active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments the salton sea geothermal system california the salton sea geothermal system is an area of active hydrothermal metamorphism of pliocene to pleistocene deltaic lacustrine and evaporitic sediments deposited in a modern continental rift zone temperatures up to 365 degrees c hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low pressure greenschist and amphibolite facies metamorphism at depths less than 3 2 km drill cores recovered by the salton sea scientific drilling project shed new light on the nature and origin of ore formation in the eastern portion of the salton sea geothermal system base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system type 1 veins are dominated by carbonate contain pyrrhotite and other sulfides are completely sealed and occur in wall rock showing minimal alteration fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system type 2 veins are dominated by silicates and hematite are permeable and occur in wall rocks showing pervasive chloritization and epidotization fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient fluid inclusions record an apparent steep progressive salinity increase in the vein systems in the upper 2 km of the system however fluid production data from salton sea geothermal wells indicate that reduced metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized metal poor fluid averaging 5 wt percent total dissolved solids a sharp interface exists between the fluids at 1 to 2 km paralleling the 250 degrees c isothermal surface a model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids magmatic intrusion induces fracturing heating and the expulsion of reduced connate fluids to cause early type 1 vein mineralization following the rising thermal front deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface forming type 2 veins the hypersaline brine rises to a level of density temperature stabilization causing pervasive greenschist metamorphism of the host sediments the diapir then begins to cool and descend shallow oxidized brine is drawn in as the hypersaline brine descends promoting late stage hematite in type 2 veins the eastern salton sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone deposition of metal bearing deltaic sediments to form a closed basin sedimentary environment episodic lacustrine evaporite formation and injection of heat and elements by rift related magmatic intrusions at depth the presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform fe zn pb cu sulfide and oxide deposits in paleorift zone environments

the context of lowstand events in the kimmeridgian late jurassic sequence stratigraphic evolution of the wessex weald basin southern england abstract a depositional sequence stratigraphic framework for the kimmeridgian stage of the wessex weald basin southern england is applied in the investigation of the sedimentary and erosional response to proposed lowstand events this framework recognises 13 sequence boundaries km1 km13 and is based on the integrated assessment of geophysical sedimentological palaeontological and geochemical data changes in the nature and continuity of kimmeridgian unconformity surfaces record the effect of the reorganisation of the onshore basins in response to eustasy and tectonic movements during the earliest kimmeridgian the basin geometry was such that falls in relative sea level generated widespread unconformities associated lowstand events are recorded in marginal areas by a number of sand units derived from a variety of local sources a tectonically enhanced deepening event during the late early kimmeridgian eudoxus zone led to unconformities being confined in their distribution to the basin margins as sea level fell during the latest kimmeridgian a progressive reduction in the depositional area led to an amplification of the effects of lowstand processes at the basin margins where unconformity surfaces are amalgamated on the most positive areas these surfaces are overprinted by a major interregional hiatus the late cimmerian unconformity the timing and effects of kimmeridgian lowstand events were intimately linked to the tectonic evolution of the wessex weald basin thus at times when rates and amounts of eustatic change are small i e during greenhouse times the tectonic signature of a basin will be the primary signal recorded in sedimentary successions

fe zn cu pb mineralization in the salton sea geothermal system imperial valley california the salton sea geothermal system is an area of active metamorphism in a plio pleistocene deltaic sedimentary sequence within it hypersaline brines containing up to 250 000 ppm total dissolved solids and temperatures up to 365 degrees c are causing incipient sulfide and oxide mineralization we have made a detailed analysis of ore mineralization in two boreholes drilled to depths of up to 2 500 m in this system major ore minerals observed are in order of decreasing abundance pyrite hematite sphalerite chalcopyrite pyrrhotite marcasite and galena the associated nonore minerals are mainly calcite quartz epidote anhydrite adularia and chlorite ore mineralization can be divided into three main types diagenetic sulfide mineralization occurring at depths less than 760 m and temperatures less than 250 degrees c consists of fine grained stratiform iron sulfides occurring as cement in sandstone and as disseminations and bands in shale metamorphic sulfide mineralization occurring at depths greater than 760 m and temperatures greater than 250 degrees c includes the progressive development of porphyroblastic pyrite with increasing depth and temperature its replacement by other fe cu zn pb sulfides at depths greater than 1 220 m and its decomposition into skeletal aggregates at depths greater than 1 525 m concurrent with hornfelsic recrystallization of the host sediments vein and related pore filling ore mineralization is of two contrasting types hematite silicate sulfide or sulfate and sulfide carbonate or silicate the first type occurs in open porous veins in single vertical zones at least 220 m thick in each borehole the second type occurs in veins that are generally sealed and occur in relatively thin scattered vertical intervals in both boreholes thermodynamic analysis indicates that the modern reservoir brines are near equilibrium with the hematite silicate sulfide or sulfate vein assemblage at 300 degrees c with ph 5 4 and log a sub o 2 30 reactions between hematite pyrite and iron bearing silicates chlorite epidote probably control the redox state of the present brines calculated metal chloride complex solubilities for fe zn pb and cu agree well with brine analyses hematite pyrite and chalcopyrite are supersaturated galena and sphalerite are undersaturated in the modern brines the phase relations of the presently sealed sulfide carbonate or silicate veins require that earlier fluids if formed at the same temperature and ph had to be more reduced and sulfur rich than the present slightly oxidized sulfur poor brines contrasting fluid redox states may be caused by a late lateral or fault controlled influx of more oxidized surficial waters lack of acid alteration implies boiling has not occurred to produce the oxidized fluids early formed diagenetic iron sulfides are commonly replaced by later cu pb zn sulfides and iron oxides and enveloped and resorbed by veins acting as nuclei and sources of s and fe for later mineralization precipitation probably occurs when seismic or hydraulic fracturing allows the metal rich sulfur poor brines to interact with earlier pyrite sulfur in the host rocks thus the geothermal system as presently explored is best classified as an incipient stratabound sulfide deposit that is being overprinted by hydrothermal and metamorphic processes

