Orbital calibration of the early Tithonian - early Valanginian in the Neuquén Basin

Abstract

Afloating orbital scale from 10 stratigraphic sections of the early Tithonian ?early Valanginian Vaca Muerta Formation has been constructed from the southernMendoza area of the Neuquén Basin. The Vaca Muerta Formation is characterizedby decimetre-scale rhythmic alternations of marls, shales and limestones, showinga well-ordered hierarchy of cycles within the Milankovitch frequency band (elementarycycles, bundles and superbundles). According to biostratigraphic data,elementary cycles have a periodicity of ~21 ky, which correlates with theprecession cycle of Earth?s axis. A time series was built from the thickness ofelementary cycles, and corrected by removing eustatic-generated trends, inorder to isolate the orbital signal. Spectral analysis based on Blackman-Tukeymethod allows us to identify frequencies of ~90-120 ky and ~400 ky, which weinterpret as the modulation of the precessional cycle by the Earth?s orbitaleccentricity. The floating orbital scale was constructed by high and lowfrequency eccentricity cycles, and tied to the international time scale at thebase of the Windhauseniceras internispinosumand Neocomites wichmanni Zones, wherethere is largest agreement on the biostratigraphy. We obtained the followingcorrelation of ammonite zones: 1) The Virgatosphinctesmendozanus Zone is probably located at the base of the middle Tithonian(upper part of the Semiforme to Fallauxi Standard Zone), but theinconsistency with paleomagnetic data would suggest an older age. 2) The Pesudolissoceras zittelli and Aulacosphinctes proximus Zones correlateswith the middle Tithonian Fallauxi toPonti Standard Zones. 3) The Windhauseniceras internispinosum Zonewould be located at the base of the upper Tithonian Microcanthum Standard Zone. 4) The Corongoceras alternans Zone coincides with the upper Tithonian (Microcanthum Standard Zone and lowerpart of the Durangites StandardZone). 5) The Substeueroceras koeneniZone would correlate with the DurangitesStandard Zone and the lower part of the OccitanicaStandard Zone. 6) The Argentinicerasnoduliferum Zone correlates with the upper part of the Occitanica Standard Zone, and The Spiticeras damesi Zone correlates with the Boissieri Standard Zone.