differential compaction faults and their implications for fluid expulsion in the northern bozhong subbasin bohai bay basin china abstract a series of differential compaction faults was identified in the northern bozhong subbasin bohai bay basin china by using 3d seismic well log and lithology data the differential compaction faults display two obvious characteristics first they develop along a lithologic boundary which separates thick sandstone from underlying mudstones facies and thickness change such as an abrupt lithologic change between sandstone and mudstone a lateral thickness change of overlying sandstone are the basic controlling factors of these differential compaction faults second they have a definite strike direction according to the fault strike the differential compaction faults can be divided into three groups group a have an approximate orthogonal strike with the tectonic faults group b display a similar strike with the slope and group c show radial strikes on the circular basement uplift tectonic faults influence the planform patterns of differential compaction faults in their near vicinity and induce these faults displaying an approximate orthogonal intersection with the late period through going faults group a basin slope promote the formation of differential compaction faults by downslope gravity sliding and cause faults parallel to the strike of slope group b circular basement topography generates isotropic stress and leads the differential compaction faults above the basement dominated by radial strikes group c production and geochemistry data is an evidence of differential compaction faults developed in a late period when the strata reach a certain burial depth that the source rocks enter the oil generating window the presence of thermogenic fluid within shallow reservoirs represents as evidence of hydrocarbon drained and migrated through the differential compaction faults the differential compaction faults could serve as significant conduits for fluid expulsion and hydrocarbon accumulation in the bohai bay basin china

discussion on development of polygonal fault systems a test of hypotheses d m d james writes the mechanics of formation of polygonal faults are widely acknowledged as not yet fully understood and it is welcome that goulty swarbrick 2005 attempt a discriminatory test between rival hypotheses it is however important that such a test bears some relation to physical reality the test is based on the assumption that the ratio of horizontal to vertical effective stress as deduced from leak off tests in two wells in the north sea at depths of about 2 km yields the co efficient of earth pressure at rest therein and hence the residual friction governing the post nucleation development of polygonal faults apparently globally and at any depth it is not clear if the authors consider that the polygonal fault tiers at the well sites are currently active neither is it clear if they assume that the coefficient of residual friction along the fracture is a reasonable approximation to the coefficient of initial friction which governs the dip of the propagating fracture tip and the fault surface as a whole relative to the orientation of the vertical effective stress it is hardly surprising that the ratio of horizontal to vertical effective stress k is very high in the wells analysed the leak off tests are from highly overpressured horizons ratios of pore fluid pressure to total vertical stress approximately 0 80 and 0 84 and the effect of pore fluid pressure coupling hillis 2003 will inevitably decrease the value of the horizontal effective stress to lesser extent than it does the vertical effective stress it is certainly not yet demonstrated that polygonal faults require overpressures of this magnitude for their formation by any mechanism leak off tests tend to overestimate the horizontal effective stress if no correction is made for the excess mud pressure necessary to overcome the tensile strength of

the development of polygonal fault systems by syneresis of colloidal sediments abstract polygonal fault systems occur in numerous sedimentary basins worldwide are generally located on passive margins in onlap fill units and tend to comprise the finest grained sediments in this geological setting these fault systems have been most thoroughly described in the central north sea basin and the detailed structure shows a significant correlation with lithological variations both vertically and laterally extension measured in stacked decoupled tiers of polygonal faults correlates positively with both clay fraction and smectite content lateral facies variations are also observed and indicate that time equivalent sequences upslope from the smectite rich polygonally faulted sediments are coarse grained clay poor and undeformed this leads us to believe that the structure and geometry of the fault system are controlled by the colloidal nature of the sediments and that the volumetric contraction measured on seismic sections can be accounted for by syneresis of colloidal smectitic gels during early compaction syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid this process occurs due to the domination of interparticle attractive forces in marine clays dependent on environment and is governed by the change of gel permeability and viscosity with progressive compaction the process of syneresis can account for a number of structural features observed within the fault systems such as tiers of faults the location of maximum fault throw and growth components at upper fault tips as such this paper represents the first attempt to correlate microscale properties of clay rich sediments to their macroscale seismic character

polygonal faults and their implications for hydrocarbon reservoirs in the southern qiongdongnan basin south china sea the polygonal faults were identified in qiongdongnan basin south china sea by using the technique of time coherent slice and horizon flattening of high resolution 3d seismic data these polygonal faults occur in three tiers of the upper meishan formation and the huangliu formation the faults have lengths of 150 1500 m spacings of 50 3000 m throws of 10 40 m and dips of 50 90 degrees tectonic evolution in the qiongdongnan basin can be divided into a rifted stage and a post rifted stage tectonic faults are widely distributed in the rifted sequences but are not well developed in the post rifted stage few faults in the post rifted sequences might suggest the absence of a migration pathway for hydrocarbon or other fluids however the existence of polygonal faults in the post rifted sequences can serve as the pathway and promote the hydrocarbon migration and accumulation in the qiongdongnan basin during the post rifted stage c 2010 elsevier ltd all rights reserved

perturbation of polygonal fault propagation by buried pockmarks and the implications for the development of polygonal fault systems abstract evidence is presented that constrains the earliest part of the growth history of a polygonal fault tier using high resolution 3d seismic data from the west african margin we document the interaction between a shallow buried wedge shaped polygonal fault tier and pockmarks located at the base of the fault tier strongly curved patterns of perturbed polygonal faults are mapped in the shallowest portions of a tier overlying a series of pockmark craters these perturbed patterns differ markedly from the background polygonal pattern developed away from areas of pockmark influence perturbation is attributed to local stress effects relating to residual topography of the underlying pockmarks by analysing the thickness variations of mapped layers within the polygonally faulted tier we demonstrate that any residual topography was most likely healed after approximately one third of the total tier thickness was deposited hence the nucleation of the polygonal faults into their perturbed propagation trajectories must have been established within the first 100 150 m of burial from these observations of early nucleation in the basal third of a tier we propose a new model of polygonal fault growth in which individual faults nucleate near the base of a tier at an early stage of burial and then propagate preferentially upwards with basal tips restricted by an underlying mechanically competent unit the base of the tier continued growth then shifts the locus of displacement such that final displacement distributions are more axio symmetrically disposed with respect to the base and top of the tier

potential salinity driven free convection in a shale rich sedimentary basin example from the gulf of mexico basin in south texas we investigate the plausibility of salinity driven free thermohaline convection in sedimentary rocks of the south texas part of the gulf of mexico basin using salinity data rayleigh number calculations and numerical models previous studies speculated that free convection could account for high fluxes evidently required for diagenesis in the basin but low permeability shales are calculated to be a barrier to free convection in the study area salinity inversions occur either above or within the transition zone from hydropressures to overpressures the positioning of brines over less saline fluids provides a significant buoyancy force rayleigh number calculations and numerical simulations suggest that homogeneous shaly systems are unstable near the high end of the expected ranges of shale permeability 10 15 10 16 m2 numerical simulations show that the influx of brine into the permeable layers and permeability heterogeneity in the shales are both conducive to free convection simulations indicate that salinity driven free convection can occur at lower permeabilities 10 16 10 18 m2 that may approximate the permeabilities of shales in the zone of extreme overpressures over geologic time simple rayleigh numbers are inadequate to predict the occurrence of free convection in heterogeneous systems salinity driven free convection at depths in some large sedimentary basins such as the gulf of mexico may be more common than expected

development of polygonal fault systems a test of hypotheses polygonal networks of normal faults in layer bound sequences of fine grained mudstones have formed without regional tectonic extension the two leading hypotheses concerning the generic mechanism responsible for their development are 1 horizontal stresses are reduced by syneresis 2 coefficients of residual friction are very low to discriminate between these hypotheses the ratios between the minimum horizontal and the vertical effective stresses have been estimated in four north sea wells penetrating oligocene and miocene sequences that contain polygonal fault networks the effective stress ratios are c 0 8 consistent with very low coefficients of friction but not with syneresis

identification of a large upper cretaceous polygonal fault network in the hammerfest basin implications on the reactivation of regional faulting and gas leakage dynamics sw barents sea abstract we use a high resolution 3d seismic dataset covering 970 km2 over the snohvit and albatross gas fields of the hammerfest basin in the barents sea in order to investigate the basin fault architecture and identify potential fluid pathways of deeper thermogenic gas and fluids a polygonal fault pf interval not previously published is described in the three non connected blocks within campanian sequences which could imply a permeable complex network of pathways that facilitate the vertical migration of hydrocarbons along the fault planes the dominant orientation of the pfs is similar to the main regional tectonic fault trend ew suggesting a tectonic influence upon the development of polygonal faults although some individual pfs locally seem to develop orthogonally to regional 1st order tectonic faults a lithologic control on the occurrence of pf patterns is evident as none are observed within calcareous low clay sediments the pf networks present three main orientations dominant ene wsw secondary nne ssw and a minor ne sw paleocene early eocene faults peefs prevail above the pf networks with coinciding ene wsw fault strikes interpreted as preferential reactivation of the pf systems numerous amplitude anomalies linked to the escape of thermogenic fluids from the jurassic reservoir were identified we present a conceptual model where the 1st order reactivated faults and peefs acted as pathways for fluids from the deeper hydrocarbon reservoirs the peefs are partially sealing with a lateral migration component interplay between the pfs peefs and the 1st order reactivated faults focuses the fluids vertically along the polygonal fault traces creating a wider diffuse zone which form large gas cloud anomalies in the seismic data

authigenic carbonates in upper miocene sediments of the tertiary piedmont basin nw italy vestiges of an ancient gas hydrate stability zone a wide array of carbonate rich rocks has been recognized in the tertiary piedmont basin nw italy hosted in lower messinian slope deposits carbonate cements show negative 13 c values and positive 18 o values suggesting that carbonate precipitation was induced by microbial degradation of methane produced from gas hydrate destabilization two groups of rocks have been distinguished 1 lucina bearing mud breccias representing the seafloor product of an ancient seepage site and 2 lucina free concretions originating below the sediment water interface within this group two subtypes have been further distinguished stratiform concretions and cylindrical concretions stratiform concretions result from precipitation of dolomite in the pores of muddy sediments some of them display a brecciated structure others show a network of septarian like cracks that are empty filled with sediments or zoned carbonate cements their internal features are related to the formation of gas hydrates within the sediments and to their destabilization thus these rocks mark a portion of the sedimentary column located within a paleo gas hydrate stability zone cylindrical concretions represent ancient fluid conduits related to the upward migration of ch 4 rich fluids subsequent to gas hydrate destabilization the carbonate rich rocks of the tertiary piedmont basin stand as one of the first examples of methane derived rocks that record successive episodes of dissociation and re formation of gas hydrates and they provide precious elements to model the general evolution of a portion of the sedimentary column located within the hydrate stability zone

messinian carbonate rich beds of the tertiary piedmont basin nw italy microbially mediated products straddling the onset of the salinity crisis abstract the seven messinian microbial carbonate rich layers cropping out in the pollenzo section tertiary piedmont basin nw italy are interbedded with a precession related cyclic succession composed of euxinic shale marl couplets and straddle the onset of the messinian salinity crisis msc a sharp change of sedimentological compositional and geochemical characteristics was observed from the onset of the msc onward suggesting strong differences in the mechanisms responsible for carbonate precipitation pre msc beds are mainly composed of dolomite and are interpreted as the product of early diagenesis formed by bacterial sulphate reduction in the shallow subsurface along the sulphate methane interface dolomite precipitation was temporarily enhanced by an upward flux of methane rich fluids possibly sourced by gas hydrate destabilisation conversely from the onset of the msc onward carbonate beds are thinly laminated and show abundant filaments interpreted as beggiatoa like bacteria while calcite and aragonite are the dominant authigenic phases these beds are interpreted as microbialites resulting from the lithification of chemotrophic bacterial mats dominated by sulphide oxidizing bacteria the carbonate necessary for their preservation was provided by bacterial sulphate reduction these microbialites that appear as the deeper water counterpart of bottom grown selenite layers deposited in the marginal part of the basin formed on an anoxic sea bottom under a density stratified water column the high sulphate concentration in the pore waters related to the presence of concentrated brines and to the regeneration of sulphate by sulphide oxidizing bacteria is considered as the driving factor controlling the mineralogical change from dolomite to calcite and aragonite finally the sharp shift towards negative 18 o values of carbonates observed from the onset of the msc onward was probably related to fractionation processes operated by sulphate reducing bacterial communities in the pore waters this suggests that care must be taken when dealing with microbial carbonates in the interpretation of the oxygen isotope values in terms of fluctuating salinity condition of the water mass in conclusion this study suggests that the onset of the msc had a strong impact on microbial populations governing carbonate precipitation in a sector of the basin dominated by depositional conditions not suitable for gypsum precipitation

small scale faulting in the upper cretaceous of the groningen block the netherlands 3d seismic interpretation fault plane analysis and regional paleostress abstract over the last years field based studies have shown that fault surfaces can exhibit a considerable self affine topography it is reasonable to assume that similar undulations are also present in fault interpretations from 3d reflection seismic data however both the interpretation uncertainty and geophysical resolution limits hinder their analysis this study analyses a set of small scale non reactivated faults in the upper cretaceous chalk group upper ommelanden formation of the nw part of the groningen block the netherlands in a high quality pre stack depth migrated 3d seismic data set the studied faults are fully contained inside the chalk group in an area located between the major tectonic bounding faults of the nw groningen block over 200 faults with offsets in the order of 30 50 m were interpreted across an area of ca 150 km 2 showing a clear preferential orientation for strike dip and dip direction detailed interpretations and 3d fault plane analyses show undulations on the fault plane we show that these undulations are not an interpretation or gridding artefact and interpret these to indicate direction of fault slip these results were used to calculate a paleostress tensor using all faults to calculate a single stress tensor for the entire study area by numerical dynamic analysis based on the orientation position and a thickness analysis it is interpreted that these faults formed due to the tectonic reactivation of salt structures in the latest cretaceous the calculated paleostress state shows a general nw se extension with a vertical maximum principle stress and a stress ratio of about 0 3 indicating that the studied faults are not the result of dewatering this interpretation agrees both with a nearby salt tectonic reconstruction as well as field based paleostress results from the uk belgium and france a first look at other surveys from the dutch sector indicates that similar faults are present in other areas with different orientations we propose that a dedicated analysis of these faults across on and offshore europe would allow extending the stress map of the late cretaceous into areas where the chalk is not outcropping

paleomagnetism and crustal rotations along a shear zone las vegas range southern nevada in southern nevada paleozoic isopachs mesozoic structures and mountain ranges trend north south but as they approach the right lateral west northwest trending las vegas valley shear zone lvvsz they are bent clockwise up to 90 in oroflexes 30 km long in the las vegas range the declination of the characteristic magnetization of the cambrian bonanza king formation has likewise been rotated clockwise more than 50 for samples from eight sites 13 23 samples per site in the las vegas range and in its extension south of the lvvsz stepwise thermal demagnetization to 600 c removed recent and tertiary overprints revealing a nearly horizontal late paleozoic viscous partial thermoremanent magnetization vptrm which predates both the tertiary oroflexural bending and the sevier thrusting in the area the declination of the characteristic magnetization after correcting for the dip of bedding varies from 133 2 at a site 24 km north of the shear zone to 196 4 13 km north of it and 204 4 11 km south of the shear zone this parallels a similar change in the structural trend the paleomagnetic data indicate that the las vegas range is broken into rather small blocks 5 km which rotate in response to more continuous deformation at depth this deformation of the upper crust can be modelled using a thin non newtonian viscous plate model which when applied to the las vegas valley shear zone predicts rotations similar to those seen in the las vegas range

paleogeographic setting and depositional architecture of a sand dominated shelf depositional system miocene utsira formation north sea basin abstract the middle late miocene utsira sandstone of the north sea basin contains a fully preserved regional marine sand deposit that records a stable paleogeographic setting of shelf sand transport and accumulation within an epeiric shelf sea which persisted for ca 8 myr the sediment dispersal system was defined by 1 sediment input through a prograding strandplain platform coast to shelf bypass and regional basin centered transport and deposition within an elongate seaway 2 a high energy marine regime 3 very low rates of sediment supply and accumulation in the accommodation dominated seaway 4 high sediment reworking ratio and consequent abundance of autochthonous sediment 5 regional along strike sediment transport and 6 local to subregional landward sediment transport shelf systems were primarily nourished by sediment reworked from the shetland strandplain which prograded eastward into the central viking trough the viking strait lay between the constructional platform and the nondepositional horda platform southward longshore transport on the high energy shoreface and shallow shelf platform constructed the mounded south viking shelf shoal southward building sandy banner banks that formed along the recurved arc of the platform margin nourished the shoal system low angle sigmoid clinoforms with downstepping aggradational tops are distinctive architectures of the strike fed sand bodies the combination of strong marine currents and slow but continual sand supply from the shetland strandplain created regional sandy shelf depositional systems that individually covered 3500 to 6000 sq km of basin floor defining elements of the shelf shoal systems include their location within the basin axis mounded morphology stratal architecture and construction by amalgamated marine sand facies

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

using polygonal layer bound faults as tools to delimit clastic reservoirs in the levant basin offshore lebanon the levant basin offshore lebanon contains an array of layer bound normal faults in the oligocene miocene units the faults are believed to have nucleated in fine grained sedimentary rocks similar to polygonal fault systems worldwide and as a result they may be influenced by lithological heterogeneities in the host rock unit three dimensional seismic data and amplitude extraction from offshore lebanon were used to map deep water channels and fan lobes demonstrating that the distribution geometry and growth of the layer bound normal faults are affected by these sedimentary bodies which are impacting fault propagation three fault types are identified differentiated by their growth models and overall geometry type i faults in the deep basin indicate that a competent unit is present in the lower miocene and is affecting their growth type ii and iii faults in the latakia ridge and the margin respectively are smaller because of restriction by upper miocene and lower oligocene basin floor fans based on their seismic expression and on analogy with other basins containing similar faults it is very likely that the overlying and underlying units causing restriction consist of coarse grained clastic intervals therefore reservoirs offshore lebanon are probably lower miocene in the deep basin whereas in the latakia ridge and the levant margin reservoirs are in upper miocene and in lower oligocene units this study provides an additional evidence that layer bound or polygonal faults can have practical industrial application during exploration of hydrocarbons even in the absence of well data

small scale faulting as an indicator of deformation mechanism in the tertiary sediments of the northern danish central trough abstract the tertiary sediments in the northern part of the danish central trough are characterized by a large number of small faults 25 50 m offset models for the accommodation of the deformation of the sediments are presented and it is shown quantitatively that a bending model best explains the observed strain distribution from this we infer that the internal deformation of the tertiary sediments is due to differential subsidence along older fault trends and not caused by applied external stresses on the tertiary sediments the effect of rock rheology and excess fluid pressure is discussed in order to explain the size of the throw and the smaller number of reverse faults compared to normal faults although the rocks are brittle they may in regions of shortening suffer more grainscale creep deformation than in regions of extension the strain distribution is determined on seismic sections and the main limitations of the method is thus the vertical and horizontal resolution of the seismic surveys used

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

measurement of diagenetic compaction strain from quantitative analysis of fault plane dip we developed a new technique for quantifying the amount of porosity loss during diagenetic transformation of opal a to opal ct the technique is based on 3d seismic data from offshore norway where the widely developed biosiliceous sediments of the brygge formation are deformed by a polygonal fault system evidence is shown from two study areas that the dip of the polygonal fault planes reduces abruptly across the opal a to opal ct diagenetic boundary suggesting that the fault planes were passively rotated into shallower dips due to diagenesis the reduction of fault plane dip was used to quantify the vertical compaction strain due to diagenesis using an independent assessment of the magnitude of the porosity loss we are able to validate the method based on fault plane dips and also to evaluate whether the porosity loss results in an exclusively vertical strain additionally the impact of silica diagenesis on the shear strength of the sediments and fault growth is investigated we present evidence suggesting that deviations from classical mohr coulomb behaviour may be expected during the combined processes of diagenesis and porosity collapse and may indeed be sufficient to promote continued fault growth

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

jurassic septarian concretions from nw scotland record interdependent bacterial physical and chemical processes of marine mudrock diagenesis septarian concretions in the staffin shales formation kimmeridgian isle of skye allow controls on concretion rheology and septarian cracking to be investigated stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite intergranular volumes range from 77 to 88 and calcite 13c and 18o values in most concretion bodies range from 10 0 to 17 3 and 0 3 to 0 6 respectively consistent with rapid and pervasive cementation in marine pore fluids septarian rupture occurred during incipient cementation with a sediment volume reduction of up to 43 crack lining brown fibrous calcite records pore fluid re oxygenation during a depositional hiatus followed by increasing fe content and 13c related to bacterial methanogenesis brown colouration results from an included gel like polar organic fraction that probably represents bacterially degraded biomass a new hypothesis for concretion growth and septarian cracking argues that quasi rigid proto concretions formed via binding of flocculated clays by bacterial extracellular polysaccharide substances eps this provided rheological and chemical conditions for tensional failure subcritical crack growth volume contraction calcite nucleation and incorporation of degraded products into crack lining cements bacterial decay of eps and syneresis of host muds provided internal stresses to initiate rupture at shallow burial development of septarian shrinkage cracks in muds is envisaged to require pervasive in situ bacterial colonization and to depend on rates of carbonate precipitation versus eps degradation and syneresis subsequent modification of septarian concretions included envelopment by siderite and calcite microspar hydraulic fracturing associated with cretaceous shallow burial or palaeogene uplift and cementation by strongly ferroan yellow sparry calcite that records meteoric water invasion of the host mudrocks an abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products and 13c data suggest a predominantly methanogenic bicarbonate source however the wide 18o range for petrographically identical cement 1 3 to 15 6 is difficult to explain

pervasive late paleozoic triassic remagnetization of miogeoclinal carbonate rocks in the basin and range and vicinity sw usa regional results and possible tectonic implications abstract relatively unmetamorphosed paleozoic miogeoclinal carbonate rocks in the basin and range of e nevada sw nevada and adjacent california and w utah yield low inclination magnetizations that reflect pervasive regional remagnetization around the close of the paleozoic the rocks range in age from mid cambrian through pennsylvanian and lie generally in a broad belt between the mid paleozoic roberts mountain thrust and the late cretaceous sevier thrusts most of the magnetizations reside in magnetite but at one site the magnetization is evidently carried by pyrrhotite preliminary rock magnetic data suggest samples with magnetite borne remanences have wasp waisted hysteresis curves typical of remagnetized carbonates the origin of the remagnetization is problematic and probably polygenetic both the permo triassic sonoma orogeny and deformation associated with the ancestral rockies seem too spatially limited but magnetite from smectite destruction seems difficult to reconcile with the great stratigraphic extent of late paleozoic remagnetization unless combined with thermal resetting of the lowermost units a number of sites also appear to have undergone some vertical axis rotation and the sense and magnitude of these rotations are grossly consistent with independent geologic evidence however the probably large age range of the low inclination components complicates their use for resolving tectonic rotations younger intermediate stability components of magnetization probably of cretaceous or cenozoic age are also found in many sites and also probably have multiple origins at sites farther w the late paleozoic component is not found which probably reflects its destruction by later mesozoic or cenozoic heating at sites farther e on and near the colorado plateau gray carbonates yield only cenozoic magnetizations some reddish oxidized carbonates there locally contain a hematite borne magnetization of late paleozoic age however it is probably related to the development of thick continental redbed sequences in overlying strata on the plateau rather than to the remagnetization process es in the miogeocline

formation of double ring circular grabens due to volumetric compaction over buried impact craters implications for thickness and nature of cover material in utopia planitia mars 1 throughout the polygonal terrain of utopia planitia are circular grabens inferred to overlie buried crater rims thirty four of the 37 circular grabens discovered in viking and thermal emission imaging system themis images southwest of the utopia basin 21 36 n 240 265 w have two nested rings while the remaining three have a single ring all 37 circular grabens bound topographic depressions whose surface relief scales directly with diameter suggesting differential compaction of cover material within the polygonal terrain the distance of doubled circular grabens from the center of the utopia basin scales directly with the spacing between the two rings implying a relation between ring spacing and cover thickness numerical models of horizontal shrinkage during volumetric compaction of material over crater rims show two maxima of horizontal normal tension on the surface these maxima where we would expect grabens to form move away from each other with increasing cover thickness the modeled spacings of stress maxima within 1 2 km thick cover are comparable to the graben spacing observed in southwest utopia planitia implying that cover thickness in this polygonal terrain may be 1 2 km thick where cover material is thin the proximity of the two normal stress maxima may only produce a single circular graben accounting for the observation that the three single ringed grabens are most distal to the center of the basin we contend that the correlation of shrinkage model results and observed ring spacing supports a wet sedimentary origin of polygonal terrain materials consistent with the past existence of standing water in the martian lowland

a mechanical model for complex fault patterns induced by evaporite dehydration and cyclic changes in fluid pressure complex fault patterns i e faults which exhibit a diverse range of strikes may develop under conditions where a regional tectonic stress field is weak or absent e g polygonal faults the present paper considers a complex system of synsedimentary faults in the umbria marche apennines italy geometrically similar to polygonal fault systems developed during an early jurassic extensional episode this particular fault pattern differs from many extensional fault systems as it lacks structures that are developed with a classical bimodal conjugate andersonian geometry a conceptual and mechanical model is proposed to explain the development and evolution of the complex fault pattern in which it is proposed that faulting is primarily controlled by the development of volumetric instability in triassic evaporites due to dehydration processes anhydritisation during burial the lithological architecture of the triassic evaporites comprising interbedded ca sulphate layers and dolostones played a fundamental role in controlling the deformation processes cyclic fluid overpressure build up release and the coexistence of brittle and brittle ductile flow processes led to horizontal isotropic and non plane extensional strain fields within the dehydrating rock mass which favoured the development of complex deformation patterns the mechanical model proposed shows that the studied fault pattern developed under a stress field consistent with almost homogeneous stress intensities within the horizontal plane the data presented show that local strain fields and transient fluid pressure conditions have been dominant over weak regional extensional tectonics the findings are relevant to many other areas where complex faulting patterns including polygonal faults occur in association with evaporite or clay rich sedimentary sequences

a probable cave breccia in middle cambrian limestone southern nevada petrography and paleomagnetic constraints on the time of formation abstract a spectacular limestone breccia which occurs at or near the contact between the carrara formation early middle cambrian and bonanza king formation middle cambrian in the desert range southern nevada probably reflects a cave that has subsequently collapsed and healed the breccia blocks are angular represent a diverse suite of shallow water limestones that can be recognized laterally in undisputed strata and exhibit clean truncation of late diagenetic fabrics such as stylolites and blocky spar cements at clast edges in the blocks the in situ directions of a secondary magnetization have been scattered the age of this magnetization probably lies in the range late cretaceous to mid cenozoic which suggests that the time of brecciation is no older than late cretaceous the reccia must be older than late tertiary however because it is truncated by basin and range normal faulting

a method for the disaggregation of mudstones we describe a method which effectively and gently disaggregates muds and mudstones by combining a saturation freeze thaw technique with treatment by ultrasonic probe the method works well for samples containing 5 toc and which have been buried to depths less than about 3500 m it does not work for samples strongly cemented with carbonates or silicates scanning electron micrographs of size fractions and the strong mineralogical segregation between size fractions are offered as evidence of complete disaggregation

three dimensional consolidation of fine grained sediments current theories of one to three dimensional consolidation are based on the concept of gravitational loading with concomitant expulsion of pore fluid as a result of mechanical rearrangements of particles and porosity reduction however the recent discovery of extensive networks of layer bound fractures forming polygonal patterns in extremely fine grained smectitic clays and chalks suggests that current theory may not adequately explain the consolidation process in such sediments these faults develop during shallow burial and often appear to have been active at the sediment water interface radially isotropic strain suggests that the process of formation was nontectonic measured values of strain suggest that deformation resulted from volumetric contraction of the sediments we argue that the formation mechanism for these fault systems is related to the colloidal properties of the constituent sediments and that as such stresses due to gravity may not be the only factor controlling pore fluid expulsion

origin and significance of decameter scale polygons in the lower peace vallis fan of gale crater mars abstract decameter scale polygons are extensively developed in the bedded fractured bf unit of the lower peace vallis fan the polygons occur in a likely extension of the gillespie lake member north of yellowknife bay the section first drilled by the mars science laboratory msl mission we examine hypotheses for the origin of these polygons to provide insight into the history of gale crater the polygons are 4 30 m across square to rectangular and defined by 0 5 4 m wide generally straight troughs with orthogonal intersections polygon networks are typically oriented orthogonal systems with occasional nearly circular patterns hundreds of meters across potential origins include cooling of lava and for sedimentary units syneresis unloading weathering desiccation impact processes and cold climate thermal contraction cold climate thermal contraction is the hypothesis most consistent with the sedimentary nature of the bf unit and the polygon morphology geometry networks and apparent restriction to the coarse grained gillespie lake member a periglacial setting further provides the best analogs for the circular networks and is consistent with geologic context and msl data most of the decametric polygons appear to be ancient they are confined to the hesperian bf unit and only a few of their bounding fractures extend into younger or recently exposed units in this regard they differ from the majority of proposed thermal contraction polygons on mars as those are generally thought to be young features and accordingly the history of formation preservation and reactivation of the decametric polygons is likely to be more complex than that of any proposed young polygons on mars the decametric polygons in the bf unit may represent landforms developed in a cold but still comparatively wet interlude between a clement early mars and the much drier and colder planet of today

stress variability and brittle tectonic structures the problem of stress variability both in space and time during brittle deformation of rocks is faced in this note several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed a genetic partitioning of the stress tensor is proposed showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific genetic components the genetic components discussed in this note are the gravitational the tectonic the fluid the thermal and the diagenetic one each genetic component is a function of several parameters but for all of them the possible important role played by the variable time is emphasised according to numerous researches based on theoretical and experimental grounds brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small and large scale spatial variations as well as about short and long term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event the relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches

assessment of β the compression coefficient of mudstones and its relationship with detailed lithology abstract the mechanical compaction of soils and fine grained sediments is commonly described in soil mechanics by a relationship between void ratio and effective stress the relationship is defined by two parameters derived from the slope of the line relating void ratio and log effective stress and e the void ratio at a chosen effective stress e g 100 kpa and e are strongly correlated and are related to the atterburg liquid limit which is itself related in a general way to the grain size and mineralogy of the sediment in general finer grained sediments have higher initial void ratios and compact faster than coarser sediments porosity depth trends for mudstones in sedimentary basins are therefore strongly influenced by lithology steps or changes in the slope of porosity depth trends can be caused by lithological change rather than changes in the pressure regime a method is presented to evaluate for mechanically compacted mudstones using porosities determined from the density log fdc and based on the relationship between effective stress and void ratio developed in soil mechanics the method allows one to obtain rapidly approximate values for materials for which it is essentially impossible to obtain compaction parameters experimentally calculated values for mudstones in wells 8 3 2 and 30 2 1 on the norwegian margin vary between 0 2 and 0 65 the values correlate positively with the clay particle content of the mudstones which we have determined for 50 mudstone samples from the same wells the strong link between mudstone lithology and suggests that a downhole log may be a useful way to visualize downhole variations in mudstone lithology the log also shows how lithology affects the rate of porosity loss with depth or effective stress for example porosity in mudstones within both wells 8 3 2 and 30 2 1 is essentially constant over 700 m sections of normally pressured mudstones the log and grain size data indicate that this reflects changes in the lithological character and of the muds and not as commonly inferred a change in the pressure regime

modeling subsurface flow in sedimentary basins groundwater flows that arise in sedimentary basins from the effects of topographic relief buoyant convection sediment compaction erosional unloading and combinations of these driving forces can be described using quantitative modeling techniques models can be constructed to consider the effects of heat and solute transport petroleum migration and the chemical interaction of water and rocks the accuracy of model predictions however is limited by the difficulty of predicting hydrologic properties of sediments on regional dimensions estimating past conditions such as topographic relief and knowledge of how physical and chemical processes interact over gelogic time scales progress in basin modeling will accelerate as hydrologic research efforts are better integrated with those of other specialities such as sedimentology rock mechanics and geochemistry

fluid expulsion in terrestrial sedimentary basins a process providing potential analogs for giant polygons and mounds in the martian lowlands abstract on earth burial of fine grained sediments in offshore passive margins e g underwater fans and deltas commonly results in fluid expulsion features including large scale polygonal fractures mud volcanoes and pockmarks comparison of resulting offshore polygons and mud volcanoes with giant polygons and high albedo mounds in the chryse acidalia region of mars shows the terrestrial and martian features to be similar in size morphology geologic context and general co occurrence within the same basin these similarities suggest that the process of terrestrial fluid expulsion may provide an analog that could link the giant polygons and mounds in chryse and acidalia to a single process moreover while the terrestrial offshore polygons and mud volcanoes commonly develop in the same basins these features do not necessarily occur in exactly the same locations within those basins as they are independent responses to compaction and dewatering thus the fluid expulsion analog does not require that the martian giant polygons and mounds have identical distributions this is the situation in chryse and acidalia where the giant polygons and mounds are extensively developed and generally have overlapping distributions but where each set of features may occur in places without the other this fluid expulsion analog is enhanced by the fact that giant polygons and mounds in chryse and acidalia co occur in a regional sense and in a geologic setting that is consistent with a fluid expulsion model of formation implications of this analog may impact our view of the role of water in the depositional history of the martian lowlands

clastic polygonal networks around lyot crater mars possible formation mechanisms from morphometric analysis polygonal networks of patterned ground are a common feature in cold climate environments they can form through the thermal contraction of ice cemented sediment i e formed from fractures or the freezing and thawing of ground ice i e formed by patterns of clasts or ground deformation the characteristics of these landforms provide information about environmental conditions analogous polygonal forms have been observed on mars leading to inferences about environmental conditions we have identified clastic polygonal features located around lyot crater mars 50 n 30 e these polygons are unusually large 100 m diameter compared to terrestrial clastic polygons and contain very large clasts some of which are up to 15 metres in diameter the polygons are distributed in a wide arc around the eastern side of lyot crater at a consistent distance from the crater rim using high resolution imaging data we digi tised these features to extract morphological information these data are compared to existing terrestrial and martian polygon data to look for similarities and differences and to inform hypotheses concerning possible formation mechanisms our results show the clastic polygons do not have any morphometric features that indicate they are similar to terrestrial sorted clastic polygons formed by freeze thaw processes they are too large do not show the expected variation in form with slope and have clasts that do not scale in size with polygon diameter however the clastic networks are similar in network morphology to thermal contraction cracks and there is a potential direct martian analogue in a sub type of thermal contraction polygons located in utopia planitia based upon our observations we reject the hypothesis that polygons located around lyot formed as freeze thaw polygons and instead an alternative mechanism is put forward they result from the infilling of earlier thermal contraction cracks by wind blown material which then became compressed and or cemented resulting in a resistant fill erosion then leads to preservation of these polygons in positive relief while later weathering results in the fracturing of the fill material to form angular clasts these results suggest that there was an extensive area of ice rich terrain the extent of which is linked to ejecta from lyot crater

can polygonal faults help locate deep water reservoirs polygonal faults are compaction related normal faults that develop in very fine grained sedimentary successions despite their ubiquity few studies have highlighted the application of polygonal fault mapping to identifying deep water sandstone reservoirs we use three dimensional seismic and borehole data from the maloy slope offshore norway to demonstrate that the distribution cross sectional geometry and throw characteristics of polygonal faults can be used to locate deep water sandstone reservoirs we identify two tiers of polygonal faults in the cretaceous to lower paleogene succession the lowermost tier is stratigraphically restricted to the lower barremian to lowermost turonian succession and likely formed during the early turonian the uppermost tier spans the entire cretaceous succession and likely formed during the maastrichtian an abrupt decrease in the thickness of the upper tier occurs where a 92 m 302 ft thick sandstone rich slope fan is developed in the upper turonian interval furthermore the lower tips of faults in the upper tier which are defined by anomalously high throw gradients cluster at the top of the sandstone resulting in decoupling of this tier from the underlying lower turonian tier we interpret that faults in the upper tier nucleated above the reservoir across the entire slope and that the slope fan sandstone acted as a mechanical barrier to downward fault propagation resulting in abrupt thinning of the tier at the sandstone pinchout we demonstrate polygonal faults are not simply an academic curiosity mapping of these enigmatic structures can have practical applications for the delineation of a variety of reservoir types in hydrocarbon bearing sedimentary basins worldwide

mesozoic lower jurassic red stromatactis limestones from the southern alps arzo switzerland calcite mineral authigenesis and syneresis type deformation the broccatello lithological unit lower jurassic hettangian to lower parts of upper sinemurian near the village of arzo southern alps southern switzerland is a mound shaped carbonate deposit that contains patches of red stromatactis limestone within the largely bioclastic broccatello unit the stromatactis limestone is distinguished by its early diagenetic cavity system a relatively fine grained texture and an in situ assemblage of calcified siliceous sponges various demosponges and hexactinellids a complex shallow subsurface diagenetic pathway can be reconstructed from sediment petrography in combination with comparative geochemical analysis carbon and oxygen isotopes trace and rare earth elements ree y this pathway includes organic matter transformation aragonite and skeletal opal dissolution patchy calcification and lithification sediment shrinkage sagging and collapse partial ree remobilization and multiple sediment infiltration these processes occurred under normal marine essentially oxic conditions and were independent from local recurring syn sedimentary faulting it is concluded that the stromatactis results from a combination of calcite mineral authigenesis and syneresis type deformation the natural stromatactis phenomenon may thus be best explained by maturation processes of particulate polymer gels expected to form in fine grained carbonate sediments in the shallow subsurface conditions favorable for the evolution of stromatactis appear to be particularly frequent during drowning of tropical or subtropical carbonate platforms

episodic basin wide fluid expulsion from geopressured shale sequences in the north sea basin a proposed general model for the episodic dewatering of thick shale successions is based on the recognition of a pervasive polygonal extensional fault network developed in the dominantly fine grained lower tertiary of the north sea basin seismic data show that the faults are arranged in stratigraphically bound structural units tiers that are delimited vertically by almost undeformed condensed sections and are restricted in distribution to the lowest permeability slope and basin floor facies i propose an episodic three stage mechanism to explain the fault genesis involving 1 the development of basin wide overpressured compartments 2 a density inversion between the overpres sured units and the overlying seal and 3 natural hydraulic fracturing pressure bleed off and resealing of the pressure compartment leading to a repeat of the cycle

episodic basin wide hydrofracturing of overpressured early cenozoic mudrock sequences in the north sea basin a deformational fabric associated with the early cenozoic mudrock dominated sequences of the north sea is described seismic data show that the mudrock sequences are pervasively faulted over a large area of the basin axis in the central and northern north sea mapping the faults using regional two and three dimensional data shows that the faults are minor extensional structures arranged in stratigraphically bound tiers in the deep water sequences and bounded by regional condensed sequences seals the geometry of the faulting is complex faults are organized into cellular networks comprising polygonal prismatic and pyramidal forms a gravitational mechanism of downslope collapse faulting is rejected on the grounds that the distribution of the deformed sequences extends over large areas of formerly horizontal basin plain a mechanism involving the repeated development of basin wide overpressured compartments is proposed in which the cellular structure is related to deformation patterns associated with a density inversion between the overpressured units and the overlying seal hydrofracturing is invoked to account for the almost random strike distribution of the faulting the region affected by the pervasive fluid expulsion faulting encompasses almost half of the north sea basin basin wide collapse of the overpressured compartments could have resulted in a geologically rapid flux of enormous volumes of pore fluids this mechanism has implications for concepts of steady state compaction heat flow in basins hydrocarbon transport and the genesis of lead zinc ore deposits

the origin and glaciodynamic significance of sandstone ridge networks from the hirnantian glaciation of the djado basin niger the djado basin niger was located beneath the inner part of the late ordovician ice sheet the felar felar formation consists mainly of glaciomarine deposits associated with the major ice sheet recession within the glaciation and is bounded by two glacial unconformities structures corresponding to sandstone ridges are found within the felar felar formation sandstone ridges are several metres high about 10 m wide and hundreds of metres long these structures are organized in extensive anastomosed to subpolygonal networks the association of sandstone ridge networks with the later glacial unconformity and with other glacial evidence suggests sub glacial conditions for their origin sandstone ridge sedimentological characteristics indicate that sandstone ridges result from the scouring of the felar felar formation by sub glacial turbulent and pressurized meltwater then subglacial cavities were infilled with sand derived from glacial abrasion sandstone ridge networks are comparable with tunnel channels and document unusual drainage structures of the inner part of the palaeo ice sheet