Classification and analysis of palynodebris from the Palaeocene sediments of the Forties Field

Sedimentary organic matter behaves as sedimentary particles and this study attempts to interpret its occurrence in the Palaeocene sands and mudstones of the Forties Field of the North Sea. New nomenclature and classification of this palynodebris are presented and are used to make quantitative counts of the palynodebris from nearly 600 samples of known sedimentary character. The new categories are further refined by objective testing of the data by Principal Components Analysis and Cluster Analysis. It is argued that two types of sedimentary organic matter are of particular value because they are preferentially deposited in different environments: palynowafers are most commonly deposited in the submarine fan lobes and channel complexes and amorphous matter occurs mostly in the lower energy basin plain sediments.     

TOWARDS A UNIFORM CONCEPT FOR THE COMPARISON AND EXTRAPOLATION OF ROCKWALL RETREAT AND ROCKFALL SUPPLY

Rates of rockwall retreat and rockfall supply are fundamental components of sediment budgets in steep environments. However, the standard procedure of referencing rockwall retreat rates using only lithology is inconsistent with research findings and results in a variability that exceeds three orders of magnitude. The concept proposed in this paper argues that the complexity inherent in rockfall studies can be reduced if the stages of (i) backweathering, (ii) filling and depletion of intermediate storage on the rock face and (iii) final rockfall supply onto the talus slopes are separated as these have different response functions and controlling factors. Backweathering responds to preweathering and weathering conditions whereas the filling and depletion of intermediate storage in the rock face is mainly a function of internal and external triggers. The noise apparent in backweathering rates and rockfall supply can be reduced by integrating the relevant controlling factors in the response functions. Simple conceptual models for the three stages are developed and are linked by a time‐dependent ‘rockfall delivery rate’, which is defined as the difference between backweathering and rockfall supply, thus reflecting the specific importance of intermediate storage in the rock face. Existing studies can be characterized according to their ‘rockfall delivery ratio’, a concept similar to the ‘sediment delivery ratio’ used in fluvial geomorphology. Their outputs can be qualified as trigger‐dependent rockfall supply rates or backweathering rates dependent on (pre‐)weathering conditions. It is shown that the existing quantitative backweathering and rockfall supply models implicitly follow the proposed conceptual models and can be accommodated into the uniform model. Suggestions are made for how best to incorporate non‐linearities, phase transitions, path dependencies and different timescales into rockfall response functions.     

IDENTIFYING MORAINE SURFACES WITH SIMILAR HISTORIES USING LICHEN SIZE DISTRIBUTIONS AND THE U2 STATISTIC, SOUTHEAST ICELAND

Moraine ridges are commonly used to identify past glacier ice margins and so infer glacier mass balance changes in response to climatic variability. However, differences in the form of past ice margins and post-depositional modification of moraine surfaces can complicate these geomorphic records. As a result, simple relationships, such as distance from current ice margin, or linear alignments, may not necessarily indicate moraines deposited contemporaneously. These disturbances can also modify the size distribution of lichen populations, providing a distinctive signature for surfaces with similar histories and a means of identifying contemporaneous moraine surfaces. In this paper, statistical analysis of lichen size distributions is used to identify moraine surfaces with similar histories from complex suites of Little Ice Age moraine fragments in the proglacial areas of Skálafellsjökull (including Sultartungnajökull) and Heinabergsjökull, southeast Iceland. The analysis is based on a novel use of the goodness-of-fit statistic, Watson's U² which provides a measure of 'closeness' between two sample distributions. Moraine fragments with similar histories are identified using cluster analysis of the U² closeness values. The spatial pattern of the clustered moraines suggests three distinct phases of moraine deposition at Skálafellsjökull and Heinabergsjökull, four phases at Sultartungnajökull and a digitate planform margin at Heinabergsjökull. These spatial patterns are corroborated with tephrochronology. The success of the U² statistical analysis in identifying surfaces with similar histories using lichen size distributions suggests that the technique may be useful in augmenting lichenometric surface dating as well as differentiating between other surfaces that support lichen populations, such as rock avalanche deposits.     

TOMOGRAPHIC INVERSION OF NORMALIZED DATA: DOUBLE-TRACE TOMOGRAPHY ALGORITHMS1

Tomography is widely used in geophysics as a technique for imaging geological structures by means of data that are line integrals of physical characteristics. In some transmission measurements, due to various kinds of normalization, the measured data are related to two (the current and the reference) raypaths and can be expressed as a function of differences between line integrals. This is the case, for example, in seismo-acoustic emission measurements, when (since the exact start time is unknown) only the differences between traveltimes (differences between line integrals of the slowness) can be determined. Similarly the use of normalized Fourier amplitudes results in data dependent upon the difference between line integrals of the absorption coefficient (computed along the actual and the reference raypaths). In order to invert these data the ordinary tomography algorithms should be modified. Some generalizations are presented for series expansion tomography methods in order to make them applicable to reconstruction problems in which the input data are differences between two line integrals. The conjugate gradient and the simultaneous iterative reconstruction technique (SIRT) methods were adapted and tested. It is shown that the modified tomography algorithms are stable and sufficiently accurate for practical use. In the reconstruction of noise-free difference data, the conjugate gradient algorithm is found to be faster and more accurate while, in the case of noisy difference data, the modified SIRT algorithm is more stable and insensitive to noise.     

