bluuebunny/crossref_metadata_2025 huggingface dataset:
Construction and validation of extensional cross sections using lost area and strain, with application to the Rhine Graben
Authors: Groshong Richard H.
Published: April 2008

The simultaneous area balance of multiple horizons in a graben system formed above a detachment is the basis for the lost-area section construction and validation technique. Area balance requires that the area displaced below the original regional level of each bed in a graben (the lost area) be balanced by the displaced area at the boundary of the system, given by the product of the displacement times the depth to detachment. For multiple horizons, this relationship is a straight line on a plot of lost area versus depth. The slope of the line is the displacement and the depth intercept is the depth of the detachment. In general, beds within the graben system undergo both visible and homogeneous layer-parallel extension. The homogeneous component can be calculated from the width of the graben system, the lost area, and the depth to detachment. Conversely, the detachment depth may be calculated from the total layer-parallel extension and the lost area. The technique is illustrated by application to the Rhine graben. The Rhine graben is inferred to have formed with its detachment at the base of a normal thickness (31 km) of crust and to have been uplifted. Crustal necking with a detachment at 15 km depth is a viable alternative but requires three times the layer-parallel extension of 6.3% measured at the top of the Permian and older basement. The large difference between the layer-parallel extension (6.3%) and the crustal extension (29%) is in close agreement with an area-balanced full-graben model having boundaries that slope at 60° toward the centrer of the graben.
Forward modelling of compaction above normal faults: an example from the Sirte Basin, Libya
Authors: Skuce A. G.
Published: April 2008

Seismic data reveal the structure of a large normal fault that forms the western boundary of the Agedabia Trough in the central Sirte Basin, Libya. A significant monoclinal flexure in the sediments forming the hanging wall of this fault is interpreted to have been generated by their compaction. A computer forward model supports this interpretation. Characteristics of this kind of structure are a vertical synclincal axial plane above the hanging-wall cutoff of the basement and an inclined but less distinct anticlinal axial plane above the footwall cutoff of the basement. Dips increase with depth. The model also shows how compaction can cause sedimentary growth within the hanging wall rather than just at the fault plane. In some circumstances, compaction can produce anticlinal structures in normal fault hanging walls that could easily be confused with inversion structures. Structures can be produced by compaction in normal fault hanging walls even where no lithology contrast occurs across the fault. Compaction structures generally will be most obvious over large planar faults that involve basement. Nevertheless, compaction structures are also important in the case of detached listric faults: in this case the effects are often obscured by the superimposition of fault bend folds that will have similar or greater magnitudes but are typically anticlinal, whereas the compaction structures are usually synclinal. Compaction effects compound the difficulties of geometric inverse modelling.
Kinematic modelling of normal fault geometries using inverse theory
Authors: Kerr Hugh G. , White Nicky
Published: April 2008

The principal features of a general kinematic method which uses the shapes of deformed strata to calculate normal fault geometries at depth in two and three dimensions are summarized here. This method assumes that hanging-wall deformation can be represented by arbitrarily inclined bulk simple shear and it should be applicable to faults at any scale. In two-dimensional modelling, deformation is assumed to occur within the plane of section. Deformation in three dimensions is parameterized by the three Euler angles which describe the horizontal component of the slip vector, the rake (i.e. pitch) angle and the inclination of shear planes. Differential compaction is included in both cases. Since we use inverse theory to determine fault geometries, the quality of our solutions is easily investigated. Both two- and three-dimensional algorithms have been rigorously tested on synthetic data, on sand-box experiments, and on depth-converted and interpreted seismic reflection profiles. Modifications of this approach can be used to calculate fault geometries which have changed shape during or after fault displacement. A similar approach is applicable to reverse faulting.
Multiple detachment levels in thrust tectonics: Sandbox experiments and palinspastic reconstruction
Authors: Verschuren Mark , Nieuwland Dick , Gast Jim
Published: April 2008

Sandbox models of thrust tectonics with multiple detachment levels allow alternative interpretations of fault patterns and evolution, even though they are well controlled experiments. Palinspastic reconstruction resolves these ambiguities and leads to refined interpretations. It reveals that if there are thick viscous interlayers, all non-fault related contraction can be accommodated by folding, without tectonic compaction. It also confirms the complete decoupling of faulting above and below a thick weak viscous layer, and the initial straightness and regular spacing and dips of thrusts above a decollement of low basal friction. A weak elasto-plastic interlayer or decollement merely has a lubricating effect, without complete decoupling.

Many natural thrust systems are underlain by a ductile substratum and have more than one decollement level. These examples almost always exhibit a complex thrust belt geometry and are apparently formed by different thrusting events that can be difficult to interpret and to time relative to one another.

Weak decollement materials and interlayers have a strong effect on the structural style of thrust belts at the moment of detachment. However, other material properties, in particular the viscosity, play a dominant role during the structural development following detachment (the overthrust phase).

The objective of this study was to improve structural interpretation of data from areas of thin-skinned thrust tectonics with weak ductile detachment horizons (i.e. excluding large scale basement involvement). The approach involved:

•

the use of sandbox experiments to provide controlled structural models with different decollement rheologies, and

•

the refinement and validation of selected cross-section interpretations with palinspastic reconstruction.

