Bivalvos del Triásico Superior en la Formación Imón (Cordillera Ibérica, España)

Abstract

A bivalve association with elements of five orders from upper part of "Dolomías tableadas de Imón" Formation (Goy, Gómez & Yebenes, 1976) is studied. The section with the bivalves record is placed in the type area and its finding is of great interest for both biostratigraphic and paleoecology of Iberian range Upper Triassic. From a taphonomical point of view, the bivalves are preserved as little internal molds keeping external shell characters only in a few specimens. Reelaborated elements have not been recorded. According to that, the production can be recognized as authoctonous. Howewer, resedimentation can be observed in some specimens of genus Rhaetavicula with desarticulate valves and in some Myophoridae which have articulated valves and sediment filling. The benthic association consists of suspension feeding bivalves with 38% of epibyssate, and an amount of infaunal species. The assemblage shows a general dwarfed character of its specimens. A supratidal or intertidal environmnent with a probably anormal salinity is pointed out. The studied bivalve association is formed by Nuculana cf. borsoni, Rhaetavicula cf. contorta, Isognomon cf. exilis, Neoschizodus reziae, Myophoriopis isosceles, Pseudocorbula alpina, Protocardia rhaetica and Laternula cf. amicii. Rhaetian R. contorta has been recognized. The rest of the cited species has been assigned to the Rhaetian or to the Norian in different alpine localities. Some of them, as N. reziae, are very common in the lower part of "Avicula contorta levels", in the Apenine and Alpine ranges of Lower Rhaetian. Therefore, this association could characterize the Upper Triassic (Rhaetian).