Kei Ogata

Peri-Adriatic melanges and their evolution in the Tethyan realm

In the peri-Adriatic region, mélanges represent a significant component of the Apennine and Dinaride–Albanide–Hellenide orogenic belts as well as ancient and present-day accretionary wedges. Different mélange types in this broad region provide an excellent case study to investigate the mode and nature of main processes (tectonic, sedimentary, and diapiric) involved in mélange formation in contrasting geodynamic settings. We present a preliminary subdivision and classification of the peri-Adriatic mélanges based on several years of field studies on chaotic rock bodies, including detailed structural and stratigraphic analyses. Six main categories of mélanges are distinguished on the basis of the processes and geodynamic settings of their formation. These mélange types are spatially and temporally associated with extensional tectonics, passive margin evolution, strike-slip tectonics, oceanic crust subduction, continental collision, and deformation. There appears to have been a strong interplay and some overlap between tectonic, sedimentary, and diapiric processes during mélange formation; however, in highly deformed regions, it is still possible to distinguish those mélanges that formed in different geodynamic environments and their main processes of formation. This study shows that a strong relationship exists between mélange-forming processes and the palaeogeographic settings and conditions of mélange formation. Given the differences in age, geographic location, and evolutionary patterns, we document the relative importance of mélanges and broken formations in the tectonic evolution of the peri-Adriatic mountain belts.

The importance of natural fractures in a tight reservoir for potential CO2 storage: a case study of the upper Triassic–middle Jurassic Kapp Toscana Group (Spitsbergen, Arctic Norway)

Abstract In the Longyearbyen CO2 laboratory project, it is planned to inject carbon dioxide into a Triassic–Jurassic fractured sandstone–shale succession (Kapp Toscana Group) at a depth of 700–1000 m below the local settlement. The targeted storage sandstones offer moderate secondary porosity and low permeability (unconventional reservoir), whereas water injection tests evidence good lateral fluid flow facilitated by extensive fracturing. Therefore, a detailed investigation of fracture sets/discontinuities and their characteristics have been undertaken, concentrating on the upper reservoir interval (670–706 m). Datasets include drill cores and well logs, and observations of outcrops, that mainly show fracturing but also some disaggregation deformation bands in the sandstones. The fracture distribution has a lithostratigraphical relationship, and can be subdivided into: (A) massive to laminated shaly intervals, offering abundant lower-angle shear fractures; (B) massive to thin-bedded, heterogeneous, mixed silty–shaly intervals, with a predominance of non-systematic, pervasive bed-confined fractures; and (C) massive to laminated, medium- to thick-bedded, fine- to coarse-grained sandstones with a lower frequency of mostly steep fractures. These domains represent pseudo-geomechanical units characterized by specific fracture sets and fracture intensity, with indicated relationships between the bed thickness and fracture intensity, and with domains separated along bedding interfaces. We discuss the impact of these lithostructural domains on the fluid flow pathways in the heterolithic storage unit.

Late Oligocene–early Miocene olistostromes (sedimentary mélanges) as tectono-stratigraphic constraints to the geodynamic evolution of the exhumed Ligurian accretionary complex (Northern Apennines, NW Italy)

In the Northern Apennines of Italy, mud-rich olistostromes (sedimentary mélanges) occur at different stratigraphic levels within the late Oligocene–early Miocene sedimentary record of episutural/wedge-top basins. They are widely distributed along the exhumed outer part of the Ligurian accretionary complex, atop the outer Apenninic prowedge, over an area about 300 km long and 10–15 km wide. Olistostromes represent excellent examples of ancient submarine mass-transport complexes (MTCs), consisting of stacked cohesive debris flows that can be directly compared to some of those observed in modern accretionary wedges. We describe the internal arrangement of olistostrome occurrences in the sector between Voghera and the Monferrato area, analysing their relationships with mesoscale liquefaction features, which are commonly difficult to observe in modern MTCs. Slope failures occurred in isolated sectors along the wedge front, where out-of-sequence thrusting, seismicity, and different pulses of overpressured tectonically induced fluid flows acted concomitantly. Referring to the Northern Apennines regional geology, we also point out a gradual lateral rejuvenation (from late Oligocene to early Miocene) toward the SE and an increasing size and thickness of the olistostromes along the strike of the frontal Apenninic prowedge. This suggests that morphological reshaping of the outer prowedge via mass-transport processes balanced, with different pulses over a short time span, the southeastward migration and segmentation of accretionary processes. The latter were probably favoured by the occurrence in the northwestern part of the Northern Apennines of major, inherited palaeogeographic features controlling the northward propagation of the prowedge. Detailed knowledge of olistostromes, as ancient examples of MTCs related to syn-sedimentary tectonics and shale diapirism, and of their lateral variations in term of age and size, provides useful information in regard to better understanding of both the tectono-stratigraphic evolution of the Apenninic prowedge and the submarine slope failures in modern accretionary wedges.

Effects of igneous intrusions on the petroleum system: a review

Igneous intrusions feature in many sedimentary basins where hydrocarbon exploration and production is continuing. Owing to distinct geophysical property contrasts with siliciclastic host rocks (e.g., higher Vp, density and resistivity than host rocks), intrusions can be easily delineated within data sets including seismic and CSEM profiles, provided igneous bodies are larger than the detection limit of the geophysical methods. On the other hand, igneous bodies affect geophysical imaging in volcanic basins. Recent analyses of 3D seismic data, supported by field observations and lab-based experiments, have provided valuable insights into the prevailing geometries of intrusions, i.e. (1) layerdiscordant dykes, (2) layer-parallel sills and (3) saucer-shaped intrusions. Where emplaced, intrusive bodies affect all five principal components of a given petroleum system: (1) charge, (2) migration, (3) reservoir, (4) trap and (5) seal. Magmatic activity may positively or adversely affect any of these individual components, for instance by locally enhancing maturation within regionally immature source rocks, typically 30-250% of the intrusion thickness, or by causing compartmentalization of source and reservoir rocks. Site-specific evaluations, including the timing and duration of the magmatic event are needed to evaluate the overall effect of intrusions on a given sedimentary basin’s petroleum system, and these are highlighted by case studies from different volcanic basins.

Sedimentary mélanges and fossil mass-transport complexes: a key for better understanding submarine mass movements?

Melanges originated from sedimentary processes (sedimentary melanges) and olistostromes are frequently present in mountain chains worldwide. They are excellent fossil examples of mass-transport complexes (MTC), often cropping out in well-preserved and laterally continuous exposures. In this article we will show the results of the integrated study of fossil MTCs, including sedimentary melanges/olistostromes, with a focus on the Apennines of Italy. Fossil MTCs, especially the basin-wide ones, are composite and multi-event units involving the entire spectra of mass-transport processes. The down-slope motion of these bodies is enabled by the relative movement of discrete masses, with progressive stratal disruption of rocks/sediment involved and flow transformation. Three kinds of MTC are here distinguished, in which the movements are enabled by (1) shear-dominated viscous flows within a muddy matrix, (2) mud-silt-sandy matrix sustained by fluid overpressure, (3) concentrated shear zones/surfaces with advection of grains and fluid (overpressured basal carpets). These MTC types may represent end-members of a continuum of products and correspond to different kinematics of transport and emplacement and to different relationship with the substratum. These observations should result in a better knowledge of mass-transport processes and bodies, in relation with the basin floor geometries.

A global review on ambient Limestone-Precipitating Springs (LPS) : Hydrogeological setting, ecology, and conservation

Springs are biodiversity hotspots and unique habitats that are threatened, especially by water overdraft. Here we review knowledge on ambient-temperature (non-geothermal) freshwater springs that achieve sufficient oversaturation for CaCO3 -by physical CO2 degassing and activity of photoautotrophs- to deposit limestone, locally resulting in scenic carbonate structures: Limestone-Precipitating Springs (LPS). The most characteristic organisms in these springs are those that contribute to carbonate precipitation, e.g.: the mosses Palustriella and Eucladium, the crenophilous desmid Oocardium stratum, and cyanobacteria (e.g., Rivularia). These organisms appear to be sensitive to phosphorus pollution. Invertebrate diversity is modest, and highest in pools with an aquatic-terrestrial interface. Internationally, comprehensive legislation for spring protection is still relatively scarce. Where available, it covers all spring types. The situation in Europe is peculiar: the only widespread spring type included in the EU Habitat Directive is LPS, mainly because of landscape aesthetics. To support LPS inventorying and management to meet conservation-legislation requirements we developed a general conceptual model to predict where LPS are more likely to occur. The model is based on the pre-requisites for LPS: an aquifer lithology that enables build-up of high bicarbonate and Ca(2+) to sustain CaCO3 oversaturation after spring emergence, combined with intense groundwater percolation especially along structural discontinuities (e.g., fault zones, joints, schistosity), and a proper hydrogeological structure of the discharging area. We validated this model by means of the LPS information system for the Emilia-Romagna Region (northern Italy). The main threats to LPS are water diversion, nutrient enrichment, and lack of awareness by non-specialized persons and administrators. We discuss an emblematic case study to provide management suggestions. The present review is devoted to LPS but the output of intense ecological research in Central Europe during the past decades has clearly shown that effective conservation legislation should be urgently extended to comprise all types of spring habitats.

The Specchio Unit (Northern Apennines, Italy): An Ancient Mass Transport Complex Originated from Near-Coastal Areas in an Intra-Slope Setting

Within the Eocene-Oligocene syn-orogenic deposits of the Epiligurian succession (Northern Apennines of Italy), a field-based study of the Specchio Unit (lower Rupelian) reveals that this complex is made up of three distinct but amalgamated mass-transport deposits (MTDs), the largest of which reaches a maximum volume of ca. 150 km3. These bodies were deposited inside the complex system of intraslope basin systems, developed atop the submerged Ligurian accretionary prism at the collision with the Adria continental plate. The MTDs originated from catastrophic retrogressive collapses starting from the upper slope and involving progressively shallow-water environments, from distal shelfal pro-delta and delta-front sediments up to proximal coastal fan-delta deposits. These recurrent and close in time, catastrophic slope failures were probably caused by tectonic and climatic triggers, such as the enhanced tectonic activity due to incipient Apenninic continental collision and the onset of harsh climatic conditions, as suggested by oxygen isotopic maxima (e.g., Oi-1a event). Although the wedge toe/foredeep systems are generally considered the principal loci of such, usually located in deep-water settings, here we stress the importance of catastrophic mass transport events also atop the wedge, in shallow-water depositional domains. Mass transport processes also have a fundamental role in reshaping the upper physiographic profile of an evolving accretionary wedge. The correct interpretation of such mass transport processes has also important implications for geohazard forecasting in modern active continental margins, for example in terms of tsunamigenic potential.

Cone-in-cone and beef mineralization associated with Triassic growth basin faulting and shallow shale diagenesis, Edgeøya, Svalbard

Cone-in-cone (CIC) and beef (BF) carbonate lenses ornament detachment zone faults underlying Triassic growth basins on Edgeoya. Field relationships place CIC and BF growth as during early diagenesis and a transition from hydroplastic to a later brittle-style of faulting that is marked by coarser calcite veining. Deformation is constrained to have occurred at <300 m depth. Multiple models exist for CIC formation. For the Edgeoya example, textural analysis of thin-sections suggests that small tensile fractures and carbonate shell fragments nucleated development of calcite aggregates with CIC and BF morphology within unconsolidated to poorly consolidated sediment to form asymmetric antitaxial tensile aggregates subparallel to bedding and fault surfaces. The conical forms result from differential growth on stepped, cleavage-parallel faces of fibres facing host sediment, with preferential inclusion incorporation at inner corners. The preferred directions of calcite growth are attributed to local stresses and seepage flow associated with pore pressure gradients. Substantial framboidal pyrite in the sediments represents an early phase of microbially driven sulphate reduction, which may have induced calcite mineralization. The transition to brittle-style faulting was marked by development of deformation twins in CIC/BF fibres, and a transition to coarse, blocky calcite growth in relay arrays of steeply oriented microveins. This indicates local fault-related stresses substantially changed during shallow diagenesis and lithification, an evolution attributed to changing pore pressures, seepage forces and material moduli. Calcite mineralizations at Edgeoya track the very significant changes in mechanical properties and stress states that occur during synlithification deformation at very shallow crustal levels.

The Epiligurian wedge-top succession in the Enza Valley (Northern Apennines): evidence of a syn-depositional transpressive system

We here discuss the Early Oligocene–Middle Miocene evolution of the Epiligurian wedge-top basin system cropping out in the middle Enza Valley (Northern Apennines, Italy). Newly acquired stratigraphic and structural data, backed up by literature review, highlight that during the Rupelian to Serravallian time span, sedimentation was controlled by a left-lateral transpressive system. This system, here named as the Enza Valley Deformation Zone (EVDZ), is SW–NE directed and trends obliquely to the main regional NW–SE-directed structural axis characterizing this part of the Northern Apennines nowadays. The syn-sedimentary activity is testified by: (1) local to regional stratigraphic unconformities, (2) lateral variations of sedimentary facies associations, (3) thickness changes of the stratigraphic units and (4) the occurrence of mass transport deposits. This study suggests that structural lineaments like the EVDZ, transversal to the main regional tectonic trends, may have played a long-term control on the syn-orogenic sedimentation atop the evolving Apennine orogen.

A conceptual hydrogeological model of ophiolitic aquifers (serpentinised peridotite): The test example of Mt. Prinzera (Northern Italy)

&NA; The main aim of this study is the experimental analysis of the hydrogeological behaviour of the Mt. Prinzera ultramafic massif in the northern Apennines, Italy. The analysed multidisciplinary database has been acquired through (a) geologic and structural survey; (b) geomorphologic survey; (c) hydrogeological monitoring; (d) physico‐chemical analyses; and (e) isotopic analyses. The ultramafic medium is made of several lithological units, tectonically overlapped. Between them, a low‐permeability, discontinuous unit has been identified. This unit behaves as an aquitard and causes a perched groundwater to temporary flow within the upper medium, close to the surface. This perched groundwater flows out along several structurally controlled depressions, and then several high‐altitude temporary springs can be observed during recharge, together with several perennial basal (i.e., low altitude) springs, caused by the compartmentalisation of the system because of high‐angle tectonic discontinuities.