the longest delay re emergence of coral reef ecosystems after the late devonian extinctions abstract reefs are an excellent tool for tracking marine ecosystem changes especially through mass extinction transitions although metazoan reefs proliferated during the phanerozoic prolonged metazoan reef recovery intervals often occurred after extinction events here we document and review the reef recovery interval following the late devonian frasnian famennian kellwasser and end famennian hangenberg mass extinctions which eliminated the largest area of metazoan stromatoporoid coral reefs of the phanerozoic previous reports of the late visean coral bioconstructions from western palaeotethys ocean may mark the first metazoan reef proliferation after the hangenberg extinction in this study abundant coral reefs coral frameworks and coral biostromes were described in detail for the first time from the late visean strata on the south china block eastern palaeotethys ocean the occurrence of these coral bioconstructions further suggests that the late visean coral reef recovery may have been a widespread phenomenon based on the high resolution reef database constructed in this study three sub intervals of the mississippian metazoan reef recovery were distinguished which are 1 metazoan reef gap phase mrg without metazoan reefs during the tournaisian 2 metazoan reef re establishment phase mrr containing a few metazoan reefs from early visean to early part of the late visean and 3 metazoan reef proliferation phase mrp with global coral reef flourishment during the middle part of the late visean late asbian to early brigantian substages hence coral reef ecosystems proliferated and became dominant in marine ecosystems during the late asbian to early brigantian indicating a prolonged metazoan reef recovery of about 12 ma and 23 ma until the mrr and mrp respectively coral reef proliferation at this time shows that the mississippian was not solely a period dominated by microbial reefs late visean coral reef development coincided with increased nektonic and benthic diversity showing that metazoan reef recovery closely tracked overall marine ecosystem evolution even compared with other slow reef recovery intervals such as the middle late cambrian and early middle triassic with the intervals until the mrr and mrp of 5 ma and 2 ma and 15 ma and 9 ma respectively the mississippian metazoan reef recovery was the longest in reef history harsh climatic and oceanic conditions were present during the mississippian mainly including the widespread marine anoxia during the middle part of tournaisian and the following recurrent glacial and interglacial climatic episodes with frequent changes in sea level sedimentary facies and sea water surface temperature which may have stymied metazoan reef recovery during this time during the late visean marine communities flourished during a phase of relative warm conditions and high sea level and coincided with the long delayed re emergence of coral reef ecosystems after the late devonian extinctions

the tindouf basin a marine refuge during the serpukhovian carboniferous mass extinction in the northwestern gondwana platform abstract several macrofaunas and microfossils of the carboniferous saharan basins have longer stratigraphic ranges than those of other basins in the western palaeotethys realm particularly in the tindouf basin morocco algeria foraminifers are particularly abundant and diverse in the serpukhovian and basal bashkirian compared to coeval basins and some taxa have longer ranges than in the neighbouring reggan and bechar basins although this effect is more marked compared to the western palaeotethyan assemblages in europe several rugose coral species are recorded from the early bashkirian that previously were considered to have disappeared in the serpukhovian the tindouf basin as one of the most western saharan basins in north africa shows the greatest stratigraphic ranges of taxa which diminish eastwards evidence for a mass extinction event during the serpukhovian in the tindouf basin has not been clearly recognized although a possible influence of glaciation is observed in the faunal diversity eustatic sea level changes were experienced in tindouf with the cyclic pattern of sedimentation but warm water ocean currents from the palaeoequator were able to maintain tropical conditions on the platform tectonics in the area led to emerging land masses and barriers and created a partly isolated basin in this sector of the western part of the sahara platform in northern gondwana the combination of those factors controlled the environmental conditions in the area allowing the persistence of the fauna for longer stratigraphic ranges than its equivalent counterparts in the western palaeotethys

late paleozoic glacial episodes in gondwanaland reflected in transgressive regressive depositional sequences in euramerica the late paleozoic glaciation of gondwanaland comprised two short episodes in the famennian i and visean ii confined to brazil and adjacent northwest africa and a long episode that started in the namurian iiia of eastern australia and bolivia argentina expanded to cover much of gondwanaland in the stephanian asselian iiib and collapsed in the early sakmarian iiic dropstones in eastern australia indicate that small ice centers lingered to the kazanian across the belt of low latitudes north of gondwanaland short ranging fossils in widespread shallow marine and paralic deposits indicate synchronous deposition of transgressive regressive sequences in different parts of euramerica these sequences correlate with glacial events in gondwanaland at three levels a four major regressions in euramerica in the famennian 1 visean 2 namurian 3 stephanian 4 and the tastubian transgression that preceded the sterlitamakian regression 5 also recorded in gondwanaland correlate with glacial episodes i ii and iiia iiib and iiic b the time interval of cyclothemic deposition in euramerica brigantian or latest visean to sterlitamakian correlates with that of glacial episode iii and c the dominant period of the euramerican cyclothems as estimated from the middle and late pennsylvanian deposits of the mid continent of north america and of the thickest known gondwanaland glacigenic sediment the earliest permian lyons group of western australia is 0 4 ma equivalent in turn to the long orbital eccentricity period of the quaternary ice age and the dominant period of fluctuation of the late miocene antarctic ice cap the three levels of correlations confirm wanless and shepard9s 1936 hypothesis that the late paleozoic cyclothems are controlled largely by sea level fluctuations related to the gondwanaland glaciation

stratigraphical and δ13c records of permo carboniferous platform carbonates south china responses to late paleozoic icehouse climate and icehouse greenhouse transition abstract little attention has been focused on the stratigraphical imprints of the late paleozoic ice age on the equatorial eastern paleo tethys an integrated study of biostratigraphy microfacies and stable carbon isotope stratigraphy was carried out on pennsylvanian middle guadalupian carbonates of the gongchuan section in the bama platform south china it records seven cooling and two warming pulses in the pre kungurian interval comparing with ice proximal stratigraphical records and far field indirect proxies in lower latitudes a kungurian negative 13 c excursion from 4 4 at the base of the kungurian to 1 6 recorded only in the subequatorial belt is documented here it is followed by a global positive 13 c trend reaching 5 3 near the cisuralian guadalupian boundary seven third order shallowing upward cycles are reported for the first time from the kungurian succession the duration of each cycle is estimated to ca 1 myr these cycles are interpreted as being of glacioeustatic origin and mainly driven by long period modulations of obliquity 1 2 myr under major p co 2 decreasing condition this interpretation is supported by recent climate ice sheet modeling and cenozoic glaciation studies which argue that astronomical forcing will be amplified by large magnitude perturbations in the carbon cycle when only localized ice centers persist our results emphasize the need for integration of sedimentological and paleoclimatological proxies in elucidating the dynamics of the deglacial phases of the late paleozoic ice age

