Givetian-Frasnian spiriferids from the Asturian coast (Spain)
JENARO L. GARCÍA-ALCALDE
Universidad de Oviedo, c/ Jesús Arias de Velasco, s/n, 33005 Oviedo,España.
The study of the late Givetian-early Frasnian Devonian spiriferid brachiopod fauna of the Asturian coast, northern Spain is carried out. Ten spiriferid taxa are described, with five new taxa. “Adolfia” cf. sauvagei is a small form, sometimes with an asymmetrical beak, ribbed flanks, costellate sinus and dorsal fold, the sinus with a median costella and two pairs of parietals; low ventral interarea, broad delthyrium with deltidial plates joined in a short apical deltidium, small delthyrial plate within the delthyrial chamber; micro-ornamentation with subradial microcostellae forming chevrons and tubular spines with barrel-like bases. Rigauxia? cf. acutosina, is transverse, thick, frequently very asymmetrical; it shows seventeen to eighteen ribs per flank and four pairs of costellae in the sinus: one pair of primary and two to three pairs of secondary arising on either side of the primary; large, concave, apsacline ventral interarea, open delthyrium likely provided with a short delthyrial plate within the delthyrial chamber; micro-ornamentation with very fine subradial microcostellae and alternating teardrop-shaped spines; the occurrence of a delthyrial plate makes doubtful the assignment to the genus Rigauxia, although topotypic specimens of the French species seem to show the same feature. Cyrtospirifer gr. verneuili and Cyrtospirifer syringothyriformis are similar to each other, medium to large sized and transverse forms, with numerous ribs on the flanks (up to fifty or more) and sinus with fifteen or more costellae: a pair of primary and two or three orders of secondary, being the external ones parietal and simple and the internal intercalated and branched; well- developed delthyrial plate and tuberculate micro-ornamentation, but C. syringothyriformis is much more transverse, alate, and its ventral valve is hemipyramidal with a very high, catacline interarea. Tenticospirifer aff. plicatulus also resembles the above forms but it has fewer lateral ribs and less costellae in the sinus (four to ten): one pair of primary and one to three pairs of consecutive, parietal secondary; high, catacline, and flattened interarea, short delthyrial plate and micro-ornamentation consisting of subradial microcostellae expanded into teardrop-shaped granules. Tenticospirifer? sinuosus n. sp. is much smaller than the previous species, mucronate, very asymmetrical with ten to eleven ribs per flank and five to six fine costellae in the sinus: a pair of primary and two other secondary pairs on one side and the other of the primary; ventral beak curved on the interarea, which is very large and somewhat concave, narrow delthyrium with deltidial plates joined into a short apical deltidium, small delthyrial plate. Eodmitria briceae n. sp. is a small to medium-sized form, ventribiconvex, subquadrangular in outline, with a hinge shorter than the maximum width, poorly developed and defined sinus and median fold, usually with less than twenty ribs per flank and less than fifteen costellae on the sinus and fold, with divisions and intercalations that weaken towards the median plane of the shell; low and concave ventral interarea, wide and open delthyrium, short delthyrial plate; micro-ornamentation with subradial microcostellae prolonged in minute pustules. Apousiella mozarti n. sp. is a small, biconvex, and rather thick form, with a wide, well-developed sinus and median fold, it shows four to six ribs per flank, sinus provided with a strong median rib; narrow delthyrium covered by an apical deltidium, with no delthyrial plate nor dental plates; numerous, imbricate growth lamellae, without micro-ornamentation elements. Apousiella dorlodoti peranensis n. subsp. closely resembles the nominal subspecies but it has a clearly transverse, semi-elliptic to trapezoidal outline, sharp cardinal extremities and a higher number of lateral ribs (nine to eleven per flank); Apousiella belliloci aramaris n. subsp. also closely resembles the nominal subspecies, but it is larger, highly winged in outline, mucronate, with more radial ribs (nine to twelve per flank), relatively convex dorsal valve, and median fold standing out from the valve surface. Some of the studied taxa and other known forms such as Cyrtospirifer verneuiliformis, and Cariniferella dumontiana, characterize the informal biozones plicatulus, peranensis, briceae, sauvagei, aramaris, and dumontiana that allow good correlations between the Asturian stratigraphical formations of the Antromero and Perlora Synclines as well as with other Asturian and Leonese units (Portilla and Nocedo formations). Likewise, the faunal affinities with those derived from the Blacourt and Beaulieu formations, from Ferques, in the Bas-Boulonnais (France), allow establishing biogeographic and biostratigraphical links between both faunas. The exact meaning of the poorly understood