early carboniferous carbonate systems an alternative to the cainozoic paradigm abstract an alternative model for carbonate deposition is offered for the early carboniferous the major features of this interval are not explained using late cainozoic analogues early carboniferous low latitude shallow seas were apparently strongly influenced by upwelling and high nutrient levels these conditions favoured highly carbonate productive offshore systems but also resulted in the formation of extensive abiogenic carbonates as muds and ooids if correct the carboniferous serves to illustrate the dangers in extrapolating from present day systems to the geological record

waulsortian type buildups and resedimented carbonate turbidite facies early mississippian burlington shelf central missouri abstract the crinoidal carbonates of the lower mississippian osagean burlington limestone in central missouri consist of individual and multiple interconnected waulsortian type buildups and associated resedimented debris carbonate turbidite facies this association of waulsortian type buildups and turbidity current sedimentation developed on the foreslope toe of slope and basinal near toe of slope areas of the prograding burlington shelf the waulsortian type buildups contain a core facies of dolomitized bryozoan crinoid lime mudstone and a flank facies of symmetrically disposed thin bedded crinoidal packstone and grainstone the core developed where bryozoan crinoid thickets trapped carbonate fines of upslope turbidity current origin lime mud hardgrounds in the cores developed during prolonged breaks in the influx of lime mud the flank beds developed by in situ disarticulation of indigenous crinoids and minor gravity slide processes the resedimented debris facies are laterally continuous and normally graded grainstone and packstone the beds show 1 a subdivision of bouma intervals a and b 2 coarse tail grading 3 scours flutes and load casts 4 a generally random fabric except for crinoid ossicle alignment in bouma interval b and 5 no articulation of crinoid stems or calyxes these features combined with the absence of infiltration textures escape burrows shell imbrication and hummocky cross stratification are useful in distinguishing these resedimented beds from lag suspension deposits of storm origin the lower part of the burlington 5 15 m thick consists exclusively of waulsortian type buildup core and flank facies these buildups rest on a thin laminated dolomitic shale bed starved basin sediment and represent basinal near toe of slope deposition the upper and middle parts of the burlington limestone corresponding to foreslope and toe of slope depositional settings consist of waulsortian type buildups and resedimented debris facies considering approximate modern analogues the resedimented debris probably came from various linear near shelf margin sources the resedimented debris contains algal bored grains from the shelf margin and foreslope biogenic grains and clasts

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

the importance of overpressure timing and permeability evolution in fine grained sediments undergoing shear abstract we summarise new experimental data which clarify the hydrological behaviour of clayey sediments undergoing shear and which emphasise the importance of the relative timing of deformation and overpressure fluid flow in fine grained materials subjected to shear is strongly dependent on the previous stress history of the material underconsolidated sediments such as those that retain high porosities through retarded dewatering and consequent excess pore fluid pressure generation deform by bulk volume loss which reduces permeability the deformation fabrics are pervasive in style weakly developed and without a significant influence on hydrological properties in contrast sediment that has been consolidated and then subjected to excess pore fluid pressure deforms dilatively along discrete brittle shear zones which markedly affect the permeability moreover below a certain threshold of effective stress these localised zones prompt a markedly non linear increase in hydraulic conductivity a laboratory demonstration and quantification of the concept of fracture permeability

the effects of fluid escape on accretionary wedges 1 variable porosity and wedge convexity fluid flow out of accretionary wedges is a consequence of the porosity reduction of wedge material both with depth and with distance from the toe of the wedge changes in porosity result in changes in bulk density because bulk density is a function of solid density and porosity these variations in bulk density in turn affect the critical taper of the wedge the critical taper of a wedge with spatially varying strength pore pressure and or bulk density is dependent not only on the value of these parameters but on their first derivatives as well dahlen 1990 using the equations derived by dahlen we find that the arc ward decrease in porosity results in the commonly observed convex wedge surface a hypothetical model using a density depth function generated from a synthesis of accretionary wedge data bray and karig 1985 demonstrates that the surface angle can decrease between 0 5 and 1 in the first 60 km from the toe of the wedge steeper wedges exhibit a greater degree of convexity greater changes in slope the amount of change in slope is relatively insensitive to the internal strength of the wedge wide angle seismic data from the barbados accretionary wedge bangs et al 1990 imaged velocity variations from the surface down to the decollement for a distance of 100 km from the toe these data can be used to map the spatial variations in bulk density within the barbados accretionary wedge holding strength and pore pressure values constant and allowing only bulk density to vary results in a model that closely matches the observed bathymetry both in overall convexity and in shorter wavelength bathymetrie features the model allows a limited range of acceptable basal pore pressure values for the wedge the ability of the model to accurately match the majority of the observed bathymetry suggests that bulk density variations are of first order importance for determining not only the wedge shape but also the shorter wavelength features the model however does not produce the most pronounced changes in slope at the ten kilometers nearest the toe this suggests that other factors such as variations in pore pressure sediment strength or cohesion are more important influences on the wedge taper nearest the toe

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

molybdenum isotopes in late archean carbonate rocks implications for early earth oxygenation abstract we present carbonate mo isotope data of the 2 64 2 5 ga old ghaap group transvaal supergroup south africa to contribute to the debate on early earth oxygenation mo fractionation in carbonates has just recently been introduced as a potential new proxy for ocean redox chemistry and it was suggested that under favorable conditions non skeletal carbonates closely monitor the ambient seawater 98 95 mo composition the record presented here is characterized by three phases of moderately to strongly fractionated 98 95 mo with values of up to 1 6 interrupted by two shifts towards detrital levels this near continuous presence of heavy 98 95 mo suggests a quasi permanent availability of free atmospheric o 2 because fractionation relative to the continental background signature occurs exclusively when mo is present as dissolved molybdate ions moo 4 2 we compare our carbonate data to a previously published black shale mo record on samples from the same drill core the general interpretation of both records is identical a rise in o 2 preceded the commonly accepted transition to a consistently oxygenated atmosphere by 150 ma however in the ghaap group carbonates fractionated mo is found at a time when the coeval black shales still indicate detrital sedimentation a combination of detritus admixture and or redox controlled mo isotope fractionation during shale formation in suboxic to anoxic waters is suggested as cause for a period characterized by a persistent offset between lighter shale and heavier carbonate 98 95 mo the apparent offset can then be attributed to the different geochemical behavior of the two archives with respect to the local depositional conditions redox vs non redox dependent such changes in the sedimentary environment might also explain why the carbonates do not reproduce rapid fluctuations found throughout the shale record but show a continuous signature with scarce outliers thus by means of the combined archives the conspicuous 98 95 mo scatter found in the shale record could be attributed to variable depositional conditions during its formation rather than global oxic anoxic fluctuations simultaneously the carbonates provide a better approximation of the long term 98 95 mo seawater evolution this study introduces the mo isotopic composition of carbonates as a promising new tool for paleo redox studies in particular when combined with black shales

