lithostratigraphy volcanism paleomagnetism and palynology of quaternary lacustrine deposits from barombi mbo west cameroon preliminary results we present preliminary results from the study of 23 50 m core from lake barombi mbo representing the last 25 000 years the lake is in an explosion crater formed during quaternary time the very laminated sediment is composed mostly of clay containing 5 10 organic carbon each couplet is commonly composed of a basal lamina rich in quartz plant debris muscovite and sponge spicules and of a more clayey upper lamina often with siderite a perturbed section near the base of the core before ca 21 000 yr b p could be the result of a violent release of gas such co2 comparable to the recent nyos gas eruption the paleomagnetic studies exhibit high frequency oscillations interpreted as paleosecular variations of the local geomagnetic field this first record obtained on the african continent can be closely compared to the type record obtained in western europe the pollen results demonstrate the presence of a forest refuge in west cameroon during the last great arid period ca 18 000 yr b p when equatorial forest was broken up elements of montane vegetation spread to the lowlands these phenomena resulted from a drying and cooling climate

towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

origin of the lethal gas burst from lake monoun cameroun abstract on 15 august 1984 a lethal gas burst issued from a submerged 96 m deep crater in lake monoun in cameroun western africa killing 37 people the event was associated with a landslide from the eastern crater rim which slumped into deep water waters below 50 m are anoxic dominated by high fe 2 600 mg l and hco 3 1900 mg l anoxic and supersaturated with siderite which is a major component of the crater floor sediments the unusually high fe 2 levels are attributed to reduction of laterite derived ferric iron gradually brought into the lake as loess and in river input sulfur compounds are below detection limits in both water and gas gases effervescing from depressurized deep waters are dominantly co 2 with minor ch 4 having 13 c of 7 18 and 54 8 per mil respectively bacterial decomposition of organic matter may account for the methane but 14 c of lake water indicates that only 10 of the carbon is modern giving an apparent age of 18 000 years the dominant source of carbon is therefore attributed to long term emission of co 2 as volcanic exhalation from vents within the crater which led to gradual build up of hco 3 in the lake the density stratification of the lake may have been upset by an earthquake and underwater landslide on 15 august which triggered overturn of the lake and caused nucleation of co 2 in the deep water the resultant ebullition of co 2 from deep lake waters led to a gas burst at the surface and locally generated a water wave up to 5 m high people travelling through the gas cloud were asphyxiated presumably from co 2 and suffered skin discoloration from unidentified components

radiometric studies of lake nyos cameroon sediments evidence of strong mixing and excess 210pb abstract a one meter long sediment core was extracted from the middle of lake nyos six months after the august 1986 gas burst disaster major and trace elements have been determined in twenty samples including alpha and gamma spectrometry for the radioisotopes preliminary results show that 137 cs is present in uniform concentrations throughout the core the measured activity 7 3 bq kg 1 corresponds to a total deposit of approximately 1800 bq m 2 in the one meter thick sediment layer similar in magnitude to the average amounts deposited at these latitudes by atmospheric thermonuclear tests 210 pb with a half life of 22 years is also uniformly distributed showing a measured activity of 233 bq kg 1 three to four times higher than the values observed in other cameroon lakes the activity ratio of 210 pb to 226 ra is about 4 these data are interpreted as follows the uniformly distributed 137 cs and 210 pb concentrations suggest complete mixing of the top of sediments at the sample location from the amount of deposited 137 cs found in the core this sediment mixing process does not appear to have involved significantly more than that top layer the 210 pb excess not supported by 226 ra can be explained by an upwards release of radon gas 222 rn prior to the co 2 burst

late quaternary palaeoenvironments in the lake barombi mbo west cameroon deduced from pollen and carbon isotopes of organic matter abstract a sediment core 23 5 m long was recovered from a water depth of ca 110 m in lake barombi mbo a maar crater of the cameroon volcanic chain this paper presents a carbon isotopic curve of organic matter linked to the main results of the pollen analysis interpretation of this carbon isotopic curve is simplified 1 because of the weak role played by the phytoplankton in the balance of the organic particle flux and 2 because of the presumed absence of diagenetic alteration of the isotopic composition the carbon isotopic curve exhibits an almost linear correlation with that of total grass pollen which form the main part of the c4 biomass the major fluctuations of isotopic and pollen curves allow four main phases to be distinguished from ca 25 000 to 20 000 yr b p 13c values of 25 to 30 pdb are related to a forest environment associated with a montane floral element from 20 000 to 13 000 yr b p the 13c values are between 23 and 28 and are linked to openings of landscape with a mosaic of forest and savanna and a fall of lake level with colonization of the shores by cyperaceae and other aquatic plants from 13 000 to 10 000 yr b p the forest extended again reaching a maximum density during the period from 9500 until 3000 yr b p and with an average 13c value of 32 a new dry phase at around 2500 2000 yr b p with temporary openings in the forest marked by a 13c excursion above 30 but during the last 2 millennia the forest developed again with 13c values of 32

palaeosecular variations of the geomagnetic field in africa during the holocene a review the importance of the full understanding of the holocene geomagnetic field spans from human history and archaeology to palaeoclimatic changes and engineering to geomagnetic field modelling and unravelling of the geomagnetic field characteristics and anomalies unfortunately the dearth of data from large under covered areas such as oceans the african and south american continents and the southern hemisphere only 4 6 of the global datasets dramatically limits our knowledge of the temporal and spatial evolution of the geomagnetic field and its application here a review of all data from the african continent is presented in order to encourage and motivate a new generation of palaeomagnetic and archaeomagnetic studies new data will sharpen the palaeomagnetism as a dating tool improve our knowledge of local global geomagnetic features and will help to finally answer some of the fundamental questions in palaeomagnetism like the temporal and spatial distribution of the palaeointensity peaks described and the origin and evolution of the south atlantic anomaly 2020 the author s published by the geological society of london all rights reserved

middle to late quaternary sediment flux and post depositional processes between the continental slope off gabon and the mid guinean margin abstract two cores from the continental slope off gabon 2330 m of water and one core from the mid guinean marigin 400 m of water were studied in order to examine climatic and paleoceanographic fluctuations during the middle and late quaternary oxygen isotope variations in one shallow core were correlated with global isotope stages and extended to the two other cores on the basis of microfaunal and mineralogic contents interglacial stages have been characterized by high carbonate content low carbonate dissolution effect and moderate flux of organic matter derived from primary oceanic production glacial stages have included dissolution processes on carbonate tests related to increased co2 content near the seabed due to the sink of organic matter from both planktonic and benthic fauna at a depth of 4000 m of water this phenomenon has controlled the vertical carbonate distribution through time many interglacial intervals record high terrigenous inputs increased quartz and kaolinite content while some glacial intervals contain peaks of both well crystallized smectites probably reworked from mesozoic cenozoic outcroppings on the adjacent continental shelf and poorly crystallized smectites land derived at a depth of 4000 m of water quartz rich layers deposited during some glacial intervals include surface textures indicative of an aeolian origin a long term climatic change is recognized consisting of a progressive cooling of sea surface waters and of climatic conditions since 400 000 350 000 yrs b p

correction for the siderite effect on rock eval parameters application to the sediments of lake barombi southwest cameroon abstract originally developed for use in the petroleum industry rock eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions the standard rock eval parameters provide information on the amount of total organic and inorganic carbon toc and minc respectively and are usually interpreted as proxies for the source aquatic or terrestrial of the primary production of organic matter hydrogen index vs oxygen index although this method usually provides valuable evidence the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter som indeed the co2 and co released by the pyrolysis of siderite are integral to the calculation of the som related standard rock eval parameters in this study we analyze sediments from a core collected in the lake barombi southwest cameroon and describe the impact of siderite on standard rock eval parameters we propose a workflow that allows standard rock eval parameters to be corrected based on the analysis of thermograms the proposed corrections provide siderite effect free parameters accurately reflecting the changes in sedimentary organic matter composition

bulk composition of sedimentary organic matter used in palaeoenvironmental reconstructions examples from the tropical belt of south america and africa abstract the organic matter of lake sediments contains information that allows for the reconstruction of past environmental and climatic changes the elemental composition and isotopic characteristics of sedimentary organic matter enable discrimination between allochthonous vegetal matter from within the catchment and autochthonous phytoplanktonic and fixed algae components this approach makes it possible to distinguish between the inputs of organic matter from c 3 vs c 4 type land plants sedimentary records from south america and africa provide examples of applications of organic bulk composition in palaeoenvironmental reconstructions the records from two african lakes show that during the last glacial maximum lgm the distribution of c org n org 13 c pairs indicates the dominance of organic material having a c 4 terrestrial origin these inferences are supported by pollen analysis which indicates opening up of the forest at both sites consistent with drier climatic conditions than present the record from siberia lake bolivia ca 3000 m altitude shows that during the lgm the sedimentary organic matter is dominated by the phytoplanktonic fraction as indicated by the c org n org 13 c pairs the petrographic analyses show that this material is not well preserved suggestive of drier conditions consistent with interpretations from the pollen record the beginning of the holocene is marked by a shift from a scattered c 4 signal to one with a greater terrestrial c 3 and aquatic phytoplanktonic influence these results indicate climatic amelioration towards more humid and warmer climatic conditions leading to forest development in the lake catchment together with increasing lake levels conducive to greater phytoplankton production these conditions persisted during the holocene as shown by the c org n org 13 c pairs in all lakes with a terrestrial c 3 signal predominating at the carajas and siberia lakes only dom helvecio lake shows a different pattern during the beginning of the holocene in the distribution of c org n org 13 c pairs this pattern indicates that the sedimentary organic material is dominated by c 4 terrestrial material coming from savanna expansion in the lake catchment during a period of relative aridity in central brazil

the nature and chronostratigraphy of quaternary pyroclastic accumulations from lake barombi mbo west cameroon abstract the k ar dating of a limburgitic flow indicates that the barombi mbo maar was active a million years ago the morphology of the maar and the unusual high k 2 o na 2 o ratio of the limburgite suggest either that this age is too old or that this flow is not the last activity of the maar twenty three meters of sediments were cored from the bottom core bm6 of the 110 m deep lake which currently fills the maar they are composed of clayey and sideritic laminae with interstratified ash layers in the lower half of the core these ashes composed of slightly vesiculated sideromelane with olivine and labradorite phenocrysts have been dated between 21 000 and 11 000 yr b p they show no alteration and correspond to aerial fallout of alkaline sodic basaltic affinity which is common in the area but are distinct from the mafic magmas which generated the maar the origin of the tilting and uplifting of a 1 5 m sedimentary column in the lower part of the core remains controversial holocene age pyroclastites are rare and consist of a mixture of granulitic xenocrysts and typically transitional tachylite similar to volcanics from the northeast manengouba magnetite microspherules occurring in holocene layers have no volcanic equivalent they could be related to biogenic or pedogenetic processes induced by more humid climatic conditions

