Paleoecology of cambrian communities of central Sonora, Mexico: Paleoenvironmental and biostratigraphic considerations

摘要

Cambrian marine ecosystems in Mexico are constituted by diverse taxonomic groups, including trace fossils, poriferans, brachiopods, hyolithids, trilobites, echinoderms and other problematic fauna. The present research aims to identify and analyze the paleoecological characteristics of the marine ecosystems of central Sonora, Mexico; specifically, in the San Jose de Gracia area where the Proveedora, Buelna, Cerro Prieto and El Gavilan formations are exposed. In this study some ecological indices have been applied, such as Simpson's dominance and evenness, Shannon H' diversity, Menhinick species richness and Chao total species richness. Guilds and adaptive strategies show how different environments were occupied through time. The present work constitutes the first paleoecological work for the marine ecosystems of Cambrian strata in Mexico.The Proveedora Formation consists of fine-grained quartzarenite with high bioturbation and trace fossils, it shows a low species richness (4) and evenness, where suspension feeders are dominating. The paleoenvironmental analysis of this unit indicates a sandy environment, such as a coastal and sublittoral areas, sandbars or delta front. The Buelna Formation is composed of limestone interbedded with shale, containing algae, brachiopods, salterellids, hyolithids, trilobites and echinoderms. Mostly it possesses a medium species richness (9) and intermediate evenness. Functional groups are dominated by primary producers and suspension feeders. The paleoenvironmental analysis of this unit indicates a shallow marine environment. The Cerro Prieto Formation consists of an oolitic limestone strongly recrystallized with few fossils, mainly brachiopods and trilobites. It has a low species richness (2) distributed in an unevenness way since only one species occupies the whole site. It indicates a shallow marine environment of high energy. The El Gavilan Formation consists of a thick sequence of shale interbedded with limestone, having abundant fossils like poriferans, brachiopods, hyolithids and trilobites, with a species richness (16) and with a high evenness, exhibiting a great variety of functional groups, dominating the suspension feeders and nektobenthic forms. The paleoenvironmental analysis of this unit shows an open shelf environment.