The result is a set of better constraints on structural interpretation of cross-sections in thrust tectonics. In natural cases, the most common weak decollement lithologies are over-pressured shales and evaporites, in particular rock salt. In our experiments we have simulated rock salt by using silicone putty (‘SP’), a viscous material that can flow slowly as a Newtonian fluid. We modelled overpressured and/or very soft clay with an oil-water emulsion (‘OWE’), which is a ductile, weak elasto-plastic, non-viscous material in which localized shear zones can form, as in any other elasto-plastic solid.
Kinematics within the Kłodawa salt diapir, central Poland
Authors: Burliga Stanisław
Published: April 2008

The Kłodawa salt diapir consists of deformed Zechstein rock salts, with subordinate anhydrite, dolomite and shales. Upward movement of these rocks and the structural evolution of the diapir started in the Triassic and continued at least until the Tertiary. Lithological, and thus rheological variation resulted in the development of a variety of tectonic structures on different scales, which can be observed in mine galleries intersecting the diapir at several levels. The rocks are folded, faulted, fractured and boudinaged, and their behaviour is strictly related to the competence contrasts between layers of different lithology. There is a range of competence from dolomite through anhydrite, shale, clayey salt and rock salt, to the least competent, potassium salt. Kinematic indications at level 600 are provided by boudins, book-shelf structures, asymmetric folds, small-scale faults, tension gashes and veins. These were produced during different stages in the evolution of the diapir and provide evidence for various mechanisms of deformation, ranging from ductile flow to brittle fracturing. The small-scale structures allow the visualization and identification of larger folds and the depiction of their development. They record changes of shear directions along the fold limbs, and also indicate a change in folding mechanism from flexuralslip folding to shear folding. High strain is localized in bedding-parallel zones and within areas dominated by low competence rocks. These zones usually form discrete horizons separating domains of more competent rocks, which show little evidence of deformation and, occasionally, undeformed synsedimentary structures (dessication polygons). The displacements were zonal, accommodated by low competence rocks, with more competent rocks passively transported as rafts within the former. The zones of intense transport are localized within beds of rock salt and potassium salt.
Deformation and sedimentation around active Miocene salt diapirs on the Tihama Plain, northwest Yemen
Authors: Davison Ian , Bosence Dan , Alsop G. Ian , Al-Aawah Mohammed H.
Published: April 2008

The Al Salif and Jabal al Milh salt diapirs of Miocene age cut through a 4–5 km thick overburden of Miocene to Recent sedimentary rocks in the southern Red Sea. The Al Salif diapir is part of a north-south oriented diapiric wall which has caused updoming of the overburden and active extensional faulting. The halite is folded into tight to isoclinal subvertical folds, with largescale rafts of interbedded gypsum-anhydrite boudinaged on the fold limbs. There is no evidence for large-scale overburden stoping or injection of salt into overlying faults and fractures, and the upward movement of the salt dome is probably caused by the overburden being forced aside and sliding off the dome. The upper surface of the salt is an undulating smooth surface which was exposed at the sea-floor during the Quaternary. Coral reefs with
14
C ages of 3700 a have been raised up to an elevation of 17 m above present-day sea-level, giving a surface uplift rate of 4.6 mm a
−1
. At the Jabal Al Milh diapir, recumbent folding and thrusting are the main deformation features observed in the siltstones and gypsum layers of the overburden. The overburden has been rotated to the vertical at least 200 m from the diapir walls. The folding and thrusting is interpreted to be caused by overflow of salt in a namakier. This study shows that overburden sediments above the Al Salif diapir extended to allow sediments to slide from the salt dome crest by listric faulting, whereas sediments underlying the Jabal al Milh salt glacier were sheared and shortened in the horizontal direction.
Uplift rate relative to base-levels of a salt diapir (Dead Sea Basin, Israel) as indicated by cave levels
Authors: Frumkin Amos
Published: April 2008

Rapid downcutting rates in the extremely soluble salt of the Sedom diapir, Dead Sea basin, Israel, allow cave channels to become rapidly graded with respect to base level. Diapir uplift leaves the older passages high and dry above present base level. Dating these passages by
14
C allows us to estimate diapir uplift rates, taking into account previous Dead Sea levels. Maximum mean Holocene uplift rates are 6–7 mm a
−1
along the eastern fault of Mount Sedom.
Hydration diapirism: a climate-related initiation of evaporite mounds in two continental Neogene basins of central Spain
Authors: Hoyos Manuel , Doblas Miguel , Sánchez-Moral Sergio , Cañaveras Juan Carlos , Ordoñez Salvador , Sesé Carmen , Sanz Enrique , Mahecha Vicente
Published: April 2008