hydrologic properties of coal beds in the powder river basin montana i geophysical log analysis as part of a multidisciplinary investigation designed to assess the implications of coal bed methane development on water resources for the powder river basin of southeastern montana six wells were drilled through paleocene age coal beds along a 31km east west transect within the tongue river drainage basin analysis of geophysical logs obtained in these wells provides insight into the hydrostratigraphic characteristics of the coal and interbedded siliciclastic rocks and their possible interaction with the local stress field natural gamma and electrical resistivity logs were effective in distinguishing individual coal beds full waveform sonic logs were used to determine elastic properties of the coal and an attendant estimate of aquifer storage is in reasonable agreement with that computed from a pumping test inspection of magnetically oriented images of the borehole walls generated from both acoustic and optical televiewers and comparison with coal cores infer a face cleat orientation of approximately n338e in close agreement with regional lineament patterns and the northeast trend of the nearby tongue river the local tectonic stress field in this physiographic province as inferred from a nearby 1984 earthquake denotes an oblique strike slip faulting regime with dominant east west compression and north south extension these stress directions are coincident with those of the primary fracture sets identified from the televiewer logs and also with the principle axes of the drawdown ellipse produced from a complementary aquifer test but oblique to apparent cleat orientation consequently examination of these geophysical logs within the context of local hydrologic characteristics indicates that transverse transmissivity anisotropy in these coals is predominantly controlled by bedding configuration and perhaps a mechanical response to the contemporary stress field rather than solely by cleat structure published by elsevier b v

liquid carbon dioxide of magmatic origin and its role in volcanic eruptions natural liquid carbon dioxide is produced commercially from a 2 5 km deep well near the 4 500 yr old maar volcano mount gambier south australia the carbon dioxide has accumulated in a dome that is located on the extension of a linear chain of volcanic activity a magmatic origin for the fluid is suggested by the geological setting 13cpdb of 4 0 for the co2 where pdb represents the carbon isotope standard and a relatively high 3he component of the contained helium and high 3he c ratio 6 4 x 10 10 the 3he 4he and he ne ratios are 3 0 and 1 370 times those of air respectively the co2 as collected at the earth s surface at 29 5 c and 75 bar expands more than 300 fold to form a gas at 1 atm and 22 c we suggest that liquid co2 or high density co2 fluid the critical point is 31 1 c 73 9 bar of volcanic origin that expands explosively from shallow levels in the earth s crust may be a major contributor to phreatic volcanic eruptions and maar formation less violent release of magmatic co2 into crater lakes may cause gas bursts with equally disastrous consequences such as occurred at lake nyos cameroon in august 1986

electrofacies analysis using high resolution wireline geophysical data as a proxy for inertinite rich coal distribution in late permian coal seams bowen basin this paper examines the stratigraphic and geographic distribution of coal composition in main late permian coal measures in the bowen basin using coal electrofacies as a proxy for inertinite maceral group content data for this research were derived from some 26 wells in the northern part of the bowen basin a companion paper roslin and esterle 2015 in review introduced the methodology which is based on electrofacies analysis and uses high resolution wireline data including microresistivity from compact micro image cmi tool and photoelectric factor pef data in addition to conventional gamma ray density and laterolog resistivity to obtain coal electrofacies validation of the methodology was performed by comparison of the weighted average proportion of coal electrofacies to the weighted average proportion of the corresponding coal lithotypes obtained from millimetre scale logging and to the percentage gathered from maceral content analysis the weighted average proportion of the interpreted dull inertinite rich coal electrofacies independent of rank and heat effects was then analysed to determine their stratigraphic and geographic distribution from the base upward the main late permian coal bearing units are the moranbah coal measure mcm fort cooper coal measure fccm and rangal coal measure rcm and their stratigraphic equivalents across the basin the proportion of inertinite rich dull coal electrofacies increases upwards in the rcm with some thick and merged seams showing distinctive electrofacies signatures within the study area the coal measures and individual seams split from north to south reflecting increased subsidence and sediment influx into basin depocentres as the seams split the proportion of inertinite rich dull coal electrofacies decreases in the tested sample set

a preliminary palaeomagnetic study of the holocene and late wurmian sediments of lac du bouchet haute loire france summary lac du bouchet is a small maar type lake located at 45 n 4 e in central france the bottom sediments have been sampled down to a depth of 6 m with mackereth type corers a chronology has been established from pollen analytical and diatom zonations and by radiocarbon age determinations throughout the period covered which runs from 22 000 years ago through to the present time declination and inclination variations consist of oscillations of up to about 30 amplitude no abnormally large amplitude excursions in direction are recorded the main contribution of this study is the extension of the european secular variation sv records to the interval from 13000 bp years before present to 22000 bp the sediments deposited through this interval ancient dryas intc wurm originated from the erosion of the internal walls of the volcano spectrum analyses of the declination and inclination curves from 13000 to 22000 bp reveal peaks at periods of 3000 4000 1800 1150 and 700 yr cross spectrum analysis shows strong coherence of the two signals at the longest of these periods the phase relationship of all these peaks indicates clockwise rotation of the palaeomagnetic vector possibly suggesting westward drifting sources although the palaeomagnetic sv record through the holocene 0 10300 bp is compressed into 1 7m of highly organic sediments the principal features exhibited by the type curves established for uk can be identified and the resulting magnetic ages are consistent with the palynology the sense of rotation of the magnetic vector is biased equally in both senses the late glacial 10300 bp to 13000 bp is characterized by oscillating climatic conditions recent dryas allerod and bolling which resulted in poorly resolved and probably discontinuous palaeomagnetic sv records which show a strong bias to anti clockwise rotation all ages are given in radiocarbon years

sensitivity analysis in permeability estimation using logging and injection falloff test data for an anthracite coalbed methane reservoir in southeast qinshui basin china abstract this paper presents a sensitivity analysis in permeability estimation by interpreting injection falloff test and logging data for a producing coalbed methane field in southeast qinshui basin injection falloff test data are interpreted by an infinitely acting constant compressibility and radial homogeneous model log interpretation is based on the archie equation using data of shallow laterolog resistivity the cementation factor of coal is calculated using laboratory measurements based on the interpreted cementation factors and the assumed fracture geometries the sensitivity in permeability estimation using logging data is analyzed results show that the permeabilities are 0 041 0 012 and 0 035 md for coal seam 3 in wells w 1 w 5 and w 6 respectively from the interpretation of injection falloff test data the calculated coal cementation factors range widely from 0 9 to 1 8 with average of 1 3 in order to match the log interpretation results with those from injection falloff tests different fracture spacing is used in log interpretation for the three wells the fracture spacing varies from 0 39 to 0 67 mm and the average fracture aperture varies from 0 229 to 0 398 m for the three wells the cementation factor has the highest impact on estimation of permeability if the cementation factor mud filtrate resistivity and fracture spacing are varied with 10 from the base case effect of fracture aperture s heterogeneity on permeability is depending on the standard deviation and average of fracture aperture effective permeability has a stronger relationship with geometric mean of face and butt cleats aperture than with total porosity for different face and butt spacing models

palaeomagnetic study of the holocene and upper pleistocene sediments from lake barombi mbo cameroun first results abstract the analyses of the stable deposition and post depositional remanent magnetization carried by the lacustrine sediments contained in a 24 m sequence collected from the bottom of the maar lake barombi mbo cameroun yield well defined declination and inclination oscillations which can be attributed to palaeosecular variations of the local geomagnetic field the transformation to a time scale based on the interpolation and extrapolation of 14 c ages allows comparison of this record with contemporaneous records obtained in other parts of the world the successive inclination peaks can be individually identified with those found in western europe in spite of small chronological discrepancies which are in the range of the standard deviation of the ages normalization of the nrms by anhysteretic magnetization provides a profile of relative palaeointensities which shows similarities with that reconstructed from the same time interval in a sequence from western europe although an important phase shift appears

