Orbital cycles of an Early Tithonian – Early Valanginian carbonate ramp (Vaca Muerta Formation), Neuquén Basin, Argentina

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

Zaragoza

Congreso; 28 Meeting of sedimentologists; 2011

IAS

The Neuquén Basin was a Mesozoic back-arc basin located on the western margin of Gondwana. During Late Jurassic and Early Cretaceous a series of marine sequences were developed throughout the basin, in response to rapid ingression from Pacific Ocean, and are grouped under the Mendoza Mesosequence. Basin to mid-ramp cyclic facies of the Early Tithonian – Early Valanginian Vaca Muerta Formation are exposed in the southern Mendoza area of the Neuquén Basin, Argentina. In this study we analyze five sections of this unit, from north to south: La Manga Creek (173 m, partially), Loncoche Creek (316 m), Cuesta del Chihuido (185 m), La Mula Creek (176 m) and Cara Cura Creek (320 m). The Vaca Muerta Formation is characterized by a decimetre-scale rhythmic alternation of marls/shales and limestones, and consists of four main facies associations, which reflect different paleoenvironmental conditions. Facies Association I consists of black shales, deposited under anoxic conditions, below storm-wave base. Facies association II consists of alternating brown laminated shales, and lime mudstones-wackestones, deposited in a dysaerobic to aerobic low energy environment. Facies association III, dominated by marls and bioclastic wackestones, is characterized by higher contents in carbonate than the previous associations, and was deposited in aerobic to dysaerobic outer ramp environment. Facies association IV is dominated by peloidal wackestones-packstones, oyster biostromes, and marls. This facies was deposited in well-oxygenated muddy bottoms located within the storm wave-base. Cyclostratigraphic studies based on a detailed facies analysis allowed the identification of cyclic patterns with frequencies within the Milankovitch band. According to biostratigraphic data, the dominant cycle in the studied section has a period of 20 ky, which correlates with the Earth’s axis precession element. Spectral analysis based on series of compacted and decompacted cycle thickness identified a subordinate frequency of about 90-120 ky, which we interpret as the modulation of the precessional cycle by the high frequency Earth’s orbital eccentricity. Cycles are probably driven by change in carbonate exportation, as changes in shallow-water carbonate production involve changes in carbonate basinward exportation. Climatic or paleoceanographic changes controlled by low frequency orbital eccentricity seem to have a lesser importance, and those controlled by the obliquity have not been identified in this study. The strength of the precession signal together with the absence of a well-defined cyclicity attributable to the obliquity orbital cycle (i.e. ~ 40 ky) is in agreement with previous data from the Northern Hemisphere.