While many halokinetic models consider that a thick overburden is necessary before diapirism develops, other studies show that salt might begin to move soon after deposition in response to different factors (e.g. regional extension). We describe two continental Neogene basins in central Spain, characterized by three Miocene sedimentary units, where unusually early initiation of evaporite mounds or ‘incipient pillows’ occurs, with no overburden on top of them. The arguments suggesting the early bulging of the evaporites of the Lower Unit, forming a palaeorelief before and during the deposition of the Intermediate Unit, rely on certain characteristics of the palaeokarsts developed on the roof of the Lower Unit, as well as in the nature, disposition, thickness, and synsedimentary processes of the Intermediate Unit. Folds, faults, tension gashes, and collapse structures develop within the evaporite mounds, and these deformation features might be related to their uplift history. The agent of initiation suggested in this paper for the evaporite mounds ‘hydration diapirism’, or a volume increase related to the hydration of anhydrite to gypsum has not been previously recognized in the literature. A series of sedimentological and palaeontological arguments further suggest that this chemical reaction was triggered by a climatic change at the boundary between the Lower and the Intermediate Units.
Deformation in the late Permian Boulby Halite (EZ3Na) in Teesside, NE England
Authors: Smith Denys B.
Published: April 2008

Differential (mainly lateral) movement of up to a few hundred metres is interpreted from borehole data to have taken place in the late Permian Boulby Halite in Teesside, northeast England, probably when it was buried to a depth of 2000 m or more. This interpretation is based on a marked variation in the present thickness of the formation, which is thought to have been almost uniformly thick when formed, together with evidence from more than 20 borehole cores of contortion, fracturing, flow-lineation and flow-brecciation; evidence of bedding-plane slip was noted in one core. Inferred boudinage in the lower part of the formation is interpreted as evidence of local extension. Unusually for rock-salt, faults cut halite in eight of the cores, and voids in fractures and faults were noted in halite in six cores.
Salt tectonism in the Persian Gulf Basin
Authors: Edgell H. S.
Published: April 2008

The Persian Gulf Basin is an elongate, margin sag-interior sag, sedimentary basin spanning the last 650 Ma along the northeastern subducting margin of the Arabian Plate and is the largest basin with active salt tectonism in the world. This basin is asymmetrical in NE-SW cross section with sediments thickening from 4,500 m near the Arabian Shield to 18,000 m beside the Main Zagros Reverse Fault. Halokinesis in the Persian Gulf Basin originates where major intersecting basement faults cut the buoyant salt beds of the Neoproterozoic Hormuz Series (1.5 to 2.5 km thick) and the equivalent Ara Formation of Oman where salt thickness reaches 4 km. Another source of salt is the Oligo-Miocene Gachsaran Formation, forming Lali salt plug. Major basement fault trends control salt tectonism in the basin, namely the extensional N-S Arabian Trend, the sinistral NE-SW Aulitic Trend, the dextral Erythaean Trend and the extensional E-W Tethyan Trend. The main types of salt structures in the Persian Gulf Basin are salt domes, salt pillows, salt walls, salt piercements, rim anticlines, turtleback structures, disharmonic folds, orogenic fold fillings and dissolution drapes. Diapiric oil fields, account for 60% of the 600 billion barrels of recoverable oil reserves of the Persian Gulf Basin and have grown continuously since Late Jurassic, or since Permian in some large structures, such as Bahrain.
Physical modelling of fold and fracture geometries associated with salt diapirism
Authors: Alsop G. Ian
Published: April 2008

The generation of salt diapirs is associated with a protracted deformation history in the surrounding overburden, which may be difficult to image seismically due to the highly inclined and fractured nature of sediments adjacent to the salt. To further the understanding of overburden deformation related to diapir development, a series of physical models has been produced incorporating a viscous SGM polymer source layer which is overlain by a granular, glass bead overburden. Model diapirs are initially triggered (to the pillow stage) via a basal indentor, but subsequently develop by a passive downbuilding process. The maximum widths of withdrawal basins and diapir-related upturn are approximately equivalent to one another. Experiments in which the salt analogue is allowed to extend laterally result in listric normal faulting of the overburden, to define graben structures centred on the diapir. Injection of the source layer into the footwall of normal faults results in reactive dispirism on the flanks of the major structure, whilst the adjacent central sections illustrate geometries which could be attributed to both active and passive diapirism. Multiple generations of normal faults produce graben with an inward-stepping sequence of fault propagation towards the central block. Complete evacuation of the source material at the keel of the normal fault simulates localized salt welds and results in extensional displacement being transferred into the hangingwall block. Sand-clay mix layers act as relatively competent horizons and display radial fracture patterns which may also be associated with a central ring fault and circular collapse graben above the diapir. Normal faults defining graben structures converge on the radial fracture pattern overlying the diapir. Circular thrust faults surrounding the diapir are developed when more competent horizons temporarily arrest vertical growth resulting in diapiric ‘ballooning’.
Palaeoenvironments in the North Sea Basin around the Paleocene-Eocene boundary: evidence from diatoms and other siliceous microfossils
Authors: Mitlehner Alexander G.
Published: April 2008

