Les encroûtements sous-marins de Méditerranée orientale : une explication génétique

Les encroûtements sous-marins ou fonds durcis ne sont pas spécifiques à la Méditerranée, mais ils y sont particulièrement abondants, notamment dans le bassin oriental. Ils tapissent les pentes de reliefs sous-marins et représentent un exemple de diagenèse superficielle liée aux périodes de vacuité sédimentaire. Il existe divers types d'encroûtements qui peuvent être distingués en fonction principalement de l'épaisseur de la partie indurée (appelée calcaire) et de la teinte générale du matériau. Le plus souvent le calcaire surmonte une boue encore plus ou moins meuble. L'induration (ou lithification) de cette dernière résulte de l'introduction d'un ciment de calcite magnésienne et on peut distinguer différentes étapes du processus d'après la nannostructure du calcaire. Le phénomène semble purement physico-chimique et résulterait de la conjonction de plusieurs facteurs : - présence de courants profonds; - sursaturation suffisamment élevée par rapport à la calcite non magnésienne; - très faibles concentrations en inhibiteurs de croissance cristalline autres que le magnésium. Bien qu'affectant des matériaux d'âges variés, cette forme de diagenèse sous-marine paraît essentiellement quaternaire. D'après les échantillons examinés, les conditions les plus favorables à la genèse des encroûtements apparaîtraient lors des stades climatiques froids (notamment le dernier) et plus particulièrement durant leur phase terminale

Normal faulting in chalk: Tectonic stresses vs. compaction-related polygonal faulting

This paper documents normal fault sets observed in chalks exposed in widely separated localities in the UK and France. These faults are characterized by having a wide range of strikes at any one locality, are developed entirely within the chalk succession and do not seem to interconnect to deeper or shallower structures. These structures may result from two different mechanisms: (1) complex polyphase deformational histories involving contrasting stress states; or (2) a single deformational phase in which the faults develop to accommodate compactional strains. Evidence is presented from microstructural and petrographic data to support the latter interpretation. In particular, the association of calcite and marcasite mineralizations with fracture surfaces and fault zones and textural observations relating flint occurrence to early fault formation point towards fault propagation at a very early stage of burial and compaction of the chalky sediments. An analogy is drawn between these outcrop-scale structures and polygonal fault systems at a larger scale recognised from seismic observations of chalk sequences deposited at passive continental margins. The origin of these structures may be related to syneresis at an early stage of deformation followed by pressure solution phenomena that may reactivate this early-inherited polygonal fault pattern until the present day

From Sediment to rock: Diagenetic processes of hardground formation in deep-water carbonate mounds of the NE Atlantic

Modern cool-water carbonate mounds topped by corals form an extended reef belt along the NW European continental margin at 200–1200 m water depth. An essential element of mound growth are hardgrounds which provide a stable substratum for mound-building invertebrate colonisation and stabilise the inclined mound flanks. Evaluating the degree of lithification and the slope stability against erosion represents an important task within the ESF programme MOUNDFORCE under the umbrella of EUROMARGINS. Sampling of hardgrounds during RV Meteor cruises M61-1 and -3 in 2004 by means of the IFM-GEOMAR TV-grab and the Bremen ROV QUEST focused on carbonate mounds of the Porcupine Seabight and northwestern Rockall Bank off Ireland. Lithified carbonates of mid-Pleistocene age were exhumed during the Holocene and are now exposed on the top and flanks of numerous carbonate mounds showing a patchy to dense colonisation by living corals and associated invertebrates. The sediments, composed of foraminiferal–nannoplankton oozes and admixed mound-derived invertebrate skeletons, range from partly lithified chalks to dense micritic limestones. These wackestones to packstones clearly differ from bacterially induced authigenic carbonate crusts typical of hydrocarbon seep settings by showing current-induced sedimentary structures, a non-luminescing matrix indicating oxic pore fluids, and a marine isotopic signature lacking any depleted carbon regime which is typical of anaerobic methane oxidation. The carbonate lithification is driven by carbonate ion diffusion from supersaturated seawater into the pore fluids in the studied areas. Vigorous bottom currents were the ultimate control not only of carbonate cementation by enhancing the diffusion process and supporting a pumping mechanism, but also of hardground formation and mound shaping by exhuming lithified carbonates and preventing fine-grained sediment accumulation at the downslope mound flanks