Preferencje help
Widoczny [Schowaj] Abstrakt
Liczba wyników

Znaleziono wyników: 3

Liczba wyników
first rewind previous Strona / 1 next fast forward last
Wyniki wyszukiwania
Wyszukiwano:
w słowach kluczowych:  Southern Tethyan margin
help Sortuj według:

help Ogranicz wyniki do:
In the southern Tethyan margin, the Essaouira-Agadir Basin (EAB), south of Morocco, exhibits well-exposed and fossiliferous sections of Aptian–Albian age. Biostratigraphy by ammonoids and sedimentological analysis have been realized for five sections located along an E-W transect in the EAB. The studied successions were dated from the latest Early Aptian to the Early Albian and are characterized by five major sedimentary discontinuities defining at least four main sedimentary sequences. The Late Aptian–Early Albian succession can be considered a gently westward-dipping ramp, marked by a deepening upward evolution. A quantitative study of calcareous nannofossils and calcium carbonate content has been performed on three of these sections. At this time, the EAB was located in the tropical-equatorial hot arid belt. The decrease in both calcium carbonate content and Nannoconus abundances at the Aptian–Albian transition could be the result of cooler climatic conditions recognized in the EAB, and/or of the associated increasing terrigenous input and nutrients, which hindered carbonate production. In the EAB, the nannofossil productivity is higher below the deposition of dark levels, which are coeval with the Niveau Paquier, recognized as the expression in southern France of the OAE 1b (Early Albian). During the Early Albian, the EAB was characterized by nannofossil fluxes two times lower than the upwelling-influenced Mazagan Plateau (southern Tethyan margin) and eight times lower than the Vocontian Basin (northern Tethyan margin). These results show that, with respect to the northern Tethyan margin, trophic conditions in sea surface waters of the pelagic realm of the southern Tethyan margin were lower. Comparable results obtained by Heldt et al. in the neritic realm of the southern Tethyan margin have been ascribed to more arid climatic conditions.
The Aptian sedimentary succession of the Chott region in southern Tunisia was deposited on the margin of the Saharan shield, and is punctuated by numerous hiatuses that separate seven 3rd-order depositional sequences. Early Aptian deposits correspond to the Berrani Member (early Bedoulian), which was deposited contemporaneously with the large carbonate platforms with rudists that developed under oligo-mesotrophic, tropical environmental conditions on both margins of the Tethys. Late Bedoulian sediments were deposited under mesotrophic conditions characterized by seagrass, algae, abundant orbitolinids and aragonite-producing organisms. The early to late Aptian transition was marked by the temporary disappearance of carbonate platforms and an important renewal of the microfauna, whose tests and skeletons became less and less aragonite-rich and more and more calcite-rich and arenaceous. This episode is reported from both Tethyan margins. The platform was subsequently flooded, and dysoxic environments with annelids marked the end of the early Aptian carbonate platform development. An arid and probably colder episode (earliest late Aptian) induced the deposition of gypsum in an intrashelf basin. Following on top, the return to more humid conditions triggered an enhanced input of detrital material in fluvio-deltaic environments (late Aptian). Finally, the return to oligo-mesotrophic, marine conditions allowed the temporary installation of wide lagoons with rudists in the latest Aptian and probably in the earliest Albian. Long-distance correlations have been established by means of benthic foraminiferal occurrences. They highlight the importance of stratigraphic gaps linked to low sea levels, which have been tentatively estimated.
Les dépôts d'âge sénonien affleurant au Jebel M'rhila (Tunisie centrale) ont enregistré les indices d'oscillations eustatiques déterminant six séquences de dépôt de troisième ordre qui sont totalement ou partiellement corrélables avec les cycles globaux. En se basant sur la répartition de la faune, notamment celle des foraminifères planctoniques et des ostracodes, les séquences de troisième ordre correspondantes sont attribuées à l'intervalle du Coniacien supérieur à Maastrichtien p.p. Simultanément, la mise en place de ces séquences a été contrôlée par l'effet d'une tectonique distensive d'orientation NO–SE qui a prévalu dans l'ensemble de Tunisie centrale. Cette tectonique syn-sédimentaire a marqué en particulier la paléogéographie du secteur du Jebel M'rhila, en engendrant une topographie sous-marine qui s'échelonne en gradins de part et d'autre d'une zone haute ayant émergé depuis le Turonien supérieur. Cette géométrie du plancher sédimentaire caractérisant un paléoenvironnement marin de type circa-littoral, avait permis des écoulements turbiditiques épisodiques de matériels carbonatés et silico-clastiques. Ainsi, des dépôts gravitaires ont été remobilisés à partir de hauts-fonds dans des creux bordiers. Le secteur du Jebel M'rhila est un exemple local mais représentatif du reste de la marge sud-téthysienne.
first rewind previous Strona / 1 next fast forward last
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.