sedimentary response to intra arc extension controls on oligo miocene deposition sarcidano sub basin sardinia the oligo miocene sardinian rift is a rare example of a well exposed intra arc basin comprising several independent depocentres synthesis of field observations from one such depocentre the sarcidano sub basin in central sardinia not only allows a new and integrated sedimentological and tectonostratigraphic history for the area but also provides insights into controls on syn and post rift sedimentation in intra arc settings initial rifting during the late rupelian early chattian oligocene occurred on nw se trending normal faults and produced a half graben topography which controlled the dispersal of locally supplied coarse continental clastic sediments latest chattian aquitanian miocene ne sw trending normal faulting defined a more complex block topography that remained underfilled in all areas except those which received voluminous ignimbritic volcanic material subsequent marginal and shallow marine sedimentation progressively infilled the fault topography as sea level rose deposition of coarse clastic sediments eroded from fault scarps was focused in the hanging wall depocentres of the intersecting fault trends whilst platform carbonates became established in topographically higher areas protected from clastic sedimentary supply finally as the relative rise in sea level drowned sediment source areas in the burdigalian deeper marine marlstones dominated sedimentation the results demonstrate that the sarcidano sub basin was dominated by a mixed volcanic siliciclastic carbonate basin fill with abrupt facies changes developed in response to complex fault defined topography relative rise in sea level and an episodic volcanic supply whilst the rift structure largely controlled the provenance supply and accumulation of sediments relative sea level primarily controlled the types of facies that developed

exceptional fossil preservation during co2 greenhouse crises abstract exceptional fossil preservation may require not only exceptional places but exceptional times as demonstrated here by two distinct types of analysis first irregular stratigraphic spacing of horizons yielding articulated triassic fishes and cambrian trilobites is highly correlated in sequences in different parts of the world as if there were short temporal intervals of exceptional preservation globally second compilations of ages of well dated fossil localities show spikes of abundance which coincide with stage boundaries mass extinctions oceanic anoxic events carbon isotope anomalies spikes of high atmospheric carbon dioxide and transient warm wet paleoclimates exceptional fossil preservation may have been promoted during unusual times comparable with the present co 2 greenhouse crises of expanding marine dead zones oceanic acidification coral bleaching wetland eutrophication sea level rise ice cap melting and biotic invasions

the geological evolution of the falkland islands continental shelf the falkland islands are surrounded by four major sedimentary basins the falkland plateau basin to the east the south falkland basin to the south the malvinas basin to the west and the north falkland basin to the north the four main basins appear to have formed initially as triassic through earliest cretaceous extensional rifts associated with the break up of gondwana a valanginian end to rifting was followed by thermal sag there is evidence of cenozoic uplift in at least the north falkland basin possibly coincident with andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the scotia sea there is no evidence from offshore seismic and gravity magnetic data to support interpretations that the falkland islands have rotated clockwise through up to 180 during gondwana separation with the exception of the south falkland basin all the major basins probably underwent initially more or less east west extension and had a similar orientation to adjacent south american and western southern african basins the falkland basins probably shared a similar geological history with the offshore southern african and south american basins

evolution of the san jorge basin argentina the san jorge basin although small is the most important hydrocarbon producing basin in argentina remaining untested potential is high because of the presence of good source rock favorable structural complexity and multiple reservoirs reservoir quality is commonly low because of the highly tuffaceous sandstones the sedimentary fill of the basin is closely related to its tectonic history northwest southeast trending grabens formed and filled during a triassic and early jurassic early rift phase climaxing with a pervasive middle jurassic volcanic episode continued growth and filling of the basin occurred during a late jurassic earliest cretaceous late rift phase and cretaceous early and late sag phases late cretaceous early tertiary extension set up many of the present day structural traps along normal faults middle tertiary andean compression produced the narrow north south san bernardo structural belt which exhibits reversed movement on older normal graben bounding faults and on local low angle thrust faults marked early to middle tertiary erosion produced a significant unconformi y within cretaceous beds around basin margins origin of upper jurassic and lowermost cretaceous sedimentary fill is primarily lacustrine or fluvial in origin lacustrine organic rich black shales are fringed by oolitic and other limestones and fluvial deltaic sandstones derived mostly from the north a significant southern source of sand existed during the valanginian interbedded marine shales occur mostly to the west toward a presumed marine seaway connection to the northern magallanes basin middle to upper cretaceous sedimentary rocks sourced mostly from the north are mainly fluvial sandstone shale successions with some minor lacustrine influence reservoir quality glauconitic sands were deposited during a late cretaceous early tertiary marine incursion from the atlantic after development of a lower tertiary regional unco formity relatively undisturbed younger tertiary sediments completed the basin fill lower cretaceous lacustrine shales of the d 129 formation are the best source rocks yet identified however most hydrocarbon production occurs above the source in middle and upper cretaceous sandstones along the basin flanks the most obvious hydrocarbon emplacement mechanism is vertical migration along reactivated graben faults

unconfined alluvial flow processes recognition and interpretation of their deposits and the significance for palaeogeographic reconstruction abstract palaeogeographic interpretation of the sedimentary rock record depends on correct recognition from the preserved evidence of the processes responsible for transporting and depositing the sediment this in turn depends on robust knowledge transfer from previous workers and the successful exchange of ideas between workers requires consistent use of a well defined vocabulary we have identified serious breakdowns in all these interpretation steps in the case of terrestrial unconfined flow and its deposits and these failures are leading to unreliable environmental and climatic interpretation this is significant because such alluvial deposits commonly form a majority of the rock record of continental environments working from the basic principles of geomorphology and fluid dynamics we have undertaken a wide ranging analysis of the nature of out of channel flow and from this make predictions about the characteristics of its deposits we identify the range of possible locations and conditions that lead to the development of unconfined flow review the processes operating in each case and examine the range of lithological features that can be produced by these processes this allows us to evaluate the reliability of the criteria claimed for identification of out of channel flow deposits and examine how our new insights might alter palaeoclimatic and palaeogeographic reconstructions published previously by others the sedimentary record of unconfined flows is much more diverse and complex than usually portrayed the received wisdom that the record of unconfined flow consists solely of upwards fining thin beds produced from shallow waning flows is shown to be flawed a wide range of lithofacies are possible and the variation in both flow steadiness and uniformity needs to be taken into account the previously published criteria for recognition of flows of this type are not diagnostic of process or location unconfined flow deposits cannot reliably be identified from grain size or bed thickness similar lithofacies may develop in a wide range of geomorphic and climatic settings because the deposits solely reflect the local flow conditions and sediment availability we recommend that the terms sandflat and sheetflood should not be used in sedimentological accounts because there is no longer a safe informal usage for either contradictory application of these terms and lack of robust definitions is leading to significant misunderstanding of palaeogeography and process our analysis should improve reconstruction of past terrestrial environments because it reveals more clearly the true variety of possibilities for the occurrence of unconfined flow and the resultant deposits enhanced understanding of the inherent uncertainties and realisation of the wider range of plausible alternative explanations should help resolve apparent contradictions with independent indicators of climate or geographic position

sedimentary responses to sub aerial felsic volcanism from the late devonian early carboniferous northern macalister synclinorium southeastern australia abstract active sub aerial volcanism has the capability to rapidly alter both the topographic and drainage characteristics of a landscape and thus fundamentally influence resulting sedimentary facies relationships between sedimentation and volcanism are explored in this paper via an investigation of the stratigraphy and early depositional history of the upper devonian volcano sedimentary units of the northern macalister synclinorium east central victoria complex interfingering relationships exist between sub aerial felsic volcanic successions rose river volcanics and alluvial fluvial and lacustrine sedimentary units the bindaree and howitt spur formations a depositional model is presented for these units in which the rose river volcanics an outflow ignimbritic plateau facies of the tolmie caldera complex to the north co existed with a series of volcaniclastic alluvial fans and freshwater lakes a dacitic volcanic centre refrigerator gap dacite appears to have developed somewhat separately to the south in the jamieson river area these lower successions of the northern macalister synclinorium record an episode of sedimentation in close proximity to an active felsic volcanic terrain and in a landscape of significant topographic relief the conglomerates of the bindaree formation in particular record the supply of abundant felsic volcanic detritus via high gradient proximal alluvial stream systems comparisons can be made with sedimentological processes operating during deposition of overlying sedimentary successions where influences by primary volcanic activity were less important well defined channel geometry successions and a diversification in clast and grain types within these younger units indicate ongoing headward erosion into metasedimentary basement rocks and deposition within lower gradient higher order stream systems

controls on the structural architecture and sedimentary character of syn rift sequences north falkland basin south atlantic abstract the falkland islands lie in a critical location for our understanding of gondwana supercontinent break up subsurface data obtained during recent exploration activity provide important new insights into the structure and stratigraphy of the north falkland basin s late jurassic syn rift fill history results of seismic interpretation indicate that deposition was largely controlled by prominent planar normal faults which divide the basin into a series of north south trending half grabens and minor grabens well data in the form of exploration wireline logs cores petrographic thin sections major and trace element analyses indicate that their syn rift fill was dominated by abundant volcaniclastic sediment supplied through debris flows hyperconcentrated flows and pyroclastic ash fall the geochemical affinity of the volcanic sedimentary components shows a direct link to the widespread middle jurassic to early cretaceous silicic chon aike large igneous province in southern south america and antarctica diagenesis has obscured provenance information and severely reduced porosity within the syn rift sediments causing them to have poor reservoir potential the results enable a radically new interpretation of this key period in the north falkland basin s evolution which helps modify and unify previously disparate views it is now suggested that basin development was the direct result of the chon aike back arc stretching influenced in part by the underlying palaeozoic basement trends this was followed by cretaceous recent post rift thermal subsidence of the basin which was only arrested by local structural inversion related to atlantic opening consequently it is thought that the extension that created the north falkland basin was independent of both early middle jurassic rotation of the falkland islands microplate and the evolution of the south atlantic ocean in the cretaceous and cenozoic

diverse subaerial and sublacustrine hot spring settings of the cerro negro epithermal system jurassic deseado massif patagonia argentina abstract the late jurassic 150 ma cerro negro volcanic epithermal geothermal system 15 km 2 area deseado massif patagonia argentina includes two inferred volcanic emission centers characterized by rhyolitic domes linked along nw se regional faults that are associated with deeper level au ag mineralization to the nw and with shallow epithermal quartz veins and mainly travertine surface hot spring manifestations to the se some travertines are silica replaced and siliceous and mixed silica carbonate geothermal deposits also are found five hot spring related facies associations were mapped in detail which show morphological and textural similarities to pleistocene recent geothermal deposits at yellowstone national park u s a the kenya rift valley and elsewhere they are interpreted to represent subaerial travertine fissure ridge mound deposits low flow spring discharge and apron terraces high flow spring discharge as well as mixed silica carbonate lake margin and shallow lake terrace vent conduit tubes stromatolitic mounds and volcano shaped cones the nearly 200 mapped fossil vent associated deposits at cerro negro are on a geographical and numerical scale comparable with subaerial and sublacustrine hydrothermal vents at mammoth hot springs and affiliated with yellowstone lake respectively overall the cerro negro geothermal system yields paleoenvironmentally significant textural details of variable quality owing to both the differential preservation potential of particular subaerial versus subaqueous facies as well as to the timing and extent of carbonate diagenesis and silica replacement of some deposits for example the western fault associated with the eureka epithermal quartz vein facilitated early silicification of the travertine deposits in the se volcanic emission center thereby preserving high quality microbial macro and micro textures of this silica replaced pseudosinter cerro negro provides an opportunity to reconstruct paleogeographic paleohydrologic and paleoenvironmental associations in a well exposed extensive and diverse fossil geothermal system this late jurassic hydrothermal deposit will likely contribute to a better understanding of the impact of depositional and post depositional history on the development and long term preservation potential of lagerstatte in epithermal settings and more generally in extreme environments of the geological record

late palaeozoic to mesozoic structural evolution of the falkland islands a displaced segment of the cape fold belt the falkland islands lie on a displaced crustal block presently forming part of the south american plate the islands possess two roughly orthogonal structural grains the relative chronology of which is here unequivocally established for the first time d1 structures form a southerly verging permo triassic age fold belt striking e w to wnw ese which represents a displaced segment of the gondwanian orogenic belt the strike swing from e w to wnw ese is coincident with a decrease in deformation intensity toward the west on west falkland d1 folds are superimposed by the early mesozoic d2 ne sw trending hornby mountains anticline producing localized kilometre scale type i and iii fold interference patterns the hornby mountains anticline is interpreted to be the result of dextral transpressive reactivation of a pre existing ne sw basement fault if the falkland islands are rotated 180 the structure of the d1 fold belt and tectono sedimentary features of the permian age lafonian supergroup display a remarkable correlation with the structural style and tectonic evolution of the eastern cape fold belt of south africa our data therefore support a 180 tectonic rotation of the falkland islands and reaffirm their pre gondwana break up position adjacent to the southeast coast of south africa in such a reconstruction the falkland islands constrain the easterly extension and eventual lateral termination of the cape fold belt

structurally controlled fluid flow high grade silver ore shoots at martha epithermal mine deseado massif argentina abstract the role of structurally controlled fluid flow during epithermal ore shoot formation is addressed through a structural study of the high grade martha mine epithermal deposit deseado massif patagonia argentina martha is a silver rich deposit characterised by a complex mineralogy dominated by ag as sb sulphosalts and cu pb zn sulphides mineralised veins can be subdivided into two groups master structures that trend approximately nw se and second order structures that trend e w and coalesce into the master structures the martha vein system is interpreted as a hard linked step over zone between two nw se trending fault segments furthermore the geometry of ore shoots at martha mine suggests a first order structural control on their location and orientation at the deposit scale mineralisation is concentrated in the step over zone mineralisation and presumably hydrothermal fluid flow was concentrated at the intersection between the two main vein groups finally at the scale of single veins jogs and subordinate step overs were important channels for hydrothermal fluids and the formation of high grade ore shoots

la josefina au ag deposit patagonia argentina a jurassic epithermal deposit formed in a hot spring environment abstract the la josefina jurassic epithermal au ag deposit located in patagonia argentina developed in an extensional setting of a back arc environment associated with a widespread middle late jurassic calc alkaline volcanism block faulting has juxtaposed shallow level features evidenced by hot spring manifestations hydrothermal eruption breccias and au rich veins which suggest that mineralization in these veins could extend far below the depths already tested by core drilling veins are filled by quartz chalcedony opal and minor adularia and barite with massive comb cockade colloform crustiform bandings and lattice bladed textures ore minerals include electrum ag rich sulfosalts freibergite pyrite galena sphalerite chalcopyrite and specular hematite with minor arsenopyrite marcasite tetrahedrite and bornite four mineralizing stages have been identified the first two s1 and s2 are au and ag rich with temperatures ranging from 225 to 290 and salinities from up to 15 wt in s1 decreasing to 1 wt nacl in s2 the third stage s3 displays higher base metal contents at lower temperatures 200 c finally the last stage s4 is barren with temperatures lower than 100 c veins are surrounded by a proximal alteration halo of quartz pyrite adularia illite followed outwards by illite smectite interstratified clays and smectites with less chlorite to a propylitic zone stable isotope values calculated for the fluids show a mostly meteoric origin for mineralization fluids such distinct features place the la josefina deposits in a hot spring environment with evidences of being formed at a proximal position of the jurassic paleosurface and paleowater level

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

lake cyclicity as response to thermal subsidence a post camp scenario in the parnaiba basin ne brazil abstract the jurassic cretaceous transition was marked by the fragmentation of the west gondwana supercontinent and consequent opening of the atlantic ocean this event resulted in extensive lava flows in the central portion of west gondwana composing the central atlantic magmatic province in the parnaiba basin ne brazil a large lacustrine system succeeded this event nevertheless the post camp scenario and the influence of the decreasing isotherms in the mesozoic sedimentation are poorly understood in this sense cyclostratigraphy and outcrop based facies analysis were carried out in the lacustrine stratigraphic record allowing the recognition of four paleoenvironments central lake sheet like delta front lakeshore and ephemeral fluvial channels mainly organized in shallowing upward cycles the upsection transition of thin black shales and limestones to thick oxidized mudstones and stratified sandstones reflects the evolution of underfilled to overfilled lake settings the lacustrine succession is organized in four major depositional cycles characterized by millimeter to centimeter scale cycles bounded by unconformities or flooding surfaces the cycles define a retrogradational progradational retrogradational stacking pattern with random and frequent changes in lithology and cycle thickness cyclostratigraphic data suggest that the upward increasing accommodation space was controlled by the post camp thermal subsidence as well as shifts in sedimentary supply and water inflow outflow the time span of this subsidence phase lasted from even before the camp magmatism 200 ma affecting the whole deposition of the lacustrine succession and was interrupted by the cretaceous magmatic event 126 ma subsidence was also enhanced by crustal loading associated with thick subsurface jurassic intrusions and probably influenced the whole basin in consequence of flexural effects

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene

upper jurassic travertine at el macanudo argentine patagonia a fossil geothermal field modified by hydrothermal silicification and acid overprinting the deseado massif hosts numerous late jurassic 150 ma fossil geothermal systems related to an extensive volcanic event developed in a diffuse extensional back arc setting detailed mapping petrography and mineralogical observations of el macanudo outcrops verify that it represents a hot spring related travertine partially replaced by silica and delineated by six sedimentary facies these are large concentric cones f1 laminated vertical columnar structures f2 porous layers f3 shrubby and irregular lamination f4 low amplitude wavy bedding f5 and mounds and breccias f6 the macanudo norte outcrop rocks constitute a silica replaced travertine sequence with development of large conical stromatolites in a deep pool or geothermally influenced shallow lacustrine environment surrounded by a subaerial travertine apron terrace whereas the macanudo sur outcrop is a subaerial travertine mound sequence structurally controlled vent areas occur in both northern f1 and southern f6 outcrops mainly located along regional nne and ene trending faults the other sedimentary units display a concentric distribution of travertine facies with respect to the interpreted vent areas the el macanudo palaeo hot spring deposit is situated in an eroded jurassic volcanic centre and records a complex evolutionary fluid history the sediments archived three different jurassic events when large and long lasting hydrothermal systems were active across the region this relative temporal sequence was formed by 1 travertine precipitation 2 development of a silica cap where early silicification was responsible for exceptional preservation of some stromatolitic fabrics and 3 acid alteration recorded by dissolution textures and clay formation and caused by a palaeo phreatic water level drop

a large and complete jurassic geothermal field at claudia deseado massif santa cruz argentina abstract late jurassic geothermal deposits at claudia argentinean patagonia are among the largest 40 km 2 and most varied in the deseado massif a 60 000 km 2 volcanic province hosting precious metals au ag mineralization generated during diffuse back arc spreading and opening of the south atlantic ocean both siliceous sinter and travertine occur in the same stratigraphic sequence deposits range from those interpreted as fluvially reworked hydrothermal silica gels to extensive apron terraces to a clustering of high temperature subaerial vent mounds paleoenvironmentally diagnostic textures of sinters include wavy laminated bubble mat and nodular fabrics and for travertines comprise fossil terracette rims wavy laminated bubble mat spherulitic oncoidal and peloidal fabrics of special note is the presence of relatively large to 25 cm high inferred subaqueous conophyton structures in travertines which serve as analogs for some precambrian stromatolites and imply the presence of relatively deep pools maintained by voluminous spring discharges the claudia geothermal field is geographically and geologically linked to the cerro vanguardia epithermal project total resource of 7 8 million ounces au equivalent via proximity similar veins and structural linkages making it an especially large and relevant prospect for the region the combined claudia cerro vanguardia hydrothermal system likely represents a fortuitous alignment of focused fluid flow and structure conducive to forming a giant epithermal ore deposit with respect to size ore concentration and potentially duration in the late jurassic of patagonia

rare earth element systematics in hydrothermal fluids abstract rare earth element concentrations have been measured in hydrothermal solutions from geothermal fields in italy dominica valles caldera salton sea and the mid atlantic ridge the measured abundances show that hydrothermal activity is not expected to affect the ree balance of either continental or oceanic rocks the ree enrichment of the solutions increases when the ph decreases high temperature solutions 230 c percolating through different rock types may show similar ree patterns

linked sequence stratigraphic and structural evolution of propagating normal faults two distinct phases in the structural evolution of normal faults can be identified in the miocene gulf of suez rift 1 an initial growth fold stage when the fault is a buried structure and 2 a subsequent surface faulting stage during the growth fold stage strata thin and become truncated toward the fault zone and are rotated and diverge away from the buried fault into growth synclines in contrast once the fault breaks surface strata form a divergent wedge which is rotated and thickens into the fault the two tectono stratigraphic styles also occur contemporaneously along the length of a single fault segment growth folding characterizes deformation around the ends of fault segments where the fault is blind whereas the center of fault segments are characterized by surface faulting these observations suggest that marked along strike variation in stratal surfaces and facies stacking patterns will occur in depositional sequences in areas of normal faulting

tectonostratigraphy and the petroleum systems in the northern sector of the north falkland basin south atlantic abstract the north falkland basin represents one of the frontier areas for hydrocarbon exploration in the south atlantic this study presents the results of new subsurface mapping using 2d seismic data in the north of the falkland islands offshore area which has delineated a series of discrete grabens northwards of the main north falkland basin referred collectively to as the northern sector of the north falkland basin nnfb six regionally significant seismic reflectors are interpreted within this data dividing the sedimentary fill into six tectonostratigraphic packages including early syn rift late syn rift transitional unit early post rift middle to late post rift and a sag unit structural interpretation of the 2d seismic data has led to the definition of four north south orientated depocentres namely 1 the eastern graben largest of the depocentres 20 km wide by 45 km long reaching depths of 3 km 2 the eastern graben splay a smaller depocentre 10 km wide by 20 km long reaching depths of 2 2 5 km 3 the western graben splay the smallest depocentre 5 km in width and 20 km long with a basin depth of 2 km and 4 the newly defined phyllis graben which is 13 km wide and 30 km long with a basin depth of 3 km a network of nw se and ne sw trending faults controls the development of these grabens separated by a western eastern and intra basin high these grabens represent a northern continuation of the northern falkland basin to the south hydrocarbon discoveries to the south of this study area e g sea lion casper beverley zebedee isobel deep and liz confirm a working petroleum system adjacent to the northern sector this study has identified a number of seismic anomalies including amplitude brightening events which potentially correspond to an extension of this petroleum system indicating active migration pathways the main targets in terms of hydrocarbon interest in the northern sector are likely to be stratigraphically trapped hydrocarbon accumulations contained within vertically amalgamated turbidite fan sandstone reservoirs deposited within the early post rift a second yet to be tested syn rift play in which the trapping geometries are structural and the reservoirs are fluvial sandstones is also identified

albitization of detrital plagioclase in triassic reservoir sandstones from the snorre field norwegian north sea abstract detrital plagioclase an12 an28 in triassic reservoir sandstones of the lunde formation in the snorre field offshore norway has been partially to completely albitized during burial diagenesis depths of about 2 500 3 000 m 75 100 c present temperature calcite caco3 99 mole and kaolinite were formed as by products of plagioclase albitization sodium rich detrital plagioclase an2 an11 are usually unalbitized detrital k feldspar is fresh or rarely partly albitized differences in albitization behavior of detrital plagioclase and k feldspar are shown to be related to the chemistry of formation waters reactivity of kaolinite and temperature the diagenetically formed albite pseudomorphs are very pure ab 9 well ordered low albite dark luminescent vacuolized mostly untwinned and comprised of numerous euhedral albite crystals some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase the amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable ca rich plagioclase the microporosity formed during albitization is proportional to the anorthite component which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite the albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of na from pore solutions

diagenetic replacement of feldspars by quartz in sandstones abstract diagenetic replacement of detrital albite and k feldspar by quartz is common in proterozoic sandstones of the visingso group the dala sandstone formation southern and central sweden respectively and the brottum formation southern norway quartz has been precipitated as a few or numerous crystals along fractures and cleavage and twinning planes as well as in dissolution voids in the feldspar grains feldspars that are pervasively replaced by quartz appear as small relics among numerous euhedral quartz crystals authigenic clay minerals mainly illite are commonly associated with quartz in altered feldspar the silica required for precipitation of quartz has been derived from 1 sources external to the feldspars e g pressure solution of quartz grains and transformation of clay minerals but which occur within the same sandstone sample and 2 dissolution and or replacement of the feldspars by diagenetic clay minerals

structural and stratigraphic development of the newark extensional basin eastern north america evidence for the growth of the basin and its bounding structures the newark basin is a half graben bounded on its northwestern margin by a system of mostly normal slip faults many of which are reactivated paleozoic thrust faults and were active at least sporadically during the deposition of the entire basin fill as a result of along strike variations in displacement on the border fault system subsidence increased from the lateral edges toward the center of the basin and from the hinged margin toward the border fault system some intrabasinal faults also formed during the deposition of units currently preserved in their hanging walls along strike variations in the density of intrabasinal faults probably reflect changes in the dip angle of the border fault system variations in the amount of extension accommodated on these intrabasinal faults resulted in the formation of a transfer fault along strike variations in fault displacement probably produced the transverse folds in the hanging walls immediately adjacent to border faults and major intrabasinal normal faults some folds formed syndepositionally progressively younger strata onlap basement rocks along the hinged margin of the basin and younger splays of the border fault system propagated into the footwall the basin therefore grew deeper wider and longer through time

40ar 39ar ages for dykes from the falkland islands with implications for the break up of southern gondwanaland 40 ar 39 incremental heating spectra analyses were made of five microgabbro dykes from the falkland islands two of which yielded an acceptable plateau and a third a close maximum estimate all three indicate an age of dyke intrusion of about 190 ma supporting a single published k ar date this provides further support for the geological continuity of the falkland islands and the eastern karoo basin where dyke swarms of similar petrological affinities were intruded at about the same time the age constrains the previously published palaeoreconstruction of the falkland islands based on palaeomagnetic data which places the falklands immediately to the east of southern africa to be valid until at least the late early jurassic

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

diffusion creep modelling of fibrous pressure shadows abstract grain scale deformation of a bimineralic composite by normal stress driven grain boundary diffusive mass transfer has been modelled using diffusion creep software that for the first time allows different diffusion coefficients for interphase boundaries and grain boundaries as well as for different components by selecting suitable coefficients we have obtained a fully coupled mechanical and mass transfer numerical solution for the formation of fibrous pressure shadows or strain fringes at the interface between a rigid object and a deforming rock matrix of different compositions the model is two dimensional and run in pure shear velocity mode to a stretch ratio of 1 4 the first results are reported which show dissolution and face controlled antitaxial growth of a soluble polycrystalline calcite matrix at the interface with a circular and relatively insoluble pyrite object the ratio of caco3 diffusion along calcite pyrite versus calcite calcite boundaries strongly influences the extent of pressure shadow growth and the pattern of strains in the matrix a low ratio produces little growth significant perturbation and deflection of total strain axes around the pyrite an intermediate ratio produces appreciable growth little perturbation and near parallel total strain axes a high ratio produces larger pressure shadows and total strain axes that bend in towards the pyrite as if it were a weak object sliding and rotation of matrix grains are also affected these results differ fundamentally from purely physical models of ductile flow around a rigid object and demonstrate the influence of interface diffusional properties on the resulting behaviour in particular assumptions such as perturbation of the flow by the object deflection of external foliation around pre deformational porphyroblasts and relationships of pressure shadow strain to bulk strain may need to be re examined in the light of this finding

tectonic and structural controls on neogene fluid release in the patagonian continental margin abstract analysis of high resolution multi beam bathymetry 2d multi channel seismic and high resolution seismic sub bottom profiles revealed the presence of widespread fluid escape features in the middle slope of the patagonian continental margin on the sea bottom these features correspond to pockmarks and mud volcanoes whereas in the sub surface they are represented in the seismic records by several acoustic anomalies such as chimneys acoustic blanking enhanced reflectors and reverse phase enhanced reflectors among others some of these acoustic signatures can be traced to syn rift deposits that fill a deep re activated and inverted graben analysis of pockmarks elongation and pockmarks alignments show a good correlation with inverted normal faults suggesting that faults and fractures have influenced the pockmark s shape and might have acted as pathways for upward fluid migration some of the acoustic anomalies associated with pockmarks are interpreted as evidence of gas the gas observed in the seismic data seems to be thermogenic and the seismic data suggests a deep origin associated with over pressured syn rift deposits two morphometrically different sets of pockmarks were identified an older set hosted by miocene aged rocks and a younger set hosted by quaternary deposits different potential triggers are discussed for the genesis of the miocene pockmarks in relation to the seismostratigraphy structural geology and regional tectonics it is concluded that tectonic activity associated with the neogene inversion of the graben faults due to andean compression is the most likely cause for the formation these pockmarks the presence of gas charged sediment and young pockmarks also suggest that after the middle miocene tectonic climax a more recent pulse of release of fluids occurred

geyserite in hot spring siliceous sinter window on earth s hottest terrestrial paleo environment and its extreme life siliceous hot spring deposits or sinters typically form in active terrestrial on land volcanic terrains where magmatically heated waters circulating through the shallow crust emerge at the earth s surface as silica charged geothermal fluids geyserites are sinters affiliated with the highest temperature 75 100 c natural geothermal fluid emissions comprising localized lithologically distinctive hydrothermal silica precipitates that develop around geysers spouters and spring vents they demarcate the position of hot fluid upflow zones useful for geothermal energy and epithermal mineral prospecting near vent areas also are extreme environment settings for the growth of microbial biofilms at near boiling temperatures microbial biosignatures e g characteristic silicified microbial textures carbon isotopes genetic material lipid biomarkers may be extracted from modern geyserite however because of strong taphonomic filtering and subsequent diagenesis fossils in geyserite are very rare in the pre quaternary sinter record which in and of itself is patchy in time and space back to about 400 ma only a few old examples are known such as geyserite reported from the devonian drummond basin australia devonian rhynie cherts scotland and a new example described herein from the spectacularly well preserved late jurassic 150 ma yellowstone style geothermal landscapes of patagonia argentina there geyserite is associated with fossil vent mounds and silicified hydrothermal breccias of the claudia sinter which is geologically related to the world class cerro vanguardia gold silver deposit of the deseado massif a part of the chon aike siliceous large igneous province tubular filament like micro inclusions from claudia were studied using integrated petrographic and laser micro raman analysis the results of which suggest a biological origin the putative fossils are enclosed within nodular geyserite a texture typical of subaerial near vent conditions overall this worldwide review of geyserite confirms its significance as a mineralizing geological archive reflecting the nature of earth s highest temperature habitable terrestrial sedimentary environment hot spring depositional settings also may serve as analogs for early earth paleoenvironments because of their elevated temperature of formation rapid mineralization by silica and morphologically comparable carbonaceous material sourced from prokaryotes adapted to life at high temperatures

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

thermal and tectonic evolution of the falkland islands implications for hydrocarbon exploration in the adjacent offshore region apatite fission track and vitrinite reflectance data combine to suggest that the falkland islands experienced three discrete episodes of heating and cooling during the phanerozoic each of which can be related to major changes in the plate tectonic setting of the islands combined with published data from the adjacent offshore areas the results provide unique insights into the behaviour of the region during the phases of convergent and divergent tectonic movements that have affected the region since the permian initial cooling of the falkland islands in the late permian was initially restricted to west falkland and was the result of differential uplift and erosion of west falkland relative to east falkland coinciding with d2 gondwanian deformation subsequent early jurassic cooling of both east and west falkland was associated with plume related thermal uplift preceding the separation of east and west gondwana and the development of the falkland plateau basin the falkland islands experienced renewed heating during the mesozoic probably due to increased burial suggesting that the offshore basins originally extended onto the present onshore region late cretaceous early tertiary cooling of the falkland islands was contemporaneous with the development of a transpressional plate boundary along the southern margin of the falkland plateau resulting in erosion of the islands that were situated on a flexural forebulge

the crustal structure of the southern argentine margin summary multichannel reflection seismic profiles combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost argentine margin at the transition from a rifted to a transform margin and outline the extent of the north falkland graben based on these data we establish a regional stratigraphic model for the post rift sediments comprising six marker horizons with a new formation in the barremian lower cretaceous our observations support that a n s trending subsidiary branch of the north falkland graben continues along the continental shelf and slope to the argentine basin during the rift phase a wide shelf area was affected by the e w extension subsequently forming the north falkland graben and the subsidiary branch along which finally breakup occurred we propose the division of the margin in two segments a n s trending rifted margin and an e w trending transform margin this is further underpinned by crustal scale gravity modelling three different tectono dynamic processes shaped the study area 1 the triassic early jurassic extensional phase resulting in the formation of the north falkland graben and additional narrower rift grabens ended synchronously with the breakup of the south atlantic in the early valanginian 2 extensional phase related to the opening of the south atlantic 3 the transform margin was active in the study area from about hauterivian times and activity lasted until late cretaceous early cenozoic both the rifted margin and the transform margin are magma poor very limited structures may have a volcanic origin but are suggested to be post rift the oceanic crust was found to be unusually thin indicating a deficit in magma supply during formation these findings in combination with the proposed breakup age in the early valanginian that considerably predates the formation of the parana etendeka continental flood basalt provinces in brazil and namibia question the influence of the tristan da cunha hotspot during the initial formation of the south atlantic

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

uplift history of a transform continental margin revealed by the stratigraphic record the case of the agulhas transform margin along the southern african plateau the south and southeast coast of southern africa from 28 s to 33 s forms a high elevated transform passive margin bounded to the east by the agulhas falkland fracture zone affz we analysed the stratigraphic record of the outeniqua and durban thekwini basins located on the african side of the affz to determine the evolution of these margins from the rifting stage to present day the goal was to reconstruct the strike slip evolution of the agulhas margin and the uplift of the inland high elevation south african plateau the agulhas transform passive margin results from four successive stages 1 rifting stage from late triassic to early cretaceous 200 134 ma punctuated by three successive rifting episodes related to the gondwana breakup 2 wrench stage 134 131 ma evidenced by strike and dip slip deformations increasing toward the affz 3 active transform margin stage 131 92 ma during which the falkland malvinas plateau drifts away along the affz with an uplift of the northeastern part of the outeniqua basin progressively migrating toward the west 4 thermal subsidence stage 92 0 ma marked by a major change in the configuration of the margin onset of the shelf break passive margin morphology two main periods of uplift were documented during the thermal subsidence stage of the agulhas margin 1 a 92 ma short lived margin scale uplift followed by a second one at 76 ma located along the outeniqua basin and 2 a long lasting uplift from 40 to 15 ma limited to the durban thekwini basin this suggests that the south african plateau is an old upper cretaceous relief 90 70 ma reactivated during late eocene to early miocene times 40 15 ma

jurassic hot spring deposits of the deseado massif patagonia argentina characteristics and controls on regional distribution abstract the deseado massif santa cruz province argentinean patagonia hosts numerous middle to late jurassic age geothermal and epithermal features represented by siliceous and calcareous chemical precipitates from hot springs sinters and travertines respectively hydrothermal breccias quartz veins and widespread hydrothermal silicification they indicate pauses in explosive volcanic activity marking the final stages in the evolution of an extensive jurassic ca 178 151 ma volcanic complex set in a diffuse extensional back arc setting heralding the opening of the atlantic ocean published paleo hot spring sites for the deseado massif plus additional sites identified during our recent field studies reveal a total of 23 locations five of which were studied in detail to determine their geologic and facies associations they show structural lithologic textural and biotic similarities with miocene to recent hot spring systems from the taupo and coromandel volcanic zones new zealand as well as with modern examples from yellowstone national park u s a these comparisons aid in the definition of facies assemblages for deseado massif deposits proximal middle apron and distal siliceous sinter and travertine terraces and mounds with preservation of many types of stromatolitic fabrics that likely were controlled by formation temperature ph hydrodynamics and fluid compositions locally the mapped hot spring deposits largely occur in association with reworked volcaniclastic lacustrine and or fluvial sediments silicic to intermediate lava domes and hydrothermal mineralization all of which are related to local and regional structural lineaments moreover the numerous geothermal and significant epithermal those with published minable resources deposits of the deseado massif geological province mostly occur in four regional nnw and wnw hydrothermal structural belts northwestern northern central and southern defined here by alignment of five or more hot spring deposits and confirmed as structurally controlled by aeromagnetic data the northern and northwestern belts in particular concentrate most of the geothermal and epithermal occurrences hence jurassic hydrothermal fluid flow was strongly influenced by the most dominant and long active geological boundaries in the region the outer limits of the deseado massif horst itself

the jurassic succession in west central argentina stratal patterns sequences and paleogeographic evolution abstract jurassic rocks in west central argentina are predominantly marine and marginal marine siliciclastics associated with prominent but volumetrically subordinate carbonates and evaporites facies developments were ruled by paleogeographic persistence within the southern infratropical latitude belt within 40 50 s and by siliciclastic derivation from the patagonian hinterland to the southwest minor volcanic and volcaniclastic supply arrived from the west out of a magmatic belt related to circum pacific convergence timing of the main marine flooding events and general correspondence with the high rank stratal packaging recorded in western north america suggest that global eustasy was also a factor in controlling the local stratigraphic record early jurassic sedimentation occurred within a series of semi isolated depocentres linked to fault bounded triassic troughs the sinemurian toarcian deposits record depocentre expansion and coalescence these trends were coeval with progressively more widespread marine invasions from the northwest and west leading to an elongate marine seaway which connected central patagonia with the pacific domain during the aalenian bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment bathonian and early callovian were times when coarse clastics prograded into the basin while the marine embayment shrank as a result of stepwise forced regressions during late callovian to oxfordian globally rising sea level the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance that promoted cratonward onlap and inception of widespread carbonate deposition ooidal coralline carbonate development was terminated after a relatively sudden messinian style event that desiccated a large tract of the andean basin and favored massive precipitation of anhydrite in the course of the kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat salina complex linked to a widespread erg and to an ephemeral drainage system by the tithonian at a period of peak oceanic stand marine connection was reinstated and recorded as the most widespread jurassic transgression across the neuquen aconcagua embayment shelfal deposition consisted of molluscs and ooid dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations like in other prolific petroleum provinces around the world the tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

overlapping faults intrabasin highs and the growth of normal faults normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs the strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault one possible explanation for these intrabasin highs is that they result from persistent along strike deficits in fault displacement such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults we present data from active normal faults within the basin and range province and from inactive normal faults of the newark basin of eastern north america demonstrating a clear correlation between the along strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins o

the role of fault interaction and linkage in controlling synrift stratigraphic sequences late jurassic statfjord east area northern north sea examination of well constrained three dimensional seismic data demonstrates the role of fault interaction and linkage in controlling the nature of synrift sequences on the hanging wall of the statfjord east fault a typical late jurassic structure in the northern north sea brent province al though now a single fault the statfjord east fault originally consisted of several en echelon segments each of which defined individual subbasins structural and stratigraphic evidence both along and across fault strike indicates that the fault resulted from segment propagation interaction and linkage facies architecture thickness variations and the internal character of synrift formations are temporally and spatially related to the subbasin geometry variations in displacement along the fault segments exhibit characteristics of interacting en echelon faults including anomalous displacement gradients in regions of segment overlap we attribute the observed shifts in depocenters to local enhancement of displacement rates resulting from the interaction of neighboring fault segments the results have far reaching consequences for synrift plays in the northern north sea because they imply that only from the perspective of fault growth and linkage can the late jurassic structure and stratigraphy be fully understood

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

the marianas san marcos vein system characteristics of a shallow low sulfidation epithermal au ag deposit in the cerro negro district deseado massif patagonia argentina the cerro negro district within the argentinian deseado massif province has become one of the most significant recent epithermal discoveries with estimated reserves plus resources of 6 7 moz au equivalent the marianas san marcos vein system contains about 70 of the au ag resources in the district mineralization consists of upper jurassic 155 ma epithermal au and ag rich veins of low to intermediate sulfidation style hosted in and genetically related to jurassic intermediate composition volcanic rocks 159 156 ma veins have a complex infill history represented by ten stages with clear crosscutting relationships that can be summarized in four main episodes a low volume metal rich initial episode e1 an extended banded quartz episode with minor mineralization e2 a barren waning stage episode e3 and a silver rich late tectonic hydrothermal episode e4 the first three episodes are interpreted to have formed at the same time and probably from fluids of similar composition a 290 230 c fluid dominated by meteoric and volcanic waters 3 to 0 18owater with 3 nacl equivalent salinity and with a magmatic source of sulfur 1 to 2 34swater metal was mainly precipitated at the beginning of vein formation episode 1 due to a combination of boiling at 600 to 800 m below the paleowater table and associated mixing cooling processes as evidenced by sulfide rich bands showing crustiform colloform quartz adularia and chlorite smectite banding during episodes 2 and 3 metal contents progressively decrease during continuing boiling conditions and veins were filled by quartz and calcite during waning stages of the hydrothermal system and the influx of bicarbonate waters 6 to 8 5 18owater hydrothermal alteration is characterized by proximal illite adularia and silica zone with chlorite and minor epidote intermediate interlayered illite smectite and a distal chlorite halo this assemblage is in agreement with measured fluid inclusion temperatures a striking aspect of the marianas san marcos vein system is that the high grade high temperature veins are partially covered by breccia and volcaniclastic deposits of acidic composition and are spatially associated with hot spring related deposits and an advanced argillic alteration blanket a telescoped model is therefore proposed for the marianas san marcos area where deeper veins were uplifted and eroded and then partially covered by non explosive post mineral rhyolitic domes and reworked volcaniclastic deposits together with shallow geothermal features the last tectonic hydrothermal mineralization episode e4 interpreted to have formed at lower temperatures could be related to this late tectonic and hydrothermal activity

the subsurface structure and stratigraphic architecture of rift related units in the lishu depression of the songliao basin china abstract this contribution reports the basin configuration feature stratigraphy and sedimentary architecture of the lishu depression in the songliao basin china the activity rate distribution and style of local faulting demonstrate the timing and extent of regional rifting distinct episodes of compressional tectonic activity caused uplift and exposure of strata evident as the traditional syn and post rift stages of basin evolution these episodes led to the sequential denudation of the upper jurassic huoshiling formation lower cretaceous yingcheng and denglouku formations and corresponding regional unconformities acting in tandem with regional compression activity along the major boundary faults influenced the evolving basin configuration as well as seismic sequences and sedimentary patterns seismic well log and drill core data described here show subdivision sections of the lishu depression strata according to discrete phases of the traditional syn rift stage of deposition we refer to these sub stages as the initial rifting the intensive rifting and the recession phases the basin configuration shifted from a graben half graben configuration during the initial rifting phase to a dustpan shaped half graben pattern during the subsequent phase of intensive rifting and finally into a gentle sedimentary basin during the final recession phase the early seismic sequence divides into a lowstand systems tract lst transgressive systems tract tst and highstand systems tract hst evidence of the lst within the seismic sequence becomes less apparent with the intensive rifting phase while the hst occupied an increasing proportion of the section the shallow water depositional fill formed during the final recession phase consists only of tst and hst components depositional environment then shifts from alluvial fan and shallow lacustrine systems to fan delta braided delta lake and finally to a braided fluvial setting the vertical stacking pattern shifts from retrogradational to progradational to aggradational identification of sub structural units and interpretation of their genetic relationships helps clarify basin evolution and thus serves larger scale continental basin analysis

frontier exploration in the south atlantic structural prospectivity in the north falkland basin seismic stratigraphic analysis of the continental shelf north of the falkland islands confirms that it is dissected by a significant extensional fault system that broadly defines the north falkland basin the basin comprises a series of extensional subbasins that developed during two distinct rift episodes analysis of structural styles shows that while extension occurred predominantly on a series of planar normal faults with changes in rift polarity in northern areas the southern portion of the basin has listric faults that sole out along preexisting thrusts after rifting the basin largely underwent postrift thermal subsidence with the passive infill of remnant topography however evidence exists for at least two uplift events affecting the postrift sediments the earlier period of uplift tilted the basin to the south resulting in the formation of a southerly prograding forced regressive wedge the later episode of uplift tilted the basin northward resulting in truncation of earlier strata and minor compressional reactivation and inversion in southernmost parts of the basin the later tilting allows for the possibility that the original rift system once extended across the falkland islands although the lack of well control does not permit a comprehensive assessment of hydrocarbon plays the seismic data do at least demonstrate the trapping potential of the basin interpreted structural and depositional styles found within the prerift synrift and postrift sequences are similar to those found in proven hydrocarbon provinces such as the north sea and the gulf of suez the success or failure of the frontier hydrocarbon province thus is likely to depend upon factors other than structural ones

40ar 39ar dating of the jurassic volcanic province of patagonia migrating magmatism related to gondwana break up and subduction abstract the mesozoic large igneous province lip of patagonia southern south america which is one of the largest silicic provinces on earth has been investigated by the 40ar 39ar method twenty seven ages considered as valid including twenty plateau ages show that the volcanic activity ranging from 187 to 144 ma occurred between and contemporaneously with the initial break up of gondwana starting with the karoo antarctic tasmanian kat flood basalt province in the east and a subduction in the west the data display a regular decreasing of ages from the ene 187 ma to the wsw 144 ma along about 650 km apparently related to the tectonic structure in half grabens oriented nnw sse the good fitting of this trend with the opening of the rocas verdes sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the ssw in a continental disruption behind the magmatic arc the observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction the spreading and thermal effect of the kat plume may have an additional effect and also could account for the unusually large volume of magma

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions