Naturtejo UNESCO Global Geopark, the first established in Portugal, joined the European and Global Geoparks networks under the auspices of UNESCO, in 2006. It was included in the International Geosciences and Geoparks Programme of UNESCO in 2015 as a territory of 5067km2, covering seven municipalities: Castelo Branco, Idanha-a-Nova, Nisa, Oleiros, Penamacor, Proença-a-Nova, and Vila Velha de Ródão. The Naturtejo Geopark geological heritage is represented by deposits from the Neoproterozoic to the Quaternary, the Ordovician successions being among its main features, exposed in large Variscan-folded structures and representing some of its most famous geosites (e.g., the Penha Garcia Ichnological Park and the Portas de Ródão Natural Monument).
When comparing with other Ordovician series, the Upper Ordovician of the Central Iberian Zone (CIZ) remained largely unknown through more than one century of geological studies. In the second half of the 20th century, there was a great increase in the knowledge of the Portuguese Upper Ordovician, and modern lithostratigraphic schemes were established (e.g., Mitchell 1974; Romano & Diggens 1974; Paris 1979; Cooper 1980; Young 1988). Despite some advances in the last 15 years (e.g., Sá et al. 2005; Romão 2006; Vaz, 2010; Meireles 2011; Pereira 2017), we are still far from knowing the true extent and architecture of Portuguese Upper Ordovician deposits. As a result of global eustatic variations, related to tectonics, a global warming event and a glaciation (e.g., Brenchley et al. 1991; Hammann 1992; Fortey & Cocks 2005), the Late Ordovician breaks with an apparent monotony and constancy that characterized sedimentation during Early and Middle Ordovician epochs in the CIZ. Late Ordovician extensional tectonics may have controlled accommodation space of the post-Sardic sedimentation (e.g., Álvaro et al. 2016), and the related magmatism generated interbedded sequences of volcanic or volcano sedimentary rocks interbedded in Katian fossiliferous sediments (e.g., Porto de Santa Anna Formation). Erosion or non-deposition significantly reduced the representation of the Upper Ordovician within CIZ and produced a great variability of sequences. So far, in Portugal, this system has been recognized in a total of 12 structural/geographical sectors (Pereira 2017), three of them occurring within the Naturtejo Geopark’s area: the Fajão-Muradal, Vila Velha de Ródão and Penha Garcia synclines. Despite their geographical proximity, these three sequences are significantly different, being a challenge not only to correlate but especially to interpret their depositional and basinal topographic settings and geodynamic evolution. Thus, their fossil content is essential for the recognition and the improvement of our understanding of these rather enigmatic sequences.
Herein, we present the state of the art of the Upper Ordovician fossil record within the Naturtejo Geopark area and the most recent advances on the stratigraphy and palaeontology of this series. This work aims to represent a bridge between scientific research within a geopark and its recognized geoheritage, contributing to unravel and expand the geoscientific offering of Naturtejo while providing a field-laboratory for researchers.
Geological settings and background of Naturtejo Geopark Upper Ordovician sequences
The studied area belongs to the Central Iberian Zone (CIZ) of the Iberian Massif, corresponding to post-Cambrian metasedimentary sequences that lie in angular unconformity on the Beiras Group (upper Ediacaran -?lower Cambrian). Five NNW-SSE to WNW-ESE major Variscan folded structures preserving lower Paleozoic successions occur within the Geopark: the Fajão-Muradal, Vila Velha de Ródão, Penha Garcia, Castelo Branco – Unhais-o-Velho and Monforte da Beira areas. This study focuses on the first three, where Upper Ordovician levels have been recognized up to now (Figs. 1, 2).
Figure 1. Simplified geological map of the Naturtejo UNESCO Global Geopark (adapted from the 1:500000 map published by Serviços Geológicos de Portugal in 1992) with the location of the three structures indicated in the text: 1 – Fajão-Muradal Syncline; 2 – Vila Velha de Ródão Syncline; 3 – Penha Garcia Syncline.