Assemblages of siliceous microfossils (diatoms, silicoflagellates, radiolaria, and ebridians) have been examined from calcareous concretions (cementstones) in the Fur Formation, an ash-bearing diatomite which crops out around the Limfjorden region of Jutland, northern Denmark. Although microfossil assemblages (particularly diatoms) change character through the deposit, only one major turnover occurs that is reflected by all of the microfossil groups examined. This biotic event indicates a marked increase in marine influence, and probably climatic warming. It appears to mark the beginning of the early Eocene transgressive phase that affected the whole of the North Sea Basin and eventually re-established this isolated basin’s links with the North Atlantic and Tethys. Other diatom assemblages examined from offshore samples towards the centre of the basin show strong similarities with those from the Fur Formation and confirm that the lower part of this deposit correlates to the Sele Formation, whilst the upper part correlates to the Balder Formation. The lowest occurrence of the widely occurring diatom
Fenestrella antiqua
is suggested as a practical marker for the base of the Eocene in both offshore and onshore sediments in the North Sea Basin. Restricted circulation processes involving seasonal, monsoon-driven upwelling leading to sporadic basin eutrophication and water-column stratification, are considered to account for the widespread proliferation of siliceous microfossils in the North Sea Basin during the late Paleocene to early Eocene.
Deep water circulation in the Paleocene Ocean
Authors: Corfield Richard M. , Norris Richard D.
Published: April 2008

We have compiled deep water benthic δ
13
C data from the Paleocene portions of several DSDP and ODP holes and present it using the new timescale of Berggren et al. (1995).

Our data show that the north Atlantic hole DSDP 384 was the most positive site for δ
13
C in the late Cretaceous and the earliest Paleocene, suggesting that the sub-tropical north Atlantic was an important locus of deep water production during these intervals. Salinity and temperature comparisons do not support unequivocal deep water production by halothermal means in this region so we prefer to avoid the term Warm Saline Deep Water (WSDW) and employ instead the more neutral term ‘palaeo-North Atlantic Deep Water’ (palaeo-NADW). During K/T boundary time, the southern ocean apparently became the major producer of deep waters. Based on δ
13
C comparisons both the North Atlantic and Southern Ocean were deep water producers during the early Paleocene to the late Paleocene interval. In the latest Paleocene (during the ‘Paleocene carbon isotope maximum’) Southern Ocean δ
13
C was most positive, supporting a Southern Ocean deep water source. The earliest Eocene ocean was characterized by deep water production in the high southern latitudes with well developed interbasinal δ
13
C gradients.

δ
18
O data show an overall decrease from the late Cretaceous into the Early Eocene interrupted by an increase between 64 and 57 Ma. This is interpreted as an overall warming trend with a superimposed, previously undocumented, cooling phase in the early to late Paleocene.
Use of palaeosols in sequence stratigraphy of peritidal carbonates
Authors: Wright V. P.
Published: April 2008

Palaeosols (fossil soils) are common in ancient shallow marine carbonate successions. The degree of development exhibited by the palaeosol reflects the duration of subaerial exposure, with palaeosols occurring at lower order sequence boundaries commonly showing complex, polygenetic histories, and those at higher order boundaries showing less development. Palaeosols in parasequence sets commonly display systematic variations in their degrees of development reflecting the varying lengths of subaerial exposure related to changes in accommodation space creation. These properties of palaeosols can be used to reappraise the sequence stratigraphical positions of peritidal carbonates in the Lower Carboniferous of southwest Britain, serving to illustrate the dangers of assigning facies units to systems tracts based solely on simple facies relationships. The palaeosols and associated sequences do not support the view that high frequency, moderate amplitude eustatic sea-level fluctuations of Milankovitch-type were operating during the Tournaisian or early Viséan.
Slump and debris-flow dominated basin-floor fans in the North Sea: an evaluation of conceptual sequence-stratigraphical models based on conventional core data
Authors: Shanmugam G. , Bloch R. B. , Mitchell S. M. , Damuth J. E. , Beamish G. W. J. , Hodgkinson R. J. , Straume T. , Syvertsen S. E. , Shields K. E.
Published: April 2008

The sequence-stratigraphical concept of the basin-floor fan within the Vail/Exxon conceptual sea-level model is popular in hydrocarbon exploration because this model predicts that basin-floor fans are composed of sand-rich turbidites with laterally extensive reservoir geometries. Many producing deep-marine facies in the Palaeogene and Cretaceous of the North Sea have been interpreted as basin-floor fans based on their seismic (e.g. mounded forms) and wire-line log signatures (e.g. blocky log motifs). We have examined nearly 3700 m (12000 feet) of conventional core from features interpreted as basin-floor fans in the North Sea and adjacent regions to determine the sedimentary facies, which actually comprise these features. Our study reveals that basin-floor fans are predominantly composed of mass-transport deposits (mainly slumps and debris flows); turbidites are extremely rare. Sedimentary features indicative of slump and debris-flow origin include sand units with sharp upper contacts, slump folds, discordant, steeply dipping layers (up to 60°), glide planes, shear zones, brecciated clasts, clastic injections, floating mudstone clasts, planar clast fabric, inverse size grading, and moderate to high matrix content (5–30%). Calibration of these facies in long cores (120–210 m or 400–700 ft) with seismic and wire-line log signatures suggests that seismic mounds and blocky log motifs are actually a manifestation of amalgamated sand units emplaced by these mass-transport processes. The cores show that in some cases, sequence boundaries on seismic profiles actually correspond to primary glide planes (decollement) between slump sheets in cores examined. Unlike laterally continuous turbidites, slump- and debris-flow-emplaced sands are often laterally discontinuous, thus making reservoir geometry and continuity much harder to predict. Nevertheless, these mass-transport deposits still form important hydrocarbon-producing reservoirs (e.g. the Frigg Field). Because both sands and muds deposited by slumps and debris flows form features that are routinely interpreted as basin-floor fans on seismic and log data, process-sedimentological interpretation of conventional core is critical to the establishment of the true origin and distribution of sands contained in such features and thereby evaluate their hydrocarbon potential.
A sequence stratigraphical approach to the understanding of basin history in orogenic Neoproterozoic successions: an example from the central Highlands of Scotland
Authors: Glover B. W. , McKie T.
Published: April 2008

Sequence stratigraphical principles, developed largely in Phanerozoic basins, are applied here to a Neoproterozoic metasedimentary succession. The Grampian and Appin groups of the southern Monadhliath represent clastic and carbonate deposition within a multiphase rift basin. Subsequent polyphase deformation has resulted in considerable crustal shortening so that, within a comparatively small area, there is significant lateral variation in basinal settings. The succession is rationalized into a number of transgressive-regressive cycles. The northwestern area is dominated by shelfal metasedimentary rocks which exhibit a hierarchy of sequence boundaries distinguished by abrupt basinward shifts (southwards and southeastwards) in the proximality of marine facies, and onlap. The surfaces between the sequences provide the objective criteria for correlation into the basinal succession of the southeast. This sequence stratigraphical approach gives new insight into Neoproterozoic basin dynamics. Although rifting is confirmed as having a dominant role in creating accommodation for the deposition of at least two ‘supersequences’ thousands of metres thick, the basin depocentre is reinterpreted to have lain to the south and east throughout deposition of the Grampian and Appin groups. The upwards trend towards ‘cleaner’ sand from Grampian to Appin group deposition is suggested to reflect increasing sediment reworking by tides during a second-order transgression. The scale of major sequences means that patterns of deposition are likely to have been basin-wide or greater, so that it may be possible to identify similar transgressive-regressive stratal architectures in structurally isolated and disparate successions. This approach may help resolve the status of some suspect terranes.
Influence of relative sea-level on facies and reservoir geometry of the Middle Jurassic lower Brent Group, UK North Viking Graben
Authors: Jennette David C. , Riley Cheyenne O.
Published: April 2008

Detailed sequence stratigraphy of several neighbouring fields in the East Shetland basin indicates that a higher order of cyclicity is superimposed onto the lower Brent Group. Both relative falls and rises in sea-level affected the strongly progradational nearshore/barrier-island system and induced important variations in the geometry, distribution and stacking patterns of the reservoir facies. The majority of the shallow marine strata belong to the transgressive and highstand systems tract and consist of the lower-shoreface Rannoch, the upper shoreface and strand-plain Etive and the back-barrier and lagoonal Lower Ness. High frequency increments of shoreline progradation are preserved in the Rannoch as gently seaward-dipping bedsets. The small-scale units mark asymmetric cycles in grain size and mica content and together, with stratiform cement, impart a strong permeability anisotropy on the Rannoch. The nearshore Etive Formation represents a sediment-laden, barred nearshore complex and is subdivided into (1) a uniformly thick upper shoreface and (2) an overlying emergent strand plain. The two subfacies are commonly separated by a thin interval of foreshore which is locally enriched in heavy minerals. The shoreface deposits show systematic variations in stacking geometries ranging from strongly progradational to aggradational.

Two relative falls in sea-level led to the subaerial erosion of nearshore Etive deposits and the juxtaposition of coastal-plain mudstones, coals and fluvial deposits with shale-clast lags on lower shoreface Rannoch strata. Although lithostratigraphically part of the Etive, these deposits bear no resemblance to the well ordered nearshore-Etive deposits. The channel complexes are vertically and laterally heterogeneous and are confined to incised valleys separated by non-depositional interfluves. In Cormorant and Tern fields, well-performance anomalies such as early water breakthrough and unpredictable water injection and oil production rates are associated with these lowstand incised valley deposits. An enigmatic coarse-grained unit occurs in the lower Rannoch in Eider field and is provisionally interpreted as the distal expression of the lowstand shoreface deposits associated with the older incised valley. Following the lowstand deposition nearshore-marine sedimentation reestablished during the transgressive/early highstand systems tract. Under the influence of rising relative sea-level the barrier complex aggraded in place with minimal landward translation of the shoreline position. The volume of sediment supplied to the Etive shoreface was sufficiently high to keep pace with phases of increased accommodation. The shoreline system subsequently stacked with a strongly progradational geometry as the high sediment supply filled the available accommodation space during the latter part of the highstand systems tract.
A model for high resolution sequence stratigraphy within extensional basins
Authors: Howell John A. , Flint Stephen S.
Published: April 2008

