bluuebunny/crossref_metadata_2025 huggingface dataset:
Astogenetic development of some lower Ordovician graptolites from Norway
Authors: Spjeldnaes Nils
Published: April 2008

The variation of taxonomically important characters commonly used in the classification of lower Ordovician graptolites has been studied in fairly large populations of
Extensograptus ‘slemmestadi’
and
‘Trochograptus’ diffusus
from the lower Ordovician of the Oslo Region, Norway. In
Extensograptus ‘slemmestadi’
, which belongs to the
patulus-hirundo
lineage, there is a good correlation between ontogeny and supposed phylogeny, but some of the commonly used taxonomic characters are not as useful as previously supposed. An astogenetic series of
‘Trochograptus’ diffusus
shows that it started as a
Tetragraptus
stage, then acquired secondary, lateral branching, (
Schizograptus
stage), and passed through a
Trochograptus
stage before the rare, large specimens reached a
Holograptus
stage. In both species, the preserved population is—as is usual in marine invertebrates—dominated by small, young specimens in which important taxonomic characters have not been developed.
On the morphological characteristics of the
spiralis
group and the stratigraphic significance of the appearance of
Cyrtograptus
Authors: Deng Bao
Published: April 2008

The paper deals with the late Llandovery and early Wenlock
Cyrtograptus
and
M. spiralis
group discovered in Ziyand, Shaanxi. Both the appearance of
Cyrtograptus
and the morphological characteristics of the
spiralis
group are stratigraphically significant. According to the mode of preservation of the
spiralis
group, the rhabdosome is thin or elongated along its proximal parts. Sometimes the proximal part is closely coiled into a perfectly regular or an irregular planospiral shape. Distally it is coiled, arcuate, straight or irregular. The proximal thecae are small, gradually becoming larger towards the distal end. All thecae are subtriangular in form. Associates of the
spiralis
group include
Cyrtograptus lapworthi, C.
cf.
laqueus, C. shishkaticus
and other genera such as
Sinostomatograptus, Stomatograptus. C. centrifugus, C.
sp. (cf.
C. graphicus
Golikov) occur at a higher horizon. According to the analysis of the specimens, speculation on the affinities between
Cyrtograptus
and the
spiralis
group is made and the possibility of evolution from
spiralis
to
Cyrtograptus
suggested. These evolutionary changes can be used to subdivide stratigraphic units. It is suggested that the first appearance of
Cyrtograptus
should be regarded as the lower boundary of the middle Silurian.
Silurian graptolites from Serbia, Yugoslavia
Authors: Pavlović Milena Mihajlović
Published: April 2008

Eastern Serbia has black graptolitic shale from early Llandovery to early Devonian times. Tectonic disturbances prevent examination of a continuous sequence, but twenty-four zones have been established with 175 species and subspecies identified. Serbian graptolites and sediments compare well with those in Bulgaria and with the Carpathian-Balkanic Arch in particular, and in general with successions in the Alpine structural province. Closest comparisons can be made with the Bohemian assemblages.
Evidence for changes in Mesozoic and Cenozoic oceanic circulation on the south-western continental margin of Ireland: DSDP/IPOD Leg 80
Authors: de Graciansky P. C. , Wylie Poag C. , Hailwood E. A. , Knox R. W. O’B. , Masson D. G. , Montadert L. , Ravenne C. , Müller C. , Sibuet J. C. , Sigal J. , Snyder S. W. , Waples D. W. , Cunningham R.
Published: April 2008

A transect of four coreholes, drilled by the
Glomar Challenger
across the Irish continental margin at the Goban Spur, evidences a dynamic palaeoceanographic regime during the late Mesozoic and Cenozoic. Shallow marine waters invaded the rift-stage grabens of the Goban Spur in the early Barremian. Thereafter, the margin subsided rapidly, producing a pelagic depositional regime by late Barremian time. Deep marine conditions were maintained as sea-floor spreading began in the early Albian, and chiefly pelagic deposition continued to the present.

Among a series of significant post-rift oceanographic changes, one of the most notable is the familiar fluctuation of oxic and anoxic sea-floor environments during the Cenomanian and Turonian. Another marked change took place during the late Palaeocene, when cooler, oxygen-rich, northern bottom waters reached the Goban Spur as a consequence of rifting and sea-floor spreading between Greenland, Rockall Plateau, and Norway. Later during the Cenozoic, the initial production of Antarctic bottom water, several accelerations of polar icecap growth, and fluctuating eustatic sea-level produced a variety of circulatory shifts on the Goban Spur. A particularly significant sedimentological consequence of these interacting processes was the widespread creation of numerous erosional and non-depositional unconformities.
Late Cretaceous anoxic events, sea-level changes and the evolution of the planktonic foraminifera
Authors: Hart Malcolm B. , Ball Kim C.
Published: April 2008

This paper investigates the relationship, if any exists, between the evolution of the planktonic foraminifera and the presence of anoxic water masses in the water column. The relationship with changes in sea-level over the expanded Cretaceous shelves is also discussed. New data from DSDP Legs 80 and 95, coupled with that from southern England, north-east England and the North Sea Basin will be used to assess the evolution of the microfauna.
Role of climate in affecting late Jurassic and early Cretaceous sedimentation in the North Atlantic
Authors: Hallam A.
Published: April 2008

Both regional and temporal changes in the influx of terrestrial organic matter, siliciclastic sediments and kaolinite are related to climate over the continents adjacent to the opening North Atlantic, reflecting a change from humid to arid and back to humid in North America and Europe, and more persistent aridity in Africa. Small-scale sedimentary cycles of laminated organic-rich and non-laminated organic-poor claystones have previously been interpreted in terms either of periodic influx of detrital plant material, or variations in sedimentation rate. They are here interpreted as the result of periodic stirring of bottom waters related possibly to Milankovich cycles.
Organic geochemistry of Cretaceous organic-carbon-rich shales and limestones from the western North Atlantic Ocean
Authors: Meyers Philip A. , Dunham Keith W. , Dunham Pamela L.
Published: April 2008

Organic-carbon-rich black shales are found in Cretaceous sedimentary settings on the North American outer continental rise. Comparison of carbon isotope and molecular biomarker compositions of black shales with adjacent lithologies gives evidence of enhanced preservation of marine organic matter in the darker layers. In middle Cretaceous sequences, the black shales represent short episodes of exceptional preservation of organic materials within an oxygenated, deep-ocean depositional setting. Neocomian black shales show poorer preservation and were deposited within turbiditic carbonates and sandstones, containing large proportions of land-derived organic matter.
The inorganic geochemistry of Cretaceous black shales (DSDP Leg 41) in comparison to modern upwelling sediments from the Gulf of California
Authors: Brumsack Hans J.
Published: April 2008

Recent trace element data for sea water and plankton reflect the importance of biological activity for the distribution of these elements in the water column and for their accumulation in sediments underlying areas of high productivity. Many trace metals in sea water are highly correlated to nutrients like phosphate, nitrate, or silica. By considering the ability of an element to be involved in nutrient cycling, as demonstrated by its deep/surface sea water concentration ratio, and its availability in sea water, a relationship between plankton and sea water chemistry is evident.

Fifty sediment samples from the Gulf of California, an area of exceptionally high primary productivity, have been analysed for all major and 15 minor elements, including Cd, Zn, Cu, Ni, Ba, Se, Cr, As, V, Mo, Co, Pb, Tl, and Bi. The ‘excess’ of nutrient and trace elements in upwelling sediments from the Gulf of California reflects the plankton chemistry. Among those elements found enriched in the diatomaceous oozes from the Guaymas Basin Slope relative to average shale are P, S, Mo, Se, Cd, and possibly Bi. For Mo and V an additional accumulation process besides bioconcentration seems to be active during early diagenesis. For these two elements their concentration ratios to aluminum increase with burial depth in the upper 30 cm of the sedimentary column, indicating that sea water seems to be a major source, besides plankton.

The chemistry of Ba in pore-waters and solids is controlled by barite solubility. A front of barite precipitation and dissolution (the ‘barite front’) is moving through the sedimentary column in the depth range of near zero pore-water sulphate, which at a drastic reduction of the sediment accumulation rate would lead to the formation of larger barite crystals or even nodules in this depth range.

Cretaceous black shales from the Cape Verde Basin (Site 367, Cores 17 to 21) are characterized by extremely high (see maximum values in brackets) concentrations of Mo (280 p.p.m.), V (3600 p.p.m.), Zn (7000 p.p.m.), and a number of other trace elements, besides S (7.4%) and organic carbon (35.6%) which cannot be explained by upwelling. If these enrichments are of primary origin, then complete stagnation of the Cape Verde Basin during intervals in Cretaceous time seems likely. The marine environment is metal-poor, therefore stagnation and corresponding low accumulation rates of detrital material would favour the accumulation of organic carbon and trace metals.
Palaeozoic Prospects on the Northern Flanks of the London-Brabant Massif
Authors: Tubb S. R. , Soulsby A. , Lawrence S. R.
Published: April 2008

Seismic data across the northern flank of the London-Brabant Massif, between the United Kingdom and the Netherlands, have highlighted some interesting exploration leads in the sub-Permian section. Carboniferous sediments have been affected by low-amplitude Hercynian folding, Mesozoic rift faulting and Cretaceous inversion-related compression and fault re-activation. These effects have created a variety of trap situations in potential Palaeozoic reservoir units. In addition to early Permian Rotliegende sandstones, potential reservoirs are formed by late Carboniferous Barren Measures and Millstone Grit sandstones, Carboniferous Limestone carbonates, and basal or pre-Carboniferous clastics. Also Carboniferous Limestone ‘reef’ developments have been tentatively identified along a paleo hinge-line. A significant stratigraphic break is identified near the base of the Barren Measures in the late Carboniferous which has been designated the ‘Symon Unconformity’. The unconformity and depositional patterns within the Barren Measures are explained by the effect of lateral heat flow on normal basin development in the late Carboniferous. An analysis of well log data has demonstrated that Barren Measures’ sandstones constitute a good reservoir objective. Reservoir quality and sand thickness/frequency increase towards the flanks of the London-Brabant Massif.

