THE LATE ATOKAN (MOSCOVIAN, PENNSYLVANIAN) CHAETETID ACCUMULATIONS OF SIERRA AGUA VERDE, SONORA (NW MEXICO): COMPOSITION, FACIES AND PALAEOENVIRONMENTAL SIGNALS

Abstract

The La Joya Formation of the Sierra Agua Verde, Sonora (NW Mexico) is late Atokan in age, equivalent to the early late Moscovian (Podolskian) and fusulinid biozone A3. In this alternance of cherty limestones and thin shaly beds, fusulinellid or anchicodiacean ('phylloid algae') wackestones-packstones and crinoidal rudstones-gramstones are the predominant microfacies, but chaetetid boundstones are conspicuous. These chaetetid occurrences of the Sierra Agua Verde are compared with the accumulations of Arizona, Texas, Kansas and Nevada (USA), and the Cantabric Cordillera (Spain). In Sonora, the environments with chaetetids were quiet, and located below wave base. Shallower facies with staffellids and Komia generally top the chaetetids. Because of the associated micritic deposits, the chaetetids have inhabited probably a soft or firm substrate. As a result of the disphotic-aphotic reconstructed environments, the possible symbionts of the chaetetids are more probably heterotrophic bacteria than autotrophic algae. The most comparable ecological conditions exist in the Atokan Marble Falls Formation of central Texas (USA). Chaetetids are not mentioned in the southern suspect terranes of Mexico, but were possibly present because these regions were located along the probable migration way to Peru, the southernmost area where Pennsylvanian chaetetids are known.