Existing sequence stratigraphic models imply that for any given time period in a sedimentary basin, the interplay between rates of eustatic sea-level change and subsidence will dictate the type of sequence boundary formed (type-1 boundary if rate of sea-level fall exceeds subsidence and type-2 if it does not). Basin margin profile will dictate whether lowstand fluvial systems feed submarine fans (shelf-slope break profile) or detached shorelines (forced regressions on ramp margin). A conceptual model presented here, proposes that in structurally active extensional domains the expressions of eustatic sea-level change are still present in the stratigraphic record. These expressions are modified by complex basin margin topography and by significant, along strike variations in subsidence and accommodation creation. This model also illustrates that tectonically complex basins can host a spectrum of time equivalent lowstand deposits previously considered to be spatially and mutually exclusive. This model is illustrated by an example from the Oxfordian of the UK North Sea Central Graben.
Hierarchical stratigraphic cycles in the non-marine Clair Group (Devonian) UKCS
Authors: McKie T. , Garden I. R.
Published: April 2008

The Devonian lower Clair Group (
c.
300–800 m thick) is dominantly composed of fluviatile sandstones and conglomerates, aeolian sandstones and minor floodplain and lacustrine shales. These facies are organized into stratigraphical cycles which have a three-fold hierarchy: an unconformity bounded cycle representing first order retreat and advance of the fluvial drainage system; three subsidiary second order cycles bounded by minor unconformities, and numerous third order cycles. These stratigraphical cycles exhibit common characteristics: they are bounded by sharp, commonly erosive surfaces; they are composed of a fining-upwards succession overlain by a variably developed coarsening-upward element; the fining-upward element records a progressive decrease in the fluvial component, and increasing proportion of aeolian, floodplain or lacustrine facies (this trend is reversed in the upper, coarsening-upward section). The grain size of the fluvial facies, and nature of the non-fluvial facies (aeolian vs floodplain/lacustrine) within any order of cycle is dependent on it’s position within a lower order (larger scale) cycle.

The cycles are areally extensive, and are present in all wells within the Clair Basin, although spatial changes in internal facies make-up occur. The second order cycles show progressive onlap followed by offlap of the basin margin.

These hierarchical cycles are interpreted to be the product of changes in the accommodation:supply ratio of the depositional system. An increasing accommodation:supply ratio provided space to preserve the record of waning fluvial influence, enhanced aeolian reworking, preserved floodplain fines or lacustrine mudrocks. In contrast, a reducing accommodation:supply ratio resulted in filling of accommodation space and sediment bypassing. Accommodation space variations were dictated by tectonic variations in strain rate, which caused incremental uplift of the basin margin and subsidence of the basin. Sediment supply changes were influenced by source area uplift, which changed sediment yield, and climate change which affected runoff. Climatic changes are detectable within the non-fluvial elements of the Clair Group by stratigraphic variations between aeolian, floodplain and lacustrine facies. These vary systematically within and between second order unconformity bounded units, suggesting a tectonic influence on climate.

Within the second and third order cycles, those which were evolving towards, or were occurring within, semi-arid to arid climatic conditions are commonly (but not exclusively) biased towards fining-upward successions, whilst the more humid systems show a better developed coarsening-upward element. This may be the result of the changing impact of discharge versus sediment supply variations when one of these parameters became suppressed or enhanced under varying climatic conditions.
Interfluve sequence boundaries in the Grassy Member, Book Cliffs, Utah: criteria for recognition and implications for subsurface correlation
Authors: O’Byrne Ciaran J. , Flint Stephen
Published: April 2008

The Late Campanian Grassy Member of the Blackhawk Formation, Mesaverde Group, Utah consists of two unconformity-bounded fourth-order sequences within a third-order highstand sequence set. Several orders of bounding surface define the internal architecture of each sequence and component parasequences. Incised valley fill deposits, dominated by braided channel-fills updip and laterally accreted fluvial and estuarine channel-fills down dip, overlie the fourth-order sequence boundaries, truncating shoreface and shelf facies of underlying sequences. Incised valleys are separated by extensive interfluves with evidence of subaerial exposure and minor tributary channel systems. The interfluve gullies are interpreted to have transported very little bedload during lowstand and acted primarily as rainwater run-off or bypass channels lateral to the main incised valley system. They are filled with bioturbated fine-grained sandstone and siltstone and capped by marine shale. Interfluve areas provide subtle but crucial evidence for base level falls, including rare examples of roots penetrating formerly offshore marine shale, partially eroded hematitic hardpan sediments and sand or silt filled gullies containing coal and shell debris. Interfluve sequence boundaries have powerful predictive application in respect to recognition of along-strike incised valley fills. Recognition of these surfaces is commonly hampered by removal of subaerial indicators as a result of subsequent transgressive erosion. There is indirect stratigraphic evidence of an emerging salt or basement fault driven structural high, orthogonal to the Late Campanian palaeoshoreline, which controlled the locus of valley incision and associated interfluves in two successive high frequency sequences.
Consequences of post-collisional deformation on the reconstruction of the East Pyrenees
Authors: Keller P. , Gehring U.
Published: April 2008

Post-collisional structures are generally ignored in tectonic reconstructions of the Pyrenees. The South Pyrenean thrust belt is characterized by different tectonic rotations about vertical axes. A new statistical analysis of palaeomagnetic data shows that the observed rotations are homogeneous within discrete zones which only partly coincide with the tectonic units of the thrust belt. The combination of the palaeomagnetic information with structural and sedimentological data demonstrates that the actual structure of the southeastern Pyrenees is due to interference of Palaeogene (collisional) and Neogene (post-collisional) deformations. Accounting for this structural interference leads to significantly less shortening than calculated by earlier reconstructions of the southeastern Pyrenees. This example clearly shows that the small contribution made by post-collisional tectonics to the overall deformation can strongly influence the structure of orogenic belts.
Palaeomagnetic evidence of block rotations in the Matese Mountains, Southern Apennines, Italy
Authors: Iorio M. , Nardi G. , Pierattini D. , Tarling D. H.
Published: April 2008

After rigorous geological and petrological study to exclude all Miocene rock samples that may have been affected by post-deposition disturbances and selecting only sites where the Miocene rocks appear to be rigidly attached to their Mesozoic substrate, over 100 samples were collected from the Monte Matese region. Over 200 oriented specimens, cut from these samples, were subjected to rigorous thermal and alternating field demagnetization. Strict criteria for the definition and within-site consistency of remanent vectors showed that most sites were dominated below 200°C or 20 mT by a magnetization associated with the present geomagnetic field direction. In almost all sites, any higher coercivity or blocking temperature components were inconsistent with the exception of one locality near Pescorosito. Re-sampling of two outcrops at this locality, 100 m apart, provided 26 samples that met all of the geological, petrological and palaeomagnetic criteria. The low temperature/coercivity component is the same as the present geomagnetic field in the locality, but all samples from eight of the nine sites have identical remanence vectors between 200 and 400/450°C. Petrological and tectonic considerations imply that this remanence must be associated with very early diagenesis and is thus probably of Mid-Miocene but possibly Early Miocene age. As these sites are autochthonous relative to their Mesozoic basement, they indicate a 40° anticlockwise rotation of the Monte Matese block, relative to Apulia, since Mid-Miocene times. Thus most of the post-Early Cretaceous rotation of this block has occurred since Mid-Miocene times. It is suggested that similar age constraints may also apply to other rotated blocks within the Southern Apennines.
Large scale rotations in North Hungary during the Neogene as indicated by palaeomagnetic data
Authors: Márton Emő , Márton Péter
Published: April 2008

Tertiary igneous and sedimentary rocks were collected for palaeomagnetic study at about 90 localities from the North Hungarian Central Range. Rock ages range from the late Eocene to the Mid-Miocene but the majority of the samples are Early Miocene in age.

After laboratory treatment 66 localities yielded palaeomagnetic results, which fall into two groups. The first group is characterized by a westerly declination rotation of 70–90° accompanied by shallowed inclinations, and the second group is characterized by a declination rotation of only 30° in the same sense. The first group is older and comprises rocks of late Eocene to early Miocene age, whereas the second group comes from rocks of late Ottnangian to Karpatian age. These results are indicative of two successive tectonic rotations, one in the late Ottnangian and another just preceding the Badenian. The first rotation was accompanied by a northward shift of the area.
Palaeomagnetism and palaeogeography of the Western Carpathians from the Permian to the Neogene
Authors: Krs Miroslav , Krsová Marta , Pruner Petr
Published: April 2008

Geodynamic models for the Western Carpathians require evaluation of palaeomagnetic data from the Outer Carpathian flysch belt, from limestones of the Klippen Belt and from volcanic and sedimentary rocks of the Inner Carpathians. Palaeomagnetic data for the Permian to the Neogene are evaluated. The data indicate marked, mostly anticlockwise tectonic rotations of larger rock complexes and nappes. The distribution of palaeomagnetic poles is characteristic of a collision zone, and crosses the apparent polar wandering path for the African plate. Tectonic rotations are observed in both the Inner Carpathians and the Klippen Belt as well as in the Outer Carpathian flysch belt. Most drift occurred during the Permian to Triassic interval, with northward movement from initial southern equatorial latitudes. Drift decelerated from the Jurassic to the Neogene.
Ultra-fine magnetostratigraphy of Cretaceous shallow water carbonates, Monte Raggeto, southern Italy
Authors: Iorio M. , Tarling D. H. , D’argenio B. , Nardi G.
Published: April 2008