Westphalian A and B coals subcrop the Symon Unconformity, and together with the Millstone Grit, onlap the Carboniferous Limestone surface on the flanks of the London-Brabant Massif. The coals constitute a proven commercial gas source which is in direct contact with the recognised potential reservoirs. Maturation studies, based on geochemical analysis, demonstrate that gas-generative maturity is largely dependent on Jurassic burial. The source/migration/entrapment model suggests that conditions have been favourable for gas accumulation in Palaeozoic reservoirs contained within rift-related fault-blocks and more subtle stratigraphic traps.
Gas Prospects in the ‘Midland Valley’ of Northern Ireland
Authors: Illing L. V. , Griffith A. E.
Published: April 2008

A substantial sedimentary basin in southeast Co. Antrim is indicated by gravity measurements and has been explored by the Larne-2 geothermal test well which encountered 9395 ft of Permo-Triassic sediments without reaching their base. Porous sandstones near the top of the Sherwood Sandstone occur beneath thick impervious halite horizons in the Mercia Mudstone. Further potential reservoir rocks occur in Lower Permian sandstones below a 370 ft thick Zechstein salt at a depth of about 6000 ft. Potential traps in the form of tilted fault blocks have been indicated by reconnaissance Vibroseis. Suitable source rocks, the remaining essential ingredient for a petroleum prospect, are expected but have yet to be proved.

A similar Permo-Triassic basin is indicated by gravity beneath Lough Neagh. Although covered by thick Tertiary basalts and Lough Neagh Clay, its western margin has been successfully mapped by reconnaissance Vibroseis. The Permo-Triassic here is thinner and is without salt. However, the underlying Carboniferous contains Westphalian and Namurian coals, and geochemical studies confirm their potential to produce gas where suitably buried. There is a good chance that similar Carboniferous strata with coals underlie the Permo-Trias of the Larne Basin of southeast Antrim and provide the source rock for an important gas province.
The Geology of the Rough Gas Field
Authors: Goodchild M. W. , Bryant P.
Published: April 2008

The Rough Gas Field was discovered in 1968. It lies in the United Kingdom Continental Shelf blocks 47/3d and 47/8b and produces from the Permian Rotliegendes sandstone in a faulted anticlinal structure. Initial reserves were 384 BCF. The field was on production until 1980 when the British Gas Corporation began developing Rough as a gas storage facility to meet peak season demand in the mid to late 1980’s. Three main facies associations are recognised from core studies of the Rotliegendes reservoir; aeolian sands, proximal fluvial sands and conglomerates, and distal sheet flood sands. The vertical arrangement of these associations within the Rotliegendes sequence define three sedimentary units which represent semi-arid alluvial fan progradation and abandonment. These units can also be recognised on electric-logs and can be correlated across the field. Reservoir porosity and permeability is largely controlled by the texture and mineralogy of the sedimentary facies, with a concomitant diagenetic influence. The geological model of facies distribution and trends in reservoir quality, together with well test data, has been used to optimise future development locations and update reservoir simulation of field behaviour in the storage mode.
Reservoir Geology of the Sean North and South Gas Fields, U.K. Southern North Sea
Authors: ten Have A. , Hillier A.
Published: April 2008

Sean North and South, discovered in 1969 and appraised in 1982, are two separate gas reservoirs some 15 km southeast of the Indefatigable field. Located in block 49/25a, they are owned equally by Shell/Esso/Britoil/Union Texas.

The fields are northwest-southeast trending anticlinal structures, separated by a transcurrent fault. The two accumulations, which are mostly fault-bounded, have different gas/water contacts. The Permian Rotliegend reservoir occurs at a depth of around 8300 ft (2500 m) and varies in thickness between 200–265 ft (60–80 m). The reservoir rock consists of aeolian dune sandstones with excellent reservoir properties.

Development drilling started early 1985 from two platforms. Combined recoverable gas reserves are in the order of 425 BCF. First gas is expected to come ashore in October 1986.
Seismic reflections from the lower crust around Britain
Authors: Matthews D. H.
Published: April 2008

A cartoon is presented which represents a time-section typical of the first 3000 km of BIRPS profiling over the continental shelf north, west and south of Britain. It shows a Mesozoic sedimentary basin in the hanging wall of a low-angle normal fault zone. The dipping fault reflection merges with layering in the lower crust which appears as several strong but short (
c.
4 km) horizontal reflections between about 6 s two-way travel time and the base of the crust at about 10 s. The uppermost mantle is devoid of reflections except where dipping events extend from the layered lower crust towards the base of the section at 15 s two-way time. The cartoon is justified by reference to examples from profiles.

Discussion states present hypotheses that are being tested. The layering in the lower crust is real and due to lenses of flat-lying foliation about 4 km across and tens of metres in thickness in which the seismic velocity has been reduced by the presence of fluids. Brine has been suggested to account for the electrical conductivity of the lower crust, and carbon dioxide may be associated with granulites. The geometry implies that the lower crust was ductile at the time of faulting, and that the foliation became horizontal during Mesozoic stretching of West Europe.
Petrology and geochemistry of the lower continental crust: an overview
Authors: Kay R. W. , Kay S. M.
Published: April 2008

Continental lower crust comes in a limited number of types that are best distinguished tectonically. Lower crust is formed primarily in oceanic or continental zones of lithospheric plate convergence and secondarily in regions of intracontinental rifting. Formation processes involve magmatic intrusion of mantle-derived basalt and structural burial of pre-existing upper crust by thrust or reverse faults. In both cases, metamorphism, crustal melting and ductile deformation result from the accompanying heating and release of volatiles. A compositional conundrum exists for both arcs and continental rifts since the new continental crust has a basaltic rather than an andesitic (mean crustal) composition. Possible solutions to this compositional disparity include formation of more silicic lower crust in the Archaean, and lower crustal delamination which could preferentially remove tectonically immature basaltic crust.
Petrogenesis of layered gabbros and ultramafic rocks from Val Sesia, the Ivrea Zone, NW Italy: trace element and isotope geochemistry
Authors: Pin C. , Sills J. D.
Published: April 2008

Layered gabbros and ultramafics, well exposed in Val Sesia, have been interpreted as a layered complex. The lower part (LLG; up to 1 km thick) comprises well layered pyroxenites and gabbros with minor peridotite and harzburgite. The upper part consists of fairly homogeneous gabbros (MG), which grade into diorite (up to 5.5 km thick). This gabbro body intrudes already highly deformed metapelites, but is itself relatively undeformed.

These two groups (LLG and MG-diorite) are distinct on the basis of isotope, trace element and major element chemistry and cannot have been derived from the same liquid. The LLG has ɛ
Nd
ranging from +1.5 to +2.5 and ɛ
Sr
from +7 to −8 and was derived from a depleted source. The MG-diorite body has ɛ
Sr
ranging from +40 to +60 and a wide range of ɛ
Nd
from −2 to −6.

The LLG pyroxenites and gabbros are dominantly cumulates of clinopyroxene with lesser amounts of orthopyroxene. Cumulus plagioclase appears towards the top of the LLG. The LLG is strongly depleted in LREE and LIL elements and was derived from a basaltic liquid with relatively depleted trace element characteristics. It probably fractionated at moderate pressures and was emplaced into the Ivrea Zone along with the adjacent Balmuccia peridotite.

The MG comprises plagioclase ± olivine cumulates with minor cumulus apatite, ilmenite and pyroxene. The diorites additionally contain cumulus biotite. The whole body has strongly LREE-enriched patterns (Ce
N
4.5–20) and large positive Eu anomalies (Eu/Eu* from 2–12). The MG has calc-alkaline features with high Ba and low Nb. The parental magma was also LREE-enriched, with low Ni, Cr and MgO and high Ba. It was possibly an andesite.

The spread in ɛ
Nd
cannot be accounted for by contamination by any of the likely crustal contaminants. The spread in intial isotope ratios either results from a heterogeneous enriched mantle source or from a two stage process involving remelting of an intermediate mafic reservoir with moderate enrichment in Rb and LREE.

The MG-diorite body was emplaced into sediments already undergoing amphibolite-facies metamorphism at pressures of 5–7 kbar. It appears as if the development of magma chambers in the crust was an important process of crustal growth in the Ivrea Zone.
Geochemistry of granulite-facies lower crustal xenoliths: implications for the geological history of the lower continental crust below the Eifel, West Germany
Authors: Stosch H.-G. , Lugmair G. W. , Seck H.A.
Published: April 2008

Mafic meta-igneous granulite facies xenoliths from Engeln/Eifel display petrographic, chemical and isotopic features which are not easily understood in terms of igneous fractionation or granulite-facies metamorphism. These features are best explained through a metasomatic modification of many of the xenoliths, characterized by the formation of amphibole and increasing Rb/Sr and Nd/Sm ratios as well as shifts in
143
Nd/
144
Nd towards values similar to those of enriched mantle under the Eifel. Granulites which essentially escaped metasomatism have a Sm-Nd depleted mantle model age of ∼1.6 Ga whereas metasomatically altered granulites have younger and highly variable model ages. A Sm-Nd isochron for minerals from one granulite resulted in an unexpectedly young age of 172 ± 5 Ma which can be regarded as the maximum age of metasomatism. This age may be indicative of a thermal event having affected the lower crust under the Eifel during the Mesozoic.
Mineral reactions in xenoliths from the Colorado Plateau; implications for lower crustal conditions and fluid composition
Authors: Broadhurst J. R.
Published: April 2008

A study of lower crust and upper mantle xenoliths from Moses Rock Diatreme, Utah, has led to the discovery of evidence for low temperature/high pressure,
in situ
hydration of pyroxene and garnet granulites in isolated parts of the lower crust of the Colorado Plateau; one of the characteristic reactions being: Plagioclase→Zoisite+Paragonite+Quartz+Albite This was followed by a second hydration at lower temperature and pressure, under more oxidizing conditions, probably associated with the incorporation of the xenoliths into the diatreme. Under these conditions, pyroxenes were replaced by fibrous amphiboles and the plagioclases were rimmed by chlorite and epidote. Upper mantle xenoliths show similar histories of cooling and hydration. The origin of the cold hydrating fluid is linked to the presence of low T/high P group ‘C’ eclogites in the suite, xenoliths which may represent relics of a subducted/underplated fragment of the Farollan plate.
Relationships between major structures, salic volcanism and sedimentation in the Kenya Rift from the equator northwards to Lake Turkana
Authors: Williams L. A. J. , Chapman G. R.
Published: April 2008

