Bioestratigrafía de la Formación Pizarras del Sueve (Ordovícico Medio) en el sector septentrional de la escama de Laviana−Sueve (Zona Cantábrica, Norte de España)

A complete review of more than one hundred Middle Ordovician fossil localities, distributed within the north western part of the Hesperian Massif, shows that the sedimentation of the dark shales (Luarca Fo rmation and equivalents) that overlie Arenig quartzites with Cruziana (the 'Armorican Quartzite' facies) was not as uniform as it has been supposed for the whole NW Spain. These shales were mainly deposited during the Oretanian in the West Asturian-Leonese Zone and in nort h e rn Central-Iberian Zone (Domain of the Ollo de Sapo Antiform). In these zones, the top of the unit is close to the Oretanian/Dobrotivian boundary, without any fossils with proved Dobrotivian age. In the Cantabrian Zone, clay sedimentation started in the latest Oretanian and continued during the Dobrotivian. Several local or regional stratigraphic gaps are proposed and characterized for the whole study area. In addition, the most recent paleog e ographical reconstructions proposed are discussed, in accordance with new paleoecological and paleobiog e ographical data. These data indicate that sedimentation took place in open shelf areas, relative ly deeper than in the southern Central Iberian shelf, and with trough areas where some mesopelagic elements are recorded. We identified a total of 97 different fossil taxa (67 from Oretanian rocks and 45 from Dobrotivian rocks), remarkable among which are the first known appearance of certain trilobites and ostracodes, the latest record of other taxa, and also the presence of some taxa in common with Avalonia and Baltica, that were previously unknown from any area of SW Europe.

Abstrakt, pasted as per aricle: 

Nine fossiliferous sections, located in the northern part of the Laviana-Sueve thrust-sheet (North of the Ventaniella Fault) have been studied, thus enabling the lithological and biostratigraphic characteristics of the Sueve Shale Formation to be established. This 50-100 m thick formation has been formally defined as a unit consisting of two black shale members (i.e., the lower Cerracin and the upper Cofiño Members) and a middle member of sandstones and shales (Bayo Member). Resting unconformably on the Barrios Formation (Armorican Quartzite facies), the formation begins with a laterally discontinuous oolitic ironstone layer. The base of the Cofiño Member also displays an ironstone band. The unit is overlain disconformably by Upper Devonian sandstones (Ermita Formation). The fossil record of the Sueve Formation includes nearly fifty marine invertebrate species (belonging to graptolites, trilobites, ostracods, brachiopods, molluscs, echinoderms), some ichnofossils, and more than seventy organic-walled microfossil taxa (chitinozoans and acritarchs). These are all particularly abundant in the Cerracin Member. Graptolites and microfossils have supplied very precise biostratigraphic data for dating and correlation. They prove that the Cerracin Member is of late Oretanian age (upper Aberdinian in the British sense), thus suggesting a stratigraphic gap above the Arenigian quartzite unit. The Oretanian/Dobrotivian boundary is located in the Bayo Member, whereas the Cofiño Member reaches late Dobrotivian age near its top (approximately equivalent to Llandeilan to early Aurelucian in the new British chronostratigraphy). Several biozones relating to different groups have been recognized. It is noteworthy that this formation yields the oldest records of some species of trilobites, ostracods, and brachiopods, of widespread distribution in SW Europe. These include Phacopidina micheli, Placoparia (Coplacoparia) cf. tournemini, Ogginscoz acadieli, Quadrata tromelini, Bradysia cf. B. benigna and Heterorthina morganstis, which were thought to be Dobrotivian, but which are associated in the Sueve Formation with upper Oretanian graptolites of the Murchisoni Zone. With regard to the microfossils, the Laufeldochitina actuata and Linochitina pissottensis Biozones, previously considered either entirely or partly Dobrotivian (Llandeilan p.p.), here show a range concurrent with the Murchisoni graptolite Zone, of late Oretanian (Aberdinian) age.