Facies architecture and depositional processes of the Holocene–Recent accretionary forced regressive Skagen spit system, Denmark

Spit systems are seldom recognized in the pre‐Quaternary sedimentary record compared to their common occurrence along present‐day coasts and in Quaternary successions. This lack of recognition may partly be due to the lack of widely accepted depositional models describing the facies characteristics of spit systems and their subaqueous platforms in particular. The Skagen spit system is a large active system that began to form 7150 yr bp and from 5500 bp to Recent times it has prograded 4 m year^?1 and accumulated 3·5 × 10⁹ m³ of sand. The spit system provides a unique opportunity for establishing a well‐constrained depositional model because uplift and erosion have made large windows into the preserved facies, while active spit‐forming processes can be examined at the young prograding end of the same system. The depositional model presented here thus builds on excellent outcrops, surface morphology, a well‐defined palaeogeography and detailed C¹⁴ age control supplemented with observations from continuous well cores and profiles obtained by ground‐penetrating radar and transient electromagnetic surveys. The factors that have governed the development of the spit system, such as relative sea‐level change, wave and current climate, tidal range, sediment transport and depositional rates are also well‐understood. The sedimentary facies of the spit system are grouped into four principal units consisting from below of thick storm sand beds, dune and bar‐trough deposits, beach deposits and peat beds. These four units form a coarsening and shallowing upward sand‐dominated succession, up to 32 m thick, which overlies offshore silt with a transition zone and is topped by a diastem overlain by young aeolian dune sand. The sedimentary structures and depositional processes are described in detail and integrated into a depositional model, which is compared to other spit systems and linear shoreface models.     

The need for Quaternary geological knowledge in geotechnical engineering

Engineering geology, like geotechnical engineering, is a new scientific subject in Denmark. Experience in the period since 1924 is laid down in the Danish Code of Practice for Foundation Engineering, which states that the geological conditions establish the foundation class and must be determined as early as possible during the soil investigations by experienced geologists and engineers. The consequences are illustrated by some examples from geotechnical investigations performed at the Danish Geotechnical Institute:
• The economic foundation of a small wooden building on the brink of a postglacial lake and fiord basin at Vedbæk.
• The deep excavation for and foundation of a huge commercial center at Lyngby situated in the middle of a tunnel valley with more than 12 m late- and postglacial organic deposits, based on design and control in the high foundation class.
• The engineering geological modelling of the Quaternary development in the Danish North Sea. Applied geology plays an increasing part in geotechnical engineering as a prerequisite for economic and safe technical solutions. The future use of geological and geotechnical data bases will bring Quaternary geological knowledge into focus.     

A Dynamic Model of Rift Zone Petrogenesis and the Regional Petrology of Iceland

The article describes the petrochemical evolution of oceanicrocks in terms of plate tectonics with special reference toIceland. The compositional variation along the rift zone isrelated to different production rates of mantle-derived olivinetholeiite of invariant composition which is added to the crustfrom below and modified by mixing with anatectic melts in thecrust and concomitant crystal fractionation. The kinematic processes of crustal accretion cause rocks depositedin the rift zone to subside towards higher temperatures wherethey suffer hydration and progressive metamorphism before becominga part of the stable crustal plate. Rocks deposited near therift-centre assume the highest metamorphic grade (greatest depth)while rocks deposited at the rift-margins follow a shallow pathbefore being carried towards lower temperatures in the stableplate. The material transport through stationary metamorphiczones produces the layering of the oceanic crust. As the hydrated rocks cross their solidus isotherm, silicicmagma is formed by incongruent partial melting. The meltingcontinues until rocks crossing the boundary between the amphiboliteand granulite fades are finally dehydrated by the break-downof amphibole. This reaction boundary defines the surface ofthe upper mantle. The segregation and retention of crust-derived magmas withinthe rift zone results in chemical fractionation in the oceaniccrust, for its lower sections are depleted in elements enteringthe early melt fractions, which are silicic and enriched inthe dispersed elements. The last melt-increments from the samesubsiding pile are ne-normative basalts. The rift-zone rocks are shown to be mantle-derived olivine tholeiitemodified by minor amounts of crustal rhyolite and nephelinebasalt, while volcanism outside the rift zone is dominated bythe crust-derived magmas themselves. All mixtures undergo furthermineralogical evolution towards invariant compositions in thebasalt system, resembling the olivine tholeiite, quartz tholeiite,and nepheline basalt of synthetic systems. The dispersed-elementgeochemistry of the oceanic rocks is but slightly modified bycrystal fractionation, and reflects the mixing ratios of theolivine tholeiite and the different crustal magmas. The geochemistry of radiogenic isotopes is controlled by continuousprocesses of crustal fractionation separating mother and daughterelements. The oxygen-isotope geochemistry can be referred tomagma mixing, for rhyolites formed by anatexis in the hydratedcrust are enriched in light oxygen relative to the mantle-derivedmelts.     

