Identifying eroded Messinian deposits on the Maltese Islands by gypsum Sr isotopes

International audienceEvaporite sediments are known to have formed during the Messinian high salinity crisis that has been recognized in many Mediterranean areas. This event was however never identified on the Maltese Islands. In this study, a Sr isotopic characterization of gypsum, found as secondary minerals in the Oligocene-Miocene formations of Malta and Gozo is performed. The 87Sr/86Sr values are discussed on the basis of comparison with the surrounding rock Sr isotopic signature. The data outline two distinguishable 87Sr/86Sr ratios for gypsum according to their facies. Most of crack-infilling mineral data suggest a downward fluid circulation in the Miocene sediment associated with interaction in the currently outcropping limestones. Some gypsum crystals found as crystallized spherules show significantly lower 87Sr/86Sr ratios, close to those defined in the literature for Messinian evaporate sediments. Taken together, this may suggest that an evaporate formation of Messinian age had in the past covered the present-day sedimentary sequence in Malta and Gozo

Geomechanical modelling and consequences for fluid-flow in complex rifted settings: A case study in the Otway Basin, Australia

Poster presented at the EGU General Assembly 2019Geomechanical modelling of dilation tendency has been completed on more than 900 faults from nine three dimensional seismic surveys within the Otway Basin, Australia. As the in-situ stress regime within the basin is currently debated, scenarios of normal, strike-slip and reverse regimes of in-situ stress have been modelled. Additionally, the stability of natural fractures from seven wellbore image logs has been modelled under the same scenarios, with the consequences for each explored. NW-SE striking faults that define the basin’s major structural fabric are at critical risk of dilation irrespective of the regime of in-situ stress, while similarly striking fractures require very low (<5MPa under a strike-slip scenario) increases in pore pressure in order to be reactivated. N-S striking and W-E striking faults show lower risks for reactivation although their propensity to dilate is still significant. Our results in part explain why fault seal within the Otway Basin has been historically so poor, and suggest that while natural fracture networks may be optimally oriented for reactivation in order to increase secondary permeability – promising for unconventional prospectively - there is a high associated risk with respect to up-dip contamination along regional faults. This case study also provides insight into possible fluid flow pathways within other more frontier passive margin settings.Hugo Burgin, Khalid Amrouch, David Kulikowski, Simon Holford, and Philippe Robio

Layer parallel stretching? Characterising magnetic and pore-fabric styles at a rifted continental margin: New insights from the Otway Ranges, Australia

Available online 25 June 2021This study presents the first analysis of the anisotropy of magnetic susceptibility (AMS) coupled with anisotropy of P-wave velocity (APV) in sedimentary rocks from a passive continental margin. In the Otway Ranges, Australia, the Lower Cretaceous sediments deposited during a period of subsidence and structural quiescence display triaxial-magnetic fabrics typical of extensional deformation. The extensional interpretation from AMS is also supported by the orientation of the pore fabric measured using APV. The inferred extensional azimuths from both AMS and APV analyses are in good agreement with axes for renewed Late Cretaceous rifting and the continental break up of Gondwana, a period of structural evolution characterised by abnormally-high levels of paleostress in the Otway Basin. Predicated on this we introduce the concept of layer parallel stretching (LPSt), a process that describes pore-scale structuring representative of the first phase of extensional deformation during periods of intense tectonic extension, in this case the LPSt was oriented NE-SW. These extensional structural fabrics in the Otway Basin were preserved during NE-SW oriented basin inversion, a process that involved reactivation of the local detachment, the partitioning of strain, and low levels of coupling between the basement and the cover. The results highlight the applicability of AMS and APV for characterising rock anisotropy in sedimentary basins at continental margins and have distinct outcomes with respect to the structural framework of the study area in the Otway Ranges.Hugo B. Burgin, Philippe Robion, Khalid Amrouc

The conundrum of remagnetized Albian turbidites in the Basin of Mauleon, North Pyrenees.

ACTInternational audienc

Fabric development and metamorphic evolution of lower Palaeozoic slaty rocks from the Rocroi massif (French-Belgian Ardennes): new constraints from magnetic fabrics, phyllosilicate preferred orientation and illite crystallinity data

This paper presents new results of a petrofabric study from Cambrian slates cropping out within the anchiepimetamorphic core of the Rocroi massif (Ardennes Variscan fold-thrust belt of northern France and southern Belgium). This study includes measurements of the Anisotropy of the Isothermal Remanent Magnetisation (AIRM) and the phyllosilicate preferred orientation. It completes the previous results of Anisotropy of Magnetic Susceptibility (AMS) and of magnetic mineralogy which were obtained in 47 sites situated in and around this massif. These new data allow the characterisation of the different subfabrics carried by the ferromagnetic minerals (metamorphic pyrrhotite and magnetite) and the paramagnetic matrix (Fe-rich phyllosilicates). Combined with some illite crystallinity data measured throughout the massif, the petrofabric measurements evidence a polyphase tectonometamorphic evolution. A major Variscan compressional event is evidenced inducing the growth of highly anisotropic pyrrhotite, coarse-grained hematite and phyllosilicates. By contrast, the magnetite grains display a less well-organised fabric due to the existence of an inherited orientation pattern. In agreement with recent metamorphic studies, it argues for an early diastathermal metamorphic event developed during a Devonian crustal-scale extension. (C) 1999 Elsevier Science B.V. All rights reserved.status: publishe