Only the GSSPs for the Bashkirian (base of the Pennsylvanian), Visean and Tournaisian (base of the Mississippian) have been formalized, although the latter now has complications.

The Carboniferous Period

Optical thermal maturity parameters and organic geochemical alteration at low grade diagenesis to anchimetamorphism: A review

The formation of an orocline by multiphase deformation: a paleomagnetic investigation of the Cantabria–Asturias Arc (northern Spain)

Quantitative analysis of the smectite-to-illite and illite-to-muscovite transformations indicates that 17–28 wt.% SiO2 and 17–23 wt.% SiO2, respectively, are liberated during these reactions, assuming that Al is conserved. Dissolution of quartz silt in shales yields up to 6–9% SiO2 in the range up to 200°C and a further 10–15% SiO2 in the 200–500°C range. For muds altered to shales at 200°C, 14–20 wt.% silica is evolved. From 200 to 500°C, a further 18–28 wt.% silica is evolved. Additional small amounts of silica may be released in the alteration of feldspar to clay and by stylolitization of quartz silt. Thus, in the burial and temperature range of diagenesis into the epizone, major quantities of silica are released from clays and by quartz dissolution in shales. Within this range of alteration, concomitant decline of whole-rock Si/Al (SiO2/Al2O3) in the transformation of smectite to illite to muscovite suggests the liberated silica migrates from the source shale. As a result, the metamorphosed shales are more micaceous and less quartzose than their progenitor muds. In the diagenetic zone and anchizone, the evolved silica is probably a major source of quartz cement in sandstones. In the epizone, evolved silica is commonly present in quartz veins in the parent rocks. Fluid-inclusion temperatures in quartz overgrowths and fracture fillings in some sandstones suggest that some cements may have been derived from downdip basinal shales and pressure solution in sandstones.

Smectite-illite-muscovite transformations, quartz dissolution, and silica release in shales

Massive hydrothermal dolomites in the southwestern Cantabrian Zone (Spain) and their relation to the Late Variscan evolution

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756897006882

Simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism: an example from the Variscan Belt of northwest Spain

Distribution of paleotemperatures has been studied in Paleozoic rocks of the Variscan fold and thrust belt of the NW Cantabrian Zone (Spain) by a combined application of illite crystallinity (IC) and conodont color alteration index (CAI). Results show that the diagenetic‐metamorphic grade increases with the increase in rock age, as marked by decreasing IC values and increasing CAI values. Moreover, the CAI isogrades roughly reflect the structural traits of the area, indicating that maximum temperatures were attained during sedimentary burial previous to folding. The geothermal paleogradient obtained suggests conditions near the boundary between low and middle P/T conditions. There is evidence of at least two thermal events during the tectonothermal development of the belt. The main stage is interpreted as the result of the burial of the rocks characterized by late diagenesis conditions that occurred under a geothermal gradient of ∼35°C km−1. The narrow zone with low anchizonal conditions in the w...

https://www.jstor.org/stable/10.1086/319978

Transition from Diagenesis to Metamorphism Near the Front of the Variscan Regional Metamorphism (Cantabrian Zone, Northwestern Spain)

The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism.

/pdf/tectono-thermal-evolution-in-a-region-with-thin-skinned-4p3lz6q51m.pdf

Tectono-thermal evolution in a region with thin-skinned tectonics : the western nappes in the cantabrian zone (variscan belt of nw spain)

La distribucion y caracteristicas de las zonas metamorficas en la transicion diagenesismetamorfismo, establecidas mediante la cristalinidad de la illita y el indice de alteracion de color de los conodontos, han permitido distinguir dos episodios de metamorfismo varisco en la Zona Cantabrica (ZC). El primero es el metamorfismo regional del NO de la Peninsula, que esta representado en pequenas areas de las partes O, NO y SO de la ZC, donde se llegan a alcanzar condiciones epizonales, y lleva asociado un clivaje generalizado; este episodio tambien se presenta como un metamorfismo de enterramiento que en la parte basal de algunos mantos alcanza condiciones anquizonales, estando cortado por los cabalgamientos basales de las principales unidades, por lo que el pico termico es anterior al emplazamiento de estas. El segundo episodio es un metamorfismo tardivarisco con dos manifestaciones diferentes: una es de caracter diastatermico y se desarrolla principalmente en las partes central y suroriental de la ZC, llevando asociado un clivaje que corta a la mayor parte de las estructuras; la otra es un metamorfismo de contacto alrededor de pequenos cuerpos intrusivos.

Los episodios metamórficos de la Zona Cantábrica y su contexto estructural

Typescript received 15 March 2005; accepted in revised form 25 July 2006 Abstract: Conodont collections from late Visean-early Bashkirian deposits include numerous elements related to the cosmopolitan species Gnathodus bilineatus (Roundy) in the Cantabrian Mountains. Arnsbergian beds yield the taxa Gnathodus bilineatus leonicus ssp. nov., G. millarensis ssp. nov., G. truyolsi ssp. nov. and Gnathodus sp. nov. A. Almost all are also found in limestones of similar age in the Pyrenees, and the G. truyolsi conodont Zone is defined for the upper Serpukhovian of the northern Iber- ian Peninsula. In addition, numerous references to the proposed taxa come from Austria, Eastern Europe, Nor- thern Africa and Asian localities. The increase in gnathodid diversity and abundance in northern Spain is related to a local Arnsbergian drowning episode of the carbonate plat- form. G. truyolsi is possibly derived from G. b. bilineatus, and there is morphological convergence with Gnathodus kiensis Pazukhin.

Susana García-López

Papers

Simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism: an example from the Variscan Belt of northwest Spain

Transition from Diagenesis to Metamorphism Near the Front of the Variscan Regional Metamorphism (Cantabrian Zone, Northwestern Spain)

Tectono-thermal evolution in a region with thin-skinned tectonics : the western nappes in the cantabrian zone (variscan belt of nw spain)

Los episodios metamórficos de la Zona Cantábrica y su contexto estructural

Serpukhovian conodonts from northern spain and their biostratigraphic application