Silvia Blanco-Ferrera

Transition from diagenesis to metamorphism in a calcareous tectonic unit of the Iberian Variscan belt (central massif of the Picos de Europa, NW Spain)

Transition from diagenesis to metamorphism has been characterized in the central part of the Picos de Europa unit (Cantabrian Zone) by conodont colour alteration index (CAI), complemented with Kubler index (KI) data. This unit essentially comprises Carboniferous limestone. The study leads us to deduce two successive tectonothermal events. The first event generated a pattern with palaeotemperatures increasing towards the Pisuerga–Carrion unit, located south of the Picos de Europa unit. The present pattern is the result of an original dip to the north of the isothermal surfaces and the subsequent rising of the Picos de Europa unit along the frontal ramp of a deep Alpine thrust. This episode is interpreted as related to an extensional tectonic regime that occurred close to the Carboniferous–Permian boundary. The second event, which gave rise to thermal anomalies in the pattern of the first episode, was the result of hydrothermal processes in which fluid movement was facilitated by a dense network of fractures in the area close to the eastern section of the studied region. These processes gave rise to numerous mineralizations and have been related to a Permian extensional tectonic regime.

Conodont alteration and tectonothermal evolution of a diagenetic unit in the Iberian Variscan belt (Ponga-Cuera unit, NW Spain)

Tectonothermal analysis of a mainly carbonate unit located in the external part of the Variscan orogen in NW Spain is dealt with using the conodont colour alteration index (CAI) and the study of textures of these microfossils. The Kubler index of the illite (KI) is used as a complementary method. The area is characterized by a great uniformity in the CAI values, which in most cases are

Overgrowths of large authigenic apatite crystals on the surface of conodonts from Cantabrian limestones (Spain)

Conodonts from the middle to upper Paleozoic limestones of the Cantabrian zone commonly show apatite overgrowths. A large crystal microtexture observed under the SEM corresponds to local rims of euhedral to subhedral apatite crystals, which were preceded by the neoformation of smaller crystals. Four types of this microtexture (blocky, columnar, fan, and denticular) are described on different areas of the oral surface of conodonts, whereas dissolution features may be present in the basal cavity area. The distribution of these types of microtexture in different areas of conodont morphology suggests a general trend to neocrystallization, where crystal size increases towards the top of the conodont ornamentation and a chemical gradient controls the crystalline growth. This arrangement is widely related to the surface morphology and to the general conodont histology. The large crystal microtexture grows during early diagenesis from near surface to moderate burial and is linked to the known secondary apatite cement present in natural fused clusters of conodonts. The features described here are also compared to microtextures developed on conodonts during low- to medium-grade metamorphic conditions, where phosphate in solution is available.

Tectonothermal evolution of a major thrust system: the Esla–Valsurbio unit (Cantabrian Zone, NW Spain)

The tectonothermal evolution of a unit in the foreland fold-and-thrust belt of the Iberian massif is established using the conodont colour alteration index (CAI). The unit consists of two parts with different tectonothermal histories – the Esla nappe region and the Valsurbio region – separated by a synorogenic Carboniferous basin (Guardo–Valderrueda basin). The Esla nappe region evolved in diacaizonal conditions (corresponding to the diagenetic conditions of the pelites) whose palaeotemperatures were controlled by rock burial. Maximum values were reached before the emplacement of the thrust nappes, so tectonic superimposition is not registered by the CAI. Overburial due to the emplacement of the thrust units was prevented by simultaneous intense erosion. The geothermal gradient obtained for burial was c . 35 °C km −1 and the temperature reached by the older Cambrian rocks was c . 210–230 °C. The Valsurbio region was affected by an extensional tectonothermal post-orogenic event that gave rise to metamorphism with ancaizonal or epicaizonal conditions (corresponding to anchizone or epizone of the pelites). The most common maximum palaeotemperatures reached in this event fall within the range 305–415 °C, although higher palaeotemperatures could be reached locally as a consequence of contact metamorphism. This event gave rise to subhorizontal cleavage that cuts the main Variscan folds. Coal rank data indicate an increase in maximum palaeotemperatures eastwards from the Esla nappe region to the Valsurbio region through the Guardo–Valderrueda basin.

The Late Ordovician–Silurian succession in the Marimanha Massif (central Pyrenees, Spain) and comments on the first the occurrence of the conodont Kockelella walliseri in North Gondwana