a pulse in the planet regional control of high frequency changes in relative sea level by mantle convection abstract an explanation for high frequency cycles of sea level in non glacial times has remained elusive despite more than two centuries of research since lavoisier9s seminal observations were published in 1789 in the development of seismic stratigraphy in the 1970s putatively global high frequency changes in relative sea level vail third order cycles were attributed to an unknown eustatic mechanism prompting a search for mesozoic ice ages over the last decade a regional mechanism of sea level control has been developed from studies of the sedimentary record in high quality oil industry data these geological studies have supported the geophysical prediction that significant regional control of sea level is exercised by mantle induced vertical motions of the earth9s surface these vertical motions can occur over time intervals from several tens of million years to less than a million years with amplitudes of tens of metres or more even at the shorter intervals the vertical motions are not confined to regions with major hotspots there are two related controls of surface vertical motion evolution of mantle convection cells and pulsing flow within each cell the effects are evident in the sedimentary record of north atlantic basins mantle convection provides an alternative regional mechanism to eustatic control for explaining medium frequency to high frequency sea level cycles

palynostratigraphy of the classic portland and purbeck sequences of dorset southern england and the correlation of jurassic cretaceous boundary beds in the tethyan and boreal realms

bathymetric mapping of the coastal and offshore geology and structure of the jurassic coast weymouth bay uk four hundred square kilometres of 1 m binned full coverage swath bathymetry data integrated with similar resolution onshore topography have been used to generate a seamless onshore to offshore bedrock map covering an extensive area adjacent to the jurassic coast world heritage site analysis of these data provides new insights into the structural development of the purbeck monocline cenozoic inversion structure in particular variations in the expression of strain between the hanging wall block and the fault inversion zone the footwall to the basin bounding faults compartmentalized deformation and uplift and acted as a buttress to compression the data also show a limited thickness changes within the major lithostratigraphical divisions and a notable absence of basin related extensional faulting in the offshore area that is in marked contrast to the more extensively studied onshore region this indicates that prior to inversion the basin evolved by intermittent activity on a few major extensional faults this improved understanding of the development of the basin and inversion structures results from our ability to integrate and quantitatively manipulate these high resolution and spatially extensive offshore and onshore datasets

broad weak regions of the nankai megathrust and implications for shallow coseismic slip abstract deep within the nankai trough subduction zone the plate boundary thrust slips along a well imaged megasplay fault system during the megathrust earthquakes that regularly strike southwest japan the routing of the active plate boundary thrust along an upward branching splay fault causes deep underthrusting of an unusually thick section of material attached to the subducting ocean crust here we present three dimensional seismic reflection data that shows this unusually thick section is fluid rich sediment that results in broad weakly coupled regions of the megathrust down into the updip end of the seismogenic zone the weakly coupled regions lie above an underthrust low seismic impedance presumed to be low velocity and low density sediment section that is between one and two kilometers thick and covers 3300 km2 at least one eighth of the total rupture area of the 1944 mw 8 1 tonankai earthquake this underthrust sediment section is broader and much deeper than inferred at other margins sediment underthrusting into the normally well coupled seismogenic zone likely releases fluids and elevates fluid pressure which reduces inter plate coupling along portions of the megasplay fault and allows coseismic rupture to propagate to unusually shallow depths and generate large tsunami as inferred for the 1944 tonankai event splay faults may be a common yet transient mechanism for developing weak subduction zone thrusts

