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Research Item (3)
The Tafna Basin corresponds to the lowlands, which are located in front of Tessala and Traras ranges, below the Tlemcen mountains, Algeria. This basin displays a complete sedimentary cycle dominated by lagoonal-fluvial and marine deposits. The continental formations located at the base of these deposits are mainly composed of alternating sandstones and clays. An early late Miocene age has been previously attributed to them, based on direct correlations with marine deposits. Search for micromammal fossils led to the discovery of three different rodent species from a single level of the Djebel Guetaf section, located at the bottom of these deposits. The rodent assemblage indicates a late Miocene age. Combined magnetostratigraphical and biostratigraphical investigations were carried out to provide a more accurate age control of these continental deposits. Sixty-four oriented rock samples were collected for a magnetostratigraphic study along a 92 meters thick section including the fossiliferous layer. Rock magnetic investigations indicate the presence of both high and low coercivity minerals. Specimens subjected to progressive thermal demagnetization procedures show that the samples exhibit a high temperature magnetization component and display a normal polarity. Based on biostratigraphic constraints, the Guetaf section is correlated with Chron C4An, indicating an age ranging from 9.1 to 8.7 Myr. chronology, a biostratigraphic study was combined to a mag-netostratigraphic study. The purpose of this paper is to refine the dating of these deposits and to establish precise correlations between the continental deposits of this basin and other North African basins. This study is worthwhile, since it is the first to be carried out in the Neogene continental deposits of Tafna Basin. The establishment of a new correlation within the late Miocene will be also used as a reference to assess correlations between fossil faunas of numerous North African Neogene basins. GEOLOGICAL SETTING The Tafna Basin belongs to the Tellian Zone, whose structural evolution corresponds to the Alpine phase. This basin acquired its morphogenesis characteristics during the early Miocene (Fenest, 1975). During the late Miocene, the sea receded and a continental sedimentation began with the deposition of an abundant detrital material with heterogeneous series deposited in depressions (Guardia, 1975; Saint Martin, 1990). The continental deposits, which are represented by conglomerates, sandstones, blackish clays and marls (Belkebir et al., 1996), expand across throughout the basin with an average thickness of 150 m. The continental episode ended with the Messinian, which is marked by a marine transgressive episode, as a shallow sea covers the entire basin. Biostratigraphic and magnetostratigraphic studies were carried out on the continental deposits, in a place referred here as Djebel Guetaf (Fig. 2). The studied section is located on the left bank of the Tafna River at 120 km West of Oran and 30 km North of Tlemcen. This section has been chosen because of the large outcrops and the occurrence of micromammal remains in
The Early Eocene deposits of El Kohol, Algeria, have yielded numerous remains of Numi-dotherium koholense, one of the most primitive and oldest known proboscideans in Africa.The Upper Eocene proboscideans of the Fayum locality (Egypt), Barytherium sp. andMoeritherium sp., were recently interpreted as aquatic or semi-aquatic, according to thestable isotopic compositions (ı13C and ı18O) of their tooth enamel. These data led us toreinvestigate the adaptations of N. koholense. Stable isotopic analysis and observations ofhistological sections of its long bones reveal that it was essentially terrestrial. Accordingto its position within the phylogenetic tree of Eocene proboscideans, the adaptation tosemi-aquatic life appears to have evolved independently in different lineages of Middleand Upper Eocene proboscideans during their adaptive radiation in Africa. Moreover, thesenew results reopen the debate about the hypothesis that Eocene to Recent proboscideansare derived from semi-aquatic ancestors.
Despite numerous discoveries that have considerably enriched the African-Arabian Tertiary fossil record over the last decades, our knowledge of the evolutionary history of many continental African vertebrate groups during the Paleogene period remains inade-quate, particularly when it is compared with the fossil records of Europe or North Amer-ica. The Eocene Epoch in Africa is especially poorly documented, being restricted to few fossiliferous localities. Our understanding of the early Tertiary emergence, diversifi cation, and paleobiogeographic history of African-Arabian mammals has been further hindered by the lack of a precise temporal framework for these sites. We conducted magnetostratigraphic analy-ses, associated with biostratigraphic studies, in the fossiliferous sequences exposed in the northwestern Hammadas of the Saharan Platform in the Glib Zegdou area and in the Saharan Atlas at the El Kohol locality (Algeria) to further defi ne the age of these Eocene continental deposits. Based on bio-stratigraphic constraints, the six polarity zones identifi ed in the El Kohol section can be correlated with chrons C24n to C22r, pro-viding the fi rst direct age estimates for the El Kohol fossiliferous strata between 52 and 51 Ma. Correlation to the geomagnetic po-larity time scale, using previously published biostratigraphic data for the Glib Zegdou fauna, suggests an age ranging between 49 and 45 Ma for this section. The high-resolution magnetostratigraphic study of the poorly known continental Eo-cene Epoch of Algeria provides new in-sights into the early Tertiary stratigraphy of northwest Africa. The placement of the Algerian localities into a consistent chrono-logical framework constitutes considerable advancement to achieve biostratigraphic cor-relation of the Paleogene African-Arabian mammal localities.