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Toarcian-Aalenian ammonite faunas from the western Middle Atlas (Morocco): controls of their distribution

El Hammichi F., Benshili K., Elmi S.

Volumina Jurassica

2006

Vol. 4, no. 1

169-169

The SW part of the Moroccan Middle Atlas represents an area where the ammonite faunas are often abundant either in outer bioclastic and ferruginous beds or in outer distal platform marly limestones (Mibladéne Fm. and Al Yabés Fm.). Sedimentary discontinuities are frequently marked by condensed (bioclastic, ferruginous and encrusted limestones). The biostratigraphy is based on the distribution of 120 species, 46 genera and 7 families. The ammonite succession fits well with the West Tethyan standards. However, there are noticeable differences between the faunal communities of the diverse sectors (“subbasins”) of the studied region. This differentiation is largely due to a palaeostructural partitioning provoked by extensional tectonic movements that strengthen or lessen the eustatic variations. The faunas are largely cosmopolitan at the exception of those of the Gradata and Bonarellii zones having a strong South-Tethyan character with Collina, Collinites, Telodactylites and Furloceras to the lack of Haugia, Brodieia, Grammoceras, Esericeras and Hudlestonia. The main qualitative turnover occurs at the beginning of the Gradata Zone and it is followed by a near complete segregation of faunal assemblages. This well known scenario is complicated by local to regional conditions linked both to environments, tectonics and, perhaps, climates. For example, Phylloceratina are rare, but Lytoceratina can be abundant, although these two groups present great variations in proportion of their occurrence to the entire fauna (37% in the condensed sections; 22% in the expanded successions). These data must be compared with the percentages of Phylloceratina and Lytoceratina in other Tethyan localities: 20% on the neritic seamounts of Sicily and of the Southern Alps; 30% on their borders; 70% in small and strongly subsiding subbasins (umbilicus) as well as at some levels of the Djebel Nador ammonitico rosso (Western Algeria) Hammatoceratids of the Aalensis and Opalinum zones are strongly similar to those of Sicily and Southern Alps. Presence of involute and acute Pseudaptetoceras klimakomphalum (Vacek) and P. christianae (Elmi & Mouterde) documents the seaways between the Alpine Tethys and the peri-Atlantic basins (Portugal).