STRATIGRAPHY AND PALEONTOLOGY OF THE PLIOCENE OF OUED ARJAT. PALEOENVIRONMENTAL CHARACTERISTICS OF THE NEOGENE BASIN OF THE BOU-REGREG (MOROCCO)

G. Alonso-Gavilán*, J. Civis*, J.A. González-Delgado* & R. Ahmed**

*Depto. Geología, Fac. Ciencias, Universidad de Salamanca (España).
**Fac. des Sciences, Univ. Tetouan (Maroc).
 

 In the framework of the stratigraphic and paleontological studies that are being carried out on the Neogene of the Bou-Regreg  basin an analysis was made of the Oued Arjat (Fig. 1), whose sedimentological and paleontological characteristics reveal the dynamic evolution of the Bou-Regreg paleogulf during the Lower Pliocene.

 The stratigraphic section studied shows a visible thickness of 60 m (Fig.2).  The first 50 m are characterised by a succession of fining upward siliciclastic cycles which begin with very well calibrated fine grain sands, with a clayey matrix and of a light yellowish-ocre colour and can finish with sandy silts or clays.  The sequences are bioturbated and can show hydromorphism and limonitisations. The stratigraphic succession shows no determined polarity or architecture, since the cycles vary their thickness throughout the column. Carbonatation is practically non-existent in the lower levels whereas a slight increase is noted towards the top of the section. The sequences are generally massive and the macrofauna is scattered in the sediment. However, in the last ten metres the sands are medium to thick grained, loose and yellowish, and have abundant glauconite clasts and the malacofouna is concentrated in levels, over which there is a bank of calcarenite with glauconite, malacofauna and foraminifera.

 These two members are separated by an erosive surface of slight relief, and the stratum over which it develops reveals vertical bioturbation galleries filled with detritic clasts, glauconite, microfauna and remains of malacofauna.

 From a paleontological point of view, the richness of Molluscs in the upper section is outstanding. The Molluscs are very scarce in the lower silty facies with fragments of Amussium cristatum. In the upper sandy levels, there are some horizons which are very rich in malacofauna. They constitute lenticular composite shell concentrations, shell supported, with variable lateral conservation due to a selective fossil-diagenetic dissolution. The shells have preburial fragmentation (see González-Delgado et al., 1977, in this volumen). The most abundant are the Naticids and the Turritella tricarina  among the Gastropods, and Abra prismatica  and Venerids among the Bivalves.
 With regard to the Foraminifera, their excellent preservation should be mentioned and their high diversity, both in bentonic and planktonic Foraminifera. A thorough systematic and biostratigraphic study of the planktonic Foraminifera was made by Racick-El Bied ( 1990), Rakic-El-Bied & Benson (1996) and Benson  & Rakic-El-Bied (1996).
 In the Oued Arjat section, the marked predominance of planktonic Foraminifera with values reaching over 80% of the total in the lower part is outstanding, a decrease in this value being noted after sample Arjat-20.  Diversity is high and the presence of Globorotalia margaritae  is notable throughout the section, especially in the lower stretch, and after level Arjat-21 it coexists with G. punticulata , which is well represented up to the top. Also, although sporadically, Globorotalia crassaformis  appears, coexisting with the other two forms.
 The bentonic Foraminifera show good preservation and high diversity (over 90 species have been determined). Casts of glauconitic composition are frequent in the upper stretch, often conserving the original morphology of the shell. With regard to assemblages of bentonic Foraminifera, the predominance of Buliminidae, Bolivinidae and Uvigerinidae in the lower part of the basal stretch should be noted, together with a good representation of Planulina ariminensis . Just as in the Salé section (see Civis et al., 1997, in this volume) the predominance of this assemblage of Foraminifera shows fluctuations, with the predominance of Buliminidae and Bolivinidae alternating with Uvigerinidae (at times reaching values of over 25%). These characteristics are maintained up to level Arjat-9, after which an increase is noted in Cibicidoid forms (over 35% of the whole).

 In the upper member (levels Arjat 20-21 and 22) a significant change is noted in the assemblages. Together with the increase in Cibicidoids, Nonion bouanum  and Ammonia  appear (at the top, at times 70% of the assemblage). An increase of Cassidulina laevigata carinata  is also observed at points.
 The stratigraphic, sedimentological and paleontological characteristics indicate that sedimentation of the basal stretch occurred on a shoreface siliciclasitc shelf subjected to dynamic variations of the waters due to an alternation of periods of anoxicity with others of greater oxicity. These characteristics attenuate towards the top of the stretch while a change in the depth of the basin is noted. The upper stretch shows offshore circalittoral shelf conditions, subjected to the action of sporadic storms revealed by the Mollusc species present, the increase in the epifauna, lithological change and changes in the Foraminifera assemblages.
 The transition from the basal stretch to the upper stretch is marked by a major sedimentary rupture (level Arjat-20 B) since the following are observed: a lithological change (sands with glauconite and calcarenites) phosphatization of the fauna, intense vertical bioturbation and galleries filled with Foraminifera and glauconite and it corresponds to a different lithostratigraphic unit. These facts imply the existence of a probable hiatus in the sedimentation and a brusque change in the dynamic and paleoenvironmental characteristics in the sedimentary basin.
 A comparative analysis with the Salé section studied by Civis et al. (1996 and 1997) indicates equivalent characteristics between the marl-clay levels  and the lower part of the basal member of Oued Arjat  up to the Arjat-9 sample. From this level  to level Arjat-20, the Salé sands section and the upper section of Oued Arjat are characterised by a new biostratigraphic unit.
 The results obtained seem to point to two cycles of rise and fall of the sea level which could be set within the global cycles proposed by Haq et al. (1988) for the Lower Pliocene.

REFERENCES
Benson, R. H. & Rakic-El Bied, K.,(1996). Not. Mém. Serv. Géol. France, 383; 5-50.
Civis, J.; González-Delgado, J.A.; Francés, G.; Raffi, S.; Alonso-Gavilán, G. y Ben  Moussa, A., (1997). Geogaceta, 21; 77-80.
Civis, J.; Ahmed, R.; Alonso-Gavilán, G.; González-Delgado, J.A.; Ben Moussa,1977. RCANS, Abstrats.
González-Delgado, J.A.; Civis, J. y Andrés, I. (1997). RCANS, Abstrats.
Haq, B.A.; Hardenbol, J. y Vail, P. (1988). Soc. Econ. Paleont. Mineral. Spec. Pub. 42, : 71-108.
Rakic-El-Bied, K. (1990). Thesis, Univ. Bordeaux, France; 250 pp.
Rakic-El-Bied & Benson (1996). Not. Mém. Serv. Géol. France, 383; 51-141.
 

Acknowledgements.This work was carried out within the framework of Project PB 92- 0284, financed by the D.G.I.C.Y.T.  The authors are also grateful for the collaboration of Jesus Roncero Marcos in the preparation of the samples and Antonio Herrero for the graphic part.