Ice-flow patterns and dispersal of erratics at the southwestern margin of the last Scandinavian Ice Sheet: signature of palaeo-ice streams

An extensive set of proxy-data was acquired from eastern and central Denmark in order to study the dynamic behaviour of the southwestern margin of the last Scandinavian Ice Sheet. We examine the last three glacier advances of the Late Weichselian: the Main advance from central Sweden, representing the maximum ice extent at this time ( c. 21–20 ka BP), and the two succeeding Baltic advances ( c. 18–15 ka BP). Directional properties from tills and glaciotectonic overprints are used to reveal large-scale flowline patterns. Together with the geomorphological outline of ice margins, flowlines were successively more fan-shaped, indicating that the dependence of the subglacial topography increases as ice advances become younger. It is suggested that while the ice thickness decreases, more lobate configurations of ice margins are created as a result. Clast-compositional data derived from the fine-gravel fraction in tills are used to reconstruct dispersal patterns of erratic material. The dispersal patterns during the three advances show a gradually diminishing influence of local Pre-Quaternary bedrocks and older glacial deposits, and progressively longer transport distances of far-travelled erratics. We speculate that the principal factor governing this development is a successively decreasing interaction between the ice and its bed, which influences the concentration of erratics, debris comminution processes and the basal ice velocity. We envisage the Main advance from central Sweden as a slow-flowing inter-stream ice body with high bed interaction compared to the succeeding Baltic advances, which we regard to have been rapid flowing ice streams with limited bed interaction.     

OPTIMIZED FAST HANKEL TRANSFORM FILTERS1

In the linear digital filter theory for calculation of Hankel transforms it is possible to find explicit series expansions for the filter coefficients. A method is presented for optimizing the Hankel filters calculated in this way. For a certain desired accuracy of computation, the sampling density and filter length are minimized by choosing the parameters determining the filter characteristics according to the analytical properties of the input function. A new approach to the calculation of the filter coefficients has been developed for these optimized filters. The length of the filters may be further reduced by introducing a shift in the sampling scheme.     

Emergence of Civilization,Changes in Fluvio‐Deltaic Style,and Nutrient Redistribution Forced by Holocene Sea‐Level Rise

During the mid‐Holocene, the first large‐scale civilizations emerged in lower alluvial systems after a marked decrease in sea‐level rise at 7–6 kyr. We show that as the landscapes of deltas and lower alluvial plains adjusted to this decrease in the rate of relative sea‐level rise, the abundance and location of resources available for human exploitation changed as did the network of waterways. This dynamic environmental evolution contributed to archaeological changes in the three fluvio‐deltaic settings considered herein: Egypt, Mesopotamia, and the Huang He in China. Specifically, an increase in the scale and intensity of agricultural practice, and the focussing of power toward a single city can be interpreted as responses to these environmental changes. Other archaeological observations, and the cultural trajectories leading to the formation of the Primary States also need to be considered in light of these evolving landscapes.     

Distribution of erosion across bedrock channels

Lateral erosion in bedrock rivers is an important control on the shape of channel cross‐sections, and the coupling of channels and hillslopes. Recent observations link lateral erosion to the variability of flow. We propose two mechanisms to explain this. One is based on changing shear stress distributions within the channel with varying flood level, the other on the competition between cover and tool effects in fluvial bedrock erosion. We assess these processes for the Liwu River, Taiwan, and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

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.