bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic mineralogy of sandstones from the Lunde Formation (late Triassic), northern North Sea, UK: origin of the palaeomagnetic signal
Authors: Hounslow Mark W. , Maher Barbara A. , Thistlewood Laurence
Published: April 2008

The minerals responsible for the magnetic properties of the Late Triassic Lunde Formation have been examined using a combination of magnetic mineral extraction, microscopy and rock magnetic measurements. The magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis, by a process of differential preservation and modification of the original detrital minerals. The magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material, which contains most of the magnetic minerals. Paramagnetic minerals, both of depositional origin and produced during diagenesis, are largely responsible for the magnetic susceptibility. A suite of residual ferrimagnetic oxides (chromite and Mn-substituted magnetite) is responsible for the remanence properties in grey sandstones, because most of the originally deposited magnetic Fe-Ti oxides have been removed by reducing conditions during diagenesis. However, a small number of reddened horizons preserve a higher proportion of original detrital Fe-Ti oxides (ferrimagnetic ilmenite, ferrimagnetic chromite and magnetite), although authigenic hematite has replaced most of the original mineralogy in the reddened horizons. Magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal, particularly in horizons which have a low content of discrete detrital ferrimagnetic grains, either through diagenetic dissolution, or paucity of the silt and clay fraction. Within the grey sandstones, the Triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates, with a contribution from detrital ferrimagnetic chromite. A model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed.
Results from a palaeomagnetic investigation of the Brent Group sediments in wells 34/10–16 and 34/10–17 showing evidence for complete remagnetization of the sediment
Authors: Beyer Claus
Published: April 2008

Cores from wells 34/10–16 and 34/10–17 in the Tampen Spur area, Norway were studied to establish a magnetic polarity stratigraphy for the Brent Group in the area. Approximately 100 samples from each well were treated by traditional thermal and alternating field demagnetization procedures and the apparent polarities determined. Thin sections were studied and scanning electron microscopy carried out with the purpose of verifying a primary origin of the remanent magnetization.

Correlation of the two wells based on the polarity logs was in agreement with the lithoand biostratigraphic correlations. However, the mineralogical analysis did not determine any primary magnetic minerals, although potential remanence carrying diagenetic minerals were observed. This, combined with magnetic data indicating extensive remagnetization of the sediment, lead to the conclusion that the remanent magnetization of these sediments is late diagenetic, probably of Tertiary age.

The results are not encouraging with respect to the use of palaeomagnetism as a chronostratigraphic tool for these sediments. It may, however, be useful for dating diagenetic events and, as a result of the strong secondary magnetization, magnetic orientation techniques can be successfully applied to the core material.
Application of magnetostratigraphy to Brent Group reservoir zonation in the Visund Field
Authors: Hauger Eirik , Veen Paul Van
Published: April 2008

Magnetostratigraphy has been applied to an increasing number of oil fields in the North Sea region with promising results for detailed chronostratigraphic reservoir correlation. However, the magnetization processes in hydrocarbon reservoir successions, and the potential use of magnetostratigraphy for reservoir zonation purposes, are not very well known. Therefore, aspects of these subjects are discussed on the basis of a magnetostratigraphic study of three different wells covering the Middle Jurassic Brent Group at the Visund Field, northern North Sea. Results from scanning electron microscopy and remanent coercivity analysis suggest that the primary magnetization defining the magnetozones in the Brent Group is a combination of detrital remanent magnetization and a chemical remanent magnetization, probably acquired contemporaneously shortly after deposition. Detailed thermal demagnetization has uncovered an important secondary magnetization component in most of the core samples. This magnetization is probably due to cooling of the cores from a reservoir temperature of about 110°C to surface temperature, and it causes the main disturbances in the palaeomagnetic results. Based on analyses of individual core samples, characteristic remanent magnetization components have been defined. The magnetopolarity stratigraphy for the individual wells is based on these components and the results are comparable from well to well. The well correlation covers a rock sequence with a thickness of about 200m, and both magnetostratigraphic and biostratigraphic results have been used in combination with the lithostratigraphy. The acme of
Corollina
represents the most important palyno-event, and the top and bottom of this palynozone is used as the main tie-level for two of the wells. Sixteen additional correlation lines have been derived from the magnetostratigraphic results, six of these considered as tentative. The correlation reveals that the magnetostratigraphic results are facies independent and some of the lithostratigraphic formation lines appear to be diachronous within this deltaic mega-sequence. Although the magnetostratigraphy only provides a relative time framework for the Brent Group, the results are encouraging and suggest the possibility of achieving high resolution chronostratigraphic well correlations for Middle Jurassic reservoir sequences in the northern North Sea area.
Palaeomagnetic reorientation of cores and the magnetic fabric of hydrocarbon reservoir sands
Authors: Hailwood E. A. , Ding F.
Published: April 2008

The palaeomagnetic method is established as an important complement to more traditional methods for reorienting cores drilled in hydrocarbon exploration and production programmes. The method is based on the isolation of one or more components of remanent magnetization within the core. These provide records of the direction of magnetic north, either for recent times (carried by magnetic overprints in the recent geomagnetic field at the drill site) or at the time of formation of the sediment (carried by the primary magnetization). Provided that any spurious components of magnetization associated with drilling are removed by appropriate analysis in the palaeomagnetism laboratory, then the magnetic components of interest can be separately isolated. The mean direction of stable remanent magnetization, averaged over several tens of centimetres of core, defines the direction of geographical north in that core section. For a recent component, the direction of north is specified directly, whereas for an ancient component this is given by reference to the direction predicted from published apparent polar wander curves for that plate or geographical region.

