geochronology and distribution of silicic volcanic rocks of plio pleistocene age from the central sector of the main ethiopian rift abstract plio pleistocene silicic volcanoes calderas and their eruptive products are common throughout the main ethiopian rift mer a compllation of k ar and 40 ar 39 ar dating results for the central sector of the mer demonstrate that the most voluminous and widespread ignimbrites of this region are pliocene in age between 4 2 and 3 5 ma these units occur in and around 1 the awasa and wagebeta calderas 2 the guraghe and munesa escarpments where a major buried caldera is believed to occur in the adjacent rift floor 3 the addis ababa nazret area and 4 the foothills and shoulders of the northern sector of the main ethiopian rift the older units are overlain by less volumious and in most cases rift bound late pliocene and pleistocene pyroclastic rocks the pliocene ignimbrites in this area are likely to be the near source equivalents of widespread a ash fall units associated with fossil hominid deposits of the turkana and afar sedimentary basins

mobility of rare earth and other elements during crystallization of peralkaline silicic lavas abstract pantellerite lavas of the dormant caldera volcano mayor island new zealand have thick nonhydrated obsidian selvages at the tops and bases and crystalline microspherulitic interiors the flows were chemically homogeneous on emplacement but the crystalline portions have lost significant na f cl cs y and ree percentage losses of y and ree may exceed 60 ce yb ratios decrease by as much as 20 but eu eu ratios are unchanged loss of y and ree is attributed to the expulsion at near solidus temperatures of y ree 3 chloride complexes but other complexing agents may have contributed to this mobility na is also lost in the same manner groundwater has introduced mg and sr and leached or redistributed na cs and halogens analysis of hydrated glassy plinian pumice shows that secondary hydration has not affected y and ree concentrations the elements na mg f cl sr cs y and ree are highly mobile during the crystallization of peralkaline silicic melts and should not be used in petrogenetic evaluations these elements but not y and ree are also redistributed during secondary hydration processes magmas of undersaturated peralkaline composition will be similarly affected

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

stratigraphical notes on the okote tuff complex at koobi fora kenya the okote tuff complex of the koobi fora formation in northern kenya contains many archaeological sites1 and many hominid specimens derive from levels nearby2 the complex lies between the kbs tuff and the chari tuff both of which have been precisely dated3 4 and recently mcdougall in the accompanying manuscript 5 has measured the age of pumice clasts derived from the complex the pumices however differ in composition from the tuff in which they were deposited the age is still useful because a tuff of known stratigraphical position in the shungura formation is of the same composition here we discuss stratigraphical relations between the okote tuff complex and other related strata at koobi fora in the lower omo valley and west of lake turkana based on compositional study of ash layers this allows us to estimate an age of 1 60 0 05 myr for the homo erectus skeleton recovered at nariokotome in 1984 see accompanying article6

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

pleistocene paleosol development and paleoenvironmental dynamics in east africa a multiproxy record from the homo bearing aalat pedostratigraphic succession dandiero basin eritrea abstract the climatic changes during the early middle pleistocene transition are a key to understand the ecosystem dynamics that involved the homo erectus ergaster distribution the aalat pedostratigraphic succession represents a continental archive in the african rift valley eritrea where remains of homo around 1 ma were identified high resolution magnetostratigraphy dated this succession between the base of the jaramillo subchron and the lower brunhes chron despite the present arid desert climate the aalat section records a persistence of water driven fluvio lacustrine environments which suggests a major tectonic control on sedimentation although climate changes are clearly overprinted macro and micromorphological physico chemical mineralogical and geochemical features up to now poorly available for pleistocene paleosols in east africa depict a poor to moderate degree of development although calcic and petrocalcic petrogypsic horizons at different stratigraphic heights indicate phases of geomorphic stability the concurrent alternation of these horizons with iron stained layers suggests cyclical changes from dry to wet conditions which fit well with aeolian dust fluxes and marine isotope stages of glacials and interglacials at higher latitudes stable isotope data are consistent with these climatic cycles and suggest a succession of monsoonal and non monsoonal conditions the homo erectus settlement lasted apparently for a short time span because a long high discharge fluvial sedimentation and or an aridity phase at the base of the fluvial facies could have made the area less suitable for human settling and could have hindered preservation of fossils and artifacts

chronostratigraphy and correlation of the plio pleistocene tephra layers of the konso formation southern main ethiopian rift ethiopia abstract the plio pleistocene konso formation is widely exposed in the konso area located at the southwestern end of the southern sector of the main ethiopian rift it is known for its rich acheulean archeological occurrences and abundant vertebrate fossils including those of a boisei and h erectus the formation also contains more than 30 tephra layers the lithologic characterization and major element compositions of discrete glass shards of these tephra combined with single crystal 40 ar 39 ar dating of seven tuffs have established the tephrostratigraphy and age range of the 200 m thick formation the succession at konso was deposited between approximately 1 9 and 1 4 ma and is divided into the sorobo turoha kayle and karat members in ascending stratigraphic order two of the konso tephra are correlative to the pliocene kbs and the early pleistocene chari tuffs of the omo turkana basin along the ethiopian and kenyan border this correlation enables precise time controlled comparisons of faunal and archeological remains between the two regions

correlation of plio pleistocene tephra in ethiopian and kenyan rift basins temporal calibration of geological features and hominid fossil records abstract the 200 m thick fossiliferous konso formation and overlying terrace deposits which crop out at the end of the southern sector of the main ethiopian rift mer contain more than 30 distal tephra layers local and regional tephra correlations of more than 20 tephra units were established using major and trace element data of discrete and purified bulk glass samples within the konso study area eleven correlative marker tuffs were recognized in stratigraphic sections of both the konso formation and the omo turkana basin sediments in southern ethiopia and northern kenya the turoha hope ivory bright white and boleshe tuffs in the konso formation and the upper white tuff in the overlying terrace deposits are securely correlated with the kbs akait lokapetamoi chari lower nariokotome and silbo tuffs of the omo turkana basin using least mobile major elements cao fe2o3 and tio2 and geochronology preliminary correlations are also suggested between the konso formation distal tephra and proximal units of the quaternary caldera forming silicic centers in the central sector of the mer the strongly peralkaline tuffs of the konso formation are compositionally similar to proximal eruptions mostly located along the quaternary axial rift zone of the southern central and northern sectors of the mer the tephra correlation provides information about the temporal and spatial features of the volcanic and tectonic processes recorded in the evolving basins thickness and sedimentation rate were determined for both the konso formation and the omo turkana basin sections measured between the turoha kbs 1 91 0 03 ma and the bright white chari 1 40 0 02 ma tuffs although the lithostratigraphic sequence at the konso study area is younger sedimentation rate within the konso formation was comparable to that of the koobi fora formation higher in the nachukui formation and lower in the shungura formation local and regional differences in thickness and sedimentation rates within the plio pleistocene deposits of the konso shungura nachukui and koobi fora formations may be attributed to erosion associated with increased topographic gradients maintained by continued subsidence mostly along axial fault zones of basins and uplift along the margins of the broadly rifted zone of the omo turkana basin and the southern sector of the mer

trace element mobility during early diagenesis of volcanic ash applications to stratigraphic correlation abstract we have analyzed the relative mobility of trace elements in five rhyolitic and two basaltic tephra layers preserved in both altered and unaltered states as a means of developing a rapid whole rock method for correlating altered tephra layers the tephra layers were altered to zeolites or clay minerals in either saline alkaline pore fluids characteristic of small ephemeral lakes or slightly more dilute pore fluids associated with perennial lakes and brackish coastal groundwaters comparison of whole rock trace elements analyses from the unaltered and altered portions of each tephra layer indicates that immobile elements such as ti zr hf and ta retain their initial concentrations relatively more reactive alkaline and alkaline earth elements such as rb sr and cs are generally mobile the lanthanide elements display variable mobility the light rare earth elements lree generally retain their initial concentrations whereas the heavy rare earth elements hree are generally depleted in alkaline solutions but immobile in more dilute pore fluids we suggest that differences between lree and hree mobility result from the combined effects of lree partitioning into stable accessory minerals and hree transport as solution complexes although our data set was not designed for quantitative analysis of these proposed mechanisms where both the diagenetic environment and the accessory mineral compositions are well constrained the trace element signatures of the unaltered and altered tephra layers can be used to correlate an altered tephra layer to its unaltered equivalent and to distinguish between different altered tephra layers four of the seven altered tephra layers canbe distinguished on the basis of ratios of immobile elements e g zr ti vs hf ti the remaining three all w type rhyolitic ashes can be largely differentiated using cluster analysis on a combination of immobile elements and elements with similar mobilities

the seismic stratigraphy of okanagan lake british columbia a record of rapid deglaciation in a deep fiord lake basin this paper presents the first detailed data regarding the newly discovered deep infill of okanagan lake okanagan lake 50 00 n 119 30 w is 120 km long 3 5 km wide and occupies a glacially overdeepened bedrock basin in the southern interior of british columbia this basin and other elongate lakes of the region e g shuswap kootenay kalamalka canim and mahood lakes mark the site of westward flowing ice streams within successive cordilleran ice sheets an air gun seismic survey of okanagan lake shows that the bedrock floor is nearly 650 m below sea level more than 2000 m below the rim of the surrounding plateau the maximum thickness of pleistocene sediment in okanagan lake basin approaches 800 m forty six seismic reflection traverses and an axial profile show a relatively simple stratigraphy composed of three seismic sequences argued to be no older than the last glacial cycle 30 ka a discontinuous basal unit sequence i characterized by large scale diffractions and up to 460 m thick infills the narrow v shaped bedrock floor of the basin and is interpreted as a boulder gravel deposited by subglacial meltwaters overlying seismic sequence ii is composed of two sub sequences sub sequence iia is a chaotic to massive facies up to 736 m thick lakeshore exposures close to where this unit reaches lake level show deformed and chaotically bedded glaciolacustrine silts containing gravel lens and large ice rafted boulders the surface topography of this sub sequence is irregular and in general mimics the form of the underlying bedrock as a result of compaction this sequence passes laterally into stratified facies sub sequence iib at the northern end of the basin seismic sequence ii appears to record rapid ice proximal dumping of glaciolacustrine silt as the okanagan glacier backwasted upvalley in a deep lake a thin 60 m max laminated seismic sequence iii drapes the hummocky surface of sequence ii and represents postglacial sedimentation from fan deltas the extreme thickness of sequences i and ii in okanagan lake reflects the focussing of large volumes of meltwater and sediment into the basin during deglaciation pre existing sediments that pre date the last glacial cycle appear to have been completely eroded glaciological conditions during sedimentation may have been similar to marine based outlet glaciers calving in deep water in fiord basins in contrast to marine settings where ice bergs are free to disperse large volumes of dead ice were trapped within the basin structural evidence for sedimentation around dead ice blocks has been previously used to argue that the cordilleran ice sheet downwasted in situ we emphasize in contrast the trapping of dead ice left behind by rapidly calving lake based outlet glaciers

ages of tuff beds at east african early hominid sites and sediments in the gulf of aden the early hominids of east africa were dated by determining the ages of tuff beds at the sites despite much research using palaeomagnetic and k ar dating techniques some of those ages are still controversial1 2 to obtain independent age estimates for these tephra layers we have examined cores from dsdp sites 231 and 232 in the gulf of aden fig 1a which consist mainly of calcareous nannofossil ooze but also contain rare tephra horizons3 dated by interpolation from the established nannofossil stratigraphy fig 1b chemical analysis confirms that the identity and sequence of these horizons is the same as that at the east african sites we conclude that the age of the tulu bor tuff is 3 4 myr and hence that the hadar hominid specimens are also 3 4 myr old

surficial failures of alluvial stream banks erosion of alluvial stream banks is of scientific and economic significance the principal focus of this investigation was stream bank erosion caused by piping of sand seams which leads to collapse of overlaying strata analyses were performed to determine which parameters of the riverbank system are most important in controlling the amount and rate at which piping occurs among the failure mechanisms studied were wedge failure of large masses and surficial shearing slabbing failure of bank faces analyses showed that the most important factors governing piping were permeability and capillary suction in sand seams slope of sand seams and water in tension cracks behind the bank face flood hydrograph parameters height of rise and duration of event were less important though significant whereas unit weight cohesion angle of internal friction and water in tension cracks influenced wedge failures only unit weight and cohesion directly influenced slabbing failures however those parameters influencing piping indirectly also influenced slabbing failures results of the analyses agreed closely with field observations

major erosion surfaces in the basal wealden beds lower cretaceous south dorset the transition from evaporitic and lagoonal purbeck beds into the fluviatile basal wealden beds ryazanian valanginian has in the past been interpreted as a continuous regressive succession mupe bay and bacon hole in south dorset are the only localities in the wessex basin of southern england at which the transition strata are well exposed and not extensively faulted on the basis of facies analysis the lowermost wealden beds are interpreted as a regressive transgressive regressive package environments fluctuated between lagoon inner lagoonal shoreline and fluvial flood plain the overlying succession is remarkable for the presence of two major erosion surfaces which based on field geometries and facies relationships are interpreted as unconformities the lower unconfor mity is overlain at mupe bay by sediments of lagoonal or lacustrine origin the upper unconformity is expressed as the celebrated mupe bay palaeo oilseep an oil cemented conglomeratic sand overlain by a thick succession of fluvial sediments a level near the basal sand and the two erosion surfaces are treated as candidate sequence boundaries wbl wb3 with the thin intervening lagoonal deposits representing peaks of transgression the pattern of one minor conformable sequence boundary followed by two major unconformable sequence boundaries bears a strong resemblance to the early cretaceous sequence stratigraphy suggested in recent global cycle charts however biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas

geology and palaeontology of the late miocene middle awash valley afar rift ethiopia the middle awash study area of ethiopia s afar rift has yielded abundant vertebrate fossils 10 000 including several hominid taxa1 2 3 4 the study area contains a long sedimentary record spanning late miocene 5 3 11 2 myr ago to holocene times exposed in a unique tectonic and volcanic transition zone between the main ethiopian rift mer and the afar rift sediments along the western afar rift margin in the middle awash provide a unique window on the late miocene of ethiopia these deposits have now yielded the earliest hominids described in an accompanying paper5 and dated here to between 5 54 and 5 77 myr these geological and palaeobiological data from the middle awash provide fresh perspectives on hominid origins and early evolution here we show that these earliest hominids derive from relatively wet and wooded environments that were modulated by tectonic volcanic climatic and geomorphic processes a similar wooded habitat also has been suggested for the 6 0 myr hominoid fossils recently recovered from lukeino kenya6 these findings require fundamental reassessment of models that invoke a significant role for global climatic change and or savannah habitat in the origin of hominids

chow bahir rift a failed rift in southern ethiopia the chow bahir rift system is a major graben in a 300 km broad rift zone recognized in southern ethiopia between the kenyan and ethiopian domes where the east african rift is not well defined an extinct failed rift discovered along the omo canyon to the north and on strike with the chow bahir rift ceased activity about 4 m y ago chow bahir is in a younger stage of abandonment as the main ethiopian rift propagates south into this region

palaeochannels in mine workings in the high hazles coal westphalian b nottinghamshire coalfield england palaeochannels have numerous deleterious effects on coal mining including the formation of washouts and their detection and delineation in the subsurface is of paramount importance a palaeochannel system in the roof of the high hazles coal westphalian b at gedling colliery nottinghamshire is described the roof strata of this seam are interpreted as the deposits of a lacustrine delta complex formed on an upper delta plain with the palaeochannels constituting minor deltaic distributaries the lacustrine delta front deposits consist of a coarsening upwards sequence of siltstones and sandstones which form a sheet like body of coalescing distributary mouth bars that were laid down by friction dominated effluents the distributary channels were filled by a variety of processes giving rise to complex channel deposits bedload deposition of sand on bars collapse and sloughing of channel banks and deposition of silts and muds from suspension were important mechanisms the nature of the deposits reflects the mode of channel abandonment rather than equilibrium channel processes overbank flood deposits and prodelta lacustrine mudstones are also described from the high hazles roof based on this study criteria are suggested that may enable the successful detection and delineation of palaeochannels in mine workings elsewhere

the menengai tuff a 36 ka widespread tephra and its chronological relevance to late pleistocene human evolution in east africa abstract the east african rift preserves the world s richest middle and late pleistocene 780 12 ka geological archaeological and paleontological archives relevant to the emergence of homo sapiens this region also provides unparalleled chronological control for many important sites through tephrochronology the dating and correlation of volcanic ashes as widespread isochronous markers in the geological record there are many well characterized pliocene early pleistocene tephras that are widespread across east africa a comparable framework is lacking for the middle and late pleistocene a period characterized by spatially and temporally complex patterns of climate change as well as the emergence of modern homo sapiens and the dispersal of this species across and out of africa unraveling relationships among these spatial and temporally complex phenomena requires a precise chronology to this end we report the menengai tuff a widespread volcanic ash produced by the large scale caldera forming eruption in kenya and 40 ar 39 ar dated to 35 62 0 26 ka geochemical characterization of 565 glass shards from 36 samples by wavelength dispersive electron probe microanalysis show the menengai tuff was deposited over 115 000 km 2 and is found in the baringo chalbi elmenteita nakuru olorgesailie turkana and victoria basins all of which preserve rich late pleistocene paleoenvironmental and archaeological archives correlation and dating of the menengai tuff demonstrate that it is the most widespread tephra and largest eruption currently known from the late pleistocene of east africa as such it is a valuable marker in establishing a late pleistocene chronology for paleoclimatic archeological and paleontological records relevant to the study of human evolution