At the equator, the Kenya (Gregory) Rift Valley is a half-graben, faulted on the eastern side. It passes northwards into a deep graben with a double western margin, and then into what is essentially a downwarped depression. The deep graben sector (0°15′–1°00′N) is virtually devoid of late Miocene-Quaternary trachytic central volcanoes: their counterparts in areas of strong faulting are salic flood lavas. Moreover, the scarcity of pyroclastics in early-late Miocene ‘plateau’ phonolite sequences of the deep graben suggests that the thick tabular lava units are some distance from source. The phonolites exposed on the western side of the rift (Kamasia-Uasin Gishu) probably came from eruptive centres beyond the deep graben, whereas on the eastern side (Laikipia) a substantial part of the sequence could well belong to a lava shield on the plateau. In contrast to intra-volcanic pyroclastic sediments that occur predominantly N of latitude 1°N, the formations in the Kamasia region consist of waterlaid deposits intercalated with tabular lava flows. This spectacular development of waterlaid sediments is attributable to the absence of major eruptive centres and the existence of tectonic-geomorphic sediment traps in the deep graben sector of the rift.
Depth to metamorphic basement in the Koobi Fora region from seismic and gravity data
Authors: Swain C. J. , Skinner N. J. , Khan M. A.
Published: April 2008

A detailed 2.3 km long seismic refraction line near Koobi Fora shows velocities increasing continuously with depth, apart from a 100 m thick low-velocity layer whose top lies at a depth varying from 100 to 250 m. This is thought to be a lacustrine sequence. Total thickness of rocks overlying basement is 1200–1300 m. Interpretations of four shorter profiles in this area are also presented, one of which indicates basement at only 40 m depth. These data are used to control the interpretation of part of the Bouguer gravity map of the region in terms of an upfaulted basement block partly coinciding with the Kokoi horst.
Volcano-tectonic control on sedimentation in the Koobi Fora sedimentary basin, Lake Turkana
Authors: Watkins R. T.
Published: April 2008

It has previously been considered that the sedimentary rocks of the Koobi Fora basin, NE of Lake Turkana, lie disconformably upon older volcanic strata, local volcanism and sedimentation being separated in time. Volcanic and sediment relationships observed at the boundary of the basin reveal that major Pliocene tholeiitic flood volcanism took place after the initiation of proto-Lake Turkana. Significant lacustrine and fluviatile deposits lie beneath lavas in the upland margin of the northern part of the basin. These are lithologically similar, and may be laterally equivalent, to strata outcropping along the northern edge of the basin and on the southeastern flank of the Kokoi upland. The fine-grained lacustrine rocks formed during an extended period of slow sedimentation. The age of Lake Turkana, therefore, may be significantly greater than 4.3 Ma, which is recorded from the lowermost tuff bed within the Koobi Fora basin (McDougall 1985). Further lacustrine sediments form thin but laterally persistent intercalations between flood lavas. Concurrence of large scale subaerial eruptions and lacustrine sedimentation is explained by a model of volcano-tumescence and deflation. The early Pliocene lake was relatively shallow and of greater extent than present-day Lake Turkana. Swellings of the land surface preceding volcanic eruptions E of the lake produced significant withdrawals of the shoreline westwards. Subsidence, following lava outpourings, allowed transgression of the lake towards the former shorelines.
Evolution and sedimentary character of lake deltas fed by ephemeral rivers in the Turkana basin, northern Kenya
Authors: Frostick L. E. , Reid I.
Published: April 2008

The growth of deltas in Lake Turkana, northern Kenya, is shown to be controlled by the erratic supply of sediment, and the short- and long-term fluctuations in lake level brought about by climatic change and tectonic activity. The sparse and infrequent rainfall of this arid region dictates that all rivers draining to the lake, except the Omo, are ephemeral. Accretion along much of the lake margin is therefore discontinuous, interrupting extended periods during which littoral sediments are reworked by wave action and aeolian processes. Although fluvial activity is infrequent (rivers may run for a total of less than 20 hours per year) the sediment load is high, in common with other arid environments (measured sediment yields reach 1600 t km
−2
a
−1
) and delta construction is rapid. Numerous seismic profiles of the Tulu Bor delta—a typical example—reveal a Holocene fill that reaches 20 m. They also show changes in position and gradient of palaeo-channels at successive levels in the sediment wedge as the system has responded to rising lake levels. Older sediments of the delta have undergone early stages of diagenesis and can be differentiated easily from the younger fill. Sediments of the present delta have been sectioned to provide a catalogue of microstratigraphy at type-sites that is useful in interpreting older, Plio-Pleistocene sediments of the Turkana basin. Upper and lower distributary, overbank levée, and interbedded littoral and fluvial environments are identified by specific associations of primary structures and grain-size characteristics. Grain size is also used to trace source areas in the hinterland and so establish sediment transport pathways.
The distribution of coarse-grained sediments in modern Lake Turkana, Kenya: implications for clastic sedimentation models of rift lakes
Authors: Cohen A. S. , Ferguson D. S. , Gram P. M. , Hubler S. L. , Sims K. W.
Published: April 2008

