Atle Mørk

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.

Evolution of the Triassic shelf in the northern Barents Sea region

The interpretation of an unpublished data set of shallow stratigraphic cores and deep, seismic profiles from the northern Barents Shelf has provided new information about the Middle and Late Triassic development of the Barents Shelf and Svalbard. At that time, sediment sources along the eastern and south-eastern margins of the Barents Sea controlled the infilling of a previously deeper shelf area, gradually converting it into a paralic platform. Compared with the eastern source, sediment volumes from other areas were small. In our data, there are no indications of a provenance area north of Svalbard. Progradation from the ESE resulted in diachronous lithostratigraphic boundaries. The organic-rich shales of the Botneheia and Steinkobbe formations were deposited in the remaining deeper shelf areas in the western and north-western Barents Sea shelf, from the Olenekian to the latest Ladinian, by which time the progradation from the ESE had reached eastern Svalbard. In mid-Carnian times, the area of paralic deposits extended from the eastern Barents Sea into the Svalbard Archipelago.

Triassic transgressive-regressive cycles in the Sverdrup Basin, Svalbard and the Barents Shelf

Nine transgressive-regressive (T-R) cycles have recently been recognized in the Triassic succession of the Sverdrup Basin, Arctic Canada (Embry, 1988). The transgressions which initiated the cycles are dated as earliest Griesbachian, earliest Smithian, late Smithian, earliest Anisian, early Ladinian, earliest Carnian, mid-Carnian, earliest Norian, latest Norian and earliest Jurassic.

Biostratigraphy and sequence stratigraphy of the lower and middle Triassic deposits from the Svalis Dome, Central Barents Sea, Norway

Abstract Material from shallow cores drilled through the uplifted and truncated deposits near the Svalis Dome in the Barents Sea contains Lower and Middle Triassic palynomorphs and ammonoids. Eight miospore assemblage zones have been established in this paper, and six of them are calibrated by ammonoids: Svalis‐1 is dated by ammonoids of the late Griesbachian commune Zone. The assemblage is recovered from the Havert Formation overlying the Permian limestone. Svalis‐2 is dated by ammonoids to the late Smithian tardus Zone. The deposits belong to the lower and middle parts of Klappmyss Formation. The unit represents the transgressive systems tract of a Smithian‐early Spathian T‐R Sequence. Svalis‐3 is recorded from the youngest part of Klappmyss Formation which is missing age conclusive faunal evidence. The palynological assemblage is correlated as early Spathian in age. The deposits represent the regressive systems tract of the Smithian‐early Spathian T‐R Sequence. Svalis‐4 is recovered from rocks deposite...

Sequence stratigraphically controlled diagenesis governs reservoir quality in the carbonate Dehluran Field, southwest Iran

The Sarvak Formation (late Albian–early Turonian) is one of the major oil and gas reservoir units in southwestern Iran and was deposited in a wide carbonate ramp marine setting. Three major transgressive–regressive sequences are interpreted in the upper part of the Sarvak Formation in the Dehluran Field. Sequence boundaries are recognized from facies shifts and diagenetic effects related to sea-level fall, whereas maximum flooding episodes are indicated by deep-water facies characterized by abundant bioturbation and high gamma-ray log responses. Diagenesis is the main controlling factor on reservoir quality and plays both a constructive and a destructive role. Permeability is decreased by cementation, which fills primary porosity and disconnects pore throats, while compaction decreases porosity by establishing tighter intergrain contacts. Conversely, the dissolution of unstable minerals (mainly aragonite) is the major process that improves porosity and then permeability by enlarging pores and pore throats. Dolomitization, when associated with dissolution, creates the best reservoir intervals in the Sarvak Formation, although this is not a widespread phenomenon. These diagenetic processes are controlled by sea-level fluctuations, and thus the sequence stratigraphic development. Dissolution, early cementation and exposure-related dolomitization took place during falling sea-levels, mainly in the uppermost, regressive parts of the major sequences. Dolomitization is recognized in the transgressive and in the regressive systems tracts. Stylolitization and fracturing are independent of sea-level fluctuations and are most likely to have formed through a combination of compaction and tectonic events.

Mesozoic source rocks on Svalbard

A review of the source rock potential of the Mesozoic formation on Svalbard is based on organic geochemical analyses of approximately 1000 samples collected during expeditions from 1976 to 1979. Regional and stratigraphic trends have been established for richness, quality and maturity of the potential hydrocarbon source rocks.

Biomagnetostratigraphy of the Vikinghøgda Formation, Svalbard (Arctic Norway), and the geomagnetic polarity timescale for the Lower Triassic

A bio-magnetostratigraphy for the Lower Triassic is constructed, using the ammonoid biostratigraphy from arctic Boreal successions. Combined thermal and alternating field demagnetisation determines the Triassic magnetic field polarity in 86% of specimens, with 36% showing linear trajectory line-fits and the remainder great circle trends towards the characteristic magnetisation. Mean pole directions for the Deltadalen (=50°, φ=159°, dp/dm=3.9°/5.1°), Lusitaniadalen (=56°, φ=163°, dp/dm=4.4°/5.4°) and Vendomdalen (=57°, φ=143°, dp/dm=4.4°/5.4°) members fall close to the European Lower Triassic apparent polar wander path. Mean directions for two of these member-means pass the reversal test. The remanence is predominantly carried by magnetite. The polarity stratigraphy, when integrated with the ammonoid and meager conodont data is similar to that determined from successions in the Sverdrup Basin (Canada). The Permian-Triassic boundary post-dates a pronounced palynofloral turnover, and pre-dates a short duration reverse magnetozone (LT1n.1r). In the correlated Shangsi section (in S. China) LT1n.1r occurs after the FAD of H. parvus, but in the arctic is within the Otoceras boreale Zone. The late Griesbachian to early Smithian is mostly reverse polarity, with three normal polarity intervals, overlain by mid and late Smithian normal polarity. The Spathian contains four reverse polarity intervals, the oldest one within the early Spathian with the remainder in the late Spathian. Transition into the Anisian is within the uppermost reverse magnetozone, a feature documented in other sections of this age. The polarity pattern is correlated to other marine sections, indicating the robustness of the bio-magnetostratigraphic composite and its utility in calibrating Lower Triassic time.

The environmental significance of the trace fossil Rhizocorallium jenense in the Lower Triassic of western Spitsbergen

The 500 m thick Lower Triassic succession of western comprises two shale-dominated formations, which both show upward-coarsening motifs. These reflect repeated coastal basin dominated by low energy fine-clastic sediments. The track fossils Rhizocorallium jenense and Skolithos are found in the coarser part of these units and variations in size and orientation of R. jenense give important palaeoenvironmental information. Rhizocorallium jenense occurs in storm-generated siltstones and stones, whose deposition interrupted prevailing intermediate energy levels. Size variations and trace fossil abundance suggest an optimal habitat in the shoreface zone, with poorer adaptation to both offshore and shallower environments. Age-equivalent marine sediments on north-eastern Greenland also contain local abundant occurrences Rhizocorallium. These Arctic occurrences contrast with the same trace fossils distribution in the Jurassic of Britain and France, where it characterizes shallower and higher energy environments; such sequences on Spitsbergen show an ichnofauna dominated by Skolithos and bivalve escape shafts. Orientations shown by the R. jenense U-tubes show a generally, but not solely, unimodal distribution, with the curved distal entedusually oriented toward onshore. Presumed aperture lineations show strongly unimodal trends, probably related to longshore currents. Burrows in bed at the top of individual storm lobe units show more complex ably patterns probably reflecting both current and wave reworking following lobe abandonment. All finds suggest early colonization by the burrowing organisms. These were not followed by other burrowers, either because of the nutrient-poor nature of the sediment or because of high sedimentation rates.

Triassic conodonts from Svalbard and their Boreal correlations

Conodont faunas are described from Triassic sections of Svalbard, and their occurrences are locally correlated with established ammonoid zones. With a synthesis of previous conodont-based publications, the current work presents a taxonomically up-to-date compilation of conodont data for the Triassic of Svalbard that is used to construct a conodont-based biochronology, indexed to the current lithostratigraphic nomenclature. Twenty-eight taxa spanning the earliest Griesbachian to the earliest Carnian are presented in a range chart. The examined conodont faunas are correlated with well-established conodont zonations of the Canadian Arctic, and in turn also form the basis for regional correlations.

Ichnology of a marine regressive systems tract : the Middle Triassic of Svalbard

The Middle Triassic succession of Svalbard forms a pronounced second-order transgressive–regressive sequence. This is represented by deltaic sediments in western Spitsbergen, grading to deep restricted shelf deposits in central and eastern parts of the archipelago. Nine ichnogenera have been recognized, which form three local ichnofacies or trace fossil assemblages: a Thalassinoides assemblage that is dominant in low-energy shelf settings, a Taenidium– Rhizocorallium assemblage that occurs in intermediate-energy deltaic and shelf environments, and a Polykladichnus assemblage that dominates high-energy deltaic environments. These three trace fossil assemblages overlap, both as a result of fluctuations in energy level with time and because of differential preservation of the different tiers. The main control of the distribution of the assemblages is an upwards increase in energy regime during progradation of the deltaic sediments along western Spitsbergen, and a contemporaneous decrease in energy regime more distally. The succession has also experienced fluctuating oxygen levels during deposition, as evidenced by very high organic matter contents and mass mortality of juvenile bivalves. These anoxic periods have been interrupted by periods of bioturbation, with the development of extensive tiered ichnocoenoses. Phosphatization of Thalassinoides fills and subsequent modification of the phosphatic fill by compaction has brought about the formation of phosphate nodules. The typical Thalassinoides framework may be recognized on well-exposed bedding surfaces. The phosphate nodules also occur as conglomeratic lag deposits, commonly occurring at the base of siltstone beds, as a result of episodic heavy storms.