The AC-geoelectrical sounding method: a combined electric/electromagnetic prospecting tool

Since 1979 the Laboratory of Geophysics, University of Aarhus, has been developing a new prospecting tool for obtaining information on the topmost 100 m of the earth. The method is an extension of the conventional geoelectric sounding method, but instead of direct current (DC) the AC-geoelectrical sounding method uses alternating current (AC) with frequencies in the range 100 Hz to 20,000 Hz. The use of alternating current adds an inductive contribution to the ordinary galvanic electric field, thus producing two different sorts of information about the underlying earth structure. These two sets of information are, in many cases, of complementary nature, which enables determination of the ground parameters much more accurately than would otherwise be possible from ordinary DC-geoelectrical soundings. Among these cases is the high resistivity equivalence which appears so frequently in Danish Quaternary deposits.     

Recent volcano-ice interaction and outburst flooding in a Mars polar cap re-entrant

Formation of chasms in the polar ice caps of Mars has been attributed to meltwater outburst floods, but the cause of melting has remained uncertain. In a cap re-entrant enveloping Abalos Colles, west of Casma Boreale in the north polar cap, we have found possible evidence of recent volcano-ice interaction and outburst flooding. In this paper we demonstrate that these two mechanisms can have acted together to form or expand the Abalos re-entrant. Flat-topped ridges and circular rims protruding above the ice cap surface in the re-entrant apex may be lava ridges and volcano craters, and can have caused melting of 3.3 to 7.7×¹⁰³ km³ of ice. The surrounding cap surface appears to have subsided and the likely volume of missing ice matches the melt estimate. Outburst flooding from this area may have reached peak discharges of 0.3 to according to scour patterns in one of the re-entrant channels. This required ponding of melt water during lava eruption and catastrophic release through a sub- or englacial melt water tunnel, the collapse of which has left a chasm in the ice cap margin. The flood features are geologically recent, and volcano-ice interaction may have occurred within the last 20,000 years.     

A demonstration of the importance of bedload transport for fluvial bedrock erosion and knickpoint propagation

We provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation. Using aerial photographs, field surveys, and hydrological data, we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by Da'an River in western Taiwan. This reach of the river experienced 10 m of uplift during the 1999 Chi‐Chi earthquake. For five years following the earthquake, bedload was prevented from entering the uplift zone, the knickpoint was static and little incision took place. Bedload transport across the uplift zone resumed in 2004, initiating extremely rapid incision, with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008. This change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation. Once bedload tools became available, knickpoint propagation was influenced by geological structure, lithology, and drainage organization. In particular, a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity. Copyright © 2012 John Wiley & Sons, Ltd.     

Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism?

Talc–kyanite schists (whiteschists), magnesiohornblende–kyanite–talc–quartzschists and enstatite–sapphirine–chlorite schistsoccur at Mautia Hill in the East African Orogen of Tanzania.They are associated with metapelites and garnet–clinopyroxene–quartzmetabasites. Geobarometry (GASP/GADS equilibria) applied tothe latter two rock types indicates a peak pressure of P = 10–11kbar. These results are confirmed by the high fO₂ assemblagehollandite–kyanite–quartz and late-stage manganianandalusite that contains up to 19·5 mol. % Mn₂SiO₅. Maximumtemperatures of T = 720°C are inferred from late-stage yoderite+ quartz. A clockwise P–T evolution is constrained byprograde kyanite inclusions in metapelitic garnet and late-stagereaction rims of cordierite between green yoderite and talcthat reflect conditions at least 3–4 kbar below the peakpressure. Oxidizing conditions are recorded throughout the metamorphichistory of the whiteschists and chlorite schists, as indicatedby the presence of haematite coexisting with pseudobrookiteand/or rutile. Increasing water activity near peak pressuresis thought to have led to the breakdown of the high-pressureassemblages (Tlc–Ky–Hem and Mg-Hbl–Ky–Hem)and the subsequent formation of certain uncommon minerals, e.g.yellow sapphirine, Mn–andalusite, green and purple yoderite,piemontite and boron-free kornerupine. The proposed increasein water activity is attributed to fluid infiltration resultingfrom the devolatilization of underlying sediments during metamorphism. KEY WORDS: fluid infiltration; high-pressure amphibolite facies; East African Orogen; Pan-African; whiteschist     

Coastal morpho-stratigraphy and Holocene relative sea level changes at Tuapaat, southeastern Disko Island, central West Greenland

The geomorphology of a coarse clastic coastal landscape at Tuapaat (69°24'N 52°36'W), southeastern Disko Island, central West Greenland, is described, and a coastal morpho-stratigraphy of the landscape is constructed. ¹⁴C ages on marine shells, whale bones, peat and gyttja are used to construct relative sea level changes throughout the Holocene. The emergence of SE Disko Island occurred in the early part of the Holocene. The Holocene marine limit is situated ca. 80m a.s.l. Between 4.7 and 1.0ka BP, the relative sea level approached the present sea level and it has probably been below present sea level between 4.7 ka BP and the present. The morpho-stratigraphy in the lowest part of the coastal landscape at Tuapaat suggests a complex late Holocene relative sea level history which includes at least 3-4 transgressions during the last ca. 2.5 ka.     

Diatoms from the Middle and Late Weichselian and the Early Flandrian period on Andøya, north Norway

BOREAS Foged, N. 1978 03 01: Diatoms from the Middle and Late Weichselian and the Early Flandrian period on Andøya, north Norway. Boreas, Vol. 7, pp. 41–47. Oslo. ISSN 0300–9483.
From cores from a depth of 9.9 m up to 5.1 m below the present surface of a mire situated approx. 36 m above sea level on Andøya, north Norway, 47 samples were analysed for diatoms. Some 240 taxa were recorded, chiefly in Late Weichselian and Early Flandrian material. They were subdivided according to their halobion and pH relation. On the whole, the pH reaction of the environment was neutral, but it changed from faintly acid to faintly alkaline during the sedimentation of the Late Weichselian material.     

A symposium on Nordic Quaternary research in 1986

Different topics of Quaternary geology were dealt with at a symposium arranged in 1986 in Århus, Denmark: (1) reviews on recent Quaternary research in the Nordic countries, (2) studies of stratigraphy and geological processes in the Fennoscandian Border Zone, which demonstrate that the Quaternary deposition pattern is highly influenced by tectonic movements, and (3) Quaternary research as it can be applied to geotechnical investigations in the search for raw materials, in connection with pollution, and with conservation.