relations between fluid inclusion trails and regional stress field a tool for fluid chronology an example of an intragranitic uranium ore deposit northwest massif central france abstract episyenites within the uranium ore bearing le bernardan granite northwest french massif central were formed through the dissolution of quartz by hydrothermal solutions these solutions may now be found fossilized as fluid inclusions f i the f i form rectilinear trails along microcracks which trapped the percolating fluids simultaneous study of the orientations of the trails and the chemistry of the inclusions shows that 1 1 the trails may be grouped according to direction each related to successive regional stress fields 2 2 the fluids of each group are compositionally distinct marking different stages of the hydrothermal event this example shows that the geometric relations between secondary f i trails and regional stress fields may be used to establish the chronology of both during a region s geological evolution

structural controls on fe mineralization at thabazimbi mine south africa abstract chemical sedimentary rocks of the transvaal supergroup along the northern margin of the transvaal basin were imbricated during formation of the mohlapitsi fold and thrust belt between ca 2 55 and 1 93 ga high grade 63 fe stratiform fe ore bodies occur at or close to the base of the banded ironstone dominated penge formation which forms a distinct e w trending mountain range in the area the penge formation is separated from underlying malmani dolostone by a thin laterally continuous rheologically weak carbonaceous shale layer that has been intensely sheared this horizon denotes the position of a regional top to n low angle detachment that has been variably deformed during progressive development of the mohlapitsi belt several of kumba s thabazimbi iron ore deposits including kwaggashoek east donkerpoort west kumba donkerpoort west and buffelshoek west are confined to a zone directly above this detachment recent studies of ore petrography fluid alteration fluid composition and stable isotope geochemistry link economic iron ore mineralization to hydrothermal or hypothermal processes along with localized high grade mineralization a structural control on fluid flow has been suggested although a lack of structural data has hampered more concise mineralization models thus far in this study we present and analyse data from detailed structural mapping over a protracted period of mining across four opencast pits this mapping is combined with drillhole data to produce accurate intergrated 3d models of these deposits which reveal the true geometry of these ore bodies data and resultant models indicate that enhanced hydrothermal fluid flow associated with significant iron mineralization is linked to thrusting shearing in response to 1 fold amplification and flexural flow along the e w trending detachment between the malmani dolostone and banded iron formation 2 subsidiary fold accommodation structures such as conjugate shear joints normal and reverse faults and thrusts associated with inner and outer arc shortening and extension respectively 3 localization of fe at the intersections of major reverse faults the detachment and stratigraphically higher low angle thrusts which are preferentially developed at the contacts between diabase and banded iron formation collectively these features both aided and compartmentalized regional pervasive fluid flow along the detachment and in stratigraphically higher sites subsequent ore preservation was enhanced by reactivation of nw to nnw trending mesozoic normal faults and concomitant downthrow of fault bounded blocks structural relationships and timing of high grade iron ore mineralization further suggest that associated top to n kinematic fold and thrust formation of the mohlapitsi belt and subsequently the main detachment between malmani dolostone and banded iron formation of the penge formation could have been active during and immediately after intrusion of the bushveld complex at ca 2 05 ga

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

intensified oceanic circulation during early carboniferous cooling events evidence from carbon and nitrogen isotopes abstract the early carboniferous was a climatic transition interval from a mid paleozoic greenhouse world into the late paleozoic ice age it was marked by a long term cooling trend that was punctuated by short glaciations in the tournaisian and visean early to mid early carboniferous here we generated organic carbon and nitrogen isotope profiles for two widely separated lower carboniferous sections arrow canyon range nevada and namur dinant basin belgium in order to assess the global nature and timing of changes in the carbon nitrogen cycles linked to oceanic productivity and redox conditions during this climatic transition interval the carbon and nitrogen isotope profiles of both study sections record major perturbations during the mid tournaisian and visean a mid tournaisian event tice is marked by increases in 13ccarb 13corg and 15nbulk at arrow canyon from 0 6 to 7 0 25 2 to 23 7 and 0 6 to 8 9 respectively and namur dinant from 0 3 to 4 7 27 9 to 22 2 and 5 7 to 5 2 respectively an early visean event vice named herein is marked by increases in 13ccarb 13corg and 15nbulk at arrow canyon from 0 6 to 3 1 28 4 to 23 6 and 1 4 to 9 4 respectively and namur dinant from 2 1 to 4 3 26 7 to 25 0 and 1 4 to 3 3 respectively the positive excursions in all three isotopic records during these events are consistent with increased fractional burial of organic carbon and enhanced denitrification implying intensification of marine productivity and expansion of hypoxia in the global ocean given that each event coincided with lowering of sea surface temperatures and increased glaciation as documented from published conodont 18o and sea level records we hypothesize that global cooling led to intensified oceanic circulation and upwelling on continental margins triggering increased marine productivity and attendant redox changes within the affected upwelling zones our results provide new insights into changes in early carboniferous oceanic conditions in response to the initial stages of cooling leading into the late paleozoic ice age

