Palaeogeography, physiography, palaeoenvironments and brachiopod communities. Example of the Liassic brachiopods in the Western Tethys

Abstract

Within the Western different and characteristic brachiopod communities, as well as other benthic and necto-benthic faunas, have evolved in various palaeoenvironments resulting from the differentiation of a highly controlled submarine topography. This physiography depends on palaeogeographical conditions which were induced by eustatic changes, exaggerated or lessened by local tectonic movements. The characteristics of brachiopod communities, their location in the tectono-sedimentary context, the reconstruction of biotooe conditions from the study of sediments, allow us to propose a pattern in which the relationships between the various characteristic brachiopod communities and the different palaeoenvironments are established. These environmental settings include: (1) inner carbonate platforms; (2) protected carbonate ramps and mud flats; (3) hinge zones between mud flats and basins; (4) low-energy outer platforms and gentle slopes; (5) medium- to high-energy outer platforms; (6) episodically waved and winnowed outer platforms and shoal borders; (7) slopes (transitional to basin); (8) umbilicus and narrow troughs; (9) epioceanic basins. Then, according to this model, the recognition and the distribution through time and space of these different brachiopod communities, makes it possible to reconstruct the palaeogeographical evolution, and to give a more accurate of the tectonic framework (buried antithetic faults) when the palaeogeographical evolution is mainly due to tilted-blocks tectonic (example of the Portuguese Lias).