origin of the copper cobalt deposits of the zambian copperbelt an epigenetic view from nchanga the zambian copperbelt is arguably the most significantly mineralized neoproterozoic basin on earth preserving a truly spectacular scale of mineralization in excess of 1 109 t of ore at 2 7 copper has been extracted to date and there are also major cobalt accumulations the origin of these deposits has been hotly debated for more than six decades yet the driving forces that generated this system are poorly understood in particular the relationships between tectonics paleo fluid circulation and ore deposition we present new field and isotopic data for the nchanga deposits in which the bulk of the mineralization is hosted by shale capped feldspathic arenites and arkoses that have undergone recrystallization and hydrothermal alteration within a host rock package controlled by low angle thrust faults by using in situ laser combustion we show for the first time that the range of 34s for copper cobalt ore sulfides 1 to 18 cannot have the same source as diagenetic pyrite 1 to 17 we suggest a new epigenetic model for the formation of these spectacular nchanga orebodies that involves the introduction of metal and sulfate bearing hydrothermal fluids into quartzofeldspathic units during basin inversion with sulfide derived from thermochemical reduction of the sulfate near the site of deposition

genesis of calcretes in the devonian wood bay group dicksonland spitsbergen abstract a continuous pedogenic calcrete and numerous glaebular calcretes are exposed over approximately 150 km 2 in the devonian wood bay group dicksonland spitsbergen the prominent continuous calcrete horizon is up to 4 m thick and exhibits both prismatic and knobby weathering outcrops the prismatic calcrete outcrop of approximately hexagonally arranged vertical fractures usually shows gradational contacts with the underlying sediment the calcrete and clastic sediments immediately below the gradational contact show petrographic evidence for both replacive and displacive carbonate crystallization in contrast knobby calcrete is generally lenticular and probably represents an intra formational conglomerate both the knobby and prismatic calcretes have a brecciated fabric in which low ferroan calcite fragments are cemented and bound by dolomite some of the calcite fragments are coated by concentric laminae of dolomite the glaebular calcretes are composed of low ferroan calcite nodules ranging in size from a few millimeters to approximately 6 cm and show elongation ratios of approximately 1 5 normal to bedding these nodular zones are most common in floodbasin or levee sediments in both the stjordalen formation and the upper part of the keltiefjellet formation nodular horizons occur singly and in groups and vary from a single layer of nodules to beds several meters thick calcretes are common in the old red sandstone of great britain and rare in equivalent strata in spitsbergen suggesting differences in climate or other aspects of the depositional environment

development of framboidal pyrite during diagenesis low grade regional metamorphism and hydrothermal alteration distribution patterns microstructural relationships and compositional variations for framboidal pyrite in the paleozoic sedimentary host rocks to gold deposits on the northern carlin trend nevada and at fosterville in central victoria collectively demonstrate that it is not all of synsedimentary early diagenetic origin framboidal pyrite also nucleated in hydrothermal veins formed at several kilometers depth and along dissolution surfaces stylolites and crenulation cleavages formed during metamorphism at up to anchizonal grade i e prehnitepumpellyite facies there are no obvious differences in either the size range or internal morphology of framboids formed in these various settings suggesting that a single relatively simple crystallization path may operate over a wide depth range in the upper crust at fosterville anomalously metal rich including au framboidal pyrite is most abundant adjacent to laminated shear veins that developed by slip along bedding surfaces during folding of the sedimentary host rocks microstructural relationships and pb isotope compositions for the framboidal pyrite and other similarly distributed early formed sulfides support development in response to the localized episodic influx of metal enriched h 2s bearing hydrothermal fluids during regional deformation between 440 and 400 ma these findings may have implications for other similarly deformed metasedimentary successions where the discovery of au bearing framboidal pyrite has recently been cited as evidence for significant au enrichment during sedimentation 2009 society of economic geologists inc

climatic versus tectonic controls of fan sequences lessons from the dead sea israel tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated the pleistocene fans of the dead sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin exposures are excellent the rivers are currently incising in response to the low lake levels of the holocene producing cliffs 50m high these reveal an interdigitation of lake deposits dominantly fine grained carbonates and the coarse river gravels which characterize the fan deposits the bounds of each facies are sharp river sediments are superimposed on lake sediments with little evidence of either reworking or loading in turn aragonite crusts are draped over alluvial gravels indicating the sudden return of still water conditions neither can be explained by gradual adjustment of the hinterland river system after tectonic shock instead it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment in interpreting basin development in ancient settings it is always tempting to assume that coarse deposits are indicative of tectonism however the dead sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence

fossil sulphate reducing bacteria in the bleiberg lead zinc deposit austria polished ore samples from the world class bleiberg lead zinc deposit in austria were studied by field emission scanning electron microscopy fesem at magnifications in the range of 105 106 the zinc ore shows nano sized sphalerite filaments and spherules which are morphologically similar to recent biofilms of sulphate reducing bacteria the activity of sulphate reducing bacteria is suggested by a large 34s difference of 40 between coeval seawater sulphate 16 and sulphide sulphur 25 in the bleiberg ores and by variable sulphur valences peloids of sphalerite zn bearing calcite and pyrite have features typical of bacterial colonies combined with geological and mineralogical evidence a significant role of bacteria during ore deposition at bleiberg is likely