cumulative deformation and original geometry of the bushveld complex abstract a structural review and re interpretation of the bushveld complex based on 254 georeferenced maps from research technical reports and geophysical surveys suggests that its current geometry differs markedly from its original geometry broadly n s flattening left lateral transpressional shearing along an interconnected fault network steepening of cumulate layering and 10 km of uplift to the north of the northern margin of the complex s main chamber were accompanied by an eastward verging escape structure burgersfort bulge and syn cooling gap areas on its western limb southward verging deformation was transmitted from the villa nora fragment via thrusting in the rooiberg area generating 50 70 km of southward displacement uplift and erosional loss of the northern limb s western portion a string of anomalous occurrences of bushveld complex units and dense areas outlined by unconstrained gravity isoshells suggest a further 170 km southward displacement through the center of the main chamber the southern limb underwent predominantly right lateral ductile offset from the western limb along the crocodile river and rustenburg faults expressed by an in line series of nnw trending magnetic highs to the south of their mapped terminations left lateral shearing and progressive splaying of a nne trending dyke swarm to the west of an inflection point provides strain markers for this offset and 35 anticlockwise rotation of the burgersfort bulge and central parts of the main chamber although recent work suggests that the complex cooled within 1 myr late intrusion syn cooling sse wards movement of its deeper ductile central parts within at least 5 10 myr after its initial intrusion the transvaalide fold and thrust event and the expulsion of fluids to form the high ti igneous suite hitis and related bodies appear to have closely post dated the complex s intrusion removal of the effects of cumulative strain produces a more equant complex shortly after its intrusion occupying an orthogonal network of normal faults and perpendicular strike slip transverse faults

rapid onset of late paleozoic glaciation on gondwana evidence from upper mississippian strata of the midcontinent united states direct evidence of the late paleozoic glaciation of gondwana from glacial deposits suggests that geographically extensive continental glaciation began some time in the namurian late mississippian however the timing and characteristics of the onset of glaciation are poorly understood because of a lack of reliable paleontological control and reworking of initial glacial deposits by subsequent glacial advances indirect evidence of glaciation preserved in unconformity bounded low latitude ramp sequences in the illinois basin united states suggests that geographically extensive continental glaciation of gondwana actually began in the late visean an abrupt change from carbonate dominated sequences bounded by disconformities with little evidence of erosion to mixed carbonate siliciclastic sequences bounded by unconformities with deep incised valleys was likely produced by a three fold increase in the magnitude of eustatic sea level fluctuations the increase in the magnitude of sea level fluctuations was likely driven by an equally abrupt increase in ice volume and marks the onset of the geographically extensive late paleozoic glaciation of gondwana a possible explanation for the rapid onset of glaciation is the closing of the equatorial seaway between laurussia and gondwana closing of this seaway would have led to an abrupt change in oceanic and atmospheric circulation patterns that could have initiated major continental glaciation in the southern hemisphere

global events of the late paleozoic early devonian to middle permian a review abstract the late paleozoic early devonian to middle permian was an interval of profound changes in earth surface systems reflected in dynamic interplay among the biosphere hydrosphere atmosphere and geosphere major events transpired including the colonization of landmasses by vascular plants the assembly of the supercontinent pangea two first order mass extinctions the frasnian famennian and devonian carboniferous boundary events and the most severe icehouse climate mode of the phanerozoic the late paleozoic ice age lpia the goals of the present review are 1 to summarize major global developments in climate oceanography and paleobiology during the late paleozoic 2 to examine the roles of land plant evolution global tectonics and large igneous province magmatism in driving these developments and 3 to serve as an introduction to the 23 contributions to this special issue of palaeogeography palaeoclimatology palaeoecology indicating how they advance our knowledge of various scientific issues related to these developments

redefinition of the meramecian chesterian boundary mississippian the meramecian chesterian mississippi boundary in the type area is currently defined as the highest occurrence of the crinoid platycrinites penicillus and the lowest occurrence of the crinoid genera agassizocrinus and talarocrinus because these taxa have not been reported outside eastern north america attempts have been made to use conodonts and foraminifera to extend the meramecian chesterian boundary outside the type area unfortunately changes in conodont and foraminiferal assemblages do not coincide with the currently defined meramecian chesterian boundary and use of these fossil groups does not allow exact placement of the boundary outside of eastern north america the authors suggest that the meramecian chesterian boundary be redefined as the boundary between foraminiferal zones 15 and 16i of the mamet scheme irrespective of the occurrence of platycrinites penicillus this change in definition places the ste genevieve limestone genevievian stage in the lowermost part of the chesterian series with which it is biotically and sedimentologically more allied than with the underlying st louis limestone this change provides a sound biostratigraphic base for correlating the meramecian chesterian boundary outside of the type area foraminifera conodonts brachiopods and corals all show significant changes at or slightly below the st louis ste genevieve contact rather than at the genevievian chesterian contact more 52 references less

how to resolve the controls on mesothermal vein systems in a goldfield characterized by sparse kinematic information and fault reactivation a structural and graphical approach current mine exposures of auriferous veins in the charters towers goldfield ctgf are characterized by a scarcity of kinematic indicators furthermore the dip direction of exposed and historically mined orebodies largely coincides with the orientation of pre existing discontinuities consequently the reactivation of earlier structures during gold deposition is strongly indicated both factors complicate the structural analysis of the auriferous vein systems as slip on earlier faults likely occurred before new rock failure and evolution of fault networks reflecting the stress field present at the time of gold deposition despite access to drillcore open pits and underground workings extracted data were limited to ten occurrences of wear groove striations and three reverse separations of dykes and a pegmatite cut by sulphide bearing quartz veins because of the scarcity of kinematic information the structural analysis focused on the geometry of veins and ore shoots also applying basic graphical and kinematic methods

new geochronological results and structural evolution of the pataz gold mining district implications for the timing and origin of the batholith hosted veins abstract the paleozoic pataz parcoy gold mining area is located in a right stepping jog on the regional cordillera blanca fault in northern peru most of the 8 million ounces of gold production from this area has come from quartz carbonate sulfide veins hosted by the pataz batholith despite a subduction zone setting since at least the cambrian the area records several periods of extension and its present structure is that of a rift and graben terrain the pataz district the northern part of the pataz parcoy area is dominated structurally by northwest to north northwest striking nw nnw faults and northeast to east northeast striking ne ene lineaments both of which have been active periodically since at least the mississippian early carboniferous nw nnw faults control the margins of a central horst that exposes basement schist and the pataz batholith and step across ne ene lineaments the lavasen graben to the east of the central horst contains the lavasen volcanics and the chagual graben to the west contains an allochthonous sedimentary sequence derived from the western andean cordillera new shrimp zircon geochronological data indicate emplacement of the pataz batholith during the middle mississippian at around 338 336 ma approximately 10 ma earlier than previous estimates based on 40 ar 39 ar geochronology the calc alkaline i type batholith comprises diorite and granodiorite the latter being the major component of the batholith and was emplaced as a sill complex within the moderately ne dipping sequence of the eastern andean cordillera moderate to high temperature ductile deformation took place on the batholith contacts during or shortly after emplacement following emplacement of the batholith differential uplift occurred along nw nnw faults forming the lavasen graben into which the lavasen volcanics were deposited shrimp u pb in zircon ages for the lavasen volcanics and the esperanza subvolcanic complex which was intruded into the western margin of the graben are within error of one another at ca 334 ma the ductile batholith contacts were cut by renewed movement on nw nnw faults such that the margins of the batholith are now controlled by these steep brittle ductile faults the nw nnw faults were oriented normal to the principal axis of regional shortening ene wsw during formation of the batholith hosted gold bearing quartz carbonate sulfide veins the misoriented faults were unable to accommodate significant displacement leading to high fluid pressures vertical extension in the competent batholith and formation of gold bearing veins brittle failure of the batholith was most extensive in the northern pataz district where the fault controlled western contact of the batholith is offset by a swarm of ne ene lineaments the timing of vein formation is not established despite published 40 ar 39 ar ages of 312 to 314 ma for metasomatic white mica which are interpreted as minimum ages of formation gold bearing veins formed during or shortly after uplift of the pataz batholith and formation of the lavasen graben they were therefore broadly coeval with deposition of the lavasen volcanics and emplacement of the esperanza subvolcanic complex these k rich weakly alkalic ferroan a type magmas may provide a viable source for the ore fluid that deposited gold in the pataz batholith

sea level curve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america at least 55 cycles of marine inundation and withdrawal are recognized in the mid desmoinesian to mid virgilian midcontinent outcrop sequence in north america they range from widespread major cycles classic cyclothems with deep water facies extending across the northern shelf through intermediate cycles persisting as marine horizons across the shelf to minor cycles developed on the lower shelf or as parts of major cycles biostratigraphic differentiation of the cycles should establish interbasinal correlation on a scale fine enough to allow evaluation of differential tectonics and sedimentation sequential groupings of cycles are more irregular than proposed megacyclothems or mesothems but they may be obscured by the distinctness of the major cyclothems estimates of cycle periods range from about 40 to 120 x 10 sup 3 yr for the minor cycles up to about 235 to 400 x 10 sup 3 yr for the major cyclothems the range for all cycles corresponds well to the range of periods of earth s orbital parameters that constitute the milankovitch insolation theory for the pleistocene ice ages and it further supports gondwanan glacial control for the pennsylvanian cycles even though the dominant period of the major pennsylvanian cyclothems is up to four times longer than the dominant 100 000 yr period inmore the pleistocene the shapes of both curves display rapid marine transgression rapid melting of ice caps and slow interrupted regression slow buildup of ice caps which suggest similar linkages between the climatic effects of the orbital parameters and ice cap formation and melting at the two different scales widely separated in time 28 references 3 figures less

the role of fluid pressure in the formation of bedding parallel thrust hosted gold deposits sabie pilgrim s rest goldfield eastern transvaal abstract shallowly dipping thrust hosted gold veins occur within the paleoproterozoic transvaal supergroup rocks in the eastern transvaal of south africa internally the veins record episodic events of fluid infiltration and subsequent mineral deposition within discrete local scale thrust faults evidence also exists for subvertical dilation within the individual veins and textures which record open space filling are preserved the probable depth of formation suggests lithostatic pressures in excess of 2 2 kbar and the vein textures indicate numerous reactivation events associated with individual thrust faults occurring in a terrain characterised by low levels of preserved strain fault reactivation may have occurred under conditions of low differential stress in the presence of high pressure fluids with p fluid approaching or exceeding lithostatic levels

linked sequence development and global climate change the upper mississippian record in the appalachian basin the character and relative stratigraphic position of paleoclimatic indicators within upper mississippian strata of southern west virginia suggest a link between eustasy and patterns of continental to global scale atmospheric circulation at the cyclothem scale annual rhythms in marine facies and paleovertisols and lacustrine carbonates in terrestrial units indicate that seasonal semiarid climatic conditions prevailed during highstand progradation in contrast leached paleosols and coals that underlie sequence boundaries and occur within transgressive heterolithic facies are suggestive of humid climatic conditions during late highstand through early transgression milankovitch band glacial interglacial cyclicity may explain both sequence development and the evidence for late mississippian climate fluctuations in the appalachian basin shifts from seasonal to humid climatic conditions are attributed to systematic variation in monsoonal circulation whereby seasonal moisture became restricted to the equatorial belt during the lowstands of each 400 k y glacial interglacial cycle

bedding parallel fibrous veins beef and cone in cone worldwide occurrence and possible significance in terms of fluid overpressure hydrocarbon generation and mineralization abstract bedding parallel fibrous veins are common worldwide in sedimentary basins especially within strata of low permeability the term beef refers to bedding parallel veins of fibrous minerals where the fibres are mutually parallel and have formed quasi vertically more complex on a smaller scale are cone in cone structures yet these are also common within bedding parallel veins for both beef and cone in cone we have compiled a worldwide catalogue 157 localities typically the veins consist mainly of white gangue minerals for example calcite gypsum or quartz but may also contain accessory minerals of economic interest for example bitumen sulphides emerald pitchblende or gold fluid inclusions may contain oil or gas calcite beef 110 localities is common in organic rich shale of marine carbonate origin especially of 1 cambrian ordovician 2 devonian carboniferous 3 early jurassic or 4 cretaceous to palaeogene ages gypsum beef 30 localities is common in evaporitic or lacustrine strata of continental origin especially of triassic or neogene ages quartz beef 17 localities is common within meta turbidite sequences especially of ordovician or proterozoic ages because these modal ages seem to reflect climatic controls we infer that the fibre forming mineral species have not travelled far vertically the same conclusion holds for accessory minerals typical temperatures of formation are 1 up to 60 c for gypsum beef 2 70 c to 120 c for calcite beef and 3 200 c to 350 c for quartz beef hydrocarbon bearing calcite beef may be a good indicator of a petroleum system in which oil or gas migrate together with aqueous solutions we argue that beef and cone in cone layers result from tensile fracturing and vertical dilation coeval with fibre growth possible causes are either 1 force of crystallization or 2 seepage forces due to fluid overpressure for layers that form at depths of several km fluid overpressure is the more likely cause

flexural slip structures in the bushveld complex south africa this paper investigates structures in the layered critical zone of the bushveld complex in south africa that have detrimentally affected mining operations the common structures are layer parallel faults with reverse dip slip and ramp faults with curved slip planes and prominent striations geometric variations include duplex ramps contained within floor and roof layer parallel faults and linking ramps connecting separate layer parallel faults hidden in the footwall and hangingwall the orientations geometries displacements and shear senses of the layer parallel faults and ramp faults are interpreted to be flexural slip structures formed during bending of the originally horizontal bushveld complex into a basin fold geometry during crustal loading

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

anoxia and fluctuating climate recorded from the devonian carboniferous black shales tethys himalaya india a multi proxy approach due to inaccessibility and tough conditions high altitude low oxygen region very less information is available about the indian tethys himalayan sequences particularly geochemical data are scarce an attempt is made in the present study to evaluate the sediment geochemistry and paleo redox conditions of devonian carboniferous black shales from the spiti region tethys himalaya using multi proxy approach besides black shales this paper also provides the first in depth study of the geochemistry of phosphatic nodules hosted by the black shales that will allow primary and secondary processes to be understood geochemical investigations particularly redox sensitive trace element ratios for e g v v ni 0 5 ni co 4 3 7 2 v cr 2 and positive ce anomalies suggest deposition of these black shales under sub oxic to anoxic marine conditions thus recording an ocean anoxic event in the region these ocean anoxic events of devonian carboniferous spiti black shales which possess distinctive geochemical signatures can very well be correlated with global events such as hangenberg events and the lower alum shale event the span of the chemical index of alteration cia values 54 78 variation of 4si parameter 82 3 and c values 0 1 0 6 recorded in the black shales strongly indicate varying degrees of weathering conditions in the source area which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in devonian carboniferous period in the spiti region the distinct geochemical characteristics such as y sr and rare earth element ree enrichments particularly middle ree enrichments with a hat shaped pattern with high p2o5 contents up to 14 wt distinguish the nodules from the host rocks black shales sem scanning electron microscope results supplemented by xrd x ray diffraction studies of the nodules show the dominance of calcium silica and phosphatic phases the diversity of clay minerals significant variations in mineralogical composition and lack of any continuous vertical trend in the nodules suggest a relatively low impact on the primary environmental signal by burial diagenesis

stratigraphic and structural controls on gold mineralization in the pilgrim s rest goldfield eastern transvaal south africa abstract since 1872 approximately 185 metric tons of gold and a significant amount of silver have been won from numerous stratiform ore bodies within the pilgrim s rest goldfield the epigenetic stratiform ore bodies are found primarily in the malmani dolomite subgroup a laterally extensive lower proterozoic carbonate platform succession a strong relationship exists between the presence of stratiform ore bodies and environment of deposition and structural deformation analysis of the host sediments indicates deposition in a retrogradational followed by an aggradational sequence that comprises supratidal sabkha siliciclastic sediments to midshelf carbonates the retrogradational and aggradational sequences are characterized by numerous higher frequency transgressive and regressive cycles mineralization is with few exceptions restricted to shallow water siliciclastic sediments deeper water carbonates are barren more importantly siliciclastic mudstones and sandstones have acted as zones of detachment for thin skinned thrust fault deformation resulting in the development of conformable passageways in which fluids concentrated their auriferous lodes evidence of shallow hinterland dipping duplex antiformal stack and imbricate thrust systems abounds in several underground and field exposures the gold silver emplacement model for the pilgrim s rest goldfield infers fluids derived from a deep seated magmatic source oxygen isotopic compositions of the mineralizing fluids recalculated from 18o quartz 12 1 to 19 5 point to a magmatic source and subsequent mixing with saline evolved formation water a magmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions 100 c km the bushveld igneous complex rustenburg layered suite to the west of pilgrim s rest is thought to be the source of the auriferous hydrothermal solutions and its emplacement was the driving mechanism of thrust fault deformation in the pilgrim s rest goldfield

intrusion hosted mineralization in the charters towers goldfield north queensland new isotopic and fluid inclusion constraints on the timing and origin of the auriferous veins auriferous quartz sulfide veins of the charters towers goldfield are mainly hosted in oxidized i type granitoids of the ravenswood batholith k ar and ar ar isotope age data of hydrothermal muscovite from alteration envelopes of veins at charters towers and the hadleigh castle mine 40 km east of charters towers are indistinguishable within error suggesting broadly synchronous gold deposition between 410 and 404 ma and across a significant segment of the ravenswood batholith published geochronological data indicate that several granitoid bodies were emplaced into the ravenswood batholith at the time of gold mineralization despite their association in space and time with igneous rocks the gold deposits lack an obvious causative intrusion moreover published lead isotope studies indicate that the lead in the ore was not acquired from any of the exposed intrusions a distal origin of the ore forming fluids also may be inferred from nitrogen isotope values of hydrothermal sericite suggesting wall rock interaction with metamorphic fluids or fluids that were in equilibrium with metamorphic rocks during the paragenetic stage of pyrite and arsenopyrite deposition stage ii veins of the charters towers goldfield contain three different types of fluid inclusions which are distinguishable by petrography and microthermometry this study is the first to report saline fluid inclusions in sphalerite 18 9 28 3 wt nacl equiv and vein quartz 20 9 24 7 wt nacl equiv trapped during the stage of gold deposition stage iii the range of temperatures and salinities particularly of the saline inclusions in sphalerite could indicate mixing between hotter more saline fluids e g deep seated magmatic and cooler more dilute solutions e g modified meteoric as the cause of gold deposition the geological and geochemical data are not compatible with derivation of fluids metals and ligands from individual plutons similarity of host rocks ore element associations alteration assemblages structural controls and tectonic settings strongly suggest that the auriferous veins of the charters towers goldfield belong to a group of granitoid hosted lode gold deposits that are generally classified as orogenic

thrust controlled gold mineralisation at the elandshoogte mine sabie pilgrim s rest goldfield south africa stratiform quartz sulphide gold veins locally termed reefs are hosted within the proterozoic transvaal sequence sedimentary succession in the sabie pilgrim s rest goldfield eastern transvaal these deposits have produced about 180 tonnes of gold and share many characteristics with those of telfer western australia detailed examination of the elandshoogte mine shows that gold deposition occurred in two stages both linked to bedding parallel thrust faulting within the sedimentary pile deformation being concentrated within incompetent shale beds interlayered within more competent units the majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early formed sulphide minerals deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone from early shearing in shales to later duplex faulting of the quartz reef fluids accompanying faulting are implied to have transported gold and a magmatic source of mineralisation is suggested

accommodation and sedimentary architecture of isolated icehouse carbonate platforms insights from forward modeling with carb3d abstract a fundamental control on the evolution of carbonate platforms is accommodation the space available for sediment deposition here we adopt a sensitivity analysis approach using the process based forward model carb3d to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy subsidence and surface dissolution sediment production the mechanism invoked to fill accommodation is also investigated fourth order milankovich driven sea level cycles dominate icehouse carbonate sedimentology with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present both overwhelm higher frequency lower amplitude fifth order cycles which are rarely preserved on the platform top subsidence is a fundamental control on sedimentary stacking patterns sequence boundary frequency sbf and platform geometry with rapid subsidence increasing the preservation of platform top sequences most icehouse platforms are subject to extended periods of platform top exposure missed beats when the platform top remains emergent at a sea level maximum are common and create non milankovich sbfs furthermore during platform emergence surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event with surface dissolution rates in humid climates comparable to moderate rates of subsidence however accommodation generation by surface dissolution is self limiting in as much as accommodation generated by dissolution is rapidly refilled on flooding sediment production is a key control on unfilled accommodation which our simulations suggest is considerable on icehouse platforms increased shallow water 3 m depth sediment production enhances intra platform bathymetric relief between the margin and interior this is driven in large part by a higher sedimentation rate at the margin than in the interior platforms can be flooded to depths up to 45 m during marine transgressions and accommodation often remains unfilled there are two distinct phases of platform top sedimentation during the transgression and early highstand platform margins accumulate at a greater rate than interior sediments generating differential intra platform bathymetry subsequently accommodation in the interior fills by forced regression and progradation from the margins into the platform interior numerical forward models can augment our understanding of icehouse carbonate platform evolution by identifying the nature of the sedimentary response to individual controls they can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms such as the large hydrocarbon fields in the north caspian and s e asia

deposition of a saline giant in the mississippian windsor group nova scotia and the nascent late paleozoic ice age abstract saline giants are vast marine evaporite deposits that currently have no modern analogues and remain one of the most enigmatic of chemical sedimentary rocks the mississippian windsor group ca 345 ma maritimes basin atlantic canada is a saline giant that consists of two evaporite rich sedimentary sequences that are subdivided into five subzones sequence 1 is composed almost entirely of thick halite belonging to subzone a osagean sequence 2 is in unconformable contact and comprised of stacked carbonate evaporite peritidal cycles of subzones b through e meramecian subzone b the focus of research herein documents the transition from wholly evaporitic to open marine conditions and thus preserves an exceptional window into the processes forming saline giants lithofacies stacking patterns in subzone b reveal that higher order fluctuations in relative sea level produced nine stacked parasequences interpreted to reflect high frequency glacioeustatic oscillations during the onset of the late paleozoic ice age each parasequence reflects progradation of intertidal and sabkha sediments over subtidal carbonate and evaporite deposits dissimilarities in cycle composition between sub basins imply the development of contrasting brine chemistries from differing recharge rates with the open ocean what the windsor group shows is that evaporite type is ostensibly linked to the amplitude and frequency of sea level rise and fall during deposition true saline giants like the basinwide evaporites of sequence 1 apparently require low amplitude long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater by contrast the high amplitude short frequency glacioeustatic variability in sea level that controlled the accumulation of peritidal evaporites in subzone b produce smaller subeconomic deposits with more complex facies relationships

water sill hypothesis for the origin of certain veins in the meguma group nova scotia canada certain bedding parallel quartz veins some gold bearing in the cambrian ordovician meguma group nova scotia are believed to indicate the former presence of water sills the veins contain primary subhedral quartz crystals which tend to be oriented normal to the vein walls and display brazil twins twinned crystals commonly show hexagonal sections with dark solid inclusion rich centers and white fluid inclusion rich rims viewed in a plane perpendicular to bedding the dark centers of crystals exhibit millimetre thick quartz plates bounded by bedding parallel micrometre thick inclusion bands of white micas the inclusion bands are aligned parallel to the vein walls and terminate within tapered crystals that have an outer zone of milky quartz the quartz veins are on limbs and hinges of folds particularly anticlines from these observations we conclude that 1 the solid inclusion banded quartz resulted from episodic hydraulic extension fracturing and incremental growth of dispersed quartz bridges across water sills 2 the milky quartz filled the vein by growing in optical continuity with the solid inclusion banded quartz 3 localization of certain bedding parallel quartz veins is due to ponding of water beneath aquacludes in very broad domes formed by differential compaction in the meguma group 4 during tectonic shortening these intensely veined rocks localized some of the major anticlines

textures paragenesis and wall rock alteration of lode gold deposits in the charters towers district north queensland implications for the conditions of ore formation ore deposits of the charters towers goldfield ctgf are mainly hosted by fault fill veins extensional 8 of all veins and stockwork like 3 veins are less common and of little economic significance crosscutting relationships and published structural and geochronological data indicate a late silurian to early devonian timing of gold mineralization coincident with regional shortening d4 and i type magmatism paragenetic relationships which are uniform in veins everywhere within the ctgf suggest that vein formation commenced with the deposition of large volumes of buck quartz stage i followed by buck and comb quartz and significant pyrite and arsenopyrite precipitation stage ii gold was introduced during stage iii after earlier sphalerite and coincident with galena and chalcopyrite narrow discontinuous calcite veins of stage iv mark the waning of gold related hydrothermal activity or a later unrelated episode ore zones within the veins are everywhere composed of comb and or gray quartz calcite and or ankerite and bands or clusters of fractured pyrite that are spatially associated with galena sphalerite or chalcopyrite low grade or barren vein sections on the other hand are mainly composed of milky buck quartz with little evidence for modification overprinting or interaction with later fluids gold related hydrothermal wall rock alteration is symmetrically zoned displaying proximal sericite ankerite and distal epidote chlorite hematite assemblages that may be taken to imply wall rock interaction with near neutral fluids ph 5 6 isocon plots assuming immobile al p ti y and zr consistently indicate as k pb s and zn enrichment and na si and sr depletion in altered wall rock specimens relative to the least altered rocks alteration assemblages quartz textures fault rocks and published fluid inclusion and stable isotope data imply that the veins were formed under conditions of episodic fluid overpressuring 0 9 3 8 kbar at a depth of 7 km and a temperature of 310 c the published fluid inclusion data also imply that gold precipitation may have been brought about by fluid mixing however physi and chemisorption of gold complexes onto sulfide surfaces may have been important depositional processes and controls on gold enrichment at the millimeter to centimeter scale given that most gold particles are attached to the surfaces of pyrite crystals of stage ii or to etch pits and fracture surfaces within the earlier pyrite

sequence architecture of the bighorn dolomite wyoming usa transition to the late ordovician icehouse abstract although major late ordovician glaciation was confined to the terminal ordovician hirnantian stage lasting less than 2 million years recent isotopic evidence suggests a prolonged interval of slow growth in polar ice caps tropical carbonate platforms from passive margins and epicontinental seas offer the opportunity to monitor changes in eustatic sea level changes that accompanied this transition from a greenhouse towards an icehouse climate the late ordovician bighorn dolomite of northern wyoming and adjacent states preserves such a transition the bighorn dolomite was deposited on a northeastward facing subequatorial platform and it consists of three third order depositional sequences each is bounded by a regional erosional surface commonly with erosional truncation visible at outcrop scale and commonly mantled by a distinctive brecciated horizon the bighorn dolomite is dominated by highly bioturbated and fossiliferous open marine shallow subtidal facies but portions also contain dolomitic bioturbated restricted marine shallow subtidal and laminated peritidal facies as well as thin bedded argillaceous and fossiliferous deep subtidal limestone the three depositional sequences preserve a transition in their internal cyclicity that is consistent with computer modeling of the stratigraphic signal of increasing period and amplitude of eustatic sea level changes the lowermost sequence which spans the lander and steamboat point members consists of homogeneous acyclic shallow subtidal facies with a few meter scale peritidal parasequences near its top the middle sequence which corresponds to the leigh member contains acyclic to weakly cyclic restricted shallow subtidal and peritidal facies the uppermost sequence consisting of the horseshoe mountain member is dominated by five decameter scale parasequences composed of open marine shallow subtidal restricted shallow subtidal and peritidal facies overall the bighorn dolomite is interpreted to represent deposition during a shift from low amplitude a few meters short period 20 kyr cyclicity to moderate amplitude 10 m long period 100 kyr cyclicity indicating a shift from greenhouse conditions towards icehouse conditions a shift that agrees with recent isotopic evidence for changing late ordovician climate

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

regional folding low angle thrusting and permeability networks structural controls of gold mineralization in the hope reef at fairview mine barberton greenstone belt south africa abstract the hope reef forms part of a number of high grade mineralized but hitherto undocumented auriferous shear zones at fairview mine in the barberton greenstone belt in south africa collectively referred to as the hanging wall reefs the reefs are hosted by steeply dipping greywacke units of the fig tree group on the western limb of the regional scale ulundi syncline the currently mined shallow hope reef represents a top to the northwest low angle thrust that formed as a secondary accommodation structure during continued nw subhorizontal shortening after the lock up of the ulundi syncline northwest shortening strains correspond to the main phase of upright regional folding d2 3 in the greenstone belt the formation of shear and extensional vein sets documents fault valve behaviour along the low angle thrust driven by close to lithostatic fluid pressures and under very low differential stresses the hanging wall reefs at fairview mine underline the formation of mineralized reefs in the barberton gold district with vastly different orientations structural and lithological controls either in sequence or broadly contemporaneous but during the late stage shortening of the greenstone belt the absolute timing of this deformation and associated hydrothermal events remain unclear

the geochemistry of black shales from the chuniespoort group transvaal sequence eastern transvaal south africa geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the chuniespoort group of the late archean early proterozoic transvaal sequence the black shales are fine grained and generally argillaceous and contain on average 1 2 percent total c organic 0 7 percent s 7 5 ppb au 60 ppm cu 330 ppm cr and 125 ppm ni these metal concentrations are similar to or slightly higher than the values given for the u s geological survey black shale standard sdo 1 all other heavy and transition metals however are depleted in the chuniespoort shales relative to sdo 1 it is well known that high concentrations of cr and ni occur in archcan shales the enhanced cr and ni values found in the chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and or diagenesis the chuniespoort shales are characterized by excess fe with respect to that required to form pyrite and have extremely variable c s ratios as well this together with the wide ranging s isotope ratios is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source whole rock s isotopes further indicate that s could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals rather element associations indicate that v and cr are preferentially contained in illite hydromuscovite and that ni co zn cu fe pb and mo probably reside in pyrite and or chalcopyrite gold is only weakly correlated with other elements which may indicate that its distribution is not solely determined by the presence of sulfide minerals the fact that au cu and s c ratios show an increase in samples with delta 34 s values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin the limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin other factors such as the low metal potential of the provenance the rate of sedimentation and the degree of oxygenation of the bottom waters of the basin may have been additional factors limiting the concentration in and extraction of metals from seawater the fact that the chuniespoort shales host significant epigenetic gold occurrences particularly in the sabie pilgrim9s rest gold field is largely related to their chemical characteristics which promoted permeability and high absorption capacities for metals in hydrothermal fluids because of their low metal concentrations elsewhere in the basin the shales cannot however be considered the source rocks for gold mineralization