Having gone unnoticed during several decades of geological studies in Portugal (e.g., Delgado 1908; Costa 1931; Teixeira 1955), the Upper Ordovician of the Naturtejo area has been discovered little by little. In the Fajão-Muradal Syncline, Thadeu (1951) correctly recognized some ‘clay-sandstone shales’ and ‘dolerites’ as being ‘Caradocian’ in age (current Berounian), but other important successions had to wait much longer to be identified: in the Penha Garcia Syncline the Upper Ordovician was being progressively recognized from the seventies on (Perdigão 1971; Romano 1982; Young 1985) and in the Vila Velha de Ródão Syncline only recently (Metodiev et al. 2009). Among these, only the chronostratigraphic assignment of the Penha Garcia sequence was supported by fossils (Young 1985), the remaining sequences being attributed to the Upper Ordovician based solely on their stratigraphic position and regional lithostratigraphic correlation.
In the last decade, work conducted by the authors led to the discovery of the first Upper Ordovician fossil assemblages from the Vila Velha de Ródão and Fajão-Muradal synclines (Pereira et al. 2015; Pereira 2017; Pereira et al. 2017) and the revelation of more complete sequences than were previously thought to exist. The Upper Ordovician of Naturtejo region, often considered absent or poorly represented, is here demonstrated to have a great potential to improve the knowledge of the geodynamic evolution of the CIZ.
Figure 2. Synthetic chart of the Upper Ordovician litho-chronostratigraphic units recognized in the studied structures of the Naturtejo UNESCO Global Geopark (it does not represent a stratigraphic log); the main lithostratigraphic contributions are cited for each sector.
Stratigraphy and Paleontology: new data and research lines
Only the southern half of this long (32 km) and narrow (about 1 km) Variscan structure lies within the Naturtejo area, roughly south of the Zêzere River (site 1 in Figure 1). In the most detailed work on this syncline (Metodiev et al. 2010), the Paleozoic succession is considerably more incomplete in the southern sector, represented mainly by the ‘Armorican Quartzite’ (Serra do Brejo Formation; Arenigian stage, Lower-Middle Ordovician). In the northern sector, the sequence is preserved up to the lower Silurian. Herein we present a mostly continuous section west of Orvalho, allowing the recognition of a more complete Ordovician record within a previously already established geosite (Ribeira do Orvalho geological section; Neto de Carvalho et al. 2013). Materials derived from the Ferradosa Formation were recognized for the first time in the Fajão-Moradal Syncline, remobilized within a diamictitic Hirnantian unit (Casal Carvalhal Formation). They are represented by limestone clasts bearing a diverse fossil assemblage of Kralodvorian age (upper Katian), including brachiopods, trilobites, machaeridians, bryozoans and echinoderms (Fig. 3S-W). Deposition of the Ferradosa Formation is related to the global warming BODA event (Fortey & Cocks 2005). Because of rising temperatures, carbonate sedimentation encroached upon high-latitude peri-Gondwana and low-latitude taxa (e.g., brachiopod genus Nicolella, Figure 3S) expanded their geographical distribution into this province (Colmenar et al. 2017). The Portuguese Kralodvorian sedimentary record is extremely scarce, being mostly absent in the remaining CIZ of central Portugal. The new data suggest that its absence is more related to an important Late Ordovician erosion event (probably associated with the Late Ordovician Glaciation), rather than to non-deposition. Furthermore, this section also provides new insights on the basin paleotopography and sediment source of post-Kralodvorian deposits within the CIZ. All regional and global Upper Ordovician stages are now known to be represented in the Fajão-Muradal syncline (Figure 2): the Louredo Formation (middle Berounian; Sa2-Ka1), ?Ribeira da Laje Formation (upper Berounian, K2), Porto de Santa Anna Formation (upper Berounian-?Kralodvorian; Ka2-3), remobilized Ferradosa Formation (Kralodvorian; Ka3-4), and Casal Carvalhal Formation (Hirnantian). Thus, this sector preserves the second most complete Upper Ordovician record of Portugal.