penecontemporaneous oil seep in the wealden early cretaceous at mupe bay dorset u k abstract investigation of an oil globule enveloped in laminated silty clay shows that oil derived from the lias impregnated early wealden alluvial sediments penecontemporaneously and later some of the earlier oil reached the surface of the floodplain and seeped from the eroding bank of an ephemeral river lag sediment in the channel comprising debris from the oilyalluvium was invaded by later oil or oils the palaeoseep was geologically short lived relevant geochemical and petrological data and the geological context are presented process models based on fault and or water table controls on oil migration are briefly proposed

source rock and seep oil maturity in dorset southern england abstract analysis of the steranes and triterpanes in the oil seeps of the dorset coast of southern england mupe bay osmington mills shows both the oils to derive from a similar mid late mature source rock measured maturity of the jurassic source rocks at outcrop are uniformally immature 0 3 0 5 r despite a history of mesozoic burial and tertiary inversion and uplift evidence from oil cemented sandstone clasts within oil stained lower cretaceous fluviatile channel sands at mupe bay suggests that oil was already seeping during the lower cretaceous detailed work comparing the extracts of the oil stained clasts and the surrounding oil stained sandstones shows the clasts to contain oil derived from a slightly but significantly less mature source rock this is consistent with lower maturity oil seeping first into the sand that was then incorporated as rip up or bank collapse clasts into the channel sand at a later stage with further source rock burial more mature oil entered the matrix sandstone locally calibrated time temperature thermal geohistory modelling can just simulate source rock maturities appropriate to the seep oils if paleo geothermal gradients and surface temperatures were locally higher in the jurassic than at the present day a volumetric balance between oil in place in the wytch farm field and the available area of source rock draining into the structure suggests that an anomalously thick and or organic rich rhaeto liassic section must have developed in the bournemouth bay basin and that cretaceous palaeo geothermal gradients were higher in this specific area

correlation of pliocene and pleistocene tephra layers between the turkana basin of east africa and the gulf of aden abstract electron microprobe analyses of glass shards from volcanic ash in pliocene and pleistocene deep sea sediments in the gulf of aden and the somali basin demonstrate that most of the tephra layers correlate with tephra layers known on land in the turkana basin of northern kenya and southern ethiopia previous correlations are reviewed and new correlations proposed together these data provide correlations between the deep sea cores and to the land based sections at eight levels ranging in age from about 4 to 0 7 ma specifically we correlate the moiti tuff 4 1 ma with a tephra layer at 188 6 m depth in dsdp hole 231 and with a tephra layer at 150 m depth in dsdp hole 241 the wargolo tuff with a tephra layer at 179 7 m in dsdp hole 231 and with a tephra layer at 155 3 m depth in dsdp hole 232 the lomogol tuff defined here with a tephra layer at 165 m in dsdp hole 232a the lokochot tuff with a tephra layer at 140 1 m depth in dsdp hole 232 the tulu bor tuff with a tephra layer at 160 8 m depth in dsdp hole 231 the kokiselei tuff with a tephra layer at 120 m depth in dsdp hole 231 and with a tephra layer at 90 3 m depth in dsdp hole 232 the silbo tuff 0 74 ma with a tephra layer at 35 5 m depth in dsdp hole 231 and possibly with a tephra layer at 10 9 m depth in dsdp hole 241 we also present analyses of other tephra from the deep sea cores for which correlative units on land are not yet known the correlated tephra layers provide eight chronostratigraphic horizons that make it possible to temporally correlate paleoecological and paleoclimatic data between the terrestrial and deep sea sites such correlations may make it possible to interpret faunal evolution in the lake turkana basin and other sites in east africa within a broader regional or global paleoclimatic context