oxygen isotope and mg ca composition of late visean mississippian brachiopod shells from sw iberia palaeoclimatic and palaeogeographic implications in northern gondwana abstract latest visean brachiopod shells gigantoproductus sp from the pedroches basin of sw spain were sampled in order to obtain chemical and isotopic evidence that may help first to assess if primary geochemical and isotopical signals are preserved in these shells and second to reconstruct temporal variations in palaeoceanographic and palaeoclimatic conditions in northern gondwana during this time span microtexture cathodoluminescence cl and ultrastructure under sem of the brachiopods were examined to avoid sampling of shell portions altered after deposition shell portions classified as non luminescent nl and non luminescent to slightly luminescent nl sl present geochemical signatures within the ranges considered normal for recent brachiopods that have not undergone significant diagenetic modification these include low to undetectable fe and mn contents high sr values and 18o values similar to those of carbonates in equilibrium with seawater on the other hand valve areas that are luminescent l or moderately luminescent with luminescent portions ml l present higher fe and mn contents lower sr concentrations and lower 18o values than those corresponding to nl and nl sl shells this suggests a greater diagenetic alteration of these portions and that they acted as microzones of reaction with the diagenetic fluids temporal trends through the studied interval show a spread in 13c values much lower than that observed for contemporaneous samples up to 1 6 thus obscuring the 13c global signal at the time scale resolution of this study in contrast the 18o record of brachiopod calcite 18oc shows an overall increase of 1 relative to v pdb from the oldest to the youngest samples this is accompanied by a 30 decrease in mg ca ratios of the brachiopod calcites confirming that at least part of this shift in 18oc was related to cooling combination of mg ca and 18oc proxies established in this paper although not full proof for brachiopods yet allowed for the calculation of a temperature drop of 3 3 c and a change in seawater 18ow that becomes 0 4 more positive during this interval the data are consistent with regional seawater cooling that accompanied an increase in continental ice volume over a relatively short time period of

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

the magnitude of late paleozoic glacioeustatic fluctuations a synthesis a comprehensive literature review shows that the magnitude of eustatic fluctuations varied throughout the carboniferous and permian and that at least eight distinct phases can be recognized facies juxtapositions in carbonate successions and erosional relief in clastic successions indicate that glacioeustatic fluctuations of 20 25 m and occasionally as much as 60 m took place throughout the early mississippian tournaisian a widely recognized glacial period middle mississippian mid chadian through holkerian shallow marine carbonate and clastic successions indicate that eustatic fluctuations were 10 25 m a decrease that matches well with the paucity of coeval glacial deposits late visean asbian through mid brigantian glacioeustatic fluctuations of 10 50 m record the initial phases of ice accumulation in advance of the widespread mid carboniferous glacial event the latest mississippian earliest pennsylvanian mid brigantian through langsettian was a time of widespread glaciation and strata of this age commonly exhibit evidence of glacioeustatic fluctuations of as much as 40 100 m although middle pennsylvanian duckmantian through asturian glacial deposits are present in eastern australia paleovalley depths suggest that coeval glacioeustatic fluctuations were less than 40 m glacioeustatic fluctuations of as much as 100 120 m have been widely reported from late pennsylvanian earliest permian stephanian through mid sakmarian successions in north america an increase that corresponds to the growth of large ice sheets across much of gondwana and the accumulation of ice in the northern hemisphere incision and facies juxtaposition in early middle permian mid sakmarian through kungurian successions in eastern australia indicate that glacioeustatic fluctuations of 30 70 m occurred during the waning stages of major glaciation erosional relief in paleoequatorial carbonates and the presence of coeval glacial deposits in australia suggests that eustatic fluctuations of 10 60 m occurred during the final stages of glaciation in the middle to late permian roadian through capitanian but the modest size of most of these fluctuations makes it difficult to isolate the glacioeustatic signature this review demonstrates that far field cyclic successions record changing glacial conditions in gondwana that the magnitude of glacioeustatic fluctuations was directly related to the volume of glacial ice that carboniferous permian glacioeustasy was more variable than previously recognized and that generalizations from short temporal intervals are probably not representative of the late paleozoic ice age as a whole although any attempt to quantify the magnitude of ancient eustatic changes is based on caveats and assumptions this review incorporates the results of over 100 published papers on the topic in an attempt to minimize the errors inherent in any one study

south eubank field haskell county kansas a case of field redevelopment using subsurface mapping and 3 d seismic data a portion of recent field development efforts in the hugoton embayment of western kansas has focused on deeper less drilled mississippian sandstone reservoirs of the chesterian series such efforts have yielded particular successes in the case of south eubank field haskell county kansas new core data combined with subsurface mapping and three dimensional 3 d seismic information have led to over 30 new producers being drilled in this field reservoirs are best developed in fine grained well sorted chesterian sandstones filling a narrow incised paleovalley system that runs north south through the field although new core information contributed to improved reservoir characterization in the area 3 d seismic data were required to accurately delineate the paleovalley system and thus locate specific prospects a number of such prospects remain to be drilled success at south eubank suggests that other portions of the same valley system also might be similarly developed this system extends at least 50 mi 80 km from northern haskell county to the oklahoma border through a number of existing field areas

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets

tropical marine climate during the late paleozoic ice age using trace element analyses of brachiopods abstract the late paleozoic ice age can be considered an important analogue to the modern ice age but comparisons between the two time intervals have been hampered by the difficulty of resolving climatic changes that occur over short e g seasonal time scales in the paleozoic record as a first step toward overcoming these limitations this study employed mg ca trace element ratios sampled across the growth bands of six specimens of the brachiopod composita to assess differences in tropical marine mean temperature and seasonality in visean and moscovian time these time intervals bracket the onset of the late paleozoic ice age which occurred in mid serpukhovian time and thus provide a snapshot of climate changes from a greenhouse to an icehouse world shells were screened for diagenetic alteration by cathode luminescence and by comparing the values of mn sr na and b to those reported for modern brachiopods in addition a 87sr 86sr value measured for one specimen agreed with a published strontium isotope curve for the late paleozoic era both the mean and range of mg ca values were larger in visean than moscovian time suggesting colder and less seasonal temperatures during the late paleozoic ice age the decline in seasonality was due to a greater proportional reduction of maximum values which presumably represent summer temperatures than minimum values these relative differences between the pre ice age and ice age tropical climate provide a first step toward understanding the broad patterns of seasonal scale changes in temperature during a critical time in earth s history