Candás Formation, in the coastal area, is herein clarified. This formation is typified by the succession at the Peran Cove (Southeast of Candas) of 190 m thick limestones and lutites, mainly characterized by conodonts and brachiopods. The Candás Formation comprises four members, A, B, C, and D (approximately equivalent to the Castiello, Perán, Cantera, and Carranques Members). The marked change of facies and faunas in the upper part of this succession (in the Members C and D transition) was considered as the boundary between the so-called “lower Candas limestones” and the “upper Candas limestones”, which would represent the Givetian/Frasnian boundary. It is, certainly, a singular horizon that records an important global faunal turnover related to the so-called Taghanic Event. This Event is understood in the present work as a conjunction of the changes that occurred at the end of the Taghanic Biocrisis (middle-late Givetian boundary), at the beginning of the T-R IIa depophase (Geneseo Event, lower hermanni conodont Biozone, late Givetian), where abundant bioevents are recorded, including the first occurrence of all species described here. The Event has been also recognized by magnetic susceptibility data in the Antromero Syncline (Asturias) and in the Unit C, near the top of the Portilla Formation on the northern flank of the Alba Syncline, Leon province. On the other hand, the Givetian/Frasnian boundary, in its modern meaning, is located higher up, practically at the very top of the Candas Formation, according to the conodont fauna (falsiovalis Biozone, Mesotaxis falsiovalis, Ancyrodella binodosa, and A. pristina) and brachiopods (transition between the briceae–aramaris zones). Likewise, the meaning of the Piñeres Formation is clarified here. The Piñeres Formation would be composed of at least two members: P1 and P2. P1 (proposed here), comprises about 200 m thick of highly fossiliferous sandstones, shales, marls and sandy limestones early to middle Frasnian in age, deposited in a shallow marine environment, and intensely folded in the studied region. It was previously misconsidered by several authors as part of the Candás Formation. P2 is equivalent to the original Piñeres Formation and in the Antromero Syncline consists of more than 380 m thick of sandstone and microconglomeratic quartzite, deposited in littoral to supralittoral conditions. Unfortunately, the top of the Member is covered by Cretaceous rocks. The lack of representative fossils makes it difficult to date P2, but the facies analysis, stratigraphic position and distribution suggest being equivalent to the upper Famennian Ermita Sandstone, from the Leon province. The outstanding decrease in thickness of the Piñeres Formation recorded between the Antromero and Perlora Synclines is due, as previously established in other Cantabrian Mountains areas, to the occurrence of intense vertical eustatic changes at the beginning of the Famennian, during a global regressive phase which triggered the emersion and erosion of a large part of the Cantabrian marine continental shelf. The posterior tilting of the peneplaned surface in the upper Famennian allowed the progradation of a greater or lesser thickness of marine clastic sediments. In the Perlora Syncline, in particular, a large part of Member P1 and the entire P2 member disappeared, the latter being replaced by a highly condensed succession of bioclastic limestones (Baleas or Candamo Formation) from the latest Famennian (Siphonodella praesulcata to Scaliognathus anchoralis conodont biozones). The stratigraphic hiatus which includes a large part of the Frasnian and Famennian, has been previously recognized in Leon province, north of the Somiedo-Correcillas Nappe (Bregón Thrust, in Torrestio and Beberino) and in the Esla Nappe relative autochthonous (in Aleje). One of the most debatable concepts related to the studied area is the presence of the so-called “Surco de Luanco”. This structure would be a sedimentary through in the Luanco area, with a very high subsidence rate during the Devonian (Givetian to Famennian) which would produce a much thicker and deeper lithostratigraphical series in Luanco than in nearby localities. The tectonostratigraphic evolution of the basin at the Variscan orogeny, would account for the apparent anomaly, in particular, for the disparate thickness of the Piñeres Formation between the Antromero and Perlora Synclines. Other differences in thickness pointed out by previous authors were mainly due to mistaken correlations and unprecedented tectonic phenomena that the new biostratigraphical data provided have contributed to resolve.
Key words: Middle–Upper Devonian, brachiopods, Asturias, correlation..
How to cite: García-Alcalde, J. L. 2022. Givetian-Frasnian spiriferids from the Asturian coast (Spain). [Espiriféridos del Givetiense–Frasniense de la costa asturiana (España)]. Spanish Journal of Palaeontology, 37(2).
Received 26 May 2022, Accepted 26 September 2022, Published online 10 November 2022