LAYERED PERMEABLE SYSTEMS1

Permeability is a second rank tensor relating flow rate to pressure gradient in a porous medium. If the permeability is a constant times the identity tensor the permeable medium is isotropic; otherwise it is anisotropic. A formalism is presented for the simple calculation of the permeability tensor of a heterogeneous layered system composed of interleaved thin layers of several permeable constituent porous media in the static limit. Corresponding to any cumulative thickness H of a constituent is an element consisting of scalar H and a matrix which is H times a hybrid matrix function of permeability. The calculation of the properties of a medium equivalent to the combination of permeable constituents may then be accomplished by simple addition of the corresponding scalar/matrix elements. Subtraction of an element removes a permeable constituent, providing the means to decompose a permeable medium into many possible sets of permeable constituents, all of which have the same flow properties. A set of layers of a constituent medium in the heterogeneous layered system with permeability of the order of 1/h as h→ 0, where h is that constituent's concentration, acts as a set of infinitely thin channels and is a model for a set of parallel cracks or fractures. Conversely, a set of layers of a given constituent with permeability of the order of h as h→ 0 acts as a set of parallel flow barriers and models a set of parallel, relatively impermeable, interfaces, such as shale stringers or some faults. Both sets of channels and sets of barriers are defined explicitly by scalar/matrix elements for which the scalar and three of the four sub-matrices vanish. Further, non-parallel sets of channels or barriers can be ‘added’ and 'subtracted’ from a background homogeneous anisotropic medium commutatively and associatively, but not non-parallel sets of channels and barriers reflecting the physical reality that fractures that penetrate barriers will give a different flow behaviour from barriers that block channels. This analysis of layered media, and the representations of the phenomena that can occur as the thickness of a constituent is allowed to approach zero, are applicable directly to layered heat conductors, layered electrostatic conductors and layered dielectrics.     

Extent,age and dynamics of Marine Isotope Stage 3 glaciations in the southwestern Baltic Basin

Houmark‐Nielsen, M. 2010: Extent, age and dynamics of Marine Isotope Stage 3 glaciations in the southwestern Baltic Basin. Boreas, 10.1111/j.1502‐3885.2009.00136.x. ISSN 0300‐9483 The southwestern Baltic region is known as a major crossroad for the expansion of Pleistocene glaciers from the Scandinavian Ice Sheet (SIS). At the peak of the Last Glacial Maximum (LGM, 25–20 kyr BP), steady‐flowing inter‐stream glaciers expanded radially from the major ice divide over central Scandinavia. During the subsequent deglaciation phase (20–15 kyr BP), streaming ice was flowing through the Baltic gateway onto the North European lowland. The lithology and directional ice‐flow properties of pre‐LGM till formations of Baltic provenance in Denmark (the Ristinge till and Klintholm till) suggest that the ice‐sheet dynamics during the Marine Isotope Stage (MIS) 3 glacier expansion were similar to those for the post‐LGM advances. Increasing geological evidence indicates that glaciers extended onto the Circum‐Baltic lowlands during MIS 3. Reconstructions of flow paths and estimates of the basal ice‐sheet coupling in Denmark suggest that southward flow of the SIS through the Baltic was probably the result of ice streaming. Despite methodological uncertainties, available OSL and ¹⁴C dates indicate that glaciers advanced at least twice during the mild second half of the Middle Weichselian (c. 75–25 kyr BP), most probably in connection with Dansgaard‐Oeschger (D‐O) events 14–13 (54–46 kyr BP) and 8–5 (35–30 kyr BP). The chronology and dynamics of glacier expansion in the southwestern Baltic in response to long‐term cooling trends, the contemporary presence of a low Arctic biota in large parts of Scandinavia and of possible leads or lags in relation to North Atlantic climate changes during MIS 3 are discussed.     

A model for debris accumulation and mobilization in steep mountain streams

Abstract

Previous work on the initiation of debris flows has emphasized the roles played by material strength, stream gradient, and fluid pressure, but in most published models the friction angle (φ′) of the channel material is assigned some characteristic or constant value. The model presented here retains gradient and pressure as variables, and considers the probable changes in φ′ and hydraulic conductivity, K, of channel debris over time. Preliminary results from the Howe Sound area in southwest British Columbia suggest that stream reworking may lead to small increases in φ′ and large increases in K, rendering channel debris more stable with time. This is partially offset by a local increase in channel gradient as debris accumulates. These factors favour the growth of large, marginally stable debris deposits, and may lead to high-magnitude, low-frequency debris torrents in channels not steep enough to produce torrents directly from hillslope failure events.     

SEISMIC TESTING OF A BUILDING STRUCTURE WITH A SEMI-ACTIVE FLUID DAMPER CONTROL SYSTEM

This paper describes shaking table tests of a multi-storey scale-model building structure subjected to seismic excitation and controlled by a semi-active fluid damper control system. The semi-active dampers were installed in the lateral bracing of the structure and the mechanical properties of the dampers were modified according to control algorithms which utilized the measured response of the structure. A simplified time-delay compensation method was developed to account for delays within the control system. The results of the shaking table tests are presented and interpreted and analytical predictions are shown to compare reasonably well with the experimental results. © 1997 John Wiley & Sons, Ltd.