Chronology and evolution of the northern Iceland Plateau

New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone.     

Petrochemistry of Jurassic and Cretaceous tholeiites from Kong Karls Land, Svalbard, and their relation to Mesozoic magmatism in the Arctic

A study of the whole-rock geochemistry and mineral chemistry of high-TiO₂ Upper Jurassic and medium-TiO₂ Lower Cretaceous basalts from Kong Karls Land, Svalbard, is presented. Geochemical criteria indicate that the basalts are initial rifting tholeiites with weak signs of crustal contamination. The Upper Jurassic basalts appear to be associated with the Olga Rift, part of a trans-Barents rift system which failed to link the proto-Atlantic and proto-Arctic basins. The Lower Cretaceous basalts may be more closely related to initial rifting tholeiites on Franz Josef Land and Spitsbergen generated during the rifting stage of opening of the Canada Basin. During break-up of the Barents Shelf, the sequence of magma types corresponds to the pre-, syn- and post-rifting stages established in other areas of continental break-up. Evidence for a possible hot-spot or plume trail, extending from Siberia to the Yermak Plateau over 250 Ma, is assembled.     

CONSTRUCTION OF COMPONENT MAPS FROM AEROMAGNETIC TOTAL FIELD ANOMALY MAPS1

Total field anomalies as defined from normal aeromagnetic surveys give good approximations of the anomalous components along the direction of the main geomagnetic field, which is generally much larger than the anomalous field. Using the relations between vertical and horizontal components of the field, the total field anomaly is related to any vertical or horizontal component and the corresponding horizontal and vertical derivatives. An example from the Siljan impact structure indicates that such directional filters may be applied to extract useful information from magnetic maps.     

END CORRECTIONS IN POTENTIAL FIELD MODELING*

A general treatment of end corrections in gravity and magnetic modeling is presented. The well-known 2-D formulae of Talwani and Talwani and Heirtzler become slightly changed to represent a 2 1/2–D body, i.e. a body of polygonal cross section with the tails In the strike direction cut off.     

The West Bornholm basin in the Baltic Sea: geological structure and Quaternary sediments

KÖgler, F.-C. & Larsen, B. 1979 03 01: The West Bornholm basin in the Baltic Sea: geological structure and Quaternary sediments. Boreas . Vol. 8, pp. 1–22. Oslo. ISSN 0300–9483.
The West Bornholm basin is an approx. 1000 km² subbasin of the Bornholm basin just north of Bornholm. The basin has been mapped by acoustic profiling and sampling of the sea floor. The basin is eroded down into Mesozoic sediments which are downfaulted between basement horsts in the Fennoscandian Border Zone. The development of the Quaternary morphology is illustrated by maps of the surface of the bedrock, the glacial landscape beneath the varved clays, the recent topography combined with isopach maps of late Glacial and Holocene formations. Quaternary formations are defined and described. The brown, very fine grained varved clay is deposited as a conformable cover on the substratum. It is probably deposited from suspensions carried in the whole water body, while turbidity currents were of minor importance. The recent sedimentary environment is an example of a restricted, but not totally anoxic basin. The recent sediment is chiefly mud rich in organic matter (ca. 4% C_org). According to a rough estimate, the long-term mean sedimentation rate of organic carbon is 6 g/m²/year.     

Evidence for a very high carbon/iridium ratio in the Tunguska impactor

Abstract— We have measured excess Ir and depletion of ¹⁴C, two independent indicators of cosmic material, in peat cores from the central Tunguska impact site. Both Ir and ¹⁴C show pronounced anomalies in the same stratigraphical depth interval. We have estimated an integral deposition of nonradioactive cosmogenic C of 6.8 ± 1.0 mg C cm^?2, and an integrated Ir deposition of 5.9 ± 1.2 pg Ir cm^?2. The very high C/Ir ratio and a deduced δ¹³C value of +55 ± 10% relative to V Pee Dee Belemnite (VPDB) of the impactor material found in this study points towards a cometary type impactor, rather than a chondritic or achondritic asteroidal type impactor.