'Autochthonous' bivalve assemblages and palaeoecologic interpretation in the upper Jurassic-Lowermost Cretaceous La Caja Formation from the Canon de San Matias (Zacatecas, Mexico)

The present research approaches the precise palaeoecologic interpretation based on the recognition of 12 'autochthonous' bivalve assemblages among 52 registered from the Upper Jurassic-Lowermost Cretaceous La Caja Formation in the Canon de San Matias section (Mexican Altiplano). Fine-grained deposits together with the near-exclusive record of suspension-feeders are considered the evidence for low-energy and nutrient-rich environments. The presence of oxygen-dependent bivalves, mainly Buchia and Lucina, supports the interpretation of oxygenation as a major factor controlling ecology and therefore the composition of the 'autochthonous' bivalve assemblages studied. Significant changes in the composition of bivalve assemblages show no relation between fluctuations in ecological factors and background depositional conditions. (C) 2000 Academie des sciences / Editions scientifiques et medicates Elsevier SAS