The geodynamic importance of the Late Cambrian to Late Devonian reworked palynomorphs from the borehole SDJ1, Santa Susana region, Ossa Morena Zone (OMZ), Portugal

摘要

The detail study of the borehole SDJ1 from the Santa Susana coal basin western border ofthe OMZ, Portugal, proved that most of the volcano sedimentary lithologic sequenceyielded palynomorphs of mid late Visean age (NM Biozone) i.e., they belong to the Toeada Moura Volcano Sedimentary Complex (TMC). The deepest few meters of the drill corehave shales of mid Moscovian (Biozona SL) (see Pereira et ai., in this volume).Togetherwith the miospore assemblages that provide the age of the SDJ1 strata, mid late Viseanand Moscovian age, associations of exceptional well preserved Lower Palaeozoicacritarchs and spores were identified and interpreted as reworked into the Carboniferousocks. Four ages of reworked associations were identified: 1 - Late Cambrian acritarchs:Aeanthodiaerodium ?petrovii, Cristallinium randomense, Eliasum sp., Timofeeviaphosphoritica; 2 - Ordovician acritarchs: Navifusa punctata, Stelliferidium striatulum,Striatotheea rarirrugulata; 3 - Mid Late Silurian to Lower Devonian, spore taxa:Ambitisporites sp., Arehaeozonotriletes ehulus, Brochotriletes sp., Dietyotriletessubgranifer; Emphanisporites sp. and Synorisporites Jabeonis, cryptospores taxaaevolaneis sp., Tetrahedraletes medinensis and Quadritisporites variabilis, acritarch taxasuch as Ouvernaysphaera aranaides, Micrhystridium stellatum, Multiplicisphaeridiumramusculosum; 4 - Late Devonian miospore taxa: Retispora lepidophyta and Rugosporaf1exuosa and acritarch taxa Craterosphaeridium ?sprueegrovense, Gorgonisphaeridiumohioense, Stellinium eomptum, S. mieropolygonale, Vil/osaeapsula eolemanii,Pterospermella sp.. Detail examination of the reworked assemblages, still in progress,indicates that these are clearly dominated by Late Cambrian, Early Ordovician and LateDevonian acritarchs. In minor amount are Mid Silurian to Lower Devonian palynomorphsand the typical Middle Devonian species are very scarce. The TMC sediments areinterpreted as had been deposited in an intra-arc basin in close association with theigneous rocks of the Beja Massif, a magmatic arc installed at the south border of the OMZ(Oliveira et al. 2006). The ages of the reworked palynomorphs suggest that considerableerosion of exposed Lower Palaeozoic OMZ basement occurred during Carboniferoustimes. The exposure of these rocks may have occurred during the first phases of theVariscan Orogeny in the OMZ. Within the OMZ the rock formations that may constitute thesources for the reworked palynomorphs are: the Early Ordovician Phyllodoeites Shales(Pigarra et aI., 2011); the Raiados and Nodulos Shales Fm. and the Terena Fm. of Midate Silurian to Early Devonian, age (Pereira et aL, 1999; Lopes et ai., 2009). The LateDevonian association could have come from the South Portuguese Zone, where lateamennian palynomorphs are very well documented. Lastly, there are no units of provenlate Cambrian age in the OMZ. The occurrence of reworked Late Cambrian palynomorphsin the Santa Susana region indicates that elsewhere in the OMZ, sedimentation of this agemust have existed.