatus Zone appears.For the Los Blancos section, calcium carbonate content measurements allow a comparison with the distribution of organic matter content. An increase of TOC in the Bifircatus Zone is due manly associated to a decrease of calcium carbonate.These changes might have been due to fluctuations either in the primary productivity or in the rate of dissolution of the carbonate, but in this case the enrichment of organic matter is consequence of an increase in detrital organic matter input. Anoxic conditions were developed within sediment as suggested by the presence of pyrite. An expanded oxygen minimum zone might destroy deeper habitats of planktic foraminifera.