3D geometry and kinematics of the Northern Variscan Thrust Front in Northern France: new insights based on reprocessing and interpretation of seismic reflection profiles.

<p>In NW Europe, the Upper Carboniferous Variscan collision between Avalonia and the Armorica-Gondwana accretion complex led to the progressive tectonic inversion of the southern Avalonian margin and the development of a crustal-scale north-vergent thrust system propagating outward from the Late Mississippian to the Middle Pennsylvanian (330-305 Ma). The northern Variscan thrust front spreads over 2,000 km across NW Europe. In the Nord-Pas-de-Calais (NPC) coal district area (northern France), its 3D geometry and kinematics have been investigated through the reprocessing and interpretation of 532 km in length of industrial seismic reflection profiles acquired in the 1980s. The seismic interpretations point out the major compressional and extensional tectonic features affecting this fossil, deeply eroded, mountain front, highlighting its very atypical structure and kinematics.</p><p>The deformation front is characterized by a main frontal thrust zone localizing most of the northward displacement (i.e. several tens of kilometers) of the Ardennes Allochthonous Unit above the slightly-deformed part of the Avalonian margin, referred to as the Brabant Para-autochthonous Unit. This large displacement induced the underthrusting of the molassic foreland basin (NPC coal basin) over nearly 20 km and was associated to the out-of-sequence dislocation of the mountain front. The underthrust Brabant Para-autochthonous Unit, made of both the Namurian-Westphalian (330-305 Ma) molassic foreland basin and the underlying Mid-Upper Devonian (390-360 Ma) and Dinantian (360-330 Ma) carbonate platform, is deformed by a series of second-order north-vergent thrust faults, often associated with ramp-related folds. These thrust faults are rooted in décollement zones located either at the transition between the Namurian shales and the Dinantian carbonates or in the Famennian shales.</p><p>The 3D integration of the seismic interpretations led to the characterization of a major lateral ramp oriented NW-SE, affecting both the main frontal thrust zone and the basal thrust of some Overturned Thrust Sheets developed at its footwall. This lateral ramp represents a major zone of relay along the thrust front, in between two major segments, oriented respectively ENE-WSW to the east and WNW-ESE to the west. At the base of the underthrust Brabant Para-autochthonous Unit, the Mid-Upper Devonian platform is shown to be structured by synsedimentary normal faults responsible for the southward deepening and thickening of the southern Avalonian margin. These faults are oriented along two main directions i.e. N060-080° and N110-130°, that is the general orientation of the future Variscan structures. Overall, the results indicate that the Devonian pre-structuration of the southern Avalonian margin exerted a primary control on the dynamics and segmentation of the Northern Variscan Front in northern France by localizing both the frontal and lateral ramps within the thrust wedge.</p>

Upper Cretaceous paleoenvironmental changes and petrophysical responses in lacustrine record (Songliao Basin, NE China) and marine sedimentary deposit (Goban Spur Basin, NW Europe)

The Cretaceous interval is marked by several important geological changes whose prints are buried in both continental and marine sytems. Although significant paleoenvironmental details of this period have been inferred from biological and geochemical indicators, little is known about the physical proxies. Through scientific borehole data, petrophysical properties of Upper Cretaceous Songliao Basin (SB) in NE China and Goban Spur Basin (GSB) in NW Europe were intercorrelated to investigate the critical geological paleoenvironmental shifts and their petrophysical responses, through statistical, wavelet and spectral approaches. The results demonstrated that petrophysical features, particularly gamma-ray and resistivity reactivities, were responsive to past environmental changes in both terrestrial and marine systems. Shifts in organic-rich shale deposition and brine bearing shale showed a correlation to a probable period of seawater incursion in SB, while the gamma log, resistivity and density reactivities were interrelated to the basin paleo-structuration. At GSB, the gamma-ray and resistivity reactivities are tied-up to the Mid-Atlantic seabed motion, marine-water level shifts and paleoceanographic instabilities. In both paleo-basins, a decrease in the gamma-ray reactivity occurred from Turonian to Maastrichtian and is consistent with a regional or global increase in hydrodynamic energy. The oceanic/lacustrine anoxic events related to low sedimentation rate occurred in both basins and are associated with high gamma-ray and resistivity signals (SB); high gamma-ray and low resistivity signals (GSB). These changes correlated with geochemical evidence, suggesting that gamma-ray and resistivity can represent alternative means for marine and continental paleoenvironmental comparison.

About this title - Subsurface Sand Remobilization and Injection

Sand injectites form during shallow-crustal deformation. Short periods of elevated pore-fluid pressure, which developed regionally, triggered formation of hydrofracture networks into which sand was sometimes injected. Sand injection complexes preserve a record of this process and sandstone intrusions are significant reservoirs in many petroleum systems. Most known subsurface sand injection complexes are from offshore NW Europe and associated with Paleogene strata. Outcrop occurrence is global. Sand injection into unconventional host rocks, including granitoid and metamorphic basement and coal seams, raises awareness of the breadth of geological environments in which sand injection may occur. Discordance between sandstone intrusions and sedimentary hosts occurs on a scale from millimetres to kilometres and is a fundamental diagnostic of intrusions. Microscale textural characterization provides new opportunities to establish possible additional criteria for differentiating intrusions from depositional sandstone. The significance of sand injection complexes in shallow crustal evolution is exemplified by the wide range of lithological hosts and diverse tectonostratigraphic settings documented in this volume. Potential for original research still remains.

New evidence on the earliest domesticated animals and possible small-scale husbandry in Atlantic NW Europe

The distribution of the first domesticated animals and crops along the coastal area of Atlantic NW Europe, which triggered the transition from a hunter-gatherer-fisher to a farmer-herder economy, has been debated for many decades among archaeologists. While some advocate a gradual transition in which indigenous hunter-gatherers from the very beginning of the 5th millennium cal BC progressively adopted Neolithic commodities, others are more in favor of a rapid transition near the end of the 5th millennium caused by a further northwest migration of farmers-herders colonizing the lowlands. Here, radiocarbon dated bones from sheep/goat and possibly also cattle are presented which provide the first hard evidence of an early introduction of domesticated animals within a hunter-gatherer context in NW Belgium, situated ca. 80 km north of the agro-pastoral frontier. Based on their isotope signal it is suggested that these first domesticates were probably not merely obtained through exchange with contemporaneous farmers but were kept locally, providing evidence of small-scale local stockbreeding in the lowlands maybe as early as ca. 4800/4600 cal BC. If confirmed by future in-depth isotope analyses, the latter testifies of intense contact and transmission of knowledge in this early contact period, which is also visible in the material culture, such as the lithic and pottery technology. It also implies direct and prolonged involvement of farmer-herders, either through visiting specialists or intermarriage, which follows recent genetic evidence demonstrating much more hunter-gatherer ancestry in early farmer’s genes in western Europe compared to central and SE Europe.

New ichnofabrics of the Cenomanian–Danian Chalk Group

ABSTRACT The Cenomanian–Danian Chalk Group of NW Europe is characterized by distinct trace-fossil assemblages dominated by Thalassinoides isp., Planolites isp., Zoophycos isp., and Chondrites isp., whereas ichnogenera such as Taenidium and Phycosiphon are rare. The trace fossils form a complex tiering arrangement, which reflects burrowing activities of diverse benthic associations that operate at different levels in the sediment column, dynamic sedimentation rates, and changes in substrate hardness during progressive burial, forming intricate ichnofabrics. In the Danish Basin, studies of chalk ichnofabrics have focused mainly on the Maastrichtian. Studies of the shallower, grain-rich Danian chalk have revealed similar trace-fossil assemblages, whereas the ichnology of the fine-grained, deeper-water Danian deposits is poorly known. Based on detailed facies and ichnofabric analysis of a mid-Danian silica-rich, pelagic chalk located in the central, deeper shelf area of the Danish Basin, four facies types, eight ichnotaxa, and two ichnofabrics are recognized. Most conspicuous and abundant are randomly distributed, variously sized meniscate burrows attributed to Bichordites isp. and Taenidium isp., whereas other common chalk trace fossils are rare or absent. This trace-fossil assemblage outlines two new ichnofabrics in the NW European chalk, which are dominated principally by upper-tier traces. The producer of the abundant Bichordites isp. and Taenidium isp. burrows is identified as a sea urchin on the basis of an exceptionally preserved Bichordites isp. trace aligned with an irregular echinoid body fossil. The identified ichnofabrics controlled early silicification and produced a more complex distribution of silica concretions compared with chalk successions elsewhere. This results in volumetrically thick silica concretion-rich units rather than distinctive silica bands as seen in other Upper Cretaceous and Danian chalk units.