fluid overpressure estimates from the aspect ratios of mineral veins abstract several hundred calcite veins and mostly normal faults were studied in limestone and shale layers of a mesozoic sedimentary basin next to the village of kilve at the bristol channel sw england the veins strike mostly e w 239 measurements that is parallel with the associated normal faults the mean vein dip is 73 n 44 measurements field observations indicate that these faults transported the fluids up into the limestone layers the vein outcrop trace length 0 025 10 3 m and thickness 0 1 28 mm size distributions are log normal taking the thickness as the dependent variable and the outcrop length as the independent variable linear regression gives a coefficient of determination goodness of fit of r2 0 74 significant with 99 confidence but natural logarithmic transformation of the thickness length data increases the coefficient of determination to r2 0 98 indicating that nearly all the variation in thickness can be explained in terms of variation in trace length the geometric mean of the aspect length thickness ratio 451 gives the best representation of the data set with 95 confidence the true geometric mean of the aspect ratios of the veins lies in the interval 409 497 using elastic crack theory appropriate elastic properties of the host rock and the mean aspect ratio the fluid overpressure that is the total fluid pressure minus the normal stress on the fracture plane at the time of vein formation is estimated at around 18 mpa from these results and using the average host rock and water densities the depth to the sources of the fluids below the present exposures forming the veins is estimated at between around 300 m and 1200 m these results are in agreement to those obtained by independent isotopic studies and indicate that the fluids were of rather local origin probably injected from sill like sources water sills inside the sedimentary basin

modelling palaeostress magnitude and age in extensional basins a case study from the mesozoic bristol channel basin u k analysis of normal faults and extensional veins developed in the upper triassic and lower jurassic succession exposed along the bristol channel coast indicate a protracted period of rift related deformation from late triassic to early cretaceous during this period it is demonstrated that a consistently oriented stress system operated with 3 oriented ne sw but with stress ratios 2 1 3 varying from 0 9 0 1 two approaches are described to estimate rifting stress magnitudes the first involves data from synsedimentary faults and yields for late triassic rifting 1 0 714 mpa 2 0 169 mpa and 3 0 033 mpa the second is developed for faulting in the lower jurassic section where no direct evidence of age is available the method calculates stress magnitudes with the rifting stress ratio of 0 9 for varying increments of overburden load each increment represents the possible magnitude of tectonic stresses at the time indicated by the amount of burial by incrementally adding the estimated remaining lower jurassic lower cretaceous overburden load to the stress magnitudes of each case a plot of reducing stress ratio for each age of faulting is determined and that which best reproduces the range of stress ratios calculated represents the modelled estimate of stress magnitude and timing the results suggest an end early jurassic onset of faulting with principal stress magnitudes of 1 12 98 mpa 2 12 56 mpa 3 8 80 mpa several simplifying assumptions used in the analysis are discussed

trace fossils from continental triassic red beds of the gondwana sequence pranhita godavari valley south india abstract three types of trace fossils a horizontal burrows b inclined passageways between successive galleries of horizontal burrows and c vertical to slightly inclined burrows occur in the triassic red beds of the gondwana sequence in the pranhita godavari valley south india as is the case with marine trace fossils morphologies of these continental burrows are found to vary with the sub environments of the fluvial domain i e channels and flood plains whereas the traces tend to be vertical in the shifting conditions of the channel facies those in more stable parts of the flood plain facies are generally horizontal trace fossils in the continental environment are as effective as those of the marine environments in the recognition of the various environmental parameters

changing fluvial environments across the permian triassic boundary in the karoo basin south africa and possible causes of tetrapod extinctions abstract the karoo supergroup is a 12 km thick succession of sedimentary rocks that accumulated in a large intracratonic retroarc foreland basin in southwestern gondwana the strata record 100 million years of almost continuous sediment accumulation from the permo carboniferous 300 ma through to the early jurassic 190 ma under a range of climatic regimes and within several tectonically controlled sub basins alluvial sediments dominate the succession from the late permian onwards fossils of synapsid reptiles and early dinosaurs are sufficiently common to be used in a ten fold biostratigraphic subdivision of these strata the permian triassic boundary in the karoo succession is marked by a major extinction of the herbivorous dicynodonts which co incides with a rapid change in fluvial facies this study uses field observations of the sedimentary facies palaeosols and in situ fossils of well exposed p tr boundary sequences in the southern karoo basin to provide evidence of environmental changes that may have caused the tetrapod extinctions in the main karoo basin below the p tr boundary sequence the sediments comprise drab greenish grey mudrocks with a few thick 3 12 m multistoried laterally accreted fine grained sandstone bodies the mudrocks host hydromorphic palaeosols with palustrine carbonate horizons and mostly disarticulated skeletal remains of dicynodon assemblage zone fauna above a transition zone of approximately 20 m thickness these strata change into reddish brown mudrocks with numerous thin 0 5 5 m ribbon sandstone bodies and more continuous sheet sandstones calcic palaeosols with rhizocretions nodular horizons and pervasive calcareous encrustation of articulated skeletons of lystrosaurus and associated fauna of the lystrosaurus assemblage zone within a 20 m thick interval of the transitional strata fossils of dicynodon and lystrosaurus occur together the facies transition that co incides with the latest permian dicynodont extinction is interpreted as a change in fluvial style from meandering to low sinuosity channels with general drying of the floodplain habitats and redistribution of vegetation belts the horsetails in particular became significantly reduced in areal distribution the changes in fluvial style in the karoo basin at the end of the permian were triggered by a pulse of thrusting in the southerly source area dated at 247 ma 2 ma which brought about rapid progradation of a large sandy braided fan system the katberg sandstone fm into the central parts of the basin a concomitant basinward migration of depositional environments allowed the lystrosaurus communities that were adapted to dry floodplains to colonise the entire basin to the detriment of most of the wet floodplain dicynodon fauna that lived there

genesis of calcretes in the devonian wood bay group dicksonland spitsbergen abstract a continuous pedogenic calcrete and numerous glaebular calcretes are exposed over approximately 150 km 2 in the devonian wood bay group dicksonland spitsbergen the prominent continuous calcrete horizon is up to 4 m thick and exhibits both prismatic and knobby weathering outcrops the prismatic calcrete outcrop of approximately hexagonally arranged vertical fractures usually shows gradational contacts with the underlying sediment the calcrete and clastic sediments immediately below the gradational contact show petrographic evidence for both replacive and displacive carbonate crystallization in contrast knobby calcrete is generally lenticular and probably represents an intra formational conglomerate both the knobby and prismatic calcretes have a brecciated fabric in which low ferroan calcite fragments are cemented and bound by dolomite some of the calcite fragments are coated by concentric laminae of dolomite the glaebular calcretes are composed of low ferroan calcite nodules ranging in size from a few millimeters to approximately 6 cm and show elongation ratios of approximately 1 5 normal to bedding these nodular zones are most common in floodbasin or levee sediments in both the stjordalen formation and the upper part of the keltiefjellet formation nodular horizons occur singly and in groups and vary from a single layer of nodules to beds several meters thick calcretes are common in the old red sandstone of great britain and rare in equivalent strata in spitsbergen suggesting differences in climate or other aspects of the depositional environment