AbstractPurposeThe study of the Upper Ordovician to Silurian stratigraphic succession in the metamorphic rocks of the Marimanha Massif (central Pyrenees) includes a regional pre-Upper Ordovician unconformity, a locally described pre-Hirnantian unconformity and a pre-Wenlock, Silurian hiatus. Silurian metamorphic carbonates correspond to a condensed succession that comprises pelmatozoan and cephalopod-bearing marbles, with reduced siliciclastic input. The occurrence of Silurian condensed carbonate succession is unusual in the deep-water anoxic shale sedimentation with mainly pelagic faunas observed in the northern margin of the central part of Gondwana.MethodsThe present study includes 6 stratigraphic sections and 8 samples with poorly preserved Silurian conodont among 30 collected samples.ResultsMarbles yield Sheinwoodian (Wenlock) conodonts such as Kockelella cf. corpulenta, K. ortus, K. ranuliformis and K. walliseri, and the Ludlow K. cf. variabilis. The occurrence of K. walliseri is the first record in the southern Europe and North Africa. This species was described from shallow-water facies in low latitudes of Laurentia, Baltica and eastern Gondwana.ConclusionsThe Upper Ordovician to Silurian sedimentary gaps in the studied succession are relate to the Late Ordovician rifting and the Hirnantian glaciation described in north Gondwana. Wenlock carbonate sedimentation occurred in a local shallow-water topographic palaeo-high. Conodont content suggest relatively shallow and warm water environment and a moderate to low latitude location in relation to the Silurian northward drift of areas derived from the Gondwanan margin as the Pyrenees.ResumenObjetivoEl estudio de la sucesión estratigráfica del Ordovícico Superior al Silúrico en las rocas metamórficas del Macizo del Marimanha (Pirineos centrales) incluye una discordancia regional pre-Ordovícico Superior, una discordancia pre-Hirnantiense y un hiato Silúrico pre-Wenlock. La sucesión silúrica condensada está formada por mármoles con abundantes pelmatozoos y otros con cefálopodos. La presencia de sucesiones carbonatadas silúricas es inusual en el margen norte de Gondwana, donde la sedimentación está caracterizada por lutitas depositadas en fondos anóxicos y con faunas de de aguas profundas.MétodosSe han realizado 6 secciones estratigráficas y recogido 30 muestras para conodontos que han propocionado especímenes silúricos en 8 de ellas.ResultadosLos conodontos extraídos corresponden a especies del Sheinwoodiense (Wenlock) como Kockelella cf. corpulenta, K. ortus, K. ranuliformis and K. walliseri, y del Ludlow como K. cf. variabilis. El hallazgo de K. walliseri es el primero en Europa meridional y el Norte de Africa. Esta especie sólo se había descrito en facies de aguas someras y en latitudes bajas de Laurentia, Baltica y Gondwana oriental.ConclusionesLos hiatos reconocidos en la sucesión estudiada son relacionados con las cuencas extensionales formadas en el norte de Gondwana entre el el Ordovícico Superior–Silúrico y la glaciación del Hirnantiense. La sedimentación carbonatada del Wenlock tuvo lugar sobre una paleotopografía elevada. Los conodontos estudiados sugieren un medio de aguas cálidas y en plataformas de paleolatitudes bajas a moderadas que se relaciona con el desplazamiento hacia el norte de areas derivadas del margen de Gondwana, como los Pirineos.

The apparatus of the Carboniferous conodont Vogelgnathus simplicatus and the early evolution of the genus

Abstract. The apparatus of Vogelgnathus simplicatus (Rhodes, Austin, and Druce, 1969) is reconstructed from discrete elements from a sample of limited diversity from the Limerick Province (Ireland). The apparatus is typical of the order Ozarkodinida and the P1 element was previously placed within Gnathodus. Here we assign it to Vogelgnathus by applying a multielemental concept rather than using P1 element morphology. The holotype and paratypes are re-illustrated and the species distribution revised based on published data. Vogelgnathus simplicatus ranges from the late Tournaisian to the early Viséan (Mississippian, Carboniferous), with common occurrences relating to the growth of Waulsortian bank complexes in a high-stand sea-level along the southern and western margins of the Laurussian landmass (Belgium, the British Isles, the Republic of Ireland, and USA). Vogelgnathus simplicatus appears to represent the rootstock from which deep-water and shallow-water Viséan species of Vogelgnathus evolved in the Mississippian.

Constraining the Timing and Amplitude of Early Serpukhovian Glacioeustasy With a Continuous Carbonate Record in Northern Spain

During the Late Paleozoic Ice Age (LPIA, 345–260 Ma), an expansion of ice house conditions at 330 Ma caused a nearly synchronous, global unconformity. Subaerially exposed paleotropical carbonates were dissolved by meteoric waters, mixed with the light terrestrial carbon, and recrystallized with overprinted, diagenetic dC values. In Northern Spain, development of a rapidly subsiding foreland basin kept local sea level relatively high, allowing continuous carbonate deposition to record dC without meteoric overprint. The Spanish sections show a 2& increase in dC that can be modeled as the ocean’s response to the creation of a significant light carbon sink through widespread meteoric diagenesis of marine carbonates during the near-global hiatus. About 15–35 m of sea level fall would have exposed a large enough volume of carbonate to account for the positive excursion in dC of oceanic DIC. Combining the dC data with high resolution biostratigraphy and new ID-TIMS U-Pb zircon ages from interbedded tuffs, we calculate that the depositional hiatus and glacioeustatic fall caused by the early Serpukhovian phase of ice growth lasted for approximately 3.5 My.