bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Mesozoic carbonaceous claystones in the western and eastern North Atlantic (DSDP Legs 76, 79 and 93)
Authors: Rullkötter Jürgen , Mukhopadhyay Prasanta K.
Published: April 2008

Mesozoic carbonaceous claystones were recovered on both margins of the North Atlantic Ocean in a number of holes drilled by the Deep Sea Drilling Project (DSDP). Geochemical investigation revealed that the organic matter in these sediments belongs to different organofacies types, as indicated by varying degrees of preservation and mixing of marine and terrigenous material. The intensity of terrigenous organic matter supply, coastal upwelling, downslope transport events, and local oxygen depletion are major factors controlling the composition and preservation of the organic matter in most of the Mesozoic carbonaceous claystones, whereas an ocean-wide anoxic event, if at all, can only be invoked for the organic-matter-rich sediments deposited at the Cenomanian-Turonian boundary. Examples discussed are from the Mazagan Escarpment (Site 547, DSDP Leg 79), the Blake Bahama Basin (Site 534, DSDP Leg 76), and the continental rise off Cape Hatteras (Site 603, DSDP Leg 93).
Organic-rich sedimentation at the Cenomanian-Turonian boundary in oceanic and coastal basins in the North Atlantic and Tethys
Authors: Herbin J. P. , Montadert L. , Müller C. , Gomez R. , Thurow J. , Wiedmann J.
Published: April 2008

One of the most striking results of the Deep Sea Drilling Project is the proof that organic-rich sediments have a widespread geographical distribution during the period from Upper Cenomanian to Middle Turonian. Such sediments were drilled at North Atlantic DSDP Sites: 105, 135, 137, 138, 367, 398, 551, 603. They are also present (from outcrops or oil wells) on the shelf of the African continental margin (Senegal, Tarfaya and Agadir Basins), in the Tethys (former Alboran Block, Southern Spain, Algeria, Tunisia and Umbrian Apennines/Italy), and in the North Sea. Although these sediments have different lithologies and depositional environments (from shelf areas to the deep sea), their study, mainly based on organic geochemistry with additional data on sedimentology, biostratigraphy and palaeo-bathymetry, suggests that a unique ‘pulse’ of organogenic accumulation characterizes the Cenomanian/Turonian Boundary Event (CTBE).

The content and type of organic matter are related to the depositional environment and organic preservation. The organogenic accumulation is distributed according to various trends. Off the African continental margin the organic content increases from onshore areas to the shelf (Casamance area), and, moreover, increases also in deep sea areas, with a gradual transition from terrestrial type III to marine type II (the best preservation of the organic matter being in the deepest areas, i.e. Site 367). Off the American continental margin Site 603 shows the same TOC and type of organic matter as at Site 105. The CTBE is also well recorded in the northern part of the Atlantic (Celtic margin, North Sea) by a drastic lithological change (black shales within chalks), but the type of organic matter is mainly terrestrial. In the Tethyan area the organic matter is of marine origin and well preserved. Results are compared with those of Pratt (1984) from the Western Interior Basin of the USA. Different hypotheses to explain this synchronous widespread accumulation of organic matter are discussed.
Mid-Cretaceous of the Gibraltar Arch Area
Authors: Thurow J. , Kuhnt W.
Published: April 2008

Mid-Cretaceous depositional environments of the Gibraltar Arch and adjacent areas exhibit at the Cenomanian-Turonian boundary a special type of organic carbon-rich siliceous sedimentation which is intercalated in all the different environments—from the shelf down to the deep—of this area. This will be called the Cenomanian-Turonian Boundary Event (CTBE). Summarizing the most important features of the CTBE it can be pointed out: (1) the initiation of the CTBE is nearly coeval; (2) very high TOCs, especially at the base of Turonian; (3) kerogen is exclusively of type (I-) II in the undiluted strata; (4) independent of given palaeogeographic setting (e.g. E. and W. margin of the Atlantic ocean); (5) bituminous sedimentation is also very pronounced in environments with high detrital input and strong dilution (e.g. deep sea fans). Here too, most strata comprise kerogen of type II; (6) as far as we know, there is a striking co-occurrence of these sediments with strong enrichment in biogenic silica from diagenetically altered radiolaria, but there is no link between good preservation of radiolaria and bituminous sediments (e.g. Scaglia-facies); (7) rich radiolarian faunas are common in the carbon-rich and carbon-free sediments of shallow marine and deep sea environments; (8) the important change in the evolution of planktonic organisms (foraminifera/radiolaria) which marks the Cenomanian-Turonian boundary coincides with the peak of anoxic conditions and biogenic silica-rich sedimentation.

All these features of the CTBE observed in the Gibraltar Arch area are comparable with coeval DSDP-facies in the North Atlantic and allow on-shore-off-shore tracing of the CTBE.
Development of N.W. Europe’s Southern Permian Gas Basin
Authors: Glennie K. W.
Published: April 2008

The Southern Permian Basin contains over 4.1 × 10
12
m
3
of recoverable non-associated gas, most of which is found in Early Permian Rotliegend sandstones. The most important source of this gas is Carboniferous coal deposited in a proto-Tethys foredeep basin. Where the coals were overlain by Rotliegend desert sands and by Late Permian Zechstein halite, the scene was set for creating important reservoirs for gas.

Deformation of the reservoir and seal to create a trap, and the generation and migration of Carboniferous gas, resulted from the structural history of the area, which was shaped in part by underlying crustal blocks and zones of crustal weakness. Much of this history was related to events most clearly expressed beyond the limits of N.W. Europe in the North Atlantic and Tethys:

(1)

E.-W. tension, which gave rise to the oblique-slip-induced subsidence of many sub-basins within the Southern Permian Basin, caused the consecutive creation of the Viking-Central graben system and Rockall Trough, and ended with the crustal spreading of the Atlantic Ocean; and

(2)

the early Mesozoic opening of Tethys, its Cretaceous closure and ensuing Alpine orogeny, which induced inversion in many areas of the Southern Permian Basin that are now gas bearing.
Strike-slip Basins and Inversion: a possible model for the Southern North Sea Gas Areas
Authors: Gibbs A. D.
Published: April 2008

This paper examines the structural style of strike-slip zones and of the associated inverted basins and presents a tectonic model for their evolution as part of a linked system. The structure of strike-slip zones may be modelled geometrically by turning an extensional or compressional dip-slip system on its side. The duplex geometries (Dahlstrom 1969a) become braided wrench zones and the transfer components (Gibbs 1984) become linking ‘flats’ with dip-slip displacements between steep strike-slip belts. The stratigraphic implications of this model are not discussed in detail.

The model developed in this paper suggests that in an area such as the Southern North Sea Gas Area, the pattern of inversions at a regional scale can be related to major crustally linked systems of steep faults which ride at different levels on flat detachments. Where the detachments step up, or down the over-lying block or leaf will invert. At a more local scale individual structures may invert by progressive rotation of individual faults or fault bounded blocks. Simple shear geometries of both extension and shortening above a relatively simple basement shear zone can be modelled by reference to the strain ellipse. The rotation of strained elements gives rise to the classic ‘flower structure’ seen in cross-section. The Gas Area Basin is a particularly good area to study the effects of rotational simple shear because of the low strength salt decoupling cover from basement and for studying the long range regional inversions across crustal shear zones and the postulated detachments on long regional seismic lines. The data presented in this paper are intended to illustrate the variety of structural styles produced by progressive evolution of structure on a linked system of strike-slip and detachment faults.
An Interpretation of Vitrinite Reflectance Data From the Southern North Sea Basin
Authors: Cope Michael J.
Published: April 2008

Vitrinite reflectance data for the Carboniferous in 68 Southern North Sea Basin wells have been used to determine a maturity-depth relationship for the basin and to provide an estimate of basement inversion. The derived maturity-depth relationship is comparable with maturity gradients in other Paleozoic coal and gas basins known to have had similar geothermal histories. The earliest onset of gas generation in the Southern North Sea was in the late Jurassic following the accumulation of ± 11700 ft (3500 m) of sediment above the Carboniferous source beds. Estimates of inversion of the Sole Pit area using vitrinite data are similar to those based on shale velocity methods (± 1500 m maximum). In some areas the amount of inversion of the Carboniferous basement apparently exceeds that of the post-Zechstein section which may be attributed to removal of significant amounts of late Carboniferous strata during late Hercynian times; these areas are also associated with the principal gas discoveries in the basin.
Evaluation Procedures in the Carboniferous of Northern Europe
Authors: Draxler J. K. , Edwards D. P.
Published: April 2008

Formation evaluation in the Carboniferous has always been an extremely difficult task due to complex formation properties coupled with adverse logging conditions. A study was undertaken in co-operation with oil companies to propose optimum data acquisition techniques and formation evaluation procedures.

The study covers a 300 kilometre-wide area of North Continental Europe. It includes data from 12 wells with different logging conditions and depths ranging from 2200 to 6700 meters.

Cores were analysed for porosity, permeability and formation factor under reservoir pressure conditions. Thin sections, SEM photographs and X-ray diffraction results, have been utilised for clay typing and to establish the lithological model best suited to Carboniferous evaluation. This model includes: quartz, ferroan dolomite, haematite and mica. Illite and kaolinite are the two main clays.

Formation water salinity was found to decrease from 230 Kppm in the uppermost formations, close to the Zechstein cap rock, to 26 Kppm in the oldest Westphalian horizons. Determination of the true formation resistivity takes account of the influence of the so-called Groningen effect.

Core and log observations pointed to the presence of a dual porosity system with variations of cementation exponent with overburden pressure. Rock mechanical property computations, estimation of in-situ stress directions and fracture detection completed the study.
Application of Image Ray Tracing in the Southern North Sea Gas Fields
Authors: Thorn S. A. , Jones T. P.
Published: April 2008

The application of Time Migration to seismic data has won a great deal of popularity due to its ability to better locate seismic reflection events. Commercially available Time Migration algorithms all involve different approximations to the Wave Equation. In practise these simplifying assumptions work effectively in areas such as the Northern North Sea, where the shallowest reflection with a significant structural dip is often close to the target horizon. In the Southern North Sea Gas Basin, however, the section which overlies the Top Rotliegendes target horizon commonly shows a large degree of structural dip. Due to the resulting high lateral velocity variation the wave equation approximations of Time Migration are no longer valid, and this results in the incomplete imaging of the seismic data. Depth Migration potentially offers a complete solution to full imaging of seismic data beneath a section with laterally varying velocities. However, in either its pre-stack or post-stack mode, Depth Migration is an expensive iterative process. In certain circumstances incomplete imaging of the Time Migrated data can be accommodated by application of Image Ray Tracing techniques. The repositioning of subsurface structures demonstrated by both techniques could potentially influence reserves, development plans, and equity determinations.