An important attribute of the palaeomagnetic core reorientation method is that the measurements are made on samples from the cores themselves, rather than by downhole instruments such as multishot orientation tools, dipmeters or imaging tools. This makes the technique particularly cost-effective and frees it from reliance on the proper performance of downhole tools or the correlation of planar features observed in the cores with those in the well. Consequently, there is no requirement for the well to be deviated or for there to be a significant natural dip to the formation (although this does help in fitting adjacent core pieces together to allow extrapolation of the reorientation data to longer intervals of core). Furthermore, as the determinations are made directly on the cores, the precision of palaeomagnetic reorientation is inherently greater than for methods based on measurements made in the well. It is possible to apply the method to cores for which the need for orientation was not apparent at the time of drilling, but for which this need arises as a result of later developments. The technique can be applied equally well to recently drilled cores or cores which have been stored for many years.

An account is given of the palaeomagnetic reorientation method and its applications to (1) defining fracture orientations and (2) establishing sediment transport directions in a channel sand body. The transport directions are inferred from magnetic fabric measurements and the principles of the anisotropy of magnetic susceptibility technique are briefly reviewed.
Magnetic susceptibility of a 180 m sediment core: reliability of incremental sampling and evidence for a relationship between susceptibility and gamma activity
Authors: Løvlie Reidar , Van Veen Paul
Published: April 2008

The magnetic susceptibility of a 180 m long core from the North Sea, covering the Middle Brent Group to Lower Cretaceous, has been measured at 2 cm intervals with a whole core sensor loop (MS2C) and on standard paleomagnetic samples collected at approximately 1 m intervals. The stratigraphic variations between the data sets reveal comparable features, suggesting that the sampling interval used yields a reliable record along this core. A relationship is demonstrated between gamma activity and susceptibility, the latter exhibiting more pronounced variations than the gamma logs, suggesting that susceptibility may be a useful lithomagnetic parameter for quantitative lithostratigraphic correlation. Non-linearity between gamma activity and magnetic susceptibility suggest that the latter may be treated as an independent lithological parameter which may be used for lithostratigraphic correlation. Comparison between corresponding plug and whole core susceptibility values indicates some degree of lateral inhomogeneity in the mineralogy of the core.
Magnetic anisotropy of borehole core samples
Authors: Tarling D. H. , Shi H.
Published: April 2008

All rocks have petrofabrics that can be determined rapidly and precisely by magnetic anisotropy as this depends on the net alignment of the crystallographic properties and the shape orientations of all grains within the rock, i.e. the grains themselves act as fabric markers. When ferromagnetic minerals are present in quantities >0.0001%, their properties dominate the observed fabric. As most rocks contain at least this level of ferromagnetic content, most techniques reflect the petrofabric of the ferromagnetic minerals, although rocks where these are absent still have petrofabrics that can be measured quickly and precisely by magnetic methods. Such magnetic fabrics can be used to define the direction of flow and velocities of the currents when the sediment was being deposited. The original fabric may be modified during shallow burial, enabling estimation of the degree of bioturbation. Subsequent fluid migration (gases, water, hydrocarbons, etc.) through the sediments superimposes magnetic fabrics than can be used to define the three-dimensional nature of such secondary channels. The very high sensitivity of the technique also enables it to detect the onset of anchimetamorphic changes at lower levels than detectable by standard techniques.
Recovery as a function of community structure
Authors: Krassilov Valentin A.
Published: April 2008

According to the climax cut-off model, ecological succession can be truncated at an early stage by environmental factors thus causing elimination of climax species and their replacement by new dominants derived from successional species. Such profound restructurings are caused by prolonged stresses rather than by episodic impacts. Recovery of a dominant group is due to its representatives in the early stages of ‘mixed’ successions. In the ‘graded’ successions with the serial and climax species belonging to different evolutionary grades (e.g. dinosaurs and mammals in the terminal Cretaceous communities) such recoveries are less probable.
Ostracode speciation following Middle Ordovician extinction events, north central United States
Authors: Swain F. M.
Published: April 2008

Studies by several workers show that major extinctions of trilobites, echinoderms, brachiopods and gastropods (but not conodonts) occurred at the horizon of a middle Ordovician K-bentonite (Deicke metabentonite) in the north central United States. Extinctions of Ostracoda are also represented at the Deicke horizon. The metabentonite, a Rocklandian stage ash fall the age of which is approximately 454 Ma, middle Caradocian, originated in the eastern United States.

About 160 species of Ostracoda have been recorded in the middle and upper Ordovician of the north central USA. Ostracode speciation was low prior to the Deicke extinction event, consisting of a few eoleperditiids, leperditellids, drepanellids, aparchitids and bairdiids of the Glenwood and Platteville formations (Blackriveran-early Rocklandian). Following the extinction event, during the next approximately 1.5 Ma, 85 ostracode species appeared in the uppermost Platteville Limestone and in the overlying Decorah Shale (late Rocklandian-early Shermanian). The known Ostracoda species durations averaged about 2 Ma, although some are only half that length of time.

Additional episodes of ash falls, following the Deicke and associated episodes, occurred in the next approximately 7.5 Ma in the region. The preceding Decorah ostracode fauna disappeared from the area and about 50 recorded new or immigrant taxa appeared following the last volcanic event in the late Edenian and Maysvillian stages (Dubuque and Maquoketa formations). Species duration during this time interval apparently averaged less than in the Decorah, about 1 Ma.

The volcanic episodes appear to have provided nutrient-rich habitats that resulted in evolutionary spurts in the Ostracoda, following at least two of the episodes. Duration of the species was perhaps related to the total nutrient budget provided by the ash falls as well as to salinity regimes, palaeoclimates and other factors.
Diachronous recovery patterns in Early Silurian corals, graptolites and acritarchs
Authors: Kaljo Dimitri
Published: April 2008

The extinctions that occurred in the latest Ordovician and earliest and latest Wenlock, were the most impressive in the biotic history of this period. They were probably caused by glaciations. Recovery processes, more or less, follow the classical scenario: extinction-low-diversity survival interval-recovery through radiation events. Rather often there occurs diachrony of the phases of this scenario in the different groups discussed. Graptolite recovery was the most rapid with a diversity maximum in the mid-Llandovery. Their main extinction and low-diversity interval were in the latest Ordovician. The evolution of corals and acritarchs was slower — after a few lower-scale origination events a diversity burst was reached in the late Llandovery. An analogous but lower-scale pattern was noted at the very beginning and in the late Wenlock. The difference was caused by evolutionary and ecological reasons. A good correlation between diversity changes and terrestrial environmental events (glaciations, sea-level movements, stable isotope records) is noted.
The mid-Carboniferous rugose coral recovery
Authors: Kossovaya O. L.
Published: April 2008

The taxonomic diversity of the rugose corals following the mid-Carboniferous ‘lesser mass extinction event’ (Alekseev 1989) may be subdivided into two intervals. The survival interval includes long-ranging survivors, short-ranging survivors (
Dibunophyllum, Palaeosmilia, Diphyphyllum
), survivors with post-crisis ontogenetic changes (
Caninia, Bothrophyllum
), newly-appeared taxa with hypothetical ancestors (
Profischerina, Protodurhamina
) and those of cryptogenic origin (
Lytvophyllum
). The lower limit of the survival interval coincides with the
Eumorphoceras/Homoceras
Zone and can be identified in the Donetz Basin, Gornaya Bashkiria, Novaya Zemlja, North Timan, Cantabrian Mountains (Rodriguez
et al.
1986) and the Western Interior of the USA (Sando 1989).

The recovery interval is established by the appearance of the immigrant genus
Petalaxis
and by increasing specific diversity in pre-existing lineages. Its lower limit corresponds to the
Pseudostaffella praegorsky—Profusulinella staffelliformis
boundary, the lower boundary of the Westphalian A in the Cantabrian Mountains and the lower boundary of the Atokan Series in the Western Interior of the USA.

The radiation interval was marked by the appearance of astreoid colonies in the Petalaxis—Ivanovia lineage at the
Fusulinella calaniae—F. vozhgalensis—F. kamensis
lower boundary.
Recoveries and Radiations: Gastropods After the Permo-Triassic Mass Extinction
Authors: Erwin Douglas H. , Hua-Zhang P.
Published: April 2008

The biotic recovery following the end-Permian mass extinction begins with a long lag phase characterized by a low-provinciality, depauperate biota. Normal marine communities do not reappear until the latest Early Triassic (Spathian) and do not become common until the early Middle Triassic (Anisian). Three explanations have been offered for this pattern: first, that the end-Permian extinction so disrupted marine ecosystems that community assembly rules needed to be rewritten prior to the re-establishment of normal community structures; second, that the recovery was delayed by continuing harsh environmental conditions; and third, that the preservational and sampling bias is significant and the pattern more apparent than real.

