dolomites and dolomites in mesozoic platform carbonates of the southern apennines geometric distribution petrography and geochemistry abstract a multidisciplinary study of the dolomitized bodies present among the late triassic to late cretaceous carbonate deposits of the monti lattari belt southern apennines of italy compares the different dolomitization styles present within this succession of these mesozoic platform carbonates and furnishes an overview of their formative genetic models the revealing traces of the diverse modes of diagenesis in this region include both surface and subsurface mechanisms utilizing a geometric reconstruction of the dolomitized bodies an integrated facies and petrographic analysis of the components and a geochemical characterization of the sediments including detailed mineralogy and associated stable isotope values the data presented demonstrate that some intervals are characterized by surface related dolomitization processes middle late triassic and cretaceous while others latest triassic to lower middle jurassic and are mainly dominated by later large scale subsurface dolomitization processes all of the middle late triassic norian successions are pervasively dolomitized exhibit fabric preserving textures and have a distinctly heavy stable isotope signature their origin has been related to diagenesis due to particular climatic and palaeoceanographic conditions leading to the formation of extended blankets of shallow warm saline seawater this setting was terminated by climatic and palaeoceanographic changes during the latest triassic which dramatically reduced the significance of surface related dolomitization on a regional scale the overlying latest triassic rhaetian outcrops consist of a widespread coarse fabric destructive massive dolomite with a lighter isotopic signature these dolomites seem genetically related to the ones of the lower middle jurassic which share the same geometrical petrographic and geochemical characteristics caused by large scale fluid circulation this diagenetic pattern apparently started in the early jurassic during burial driven by thermal convection due to the different temperatures between the oceanic waters and the warmer carbonate platform the middle upper jurassic units feature local episodes of early dolomitization that partially replaced the muddy carbonate facies due to tidal pumping and reflux mechanisms dolomitization is not a pervasive feature during this phase of deposition in the cretaceous portion of the succession dolomites have a totally different and distinct character and are generally related to bedding and show variability in textures and mg ca ratios local association with evaporites geometry of dolomitized bodies and stable isotope geochemistry suggest processes comprising evaporative pumping and reflux all related to the surface environment this evidence points to a renewal of a climatic context more favorable for early dolomitization thus this study permits recognition of the presence of three quite different dolomitization styles within the carbonate platform succession of the southern apennines fold thrust belt one ascribed to the norian one to the rhaetian lower jurassic and one to the cretaceous

itrf2014 a new release of the international terrestrial reference frame modeling nonlinear station motions for the first time in the international terrestrial reference frame itrf history the itrf2014 is generated with an enhanced modeling of nonlinear station motions including seasonal annual and semiannual signals of station positions and postseismic deformation for sites that were subject to major earthquakes using the full observation history of the four space geodetic techniques very long baseline interferometry vlbi satellite laser ranging slr global navigation satellite systems gnss and doppler orbitography and radiopositioning integrated by satellite doris the corresponding international services provided reprocessed time series weekly from slr and doris daily from gnss and 24 h session wise from vlbi of station positions and daily earth orientation parameters itrf2014 is demonstrated to be superior to past itrf releases as it precisely models the actual station trajectories leading to a more robust secular frame and site velocities the itrf2014 long term origin coincides with the earth system center of mass as sensed by slr observations collected on the two lageos satellites over the time span between 1993 0 and 2015 0 the estimated accuracy of the itrf2014 origin as reflected by the level of agreement with the itrf2008 both origins are defined by slr is at the level of less than 3 mm at epoch 2010 0 and less than 0 2 mm yr in time evolution the itrf2014 scale is defined by the arithmetic average of the implicit scales of slr and vlbi solutions as obtained by the stacking of their respective time series the resulting scale and scale rate differences between the two solutions are 1 37 0 10 ppb at epoch 2010 0 and 0 02 0 02 ppb yr while the postseismic deformation models were estimated using gnss gps data the resulting parametric models at earthquake colocation sites were applied to the station position time series of the three other techniques showing a very high level of consistency which enforces more the link between techniques within the itrf2014 frame the users should be aware that the postseismic deformation models are part of the itrf2014 products unlike the annual and semiannual signals which were estimated internally with the only purpose of enhancing the velocity field estimation of the secular frame

itrf2008 contribution to glacial isostatic adjustment and recent ice melting assessment 1 we investigate what information station vertical velocities of the itrf2008 provide on global geodetic parameters and by extension on glacial isostatic adjustment gia and recent ice melting rim processes we infer degree 2 spherical harmonic coefficients shc of the earth figure change and thej2 gravity rate j 2 which we compare with five gia models we find j 2 to be 0 0 2 4 10 11 yr 1 which is consistent with recent studies that propose a j 2 change in the 1990s due to rim whose contribution to the j 2 would be today around 3 5 4 0 2 4 10 11 yr 1 such results favor peltier 2004 vm2 or paulson et al 2007 gia models the itrf2008 shc that are directly impacted by the gia rotational feedback confirm with a good precision recent results from grace mission that initiated a debate on gia rotational feedback and about peltier gia model quality we find a coefficient consistent with paulson s and other model and more than 7 times smaller than coefficients in peltier s models two explanations are possible 1 if the model of peltier 2004 vm2 were to be correct then the strong rotational feedback in the model must be counteracted by a strong rotational feedback in the opposite direction generated by current ice loss 2 if the model of paulson et al 2007 were to be correct therefore gia and rim separately induce negligible rotational feedbacks both answers are quite extreme and call for more investigation on gia modeling and rotational feedback

early permian rotliegendes palaeowinds of the north sea abstract the classical desert sediments of the early permian upper rotliegendes were deposited in a post variscan basin that extends from eastern england to the russo polish border which has been referred to as the southern permian basin these sediments comprise four main depositional facies fluvial wadi aeolian sabkha and lacustrine the lacustrine facies includes important bedded halites the southern permian basin is separated from the smaller northern permian basin by the fragmented mid north sea ringkobing fyn system of highs and beyond the grampian spur lies the even smaller moray firth basin these latter basins contain rock sequences similar to those of the southern basin with the exception that bedded halite has not been recognised in the lacustrine facies the rotliegendes of the southern basin apparently conforms to deposition in a northern hemisphere tradewind desert similar to the modern southern sahara apart from regional palaeomagnetic considerations and the occurrence of sediments of a non arid nature south of the variscan highlands the strongest evidence supporting this interpretation is the pattern of palaeowind directions deduced from the orientations of dune bedding this is seen in both outcrop and core and derived from the continuous dip meter logs of many north sea wells by plotting the poles to the bedding attitudes on polar nets palaeowind directions have also been deduced from the dip meter logs of wells drilled in the northern and moray firth basins in general they indicate winds that blew in a direction opposed to those of the southern basin thus an area of high barometric pressure seems to have been located over the mid north sea high when the bedding attitudes in a well are roughly unidirectional an origin on a transverse dune is normally indicated with some wells the indicated palaeowind direction is regionally anomalous in many cases this can be rectified if the well is presumed to have drilled down through one flank of a seif dune the indicated palaeowind is thus changed by up to 90 and then commonly fits the regional pattern these early permian desert basins were smaller and closer to the equator than the modern sahara their location in the centre of a continent between the caledonian and variscan mountain ranges suggests the analogy of a more tropical equivalent of the central asian takla makan desert

a note on the distribution and significance of metamorphic limestones in the moine and lewisian of the scottish highlands and islands

