Search

A promising primary reference material for U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) carbonate dating is analysed and reported here. The new reference material (RM) is a botryoidal cement (C1) from sample RA138. The sample was collected in outcrop strata of mid-Carboniferous age in northern Spain near La Robla, and multiple aliquots have been meticulously prepared for distribution. RA138 is characterized by variable U/Pb ratios (from ∼ 1 to ∼ 19) and a relatively high and homogeneous U content (∼ 4 ppm). This material exhibits a low age uncertainty (0.2 %, 2 s; unanchored; ID-TIMS), allowing for the establishment of a well-defined isochron, particularly when anchored to the initial Pb ratio using LA-ICP-MS. Isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses of micro-drilled C1 cement (17 sub-samples) produce a lower-intercept age of 321.99 ± 0.65 Ma, an initial 207Pb/206Pb ratio of 0.8495 ± 0.0065, and a mean square weighted deviation (MSWD) of 5.1. The systematic uncertainty of 1.5 % observed in repeated LA-ICP-MS analyses challenges previous estimations of 2 %–2.5 % based on repeated analyses of ASH-15D and JT using WC-1 as primary reference material, underscoring the precision and reliability of RA138 for U–Pb dating applications. [ABSTRACT FROM AUTHOR]

The final stages of the Variscan orogeny (mostly Carboniferous) in the Western European Variscan belt involved the development of the Iberian-Armorican arc, which is cored by the Cantabrian zone (NW Iberia). The Cantabrian zone is the foreland of the Western European Variscan belt, and it is interpreted to record the waning stages of the closure of the Rheic Ocean. The distalmost tectonic units within the Cantabrian zone (the Cuera Unit and the Picos de Europa Province) were the last tectonic units emplaced at the core of the Western European Variscan belt orocline. Together, they form an imbricate system and associated wedge-top basins that are key to understanding the development of the orocline. The emplacement of the Cuera Unit and the Picos de Europa Province occurred in the latest Penusylvanian, between Moscovian and Gzhelian times. New detailed mapping together with stratigraphic, sedimentologic, and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system. Our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the Cantabrian Variscan foreland fold-and-thrust belt, most probably during the oroclinal bending of the SW European Variscan belt that formed the Iberian-Armorican arc. The N-S--directed imbricate system was characterized by a shallow dip of the topographic surface ([alpha] < 1[degrees]), allowing for the development of wedge-top basins. The large amount of shortening (>150 [+ or -] 15 km) and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying Gondwana lithosphere and may reflect the closure of the Iberian-Armorican arc.

The Carboniferous carbonate rocks cropping out in the NE of the Cantabrian Zone formed part of a microbial boundstone-dominated carbonate platform characterized by high-relief mar- gins and steep depositional slopes. Although the most continuous and best-preserved cross-sections of this carbonate platform, showing its seismic-scale depositional profile, have been intensely studied (Sierra del Cuera and Las Llacerias), other are poorly documented from a sedimentological point of view. This paper describes the San Antolín-La Huelga section, where the excellent exposures and the well-preserved depositional fabrics make it a reference section for the characterization of the lithofaties belts forming this particular depositional system. [ABSTRACT FROM AUTHOR]

Seismic-scale continuous exposures of an Upper Carboniferous (Bashkirian–Moscovian) carbonate platform (N Spain) provide detailed information about the lithofacies and stratal geometries (quantified with differential global positioning system measurements) of microbial boundstone-dominated, steep prograding and aggrading platform margins. Progradational and aggradational platform-to-slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable, high-relief depositional systems. The Bashkirian is characterized by rapid progradation at rates of 415–970 m My−1. Characteristic outer-platform facies are high-energy grainstones with coated intraclasts, ooids and pisoids, moderate-energy algal-skeletal grainstones to packstones and lower energy algal packstone and boundstone units. The Moscovian aggradational phase is characterized by aggradation rates of 108 m My−1. Coated-grain shoals are less common, whereas crinoidal bars nucleated in well-circulated settings below wave-base. Boundstones form a belt (30–300 m wide) at the platform break and interfinger inwards with massive algal-skeletal wackestones (mud-rich banks). The progradational phase has divergent outer-platform strata with basinward dips of 12° to 2°. Steep clinoforms with dips of 20–28° are 650–750 m in relief and possibly sigmoidal to concave in the lower part. The basinward-dipping outer-platform strata might be depositional for less than 6°, consistent with lithofacies deepening seaward. The basinward dip is attributed to the downward shift of upper-slope boundstone, forced by late highstand and relative sea-level fall, and to compaction-induced differential subsidence during progradation. The aggradational phase is characterized by horizontally layered platform strata. Clinoforms steepen to 30–45° reaching heights of 850 m and are planar to concave. The evolution from progradation to aggradation, at the Bashkirian–Moscovian boundary, is attributed to increased foreland-basin subsidence and decreased boundstone accumulation rates. Progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top. Within the major phases, aggradational–progradational increments are produced by third- to fourth-order relative sea-level fluctuations. [ABSTRACT FROM AUTHOR]

Abstract Positive shifts in global seawater δ[sup 13] C[sub DIC] are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ[sup 18] O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ[sup 13] C and δ[sup 18] O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and [sup 13] C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate [sup 13] C and [sup 18] O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas. [ABSTRACT FROM AUTHOR]