Data on the stratigraphic, biogeographical and environmental distribution of Wordian-Anisian marine gastropod genera and species were used to evaluate these hypotheses. The results demonstrate (1) an initial removal of endemic (narrowly distributed) genera during the Wordian and Capitanian; (2) the presence of a low provinciality gastropod assemblage during the Djulfian and Changxingian; and (3) the persistence into the earliest Triassic of geographically widespread, environmentally tolerant genera, as well as the apparent (but unobserved), occurrence of numerous ‘Lazarus’ taxa. The data indicate that while preservational bias was significant, environmental dampening was also important.
Planktonic foraminifera recovery from the Cenomanian-Turonian mass extinction event, northeastern Caucasus
Authors: Tur Nataliya A.
Published: April 2008

The changing structure of the planktonic foraminiferal assemblages is examined from the Upper Cenomanian-Lower Coniacian succession of ‘Carbonate Dagestan’.

The analysis of the morphological and taxonomical diversity shows that the deep-water forms are most severely affected by the mass extinction event. The repopulation of their post-crisis assemblages is rapid. The composition of the shallow-water population remains more or less stable.

The foraminiferal dynamics suggests the main developmental phases. The Extinction Phase occurs during the
R. cushmani
Zone, when the bathypelagic rotaliporids became extinct as a result of an expansion of the oxygen-minimum conditions. During the
D. hagni
Zone the survival community is characterized by the primitive forms and forms with an incipient keeled structure. The Early Recovery Phase, occurring during the
H. helvetica, M. coronata
and early
M. tarfayaensis
Zones, is characterized by the appearance of the first marginotruncanids. The Later Recovery Phase, beginning in the later
M. tarfayaensis
Zone, involves a maximum of morphological diversity of marginotruncanids in the
M. concavata
Zone.
Validation of seismic data processing and interpretation with integration of gravity and magnetic data
Authors: Horscroft T. R. , Bain J. E.
Published: April 2008

integration of gravity and magnetic data with seismic interpretations provides a definitive means of section validation. Traditionally this has been achieved as a two stage and separate process — seismic interpretation in the time domain and the potential fields model in the depth domain.

Recent advances in software technology now enables interactive simultaneous real-time modelling of seismic gravity and magnetic data. In addition to conventional interpretations, modern uses of modelling now includes: quality control of seismic depth conversion, control of the potential field model using offset raytracing; derivation of a high quality depth model as a starting point for processes like pre-stack depth migration or turning wave analyses. Integration too can be used to differentiate between geometrically similar but lithologically different seismic structures.

This paper describes the advantages of dynamically linking velocity, density and susceptibility in the modelling process illustrated by three different North Sea geological examples.

(a)

Depth-velocity modelling of salt structures in the Central Graben.

(b)

Structural interpretations of the Rattray volcanic rocks in the Outer Moray Firth.

(c)

Shallow sand channels and velocity models from high resolution aeromagnetic data.
Discussion of potential errors in fault heave methods for extension estimates in rifts, with particular reference to fractal fault populations and inherited fabrics
Authors: Morley C. K.
Published: April 2008

Fault heave methods for estimating extension (including balanced cross-sections and the flexural cantilever model) are important because they are the only methods that are universally applicable to estimating extension from the syn-rift stratigraphic section. One of the main criticisms of the methods when based on reflection seismic data is the failure to account for crustal extension along sesimically invisible faults (displacements up to a few tens of metres). The fractal distribution of several fault populations has been cited as evidence for the existance of such minor faults. It has been suggested by several workers that the discrepancies between extension estimates for the North Sea rifts calculated from the syn-rift section and by backstripping (using the McKenzie model) can be explained by a substantial amount of crustal extension having taken place on seismically invisible faults. This paper examines whether it is reasonable to extrapolate such conclusions to other rifts.

Estimates for the contribution of minor faults to crustal extension based on fractal distributions of faults need to be treated with caution for several reasons: (1) in some rifts large boundary faults (several kilometres displacement) may account for 80–95% of the total extension amount. In such rifts minor faults may not contribute significantly to crustal extension (less than 5%). (2) The different distribution of fault size and spacing between different zones within rifts suggests that a single fractal distribution along a regional rift traverse is commonly unrealistic. (3) Fractal fault populations measured in syn-rift sedimentary sequences may be measuring faults involved in gravity tectonics and compaction, not just those faults that contribute to crustal extension.

Regions with very complex pre-rift structural histories (e.g. North Sea) appear to have significant amounts of extension on minor faults. This contrasts with rifts developed on crusts with a relatively stable history (e.g. East African rift), where much of the extension is accommodated on a few large faults. The differences between these two rifts (despite both rifts having undergone similar amounts of extension) illustrate that pre-existing fabrics are extremely significant for understanding fault populations, fault orientations, partitioning of extension between different sizes of fault, the types of linkage between faults (transfer zones), syn-rift basin subsidence and fault lengths. An appreciation of this variability should lead to more accurate prediction of the contribution of seismically invisible faults to extension on balanced cross-sections.
Finite element modelling of the competition between shear bands in the early stages of thrusting: Strain localization analysis and constitutive law influence
Authors: Barnichon J. D. , Charlier R.
Published: April 2008

Finite element simulation of a sandbox model of thrusting is performed using large strain analysis and two different non-associated elastoplastic constitutive laws (namely Drücker-Prager and Van Eekelen criteria). The analysis of strain localization using the Rice bifurcation criterion coupled with a kinematic indicator shows that, in the early stages of imbricated thrusting, the development of major shear bands can be influenced by some competition with second order bands. The influence of the vertical/horizontal stress ratio is checked, as well as the influence of Lode angle in the constitutive law. Significant differences are found between a classical plasticity criterion (Drücker-Prager) and a more realistic one (Van Eekelen) regarding the resistance of the model and the stress paths. These differences may result in erroneous fault type prediction.
Mathematical modelling of growth strata associated with fault-related fold structures
Authors: Hardy Stuart , Poblet Josep , McClay Ken , Waltham Dave
Published: April 2008

Previous geometric models of fault-related folding have been limited in their ability to model sedimentation contemporaneous with contractional deformation. Waltham’s 1992 general forward modelling equation allows more realistic models of sedimentation to be combined with mathematical models of fault-bend folding, fault-propagation folding and limb-rotation associated with detachment folding. Using average rates of fault slip and sedimentation derived from the literature, the mathematical models are used to predict stratal geometries associated with each of the major styles of fault-related folding in both sub-marine and sub-aerial settings. Both background sedimentation and local erosion, transport and deposition as a result of fold growth are modelled. Comparison is made with natural examples and the utility of growth strata architectures in the assessment of fault-fold kinematics assessed.
An insight on salt tectonics in the Angola Basin, South Atlantic
Authors: Spathopoulos Fivos
Published: April 2008

Salt movements have dominated the post-Aptian tectonic process of the West Africa Salt Basin, which extends from northern Namibia to southern Cameroon. The thick salt layer was deposited during the Aptian within the shallow rift basins of the proto-South Atlantic and was overlain by an extensive carbonate platform. As a result of the basinward tilt of the margin, the carbonate platform and the overlying Upper Cretaceous sequence slid on the salt layer and broke up into smaller blocks. This process caused a mobilization of the salt and the formation of salt rollers, salt diapirs and a whole suite of raft tectonics structures such as turtlebacks, carbonate rafts and severe folding at the toe of the salt basin. Although this process is seen throughout the Angola Salt Basin, the areal distribution of the salt movement-induced structures shows a significant differentiation. Burial history curves also show important differences along the margin in the post-Aptian subsidence. Analysis of recent seismic data suggests that the thin-skinned extension needed for salt and raft tectonics is accommodated basinward by folding of the overburden up to a point where all the extension has been taken up. From there to the toe of the salt layer, the overburden is largely undisturbed. The salt and raft tectonics in the Angola Basin was the main factor that influenced the distribution of oil reserves in the region and its understanding is very important for hydrocarbon exploration there.
Controls on Eocene sedimentation in the central North Sea Basin: results of a basinwide correlation study
Authors: Joy Aidan M.
Published: April 2008

The Eocene of the central North Sea Basin consists of mudstones with locally developed deltaic and gravity-flow sandstones, some of which are important hydrocarbon reservoirs. The pronounced lateral variability in sandstone thickness, combined with the difficulties of seismic interpretation in the Eocene, obliges explorationists to develop a regional understanding of Eocene depositional systems in order to predict sandstone distribution. However the development of such a regional understanding is hampered by the need to correlate between widely spaced wells. The correlation of basinal sections with their marginal equivalents has proved to be a particularly intractable problem.

This study utilizes a simple subdivision of the Eocene into two sequences based upon the recognition of a small number of areally extensive wireline-log and seismic markers. Though the sediments belonging to these two sequences are very similar, the nature and distribution of the large-scale depositional systems developed during the two intervals are quite different. During sequence I time (early to mid Eocene) sand supply was dominated by two deltas, one in UK Quadrant 9 and one in UK Quadrant 21. The late Paleocene Moray Delta formed a positive bathymetric feature in UK Quadrant 15 at this time, suppressing delta development in this region. By sequence II time (mid to late Eocene) a rise in relative sea level had created space in Quadrant 15, allowing a single major deltaic depocentre to develop here; this delta dominated sand supply during sequence II time. Both the location and the morphology of this delta suggest that it was fluvially dominated.

This interpretation suggests that the availability of accommodation space for delta development was a fundamental controlling factor in the supply of sand to the basin. There is no evidence for a eustatic control on sand supply.
Mammalian biostratigraphy across the Paleocene-Eocene boundary in the Paris, London and Belgian basins
Authors: Hooker J. J.
Published: April 2008

Problems of resolution and poor superpositional evidence in mammalian biostratigraphy through Paleocene-Eocene boundary strata in NW Europe are solved by applying parsimony analysis to taxa shared between localities. On this basis, five biozones are established in the area for the interval formerly delineated by mammalian biostratigraphers as MP7-MP9. Integration with other biostratigraphies (dinocyst, calcareous nannoplankton, charophytes) aids correlation between the London, Belgian and Paris Basins, and supports the earlier idea of diachronism of the ‘argile à lignites’ facies. The advent of ‘Sparnacian’ mammal faunas in Europe may coincide with a carbon isotope excursion recently recognized in the Paris Basin. This would support recent views on essential synchronism of the beginnings of both the North American Wasatchian and European ‘Sparnacian’ land mammal ages.
A forward-modelling analysis of the controls on sequence stratigraphical geometries
Authors: Burgess P. M. , Allen P. A.
Published: April 2008

The sequence stratigraphical depositional model has concentrated primarily on eustasy and tectonic subsidence as the dominant controls on sequence development, with variations in other factors such as sediment supply playing only a modifying part. Little consideration has been given to the significance of the uniqueness problem to the model predictions. Thus it is often assumed that if the sequence stratigraphical model can provide a simple fit with the available data, then it must be the correct and unique solution. This is rarely likely to be the case. Using a quantitative forward model it is possible to reproduce the basic geometries of type 1 and type 2 sequences and then to assess, via a series of sensitivity tests, the significance of some other controls. Use of this forward-modelling approach suggests that fluvial profile behaviour is a first-order control on sequence geometries. It also suggests that variations in the magnitude of sediment supply can significantly alter the development of transgressive ravinement surfaces. These two examples highlight the importance of the uniqueness problem to the sequence stratigraphical model.
High resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio-Pleistocene
Authors: Armentrout John M.
Published: April 2008

High resolution sequence biostratigraphy provides a framework for the construction of palaeogeographic maps for each phase of relative sea-level change. The late Pliocene and Pleistocene of the Gulf of Mexico provides a case history of the integration of this methodology. The strata are abundantly fossiliferous and stratigraphically thick so that exploration well samples provide a record of glacial/interglacial cycles with a 200–300 ka frequency. The methodology developed is suitable for other stratigraphic intervals where microfossils are relatively abundant.
High frequency sequence stratigraphy of a siliciclastic influenced carbonate platform, lower Moscovian, Amdrup Land, North Greenland
Authors: Stemmerik Lars
Published: April 2008

The Moscovian succession in southern Amdrup Land, North Greenland consists of seven, 40–135 m thick, third-order depositional sequences of mixed carbonates, siliciclastics and evaporites. The sequences have sheet-like or wedge-shape geometries and are composed of stacked high frequency sequences with a duration of
c.
100 000 years. The Kap Jungersen 2 Sequence delineates an up to 135m thick carbonate platform composed of 10 high frequency sequences. Each individual high frequency sequence shows great lateral facies and thickness variations. In the platform area they are dominated by bioclastic grainstones,
Palaeoaplysina
-phylloid algae build-ups and bryozoan build-ups. Deposition took place mainly during sea-level highstands; the transgressive systems tracts are generally thin and shaley, and sea-level lowstand led to subaerial exposure and karstification.
Inclination error linked to sedimentary facies in Miocene detrital sequences from the Vallès-Penedès Basin (NE Spain)
Authors: Garcés M. , Parés J. M. , Cabrera L.
Published: April 2008

Palaeogeographic reconstructions based on kinematic and other integrated models are rarely in perfect agreement with palaeomagnetic data. Furthermore, the reliability of rocks that record the geomagnetic field is often questioned, particularly for detrital remanent magnetization (DRM). Among the diverse types of magnetization bias,
inclination shallowing
caused by several mechanisms such as gravitational torques upon deposition and post-depositional compaction can be a major source of error in palaeolatitudinal estimations. Inclination shallowing for a variety of fine-grained alluvial sediments from the Neogene Vallès-Penedès Basin (NE Spain) has been observed to be dependent on sedimentary facies. The correlation of inclination shallowing to sediment fabric is investigated in samples from 11 stratigraphic profiles. Each profile has a dominant lithology, from thinly laminated to edafized silts and massive muds and breccias. A measure of particle alignment in sediment can be determined by means of the
degree of anisotropy (P)
and
shape symmetry (T)
AMS parameters. Since each sedimentary facies shows a particular range of
P
and
T
, a representative mean AMS ellipsoid can be estimated from each lithologically homogeneous profile. Estimated mean remanence inclination (
I
NRM
) has a strongly consistent relationship to the mean AMS ellipsoid and a positive linear correlation is inferred between ln (
P
) and ln tan (
I
NRM
). Given the extreme case of no sediment anisotropy, i.e.
P
= 1, inclination error would be cancelled and
I
NRM
would record the true magnetic field inclination of the sampling site. The regression function predicts a corrected magnetic inclination of 60°, consistent with that of the expected geomagnetic dipole for the studied site and age.
A magnetostratigraphic study of the onset of the Mediterranean Messinian salility crisis; Caltanissetta Basin, Sicily
Authors: McClelland E. , Finegan B. , Butler R. W. H.
Published: April 2008

The Messinian ‘salinity crisis’ marked a dramatic climatic change in the Mediterranean region. The timing and extent of this event remain controversial, with conflicting models ranging from predictions of rapid and catastrophic dessication of all of the Mediterranean basin at once, to progressive draw-down of sea-level causing gradual onset of evaporite formation starting at the margins of the Mediterranean Sea. In this paper the preliminary magnetostratigraphy is described of two sections in the Neogene Caltanissetta basin, Sicily, containing the early Messinian Tripoli Formation, which pre-dates evaporite formation, and the overlying Calcare di Base shallow water carbonates that mark the first evaporite phase. The magnetisation of the sediments has been extensively overprinted by recent weathering, but primary or early diagenetic remanences which pass reversal and fold tests have been isolated. This has been possible because the sections have been tilted by Pliocene tectonic movements, thus rotating the pre-tilt remanence direction away from the later overprint direction, allowing resolution of the components by great circle analysis. The sections have been correlated with Krijsman
et al
.’s revision of the geomagnetic polarity time scale (GPTS) of Cande & Kent. This was achieved by comparing variations in the sediment rate derived from the alternative matches with the GPTS with the observed proportions of diatomite (relatively fast deposition) and clay (slow deposition). The proposed correlation puts the base of both sections at or close to the start of the Messinian, but predicts diachroneity in the onset of Calcare di Base deposition between the two sections, as the base of the Calcare di Base falls in opposite polarity chrons in the two sections. The sections lie in two separate sub-basins controlled by thrust anticlines of the Maghrebian thrust belt. Structural and sedimentological evidence suggests that the onset of evaporite formation in these perched basins was strongly controlled by the extent and development of the emerging anticlines. The magnetostratigraphy provides a first positive test of this prediction of tectonically controlled diachroneity of facies. Further palaeomagnetic work is in progress to assess the relative timing of evaporite formation on Sicily in more detail.
Palaeomagnetic investigations in Northern Albania and their significance for the geodynamic evolution of the Adriatic-Aegean realm
Authors: Mauritsch H. J. , Scholger R. , Bushati S. L. , Xhomo A.
Published: April 2008

Palaeomagnetic results from the Inner and Outer Albanides south of the Shkoder-Pec line have established a clockwise rotation of about 45° for all tectonic units investigated so far. In Northern Albania, different tectonic zones surrounding the Shkoder-Pecline were sampled at 56 sites consisting of about 400 samples. Less than 50% of the samples had sufficiently high NRM intensities for thermal demagnetization and magnetization vector analysis. Fold and reversal tests support the results. The palaeomagnetic directions demonstrate a strong tectonic disturbance in the central part of the study area. The northernmost sites in the Albanian Alps show counterclockwise rotation (with respect to present north) in agreement with results from the Southern Dinarides. The pattern favours the idea that the Shkoder-Pec line is the transition zone between the counterclockwise rotation in the north and the clockwise rotation in the south.
Implications of ophiolite palaeomagnetism for the interpretation of the geodynamics of Northern Greece
Authors: Feinberg H. , Edel B. , Kondopoulou D. , Michard A.
Published: April 2008

Independent studies undertaken in Northern Greece (Axios-Chalkidiki belt and Vourinos massif), mainly on ophiolites but also on granites and metasediments, have shown that despite the occurrence of widespread remagnetizations it is possible to find a consistent arrangement of palaeomagnetic directions. Application of the fold test to four sites in the Kassandra peninsula, comprising pillow lavas and overlying metaflysch deposits demonstrates that the main regional magnetic component (
D
= 47.7°,
I
= 47.7°, α
95
= 8.8°) is a post-folding remagnetization. A comparison with directions found in the Sithonia granite shows that this component was probably acquired during the late-metamorphic emplacement of this granite (in the Eocene). An older component (
D
= 325.4°,
I
= 34.1°, α
95
= 18°) defined at a site in the sediments of the Kassandra area is also present in the ophiolites. This direction has probably been acquired during Late Jurassic-Early Cretaceous times. Further comparisons with the directions found in the Axios-Chalkidiki-Vourinos ophiolites and Florina granite show that the Mesozoic component is consistently found over a broad area.

The Cenozoic clockwise rotation of about 45° shown by the Eocene remagnetisation has been widely described by various authors, but the Mesozoic rotation indicated by the oldest component has been less investigated. This counterclockwise rotation is in the same sense as the rotation of the African plate during the Mesozoic, but is of larger amplitude. This suggests that: (i) Northern Greece was part of the African foreland before the Alpine collision; and (ii) oceanic spreading south of the Pelagonian zone during Cretaceous times would have increased the rotational movement of the studied area with respect to Africa.
Magnetic stratigraphy procedures in volcanic areas: the experience at Vesuvius
Authors: Incoronato Alberto
Published: April 2008

A procedure involving stringent criteria for the definition of a linear vector, combined with visual assessment, is demonstrated to be essential for magnetic dating of volcanic lavas during historical times and that the partial demagnetization process must be extended to where the entire spectrum has been destroyed. As a result, it is demonstrated that some magnetic dating of Medieval lavas on Vesuvius are incorrect. Five flows are shown to have identical directions of magnetization,
D
= 13°,
I
= +64° (α
95
= 0.9°) which can be shown to date to the
AD
1631 ‘big eruption’, confirming that this eruption produced several lava flows, despite recent reports to the contrary. It is also shown, mainly using data from Etna, that five other flows on Vesuvius correspond to separate eruptions during a time span between the nineth and eleventh centuries.
Correlation of the Salawati and Tomori Basins, eastern Indonesia: a constraint on left-lateral displacements of the Sorong fault zone
Authors: Charlton Tim R.
Published: April 2008

The Salawati Basin of western New Guinea and the Tomori Basin of eastern Sulawesi, Indonesia, are two sedimentary basins located either side of the main fault strands in the Sorong fault zone strike-slip system. It is suggested that prior to displacement on the Sorong system the two formed a single sedimentary basin. Movement on the Sorong system occurred largely during the latest Miocene-Quaternary, contemporaneous with deposition of a clastic sedimentary succession. An older basinal sequence, essentially Miocene in age, is composed predominantly of carbonate sediments, and this may have formed part of the foreland basin sequence related to the east Sulawesi orogenic belt. Correlation of the Salawati and Tomori Basins implies a left-lateral displacement of about 900 km on part of the Sorong fault zone.
Evidence for microbial influence on the development of Lower Carboniferous buildups
Authors: Pickard Neil A. H.
Published: April 2008

Following a series of mass extinction events towards the end of the Devonian (Frasnian), true skeletal reef-building organisms were effectively lost from latest Devonian (Famennian) and Lower Carboniferous (Dinantian) marine ecosystems. Nevertheless, nonskeletal, mud-dominated buildups (mud-mounds) are common throughout this geological interval, and microbial activity (fungi, algae and/or bacteria) is considered to have played a significant role in the development of these buildups. Evidence of microbial activity in Dinantian buildups may be divided into two broad categories: (a) direct evidence, such as the development of stromatolitic and thrombolitic macrostructures, presence of cryptic incrustations and preservation of calcimicrobes such as
Renalcis
; (b) cryptic evidence, in the form of peloidal ‘mud’ which forms the matrix of many Dinantian buildups. The matrix peloidal micrites closely resemble modern microbially mediated (algal and bacterial) carbonate precipitates. Precipitation of the peloidal fabrics is considered to have occurred within a microbial mat that bound the accreting buildup surface, thereby allowing steep depositional slopes to develop in deep water buildups and protecting shallow water buildups from erosion. Extensive cavity systems, often developed in Dinantian buildups, may also contain stromatolitic mats and micritized cavity walls. These fabrics indicate the additional presence of internal heterotrophic microbial communities and suggest that a diverse range of microbes were present in Dinantian buildups.

Controls on the development of Dinantian buildups are likely to have been multiple. Nutrient-rich (mesotrophic) water masses, in combination with relatively low sedimentation rates, may have promoted buildup initiation, while regional palaeogeography and tectonic events played a role in their location. Tournaisian buildups commonly developed in a wide range of water depths on carbonate ramps, whereas Viséan buildups, particularly in northwestern Europe, were often restricted to relatively shallow water over tectonically controlled topographic highs.

Evolution of new organisms during mid-Carboniferous time led to a diversification of buildup types. However, microbial fabrics identical to those developed in Dinantian buildups are still present in many late Carboniferous and Permian phylloid algal,
Palaeoaplysina
and bryozoan buildups. The role of microbial activity in the development of these late Carboniferous and Permian buildups may have been greatly underestimated.
Constituent composition of Early Mississippian carbonate buildups and their level-bottom equivalents, Sacramento Mountains, New Mexico
Authors: Ahr Wayne M. , Stanton Robert J.
Published: April 2008

The Alamogordo Member of the Lake Valley Formation (Lower Mississippian-Tournaisian to early Viséan) was deposited on a homoclinal ramp at depths below the fair-weather wave-base and mainly below the photic zone. Depth increased toward the present southwest, and the level-bottom biota of the Alamogordo Member is present in assemblages that parallel the depth gradient (Jeffery & Stanton this volume). Waulsortian-like mounds are abundant in the Alamogordo, Nunn and Tierra Blanca Members of the Lake Valley Formation on the distal portion of the homoclinal ramp. These mounds contain essentially the same biota as the Waulsortian buildups of Europe. Alamogordo mounds are low, tabular features composed mainly of spiculiferous micrite, while Nunn and Tierra Blanca mounds are more domical and are composed largely of fenestrate bryozoan cementstone.

Lithology and biota of coeval, level-bottom beds and mounds differ significantly because of habitat differences (cavities and hard substrates in and on mounds), and because of taphonomy, an example of which is the preservation of fenestrate bryozoan sheets in radiaxial fibrous calcite cements on mounds as compared with comminuted and dispersed fenestrate hash in level-bottom muds. Lake Valley mound constituents, although essentially identical to those in the European Waulsortian, do not occur in regular growth ‘phases’ like those described by Lees & Miller (1985). The difference is interpreted to mean that the New Mexican mounds lacked enough vertical relief above the sea floor in deep water to have been affected by any environmental gradients that may have existed across the entire platform.