Search

Observations of comet nuclei indicate that the main constituent is a mix of ice and refractory materials characterized by high porosity (70–75%) and low bulk strength (10 −4 –10 −6 MPa); however, the nature and physical properties of these materials remain largely unknown. By combining surface inspection of comet 67P/Churyumov–Gerasimenko and three-dimensional (3D) modeling of the independent concentric sets of layers that make up the structure of its two lobes, we provide clues about the large-scale rheological behavior of the nucleus and the kinematics of the impact that originated it. Large folds in the layered structure indicate that the merging of the two cometesimals involved reciprocal motion with dextral strike–slip kinematics that bent the layers in the contact area without obliterating them. Widespread long cracks and the evidence of relevant mass loss in absence of large density variations within the comet’s body testify that large-scale deformation occurred in a brittle-plastic regime and was accommodated through folding and fracturing. Comparison of refined 3D geologic models of the lobes with triaxial ellipsoids that suitably represent the overall layers arrangement reveals characteristics that are consistent with an impact between two roughly ellipsoidal cometesimals that produced large-scale axial compression and transversal elongation. The observed features imply global transfer of impact-related shortening into transversal strain. These elements delineate a model for the global rheology of cometesimals that could be possible evoking a prominent bonding action of ice and, to a minor extent, organics.

Early Jurassic shallow water carbonate environments of Tethys experienced important changes in a context of rifting that climaxed with the final break-up of Pangea. This time was also characterized by major perturbations of the global carbon cycle, some of which likely linked to the emplacement of large igneous provinces. At the Sinemurian−Pliensbachian (Early Jurassic) transition a globally recognized carbon isotope perturbation named “S−P Event” was found. In the western Tethys, this coincided with important architectural changes in the carbonate platforms which switched from peritidal flats to subtidal rimmed lagoons. A similar evolution in carbonate systems was observed during the Carnian Pluvial Episode (Late Triassic) when a global carbon isotope perturbation coincided with the demise of microbial carbonate platforms in the Tethys. In this paper, new carbonate and organic carbon isotope records and quantitative petrography data (modal analysis) from the deep- and shallow-water Sinemurian− Pliensbachian sedimentary successions exposed in the central Southern Alps of Italy are presented. Data show that across two negative carbon isotope excursions the carbonate factory abruptly changed and a drop in the microbial fraction of the carbonate occurred. This suggests that environmental modifications associated with these isotope excursions impacted the type of carbonate production and were a key-factor in determining the evolution of Tethyan carbonate platforms. New results and a comparison between the S−P Event and the Carnian Pluvial Episode highlight striking commonalities and imply that the demise of microbial carbonate systems coincident to isotope perturbations is not an isolated event in geologic history. Microbialites, therefore, do not necessarily represent “crisis facies” following extinction events. Rather, their development or demise appears linked to yet poorly understood mechanisms governing the interaction between global climate changes and shallow-water carbonate sedimentation.

The potential of Terrestrial Laser Scanner imaging (TLS) as a tool to map chert, an amorphous variety of silica diffused in sedimentary rocks, is here discussed together with an original method for its automatic detection. Reflectance measurements in the VIS-NIR band (400-2500. nm) show that chert displays low reflectance in the IR wavelengths that are operated by several commercial TLS. To develop and test a recognition method an outcrop of limestone with chert nodules was scanned with an IR (1541. nm) TLS. The intensity information, after proper distance correction, was coupled with geometric and intensity descriptors for training Support Vector Machines (SVM) to separate vegetation from rock and limestone from chert. Results, cross inspected in the field and with reference pictures, demonstrate that TLS data can be efficiently exploited to map chert when the monochromatic information of the intensity is integrated with feature descriptors and SVM classifiers. © 2014 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS).