Bioestatigrafía del tránsito Triásico-jurásico en Asturias: ammonoideos y bivalvos entre Aviles y Caravia. (España)

Abstract

The finding of ammonoids and bivalves in the deposits of the Triassic-Jurassic (T-J) transition in the Gijón Formation (Asturias) allowed improvement of the biostratigraphy and chronostratigraphy of the Rhaetian and the Hettangian. Among the ammonoids, the record of an Arcestidae ? (Rhae- tian) and several Psiloceras (Hettangian, Planorbis Zone) in the upper part of the Solis Member can be stand out. The dominant Rhaetian bivalves are Isocyprina concentrica and Bakevellia praecursor. These species, together with Isocyprina ewaldi, "Pteromorphus" elongatus, Pteromya longportensis, Placunopsis cf. alpina, Modiolus minimus and Paleocardita cf. austriaca, represents a similar assemblage to that found in the Westbury and Lilstok formations (Penarth Group) in the late Rhaetian of southern England. The Hettangian bivalves are represented by the widely distributed species Pteromya tatei, which is associated to Cuneigervillia rhombica, Parallelodon hettangiensis and Eomiodos menkei. This assemblage is known in the Hettangian Planorbis Zone of France and Germany. As a consequence, the T-J boundary has been located within the upper part of the Solis Member of the Gijón Formation.