bluuebunny/crossref_metadata_2025 huggingface dataset:
Textures and microstructures of naturally deformed amphibolites from the northern Cascades, NW USA: methodology and regional aspects
Authors: Leiss Bernd , Gröger Heike R. , Ullemeyer Klaus , Lebit Hermann
Published: April 2008

Neutron texture analyses of quartz-bearing and quartz-free amphibolite mylonites from the Windy Pass thrust, Cascades Crystalline Core (Washington/USA) reveal pronounced textures of plagioclase and clino-amphiboles (hornblende, cummingtonite) but no preferred orientation of quartz. A reliable strategy for amphibolite fabric analysis is presented by a systematic analytical approach to the experimental diffraction data processing. Clino-amphiboles show transitional textures between ideal single crystal orientations and axial symmetric great circle distributions. Plagioclase reveals
a
-axes distributions scattering along a great circle approximating the foliation plane as well as
a
-axes maxima close to the macroscopic lineation. Correlation of the textures with grain shape anisotropies of horn-blende and plagioclase and comparison with data from the literature suggest that the texture variations are due to different strain regimes rather than due to different crystallographic reorientation mechanisms. The kinematic directions deduced from the microfabric correlate well with the regional tectonic interpretations. In contrast, individual deformation paths are not yet established for the different tectonic units, as the significance of the separating Windy Pass thrust requires further structural analysis and fabric studies.
GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide
Authors: Jol Harry M. , Bristow Charlie S.
Published: April 2008

Within sedimentological studies, ground penetrating radar (GPR) is being used with increasing frequency because it yields images of the shallow subsurface that cannot be achieved by any other non-destructive method. The purpose of this paper is to provide an introduction to the collection, processing and interpretation of GPR data so that future sedimentary studies can be improved. With GPR equipment now being lightweight, robust and portable, proper data collection and survey design methods need to be followed in order to acquire high resolution, subsurface digital data. Various factors are discussed including: reflection profiling, velocity soundings, test surveys, topography, logistics, data quality and extreme environments. Basic data processing and visualization are then reviewed, followed by a discussion on GPR interpretation strategies including a background to radar stratigraphy. For the sedimentary geologist or geomorphologist, GPR offers unique data of the shallow subsurface including stratigraphy, geometry, architecture and structure.
Evidence for dune reactivation from GPR profiles on the Maputaland coastal plain, South Africa
Authors: Botha Greg A. , Bristow Charlie S. , Porat Naomi , Duller Geoff , Armitage Simon J. , Roberts Helen M. , Clarke Brendan M. , Kota Mxolisi W. , Schoeman Philo
Published: April 2008

The Maputaland area of northeastern South Africa is characterized by extensive dunefields which developed during polyphase reworking of regional aeolian cover sand from the Mid-Pleistocene to the Holocene. Extended parabolic dunes, many preserved only as wind-rift trailing limbs, as well as areas of sinuous crested dunes, hummocky dune systems and the high, composite, accretionary coastal barrier dune cordon are the dominant dune forms. There are few natural sections exposing the stratigraphic succession and unequivocal relative age relationships between dune systems are uncommon. A ground penetrating radar (GPR) survey of dunes and representative aeolian sand stratigraphic units was undertaken in order to investigate the internal structure of the different dune forms and identify stratigraphic relationships between buried sedimentary units. The GPR profiles revealed that the trailing limbs of almost all the parabolic dunes that were surveyed comprise stacked sand units, separated by low-angle reflections interpreted as bounding surfaces, which accumulated through polyphase vertical accretion. Most extended parabolic dunes are aligned north-south and the upper parts of the dunes are characterized by inclined reflections in GPR profile interpreted as large-scale sets of cross-stratification with apparent dips toward the west. A hummocky dune revealed cross-stratified aeolian sand superimposed on a truncated dune form and probably formed through deflation of pre-existing dunes. Using 100 MHz and 200 MHz antennae, it is clear that GPR is capable of imaging very fine sedimentary structures and buried erosional surfaces in the homogeneous aeolian sand of Maputaland. At some of the sites investigated, the buried sand units identified were sampled by hand-augering for infrared-stimulated luminescence dating. The age determinations on these samples suggest that vertical accretion of up to 7 m of sand occurred intermittently over variable time scales up to 25 000 years on some parabolic dune limbs during the Late Pleistocene. In some complex dunefields, adjacent dunes were mobilized at different times, suggesting that remobilization was localized. The implications of the complex internal structure and vertical accretion of extended parabolic dunes are discussed in the context of changes in vegetation cover and water table due to seasonal and short-term cyclical climate variations as well as long-term climate change patterns during the last glacial cycle and the Holocene.
GPR investigation of multiple stage-5 sea-level fluctuations on a siliciclastic estuarine shoreline, Delaware Bay, southern New Jersey, USA
Authors: O’Neal Michael L. , Dunn Richard K.
Published: April 2008

