Antérieurement attribuées à l'Albo-Aptien et considérées comme résultant du diapirisme des évaporites triasiques, les brèches de Bosmendiette se seraient, pour certains auteurs récents, mises en place au Sélandien (Paléocène), dans des canyons sousmarins établis au sein d'un profond sillon, le long de la chaîne des Pyrénées, du Roussillon jusqu'à l'Atlantique. Leur âge serait fondé sur la découverte d'une riche microfaune pélagique de globigérinidés. Nos recherches montrent qu'azoïques à leur base, pourvues de radiolaires et de rares foraminifères benthiques au sommet, ces brèches résultent en réalité de la destruction de boucles de slumps affectant l'« Infralias ». Recouvertes en onlap par les calcaires et marnes de l'Albo-Aptien, elles traduisent, conformément à nos précédentes interprétations, l'effondrement par collapse et le glissement vers le nord, en milieu marin graduellement ouvert, de la plate-forme jurassique, lors de la mise en place du diapir de Béloscare-Apoura, au cours du Crétacé inférieur.
The Early Cretaceous series of Site 549 have been analyzed from both an ecological and a chronostratigraphical viewpoint. Lithology and microfaunal assemblages allow the division of these deposits into two parts. The lower part extends from the lower Barremian to the basal upper Barremian and possibly down into the upper Hauterivian. Three foraminiferal associations suggest an orderly evolution from a low-energy, littoral environment, in which continental influences dominated, to progressively more open marine conditions. A dolosparite bed (which has not yet been dated) separates these synrift deposits from lower to basal middle Albian postrift sediments that developed in a pelagic bathyal environment on the middle to nearby lower continental slope.
This study is devoted to the lower series (Barremian-lower Albian) of Hole 549, just west of the continental shelf of northwestern Europe (Leg 80 of DSDP-IPOD). The lithological analysis is correlated with downhole logging, X-ray mineralogy, foraminiferal assemblages, and biological content. Biosedimentary environments and general pattern of distribution are interpreted and compared with Cretaceous exposures of northern Spain. Two successive paleogeographic phases are identified: a short transgressive episode giving a shallow environment (Barremian), and the opening (lower Albian) of a long period of pelagic sedimentation in a bathyal environment. They were separated by a gap in deposition, so that the record of intervening events is lost. The lower series begins with a record of marine transgression. The sporadic occurrences of oysters indicate a lowering of salinity. As the transgression became more extensive, the sediments became more calcareous, forming part of a carbonate platform. Trocholina species are associated with encrusting and other benthic foraminifers, bryozoans, corals, and algae. Subsequent deposition could not keep pace with subsidence, and the water deepened, as indicated by the presence of the planktonic foraminifer Hedbergella in association with small benthic foraminifers in a clayey-silty sediment interpreted as a quiet muddy seafloor. Clastic material is very mature, implying provenance in a very weathered area. In the lower part of the column kaolinite was the main mineral in the fraction smaller than 2 µm. In the carbonate sediments, kaolinite has been replaced by smectite. The second phase is characterized by planktonic foraminifers of the genus Hedbergella and by radiolarians, reflecting increased subsidence and bathyal conditions. The general trend of this evolution is chronologically in accordance with that known from the southern margin (Spain margin) of Biscay Bay.
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