THE BENTHIC COMMUNITIES (MOLLUSKS AND FORAMINIFERS) IN THE LOWER PLIOCENE OF THE OUED ARJAT SECTION (BOU REGREG BASIN, NW OF MOROCCO)

J.A. González Delgado, J. Civis & I. Andrés

Dpt. of Geology (Paleontology). Faculty of Sciences. University of Salamanca (Spain)

 In this work we study the molluscan and foraminifer assemblages of the Oued Arjat section, near Rabat (Morocco). The section are located in the NW of the Bou Regreg Basin, which formed during the Upper Miocene the well known South Rifian Corridor, an strait of Atlantic and Mediterranean water exchange before the Messinian salinity crisis. The thickness of the section is 52, and the uppermost levels (named 20 to 22, see Civis et al, 1997 for a detailed description) are fine sandy and very rich in molluscs, with 160 taxa. This is one of the richest malacological sections in the Neogene of Morocco.
 The Molluscs (level 21) are arranged in a shell concentration, of the composite type (Kidwell, 1991), of 30 cm. thick, shell supported (they are groups of shells stacked), with more than 55% of volume abondance of shells in cross section. The base is erosive or with molluscs infilling burrows. The general abrasion of the shells is low, and the pre-burial fragamentation is moderate. Shell sorting is high. Articulation, bioerosion and bioencrustation are low. Itís striking the little size of the malacofauna, except the Scaphopods. This taphonomic signature and the sedimentary setting suggest a distal stormy deposit.
 The level 20 has Globorotalia margaritae; the 21 is very rich in G. puncticulata and scarce G. margaritae, and the upper level (22) have both, and scarceG. crassaformis. This suggest a Lower Pliocene age for the upper levels.
 The abondance of benthic foraminifers is variable. Level 20 has Cibicidoides (more than 30%; mainly C. pseudoungerianus  and  C. dutemplei). Nonion boueanum, Cassidulina laevigata carinata  y Anomalinoides flinti   are also importants (5%). Level 21, has Cassidulina laevigata carinata (more than 25%), and Nonion boueanum, Ammonia beccarii, Bulimina elongata  and Lobatula lobatula. Level 22 is very rich in Nonion  and Ammonia, with more than 70% of the assemblage.
 In the outcrop we are found for the moment 65 taxa of Bivalves, 5 of Scaphopods, and 90 of Gastropods. The abondance of individuals is higher in Bivalves than in Gastropods and Scaphopods. Itís striking the litle abondance of Pectinids (althoug is present Palliolum excisum) and Ostreids.
 Among the Bivalves, Veneridae, Tellinidae, Nuculanidae and Lucinidae are the groups with major diversity. Abra prismatica, Venus lamellosa, Corbula gibba  and Pitar rudis  are the most abundants. They represent a taxocoenosis mainly infaunal and suspensivorous.
 The Gastropod families Turridae, Nassariidae and Pyramidellidae have the more taxa. The Naticids, Turritella tricarinata , Nassarids, Gibberula miliaria  and Ringicula ventricosa  are the most abundants.
 The malacological and foraminiferal data agree with communities of a circalittoral battimetry; they are taxa like Abra prismatica, Turritella tricarinata  and Melanella polita  which are characteristic of DC or VTC circalittoral biocoenoses (Péres et Picard, 1964). The increase in Cassidulina, and the abondance of Turritella tricarinata  would be related with an increasing of the nutrients and relative lower temperatures than the actuality.
 The Oued Arjat area have more Molluscan taxa than that of the Asilah basin, also in the NW of Morocco, of the same age but of infralittoral bathymetry, and where we have found 91 Molluscan taxa, belonging SFBC biocoenosses (González Delgado et al, 1996). Nevertheless, the number of taxa is half than the Huelva area, in the W of the Guadalquivir basin (Spain), (SFBC and DC biocoenosses: González Delgado et al, 1994).

REFERENCES
Civis, J Ahmed, R., Alonso-Gavilán, G, González-Delgado, J.A. & Ben Moussa A. (1997) The associations of benthonic foraminifera in the neogene of Salé (Bou-Regreg, southern rift corridor, Morocco). Second Congress R.C.A.N.S., Abstracts
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González Delgado, J.A., Civis, J., Alonso Gavilán, G., Andrés, Y. & Ben Moussa, A. (1996).- Paleontología y estratigrafía del Plioceno inferior de la Cuenca de Asilah (NW de Marruecos). Geogaceta 20 (1): 231-234
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