Ground penetrating radar (GPR) surveys were used to identify three distinct sea-level highstands within the oxygen isotope stage (OIS) 5-equivalent sediments of the Quaternary Cape May Formation in southern New Jersey. The three units, QCM1a, QCM1b, and QCM1c, are designated as submembers of the Cape May 1 member. The transgressive ravinement surfaces (TRS) forming the bounding unconformities of these units are recognized as laterally extensive reflections on the radar records and have been corroborated as gravel lag deposits and associated lithological discontinuities in continuous vibracores and hand-augered boreholes. GPR and lithological data have been used to interpret these highstand deposits in a sequence-stratigraphic framework. These units are thin (3 m or less), lithologically similar and present limited geomorphic expression of their individual existence beneath a low terrace (+6 m to mean sea level/MSL) on the northern margin of Delaware Bay. In the upper two units, QCM1b and QCM1a, radar surveys across subtle, bay-margin parallel, sublinear topographic highs of 3 m relief or less in the Jones Island area reveal internal structures consistent with geomorphic interpretation as stranded estuarine shorelines. The newly identified shoreline complexes are located well bayward of, and lower in elevation than, the commonly accepted +5 m to +6 m MSL OIS 5e shoreline. This older shoreline abuts the Cedarville Scarp and is attributed to the sea-level highstand associated with QCM1a. GPR was also used to accurately place vibracores for gathering samples of the three units for optically stimulated luminescence dating. Together, these three highstand deposits record sea-level fluctuations within OIS 5, at time scales shorter than those commonly recognized in the mid-Atlantic region.
Sedimentary architecture and post-glacial evolution of Cheekye fan, southwestern British Columbia, Canada
Authors: Ékes Csaba , Friele Pierre
Published: April 2008

The purpose of this paper is to examine the internal architecture and post-glacial evolution of Cheekye fan, British Columbia, Canada. Analysis of a large database of ground penetrating radar (GPR) profiles has allowed the identification of ten reflection configurations that characterize this high-energy environment. GPR profiles augmented with test-pit, well-log and radiocarbon data provided detailed subsurface information and revealed the large-scale internal architecture and Holocene sedimentation history of Cheekye fan. Based on a shift in reflection configuration with depth, GPR data appears to record a change in the mode of deposition through time. This data suggests that Cheekye fan is a paraglacial fan, largely a product of the geological past. This fact should be taken into consideration when making future hazard estimates.
GPR survey of a lobate rock glacier in Yankee Boy Basin, Colorado, USA
Authors: Degenhardt John J. , Giardino John R. , Junck M. Brian
Published: April 2008

The internal structure of a lobate rock glacier located in the San Juan Mountains of southwest Colorado was investigated using ground penetrating radar (GPR). A 440 m, 25 MHz longitudinal profile oriented along the central axis of the rock glacier shows moderate to strongly coherent reflection horizons or layers that can be recognized clearly to a depth of 30–35 m. The layers are interpreted as representing ice-supersaturated sediments and coarse, blocky rockslide debris that are the result of flow, perhaps generated by seasonal snow pack covered by episodic debris flows or high-magnitude discharges of talus from the cirque headwall. Profiles collected at 50 MHz indicate that, in the upper 20 m thickness of the rock glacier, many of these layers are laterally continuous. The total depth of penetration (∼40 m at 25 MHz) was sufficient to detect the rock glacier-cirque-floor contact, which is composed of underlying moraine. Several prominent reflection events that subdivide the profile into broad 10–15 m-thick layers represent contacts between major depositional units. These units are believed to be individual flow lobes that were initiated at various cirque-headwall locations. We interpret this rock glacier to be a composite feature that formed by a process involving the development and subsequent overlap of discrete flow lobes that have over-ridden older glacial moraine and protalus rampart materials. The latter materials have been incorporated into the present flow structure of the rock glacier.
GPR images of periglacial slope deposits beneath peat bogs in the Central European Highlands, Germany
Authors: Leopold Matthias , Völkel Jörg
Published: April 2008

This research investigates the use of ground penetrating radar (GPR) to determine the continuity of periglacial slope deposits under peat bogs. In the Central European Highlands, periglacial slope deposits are a common parent material for soils and play a critical role in determining the nature of chemical and physical processes in soil development. There is a common agreement on the general distribution of these slope deposits, but there is an ongoing debate in German literature concerning the actual age of these deposits. In particular, the youngest head can usually be found under peat bogs of Late Glacial age (
14
C and pollen stratigraphy) and has lead to a minimum age determination of pre-Alleröd. Stratigraphic investigations were made using drill cores, and GPR was used to define the continuity of the head under the bog. Several examples from southeastern Bavaria are given.
Soil effects on GPR detection of buried non-metallic mines
Authors: Hendrickx Jan M. H. , Hong Sung-Ho , Miller Tim , Borchers Brian , Rhebergen Jan B.
Published: April 2008

Landmines are a major problem in many areas of the world. In spite of the fact that many different types of landmine sensors have been developed, the detection of non-metallic landmines remains very difficult. The objective of this contribution is to synthesize our work related to the effects of soil texture and soil water content on the detection of non-metallic anti-tank landmines using ground penetrating radar (GPR). We present a model for the prediction of radar response under a wide range of soil texture and soil water content conditions. Next we present measurements from field experiments in a sand and a silt loam which validate the radar response model. The spatial variability of soil water content in field soils may strongly decrease the effectiveness of GPR for anti-tank mine detection. Uniform wetting of sand and loam soils by irrigation or rain will improve the radar signal. Radar detection in clay soils will be most effective under dry conditions.
The use of vertical radar profiling (VRP) in GPR surveys of ancient sedimentary strata
Authors: Pringle J. K. , Westerman A. R. , Clark J. D. , Guest J. A. , Ferguson R. J. , Gardiner A. R.
Published: April 2008

Vertical radar profiling (VRP) is an application of ground penetrating radar (GPR) technologies that can extract important subsurface information from suitable outcrops. Using standard GPR equipment, a site-specific time-depth calibration can be obtained, along with correlation of observed sedimentological horizons exposed on cliff-faces. These horizons may then be correlated with subsurface reflection events imaged on fixed-offset profiles. Summaries of six GPR study sites, where the VRP technique was used, are detailed. Where possible, CMP and VRP velocities have been compared, and show good correlations. Geochemical analysis of selected sedimentary rocks shows that increasing grain size and quartz mineral percentages generally lead to increased GPR velocities. Reflection events tend to be associated with sandstone/shale boundaries.
Downhole GPR for high-resolution analysis of material properties near Fairbanks, Alaska
Authors: Hunter Lewis E. , Delaney Allan J. , Lawson Daniel E. , Davis Les
Published: April 2008

In August 2000, we used cross-borehole ground penetrating radar (GPR) measurements to investigate sites on Fort Wainwright, Alaska, and east of the Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel near Fox, Alaska. The sites are characterized by perennially frozen soils characteristic of much of interior Alaska. The purpose of this investigation was to define variations in GPR signal velocity and attenuation that may be indicative of hydrocarbon contamination and to determine if downhole GPR methods could detect petroleum contaminants. To acquire background information for comparison, we conducted detailed investigations in non-contaminated areas to define general conditions before we profiled at the Fort Wainwright Tank Farm where extensive contamination has been documented.

Results showed that cross-borehole GPR is a useful tool for detecting changes in electrical characteristics in permafrost and contaminated environments. Bulk electrical properties (velocity and attenuation) were observed to vary systematically between frozen and unfrozen materials, and produced distinct signal responses to frozen/unfrozen sediments, bedrock and massive ground ice. There is considerable overlap in both velocity and attenuation for soils and rock. Fine-grained silts exhibit higher attenuation (9–10 dB m
−1
) than coarser sand and gravel (1.5–2 dB m
−1
). Bedrock values are intermediate (2–7 dB m
−1
). Unfrozen sediment and rock exhibit velocities between 0.05 m ns
−1
and 0.1 m ns
−1
and attenuation ranges from 4 dB m
−1
to 11 dB m
−1
at 100 Mhz while frozen materials are between the velocities of 0.1 m ns
−1
and 0.18 m ns
−1
and 1.5 dB m
−1
and 10 dB m
−1
at −0.3 °C. The general trend is for frozen materials to have higher velocities than their unsaturated or saturated counterparts. Preliminary results where petroleum contamination was pervasive show that the attenuation decreases from 6 dB m
−1
to 7 dB m
−1
in uncontaminated schist to 4 dB m
−1
in the hydrocarbon-contaminated schist. Additional measurements are required to determine that the observed decrease in attenuation is primarily related to petroleum contamination.
The use of GPR to image three-dimensional (3-D) turbidite channel architecture in the Carboniferous Ross Formation, County Clare, western Ireland
Authors: Pringle J. K. , Clark J. D. , Westerman A. R. , Gardiner A. R.
Published: April 2008

Petroleum reservoir models are currently built from two-dimensional (2-D) information. An understanding of both the large-scale and internal three-dimensional (3-D) architecture of turbidite channel deposits is important for both hydrocarbon exploration and production. A ground penetrating radar (GPR) survey was undertaken on a study site exposing Upper Carboniferous Ross Formation deposits in western Ireland. Both channel margins and intrachannel fill were imaged in 3-D. Constant-offset, 2-D reflection sections were calibrated by vertical radar profiles. GPR data were integrated with sedimentary and survey data to produce a 3-D model of the study site.
Impact of periodicity on sediment flux in alluvial systems: grain to basin scale
Authors: Frostick Lynne E. , Jones Stuart J.
Published: April 2008

Periodicity is a common component of many sedimentological processes, but seldom is it considered across all scales of fluvial processes in order to understand the complete impact on sediment supply to basins. Temporal changes in sediment supply within drainage systems and sedimentary basins are a consequence of the inherent instability in transport processes. The causes of fluctuations are of 2 main types: (i) changes in factors endemic to the supply of sediment but which are at least partly independent of erosive forces and (ii) changes in the magnitude of forces available to transport sediment. Fluctuations at spatial scales from grain — through reach — to basin — scales and at temporal scales from minutes to millennia are discussed and evaluated. All fluctuations are reflected in sedimentary deposits in some way. For example, irregular patterns of bed break-up during erosion can generate bedforms that are recorded in deposits, the passage of waves of sediment can cause cycles of incision and aggradation in a reach; large flood events will flush sediment into coastal regions and will be recorded as an identifiable ‘package’ in the deposits. Many models of basin processes and products assume a consistent supply of sediment which is far from the case in nature. One of the challenges in the coming decade is to move away from using long-term averages of sediment supply and to link models directly into geomorphic processes.
Annual sediment budgets in an unstable gravel-bed river: the River Coquet, northern England
Authors: Fuller I. C. , Passmore D. G. , Heritage G. L. , Large A. R. G. , Milan D. J. , Brewer P. A.
Published: April 2008

