Tephra layers in the marine environment: A review of properties and emplacement processes

This review focusses on the recognition of volcanic ash occurrences in marine sediment cores and on using their appearance and properties to deduce their origin. Widespread marine tephra layers are important marker horizons for both volcanological as well as general geological investigations. We describe ash detection by visual inspection and logging of sediment cores. Ash layer structure and texture, particle morphologies and lithological compositions of primary volcanic deposits are summarized and processes modifying them are discussed, both natural processes acting on and in the seafloor, i.e., erosion and bioturbation, and man-made modifications during drilling/coring and core preparation. We discuss primary emplacement processes of marine fall and flow tephra deposits derived from either subaerial or submarine sources in order to identify distinguishing properties. We also elaborate on processes generating secondary, resedimented volcaniclastic layers such as submarine landslides and shelf erosion as well as fluvial input and ice-rafting, and how they can be distinguished from primary volcaniclastic deposits, which is essential in tephrostratigraphy. Finally, methods of tephra correlation between cores and on-land deposits/volcanoes are illustrated because they allow us to extend the 1-D information from single cores to 3-D distribution and facies changes of tephras and to bridge the land-sea gap.

Differentiated Archean Dolerites: Igneous and Emplacement Processes that Enhance Prospectivity for Orogenic Gold

Magnetite-bearing granophyre and quartz dolerite are the evolved fractions of differentiated dolerite (diabase) sills and are an important host to Archean gold deposits because they are chemical traps for orogenic fluids. Despite their economic importance, there is a poor understanding of how melt composition, crystal fractionation, sill geometry, and depth of emplacement increase the volume of host rock that is most favorable for gold precipitation during orogenesis. We use drill core logging, whole-rock geochemistry, magnetic susceptibility, gold assay, and thermodynamic modeling data from 11 mineralized and unmineralized ca. 2.7 Ga differentiated dolerites in the Eastern Goldfields superterrane (Yilgarn craton, Western Australia) to better understand the influence of igneous and emplacement processes on gold prospectivity. Orogenic gold favors differentiated dolerites, derived from iron-rich parental magmas, that crystallize large volumes of magnetite-bearing quartz dolerite (>25% total thickness). Mineralized sills are commonly >150 m thick and hosted by thick and broadly coeval sedimentary sequences. Sill thickness is an important predictor for gold prospectivity, as it largely controls cooling rate and hence fractionation. The parental melts of gold mineralized sills fractionated large amounts of clinopyroxene and plagioclase (possibly up to 50%) at depth before emplacement in the shallow crust. A second fractionation event at shallow levels (<3 km) operated both vertically and laterally, resulting in an antithetic relationship between quartz (magnetite) dolerite and cumulates (pyroxenites and peridotites). By comparison with younger mafic sills emplaced in synsedimentary basins, we argue that the geometry of these high-level sills was more irregular than the often-assumed tabular form. Any irregularities in the lower sill margin act as traps for early formed (dense) ferromagnesian minerals, now represented by pyroxene and peridotite cumulates. In contrast, irregularities in the upper sill margin trap the buoyant fractionated liquids when the sill is more crystalline, through magma flow on the scale of <1 km. Sills derived from iron-poor melts are rarely mineralized and, all else being equal, probably have to be thicker than Fe-rich sills to be similarly prospective for orogenic gold. Finally, we provide a list of quantifiable parameters that can be incorporated into an exploration program targeting differentiated dolerites that host orogenic gold.

Diamondiferous alluvial deposits of the Longatshimo Valley, Kasai Province, southern DRC: a sedimentary and economic model of a central African diamond placer

The Kasai alluvial field in southern Democratic Republic of Congo (DRC) is part of central Africa’s largest diamond placer that has produced more than 200 million carats, mainly derived from Quaternary deposits. A small part of these deposits, along and within the Longatshimo River, is the subject of this study providing a glimpse into the alluvial history of the Kasai diamond placer. This work documents their sedimentological and diamond mineralization attributes, as well as their emplacement processes, which can inform future exploration models. The key controls of this placer formation, notably Quaternary climatic variations, fluvial landscape evolution and bedrock conditions are also evaluated. A consequence of the interplay among these processes is that diamond supply (from Cretaceous alluvial sources), recycling and concentration were most pronounced and consistent, in the Late Quaternary. Alluvial diamond mineralization in this central African region thus evolved differently to those in southern Africa. Based on exploration results in the Longatshimo Valley, diamond concentration improves but diamond size diminishes with decreasing deposit age, and thus the modern river sediments contain the highest abundance but smallest diamonds. This is opposite to the grade and diamond size trend that characterises southern African fluvial diamond placers. The Longatshimo River study offers insight into the Kasai alluvial field, and its placer model is expected to be applicable to the exploration of other central African diamond placers.

From morpho-stratigraphic to geo-stratigraphic units: the PLANMAP contribution.

<p>Geological units on Earth are defined by several parameters besides the stratigraphic ones, such as rock textures, lithology, composition, and environmental conditions of their origin (numerous and diverse magmatic, volcanic, metamorphic and sedimentary environments). On the other hand, from the Apollo era onward, planetary ‘geologic’ mapping has been carried out using a photo-interpretative approach mainly on panchromatic and monochromatic images. This limited the definition of geological units to morpho-stratigraphic considerations so that units were mainly defined by their stratigraphic position, surface textures and morphology, and attribution to general emplacement processes (a few related to magmatism, some broad sedimentary environments, some diverse impact domains, and all with uncertainties of interpretation). Hence, the two products are still separated by an important conceptual and effective gap which makes the traditional planetary morpho-stratigraphic maps unable to satisfy fully the needs of modern planetary exploration, i.e. an optimised product to define mission strategy in terms of target selection, exploration traverse definition and resource evaluation for ISRU purposes. One of the approaches that might close this gap is to integrate spectral, color and compositional information into morpho-stratigraphic maps, thus generating spectro-morphic or geo-stratigraphic maps.</p><p>The PLANMAP team has explored diverse methods for the integration of color variation and spectral information into planetary geological maps that diverge on the bases of the data available, the planetary surface under consideration (Moon, Mars and Mercury),the  geological environments and the scale of mapping.</p><p> </p><p>PLANMAP received funding from the European Union Horizon 2020 research and innovation program under grant agreement N. 776276.</p>

Unravelling the dynamics of magma emplacement through palaeomagnetic backstripping of intrusion-induced host rock deformation: Analysis from the Sandfell Laccolith, SE Iceland

<p>Injection and inflation of magma in the shallow crust is commonly accommodated by uplift of the surrounding host rock, producing intrusion-induced forced folding that mimics the geometry of the underlying intrusion. Whilst such forced folds have previously been described from field exposures, seismic reflection images, and modelled in scaled laboratory experiments, the dynamic interaction between progressive emplacement of hot magma, roof uplift, and any associated fracture/fault development remains poorly understood. Analysis of ancient examples where magmatism has long-since ceased typically only provides information on final geometrical relationships, while studies of active intrusions and forced folding only capture brief phases of the dynamic evolution of these structures. If we could unravel the spatial and temporal evolution of ancient forced folds, we could therefore acquire critical insights into magma emplacement processes and interpretation of ground deformation data at active volcanoes.</p><p> </p><p>We put forth a new hypothesis suggesting that thermoremanent magnetization records progressive deflection of the host rock during laccolith construction where these measurements can be used to measure the rate and dynamics of the magma emplacement of. Our test site is located within the basaltic lava pile of the ~800 m wide structural aureole surrounding the rhyolitic Sandfell Laccolith in SE Iceland, which intruded <1 Km below the palaeosurface at ~11.7 Ma. Our results show heat from the laccolith resets the remanence from samples within 50 m of the contact. Several variations in thermoremanent vectors observed further outward along the structural aureole reflect stepwise folding from incremental injection of magma suggesting as and the laccolith develops, different sections of the host rock are incrementally tilted and possibly reheated. This procedure could be tested in other ancient structure aureoles to investigate whether single or multiple thermal [email protected] coupled with structural observations could be used a proxy for ground deformation patterns in volcanic hazard assessment.</p>

Making a “Bangladeshi diaspora”: Migration, group formation and emplacement between Portugal and Bangladesh

In 1996, Appadurai argued that imagination is an essential element in the creation of cross-border political forms.Electronic media, for example, establishes links across national boundaries, linking those who move and those who stay.In his argument, these diasporic public spheres were examples of post-national political worlds and revealed the erosion of the nation-state in the face of globalisation and modernity. In this paper, I draw inspiration on this concept of diasporicpublic sphere but to show how these imaginaries are intimately tied to forms of group making and emplacement in several contexts. This argument is based on an ethnographic research about the creation of a transnational federation ofBangladeshi associations – the All European Bangladeshi Association (AEBA) – in the past decade, its main objectivesand activities. Through the analysis of an AEBA event that took place in Lisbon, I want to show the productive dialecticbetween diasporic imaginaries, group formation and emplacement processes between Portugal and Bangladesh.

Bathymetry and Shallow Seismic Imaging of the 2018 Flank Collapse of Anak Krakatau

The flank failure and collapse of Anak Krakatau on December 22nd, 2018 triggered a destructive tsunami. Whether the prior activity of the volcano led to this collapse, or it was triggered by another means, remains a challenge to understand. This study seeks to investigate the recent volcano submarine mass-landslide deposit and emplacement processes, including the seafloor morphology of the flank collapse and the landslide deposit extent. Bathymetry and sparker seismic data were used during this study. Bathymetry data collected in August, 2019 shows the run-out area and the seafloor landslide deposit morphology. Bathymetry data acquired in May, 2017, is used as the base limit of the collapse to estimate the volume of the flank collapse. Comparisons between seismic data acquired in 2017 and 2019 provide an insight into the landslide emplacement processes, the deposit sequence, and structure below the seafloor. From these results we highlight two areas of the submarine-mass landslide deposit, one proximal to Anak Krakatau island (∼1.6 km) and one distal (∼1.4 km). The resulting analysis suggests that the submarine-mass landslide deposit might be produced by a frontally compressional, faulted, landslide, triggered by the critical stability slope, and due to the recent volcanic activity. Blocky seabed features clearly lie to the southwest of Anak Krakatau, and may represent the collapse blocks of the landslide. The seismic analysis of the data acquired in August, 2019 reveals that the blocky facies extends to ∼1.62 km in the width around Anak Krakatau, and the block thicknesses vary up to 70.4 m. The marine data provides a new insight into the landslide run out and extent, together with the landslide deposit morphology and structure that are not available from satellite imagery or subaerial surveys. We conclude that the landslide run out area southwest of the recent collapse, is ∼7.02 ± 0.21 km2.