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

the geomorphological and geological importance of palaeosurfaces this special publication is a collection of papers which identifies describes and interprets the occurrence of palaeosurfaces in the geological record the concept of a palaeo surface is one which is generally understood across the geomorphological and geological sciences but according to discipline and application often differs in the emphasis and detail of its interpretation in order to encompass the widest range of contributions and views upon the subject of palaeosurfaces a broad remit was deliberately adopted during the preparation of this volume cations of ancient palaeosurfaces the research areas represented are geographically widespread and include examples from australia africa south america india and europe

a sequence stratigraphy of the kimmeridgian and bolonian stages late jurassic wessex weald basin southern england a sequence stratigraphic framework for the late jurassic kimmeridgian and bolonian stages of the wessex weald basin southern england is proposed based on the integration of sedimentological geophysical and geochemical data the nerc funded rapid global geological events rgge boreholes of swanworth quarry 1 and 2 and metherhills 1 are used as reference sections eleven complete depositional sequences and their component systems tracts are recognized within the kimmeridge clay formation bounded by 12 sequence boundaries km1 12 seventy four boreholes have been used in this study during the kimmeridgian a major transgression associated with tectonic movements led to a marked change in basin geometry major deepening of the sea during the late kimmeridgian eudoxus zone led to a change from unconformities of basinwide extent to those confined to the basin margins the progressive deepening of the basin towards a highstand in the mid bolonian wheatleyensis to pectinatus zones was associated with a broad scale change in sedimentary faunal and geochemical characters the layer cake basin architecture and thickening of the kimmeridge clay formation towards the basin centre suggests a dominantly aggradational system the partially enclosed nature of the kimmeridgian and bolonian seas generated basin dynamics more analogous to those of modern day meromictic lakes consequently sequence stratigraphic approaches derived from continental shelf margins are inappropriate in epicontinental basins of this type

geochim cosmochim acta m a a schoonen and h l barnes erratum to schoonen and barnes 1991 mechanisms of pyrite and marcasite formation from solution iii hydrothermal processes 55 3491 3504

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

the portland purbeck junction portlandian berriasian in the weald and correlation of latest jurassic early cretaceous rocks in southern england ammonites and palynology are described at the portland purbeck junction in the weald a major disconformity exists between glaucolithus zone portland beds and the overlying basal purbeck facies the basal purbeck beds of the weald are correlated with the middle purbeck strata of dorset the cinder bed of dorset and wiltshire is described as one of several marine incursions entering wessex from further south in the anglo paris basin not from the spilsby basin the use of cinder bed as a convenient jurassic cretaceous portlandian ryazanian boundary is critically reviewed as is the placing of the base of the berriasian stage in the type purbeck section the rival biostratigrahic schemes based on ostracods and palynology are discussed and the discrepancies between the two schemes in southern england are examined

uranium thorium series radionuclides in brines and reservoir rocks from two deep geothermal boreholes in the salton sea geothermal field southeastern california naturally occurring u and th series radionuclides have been analyzed in high temperature brines 300 c 25 wt dissolved solids and associated rocks from two deep geothermal wells located on the northeastern margin of the salton sea geothermal field ssgf these data are part of a study of the ssgf as a natural analog of possible radionuclide behavior near a nuclear waste repository constructed in salt beds and permit evaluation of some characteristics of water rock interaction in the ssgf rock brine concentration ratios rc dpm g rock dpm g brine were found to vary from near unity for isotopes of ra pb and rn to about 5 105 for 232th the high sorptivity of 232th is closely followed by that of 238u and 234u rc 5 104 suggesting that u is retained in the 4 oxidation state by the reducing conditions in the brines the relatively high solubility of 210pb and 212pb is attributed to formation of chloride complexes while the high ra solubility is attributed to chloride complexing a lack of suitable adsorption sites due to the high brine salinity and temperature and the reducing conditions that prevent mno2 and raso4 from forming the 228ra226ra ratios in the brines are approximately equal to those of their parents 232th230th in associated rocks indicating that ra equilibration in the brine rock system is achieved within the mean life of 228ra 8 3 years the 224ra228ra ratios in these brines are about 0 7 indicating that either 1 brine composition is not homogeneous and 224ra decays in fracture zones deficient in ra and th as the brine travels to the wellhead or 2 ra equilibration in the brine host rock system is not complete within the mean life of 224ra 5 2 days because the desorption of 224ra from the solid phase is impeded the 228ac228ra activity ratio in the ssgf brines studied is 0 1 and from this ratio the residence time of 228ac in the brine before sorption onto solid surfaces is estimated to be 70 minutes indicating the potential for rapid removal of reactive isotopes fom brines the brine is greatly enriched in 226ra 2 3 dpm g about 104 105 times that of its parent 230th while reservoir rocks are about 10 deficient in 226ra relative to 230th material balance calculations for 2226ra and 18o suggest that brines reside in the reservoir for 102 103 years that the ssgf formed 10 000 40 000 years b p and that porosity cannot be more than 20