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The Triassic-Jurassic transition in Asturias (northern Spain): ammonoids, bivalves and palynomorphs

Barrón E., Gómez J., Goy A., Márquez-Aliaga A.

Volumina Jurassica

2006

Vol. 4, no. 1

275-276

A study of the Triassic/Jurassic transition in Asturias (northern Spain) was performed studying 5 surface sections and 2 boreholes (Fig. 1). Four lithological units were differentiated and correlated. The lower unit is composed of lutites and evaporites (equivalent to the Upper Triassic Keuper facies). The middle unit, which contains the Triassic/Jurassic boundary, corresponds to the well-bedded carbonates of the Solis Member of the Gijon Formation (Fig. 2). The upper unit (Barzana Member) is composed of lutites, evaporites and carbonates. The Fabares Member, overlying or representing a time equivalent of the Barzana Member is composed of a carbonate breccia with a lutitic matrix, formed by the dissolution and the collapse of the Barzana Member. The finding of ammonoids and bivalves in the Solis Member allowed refinement of the biostratigraphy and chronostratigraphy of the Rhaetian and the Hettangian. Among the ammonoids, the record of an (?)Arcestidae (Rhaetian) and several Psiloceras (such as Caloceras pirondii (Reyn?s) of the Hettangian, Planorbis Zone, in the upper part of the Solís Member can be stand out. The dominant Rhaetian bivalves are Isocyprina concentrica and Bakevellia praecursor, which together with Isocyprina ewaldi, "Pteromorphus" elongatus, Pteromya longportensis, "Placunopsis" cf. alpina, Modiolus minimus and Paleocardita cf. austriaca, represent a similar assemblage to that found in the Westbury and Lilstok formations of the Late Rhaetian age of the UK. The Hettangian bivalves are represented by the widely distributed species Pteromya tatei, associated with Cuneigervillia rhombica, Parallelodon hettangiensis and Eomiodos menkei. This assemblage is known in the Hettangian Planorbis Zone of France and Germany. The palynological study allowed identification of 20 spores taxa, 24 pollen taxa, 1 acritarch, 2 prasinophytes and 2 dinoflagellate cyst. Three palynological assemblages (PA) have been distinguished. PA1 is typically Rhaetian and corresponds to the Rhaetipollis germanicus Zone. It is characterized by the presence of Corollina meyeriana, Rhaetipollis germanicus, Ovalipollis pseudoalatus and Tsugaepollenites pseudomassulae. PA2, identified in the Solis Member, is dominated by Corollina pollen grains together with a few spores taxa, acritachs and prasinophytes and can be Rhaetian and/or Hettangian in age. PA3 represented in the upper part of Solis Member and in the Barzana Member contains Hettangian pollen assemblages characterized by Corollina meyeriana, C. torosa, Kraeuselisporites reissingeri, Ischyosporites variegatus and Cerebropollenites thiergartii. Both PA2 and PA3 can be related to the Kraeuselisporites reissingeri Zone. The palynology of the Asturian sections can reasonably be correlated with that of St. Audrie's Bay (UK).