This paper contains Image Ray Tracing examples from the Southern North Sea Gas Area. It discusses a practical procedure to incorporate the modelling results, and highlights situations where this technique has proven to be successful, and others where problems with interpreting the results of modelling still exist.
Reservoir Geology of the Morecambe Field
Authors: Bushell T.P.
Published: April 2008

The Morecambe Gas Field, located in the Irish Sea Basin, came on stream on 1 January 1985 and is to be used as a seasonal storage facility. Structurally the field is a northwest-southeast trending faulted anticline divided into two parts by a narrow southwest-northeast graben system. Gas is produced from the upper part of the Lower Triassic Sherwood Sandstone Group. The reservoir sandstones were deposited predominantly by the action of low sinuosity braided streams during periods of high stage flow. Facies associations recognised from core and wireline log data include: Channel Sandstones, Secondary and Ephemeral Channel Sandstones, Sheetflood Sandstones, Floodplain fine member and Channel abandonment deposits. Reservoir quality is dependent on grain size, shape, sorting, facies control and the distribution and abundance of authigenic cements. Quartz and dolomite cements are a major factor in causing porosity reduction, whilst authigenic illite controls effective permeability. Platy illite only occurs below an identifiable datum believed to represent a ‘paleo’ gas-water contact. Below the illite datum permeability is impaired by up to one hundred fold. A change in illite crystal morphology in the lower parts of the reservoir results in a basal zone which exhibits improved quality. The main producing horizons are stacked sandbodies within the Keuper Waterstones and Keuper Sandstone Frodsham Member. On the basis of facies modelling a three dimensional reservoir model has been developed for simulation purposes. A simplified three layer model which incorporates the influence of authigenic illite has also been erected and will control future field development. A phased development programme is planned for the field over the next ten years. The first stage is currently underway, utilising slant drilling technology to economically drain the reservoir. Maximum gas production from the field will be 1200 million cubic feet per day.
Esmond Gas Complex
Authors: Bifani Ron
Published: April 2008

Esmond Gas Complex consists of three gas fields (Esmond, Forbes and Gordon Fields), located in Quadrant 43 of the Southern North Sea. These fields are all simple anticlines caused by deep sale piercement and are relatively small in areal extent. The reservoir, composed of a 400 ft to 500 ft layer of Lower Triassic, Bunter Sandstone, consists of predominantly very fine to fine grained sandstone deposited by fluvial processes in an arid to semi-arid environment.

Porosity and permeability are controlled by original texture and by subsequent diagenesis. The presence of considerable quantities of halite, as a cementing agent, resulted in a very large apparent ‘gas effect’ on the FDC-CNL log in certain low porosity intervals. Variation in halite content throughout the reservoir required customised log analysis and special consideration in core preparation in order not to dissolve halite or alter the delicate pore geometry. This paper deals with techniques used in identifying this problem, and in procedures used to convert the data into useable information from which definitive reserve and deliverability determinations were made.

Field Development was approved by the Department of Energy and production commenced in June 1985.
The Leman Sandstone Formation in U.K. Block 49/28 Sedimentation, Diagenesis and Burial History
Authors: Arthur T. J. , Pilling D. , Bush D. , Macchi L.
Published: April 2008

Block 49/28 lies on the northeastern side of the Sole Pit Inversion. The Leman Sandstone Formation increases in thickness from 280 ft to 702 ft southeast wards across the block. The section can be subdivided by facies and by the recognition of major bounding surfaces between aeolian sequences. Transverse dunes up to 100 ft in height have been identified. Early secondary porosity development accompanied by growth of mixed layer clays, feldspar, dolomite and silica cements with minor anhydrite was followed by burial related illitisation of clay minerals and late kaolinite authigenesis.

The most important control on reservoir quality in the block is the development of illite, which reduces permeability. The extent of fibrous illite growth was controlled by pre-Cretaceous burial. Late kaolinite, related to the migration of pore waters accompanying gas, is restricted to gas bearing structures. The relative timing of authigenic mineral growth and the structural history of the area can be combined to pinpoint the timing of gas accumulation as Middle to Late Jurassic.
Twenty years of deep seismic reflection profiling in Germany—a contribution to our knowledge of the nature of the lower Variscan crust
Authors: Meissner R.
Published: April 2008

Near-vertical reflection studies in Germany have concentrated on the following objectives:

(1)

The nature of crustal reflectors

(2)

A detailed velocity structure

(3)

Zones of increased tectonic activity, e.g., faults.

