The exhumation history of basement areas is poorly constrained because of large gaps in the sedimentary record. Indirect methods including low temperature thermochronology may be used to estimate exhumation but these require an inverse modeling procedure to interpret the data. Solutions from such modeling are not always satisfactory as they may be too broad or may conflict with independent geological data. This study shows that the input of geological constraints is necessary to obtain a valuable and refined exhumation history and to identify the presence of a former sedimentary cover presently completely eroded. Apatite fission-track (AFT) data have been acquired on the northern part of the Ardenne Massif close to the Variscan front and in the southern Brabant, in particular for the Visean ash-beds. Apatite fission-track ages for surface samples range between 140 ± 13 and 261 ± 33 Ma and confined tracks lengths are ranging between 12.6 ± 0.2 and 13.8 ± 0.2 μm. Thermal inversion has been realized assuming that (1) samples were close to the surface (20–40 °C) during Triassic times, this is supported by remnants of detrital Upper Permian–Triassic sediments preserved in the south of the Ardenne and in the east (border of the Roer Graben and Malmédy Graben), and (2) terrestrial conditions prevailed during the Early Cretaceous for the Ardenne Massif, as indicated by radiometric ages on paleoweathering products. Inversion of the AFT data characterizes higher temperatures than surface temperatures during most of the Jurassic. Temperature range is wide but is compatible with the deposition on the northern Ardenne of a significant sedimentary cover, which has been later eroded during the Late Jurassic and/or the Early Cretaceous. Despite the presence of small outliers of Late Cretaceous (Hautes Fagnes area), no evidence is recorded by the fission-track data for the deposition of a significant chalk cover as highlighted in different parts of western Europe. These results question the existence of the London-Brabant Massif as a permanent positive structure during the Mesozoic.
Abstract In the aftermath of the end‐ P ermian mass extinction, E arly T riassic sediments record some of the largest P hanerozoic carbon isotopic excursions. Among them, a global S mithian‐negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the S mithian– S pathian boundary ( SSB ; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the L ate S mithian. It is commonly assumed that S mithian and S pathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the T haynes G roup ( U tah, USA ) to evaluate the extent to which the E arly T riassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ 13 C carb variations obtained here reproduce the known S mithian δ 13 C carb ‐negative excursion. However, the δ 13 C signal of the bulk organic matter is invariant across the SSB and variations in the δ 34 S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that M iddle to L ate S mithian δ 13 C carb signal in the shallow marine environments of the T haynes G roup does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.
Abstract The Lower Triassic Mineral Mountains area (Utah, USA ) preserves diversified Smithian and Spathian reefs and bioaccumulations that contain fenestral‐microbialites and various benthic and pelagic organisms. Ecological and environmental changes during the Early Triassic are commonly assumed to be associated with numerous perturbations (productivity changes, acidifica‐tion, redox changes, hypercapnia, eustatism and temperature changes) post‐dating the Permian–Triassic mass extinction. New data acquired in the Mineral Mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures. These data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the Smithian–early Spathian. The studied section records a large‐scale depositional sequence during the Dienerian(?)–Spathian interval. During the transgression, depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral–microbial limestones, shallow subtidal marine sponge–microbial reefs to deep subtidal mud‐dominated limestones. Storm‐induced deposits, microbialite–sponge reefs and shallow subtidal deposits indicate the regression. Three microbialite associations occur in ascending order: (i) a red beds microbialite association deposited in low‐energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure; (ii) a Smithian microbialite association formed in moderate to high‐energy, tidal conditions where microbialites include stromatolites and associated carbonate grains (oncoids, ooids and peloids); and (iii) a Spathian microbialite association developed in low‐energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes. Data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase in accommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and the traditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned.
La reconstitution de la geodynamique et de l’evolution paleogeographique des massifs de Boheme et de l’Ardenne se place dans la comprehension des mecanismes de la propagation de la deformation dans l’avant-pays nord-alpin. Les episodes de mouvements verticaux les plus superficiels de la lithosphere continentale ont ete identifies et replaces chronologiquement grâce a l’application des methodes de traces de fission et (U-Th)/He sur les cristaux d’apatite. Les phases d’exhumations meso-cenozoiques sont liees a la reactivation des structures varisques, mais aussi a la propagation du front de deformation alpin sur la marge europeenne. L’histoire thermique post-varisque confirme qu’une couverture sedimentaire d’epaisseur kilometrique, d’âge jurassique, cretace superieur et neogene inferieur, a existe sur le soubassement paleozoique des massifs ardennais et bohemien, et que cette couverture a ete discontinue et d’epaisseur inegale en raison des paleoreliefs (multiplicite des depocentres). Cette couverture sedimentaire a ete erodee suite aux episodes d’inversion tectonique. Le centre de ces massifs, relativement plus stable, a connu un recouvrement sedimentaire superficiel par rapport a leurs regions bordieres. Au regard des resultats thermochronologiques basse temperature de ce travail, des donnees multi-methodes de la litterature, la deformation de l’avant-pays alpin subit un style cassant plutot que ductile. Dans le contexte tectonique des blocs structuraux du domaine bohemien et de l’Europe centrale, l’hypothese d’un flambage de la lithosphere n’est pas le seul processus pour expliquer la propagation des deformations ainsi que la repartition des terrains exhumes.
