bluuebunny/crossref_metadata_2025 huggingface dataset:
Cretaceous foraminiferal events
Authors: Hart Malcolm B.
Published: April 2008

Bioevents range in importance from the first/last appearance of a single taxon, through zonal boundaries of local/regional/international significance, major non-sequences, to (at the highest level) global bioevents. The UK mid-Upper Cretaceous succession is now sampled at better than a 1 m spacing from many localities providing over 2000 samples for detailed analysis and development of high resolution stratigraphy. Examples of detailed correlation are provided for the Albian/Cenomanian boundary, the mid-Cenomanian and the uppermost Cenomanian, while consideration of the complete faunal succession provides information necessary for the monitoring of global environmental change in the Cretaceous.
Devonian goniatite biostratigraphy and timing of facies movements in the Frasnian of eastern North America
Authors: House Michael R. , Kirchgasser William T.
Published: April 2008

The conjunction of detailed goniatite and conodont biostratigraphy has enabled correlation of the Frasnian and adjacent strata of New York with the international standards. A regional goniatite zonation is proposed and as part of the documentation a new Frasnian goniatite assigned to the Triainoceratidae is described as
Wellsites tynani
n.gen. n. sp. The zonation has enabled documentation of the timing and extent of facies movements associated with the Catskill Delta and more easterly deposits in New York State. The broad pattern is one of westward progradation of shallower, more clastic facies. Pulses of deeper water sedimentation, represented by black shale facies indicating anoxic/hypoxic conditions, punctuate the succession and give marker levels. Typically, these black shales initiate shallowing upward cycles. The major pulses are represented by the Tully, Geneseo, Renwick, Genundewa, Middlesex, Rhinestreet, Pipe Creek and Dunkirk units. The cycles enable estimates to be made of deepening against a subsiding shelf; the Rhinestreet Shale represents the greatest. Evidence is given on the regional extent of some of these. International comparison suggests some of these cycles represent global, rather than local events. Microcycles at the Milankovitch Band level also show thin black shales suggesting that climatic control has a bearing on facies developments. General comment is made on the factors controlling local and eustatic changes.
Tectonics and seismic sequence stratigraphy: an introduction
Authors: Williams G. D.
Published: April 2008

The application of sequence stratigraphic models to seismic data is restricted by the vertical and spatial resolution of the data. The fundamental stratigraphic unit of the sequence stratigraphic technique is the seismically distinguished ‘sequence’ although the concept of tectonically related ‘megasequences’ is useful. Basin stratigraphy is controlled to varying degrees by eustatic sea-level change (or base level in lakes), subsidence/uplift (tectonics) and sediment supply. Three main basin types, rift-, wrench-, and thrust-related basins, have distinctive gross stratigraphic architectures. Localized, tectonically controlled subsidence and uplift has a significant control on three-dimensional stratigraphic patterns.
Wind-speed profiles over a reversing transverse dune
Authors: Burkinshaw Jennifer R. , Illenberger Werner K. , Rust Izak C.
Published: April 2008

Wind-speed profiles were measured within 1.5 m of the surface on the windward slope of a 7 m high transverse dune that reverses seasonally with respect to summer easterly and winter northwesterly winds. The profiles were measure along section lines normal to the dune trend at different stages of dune reversal. An independent weather station recorded relatively undisturbed flow at 6 m elevation above the surface.

Flow deceleration at the base of the windward slope and flow acceleration against the middle to upper slope generate non-logarithmic wind-speed profiles. The shape of the dune and the strength of the wind play a major role in determining the behaviour of the wind-speed profile. Airflow compression is enhanced by steeper slopes, resulting in high surface shear stress that causes rapid erosion of the windward slope.

The wind-speed profile can be divided into three zones: a surface shear stress layer, an amplification layer and a recovery layer. It seems that the shear stress that governs sand transport is developed within 5 cm of the surface, in the surface shear stress layer. Future work should concentrate on measurements within this zone.
Luminescence dating of aeolian sands: an overview
Authors: Wintle Ann G.
Published: April 2008

Aeolian sands are ideal materials for the application of luminescence techniques. Their primary mineral constituents, quartz and feldspars, exhibit a variety of luminescence properties and can be separated for mineral-specific measurements. Individual grains are usually well exposed to light prior to deposition thus enabling their signal to be zeroed at the time of interest.

Luminescence is produced either by heating the grains to produce thermoluminescence (TL) or by stimulating at room temperature with wavelengths specific for each mineral component. Only light-sensitive signals are observed in measurements of optically stimulated luminescence (OSL). For feldspars, infra-red stimulated luminescence (IRSL) can be obtained using IR-emitting diodes.