Sediment budgeting procedures based on analysis of three-dimensional morphological change provide a useful mechanism by which rates and patterns of fluvial sediment erosion, transfer and deposition can be monitored. This paper presents results from an annual sediment budgeting programme established in a 1-km long piedmont reach of the gravel-bed River Coquet in Northumberland, northern England. The study reach has a locally braided channel planform and has experienced lateral instability over at least the past 150 years. Annual sediment budgets for 1997–1998 and 1998–1999 have been based on tacheometric survey of: (i) 15 monumented channel cross-profiles; and (ii) channel margins and gravel-bar morphology. Survey data have been analysed for each discrete morphological unit (differentiating channel and complex bar assemblages) within 17 sub-reaches of the study reach using Arc/Info
™
GIS. The morphological sediment budgeting techniques used to generate these reach-scale budgets may be particularly valuable in unstable gravel-bed rivers due to the inherent difficulties in measuring bed-load transport. The results show considerable variability in rates and patterns of within-reach sediment transfer between the successive surveys. The channel at Holystone is characterized by substantial within-reach sediment transfer, with minimal net export downstream. This behaviour appears to be characteristic of UK gravel-bed piedmont rivers.
Drainage basin structure, sediment delivery and the response to environmental change
Authors: Richards Keith
Published: April 2008

The purpose of this paper is to explore the role that drainage basins and networks play in filtering and modulating the effects of environmental change. The record of this change is often inferred from the properties of alluvial stratigraphy in river valleys. However, the variable time lags and smoothing effects caused during the flux of sediment through a drainage basin are very poorly understood, and need to be clarified before the depositional record can be linked to the causes of change. Interactions amongst drainage network elements — main valleys and tributaries — result in complex patterns of storage and delivery in time and space, and these interactions are reviewed in this paper. While the combination of cosmogenic isotopic dating of erosional records, and the use of shallow seismic and ground penetrating radar methods may provide data on the empirical linkages between sediment sources and sinks, a significant research need is for a modelling framework to enable cut-and-fill records to be simulated, and to provide a rigorous connection between the valley-fill depositional record and the environmentally-driven variations in sediment production. This will require a return to the long-neglected analysis of drainage network structure, and an improved understanding of the within-catchment variations in the sediment delivery ratio. Until this is achieved, it will be premature to assume direct connections between environmental change and the record of alluviation in many valleys.
Metamorphism and thermodynamics: the formative years
Authors: Fritscher Bernhard
Published: April 2008

Between 1890 and the 1920s petrologists and mineralogists began to apply concepts of theoretical chemistry — in particular, the concept of chemical equilibrium — to the study of metamorphic rocks. The majority of the petrological community, however, hesitated to apply the new method on a large scale to metamorphism. Focusing on the works of Becke, Goldschmidt and Eskola, some early approaches to a linkage of metamorphic petrology and theoretical chemistry are reviewed. The controversial discussion, particularly of Goldschmidt’s classical study of the Christiania area, led Miyashiro to distinguish two paradigms of early twentieth-century metamorphic petrology. With regard to the contemporary discussion, as well as to Miyashiro’s interpretation, this paper is concluded by an epilogue on ‘image and logic’, which is intended to relate the paradigms of early modern metamorphism to different cultures, and ‘national styles’ of Earth sciences in the nineteenth and early twentieth centuries.
Marie Tharp, oceanographic cartographer, and her contributions to the revolution in the Earth sciences
Authors: Barton Cathy
Published: April 2008

In the early 1950s, two American geologists, Bruce Charles Heezen (1924–1977) and Marie Tharp, began mapping the sea floor to improve understanding of ocean-basin geology and to connect the oceans to the continents theoretically. Both were researchers at the Lamont Geological Observatory of Columbia University, now Lamont-Doherty Earth Observatory. Heezen and Tharp used the ‘physiographic mapping’ technique, which makes it possible to relate topographic features to underlying geology. The diagrams mostly utilized light and texture, rather than colour, and were sketched using a hachuring technique. Heezen collected data for research purposes and Tharp used his information to compile their physiographic diagrams. During this process, she confirmed previous predictions when she made an important discovery: a rift on the Mid-Atlantic Ridge. Tharp’s visual interpretations of the sea-floor data contributed to the reintroduction of continental drift theory and the 1960s geological revolution. At a time when most women were excluded from scientific careers, Tharp, initially a research assistant, succeeded in this competitive arena. Working with Heezen as a geologist and cartographer, she had an unusual opportunity to participate in the era’s exciting discoveries; and her contributions were acknowledged. While their data-gathering activities and analyses stimulated change and contributed to the revolution in the Earth sciences, Heezen and Tharp were not directly involved in the plate-tectonics revolution, but favoured expanding-Earth theory.
Some personal thoughts on stratigraphic precision in the twentieth century
Authors: Torrens Hugh S.
Published: April 2008

Surprisingly, most of the major elements of today’s stratigraphic column were in place by 1850. By then, the ideas that stratigraphy concerned geological time relations, and that a palaeontological identity of ‘best’ fossils (like ammonites) was an indication of time-equivalence, were starting to be accepted. By 1900, thanks to the work of people like Henry Shaler Williams (USA) and Sydney Savory Buckman (UK) stratigraphy was starting to concern itself with the precision with which biochronological time-scales could be created, especially in the Jurassic. By then, Buckman had demonstrated the great extent to which particular lithologies could cross time-lines and equally how well such rapidly evolving fossils as ammonites could be used to discriminate time. But from 1960, facing new demands for energy, and the growth of new ‘earth science’, focussing on numerical methods in geophysics and geochemistry using computers, such field-based ‘historical geology’ was progressively perceived as boring, out-dated, and expensive. Many new techniques, which ignored, or worse, assumed time-equivalence, now evolved. Fossils by their unique nature had given unique signatures to discriminate time. But some of the new methods relied on binary repetitions, not unique to time, and may suggest a false precision. This paper attempts a, now near-impossible, investigation of the temporal precisions that stratigraphic methods, both old and new, might attain. It concludes that we need to pay greater attention to the incompleteness of the stratigraphic record and to the chronological precision with which we can investigate that record. It now seems almost axiomatic that the harder you look at a rock the more incomplete the record of its stratigraphy appears to become.
Brachiopods: Cambrian-Tremadoc precursors to Ordovician radiation events
Authors: Bassett Michael G. , Popov Leonid E. , Holmer Lars E.
Published: April 2008

Brachiopod-dominated palaeocommunities incorporating a structure typical of faunal groups within the Palaeozoic Evolutionary Fauna were already present in North and East Gondwana and associated terranes as early as the mid-Cambrian, confined exclusively to shallow marine, inshore environments. The late Cambrian and Tremadoc record of these faunas is incomplete, because of pronounced global sea-level lowstand and subsequent break-up and destruction of the Cambrian Gondwanan margin. It is likely, however, that those groups later forming the core of the Palaeozoic Evolutionary Fauna evolved originally in shallow-water environments of low-latitude peri-Gondwana, and dispersed widely when favourable ecological conditions developed. Conspicuous sea-level rise through the early to mid-Arenig provided newly available habitats in the expanding epeiric seas, where the new faunas evolved and diversified by the mid-Ordovician, when rapid drift separated the early Palaeozoic continents. Relatively short-lived precursor and transitional brachiopod assemblages can be identified on most of the main palaeocontinents prior to the Ordovician radiation of the Palaeozoic Evolutionary Fauna.
Early Ordovician rhynchonelliformean brachiopod biodiversity: comparing some platforms, margins and intra-oceanic sites around the Iapetus Ocean
Authors: Harper David A. T. , Niocaill Conall Mac
Published: April 2008

During the Arenig-Llanvirn interval a series of radiations across all the major clades, established the Brachiopoda as a major component of the Palaeozoic benthos. Radiations on Baltica and Laurentia during the Arenig formed the basis for two distinct biogeographical provinces with contrasting articulated brachiopod faunas. These platform provinces were supplemented by the marginal Celtic and Toquima-Head provinces; the latter included both marginal and intra-ocean island sites. These marginal and intra-oceanic sites may have served as both ‘cradles and museums’ alternately providing sources for radiations on the platforms and refugia for otherwise relict taxa. Such terranes also partitioned oceanic circulation patterns within the Iapetus Ocean and provided mosaics of rapidly changing, nearshore unstable environments. In contrast to later Ordovician brachiopod faunas, many early Ordovician genera are reported from only one or two sites in the Iapetus region; narrow geographical ranges are characteristic of many taxa. The strong biogeographic differentiation at the generic level is less marked at the familial and higher levels suggesting a series of late Cambrian and early Ordovician migrations prior to the more regional development of the Arenig-Llanvirn brachiopod biofacies across the Iapetus terranes. However, the origination of many brachiopod taxa apparently occurred outside the Iapetus region suggesting that the initial stages of the Ordovician radiations here were first spiked by a series of immigrations.
The spatial and temporal diversification of Early Palaeozoic vertebrates
Authors: Smith M. Paul , Donoghue Philip C. J. , Sansom Ivan J.
Published: April 2008