facies and cyclicity of the late permian bainmedart coal measures in the northern prince charles mountains macrobertson land antarctica the late permian bainmedart coal measures form part of the permo triassic amery group which crops out in the beaver lake area of the northern prince charles mountains macrobertson land antarctica the exposed strata are believed to have formed in graben or half graben sub basins on the western edge of the lambert graben a major failed rift system sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly flowing rivers of low sinuosity represented by facies a1 and a2 flowed northward down the axis of the basin and were associated with waterlogged floodbasin and peat forming wetlands facies b1 b4 a third facies association comprising facies c1 c3 interpreted as the deposits of lake floor and delta environments is exclusively developed within a distinctive fine grained interval here named the dragon s teeth member the proportion of association b facies within the succession increases markedly above the level of the dragon s teeth member at about 300 m above the base of the formation flat low angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical presence of massive and chaotic intervals of sandstone further implies some deposition from high concentration aqueous flows alluvial channel bodies show evidence of incision into underlying substrates both during initiation and at later stages in channel belt construction the lack of interfingering between channel deposits and coals suggests that thick peats formed only in areas and at times of minimal clastic sediment supply analysis of well developed cyclicity within the coal measures suggests that the dominant control on sequence architecture was climatic related to precessional milankovitch fluctuations of c 19 kyr periodicity cycles began abruptly with the deposition of coarse grained material in high energy alluvial channels which contracted with time in response to changes in water supply rainfall upper parts of cycles are dominated by finer grained sediments and then coal indicative of progressively reduced coarse sediment input tectonic processes overprinted this pattern at least once during the period of sediment accumulation to form the dragon s teeth member

depositional facies and environments in the permian umm irna formation dead sea area jordan abstract the permian umm irna formation in central jordan consists of a 60 m thick sequence of clastic sediments which can be divided into two fluvial sedimentary facies the lowermost facies 1 is characterized by the presence of five sandstone dominated fining upward sequences each sequence comprising an erosively based coarse to fine grained trough cross bedded sandstone overlain and laterally intertonguing with maroon siltstone and silty shale containing locally abundant carbonaceous plant material the overlying facies 2 also consists of up to five fining upward sequences each comprising an erosively based coarse grained pebbly sandstone grading up through fine grained sandstone and siltstone into silty shale the sandstones are tabular and laterally persistent internally complex units structured by erosively bounded trough cosets the thickness and grain size of the sandstones and the proportion of siltstone and silty shale is much greater than in facies 1 it also contains abundant ferruginous concretions glaebules deposition occurred on an unconfined braidplain sloping northwestwards away from an elevated provenance arabo nubian shield in the south and east the lowermost facies 1 is regarded as a distal environmental equivalent of the overlying facies 2 deposition was largely controlled by period c shifts of the active channel tract influenced by a gradual increase in the elevation of the source area thereby promoting basinwide progradation and vertical stacking of the two major facies components the ferruginous concretions glaebules in facies 2 are thought to have resulted from diagenetic processes in the vadose zone of a humid possibly tropical climate characterized by alternating wet and dry seasons and strong leaching the fluvial depositional system drained basinwards into a shallow water marine depository along the southern margin of the tethys ocean the ocean may have exerted a maritime influence on the climate of the alluvial plain

isotopic composition of displacive fibrous calcite veins reversals in pore water composition trends during burial diagenesis abstract stable isotope and petrographic analyses of diagenetic calcite cone in cone and beef veins from british jurassic and lower cretaceous shales have been used to determine the environment of precipitation of fibrous calcites successive growth took place by antitaxial displacive addition at the vein margins away from primary sedimentary laminations or early diagenetic concretions carbon and oxygen isotopic ratios 13c 0 and 18 4 to 11 indicate a relatively late dingenetic origin for the veins after tens or probably hundreds of meters of burial and after cessation of bacterial activity nd considerable modification of the oxygen isotopic composition of the pore water vein growth was discontinuous and took place in waters of changing isotopic and trace element composition changes are not unidirectional and are unlikely to result from the simple evolution of a single conhate pore water reversals in isotopic trends indicate that precipitation took place during periods of renewed lateral groundwater flow tapping different sources of bicarbonate bearing solution

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

brittle deformation relating to the carboniferous cretaceous evolution of the lambert graben east antarctica a precursor for cenozoic relief development in an intraplate and glaciated region abstract stress inversions of kinematic data measured from brittle deformation structures exposed in the southern prince charles mountains have been carried out to elucidate the drivers of intracontinental deformation responsible for the development of the lambert graben east antarctica four palaeostress fields d 1 d 4 were identified from these data which when coupled with pre existing geochronological and sedimentological evidence provide a clear deformation framework the architecture of the lambert graben is dominantly defined by north south trending d 2 and northeast southwest trending d 3 faults that formed respectively during 1 carboniferous permian intracontinental extrusion and 2 cretaceous transtensional tectonics related to the dispersion of east gondwana interestingly sub ice topography in the southern prince charles mountains and further south towards the gamburtsev mountains is defined by bedrock lineaments that parallel the orientations of the main fault arrays previous work shows that the development of relief in the southern prince charles mountains is tertiary in age and probably not associated with carboniferous permian or cretaceous tectonic events based on a clear spatial link between the orientations of faults and bedrock lineaments we suggest that inherited fault zones were integral in accommodating uplift driven by plate flexure as a response to crustal unloading during glacial incision

regular and irregular climatic cycles and the belemnite marls pliensbachian lower jurassic wessex basin time series of carbonate and organic carbon values are reported for 10 5 m of the belemnite marls the light and dark marl beds form decimetre scale couplets and metre scale bundles light marls are carbon poor and carbonate rich whereas dark marls and laminated shales are carbon rich and carbonate poor sediment composition appears to have been indirectly controlled by climate the regular couplets may record orbital precession cycles 20 ka however the bundles cannot be linked to changes in eccentricity rather they apparently record irregular large amplitude climatic variations with periods of a few hundred thousand years it should not be assumed that 100 400 ka cyclicity in climate related sedimentary sequences can be explained only by orbital forcing

hybrid veins from the southern margin of the bristol channel basin uk sinistral and dextral en echelon hybrid calcite veins are exposed on the southern margin of the bristol channel basin north somerset on a liassic carbonate platform these hybrid veins are transitional between pure extensional mode i and shear fractures mode ii mode iii and experienced both extensional and shear displacements during their formation in the study area they occur in conjugate sets with the conjugate angle ranging between 10 and 50 based on the kinematic analysis of these veins a new model for the opening of the bristol channel is proposed

the first record of the permian glossopteris flora from sri lanka implications for hydrocarbon source rocks in the mannar basin strata exposed near tabbowa tank tabbowa basin western sri lanka have yielded thefirst representatives of the distinctive permian glossopteris flora from that country the assemblageincludes gymn

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

from orientation to magnitudes in paleostress determinations using fault slip data abstract determinations of reduced stress tensors using fault slip data yield the orientation of principal stress axes and the ratio of the differences between principal stress magnitudes the use of rupture and friction laws allows determination of the two remaining unknowns that is the reconstruction of the complete stress tensor taking into account the depth of overburden brings an additional constraint the method is applied and discussed in the case of the hoover dam site western u s a where large data sets and rock mechanics information are available differences between intact sample and rock mass properties account for apparent disagreements between paleostress levels determined in similar tectonic environments pore pressure plays an important role where information about pore pressure is absent zero and hydrostatic pore pressure cases should be considered as limits

structural evolution of the pematang reservoirs kelabu dingga gas fields sumatra the kelabu jingga area located in the kiri trough of the central sumatra basin produces gas from the paleogene pematang group the pematang group consists of sandstones claystones organic rich shales and conglomerates deposited in fluvial and fresh water deltaic and lacustrine environments deposition occurred during regional tectonism associated with a major plate reorganization in the pacific and indian oceans during the paleogene subsequent rifting and basin development occurred in the kiri trough area in central sumatra syngenetic listric faults and associated rollover folds formed during rifting the pematang group was deposited in deep transtensional pull apart grabens and shallow extensional rifts tectonics strongly influenced the geometry and lateral continuity of the fluvial deltaic sands during the neogene oblique convergence of the indian ocean plate with the asian plate resulted in a regional dextral wrenching event in the central sumatra basin that overprinted the earlier extensional style of faulting in the kiri trough both extensional and compressional features are evident interpretation of a 3d seismic survey wireline logs including dipmeter and fms data and core data has delineated fault styles and depositional trends within the pematang group in the kelabu jingga fields the resultant maps and cross sections reveal hydrocarbon reserves and new drilling opportunities

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

aeolian sands in continental red beds of the middle buntsandstein lower triassic at the western margin of the german basin abstract aeolian sands occur widespread in continental red beds of the middle buntsandstein lower triassic at the western margin of the german basin middle europe in the eifel area cross bedded sands were mainly deposited by grainfall on lee slopes of barchanoid type dunes in unimodal wind regime in both low energy and high energy environments occasionally sets are internally truncated by reactivation surfaces crest height of solitary set dunes varies in the range of some meters locally sequences of cosets both of tabular planar and wedge planar type capped by flat or gently windward dipping topset beds represent larger dune complexes of about 10 20 m in height and at least 80 100 m in lateral extent horizontal laminated deposits originated as sand sheets in interdune areas the dune sands were deposited by trade winds of the northern hemisphere in low palaeolatitude predominantly by southeasterly and southwesterly trade winds in summer when the intertropical convergence zone was shifted to the north aeolian sands built up an extensive dune belt in the eifel intersected by braided to anastomosing rivers fluviatile incursions or heavy ephemeral rainfall led to aquatic redeposition of aeolian sediments and origin of shallow lakes in interdune depressions distribution of aeolian and fluviatile sediments in karlstal schichten is different in the southern western and northern eifel middle buntsandstein sequence in the southern eifel reflects an evolution of the fluviatile depositional environment also incorporating the occurrence and distribution of aeolian sands evolution is characterized by increasing spacing and sinuosity of channels weakening supply of coarse detritus from the source areas and decreasing braiding of the river systems aeolian sands occur in the final phase of this evolution which led from local alluvial fans via cobbly and pebbly braided rivers to sandy braided streams and finally to an intertonguing of aeolian dunes and braided to anastomosing rivers matching with the climax of regional diversification of depositional milieu the aeolian sand sea in the eifel covered an area of nearly 2500 km2 at the time of its largest extension the dune belt ended in the northern eifel the alternating sequence passed into an entirely alluvial succession which graded northwards by continuously diminishing grain size into the central playa lacustrine to marine sediment series in northwest germany palaeogeography sedimentology and genesis of the middle buntsandstein at the western margin of the mid european basin and of rotliegendes in northwest germany appear to be partially similar as is revealed by certain characteristics of the palaeogeographical reconstructions concurrence of middle buntsandstein palaeogeographical models for both eifel and the whole german basin most clearly delineates the key position of the eifel for interpretation of the buntsandstein sedimentary environment and the palaeogeography in middle europe

depositional setting and paleobotany of permian and triassic permineralized peat from the central transantarctic mountains antarctica abstract silicified peat is known from two sites in the central transantarctic mountains both are within a 2 km thick permo triassic sandstone shale sequence that was deposited by braided streams in a rapidly subsiding foreland basin along the paleo pacific margin of antarctica upper permian permineralized peat occurs as scattered boulders just above a channel form sandstone in the upper part of the buckley formation on skaar ridge overlooking the beardmore glacier these boulders are erosional remnants of fine grained deposits that accumulated in shallow lakes or swamps on a flood plain at fremouw peak the peat occurs near the top of the fremouw formation in strata that are probably middle to late triassic in age the peat consists of large blocks that were rafted into a sandy braided stream channel during a flood and then stranded and buried as flood waters receded both sites are characterized by exceptionally well preserved plant material although the composition and diversity of the two floras are very different permineralization apparently took place rapidly and was enhanced by the dissolution of siliceous volcanic detritus that is abundant in both formations

cenozoic landscape evolution of an east antarctic oasis radok lake area northern prince charles mountains and its implications for the glacial and climatic history of antarctica abstract ice free areas antarctica reveal a multi million year history of landscape evolution but most attention up to now has focused on the transantarctic mountains the amery oasis in the northern prince charles mountains borders the lambert glacier amery ice shelf system that drains 1 million km2 of the east antarctic ice sheet and therefore provides a record of fluctuations of both local and regional ice since the ice sheet first formed in early oligocene time this glacial record has been deciphered by i geomorphological mapping from aerial photographs and on the ground ii documenting the relationship between thick well dated uplifted glaciomarine strata and the underlying palaeolandscape iii examining surficial sediment facies and iv surface exposure dating using 10be and 26al the se amery oasis records at least 10 million years of landscape evolution beginning with a pre late miocene phase of glacial erosion followed by deposition of glaciomarine strata of the battye glacier formation pagodroma group in late miocene time a wet based ice sheet next expanded over the se amery oasis following which deposition of the glaciomarine pliocene bardin bluffs formation pagodroma group took place both formations were uplifted by at least 500 and 200 m respectively their tops are characterised by geomorphological surfaces upon which intensive periglacial activity took place higher level bedrock areas were subjected to deep weathering and tor formation early pleistocene time was characterised by expansion of a cold based ice sheet across the whole area but it left little more than patches of sandy gravel and erratic blocks late pleistocene expansion of local ice the battye glacier saw deposition of moraine mound complexes on low ground around radok lake and ice dammed lake phenomena subglacial drainage of the lake escaped to the east exhuming the sediment filled gorges holocene landscape modification has been relatively superficial overall the landscape of the amery oasis evolved primarily under the influence of wet based probably polythermal glaciers in miocene and pliocene times whereas the quaternary period was characterised mainly by cold based glaciers that had comparatively little impact on the landscape

eolian permian deposits in west and northwest argentina abstract the sedimentary and stratigraphic characteristics of eolian permian deposits exposed in mendoza la rioja and san juan provinces west and northwest argentina are described in this paper the eolianites are fine and medium sandstones with large scale cross bedding multiple parallel truncation planes and some asymmetrical ripples three genetic types of deposits have been identified dune facies eolian sand sheet facies and mixed fluvial eolian facies fine and medium sandstones with large scale cross bedding and multiple parallel truncation planes are here interpreted as dune deposits mainly crescentic dunes and unstratified or flat bedded sandstones as eolian sand sheet deposits mixed fluvial and eolian sequences composed of sandstones mudstones and some matrix supported conglomerates represent a transitional facies between those formed in eolian and fluvial environments this considerable deposition of eolian sediments was probably brought about by the existence of an extensive medium to low latitude continent and the withdrawal of marine environments the environments with highest aridity occurred towards the south and west of the region the eolian circulation pattern was controlled by a long ensialic volcanic arc emerging towards the west of the basin

oblique extensional tectonics in the malawi rift africa the malawi rift is the southern segment of the western branch of the east african rift system there has been little agreement as to whether this rift is opening perpendicularly or obliquely to its north south axis and little agreement on a mechanism for the formation of the asymmetric basins that make up the rift our analysis combines landsat image interpretation and field measurement of the latest slip direction on faults to produce a kinematic model of rift evolution the fault slip data are consistently compatible with a late cenozoic northwest southeast directed divergence between the nubian and somalian blocks plates across the rift the northwest striking livingstone fault at the northern limit of the malawi rift and the northwest striking zambezi fault zone to the south serve as dextral intracontinental transform zones between left stepping segments of the western branch of the east african rift system within the rift second order northwest striking transfer faults separate adjacent asymmetric basins the transform and transfer faults form a pattern that is similar to overlapping right stepping strike slip faults with a releasing geometry fault surfaces with two sets of striations indicate a progression from predominantly strike slip toward oblique normal faulting we hypothesize that during initial rift formation most northwest striking faults were dextral and that fault interaction between these structures resulted in curved stress trajectories and a pull apart component arcuate north to northeast striking border faults and subsiding basins then formed between these northwest striking faults in response to the right stepping dextral shear the maximum distance over which fault interaction is significant may control basin dimensions as opening of the rift proceeds slip partitioning and gravity failure result in increasing normal slip perpendicular to major escarpments and decreasing strike slip parallel to them

massive sandstone facies in the hawkesbury sandstone a triassic fluvial deposit near sydney australia abstract massive sandstone is a common facies in the hawkesbury sandstone a triassic formation attributed to deposition by a large braided river the massive sandstone occurs as sheets and in elongate depressions trending perpendicular to the paleoslope as indicated by the unidirectional mean orientation of abundant cross strata some of the depressions filled with massive sandstone also contain angular mudstone intraclasts up to several meters in length massive sandstone is also associated with deformed cross strata formed by progressive loss of lamination during mass movement down foreset slopes foreset failure is attributed to liquefaction due to falling water level or to collapse of adjacent mud banks the latter mechanism resulted in rapid loading of bedform foresets and introduce large mudclasts into the massive sand sand and mudclasts accumulated in transverse scours at the foot of the foreset slopes but also travelled along the scours or spilled out to form massive sheet sands the abundance of massive sandstone in the hawkesbury sandstone is attributed to deposition in a large river in which flood stage bedforms were probably up to 15 m high because of the size of the bedforms large volumes of liquefied sand were generated when foreset slopes failed

the structure and origin of prydz bay and macrobertson shelf east antarctica abstract a marine geophysical survey in early 1982 conducted by the australian bureau of mineral resources indicated that much of prydz bay is underlain by a sedimentary basin severe seismic multiples preclude an accurate estimate of total sediment thickness but interpretation of the seismic and magnetic data suggest that it is probably at least 5 km the trends of the southeast basin margin and of mild faulting and folding in the southwest indicate an overall nne trend roughly orthogonal to the continental margin in the south of prydz bay two series of seismic sequences are evident separated by a mildly erosional unconformity the lower series ranges from poorly to well stratified has minor folding and faulting and probably derives from continental and perhaps shallow marine pre breakup sediments the upper series is generally well stratified and prograded near the shelf edge it probably represents shallow marine post breakup sediments the seabed is distinctly unconformable with the underlying sediments implying both that much of the upper series sediments and some of the lower series sediments have been bulldozed off by advances of the amery ice shelf and that present sedimentation rates are very low possible thin moraines or tillites in the northeast part of the prydz bay are also attributed to these glacial advances the continental slope and rise sedimentary section ranges from at least 3 km thick off prydz bay to thin off the macrobertson shelf to the west reflecting the more prolific sediment source in prydz bay the deep water section includes several seismic sequences the most distinctive being interpreted as sheet volcanics and turbidite fans the deepest visible unconformity is locally strongly faulted and may separate the pre breakup and post breakup sediments indo antarctic breakup has been tentatively dated as early neocomian 130 ma and the e w orientation of the resultant antarctic coast invites interpretation of the lambert glacier prydz bay structure as a possible failed rift arm of a triple junction direct information on the age and nature of the sediments under prydz bay is lacking but permian continental sediments cropping out at beaver lake to the south may correspond to the lower pre breakup series this interpretation gains some support from analogies with fault bounded intracratonic basins in india which contain permian to triassic continental strata and which may have been juxtaposed prior to breakup the upper series probably consists of upper cretaceous and cainozoic sands and shales with moraines or tillites at the top of the section

log linear models markov chains and cyclic sedimentation abstract quantitative techniques employing markovian processes have been used by a number of investigators to test stratigraphic sections against a null hypothesis of a random succession of sedimentary facies rejection of the null hypothesis is used to imply presence of a markov chain in a succession however many of these techniques possess serious statistical flaws arising from the structuring of facies transitions the result is that presence of preferred facies and regular cyclic behavior in sedimentary successions cannot be rigorously supported log linear models in conjunction with a stepwise procedure for identification of significant transitions provide a rigorous method for determining the presence and extent of the markov property in stratigraphic sections a wide variety of geologic problems that are structured in terms of temporal succession can be examined as a markov process using log linear models

inorganic chemistry petrography and palaeobotany of permian coals in the prince charles mountains east antarctica sampled outcrops of permian coal seams of the bainmedart coal measures in the lambert graben eastern antarctica have been analysed for their proximates ultimates ash constituents and trace elements a similar series of samples has been analysed for their principle maceral and microlithotype components and vitrinite reflectance the coals are sub bituminous to high volatile bituminous in rank maturity increases markedly in southern exposures around radok lake where the oldest part of the succession is exposed and some strata have been intruded by mafic dykes and ultramafic sills the coal ash is mostly silica and aluminium oxides indicating that the mineral ash component is mostly quartz and various clay minerals the ratio of silica to aluminium oxides appears to increase in an upward stratigraphic direction the coal macerals include a relatively high liptinite content mainly sporinite that is significantly higher than for typical gondwana coals greater degrees of weathering within the floodbasin peat mire environments associated with climatic drying towards the end of the permian might account for both preferential sporopollenin preservation and increased silica aluminium oxide ratios up section correlation of the coal maceral components to adjacent peninsula india coals indicates the closest comparative coals of similar age and rank occur within the godavari basin rather then the mahanadi basin which is traditionally interpreted to have been contiguous with the lambert graben before gondwanan breakup the petrological characteristics suggest that either previous interpretations of palaeozoic basin alignments between antarctica and india are incorrect or that environmental settings and post permian burial histories of these basins were strongly independent of their tectonic juxtaposition a permineralized peat bed within the succession reveals that the coals predominantly comprise wood and leaf rich debris derived from low diversity forest mire communities dominated by glossopterid and noeggerathiopsid gymnosperms

variations in the composition of the clay fraction of the cenozoic pagodroma group east antarctica implications for determining provenance abstract the cenozoic pagodroma group in the northern prince charles mountains east antarctica is a glaciomarine succession of fjordal character comprising four uplifted formations of different ages the composition of the the assemblage of the upper oligocene to lower miocene mount johnston formation is characterised by the dominance of illite and intermediate concentrations of chlorite similar to that assemblage is the clay mineral suite of the middle miocene fisher bench formation where illite and chlorite together account for 95 of the clay minerals the middle to upper miocene battye glacier formation is the only formation with significant and persistent smectite concentrations although illite is still dominant the kaolinite concentration is also high and is even higher than that of chlorite the clay fraction of the upper pliocene to lower pleistocene bardin bluffs formation is characterised by maximum kaolinite concentrations and relatively low illite and chlorite concentrations the bulk of the clay fraction in each formation can be explained by the physical weathering and erosion of a nearby source under glacial conditions in the case of mount johnston formation and fisher bench formation this source may be situated in the metavolcanic and gneissic rocks of fisher massif the sediments of the bardin bluffs formation indicate a local source within the amery oasis where proterozoic granitoid rocks and gneisses and permo triassic fluvial rocks of the amery group are exposed these results suggest a strong local imprint on the glacial sediments as northwards flowing ice eroded the bedrock in these areas the origin of the clay fraction of the battye glacier formation is a matter of debate the smectite and kaolinite content most easily can be explained by erosion of sources largely hidden beneath the ice upstream less likely these clay minerals reflect climatic conditions that were much warmer and wetter than today facilitating chemical weathering

paleosols in permo triassic continental sediments from prydz bay east antarctica abstract odp leg 119 drilled a sequence of permo triassic continental red beds 58 m thick containing paleosols in prydz bay east antarctica the paleosols mainly in overbank siltstones and mudstones are characterized by extensive destruction of sedimentary structures and the presence of mottles rootlets and rare chalky caliche because of the shallow depth of burial and limited compaction and diagenesis the paleosols have retained most of their original microstructure and fabric micromorphological analysis of the paleosols reveals the presence of phreatic monopodial root systems nodules and a variety of soil structures and fabrics as well as a micritic root structure with alveolar texture most of the paleosols are weakly developed with small incomplete ped structures and lack of illuviation they resemble weakly developed alluvial soils with a c and b c profiles and are similar to modern base deficient inceptisols the nature of the paleosols suggests relatively high permeabilities and water circulation during soil formation and a relatively mobile water table with little reduction of iron except locally on the less well drained parts of the alluvial plain extensive pedogenic carbonate formation was precluded by the soil moisture regime and relatively wet climate during late permian and early triassic times prydz bay which lay some 30 south of the equator experienced climatic conditions broadly similar to parts of the contemporary tropics dominated by a ubtropical high pressure system easterly winds and seasonal rainfall

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

the hawkesbury sandstone south of sydney australia triassic analogue for the deposit of a large braided river abstract the hawkesbury sandstone is a triassic sheet sandstone extensively exposed in the sydney basin new south wales particularly along the coast near sydney unidirectional paleoflow in the sandstone its freshwater biota and abundant mudrock intraclasts indicate fluvial deposition sheet morphology low paleocurrent variance abundant erosion surfaces and the paucity of in situ mudrocks point to a braided fluvial system three facies assemblages have been recognized stratified sandstone massive sandstone and a minor mudrock assemblage the stratified sandstone assemblage is dominated by stacked sets of planar cross strata and minor trough cosets in sequences 6 23 m thick bounded by erosion surfaces significant paleocurrent changes between channel sequences indicate that they were initiated by the avulsion of major channel systems in some cases the channel sequences fine upward with planar cross stratal sets overlain by trough sets that decrease in magnitude upwards fining up into mudrocks the massive sandstone assemblage occurs principally as massive sandstone in elongate erosional features oriented transverse to paleoflow the massive sandstone commonly contains large mudrock intraclasts and is attributed to failure of high channel banks and or large bedforms during falling water stages the mudrock assemblage comprises rippled and horizontally laminated fine sandstone siltstone and shale with minor mudstone it is attributed primarily to floodplain deposition but abandoned channel fills are also present the hawkesbury sandstone does not conform to existing models for braided fluvial deposition in that planar cross strata accumulated in deeper parts of channels whereas trough cross strata formed in shallower water the large scale of planar cross strata sets up to 7 5 m thick mudrock intraclasts up to 38 m long bank collapse scars up to 11 m deep and abandoned channel fills up to 18 m deep show that the hawkesbury river was very large with deep main channels and high but variable discharge

the carnian humid episode of the late triassic a review from 1989 to 1994 a series of papers outlined evidence for a brief episode of climate change from arid to humid and then back to arid during the carnian stage of the late triassic epoch this time of climate change was compared to marine and terrestrial biotic changes mainly extinction and then radiation of flora and fauna subsequently termed albeit incorrectly the carnian pluvial event cpe by successive authors interest in this episode of climatic change has increased steadily with new evidence being published as well as several challenges to the theory the exact nature of this humid episode whether reflecting widespread precipitation or more local effects as well as its ultimate cause remains equivocal bed by bed sampling of the carnian in the southern alps dolomites shows the episode began with a negative carbon isotope excursion that lasted for only part of one ammonoid zone a austriacum however that the carnian humid episode represents a significantly longer period both environmentally and biotically is irrefutable the evidence is strongest in the european middle eastern himalayan north american and japanese successions but not always so clear in south america antarctica and australia the eruption of the wrangellia large igneous province and global warming causing increased evaporation in the tethyan and panthalassic oceans are suggested as causes for the humid episode

evolution of buntsandstein fluvial sedimentation in the northwest iberian ranges central spain abstract the buntsandstein sediments in central spain permian and triassic are mainly continental redbeds that form the base of a sequence that passes up into siliciclastic and carbonate tidal sediments the muschelkalk facies buntsandstein sedimentation began with conglomeratic and sandy fluvial facies whose detailed sedimentological analysis is the main subject of this work massive sheets are the most important conglomerate facies they were formed by deposition as longitudinal bars channel fill conglomerates are the second most important facies lateral accretion deposits also occur in the conglomerates they evolved from longitudinal bars because of decreasing flow or because the bars had reached a certain height sandstones are composed of eleven different facies large linguoid or transverse bars were the main bedforms in this environment these bedforms may be related downstream to large oblique troughs developed when the tabular bedform grew to a certain size or when a tabular bedform reached another one that acted as an obstacle compound and composite compound bars consisting of large sets of trough cross bedding with scoured wavy bases are al o recognized in the sandstone facies two major cycles occur in the complete conglomerate sequence which correspond to different physiographic models the lower cycle was mainly formed by channels and bars that were smaller than those in the upper cycle the upper cycle is more extensive than the lower one and has characteristics that indicate a higher stability paleocurrents for both cycles have a north northeasterly trend the vertical evolution of these sediments is clearly related to the different fault systems that controlled their distribution the paleogeography of the sandstone facies together with paleocurrents perpendicular to the conglomerate paleoccurents indicate deposition from a separate dispersal system that extensively overlies the previous one this unit which can be followed extensively over most of the iberian ranges and even outside the iberian ranges represents the growth of a fluvial braidplain over a large area of iberia during the lower triassic

strike slip fault bridge fluid pumping mechanism insights from field based palaeostress analysis and numerical modelling abstract we present a finite element study of stress perturbation in evolving compressive and extensional strike slip fault bridges the results are compared with a fracture study of a compressive bridge at st donats uk horizontally interbedded calcareous mudstone and bioclastic calcilutite at st donats have a distinct vertical permeability anisotropy this sedimentary sequence behaves as a set of horizontal aquifers the fluid flow in these aquifers is sensitive to mean stress gradients paleostress analysis of field fracture data verified by finite element modelling indicates a rotation of 1 towards parallelism with boundary faults inside the growing compressive bridge boundary faults and bridge faults recorded numerous fluid flow events the modelled mean stress pattern shows a regional maximum within the bridge and local maxima minima pairs at boundary fault tips finite element modelling of an extensional bridge indicates that 3 rotates towards parallelism with boundary faults the mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged the stress patterns are reestablished by each stress build up preceding the rupturation of the boundary faults throughout the development stages of strike slip fault bridges mean stress gradients developed pre failure control the fluid flow in fractures of the strike slip fault system at and after the end of each stress build up and the fluid flow in boundary faults post failure fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge rupture and sealing of the main bounding faults is a step wise process that opens and closes fluid conduits between areas with different pressures

geological evidence for intra jurassic faulting in the wessex basin and its margins geological observations on jurassic outcrops close to major faults in the wessex basin mendip area reveal the local presence of ammonite and brachiopod bearing sediments penetrating underlying strata toarcian and bajocian neptunian dykes and particularly sills are associated with the eypemouth fault and bajocian sills with the bride fault and mere fault in the mendip area numerous neptunian dykes of hettangian sinemurian pliensbachian and bajocian ages cross cutting carboniferous limestone are recorded typically also associated with major basement faults e g cranmore and leighton faults these periods of assumed sediment injection are taken as indicating times of displacement along the faults in question variations in facies hettangian sinemurian toarcian bajocian uppermost oxfordian kimmeridgian spatially linked to faults are documented from some areas and boreholes reveal considerable fault controlled thickness changes in hettangian sinemurian bajocian and kimmeridgian sediments the timing of jurassic faulting in the wessex basin mendip area thus polarizes into two intervals hettangian bajocian and latest oxfordian onwards correlating with the early rifting phases of the central and north atlantic respectively

the post variscan development of the british isles within a regional transfer zone influenced by orogenesis abstract the break up of pangaea after the variscan orogeny included rifting extending southwards from the barents sea via the norwegian greenland rift and into the north sea and northwards from the central atlantic these two major rift systems interacted to form an approximately 1200 km wide transfer zone across the british isles where a complex network of basins developed during the mesozoic fault patterns were commonly controlled by reactivation of precambrian caledonian and variscan structures the two main rift systems were unable to breach this regional transfer zone where the crust had been thickened by the caledonian and variscan orogenies until the eocene breaching did not occur down the north sea and through the english channel because of alpine contraction in nw europe instead breaching occurred around the west of ireland and nw scotland so the british isles remained connected to europe rather than to the north american plate

late cretaceous to early tertiary structural elements of west kalimantan abstract recent geological and geophysical mapping in west kalimantan has allowed the definition of three domains with distinct geological and geophysical characteristics which were imprinted on the region by early cretaceous to eocene convergent tectonic events subduction related granitoids intruded low grade regional metamorphic rocks during the early cretaceous forming the schwaner mountains domain extending east west across the southern and central part of west kalimantan this is flanked in the northwest by a complex sequence of sedimentary and volcanic rocks ranging in age from late carboniferous to cretaceous which comprise the northwest kalimantan domain deformed cretaceous flysch deposits east of the northwest kalimantan domain form an accretionary complex representing the southern continuation of the extensive dominantly tertiary accretionary wedge in sarawak the boundary between the cretaceous accretionary domain and rocks of the northwest kalimantan domain is a fault which we interpret as a transform that marked the western limit of late cretaceous southward subduction beneath west kalimantan growth of the accretionary complex resulted in uplift of a ridge of melange and flysch deposits on which a half graben extensional basin formed and accumulated thick lacustrine deposits a sedimentary basin developed between continental rocks to the south and the emergent ridge comprising accretionary complex rocks and outer shelf deposits to the north in the position of a forearc basin as accretion proceeded the locus of underthrusting migrated northward and a second melange ridge and sedimentary basin developed farther to the north the evidence for accretionary processes and the association of these with sedimentary basin formation supports the hypothesis that an ancient south dipping subduction zone existed in the eastern part of west kalimantan during late cretaceous to early tertiary times

sedimentology of the permian radok conglomerate in the beaver lake area of macrobertson land east antarctica the mid to upper permian radok conglomerate the lowermost formation of the permo triassic amery group crops out in the beaver lake area of the northern prince charles mountains east antarctica outcrop is confined to a north south elongate fault bounded corridor interpretedas a remnant of a continental extensional basin formed during late palaeozoic times this basinforms a small part of the much larger lambert graben a major continental rift system the radokconglomerate consists of interbedded conglomerates argillaceous sandstones siltstones and minor thin carbonaceous siltstones and coals textural petrographic palaeocurrent and other data suggestlocal derivation from precambrian massifs to the immediate west during a period of fault activity the unit is a minimum of 400 m thick the base being unexposed and grossly fines upward it isabruptly overlain by quartzo feldspathic sandstone dominated rocks of the upper permian bainmedart coal measures seven recurrent lithofacies have been recognized with the radok conglomerate and are interpreted as the products of poorly confined stream flow sheet flow and sediment gravity flow processes suspension fallout in shallow standing water and organic sediment accumulation in peat forming wetlands the unit as a whole is interpreted as having accumulated as a coarse alluvial apron along the western margin of a graben extensional trough similar though poorly exposed facies are exposed on the eastern margin of the basin and may reflect similar depositional systems towards the top of the radok conglomerate typical radok lithologies are interbedded with quartzo feldspathic sandstones derived from the south precursors of the overlying bainmedart coal measures interference between transverse radok and axial bainmedart drainage is possibly related to progressive infilling of extensional topography thereby allowing axially flowing rivers to avulse increasingly into the beaver lake region from the main lambert graben

the south celtic sea bristol channel basin origin deformation and inversion history abstract the south celtic sea bristol channel basin is a relatively narrow and elongated sedimentary trough located within the northern part of the hercynian fold belt and filled with permo triassic redbeds to jurassic marine sediments its present configuration is the result of a three fold sequence of tectonic phases initiated during the late palaeozoic the origin of the basin appears to be related to localised extensional reactivation of hercynian thrust zones during the permian and early triassic followed by regional subsidence during the late triassic and jurassic uplift of the basin margins together with dextral strike slip movements in an east west sense and large scale erosion during the late jurassic to early cretaceous resulted in the present elongated configuration of the lower mesozoic basin regional subsidence from aptian time onwards resulted in widespread deposition of a relatively thin section of wealden elastics and upper cretaceous chalk unconformably overlying folded pre cretaceous strata sedimentation was interrupted during the early tertiary by a series of tectonic events that included emplacement of the lundy igneous mass during the early eocene and mild basin inversion during the oligo miocene inversion was accompanied by dextral reactivation of the nw se oriented sticklepath fault zone the total amount of uplift probably not exceeding 350 m reflects the rather stable configuration of the underlying lithospheric crust as indicated by seismic reflection and refraction data

fluvial sedimentology and revised stratigraphy of the triassic flagstone bench formation northern prince charles mountains east antarctica the flagstone bench formation ranges in age from earliest triassic to norian late triassic and is exposed in the beaver lake area of the northern prince charles mountains this sandstone dominated formation rests conformably on the bainmedart coal measures and represents the upper part of the permian triassic amery group it is divisible into three members the ritchie jetty and mckelvey members in ascending order nine sedimentary facies assignable to three facies associations major channel crevasse fan and flood basin deposits are recognized within the formation ritchie member sedimentation took place during a transition from consistently hygric to seasonally dry conditions and the unit comprises sandstone dominated sheet like channel deposits interspersed with few thin mottled haematite rich flood basin siltstones deposition took place under fluctuating discharge conditions chiefly within the channel tracts of axially northwesterly northeasterly flowing low sinuosity braided rivers the jetty member shows a gross upward fining profile dominated in the lower part by poorly sorted pebbly sandstones and in the upper part by ferruginous mudcracked siltstones mottled palaeosols calcrete and thin massive sandstone sheets this unit reflects deposition of easterly directed alluvial fans and extensive flood basin silt under a semi arid climatic regime the upper triassic sandstone dominated mckelvey member shows a return to axial drainage along the lambert graben with sedimentation occurring primarily within low sinuosity braided channel tracts under wetter climatic conditions

the association between tectonic deformation inorganic composition and coal rank in the bituminous coals from the south wales coalfield united kingdom analysis of vertical distribution of coal rank in three collieries and one opencast mine in the se of the south wales coalfield has revealed deviations from hilt s law in both proximate analyses and in vitrinite reflectance the variations in rank parameters studied are closely correlated with variations in the intensity of tectonic deformation measured using a numerical deformation index ndi three seams in the sequence were also analysed for major and trace element geochemistry and rock eval pyrolysis variations in these parameters are related to thrust inclusion of floor and or roof mudrocks into the seams a detailed conceptual model of a major thrust and associated in seam thrust detachments and lag faults is developed to explain the vertical rank profile by post coalification faulting it is concluded that although the model can explain the vertical pattern of rank the complexity of the required faults render the model improbable the alternative mechanisms for enhancement of the rank parameters associated with tectonic deformation including shear stress control frictional heating and localised fluid flow are discussed and are thought more likely to be responsible for the deviations from hilt s law in the vertical rank profile rank suppression shown by three seams two of which are overlain by marine bands is thought to be related to biochemical activity in the peat environment resulting from the influx of saline waters

the stratigraphy and sedimentology of the fundy group triassic of the st martins area new brunswick triassic sediments exposed along the south shore of new brunswick near st martins were deposited primarily by a series of alluvial fans prograding eastward from the western margin of the fundy half graben and are interbedded with fluvial conglomerates deposited by an axially flowing river system the sediments are divisible into three formations which are described here formally for the first time these are from base to top the honeycomb point quaco and echo cove formations the basal honeycomb point formation consists of approximately 990 m of red beds exposed in several fault blocks the western outcrops consist of coarse breccias of the proximal fan the browns beach member which fines eastward into sheetflood deposits of the mid to distal fan the mccumber point member both units contain eolian sands that indicate paleowind direction from the northeast these are overlain by fluvial conglomerates of the quaco formation 190 300 m thick which were deposited by a large braided river system

the age origin and tectonic significance of mesozoic sediment filled fissures in the mendip hills sw england implications for extension models and jurassic sea level curves in the eastern mendip hills on the northern margin of the wessex basin sw england the carboniferous limestone is cut by numerous fissures that are filled with mesozoic sediments sedimentary dykes neptunian dykes the fissures contain a record of triassic lower jurassic sediments that are only sparingly preserved in their normal stratigraphical position between the carboniferous limestone and the unconformably overlying upper inferior oolite of bajocian age detailed analysis of cross cutting relationships facies analysis biostratigraphy lithostratigraphy and strontium isotope ages of relevant mesozoic sediments has allowed the construction of an upper triassic lower jurassic fissure fill stratigraphy for the eastern mendip area most fissures were clearly formed by rapid influx of unlithified sediment from the land surface or sea floor some smaller cavities or larger cavities with restricted access to the unconformity were apparently filled by sediment that trickled down into the fissure system the vast majority of the mendip fissures are interpreted as having formed as a response of the carboniferous limestone north of major basin bounding faults to pulses of tectonic extension during ladinian norian rhaetian late hettangian early sinemurian late sinemurian early pliensbachian late pliensbachian and bajocian times triassic earliest jurassic fissures have a broad spread of strike from e w to nw se to n s accommodating extension in a roughly ne sw direction younger jurassic fissures show well defined e w and n s trends with the former becoming dominant through time total extension of 4 7 n s and 0 6 e w was produced by the formation of triassic jurassic fissures within the carboniferous limestone such patterns of extension are thought likely to be characteristic of the subsurface geology in much of southern england and wales major implications of this study are that 1 the presence of seismically unresolvable sediment filled fissures in supposedly rigid fault blocks can lead to a significant underestimate of regional extension based on the restoration of motion on normal faults on seismic reflection profiles and 2 the isolation of pulses of tectonic activity with a temporal resolution of 10 5 10 6 years may provide a means of identifying a tectonic signal in relative sea level curves derived from the jurassic sedimentary record

tributary distributary and other fluvial patterns what really represents the norm in the continental rock record abstract this paper evaluates the recent claim made by weissmann et al 2010 that deposits of distributive fluvial systems dfs may form the bulk of the continental fluvial record weissmann et al 2010 define dfs as a pattern of channel and floodplain deposits that radiate outward from an apex that is located where the river enters the sedimentary basin as such dfs are fan like systems on which multiple channels coexist and distribute water and sediment across the fan published criteria for the recognition of dfs weissmann et al 2010 are 1 a radial pattern of channels from the dfs apex 2 downslope decreases in channel size 3 down dfs grain size decreases and 4 a lack of lateral channel confinement however in weissmann et al 2010 and subsequent papers only the first of these criteria is applied rigorously thus allowing a variety of types of fluvial system including avulsive incised anabranching and coastal plain distributary systems to potentially be classified uncritically as dfs an ancient succession formed by dfs should preserve mostly small channel bodies with a limited range of dimensions and no outsized channel bodies channel bodies in dfs dominated successions should also decrease in size in a down palaeoslope direction and show palaeocurrent relationships indicative of radial dispersal in order for the term dfs to be applied these characteristics should also be established within stratigraphic bodies that formed coevally however very few examples have yet been published that satisfy these criteria if the claim that dfs dominate the alluvial stratigraphic record is valid then few if any sedimentary bodies formed by large channels should be preserved a review of published research demonstrates however that the alluvial stratigraphic record contains a broad diversity of fluvial network styles including abundant representatives of tributary avulsive anabranching and incised in addition to distributary types it is also clear that the deposits of large main stem rivers that are comparable to the big rivers of the modern world are abundantly preserved in the stratigraphic record an analysis of the planimetric area of modern tributary and distributary fluvial systems is presented demonstrating that tributary systems are also likely to represent a far larger proportion of the ancient sedimentary record than dfs the body of evidence presented herein from modern and ancient alluvial systems thus suggests that the hypothesis of weissmann et al 2010 should be wholly rejected dfs do not dominate the continental stratigraphic record

early triassic early olenekian life in the interior of east gondwana mixed marine terrestrial biota from the kockatea shale western australia abstract a new terrestrial marine assemblage from the lower beds of a thin outcrop section of the kockatea shale in the northern perth basin western australia contains a range of fossil groups most of which are rare or poorly known from the lower triassic of the region to date the collection includes spinose acritarchs organic cemented agglutinated foraminifera lingulids minute bivalves and gastropods ammonoids spinicaudatans insects austriocaridid crustaceans actinopterygians a temnospondyl like mandible plant remains and spores and pollen of these groups the insects crustaceans and macroplant remains are recorded for the first time from this unit palynomorphs permit correlation to nearby sections where conodonts indicate an early olenekian smithian age the locality likely represents the margin of an early triassic shallow interior sea with variable estuarine like water conditions at the southwestern end of an elongate embayment within the east gondwana interior rift sag system preserved along the western australian margin monospecific spinose acritarch assemblages intertwined with amorphous organic matter may represent phytoplankton blooms that accumulated as mats and suggest potentially eutrophic surface waters the assemblage represents a mixure of marine and terrestrial taxa suggesting variations in water conditions or that fresh brackish water and terrestrial organisms were transported from adjacent biotopes some of the lower dark shaly beds are dominated by spinicaudatans likely indicating periods when the depositional water body was ephemeral isolated or subjected to other difficult environmental conditions the biota of the kockatea shale is insufficiently known to estimate biotic diversity and relationships of individual taxa to their permian progenitors and triassic successors but provides a glimpse into a coastal zone from the interior of eastern gondwana specialist collecting is needed to clarify the taxonomy of many groups and comparisons to other lower triassic sites are required to provide insights into the pattern of biotic decline and recovery at the end permian crisis

primary versus diagenetic origin of blue lias rhythms dorset uk evidence from sulphur geochemistry the question of a primary versus diagenetic origin for the limestone shale rhythms of the blue lias has been addressed through a study of pyrite abundance and isotopic composition pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent less calcareous sediment the abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation the isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate part of which was subsequently reduced back to pyrite before preservation by burial acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution hence supersaturation of sulphides could be maintained by contrast in adjacent less calcareous sediments carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron rich pore solutions continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and with burial below the zone of bioturbation the alkalinity so generated caused cementation of the limestones diagenetic cementation would be enhanced during an hiatus in sedimentation an event which might be related to a milankovitch forcing mechanism but which would not be recorded in bioturbated less calcareous sediment thus leaving an imperfect record only cyclicity in pre diagenetic sedimentation patterns may be safely related to a milankovitch forcing mechanism as proposed by weedon

the cambrian araba formation in northeastern egypt facies and depositional environments abstract the cambrian araba formation exposed in gabal el zeit and in the sinai peninsula unconformably overlies precambrian basement rocks and is in turn overlain unconformably by the ordovician naqus formation the araba formation has been subdivided into three informal lithologic units lower middle and upper from field observations seven sedimentary facies assignable to three facies associations fluvial tidal and coastal to open marine are recognized within the araba formation the lower unit comprises two main facies matrix supported conglomerate facies a in the south gabal araba and wadi feiran and interbeds of granulestone and sandstone facies b in the north gabal dhalal and taba deposited in fluvial conditions the middle unit includes four facies cross bedded sandstone facies c thin laminated sandstone facies d burrowed massive sandstone facies e and rhythmically bedded sandstone mudstone facies f deposited mainly under tidal conditions in the coastal zone the upper unit consists mainly of interbeds of burrowed mudstone siltstone with skolithos probably deposited in open marine environment the sequence records the southward transgression of the tethys ocean over a horst and graben system developed in the late precambrian on the northern margin of the arabo nubian continent

inferring stress from faulting from early concepts to inverse methods abstract we review the evolution of concepts on and methods of estimating the state of stress from fault movements theories of failure in isotropic materials suggested a simple geometrical construction of optimal principal stress directions from a fault plane and its associated slip these optimal directions align shear stress and slip directions and maximize the difference between shear stress and frictional resistance on the fault plane optimal stress directions for calcite twinning are obtained by a similar construction with the difference that they maximize shear stress force representation of seismic sources independently introduced pressure p and tension t axes at positions that also maximize shear stress on both nodal planes frictional slip theory and the constraint that slip and shear stress directions be parallel allowed to address reactivation of pre existing faults this suggested that stress could also be inverted from reactivated fault and slip data or earthquake focal mechanisms early methods relied on geometrical constructions as a substitute for calculations whereas later methods relied on software as these calculations became tractable with the help of computers similar methods were developed for the inversion of stress from crystal twin gliding with non optimal geometry with a different criterion that relies on a threshold of the component of shear stress along the gliding line even though these methods seek a common stress tensor compatible with fault and slip data their main use is to separate polyphase data into homogeneous subsets and help deciphering complex tectonic histories fault and slip data can also be analyzed to constrain the strain rather than the stress tensor in most cases this involves a summation and yields an average strain for the considered rock volume stress inversion thus appears better suited for differentiating heterogeneous data whereas strain analysis appears better suited for homogenizing them

fluvial conglomerates in continental red beds of the buntsandstein lower triassic in the eifel f r g and their palaeoenvironmental palaeogeographical and palaeotectonic significance abstract fluvial conglomerates occur in a variety of facies within the continental red bed sequence of the buntsandstein lower triassic in the eifel at the western margin of the mid european triassic basin the alluvial rudites are classified into channel floor lag channel bar channel top lag crevasse splay and floodplain lag conglomerates the conglomerates were predominantly deposited in scott type and in subordinate amounts also in donjek type and south sasketchewan type braided rivers the vertical distribution of clast size and frequency within the megacycles reflects the palaeotectonic sequence of uplift events in the source area resulting in supply of large amounts of coarse detritus which cause recurrences in fluvial style gradually upwards diminishing size and abundance of gravel indicate continuous lowering of source area relief during weakening endogenic activity or even tectonic quiescence after final peneplanation the palaeoenvironmental interpretation of the individual members and the palaeotectonic analysis of the conglomerate successions are combined with a reconstruction of the depositional history of the eifel buntsandstein which comprises an evolution of fluvial deposition the terminal stage of the palaeoenvironmental evolution is highlighted by a climax of regional diversification of conglomerate sedimentary milieu the horizontal distribution of clast size and abundance delineates the palaeogeographical evolution of the elongated eifel basin giving evidence of a progressive sidewards extension of the depositional area with time lateral supply from parts of the western margin and mixing of proximally derived coarse clasts with distantly delivered fine detritus result in highly inhomogeneous gravel distributions within the sedimentary region often not being characterized by gradually decreasing size and abundance of gravel in downstream direction the stepwise southwestwards shift of the marginal centre of supply of coarse gravel with time during the course of the lateral extension of the basin results in ascension of the fluvial conglomerate wedges into successively younger formations comprising consecutively middle buntsandstein upper buntsandstein lower muschelkalk middle muschelkalk and finally middle keuper the extraformational conglomerates mainly delineate the palaeogeographical framework whereas the intraclastic rudites primarily give evidence of processes operating within the depositional area reflecting erosion and reworking of overbank sequences with floodplain fines palaeosols and aeolian dune sands during lateral channel shift the brockelbank intraformational carbonate breccias to conglomerates originating from reworking of calcrete palaeosols are a peculiar facies element with particularly high palaeoenvironmental significance they document the frequency spatial extension and temporal persistence of pedogenesis within the alluvial plain the size of the extraformational clasts gives evidence of the range of current velocities during aggradation of the channels the distribution of conglomerate type and clast characteristics within cyclothems indicates the presence of stage fluctuations and variations in discharge in parts of the sequence the preservation of complete cyclothems or the multistoreying of channel bar conglomerates reflects the differential importance of the two main mechanisms controlling accumulation of sediments in the alluvial plain primary depositional restriction to suppression of formation and secondary erosional removal to destruction of finer grained channel sediments floodplain deposits and aeolian dune sands or palaeosols condensation of a series of depositional events within monotonous stacked conglomerate sequences obliterates the full range of sedimentary processes and mimics periods of long term stability of conglomerate formation waning flow mud drapes are particularly significant instruments to split up the individual genetical units of formation of fluvial conglomerates often comprising discrete flood phases separated by stage fluctuations of different magnitude the homogeneous or microcyclic composition of the coarse member reflects uniphase or multiphase aggradation of watercourses with gradually declining or abruptly waning individual flood periods the fluvial rudites thus are of high significance in the reconstruction of the depositional milieu in the eifel buntsandstein and provide a key to detailed interpretation of palaeoenvironmental processes palaeogeographical constellations and palaeotectonic events which could not have been identified to such an extent in exclusively sandy sequences

a high latitude gondwanan lagerstatte the permian permineralised peat biota of the prince charles mountains antarctica abstract the toploje member chert is a roadian to wordian autochthonous parautochthonous silicified peat preserved within the lambert graben east antarctica it preserves a remarkable sample of terrestrial life from high latitude central gondwana prior to the capitanian mass extinction event from both mega and microfossil evidence that includes cryptic components rarely seen in other fossil assemblages the peat layer is dominated by glossopterid and cordaitalean gymnosperms and contains moderately common herbaceous lycophytes together with a broad array of dispersed organs of ferns and other gymnosperms rare arthropod plant and fungal plant interactions are preserved in detail together with a plethora of fungal morphotypes peronosporomycetes arthropod remains and a diverse coprolite assemblage comparisons to other palaeozoic ecosystems show that the macroflora is of low diversity the fungal and invertebrate plant associations demonstrate that a multitude of ecological interactions were well developed by the middle permian in high latitude forest mires that contributed to the dominant coal deposits of the southern hemisphere quantitative analysis of the constituents of the silicified peat and of macerals within adjacent coal seams reveals that whilst silicified peats provide an unparalleled sample of the organisms forming permian coals they do not necessarily reflect the volumetric proportions of constituents within the derived coal the toploje member chert lagerstatte provides a snapshot of a rapidly entombed mire climax ecosystem in the closing stages of the palaeozoic but prior to the onset of the protracted crisis that engulfed and overthrew these ecosystems at the close of the permian

sheet flow deposits of graded beds and mudstones on an alluvial sandflat playa system upper triassic blomidon redbeds st mary s bay nova scotia the 400 m of blomidon redbeds accumulated in a semi arid rift valley in the subtropics at st mary s bay these redbeds are 64 sandy mudstone playa mudflats 25 graded beds sandflats at the toes of alluvial fans 10 fissile claystone playa lakes and 1 channel sandstone stream channels flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north east as sheet flows on sandflats and playa mudflats deceleration of the sheet flows deposited graded beds 2 83 cm in thickness on the sandflats and thin layers of mud on the playas nine sequences consisting of arrangements of six lithologies compose 90 of the graded beds in order of decreasing abundance these are ripple cross laminated siltstone horizontally laminated mudstone fining upward ripple cross laminated siltstone ripple cross laminated sandstone horizontally laminated mudstone cross bedded sandstone horizontally laminated mudstone ripple cross laminated sandstone ripple cross laminated siltstone rippledrift cross laminated siltstone horizontally laminated mudstone fining upward ripple drift cross laminated siltstone cross bedded sandstone ripple cross laminated siltstone and cross bedded sandstone ripple cross laminated siltstone horizontally laminated mudstone the sheet flows perhaps up to 1 m in depth had a high concentration of suspended load deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration there are 32 thinning and fining up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone the cycles range in thickness from 1 3 to 13 3 m averaging 4 6 m each cycle is initiated by avulsion to a new active channel network on a fan gradual abandonment of the channel network produces the thinning and fining up cycle the cycles are grouped in three 60 70 m fining up megacycles upwards within each megacycle the packages of sandy mudstone compose a progressively larger proportion of the cycles each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor subsequent erosion gradually lowered the relief to yield a fining up megacycle