Luminescence dating techniques can be applied to inland sand seas, small dunefields, coastal dune systems or periglacial sand sheets. Natural variation in radioactive content in different geographical areas can be used to advantage — sands with a high radioactive content, as found in areas dominated by granitic rocks, are best suited for applications to Holocene dunefields (such as Kelso, California), whereas sands with a low radioactive content, as found in areas containing little feldspar-bearing rock and with much recycled sediment (e.g. Australia), are best suited to applications spanning the last 100 ka.
Entrada Sandstone: an example of a wet aeolian system
Authors: Crabaugh Mary , Kocurek Gary
Published: April 2008

Wet aeolian systems are those in which the water table is shallow and the floors of the interdune flats are within the capillary fringe. Accumulation within a wet aeolian system occurs because of a relative rise in the water table, and these accumulations are characterized by the presence of both dune and interdune-flat deposits. The Jurassic Entrada Sandstone, studied along a 2.7 km traverse in NE Utah, is interpreted as having been formed in a wet aeolian system because of a preponderance of features indicating a shallow water table throughout accumulation of the unit, and regional correlation to marine-sabkha units that vertically stack, then onlap the aeolian-dominated strata. Features indicating a shallow water table and/or evaporites include subaqueous ripple deposits, contorted strata, breccias, collapse features, wavy bedding, foundered sets, loaded set bases, corrugated surfaces, polygonal fractures, ball-and-pillow structures, and trains of small dunes ‘frozen’ in place. Along the traverse, oriented roughly parallel to the predicted palaeoflow from the NNE, sets of cross-strata, bounding surfaces and ‘flat’-bedded strata climb to the SW, while other surfaces and ‘flat’-bedded strata are horizontal. Climb is interpreted to result from the migration of dunes and interdune areas during periods of a rising water table, whereas horizontal surfaces represent super-bounding surfaces formed during periods of static or falling water table, and horizontal ‘flat’ strata represent vertical sabkha accretion. The cross-stratified units consist of compound sets with two scales of cyclicity. The larger, most prominent cyclicity is formed of scalloped bounding surfaces systematically truncating foresets to the SW, and results from superimposed dunes migrating NW and SE along-slope larger bedforms that migrated toward the SW. The smaller scale of cyclicity consists of packages of wind-ripple laminae and grainflow strata truncated by bounding surfaces, and these cycles are thought to represent annual variations in wind direction. Some cross-stratified units are co-sets in which the foresets of the main set can be traced downward into bottomsets consisting of small sets separated by corrugated surfaces. The basal sets are thought to represent small satellite dunes moving along the base of the larger bedforms and across damp interdune floors. The recognition of super surfaces within the Entrada allow the identification of four genetic packages that comprise the formation. Within genetic packages the available sand supply can be estimated, and is considered as a function of the rates of sediment supply and water table fluctuations. These relative rates, coupled with close proximity of the study area to the Jurassic palaeoshoreline and the regional stratigraphic relationships, suggest that fluctuation in relative sea-level controlled the facies architecture seen in the Entrada Sandstone of NE Utah.
Foredune morphology and sediment budget, Perdido Key, Florida, USA
Authors: Psuty Norbert P.
Published: April 2008

Along-shore foredune morphology on the eastern portion of Perdido Key is fitted to a model of foredune development related to sediment budget. The Perdido Key barrier island has a long history of shoreline erosion, up to 2 m a
−1
for over a century. Sediment availability was altered in 1985 because of beach nourishment. Subsequent re-working of the beach fill has transferred sediment in the along-shore direction as well as cross-shore. Over three years of survey data, including passage of three hurricanes, and aerial photo coverage portray the morphological changes in the foredune system and the displacement of the shoreline. Measurements of the changes in a spatial context record sediment redistribution from the nourishment area. Measurements of cross-sectional area changes support the model of foredune enhancement associated with negative sediment budget in the beach. Net along-shore changes of the foredune compare well with the barrier-island foredune development model.
Wind regime and sand transport on a coastal beach-dune complex, Tentsmuir, eastern Scotland
Authors: Wal Abhilasha , McManus John
Published: April 2008

Aeolian processes play a key role in the exchange of sediment between beaches and foredunes in coastal embayments. The present contribution examines the wind regimes characterizing one stretch of temperate coast (Tentsmuir, Scotland), the routing and volumes of the resultant aeolian sediment transport on the beaches and foredunes and presents a summary of wind-induced aeolian bedforms along the coast.

The wind regimes identified in the Tentsmuir area include high-energy seasonal ‘unimodal’ (offshore or onshore) and ‘bimodal’ (both offshore and onshore) patterns. Landward aeolian sand transport results in foredune accretion, whereas seaward and longshore components contribute to beach growth despite reworking by waves at high water. Under the influence of the prevalent wind regimes and the resultant aeolian sediment transport, foredune accretion at Tentsmuir leads to the burial of the landward margin of the beach face.
Genesis and sedimentary structures of late Holocene aeolian drift sands in northwest Europe
Authors: Koster Eduard A. , Castel Ilona I. Y. , Nap Ron L.
Published: April 2008

