bluuebunny/crossref_metadata_2025_split huggingface dataset:
Volcano-tectonic control on sedimentation in the Koobi Fora sedimentary basin, Lake Turkana
Authors: Watkins R. T. 
Published: April 2008

Summary
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

Summary

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

Summary
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

Summary

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

Summary
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

Summary
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

Summary
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

Summary

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

Summary
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

Summary
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

Summary
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

Summary
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

Summary
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

Summary
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.
The Cenomanian-Turonian Oceanic Anoxic Event, II. Palaeoceanographic controls on organic-matter production and preservation
Authors: Arthur M. A. , Schlanger S. O. , Jenkyns H. C. 
Published: April 2008

Summary

Correlation of the
δ
13
C spike with the well dated occurrences of strata rich in organic carbon detailed in Schlanger
et al.
(this volume), indicates that a global episode of intense organic carbon (
org
C) burial took place during the latest Cenomanian-earliest Turonian ‘Oceanic Anoxic Event’ (OAE) (
A. plenus
through
I. labiatus
macrofossil zones and upper
R. cushmani
TRZ through
W. archecretacea
PRZ foraminiferal zones) over a period of no more than 1 million years (m.y.). The shape of the
δ
13
C curve indicates that rates of
org
C burial gradually increased in the early part of the late Cenomanian, increased more rapidly in the later Cenomanian, and levelled off at peak values in latest Cenomanian-early Turonian time during the maximum rate of
org
C burial. The δ
13
C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in
δ
13
C reflects decreasing rates of
org
C burial following the Cenomanian-Turonian ‘oceanic anoxic event’ as well as the probable oxidation and return of significant amounts of
org
C to the oceans following regression and re-oxygenation of much of the deeper water masses in contact with the seafloor.

The Cenomanian-Turonian OAE coincided with a maximum sea level highstand. We suggest that sea level, which may be responding to some volcano-tectonic event, is the common link and ultimately the driving force for
org
C deposition in globally distributed basins under different climatic and ocean circulation regimes. The rate of production of warm, saline deep water may have been proportional to the area of shelf flooding such that the maximum occurred near the Cenomanian-Turonian boundary. As rates of deep-water formation increased, rates of upwelling of deeper oceanic water masses must also have increased thereby increasing sea-surface fertility and productivity. In somewhat restricted higher latitude basins, such as the Cretaceous Interior Seaway of North America, periodic high rates of freshwater runoff coupled with deepening seas during the transgression created periodic salinity stratification, oxygen depletion in bottom waters, and resultant enhanced
org
C preservation.

The disappearance of some types of keeled planktonic formainifers and ammonites at the Cenomanian-Turonian boundary is probably due to the rather sudden but short-term disappearance of suitable shallow midwater habitats because of widespread severe oxygen depletion in these levels. This interpretation is strengthened by the occurrence of benthic-free zones or depauperate benthic faunas near the Cenomanian-Turonian boundary in many localities.