PALEOENVIRONMENTAL RECONSTRUCTION OF AN EXTRAORDINARY MID-CRETACEOUS BIOTA, TEPEXI DE RODRIGUEZ, PUEBLA, MEXICO

Statistically based microfacies-transition analysis as well as spectral analysis of depth-series measurements were combined to study the paleoenvironments of Tepexi de Rodrigues (Tepexi), an exceptionally fossiliferous quarry in Puebla, Mexico, exposing finely laminated carbonates of mid-Cretaceous age. A 30 m core cut from the quarry face served as the basis of this study.

Tepexi is composed of a complex amalgam of primary and diagenetically altered fabrics. Analysis of microfacies successions via a log linear model of quasi-independence allowed the reconstruction of repetitive patterns among primary microfacies. The reconstructed fining-upward microfacies sequence -- wackestone->mudstone with mosaic appearance->mudstone -- is comparable to the distal parts of fine-grained turbidites and/or suspension deposits related to episodic events.

Spectral analysis of depth-series measurements of magnetic susceptibility and RGB visible color indicate repetitive bedding in the sedimentary sequence that is concordant with patterns of Milankovitch cyclicity, implying external Milankovich forcing of sedimentary processes. Strong eccentricity, obliquity, and semi-precessional signals are recognized; however, a statistically significant precessional signal is absent. This unique suite of Milankovitch cyclicities is attributed to double-monsoon influences on Tepexi from both hemispheres.

Our observations are at odds with the binary bedding types and high rates of deposition reported from other localities of exceptional preservation, such as Solnhofen and the Hamilton Lagerstätte. The overall thickness of Tepexi coupled with the presence of Milankovitch rhythmicity allows us to calculate an average rock accumulation rate of 1.9 cm/year.

A possible taphonomic interpretation of Tepexi would be that the exceptional preservation at Tepexi was caused by dysaerobic or anaerobic conditions due to stagnation at the bottom of a relatively deep basin. If so, the diverse reef-associated biota which must have lived in marginal reef banks, as well as the terrestrial organisms found with it, were episodically carried into the basin by floatation.