Advances in the Lower Ordovician of the western Iberian Chain, NE Spain

Abstract

The western Iberian Chain of NE Spain represents a lateral prolongation of the West Asturian-Leonese Zone. As other proximal sectors fringing NW Gondwana, the chain comprises a conformable and continuous Furongian-Tremadocian transition, comprising a thick (3600-4500 m) Lower Ordovician sedimentary succession, exclusively composed of siliciclastic strata and deposited in mid-latitude (temperate) waters. Although the shelly fossil record is not abundant, some key trilobites and brachiopods allow the identification of distinct biogeographic links with the Argentinian margin of West Gondwana, Oaxaca (Mexico), and the neighbouring Montagne Noire (France), with which a common biostratigraphic biozonation can be envisaged. Two phosphoritic interbeds rich in linguliformean brachiopods punctuate the Valconchán and Borrachón formations, and represent event beds related to condensation processes and sedimentation of explosive ignimbritic tuffs, respectively.

Headnote

Keywords:

brachiopods, trilobites, echinoderms, ichnofossils, Ordovician, NW Gondwana

ABSTRACT

The western Iberian Chain of NE Spain represents a lateral prolongation of the West Asturian-Leonese Zone. As other proximal sectors fringing NW Gondwana, the chain comprises a conformable and continuous Furongian-Tremadocian transition, comprising a thick (3600-4500 m) Lower Ordovician sedimentary succession, exclusively composed of siliciclastic strata and deposited in mid-latitude (temperate) waters. Although the shelly fossil record is not abundant, some key trilobites and brachiopods allow the identification of distinct biogeographic links with the Argentinian margin of West Gondwana, Oaxaca (Mexico), and the neighbouring Montagne Noire (France), with which a common biostratigraphic biozonation can be envisaged. Two phosphoritic interbeds rich in linguliformean brachiopods punctuate the Valconchán and Borrachón formations, and represent event beds related to condensation processes and sedimentation of explosive ignimbritic tuffs, respectively.

Introduction

In the Iberian Massif, the Ollo de Sapo ('toad eye') magmatism has long been recognized as an enigmatic Furongian-Early Ordovician (495-470 Ma) assemblage of felsic (sub-)volcanic and plutonic rocks. The main exposures of this event crop out along the core of a 600 km long antiform fringing the northeastern edge of the Central Iberian Zone (Fig. 1A). Penecontemporaneous felsic byproducts locally occur in the neighbouring Ossa-Morena Zone and the Armorican and Occitan domains, reflecting the onset of a large igneous province (LIP) of ca 200 000 km3 (García-Arias et al. 2018). The Ollo de Sapo magmatic event is also contemporaneous with the development of the so-called Toledanian Phase and gap, which place upper Tremadocian-Floian rocks overlapping with the inherited palaeorelief of tilted Ediacaran-Cambrian blocks. Their unconformable contact, ranging from paraconformities to angular discordances, involves a stratigraphic gap of ca 22 m.y. This unconformity can be correlated with the 'Furongian gap' of the Ossa-Morena Zone and the Anti-Atlas of Morocco, and the Norman gap of the Central and North Armorican domains (Álvaro et al. 2021). The Toledanian Phase, however, is absent in the most proximal sector of the Iberian margin, i.e. the West Asturian-Leonese and Cantabrian zones, and their lateral prolongations into the Sierra de la Demanda and Iberian Chains (Fig. 1A, B). In the latter, a continuous and conformable Furongian-Lower Ordovician transition has traditionally been documented, unaffected by the Toledanian uplift and erosion. The Furongian-Tremadocian trilobite, echinoderm and brachiopod assemblages of the western Iberian Chain are updated and summarized in this short communication.

Results and discussion

The Iberian Chains, located in the NE of the Iberian Peninsula (Fig. 1A, B), represent a lateral prolongation of the West Asturian-Leonese and Cantabrian zones. There, the Furongian-Lower Ordovician strata consist of successive alternations of shale- and sandstone-dominated sedimentary units, and comprise, from bottom to top, the Valtorres Formation (Acón Group), the Valconchán, Borrachón, Dere and Santed formations (Ateca Group, broadly coinciding with the 'Iberian Series' of Schmitz 1971 and Josopait 1972), and the Armorican Quartzite Formation (Fig. 1C).

The Valtorres Formation, 200-350 m thick, consists of monotonous green shales, locally punctuated by sandstone interbeds and silica concretions. Towards its top, there is a gradual increase in the thickness and abundance of sandstone interbeds. The overlying Valconchán Formation, 30-300 m thick, consists of thin- to thick-bedded sandstones and quartzites with shale and conglomerate interbeds. Trilobites of the Valtorres Formation and the lowermost part of the Valconchán Formation (Josopait 1972; Wolf 1980; Shergold and Sdzuy 1991) form a Furongian association, which includes agnostids (Pseudagnostus sp.) and trilobites, such as Aphelaspidine aff. Aphelaspis rara, Elegantaspis cf. beta, Parachangshania? sp., Punctaspis? schmitzi, and Valtorresia volkeri (Shergold and Sdzuy 1991), associated with the brachiopods Billingsella jalonensis and B. perarea (Havlíček and Josopait 1972). According to Shergold and Sdzuy (1991), this fauna may be considered to be approximately equivalent to the Parabolina spinulosa Zone of Baltica. In contrast, the upper part of the Valconchán Formation has yielded the trilobite Pagodia (Wittekindtia) alarbaensis (Shergold and Sdzuy 1991), the pelmatozoan holdfast Oryctoconus lobatus (Álvaro and Colchen 2002; Zamora et al. 2009), and the brachiopod Protambonites primigenius (Havlíček and Josopait 1972). Based on the occurrence of their respective genera and subgenera in Afghanistan, Turkey and the Cantabrian Zone, this endemic assemblage has traditionally marked the Furongian-Tremadocian transition in the Iberian Chains. Near Valconchán village, the topmost part of the Valconchán Formation contains a condensation phosphorite level, up to 10 cm thick, composed of disarticulated and broken linguliformean brachiopods (Fig. 2A), and separating two shallowing-upward shoaling cycles, up to 2.4 m thick.

The overlying Borrachón Formation, 320-900 m thick, comprises mainly green shales with subsidiary sandstone interbeds, which have yielded trilobites (e.g., Angelina aff. hyeronimi, Asaphellus sp., Conophrys pusilla, and Dikelokephalina sp.), acritarchs and ichnofossils (Cruziana semiplicata, C. furcifera and C. rugosa) (Schmitz 1971; Josopait 1972; Kolb and Wolf 1979; Wolf 1980). The occurrence of age-diagnostic acritarchs allows its assignation to the Tremadocian. The presence of the olenid Angelina aff. hyeronimi (reported by Hammann et al. 1982, 24; and illustrated in Fig. 3A, B) allows taxonomic and biogeographic links with A. hyeronimi, a key species crossing the Furongian-Tremadocian boundary interval in Argentina (where the stratigraphic range comprises the Cordylodus intermedius, C. lindstromi and C. angulatus conodont-based zones; Tortello 2003) and the basal Tremadocian of Mexico (Robison and Pantoja-Alor 1968), which would suggest that the Furongian-Tremadocian boundary should be tentatively located within the lowermost part of the Borrachón Formation. C. pusilla (Fig. 3C, D) allows correlation with the homonymous Tremadocian biozone from the southern Montagne Noire (Vizcaino and Álvaro 2003). A rhyolitic tuff located in the vicinity of Bubierca (Figs 1B, 2B, C) has yielded broken linguliformean brachiopods, such as Ectenoglossa sp. and Lingulepis cf. acuminata (Wolf 1980).

The overlying sandstones and quartzites of the Dere Formation are 420-850 m thick and contain ichnofossils (Cruziana semiplicata and C. rugosa), poorly preserved linguliformean brachiopods, trilobites (e.g., Asaphellus sp.) and molluscs, and acritarchs (Scheuplein 1970; Schmitz 1971; Havlíček and Josopait 1972; Wolf 1980).

The shale-dominated Santed Formation, 200-950 m thick, has yielded ichnofossils, trilobites, brachiopods, echinoderms and acritarchs (Schmitz 1971; Josopait 1972; Wolf 1980; Zamora et al. 2009). The Tremadocian - 'Arenig' (Floian) boundary interval is currently placed within this formation. Josopait (1972), Wolf (1980) and Hammann et al. (1982) reported the youngest Tremadocian and oldest 'Arenig' trilobites in the vicinity of the Tranquera dam (Fig. 1B). The uppermost Tremadocian fossil assemblage occurs within a dacitic tuffitic bed (Josopait's Fp 32; Álvaro et al. 2008), which has yielded, among others, the trilobites Euloma cf. filacovi and Prionocheilus cf. languedocensis, whereas the lowermost 'Arenig' assemblage (Josopait's Fp 33) contains Prionocheilus cf. languedocensis and Megistaspis (Ekeraspis) cf. filacovi (Fig. 3E-J). Wolf (1980) correlated both levels biostratigraphically with the 'faunizones' E and F of the southern Montagne Noire, which were re-evaluated by Vizcaino and Álvaro (2003) as two different fossil assemblages (not yet found in the same section of the Montagne Noire) belonging to the Euloma filacovi Zone, which would include the Tremadocian-Floian boundary interval. Finally, the overlying Armorican Quartzite Formation, 450-650 m thick, consists of two thick-bedded quartzite and sandstone members separated by a shale-dominated middle member. The formation has yielded rare brachiopods, conularids, bivalves and trilobites (Babin and Hammann 2001), and ichnofossils (Cruziana rugosa and C. imbricata). A recent U-Pb dating of K-bentonites embedded in the Armorican Quartzite (upper Barrios Formation) of the Cantabrian Zone, northern Spain, has yielded an age of477.47 ± 0.93 Ma (Gutiérrez-Alonso et al. 2016), a result extremely close to the currently accepted interpolated age for the Tremadocian-Floian boundary GSSP at 478.6 ± 1.7 Ma.

Conclusions

The western Iberian Chain of NE Spain comprises a thick (3600-4500 m) and conformable Lower Ordovician sedimentary succession, exclusively composed of siliciclastic strata and deposited in mid-latitude (temperate) waters fringing NW Gondwana. The Furongian-Tremadocian boundary interval lies within the Borrachón Formation, where the trilobite Angelina aff. hyeronimi supports a direct correlation with the Argentinian margin of Gondwana and Oaxaca. The Tremadocian-Floian boundary interval is placed within the Santed Formation, where the trilobites Euloma cf. filacovi, Prionocheilus cf. languedocensis and Megistaspis (Ekeraspis) cf. filacovi allow a direct correlation with the southern Montagne Noire, France. Two phosphoritic interbeds rich in linguliformean brachiopods punctuate the Valconchán and Borrachón formations, and represent event beds related to condensation processes and sedimentation of explosive ignimbritic tuffs, respectively.

Acknowledgements

The authors appreciate the revision made by Olle Hints and Ian Percival, and the technical editing by Margit Sepp. This study is a contribution to IGCP Project 735 'Rocks and the Rise of Ordovician Life' and was supported by the Spanish Project PID2021-125585NB-I00. The publication costs of this article were partially covered by the Estonian Academy of Sciences.

Sidebar

Received 30 March 2023

Accepted 16 April 2023

Available online 9 June 2023

Corresponding author:

J. Javier Álvaro

[email protected]

Citation:

Álvaro, J. J. and Martínez-Benítez, B. 2023. Advances in the Lower Ordovician of the western Iberian Chain, NE Spain. Estonian Journal of Earth Sciences, 72(1), 6-9. https://doi.org/10.3176/earth.2023.15

References

References

Álvaro, J. J. and Colchen, M. 2002. Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited. Eclogae GeologicaeHelvetiae, 95(3), 451-459.

Álvaro, J. J., Ezzouhairi, H., Ribeiro, M. L., Ramos, J. F. and Solá, T. 2008. Early Ordovician volcanism in the Iberian Chains (NE Spain) and its influence on the preservation of shell concentrations. Bulletin de la Société Géologique de France, 179(6), 569581.

Álvaro, J. J., Casas, J. M. and Quesada, C. 2021. Reconstructing the pre-Variscan puzzle of Cambro-Ordovician basement rocks in the southwestern European margin of Gondwana. In Pannotia to Pangaea: Neoproterozoic and Paleozoic Orogenic Cycles in the Circum-Atlantic Region (Murphy, J. B., Strachan, R. A. and Quesada, C., eds). Geological Society, London, Special Publications, 503, 531-562.

Babin, C. and Hammann, W. 2001. Une nouvelle espece de Modiolopsis (Bivalvia) dans l'Arénig (Ordovicien inférieur) de Daroca (Aragon, Espagne): réflexions sur la denture des bivalves primitifs (A new species of Modiolopsis (Bivalvia) in the Arenig (Lower Ordovician) of Daroca (Aragon, Spain): reflections on the dentition of primitive bivalves). Revista Española de Paleontología, 16(2), 269-282.

García-Arias, M., Díez-Montes, A., Villaseca, C. and BlancoQuintero, I. F. 2018. The Cambro-Ordovician Ollo de Sapo magmatism in the Iberian Massif and its Variscan evolution: A review. Earth-Science Reviews, 176, 345-372.

Gutiérrez-Alonso, G., Gutiérrez-Marco, J. C., Fernández-Suárez, J., Bernárdez, E. and Corfu, F. 2016. Was there a super-eruption on the Gondwanan coast 477 Ma ago? Tectonophysics, 681, 85-94.

Hammann, W., Robardet, M. and Romano, W. 1982. The Ordovician System in southwestern Europe (France, Spain, and Portugal). Correlation chart and explanatory notes. IUGS Publications, 11, 1-46.

Havlíček, V. and Josopait, V. 1972. Articulate brachiopods from the Iberian Chains, northern Spain (Middle Cambrian-Upper Cambrian-Tremadoc). Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 140, 328-353.

Josopait, V. 1972. Das Kambrium und das Tremadoc von Ateca (Westliche Iberische Ketten, NE Spanien) (The Cambrian and Tremadocian of Ateca (western Iberian Chains, NE Spain)). Münstersche Forschungen zur Geologie und Palaeontologie, 23, 1-123.

Kolb, S. and Wolf, R. 1979. Distribution of Cruziana in the Lower Ordovician sequence of Celtiberia (NE Spain) with a revision of the Cruziana rugosa-group. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte, 1979(8), 457-474.

Robison, R. A. and Pantoja-Alor, J. 1968. Tremadocian trilobites from the Nochixtlan Region, Oaxaca, Mexico. Journal of Paleontology, 42(3), 767-800.

Scheuplein, R. 1970. Stratigraphie und Tektonik der Schichten-folge in Raume Daroca-Calamocha (Westliche Iberischen Ketten, NE Spanien) (Stratigraphy and tectonics of the strata in the DarocaCalamocha area (western Iberian Chains, NE Spain)). PhD thesis. University of Würzburg, Germany (unpublished).

Schmitz, U. 1971. Stratigraphie und Sedimentologie im Kambrium und Tremadoc der Westlichen Iberischen Ketten nördlich Ateca (Zaragoza), NE Spanien (Stratigraphy and sedimentology in the Cambrian and Tremadocian of the western Iberian Chains north of Ateca (Zaragoza), NE Spain). Münstersche Forschungen zur Geologie und Paläontologie, 22, 1-123.

Shergold, J. H. and Sdzuy, K. 1991. Late Cambrian trilobites from the Iberian Mountains. Zaragoza Province. Spain. Beringeria, 4, 193-235.

Tortello, M. F. 2003. Biostratigraphic significance of the latest Cambrian-earliest Ordovician agnostoid trilobites from Northwestern Argentina. Geologica Acta, 1, 61-72.

Vizcaino, D. and Álvaro, J. J. 2003. Adequacy of the Early Ordovician trilobite record in the southern Montagne Noire (France): biases for biodiversity documentation. Transactions of the Royal Society of Edinburgh: Earth Sciences, 93, 393-401.

Wolf, R. 1980. The lower and upper boundary of the Ordovician system of some selected regions (Celtiberia, eastern Sierra Morena) in Spain. Neues Jahrbuchfür Geologie und Paläontologie, Abhandlungen, 160, 118-137.

Zamora, S., Álvaro, J. J. and Vizcaino, D. 2009. Pelmatozoan echinoderms from the Cambrian-Ordovician transition of the Iberian Chains (NE Spain): early diversification of anchoring strategies. Swiss Journal of Geosciences, 102, 43-55.