Interpretation of Saccocoma microfacies and their significance in the Tithonian of the Neuquén Basin, Vaca Muerta Formation, Mendoza, Argentina

KIETZMANN, D.A.; PALMA, R.M.; FERRÉ, BR.

Bahía Blanca

Simposio; IV Simposio Argentino del Jurásico y sus Límites; 2010

Universidad Nacional del Sur

Microfacies studies conducted in the Vaca Muerta Formation have recorded the presence of saccocomid microcrinoids in the Neuquén Basin. This finding is the first record of Saccocomidae outside the Tethys/North Atlantic area. Samples with saccocomids come from the middle and upper Tithonian (Pseudolissoceras zitteli to Corongoceras alternans zones) of the Río Salado, Arroyo La Manga, Arroyo Loncoche, Bardas Blancas, Sierra Azul and Sierra de Cara Cura sections. A northeast-southwest transect of Sequence-1 is used here to establish the distal proximal gradient of the carbonate ramp. Saccocomid remains occur in laminated bioclastic wackestones, wackestones with HCS as sparse elements, as well as in sedimentary concentrations (Saccocoma microfacies) forming laminated packstones with intralaminar normal gradation. These deposits are interpreted as proximal and distal tempestites. Sparse elements are typical of middle-ramp and proximal outer-ramp facies in HST deposits, whereas “Saccocoma microfacies” is typically found in outer-ramp facies associated with TST deposits. Taphonomic and sedimentologic studies clearly demonstrate that the differential hydrodynamic behaviour of the different elements is minimal. However, the dominance (relative to the ideal ratio in Saccocoma tenella) of primaxilar (IBr2) and secundibrachials (IIBr2, IIBr4-7 and IIBr8-17) on radials (R), primibrachials (IBr1), and non-ornamented secundibrachials (IIBr1 and IIBR 3), could be used as a distally index. Additionally, future detailed studies of the temporal distribution of genera Saccocoma and Crassicoma could contribute to the biostratigraphy of the Tithonian of the Neuquén Basin.