In northwest Europe local resedimentation of terrestrial deposits by wind from the Neolithic to the present has resulted in widespread accumulation of (aeolian) drift sands. These deposits consist mainly of fine, well-sorted and well-rounded unimodal sands; their mineralogical composition usually reflects local provenance. Palynological studies combined with radiocarbon analyses of the topmost part of buried peat sections reveal no distinct regional phases in drift sand accumulation: ages mainly vary from
ad
500 to 1700. Small remnants of current actively moving drift sands provide evidence of sub-recent deflation and accumulation processes in relation to relief, topsoil and aerodynamic conditions. These are reflected in the sedimentary structures. Principal structures show dune foreset cross-bedding as well as even or wavy sub-horizontal lamination, while occasional strings of granules or small pebbles represent deflation phases. Primary structures resulting from tractional deposition (mainly sub-critical climbing ripple migration) and grainfall deposition, secondary structures (mainly slump, scour-fill and adhesion structures) and non-aeolian structures (mainly caused by rain impact, convolution structures, wetted sand crusts, and foot- or hoof-imprints) have a distinct appearance in lacquer peels and in SEM micrographs. Transport and accumulation of these temperate aeolian sands appear to have occurred under dry, moist and wet conditions.
Tectonic controls upon drainage basin development, river channel migration and alluvial architecture: implications for hydrocarbon reservoir development and characterization
Authors: Leeder M. R.
Published: April 2008

Hydrocarbon reservoirs of fluviatile origin are the product of a number of geological processes acting on a variety of scales. Reservoir characterization must be approached holistically in order to understand these processes fully. Tectonic style produces the macro-architectural framework and influences the behaviour of rivers at a mesoarchitectural scale. Tectonics controls the magnitude, position and development of drainage basins and thus, with climate and local geology, controls the flux of sediment fed into any fluviatile system. Migration of normal faulting may lead to the initiation of drainage basins in previously depositional areas, erosion and reworking thus enhancing reservoir quality. Sediment may be introduced as axial or transverse fluxes. Deducing these contributions in ancient basins establishes basin geometry and the basis for reservoir characterization. Individual river channels are highly susceptible to gradient changes caused by tectonic tilting. This causes slow channel belt migration, incision or sudden diversion (avulsion), depending, in a poorly known way, upon the magnitude of the gradients involved. The alluvial architecture of both tilt-block and growth fold basins is discussed, the latter illustrated by recent studies in the Southern North Sea Basin.
Quantitative regime modelling of fluvial depositional sequences: application to Holocene stratigraphy of humid-glacial braid-plains (Icelandic sandurs)
Authors: Maizels Judith K.
Published: April 2008

This paper introduces a forward, 1–2 dimensional model that predicts fluvial depositional sequences in response to (a) variations in runoff and sediment supply regimes of the source catchment, as well as to (b) fluctuations in relative sea-level controlled by eustatic and isostatic changes. Variations in sedimentology and aggradational history are simulated from accommodation principles and unit power relations of flows of varying sediment concentration. The model is applied to the sandurs of Iceland, which have been subject to large-scale environmental changes over the past 10 ka, associated with deglaciation, isostatic uplift and fluctuations in runoff regimes. Flows include not only normal seasonal ablation runoff from glaciers, but also repeated glacier burst floods generated by sudden volcanic activity. Simulation of 100-year runoff events has allowed prediction of the stratigraphy, sedimentology and geomorphic history of the sandurs. The model has been successfully tested at the smaller Solheimasandur, and then applied to Skeidararsandur, the largest of the Icelandic outwash plains. In both deposits, the predicted stratigraphy and geomorphic history highlight the significance of extreme events for the development of the alluvial sequences. The results indicate that, since alluvial stratigraphy can be so highly dependent on fluctuations in runoff and sediment supply regimes, these factors should be essential components in development of quantitative models of alluvial sequences used for reservoir exploration.
Reservoir characteristics of a braid-plain depositional system: the Upper Carboniferous Pennant Sandstone of South Wales
Authors: Jones Julie A. , Hartley Adrian J.
Published: April 2008

Braid-plain depositional systems form important hydrocarbon reservoirs in Palaeozoic, Mesozoic and Cenozoic basins throughout the world. Production from these reservoirs is frequently complex due to the nature and scales of permeability barriers inherent to braid-plain deposits. An analysis of the nature and distribution of permeability barriers from the well exposed braid-plain deposits of the Upper Carboniferous Pennant Measures of South Wales has revealed a variety of scales of heterogeneity. Potential permeability barriers include: (1) conglomerate channel lag deposits, (2) thick accumulations of plant detritus, (3) distribution of fines on foreset, set and coset boundaries, (4) laterally discontinuous mudstone bodies parallel or transverse to channel azimuth and (5) periodic basin-wide coal and seatearth development. Comparison of the Pennant outcrop with producing braid-plain reservoirs indicates that these permeability barriers are commonly encountered during production and are likely to be present (to some extent) in all potential braid-plain reservoirs. Account of heterogeneity distribution during production/development of braid-plain reservoirs will aid cost-effective production strategy.
Cyclical discharge variation recorded in alluvial sediments: an example from the Devonian of southwest Ireland
Authors: Kelly S. B.
Published: April 2008

Cross-stratification is a common sedimentary structure in alluvial deposits and can often be recognized in core. The quantitative study of cross-strata can potentially yield information concerning the behaviour of alluvial systems through time. Such information is valuable to a reservoir geologist when assessing the architecture of an alluvial reservoir.

