BARREMIAN PLANKTIC FORAMINIFERAL BIOSTRATIGRAPHY OF THE SAN LUCAS FORMATION, MICHOACAN (SW MEXICO)

Most of southwestern of Mexico is characterized by magmatic volcaniclastic and sedimentary sequence typical of island arcs. The San Lucas Formation crops out in the Tiringueo section near San Lucas town (100º 55’, 100º 45’W and 18º 40’, 18º 30’N). This section is a flysch sequence that consists of a frequently rhythmic deposit of shale, siltstone and fine to medium graded sandstone interbedded with limestone. The San Lucas succession contains ammonites, foraminifers and radiolarians. The foraminiferal assemblage is usually composed by a few specimens poorly preserved. However, some important events as the first occurrence of Hedbergella similis and Globigerinelloides blowi could be correlated with ammonite zones. Biostratigraphical data from the foraminiferal study, permit to establish two planktic zones of Barremian age. The Hedbergella similis Zone (SL-0 to SL-2C) was defined by the first occurrence of the nominal taxon with Gorbachikella kugleri, Hedbergella kuznetzovae, H. aptica, H. sigali, H. delrioensis, H. aptiana and Globigerinelloides sp. The size of the planktic foraminifera was about 125 -150 micras (Sample SL-0). In this interval the benthic foraminifer Gavellinella barremiana and abundant radiolarians have been observed (Sample SL-2A). Hedbergella similis appeared in the early Barremian (Kotetishvilia nicklesi Zone) and the fauna was abundant and increased its size. The Globigerinelloides blowi Zone (SL-2C´ to SL-11) was delimited by the first appearance of Globigerinelloides blowi and Hedbergella excelsa. In this zone, Hedbergella aptiana, H. sigali, H. similis and H. delrioensis are also present (Sample SL-2C´). Globigerinelloides blowi first appears at the middle of the ammonite Ancyloceras vandenheckii Zone in the late Barremian. Globigerinelloides aptiense and acme zone of the same genus were setting in the ammonite Gerhardtia sartousiana Zone (Sample SL-11). The San Lucas Barremian sequence was deposited in pelagic environmental conditions based on the faunal assemblage and the distinctive lithological characteristics.