the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

fractured reservoirs with fracture corridors outcrop studies reveal a common occurrence of tabular zones of significantly increased fracture intensity affecting otherwise well lithified rocks these zones called fracture corridors can have a profound effect on multi phase fluid flow in the subsurface using standard geo modelling tools it is possible to generate 3d realizations of reservoirs that contain distributions of such fracture corridors that are consistent with observations including the vertical frequency in pseudo wells inserted into the model at random locations these models can generate the inputs to flow simulation the approach adopted here is to run the flow simulations in a single porosity representation where the flow effects of fractures are upscaled into equivalent cell based properties preserving a clear spatial relationship between the input geology and the resulting cellular model the simulated reservoir performance outcomes are very similar to those seen in real oilfields extreme variability between wells early water breakthrough disappointing recoveries and patchy saturation distributions thus a model based on fracture corridors can provide an explanation for the observed flow performance of a suitable field however the use of seismics to identify fracture corridors is not an easy task new work is needed to predict the seismic responses of fracture corridor systems to be able to judge whether it is likely that we can robustly detect and characterize these flow significant features adequately

cathedral cave wellington caves new south wales australia a multiphase non fluvial cave cathedral cave is an outstanding example of a class of multiphase caves with largely non fluvial origins it contains large cavities such as cathedrals and cupolas characteristic of excavation by convection currents in rising waters smaller scale features such as rising half tubes pseudonotches curved juts projecting corners blades and bridges indicate intersection and exhumation of older cavities during the formation of younger ones it is possible to recognize at least ten significant phases of speleogenesis by morphostratigraphy in addition to the four generations of cave filling palaeokarst deposits intersected by the cave the cave we see today results from the progressive integration of a number of previously disconnected or poorly connected solution cavities copyright 2007 john wiley sons ltd

a suggestion for the origin of mesoscopic fabric in accretionary melange based on features observed in the chrystalls beach complex south island new zealand the chrystalls beach complex a probable accretionary melange of triassic age cropping out in south island new zealand contains a distinctive set of mesoscopic structures all of which contribute to the typically chaotic appearance of the melange these structures are 1 soft sediment bedding pull aparts 2 weakly developed slaty cleavage 3 folds in dismembered bedding with slaty cleavage as the axial surface foliation and 4 quartz fiber lineated shear surfaces analysis of overprinting relations indicates that these structures formed during a single progressive deformation rather than in several discrete events as such they record a change in response to incremental strain from initial ductile behavior toward one dominated by brittle failure it is suggested that this variation resulted from progressive dewatering during shallow level accretion at the base of an inner trench slope

discovering geochemical patterns using self organizing neural networks a new perspective for sedimentary provenance analysis abstract an unsupervised neural network technique growing cell structures gcs was used to visualize geochemical differences between sandstones of four different sedimentary provenance groups p1 mafic p2 intermediate p3 felsic and p4 recycled multidimensional data of four sandstone data sets comprising major elements log normalized major elements trace elements and high field strength elements hfse were projected into colored two dimensional maps that can be visually and quantitatively interpreted the cluster structure and variable distributions produced show that each sedimentary provenance group can be distinguished in the neural maps according to a unique combination of major or trace element concentrations in these terms the distinguishing features of each provenance group are p1 high fe2o3t tio2 mgo mno cao p2o5 sc v cr and cu p2 intermediate fe2o3t tio2 mgo mno cao sc v and cu p3 intermediate to high k2o intermediate sio2 and al2o3 low fe2o3t and tio2 and intermediate to low nb rb and th p4 high sio2 y nb rb th ba and zr coupled with low al2o3 cao na2o fe2o3t mgo mno and tio2 the elemental associations in p1 p2 and p3 reflect petrogenetic evolution of first cycle sources whereas the associations in p4 are compatible with the combined effects of recycling weathering and heavy mineral concentration

interplay between fluid flow and fault fracture mesh generation within underthrust sediments geochemical evidence from the chrystalls beach complex new zealand the chrystalls beach complex in the otago schist on the south island of new zealand is a melange comprising sheared trench fill sediments and fragments of oceanic crust it represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces the melange is cross cut by a fault fracture mesh comprising subvertical extension veins and subhorizontal slickenfibre coated shear surfaces both shear and extension veins have a crack seal microstructure indicating episodic growth shear veins are associated with pressure solution selvages along the shear surface whereas wall rock alteration is not observed adjacent to extension veins electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins and therefore these slickenfibres probably grew by local dissolution precipitation of silica on the contrary no depletion or addition of silica is detected around extension veins indicating these veins grew by precipitation from advecting fluids oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 18o 10 consistent with a fluid derived from low grade metamorphic dehydration reactions fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable melange with increasing metamorphic grade and decreasing porosity fault fracture mesh generation therefore involved localized shear assisted by dissolution precipitation creep and concomitant extension fracturing this led in turn to transient permeability associated with a fluid pressure drop allowing episodic vein growth this process may be analogous to geophysically observed episodic tremor and slow slip which also involves a mixture of deformation styles that put together achieve shear slip along the subduction thrust interface

two million years of river and cave aggradation in ne brazil implications for speleogenesis and landscape evolution abstract this study characterizes and provides ages for an extensive sedimentary record occurring in lapa doce and torrinha caves ne brazil with 40 km of surveyed passages these caves integrate a distributary cave system fed by allogenic recharge from the surrounding sandstone plateaus sediment petrography together with descriptions of depositional facies and architectural elements shows four depositional units related to fluvial and standing water environments these include from bottom to top 1 a channel unit including lateral bars deposited during an ordinary flood regime 2 a sandy flood unit including minor channel and scour fills derived from bank full equivalent flood events 3 mud caps deposited in standing water that often reach the ceiling and 4 intraclast breccias associated with collapse of the mud caps under saturated conditions the deposits were dated using a combination of cosmogenic nuclide burial dating and u series dating of flowstone cosmogenic nuclide data point to fluvial aggradation being active since 1 91 0 12 my and extending until 0 36 0 08 my with intensive cave and valley aggradation events between 0 78 0 10 my and 0 44 0 12 my long term alluviation of the cave system seems to be important in forming passages determining their configuration and setting up a general distributary pattern evident in passage morphology and sedimentary sequences mud caps overlapping the fluvial deposits are interpreted as the products of successive rising and lowering of the water table static level radiometric ages of interstratified flowstones and speleothems show that these oscillations were active at least since 115 ky ago and finally ceased around 12 ky ago indicated by the recurrent age obtained from uneroded capping flowstones these long term water table oscillations may drive paragenetic expansion of the whole cave system valleys caves and other landforms in our study area are part of an ancient landscape with multiple episodes of burial and exhumation despite the possibility of much older sediment being preserved somewhere in the cave system our data indicate a younger quaternary age for the bulk of the sediment filling the study caves

morphological record of oxygenic photosynthesis in conical stromatolites conical stromatolites are thought to be robust indicators of the presence of photosynthetic and phototactic microbes in aquatic environments as early as 3 5 billion years ago however phototaxis alone cannot explain the ubiquity of disrupted curled and contorted laminae in the crests of many mesoproterozoic paleoproterozoic and some archean conical stromatolites here we demonstrate that cyanobacterial production of oxygen in the tips of modern conical aggregates creates contorted laminae and submillimeter to millimeter scale enmeshed bubbles similarly sized fossil bubbles and contorted laminae may be present only in the crestal zones of some conical stromatolites 2 7 billion years old or younger this implies not only that cyanobacteria built proterozoic conical stromatolites but also that fossil bubbles may constrain the timing of the evolution of oxygenic photosynthesis

breccias and ores part 1 history organization and petrography of breccias abstract although the term breccia is one of the oldest rock names still in use and proliferating in its application a satisfactory definition remains problematic and an overall classification of breccias is non existent the names of breccia varieties several hundred please the heart rather than the computer and there is little interdisciplinary correlation among the more than fifty specialized schools of breccia study this contribution reviews briefly the history of breccia research and provides a list of the present study trends complete with references with emphasis on ore associated breccias wherever possible conventional breccia definitions and their shortcomings are discussed and a case is made for the introduction of the petrographic category of coarse fragmentites of which the typical sedimentary breccias and conglomerates as well as volcanic tectonic hydrothermal impact and other fragmental materials are members whether designated or not as breccia in the conventional literature this is followed by a proposed rational organization classification of coarse fragmentites and breccias based on the regime of formation and architecture fabric gefuge two fundamental end member sequences a b and a transitional suite c can be recognized a disaggregation fragmentites d breccias produced from a solid progenitor by increasing rate of fracturing expansion rotation mixing and attrition the transitional sequence is crackle mosaic rubble melange milled breccia b aggregation fragmentites a breccias generated by gathering aggregation and accumulation of originally dispersed fragments at the site of deposition these materials form a sequence ranging from tightly packed fragment supported matrix free fragmentites through matrix infilled matrix supported fragmentites to lonestone rocks c in transport fragmentites t breccias formed by redeposition of an aggregate gathered in an earlier phase of development as an a fragmentite or by transportation and redeposition of an advanced member i e rubble melange or milled fragmental aggregate of a d fragmentite examples of metallic mineralizations coeval with or superimposed on the fragmentite sequences are reviewed

facies stratigraphy and evolution of a middle ediacaran carbonate ramp khufai formation sultanate of oman the khufai formation is the oldest carbonate platform of the cryogenian to lowermost cambrian huqf supergroup a stratigraphic characterization of this unit includes detailed facies descriptions a sequence stratigraphic interpretation and evaluation of lateral heterogeneity and overall ramp evolution the khufai formation comprises one and one half depositional sequences with a maximum flooding interval near the base of the formation and a sequence boundary within the upper peritidal facies most of the deposition occurred during highstand progradation of a carbonate ramp facies tracts include outer ramp and midramp mudstones and wackestones ramp crest grainstone shoal deposits and extensive inner ramp microbially dominated peritidal deposits outcrops in the oman mountains are deep water deposits including turbiditic grainstone and wackestone interbedded with siliciclastic rich siltstone and crinkly laminite facies patterns and parasequence composition are variable both laterally across the outcrop area and vertically through time because of a combination of ramp morphology siliciclastic supply and possible syndepositional faulting the lithostratigraphic boundary between the khufai formation and the overlying shuram formation is gradational and represents significant flooding of the carbonate platform the stratigraphic characterization presented here along with the identification of key facies and diagenetic features will help further future exploration and production of hydrocarbons from the khufai formation

