High-resolution cyclostratigraphy analysis from a Tithonian alternating marl-limestone succession, Vaca Muerta Formation, Neuquén Basin, Mendoza, Argentina.

2. PALMA, R., MARTIN-CHIVELET, J., LÓPEZ GÓMEZ, J., CONCHEYRO A. Y LESCANO, M. 2008

- Hammamet (Tunisie)

Simposio; 5 International Symposium of IGCP 506; 2008

IUGS-IGCP

The Neuquen back-arc Basin is located on the west margin of South American platform between latitude of 36O and 40° S. The basin is worldwide famous for its continuous sedimentary from the Late Triassic to Cenozoic record comprising continental and marine clastic, carbonate and evaporitic deposits which are up to 2,600 m in thickness. The Tithonian-Valanginian deposits are represented by the Vaca Muerta Formation, which is widely distributed over all the Neuquen Basin, and consists of dark bituminous shales, marls and limestones cycles, which have been recently  become the focus of a regarding interpretation of the cycles as responses to Milankovitchian influences. Preliminary results of cyclostratigraphy based on sedirnentological and biostratigraphical studies on a well exposed Tithonian Vaca Muerta outcrops of three selected sections (Loncoche Creek, Salado River, and Quebrada Amarilla) suggest that the whole section developed during a time-interval spanning from the Lower Tithonian (Virgatosphinctes mendozanus) to the Upper Tithonian (Substeueroceras koeneni Zone). Nevertheless, the recognition of detailed studies of ammonite biostratigraphy from the Salado River section (184 m) shows lower Upper Tithonain (Corongoceras alternas Zone) up to Upper Tithonian (Substeueroceras koeneni). The three sections are characterized by a cmldm scale alternation of limestones and marls. From the beginning of Tithonian, beds in the Quebrada Amarilla shows no signs of fauna1 mixing and seems to be stratigraphically complete, but it represents a stratigraphical condensed section in relationships with another studies successions (Loncoche Creek, and Salado  River). The nannofossil assemblage is typical for the Upper Jurassic that include Cyclagelosphaera deflandrei, C. margerelii, Diazomatolithus lehmanii, D. galicianus, Watzanueria barnesiae, W. britannica, W. fossacincta, W. ovata, Zeughrabdotus embergerii, and Z. erectus. Polycostella beckmanii, a Jurassic reliable marker has been also found. Spectral analysis of the Tithonian succession reveals the same signal for the studies outcrops. In the Salado River section (184 m), the couplet thickness varies between 6 and 78 cm, at a rate of 24 rnlMa. The detected periodicities are 147 ka, 118 ka, 102 ka, 54 ka, 48 ka, 36 ka, 20 ka, and 22 ka. The Loncoche section (154 m) the couplet thickness varies between 8 and 81 cm, at a rate of 96 m1Ma. The detected periodicities are 398 ka, 127 ka, 91 ka, 78 ka, 34 ka, 18 ka and 22 ka. The Quebrada Amarilla section (89m) the couplet thickness varies between 5 and 45 cm, at a rate of 17m IMa. The detected periodicities are 77 ka, 58 ka, 44 ka, 14 ka, and 23 ka. The results of the  spectral analysis are strong indications that the Vaca Muerta succession in the Neuquen Basin was deposited under the influence of climatic changes induced by the orbital parameters. The peaks of 398 ka, 147 ka and 102 ka are related to eccentricity to the earth orbit. The peaks of 77 ka, 58 ka, 44 ka and 46 ka could be ascribed to the obliquity of the earth axis. The peaks of 22 ka, 18 ka, ka 23 ka, and 14 ka cycles are very close to them obtained from the precession values.W. ovata, Zeughrabdotus embergerii, and Z. erectus. Polycostella beckmanii, a Jurassic reliable marker has been also found. Spectral analysis of the Tithonian succession reveals the same signal for the studies outcrops. In the Salado River section (184 m), the couplet thickness varies between 6 and 78 cm, at a rate of 24 rnlMa. The detected periodicities are 147 ka, 118 ka, 102 ka, 54 ka, 48 ka, 36 ka, 20 ka, and 22 ka. The Loncoche section (154 m) the couplet thickness varies between 8 and 81 cm, at a rate of 96 m1Ma. The detected periodicities are 398 ka, 127 ka, 91 ka, 78 ka, 34 ka, 18 ka and 22 ka. The Quebrada Amarilla section (89m) the couplet thickness varies between 5 and 45 cm, at a rate of 17m IMa. The detected periodicities are 77 ka, 58 ka, 44 ka, 14 ka, and 23 ka. The results of the  spectral analysis are strong indications that the Vaca Muerta succession in the Neuquen Basin was deposited under the influence of climatic changes induced by the orbital parameters. The peaks of 398 ka, 147 ka and 102 ka are related to eccentricity to the earth orbit. The peaks of 77 ka, 58 ka, 44 ka and 46 ka could be ascribed to the obliquity of the earth axis. The peaks of 22 ka, 18 ka, ka 23 ka, and 14 ka cycles are very close to them obtained from the precession values.