ichnology paleosols and palimpsests surfaces in the tinat member of the nuayyim formation unayzah group subsurface saudi arabia trace fossils and genetic stratigraphy in continental settings abstract the nuayyim formation of the unayzah group is widespread in subsurface central saudi arabia and constitutes a laterally variable stratigraphic unit with a complex mosaic of facies sedimentation for the tinat member took place majorly in a continental setting dominated by aeolian processes that resulted in dunes interdunes ephemeral stream deposits terminating with regional development of paleosols facies ichnofacies and palynological analysis suggest an arid to semi arid setting with biogenic activity in the form of continental fauna and plants in deposits associated with pedogenetical alteration in paleosol horizons therein burrowing exhibits elements of the scoyenia ichnofacies in beds with sparse to abundant bioturbation tectonic uplift and subaerial exposure resulted in the exhumation of previously buried substrates leading to the formation of burrowed palimpsests surfaces associated with paleosols at the end of unayzah deposition prior to the onset of the khuff formation progressive thermal updoming is interpreted to have occurred during the latest portion of the unayzah group sedimentation cycle in the arabian plate culminating in the regional pre khuff unconformity at the top of this megacycle recognized in this study by biogenically demarcated diastems ichnological and sedimentological observations outlined in this study are significant because they i refine existing interpretations of the paleoenvironmental history in the study area ii document diastems marking breaks in sedimentation and iii outline several soil forming cycles that allowed the establishment of a trace fossil suite with continental affinities thus demarcating the end of an entire sedimentation cycle the recognition and delineation of burrowed horizons associated with paleosols provides a tool to understand the tectono stratigraphic evolution of similar successions in the rock record

event stratigraphy paleoenvironment and chronology of se arabian deserts the geological record of the se arabian desert is exhibited in a variety of geomorphic features and their characteristic sediments ranging from alluvial fans to inland and coastal dunes and sabkhas together they suggest a large range of environmental and climatic variability between periods of relative aridity and humidity sedimentary events in the emirates appear to correlate broadly with high latitude glaciations which affected both the shamal winds and the extent of exposure of the arabian gulf in general the aeolian sands in the rub al khali responded to glacial events as a consequence of exposure of the floor of the arabian gulf during glacials when global sea level was low quartz sands from across the exposed floor of the arabian gulf were transported by the shamal winds south eastwards to the emirates and then south to the rub al khali the supply of shamal transported sand from the floor of the arabian gulf was cut off by high stands of sea level during interglacial periods the new coastal dunes miliolite rich in foraminifera and other cement assisting calcareous shell fragments were deflated down to the water table as their sand was transported further to the south leading to the creation of coastal sabkhas studies on aeolianites and the lacustrine deposits provide evidence of phases of enhanced humidity prior to the last glacial maximum and during the holocene in contrast the bedding attitudes of miliolites in se oman suggest that the wahiba sands were transported from south to north by a branch of the sw monsoon the depositional ages of these dunes indicate a close connection between the winds and coeval fluctuations of the sw monsoon the interdunal lakes and underlying dune sands reflect an active sw monsoon during the holocene this is also indicated by the aeolian and lacustrine records of the thar desert in india which likewise are controlled by the sw monsoon south and west of the oman mountains is a broad alluvial fan bajada that is mostly inactive today the oldest fluvial sediments are undated but preliminary age estimates suggest that some correlate with more humid earlier interglacials

sedimentology of a proterozoic erg the venkatpur sandstone pranhita godavari valley south india reappraisal of the late proterozoic venkatpur sandstone indicates that the bulk of the sandstone is aeolian in origin aeolian stratification types namely i inverse graded translatent strata ii adhesion laminae iii grainflow strata and iv grainfall strata are present throughout the outcrop belt nine facies have been identified that represent both aeolian and related aqueous environments within a well developed erg cosets of large cross beds at the bellampalli section in the nw of the study area record dune fields in the interior of the sand sea to the se at the godavari river and ramgundam sections a progressive increase in the relative proportion of the flat bedded to cross bedded facies and intercalated non aeolian facies delineates the transition from the dune field to sand sheet environment an alternating sequence of aeolian and marine sediments at laknavaram in the extreme se marks the termination of the sand sea palaeocurrent data suggest that the nw se trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction abundant horizontally stratified units in the vankatpur sandstone do not always represent the interdune sediments on the basis of the thickness and geometry of the units nature of bounding surfaces and associated facies sequence the facies is variously interpreted to represent interdune inland sabkha sand sheet and coastal sand flat deposits

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

carboniferous and early permian biogeography during the carboniferous changes in the biogeographical distribution of shelf dwelling benthic marine invertebrates were made in response to changes in physical paleogeography and climatic variations calcareous foraminifers and bryozoans are principal examples of the general trends during the early carboniferous which show that tournaisian and early and middle visean faunas were broadly cosmopolitan in a circumequatorial belt and that latitudinal diversity gradients were relatively minor during the later part of the visean and early part of the namurian the hercynian orogeny caused by the collision of euramerica with gondwana disrupted these cosmopolitan equatorial faunal patterns this was also a time of progressively cooler temperatures throughout the world of dramatic reduction in faunal diversity and of high rates of extinction of both species and genera during middle carboniferous time strongly provincial faunas were common spasmodic but limited dispersals gave a few widespread genera significantly different stratigraphic ranges in different provinces so that tethyan and non tethyan distributions are recognizable faunal diversity gradually increased during the middle carboniferous and then declined at the end of the epoch with additional high levels of extinction of species and genera during late carboniferous time several new faunal lineages became well established some filled vacant ecological niches however others took advantage of the increase in the number of niches that became available because of gradually warming climates provinciality continued to be pronounced diversity gradually increased and continued to do so through the early permian

isotope systematics of precambrian marbles from the lewisian complex of northwest scotland implications for pbpb dating of metamorphosed carbonates abstract we present pb isotopic data from the late archaean lewisian marbles of tiree iona and glenelg each locality representing a different tectonic history since the mid proterozoic data from tiree unaffected precambrian foreland to the caledonian yield an age of 1656 23 ma which is within error of broadly defined laxfordian reworking events both locally and throughout the lewisian nd isotopic data confirm a late archaean protolith although low model 1 values require considerably lower pre laxfordian u pb ratios an age pf 1418 56 ma is obtained from the iona marble foreland buried beneath caledonian nappes again with a low 1 and negative nd t this age is too young for the laxfordian and might indicate some disturbance of u pb systematics during caledonian events no meaningful age data are obtained from the glenelg marbles lewisian inlier in caledonian thrust sheet where grenvillian 1 1 ga eclogite metamorphism preceded caledonian events and pb isotopic compositions appear not to have been homogenised leaching experiments for the tiree and iona marbles reveal that radiogenic pb is hosted in silicate phases with unradiogenic pb in the carbonates a reversal of the situation commonly observed in sedimentary carbonates if a sedimentary precursor is assumed redistribution of u and pb probably occurred during high grade metamorphism recorded by the age regression accompanied by significant u pb fractionation to yield higher post metamorphic u pb ratios the significance of non zero th u ratios between 1 and 3 post laxfordian remains unclear but may not have any bearing upon the age or nature of the protolith our study provides further evidence that pb isotopic studies of marbles can yield reliable geochronological information on metamorphic events in the absence of later tectono metamorphic overprinting and is therefore a potentially useful technique for reconnaissance geochronology in metasupracrustal belts however caution should be exercised in applying this technique to terrains with complex histories