Approximately 20 years ago, the identity between reflectors in the near-vertical and the wide-angle range was established, wide-angle events being strongly influenced by velocity gradients. A pronounced, but limited low-velocity body below the Urach geothermal anomaly was detected by using an especially long spread length. An alternating polarity of deep crustal reflectors could be demonstrated. A deep fault zone, cutting the whole crust at steep angles, and shallow-angle overthrust structures were detected in the Variscan area. There is generally a strong reflectivity in the Variscan lower crust between the Conrad level at 6 s two-way travel time, i.e., 18 km, and the Moho at 9–10 s, i.e., 27–30 km depth. The lower crust seems to consist of thin lenses or lamellae with alternating velocity. They were probably generated by an extensive syn- and post-orogenic melting process leaving behind mafic and ultramafic ‘splinters’ in the lower crust and a compact ultramafic residue in the uppermost mantle, the Moho being formed as a new and uniform boundary. The poorly reflective upper crust is probably dominated by vertically oriented plutons in a rigid environment.
A physical model of the lower crust from North America based on seismic reflection data
Authors: Smithson S. B.
Published: April 2008

In speculations about crustal composition, structure, and genesis, the nature of the deep crust is always the major unknown. Seismic reflection data indicate a complex, finely (about 30–100 m) layered lower crust in some areas and an apparently transparent lower crust in other areas. Lower crustal layering is folded to sub-horizontal. The number of seismic events may be highly influenced by recording conditions. Variations in lower crustal structure appear extensive. Best lower crustal reflections are found in areas of young extension in the western US, ancient gneiss in Minnesota, and in offshore Long Island. The lower crust may be affected by a wide range of processes after its formation. Variability and complexity of the lower crust is consistent with postulated exposed deep crustal sections.
The physical properties of layered rocks in deep continental crust
Authors: Hall J.
Published: April 2008

Normal incidence seismic reflection profiles often show strong reflectivity—indicating sub-horizontal layering— in the deep continental crust, below poorly reflective upper crust. An example from the WINCH profile across the Caledonides of NW Britain shows necking of the lower crustal reflective layer near Islay. Combining the seismic data with gravity modelling constrains the lower crustal density to be about 3.1 Mg m
−3
and the P-wave velocity to be about 7.3 km s
−1
. This layer can be correlated with the deepest crustal layer of the parallel LISPB refraction profile where the layer has a similar P-wave velocity, Poisson’s ratio of 0.25 and electrical conductivity of more than 0.003 S m
−1
. These data suggest that a) the lower crust is heterogeneous, b) its mean composition is like that of very basic igneous rocks, and c) hydrated minerals and possibly free water are significant components. Aqueous fluids are shown to permeate deep crustal rocks even more effectively than they do sedimentary rocks and a mechanism of water capture in the lower crust is presented. Layering may result from interleaving of mantle and crust, fabric re-orientation due to shearing or, most likely, by intrusive underplating. Reflectivity may be enhanced by layer-variable water content.
Diversity in the lower continental crust
Authors: Dewey J. F.
Published: April 2008

Arguments based upon tectonic style and process suggest that the lower continental crust is laterally exceedingly inhomogeneous on the scale of tens and hundreds of kilometres. At least twenty basic variations exist apart from unmodified Archaean nuclei. Some 80% of the continental crust was generated by 2.5 × 10
9
years and most of this has been modified and/or redistributed by tectonic and sedimentary processes. Two general secular trends have occurred to increase homogeneity in the continental crust; addition of mafic igneous rocks to the lower crust, and differentiation, in both extensional and shortening environments, into a more mafic, refractory lower crust and an upper, wetter, more silicic, crust enriched in lithophile and radiogenic elements.
Continental lithosphere strength: the critical role of lower crustal deformation
Authors: Kusznir N. J. , Park R. G.
Published: April 2008

A thermo-rheological model of lithosphere deformation, incorporating the elastic, ductile and brittle behaviour of lithosphere material, has been used to examine intraplate continental lithosphere strength, brittle-ductile transition depth and flexural rigidity. These parameters are critically dependent on crust and mantle rheology and consequently on geothermal gradient, crustal thickness and lower crustal composition.

For lithosphere subjected to a lateral tectonic force, creep in the lower crust and mantle leads to stress release, and the subsequent stress redistribution generates stresses in the upper lithosphere sufficient to cause brittle fracture. The extent of creep in the lower crust and mantle and the degree of upper lithosphere stress amplification (which together determine bulk lithosphere strength) increase with geothermal gradient. For significant lithosphere extension, under maximum likely levels of available tectonic stress, a lithosphere surface heat flow of 60 mW m
−2
or greater is required, while for compressive lithosphere deformation, heat flow must exceed 75 mW m
−2
. Similarly flexural rigidity increases with increase in the thermal age of the lithosphere at the time of loading.

The depth of the brittle-ductile transition decreases with increase in geothermal gradient. For a limited range of gradients (expressed by heat flow
q
= 50–55 mW m
−2
) multiple brittle-ductile transitions may exist in the middle and lower crust and upper mantle, with important tectonic implications (e.g. for intra-crustal detachments and crust-mantle decoupling).

Lithosphere strength in extension and compression, and flexural rigidity, are both controlled by the quartzo-feldspathic rheology of the crust for thermally young lithosphere and by the olivine rheology of the mantle for older lithosphere. Lithosphere strength is therefore critically influenced by the thickness of the crust (decreasing with increase in crustal thickness) and by the composition of the lower crust, particularly for lithosphere with intermediate heat flows.
Crustal structure and evolution of the central Australian basins
Authors: Lambeck K.
Published: April 2008

The basins of central Australia formed over a period of 600 Ma or more from late Proterozoic to Carboniferous time. These basins appear to overlie crust of normal thickness whereas the crust of the intervening arches has been uplifted and reduced in thickness. The Moho, in consequence, varies in depth by up to ± 10 km about its mean depth, over horizontal distances of 50 km or less. The stresses associated with this lower crustal structure are of the order of a few kbars and extend throughout the crust. These stresses have existed since at least late Proterozoic time, and because the Moho undulations appear to follow the near surface deformations, this rules out the existence of a mechanically weak lower crust. The region is out of isostatic equilibrium and this state must be maintained by either a mechanically strong lithosphere of more than 150 km thickness or by horizontal compressive stresses, of the order of a few kbars. The former possibility is ruled out by the available mantle flow laws and geotherms proposed for the central Australian shield. In the latter case the horizontal compressive forces have also played the key role in shaping the basins and crustal structure. The horizontal compression must act down to at least the Moho and must characterize the dominant state of stress since late Proterozoic time.
The chemical composition of the Archaean crust
Authors: Taylor S. R. , McLennan S.M.
Published: April 2008

Three separate observations, from sedimentary rock compositions, subduction zone igneous rocks and heat flow constraints combine to suggest that the composition of the Archaean crust was distinct from that of the present-day continental crust. The sedimentary REE patterns are consistent with derivation of the upper crust from a 1:1 mixture of the Archaean bimodal basic-felsic suite. The andesite model, while adequate to account for present-day continental growth does not provide enough Cr and Ni for the Archaean crust. Average crustal component heat flow values of 14 mW m
−2
for Archaean terrains are significantly lower than the post-Archaean average of 23 mW m
−2
for crustally produced heat.

Correlations from the sedimentary trace element data provide Archaean upper crustal values of 1.5% K, 5.7 ppm Th and 1.5 ppm U. Heat flow constraints restrict such compositions to the upper layers. The total crustal values are consistent with a 2:1 basic/felsic composition, and give values for the total crust of 0.75% K, 2.9 ppm Th and 0.75 ppm U, which provide crustal heat flow components of 14 mW m
−2
for a 30 km thick crust and 19 mW m
−2
for a 40 km crust.
The structural evolution of the lower crust of orogenic belts, present and past
Authors: Windley B. F. , Tarney J.
Published: April 2008

The lower crust of orogenic belts may be exposed by post-growth uplift in magmatic arcs or by tectonic uplift on post-collisional crustal-scale thrusts. Each tectonic zone of a belt can be expected to have a different type of lower crust, and there is a similar tectonic zonation in collisional belts ranging from the early Proterozoic to the Tertiary. Tonalitic gneisses make up the bulk of the lower crust of Andean-type arcs. Deep sections of sutures may expose nepheline gneisses, blueschists and granulites. Exposed basic dyke swarms represent the feeders to eroded lavas in aulacogens in continental forelands. Post-collisional thrusts uplift earlier lower crust of amphibolite facies gneisses or granulites, of either igneous or sedimentary parentage. The result of the many factors controlling variable uplift in the different tectonic zones of an orogenic belt is that a deep-seated cross-section can only be constructed by compilation of type sections from different belts.

Crustal generation in the Archaean was dominantly magmatic and broadly similar to that seen in the deeper parts of Andean-type arcs, but on a much more extensive scale. Magmatic underplating, associated with strong horizontal thrusting, produced a strongly foliated lower crust with interlayering of mafic and silicic components. Archaean crust probably had a primary vertical zonation with respect to the proportion of these components and of heat-producing elements.
Evolution of the late Archaean lower continental crust in southern West Greenland
Authors: Robertson S.
Published: April 2008

Accretion of new continental crust onto the base of pre-existing sialic crust is indicated by field, geochemical and isotopic data from two suites of Archaean gneisses from the Inner Godthåbsfjord region of southern West Greenland. Crustal accretion combined with horizontal tectonic interleaving of existing rock suites thickened the crust resulting in high-grade metamorphism. Enrichment in LIL elements resulted from the depletion in these elements of granulite-facies gneisses below the present level of erosion. Related fluid movements were significant in leading to the anatexis of rocks enriched in LIL elements. The present-day distribution of rock suites is at least in part a function of the juxtaposition of different levels of the late Archaean crust.
Granulitic xenoliths from the French Massif Central—petrology, Sr and Nd isotope systematics and model age estimates
Authors: Downes H. , Leyreloup A.
Published: April 2008

The protoliths of granulitic xenoliths found in Tertiary/Quaternary alkali volcanics from the French Massif Central are shown to have been principally immature sediments and calc-alkaline intrusives. The association may indicate formation of the crust in an island-arc or continental margin environment. Pressure and temperature estimates indicate that they have undergone intermediate to high pressure granulite-facies metamorphism (9–15 kbar) followed by uplift to lower pressures (4–9 kbar). Rb-Sr and Sm-Nd isotopic data and REE abundances are presented for both meta-igneous and meta-sedimentary suites. ɛSr present-day values for the granulitic xenoliths range from +2.7 to +325, with ɛNd values between +0.2 and −12.0. There are no isochron relationships within the Sr and Nd data, but possible mixing lines in the Rb-Sr system may indicate sedimentation at 700–1000 Myr. T
Nd
DM
model ages indicate a Proterozoic (800–2100 Myr) origin for the protoliths of the meta-igneous samples and a similar provenance age for the meta-sediments.
Xenoliths from southern Africa: a perspective on the lower crust
Authors: van Calsteren P. W. C. , Harris N. B. W. , Hawkesworth C. J. , Menzies M. A. , Rogers N. W.
Published: April 2008

Granulite-facies xenoliths from southern African kimberlites yield temperatures from 780–900°C and pressures from 10–15 kb. They have Nd model ages of 1.4–3.7 Ga and retain Rb-Sr mica and Sm-Nd garnet ages from 0.14–1 Ga, significantly older than kimberlite emplacement ages. It is argued that low abundances of U, K and Rb in lower crustal rocks can be either primary features resulting from plagioclase-dominated cumulate processes, or secondary effects caused by the granulite-facies breakdown of mica in wet intermediate-acid rocks. Combined trace element and isotope arguments suggest that contamination with lower crustal material does not significantly affect the geochemistry of flood basalts from the Karoo, the Parana and the Deccan.
The lower crust in eastern Australia: xenolith evidence
Authors: Griffin W. L. , O’Reilly S. Y.
Published: April 2008

Hundreds of Mesozoic to Recent xenolith-bearing basalt flows, diatremes, cinder cones and maars have erupted through the low-grade Palaeozoic metamorphic rocks of the Tasman Fold Belt, which makes up the eastern third of the Australian continent. Granulite-facies xenoliths, interpreted as lower-crustal material, have been found at more than 40 of these localities. A palaeogeotherm, based on garnet pyroxenite xenoliths from Victoria, also fits available data from New South Wales and Queensland. A crustal thickness of 25—30 km, and lower-crustal temperatures of 700–850°C, have been derived by referring T estimates for spinel lherzolite xenoliths to this geotherm. The lower-crustal xenolith suites are dominated by basic pyroxene granulites and garnet granulites; more silicic xenoliths are very rarely reported. The granulites cover a range in composition typical of intraplate basaltic magmas and cumulates. Granoblastic microstructures are typical, but common relict igneous features suggest that all the mafic granulites originated as igneous rocks. The lower crust in this area probably formed through multiple intrusion of basic magmas near the crust/mantle interface over a long time span. The strongly layered ‘lower crust’ seen on seismic reflection studies in eastern Australia probably represents this thick crust-mantle transition zone. The seismic ‘Moho’ reflects the depth at which the proportion of basic to ultrabasic rocks drops below a critical value, and probably lies well below the
petrographically
defined base of the crust.
Geophysical controls on sedimentation within the African Rift Systems
Authors: Fairhead J. D.
Published: April 2008

Gravity and seismological data are used, together with unpublished oil company data, to investigate the lithospheric structures associated with the East, and the West and Central African Rift Systems. Although these two rift systems are the result of extensional tectonics, they appear to be tectonically very different, with uplift and volcanism in East Africa, and subsidence and sedimentation in West and Central Africa. These differences can be explained in terms of the amounts of crustal extension (and thinning) that these rift systems have undergone and the isostatic response of these crustal structures during, and subsequent to, the extensional phase. In West and Central Africa, gravity modelling indicates crust extensions in the order of 70 km, while in East Africa extension across the Kenya Rift could be as little as 10 km. The implications of these lithospheric models for the formation of sedimentary basins are discussed, as well as how the rift structures relate to the crustal structure of continental margins.
Structural expressions of rifting: lessons from Lake Tanganyika, Africa
Authors: Rosendahl B. R. , Reynolds D. J. , Lorber P. M. , Burgess C. F. , McGill J. , Scott D. , Lambiase J. J. , Derksen S. J.
Published: April 2008

The fundamental building blocks of Lake Tanganyika are half-graben that tend to be arcuate or crescent-shaped in plan view. When combined with the asymmetric subsidence of the half-graben, this geometry creates horizontal components of motion which are expressed as shearing and rotation along the ends of the units. The key to understanding rift-basin morphology is recognizing the various ways in which these fundamental units can be linked together in time and space. Although many modes of linking occur, the basic theme is one of alternating half-graben polarities along the strike of the rift. Where adjacent half-graben face in opposite directions and do not overlap with each other, they are usually separated by interbasinal ridges trending oblique to the rift axis. In cases where facing half-graben overlap, the apparent morphology can be that of a full-graben with a central high or a half-graben abutting a structural platform, depending upon the orientation of the observation line relative to the overlap geometry. Mechanically, such full-graben should be viewed as opposing half-graben which hinge away from the central high. Examination of the linking models in Tanganyika has enabled us to derive an empirical model of rifting. The model is extremely useful in understanding the stratigraphy of Tanganyika, and it seems to explain many of the styles and patterns of deposition which are peculiar to rift basins. If the model has general applicability to continental rifts, it can also be useful to those who deal with the mechanics of rifting and the deep structure of passive margins.
Tectonics and volcanism of the southern Kenya Rift Valley and its influence on rift sedimentation
Authors: Baker B. H.
Published: April 2008

Volcanism in the northern half of the Kenya Rift Valley began at 30 Ma, and in the southern half it began at 15 Ma. In the southern rift tectonic development started with gentle downwarping followed by repeated flooding of the depression with lavas. A half-graben formed at 7 Ma by faulting on the W, succeeded by further eruptions in the rift floor. By 4 Ma a graben had formed with faulting of the flexed eastern margin of the depression. Succeeding periods of faulting and volcanism migrated inwards, creating step-fault platforms and a narrow inner graben, but in the Naivasha-Nakuru sector voluminous trachytic volcanism completely filled the rift depression at intervals between 6 and 2 Ma. The inner graben was cut by dense swarms of minor faults from 2 Ma; and in the last 0.5 Ma a series of trachytic caldera volcanoes was built axially in the inner graben. Large sedimentary basins are found in broad half-graben in the northern and southern splay-fault sectors, and are subdivided into local basins where volcanic damming has occurred. Minor basins formed on back-tilted step-fault platforms, and numerous smaller lakes existed in graben of the rift floor. In the central sector lake basins spread broadly over pyroclastic deposits between volcanic piles. Ramp and step-ramp structures locally divert drainage from the high rift flanks into rift basins. Hypersaline lakes with trona evaporites in the Natron-Magadi basin were caused by the arid climate, eruption of sodium carbonate-rich ash, and recirculation of alkaline groundwater by hot springs into closed structural basins.
Slope processes, sediment derivation and landform evolution in a rift valley basin, northern Kenya
Authors: Reid I. , Frostick L. E.
Published: April 2008

The Plio-Pleistocene sediments of the Lake Turkana basin are the product of episodic deposition interspersed with long periods of reworking. Their stratigraphy is complex and includes a wide range of lithotypes from fine lake clays to extensive coarse gravel sheets. The current phase of landscape evolution is dominated by fluvial incision and slope retreat under semi-arid conditions. Rates of sediment derivation are high, with measured values of up to 1600 t km
−2
a
−1
, and a catalogue of evidence of surface processes (rainsplash pillars, mobile soil mantles, etc.) illustrates the erosional dynamism of an area that currently receives a rainfall of only 300 mm a
−1
. Detailed process studies reveal the ten-fold decline in infiltration capacity of silty regoliths that results from impaction of the surface by the first fall of a rain season. They also explain the rapid onset of overland flow which only accounts for between 2 and 7% of stormwater but which ravages the soil, causing erosion that ranges from 0.2 to 0.5 kg m
−2
a
−1
. Shallow gradient slopes are shown to export fine material (D
50
= 0.2 mm) and to concentrate coarse fragments > 4 mm as a surface lag. But studies on steeper gradient slopes where the regolith is typically a talluvium, and where processes are gravity-dominated, sliding or rolling movements indicate a direct relationship between particle size and downslope movement towards the stream system. Despite the variable importance of different processes on different type-slopes, a clearcut relationship emerges between slope gradient and erosion rate that allows us to model slope retreat and landscape evolution. A volumetric analysis gives an age to the present basin landform of 0.11 Ma. Analysis of sequential links in the sediment transport train permits us to match clastic materials deposited in the accumulation zones of rift valley basins (riverine floodplain, lake delta, beach) with those of the erosional source areas.
Lake Magadi, Kenya: a model for rift valley hydrochemistry and sedimentation?
Authors: Eugster H. P.
Published: April 2008

Previous studies on Lake Magadi, Kenya, and the Green River Formation of Wyoming, are reviewed in order to delineate the differences between these two settings, to determine whether Lake Magadi can serve as a reference for rift valley environments and to define a set of diagnostic criteria for recognizing saline alkaline lake deposits in the geologic column. Magadi inflow waters are influenced strongly by weathering of volcanic rocks and are characterized by Na≈Ca and (Ca + Mg)/HCO
3
≪1. Compared to the Green River Formation, calcite production is low to moderate and occurs largely in the soil zone. Sediments are dominated by volcaniclastic material transported during flood events. In contrast, Eocene Lake Gosiute and its mudflats had more gentle gradients, and inflow chemistry was such that vast amounts of calcite and dolomite were produced before brines became alkaline. Sedimentation in both cases is represented by the ephemeral floodplain-saline lake complex; a preliminary subfacies map for Magadi has been constructed. Magadi possesses the typical attributes of a rift valley basin in terms of hydrology, hydrochemistry and sedimentation, as well as the indices characterizing saline alkaline lakes: sodium carbonate evaporites or their pseudomorphs, Magadi-type chert, and tuffaceous rocks altered to zeolites and K-feldspar. Many similar environments have been recognized in the Pleistocene lake beds of North America. Magadi-type cherts were found from the Pleistocene to the Jurassic, and are also present in the Cambrian Officer Basin of Australia. Proterozoic Magadi-Green River-type environments have been suggested for the Duruchaus Formation of Namibia and HYC shale of the McArthur River area of North Australia. Many more Proterozoic examples remain to be discovered.
Palaeoenvironment of Lake Abijata, Ethiopia, during the past 2000 years
Authors: Bonnefille R. , Robert C. , Lezine A. M. , Perinet G. , Delibrias G. , Elenga C. , Herbin J. P. , Tiercelin J. J.
Published: April 2008

Multidisciplinary studies, including the examination of sedimentology, palynology, diatoms and organic matter have been undertaken on a 6 m core from Lake Abijata, Ethiopia. The bottom of this core has been dated at 1720 ± BP. Analyses have shown four distinct stages in lake evolution. Two transgressive phases that have been recognized can be correlated with other lake level evidence. The transgressions result from changing inputs from lowland and highland regions within the catchment. They are most probably related to changes in seasonal rainfall patterns.
Palaeolimnology of Lake Bogoria (Kenya) during the 4500 BP high lacustrine phase
Authors: Vincens A. , Casanova J. , Tiercelin J. J.
Published: April 2008

Lake Bogoria (altitude 990 m) lies within the Kenya Rift Valley (0°15′N, 36°10′E). A high lake stage is marked by a double stromatolite belt: the first, occurring at 999 m, is radiocarbon dated 4140 ± 60 BP and the second, at 995 m, is dated 3880 ± 60 and 3750 ± 180 BP. Palynological data from shoreline stromatolites and lacustrine sediments recovered in three 15 m cores permit a reconstruction of former limnological conditions. They show that the lacustrine extension resulted from a positive water balance brought about by runoff under climatic conditions more humid than today but drier than those prevailing during the early and mid-Holocene.
Role of tephra in the preservation of fossils in Cenozoic deposits of East Africa
Authors: Hay R. L.
Published: April 2008

Volcanism has been particularly important in preservation of fossils at some East African sites. It has supplied sediment for burial, and carbonatite ash which has chemically aided fossilization. Moreover, subaerial tuffs have preserved the remains of faunas unlike those of riverine and lacustrine environments. Examples include faunal sites at Napak, Uganda; Songhor and Koru, Kenya; and Laetoli, Tanzania. Some, and perhaps most, of the carbonatite ash erupted from East African volcanoes was natrocarbonatite, alkali-rich and broadly similar to the modern natrocarbonatite. The pre-modern natrocarbonatites have been leached of their alkalies by meteoric water. In some examples the original natrocarbonatite composition can be reliably inferred from textural and chemical evidence; in other examples only the possibility or probability of former natrocarbonatite ash can be demonstrated. Fossiliferous tuffs at Napak, Koru, Songhor, and the Footprint Tuff at Laetoli contain good evidence of natrocarbonatite ash; evidence is suggestive for Rusinga, and for the fossiliferous aeolian tuffs of the Laetolil Beds at Laetoli.
The genesis and palynofacies characteristics of marine petroleum source rocks
Authors: Tyson R. V.
Published: April 2008

This paper attempts to demonstrate that the multi-disciplinary philosophy behind palynofacies investigations is of great value to the meaningful interpretation of the origins and palaeoenvironment of marine petroleum source rocks (MPSRs).

The key control on the formation of MPSRs is identified as bottom water oxygenation (correlated with the location of the Eh interface and the intensity of macrobenthic activity). Particular emphasis is placed on the necessity for greater accuracy in the terminology used to describe levels of oxygenation. Aerobic environments are characterized by organically lean sediments (0–3.0% TOC) with Type III kerogen assemblages composed of relatively refractory land plant debris or highly degraded, marine-derived, amorphous organic matter (AOM). Their organic richness is largely dependent on sediment accumulation rate and proximity to sources of terrestrial organic matter supply. They produce mainly gas at maturation. Dysaerobic to anoxic environments are characterized by MPSR facies with high TOC values and Type II kerogen assemblages dominated by relatively lipid-rich AOM. These represent the classic ‘black shale’ source rocks and are the main source of petroleum in marine basins.

Except where they are redeposited, dinocysts are characteristically absent or rare in marine ‘black shales’. They are mainly produced in unstable, seasonally mixed water masses and may consequently be regarded as indices of hydrographic stability. Prasinophycean phycomas, which differ from dinocysts in their function, are often the dominant, or most conspicuous, marine palynomorphs of pelagic sediments and stably stratified ‘black shale’ basins.
Pyrite formation in Jurassic shales of contrasting biofacies
Authors: Fisher I. St. J. , Hudson J. D.
Published: April 2008

The quantity and composition of organic matter passing into the hydrocarbon window is dependent on the extent of organic degradation occurring during early diagenesis. An important early diagenetic reaction in which organic matter is consumed is sulphate reduction. Relationships exist between pyrite sulphur (a product of sulphate reduction), organic carbon, and degree of pyritization of available iron, which can be used to help characterize the source-rock potential. In Jurassic, marine, organic-rich shales, these parameters are related to the environment of deposition, as indicated by biofacies analysis, and to variations in bottom water oxygenation and supplies of organic carbon, sulphate and iron. The best preservation of organic matter, for reaction in subsequent diagenetic zones, does not necessarily occur in the most anoxic sediments, but in those in which the organic matter buried in the sediment is more than sufficient to satisfy the supply of sulphate by diffusion. In the shales studied this is indicated by the most positive (> −28‰) δ
34
S values for fine grained pyrite.