Discovery of the messaoudensis trifidum acritarch assemblage ( upper Tremadocian lower Floian , Lower Ordovician ) in the subsurface of Morocco

The upper Tremadocian to lower Floian messaoudensistrifidum acritarch assemblage was first described from the Skiddaw Group of England and subsequently from several localities on the Gondwanan margin that were positioned in high southern latitudes during the Early Ordovician. It is here reported for the first time from North Africa, from the Fezouata formations (Tremadocian to Floian) in the AZ-1 borehole, southeastern Morocco. The assemblage is comparable with that from the Skiddaw Group, with Cymatiogalea deunffii, C. messaoudensis, C. velifera, Caldariola glabra glabra, Stelliferidium trifidum and Veryhachium lairdii s.l. The Moroccan assemblage indicates a late Tremadocian age.

The Lower and Upper Fezouata formations consist mainly of argillites and, together with the overlying Zini Sandstones and the Tachilla Formation, comprise the Outer Feijas Shale Group (Tremadocian to Darriwilian).The Fezouata formations range in age from the Tremadocian to the early Floian, with the lower formation disconformably overlying Cambrian sediments.Outcrops of the Fezouata formations are found in the Draa valley near the city of Zagora, in the central Anti-Atlas region, and they are also known to extend further to the southwest in the subsurface (Destombes et al. 1985).Recently the formations gained attention after the discovery of fossils with exceptional preservation (Van Roy et al. 2010) and are currently being studied under the auspices of the French 'Agence Nationale de la Recherche' (ANR) RALI (Rise of Animal Life) project.Palynomorphs from the Fezouata formations have previously been studied by Deunff (1968a, 1968b, in Destombes et al. 1985), Elaouad-Debbaj (1984, 1988) and Snape (1993).

MATERIALS AND METHODS
The AZ-1 (or Adrar Zouggar-1) borehole was drilled for petroleum exploration by Petrofina in 1963 to 1964 on Adrar Zouggar Mountain, about 300 km southwest of Zagora.It had a total depth of 3398.13 m.The interval between 624 and 1134.8 m was assigned to the Fezouata formations without discrimination between the lower and upper parts.Sixteen well cutting samples from that interval (see Table 1) were each treated partly according to standard palynomorph extraction procedures and partly using a low manipulation technique similar to that described by Butterfield & Harvey (2012).The residue was filtered at 51 µm (for the standard technique) or 63 µm (for the low manipulation technique) and 15 µm mesh sizes.Slides were produced from the 15-51 µm or 15-63 µm fractions and examined under a light microscope.
All specimens figured herein are housed at the Evo-Eco-Paleo Department of University Lille 1 (France).

RESULTS
The lowermost sample (1129.5 m) and the samples from 905 m upwards yielded no or very few, poorly preserved acritarchs (see Table 1).The seven remaining samples from 1100.65 to 930.0 m provided comparatively rich and diverse acritarch assemblages.Chitinozoans are also abundant in all levels except for the lowermost sample.Graptolite prosiculae and nema fragments occur sporadically.Scolecodonts are very rare.The residues produced by both techniques are comparable in terms of palynomorph richness.

DISCUSSION
The presence of the messaoudensis-trifidum assemblage in Morocco adds further evidence to the importance and wide distribution of this assemblage in high southern latitudes during the Early Ordovician.It allows for correlation of the studied part of the AZ-1 borehole with surface samples from the Zagora area and with other regions.The late Tremadocian age in the borehole fits well with the established age of the Lower Fezouata Formation from the outcrop areas (Destombes et al. 1985).Similarities between surface and borehole samples (Snape 1993) also support the assumption that the respective sediments are contemporaneous and palaeoecologically comparable.The full range of diversity of acritarchs and other palynomorph groups present as well as their potential for high-resolution biostratigraphy are planned to be reported in a future publication.

Table 1 .
Occurrences of palynomorph groups and selected acritarch species in the Fezouata formations of borehole AZ-1