quaternary climatic changes over southern arabia and the thar desert india the distribution of sand dunes over the southern half of arabia conforms to the influence of two wind systems the northern shamal which is a strong wind that blows to the sse down the persian arabian gulf and then swings to the sw across the hyperarid rub al khali towards north yemen and the strong winds of the sw monsoon system which were responsible for forming linear dunes that trend north south in the wahiba sands of eastern oman and sw ne in the thar desert nw india in the thar desert the sw monsoon alternates with the weaker ne monsoon the dating of exposures of older dune systems by isotopic radiometric and optically stimulated luminescence osl analyses has shown that the shamal was active throughout the latter part of the quaternary period and probably as long ago as mid miocene time c 15 ma at times of glacial maxima when global sea level was some 100 120 m or more lower than now siliciclastic and carbonate grains were deflated from the exposed surface of the persian gulf and transported into the ne rub al khali within the united arab emirates it is suspected that occasionally the shamal also transported some quartz sands from the nw onto the exposed narrow continental shelf of se arabia with silt size particles being carried into the arabian sea the sw monsoon on the other hand was re established over the coast of se arabia several millennia after the last glacial maximum and was fully established near the coast of se arabia during the early holocene interglacial after the atmospheric high pressure system associated with the glacial period had become weaker early during the holocene interglacial periods when the sw monsoon dominated a combination of quartz and carbonate sands was deflated from the exposed continental shelf and transported to the north into the wahiba sands aeolian activity in the thar desert also peaked during this period of transition from full glacial to interglacial conditions the dune systems of se arabia overlie the distal edges of older alluvial fans that in oman date back at least 350 ka the sediments of some of these fluvial sequences in oman reached the arabian sea via wadi batha only to be removed by along shore currents driven by the sw monsoon in the thar desert the supply of aeolian sediment is mostly from fluvial sources marine sediments from the arabian sea between arabia and thar record the contrasting effects of the shamal and the sw monsoon the former mostly as a source of wind blown dust from arabia and the latter by causing upwelling of nutrient rich waters leading to organic blooms

characterization of the aeolian terrain facies in wadi araba desert southwestern jordan abstract the sand dunes in wadi araba desert southwestern jordan conform to the influence of two main wind systems 1 the shamal north wind the main determinant of dune patterns and 2 the southerly winds caused by the red sea trough and khamasin winds wadi araba is a narrow elongated morphotectonic depression bordered by the high eastern and western mountain ranges that would obstruct most of westerlies in winter and the hot dry easterlies in summer wadi araba desert is caused by the rain shadow of the eastern and western topographic highs with an arid hyperarid climate and moisture index between 0 1 and

geochemistry and tectonic setting of lewisian clastic metasediments from the early proterozoic loch maree group of gairloch nw scotland abstract the early proterozoic 2 0 ga loch maree group lmg of the lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the archaean basement the metasediments comprise three main mapped units the flowerdale schists kerrysdale schists and charlestown schists together with numerous narrower bands the dominant clastic metasediments within the schist units semipelites and psammites are chemically distinguished as a series of greywackes and lithic sandstones with variable lil and transition element abundances fractionated ree patterns and overall compositions similar to paas three chemical groups are recognized whose range of composition largely reflects the mixing between two main source components lewisian gneissose basement with an intermediate acid continental crust composition and local rift related amphibolites with a primitive tholeiitic composition which also incorporates components not preserved within the lmg the geochemistry of the clastic metasediments confirms a previous tectonic model for the lmg as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement as well as from internal volcanogenic sources

the action of wind and water in a mid cretaceous subtropical erg margin system close to the variscan iberian massif spain aeolian processes and ephemeral water influx from the variscan iberian massif to the mid cretaceous outer back erg margin system in eastern iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements remains of aeolian dunes encased in ephemeral fluvial deposits aeolian pods demonstrate intense erosion of windblown deposits by sudden water fluxes the alternating activity of wind and water led to a variety of facies associations such as deflation lags desert pavements aeolian dunes pebbles scattered throughout dune strata aeolian sandsheets aeolian deposits with bimodal grain size distributions mud playa ephemeral floodplain pebble sand and cobble sand bedload stream pebble cobble sand sheet flood sand bedload stream debris flow and hyperconcentrated flow deposits sediment in this desert system underwent transport by wind and water and reworking in a variety of sub environments the nearby variscan iberian massif supplied quartzite pebbles as part of mass flows pebbles and cobbles were concentrated in deflation lags eroded and polished by wind driven sands facets and ventifacts and incorporated by rolling into the toesets of aeolian dunes the back erg depositional system comprises an outer back erg close to the variscan highlands and an inner back erg close to the central erg area the inner back erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits in the inner back erg area ephemeral wadis desiccated after occasional floods were mud cracked and overrun episodically by aeolian dunes subsequent floods eroded the aeolian dunes and mud cracked surfaces resulting in largely structureless sandstones with boulder size mudstone intraclasts floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes the remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands upon desiccation deflation resulted in lags of coarser grained sediments a renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions synsedimentary tectonics caused the outer back erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands ephemeral water flow to the outer back erg area supplied pebbles eroded aeolian dunes and produced hyperconcentrated flow deposits fluidization and liquefaction generated gravel pockets and recumbent folds dune damming after sporadic rains the case of the namib desert monsoonal water discharge thar desert and meltwater fluxes from glaciated mountains taklamakan desert are three potential non exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the cretaceous desert margin system an increase in relief driven by the aptian anti clockwise rotation of iberia led to an altitude sufficient for the development of orographic rains and snowfall which fed melt water fluxes to the desert margin system quartzite conglomerates and sands dominantly consisting of quartz and well preserved feldspar grains which are also observed in older cretaceous strata indicate an arid climate and the mechanical weathering of precambrian and palaeozoic metamorphic sediments and felsic igneous rocks unroofing of much of the cover of sedimentary rocks in the variscan iberian massif must therefore have taken place in pre cretaceous times

the late permian climate what can be inferred from climate modelling concerning pangea scenarios and hercynian range altitude abstract a major unsolved geodynamic problem is the permian pangea plate configuration at the end of paleozoic era around 255 ma while consensual geology indicates a plate arrangement close to that of the jurassic prior to the atlantic opening pangea a paleomagnetic data indicates a nearly 3000 km more eastward position of gondwana with respect to laurussia leading to major plate and relieves reorganisation during permo triassic times pangea b using an atmospheric general circulation model agcm we simulate the climatic response to both configurations we also test the fundamental role of paleo elevations through sensitivity experiments each simulated climate is then compared with the aim of constraining the best fit to a particular paleogeographic scenario main trends of simulated late permian climate in agreement with paleoclimatic indicators are 1 a warm temperate climate accompanied by a monsoon circulation over the eastern side of gondwana 2 a cold temperate climate marked by strong annual thermal amplitude at high latitudes in gondwana and in siberia 3 an arid belt in the subtropics over the western side of gondwana and laurussia and 4 strong climatic differences over both hemispheres respectively main variance is found over laurussia with a simulated tropical climate while an arid climate is suggested by paleodata simulating different paleo elevations of the appalachian and the variscan fold belts can solve this discrepancy the best model data fit is reached for a mean altitude of 4500 m in the appalachians and appears to be dependent of pangea configuration with a southern europe variscan range of some 3000 m in a pangea a configuration or only 2000 m in for pangea b taking into account the geodynamic context we argue that pangea b appears to be the more probable configuration

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

aeolian to marine transition in cambro ordovician cratonic sheet sandstones of the northern mississippi valley u s a cratonic quartz sandstones have presented several intractable problems besides their extreme textural and compositional maturity and paucity of shale their sheet like geometry is particularly notable if the sandstones were entirely marine as long supposed such geometry is difficult to explain in terms of modern shelf sediments which are generally held to be either relict or only slightly reworked by the holocene transgression palimpsest re study of two quartz sandstones in the northern mississippi valley region reveals evidence for significant non marine deposition followed by varying degrees of marine reworking during transgressions facies patterns are similar in the cambrian wonewoc and ordovician st peter sandstones both of which overlie unconformities in both a large scale cross stratified facies believed to represent aeolian ergs passes laterally into a planar and channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits criteria of aeolian deposition in both facies include adhesion structures large ripple index fine climbing translatent lamination grainfall and grainflow stratification criteria of braided fluvial deposition include shallow channels containing sequences of thinning upward sets of trough cross stratification reactivation surfaces low index ripples and polygonal cracks probable aeolian sand sheets contain flat bedding punctuated by small channels adhesion structures and coarse sand ripples with large index there is a conspicuous absence of trace and body fossils from these inferred non marine deposits in contrast is a burrowed and trough cross stratified facies characterized by medium scale cross bedding alternating with bioturbated intervals and rare brachiopod or trilobite mould coquinas which is interpreted as shallow marine in both formations this last facies replaces laterally and overlaps the other two reflecting transgression and variable reworking the main areas of non marine deposits in both formations are capped by a thin burrowed subfacies that represents the culmination of each transgression that is a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense it is suggested that the sheet like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes that such processes must have been orders of magnitude more important on pre devonian vegetation free landscapes than they have been since not only helps to explain the sheet like character but also the exceptional maturity of the older cratonic sandstones

carbonate rocks and related facies with vestiges of biomarkers clues to redox conditions in the mesoproterozoic ocean abstract the raipur group of the chattisgarh basin preserves two major late mesoproterozoic carbonate platforms the lower platform is about 490 m thick separated from the upper platform 670 m thick by a 500 m thick calcareous shale carbonate strata cover almost 40 of the chattisgarh basin outcrop and represent two major platform types a a non stromatolitic ramp the charmuria sarangarh limestone and b a platform developed chiefly in the intertidal to shallow subtidal environment with prolific growth of stromatolites the chandi saradih limestone the first platform consists primarily of the black timarlaga limestone that is locally replaced by early diagenetic dolomite this carbonate platform experienced strong storm waves and was subsequently drowned by a major transgression during which extensive black limestone marl rhythmite was deposited followed by deposition of the gunderdehi shale the carbonate factory was later re established with development of an extensive stromatolite dominated charmuria sarangarh platform that ranged from restricted embayment to open marine conditions sea level change played a major role in controlling the broad facies pattern and platform evolution the 13 c signatures of the chattisgarh limestones falling within a relatively narrow range 0 to 4 are typical for upper mesoproterozoic carbonate rocks 18 o values however have a greater range 5 7 to 13 3 indicating significant diagenetic alteration of some samples likely dysoxic or anoxic conditions prevailed during deposition of the black timarlaga limestone and well oxygenated conditions during deposition of the gunderdehi shale and saradih chandi stromatolite the lack of 17 21 moretanes and high t max values suggest mature organic matter in the non stromatolitic ramp a paucity of diagnostic eukaryotic steroids indicates that algae were rare in the chattisgarh basin a high content of hopanes supports a generally bacterially dominated proterozoic ocean in which various stromatolites flourished

monitoring surface change on a namib linear dune in tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large scale deductive models to smaller scale field studies of individual dunes this paper reports a study of surface change on a large complex linear dune in the namib desert southern africa the dune surface responds to a markedly seasonal wind regime in summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest in winter this pattern is reversed easterly winds erode sand from the east slope and deposit it on the west slope the crest therefore moves back and forth some 15 m each year returning at the end of a year s cycle to its position at the beginning the position of the base of the dune appears to remain fixed even though sand is moving throughout the dune system the dune does extend northward along some resultant of the westerly and easterly winds despite relatively high levels of activity especially at the dune crest there is no evidence of the breakdown of the linear dune form the conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested

the composition of the precambrian ocean waters abstract the composition of the precambrian oceans was different from the present composition this is a direct consequence of a co2 atmosphere depleted in oxygen at the beginning the mineralization was lower the oceans waters were chloro dicarbonated with a lower tenor of chlorine richer in phosphates and they were depleted in sulphates sulphur was in the reduced state of sulphides the abundance of phosphates supported the expansion of life the cations na and mg2 coexisted with ca2 fe2 and ba2 whose levels were higher than they are today the main trends of the evolution are schematized in the figure

genesis of the buntsandstein lower triassic in the western eifel germany abstract the buntsandstein lower triassic of the western eifel germany is composed of sediments of different origin these are alluvial fan fluviatile aeolian lacustrine and deltaic deposits the fluviatile deposits are subdivided into channel lag bar and topstratum deposits alluvial fan deposits occur in local depressions of the pre triassic morphology aeolian sediments build up the main part of the middle buntsandstein locally interfingering with fluviatile deposits paleowind and paleocurrent directions generally coincide the upper buntsandstein consists of fluviatile sediments only which can be subdivided into well developed small cyclothems average thickness of sedimentation units 3 8 m capped by paleosoils the uppermost buntsandstein is of deltaic origin the ingression of the lower muschelkalk sea terminates the terrestrial buntsandstein sequence

150 million years of climatic stability evidence from the atacama desert northern chile the sedimentary succession in the atacama desert records deposition under an arid to semiarid climate from the late jurassic 150 ma to the present day palaeomagnetic data indicate no significant latitudinal movement of this area since the late jurassic the present day location of the atacama within the dry subtropical climate belt is the principal cause of aridity this situation is likely to have prevailed since the late jurassic supplemented by 1 the continentality effect enhanced by the gondwanan landmass and 2 the presence offshore of a cold upwelling current from at least the early cenozoic onwards and possibly earlier resulting in conditions promoting climatic stability and desert development rapid and extreme climatic fluctuations during the plio pleistocene were not sufficient to destabilize the climate within the atacama comparison with other long lived deserts e g sw usa namib sahara and australia suggests that the atacama is the oldest extant desert on earth

early diagenesis atherton formation quaternary a guide for understanding early cement distribution and grain modifications in nonmarine deposits abstract the atherton formation represents an extensive sheet of outwash and related materials of quaternary age these sand q77f8r15 and gravel sediments are locally lithified and present an excellent opportunity for evaluating the effects of early diagenesis on a terrigenous deposit the principal cement present is calcium carbonate 98 occurring as euhedral to anhedral grain contact pore lining and porefilling varieties limonite and gypsum are present as rare cements cement distribution was controlled at various levels by 1 stratification character 2 grain size and sorting 3 framework grain composition and 4 proximity to near surface processes initial cement development and subsequent early growth were primarily parallel to laminae within beds this precipitation site preference is related to initially higher relative permeabilities estimated at 2 to 2 5 x parallel versus normal to stratification when associated with trough cross strata cements preferentially are located in the lower portion of troughs or directly below trough set boun aries there is a slight preference for early formed cements to be present along the axis instead of the flanks of a trough about 50 of pebble gravel and sandy pebble gravel units contain locally cemented zones while 17 of finer grain size strata are locally lithified over 95 of locally cemented units are within 7 m of the present ground surface suggestive of a causal relation to near surface perhaps soil related processes the sources for the various constituents composing cements are related to the dissolution of previously cemented layers and framework components such as carbonate rock fragments calcareous pelitic rock fragments and iron bearing silicate grains the examination of oversized voids elongate pores and intraparticle porosity suggests that at least locally multiple episodes of cementation followed by dissolution have taken place reactions between carbonate cement and limestone rock fragments have destroyed a high percentage of detrital carbonate grain boundaries such reactions combined with early dissolution can result in an information loss in excess of 50 of the limestone rock fragment component in a sandstone a combination of organic acids and co2 mixed with meteori waters is likely responsible for the dissolution and transportation of cement constituents

the origin of alteration orangettes in dhofar 019 implications for the age and aqueous history of the shergottites the shergottites are the largest group of martian meteorites and the only group that has not been found to contain definitive evidence of martian aqueous alteration given recent reports of current liquid water at the surface of mars this study aimed to investigate in detail the possibility of martian phyllosilicate within shergottite dhofar 019 optical and scanning electron microscopy followed by transmission electron microscopy confirmed the presence of alteration orangettes with a layered structure consisting of poorly ordered mg phyllosilicate and calcite these investigations identified maskelynite dissolution followed by mg phyllosilicate and calcite deposition within the dissolution pits as the method of orangette production the presence of celestine within the orangette layers the absence of shock dislocation features within calcite and the mg rich nature of the phyllosilicate all indicate a terrestrial origin for these features on dhofar 019

are stable isotopic trends in amphibolite to granulite facies transitions metamorphic or diagenetic an answer for the arendal area bamble sector southeastern norway from mid proterozoic carbon bearing rocks the isotopic 3 i3 c and 3 18 0 compositions of primary marbles and secondary veins calcites and primary fahlbands and metapelites and secondary veins graphites sampled across the am phibolite to granulite facies transition in the bamble sector southeast ern norway have been investigated the primary calcites display a regional trend in a 3 i3c 3i80 plot which is the reverse of the expected trend the granulite grade primary calcites essentially preserved most of their initial sedimentary compositions whereas the amphibolite grade calcites have increasingly lower 3 i3 c values the 3 18 0 composi tions of amphibolite grade primary calcites and of some vein tlpes appear to be buffered by the o reservoir of the country rock the 3 1 c of graphite of different types is specific for a certain lithology but not spatially related to the metamorphic zoning reaction progress calculations and rayleigh fractionation model ling prove that prograde metamorphic decarbonation reactions alone cannot be held responsible for the isotopic trend in the marbles the reversed isotopic trend in the marbles themselves combined with the lack of any trend in the primary graphites is not compatible with pervasive flushing with a homogeneous mantle derived co2 rich fluid at granulite facies metamorphism the marble trend and the constant 3 i3 c composition of j rimary graphite throughout the metamorphic zoning are best explamed by low grade processes that did affect the stable isotopic compositions of the marbles but left the graphites unaffected the processes may even occur under diagenetic conditions later amphibolite granulite facies metamorphism preserved the trend despite the production of calcsilicates subsequent retrograde hydro thermal action produced graphite quartz veins and several types of calcite veins without resetting stable isotope systematics on a regional scale

the geochemistry of lewisian marbles summary to rectify the dearth of published petrographical and analytical data for lewisian marbles 307 samples from nearly all outcrops known have been examined in thin section and 114 samples analysed for 35 elements lewisian marbles can be divided texturally into 1 macrocrystalline generally white pure to silicate rich eg from glenelg 2 microcrystalline often coloured ornamental pure to silicate rich eg from iona 3 microbrecciated grey crushed fine grained silicate opaque rich eg from the loch maree group lmg lewisian marbles are geochemically typical of ancient carbonates tending to have lower contents of trace elements notably sr than younger eg scottish dalradian marbles whereas all analyzed marbles from the south harris and coll foreland outcrops and from the glenelg and kintail inliers are dolostones mgo 15 20 sio 2 5 15 those of the eastern inliers glen strathfarrar shinness are pure limestones mgo 2 eg scardroy inlier microbrecciated lmg marbles flowerdale belt of gairloch furnace belt of loch maree have notably higher chalcophile and siderophile element contents than macrocrystalline marbles in adjacent belts cloiche of gairloch gleann tulacha of loch maree aside from such local differences however limestones and dolostones of the inliers lmg and remaining foreland reveal few non trivial differences in composition moreover lewisian marbles collectively show strong pearson and spearman correlations between elements concentrated in accessory phases eg y la ce k rb al fe co v given established secular variations in the chemistry of global carbonate rocks these relationships between both marble groups and chemical variables are difficult to explain if some lewisian marbles eg s harris represent late scourian and others eg lmg early proterozoic sediments they may provide a tentative hint that most lewisian marbles were deposited contemporaneously

high resolution stratigraphic architecture and lithological heterogeneity within marginal aeolian reservoir analogues marginal aeolian successions contain different lithological units with variable geometries dimensions and spatial distributions such variations may result in considerable heterogeneity within hydrocarbon reservoirs developed in successions of this type which poses a high risk to their efficient development here such heterogeneity is described and characterized at inter well 1 km scales using two well exposed outcrop analogues of end member marginal aeolian deposits from the permian cedar mesa sandstone and jurassic page sandstone of south central utah usa the sedimentology and stratigraphic architecture of the cedar mesa sandstone was studied in a 1 2 km2 area in the indian creek region of southern utah where the interval consists of interbedded fluvial and aeolian deposits representative of a fluvial dominated erg margin the page sandstone was studied in a 4 3 km2 area near escalante close to the utah arizona border where it consists of interbedded sabkha and aeolian deposits representative of a transitional marine erg margin the three dimensional stratigraphic architectures of both reservoir analogues have been characterized in order to establish the dimensions geometries and connectivity of high permeability aeolian sandstones facies architecture of the aeolian sabkha deposits is characterized by laterally continuous aeolian sandstone layers of relatively uniform thickness that alternate with layers of heterolithic sabkha deposits aeolian sandstones are thus likely to form vertically unconnected but laterally widespread flow units in analogous reservoirs facies architecture in the aeolian fluvial deposits is more complex because it contains alternating intervals of aeolian sandstone and fluvial heterolithic strata both of which may be laterally discontinuous at the studied length scales aeolian sandstones encased by fluvial heterolithic strata may form small isolated flow units in analogous reservoirs although the limited continuity of fluvial heterolithic strata results in vertical connectivity between successive aeolian sandstones in other locations these architectural templates may be used to condition zonation schemes in models of marginal aeolian reservoirs

multiple episodes of aggradation and calcrete formation in late quaternary aeolian sands central thar desert rajasthan india abstract a 12 m thick section in a dune sandy plain terrain of the central thar in rajasthan has provided a near continuous record of environmental change for the past 160 ka the site presently receives 280 mm rainfall almost entirely from the summer monsoon the base of this section comprises a gravel bed of an ephemeral stream and the overlying six litho units each with discrete boundaries comprise a succession of aeolian sands luminescence dating provided an estimate of the timing of the sand aggradation periods to 160 90 60 27 and 17 14 ka and helped constrain the timing of calcrete formation periods in each aggradation unit discrete nodular calcretes formed by the leaching of carbonate from the overlying solum this is analogous to present day conditions in sandy plains during periods of increased rainfall and landscape stability several of these calcretes are however devoid of their donor solum suggesting solum removal during a subsequent period of decreased rainfall and resultant surface instability this is supported by the presence of reworked nodules on the surfaces of some calcretes a prominent phase of calcrete development followed the aeolian sand aggradation at 60 ka suggesting climate amelioration that also caused the formation of groundwater related calcrete and mottling the study suggests that stage ii calcrete nodules form in a time frame of 10 20 ka and confirms limited data on the duration and stage of calcrete development in the literature the 13c values of calcrete carbonate lie in a narrow range 0 5 to 1 1 suggesting formation under soils with c4 vegetation this implies that even during phases of climatic amelioration the high temperatures and increased seasonality of rainfall did not permit significant development of c3 plants in the central thar

contrasting siliciclastic evaporite strata in subsurface and outcrop an example from the permian nippewalla group of kansas u s a abstract geologists tend to expect that rocks as they appear in outcrop are a very good approximation of the same strata in the subsurface but this is not the case in mixed siliciclastic evaporite units strata that contain both a siliciclastic and an evaporite mineral component distinct differences in lithologic and stratigraphic descriptions of ancient siliciclastic evaporite strata exist between outcrop and core these differences present challenges in stratigraphic nomenclature lithologic correlation age determinations and interpretations about past depositional environments diagenetic history and paleoclimate an example of siliciclastic evaporite strata is the middle permian to early triassic gypsum and or anhydrite hereby referred to as gypsum anhydrite and halite bearing red beds throughout the midcontinent of north america and equivalent strata from other pangean deposits here we use core and field observations of the middle permian nippewalla group in kansas to show the range of similarities and differences in lithologies and thicknesses of specific lithofacies in outcrop and at various depths for example the displacive halite lithology consisting of red mudstones dominated by randomly oriented halite crystals is abundant in cores and interpreted as saline mudflat deposits the outcrop counterpart is less common friable massive red mudstones with some gypsum anhydrite pseudomorphs after displacive halite crystals we interpret these lithologic differences of this saline mudflat lithofacies as the result of excellent preservation of buried displacive halite and late stage dissolution of near surface halite additionally bedded halite and intergranular halite cements in siltstones and sandstones are also vulnerable to late stage dissolution near the surface further modifying lithologies and thicknesses thickness of the nippewalla group in the amoco rebecca k bounds core of western kansas is 931 feet 284 meters thick but it is estimated to be only half as thick in the outcrop belt of south central kansas to estimate amount of rock lost to late stage dissolution we conducted dissolution experiments on displacive halite units from cores which resulted in 60 95 loss of rock thickness and mass due to dissolution of halite the unconsolidated red sediment resulting from these dissolution experiments compares favorably with much of the fine massive red siliciclastics that make up the greater part of the nippewalla group outcrops we propose that major lithologic differences resulting from near surface diagenetic processes must be expected in any evaporite bearing siliciclastic unit besides guiding correlation attempts between surface and subsurface this knowledge is vital for making accurate interpretations of depositional and diagenetic history of siliciclastic evaporite units

a comparative geochemical study of pelitic schists and metamorphosed carbonate rocks from south central maine usa whole rock major element chemical analyses of progressively metamorphosed impure carbonate rocks and pelitic schists collected from the same metamorphic terrain reveal similarities and differences in the chemical response of these rock types to the metamorphic event relative to a constant aluminum reference frame both schist and carbonate exhibit no detectable change in their contents of fe mg ti si and ca with change in metamorphic grade carbonate rocks become progressively depleted in k and na with increasing grade of metamorphism while schists exhibit no statistically significant change in their contents of k and na both rock types become depleted in volatiles principally co2 and h2o with increasing grade

chemistry of the dalradian vendian cambrian metalimestones british isles abstract based on 600 300 new analyses for up to 39 elements chemical variations in all major dalradian carbonate formations have been investigated based on actual sio 2 and mgo distributions samples are divided into pure carbonate rocks 2 70 of analyses impure carbonates 25 40 sio 2 22 of analyses and skarns ca mg fe silicate rocks 40 sio 2 8 of analyses pure carbonates are divided into limestones dolostones 7 5 mgo 27 of analyses and ophicalcites 25 mgo 2 of analyses impure carbonates and skarns have often been metasomatically altered from sediment compositions only pure carbonates were assessed statistically element distributions are non gaussian e g bimodal for si ca sr and mg positively skewed near poisson for elements concentrated in accessory minerals outlying values are found for many especially metallic elements probably reflecting local mineralisation graphical and non parametric statistical techniques were therefore used to delineate the following stratigraphical and lateral variations 1 1 except for thin dolostone beds in the crinan and islay subgroups dolostone limestone ratios decrease up the succession from 2 1 to near zero lower to upper dalradian 2 2 dolostones have stochastically higher si fe v cr zn y zr ba and pb than limestones suggesting that dolomitisation has preferentially affected impure carbonate sediments dolostones have lower sr because dolomite tolerates less sr than calcite 3 3 many supposed laterally equivalent carbonate formations are chemically heterogeneous especially in the blair atholl subgroup where facies changes readily occurred in a shallow water environment 4 4 in the loch tay limestone confidently traceable for 225 km along strike ca zn rb pb and th tend to decrease and si and mg to increase from sw to ne metamorphic grade increases slightly in the same direction but since non volatile element contents all increase with prograde decarbonation these changes are likely to be pre metamorphic 5 5 if ranks are assigned to major carbonate horizons from 1 oldest e g ballachulish limestone to 8 youngest e g largie limestone v cr co ni zn and pb contents of 90 dolostones are found to increase up the dalradian sequence 6 6 by contrast maximum contents of 16 elements e g v rb al ti in 350 limestones increase inwards from lower and upper towards less pure middle dalradian limestones observations 4 and 6 are consistent with the greater instability and incidence of volcanism in the middle dalradian basin observation 5 appears to reflect an association of dolomitisation with increasing syngenetic pb zn mineralisation many stratigraphical correlations suggested by field workers e g lismore blair atholl culdaff tayvallich limestones are upheld by chemical comparison whereas others may require revision e g sandend group may correlate with the ballachulish not lismore limestone the kinlochlaggan limestone is most probably lower dalradian

internal structure and surface geometry of longitudinal seif dunes abstract a mutual relationship exists between the morphology of longitudinal seif dunes and their dynamics and internal structure the basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank the magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line after heavy rain exposed the internal structure of the dune two kinds of depositional unit were found the main unit forms in the meandering area on both sides of the dune the other secondary unit is a narrow belt on both sides and parallel to the crest line the two units have different strikes because the laminae deposited on the dune are of the three primary stratification types climbing translatent strata sand flow cross strata and grainfall laminae the range of dips is wide 10 to 30 these dips accord with topographic profiles across the dune a model of the internal structure of a longitudinal dune has been developed from the field data the model shows the erosional and depositional areas along this linear dune

effects of glacial transport and neomorphism on precambrian dolomite crystal sizes late precambrian glacial rocks are notable for their association with dolostones1 and this has been regarded as a palaeoclimatological paradox2 5 however little detailed work has been carried out to determine the mode of origin of the dolomite in north east spitsbergen svalbard the vendian6 7 succession contains two glacial stratigraphical units8 9 which are dolomitic and are underlain and overlain by dolostones evidence from sedimentary structures and gross textures indicates that dolomite within the glacial units is detrital whereas at other levels it is a precipitate10 accordingly dolomite crystal sizes were analysed to determine whether evidence of glacial erosion and transport rock flour has been preserved allowing for the effects of later crystal enlargement aggrading neomorphism which vary within and between samples the results show a markedly higher proportion of very fine crystals in the inferred detrital dolostones compared with the precipitated dolostones the very fine crystals are interpreted as relics of rock flour the preservation of which demonstrates a novel means of testing the origin of dolostones associated with glacial sequences inorganic precipitation of dolomite during glaciation or secondary dolomitization of till are not required to explain the dolomite tillite association but pronounced climatic changes at the onset and close of glacial periods are indicated