digital characterization of thrombolite stromatolite reef distribution in a carbonate ramp system terminal proterozoic nama group namibia the stratigraphic architecture of a terminal proterozoic carbonate ramp system ca 550 ma nama group namibia was mapped quantitatively with digital surveying technologies the carbonate ramp consists of a shoaling upward ramp sequence in which thrombolite stromatolite reefs developed at several stratigraphic levels the reefs are associated with grainstone and heterolithic facies and exhibit diverse geometries and dimensions related to the position in the sequence stratigraphic framework laterally extensive reefs with a tabular geometry developed when accommodation was relatively low whereas discontinuous oblate dome shaped reefs developed during times when accommodation space was relatively high collecting sedimentological and stratigraphic data digitally in an extensive canyon system allowed a comprehensive documentation of the three dimensional 3 d architecture and dimensions of the reefal buildups both deterministic and stochastic methods were used to extend outcrop observations to construct 3 d models that honor the observed stratigraphy in particular the accuracy with which dimensions of reefal buildups can be measured is critically important in the statistical modeling of the dome shaped buildups calculations and corrections can be applied directly to the digital data set and serve as input during model building the final 3 d model faithfully reproduces the outcrop distribution of facies and geological objects and has a high spatial resolution compared with petroleum industry reservoir models the organization of the reefal buildups in the stratigraphic framework has direct implications for reservoir continuity and connectivity in analogous settings the digital characterization and 3 d outcrop models presented in this article can be subsequently used to condition dynamic reservoir simulation modeling of geologically similar areas

the permo carboniferous dwyka formation of southern africa deposition by a predominantly subpolar marine ice sheet abstract the 800 m thick glacigene dwyka formation was deposited along the northern margin of the permo carboniferous dwyka basin which covered an area of approximately 2 10 6 km 2 in southwestern gondwana the palaeogeographic setting geochemical data of the mudrocks and diamictites and the palaeontology indicate marine conditions during sedimentation ice lobes from spreading centres in the north east and south coalesced in the basin to form an extensive ice cover from the westphalian to the late sakmarian lodgement rain out and subaqueous debris flow diamictons subaqueous and subglacial melt water sands suspended mud and turbidity current sands and silts accumulated in the dwyka basin sedimentation started on the continental shelf during a grounded ice sheet stage predominantly lodgement processes then a floating ice stage predominantly debris rain out and finally an ice sheet disintegration stage debris rain out sand fall out and suspension settling of mud the palaeogeographic setting presence of marine conditions in the basin and the scale of glaciation indicate deposition from a predominantly mid latitudinal marine ice sheet the overall characteristics of the glacial sequence are neither typical of a polar nor a temperate setting and for such ancient glaciations a subpolar setting with the presence of unstable ice shelves is suggested

modeling of fluid flow and pore pressure at the toe of oregon and barbados accretionary wedges fluid venting was recently found on the central oregon accretionary wedge and evidence of fluid flow along the decollement were obtained in the barbados wedge the chemistry of hydrocarbons contained in the migrating fluids is different in these two areas and indicates that the main source is deep for the barbados decollement and shallow for the oregon vents we apply a finite element model with imposed thrusting kinematics and nonlinearities in the compaction equation to the toe of both wedges no deep fluid source is included thus the main source is the footwall just below the frontal thrust the model indicates the following 1 pore pressure build up would be essentially controlled by kn the equivalent permeability in the direction normal to the strata in spite of very different sediment lithologies kn is of the order of 10 17 m2 for both oregon turbidites horath 1989 and barbados muds taylor and leonard 1990 and no high pore pressures can be obtained inside the offscraped sediments 2 permeability anisotropy greatly modifies fluid flow paths a fault with a permeability anisotropy of 103 would channel fluids efficiently but leave the footwall pressure almost unchanged a stratigraphic permeability anisotropy of 102 as documented for the oregon turbidites would redirect the flow of pressurized fluids in the footwall along strata into the trench if clogging of the frontal thrust occured fluids expelled from the footwall would move across the frontal thrust migrate along permeable strata in the hanging wall and then be vented near the top of the eroded anticline as observed in oregon 3 comparison of published estimates from data of fluid explusion fluxes with the modeling results suggests that a deeper and larger fluid source is needed at the barbados site but is not required for the oregon site

primary haematite formation in an oxygenated sea 3 46 billion years ago the timing of the origin of photosynthesis remains controversial the discovery of ancient haematite crystals that formed in a jasper formation in australia which was created in a marine setting suggests that oxygen was being produced at least locally by photosynthesis as early as 3 46 billion years ago

cyclic ramp to basin carbonate deposits lower mississippian wyoming and montana a combined field and computer modeling study abstract the lower mississippian lodgepole lower madison formations 20 225 m thick developed along a broad 700 km storm dominated pericratonic ramp three types of fifth order upward shallowing cycles are recognized across the ramp to basin transition peritidal cycles consist of very shallow subtidal facies overlain by algal laminated tidal flat deposits which are rarely capped by paleosol breccia layers shallow subtidal cycles consist of stacked ooid grainstone shoal deposits or deeper subtidal facies overlain by ooid skeletal grainstone caps deep subtidal cycles located along the outer ramp consist of basal sub storm wave base limestone argillite overlain by storm deposited limestone which are capped by hummocky stratified to massive skeletal ooid grainstone deep subtidal c cles pass downslope into ramp slope facies composed to rhythmically interbedded limestone and argillite with local deep water mud mounds no upward shallowing cycles occur within the ramp slope facies average cycle durations calculated along the outer ramp are between 30 110 ky the fifth order cycles are stacked to form three third to fourth order depositional sequences which are recognized by regional transgressive regressive facies trends and cycles stacking patterns ramp margin wedges rmw developed during long term sea level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies which pass downdip into deep subtidal cycles transgressive systems tracts tst which onlapped the ramp during long term sea level rise include thick shallow and deep subtidal cycles peritidal cycles are restricted to the inner ramp highstand systems tracts hst developed during long term sea level highstand and fall and along the ramp are composed of early hst shallow subtidal cycles overlain by late hst peritidal cycles shallow through deep subtidal cycles characterize the hst along the ramp slope two dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies fifth order sea level oscillations of at least 20 25 m were required to generate deep subtidal cycles along the ramp slope synthetic sequences run with fifth order sea level amplitudes end page 1194 and tst deposition a feature not observed in the actual sequences other factors in addition to fifth order sea level oscillations likely played a role in generating synchronous peritidal and deep subtidal cycles during hst deposition these factors may include short term climatic changes which influenced the depths to storm wave reworking the moderate amplitude sea level oscillations suggested by the cyclic sequences may reflect the initial effects of carboniferous glaciation that was occurring in gondwana

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

decollement processes at the nankai accretionary margin southeast japan propagation deformation and dewatering the decollement zone expressed on seismic profiles and observed in drill cores from the nankai accretionary margin off the southeast coast of japan reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the decollement the extension of this fault zone well in front of the tectonic deformation front and the absence of features indicative of precursory shear for example folded sediments shear bands and penetrative mineral fabrics these characteristics suggest that the mode of formation and evolution of this decollement zone may be unique from that of most thrust faults we propose that the decollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures to test this possibility physical property measurements and clay mineral fabrics were obtained for several samples from the nankai decollement zone using computed tomography and x ray texture methods our findings suggest that deformation within the decollement zone is partitioned into a volumetric component preserved as reduced porosities within coherent fragments and a localized shear component evidenced by mineral preferred orientations along discrete slip surfaces we suggest that the reduced porosities result from the destruction of cementation in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures the nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward and the weakened protodecollement subsequently accommodates shear displacements along discrete fractures

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

upper devonian frasnian stromatactis bearing mud mounds western alberta canada reef framework dominated by peloidal microcrystalline calcite

structural styles chronology rates of deformation and time space relationships in the umbria marche thrust system central apennines italy structural interpretation of geological and geophysical data available for the central apennines italy allowed us to draw balance and restore three geological profiles across the external zones of the umbria marche thrust system and to evaluate the timing and rates of deformation in a roughly 4 ma time window from late messinian to late pleistocene time from this data set we established a general correlation between spacing l and thickness d of the thrust sheets which was used as a depth correction factor for deriving local versus regional depths to the sole thrust and a time space relationship between the closing time of activity of each thrust and the distance x from a reference point within the system

conditions for hydrofracture and the fluid permeability of accretionary wedges abstract the dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation episodic hydrofracture in these zones enhances permeability enabling fluid flow to be faster than the speed of underthrusting here i investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode the physical properties of model rock types are chosen to represent uncemented mud or cherty marly mudstones that have undergone significant diagenesis during thrust faulting hydrofracture always requires pore fluid pressures in excess of the lithostatic load whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load all three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper the model computations indicate that in the upper 80 120 m of a section of soft mudstone hydrofracture inevitably accompanies wrench or normal faulting hard mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting these differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges they influence hydrocarbon trapping and play a role in the localisation of mud diapirs and mud volcanism

the cycling and redox state of nitrogen in the archaean ocean organisms that produce oxygen through photosynthesis existed during the late archaean eon about 2 500 million years ago but controversial evidence suggests that they may have evolved several hundred million years earlier oxygen is expected to react with oceanic nitrogen altering its redox state the reaction leaves a signature in the isotopic composition of the nitrogen bound in organic matter here we present a record of the nitrogen isotopic composition of kerogen extracted from minimally altered shales from the campbellrand malmani platform in south africa between the palaeo archaean and about 2 670 million years ago the 15n values of the kerogen rose by about 2 we interpret this increase as an indication of the onset of coupled nitrification and denitrification or anammox reactions in the surface oceans which require the presence of free oxygen a second increase in nitrogen isotopic composition around 2 520 million years ago implies instability of the n cycle with loss of fixed n this evidence for available oxygen in the oceans occurs at least 200 million years before geochemical indications of the presence of significant levels of atmospheric oxygen we suggest that coupled nitrification and denitrification drove the loss of fixed inorganic nitrogen leading to nitrogen limitation and conclude that the low levels of biologically available nitrogen limited the growth of oxygen producing plankton delaying the accumulation of oxygen in the atmosphere the initial production of oxygen in early earth s oceans altered the redox chemistry and cycling of nitrogen a record of nitrogen isotopes from preserved organic matter indicates nitrogen cycling in the presence of free oxygen 2 67 billion years ago about 200 million years before the first geochemical evidence for atmospheric free oxygen

growth dynamics of pennsylvanian carbonate mounds from a mixed terrigenous carbonate ramp in the puebla de lillo area cantabrian mountains northern spain abstract carbonate mounds are widespread in many places around the world and are especially well exposed in the cantabrian mountains northern spain this study focuses on the composition and growth dynamics of two large mound complexes well exposed near the village of puebla de lillo 70 km northeast of leon and evaluates the impact of sea level accommodation and siliciclastic input on mound growth the mound substrate consists of bedded dark skeletal bindstone the core facies exhibits patchy growth forms of alternating massive microbial algal boundstone and skeletal and nonskeletal wackestone reflecting the overall mound internal composition of smaller coalescent bodies microbial activity as inferred from the volumetrically important clotted peloids appears to be the main factor leading to the high accumulation rate in the cores the steeply dipping flanks presumably resulted from early cementation and subsequent lithification and stabilization induced mainly by microbial activity in the intermound strata carbonate beds and small mounds alternate with fine grained sandstone siltstone and shales the intermound carbonates consist of skeletal and intraclastic mudstone to wackestone whereas algal microbial boundstone is a minor component overall facies transition from the mound core towards the flanks or the mound capping beds is gradual however the volume of bioclasts and intraclasts increases towards the flank and capping facies inversely to amount of clotted peloids or micrite the substrate and the cores of the two mound complexes likely formed in the photic zone as inferred from the common calcareous algae carbonate precipitation favored by microbial activity and algal growth along with gain in accommodation space led to mound core accretion the growth of the small mound complexes was most likely stopped by recurrent sea level fall and siliciclastic input the large high relief mound complex actually kept growing until reaching shallow water and high energy conditions unlike many late paleozoic counterparts these mounds were not subaerially exposed during growth the great thickness and the aggradational mound growth are indicative of a high subsidence rate at a later stage however siliciclastic input also buried the large mound complex rapid subsidence sea level fluctuations and siliciclastic input were thus decisive factors controlling carbonate production this study reveals that mound complexes nucleated and grew on preexisting paleohighs maintaining consistent growth in these specific settings paleohighs recurrent sea level fluctuations presumably linked to glacio eustasy and episodic siliciclastic input are the principal factors that controlled the shape size and growth evolution furthermore the internal architecture as a mosaic of juxtaposed small bodies composing the complexes represents an important finding for better understanding of mound growth and thus for better constraining reservoir heterogeneity particularly for upper paleozoic deposits

influence of early strike slip deformation on subsequent perpendicular shortening an experimental approach we used sandbox analogue models to study the influence of previous pure strike slip and transpressional structures on subsequent perpendicular compression we performed also a brittle viscous system in order to analyse the presence of a viscous basal level experimental results show that a previous pattern generated under pure strike slip and transpressive regime causes the reactivation of structures with favourable orientation and the nucleation of oblique thrusts and non rectilinear deformation fronts under compression perpendicular to the first fault system the presence of a viscous basal level with the imposed strain rate which does not favour the coupling between the viscous layer and cover inhibits the reactivation of previous structures comparison with a sector of northern chile margin 24 25 s located in the forearc trench parallel region supports the experimental results

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

application of 24 norcholestanes for constraining source age of petroleum abstract two c26 sterane ratios the 24 nordiacholestane ratio ndr and the 24 norcholestane ratio ncr are useful for assessing petroleum source age values for these parameters generally increase in the cretaceous when diatoms were rapidly radiating and further in the oligo miocene due to widespread high latitude deposition of diatomaceous sediments sources with shale lithologies tend to exhibit the highest ratios while carbonate and terrigenous rich lithologies generally show lower ratios on the north slope of alaska elevated ratios are observed for cretaceous hrz oils and mixed oils containing cretaceous input compared to older triassic and jurassic derived oils tertiary sourced oils from romania and bulgaria have much higher ratios relative to jurassic sourced oils in the black sea region oleanane was observed in comparable levels in some black sea tertiary and jurassic oils and source rocks ndr and ncr coupled with other age diagnostic indicators such as oleanane and dinosterane provide powerful tools for differentiating age of source and organic facies for oils from the black sea and sub andean basins of peru and ecuador respectively oils derived from sources deposited in low tropical latitudes tend to have little diatom input and as a consequence do not display high values regardless of age

application of new diterpane biomarkers to source biodegradation and mixing effects on central llanos basin oils colombia llanos basin oils have been commonly attributed to the prolific upper cretaceous age source formations outcropping in the eastern cordillera recently however variable oleanane indices have been reported for llanos oils suggesting a contribution from tertiary source sequences largely on the basis of the presence of high relative abundance of oleanane a tertiary age diagnostic biomarker a homologous series of 25 norhopanes in many central llanos basin oils indicates that heavy biodegradation is particularly common to the initial oil charging reservoirs from upper cretaceous marine sources one homologue 20s 25 norhomohopane c30 25 norhopane co elutes with oleanane and may thus contribute to the peak attributed to oleanane also confusing the source identification is the possibility that some cretaceous facies also contain oleanane three plant diterpanes which may be isomers of fichtelite and are common to the central llanos oils have been observed in an eocene source rock sample but not in cretaceous rocks or oils providing additional strong evidence for a tertiary contribution to some llanos basin oils in mixed oils with low to moderate oleanane contents and with a strong co eluting c30 25 norhopane the fichtelite isomers are the more reliable indicators of tertiary source input

seismic stratigraphy and geological evolution of the cenozoic cool water eucla platform great australian bight seismic images of the continental margin in the western great australian bight reveal the internal anatomy of a long lived cenozoic cool water carbonate shelf the cenozoic succession is divisible into seven seismic sequences that reflect four depositional phases phase a paleocene middle eocene is a progradational siliciclastic wedge deposited in a depositional sag and represents lowstand and transgressive sedimentation phase b middle eocene earliest middle miocene contains cool water ramp carbonates with biogenic mounds eocene oligocene overlain by a warm water flat topped platform rimmed by the early middle miocene little barrier reef coeval deep water carbonate deposition formed a multi lobed sediment apron this phase was terminated by gentle uplift and tilting throughout southern australia in the late middle miocene phase c late miocene early pliocene represents cool water lowstand wedge and ramp deposition and contains numerous biogenic mounds in the youngest sequence this phase is terminated by an unconformity attributed to marine erosion phase d pliocene quaternary is a thick succession of cool water carbonates with spectacular clinoform ramp geometry that forms most of the modern outer shelf and contains large deep water biogenic mounds this platform the first large cool water carbonate shelf imaged by high quality seismic reflection data demonstrates the interaction between regional tectonic and local and global paleo oceanographic processes as australia drifted northward during the cenozoic the great australian bight moved from high to middle latitudes and the regional oceanographic regime remained cool water largely because of coeval development of the antarctic circumpolar current in the evolving southern ocean short episodes of warm water deposition probably reflect incursions of a proto leeuwin current from the indian ocean whereas growth of the miocene coral algal little barrier reef resulted from a short term global increase in sea surface temperatures this platform dominated by stacked carbonate ramps is most similar to the west florida shelf but contains many more biogenic mounds the western great australian bight carbonate platform is an excellent modern analog for the mesoscale structure of cool water platforms in the older geologic record

stable isotope chemostratigraphy as a tool to correlate complex mississippian marine carbonate facies of the anadarko shelf oklahoma and kansas the mississippian section of the united states mid continent anadarko basin oklahoma and kansas has been a prolific hydrocarbon reservoir since the 1920s yet large scale regional correlations between individual stratigraphic units remain difficult because of the complex and heterogeneous nature of the carbonate facies new sedimentologic and carbon isotopic data from a nearly continuous mississippian core pan american 1 albert severin from the anadarko basin oklahoma garfield county united states provides insight into the potential of carbon isotope chemostratigraphy as a correlation tool in complex stratigraphic successions for which biostratigraphic data are not available the carbon isotopic composition of whole rock samples was analyzed to determine if stratigraphic trends reflect global changes in the carbon cycle a large positive shift 5 6 in the lower tournaisian kinderhookian consistent values averaging 2 3 in the upper tournaisian through middle visean upper kinderhookian meramecian and a negative shift 2 3 in the uppermost visean lower chesterian correspond to trends in the carbon isotopic compositions values from other regional data sets including the global type section at arrow canyon nevada further analysis of the data reveals that isotopic compositions are not facies dependent suggesting that marine chemistry and depositional changes in the anadarko basin reflect global environmental changes during the mississippian these inferences demonstrate the potential of the pan american 1 albert severin core to be a mississippian type section for the anadarko basin and that stable isotope chemostratigraphy can be used as a correlation tool to better understand the subsurface in complex successions such as the mississippian limestone of the united states mid continent

cenozoic biogenic mounds and buried miocene barrier reef on a predominantly cool water carbonate continental margin eucla basin western great australian bight the southern continental margin of australia is the largest area of cool water carbonate shelf deposition on the globe interpretation of 5495 km of airgun seismic reflection data in the western part of the great australian bight indicates that the 700 m thick cenozoic section of the offshore eucla basin was deposited largely as a prograding cool water middle to high latitude carbonate ramp characterized by widespread development of broad low relief biogenic bryozoan sponge shelf and upper slope mounds the succession also contains a spectacular and extensive 475 km long buried middle miocene barrier reef the miocene little barrier reef parallel to the modern shelf edge this rimmed carbonate platform margin represents an episode of warm water sedimentation during a global climatic optimum probably coupled with strong eastward flow of a proto leeuwin current the late miocene eustatic sea level fall produced an areally restricted debris apron sequence at the foot of the reef escarpment the carbonate platform is capped by a neogene cool water carbonate ramp succession typified by aggradational to sigmoidal sequences punctuated by periods of cold water sea floor erosion interpretation of this succession in the light of global and local tectonic and oceanographic events illustrates the dominant influence of water temperature on carbonate platform and reef growth throughout the cenozoic

oil and its biomarkers associated with the palaeoproterozoic oklo natural fission reactors gabon abstract the 2 1 ga fa formation sandstone of the franceville basin in gabon that hosts the oklo natural fission reactors has been discovered to contain abundant palaeoproterozoic oil bearing fluid inclusions inside which syngenetic biomarkers are preserved the oil occurs within h2o and co2 dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and co2 with liquid hydrocarbons prior to trapping textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca 115 to 200 c soon after deposition of the sequence at ca 2 1 ga and secondly during a hydrothermal event at 200 to 285 c linked to the operation of the reactors ca 1 98 ga a series of sequential outside rinse blanks of framework grains analysed by gas chromatography mass spectrometry gc ms prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co occurring solid bitumens which have a different geochemical distribution compared to the fluid inclusion oil the crush leach extract containing oil from fluid inclusions is markedly richer than the final outside rinse blank displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen the fluid inclusion oil has a high maturity is non biodegraded and lacks clear effects of radiolysis it is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor heated fluids rather than by the criticality event itself biomarkers include hopanes 2 methylhopanes terpanes and steranes indicative of bacteria including cyanobacteria and of eukaryotes the relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution trace amounts of c30n propylcholestanes indicate a contribution from marine algae the oil was likely derived from conformably overlying marine black shales that were deposited during the great oxidation event during which the earth s atmosphere became oxygenated the biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2 7 and 1 4 ga it is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period despite apparent fluctuations of atmospheric o2

supply of allochthonous sediment and its effects on development of carbonate mud mounds mississippian lake valley formation sacramento mountains south central new mexico u s a abstract carbonate mud mounds in the mississippian lake valley formation new mexico can be classified according to their external morphology internal growth phases and paleogeographic location along the lake valley ramp most of the lake valley mounds grew during a complete third order accommodation cycle alamogordo nunn tierra blanca interval 3 6 myr duration which directly influenced mound biota facies types and the physical interactions between the mounds and sediment gravity flows that transported skeletal sediment around and over the mounds during the regressive phase of the cycle lenticular mounds are found in updip parts of the preserved lake valley ramp which largely consists of outer ramp facies most lenticular mounds grew exclusively during deposition of the transgressive alamogordo member when outer parts of the ramp were somewhat sediment starved coarse grained carbonate gravity flow deposits nunn and tierra blanca members typically onlap lenticular mounds and only locally interfinger with mound core facies these relationships indicate lenticular mounds were buried by carbonate gravity flow deposits which accumulated much faster than mound cores could aggrade the sediment gravity flows constructed shingled lobes sand sheets and oblique clinoforms and were erosive at times which also probably helped to shape lenticular mounds transitional mounds are typically located downdip from the lenticular mounds and had an initial lenticular growth phase which was followed by aggradational growth of mound cores aggradational mound cores were locally modified by submarine erosion or failure which created sharp stratal truncation surfaces that were later onlapped by gravity flow deposits after the aggradational phase and stratal onlap mounds with remnant seafloor relief served as substrates for continued growth during deposition of the tierra blanca member this final growth phase in transitional mounds records minor aggradation and significant typically asymmetric lateral accretion which suggests that 1 the mounds had grown to some accommodation limit possibly storm wave base and 2 the complex dispersal and accumulation patterns for the gravity flows also influenced how mound cores accreted laterally for example the upcurrent sides of some transitional mounds were buried preferentially by onlapping tierra blanca gravity flow deposits whereas their downcurrent sides locally accreted for significantly greater distances and with very different stratal geometries hemispherical mounds are located farthest downdip on the lake valley ramp they are the largest mounds and contain all three lenticular aggradational and accretionary growth phases later growth phases of hemispherical mounds have steeply dipping mound core to flank relationships with abrupt facies transitions because of their distal position along the lake valley ramp and greater distance from the progradational sand bodies the hemispherical mounds were the dominant sources for coarse grained carbonate sediment to adjacent non mound environments during nunn and tierra blanca time thus the distance of lateral accretion and overall morphology of hemispherical mounds were not affected greatly by allochthonous sediment from updip parts of the ramp

late archean molecular fossils from the transvaal supergroup record the antiquity of microbial diversity and aerobiosis abstract cores recovered during the agouron griqualand drilling project contain over 2500 m of well preserved late archean transvaal supergroup sediments dating from ca 2 67 to 2 46 ga bitumen extracts of these strata were obtained using clean drilling sampling and analysis protocols that avoided overprinting syngenetic molecular fossil signatures with contaminant hydrocarbons comparisons of biomarker contents in stratigraphically correlated intervals from diverse lithofacies in two boreholes separated by 24 km as well as across a 2 gyr unconformity provide compelling support for their syngenetic nature the suite of molecular fossils identified in the late archean bitumens includes hopanes attributable to bacteria potentially including cyanobacteria and methanotrophs and steranes of eukaryotic origin this molecular fossil record supports an origin in the archean eon of the three domains of cellular life as well as of oxygenic photosynthesis and the anabolic use of o2

structural evolution of a foreland fold and thrust belt the umbria marche apennines italy abstract outcrop scale structures that record a progressive sequence of deformation can be used to clarify the kinematic evolution of an entire fold and thrust belt the temporal progression of the compressional deformation as reconstructed by means of a mesoscopic structural analysis provides a key for unraveling the kinematic history of the umbria marche apennines central italy overprinting relationships at outcrop scale allow for the recognition of three distinct structural stages which are in sequence a layer parallel shortening b folding c thrusting individual stages are explained in the framework of a progressive deformation model where shortening of the sedimentary cover was continuous and occurred during a single contractional episode as a consequence of tip line folding processes a comparison of this history with those reported from other fold and thrust belts shows that layer parallel shortening folding and thrusting are sequentially dominant processes in areas which have experienced compressional deformation

evidence of archean life stromatolites and microfossils abstract fossil evidence of the existence of life during the archean eon of earth history 2500 ma is summarized data are outlined for 48 archean deposits reported to contain biogenic stromatolites and for 14 such units that contain a total of 40 morphotypes of described microfossils among the oldest of these putatively microfossiliferous units is a brecciated chert of the 3465 ma apex basalt of western australia the paleoenvironment carbonaceous composition mode of preservation and morphology of the apex microbe like filaments backed by new evidence of their cellular structure provided by two and three dimensional raman imagery support their biogenic interpretation such data together with the presence of stromatolites microfossils and carbon isotopic evidence of biological activity in similarly aged deposits indicate that the antiquity of life on earth extends to at least 3500 ma

fracture cleavage in the duluth complex northeastern minnesota an unusual cleavage is developed in troctolitic rocks of the late precambrian duluth complex in northeastern minnesota it is localized in a zone 250 m wide and it is parallel to the axes of gentle folds in rhythmic banding the rocks show no textural signs of ductile deformation and the cleavage is defined by fractures that are fairly straight and evenly spaced but on a fine scale somewhat irregular multiple anastomosing and locally crosscutting textures indicate a purely extensional mode of development cleavage refraction across interfaces of bands of different composition is common serpentine coats the fracture surfaces and suggests that fluids were present to produce this alteration during formation of cleavage the geological relationships and serpentinization suggest conditions of cleavage formation of 1 5 1 8 kb and 350 to 500 c the presence of extensional fractures indicates that cleavage formation involved crack propagation at high fluid pressures the absence of shear fractures or ductile deformation features and the nature of the cleavage refraction indicate that differential stresses were low localization of the cleavage seems to be due to the presence of a structure probably a dike below the layered intrusive rocks this could have served to localize stresses and reactive fluids at high pressure during a late stage of continental rifting

tectonic control on sea floor relief and the localization of lower mississippian waulsortian mounds new mexico lower mississippian carbonate mud rich bioherms generally referred to as waulsortian mounds are commonly associated with low paleolatitude carbonate ramp settings and have recently been recognized as important hydrocarbon reservoirs the factors controlling localization of waulsortian mounds have heretofore been poorly understood stratal relations exposed in the alamogordo member of the lake valley formation in the sacramento mountains of new mexico illustrate the effects of tectonism on carbonate sedimentation prior to during and after mound growth they indicate that mound initiation and localization were strongly controlled by tectonically generated intraramp sea floor topography these observations bear strongly on understanding the controls on localization and growth of mud mounds in general stratal geometries observed in the underlying andrecito member indicate that this topography was modified by erosional and depositional processes prior to mound initiation mounds formed on the surfaces and margins of the intraramp topography as the result of aggradational in situ accumulation of biogenic sediment differences in growth geometry of stratal units within individual mounds and differences between mounds are correlated with position of the mound on the ramp and the deformation occurring immediately prior to mound growth it is probable that local tectonism continued during mound growth and that local differences in the amount of relative uplift resulted in different amounts of space for growth of individual mounds and thus determined differences in mound size and geometry

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

insights into shallow level processes of mountain building from the northern apennines italy recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations in physical conditions as they progressively deform and lithify the early stages of mountain building similar on land evidence is comparatively sparse as a result of difficulties of exposure and of early features having been obscured here we describe aspects of three structures from the northern apennines of italy that corroborate these notions of complex shallow level orogenic processes the upper cretaceous castiglioncello melange formed during the subduction phase of apennine orogenesis the major cervarola thrust and the smaller intra formational balduini thrust both of tortonian age represent the ensuing collisional phase all three structures formed at burial depths less than 2 or 3 km and are syn diagenetic although differing in detail all three show features such as clay fabrics and mineralized veins that record complex histories of fluid flow overpressure and deformation the clays show varying types of scaliness some of which had to involve pore dilation and collapse operating in cycles the veins typically evolved from networks of bedding parallel and bedding perpendicular veinlets through dilation breccias and mosaic textures to shear and extensional fibre growth during hydrofracture such observations imply fluctuating physical conditions and dynamic tectonic processes operating on shallow level sediments in a way that is highly heterogeneous with depth areally and through time

negative polarity seismic reflections along faults of the oregon accretionary prism indicators of overpressuring either thrusting of higher over lower impedance sediment or reduction of impedance locally in the fault zone can produce negative polarity reflections along the protothrusts and the frontal thrust of the oregon accretionary prism the vertical displacement of protothrusts averages only 27 m is uncorrelated to the occurrence of polarity reversals and is insufficient to generate the impedance contrasts associated with the negative polarity reflections the negative polarity reflections from the protothrusts are probably due to localized impedance reduction in the fault zone waveform modeling of these negative polarity reflections indicates they develop due to a single interface with negative impedance contrast this seismic signal could be produced by a fault zone with a sharp upper boundary with low impedance fault material and a transition down section to sediment of higher impedance an impedance inversion due to the 1 km throw on the frontal thrust could create the observed negative polarity reflections interval velocities do not decrease across the frontal thrust but core scale velocity measurements indicate a velocity decrease in the frontal thrust therefore the negative polarity reflections are apparently produced by a localized decrease in impedance waveform modeling of frontal thrust reflections suggests the low impedance interval is about 20 m thick with discrete upper and lower boundaries moderately dipping faults are an efficient path for fluid expulsion across the turbidites with low equivalent vertical permeability fluid migration from the over pressured section up the faults can locally reduce the velocity sufficiently to cause the negative polarity reflections

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

transected folds and transpression how are they associated abstract folds transected by cleavage are a distinctive structural feature in the caledonian appalachian orogenic belt with the advent of a tectonic model of transpression for this belt there has been an assumption that transected folds are a symptom of transpressive deformation in particular that clockwise transection signifies sinistral transpression we examine this tenet and conclude that there is no unique explanation for folds transected by a synchronous cleavage explanations can be found for either clockwise or anticlockwise transection for sinistral or dextral transpression each requiring assumptions about the inclination of bedding to the transpression vector or requiring that cleavage does not track the finite xy plane a further explanation which may fit the regional data best requires a non constant transpressive deformation with initial pure shear dominated deformation giving way to increasing transcurrent movement in time none of these models can be assumed to be right without supporting data on the incremental and finite strain histories of such regions

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

influence of along strike pre orogenic sedimentary tapering on the internal architecture of experimental thrust wedges the occurrence of along strike thickness variations in pretectonic sedimentary packages is expected to influence the structural architecture of doubly verging thrust wedges to test this hypothesis we used laboratory sandbox experiments model results show that longitudinal tapering of pretectonic sediments causes a great complexity in the internal tectonic fabric of the wedge particularly in the pro wedge dominated by highly segmented curvilinear thrusts the along strike variation of the mode by which the same amount of bulk contraction is accommodated in different regions of the orogen produces the obliquity of the deformation fronts in both the pro wedge and the retro wedge comparison with the overall architecture of the lesser antilles and manila accretionary systems validates our experimental results

do principal surfaces of stress and strain always exist variations of stress and strain are commonly expressed by patterns of stress or strain trajectories three mutually orthogonal families of continuous lines parallel to maximum intermediate and minimum stress or strain axes it might be assumed that there are equivalent continuous principal surfaces of stress or strain for any state of continuously varying stress or strain we demonstrate that this will not generally be the case for three dimensionally varying states of stress or strain whether or not principal surfaces of stress or strain exist is governed by the abnormality of the vector field of principal trajectories we consider the z vector fields for examples of many types of three dimensional heterogeneous deformation and show that most of these do not lead to definable principal xy strain surfaces an alternative geometric test is presented termed the continuity loop for simply demonstrating the existence or not of principal surfaces using geometrical and orientational information it is important to the understanding of geological structures to know which kinds of heterogeneous deformation give rise to principal surfaces of stress or strain we conclude with examples of structures which might be indicative of the absence of continuous principal surfaces of stress segmented faults echelon veins and dykes a discussion of the implication for strain fabrics and foliations and a warning that foliation trace trajectories on maps or sections may not necessarily indicate the existence of real foliation surfaces in three dimensions

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

the antiquity of oxygenic photosynthesis evidence from stromatolites in sulphate deficient archaean lakes the tumbiana formation about 2700 million years old was largely deposited in ephemeral saline lakes as judged by the unusual evaporite paragenesis of carbonate and halite with no sulfate stromatolites of diverse morphology occur in the lacustrine sediments some with palimpsest fabrics after erect filaments these stromatolites were probably accreted by phototropic microbes that from their habitat in shallow isolated basins with negligible sulfate concentrations almost certainly metabolized by ozygenic photosynthesis

fluid flow pathways in actively deforming sediments the role of pore fluid pressures and volume change abstract the manner in which sediments respond to burial and shear depends largely on the ease with which fluids can escape rapid increase of overburden pressure or shear prevents efficient dewatering and can lead to the generation of excess pore fluid pressures a principal control on the development of overpressure and hence mechanical strength is the sediments permeability observations from modern plate margin accretionary prisms indicate that many faults are associated with both overpressuring and efficient fluid flow that operates in pulses indicative of permeability that is constantly changing we present the results of laboratory experiments exploring how the permeability of sediment retrieved from the barbados accretionary prism and laboratory analogues varies with progressive strain volume changes are linked to permeability fluctuations and the drainage capacity of the material we investigate the importance of the previous stress history imparted on the sediment and quantify the extent to which different deformation structures can acts as conduits for fluid flow under conditions of fluctuating effective stress material that has been initially consolidated and then subjected to reduction of effective stress either by raised fluid pressure or reduction of load inducing overconsolidation dilates during shear zone formation and displays an increase in permeability during strain shear fabrics can localise flow further where the principal mode of deformation is intensely localised and brittle and effective stress approaches zero conversely normally consolidated sediments deform by bulk volume loss without generating discrete fabrics and hence do not enhance flow under reduced effective stress conditions permeability can increase during strain by up to one order of magnitude when deformation results in sudden porosity collapse and pore water expulsion microstructural analysis and permeability testing before during and after shear have helped constrain the principal factors that control the evolution of flow conduits in fine grained low permeability sediments

waulsortian type bioherm development and response to sea level fluctuations upper visean of bechar basin western algeria abstract an exceptionally well developed and well preserved upper visean succession rich in sponge bryozoan crinoid mounds in the foreland carboniferous bechar basin northwestern algerian sahara is described and discussed the succession is composed of recurrent facies mosaics forming individual superposed members an ideal member is made up of two distinct fades assemblages the lower assemblage forms the actual mounds and is composed of sponge bafflestone wackestone at the base overlain by massive sponge fenestellid bafflestone wackestone and capped by massive crinoid wackestone with bedded flanks of lithoclastic wackestone in contrast to massiveness of the mounds the upper facies assemblage is composed of well bedded crinoid packstone grainstone and ooid grainstone with local rugose coral and algal foraminiferal banks on the basis of benthic assemblages and nature of substrate seven bathymetric zones are defined and indicate that each member of the bioherm rich formations is a shallowing upward parasequence controlled by an asymmetrical transgressive regressive cycle the local curve of relative sea level for the late visean shows 13 cycles interpreted as fourth order eustatic sea level changes each cycle averaging half a million years in duration third order cycles could not be identified the platform model proposed is a distally steepened ramp 15 20 km wide that developed in a rapidly subsiding foreland basin lateral distribution of facies on the ramp depended strongly on local tectonic setting whereas vertical development was controlled by sea level fluctuation the deep water mud rich sponge bryozoan crinoid mounds developed in 100 150 m water depths during phases of sea level highstand whereas deposition of shallow water facies occurred on top of the mounds in less than 70 80 m water depths during regressive phases the bechar mounds share similarities with the classical lower carboniferous waulsortian mounds but they differ in two aspects the abundance of large sponges which is unique and their vertical zonation wherein basal facies consist of sponge dominated assemblages the proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbon reservoirs in the bechar basin because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migration and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

early earth microbes and the rise of oxygen reconstructions of atmospheric chemistry and microbial life early in the earth s history have been contentious observations increasingly point to the evolution of complex and variable environments earlier in time

tectonic dewatering and mechanics of protothrust zones example from the cascadia accretionary margin recent analyses of multichannel seismic data for the cascadia margin off central oregon show a systematic landward increase in velocity increase in sediment dewatering within a protothrust zone about 10 km wide seaward of the toe of the accretionary wedge cochrane et al 1994 here we analyze the mechanical conditions for the transmission of compression from the rear of the accretionary wedge to the area seaward of the frontal thrust to create the protothrust zone and to cause sediment dewatering the analysis assumes that sediments are elastoplastic coulomb material i e they behave elastically up to a yield limit at which failure occurs we show that the necessary conditions for the transmission of compression from the accretionary wedge to the seaward area are 1 the presence of incipient decollements seaward of the frontal thrust and 2 the presence of high pore pressures within the incipient decollement we also show that the magnitude of pore pressure within the incipient decollement may control the width of the protothrust zone we suggest that the latter determines the location of the next frontal thrust and hence the ramp spacing between the imbricated thrusts within accretionary wedges thus pore pressures within incipient decollements may significantly influence the evolution of the internal structures of accretionary wedges

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

mississippian carbonate ramp to basin transitions in south central new mexico sequence stratigraphic response to progressively steepening outer ramp profiles abstract the sequence stratigraphic architecture of mississippian strata from the sacramento and san andres mountains south central new mexico illustrates the combined effects of relative sea level change and progressively increasing depositional gradients on sedimentary processes and resultant stratal geometries along the outer parts of a carbonate ramp to basin transition mississippian strata can be subdivided into three progradational sequences sequences 1 3 and a fourth basin restricted onlapping sequence sequence 4 sequences 1 3 contain wackestone to mudstone dominated transgressive systems tracts that are overlain by packstone to grainstone dominated progradational systems tracts progradational systems tracts created distinct ramp to basin transitions outer ramp margin that are characterized by basinward stratal thinning and a concomitant change to muddy lithofacies sequence 4 consists of onlapping fining upward stratal packages of peloidal skeletal grainstone that grade upward to laminated spiculitic lime mudstone the outer ramp margin facies of sequences 1 through 3 gradually steepened the outer ramp profile until the cumulative effects of the steepening caused the depositional gradient to surpass an equilibrium grade i e the gradient at which deposition and erosion bypass are about equal once the depositional gradient exceeded the equilibrium grade for the caliber of sediment supplied to the outer ramp margin widespread submarine erosion and sediment bypass on the outer ramp was initiated resulting in deposition of basin restricted strata of sequence 4 similar relationships and inferred processes have been recognized in progradational siliciclastic shelf deposits and are collectively described by previous workers as slope readjustment the mississippian ramp to basin transition in south central new mexico illustrates how sediment deposition dispersal and erosion responded to evolving depositional gradients and in turn controlled resultant large scale stratal geometries

documentation and significance of progressive microfabric changes in middle ordovician trench mudstones middle ordovician mudstones of the greenwich slice of the hamburg klippe an inferred early paleozoic subduction complex in the central appalachians comprise three mesoscopically distinct fabric zones that reflect progressive development of scaly foliation from undeformed zone mudstones through deposits of the transition zone into stratally disrupted deformed zone mudstones scanning electron microscopic analysis of these deposits demonstrates an over all decrease in porosity from undeformed mudstone into the deformed zone undeformed zone mudstones illustrate a porous clay fabric that becomes more closely packed in the transition zone transition zone mudstones also show 1 discrete discontinuous shear zones characterized by compact domains of oriented platy grains and 2 fracture surfaces along which there has been no grain reorientation the bulk of the mudstone however has apparently been compacted by distributed fabric collapse rather than shear related compaction mudstones of the deformed zone display a complex array of continuous well defined in some cases striated or grooved scaly folia bound on one side by very compact domains of strongly oriented platy grains that pass outward into a more open or porous clay fabric deformation of greenwich slice mudstones can be explained by strain hardening of accreted trench fill deposits at the toe of the early paleozoic accretionary prism initial deformation was probably concentrated in weak water rich sediment horizons sandwiched between more consolidated strata strain hardening of the early deformed sediment in response to shear related fabric collapse and dewatering as evidenced by the scaly fabric of deformed zone mudstone probably resulted in lateral propagation of deformation and progressive scaly fabric development the extent to which stratal disruption propagated may have been controlled by the slope of the water content gradient of the deforming sediment that is the deformation front migrated outward from the initially deformed water rich sediment to that position where the strength of the sediment as determined by its relatively low water content precluded further sediment yield the microscopic shear zones and fractures within transition zone mudstones suggest that these deposits were dry and therefore strong enough to arrest further stratal disruption thus the final width of a deformation zone characterized by stratal disruption and scaly clays may be inversely proportional to the slope of the water content gradient that existed between deforming water rich underconsolidated sediment and stronger more consolidated deposits

discontinuous fault zones abstract many tectonic faults and tension fractures are at least initially composed of separate segments this note deals with a little explored reason for this phenomenon which in faulting has obvious implications both for the migration of hydrocarbons and for the sealing capacity of faults theoretical arguments based on coulombmohr s theory of shear failure and on a theorem for the integrability of vector fields lead to the expectation that in general non uniform and truly three dimensional stress fields will impede the formation of smooth coherent fault surfaces this is in contrast to the stress fields that are associated with plane deformation examples are given and special attention is drawn to the role of tectonic stress fields with horizontal principal stresses that change with depth in magnitude and direction

controls on the evolution of carbonate mud mounds in the lower cretaceous cupido formation northeastern mexico abstract the lower cretaceous cupido formation in northeastern mexico represents part of an extensive post rift carbonate platform system that stretched across much of the gulf of mexico passive margin numerous sponge microbial mud mounds are found within the lower part of the cupido formation four mound types were identified at bustamante canyon and other localities in the state of nuevo leon mexico type 1 siliceous sponge microbial mounds contain boundstone core facies are the oldest mounds and are associated with adjacent deep ramp to basinal lime mudstone facies type 2 sponge microbial mounds also have boundstone core facies are associated with outer ramp wackestone packstone facies and overlie strata that contain type 1 mounds type 3 sponge microbial coral mounds contain only 40 50 boundstone core facies and overlie or are locally interstratified with type 2 mound bearing strata although type 3 mounds are associated with outer ramp packstone facies type 4 calcisponge microbial coral biodetrital dominantly skeletal packstone mounds are the youngest mounds and are associated with outer ramp packstone and ramp crest rudist packstone facies both the length and height i e synoptic relief of individual types 1 2 and 3 mounds range from a few meters to tens of meters only type 4 mounds reach several hundred meters in length and are up to 90 m thick although synoptic relief on type 4 mounds was minor systematic changes in the biota morphologies dimensions abundance stratigraphic distribution and facies associations of the lower cupido mounds were largely controlled by the initial ramp like depositional profile of the lower cupido platform and the long term second order accommodation trends that affected ramp evolution during early cretaceous time the ramp like profile was not conducive to transport of large volumes of allochthonous sediment from updip source areas to mound locations on the outer ramp this allowed slowly accumulating types 1 2 and 3 mounds to build synoptic relief because adjacent outer ramp facies accumulated at even slower rates in contrast type 4 mound cores contain abundant coarse grained skeletal sediment and also developed the least synoptic relief of all lower cupido mounds because of higher accumulation rates associated with adjacent grain rich ramp crest lithofacies we suggest that these systematic changes in mound attributes largely reflect long term second order accommodation loss and regional progradation of the lower cupido ramp during early cretaceous time accordingly the upward transition from type 1 to types 2 and 3 mounds and ultimately to type 4 buildups can be explained by the progressive shallowing of outer ramp environments as more updip facies tracts prograded eastward from late hauterivian to early aptian time there are few stratigraphic successions in the rock record that provide better evidence for the links between large scale stratigraphic relationships and mound evolution along low gradient carbonate ramps

segmented curved faults the example of the balduini thrust zone northern apennines italy abstract meso and micro structural studies of the well exposed balduini thrust northern apennines italy indicate that the structure formed during a single folding event contemporaneous with diagenesis and is a zone comprising curved en echelon fault segments the geometry of each segment is arcuate with pure compression at one end and right lateral displacement along the other the thrust developed during the tortonian within a single mud rich formation the upper eocene upper oligocene scaglia cinerea but rheological variations within the unit led to differences in deformation style zones of scaly fabric are discontinuous and calcite veins vary in abundance the mesoscopic morphology of the veins and the distribution of calcium carbonate along the formation indicate variations in the distribution of fluids at the time of deformation which affected both diagenesis and the structural response of the material systematic variations of mechanical properties within the thickness of the scaglia cinerea formation account for the curvature of the propagating thrust together with the heterogeneity of the stress field the confinement of the arcing thrust to this single weak unit lead to repeated initiation of new fractures and hence segmentation of the propagating thrust geometric analysis of the calcite veins and their microscopic characters suggests that hydrofracturing was involved with the scaglia cinerea formation experiencing high fluid pressure followed by rapid water expulsion the balduini thrust is therefore an example of a fluid driven refracted compound shear zone the analysis presented here provides insights into the three dimensional arrangement of fault zones and fluid migration patterns during regional faulting