Evidence of precessional and eccentricity orbital cycles in a Tithonian source rock: The mid-outer carbonate ramp of the Vaca MuertaFormation, northern Neuquén Basin, Argentina

KIETZMANN, D.; CHIVELET, M.; PALMA, R.; LOPEZ GOMEZ, J.; LESCANO, M.; CONCHEYRO, A.

AAPG BULLETIN

AMER ASSOC PETROLEUM GEOLOGIST

Año: 2011 vol. 95 p. 1459 - 1474

0149-1423

Basin to mid-ramp cyclic facies of the Tithonian Vaca Muerta Formation are exposed in the Loncoche creek section of the Neuquén Basin, Mendoza province, Argentina. This unit is characterized by a decimetre-scale rhythmic alternation of marls/shales and limestones, and extends from the early Tithonian to late Berriasian. 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 caused by the Earth?s orbital eccentricity. 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.