geochemistry of metasediments from the precambrian hapschan series eastern anabar shield siberia abstract the precambrian hapschan series in the anabar shield in northern siberia is composed of garnet paragneisses pelitic graywackes marbles calciphyres and calc silicate rocks carbonates and carbonate rich clastic sediments and a minor amount of enderbite and mafic granulite trace element distributions in the paragneisses especially ree patterns and low cr and ni suggest felsic sources for the graywacke protoliths with exception of mn pb and sr trace elements in hapschan carbonates and calc silicates are housed chiefly in chiefly silicate contaminant phases when normalized to al2o3 to partially compensate for trace elements contained in al silicates most trace elements are depleted in hapschan carbonates relative to average phanerozoic marine limestone pml rb cr ni and ti are strikingly depleted in hapschan as well as other precambrian high grade carbonates relative to pml the element depletions in hapschan marbles may be explained by some combination of loss during granulite facies metamorphism or by removal in another depository such as bif geologic and geochemical data are consistent with deposition of the hapschan series in a cratonic or passive continental margin setting during the early proterozoic basement to this series may be the high grade rocks of the daldyn and upper anabar series from which a zircon u ion probe age of 3 3 ga has been reported

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

norphlet formation upper jurassic of southwestern and offshore alabama environments of deposition and petroleum geology upper jurassic norphlet sediments in southwestern and offshore alabama accumulated under arid climatic conditions the appalachian mountains of the eastern united states extended into southwestern alabama to provide a barrier for air and water circulation during the deposition of the norphlet formation these mountains produced topographic conditions that contributed to the arid climate and they affected sedimentation norphlet paleogeography in southwestern alabama was dominated by a broad desert plain rimmed to the north and east by the appalachians and to the south by a developing shallow sea the desert plain extended westward into eastern and central mississippi norphlet sedimentation initiated as a result of basin subsidence accompanied by erosion of the southern appalachians norphlet conglomerates were deposited in coalescing alluvial fans in proximity to an appalachian source the conglomeratic sandstones grade downdip into red beds that accumulated in distal portions of alluvial fan and wadi systems quartz rich sandstones were deposited as dune and interdune sediments on a broad desert plain the principal source of the sand was updip alluvial fan and plain and wadi deposits wadi and playa lake sediments also accumulated in the interdune areas a marine transgression was initiated during the late phase of deposition of the norphlet formation resulting in the reworking of previously deposited norphlet sediments norphlet hydrocarbon potential in southwestern and offshore alabama is excellent six oil and gas fields already have been established petroleum traps discovered to date are primarily structural traps involving salt anticlines faulted salt anticlines and extensional fault traps associated with salt movement reservoir rocks consist primarily of quartz rich sandstones that are eolian wadi and marine in origin porosity is principally secondary dissolution with some intergranular porosity smackover algal carbonate mudstones were probably the source for the norphlet hydrocarbons jurassic oil generation and migration probably were initiated in the early cretaceous

significance of aeolian limestone lenses in quartz sand formations an interdigitation of coastal and continental facies perth basin southwestern australia abstract in the quaternary of the perth basin of southwestern australia the transition from coastal dune ridges to continental siliciclastic sand hinterland is a zone of calcareous sand or limestone lenses on or within siliciclastic sand in the holocene sequences the transition is a zone of landward extended and detached parabolic dunes which in section appear as lenses of calcareous sand resting on pleistocene quartz sand and limestone in the pleistocene sequences aeolian limestone lenses are common in thick sections of yellow quartz sand using the holocene coast to hinterland transition as an analogue the limestone lenses are interpreted to be buried outliers of attenuated parabolic calcareous dunes that migrated inland over an undulating plain of yellow quartz sand from a pleistocene coastal zone these dunes were later buried by influxes of aeolian yellow quartz sand during glacial age desert phases of the pleistocene in section these buried dunes form a zone of limestone lenses within aeolian quartz sand formations thus in a given time interval limestone lenses represent the transition between coastal and hinterland continental desert environments and record the extent that coastal dunes penetrated inland the pleistocene stratigraphic model of aeolian limestone lenses set within a sequence of aeolian siliciclastic sand is a distinctive style of coastal to continental transition which can be used to unravel similar sequences in the stratigraphic record where coastal aeolian calcareous facies interdigitate with desert aeolian siliciclastic facies

tectonic and climatic controls on continental depositional facies in the karoo basin of northern natal south africa abstract the eastern karoo basin south africa contains a thick sequence of terrigenous clastic sediments comprising a meanderbelt facies braided channel facies divided into coarse and fine subfacies fluviolacustrine facies and aeolian facies depositional trends and changes in fluvial style reflect a progressive increase in aridity of the climate under stable tectonic conditions interrupted by two phases of source area tectonism and the development of fine and coarse clastic wedges of the braided channel subfacies the latter signifying a short interlude of cool wet conditions the fine braided channel subfacies occurs in the upper part of the meanderbelt facies which was deposited by ephemeral meandering mixed load streams of variable discharge and sinuosity under dry semi arid climatic conditions these deposited complex internally discordant channel sands and well developed levee deposits following deposition of the coarse braided channel subfacies semi arid conditions returned and fluvial deposition was dominated by ephemeral straight to slightly sinuous mixed load streams characterised by simple channel sand bodies as the aridity of the climate increased the streams became more localised and carried an increasing proportion of fines interbedded with and overlying the fluvial deposits is a mudstone dominated lacustrine sequence grading up into aeolian sands suggesting a playa lake type situation the general absence of evaporites from these sediments is attributed to the fresh nature of the lake waters as evidenced by the freshwater aquatic organisms and clay mineral suite the lack of adequate inflow for solute accumulation and the removal of dust impregnated by salts from the surface of the dry lake bed during the dry season by superheated upward spiralling columns of air broadly similar environments to the fluvio lacustrine and aeolian facies sequence are to be found in the lake eyre basin of central australia and the okavango delta of northern botswana the okavango delta model has an important bearing on patterns of fluvial sedimentation in arid regions since it shows many characteristics of temperate well vegetated anastomosed fluvial systems despite its location in the kalahari desert

geochemistry and origin of the archaean beit bridge complex limpopo belt south africa the beit bridge complex bbc in the central zone of the limpopo belt in southern africa includes metasedimentary rocks of archaean age chiefly metapelites quartzites and carbonates amphibolites chiefly mafic dykes and sills and granitoids which comprise 50 of the complex the amphibolites are tholeiites with relatively unfractionated ree patterns and pronounced subduction zone geochemical signatures in terms of relatively immobile incompatible elements tonalites and granites which also exhibit such a signature can be formed by batch melting of a mafic source with residual garnet or amphibole most granites have negative eu anomalies and appear to represent residual liquids bbc metapelites have negative eu anomalies and other immobile element distributions similar to phanerozoic shales one group of metapelites that shows significant depletion in ca na and sr appears to have been derived from intensely weathered sources the negative eu anomalies in bbc metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of southern africa by about 3000 ma bbc quartzites are quartz rich sandstones that are depleted in hfs elements ba and sr relative to phanerozoic counterparts a feature that probably reflects less recycling during the early archaean bbc banded iron formation bif is similar to other archaean bif and shows large positive eu anomalies bbc carbonates are dolostones depleted in ree hfs elements u th ba sr ni sc v and y and enriched in fe mn na rb and cs relative to phanerozoic marine carbonates