The sedimentological and palaeomagnetic parameters of bore cores of shallow-water Hauterivian and Barremian carbonates have been defined at 1 cm and 2 cm intervals, respectively. The stability of remanence to alternating fields has been established and a composite 88 m sequence compiled. This shows fine structure polarity changes, with the Hauterivian section being almost entirely of Normal polarity and the Barremian including thick zones of both predominantly Normal and Reversed polarity. Narrow zones of opposite polarity occur throughout. The sedimentological and palaeomagnetic properties are found to exhibit clear cyclicities, which, when normalized, show an extremely high correlation to each other and to the predicted orbital variations for this period. This enables a study of polarity transitions and secular variations using a chronology in which 1 cm is equivalent, on average, to 285 ± 8 years.
Revised magnetostratigraphy and rock magnetism of Pliocene sediments from Valle Ricca (Rome, Italy)
Authors: Florindo F. , Sagnotti L.
Published: April 2008

This is an extended abstract of the paper ‘Palaeomagnetism and rock magnetism in the upper Pliocene Valle Ricca (Rome, Italy) section’ by Florindo & Sagnotti (1995). It is shown that a normal polarity event reported from a sedimentary section in the Tini quarry is associated with a different mineralogy than the rest of the sequence. Combined thermal and AF demagnetization reveals that there is a pervasive reversed polarity component throughout the sequence, but this is obscured by a strong normal component in sediments close to an ash bed. A variety of techniques show that the reversed component is carried by magnetite and likely to be primary. The normal component is associated with greigite and is likely to be of late diagenetic origin or possibly associated with self-reversal behaviour. The previous correlation between the normal polarity zone and the Reunion Event is therefore invalid and the entire sequence is considered to correlate with the C2r. 1r chron, between the Olduvai and Reunion Events.
Palaeomagnetic database: the effect of quality filtering for geodynamic studies
Authors: Scalera Giancarlo , Favali Paolo , Florindo Fabio
Published: April 2008

The Global Palaeomagnetic Database (GPMDB), now updated to 1992, contains about 7000 palaeomagnetic data, which are fundamental tools to define regional and global geodynamic models. A software developed at the Istituto Nazionale di Geofisica allows the selection of data on the basis of space, time, and quality. Six quality classes have been proposed. The African and European Apparent Polar Wander Paths (APWPs) have been computed and the role of the statistical uncertainties is discussed. Some examples from the Tethys Belt have been chosen to demonstrate the effect of the quality filtering in geodynamic studies.
Palaeomagnetism as a tool for testing geodynamic models in the North Aegean: convergences, controversies and a further hypothesis
Authors: Kondopoulou D. , Atzemoglou A. , Pavlides S.
Published: April 2008

The north Aegean region is a geologically complicated area, in which numerous studies have focused during the last decade. Palaeomagnetism can give a decisive contribution in selecting possible models. In the present study all available palaeomagnetic data from the north Aegean and surrounding area (39.5°N to 42°N and 20°E to 26°E) are compiled, classified and evaluated. Their significance in terms of the rotational behaviour is given as follows.

In various plutonic, volcanic (Eocene-Oligocene) and sedimentary (Mio-Pliocene) formations, systematic easterly declinations are observed. The rare exceptions are caused either by remagnetized material or by very local phenomena. The declination gradually increases from recent to older periods, varying between approximately 20° and 40°. This distribution is seen in the area between south Albania and the western Greek Rhodope. In contrast, the eastern Greek Rhodope appears to be unrotated. The inclinations diverge systematically from the expected values. This could be due to either an alternative pole position for the Eocene (Westphal 1993) or possible tilting of blocks during the Oligocene-Miocene, associated with a NE-SW brittle-ductile extensional stress field. The data partially confirm geodynamic models proposed for the area by Taymaz
et al.
(1991), Sokoutis
et al.
(1993) and Jolivet (1993). The model proposed by Dinter & Royden (1992) does not predict rotations and is not supported by the palaeomagnetic data.

When the published volcanological, structural, seismic and palaeomagnetic data are combined they suggest that the north Aegean region during the Early Tertiary was tectonically similar to the present day active south Aegean subduction zone.
Palaeomagnetic study of crustal deformation across an intracontinental transform: the North Anatolian Fault Zone in Northern Turkey
Authors: Piper J. D. A. , Moore Joanna M. , Tatar O. , Gursoy H. , Park R. G.
Published: April 2008

Eocene volcanic rocks spanning the North Anatolian Fault Zone in north central Turkey have a common reversed polarity and appear to record a short term volcanic episode useful for identifying subsequent tectonic rotations. Although regional differences are present, no distributed clockwise rotations caused by dextral motion across the fault zone since mid-Miocene times are found. Instead variable anticlockwise block rotations demonstrate that this fault system does not obey theoretical models for crustal behaviour across continental transforms. Deformation is found to be highly inhomogeneous with a narrow zone of intense clockwise rotation recognised within blocks bounded by strike-slip faults above, and parallel to, the fundamental lineament. Further from the lineament no systematic rotations with respect to the major bounding plates are detected. A zone of
c.
30° anticlockwise rotation in the east may be either a consequence of emplacement of the Pontides or an ongoing consequence of continental collision. Slightly larger rotations south of the fault probably record block rotations into Anatolia as this region is being extruded westwards by continuing impingement of Afro-Arabia into the Eurasian Plate.