the role of tephrachronology in recognizing synchronous caldera forming events at the quaternary volcanoes longonot and suswa south kenya rift

a 1 4 million year old bone handaxe from konso ethiopia shows advanced tool technology in the early acheulean in the past decade the early acheulean before 1 mya has been a focus of active research acheulean lithic assemblages have been shown to extend back to 1 75 mya and considerable advances in core reduction technologies are seen by 1 5 to 1 4 mya here we report a bifacially flaked bone fragment maximum dimension 13 cm of a hippopotamus femur from the 1 4 mya sediments of the konso formation in southern ethiopia the large number of flake scars and their distribution pattern together with the high frequency of cone fractures indicate anthropogenic flaking into handaxe like form use wear analyses show quasi continuous alternate microflake scars wear polish edge rounding and striae patches along an 5 cm long edge toward the handaxe tip the striae run predominantly oblique to the edge with some perpendicular on both the cortical and inner faces the combined evidence is consistent with the use of this bone artifact in longitudinal motions such as in cutting and or sawing this bone handaxe is the oldest known extensively flaked example from the early pleistocene despite scarcity of well shaped bone tools its presence at konso shows that sophisticated flaking was practiced by 1 4 mya not only on a range of lithic materials but also occasionally on bone thus expanding the documented technological repertoire of african early pleistocene homo

anatomy of a channel bank collapse structure in tertiary fluvio lacustrine sediments of the lower rhine basin germany an exceptionally well preserved synsedimentary deformation feature is described from uppermost miocene fluvio lacustrine coastal plain sediments exposed in lignite mines of the lower rhine basin germany the feature is interpreted as a complex multi stage channel bank collapse structure rotational slumping of bank material into the channel along two failure surfaces extending below the channel toe generated a series ofminor compressional thrust backthrust structures within the muddy channel fill sediments a reconstruction of the failure event is proposed which is linked toa lowering of river stage and subsequent channel abandonment the accurate description of this unusually well preserved channel bank collapse structure may have general application in the interpretation of similar but less well exposed preserved phenomena elsewhere

use of single grain geochemistry of cryptic tuffs and volcaniclastic sandstones improves the tephrostratigraphic framework of olduvai gorge tanzania single grain geochemical composition of volcaniclastic sandstones can be a potential tool to improve correlations of mixed pyroclastic epiclastic deposits to test this trachytic tuffs of the paleoanthropologically important flk flk n and flk nn sites of pleistocene olduvai gorge bed i tanzania are used as an established tephrostratigraphic framework against which to test volcaniclastic sandstone correlations fluvio lacustrine sandstones and tuff samples were collected from eight archeological trenches between tuffs ib and id across a 500 m transect including leakey s famous zinjanthropus flk and oh 7 oh 8 flk nn sites a previously unknown thin fine mineralogically unique black trachyandesitic fallout ash was discovered below tuff ic compositions of individual augite feldspar and titanomagnetite grains from sandstones between tuffs ib and ic reveal some ib equivalent material and a new compositional assemblage distinct from the sandwiching marker tuffs mineral compositions of the tripartite volcaniclastic sandstone between tuffs ic and id are similar to id volcaniclastic sandstone grain fingerprints further refine correlations between fluvio lacustrine sections within the area providing support for proposed high resolution stratigraphic reconstruction of the zinjanthropus and oh 7 oh 8 land surfaces this method might be applied to other sections where pyroclastic particles are admixed but distinct tuffs are not preserved

lithostratigraphy and sedimentary environments of the hominid bearing pliocene pleistocene konso formation in the southern main ethiopian rift ethiopia abstract the pliocene pleistocene konso formation defined here is 180 m thick and is extensively exposed in the konso area at the southernmost part of the ganjuli graben of the main ethiopian rift the konso area is known for its abundant vertebrate fossils including those of homo erectus and australopithecus boisei and its rich acheulean archaeological assemblages the fluvial floodplain lake margin and lacustrine sediments of the formation were mostly deposited between 1 9 and 1 4 ma based on single crystal 40ar 39ar ages of interbedded marker tuffs the formation is subdivided into the sorobo turoha kayle and karat members in ascending stratigraphic order each member contains dark brown or dark grey clay beds of lake margin and or lacustrine origin suggesting the recurrence of lake environments most of the fossils and artifacts derive from whitish grey or brown silt sand and gravel beds widely exposed between the finer sediments these beds appear to have been deposited in an emerging marginal floodplain following repeated recession of the palaeo lake the depositional history of the konso formation reveals aspects of quaternary rifting at the southern terminus of the main ethiopian rift rifting subsidence and sedimentation in the ganjuli graben occurred in the konso area mainly between 1 9 and 1 4 ma while active faulting with associated volcanism during the pleistocene was mostly confined to the middle part of the graben between lakes abaya and chamo and to the segen basin east of konso a shift of rift related faulting and subsidence from the konso area eastward to the segen basin is likely to have resulted in the erosion and exposure of the fossiliferous konso formation

the fossil oil seep in mupe bay dorset a myth investigated abstract the story of the mupe bay boulder bed has assumed mythical proportions in the british oil industry it is supposed to demonstrate a seep which was active at the time of deposition of the lowermost wealden sandstones indicating that the lias source rocks offshore to the south had already entered the oil window at that time the boulder bed contains sandstone clasts which are apparently oil cemented in a sandstone matrix which shows lighter oil staining the boulders were obviously soft when deposited as they were deformed during deposition this study suggests that there is no difference in the maturity or source of the oils from the clasts and the host channel sand that there is no significant difference in the diagenetic sequence or in the authigenic mineral assemblage of the two sandstones the conclusion is that the clasts are not contemporaneous oil cemented boulders as previously supposed the only differences noted in this study between the clasts and their host sandstone is in their grain sizes and the degree of biodegradation of the oils this difference in biodegradation can logically be ascribed to the difference in permeability associated with the change in grain size and the resulting penetration of meteoric water the source of the oil in both of the sandstones can be correlated to the lias aromatic biological markers for thermal maturity resistant to biodegradation indicate a maturity equivalent to about 1 vitrinite reflectance or higher for both oils these conclusions remove the constraint of early wealden generation of oil previously imposed on burial history reconstructions for the timing of oil generation and migration in south dorset it is further proposed that osmington bran point lulworth cove mupe bay worbarrow bay area was the site of a large oilfield with reservoirs within the wealden and extending down to the bencliff grits and bridport sandstones of the same order of size as the wytch farm accumulation this oilfield was uplifted and breached during the erosion which followed tertiary compression

tephra sources and correlations in ethiopia application of elemental and neodymium isotope data abstract combined elemental and neodymium nd isotope data for proximal and distal pyroclastic deposits from the ethiopian volcanic province illustrate the utility of these parameters for establishing potential source regions for distal tephra and for correlation of widely separated distal deposits in the ethiopian volcanic province nd isotopic compositions and to some extent elemental characteristics of volcanic products from distinct geographic tectonic sub provinces allow first order indentification of geochemical domains these domains are used to target potential source regions for the numerous distal tephra deposits exposed within the fossiliferous sedimentary sequences of the afar and omo turkana basins pliocene distal deposits from the hadar and middle awash regions of afar ethiopia and the omo turkana region of southern ethiopia northern kenya are examined combined elemental and nd isotope data are consistent with derivation of these tephra from sources within or adjacent to the central sector of the main ethiopian rift or within the central to northern afar the data presented in this paper also point to the need for a better understanding of the physical and chemical characteristics of the eruptions giving rise to many of the distal deposits until information regarding the range of chemical and isotopic variability in these magmatic systems is available it is difficult to link a specific set of geochemical characteristics to one particular eruption or vent

revised ages for tuffs of the yellowstone plateau volcanic field assignment of the huckleberry ridge tuff to a new geomagnetic polarity event 40 ar 39 ar ages were determined on the three major ash flow tuffs of the yellowstone plateau volcanic field in the region of yellowstone national park in order to improve the precision of previously determined ages total fusion and incremental heating ages of sanidine yielded the following mean ages huckleberry ridge tuff 2 059 0 004 ma mesa falls tuff 1 285 0 004 ma and lava creek tuff 0 639 0 002 ma the huckleberry ridge tuff has a transitional magnetic direction and has previously been related to the reunion normal polarity subchron dating of the reunion event has been reviewed and its ages have been normalized to a common value for mineral standards the age of the huckleberry ridge tuff is significantly younger than lava flows of the reunion event on re union island supporting other evidence for a normal polarity event younger than the reunion event