desert sedimentary environments present and past a summary abstract tropical deserts develop where the potential rate of evaporation exceeds that of precipitation where rainfall is too low or spasmodic to support vegetation these usually occur in the trade wind belts north and south of the equator and in the rain shadow of mountain ranges about half the area of deserts comprise outcrop subject to erosion and deflation the rest consists of sediments of fluvial wadi lacustrine salina paralic coastal sabkha and aeolian origin and of evaporites ephemeral streams deposit sands and gravels over basin margin alluvial fans and over desert plains where sub surface cementation commonly takes place evaporation of basin centre temporary lakes results in a crust of halite inland sabkha fluvial sands are an important source of dune sand the finer fractions are removed from the desert in suspension to become loess and the coarser grain remain as a deflation lag the axes of sand dunes are transverse to moderate winds and parallel to strong winds giant forms of the latter dune type were prevalent during the last ice age and in coastal areas carbonate cemented dune sands extend below present sea level this suggests a causal relationship between polar glaciations and strong desert winds desert fluvial sediments rarely reach the sea so the warm and clear coastal waters are ideal for the manufacture of organic carbonates longshore currents sweep them into offshore bars behind which evaporitic lagoonal conditions develop the lagoons become sabkhas as they are infilled with algally bound marine and wind blown sediment although there are desert sequences as old as about 2000 ma they seem to have occurred only sporadically during the earth s history it seems likely that their occurrence was controlled by the past size of continents their location relative to the equator by the freedom of circulation of global oceanic water and by the presence of ice caps the permo triassic was a time of extensive evaporite deposition and dune activity by implication other known occurrences of evaporites may be marginal to wadi and dune sequences that have yet to be identified

the interaction of aeolian and fluvial processes during deposition of the upper cretaceous capping sandstone member wahweap formation kaiparowits basin utah u s a abstract detailed examination of the upper cretaceous capping sandstone member of the wahweap formation in the kaiparowits basin utah reveals the presence of aeolian stratification deposition by aeolian processes is recognized and distinguished from subaqueous deposition by the presence of centimeter scale inversely graded wind ripple stratification the aeolian stratification in the wahweap formation is the first occurrence reported from upper cretaceous strata on the colorado plateau of the western u s a aeolian stratification occurs as thin wind reworked caps of fluvial bar sandstones deposited within low sinuosity braided streams and as more extensive deposits of small scale dunes that developed in geographically restricted dune fields aeolian reworking of bars took place during low stage flow and was possibly controlled by intermittent seasonal discharge variations prolonged aridity led to increased sand supply entering the aeolian system dunes nucleated and grew rapidly between the braided stream systems these small scale dunes rarely developed extensive or tall slip faces as evidenced by the rare preservation of grain flow strata and the dominance of wind ripple stratification the dune field was characterized by sinuous crested dunes probably barchans amid and linked to the extensive braided stream systems based on modern analogs the resulting change in sand storage supply probably is related to a short term shift to a more semi arid arid climate possibly seasonally however a longer term climate change to semi arid arid may be indicated for the dune complex near the boundary with the overlying kaiparowits formation

desert sediments ancient and modern tropical deserts have existed sporodically on our planet from the precambrian to the present and seem not to have been a permanent feature of its surface glennie 1987 depending on their definition tropical deserts and semi deserts currently occupy between approximately one fifth and one third of the earth s land surface of which only about 4 is covered by that popular concept of what a desert consists of sand dunes the remaining area comprises barren rock both hill and plateau with a variable cover of sediments transported by ephemeral streams wadis arroyos to form the deposits at the terminal points of these streams varying with the ratio of water supply to the annual potential rate of evaporation those terminal areas may be occupied by desert lakes that are generally of a temporary nature permanent only if the water is provided from beyond the margins of the desert or is fed by groundwater and become more saline as they become desiccated the end product of such a situation is a salina or sabkha area of sand silt or clay commonly encrusted with halite there is no universally accepted definition of a desert in its simplest form it can be defined as a barren tract of land over which rainfall is too limited or spasmodic to support vegetation adequately very few desert areas are completely devoid of vegetation and many areas that fall within a desert in terms of average annual rainfall may have an even though sparse cover of plants that have adapted to the relatively arid environment in which they live in this context some writers define deserts as areas that have an average upper limit of 250 mm of annual rainfall even though it may all fall in one storm and rainfall may not recur for several years perhaps more important than the rainfall itself is the ratio between it and the potential rate of evaporat ion the aridity or desiccation factor this is about 1 10 in some parts of the australian desert and up to 1 500 in areas of the sahara cooke warren 1973 tropical deserts exist either because the winds that cross their surfaces have a low relative humidity or there are no hills to push the winds up to colder cloud forming altitudes the southwest monsoon gives torrential rain over the western ghats of peninsular india but little over the arid low lying rajasthan desert further to the n drawn from the high pressure area of the mediterranean sea the north african trade winds blow south and southwestward under clear skies warming as they cross the sahara and so becoming capable of absorbing more moisture without condensation by contrast the southern hemisphere coastal deserts of the namib and atacama are often shrouded in mist but rarely experience rain cold deserts also occur in areas of low precipitation but lack a protective cover of vegetation because the moisture necessary for growth is frozen for a large part of the year allied to the low temperature peri glacial dune fields exist because of adequate supplies of fluvioglacial sediments that are unprotected by vegetation and have a grain size that is suitable for deflation once the summer melt floods have ceased dunes are formed because strong dry winds can transport sand size particles as a saltation load within a few metres of the land surface sand storm because winds can drive sand over a hard immobile surface faster than over one of loose sand bagnold 1941 sand tends to accumulate in areas that are already sand covered these sand patches grow in size whenever the velocity of the wind lessens and is no longer capable of keeping the grains in motion dunes eventually result silt and clay size particles however can be carried in suspension dust storm and by this means may be transported right out of the desert to be deposited elsewhere as a marine horizon in the ocean or on land as loess a cover of vegetation on the land surface tends to inhibit the aeolian transport of sand some ancient dune sequences that were deposited before higher plants began to colonize extensive areas of land during the devonian eg precambrian dunes ross 1983 may reflect the ease with which sand could be moved by the wind in the absence of vegetation rather than any long term aridity the devonian old red sandstone sequences of britain for instance are dominated by fluvial and lacustrine sediment that must indicate a fairly high annual rainfall and yet dune sands have accumulated locally mykura 1983 carruthers this volume to confirm a true desert origin for ancient sequences additional evidence of aridity is needed in hot deserts this is most easily recog

the la ce geochronometer a new dating method the isotope 138la 0 089 of natural lanthanum decays by and electron capture to 138ce and 138ba with a long half life we present here the first age determination using la ce dating an internal la ce isochron of a gabbro from the upper zone of the bushveld south africa yields an age of 2 390 480 2 myr based on the partial decay constant 2 58 10 12 yr 1 this age agrees with the sm nd isochron age 2 050 90 2 myr which was obtained from an aliquot of the same sample solution within the uncertainties of 138la half life and isotopic measurement

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic