Autochthonous to parautochthonous bivalve concentrations within transgressive marginal marine strata of the Upper Jurassic of Portugal

Bivalve concentrations are a conspicuous feature of the Upper Jurassic rock succession of the central Lusitanian Basin. In order to illustrate the mode of their formation and their palaeoecological and sequence stratigraphic significance, we describe and interpret in detail 27 assemblages of bivalves based on their taphonomic and ecological features. Bivalve concentrations are particularly widespread in marginally marine, salinity-controlled environments and less common in open shelf settings. Only the Eomiodon and Myophorella concentrations are dominated by a shallow infaunal bivalve; all other concentrations are composed mainly of epi- or endobyssate [Isognomon (Rostroperna), I. (Isognomon), Pteria, Alaperna, Juranomia] and cementing taxa (Praeexogyra, Actinostreon). Most of the concentrations are mono- to paucispecific, except for the highly diverse Alaperna polita-Pteria credneriana concentrations, which occur in nearshore shelf environments. Lack of sorting, a high percentage of articulated shells, and shells preserved in growth position are evidence that, with few exceptions, the concentrations are autochthonous. Main factors leading to their formation are, apart from a gregarious life habit, a high productivity and a reduced rate of sedimentation. Autogenic and allogenic successional stages can be recognized on the basis of their spatial separation. The time involved in the formation of the concentrations is generally in the order of 102-103 years. Due to the lack of compositional changes within concentrations, time-averaging played no or only a minor role, except in the Alaperna polita-Pteria credneriana concentrations. In sequence stratigraphic terms, most of the concentrations are the product of the maximum flooding zone, but in some cases they constitute the transgressive systems tract. Invariably, they are useful tools for defining depositional sequences. Several of the concentrations described here are not restricted to the Lusitanian Basin but also occur elsewhere in the epicontinental seas bordering the northern margin of the Neotethys.