Henning Dypvik

Geochemical facies analysis of fine-grained siliciclastics using Th/U, Zr/Rb and (Zr+Rb)/Sr ratios

Abstract Abundance of the trace elements Zr, Rb, Sr, Th and U may provide insight into the sedimentological origin of fine-grained siliciclastic sediments, provided that the units are derived from similar source areas, as demonstrated here with suites of the fine-grained Jurassic and Cretaceous rocks from the Barents Sea and Svalbard. Zr/Rb ratios can be interpreted to indicate grain size variations, while (Zr+Rb)/Sr values reflect variations in the siliciclastic versus carbonate contents. The Th/U ratio can be applied as an indicator of the redox conditions. In 3D plots such as presented here, the interplay between these geochemical factors can be interpreted in terms of the sedimentological evolution of the depositional basin. The geochemical facies analyses show that similar depositional conditions existed on the Jurassic shelf in the two studied localities. The succeeding prodeltaic Cretaceous beds were generally coarser grained and deposited in ventilated depositional conditions.

Mjølnir structure: An impact crater in the Barents Sea

A systematic search for impact indicators was conducted on a core of Late Jurassic-Early Cretaceous sedimentary strata from the vicinity of the proposed Mjolnir impact structure, Barents Sea. A 0.8 m-thick section of the core was found to contain unequivocal indicators of meteoritic impact: shocked quartz grains and a strong enrichment in iridium. The ejecta-bearing strata were discovered only 30 km north-northeast of the structure, within a stratigraphic interval corresponding to the seismically defined deformation event at Mjolnir. Further study of this unusually well presented impact-crater-ejecta-layer pair may help constrain poorly understood aspects of large-magnitude meteorite impacts into the oceans. 14 refs., 4 figs.

Sedimentary signatures and processes during marine bolide impacts: a review

Abstract Studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts. This results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters. In subaerial impacts, the targets are commonly hard rocks, frequently of igneous and/or metamorphic origin, whereas in submarine impacts, the targets are mostly unconsolidated or poorly lithified sediments, or sedimentary rocks, with high volumes of pore water. Such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area. Impacts in shallow-water marine (neritic) environments produced craters with low or absent rims and wide and shallow brims, as characterize by both the Montagnais (on the Scotian shelf), the Mjolnir (in the Barents Sea), 45 and 40 km in diameter, respectively, and the Chesapeake Bay (90 km in diameter). Lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity, as the water in the crater is vaporized. During this process, resurge gullies can be cut across the rim, while mass- and debris-flows, turbidites, and other gravity deposits are produced as results of tsunami and crater-wall and central high collapse, during and after the crater excavation stage. Such deposits are found both within and outside the crater structure. The only difference between gravity deposits triggered by an impact or other rare events, such as earthquakes, is the admixture of various melt particles and possible enrichments in iridium in the former. Impacts near the shelf edge may cause partial collapse of the continental margin as shown by the Montagnais and Chicxulub impacts. Some of the gravity and debris flows generated by margin collapse may be channelized, with final deposits up to several hundred meters thick, extending for hundreds of kilometers from the impact site. Other impact features such as shatter cones, tektites, spherules and Ni-spinels, shocked quartz, isotropication, and partial melts, are common to both submarine and subaerial impacts. Theoretical calculations of the destructive forces of mega-tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence. A dearth of gigantic tsunami evidence for the Chicxulub impact outside of the Gulf of Mexico, where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep North Atlantic basin should have been >1 m/s, could be the result of the mitigating effect of the bathymetry of surrounding area, causing wave diffraction and interference. Calculated maximum horizontal orbital velocity near the seafloor at the shelf for the Montagnais impact is 22 m/s for a 200-m-high wave, 5.5 m/s for a 50-m-high wave at 500 km, decreasing to 0.5 m/s at 1000 km, strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record. However, lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows.

Provenance interpretation of quartz by scanning electron microscope–cathodoluminescence fabric analysis

We used a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM) to study patterns of variable-intensity CL in quartz grains from a variety of igneous, metamorphic, sedimentary, and shock-deformed (meteorite-impact) rocks. Distinctive fabrics in quartz grains revealed by SEM-imaged differential CL include zoning, healed fractures, complex shears, planar features (shocked quartz), dark CL streaks and patches, indistinct, mottled texture, and nondifferential (low-contrast) CL. Zoning is common in volcanic quartz and some plutonic quartz. Zoned plutonic quartz is distinguished from volcanic quartz by the presence of closed fractures and dark CL streaks and patches. Metamorphic quartz displays either an indistinct, mottled texture, or nearly uniform (nondifferential) CL. Quartz from rocks severely deformed by tectonism displays a complex pattern of multiple, small-scale shears. Quartz from meteorite-impact sites and some system boundaries is characterized by intricate patterns of planar features, presumably created by shock metamorphism. Thus, the SEM-CL fabric-analysis technique provides a rapid method for distinguishing quartz from a variety of source rocks.

Depositional conditions of the Bathonian to Hauterivian Janusfjellet Subgroup, Spitsbergen

Abstract The Janusfjellet Subgroup was formed under shallow marine shelf conditions dominated by sedimentation of clay, but silt and sand also accumulated in significant amounts. The sand bodies recognized in the Agardhfjellet Formation (Bathonian-B) are redeposition of sediments originating from earlier deltaic/coastal progradation. The Rurikfjellet Formation (Berriasian-Hauterivian) reveals an upwards increasing sand content reflecting built-out of a delta system. The prodeltaic material was partly reworked by storm currents. The shales of the Agardhfjellet Formation were in part deposited under oxygen-depleted conditions.

Neogene-Holocene rift propagation in central Tanzania: Morphostructural and aeromagnetic evidence from the Kilombero area

Based on field studies supplemented by remote sensing and aeromagnetic data from central Tanzania, a Phanerozoic structural history for the region can be developed and placed in a broader rift context. The major contribution of this work is the recognition of rift morphology over an area lying 400 km beyond the southern termination of the Eastern, or Kenya, Rift. The most prominent rift structures occur in the Kilombero region and consist of a wide range of uplifted basement blocks fringed to the west by an east-facing half-graben that may contain 6–8 km of sedimentary strata. Physiographic features and river drainage anomalies suggest that Holocene/Neogene deformation occurs along both rift-parallel and transverse faults, in agreement with the seismogenic character of a number of oblique faults. The present-day rift pattern of the Kilombero extensional province results from the complete overprinting of an earlier (Karoo) rift basin by Neogene- Holocene faults. The Kilombero rift zone is assumed to connect northward into the central rift arm (Manyara) of the Eastern Rift via an active transverse fault zone. The proposed rift model implies that incipient rifting propagates throughout the cold and strong lithosphere of central Tanzania following Proterozoic basement weakness zones (N140°E) and earlier Karoo rift structures (north-south). An eventual structural connection of the Kilombero rift zone with the Lake Malawi rift further south is also envisaged and should imply the spatial link of the eastern and western branches of the East African Rift System south of the Tanzanian craton.

JURASSIC AND CRETACEOUS BLACK SHALES OF THE JANUSFJELLET FORMATION, SVALBARD, NORWAY

Abstract The Upper Jurassic-Lower Cretaceous Janusfjellet Formation, a ca. 400 m thick shaly unit, divisible into the Agardhfjellet and Rurikfjellet Members, has been studied in the field and by petrographical and geochemical means. The Bathonian-Kimmeridgian Agardhfjellet Member was deposited in an open shelf environment during a transgressive phase, and is characterized by more stagnant conditions than the Berriasian-Hauterivian Rurikfjellet Member. Total organic carbon values range from 1 to 4%, exhibiting a maximum of 12% in the Callovian paper shales. The Rurikfjellet Member, slightly enriched in volcanogenic components compared to the Agardhfjellet Member, displays regressive shelf sedimentation, ending in prodeltaic to deltafront depositional environments. Based on the sedimentpetrographical analyses, each member is divided into minor regressive and transgressive phases, probably reflecting tectonic activity and/or eustatic sea-level variations.

Cratering in Marine Environments and on ice

Impacts into Marine and Icy Environments.- Biotic Responses to the Mjolnir Meteorite Impact, Barents Sea: Evidence from a Core Drilled within the Crater.- Near-field Erosional Features at the Mjolnir Impact Crater: the Role of Marine Sedimentary Target.- Global Effects of the Chicxulub Impact on Terrestrial Vegetation - Review of the Palynological Record from New Zealand Cretaceous/Tertiary Boundary.- The Neugrund Marine Impact Structure (Gulf of Finland, Estonia).- Structure-filling Sediments of the Wetumpka Marine-target Impact Structure (Alabama, USA).- Krk-breccia, Possible Impact-Crater Fill, Island of Krk in Eastern Adriatic Sea (Croatia).- Did the Puchezh-Katunki Impact Trigger an Extinction?.- Geochemistry of a Langhian Pelagic Marly Limestone Sequence of the Conero Riviera, Ancona (Italy) and the Search for a Ries Impact Signature: A Progress Report.- Titan: A New World Covered in Submarine Craters?.- Estimating Crater Size for Hypervelocity Impacts on Small Icy Bodies (e.g. Comet Nucleus).- Survivability of Bacteria in Hypervelocity Impacts on Ice.- Impact Cratering of Icy and Rocky Targets in Planetary Sciences and in the Laboratory.- Paleomagnetism and 40Ar/39Ar Age Determinations of Impactites from the Ilyinets Structure, Ukraine.- Cathodoluminescence, Electron Microscopy, and Raman Spectroscopy of Experimentally Shock Metamorphosed Zircon Crystals and Naturally Shocked Zircon from the Ries Impact Crater.- A Brief Introduction to Hydrocode Modeling of Impact Cratering.

Jurassic and Cretaceous palaeogeography and stratigraphic comparisons in the North Greenland-Svalbard region

We present sedimentological comparisons and stratigraphical correlations of the Jurassic and Cretaceous epicontinental shelf deposits of Svalbard and updated descriptions of the shallow-marine North Greenland sediments of East Peary Land and Kronprins Christians Land (Kilen). The Callovian to Volgian Agardhfjellet Formation of Svalbard is correlated to the lower part of the Ladegårdsåen Formation on East Peary Land, and to the Birkelund Fjeld, Splitbæk, and Kuglelejet formations of Kronprins Christian Land (Kilen). The Berriasian to Hauterivian Rurikfjellet Formation (Svalbard) correlates with the Dromledome and Lichen Ryg formations from Kilen and the middle part of the Ladegårdsåen Formation from East Peary Land. The Galadriel Fjeld Formation from Kilen and the upper part of the Ladegårdsåen Formation (East Peary Land) are comparable to the Helvetiafjellet and Carolinefjellet formations of Svalbard. These comparisons between Svalbard and North Greenland are combined with stratigraphical information from neighbouring regions in palaeogeographical reconstructions. Five selected time slices are presented within a setting of the most recent plate tectonic reconstructions for the area.

Clay Mineral Transformations in Tertiary and Mesozoic Sediments from North Sea

Clay mineral transformations of 10 North Sea wells have been studied, and several transformation stages picked out and correlated with geothermal variations: smectite disappears at 149 to 167°F (65 to 75°C), whereas transformations in the mixed-layer illite-smectite clay minerals are observed between 113 and about 212°F (45 and about 100°C). At temperatures above 113 to 149°F (45 to 65°C), more than 30% illitic layers are found in the mixed-layered clay minerals, whereas temperatures from 140 to 167°F (60 to 75°C) indicate the transition zone toward more than 50% dehydrated lattice positions. Temperatures above 176 to 212°F (80 to 100°C) mark the lower limit for mixed-layer clay minerals containing more than 70% illitic la ers. The major part of the clay mineral transformations apparently takes place in the mesodiagenetic zone, in which most of the hydrocarbons are also formed.