The nearshore environment of Lake Turkana, Kenya was studied to determine how coarse clastic sediments are being distributed in the modern lake. Four lake margin study localities were examined in detail: (1) an area of basaltic colluvial headlands; (2) an area of sand, sandy silt and silty mud substrates; (3) an area of highly variable substrate conditions; and (4) an area of alternating basaltic headlands and gravel/shingle terraces. Based upon quantitative observations in these areas and qualitative observations elsewhere in the lake, we believe that two interacting factors have the greatest influence on the distribution of nearshore clastics. The first is a sediment distribution system which is primarily dependent on localized supply, via rockfall and intermittent streams. The second is the presence of physiographic barriers, principally faulted and eroded volcanic flows, which border on local coarse sediment delivery systems and isolate them from adjacent systems. Volcanic barriers may operate at various scales in regulating the delivery systems of clastic sediments in rift lakes, from localized facies disruption, as observed in Lake Turkana, to the isolation of entire sedimentary basins. At the largest levels the presence or absence of volcanic activity may have an important bearing on the controversy of hydrocarbon accumulation in rift lakes.
Mineral precipitation and diagenesis in the sediments of the Lake Bogoria basin, Kenya Rift Valley
Authors: Renaut R. W. , Tiercelin J. J. , Owen R. B.
Published: April 2008

Lake Bogoria is a perennial, saline, alkaline closed lake fed by hot springs and ephemeral runoff. Detailed studies of the sedimentology, mineralogy and chemistry of the late Quaternary sediments, both from exposures around the shoreline and from lake bottom cores, have provided abundant evidence for early diagenesis and mineral precipitation. Marginal fan-deltaic clastics exhibit carbonate (calcite, dolomite), iron oxide, opaline silica and zeolite (analcime, natrolite) cements, especially where groundwater is shallow and subject to evaporative concentration. Travertines associated with microbial activity around hydrothermal springs are predominantly calcite; high temperature, inorganically precipitated travertines are aragonitic. Most are relic deposits. In places, spring carbonates have been silicified (opal-A) or altered to fluorite in response to changing groundwater composition. Fan-delta conglomerates have been hydrothermally altered by alkaline spring waters yielding zeolites (analcime, thomsonite, clinoptiloite, mordenite), clay minerals, opaline silica and magadiite. The lacustrine sediments include prodeltaic clastics, organic oozes and sodium carbonate evaporites. Trona and nahcolite predominate in the central sub-basin. The local occurrence of thermonatrite may be associated with sub-lacustrine hydrothermal activity. In the north subbasin, evaporites are exclusively gaylussite. Sodium silicates (magadiite, kanemite) occur in black organic oozes and formed in an anoxic environment rich in sodium carbonate and dissolved SiO
2
. Other authigenic minerals in the lacustrine sediments include pyrite, siderite, fluorite, zeolites (analcime, mordenite, clinoptilolite), opaline silica (formed from a gel precursor around sub-lacustrine springs?) and vivianite.
East African Rift stromatolites
Authors: Casanova J.
Published: April 2008

The East African Rift System contains many lacustrine basins in which biologically precipitated stromatolite carbonates are found. The development of these carbonates results from low-Mg calcite precipitation by cyanophytes and bacteria; the binding and trapping of sediments are not involved. A general outline for the distribution of stromatolites has been established by analysing data from different sites in East Africa. In the fluviatile environment, travertines are associated with a biotype of flowing waters with high calcium content. During exceptional rainy seasons, inundation of floodplains promotes seasonal growth of oncolites. Chimneys and pool-rim dams associated with hydrothermal springs may be sub-lacustrine, or may occur along lake shores. In the lacustrine environment, stromatolites form shorelines that indicate former high stands. The study of Quaternary stromatolites and their modern equivalents reveals their exceptional potential for reconstructing sedimentary palaeoenvironments in rifts. Stromatolites represent a particular continental carbonate facies, somewhat analogous to marine reefs. Furthermore they constitute an important source of other geological data: material for isotopic dating; determining altitudes of former lake levels; and providing evidence for hydrological and physico-chemical conditions in palaeolakes. All these may be combined to assist palaeoclimatic reconstruction.
The Pliocene Hadar Formation, Afar depression of Ethiopia
Authors: Tiercelin J. J.
Published: April 2008

Along the western margin of the Afar depression there is a fluvio-lacustrine sequence, well known for its hominid fossils, which consists of a 150–300 m thick layer of coarse to fine detrital sediments with interbedded pyroclastics and basalt. This series, deposited between at least 4.2 and 2.4 Ma, is called the Hadar Formation. Sedimentary and microfossil evidence allows the reconstruction of sedimentary, volcano-tectonic and climatic events in the evolution of the Hadar basin. During 1.8 Ma, the palaeogeography of the Hadar region appears to have been an intricate succession of fluvial, marshy and lacustrine environments, subject to rapid lake-level fluctuations and a wide range of hydrological conditions. Tectonic activity is confined to subsidence. However, important vertical movements of the western escarpment occur towards the end of the sedimentary succession in the Hadar basin.
Sedimentary stratigraphy of the Tugen Hills, Baringo, Kenya
Authors: Hill A. , Curtis G. , Drake R.
Published: April 2008