Recent discoveries have dramatically altered traditional views of the stratigraphic distribution and phylogeny of Early Palaeozoic vertebrates and permit a reappraisal of biogeographic patterns and processes over the first 120 million years of vertebrate evolution. Stratigraphic calibration of the phylogenetic trees indicates that most of the pre-Silurian record can be inferred only through ghost ranges. Assessment of the available data suggests that this is due to a shift in ecological niches after the latest Ordovician extinction event and a broadening of geographical range following the amalgamation of Euramerica during the early Silurian. Two major patterns are apparent in the biogeographic data. Firstly, the majority of jawless fishes with dermoskeletal, plated ‘armour’ were highly endemic during Cambrian-Ordovician time, with arandaspids restricted to Gondwana, galeaspids to China, and anatolepids, astraspids and, possibly, heterostracans confined to Laurentia. These Laurentian groups began to disperse to other continental blocks as the ‘Old Red Sandstone continent’ amalgamated through a series of tectonic collisions. The second major pattern, in contrast, encompasses a number of microsquamous and naked, jawed and jawless primitive vertebrates such as conodonts, thelodonts, placoderms, chondrichthyans and acanthodians, which dispersed rapidly and crossed oceanic barriers to attain cosmopolitan distributions, although many have Laurentian origins. A clear difference in dispersal potential exists between these two types of fishes. Overall, the development of biogeographic patterns in Early Palaeozoic vertebrates involved a complex interaction of dispersal, vicariance and tectonic convergence.
Integrating the present and past records of climate, biodiversity and biogeography: implications for palaeoecology and palaeoclimatology
Authors: Markwick Paul J.
Published: April 2008

A geographic information system (GIS) based, integrated dataset of Recent North American, European, southern African and Australian non-avian tetrapod faunas is used to examine the macroscale relationship between climate, biogeography and terrestrial taxonomic and functional species diversity (richness). The results support a modified form of the species-energy hypothesis, with the pattern of terrestrial biodiversity reflecting the manner in which species procure energy, rather than only the absolute amount of ‘available energy’. Area and history are also found to be important. Ectotherms show the simplest relationship with environmental variables (and strongest latitudinal diversity gradients), and endotherms the most complex. A strong linear relationship is found between the proportion of each fauna represented by ectotherms and temperature (mean annual temperature and coldest month mean temperature). This relationship is used in an experiment to retrodict the palaeotemperature for the Middle Eocene lagerstätten fauna from Messel, Germany. Results compare well with interpretations based on other climate proxies.
Paleogene plate tectonic evolution of the Arabian and Eastern Somali basins
Authors: Royer Jean-Yves , Chaubey A. K. , Dyment Jérome , Bhattacharya G. C. , Srinivas K. , Yatheesh V. , Ramprasad T.
Published: April 2008

We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61–46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudofaults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c.54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a−1. Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.
Paleogene magnetic isochrons and palaeo-propagators in the Arabian and Eastern Somali basins, NW Indian Ocean
Authors: Chaubey A. K. , Dyment Jérome , Bhattacharya G. C. , Royer Jean-Yves , Srinivas K. , Yatheesh V.
Published: April 2008

We present a revised magnetic isochron map of the conjugate Arabian a and Eastern Somali basins based on an up-to-date compilation of Indian, French, and other available seasurface magnetic data. We have used the magnetic anomaly and the modulus of the analytical signal computed from the magnetic anomaly to identify and precisely locate the young and old edges of magnetic chrons in both basins. In addition to the major, well-defined anomalies, we have also used correlatable second-order features of the magnetic anomalies, the ‘tiny wiggles’, to strengthen the interpretation. The resulting isochrons and tectonic elements have been validated using the stochastic method of palaeogeographical reconstruction. The magnetic anomaly pattern in both basins depicts clear oblique offsets, characteristics of pseudofaults associated with propagating ridge segments. Our tectonic interpretation of the area revealed: (1) a complex pattern of ridge propagation between Chrons 28n (
c.
63 Ma) and 25n (
c.
56 Ma), with dominant eastward propagation between Chrons 26n (
c.
58 Ma) and 25n; (2) numerous, systematic westward propagations between Chrons 24n (
c.
53 Ma) and 20n (
c.
43 Ma); (3) asymmetric crustal accretion (caused by ridge propagation and asymmetric sea-floor spreading) in the conjugate basins during the whole period; (4) a slowing of India-Somalia motion after
c.
52 Ma.
Basalt and peridotite recovered from Murray Ridge: are they of supra-subduction origin?
Authors: Burgath Klaus-Peter , Von Rad Ulrich , Van Der Linden Willem , Block Martin , Khan Athar Ali , Roeser Hans Albert , Weiss Wolfgang
Published: April 2008