tectonic and sedimentary evolution of a cretaceous continental arc backarc system in the korean peninsula new view abstract this paper focuses on the tectonic and sedimentary evolution of the cretaceous volcanic rocks and nonmarine successions in the southeastern part of the korean peninsula which constitute an ancient continental arc system the arc system gyeongsang arc system comprising an arc platform gyeongsang volcanic arc and a backarc basin gyeongsang backarc basin was a southwestward extension of the japanese arc formed by oblique northward subduction of the proto pacific izanagi plate under the asian continent the backarc basin was initiated in the southeastern part of the korean peninsula in the early cretaceous as a narrow ns trending trough the basin was bounded by a major fault in the northern part of the basin from which sediments largely emanated sediments were deposited in streamflow dominated alluvial fans adjacent to the fault bounded basin margin in the north and low gradient fluvial systems of braided channels that extended southward and southeastward for tens of kilometers sediments were also derived from the western highland margin draining precambrian to jurassic basement rocks the initially narrow trough progressively expanded toward the east resulting in eastward migration of depocenters that eventually generated a broad fluvio lacustrine plain fringing the volcanic arc platform the arc platform played an important role for the derivation of volcanogenic materials and accreted sediments into the backarc basin via extensive fluvial network pyroclastic density currents and landslides which originated from the arc platform also entered the basin in addition extrusion of basaltic volcanic rocks was continual within the basin during basin expansion the resultant succession of mixed sedimentary and volcanogenic rocks is generally indicative of a temporal increase in volcanic activity in the arc platform and in subsidence rate of the basin in the late cretaceous andesitic to rhyolitic volcanism became climactic in the arc platform producing a number of calderas or volcanotectonic depressions volcaniclastic aprons fringing the arc platform encroached upon the basin fill intraarc basins produced by sinistral shearing of the arc platform received sediments from active volcanoes and footwalls of contemporaneous faults forming small scale streamflow dominated alluvial fans and floodplains with later development of a deep lake overall the gyeongsang arc system formed under an extensional or transtensional sinistral strike slip stress regime suggesting that the subducting and the overriding plates were not strongly coupled both the volcanic arc and the backarc basin ceased to develop as volcanic activities shifted progressively eastward in the paleogene accompanied with rollback of the subduction of the izanagi plate

large scale facies sequences in alluvial overbank environments abstract gradational coarsening upwards or fining upwards sequences order of meters thick in overbank environments can be produced by progressive crevasse splay levee progradation or abandonment while the channel belt remains unchanged in position in contrast meter scale facies sequences related to periodic channel bell avulsion may show relatively abrupt changes in facies and paleocurrent direction simulation models suggest that the number of such avulsion related facies changes in any overbank sequence bounded above and below by channel belt deposits is on the order of 1 to 10 this is confirmed using an ancient example in this example the average thickness of each avulsion bounded overbank facies sequence was 3 m assuming an average recurrence interval for avulsions of about 1 000 ye rs the mean overbank deposition rate was about 3 mm per year

sedimentation and evolution of the cretaceous gyeongsang basin southeastern korea the sedimentation and evolutionary history of the gyeongsang basin and their implications for plate tectonics are reviewed based mainly on sedimentological work on the gyeongsang strata and the regional geology of the japanese islands during the cretaceous e asia was characterized by an andean type continental margin formed by subduction of the kula plate and kula pacific ridge the marginal zone was subjected to precursory extensional tectonism of the magmatic arc so that fault bounded continental depressions developed including the proto gyeongsang basin the gyeongsang strata about 9 000 m thick consist in ascending order of the sindong the hayang with volcanic9 stics and volcanic yuchon groups during the sindong period extensional tectonism prevailed a the proto gyeongsang basin was a graben occupied by alluvial fan fluvial plain and lake environments from the margins to the centre of the basin streams flowed from the upthrown lip areas to the centre alluvial fans expanded periodically with rejuvenation of fault activity while the fluvial plaid lake boundary shifted in response to climatic changes during the hayang period the faults were no longer active and consequently the basin extended beyond them probably by back faulting the water level of the lake s at the basin centre fluctuated with changes of the evaporation precipitation ratio during parts of the period the marginal fluvial plain was drained by ephemeral streams which disappeared before reaching the lake s depositing fluvial sequences lacking channel sandstones localized volcanism formed volcanic terrains and supplied pyroclastics this volcanism was climactic during the yuchon period and terminated the sedimentation granitic plutons intruded all the gyeongsang strata at about the end of the cretaceous

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

sem imaging of bacteria and nannobacteria in carbonate sediments and rocks abstract bacteria and nannobacteria 0 1 m dwarfed forms are abundant in both non etched and gently etched travertines deposited by hot springs in viterbo italy they are also plainly visible in bahama ooids and hardgrounds and in great salt lake aragonite ooids and cements in addition nannobacteria are found in paleozoic and mesozoic rocks because of their extraordinary abundance in some specimens they probably play a prominent role in catalyzing the precipitation of carbonate materials

a small scale flood plain a miniature 9 m wide floodplain developed along a gravel washing effluent stream shows features such as levees crevasse splays and floodbasins which compare with their larger scale counterparts for sediments deposited overbank median size decreases exponentially with distance from the channel whilst sorting increases with coarser sediment on the outside of a meander bend overbank flows are only a few grain diameters in depth near the channel this study shows potentially useful systematic relationships in floodplain sediment textures but it involves only one of a possible variety of floodplain types dominated by overbank sedimentation this suggests that further exploration of overbank depositional processes is desirable as an aid to field interpretation

contrasting development patterns of crevasse channel deposits in cretaceous alluvial successions korea abstract the eumsung and pyonghae basins both of cretaceous age in korea comprise a thick alluvial sequence of conglomerate sandstone and purplish siltstone laterally impersistent gravelly sandstone beds can be classified into two types based on bed geometry internal structure and occurrence a vertically stacked sheetlike bodies facies 1 and b migratory ribbon shaped bodies facies 2 facies 1 is characterized by symmetrical channel geometry with a large ratio of width to thickness of sandstone beds which are mostly stratified and bounded by relatively distinct lower and upper boundaries they are frequently found in aggradational style facies 2 shows asymmetric geometry with a relatively small ratio of width to thickness of sandstone beds it comprises massive cross stratified units bounded by sharp basal contacts and gradational upper boundaries which resulted from laterally shifting mode both types represent crevasse distributary channels developed in floodplain environments facies 1 is suggestive of gradual migration of crevasse distributary channels under the conditions of overall aggradation whereas facies 2 represents discrete abrupt diversion of crevasse channels these features are due to differences in floodplain gradients paleohydrology and trunk channel behaviour

sandy high energy flood sedimentation some criteria for recognition with an example from the devonian of s w england abstract many modern sand deposits resulting from episodic floods in semi arid zones are dominated by parallel laminations but parallel laminated sands are not a major feature in perennial streams single flood events may deposit over 1 5 m of parallel laminated sand hence ancient parallel laminated sand dominated alluvial deposits may be the product of ephemeral flows the trentishoe formation hangman sandstone group middle devonian of north devon consists in part of laterally extensive beds of parallel laminated fine grained red sandstones that build multistorey sand bodies individual beds are little more than 1 m thick but may extend laterally for hundreds of metres a vertical sequence of erosion surface sparse silt clasts parallel laminations silt drape is often found suggesting high flow stage deposition of plane bedded sand followed by a very rapid waning of flow with silt fallout in the final stages of the flood this sequence bears marked similarity to certain modern ephemeral flow sands

subaerial debris flow deposition in the upper paleozoic cutler formation western colorado abstract outcrops of the lower part of the cutler formation near gateway colorado contain proximal alluvial fan deposits of two principal lithofacies the diamictite lithofacies d consists of a variety of sharply bounded massive muddy beds which are interpreted as deposits of subaerial debris flows the gravelly sandstone lithofacies s consists of amalgamated beds of laminated and cross bedded conglomeratic sandstones which are interpreted as water deposited sediments clasts in s are rarely larger than 0 5 m in maximum diameter but clasts in d are commonly much larger up to 8 m both normal and inverse grading are visible in d but grading is generally not well developed in either lithofacies imbrication of clasts although more pronounced and more consistent in s is fairly commo in both lithofacies four subfacies of diamictite are distinguishable on the basis of clast packing and grading massive matrix supported dram graded matrix supported dmg inversely graded clast supported dci and massive clast supported dcm given the hydrodynamic and rheologic properties of debris flows as reported by previous workers four debris flow regimes are postulated on the basis of varying water content and clast interaction subfacies dram and drag typify deposits of flows having viscous clast interactions subfacies dci represents deposits of flows having inertial or colliding clast interactions subfacies dram comprises deposits of flows with demonstrably high yield strengths subfacies dcm represents deposits of flows in which yield strength was insufficient to support most clasts this classification specifically recognizes turbulence selective deposition and bedload transport of clasts as essential characteristics of certain dilute debris flows it also accommodates as debris flow deposits some beds which have features usually associated with aqueous deposition including clast frameworks channelization imbrication and normal grading

extreme 34s depletions in zns at the mike gold deposit carlin trend nevada evidence for bacteriogenic supergene sphalerite we identified submicrometer sized framboidal sphalerite zns below the base of supergene oxidation in a carlin type gold deposit of eocene age in nevada united states where the framboidal sphalerite forms a blanket like body containing 400 000 metric tons of zinc framboidal sphalerite 0 1 mum in diameter formed in the early miocene ranges from 0 1 to 0 35 mol fes the delta 34 s values range from 25 to w 70 the lowest values measured in a marine or terrestrial environment these s isotope data demonstrate the involvement of sulfate reducing bacteria and provide the first documentation that sphalerite can form significant supergene sulfide enrichment blankets

the cretaceous pyonghae basin southeast korea sequential development of crevasse splay and avulsion in a terminal alluvial fan abstract the pyonghae basin early cretaceous southeast korea comprises a thick sequence of purplish siltstone with subordinate gravelstone and gravelly sandstone deposited in a terminal alluvial fan environment based on bed geometry and facies assemblage the gravelly sandstone bodies are divided into four types type a deposit consists of disorganized gravel patches and massive sandstone lenticules a few decimetres thick and a few metres wide most likely deposited from overflows type 2 deposit is characterized by crudely stratified and massive sandstone beds up to 2 m thick 20 m wide and comprises lower disorganized gravel layers and upper stratified gravelly sandstone beds forming a fining upward trend it probably represents crevasse channel distributary channel fills type 4 deposit 3 5 m thick 20 m wide consists of stratified cross stratified gravelly sandstone beds which are interpreted as main braided channel fills aggradation of the terminal fan was associated with frequent shift of channels which range from crevasse splay channel type 2 through crevasse channel distributary channel type 3 to main channel type 4 the crevassing or avulsion was preceded by overflows type 1 the avulsion processes in the terminal fan were achieved through stepwise enlargement of crevasse channels which reflect different channel responses to floods of different magnitude and frequence

uncovering framboidal pyrite biogenicity using nano scale cnorg mapping framboidal pyrite has been used as a paleo redox proxy and a biomarker in ancient sediments but the interpretation of pyrite framboids can be controversial especially where later overgrowths have obscured primary textures here we show how nano scale chemical mapping of organic carbon and nitrogen cn org can detect relict framboids within precambrian pyrite grains and determine their formation mechanism pyrite grains associated with an ediacaran fossil lagerstatte from newfoundland ca 560 ma hold significance for our understanding of taphonomy and redox history of the earliest macrofossil assemblages they show distinct chemical zoning with respect to cn org relict framboids are revealed as spheroidal zones within larger pyrite grains whereby pure pyrite microcrystals are enclosed by a mesh like matrix of pyrite possessing elevated cn org replicating observations from framboids growing within modern biofilms subsequent pyrite overgrowths also incorporated cn org from biofilms with concentric cn org zoning showing that the availability of cn org progressively decreased during later pyrite growth multiple framboids are commonly cemented together by these overgrowths to form larger grains with relict framboids only detectable in cn org maps in situ sulfur isotope data 34 s 24 to 15 show that the source of sulfur for the pyrite was also biologically mediated most likely via a sulfate reducing microbial metabolism within the biofilms relict framboids have significantly smaller diameters than the pyrite grains that enclose them suggesting that the use of framboid diameters to infer water column paleo redox conditions should be approached with caution this work shows that pyrite framboids have formed within organic biofilms for at least 560 m y and provides a novel methodology that could readily be extended to search for such biomarkers in older rocks and potentially on other planets

biogeochemistry of the 1640 ma mcarthur river hyc lead zinc ore and host sediments northern territory australia abstract the formation of the mcarthur river lead zinc deposit involves thermogenic or biologic oxidation of sedimentary organic matter the products of which generated a massive stratiform sulfide ore body and secondary carbonate and silica precipitates formed within the sediment pile down the flow pathway and above the reaction zone the fine grained texture of the mineralization indicates that primary ore texture is preserved and coupled with the regional thermal maturity indicate that this deposit is an ideal location to study organic matter signals related to ore formation and the sedimentary environment biomarker data point to a marine environment of deposition and are consistent with data previously collected from the host barney creek formation in the adjacent glyde subbasin an unusual biomarker distribution found in some samples from within two orebody is considered to be related to the presence of sulfide oxidizing bacteria these organisms flourished after turbidite deposition when oxygen in the upper part of the water column was mixed down to the sediment water interface the biomarker data are supported by micropalaeontologic observations from the same samples and are consistent with intermittent oxygenation of the water column to the sediment water interface this observation suggests an extension of the known occurrence of sulfide oxidizing bacteria back in time by 800 million years to 1640 ma

microbial structures and possible bacterial sulfide fossils in the giant jinding zn pb deposit yunnan sw china insights into the genesis of zn pb sulfide mineralization abstract the jinding zn pb sulfide deposit lanping yunnan china is one of the youngest sediment hosted giant zn pb deposit in the world it is also a well known deposit in china the large scale of the mineralization and especially the huge volume of h 2 s that was required to precipitate the zn pb sulfide are of great interest to ore geologists ore microscopy shows microbial structures such as framboidal sphalerite spherical aggregates of galena sphalerite oolite sphalerite dendritic colloidal sphalerite pyrite metacolloidal sphalerite pyrite multinuclear metacolloidalring like sphalerite pyrite botryoidal sphalerite pyrite framboidal pyrite and bacterial plate like of galena sphalerite detailed observations and analyses of the microbial structures at micron to nanometer scales by field emission scanning electron microscopy fesem and energy dispersive spectrometry eds reveal that the bacteria fossils are composed of sphalerite galena or pyrite or of multiple sulfide minerals with calcite and that they are distributed in bacterial colonies in the ore the individual bacteria fossils are spherical botryoidal or ring like with a diameter of 200 500 nm filamentous mucoid and tubular extracellular polymeric substances eps are commonly observed that are closely associated with and have the same composition as the bacteria fossils secondary ion microprobe analysis sims by in situ sulfur micro isotopic analysis of the microbial sulfide structures and bacteria fossils yield 34 s v cdt values varying from 48 6 to 9 5 per mil indicating that there was a high percentage of sulfate reducing bacteria in the zn pb sulfide mineralization process previous analyses have shown that the zn pb mineralization was formed at a shallow depth and low temperature with barely visible hydrothermal alteration the presence of evaporitic sulfate layers and paleo oil reservoir characteristics the authors believe that bacterial sulfate reduction might be the key h 2 s production mechanism in the jinding dome that was required to form this giant pb zn sulfide ore body

controls on synorogenic alluvial fan architecture beartooth conglomerate palaeocene wyoming and montana late palaeocene uplift of the beartooth range in northwestern wyoming and southwestern montana generated the beartooth conglomerate along the eastern and northeastern flanks of the range systematic unroofing sequences and intraformational unconformities folds and faults in the conglomerate attest to deposition during uplift along the eastern flank at least three ancient alluvial fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies the two southern fans consist of 700 m of sedimentary clast conglomerate and subordinate sandstone dominated by hyperconcentratedflow and stream flow facies the next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud matrix supported debris flow facies as well as stream flow facies the northernmost depositional system consists of arkosic channellized fluvial conglomerate and sandstone overbank mudstone and crevasse splay sandstone units palaeocurrent data indicate eastward dispersal away from the beartooth range outstanding exposure of the beartooth conglomerate allows facies to be mapped on lateral photographic mosaics a seven fold hierarchy of bounding surfaces and enclosed lithosomes exists in the beartooth conglomerate first through fourth order surfaces are analogous to first through fourth order surfaces that recently have been documented in sandy fluvial facies with one exception sediment gravity flows are bounded by first order surfaces fifth order surfaces are either erosional e g lateral migration of fanhead trench or accretionary e g aggradation of fan surface during backfilling of trench and construction of lobes on lower fan during entrenchment on upper fan some fifth order surfaces coincide with intraformational angular unconformities and are thus the result of long term fanhead entrenchment following uplift of the upper part of the fan sixth order surfaces bound individual fan packages that are several hundred metres thick and 10 km2 in area the enclosed sixth order lithosomes are distinguishable in terms of petrofacies and lithofacies a single seventh order surface bounds the entire beartooth conglomerate lower order lithosomes are produced by intrinsic processes of fan construction fifth order lithosomes can be attributed to both extrinsic and intrinsic controls sixth and seventh order lithosomes are generated by extrinsic controls

splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central australia abstract splays are common features of many rivers and deltas forming as a result of overbank flooding and sediment deposition but they have rarely been described from modern dryland settings this paper investigates splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central australia here splays vary from small lobate or tongue shaped features 1 km long to larger elongate features up to 4 km long and are supplied with rare floodwater and sediment through well defined breaches in the upper parts of the main channel banks in proximal and medial parts splay channels are incised up to 2 m into pre splay fine grained floodplain sediments but remain elevated between 0 5 m and 2 m above the main channels only in the very distal parts are splay channels less deeply incised either grading into the floodplain surface or terminating as sediment lobes that are deposited upon the floodplain aerial photographs and field observations indicate that many splays develop rapidly during large floods and are modified during infrequent subsequent floods four main stages of splay development are proposed 1 aggressive overbank flow results in bank line breaching and channel scour on the floodplain but little sediment deposition 2 coarse bed sediment is diverted from the main channel preventing further deepening at the bank line breach and slowing channel scour 3 increasing coarse sediment transport through the bank line breach and scoured channels leads to the formation of distal sediment lobes 4 subsequent floods divert around these distal lobes sometimes leading to further channel scour and lobe formation on the northern plains splays contribute to downstream decreases in the discharge and size of the main channels and splay deposits form local accumulations of gravelly sand in the predominantly fine grained floodplain successions some splays ultimately may evolve into distributaries or anabranches the dominantly erosional bases of these splay channels contrast with typical descriptions of many humid region splays but they do not display any geomorphological or sedimentological features that are necessarily diagnostic of formation in a dryland setting

peloids in stratabound zn pb deposits and their genetic importance peloids from sediment hosted zn pb deposits in belgium ireland and poland are composed of zn calcite siderite smithsonite silica pyrite melnikovite sphalerite galena thiosulphates and zn as bearing limonite the size of peloids is between 7 and 60 m the core and the shell of peloids are composed of different minerals and the shell is usually coarsely crystalline peloids are present in collapse breccias of karst cavities stromatactis cavities debris flow breccias and fractures cross cutting carbonates hosting the mineralization in places peloids form a major microtexture of the sulfide mineralization peloids are cemented and replaced by sulfides organic acids extracted from sulfide peloids are composed of amido acids considered to be characteristic of bacterial origin the size and specific texture of peloids are independent of mineralogy location and age of the mineralization therefore the peloids disussed are considered as forms induced by bacterial activity rather than forms precipitated inorganically

a small alluvial crevasse splay abstract a crevasse splay deposited on the floodplain of the clarence river in northeastern new south wales australia by a small ephemeral tributary provides a guide to the geometry and sedimentary structures of crevasse splays in general and to the processes affecting their final form the splay consists of a main lobe of sand with smaller sand fingers that end in slipfaces around its edges water seeping out of the channel reworks part of the splay into a gently thinning sheet small scale facies models derived from different parts of this splay explain the origins of solitary planar cross sets and gently inclined bedding in some ancient crevasse splay sandstones

the nature of fossil bacteria a guide to the search for extraterrestrial life in an attempt to establish reliable criteria for the identification of potential fossil life in extraterrestrial materials the fossilizable characteristics of bacteria namely size shape cell wall texture association and colony formation are described and an overview is given of the ways in which fossil bacteria are preserved as compressions in fine grained sediments preservation in amber permineralized by silica replacement by minerals such as silica pyrite fe mn oxides calcite phosphate and siderite or as molds in minerals the problem of confounding minerally replaced bacteria with non biological structures having a bacterial morphology is addressed examples of fossilized bacteria from the early archaean through to the recent are used to illustrate the various modes of preservation and the morphology of fossil bacteria

fluvial channel and overbank deposits from the westphalian of the durham coalfield ne england the westphalian upper carboniferous coal measures of the durham coalfield in ne england were deposited in lower and upper delta plain environments distributary channels crossed the plain and were separated by shallow interdistributary lakes and bays detailed observation of three dimensional 3 d opencast surface mine exposures in collaboration with subsurface borehole analysis has revealed the existence of five varieties of channel deposits and two associated overbank facies within the durham coal measures major distributary channels were the major avenues of sediment transport across the coal measures plain were variably sinuous mostly 1 2 km wide and deposited elongate belts of sand mostly up to 5 km wide proximal major crevasse splay channels formed by the breaching of major channel banks during flood events were straight sand filled and up to 400 m wide minor distributary channels formed by the sustained operation of such crevasses varied from straight to highly sinuous and deposited shoestring sand mud belts up to a few hundred metres wide minor crevasse channels generally straight and up to 50 m wide were formed through bank breaching of minor and in a few cases major distributaries distal feeder channels formed down palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems were generally straight and up to 200 m wide of the two types of channel overbank levee deposits recognized one comprising thinly interbedded fine grained sandstone and siltstone claystone is mostly though not exclusively associated with major distributary channels the other consisting of massive siltstones with regularly spaced thin claystone bands is uniquely developed at the margins of minor distributary channels the lower part of the westphalian a succession in the northern pennines records a change in the depositional environment upwards from a lower to upper delta plain through this transition major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology channel sedimentation was profoundly influenced by regular possibly seasonal variations in flow stage and sediment load

bacteria as nucleation sites for authigenic minerals in a metal contaminated lake sediment abstract sediment samples contaminated with metals arising from mine tailings drainage were obtained from lower moose lake in the onaping region near sudbury ontario canada the samples were examined by electron microscopy selected area electron diffraction and energy dispersive x ray spectroscopy individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic fe al silicate the principal metal sulfide species associated with the bacteria were amorphous mackinawite fes 1 x and microcrystalline millerite nis trace amounts of cu and zu were also detected in some of the sulfide precipitates at least two structural forms of the fe al silicate were present and energy dispersive x ray spectroscopy point analyses revealed corresponding differences in chemical composition poorly ordered limonitic clay type phases had a granular morphology and contained less fe than well developed crystalline material which generated hexagonal diffraction patterns with reflections d 4 60 and 2 55 characteristic of an interstratified chamositic clay

biodegradation of hydrocarbons and biogeochemical sulfur cycling in the salt dome environment inferences from sulfur isotope and organic geochemical investigations of the bahloul formation at the bou grine zn pb ore deposit tunisia abstract combined organic geochemical and stable isotope s analyses of samples from the cretaceous bahloul formation tunisia provide insight to oil accumulation processes biogeochemical alteration of hydrocarbons microbial sulfate reduction and mineral deposition at the flanks of the triassic jebel lorbeus diapir forming the bou grine zn pb deposit the sulfur isotopic composition of the metal sulfides correlates with the degree of biodegradation of hydrocarbons with the base metal content and with the proportion of aromatics in the organic extracts the 34 s values are interpreted to reflect bacterial sulfate reduction in a more or less closed system rather than a thermogenic contribution the extent of h 2 s production by the activity of the sulfate reducing bacteria probably was limited by the availability of sulfate which in turn was governed by the permeability of the respective sedimentary sequence and by the distance to the anhydrite cap rock evidence is provided that biodegradation of hydrocarbons and microbial sulfate reduction contribute to the formation of the high grade mineralization inside the bahloul formation at the contact with the salt dome cap rock the metals probably were derived through leaching of deeper sedimentary sequences by hot hypersaline basinal brines evolved by dissolution of salt at the flanks of the diapirs these hot metalliferous brines are proposed to migrate up around the diapir finally mixing with near surface sulfate rich brines in the roof zone when the fluids came in contact with the organic rich sediments of the bahloul formation the dissolved sulfate was reduced by the sulfate reducing bacteria hydrocarbons generated or accumulated in the bahloul formation were utilized by sulfate reducers the occurrence of high amounts of native sulfur in high grade ore samples suggest that the production rate of h 2 s by bacterial sulfate reduction exceeded its consumption by metal sulfide precipitation the supply of dissolved metals probably was the limiting reaction the trend of 34 s of native sulfur s 0 towards lower values to the top of the highly mineralized bahloul profile provides evidence for a biogenic origin of s 0 by sulfide oxidizers the variation in isotopic fractionation between native sulfur and sulfide sulfur in the ore minerals reflects the kinetic isotope fractionation factor connected with metabolic processes and the respective reaction rate during biogenic oxidation of sulfur