timing and mechanisms for the generation and modification of the anomalous topography of the borborema province northeastern brazil abstract the results of apatite fission track analysis in 14 granitic gneissic samples from two regional transects across the borborema plateau northeastern brazil show evidence for two dominant paleothermal events a late cretaceous cooling event beginning sometime between 100 and 90 ma and a second cooling event in the neogene the distribution of the fission track results suggests that the cooling events have a broad regional expression and are consistent with the geologic record in the araripe basin western borborema province which attests to a post albian uplift of the whole region we hypothesize that the first event is due to the uplift and denudation of regional permanent topography generated after the breakup of brazil and africa such topography is predicted by models of continental margin extension in which continental lithosphere thinning is followed by thickening of the adjacent hinterland lithosphere and crust kusznir n j karner g d 2007 continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins in karner g d manatschal g pinheiro l eds imaging mapping and modeling continental lithosphere extension and breakup special publication 282 geological society london pp 389 419 in northeastern brazil this extension engendered topography may have been amplified by magmatic underplating related to the saint helena and ascension plumes the miocene cooling event 20 0 ma occurred at a time characterized by the transition from carbonate ramp to progradational clastic systems on the pernambuco paraiba margin and the offshore potiguar basin this same stratigraphic response characterizes the neogene stratigraphy of many passive margins and attests to a global increase in the delivery of clastics to margins the simplest explanation of which is a climate change that accentuated erosion of pre existing topography thus the present rugged landscape of northeastern brazil is interpreted to be a product of this younger denudation event a corollary of this study is that the history distribution and delivery of clastics to the northern and northeastern margins of brazil are a function of the regional development of the continental landscape during the late cenozoic

carbon and oxygen isotope profiles across meso neoproterozoic limestones from central brazil bambui and paranoa groups we present carbon and oxygen isotopic data on carbonates along three profiles of the neoproterozoic bambui group in central brazil this unit covers an area of more than 300 000 km2 and comprises carbonate silicliclastic sediments at the base that grade into siliciclastic sediments towards the top the bambui group overlies by unconformity the paranoa group which consists mostly of siltstone quartzite and minor limestone the data presented here improve the stratigraphic correlation within the bambui basin and show that it evolved in an environment significantly different from that of the paranoa basin our data show large fluctuations of 13cpdb in limestones from the bambui group from 0 8 to 13 5 in all the three studied areas some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale this observation is relevant considering the lack of fossil record and other stratigraphic markers in neoproterozoic sequences we also present the first isotopic profiles along the paranoa bambui transition which shows that the 13cpdb values grade from 1 0 in the paranoa group to 2 6 in the lower portion of the bambui group increasing up to 12 in the upper part of this unit based on our carbon isotope data as well as other geological mineralogical and nd isotope studies we argue that the sediments of the paranoa group were deposited on an open platform that was fully connected to the ocean on the other hand the sediments of the bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790 600 ma this unit displays an increased amount of clastic sediments upwards we argue that the high carbon isotope values observed in limestones and marlstones from the bambui group are correlated to worldwide high carbon isotope values reported for the neoproterozoic however we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the bambui carbonates

continuous metamorphic gradient documented by graphitization and k ar age southeast otago new zealand the chrystalls beach brighton coastal section southwest of dunedin new zealand exposes otago schist including the weakly metamorphosed triassic accretionary melange of the chrystalls beach complex and higher grade rocks of a largely felsic arc derived provenance melange zones contain pelagic sediments some of which are manganiferous and metabasites the grade of metamorphism increases progressively from southwest to northeast four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks pumpellyite chlorite pumpellyite actinolite epidote actinolite and biotite d 002 data for carbonaceous material b 0 values for phengite and metamorphic phengite k ar ages are reported for 32 pelitic and semipelitic samples remarkably tight correlations are revealed between d 002 for progressively graphitized carbonaceous material k ar ages that range from 184 6 to 138 7 ma and linear distance along the section both the d 002 values and the k ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes all rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements a convex trend for phengite b 0 values plotted against advancing graphitization suggests p t conditions during metamorphism as in the high pressure intermediate facies series the age of the metamorphic peak is inferred to be 175 155 ma middle to late jurassic with cooling to the mica closure temperature at 155 135 ma late jurassic to early cretaceous unloading denudation and cooling were accompanied by localized hydrothermal events the oldest covering strata were deposited at ca 100 ma and denudation was completed by ca 75 80 ma middle triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 ma associated turbidites contain suspected late middle to early late triassic tube fossils

stratigraphy and environmental conditions of the terminal neoproterozoic cambrian period in oman evidence from sulphur isotopes terminal neoproterozoic and cambrian evaporite rocks worldwide record a marked sulphur isotope excursion to very heavy 34 s isotopes that reflect a period of significant burial of reduced sulphur new 34 s data from anhydrite and pyrite of the ara group in oman extend this record to arabia depositional history of the ara group spans c 8 ma across the terminal neoproterozoic cambrian boundary therefore these rocks provide a detailed record of sulphur isotopic evolution in this important time interval ara group sulphur isotope values include a mean 34 s anhydrite of 39 4 2 6 1 range 32 4 46 4 n 61 and a mean 34 s pyrite of 1 4 7 3 1 range 10 8 to 11 4 n 12 the isotope signatures were not altered significantly during diagenesis and are representative of the isotopic composition of parent waters in the basin marine bromine concentrations in halite and marine fossils in associated carbonate rocks indicate that these waters had a marine origin bacterial sulphate reduction was the dominant process leading to the segregation between isotopically heavy sulphur incorporated in evaporite rocks and isotopically light sulphur buried in pyrite and organic matter strong sulphate reduction operated in a hypersaline and partly anoxic basin these environmental conditions were also conducive to the preservation of organic material and make the ara group a prolific hydrocarbon province significant variation of the observed values is attributed to the inherent variability of bacterial sulphate reduction including partially closed system behaviour disproportionation processes limited diagenesis and possibly secular variation of sulphur isotopes within the basin ara group 34 s anhydrite values overlap with data from other terminal neoproterozoic to cambrian basins but the mean 34 s anhydrite is highest in oman this observation cannot be attributed solely to the specific anoxic conditions and strong bacterial sulphate reduction in oman because such conditions were widespread in most evaporite basins of the terminal neoproterozoic cambrian periods instead slight age differences between these basins can introduce a bias in the sulphur isotope curve excellent preservation of ara group deposits and in particular the precise age constraints make the ara group representative for the seawater sulphur isotopic composition at the terminal neoproterozoic cambrian boundary as a result of the inherent variability of bacterial sulphate reduction 34 s values at the lower end of the spectrum should be most representative for the global signal based on results of this study the global signal is estimated to be approximately 35 40

combining finite strain analysis and illite crystallinity to examine strain variation in a shale detachment zone abstract determining the physical conditions and material properties of low grade sub green schist facies deformation accurately is problematic due to the lack of adequate strain indicators and mineral equilibrium assemblages with established relationships to pressure and temperature conditions for low grade deformation proxies such as illite crystallinity are often used to provide broad constraints on these conditions the relationship between illite crystallinity and temperature remains qualitative however it has been suggested that illite crystallinity may be a more appropriate proxy for tectonic strain at low temperatures recent work on an exposed shale detachment zone in the khao khwang fold thrust belt has described strain partitioning into shale dominated shear domains which delineate coarser grained fault domains this strain partitioning is a key influence in the development of the complex three dimensional shear zone networks which characterise the multiple shale detachment zones in the area here we apply both the fry method of finite strain analysis and illite crystallinity by x ray diffraction analysis to examine the potential strain variations between different shear and fault domains shear domains show less variable and higher grade illite crystallinity values while the fry method results show higher strain values in these shear domains this is confirmed by qemscan mineral mapping of each domain type where shear domains show a far more developed tectonic fabric indicating a much higher degree of strain is accommodated continually throughout these relatively thin shear zones the fault domains they delineate by contrast show far less internal strain which is accommodated discontinuously by discrete low offset fault planes data from our results also show a remarkably strong correlation between illite crystallinity and finite strain and while the small number of samples involved in this study leads us to caution the over interpretation of these results it points to a strong relationship between illite crystallinity and finite strain at low temperatures if further investigated and quantified this relationship could provide a powerful and inexpensive tool for strain determination in fine grained rocks

whitings a sedimentologic dilemma whitings drifting clouds of water milky because of suspended carbonate have been claimed to originate from either the action of bottom feeding fish or direct precipitation of calcium carbonate five cruises during different seasons were made to the great bahama bank to collect data pertinent to the controversy measurements of particulate concentrations average 10 mg liter with a maximum of 20 mg liter of carbonate sediment suspended in whiting water compared with an average of 1 5 mg liter for clear water outside the whitings the particles are dominantly acicular aragonite but mg calcite composes as much as 20 percent of some whitings sedimentation rates measured with fixed and drifting sediment traps were as great as 34 g m 2 hr sediment suspended in whitings aggregated into silt and sand size fioccules and settled to the bottom of settling tanks within six hours even on a rocking ship sediment in artificial whitings created by stirring sediment from the bottom with a shrimp trawl settled to the bottom in about the same time natural whitings on the other hand were never observed to dissipate because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours we conclude that the natural whitings are continually replenished by direct precipitation the search for fish in whitings utilized sidescan sonar and fathometer imaging shrimp trawls rotenone remote video and direct scuba observation these methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most bahamian whitings several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend the distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish stable carbon and oxygen isotopic analyses and delta 14 c activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension estimates indicate that the amount of new carbonate produced in whitings on the great bahama bank is substantially higher than that arising from algal production consequently the amount of sediment transported to deep water may be much greater than previously thought

tectonic significance of ductile deformation in low grade sandstones in the mesozoic otago subduction wedge new zealand this study demonstrates how ductile strain measurements can determine the tectonic evolution of a large and long lived subduction wedge we provide a synthesis of the geology of the south island of new zealand with an emphasis on a wedge tectonics perspective that contrasts with a traditional view that interprets new zealand geology in the context of terrane collision and accretion we argue that the otago subduction wedge evolved in a steady fashion throughout its 290 to 105 ma history in response to accretion of trench fill and abyssal plain sediments and slow erosion of a subaerially exposed forearc high maximum temperatures for rocks in the flanks of the forearc high were no greater than 150 to 300 c with solution mass transfer active as the dominant ductile mechanism the 54 studied samples provide information about the absolute ductile strains acquired all along their flow path from the site of accretion to exhumation in the forearc high we use tensor averages to estimate strain at a regional scale these show plane strain uniaxial flattening given that the tensor averages for sy and sx are close to one on average sz is approximately 0 77 and this is balanced by a mass loss of about 23 percent the average z direction is sub horizontal in the prowedge and moderately plunging in the retrowedge a difference attributed to spatial variations in the mode of accretion we infer that rocks presently in the pro side of the otago high were sourced by frontal accretion and those in the retro side were underplated this result highlights the important role of accretion in determining the style of within wedge deformation and also demonstrates the benefit of using a tensor averaging approach to examine regional strain

depositional facies and platform architecture of microbialite dominated carbonate reservoirs ediacaran cambrian ara group sultanate of oman intrasalt carbonates of the ediacaran cambrian ara group constitute a significant reservoir element of the intrasalt stringer play in oman in which dolomitic carbonates are encased in salt at depths of 3 to 7 km 1 9 to 4 3 mi these reservoir carbonates have significant microbial influences although ara group reservoirs are mostly latest precambrian the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded in lacustrine settings where calcified invertebrates are not a significant source of carbonate or after periods of mass extinction before faunal recovery a broad range of carbonate facies provides the context in which to understand the origin of the microbialite dominated reservoirs developed across both ramp and rimmed shelf profiles major facies associations include carbonate evaporite transition zone deep ramp and slope subtidal microbialites clastic textured carbonates and restricted peritidal carbonates microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties and that help in predicting the location of reservoir fairways microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities 50 shallow subtidal pustular laminites with cm scale variability of lamina morphology deeper subtidal crinkly laminites that show mm scale variability of lamina morphology and intertidal tufted laminates that show mm to cm scale tufted textures other reservoir facies are more conventional grainy carbonates including ripple cross stratified grainstone packstone hummocky cross stratified grainstone packstone flat pebble conglomerate ooid and intraclast grainstone packstone and cloudina grainstone packstone these facies are almost invariably dolomitized and all have moderate to excellent reservoir quality these facies comprise carbonate platforms broken up during salt tectonics that range up to 160 m 525 ft in thickness and extended laterally prior to halokinesis for tens to over 50 km 31 mi the distribution of reservoir facies follows sequence stratigraphic predictions with microbialites occurring in every accommodation profile late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build ups whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate

fractal vein distributions within a fault fracture mesh in an exhumed accretionary melange chrystalls beach complex new zealand a well developed fault fracture mesh is observed in the chrystalls beach complex an accretionary melange within the otago schist on the south island of new zealand in this study an analysis of vein thicknesses and clustering of veins is presented both shear and extension veins have a power law thickness distribution measures of vein spacing best fit a power law distribution but a small data set limits this interpretation to a small fractal range vein clustering varies from random to moderately clustered between outcrops and is the greatest where a large proportion of relatively competent blocks occurs within the melange fractures are distributed within the melange matrix and this localized deformation requires heterogeneity in rheology and or fluid pressure distribution whereas pervasive distributed deformation occurs in relatively homogeneous rock the overall trend of this deformation being mainly accommodated by thin veins required that new fractures formed preferentially over refracturing existing veins which highlights the distributed nature of deformation within a fault fracture mesh the predominance of new fractures may result from vein material being stronger than the cleaved wall rock such that wall rock failure occurred instead of reopening of pre existing shear and extension veins

pb pb dating and pb isotope geochemistry of neoproterozoic carbonate rocks from the sao francisco basin brazil implications for the mobility of pb isotopes during tectonism and metamorphism abstract a u pb study was carried out on carbonate rocks from the neoproterozoic bambui group in the southern part of the sao francisco basin brazil pb isotopic compositions and u and pb concentrations were determined on more than 90 samples from different parts of the basin these samples were found to contain four distinct types of pb here called types i ii iii and iv type i pb was found in samples with low pb concentrations and relatively high u concentrations it represents in situ growth of radiogenic pb and has a large enough variation to define pb pb isochron ages type ii pb is present in samples with relatively high pb concentrations and low u concentrations it is nonradiogenic crustal pb that could either represent average crustal pb at the time of deposition or as interpreted here at the time of deformation of these rocks type iii pb is also found in samples with high pb concentrations and low u concentrations but it apparently is radiogenic crustal pb from the archean or paleoproterozoic basement that was incorporated into the carbonates during the 600 ma brasiliano orogeny type iv pb is intermediate in composition between type iii and type i pb it represents a mixture of these two types pb pb isochron ages obtained from mesoscopically undeformed carbonates containing type i pb range from 686 69 to 520 53 ma the older age is the minimum depositional age for carbonate rocks from the sete lagoas formation during the interval from 690 to 500 ma the pb isotope system of carbonates from the sao francisco basin was disturbed and in some areas it was totally reset type iii pb compositions define a straight line type ii pb falls on the lower end of this line which intercepts the stacey and kramers s k pb growth curve at about 520 and 2100 ma this line is interpreted as representing a stage of crustal pb evolution beginning about 2100 ma and ending about 550 to 500 ma when variably radiogenic pb from the basement was incorporated into these carbonates the ages determined in this study are in agreement with most published ages for brasiliano fold belts marginal to underlying sao francisco craton showing that isotopic systems of sao francisco basin rocks were largely affected by brasiliano tectonism

tectonic and paleobiogeographic significance of radiolarian microfaunas in the permian to mesozoic basement rocks of the north island new zealand abstract from the terranes sutured to the new zealand gondwana margin in mid cretaceous time the murihiku waipapa torlesse and mata river terranes have yielded tectonically and paleographically significant radiolarian faunas a late jurassic fauna from the murihiku forearc terrane is more cosmopolitan than coeval faunas from the waipapa faunas from the terranes east of the murihiku which all show evidence of accretion tectonics can be subdivided into ocean floor assemblages and assemblages of terrigenous clastic deposits waipapa ocean floor assemblages are typically tethyan while the terrigenous clastic assemblages are non tethyan possibly of southern hemisphere high latitude origin ocean floor assemblages show the same eastward younging trends opposed to the westward sedimentological younging due to imbrication as do the assemblages of the terrigenous clastics but are always a few tens of million years older radiolarians from ophiolites tangihua volcanics and matakaoa volcanics indicate that significant portions of the ocean floor obducted onto new zealand in late oligocene to earliest miocene time were of late cretaceous to early tertiary age

stratigraphic controls on vertical fracture patterns in silurian dolomite northeastern wisconsin vertical opening mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed silurian dolomite of northeastern wisconsin our two stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons e g organic partings or cycle bounding mud horizons in terminating opening mode fractures first the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces the two methods show similar results confirming the validity of qualitative visual inspection the second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy for example 63 of cycle bounding mud horizons within the inner middle and middle shelf facies associations serve as mechanical interfaces these empirical percentages are input to a monte carlo analysis of 50 stochastic realizations of mechanical stratigraphy comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33 this method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces the methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing

structural control on karst water circulation and speleogenesis in a lithological contact zone the bossea cave system western alps italy abstract geological surveys structural mapping and thin section analysis helped to unravel the speleogenesis and water circulation in the bossea cave northern italy an unconventional karst system at the contact between carbonates and underlying insoluble rocks the surveys within the cave and in the surrounding surface outcrops revealed a complex structural setting caused by the coupling of lithological mechanical heterogeneities with two major regional left lateral ese wnw strike slip faults transpression associated with the faults led to a disharmonic deformation accommodated by a detachment surface between the buckled basement meta volcanics and the disharmonically folded marble sequence laying on top low angle shear zones recumbent or drag folds and small stacks of duplexes are localized next to this detachment the preferential pathways for water circulation inside the vadose zone are the folded bedding interfaces in the meta carbonate sequence fracture clusters at slipping bed terminations on the other hand provide through sequence connectivity once underground stream water flow reached the cataclastic and highly deformed meta volcanics of the detachment core and damage zones erosion started removing large volumes of rocks the downward erosion of the permian basement rocks caused gravitational instability and subsequent roof collapses aided by structural control in the damage zone above the detachment surface slip surfaces in between duplexes or drag folds this gravitational instability ultimately led to the formation of the giant halls that are characteristic of the downstream part of the bossea cave

paleomagnetism and geochronology of the bebedouro cap carbonate evidence for continental scale cambrian remagnetization in the sao francisco craton brazil abstract new pb pb ages rock magnetic and paleomagnetic data were obtained on neoproterozoic carbonates of the salitre formation which conformably overlie the glacial rocks of the bebedouro formation in undeformed areas of the irece basin sao francisco craton these data give new supporting evidence for a large scale remagnetization and resetting of the u pb isotopic system at cambrian times along the sao francisco and irece basins which cover more than 300 000 km2 of the sao francisco craton surface a 207 pb 206 pb isochron age of 514 33 ma was on undeformed stromatolitic carbonates from the salitre formation although samples from other sites give almost non radiogenic pb signatures they fall at the lower end of this line rock magnetic properties obtained for most of the sites are typical of remagnetized carbonates and include wasp waisted hysteresis loops contradictory lowrie fuller and cisowski tests and anomalously high hysteresis ratios some samples however have presented only some of these magnetic characteristics four magnetic components a b c and d were isolated from the salitre carbonates three of them a b and c are comparable to previous results from the correlative bambui carbonates which crop out ca 600 km southward the component c carried by fine grained sd psd magnetite was identified at 23 sites it directs steeply positive to the northeast giving a paleomagnetic pole sac at 33 n 323 e 95 4 0 k 23 6 the sac pole overlaps with that obtained from component c of bambui carbonates bac pole 30 n 321 e 95 3 8 the paleopoles from component b of salitre sab and bambui bab also lie close but are statistically different after rotation of south america to africa all these poles fall close to the 520 ma segment of the gondwana apparent polar wander path suggesting that the resetting of the isotopic and magnetic systems in the neoproterozoic carbonates of the sao francisco craton occurred at that time the single polarity directions found along the whole sedimentary succession in both areas give additional support for rapid remagnetization events we suggest that paleomagnetic data and most of the pb pb isochron ages in the sao francisco carbonates recorded a regional scale fluid migration event in the aftermath of the brasiliano panafrican collision this fluid flow event may also have concurred for the genesis of the pb zn mineralizations found across the basin

sulfuric acid caves of italy a review abstract in italy especially along the apennine chain numerous active and inactive sulfuric acid speleogenetic sas caves have been documented in the last two decades here we present an overview of these peculiar hypogene systems illustrating their main geomorphological and mineralogical features and the microbial signatures observed in the active underground environment sas caves are widely distributed in the northern and central apennines whereas they are less abundant in the southern apennines in the apulian foreland in sicily and in sardinia their location is significantly influenced by lithological and structural rock properties as they occur in carbonate areas where acidic fluids deriving from the interactions with deep seated sulfates and or sulfides rise through deeply rooted geological structures geomorphological observations demonstrated sub horizontal maze passages to be one of the most favorite arrangements for sas caves whereas gypsum sulfur and alunite supergroup minerals are the most typical sulfuric acid by products the 34 s results demonstrated the deep seated h2s sources to be mainly related to the interaction between triassic evaporites and hydrocarbons

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

back scattered electron imagery of the tectonic fabrics of some fine grained sediments implications for fabric nomenclature and deformation processes back scattered electron investigations of argille scagliose mudstones from the barbados forearc sampled during ocean drilling program leg 110 and the matrix of the okitsu melange southwest japan indicate a variety of microfabric geometries and deformation mechanisms despite the similarity of their mesoscopic scaly fabrics in the okitsu melange the scaly fabric is the mesoscopic expression of an anastomosing microfabric whereas the detailed geometry of scaly fabrics is unrelated to the microfabric in the argille scagliose and leg 110 mudstones care must be taken in the classification and interpretation of scaly fabrics in light of these data and future studies should focus on microstructural investigations

condensation corrosion speleogenesis above a carbonate saturated aquifer devils hole ridge nevada abstract the devils hole ridge a small block of paleozoic carbonate rocks surrounded by the amargosa desert in southern nevada is located at the discharge end of the ash meadows regional carbonate groundwater flow system the massif hosts the famous devils hole a near vertical extensional fracture which intersects slightly thermal groundwater 33 6 c and whose walls are coated below the water table by thick mammillary calcite the latter was deposited continuously over the last ca 600 kyr indicating that the water underneath devils hole ridge was supersaturated with respect to calcite during this time span rocks located more than 9 m above the present day water table remained in the unsaturated vadose zone continuous long term presence of thermal water and the extensional tectonic setting creating underground thermal lakes in open fractures is associated with intense dissolution above the water table condensation corrosion is responsible for modifications of the morphology of the subaerial parts of the tectonic caves we also describe for the first time the devils hole prospect cave which was created almost entirely by condensation corrosion caves and cavities in the devils hole ridge are an interesting example of hypogene speleogenesis by condensation corrosion operating above an aquifer which was demonstrably supersaturated with respect to calcite for hundreds of thousands of years

the development of the sw pacific margin of gondwana correlations between the rangitata and new england orogens prior to formation of the tasman sea the late paleozoic mesozoic rangitata orogen of new zealand and new caledonia abutted the paleozoic new england orogen of eastern australia comparison of the record of permian cretaceous igneous and deformational events from the two orogens suggests that their tectonic evolution was interrelated and is a consequence of convergent plate interaction along the southwest pacific margin of gondwana the following relations are proposed 1 termination of arc volcanism and widespread sedimentation in new england together with the onset of regional deformation and crustal anatexis were synchronous with the commencement of volcanism and sedimentation within the rangitata orogen 2 early permian andesitic volcanism in eastern new england represents an along strike extension of the brook street terrane of new zealand 3 late permian regional deformation in new england coincides with both a break in subduction related igneous activity in the new england and rangitata orogens and a shift in the locus of this activity 4 late permian triassic calc alkaline igneous activity in new england correlates with a phase of relatively continuous accumulation of pyroclastic material in the forearc basin of the rangitata orogen 5 cessation of plutonism in new england corresponds with commencement of formation of the esk head melange in new zealand and the probable commencement of juxtaposition of the te anau and alpine assemblage 6 late cretaceous epizonal plutons intruded into the new england orogen are similar in character and age to those emplaced during the final phases of rangitata orogenesis and both appear to mark initial stages of rifting associated with formation of the tasman sea the generation of permian and triassic igneous activity in eastern new england by convergent plate interaction results on present reconstructions of the gondwana margin in an excessively wide arc trench gap succession for the position of the trench is constrained to east of new caledonia from the permian onward this suggests there may have been some rearrangement of tectonic elements along the margin resulting in widening of the lord howe rise between australia and new caledonia in the period between termination of igneous activity in new england and formation of the tasman sea in the late cretaceous

carbonate deposition and hydrocarbon reservoir development at the precambrian cambrian boundary the ara group in south oman abstract the ediacaran early cambrian ara group in the south oman salt basin consists of six evaporite carbonate cycles a0 a1 to a6 that record tectono eustatic sea level changes the a4 cycle developed at the precambrian cambrian boundary 542 ma it forms a single depositional sequence with evaporites deposited in a lowstand systems tract lst overlying carbonates represent an open marine shallowing upward ramp succession that developed in transgressive tst and highstand systems tracts hst the low energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high the tst facies include sulfates evaporite carbonate laminites and organic rich carbonate laminites that record initial flooding deepening of the basin and establishment of an outer ramp depositional environment carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity subsequent hst facies comprise mostly fine grained clastic carbonates and stromatolites that formed in middle and inner ramp settings these facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp sediment was redistributed in suspension by muddy turbidity currents muddy debris flows storm and shallow water currents during the late hst carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying lst evaporite unit a combination of strong tectonic subsidence and transient flooding caused significant shallow water evaporite deposition to occur not only down dip but also on top of the former carbonate platform where several hundreds of meters of evaporites accumulated the transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit the presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties distribution of organic material and carbonate mud can be linked to specific environmental conditions low physical and biogenic disturbance of sediment and stratified water mass and the sequence stratigraphic position low flux of fine grained carbonate during tst in contrast diagenetic evaporite formation has largely degraded reservoir quality in porous shallow water facies near the top of the a4c

isotope wallrock alteration associated with hypogene karst of the crimean piedmont ukraine although it is commonly perceived that isotopic alteration of carbonate rock is suppressed at low temperatures below ca 80 100 c this study demonstrates that alteration halos do develop in permeable limestones of relatively low textural maturity the study of paleogene carbonate bedrock constituting the walls of hypogene karst conduits of the crimean piedmont ukraine revealed several events of water rock interaction each resulting in isotopic alteration of the bedrock early development of hypogene karst conduits shifted 18o and 13c toward lower values by 1 2 and 1 3 respectively the width of the alteration zone locally exceeds several meters the case hardening associated with cavernous weathering shifted 18o and 13c of the bedrock toward slightly more positive values by up to 3 and 1 respectively the thickness of isotopic halos ranges from a few centimeters to tens of centimeters during late stage hypogene karstification the 18o values of the bedrock was shifted toward lower values by 5 6 forming a thin 5 15 mm alteration halo in most cases 13c was also shifted toward lower values by 6 9 but in one case a shift toward higher values by ca 2 was observed water involved in early hypogene karst and in the origin of cavernous weathering structures was enriched in 18 such waters are known in crimea in association with mud volcanoes on the kerch peninsula we suggest that similar deep seated water affected the crimean piedmont during the middle miocene water involved in late hypogene karstification had d values of 75 to 68 and 18o values of 10 to 9 fluid inclusion data similar to present day spring and well water in crimea isotopic calculations and fluid inclusion data indicate that the late hypogene karst processes occurred at low temperatures ca 50 c these processes ceased before 130 ka when the crimean piedmont was uplifted dissected by erosion and acquired its modern morphology

late quaternary paleoclimate in semiarid northeastern brazil from u series dating of travertine and water table speleothems abstract fossil travertine and subaqueous speleothems in presently dry caves suggest periods of enhanced ground water recharge during the quaternary period in semiarid northeastern brazil travertine deposits and water table speleothems were dated by the 230 th 234 u technique yielding new evidence for the timing of past periods of increased rainfall the travertine was deposited in two distinct phases extensive in situ deposits during marine isotope stage 2 and fragmentary indurated deposits dating to ca 400 000 yr b p possibly marine isotope stage 10 or 12 dating of water table speleothems showed that the regional water table was 13 1 m above the present level at the last glacial maximum a second site yielded ages suggesting a still higher water table during marine isotope stage 6 these new data demonstrate that northeastern brazil did not follow the general pattern of a dry last glacial maximum as suggested by other continental records in amazonia and southeastern brazil and as depicted in recent general circulation model gcm simulations our results thus indicate that significant variability in regional rainfall patterns occurred in the area during glaciations and caution against overgeneralization of paleoclimate patterns

chemostratigraphic correlation of neoproterozoic successions in south america abstract the neoproterozoic successions of south america largely accumulated during extensional events associated with the breakup of rodinia these units are preserved as thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments on passive cratonal margins which have been intensely deformed in mobile belts surrounding the craton volcaniclastic and siliciclastic materials are associated with some of the marginal fold belts three mega sequences are represented in the cratonal area glaciogenic carbonate and molasse at least two transgressive regressive sea level cycles occurred during the evolution of the carbonate mega sequence which lies above iron cemented glacio marine diamictite of probable sturtian age we report high resolution carbon oxygen strontium and sulfur isotope trends from analyses of well preserved sample sets and use principally strontium and carbon isotopes from the best preserved samples in concert with lithostratigraphic and biostratigraphic observations to provide detailed correlations of the south american neoproterozoic successions lead zinc deposits some of which with important metal reserves seem to be restricted to the sturtian carbonate succession and associated with a sequence boundary at the end of the first transgressive regressive cycle phosphate deposits are found in both sturtian and varangerian marinoan carbonate successions above glaciogenic sequences

pedogenic calcretes within fracture systems and beddings in neoproterozoic limestones of the irece basin northeastern brazil abstract calcretes or caliches are continental limestones developed by surficial weathering process that takes place mostly in arid and semi arid regions in the irece basin northeastern brazil in addition to the regular occurrence of pedogenic calcretes a peculiar type of structurally controlled calcretes occurs on neoproterozoic limestones these peculiar calcretes developed near the surface and occur 1 between layers 2 inside fractures and 3 within major thrust faults fieldwork on selected outcrops was integrated with petrographic mineralogic geochemical density and mercury intrusion porosity analyses to constrain the environment of formation and their petrophysical properties the results revealed that this type of calcrete is the product of multiepisodic events of dissolution and precipitation occurring during the wet and dry seasons in the region along fractures and beddings based on the petrophysical results we suggest that these calcretes may have an important role in the migration of fluids through the impermeable host carbonate rock and that they act as a conduit for fluid flow as revealed by their high porosity mean value 26 and remarkable pore connectivity

superposed folding and associated fracturing influence hypogene karst development in neoproterozoic carbonates sao francisco craton brazil abstract porosity and permeability along fractured zones in carbonates could be significantly enhanced by ascending fluid flow resulting in hypogene karst development this work presents a detailed structural analysis of the longest cave system in south america to investigate the relationship between patterns of karst conduits and regional deformation our study area encompasses the toca da boa vista tbv and toca da barriguda tbr caves which are ca 107 km and 34 km long respectively this cave system occurs in neoproterozoic carbonates of the salitre formation in the northern part of the sao francisco craton brazil the fold belts that are around and at the craton edges were deformed in a compressive setting during the brasiliano orogeny between 750 and 540 ma based on the integrated analysis of the folds and brittle deformation in the caves and in outcrops of the surrounding region we show the following 1 the caves occur in a tectonic transpressive corridor along a regional thrust belt 2 major cave passages at the middle storey of the system considering both length and frequency developed laterally along mainly a ne sw to e w and b n to s oriented anticline hinges 3 conduits were formed by dissolutional enlargement of subvertical joints which present a high concentration along anticline hinges due to folding of competent grainstone layers 4 the first folding event f1 was previously documented in the region and corresponds with nw se to n s trending compression whereas the second event f2 documented for the first time in the present study is related to e w compression and 5 both folding events occurred during the brasiliano orogeny we conclude that fluid flow and related dissolution pathways have a close relationship with regional deformation events thus enhancing our ability to predict karst patterns in layered carbonates

influence of fracture stratigraphy on hypogene cave development and fluid flow anisotropy in layered carbonates ne brazil abstract fractured and karstified carbonates are often major aquifers and hydrocarbon reservoirs the toca da boa vista tbv and toca da barriguda tbr caves in ne brazil which form the longest cave system in south america developed in a gently folded layered and fractured neoproterozoic carbonate sequence made of different lithological units limestones versus siliciclastic rocks and marls with variable bed properties bed thickness surface roughness of bed to bed interlayers and fracture attributes spacing height the functional organization of the hypogenic cave system has been recently revealed with subdivision in three speleo stratigraphic storeys which from bottom to top acted as recharge lateral redistribution and outflow components of the rising fluid flow system respectively structural analysis and new fracture data from tbv and tbr indicate that the stratigraphic setting controlled the fracture pattern development during two tectonic phases which determined the fluid flow anisotropy during speleogenesis we found that mechanical and fracture stratigraphy in conjunction with bed thickness and bed to bed interlayer characteristics controlled the spacing and height of fractures and as a result strongly influenced the fluid flow pattern and functional organization of the cave system in particular bed normal rising fluids localized along m to tens of m spaced throughgoing fractures developed in the lower and upper storeys whereas horizontal and bed parallel fluid flow was focused in a package of thin carbonates siliciclastics and marls with cm spaced strata bound fractures this fluid flow anisotropy is attained also due to the presence of rough walled bedding parallel interlayers i e burial stylolites in the lower massive grainstone which hampered bed parallel flow and smooth planar bedding parallel interlayers which enhanced lateral fluid movements in the well layered and fractured middle storey units therefore we conclude that in heterolithic layered carbonate packages the intricate hypogenic karst pattern is related to heterogeneous fluid flow behaviour associated with the structural complexity imparted by the mechanical versus fracture stratigraphy

frequency size distribution of competent lenses in a block in matrix melange imposed length scales of brittle deformation 1 this study explores the frequency size distribution of competent lenses phacoids in the chrystalls beach complex a melange shear zone within the otago schist accretion collision assemblage new zealand the distribution of phacoids within a cleaved pelitic matrix follows a power law distribution with exponent 1 18 d 2 37 2 d at outcrop scale whereas distributions at thin section scale have higher d than the outcrop they were sampled from d relates to volume fraction and distribution of competent material where high d correlates with small aspect ratio phacoids and high matrix volume fraction in outcrops which experienced mixed continuous discontinuous deformation d 1 4 0 2 it is suggested that d 1 4 indicates dominant continuous deformation whereas d 1 4 is indicative of localized deformation on shear discontinuities in this melange d therefore appears a function of dominant deformation style actively deforming melange shear zones likely exhibit a range of seismic styles and earthquake magnitudes at length scales limited by lithological layering and shear zone thickness a hypothesis is suggested where strongly coupled shear zone segments may be distinguished by a high volume fraction of competent material low d while aseismically creeping regions are characterized by a high proportion of incompetent rock high d as the b value in the gutenberg richter frequency magnitude relationship also tends to be high in coupled and low in creeping fault segments it may be that d is analogous to b and the b value is also related to the ratio of incompetent competent material and dominant fault zone deformation style

foliated breccias in the active portuguese bend landslide complex california bearing on melange genesis the active portion of the portuguese bend landslide complex is approximately 3 km2 in area and 30 50 m thick measured displacement rates range from less than 1 to greater than 30 mm day on different parts of the landslide with total displacements over the last 30 yrs ranging from about 10 to greater than 150 m six types of breccia each locally with a foliated matrix were recognized in the active landslide complex and are absent outside the landslide complex slide body breccias are of two types the first formed by extensional fracturing during bulk pure shear at the top of the landslide slide top breccia and the second by flow of tuffaceous shales and fracture of embedded siliceous shales during simple shear deep in the landslide to the basal decollement slide bottom breccias slide margin breccias also in simple shear are produced on the lateral margins of individual slide blocks accompanying wrench fault motion other breccias fault ramp breccias are formed during motion over ramps coll

the influence of bedrock derived acidity in the development of surface and underground karst evidence from the precambrian carbonates of semi arid northeastern brazil very extensive cave systems are developed in precambrian una group carbonates in the campo formoso area eastern brazil in contrast the area is largely devoid of significant surface karst landforms as would be expected given its semi arid climate the caves in the area display many morphological features characteristic of deep seated hypogenic caves such as lack of relationship with the surface ramiform network pattern abrupt variations of passage cross sections and absence of fluvial sediments but do not show evidence of vertical passages marking the ascending path of acidic water nor present extensive gypsum or acid clay mineral deposits hydrochemical analyses of present day ground water indicate that oxidation of bedrock sulphide is an active process and sulphuric acid may be the main agent driving carbonate dissolution in the area a shallow mode of speleogenesis is thus proposed in which sulphuric acid produced through the oxidation of sulphide beds within the carbonates controls cave initiation and development moreover the geological situation of the area in an ancient stable passive margin precludes the possibility of deep seated sources of acidity under dry climate due to the absence of recharge solutional landforms will be largely subdued in the surface hypogenic processes if present are likely to predominate producing a landscape characterized by a marked disparity in the comparative degree of development between surface and underground landforms rates of karst landform development have traditionally been analysed through a climatic perspective runoff being the main controlling factor in promoting karst development this view needs to be reassessed in the light of the growing awareness of the importance of climate independent processes related to hypogenic sources of acidity copyright 2003 john wiley sons ltd

morphogenesis of hypogenic caves abstract hypogenic speleogenesis is the formation of solution enlarged permeability structures by waters ascending to a cave forming zone from below in leaky confined conditions where deeper groundwaters in regional or intermediate flow systems interact with shallower and more local groundwater flow systems this is in contrast to more familiar epigenic speleogenesis which is dominated by shallow groundwater systems receiving recharge from the overlying or immediately adjacent surface hypogenic caves are identified in various geological and tectonic settings formed by different dissolutional mechanisms operating in various lithologies despite these variations resultant caves demonstrate a remarkable similarity in patterns and meso morphology which strongly suggests that the hydrogeologic settings were broadly identical in their formation hypogenic caves commonly demonstrate a characteristic suite of cave morphologies resulting from rising flow across the cave forming zone with distinct buoyancy dissolution components in addition to hydrogeological criteria hydrostratigraphic position recharge discharge configuration and flow pattern viewed from the perspective of the evolution of a regional groundwater flow system morphogenetic analysis is the primary tool in identifying hypogenic caves cave patterns resulting from ascending transverse speleogenesis are strongly guided by the permeability structure in a cave formation they are also influenced by the discordance of permeability structure in the adjacent beds and by the overall hydrostratigraphic arrangement three dimensional mazes with multiple storeys or complex 3 d cave systems are most common although single isolated chambers passages or crude clusters of a few intersecting passages may occur where fracturing is scarce and laterally discontinuous large rising shafts and collapse sinkholes over large voids associated with deep hydrothermal systems are also known hypogenic caves include many of the largest by integrated length and by volume documented caves in the world more importantly hypogenic speleogenesis is much more widespread than it was previously presumed growing recognition of hypogenic speleogenesis and improved understanding of its peculiar characteristics has an immense importance to both karst science and applied fields as it promises to answer many questions about karst porosity especially as deep seated settings are concerned which remained poorly addressed within the traditional epigenetic karst paradigm

storm dominated mixed carbonate siliciclastic shelf sequence displaying cycles of hummocky cross stratification late proterozoic wonoka formation south australia abstract the regressive late proterozoic wonoka formation of the adelaide geosyncline in south australia contains a thick mixed carbonate siliciclastic interval of middle to inner shelf origin the shelf was storm dominated and displays abundant hummocky cross stratification hcs of varying scales and other wave formed sedimentary structures some parts of this sequence are notably cyclic two kinds of cycles are described here the first average cycle 8 m thick consists of the alternation of silty and argillaccous limestone with fine grained hummocky cross stratified calcareous sandstone climatic cyclicity e g milankovitch cycles involving alternate wet and dry periods or minor sealevel fluctuations along a low gradient shoreline are considered the most likely cause of the observed sediment cyclicity carbonate deposition predominated during dry periods or high sealevel followed by influxes of sand from deltaic sources during wet periods or times of low sealevel the prevalence of hcs in the sandstones is most likely due to much slower rates of lithification in comparison to the limestones which give evidence of rapid lithification on the seafloor the second type of cycle average cycle 5 10 m thick is characterised by the cyclic appearence of red hummocky cross stratified glauconitic and phosphatic limestones within a sequence dominated by grey green stylonodular limestone tempestites the most likely origin of the red limestone involves contributions of volcanic ash fall material the resulting sediment did not undergo the rapid seafloor lithification of the associated grey green limestones and thus was involved in considerable storm wave reworking before deposition the micritic shelf carbonates of the wonoka formation are considered to represent direct inorganic precipitation into the middle shelf environment rather than reworking of detrital material from shallow carbonate banks

speleogenesis geometry and topology of caves a quantitative study of 3d karst conduits karst systems are hierarchically spatially organized three dimensional 3d networks of conduits behaving as drains for groundwater flow recently geostatistical approaches proposed to generate karst networks from data and parameters stemming from analogous observed karst features other studies have qualitatively highlighted relationships between speleogenetic processes and cave patterns however few studies have been performed to quantitatively define these relationships this paper reports a quantitative study of cave geometries and topologies that takes the underlying speleogenetic processes into account in order to study the spatial organization of caves a 3d numerical database was built from 26 caves corresponding to 621 km of cumulative cave passages representative of the variety of karst network patterns the database includes 3d speleological surveys for which the speleogenetic context is known allowing the polygenic karst networks to be divided into 48 monogenic cave samples and classified into four cave patterns vadose branchwork vb water table cave wtc looping cave lc and angular maze am eight morphometric cave descriptors were calculated four geometrical parameters width height ratio tortuosity curvature and vertical index and four topological ones degree of node connectivity and graph indices and ramification index respectively the results were validated by statistical analyses kruskal wallis test and pca the vb patterns are clearly distinct from am ones and from a third group including wtc and lc a quantitative database of cave morphology characteristics is provided depending on their speleogenetic processes these characteristics can be used to constrain and or validate 3d geostatistical simulations this study shows how important it is to relate the geometry and connectivity of cave networks to recharge and flow processes conversely the approach developed here provides proxies to estimate the evolution of the vadose zone to epiphreatic and phreatic zones in limestones from the quantitative analysis of existing cave patterns

characteristics and formation mechanisms of the unconformity related paleokarst reservoirs in the upper sinian northwestern tarim basin china abstract in the late sinian the tarim basin experienced regional tectonic uplift and subaerial exposure a regional unconformity was extensively distributed during the succession from the sinian to cambrian as a result a large number of secondary pores were developed in the upper sinian qegebulake formation an integrated study based on petrographical and petrophysical analysis geochemistry karstology and fluid inclusions was performed to provide insight into the characteristics and formation mechanism of the upper sinian carbonate reservoirs the reservoir rocks mainly include microbial dolomite crystalline dolomite and breccias reservoir matrix porosity and permeability range from 0 30 to 11 06 and from 0 00156 md to 1 49 md respectively geological and geochemical characteristics suggest that three phases of dissolution were determined to be responsible for the development of the reservoirs within about 70 m beneath the unconformity they are meteoric dissolution during subaerial exposure and deep seated hydrothermal dissolution and organic acid dissolution during the burial stage the upper sinian dolomite underwent strong meteoric dissolution which is supported by the appearance of the unconformity and the associated caves the geochemical characteristics of the cave filling calcite further indicate the diagenetic overprint from meteoric water including the relatively negative values of 18o 18 46 to 8 5 vpdb and wide range of 13c 8 11 to 0 24 vpdb low ree 1 77 4 23 ppm and yb la and yb ca variation the occurrence of saddle dolomite as well as analytical results such as 13c 0 67 2 52 vpdb and 18o 13 21 to 9 81 vpdb isotope signature and high homogenization temperature of fluid inclusions in quartz 87 2 to 220 c and saddle dolomite 82 3 135 4 c suggest that the reservoirs rocks experienced interaction with hydrothermal fluids only bitumen occurred in the edge of secondary pores formed by dissolution of quartz and saddle dolomite which suggests that organic acid dissolution likely occurred during the evolution of reservoirs a geological model for the evolution of the paleokarst reservoirs is proposed through the analysis of major reservoir controlling factors the primary pore types and reservoir petrophysical property of the qigebulake formation mainly depend on the sedimentation supergene karstification is the most important process for reservoir formation deep seated hydrothermal activity has both positive and negative attribution on the formation of reservoirs organic acid dissolution plays an active role in the formation and preservation of reservoirs this study provides a useful reference for hydrocarbon exploration in the deep buried sinian carbonate reservoirs

supercontinent evolution and the proterozoic metallogeny of south america abstract the cratonic blocks of south america have been accreted from 2 2 to 1 9 ga and all of these blocks have been previously involved in the assembly and breakup of the paleoproterozoic atlantica the mesoproterozoic to neoproterozoic rodinia and the neoproterozoic to phanerozoic west gondwana continents several mineralization phases have sequentially taken place during atlantica evolution involving au u cr w and sn during rodinia assembly and breakup and gondwana formation the crust dominated metallogenic processes have been overriding responsible for several mineral deposits including au pd sn ni cu zn mn fe pb u p 2 o 5 ta w li be and precious stones during rodinia breakup epicontinental carbonate siliciclastic basins were deposited which host important non ferrous base metal deposits of cu co and pb zn ag in africa and south america isotope pb pb analyses of sulfides from the non ferrous deposits unambiguously indicate an upper crustal source for the metals a genetic model for these deposits involves extensional faults driving the circulation of hydrothermal mineralizing fluids from the archean paleoproterozoic basement to the neoproterozoic sedimentary cover these relations demonstrate the individuality of metal associations of every sediment hosted neoproterozoic base metal deposit of west gondwana has been highly influenced by the mineralogical and chemical composition of the underlying igneous and metaigneous rocks

the gundahl complex of the new england fold belt eastern australia a tectonic melange formed in a palaeozoic subduction complex abstract the new england fold belt of eastern australia preserves a palaeozoic fore arc terrain with a magmatic arc fore arc basin and a subduction complex the gundahl complex is a tectonic melange of regional extent in the subduction complex the matrix and slabs of the gundahl complex have six mappable lithofacies argillite greywacke argillite greywacke argillite tuff bedded chert and greenstone the argillite matrix is pervasively sheared with many slickensided shear fractures locally the matrix is formed by highly sheared greenstone greywacke and greenstone blocks are affected by internal shear zones and the blocks themselves pinch and swell folds in places with axial surface spaced cleavage are common within those slabs comprised of well bedded sequences bedding plane shear and faulting at a high angle to bedding also occur in these slabs on a map scale much of the gundahl complex comprises slabs up to 10 km long in imbricate fault bounded slices which repeat the disrupted pre melange stratigraphic sequence elsewhere there are lithologically distinctive blocks containing thick coherent sequences which are structurally incorporated into the gundahl complex the unit is believed to have formed by accretion imbrication and subsequent tectonic disruption of arc derived sediments and less abundant pelagic sediment and greenstone in an ancient subduction complex

geology of the seismogenic subduction thrust interface a microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces the exhumed chrystalls beach complex in the otago schist new zealand may be analogous to an actively deforming underthrust rock assemblage the complex contains asymmetric competent lenses of sandstone chert and basalt enclosed in a cleaved mudstone matrix continuous fabrics such as folds boudins and asymmetric phacoids formed by distributed cataclasis and dissolution precipitation creep discontinuous deformation is evident in an extensive fault fracture mesh involving mutually cross cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces the hikurangi margin provides an example of along strike variations in seismic style possibly related to heterogeneous fluid pressure state and interface geology in both the ancient and active subduction related shear zone fluid pressure state appears to be a critical control on frictional failure which primarily occurs on weak fluid overpressured discontinuities continuous aseismic deformation occurs where other mineral deformation mechanisms such as dissolution precipitation creep are preferred the geometry and composition of the underthrust rock assemblage appear to be first order controls on megathrust fluid pressure distribution bulk rheology and dominant deformation mechanism and thus may be significant controls on megathrust seismic style

a depositional model for the terminal neoproterozoic early cambrian ara group evaporites in south oman six evaporite carbonate sequences are recognized in the terminal neoproterozoic early cambrian ara group in the subsurface of oman individual sequences consist of a lower evaporitic part that formed mainly during a lowstand systems tract overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment source of brines and basin evolution at the beginning of the evaporative phase prograding succession a shallow water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas minor amounts of highly soluble salts locally record the last stage of basin desiccation this gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow water halite from shallow water sulphate and shallow water sulphate from relatively deeper water carbonate respectively these surfaces record repeated flooding of the basin dissolution of evaporites and stepwise reduction in salinity final flooding led to submergence of the basin and the establishment of an open water carbonate ramp marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines the ara group sequences represent a time of relatively stable arid climate in a tectonically active basin strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres while tectonic barriers simultaneously provided the required restricted conditions subsidence allowed evaporites to blanket basinal and platform areas the study suggests a deep basin shallow water model for the evaporites

shear veins observed within anisotropic fabric at high angles to the maximum compressive stress some faults seem to slip at unusually high angles 45 relative to the orientation of the greatest principal compressive stress1 2 3 4 5 this implies that these faults are extremely weak compared with the surrounding rock6 laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre existing frictionally weak surface7 8 9 weakening from chemical reactions10 elevated fluid pressure11 12 13 or dissolution precipitation creep14 15 here we describe shear veins within the chrystalls beach accretionary melange new zealand the melange is a highly sheared assemblage of relatively competent rock within a cleaved anisotropic mudstone matrix the orientation of the shear veins compared with the direction of hydrothermal extension veins that formed contemporaneously indicates that they were active at an angle of 80 5 to the greatest principal compressive stress we show that the shear veins developed incrementally along the cleavage planes of the matrix thus we suggest that episodic slip was facilitated by the anisotropic internal fabric in a fluid overpressured heterogeneous shear zone a similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings we therefore conclude that incremental slip along a pre existing planar fabric coupled to high fluid pressure and dissolution precipitation creep may explain active slip on severely misoriented faults

origin of petroleum in the neoproterozoic cambrian south oman salt basin abstract the south oman salt basin sosb is host to the world s oldest known commercial deposits most of the south oman oils have been proven to be associated with the source rocks of the neoproterozoic to cambrian huqf supergroup but the assignment of oils to specific huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the entire huqf sequence possibly as a consequence of limited change in the local palaeoenvironment and biota over the time of its deposition this study was conducted to establish improved correlations between organic rich rock units and reservoir fluids in the sosb through detailed molecular and isotopic analysis of the huqf supergroup with special emphasis directed towards understanding the ara carbonate stringer play unusual biomarkers tentatively identified as a norsteranes show distinctive patterns among carbonate stringer oils and rocks different from those observed in nafun sediments and ara rocks from the athel basin these putative a norsteranes form the basis for new oil source correlations in the sosb and provide for the first time geochemical evidence of a self charging mechanism for the carbonate stringer play the paucity of markers specific to the nafun group shuram buah and masirah bay formations confounds attempts to quantify their respective contributions to huqf oil accumulations nafun inputs can only be determined on the basis of subtle differences between nafun and ara biomarker ratios the most useful geochemical characteristics delineating nafun group organic matter from ara group intra salt source rocks included low relative abundance of mid chain monomethyl alkanes x compounds low relative abundance of gammacerane 28 30 dinorhopane 25 28 30 trinorhopane and 2 methylhopanes low c 22 t c 21 t and high c 23 t c 24 t cheilanthanes ratio values based on these parameters molecular evidence for major contributions of liquid hydrocarbons from nafun group sediments shuram buah and masirah bay formations is lacking our results suggest that the majority of sosb hydrocarbon accumulations originate from within the ara group either from the carbonate stringers or from the package of sediments that comprises the thuleilat athel silicilyte and u shale formations subtle aspects of the composition of some carbonate stringer and post salt huqf oils could suggest some degree of sourcing from the nafun rocks but stronger evidence is needed to confirm this

episodic burial and exhumation in ne brazil after opening of the south atlantic it is a common assumption that elevated passive continental margins have remained high since rifting and breakup here we show that the atlantic margin of ne brazil has undergone a more complex history our synthesis of geological data landscape analysis and paleothermal and paleoburial data reveals a four stage history 1 after early cretaceous breakup the margin under went burial beneath a thick sedimentary cover 2 uplift episodes in the campanian and eocene led to almost complete removal of these deposits 3 the resulting large scale low relief erosion surface peneplain was deeply weathered and finally reburied at the oligocene miocene transition and 4 miocene uplift and erosion produced a new lower level peneplain by incision of the uplifted and re exposed paleogene peneplain previous studies have identified aspects of this interpretation but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed early cretaceous rifting and eocene oligocene peneplanation we suggest that a late sedimentary cover protected paleogene weathering profiles until the present day the uplift phases in brazil are synchronous with uplift phases in africa and the andes the andean phases coincided with rapid convergence on the western margin of south america and the campanian uplift coincided with a decline in spreading rate at the mid atlantic ridge consequently we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause which is lateral resistance to plate motion

on stress and strain in a continuous discontinuous shear zone undergoing simple shear and volume loss i summarise observations within a continuous discontinuous shear zone to discuss the local stress and strain conditions experienced within a mixed rheology shear zone undergoing volume loss and deformation approximating simple shear the chrystalls beach complex new zealand comprises phacoids formed from dismembered beds by layer parallel extension enclosed within a relatively incompetent matrix local extension is generally subparallel to the regional direction of shortening and overall it appears that layer parallel extension is a geometrical necessity in low angle shear zones where significant flattening occurs in response to simple shear accompanied by volume loss preferential stress loading of phacoids is predicted by fibre loading theory and the failure of phacoids by brittle fracture is thereby governed by fibre stresses transferred from the matrix the principal stress orientations in a phacoid are likely rotated relative to the matrix and either parallel or perpendicular to the phacoid matrix interface as preferential loading of phacoids decreases the stress level in the matrix an increased volume fraction of phacoids increases the strength of the shear zone as a whole however only small matrix volume fractions are required for the composite to act nearly as weak as the matrix 2012 elsevier ltd all rights reserved

porosity estimation and geometric characterization of fractured and karstified carbonate rocks using gpr data in the salitre formation brazil three dimensional distribution of karstic zones and estimated porosity were obtained in fractured carbonate units of the salitre formation brazil from gpr data low amplitude shadow gpr zones could be associated with mineral dissolution halos that developed around fractures and bedding planes gpr volumes show a 3d mazelike geometry of the vertical karstified carbonate zones similar to the set of passages controlled by n s and e w oriented fracture systems observed on the surface in unmanned aerial vehicle images measurements in plug samples indicate that the mineral dissolution caused bulk density and dielectric permittivity decline and incipient increase in porosity from 0 95 to 1 01 these changes in the rock properties affect the em impedance contrasts between carbonate bedding causing loss of the reflected gpr signal the correlation between dielectric permittivity and porosity could be analyzed using the complex refractive index model crim which is used to convert the relative dielectric permittivity into porosity the crim porosities are somewhat lower than plug sample porosities due to different estimation methodologies and measurement scales however the fact that porosity is slightly higher in the dissolved carbonates was maintained in addition gpr amplitude and frequency instantaneous attributes were used to estimate spatial variability in porosity throughout the whole gpr data volume the crossplot of these two gpr attributes show that altered zones generally cluster in narrow ranges of instantaneous gpr amplitude and in broad range of instantaneous gpr frequency assuming empirical correlation between physical properties and gpr attributes two predicted porosity volumes were established for the karstified carbonates in the studied area

geologic and gravity evidence for thrust emplacement of the smartville ophiolite detailed structural relationships at the margins of ophiolites can provide important information about the mode of emplacement of ophiolitic terranes in this study we use geologic mapping and gravity data to determine the structural and tectonic relations between a middle to late jurassic volcanic arc complex the smartville ophiolite block and a pre late jurassic melange terrane in the northern sierra nevada foothills the volcanic arc complex overlies the melange along a complex fault zone although this fault zone is steeply dipping wherever it is observed map patterns suggest that the smartville upper volcanic unit was originally emplaced along a subhorizontal fault that has been subsequently deformed two dimensional interactive computer analysis of gravity data from over 200 stations shows that the smartville upper volcanic unit is a relatively shallow feature with a maximum thickness ranging from 0 5 to 3 km and that it was thrust as a sheet over the previously deformed melange both units were subsequently folded giving rise to the steeply dipping faults observed at the surface recognition of low angle thrusting along the northern margin of the smartville ophiolite block has important implications for the mode of emplacement of the block the most plausible mechanism for the thrusting of this particular oceanic island arc sequence onto the continental margin involves a subduction zone dipping westward beneath the smartville ophiolite block similar to the present day situation in the molucca sea collision zone

the conduit seal system of hypogene karst in neoproterozoic carbonates in northeastern brazil abstract karst processes are of primary importance for the generation of secondary porosity and permeability in carbonate reservoirs and aquifers the topic investigated in this study are the effects of the facies and stratigraphy of different layers on the vertical distribution of solution cavities formed by hypogenic fluids in a carbonate sequence this work uses the toca da boa vista tbv and toca da barriguda tbr caves as analogues of carbonate reservoirs that are affected by karst processes these are two different caves that are 400 m apart but exhibit the same geological characteristics and occur in the neoproterozoic salitre formation 700 560 ma sao francisco craton brazil these caves form the longest cave system in south america with conduits of lengths 107 km tbv and 34 km tbr previous studies of these caves indicated their hypogene origins these caves developed by an ascending fluid flow mainly along fractures the fluid then flowed laterally and was influenced by the following five stratigraphic units from the bottom to the top 1 cross bedded oolitic grainstone 2 fine grainstone with chert nodules 3 microbial carbonate 4 interbedded fine siliciclastics and marls and 5 crystalline grainstone interfingered with chert layers units 4 and 5 formed a stratigraphic seal units 1 2 and 3 below the seal supported the lateral redistribution of flow and respective conduit development therefore these units exhibit a high degree of karstification veins related to the first fluid phases that compose a hydrothermal mineral assemblage cut across these units we conclude that stratigraphic control is important for determining the architecture of the hypogenic cave system at the local scale

diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon nevada u s a abstract a model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper paleozoic golconda allochthon of nevada u s a numerous thrust faults slice the cherts into packets each with a unique set of internal structures fold geometries boudin profiles pressure solution features etc vary from packet to packet and layer to layer analogous variations in the diagenetically mixed siliceous sediments of the miocene monterey formation california suggest the cherts of the golconda allochthon were similarly mixed when they were deformed radiolarian sediments composed of biogenic silica opal a developed ductile structures and pressure solution features because of their high porosity weak lithification and the high solubility of disordered silica lowporosity strongly lithified relatively insoluble quartz cherts the final product of silica diagenesis deformed in a brittle fashion and developed fewer pressure solution features diagenetically intermediate ct cherts and ct porcelanites exhibited transitional behavior mixed diagenetic zones produced structures with layer by layer changes in structural style the dehydration of opal a and opal ct helped create the excess pore fluid pressures responsible for thrust faults hydraulic fractures dilation breccias and elastic intrusions the presence of ductile structures and numerous pressure solution features in the oldest cherts of the havallah sequence suggests that these cherts were deformed while diagenetically immature possibly within a long lived upper paleozoic accretionary prism

the emplacement and fault history of the coolac serpentinite lachlan fold belt southeastern australia abstract the tumut synclinorial zone located in the lachlan fold belt southeastern australia forms a fault bounded belt of early silurian volcanics and flyschoid metasediments of the tumut trough the mooney mooney fault system containing an extensive ultramafic belt known as the coolac serpentinite forms the eastern margin of the trough the ultramafics together with mafic volcanics and intrusive grabbroic rocks have been previously interpreted as early palaeozoic oceanic crust dismembered and obducted during late silurian deformation of the silurian trough sequence recent work however has unravelled a more complex history involving several periods of movement the ultramafics and early silurian volcanics are intruded by early silurian gabbro and syn kinematic late silurian granodiorite these intrusive relationships indicate that the ultramafic rocks were present in approximately their present structural position prior to deformation of the trough sequence the ultramafics therefore cannot represent early silurian oceanic crust obducted during the late silurian deformation they probably represent either early silurian or cambrian ordovician mantle derived material emplaced within a strike slip fault zone during early silurian oblique extension

the geochemistry and origin of fluids in the carbonate structure of the hranice karst with the world s deepest flooded cave of the hranicka abyss czech republic abstract the origin of fluids in the hranice karst containing the deepest flooded abyss in the world has been investigated using hydrogeological hydrogeochemical and isotopic data at least a part of the co2 gas originates in the mantle as indicated by very enriched 13c dic values and from existing he isotope analyses the origin of groundwater in the karstic aquifer which is exploited at the teplice nad becvou spa is meteoric with a recharge area about 200 m above the becva river valley as indicated by depleted values of 2h and 18o compared to the river water based on detectable tritium the groundwater is from 20 to 50 years old water in the hranicka abyss and in the zbrasov aragonite caves is a mixture of carbonate aquifer groundwater with the river and or shallow groundwater comprising variable proportions of both end members water in death cave lake seems to be affected by agriculture contaminated shallow groundwater as indicated by increased nitrate concentration inverse geochemical modeling of aquifer geochemistry suggested two scenarios 1 reaction of mg rich calcite with deep hypogenic co2 about 30 mmol l plus dissolution of trace amounts of halite and sylvite and cation exchange 2 reaction of mg depleted calcite and mg silicate talc as a source of mg together with deep co2 both scenarios were calibrated using 13c dic values and gave satisfactory results a conceptual model of the site has been developed which includes a gravity driven flow system where meteoric water which has recharged in the surrounding uplands is heated at depth and acquires large amounts of hypogenic co2 which preferentially dissolves mg carbonates along the pre existing tectonic features the miocene transgression followed by the later incision of the becva valley played an important role in groundwater circulation and the origin of fluids

hypogenic origin geologic controls and functional organization of a giant cave system in precambrian carbonates brazil abstract this study is focused on speleogenesis of the toca da boa vista tbv and toca da barriguda tbr the longest caves in south america occurring in the neoproterozoic salitre formation in the sao francisco craton ne brazil we employ a multidisciplinary approach integrating detailed speleomorphogenetic lithostratigraphic and geological structure studies in order to reveal the origin of the caves their functional organization and geologic controls on their development the caves developed in deep seated confined conditions by rising flow the overall fields of passages of tbv and tbr caves represent a speleogenetically exploited large ne sw trending fracture corridor associated with a major thrust this corridor vertically extends across the salitre formation allowing the rise of deep fluids in the overall ascending flow system the formation of the cave pattern was controlled by a system of sub parallel anticlines and troughs with nne ssw dominant orientation and by vertical and lateral heterogeneities in fracture distribution three cave stratigraphic stories reflect the actual hydrostratigraphy during the main phase of speleogenesis cavities at different stories are distinct in morphology and functioning the gross tree dimensional pattern of the system is effectively organized to conduct rising flow in deep seated confined conditions cavities in the lower story developed as recharge components to the system a laterally extensive conduit network in the middle story formed because the vertical flow from numerous recharge points has been redirected laterally along the highly conductive unit occurring below the major seal a scarcely fractured unit rift like and shaft like conduits in the upper story developed along fracture controlled outflow paths breaching the integrity of the major seal and served as outlets for the cave system the cave system represents a series of vertically organized functionally largely independent clusters of cavities developed within individual ascending flow cells lateral integration of clusters occurred due to hydrodynamic interaction between the flow cells in course of speleogenetic evolution and change of boundary conditions the main speleogenetic phase during which the gross cave pattern has been established and the caves acquired most of their volume was likely related to rise of deep fluids at about 520 ma or associated with rifting and the pangea break up in triassic cretaceous this study highlights the importance of speleogenetic studies for interpreting porosity and permeability features in carbonate reservoirs

halite cementation and carbonate diagenesis of intra salt reservoirs from the late neoproterozoic to early cambrian ara group south oman salt basin late neoproterozoic to early cambrian carbonates of the ara group form important intra salt stringer reservoirs in the south oman salt basin differential loading of thick continental clastics above the six carbonate to evaporite cycles of the ara group led to the formation of salt diapirs encasing a predominantly self charging hydrocarbon system within partly highly overpressured carbonate bodies stringers these carbonates underwent a complex diagenetic evolution with one stage of halite cementation in a shallow early and another in a deep late burial environment early and late halite cements are defined by their microstructural relationship with solid bitumen the early phase of halite cementation is post dated by solid reservoir bitumen this phase is most pervasive towards the top of carbonate stringers where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics bromine geochemistry revealed significantly higher bromine contents up to 280 p p m in the early halite compared with the late halite 173 p p m the distribution patterns and the high bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval later in burial history relatively small quantities of early halite were dissolved locally and re precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements late halite cement also seals fractures which show evidence for repeated reopening initially these fractures formed during a period of hydrothermal activity and were later reopened by a crack seal mechanism caused by high fluid overpressures porosity plugging by early halite cements affects the poroperm characteristics of the ara carbonates much more than the volumetrically less important late halite cement the formation mechanisms and distribution patterns of halite cementation processes in the south oman salt basin can be generalized to other petroliferous evaporite basins

upper proterozoic carbonate stratigraphy diagenesis and stromatolitic phosphorite formation irece basin bahia brazil abstract carbonate strata of the una group represent late proterozoic platform sedimentation in the irece basin of east central brazil the irece basin contains a basal siliciclastic sequence the bebedouro formation composed of diamictites dominantly of glacial origin that is overlain by lacustrine carbonate sediments these are succeeded by the una group a marine carbonate sequence that totals several hundred meters thick the marine units consist of a basal laminated limestone and dolomitic limestone sequence that grades upward into dolostone and cherty dolostone a middle sequence dominated by gray argillaceous limestones shales and siltstones and an upper sequence of black organic rich lime grainstones stratabound phosphate and sulfide rich units are present within a 50 m thick tid l flat sequence of dolomitic limestone and cherty dolostone in the lower una group shallow water sedimentary structures include stromatolitic laminae columnar stromatolites mudcracks teepee structures collapse breccias length slow quartz nodules and other pseudomorphs after evaporitic sulfates conventional petrography supported by cathodoluminescence and scanning electron microscopy of the carbonate strata reveals a complex diagenetic history representing four major diagenetic environments marine reflux meteoric and burial carbon and oxygen isotope analyses of diagenetic carbonate generations yield distinct populations that support the environmental interpretations concentrations of fine grained carbonate fluorapatite are present in stromatolitic carbonate units in the irece lapao area three types of primary phosphate concentrations have been defined columnar stromatolitic laminar stromatolitic and intraclastic the presence of resedimented phosphatic clasts and of phosphatic units interbedded with barren dolostones suggest that the formation of stromatolitic phosphorite was an early diagenetic process carbon and oxygen isotope data for calcite dolomite and carbonate fluorapatite as well as sulfur isotope data from sulfates and sulfides suggest the presence of an anoxic environment during the formation of the irece phosphorites postglacial enrichment of the basin waters in phosphate is considered to be a critical aspect of phosphatizat on bacterial degradation of organic matter in the stromatolites probably was responsible for local phosphate enrichment of pore waters and resulted in concentration of carbonate fluorapatite by direct precipitation or by replacement of calcium carbonate

progressive development of block in matrix fabric in a shale dominated shear zone insights from the bobbio tectonic window northern apennines italy 1 block in matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary tectonic and mud diapiric processes in many exposed orogenic belts a true melange originates when this fabric is associated with mixing of rocks of different ages and provenance as that characterizing the main decollement shear zone developed between the highly allochthonous ligurian nappe and its substratum of foredeep deposits at the margins of the bobbio tectonic window trebbia valley northern apennines mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement evidence of polyphased deformations spanning from liquefaction related and hydroplastic structures to pseudo hydrofracturing features and mineralized veining have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure a novel evolutionary scheme for block in matrix fabric development is here proposed involving the progressive dismembering of already highly deformed units originally developed in non to poorly lithified conditions through a generalized simple shearing achieved during the evolution of the shear zone coupled with strain hardening due to tectonic compaction synkinematic diagenesis and fluid expulsion

a summary of the geology and oil habitat of the eastern flank hydrocarbon province of south oman abstract the eastern flank hydrocarbon province borders the southeastern edge of the south oman salt basin in the southern part of oman an intensive exploration programme by pdo over the past seven years has led to the discovery of almost 2 10 9 m 3 of oil with current production of some 15000 m 3 d from six fields in stark contrast to other oil habitats of the arabian peninsula the main play involves rock units and geological events ranging in age from late precambrian to quaternary times while trap development and charging are closely related to syn tectonic salt dissolution the principal reservoirs and seals are poorly consolidated palaeozoic clastics which drape due to salt dissolution over residual cores principally composed of either lower palaeozoic sandstones haima group or late proterozoic carbonates huqf group or some combination of these oils have been geochemically correlated with algal source rocks of the huqf eastern flank structures are considered to have been charged from late mesozoic times onwards in conjunction with progressive subsurface salt removal and the onset of maturation of local source rocks the effectiveness of charging along the retreating salt edge is indicated by the small percentage of dry wells major oil zones are found in both anticlinal and truncation traps which are commonly developed around sandstone haima group cores oils show a wide range of characteristics but are typically heavy early expulsion crudes high sulphur with little associated gas bacterial transformation is not uncommon producing reservoirs mainly occur at relatively shallow depths 600 1200 m primary recovery factors of around 7 reflect the high density and viscosity of the eastern flank crudes and have initiated interest in enhanced oil recovery eor techniques two such pilot projects involving steam and polymer flooding are due to commence in 1984 and if successful will substantially increase the reserve base of these fields

famennian upper devonian stromatolite reefs at shatang guilin guangxi south china stromatolites occurred in many reef complexes throughout the phanerozoic and microbial communities played an important role in both ancient and modem stromatolite reefs famennian upper devonian limestone outcrops at shatang northwest of guilin south china contain stromatolite reefs calcimicrobes and algae are also abundant in the reef limestones highlighting the importance of microbial communities in initial reef formation after the frasnian famennian biotic crisis in some cases the shatang stromatolites are connected to form a rigid framework showing a significant ability for construction of biohermal reefs and in other cases they occur as isolated structures the cores of stromatolite reefs were formed by massive limestone containing columnar and dichotomous branching stromatolites calcimicrobes and algae e g solenopora parachaetetes garwoodia and problematic forms are also common some of the calcimicrobes formed stromatolites and other calcimicrobes played a role in sediment stabilization and were more common in biostromal thickets shatang stromatolitic reefs grew on a shoal deposit composed of sand and pebble sized bioclasts field observation and facies analysis suggest that shatang stromatolitic reefs formed platform margin facies which together with previously described renalcis epiphyton reefs renalcis reefs ooid shoals brachiopod shell shoals and nautiloid shoals rimmed the northwest part of the guilin carbonate platform the fossil microbial algal communities together with a complex of sedimentary features allow reconstruction of the depositional history of the environment based on comparison with coeval reefs in the guilin area and the previously developed regional tectonic history a model is proposed describing the sedimentary history of famennian reefs in guilin interpretation of the shatang stromatolites as disaster biota is equivocal as other factors may also have influenced the abundance and type of microbialites in shallow platform margin reef settings in the late devonian c 2004 elsevier b v all rights reserved

petrography and geochemistry of the carboniferous triassic trinity peninsula group west antarctica implications for provenance and tectonic setting the carboniferous triassic trinity peninsula group is a metasedimentary sequence that crops out widely in the northern antarctic peninsula these are some of the most extensive outcrops in the area and hold the key to evaluating the connections of the antarctic peninsula in gondwana however they are still poorly understood here we present our provenance study of the trinity peninsula group using petrographic and geochemical approaches in combination with cathodoluminescence of detrital quartz in order to constrain its source characteristics and tectonic setting using differences in modal composition and quartz cathodoluminescence characteristics we define three petrofacies derived from the progressive uplift and erosion of a volcano plutonic continental arc which exposed the plutonic metamorphic roots as indicated by major and trace elements the source is felsic with a composition ranging from tonalitic to granodioritic the relatively unweathered condition of the source area points to a dry and cold climate at the time of deposition but this does not necessarily mean that it was glaciated deposition of the sediments occurred within an active continental margin relatively close to the source area probably along the south patagonia antarctic peninsula sector of gondwana strong chronological petrological and chemical similarities with the sediments of the duque the york complex in patagonia suggest that they were derived from the same source

the relative roles of boudinage and structural slicing in the disruption of layered rock sequences the dominant micro and mesoscopic structural features of many melanges fault zones and other disrupted rock sequences are anastomosing shear surfaces these surfaces progressively dismember and isolate sections of stiffer lithologies within more ductile matrix in a process referred to here as structural slicing this process is distinct from classical boudinage and when recognized as such may provide a means for differentiating normal boudinaged schist and gneiss terranes from metamorphosed melanges in many sheared melanges structural slicing dominates over both sedimentary processes olistostromal deposition soft sediment deformation and other tectonic processes folding boudinage in the formation of blocks and control of the overall melange geometry this is true at all scales of observation from the production of small clasts to the definition of kilometer scale knockers

schmidt hammer exposure age dating shd of late quaternary fluvial terraces in new zealand schmidt hammer sh r values are reported for surface clasts from numerically dated holocene and pleistocene fluvial terraces in the south island of new zealand the r values are combined with previously obtained weathering rind radiocarbon terrestrial cosmogenic nuclide and luminescence terrace ages to derive sh r value chronofunctions for greywacke clasts from four distinct locations our results show that different weathering rates affect the form of the sh r value versus age curve however a fundamental dependency between the two remains constant over timescales ranging from 102 to 105 years power law scaling constants suggest changes in clast weathering rates are primarily affected by climatic precipitation and temperature and sedimentologic variables source terrane petrology age uncertainties of 22 of the surface age suggest that schmidt hammer exposure age dating shd is a reliable calibrated age dating technique for fluvial terraces copyright 2013 john wiley sons ltd

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms