Morphology and growth of aragonite crystals in hot-spring travertines at Lake Bogoria, Kenya Rift Valley

Pseudohexagonal aragonite crystals are common components in some hot-spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non-crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal. Today the hot (T>95 °C) Na-HCO₃-Cl spring waters at Chemurkeu have a salinity of 5–6 g L^?1 TDS, a pH of 8·1–9·1, Ca²⁺ concentrations of <2 mg L^?1 and Mg²⁺ concentrations of <0·7 mg L^?1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T～100 °C) and a deeper aquifer (T～170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca²⁺ than today, possibly under more humid conditions during the Holocene.     

Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley

Microbial mats, located along the margins of hot-spring pools and outflow channels at Lake Bogoria, Kenya, are commonly silicified forming friable laminated crusts. Columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray. Silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small (< 2 μm) silica spheroids. Rapid silicification of the microbes, which may begin while some are still alive, can preserve sheaths and in some examples, the filaments, capsules and cells. Although this provides evidence of their general morphology, the biological features that are required for taxonomic identifications are commonly poorly preserved.
The silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes. Almost all the silica at the Loburu springs nucleates on microbial substrates. This affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts.     

Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley

Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake. The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X-ray microanalysis) or 2–18 (d-value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na₂CO₃ rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by-product of the reactions. Chemical analyses suggest that pore waters supplied some of Na⁺, and possibly K⁺ and SiO₂. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca²⁺ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.     

Impact of lake‐level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,Kenya Rift Valley

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO₃ waters containing little Ca (<2 mg l^?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO₂ degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca²⁺ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.     

The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida,Kenya Rift Valley

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO₂ is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.     

Multi‐basin depositional framework for moisture‐balance reconstruction during the last 1300 years at Lake Bogoria,central Kenya Rift Valley

Multi‐proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high‐resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. These palaeolimnological data were supplemented with information on present‐day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high‐resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture‐balance variability through time, reflecting regional hydroclimate dynamics over the past 1300 years. Between ca 690 and 950 AD , the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near‐complete desiccation triggered massive trona precipitation. Between ca 950 and 1100 AD , slightly higher water levels allowed the build‐up of high pCO ₂ leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350 AD , only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800 AD , the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria.     

Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana) eastern Rift Valley, Kenya

Tertiary sediments around Lake Rudolf (now Lake Turkana) in the East African Rift Valley have yielded abundant palaeontological and palaeoanthropological remains. The present study provides a basis for interpreting the ancient lake environment and furthering our knowledge of rift valley lacustrine deposits. Bottom sediments in Lake Rudolf are fine-grained (average 71% clay) well laminated and have montmorillonite, kaolinite and illite as the principal clay minerals. The sediments are relatively poor in silica (40–45%) but rich in Fe₂O₃ (10%). Both mineral proportions and chemical composition change systematically over the area of the lake and delineate four sedimentological provinces: (1) iron-rich, silty kaolinitic muds (Omo Delta); (2) iron-rich, fine-grained montmorillonite muds (North Basin); (3) silty montmorillonite muds rich in Na₂O and K₂O (Central Delta); and (4) argillaceous calcite silts (South Basin). Omo Delta and North Basin sediments are derived from the volcanics of the Ethiopian plateau; the source of Central Delta sediments is the Precambrian metamorphic terrain of the rift valley margin; the South Basin has a restricted detrital input. The water in the lake is alkaline (pH 9.2) and moderately saline (TDS = 2500 p.p.m.). Comparisons with influent water from the Omo River indicate a 200-fold concentration for the lake water. Models based on equilibrium between sediments and water column account for most of the non-conservative chemical components in the lake water. Sedimentation rates are high (about 1 m per 1000 years) and the dominance of detrital sediments makes Lake Rudolf unusual in comparison with other closed-basin lakes in the African Rift Valley although some similarities with ancient rift valley deposits are suggested.     

Lake Bogoria, Kenya: soda, hot springs and about a million flamingoes

Lake Bogoria is a saline, alkaline, meromictic lake in a geothermally active part of the Kenya Rift Valley. Coring of the lake floor has shown two types of sedimentation – a shallow fan–deltaic clastic zone and a deeper zone with alternating organic muds and evaporites. The organic muds formed during periods of relatively high lake level and high microbial productivity, the evaporites during more arid phases. Analyses of the cores show many environmental fluctuations during the past 30000 years, related to regional climatic changes and to local tectonic and hydrological controls.     

Major and trace element geochemistry of Lake Bogoria and Lake Nakuru,Kenya, during extreme draught

The physico-chemical properties of water samples from the two athalassic endorheic lakes Bogoria and Nakuru in Kenya were analysed. Surface water samples were taken between July 2008 and October 2009 in weekly intervals from each lake. The following parameters were determined: pH, salinity, electric conductivity, dissolved organic carbon (DOC), the major cations (FAAS and ICP-OES) and the major anions (IC), as well as certain trace elements (ICP-OES). Samples of superficial sediments were taken in October 2009 and examined using Instrumental Neutron Activation Analysis (INAA) for their major and trace element content including rare earth elements (REE). Both lakes are highly alkaline with a dominance of Na > K > Si > Ca in cations and HCO₃ > CO₃ > Cl > F > SO₄ in anions. Both lakes also exhibited high concentrations of Mo, As and fluoride. Due to an extreme draught from March to October 2009, the water level of Lake Nakuru dropped significantly. This created drastic evapoconcentration, with the total salinity rising from about 20‰ up to 63‰. Most parameters (DOC, Na, K, Ca, F, Mo and As) increased with falling water levels. A clear change in the quality of DOC was observed, followed by an almost complete depletion of dissolved Fe from the water phase. In Lake Bogoria the evapoconcentration effects were less pronounced (total salinity changed from about 40‰ to 48‰). The distributions of REE in the superficial sediments of Lake Nakuru and Lake Bogoria are presented here for the first time. The results show a high abundance of the REE and a very distinct Eu depletion of Eu/Eu* = 0.33–0.45.     

Listric growth faults in the Kenya Rift Valley

Many of the major faults in the Kenya Rift Valley are curved in section, were active over considerable periods and form sets which are related in space and time. They can, therefore, be regarded as systems of listric growth faults. The Elgeyo Fault marks the western limit of rift structures at this latitude and displaces the basement surface by up to about 6 km. The Kamasia Hills are a block rotated above this fault plane. Movement on the Elgeyo Fault has been grossly continuous since at least 16 Ma ago but deposition of volcanics and sediments has generally kept pace with the growth of the escarpment. The Kaparaina Arch is a rollover anticline on the downthrown side of the Saimo Fault on the eastern side of the Kamasia Hills. On the eastern side of the rift, the block between the Bogoria and Wasages-Marmanet Faults has shown continued rotation since about 15 Ma. The Pleistocene lavas on the rift floor here show rollover into the Bogoria Fault and have formed a facing near the top of the escarpment. Area balancing calculations suggest depths to décollement of 25 km for the Elgeyo Fault, 6 km for the Saimo Fault and 12 km for the Bogoria Fault. The most direct evidence for the listric nature of the faults is provided by microearthquakes near Lake Manyara which appear to lie on fault planes connected to surface escarpments.     

Geochemistry of High-silica Peralkaline Rhyolites, Naivasha, Kenya Rift Valley

The Recent (<15000 y) volcanic complex of southwest Naivasha,Kenya, consists of mildly peralkaline (comenditic) rhyolitedomes, lava flows, air fall pumices, and lake sediments, withminor, peripheral, basalts and hawaiites. The comendites areeither aphyric or sparsely porphyritic, few samples containing>5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon.Ferrohedenbergite and zircon are restricted to less peralkaline,and amphibole, aenigmatite, and biotite to more peralkaline,rocks. The comendites show unusually strong enrichment in Cs, F, Hf,Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletionin Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched,with large, negative Eu anomalies. Values of LIL/HFS elementratios, such as Th/Ta and Rb/Zr, are unusually high for peralkalinerhyolites, and are consistent with a substantial crustal componentin the comendites. Parameters such as LREE/HREE and Zr/Nb ratiosindicate that the Naivasha rhyolites represent several pulsesof closely related, but subtly different, magmas. Sanidine/glasspartition coefficients for Ba, Pb, Rb, Sr, U, and the REE arepresented for one specimen. Major and trace element modelling, and feldspar-rock relationships,show that closed system crystal fractionation cannot alone accountfor the overall compositional variations in the comendites.A model involving partial melting of variable crustal sourcerocks and migration of dissolved volatile-metal complexes maybe appropriate at Naivasha.     

The geology and petrology of the Hannington Trachyphonolite formation,Kenya Rift Valley

Field, petrographical and geochemical studies of a group of late Pleistocene, alkaline and mildly peralkaline trachytic and trachyphonolitic lavas from the northern Kenya Rift have been undertaken. A large number of flows were erupted from widely dispersed centres to form an extensive volcanic shield within the floor of the rift. The major element composition of most rocks was substantially modified during crystallisation, but other data show that differentiation within the suite was the result of protracted feldspar fractionation of a trachytic magma with intially very low abundances of residual trace elements.     

Neotectonics and extension direction of the Southern Kenya Rift, Lake Magadi area

The Magadi area, located in the southern part of the Kenya Rift, is a seismically active region where rifting is still in progress. The recent tectonic activity has been investigated through a seismological survey and the study of neotectonic joints found in Lake Magadi sediments, which were deposited some 5000 years ago. The structural analysis of these open fractures was combined with a quantitative analysis of the orientation and size characteristics of imagery faults. The gathered data demonstrate (1) that the majority of the systematic joints have straight and parallel trajectories with a common en echelon mode of propagation displayed through a rich variety of patterns, and (2) that there is a self-similarity in fault and joint principal directions recognised at the different telescopic scales. SPOT image (1:125,000), aerial photos (1:76,000), and outcrop fieldwork reveal two important structural orientations which are N015°E and N015°W. The N015°E regional direction is consistent with the orientation of the southern segment of the Kenya Rift. Structural analysis is supported by results of a joint microseismic investigation in the Lake Magadi area. Obtained focal mechanism solutions indicate an E–W to ESE–WNW normal faulting extension direction.     

Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley

Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO₂ conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalite–magnetite–quartz buffer)and at high fF₂ inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite [molar (Na₂O+ K₂O)/Al₂O₃ (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A     

Some constraints on the origin of phonolites from the Gregory Rift,Kenya, and inferences concerning basaltic magmas in the Rift System

Plateau-type phonolites of the Gregory Rift represent magmas with densities of about 2.3 g/cm³. These magmas must have erupted soon after their formation. The average depth of the bases of the approximately 100 to 300 magma chambers from which they erupted was between about 10 and about 24 km. The average vertical extent of the magma chambers was between about 3 and about 8 km. The aggregate volume of phonolitic magma formed beneath the Rift in Miocene times probably lies in the range of 0.5 to 1×10⁵ km³. Both the crystal fractionation model and the anatectic model for formation of this volume of phonolitic magma require the presence of a large reservoir of basaltic magma, probably picritic in character, with a volume of at least 5×10⁵ km³ and perhaps as much as 20×10⁵ km³. This reservoir presumably is now part of the dense basic intrusive complex along the Rift axis. The Miocene and Pliocene episodes of basaltic volcanism in this region may be related to eruption of evolved liquids from this reservoir.     

The origin of high bicarbonate and fluoride concentrations in waters of the Main Ethiopian Rift Valley, East African Rift system

Thermal waters in the Main Ethiopian Rift Valley are characterized by high Na, bicarbonate and fluoride concentrations, and near-neutral to alkaline pH. Sodium, bicarbonate and fluoride are positively correlated in the waters. The principal reason for the bicarbonate in the area is the high rate of carbon dioxide outgassing. This, combined with acid volcanics, geothermal heating, low Ca and low salinity, is also one of the causes of high fluoride in this part of the active volcanic zone of the East African Rift. Evaporative concentration is responsible for the high salinity, alkalinity and fluoride in the closed-basin lakes of the region. The waters are undersaturated with respect to fluoride and anhydrite. Calcium tends to be fixed in Ca bearing minerals such as calcite and epidote, which are abundant in the system. Hence, it appears that fluoride is a mobile component in acid volcanic geothermal systems.     

Geochemical trends through time and lateral variability of diatom floras in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

The Olorgesailie Formation (1.2-0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin (Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8-9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200-16,000 μS cm^− 1 and pH of 7.5-9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution.     

The Mineralogy and Petrology of Mount Suswa, Kenya

Mount Suswa, a Quaternary volcano in the Rift Valley of Kenya,is composed of sodalite-trachytes, sodalite-phonolites, andphonolites, the majority of which are mildly peralkaline. Thelavas are predominantly feldspathic with phenocrysts of alkalifeldspar and, less commonly, slightly sodic augite and fayaliticolivine. The groundmass typically contains alkali feldspar,augite, titanomagnetite, and may in addition contain sodalite,nepheline, alkali amphibole, aenigmatite, and glass. The lavasof four stratigraphically distinct episodes can be distinguishedon the basis of mineralogy and chemical composition. These lavasare the products of at least three parental magmas, none ofwhich appears to be a derivative of the other. Each ‘magmatype’ represents an independent episode of magma generation,emplacement, and eruption. The order of eruption in the finalepisode corresponds to increasing peralkalinity and undersaturationwith respect to silica, and indicates that these lavas weregenerated via the tapping of a differentiating magma, with thefirst lavas being the least differentiated. Utilizing coexistingfeldspar, residual glass, and bulk rock compositions, the derivationof peralkaline phonolitic residual liquids from a trachyticparent is shown to be a process controlled by feldspar fractionation.     

Ground subsidence and its socio-economic implications on the population: a case study of the Nakuru area in Central Rift Valley, Kenya

 Several areas of Nakuru Town and its environs often undergo subsidence along the parallel fault zones during and after heavy rainfall. During the rainy season, when most of the subsidence occurs, the overlying unconsolidated volcanoclastic sediments become oversaturated with water. The water reduces the shear strength of the sediments and also introduces extra loading through saturation leading to subterranean erosion along faults. The unconsolidated sediments then collapse into the subsurface water channels which closely follow the fault zones, leading to formation of “sinkholes”. The frequent incidences of ground subsidence in the study area, have caused several fatalities, destroyed settlements and physical infrastructure. Furthermore persistent subsidence has increased the cost of construction and the repair of the destroyed properties. Apart from being hazardous, ground subsidence degrades environment when sewage water, refuse and garbage enter into the groundwater systems through the sinkholes. The fissures formed after subsidence also stand prominently as ugly features from the rest of the terrain. Mitigation measures including control, channelizing of drainage, proper engineering practices and appropriate land use are suggested in this paper. Received: 1 December 1998 · Accepted: 8 March 1999     

东非裂谷东支肯尼亚裂谷形成演化

东非裂谷东支肯尼亚裂谷包括5个重点凹陷,除South Lokichar凹陷外,其余凹陷均无发现。肯尼亚裂谷的形成演化及动力机制认识不清是制约油气勘探的关键。通过系统分析肯尼亚裂谷不同凹陷形成年代、裂谷类型和动力机制、构造演化和沉积充填过程,得到以下几点认识:(1)South Lokichar凹陷形成时间早,地层年代老。South Lokichar凹陷主要发育晚渐新世-中中新世地层;Turkana、Kerio及North Lokichar凹陷以中中新世-全新世地层为主;Kerio Valley凹陷主要发育中-晚中新世地层;(2)阿法尔地幔柱和肯尼亚地幔柱的隆升控制肯尼亚裂谷的形成演化。South Lokichar凹陷的形成演化主要受阿法尔地幔柱控制;其余4个凹陷主要受肯尼亚地幔柱控制;(3)不同类型的凹陷构造演化和沉积充填差异较大。South Lokichar凹陷与Kerio Valley凹陷为被动型裂谷,构造演化经历初始裂陷期、主裂陷期和裂陷后期三个阶段,在主裂陷期发育优质烃源岩;Turkana、Kerio及North Lokichar凹陷为主动型裂谷,构造演化包括火山事件活动期和间歇...