Vila Velha de Ródão Syncline
This 30 km-long syncline is relatively wider (2.5 km) than the Fajão-Moradal structure, and contains a more complete Ordovician-Silurian sequence. The geological mapping produced by Metodiev et al. (2009) significantly improved the lithostratigraphical knowledge of this sector. The studied sequence is located at the Barroca da Senhora stream (Metodiev et al. 2009, Figure 2 – Cabeço da Achada), in the northeastern limb of the syncline and it is, at present, the most complete Upper Ordovician record within this syncline (Figure 2). Fossils were only found at the base of the Louredo Formation, although scarce, but including a new trilobite (Primaspis aff. primordialis; Figure 3A) also known from the Amêndoa-Carvoeiro Syncline (Pereira 2017). Despite the proximity to this latter structure (less than 20 km), the Upper Ordovician of the Vila Velha de Ródão Syncline is surprisingly different. The bryozoan-mudstone beds of the Cabeço do Peão Formation are here replaced by the sandstones of Louredo Formation, just like in the Fajão-Muradal Syncline. Nevertheless, the Kralodvorian succession of the latter is absent at Vila Velha de Ródão and the succeeding Hirnantian sequence is poorly understood: it was previously assigned to the Casal Carvalhal Formation, based on its typical spheroidal weathering (Metodiev et al. 2009), but in our fieldwork campaigns we did not recognize those levels as representing this unit.
Penha Garcia Syncline
Famous for hosting one of the most important geosites of Naturtejo, and possibly the most impressive forms of Cruziana worldwide (Penha Garcia Ichnological Park), this syncline is 21.5 km long in Portugal, with an average width of 2 km. Contrary to the previously described structures, the Penha Garcia Syncline sequence finishes in the Upper Ordovician, overlapped by Cenozoic deposits. The youngest unit recognised at present is the Ribeira da Laje Formation of upper Berounian (Katian, Ka2) age (Figure 2). A diverse fossil assemblage from the Louredo Formation is herein reported from a fossil site discovered by Neto de Carvalho et al. (2014), within the private estate of Vale Feitoso. It is dominated by trilobites (Deanaspis seunesi, Crozonaspis dujardini and Dalmanitina sp.) and brachiopods (Aegiromena aquila aquila, Svobodaina havliceki and Lingulidae indet.), with fewer molluscs (Praeleda ribeiroi, Holopea? sp.) and trepostomate bryozoans (Fig. 3B–M). Curiously, a mudstone unit (the Vaca Member) is present in the same estate, unique within the Naturtejo area and correlated with the Galhano Member of the Buçaco Syncline (Young 1988). Its fossil assemblage (Fig. 3N-R) remains unstudied (it was preliminarily presented by Young 1985) and includes some unique occurrences within Portugal, such as the brachiopods Kjaerina (Kjaerina) gondwanensis and Rafinesquina (Mesogeina) pseudoloricata. It is particularly important to understand better deeper marine communities from offshore environments of the peri-Gondwanan late Berounian, usually represented by shallower, shoreface environments. The presence of a poorly exposed thin oolitic ironstone occurring above the Vaca Member, which Young (1988) referred to the basal Leira Má Member of the Porto da Santa Anna Formation of the Buçaco Syncline, is also unique within the Penha Garcia syncline. The recent assignment of the Ribeira da Laje Formation to the upper Berounian (Vaz 2010), instead of Hirnantian, as originally proposed by Young (1988), makes this correlation problematic: this oolitic ironstone at the top of the Vaca Member is overlaid by the Ribeira da Laje Formation. Until a broader understanding of the Ribeira da Laje Formation has been achieved in its type area (Amêndoa-Carvoeiro Syncline), the age of this unit must be treated carefully. In the Penha Garcia Syncline, the Ribeira da Laje Formation shows a rare occurrence in the Upper Ordovician of CIZ of bioturbation composed by mostly mm-scale, horizontal simple burrows and Nereites missouriensis in simple shallow tiers, which seems to show sporadic colonization of dysoxic bottoms by minute foragers. Further investigation will allow clarifying what really happened during Late Ordovician in Penha Garcia area.