Document Type : European UNESCO Geoparks: Short Communication

Authors

1 Centro de Geociências, Universidade de Coimbra, Rua Sílvio Lima, 3030-790 Coimbra, Portugal

2 Department of Geology, Ghent University. Krijgslaan 281, S8. 9000 Ghent, Belgium

3 Instituto Dom Luiz, Departamento de Geologia, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal

4 Geopark Naturtejo Meseta Meridional. Centro Cultural Raiano. Avenida Joaquim Morão, 6060-101, Idanha-a-Nova, Portugal

Abstract

Middle Ordovician successions occur at Naturtejo UNESCO Global Geopark in five Variscan-folded, kilometer- to tens of kilometer-long structures. Four of them revealed to be fossiliferous in the different recognized lithostratigraphic units, middle-to-uppermost Darriwilian in age, with particular emphasis for Brejo Fundeiro (Oretanian regional chronostratigraphic stage) and Fonte da Horta (Dobrotivian regional stage) formations. Recent paleontological and biostratigraphic works have detected most of the previously recorded and new fossil sites, improving the Middle Ordovician biostratigraphy of this region and updating the knowledge of its fossil assemblages, which are fairly more diverse than previously considered. The good preservation of the fossils and their relevance to understanding the climax of the Global Ordovician Diversification Event at high paleolatitude shallow marine environments, as well as the protection or identification of some of the fossil sites as UNESCO Geopark’ geosites, make certain their interpretation and use in already existing geotrails and new interpretation centers and school programs.

Keywords

Introduction

Naturtejo UNESCO Global Geopark 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 5067 km2, covering the municipalities of Castelo Branco, Idanha-a-Nova, Nisa, Oleiros, Penamacor, Proença-a-Nova, and Vila Velha de Ródão, located in Central Portugal. Represented by deposits from the Neoproterozoic to the Quaternary, the Ordovician successions are among the main features of the Naturtejo geological heritage, exposed in large, kilometers to tens of kilometers long, Variscan-folded structures, forming Appalachian-type landforms and representing some of its most famous geosites (e.g., the Penha Garcia Ichnological Park and the Portas de Ródão Natural Monument).

The Middle Ordovician is the most fossiliferous Ordovician series across the Central Iberian Zone. The so-called “tristani beds” (Born 1916), due to the abundance of the trilobite Neseuretus tristani in these shallow marine deposits, represents the Oretanian-Dobrotivian interval (correlated with most of the global Darriwilian Stage), which hosts the most classical Portuguese Ordovician fossil sites (e.g. Valongo, Arouca and Buçaco; e.g., Romano 1982; Couto et al. 1997; Guy & Lebrun 2010). While in Northern Portugal these stages are represented by a homogeneous succession of shales and siltstones (Valongo and Moncorvo formations) overlying the ‘Armorican Quartzite’ (Santa Justa and Marão formations, Romano & Diggens 1974; Sá et al. 2005), in central Portugal this interval is represented by intercalation of mudstones and sandstones units: Brejo Fundeiro, Monte da Sombadeira, Fonte da Horta, and Cabril formations (Cooper 1980; Young 1988). Among these, the lower Dobrotivian Fonte da Horta Formation is the most fossiliferous unit, providing highly diverse assemblages (mainly brachiopods, trilobites, ostracods, molluscs, hyolithids, bryozoans, and crinoids), with fewer but frequent fossils also reported from the Brejo Fundeiro and the Cabril formations (e.g., Delgado 1908; Young 1985). Finally, the paleontological record of the Monte da Sombadeira Formation is sparse, but documented (e.g., Delgado 1908; Cooper 1980; Romão 2006).

Within the Naturtejo Geopark area, fossiliferous Oretanian and/or Dobrotivian units are represented in four of its five structural Variscan-folded structures: the Fajão-Muradal, the Vila Velha de Ródão, and the Penha Garcia synclines plus the Monforte da Beira folded structure. During the first decades of geological studies in Portugal, the Middle Ordovician of these sectors was considered poorly fossiliferous, bearing solely rare and badly preserved remains, what was interpreted as being related to deeper environments (Delgado 1908). During the second half of the twentieth century, the discovery of Middle Ordovician fossiliferous levels in Fajão-Muradal, Vila Velha de Ródão and Penha Garcia synclines changed somewhat this interpretation (e.g., Thadeu 1951; Ribeiro et al. 1965, 1967; Perdigão 1971; Young 1988), but their paleontological record was rarely figured (e.g., Neto de Carvalho et al. 2014), and the idea of a low potential for this paleontological heritage remains.

In this work, we show this potential through a selection of specimens from old and new fossil sites, improving the Middle Ordovician biostratigraphy of this region and updating the knowledge of its fossil assemblages, which are fairly more diverse than previously considered.

Stratigraphic Settings of Naturtejo Oretanian/Dobrotivian (Darriwilian) 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 ranging in age from Lower Ordovician (Tremadocian) up to, at most, lower Silurian (Llandovery), occur within the Geopark: Fajão-Muradal, Vila Velha de Ródão, Penha Garcia, Castelo Branco – Unhais-o-Velho and Monforte da Beira sectors (Fig. 1). Only the southern half of the 32 km-long Fajão-Muradal Syncline lies within Naturtejo area, roughly south of the Zêzere River (Fig. 1). For this reason, herein we will focus only on the Muradal sector when referring to Fajão-Muradal Syncline.

Figure 1. Simplified geological map of the Naturtejo UNESCO Global Geopark (adapted from the 1:500000 map published by Serviços Geológicos de Portugal, Oliveira et al. 1992) with the location of the four sectors indicated in the text: 1 – Fajão-Muradal Syncline; 2 – Vila Velha de Ródão Syncline; 3 – Penha Garcia Syncline; 4 – Monforte da Beira folded structure.

The Oretanian/Dobrotivian (middle to upper Darriwilian) succession among the Naturtejo area starts with the Brejo Fundeiro Formation (20-120 m), conformably overlying the Serra do Brejo and Penha Garcia formations (‘Armorican Quartzite’, Cooper 1980; Bayet-Goll & Neto de Carvalho 2020). This metapelitic unit is recognized throughout all Naturtejo sectors, thickening southwards, from solely 20 m in southern half of the Fajão-Muradal Syncline to 120 m in Vila Velha de Ródão, and eastwards, to about 80 m. The Brejo Fundeiro Formation is assigned to the regional Oretanian stage (middle to upper Darriwilian). In Castelo Branco – Unhais-o-Velho and Monforte da Beira exposures, the lower Paleozoic succession is preserved only up to part of the Brejo Fundeiro Formation, being impossible to estimate its original thickness. A sudden change in lithology marks the base of the following unit, the Monte da Sombadeira Formation (5-50 m), a regressive storm-generated sandstones sequence of lower Dobrotivian age (ca. uppermost Darriwilian). Its thickening and coarsening tendency south and eastwards in Portuguese Central Iberian Zone (Young 1985), is also verified in the studied area, being only 5 m thick in Fajão-Muradal, 20 m in Vila Velha de Ródão and 50 m in the Penha Garcia Syncline. Conformably overlying these micaceous sandstones is the Fonte da Horta Formation (20-65 m), consisting of mudstones with rare sandstone beds, also of lower Dobrotivian age. It was recognized throughout the synclines of Fajão-Muradal, Vila Velha de Ródão and Penha Garcia. The Fonte da Horta Formation rapidly thins southwards, from 55-65 m in Fajão sector (Young 1988; Metodiev et al. 2009) to solely 10-15 m in Muradal and Vila Velha de Ródão areas, thickening eastwards in Penha Garcia to about 50 m. Finally, this mudstone-dominated unit is overlaid by the Cabril Formation, composed of siltstones and sandstones beds exhibiting hummocky cross-stratification within a background of laminated siltstones and mudstones. This unit is assigned to the base of the upper Dobrotivian (global uppermost Darriwilian), being less than 30 m thick and poorly characterized in the Penha Garcia Syncline (Young 1985). In its type area (Buçaco Syncline) and the Amêndoa-Carvoeiro Syncline (located westwards of the studied sequences), the top of the Cabril Formation is marked by a persistent phosphatic conglomerate, not represented in any of the Naturtejo’ sectors. Likewise, the following unit in the Buçaco and Amêndoa-Carvoeiro synclines, the Carregueira Formation, is also absent in the studied sequences. Deposition conditioned by eustatic events and an erosive episode that preceded the deposition of the Upper Ordovician (probably related to the Sardic phase, a deformation event along the northern Gondwana margin; e.g., Puddu et al. 2019) may justify the incompleteness of the Cabril Formation and the absence of the Carregueira Formation in Naturtejo sectors. Furthermore, Young (1988) recognized a maximum shoaling during the deposition of the upper part of the Cabril Formation and concluded that Carregueira Formation, which shows marked thickness variations, may be absent in the “rise” areas. According to the stratigraphic data presented herein, the Fajão-Muradal Syncline marks the belt of maximum condensation of the sequences during the Middle Ordovician. Thus, the sedimentary model suggested by Young (1988) supports and justifies the verified sedimentary hiatus for this sector and south-eastwards areas.

Paleontology and Biostratigraphy

Fajão-Muradal Syncline (southern sector)

The first Middle Ordovician fossils from the Fajão-Muradal Syncline were reported by Thadeu (1951, pp. 38-39), but all the occurrences are from the northern part of the syncline (outside the studied area). Besides graptolites in the Brejo Fundeiro Formation, this author recovered numerous fragments of the trilobite Neseuretus tristani, two indeterminate orthid brachiopods and one ostracod from the mudstones of the Fonte da Horta Formation. Two decades later, Perdigão (1971) presented the only previously reported fossil occurrences from the Middle-to-Upper Ordovician of the Muradal sector. Several fossil sites for the Brejo Fundeiro Formation were documented, yielding the characteristic graptolite Didymograptus (Sarnadas de S. Simão, Picoto, Candal, Zebro and Orvalho). In the Fonte da Horta Formation, Perdigão (1971) identified one single fossil site (Candal at Sarnadas), yielding the trilobite Neseuretus tristani and indeterminate crinoids. One single specimen of a graptolite from the Muradal sector was figured (‘D. bifidus’:  Perdigão 1971, pl. 1, fig. 6) and up to now, this was the only Middle Ordovician fossil ever published from there. The recent geologic mapping survey conducted by Metodiev et al. (2010) did not provide fossil remains from the southern sector of the syncline. From a paleontological point of view, the Middle Ordovician of the Muradal region is fairly fossiliferous (Fig. 2), and the present work revealed several new fossil sites, increasing the total number of known species from three to almost 30. The Brejo Fundeiro Formation provided three fossiliferous horizons in the lower part of the unit containing an assemblage representative of the Oretanian age, yielding graptolites (Didymograptus murchisoni and Didymograptus cf. artus), echinoderms (Calix sp.), trilobites (Neseuretus cf. avus and Asaphellus sp.), bivalves (Redonia deshayesi, Praenucula?sp.), brachiopods (Orthidae indet.) and hyolitids (Figs. 2A-E). No fossils were found up to the moment in the Monte da Sombadeira Formation. The mudstones of the Fonte da Horta Formation are very fossiliferous, like in other geographic sectors of central Portugal. Its fossil assemblage is typical of lower Dobrotivian age, yielding abundant trilobites (Neseuretus tristani, Kerfornella brevicaudata, Crozonaspis cf. struvei, Plaesiacomia oehlerti, Isabelinia?sp., Phacopidina micheli, Selenopeltis sp. and Eccoptochile sp.), ostracods (Lardeuxella bussacensis, Reuentalina ribeiriana, Quadritia tromelini, Medianella sp., Quadrijugator marcoi), bivalves (Cardiolaria beirensis), brachiopods (Crozonorthis musculosa, Heterorthina sp.and Horderleyella sp.), indeterminate crinoids and the ichnospecies Tomaculum problematicum. The Cabril Formation did not provide fossils up to the moment.