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

Abstract

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 km 3 (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 discussionThe 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 andJosopait 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 Fezouata Shale Formation biota is typical for the high latitudes of the Early Ordovician—a quantitative approach

“…It is crucial to emphasize that despite our efforts to exhaustively sample information in the PBDB from the high latitudes of the Early Ordovician ecosystems, the subassemblages with the necessary occurrence information to be included are restricted geographically to Gondwana and peri-Gondwana (Iberia and Armorica) (Modliński and Szymański 2001; Tortello et al 2006; Kraft et al 2014; Van Roy et al 2015a; Álvaro and Martínez-Benítez 2023). This pattern likely derives from the impact of colonization and neo-colonization on global inequality (e.g., material resources, stability, and economic and social development), which ultimately affects occurrence data uploaded into online repositories (Amano and Sutherland 2013; Trisos et al 2021; Raja et al 2022).…”

“…The total dataset includes 491 fossil occurrences consisting of 227 different genera originating from 31 localities and 11 geological formations. The geological formations represented include: the Borrachon, the Dere, and the Santed of Spain (Álvaro and Martínez-Benítez 2023), the Couches de Barroubio, the Couches de la Maurerie, and the Saint-Chinian of France (Tortello et al 2006), the Trenice and Milina of the Czech Republic (Kraft et al 2014), the Wysockzi of Poland (Modliński and Szymański 2001), and the lower and upper Fezouata Shale of Morocco (Van Roy et al 2010, 2015a). We treat the lower Fezouata Shale (Tremadocian) and the upper Fezouata Shale (Floian) as distinct geological formations to facilitate the discussion of their temporal and taxonomic differences.…”