alluvial fans and their tectonic controls the study of alluvial fan sequences the most proximal components of the alluvial spectrum can be one of the most profitable avenues of research in basin analysis because fans are particularly sensitive to external influences and have a relatively high preservation potential tectonism has long been invoked to explain many variations in fan sediments and increasingly the presence and nature of fan sequences in the rock record is being used to denote proximity to active faults and to document movement on these faults there are however many difficulties involved with the interpretation of alluvial fan sequences not the least of which is deciding whether the sediments are truly components of an alluvial fan or the products of a different depositional system such as an alluvial braidplain many of the problems arise from a lack of knowledge about the relative roles in fan construction of allocyclic processes such as climatic change and tectonic activity and of autocyclic processes such as channel avulsion and fanhead entrenchment whilst researching alluvial fan sediments in a variety of tectonic settings and ages it has become increasingly clear to the authors that there are many inadequacies in our understanding of alluvial fans and hence weaknesses in the conclusions drawn from these sediments this prompted the one day meeting organized to promote discussion of the problems of distinguishing tectonic from other influences on the evolution of modern and ancient fans 110 industrial and academic geologists gathered at the university of bristol on 17th october 1987 to hear 16

pyrite discs in coal evidence for fossilized bacterial colonies discs of pyrite from 1 to 3 mm in diameter and 100 m thick were observed within fracture planes in coal from the black mesa coal deposit in northeastern arizona the pyrite discs were composed of aggregates of crystals which suggested that sulfide mineral diagenesis had initiated at multiple nucleation sites and occurred prior to the compaction forces occurring during coal formation stable sulfur isotope analysis of the discs 34s 31 7 supports a bacterial origin resulting from dissimilatory sulfate reduction fossilized bacteria on the disc surfaces average 27 100 m2 appeared as halos when viewed using reflected light microscopy but were lenticular by scanning electron microscopy each microfossil being 2 3 m in length a fossilized bacterial colony pyrite disc 1 mm in diameter would contain 2 1 107 microfossils these microfossils were not observed on hydrothermal pyrite coating and in filling of sulfate reducing bacteria with iron disulfide during in vitro sulfide mineral diagenesis provide mechanisms to explain the preservation of the three dimensional lenticular microfossils observed on the pyrite discs

evidence of uranium biomineralization in sandstone hosted roll front uranium deposits northwestern china we show evidence that the primary uranium minerals uraninite and coffinite from high grade ore samples u3o8 0 3 in the wuyiyi wuyier and wuyisan sandstone hosted roll front uranium deposits xinjiang northwestern china were biogenically precipitated and psuedomorphically replace fungi and bacteria uranium vi which was the sole electron acceptor was likely to have been enzymically reduced post mortem accumulation of uranium may have also occurred through physio chemical interaction between uranium and negatively charged cellular sites and inorganic adsorption or precipitation reactions these results suggest that microorganisms may have played a key role in formation of the sandstone or roll type uranium deposits which are among the most economically significant uranium deposits in the world

alluvial to lacustrine systems in a pull apart margin southwestern eumsung basin cretaceous korea abstract in the pull apart margin of pull apart basins sedimentation patterns are poorly known compared with those in rift basins the rhomb shaped eumsung basin cretaceous mid west korea was formed in an overstep of the kongju fault system and provides significant information on how sedimentary bodies dynamically filled the pull apart margin according to a detailed analysis of sedimentary facies palaeoflow directions clast compositions and downstream changes in clast size the southwestern basin fills comprise three facies associations representing distinctive depositional environments 1 facies association f a i volcaniclastics dominated alluvial fan 2 f a ii alluvial plain and 3 f a iii floodplain lake the matrix supported and clast supported conglomerate f a i is indicative of deposition in alluvial fans the hollow fill or sheetlike deposits encased in purple siltstone f a ii are suggestive of channel or sheetflood deposition in an alluvial plain the dominant purple and green mudstone f a iii indicates deposition in floodplain and lacustrine environments the distribution of facies associations represents a basinward change in depositional environments from alluvial fan to lacustrine systems the succession is divisible into three large scale lateral and partly vertical units the southernmost ochang 1 5 km thick the medial baeti 1 km thick and the northernmost kugokri units 0 5 km thick on the basis of basinal structure and changes in clast composition and lithofacies the ochang baeti transition reflects synchronous development of the alluvial to lacustrine systems in a pull apart margin this succession development is attributable to the basinal faults progressed during the basin development the baeti kugokri transition recorded the changes in clast composition and lithofacies which was due to the progressive shifting in basinal setting from a pull apart margin to a strike slip margin unlike that of the strike slip margins clast composition of the pull apart marginal fills reflects adjacent source rock lithology in the pull apart margin alluvial to lacustrine systems show a transverse to margin channel network 060 135 converging into the southwestern depocentre although the basin is of strike slip origin sedimentation patterns in the pull apart margin are suggestive of relatively synchronous developments of the alluvial to lacustrine systems reflecting the lithology of the adjacent source rock and subsurface structure of normal slip dominant faults

the influence of grain size analysis methods and sediment mixing on curve shapes and textural parameters implications for sediment trend analysis abstract to this day deterministic physical models capable of explaining the evolution of grain size distributions in the course of transport are still lacking for this reason various attributes of particle frequency distributions in particular curve shapes and textural parameters have for many decades been investigated for potential information about transport behaviour and size sorting processes of sediments in numerous environments such approaches are essentially conceptual and hence rely heavily on the validity of the assumptions on which they are based a factor which has to date been largely ignored in this context is the fact that different methods of grain size analysis e g sieving laser absorption and diffraction settling velocity measurements when applied to the same sample material produce variable curve shapes and hence incongruous textural data this is illustrated by selected examples showing the differences between sieving and settling results conversion of settling velocities into equivalent settling diameters psi phi transformations and the influences of particle shape particle density and water temperature it is demonstrated that particle size distributions are not only method dependent but also dependent on the adopted post processing procedure as a result only frequency curves generated by the same method and subsequently processed by identical computational procedures can be meaningfully compared furthermore the computation of textural parameters from bi or multimodal size distributions produces spurious results which are unrelated to the processes leading to the mixing of different size populations frequently observed in nature in such cases only the decomposition of such distributions into individual populations and the spatial comparison of such populations makes any sense because a physical explanation for the generation of size distributions is lacking a particular curve shape of a grain size population has no meaning on its own only a systematic comparison of progressively changing curve shapes and associated textural parameters of sediments collected on a closely spaced grid can yield data suitable for sediment trend analysis

extending the upper temperature limit for life the upper temperature limit for life is a key parameter for delimiting when and where life might have evolved on a hot early earth the depth to which life exists in the earth s subsurface and the potential for life in hot extraterrestrial environments a combination of geological and

extremely negative and inhomogeneous sulfur isotope signatures in cretaceous chilean manto type cu ag deposits coastal range of central chile abstract chilean manto type cmt cu ag hydrothermal deposits share a characteristic association of volcano sedimentary jurassic to lower cretaceous host rocks style of mineralization ore and associated mineralogy and geochemistry with ore grades typically 1 cu that make this family of deposits significant and interesting both academically and economically although often stratabound geological evidence supports an epigenetic origin for these deposits we present a detailed stable isotope study of la serena and melipilla naltahua lower cretaceous deposits central chile which reveals extremely negative 34 s values to 50 which are among the lowest values found in any ore deposit in addition the range of 34 s values from sulfides in the two areas is very wide 38 3 to 6 9 in la serena and 50 4 to 0 6 in melipilla naltahua these new data significantly extended the reported range of 34 s data for cmt deposits co existing sulfates range from 7 9 to 14 3 and are exclusive to la serena deposit the wide sulfide isotopic range occurs at deposit and hand specimen scale and suggests a polygenic sulfur source for these deposits where bacteriogenic sulfide dominates while sulfur isotope data for the bulk of jurassic cmt deposits northern chile suggests a predominant magmatic source in their origin mean 2 7 1 9 1 contributions of a magmatic component is only likely to be involved at melipilla naltahua deposit the 13 c values obtained for calcites associated with the mineralization range from 20 1 to 0 2 also suggesting polygenic carbon sources with the likely strong involvement of degradation of organic matter and leaching of limestone two different genetic models with involvement of hydrocarbon are proposed for both areas for melipilla naltahua a two step model can be developed as follows 1 framboidal pyrite growth with very low 34 s formed by bacterial sulfate reduction in an open system and with diagenetic degradation of oil related brines leaving pyrobitumen 2 cu bearing stage replacing of framboidal pyrite inheriting depleted sulfur as low as 50 4 together with sulfides directly precipitated from a hydrothermal fluid with 34 s close to 0 for la serena a single step model fits best without framboidal pyrite generation cu bearing sulfides were precipitated mainly in veins where cu plus base metal bearing hydrothermal fluids mixed with h 2 s generated by bacterial sulfate reduction in the host rocks isotopic evidence clearly illustrates that bacterial activity perhaps enhanced by hydrothermal activity was fed by hydrocarbon brines and sulfate remobilized from continental evaporites it is possible that variable ecological conditions led to different extents of isotopic fractionation adding to the typical sulfur isotopic heterogeneity of such bacterial systems for both areas the cu bearing stage occurred during the peak to waning stages of the very low grade metamorphism that affected the lower cretaceous sequence