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

sedimentation and palaeoenvironment in crater lake barombi mbo cameroon during the last 25 000 years lake barombi mbo is situated in a maar crater of the cameroon volcanic chain sediment cores were recovered with a livingstone corer this paper presents data on core bm6 23 5 m long from a water depth of 110 m the sediments are mainly organic matter rich clays 5 to 10 total organic carbon showing alternating brown and yellowish horizontal laminae on a cm scale the lower part of a couplet basal sublamina is rich in siliciclastic quartz mica grains sponge spicules and documents the deposition of flood detritus the upper unit upper sublamina is composed of grey to bluish clay which becomes green toward the top with frequently yellow layered concretions of siderite it corresponds to quiet conditions with settling of fine clayey particles from suspension the overall recurrence rate of these laminae is about 15 years this frequency is similar to present day large flood events in this intertropical zone from 14 000 to 20 000 yrs bp the sediment accumulation rate was quite low 15 30 g cm 2 10 3 yrs with less abundant precipitation pedogenic clay formed from mica family minerals from 12 000 yrs bp throughout the holocene the rate of accumulation increased 40 g cm 2 10 3 yrs and increasing precipitation favoured the pedogenic formation of kaolinites turbidity currents became less important to the overall sedimentation plankton production remained low documented by organic matter which is more of the exinite than the alginite type siderite concretions occur as early diagenetic products possibly related to alkalinity in anoxic waters at the sediment water interface which limited the formation of fe3 thin ash layers occur between 11 5 and 18 m depth in core bm6 bearing witness to several volcanic events during the late quaternary sediments are disturbed below 21 m with folded laminations and inverse ages these probably result from a slump glide of lower slope sediment but could also signal a possible gas event

phreatomagmatic deposits and stratigraphic reconstruction at debunscha maar mt cameroon volcano the debunscha maar dm is located on the southwest flank of mount cameroon an active stratovolcano on the cameroon volcanic line cvl here we present the physical characteristics of the pyroclastic deposits at dm with the aim of deciphering tephra emplacement mechanisms evolution of water magma interaction and reconstructing the stratigraphy beneath the maar from gps measurements the crater has long and short axes of 500 m and 320 m respectively generally the pyroclastic deposits are well stratified and present a variety of depositional bed forms including structureless massive beds massive beds with faint internal stratifications inversely graded beds lens shaped units impact sags cross lamination planar beds as well as dune like beds clast sizes include ash lapilli tuff bombs and blocks pyroclastic breccia with clast lithologies consisting of entrained lithics of porous ankaramite pillow lavas lithified sediments sandstone and shale and juvenile material the porous ankaramite pillow lavas have glassy margins and vesicle zonations typical of pillow lavas formed by subaqueous eruption the pillow fragments are more common in early formed eruption products at the base of the deposit the lithified sandstones show planar laminations and together with the shales occur predominantly in stratigraphic positions above the ankaramite pillow lavas the juvenile materials include basaltic bombs with low vesicularity b15 and moderate vesicularity 15 50 the bombs have chilled surfaces and their abundance increases towards the top of the deposit the presence of accretionary lapilli fragments of country rock and juvenile clasts with ragged surfaces as well as curved and chilled margins is unambiguous evidence in support of phreatomagmatic activity of the observed lithic clasts only the pillow lavas would appear to have the porosity necessary to furnish the required amount of water to feed the phreatomagmatic maar eruption the clast stratigraphy suggests that the maar is underlain by ankaramite pillow lava that erupted on a consolidated sedimentary substratum studying deposits resulting from maar eruptions has a direct implication for hazards assessment at areas of active maar volcanism because many surface processes occur around such volcanoes well after the eruptive activities have stopped

ecological dynamic equilibrium in an early miocene 21 73 ma forest ethiopia abstract miocene paleoecology of east africa has implications for human origins and understanding the vicariant legacy forests found today on either side of the east african rift fossil leaves preserved in 21 73 million year old lacustrine sediments from the mush valley ethiopia provide a unique opportunity to investigate forest composition and dominance diversity patterns at an ecological scale we classified and analyzed 2427 leaves in total from two to three quarries within each of six stratigraphic levels spanning 7 m of section we estimate each quarry census represents one to three centuries and 50 60 kyrs separate the oldest and youngest levels pollen phytolith and compound specific organic geochemical data were also collected in a detailed stratigraphic context to provide independent integrated lines of evidence for landscape evolution and lacustrine paleoecology of the system that preserves the macrofossils forty nine leaf morphotypes were documented and legume 1 dominated all samples nonmetric multidimensional scaling jaccard similarity analyses and diversity and evenness indices demonstrate a degree of change comparable to community ecology dynamics likely illustrating a dynamic stable state in forest vegetation surrounding the lake taxonomic assessments of leaves phytoliths and pollen are consistent with a closed canopy forest with limited palm diversity a high abundance of des a ring triterpenoid molecules diagenetic products formed by microbial degradation under anoxic conditions and very negative 13c values

on the current state of lake sediment palaeomagnetic research summary it is now possible to obtain high quality records of geomagnetic secular variations in the direction though not the magnitude of the ancient field type curves are presented for four regions namely north america west europe argentina and australia constructed by fitting cubic splines to data stacked from numbers of cores of post glacial sediments ways in which the results can be used to investigate the grosser properties of the ancient field are discussed

in situ stress pore pressure and stress dependent permeability in the southern qinshui basin this study focuses on the in situ stress pore pressure and permeability in the southern qinshui basin one of largest coalbed methane basins in china well tests show that permeability in this basin is higher than other coalbed methane reservoirs this is because it is located in an extensional basin where the normal faulting stress regime is dominated this in situ stress regime is advantageous to keep coal cleats open hydraulic fracturing tests indicate that the fracture gradient or minimum horizontal stress is much lower than the shales in the gulf of mexico and other oil basins the minimum horizontal stress model is proposed with consideration of the stress coefficient based on the uniaxial strain method this model provides a fairly good prediction on the minimum stress permeability data show that the effective stress dependent permeability is pronounced in the coalbed methane reservoir this is significant for the dual porosity and dual permeability coal reservoirs which consist of coal porous matrices and cleats the reason is that a rapid increase in effective stress can induce the closure of cleats which may cause a permanent loss of permeability in the cleats this reduces the connectivity between the cleats and coal matrices hence the coal matrices cannot deliver gas pressure to the cleats for supporting the cleat space therefore slowing down the effective stress change during production e g slowing reservoir drawdown can decelerate the permeability reduction this is particularly important for the reservoir in which the pore pressure is not significantly overpressured such that in the southern qinshui basin

pollen evidence of late quaternary vegetation and inferred climate changes in congo a detailed palynological analysis of two cores recovered from a swamp in the southern bateke plateaux congo provides information on the botanical history of this region during the last 24 000 yr b p rich and well diversified pollen counts exhibit changes in the vegetational communities related to hydrological and climatic fluctuations around 24 000 yr b p the occurrence of hydromorphous forest elements gives evidence of a humid phase between 24 000 and around 13 000 yr b p swampy herbaceous communities expanded indicating slightly drier climatic conditions from approximately 13 000 yr b p onward the beginning of a humid episode is reflected by the development of mesophilous forests the decrease of these forests is documented since 3000 yr b p grasslands are found to extend locally whereas elaies guineensis oil palm spreads out indicating both climatic change but also anthropogenic activities

identification of coal structures using geophysical logging data in qinshui basin china investigation by kernel fisher discriminant analysis abstract coal structure is closely related to the porosity and permeability of coal reservoirs which not only affects the enrichment of coalbed methane cbm but also influences the hydraulic fracturing and efficient development of cbm the accurate identification of the coal structure would be a critical issue in cbm exploration and development and is always a challenge compared with traditional methods for identifying coal structure base on borehole cores or mining seam observation geophysical logging has become the most economic and efficient technique several linear correlations have been established to describe the relationships between the coal structures and well logs however those correlations cannot accurately reflect nonlinear relations between them therefore a reliable and efficient method to identify coal structure is needed as a powerful nonlinear classifier the kernel fisher discriminant analysis kfd method has been widely used due to its strong generalization ability in this paper a new quantitative coal structure identification model was developed based on the kfd method by using geophysical logging data the model was trained tested and optimized using 178 logging data sets from 15 cbm wells in qinshui basin china the approach accounted for all the available well logging attributes and the training data sizes and kernel parameters were analyzed to get the most appropriate model in practice in addition the built model was validated individually by employing logging data of a new cbm well the results indicate that the kfd based identification model has high prediction accuracy which can be used as a reliable method for coal structure identification the kfd method exhibits strong capability during the modeling and generalization in the determination of nonlinear relationships which provides an efficient way for the coal structure prediction in basic research of coal reservoirs

evaluation of coal texture distributions in the southern qinshui basin north china investigation by a multiple geophysical logging method abstract coal texture is indicative of coal mechanical strength which is important not only for evaluating coalbed methane cbm reservoir permeability but also for evaluating the possibility of coal and gas outbursts in underground coal mining this paper begins with a foundation of a geophysical logging based coal texture evaluation method that compares the digital logging data with the results from 108 coring samples then the method was applied to the analyses of coal textures in 39 cbm wells the results of which are discussed in terms of the vertical distribution of coal textures in the study area finally a ternary diagram based on thickness proportions of coal textures was used to identify reservoir compartments in the zhengzhuang field of the southern qinshui basin the results show that undeformed coals are easily distinguished from cataclastic and granulated coals because of the high den and gr of undeformed coals meanwhile cataclastic coal has lower response values of ac and lld than granulated coal which can be used in identifying cataclastic from granulated coal a multiple geophysical logging method combining the log data of den gr ac and lld was provided for predication of the coal textures in the zhengzhuang field the predicated results show that the coal seam can be vertically divided into three to eight sub layers with a typical interbedded pattern of undeformed and cataclastic coals on the upper and lower parts and granulated coal in the middle at a relatively large thickness the study area is divided into four major regions and 11 sub regions based on the classification of coal texture types the undeformed coal dominated region is located in the southwestern part of the study area indicating a low permeability in the coal reservoir the granulated coal dominated region is mainly in the northern and eastern parts of the study area and has resulted from deep burial and multi period structural activities the cataclastic coal dominated region is in the middle of the study area the coal reservoir in this region is possibly the most favorable for cbm production

analysis of geomagnetic secular variations 10 000 to 30 000 years bp lac du bouchet france abstract cores of the bottom sediments from lac du bouchet in the haute loire france 44 9 n 3 8 e have been taken using mackereth type pneumatic piston corers the sediments have been dated by palynological control back to 15 000 years bp years before present and by radiocarbon age determinations using both conventional and accelerator methods the depth time transform provisionally used here dates the sequence from 10 000 to 30 000 years bp the stacked records of demagnetised remanent magnetisation from eight cores have been examined fourier and maximum entropy method spectral analyses show good agreement inclination and declination spectra are different in that the dominant inclination periods tend to fall in the negative part of the complex spectrum while the dominant declination periods tend to fall in the positive part the vgp path traced out by the remanent magnetisation vector shows predominantly clockwise looping consistent with essentially westward drifting geomagnetic sources the detailed form of the vgp path is affected by the degree of smoothing but clockwise looping always predominates averaging 67 over the whole time interval investigated a band of frequencies between 0 9 and 0 3 cycles per thousand years is associated with the strongest bias towards clockwise rotation 80 to 90 while a band between 0 3 and 0 1 cycles per thousand years is associated with only 50 clockwise rotation individual core records show some negative inclinations at horizons where inclination minima occur suggesting that excursions should be regarded as extreme values of secular variation rather than aborted polarity reversals of the main field