Several alluvial systems in the Munster Basin (Devonian), southwest Ireland, display the characteristics of terminal fans generated in a ‘hydrographically-closed’ basin. Analysis of three formations reveals cyclical variations (‘megacycles’) in the dimensions of cosets and sets within channel sandbodies. The ‘megacycles’ are 100–200 m thick and record increases and subsequent decreases in the dimensions of sedimentary structures. The megacycles are also reflected by the density of channel deposits such that high densities are associated with relatively large channels and low densities are associated with relatively small channels. Cyclicity is also locally developed in the flood-plain deposits as alternations of red beds with calcretes (oxidizing conditions) and green/grey beds with pyrite nodules (reducing conditions).

Established relationships between dune size and flow depth suggest that cyclical variations in the size of sedimentary structures reflect discharge fluctuations. This is supported by the close correlation between set thickness and coset thickness, the latter being closely related to sandbody thickness. The cyclical variations in channel sandbody density and flood-plain conditions are also thought to reflect discharge variation.

Available biostratigraphic data and radiometric dates suggest that the cycles represent periods of several hundred thousand years. The periodicity and regularity of the cycles together with their relationship to discharge variation suggests that they may be related to astronomical forcing of the Earth’s climate.
A review of aeolian bounding surfaces, with examples from the Permian Minnelusa Formation, USA
Authors: Fryberger Steven G.
Published: April 2008

The recognition and use of bounding surfaces continues to be a problem in the study of modern and ancient aeolian sediments. The economic problems involve the apparent partitioning of aeolian petroleum reservoirs by cementation along bounding surfaces and associated strata. Bounding surfaces have also become a significant part of outcrop description. Ideally, bounding surfaces in ancient rocks should be tied to modern analogs, of which many are now recognized.

The currently popular approach to bounding surface description involves breakdown of rock units into orders based on theoretical relationships to draa migration. As described in this report, however, there are numerous bounding surfaces within aeolian genetic units, and between aeolian and non-aeolian strata that are not the product of draa migration. Indeed, conditions in which modern bedforms climb, and are preserved in perfect sequence are relatively rare, occurring most commonly where sand seas pile up at mountain fronts. Further, aeolian deposition and preservation is more complex and episodic than can be accommodated by the restrictive requirements of the climbing bedform model.

The author’s preferred approach to the description of aeolian sequences is in terms of genetic units and subunits, with objective description of crossbed sets using either Campbell’s or Mckee & Wier’s methods. These methods tie in well to sequence stratigraphic concepts that are applied at higher heirarchial levels.

Some examples of the application of bounding surface geometries and terminologies can be found in the Permian Minnelusa Formation of Wyoming, USA. The Upper Minnelusa Formation comprises a parasequence set of at least four parasequences. Each parasequence boundary was formed by marine transgression over an aeolian dunefield. The entire sequence is bounded by unconformities at the top and based. Most oil exploration takes place within the individual parasequences. Development drilling, however, usually making use of core, focuses on bounding surfaces at a much smaller scale. Perhaps surprisingly, the most important form of bounding surface control on reservior quality that can confidently be identified is fabric-selective cementation along primary bounding surfaces and strata. Other forms of cementation and decementation, not directly tied to bounding surfaces are also important.
Mixed aeolian sandsheet and fluvial deposits in the Tumblagooda Sandstone, Western Australia
Authors: Trewin Nigel H.
Published: April 2008

Field investigation of the Late Silurian Tumblagooda Sandstone of Kalbarri, Western Australia demonstrates that the lower part of the formation comprises a unit of fluvial trough crossbedded sandstone (450 m) overlain by previously unrecognized aeolian sandsheet deposits (350 m) which include small (to 2 m) aeolian dunes. Interbedding of aeolian and fluvial facies is present and fluvial facies decrease in importance down the fluvial transport path which was to the west and northwest. Aeolian transport was to the southeast and resulted in formation of medium- to coarse-grained low-angle sandsheets and small parallel-crested dunes. Fluvial transport in aeolian-dominated areas was deflected to the southwest along interdune corridors. Interdune deflation areas were frequently flooded, probably by rising ground water. Traces fossils are abundant, including beds bioturbated by
Heimdallia
and numerous arthropod trackways which cross aeolian strata. The dominance of sandsheet relative to dune morphologies is ascribed to the following factors: (1) mediumcoarse sand grain size, (2) high water table, and (3) short aeolian transport distance between fluvial channels not permitting extensive dune formation. A lack of vegetation is essential for development of fluvial and aeolian sandsheet facies. Fluvial sandstones were cemented by quartz overgrowths with only minor compaction. Aeolian dune sandstones have only minor quartz cement but show greater chemical compaction effects. Porosity retention is better in the aeolian sandstones.

The Taylor Group of Antarctica which is thought to be of early Devonian age shows strong similarities to the Tumblagooda Sandstone in terms of trace fossil assemblages, and sedimentology.
Structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir: the Triassic Sherwood Sandstone in the Irish Sea
Authors: Meadows Neil S. , Beach Alastair
Published: April 2008

Interpretation of nearly 5000 km of seismic data, and facies analysis of approximately 900 m of core from 11 wells, has produced structural and sedimentological models of the Triassic Sherwood Sandstone in the East Irish Sea Basin. Sedimentary facies recorded include moderate to low-sinuosity braided fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and non-evaporitic playa deposits. The establishment of facies associations and correlation of these with wireline logs has enabled extrapolation of sedimentary models into uncored intervals and between wells. The identification of differing seismic character associated with the various facies has made possible the mapping of these diagnostic seismic packages using a seismo-stratigraphic approach. The resulting distribution of seismically derived facies accords with proposed structural and sedimentological models, predicts the position and course of fluvial channels operating in the basin during late Sherwood Sandstone time, and shows that these are largely controlled by fault activity. Detailed facies analysis of sandflat sequences reveals a systematic variation in the proportion of sheetflood and playa to aeolian deposits. It is suggested that this corresponds to a climatic control with sheetflood- and playa-dominated sequences representing relatively wet episodes with a high water table, and aeolian-dominated sequences representing relatively dry episodes with a lowered water table. Correlation of these climatic events may introduce a chronostratigraphic element to the analysis of these continental ‘red-bed’ sequences.
The Rough Gas Field: distribution of Permian aeolian and non-aeolian reservoir facies and their impact on field development
Authors: Ellis D.
Published: April 2008

The Rough Gas Field lies in United Kingdom Continental Shelf Blocks 47/3d and 48/8b in the Southern North Sea and is operated in a gas storage mode by BGE&P involving the annual injection and production of large volumes of gas. To achieve full efficiency in this designated role it was necessary to carry out a detailed evaluation of the main producing reservoir, the Permian Rotliegendes Sandstone, after early development drilling demonstrated a need to re-evaluate the initial geological model. The present geological model is based on the detailed analysis of nine fully cored new development wells and well productivity surveys following an original six well conventional depletion. This resulted in the successful targeting of later development wells into specific geographic areas of the field for optimal field development.

Reservoir facies development can be described in terms of complex fluvial processes associated with the basin margin and aeolian influences associated with the basin as a whole. Within the Rough Field the Rotliegendes can be subdivided vertically into three reservoir units or layers (Units A, B, and C) and laterally into regions characterized by differing degrees of aeolian or non-aeolian influence. It is observed that the highest reservoir permeabilities are associated with aeolian or reworked aeolian sands while flow profile studies indicate close correspondence between injectivity/productivity and sedimentary facies. Aeolian facies or reworked aeolian sands of the lowermost ‘A’ unit facilitate the bulk of gas injection and production while fluviatile sands in all three units show reduced or even zero flow.
Diagenesis of Triassic Sherwood Sandstone Group rocks, Preston, Lancashire, UK: a possible evaporitic cement precursor to secondary porosity?
Authors: Strong G. E.
Published: April 2008

Boreholes at Kirkham [SD 4324 3247] and Weeton Camp [SD 3888 3603] in Lancashire penetrate Triassic sandstones of the Sherwood Sandstone Group that occur near the eastern margins of the Manx-Furness (East Irish Sea) Basin. Petrological analysis of sandstone samples from cores was undertaken to determine diagenetic characteristics.

Some very porous sandstones with porosities of the order of 25–30% have cement dissolution textures indicating that earlier extensive cements have largely been removed by aggressive pore fluids, probably during the Triassic. These early cements were calcite and non-ferroan dolomite, and possibly later evaporitic cements (anhydrite, gypsum, halite) introduced during the period of deposition of the Mercia Mudstones. Non-ferroan dolomite post-dates and locally replaces early calcite. There is minor ferroan dolomite, late calcite and pyrite.

Extensive early quartz overgrowths and cements have developed in some quartz arenites. Porosity values are about 10 to 13%, but permeabilities are likely to be low. These quartzcemented quartz arenite units may correlate between the boreholes.

The clay mineral assemblages of the sandstones of each borehole are significantly different. The Kirkham sequence is characterized by illite with Fe-rich chlorite and minor smectite, whereas the Weeton Camp sequence is dominated by kaolinite with minor illite and smectite. These differences are attributed to different pore fluid histories, possibly structurally controlled.
A new lithostratigraphy and depositional model for the Upper Rotliegend of the UK Sector of the Southern North Sea
Authors: George Gareth T. , Berry Jeremy K.
Published: April 2008

The lack of a pragmatic basin-wide lithostratigraphic subdivision for the Upper Rotliegend of the Southern North Sea Basin has presented numerous problems when attempting to correlate wells or implement reservoir zonation schemes within this important gas province. This deficiency has also made it extremely difficult to predict facies distribution, and thus reservoir quality, when generating or assessing new prospects. With this in mind, a detailed study of the cores and logs from 126 released wells from the UK sector of the gas province was undertaken. The results of this study suggest that it is possible to subdivide the Upper Rotliegend into five lithostratigraphic units (Units 1–5 from oldest to youngest) based on the recognition of drying-upward cycles. These drying-upward cycles are developed throughout the basin in both the Leman Sandstone and Silverpit Claystone Formations. They are considered to reflect long term fluctuations in the Rotliegend climate and thus they have important chronostratigraphic implications. In any given well the numbers of drying-upward cycles (or alternatively lithostratigraphic units) actually developed, the nature of their bounding surfaces and their component lithofacies are controlled by the position of that well within the depositional basin. Utilizing the new subdivision as a stratigraphic framework it has been possible to construct detailed lithofacies maps for the five units. These maps have revealed the complex tectonic and sedimentary evolution of the Rotliegend basin and they have also suggested a new depositional model for the fluvial facies association. The tectonic evolution of the basin has been traced by analysing the spatial and temporal distribution of the various units and their component facies. In areas such as the Sole Pit, where the onset of sedimentation was contemporary with early basin rifting, all five units (Units 1–5) can be recognized. However, in those areas where sedimentation was delayed due to progressive tectonic evolution (e.g. back-stepping of the Dowsing Fault Zone), one or more of the earlier units may be absent. The later post-rift sequences (Units 4 & 5) display the greatest areal distribution within the basin.
Ground penetrating radar: application to sandbody geometry and heterogeneity studies
Authors: Gawthorpe R. L. , Collier R. E. Li , Alexander J. , Bridge J. S. , Leeder M. R.
Published: April 2008

Ground penetrating radar (GPR) offers a high-resolution, shallow subsurface profiling technique for use in sedimentological and reservoir analogue studies. GPR is similar to seismic reflection profiling but uses electromagnetic radiation in the 50 to 500 MHz frequency range (in geological applications). By using these relatively high frequencies, high resolution data can be obtained. Short duration pulses of electromagnetic energy are transmitted into the ground, reflected from interfaces across which there are abrupt changes in dielectric properties, and are detected by a receiver. These received signals are displayed in nanoseconds two-way time and may be recorded digitally allowing subsequent processing.

Some 1000 m of 2D GPR profiles were collected from a modern point bar on the Madison River, Montana USA and have been interpreted using an approach similar to seismic stratigraphic analysis. This has allowed identification of a number of radar sequences and radar facies. Radar sequence boundaries are identified by reflector terminations (onlap, downlap, toplap and erosional truncation) and represent episodes of erosion during the development of the point bar. In contrast, radar sequences and their component radar facies record phases of accretion of the point bar. Each radar sequence is linked to a discrete accretionary unit that can be mapped on the surface of the point bar. Mapping of the radar sequences and radar facies has allowed quantification of their 3D geometry.
Quantification of variability in laminated sediments: a role for the probe permeameter in improved reservoir characterization
Authors: Corbett P. W. M. , Jensen J. L.
Published: April 2008

Fluvial and aeolian reservoirs are commonly laminated. Cross-bedding, for example, in these environments comprises foresets of contrasting grain size and sorting resulting from periodic fluctuations in depositional current strengths. These contrasts are commonly associated with variations in permeability. The recent development of a small-scale permeability measurement device (probe permeameter) allows quantification of these contrasts. Laminated sediments can now be described numerically in terms of permeability variability. Such measurements provide an appropriate quantity for comparison of lamination styles and are useful in reservoir characterisation.

We illustrate the measurement of permeability variability with probe permeameter data from the Etive and Rannoch Formations (Middle Jurassic, lower Brent Group, North Sea). These intervals, albeit from a shallow marine setting, reveal a relationship between permeability and depositional facies that was not clear from traditional permeability measurements (core plugs). The permeability variability is related to grain size variations (Etive) and mica content (Rannoch) which are controlled by depositional processes. The coefficient of variation (a statistical measure) has been used to quantify the variability. Using this measure, we compare permeability variation in the laminated shoreface sandstones with recently published examples from aeolian and fluvial sandstones.
Jurassic and Cretaceous orogenic events in the North Karakoram: age constraints from sedimentary rocks
Authors: Gaetani Maurizio , Jadoul Flavio , Erba Elisabetta , Garzanti Eduardo
Published: April 2008

At least two major episodes of tectonic deformation are recorded in the Mesozoic sedimentary succession of the North Karakoram. The older event took place between the Liassic and the early part of the Mid-Jurassic, as documented by tens to hundreds of metres thickness of dark grey litharenites (Ashtigar Fm.). The latter are sharply overlain by 35 m to 150 m thick red sandstones, interbedded with shales and siltstones (Yashkuk Fm.). Terrigenous bodies increase in thickness from west to east, and from north to south. Petrographic composition indicates the orogenic character of these quartzo-lithic sandstones, characterized by mafic volcanic and serpentinite detritus, being replaced upwards by sedimentary and subordinate metasedimentary clasts. The Ashtigar Fm. may have been deposited in a collisional basin close to a newly-formed suture belt. Contribution from allochthonous oceanic rocks was significant in the first stages of collision, whereas detritus was subsequently derived mainly from uplifted sedimentary successions. Terrigenous units were conformably transgressed by shallow-water limestones in the Aalenian, and later on marine conditions persisted until the earliest Cretaceous.

The second event of tectonic deformation occurred between the earliest and the Late Cretaceous, when the fan delta Tupop Conglomerate, mostly derived from uplifted carbonate and clastic rocks, sealed the strongly folded and faulted underlying succession. Mid-Cretaceous granitoid plutons are also widespread in the North Karakoram. This orogenic event is interpreted as related to the final welding of the Kohistan arc and Karakoram microplate to Asia.

The highly irregular topography of the Tupop Conglomerate is onlapped unconformably by reddish to grey nodular limestones, some hundred metres thick and containing phosphatized inoceramids and nannofossils of Campanian age (Darband Fm.). These are the youngest marine sediments detected in the Karakoram to date.
Collision-related granitoid magmatism and crustal structure of the Hunza Karakoram, North Pakistan
Authors: Crawford M. B. , Searle M. P.
Published: April 2008

The Cretaceous to Miocene Karakoram Axial Batholith runs E-W across the middle Hunza Valley in north Pakistan and separates a dominantly sedimentary terrain to the north from a regional metamorphic terrain to the south. A strong phase of deformation and metamorphism associated with the
c.
65–50 Ma collision of India with Asia affected the area. The southern part of the batholith and the metamorphic rocks to the south are affected by strong south-vergent thrusting which has brought deeper level, hotter rocks over higher level, cooler rocks so that metamorphic grade increases with structural height. In contrast, the northern part of the batholith is relatively undeformed and shows an intrusive contact with an andalusite-bearing thermal aureole.

Two groups of post-collisional magmatic rocks can be recognised in the area. The first group, the Hunza dykes, occurs as a massive dyke complex intruding the early phase of the batholith as well as the sillimanite-bearing gneisses to the south. These dykes consist of a co-genetic suite of granodiorites, monzogranites and leucogranites which appear to be related by feldspar-biotite fractionation. They are volatile-depleted, non-minimum melts with unevolved crustal isotopic ratios and enriched Sr and Ba signatures similar to the Miocene Baltoro pluton further east. They are considered to represent lower-crustal magmas derived by dehydration melting of metasediment, probably promoted by intrusion of mantle-derived magmas.

The second group is represented by the Sumayar pluton which intrudes the staurolite-grade metamorphic rocks and is a homogeneous, water-saturated minimum-melt leucogranite, with a major element composition similar to that of the Manaslu leucogranite of central Nepal. Abundant tourmaline and evolved Sr isotope ratios indicate a metapelitic source. Analogy with Manaslu suggests that this source was either fluxed by fluids to promote melting or produced melt under vapour-absent conditions via muscovite breakdown. In the case of the latter, low melt fractions require that an extensive area was tapped to generate a pluton the size of Sumayar. Consideration of the petrogeneses of the two groups of magmas allow constraints to be placed on the deeper crustal structure of the Hunza Valley.
Leucocratic rocks from the Bela ophiolite, Khuzdar District, Pakistan
Authors: Ahmed Zulfiqar
Published: April 2008

This is the first detailed report of the well-developed leucocratic rocks, dominantly plagiogranites, from the Bela ophiolite, the most complete and largest ophiolitic outcrop of Pakistan. A wide range of leucocratic rock types occur within the ophiolite including leucogabbro, gabbroic pegmatite, diorite, quartz diorite, trondhjemite, epidote trondhjemite, granite, keratophyre and acidic pegmatite. Two suites of acidic leucocratic rocks are recognized: a very low potash and high soda suite containing magma-fractionated plagiogranite characteristic of most ophiolites, is accompanied by an anatectic suite with higher potash content. Each suite possesses distinct trace element and rare earth contents, together with distinctly different Sr and Nd systematics. These differences indicate differences in tectonic setting for each leucocratic suite. U-Pb isotopic measurements on zircons separated from the acidic rocks yield a crystallization age of 65 ± 1 Ma for both the rock suites. On the basis of geochemical data a suprasubduction zone origin dominated by a marginal basin is proposed for the Bela ophiolite. The plagiogranitic rock suite could have been derived from basic magma by fractional crystallization. The potash-richer granitic rocks may have been generated by crustal anatexis forming a melt that locally invaded the ophiolite.
Chemistry of chromite and associated phases from the Shangla ultramafic body in the Indus suture zone of Pakistan
Authors: Arif Mohammad , Jan M. Qasim
Published: April 2008

The partly to wholly serpentinized ultramafic rocks of the Shangla area constitute the northern part of the Mingora ophiolitic melange which lies along the Main Mantle Thrust (MMT) zone between the Kohistan island arc and the Indo-Pakistan plate. These rocks contain disseminated grains and massive bodies of segregated chromite. Field, textural, and chemical characteristics of the segregated chromite closely resemble those of podiform chromite deposits. The chemical composition of the chromite varies considerably from sample to sample with modal proportion, from grain to grain in the domain of a thin section, and from core to margin within individual grains. In general, the segregated chromites are richer in Cr and Mg, but poorer in Fe, Al and Mn than the accessory chromites. The inter- and intragranular chemical variations are much greater and more common in the case of accessory chromite due probably to a greater degree of subsolidus re-equilibration with the associated abundant silicate matrix. The intragranular variation, probably a result of alteration, mostly involves enrichment in Fe, Mn (and Ti), and depletion in Al, Mg and Cr, leading to the development of ferritchromit and, rarely, magnetite along fractures and margins of individual grains.

The studied rocks from the Shangla area display cumulus textures but contain abnormally magnesian olivine (Fo96.0–98.5). The 100Cr/(Cr + Al) ratios of their chromite suggest a complex origin. These rocks may have formed under conditions transitional between those of arc and oceanic settings. The Shangla rocks underwent a low- to ?medium-grade metamorphism as indicated by the development of talc and antigorite. This prograde metamorphic process probably also accounted for the extreme chemical zoning of the Shangla chromite.
A section through the Indian Plate, Kaghan Valley, NW Himalaya, Pakistan
Authors: Greco Antonio , Spencer David A.
Published: April 2008

A map is presented which summarizes the work of the ETH Himalayan Group in the Kaghan Valley and adjacent areas of the NW Himalaya in Pakistan. The stratigraphical, structural and metamorphic features of the lithologies belonging to a virtually complete section through the Indian plate are described. Evidence is presented which shows a stratigraphic range in ages from the Precambrian Salkhala Formation to the Eocene Murree Formation which have been metamorphosed at various metamorphic grades ranging from very low grades/unmetamorphosed to eclogite facies. An inverse metamorphism, associated with the Main Central Thrust is also described. A large-scale tectonic subdivision of the area into the classical Sub-, Lesser and Higher Himalayan elements can be correlated with the Kashmir Himalaya to the east. A tectonic model is presented which suggests that an anticlockwise rotation of the transport direction caused the refolding and stacking of the nappe piles in the Paleogene.
The tectonic evolution of the Northwestern Himalaya in eastern Ladakh and Lahul, India
Authors: Steck A. , Spring L. , Vannay J.-C. , Masson H. , Bucher H. , Stutz E. , Marchant R. , Tieche J.-C.
Published: April 2008

Geological studies along a transect across the Himalaya in eastern Ladakh and Lahul provide new insights into the Tertiary structural evolution of this region. The initiation of the Nyimaling-Tsarap Nappe is related to an early phase of underthrusting of India below Asia. In Lahul, an opposite vergent intra-continental underthrusting develops immediately after continental collision (NE-vergent Tandi Syncline and Shikar Beh Nappe). This NE-vergent nappe stack is responsible for the amphibolite-facies regional metamorphism of the lower Chandra Valley. The subsequent phase corresponds to the main thrusting of the SW-vergent Nyimaling-Tsarap Nappe, developed by ductile shear (87 km Eocene shortening). This nappe pile is responsible for the regional metamorphism of SE Zanskar (kyanite-staurolite near Sarchu). The root zone and the frontal part of the Nyimaling-Tsarap Nappe are subsequently overprinted by two NW-SE-orientated dextral transpressional shear zones. To the south of the investigated area, the Main Central Thrust has been developed as a shear zone in the regional metamorphic ductile crustal rocks below the older nappes to the north. In the Sarchu and Nyimaling regions, the following tectonic phase corresponds to NE-vergent ‘backfolding’ (Miocene). Normal faults in the Sarchu area record a late extension of approximately 14 km.
Field relations, petrogenesis and emplacement of the Bhagirathi leucogranite, Garhwal Himalaya
Authors: Searle M. P. , Metcalfe R. P. , Rex A. J. , Norry M. J.
Published: April 2008

The Bhagirathi leucogranite forms a series of low-angle en echelon, lensoidal intrusions at the top of the High Himalayan slab in the central Himalaya of Garhwal, northern India. The leucogranite comprises the assemblage: K-feldspar + quartz + plagioclase + tourmaline + muscovite ± biotite ± garnet. Compared to other High Himalayan leucogranites it is particularly rich in tourmaline. The granite is generally compositionally homogeneous although it is magmatically banded in both the upper and lower portions.

The Bhagirathi leucogranite is situated structurally above the kyanite and sillimanite gneisses of the Vaikrita Group which, in turn, overlie the north-dipping Main Central Thrust zone of inverted metamorphic isograds. A pegmatite — aplite leucogranite sill and dyke swarm is present around the margins of the leucogranite. Vaikrita Group gneisses below the leucogranite contain a pronounced northeasterly component to generally randomly orientated mineral stretching lineations. This reflects localized reorientation of early, coaxially constrained, mineral growth by later, non-coaxial deformation. Various shear criteria in the gneisses immediately below the granite document the existence of a major zone of ductile NNE-SSW-directed extension across a northeastward dipping, low-angle normal fault zone. The top of the 1500–1800 m thick leucogranite sheet exposed on the peaks of Bhagirathi, Shivling and Thalay Sagar are intrusive into Martoli Formation metasediments of the Tethyan sequence which locally contain andalusite and staurolite. The roof complex shows numerous stoped blocks, xenoliths and elongate rafts of host rock in the top 100 m together with an extensive zone of layer-parallel leucogranite veining.

The northward-dipping Bhagirathi leucogranite was intruded syn-tectonically during the ductile to brittle transition and has been deformed into linked, en echelon bodies within the extensional shear zone at the interface between the Vaikrita Group gneisses and the Tethyan sedimentary cover. The long axes of the leucogranite lenses lie parallel to the
y-z
plane of the finite strain ellipsoid for this extensional duplex. During extension, sub-orthogonal dilatational forces exceeded sub-horizontal shear stresses thus facilitating the repeated emplacement of sheeted granite melt, a process analogous to low-angle tension gash development. The final emplacement level in the crust must have been ultimately controlled by the density contrast between melt and country rocks, the thermal blanketing of the Tethyan sedimentary cover, and the extensional stress field along the top of the High Himalayan slab.