The Tugen Hills sequence within the northern Kenya Rift Valley W of Lake Baringo is unusual in providing sedimentary and faunal information over a relatively long period of time, from 14 Ma to the Pleistocene, and including some periods not yet noted elsewhere in sub-Saharan Africa. The sediments record changing depositional environments in an evolving rift system, and the enclosed fossils permit the evolutionary lineages of a number of families of animals important in the modern African fauna to be elucidated. We document the history of investigation of sedimentary units belonging to the Tugen Hills succession, and describe their current status. Information is given concerning the general sedimentary character of the units and their age is determined by faunal, isotopic and palaeomagnetic methods. Faunal change through the sequence is summarized.
Quaternary environments of the Chalbi basin, Kenya: sedimentary and geomorphological evidence
Authors: Nyamweru C. K.
Published: April 2008

The Chalbi basin is an inland drainage basin surrounded by lava uplands in arid northern Kenya. In its centre lies the Chalbi playa, a bare surface of mud subject to occasional flooding, the surface of which is at
c.
370 m above sea-level. Along the edges of the basin, basalt overlies sediments. At one site (Algas) sediments contain the fossil bones of aquatic and terrestrial species which suggest an age within the range 1.6 to 2.0 Ma. A K-Ar date of 2.5 ± 0.3 Ma has been obtained for basalt overlying the bone-bearing sediment. Between the basalt front and the Chalbi playa there is a belt of younger sediments that include sands and horizontally-bedded silts. At several localities, fossil freshwater gastropods have given
14
C dates ranging from 9530 to 11,080 BP. The gastropods may have originated within a single Lake Chalbi with a surface water level above 410 m, or, alternatively, in a number of separate ponds or swamps. The evidence of early Holocene humid conditions in the Chalbi basin agrees with that from the Lake Turkana basin and from other lake basins in East Africa.
Sedimentology, stratigraphy and palaeoenvironments of the Holocene Galana Boi Formation, NE Lake Turkana, Kenya
Authors: Owen R. B. , Renaut R. W.
Published: April 2008

Lake Turkana lies in a closed drainage basin at the northern end of the Kenya Rift Valley. During the early and middle Holocene the lake was fresh, much expanded and subject to many fluctuations in surface level. Periodically it formed one linkage in a greater Nile drainage network. Late Holocene, climatically-induced regression left a ring of sediments stranded up to 80 m above the modern lake. To the NE, these deposits are known as the Galana Boi Beds and here we propose raising them to formation status. They consist of diatomaceous silts, silty diatomites, quartz-arenites, arkoses, litharenites, coquinas and at least one air-fall tuff. Depositional environments ranged from low-energy offshore, through littoral, lagoonal and deltaic to alluvial and aeolian. A rich, diverse microfossil assemblage of diatoms, pollen and ostracods has been recorded and used in palaeoenvironmental analysis. Macrofossils include molluscs, fish debris and other vertebrates. With their excellent outcrop and diverse environments represented, the Galana Boi Formation can serve as a model for sedimentation of the margins of a large rift lake, and aid in the interpretation of similar, but poorly exposed graben sequences in the geological record.
Sedimentation and fossil preservation in the Nyanza Rift System, Kenya
Authors: Pickford M.
Published: April 2008

Two major factors led to the accumulation of fossiliferous strata in the Nyanza Rift System—volcanism and tectonism. Several volcanoes have been active in, or near, the Nyanza Rift Valley during the past 20 Ma. Arranged in rings round these volcanoes were 3-D annular lithofacies dependent for their origin on various volcanic, geomorphic and pedogenic factors. Near-vent facies are characterized by predominantly coarse clastic deposits which accumulated on steep slopes (>10°) in upland areas. They usually show few signs of pedogenesis. Further from the vent (4–15 km radially) the apron facies are represented by finer-grained volcaniclastic strata, often with significant amounts of pedogenic overprint but little evidence of deposition from, or in, water. These apron facies accumulated on gentler (3–10°) slopes, both by subaerial fallout and by radial reworking of the near-vent facies. Even further from the volcanic centre are the arena facies which are generally fine-grained and waterlain in virtually flat-lying countryside (0–3° slopes). Primary volcanic ashes are occasionally present in the arena facies. The bulk of these volcanogenic sediments accumulated on dry land. Their deposition was not especially related to existing sedimentary basins. The second major factor which was responsible for the genesis of at least part of the Nyanza sediments was the formation of basins by tectonic activity associated with rifting. Depressions were formed in which fluvial and lacustrine sediments accumulated. The bulk of these sediments represent the products of erosion of the scarp shoulders of the rift, the sloping half-graben blocks, and the upstanding volcanic forms within the rift and on its shoulders. The distribution of fossil vertebrate species in western Kenya is closely related to depositional facies. Terrestrial assemblages are particularly distinct from aquatic faunas and generally occur in strata which accumulated subaerially as part of the apron and arena facies. Aquatic faunas are found in the fluvial and lacustrine sediments. In addition, the geochemical composition of the source rocks is an important factor in fossil preservation. Sediments derived from rocks rich in lime and low in silica contain abundant vertebrate and invertebrate fossils, while those that are silica-rich and lime-poor yield remarkably few faunal remains.
Tectonic and environmental control on rodent diversity in the Plio-Pleistocene sediments of the African Rift System
Authors: Denys C. , Chorowicz J. , Tiercelin J. J.
Published: April 2008

Tectonic, sedimentological, climatic and zoo-geographic data are combined in order to explain faunal development in the East African Rift Valley during Plio-Pleistocene times. The main factors controlling the evolution of rodents are the tectonic events which influence local sedimentation and climate, and not only the global climatic glacial events which appear to have reduced effects in the rift valley.
A model of organic sedimentation on present-day continental margins
Authors: Pelet R.
Published: April 2008

A very general mathematical formalism describing biological degradation in either the water column or in sediments is proposed. Coupled with considerations of sedimentary processes, it permits the qualitative description of organic sedimentation. It is found that a simple, general relationship relating the organic-carbon content of sediments and sedimentation rates cannot exist, and that any apparent relationship is caused by the covariation of factors which are not logically linked. Consideration of the rates of degradation of organic matter in different oceanic environments leads to the conclusion that, in aerobic environments, meio- and macro- benthos are more efficient biological consumers than the free bacterial community.
Stratigraphic distribution and palaeo-oceanographic significance of European early Palaeozoic organic-rich sediments
Authors: Thickpenny A. , Leggett J. K.
Published: April 2008

Marine, organic-rich sediments of early Palaeozoic age are thicker, and were deposited over longer time-periods, than those of other Phanerozoic intervals. They occur widely throughout North and West Europe. Their palaeogeographic setting is less certain than those of their Mesozoic and Caenozoic counterparts, making genetic models difficult to construct. The stratigraphic distribution of organic-rich sediments in the area from the Ural Mountains to Portugal is briefly reviewed for the Cambrian, Ordovician and Silurian Systems. The occurrence of organic-rich sediments in different palaeoenvironments (shelf, slope, basin) is compared for the three palaeocontinents Gondwanaland, Baltica and Laurentia, using the reconstructions of Cocks and Fortey (1982). Organic-rich sediments are shown to be more widely distributed at certain times: the middle and late Cambrian, the Caradoc and the Llandovery-Wenlock intervals. Two areas are selected for more detailed discussion: the Middle and Upper Cambrian Alum Shales of Scandinavia, illustrative of shelf organic-rich sedimentation and Caradoc-Llandovery sediments of the British Isles, which include common black shales deposited in varying abyssal to shelf environments. Possible controls on black shale deposition include variations in sea level stand, degree of geographical restriction, climatic variation, oxygen levels in the atmosphere and hydrosphere, variations in organic productivity and sedimentation rates. Our ability to recognize the relative effects of these controls in the past is variable. Where information is sufficient, it suggests that high sea-level stands associated with warm climatic conditions, and low hydrospheric oxygen contents are important in the development of early Palaeozoic organic-rich sediments.
Organic-rich sediments and palaeoenvironmental reconstructions of the Cretaceous North Atlantic
Authors: de Graciansky P. C. , Brosse E. , Deroo G. , Herbin J.-P. , Müller C. , Sigal J. , Schaaf A. , Montadert L.
Published: April 2008

A survey of DSDP black shales from the North Atlantic has been carried out in order to study the evolution of Cretaceous palaeoenvironments in the region. The study involved a reappraisal of the biostratigraphy of the Cretaceous formations and considered accumulation rate, mineralogical and organic geochemical data. Special attention was paid to distinguishing between redeposited and autochthonous sediments. Three main phases of deposition are recognized which are separated by two unconformities:

The Blake-Bahama phase (Valanginian-early Aptian (Bedoulian)) corresponds to relatively uniform depositional conditions over the whole of the North Atlantic with periods of anoxia. The clay mineral assemblages reflect different sedimentary influxes from the continents into the eastern and western parts of the basin which were separated by the mid-ocean ridge.

The transition to the Hatteras phase (Event E1) is marked by a drastic fall in sediment accumulation rate and by a rapid rise in the CCD but not by any appreciable change in clay mineral fluxes.

The Hatteras phase (late Aptian-late Cenomanian) is characterized by distinctly diverse depositional conditions. In the southeastern part of the basin anoxic conditions were persistent, while in the northeast along the European margin there is no, or only very rare, evidence of anoxia. In the west, periods of oxia and anoxia alternated. Over the mid-ocean ridge conditions were oxic. A ‘barrier’ in the region of the present-day Bermuda Rise separated two areas of distinct clay mineral deposition.

The transition to the Plantagenet phase (Event E2) corresponds to low sedimentation rates. It was immediately preceded by, or was coincident with, deposition of a black shale rich in marine organic matter.

The Plantagenet phase (Turonian-Senonian) was a period of uniform sedimentation under oxic conditions.

Possible causes of these changes in sedimentation are outlined.