Petrographic and geochemical data for new basalt and peridotite samples recovered from sampling sites at the Southern Murray Ridge help to constrain models for the evolution of the Owen-Murray Ridge system, which forms the northwestern boundary of the Indian plate. Trace elements immobile during alteration (Ti, Zr, Nb, Y and rare earth elements) suggest that the altered microphyric metabasalt has affinities to magmatism of active margins (island-arc tholeiite
sensu lato
). It is distinctly different from mid-ocean ridge basalt, back-arc-basin basalt, or intra-plate Deccan Trap basalt. The sampled serpentinized harzburgite or clinopyroxene-poor lherzolite was deformed under mantle conditions and is similar to the mantle section of nearby ophiolite sequences. This association of rocks suggests that an ophiolite mélange was sampled. However, results from sampling station 462 NIOP indicate that the Murray Ridge complex also contains igneous rocks with Deccan Trap affinity. For the emplacement of the island-arc tholeiite we assume an origin in a convergent supra-subduction setting, related to the closing of a Late Cretaceous Neo-Tethyan ocean basin between the Arabian and Indian plates to the south and the Eurasian plate to the north. Since Neogene time, the Murray Ridge-Dalrymple Trough has been underlain by attenuated (?)continental crust and characterized by extensional rift tectonics.
Seismic stratigraphy of the offshore Indus Basin
Authors: Daley Tim , Alam Zaheer
Published: April 2008

In 1997 Lasmo Oil Pakistan Ltd (Lasmo) gained a significant position in the offshore Indus Basin with the award of the Indus A and B Blocks. The main hydrocarbon play comprises Miocene shelf-delta sands interbedded with intraformational shale seals and sourced by gas-prone offshore equivalents. Approximately 12 000 km of seismic data have been interpreted in the detailed evaluation of these blocks. However, only four wells have tested the preferred play type and no core or rock data were available to provide further insights into facies or age dating. Log data from two key wells in the offshore Indus area record the initial infilling of the basin by shale-dominated basinal or outer shelf sediments, followed by stacked thin-bedded sandstone-shale sequences of a shelf-delta nature. A zone of progradational sequences marks the transition between the two, but no other workable stratigraphic divisions were apparent. Regional seismic correlation established the diachronous nature of the prograding shelf package and this was matched by distinct bands of seismic progrades. A series of simple palaeogeographies of the prograding shelf margin were developed showing initial sediment input from the north and rapid progradation towards the south and west. The Oligo-Miocene basin fill of the offshore Indus Basin appears to be a ‘one-step’ fill process of a significant depocentre created between the Karachi Platform and the Murray Ridge. Canyons are a very distinct feature on seismic profiles and two main phases of development are apparent. The earlier phase is interpreted to be of Early Miocene age. Downcutting at this time rarely exceeded 400 m. The second phase of canyon development occurred during Plio-Pleistocene time, and these younger canyons often dominate the shallow section, with multiple phases of downcutting sometimes exceeding 1000 m. Where drilled, canyons of both ages have been found to be shale prone. These drilled canyons are interpreted to be on the palaeo-slope where erosion and sediment by-pass occurred during the active phase, and were subsequently filled by fine-grained deposits after abandonment. The two phases of canyoning are considered to relate to phases of tectonic activity in the collision zone between the western margin of the Indo-Pakistan plate and the Eurasian plate.
Quaternary climatic changes over Southern Arabia and the Thar Desert, India
Authors: Glennie K. W. , Singhvi A. K. , Lancaster N. , Teller J. T.
Published: April 2008

The distribution of sand dunes over the southern half of Arabia conforms to the influence of two wind systems: the northern Shamal, which is a strong wind that blows to the SSE down the Persian (Arabian) Gulf and then swings to the SW across the hyperarid Rub al Khali towards North Yemen; and the strong winds of the SW Monsoon system, which were responsible for forming linear dunes that trend north-south in the Wahiba Sands of eastern Oman and SW—NE in the Thar Desert (NW India). In the Thar Desert, the SW Monsoon alternates with the weaker NE Monsoon. The dating of exposures of older dune systems by isotopic, radiometric and optically stimulated luminescence (OSL) analyses has shown that the Shamal was active throughout the latter part of the Quaternary period, and probably as long ago as Mid-Miocene time (c.15 Ma). At times of glacial maxima, when global sea level was some 100–120 m or more lower than now, siliciclastic and carbonate grains were deflated from the exposed surface of the Persian Gulf and transported into the NE Rub al Khali within the United Arab Emirates. It is suspected that occasionally the Shamal also transported some quartz sands from the NW onto the exposed narrow continental shelf of SE Arabia, with silt-size particles being carried into the Arabian Sea. The SW Monsoon, on the other hand, was re-established over the coast of SE Arabia several millennia after the last glacial maximum and was fully established near the coast of SE Arabia during the early Holocene interglacial after the atmospheric high-pressure system associated with the glacial period had become weaker. Early during the Holocene interglacial periods when the SW Monsoon dominated, a combination of quartz and carbonate sands was deflated from the exposed continental shelf and transported to the north into the Wahiba Sands. Aeolian activity in the Thar Desert also peaked during this period of transition from full glacial to interglacial conditions. The dune systems of SE Arabia overlie the distal edges of older alluvial fans that in Oman date back at least 350 ka. The sediments of some of these fluvial sequences in Oman reached the Arabian Sea via Wadi Batha, only to be removed by along-shore currents driven by the SW Monsoon. In the Thar Desert, the supply of aeolian sediment is mostly from fluvial sources. Marine sediments from the Arabian Sea between Arabia and Thar record the contrasting effects of the Shamal and the SW Monsoon: the former mostly as a source of wind-blown dust from Arabia and the latter by causing upwelling of nutrient-rich waters leading to organic blooms.
Paleocene initiation of Cenozoic uplift in Norway
Authors: Nielsen S. B. , Paulsen G. E. , Hansen D. L. , Gemmer L. , Clausen O. R. , Jacobsen B. H. , Balling N. , Huuse M. , Gallagher K.
Published: April 2008

The timing of Cenozoic surface uplift in NW Europe relies on the assumption that the sedimentary response in basins is synchronous with tectonic processes in the source areas. However, many of the phenomena commonly used to infer recent uplift may as well be a consequence of climate change and sea-level fall. The timing of surface uplift therefore remains unconstrained from the sedimentary record alone, and it becomes necessary to consider the constraints imposed by physically and geologically plausible tectonic mechanisms, which have a causal relation to an initiating agent. The gradual reversal of the regional stress field following the break-up produced minor perturbations to the thermal subsidence on the Norwegian Shelf and in the North Sea. Pulses of increased compression cannot be the cause of Cenozoic land surface uplift and accelerated Neogene basin subsidence. Virtually deformation-free regional vertical movements could have been caused by changes in the density column of the lithosphere and asthenosphere following the emplacement of the Iceland plume. A transient uplift component was produced as the plume displaced denser asthenosphere at the base of the lithosphere. This component decayed as the plume material cooled. Permanent uplift as a result of igneous underplating occurred in areas of a thin lithosphere (some Palaeozoic and Mesozoic basins) or for lithosphere under extension at the time of plume emplacement (the ocean-continent boundary). In areas of a thicker lithosphere (East Greenland, Scotland and Norway) plume emplacement may have triggered a Rayleigh-Taylor instability, causing partial lithospheric delamination and associated transient surface uplift at a decreasing rate throughout Cenozoic time. A possible uplift history for the adjacent land areas hence reads: initial transient surface uplift around the break-up time at 53 Ma caused by plume emplacement, and permanent tectonic uplift caused by lithospheric delamination and associated lithospheric heating. The permanent tectonic uplift increased through Cenozoic time at a decreasing rate. Denudation acted on this evolving topography and reduced the average surface elevation, but significantly increased the elevation of the summit envelope. The marked variations in the sedimentary response in the basins were caused by climatic variations and the generally falling eustatic level. This scenario bridges the gap between the ideas of Paleocene-Eocene uplift versus repeated Cenozoic tectonic activity: the tectonic uplift history was initiated by the emplacement of the Iceland plume, but continued throughout Cenozoic time as a consequence of early plume emplacement, with climatic and eustatic control on denudation. The mechanism is consistent with topography, heat flow, crustal structure, and the Bouguer gravity of Norway, and may be applicable also to East Greenland.
Landforms and uplift in Scandinavia
Authors: Lidmar-Bergström K. , Näslund J. O.
Published: April 2008

The relation between Scandinavian landforms and Cenozoic uplift events is examined by analysis of digital elevation data in a regional geological context as well as in a geomorphological process perspective. Re-exposed flat sub-Cambrian and sub-Mesozoic hilly relief aids in deciphering uplift and erosional events. The highly dissected mountains of the Northern Scandes (NS) rise maximally 1500 m above a slightly titled lowest level continuing in the Muddus plains eastwards at 300–550 m above sea level (a.s.l.). This level is correlated with the lowest, slightly warped level of the Palaeic relief at 1000–1300 m a.s.l. of the Southern Scandes (SS), over which mountains of similar height rise. This lowest surface is thought to be the end result of Paleogene erosion to the general base level. Northern Scandinavia with the NS and the Muddus plains acted as a block that was progressively tilted to the SE, whereas the Southern Scandes experienced continuous doming, with a major uplift event of about 1000 m in Neogene time causing deep valley incision in the uplifted plateau. The South Swedish Dome emerged from its Palaeozoic and Mesozoic cover in Neogene time and still retains well-preserved re-exposed palaeosurfaces.
Pattern and timing of the post-Caledonian denudation of northern Scandinavia constrained by apatite fission-track thermochronology
Authors: Hendriks Bart W. H. , Andriessen Paul A. M.
Published: April 2008

Apatite fission-track thermochronology has been used to study the post-Caledonian denudation history of northern Scandinavia. Post-orogenic denudation progressively shifted from the interior of the continent towards the North Atlantic margin. The present-day area of maximum elevation in the Northern Scandes mountain range has experienced continuous denudation at least since Jurassic time. In Jurassic-Cretaceous time, the area north and east of this region experienced either no denudation at all or some denudation followed by a transient thermal event with a peak temperature in late Cretaceous time. Final denudation of the area to the east of the Northern Scandes probably started in late Cretaceous-Paleogene time and possibly accelerated in Neogene time. The denudation history of northern Scandinavia can be explained by scarp retreat of an uplifted rift flank. The pattern and timing of denudation of the Northern Scandes is different from that of the Southern Scandes, which experienced domal-style, late-stage postrift uplift in Neogene time. Geomorphological observations, offshore data from the Atlantic and Barents Sea margins, and scarce stratigraphical information from the mainland are in general agreement with the new thermochronological data.