probable burial diagenetic and provenance effects on dakota group clay mineralogy denver basin abstract illite smectites in cored dakota group cretaceous shales and bentonites of the northern denver basin identified using a modification of reynolds and hower s 1970 x ray diffraction criteria show decreasing sample sample expandability ranges lower modal expandability and a change from random to ordered mixed layering with increasing burial depths and temperatures these are results characteristic of burial diagenesis as described by perry and hower 1970 ordered illite smectites from low temperature wells and much of the kaolinite chlorite vermiculite and discrete illite from all wells are interpreted as detrital episodic but frequent admixture of highly expandable bentonitic illite smectite with detrital clays is reflected by fluctuating expandabilities within individual cores and the lack of samples containing both highly expandable illite smectite and high proportions of other clay minerals differential diagenesis and or differential sedimentation rates do not satisfactorily account for lower modal expandabilities and increased proportions of ordered illite smectite observed in shales from d and j sand stones compared with those from huntsman and skull creek shales of equivalent temperatures instead they are attributed to provenance differences which gave lower expandability illite smectites and a head start on diagenetic trends to sand associated shales eastern and western provenances identified from heavy mineral studies e g mackenzie and poole 1962 for d and j sands were not matched by similar clay mineral provenances affecting shales however decreasing proportions of kaolinite chlorite vermiculite encountered from northeastern to southeastern d locales are interpreted as the results of differing sources

chemical models of vitrinite reflectance evolution a global kinetic model of vitrinite reflectance evolution is developed to illustrate a new approach to vitrinite reflectance modelling the method utilizes quantitative pyrolysisgas chromatographic data from isolated vitrinite kerogens to derive the concentrations of structurally specific moieties alkylphenol precursors in vitrinite kerogens as a function of rank this data set is used to calibrate a gaussian distributed activation energy model for phenol precursor loss from vitrinites using published burial histories phenol yield is then correlated with vitrinite reflectance the predictive vitrinite reflectance model based on this phenol system is tested against a well from the us gulf coast area the results being compared with a lopatin model of the same well this type of vitrinite reflectance model is numerically compatible with current generation kinetic models of hydrocarbon production from source rock kerogens and has a better heating rate independence than lopatin s method

relationship between thermal maturation gradients geothermal gradients and estimates of the thickness of the eroded foreland section southern alberta plains canada abstract analysis of post laramide tertiary uplift and erosion in the southern alberta plains indicates that the greatest erosion occurred in regions where the youngest stratigraphic units are preserved this is consistent with geological structure and the few preserved stratigraphic constraints on erosional history throughout this region erosion is pervasive and the uplift history is commonly constrained by coalification data the analysis of coalification erosion and paleogeothermics are interrelated and knowledge of them is important to understand hydrocarbon generation and tectonic history several independent attempts to analyse coalification erosion and paleogeothermics in this area have given different results the most striking differences are those between estimates derived from the analysis of well profile coalification gradient and those determined using a commonly accepted relationship between the amount of eroded section and the equilibrium moisture content of near surface coals the match between present and paleogeothermal gradient fields is better when the latter technique is used correlation is expected between the late orogenic and present geothermal gradient fields because the hydrodynamically controlled advective heat flow pattern that now dominates geothermal gradients in the mesozoic and tertiary foreland succession was initiated by the growth of topography in the west throughout cordilleran orogeny well profile coalification gradients have significant though commonly unacknowledged uncertainties these uncertainties are propagated in erosion and paleogeothermal gradient calculations ideally the relationship between erosion estimates using well profile coalification gradients and near surface coal properties can be compared unfortunately the uncertainties in well profile coalification gradients prevent this

coalification in the northern wildhorn nappe and adjacent units western switzerland implications for tectonic burial histories abstract vitrinite reflectance of fourteen eocene coal samples from the northern wildhorn nappe in the kander valley have been analyzed and compared to nine agewise similar coals from adjacent tectonic units the degree of coalification decreases regularly from semi anthracite in the kander valley to lignite in the plateau molasse the degree of coalification in the tertiary coals appears to be solely dependent on their tectonic history and indicates and estimated 6 km of burial by higher pennine nappes for the kander valley the rank at this locality low volatile bituminous to semi anthracite is significantly higher than in the border chain at beatenberg high volatile a bituminous the latter locality therefore was never buried deeper than about 4 km a further decrease in rank is found in the subalpine flysch and molasse high volatile b bituminous which in turn is significantly more mature than lignites from the folded plateau molasse at linden two samples of dogger coal from the pennine prealpes medianes are medium to low volatile bituminous that is intermediate between the kander valley and beatenberg their rank however was achieved before their final emplacement north of the wildhorn nappe by comparing calculated tti time temperature indices values for time depth burial curves with measured vitrinite reflectances maximum burial temperatures were estimated for the various tectonic units for the wildhorn nappe 160 180 c were obtained in the kander valley and 100 130 c in the border chain as a result of tectonic burial due to the thrusting by the pennine prealps sedimentary burial led to temperatures of 100 120 c in the subalpine molasse below the border chain prior to its thrusting

prominent magsat anomalies over india abstract the present study confirms the well known pogo and magsat anomalies showing a broad magnetic high on the indian shield and a broad low over the himalayas a characteristic difference in magnetic signatures over the peshawar himalaya a broad high the punjab himalaya a broad low and the kumaun himalaya north of sarda depression a sharp and deeper low indicates the existence of different blocks in the himalayas the short wavelength low over the rifted region of the narmada son lineament can be interpreted as a reflection of an ancient suture zone interestingly the surface bouguer gravity picture shows an inverse correlation with magsat anomalies over peninsular india and a positive correlation over the himalayan region quantitative model estimates from the observed satellite magnetic and surface gravity data have revealed compositional changes in the peninsular india and thickening of magnetic crust in the himalayas

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

the genesis of refractory melts in the formation of oceanic crust refractory primary liquids arising in various oceanic plate tectonic settings are characterized by high mgo sio2 ca na low tio2 and generally low incompatible element abundances relative to primary liquids parental to morb we propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to morb a compositional range in the second stage melts is expected depending on the extent of previous depletion of the peridotite the temperature and pressure of melt segregation and the possible influence of volatile phases c h o present during melting

thermal diagenesis in the swiss molasse basin implications for oil generation clay mineral transformations during burial are indicators of the degree of diagenesis of sediments diagenetic zonations in numerous wells of the swiss molasse basin are defined by the disappearance of smectite and or the appearance of either corrensite or irregular 2 1 mixed layers the maximum level attained in the thickest molasse sections corresponds to organic matter maturation suitable for hydrocarbon generation reconstructed on the basis of the clay mineral transformation data a burial history of the basin is proposed

contrasting thermal evolution of thrust systems an analytical and modeling approach in the front of the western alps in foreland fold and thrust belts the study of the maturity of the organic matter and kinematic and thermal modeling techniques is useful in unraveling the thermal history and maturation of source rocks the case study presented here is the classical example the western alps this work evidences that contrasting thermal evolutions can develop in relatively closely adjacent areas of the same thrust belt analytical results show that the mesozoic rocks have undergone a wide range of temperatures up to 250c locally the higher temperature values are associated with the tectonic emplacement of early widespread allochthonous nappes that were emplaced obliquely in the northern part of the area prior to the thrust tectonics of the subalpine chains folding and offsets of the isomaturity surfaces are observed which implies that in some parts of the thrust belt the thermal peak was reached before fold and thrust deformation erosion rates vary greatly along the thrust front extensive erosion probably more than 4 km 2 5 mi of sediments from the tertiary and the prealpine nappes occurred in the north of the area studied along the bornes transect away from the area of influence of the prealpine nappes numerical modeling shows differences in the timing of maturation of the source rocks according to the area considered maturation was attained either early before thrust tectonics most of the vercors area or during the deposition of the syntectonic flexural series preceding thrust tectonics molasse basin or else during thrust tectonics most of the chartreuse area

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

vitrinite reflectance and shear induced graphitization in orogenic belts a case study from the kandersteg area helvetic alps switzerland vitrinite reflectance vr is an important indicator of incipient metamorphism and it is generally assumed that temperature and to a lesser degree time are the two main variables the role of stress in vr is controversial and will be addressed in this study in the kandersteg area north of lake oeschinen four different helvetic tectonic units are present comprising mainly limestones marls and greywackes of mesozoic and lower tertiary age vr rr rmax rmin and illite crystallinity ic were determined from a very steep section at elevations between 1700 and 2900 m a s l in general rr rmax and ic increase from tectonically higher to lower units i e from upper diagenetic 2 3 to 4 5 rr to upper anchizonal conditions 3 4 to 5 6 rmax several samples contain in addition to coaly fragments abundant semigraphitized and graphitized particles that are believed to be of authigenic origin transitional matter showsrmax values ranging from 7 to about 11 and rmin of 1 3 3 4 optical graphite showsrmax of about 15 16 and rmin of 1 6 1 8 there is good textural evidence that tectonic deformation has played a major role in the development of these strongly coalified particles the first tiny needle like crystals of graphitic material occur within narrow shear zones transecting the rock matrix the beginning of graphite formation parallels the onset of plastic deformation and dynamic recrystallization of quartz elevated vitrinite reflectance rmax and bireflectance values are found near thrust planes where simple optical observation on quartz grain morphology indicates an increase in strain intensity it appears that frictional heating associated with thin shear zones may have been responsible for the formation of authigenic graphite in the kandersteg samples

early proterozoic basaltic andesite tuff breaccia downslope subaqueous mass transport of phreatomagmatically generated tephra at bear lake in the flin flon snow lake greenstone belt of manitoba 400 m of thick to very thick bedded generally ungraded basaltic andesite tuff breccia breccia and lapilli tuff are intercalated with pillowed lava flows in the upper part of an early proterozoic submarine basaltic andesite shield volcano the fragmental rocks comprise angular amygdaloidal blocks and lapilli many with partial chilled selvages in a matrix of blocky non amygdaloidal to highly amygdaloidal vitric basaltic andesite ash and small lapilli minor thin to medium bedded commonly normally graded tuff occurs in the upper part of the sequence clasts in fragmental beds consistently have higher amygdule contents than intercalated lava flows although similar to pillow fragment breccias the bear lake fragmental rocks were produced by extended surtseyan type phreatomagmatic eruptions with associated fire fountain activity at a progressively subsiding shallow water vent periodic tephra slumping generated debris flows that transported particles down the uppe gentle slope of the volcano to a depositional site at a water depth of less than 1 km turbidity currents probably carried much fine tephra to deeper water tuff was deposited in the preserved section only after explosive volcanism ceased

the original swiss flysch a reappraisal of the type deposits in the swiss prealps abstract the flysch sequences of the swiss prealps alpine front ranges are described and interpreted in the light of a decade of study by a group research project at fribourg the geographical and geological settings are illustrated the stratigraphy and sedimentology the sedimentary petrography and the geodynamic setting are discussed correlations within and outside the prealps as well as outstanding problems are treated the flysch units and sequences covered are the gurnigel flysch the medianes flysch helminthoid flysch fouyet manche and mocausa flysch hundsruck flysch niesen flysch and the ultrahelvetic flysch the data from clast framework grain and heavy mineral analysis from micropalaeontology from facies analysis palaeocurrent measurements and stratigraphy allow the interpretation of sedimentary environments source terranes and depositional models in all cases flysch deposits are deep marine detrital accumulations and source areas were undergoing active tectonics individual models are based on the deep sea fan concept but interpretations tend to show significant modifications with respect to simple fan characteristics these departures are to some extent due to available grain size to active structuring of basins or to minor controls such as eustatic variations or climatic influence the integration of sedimentological data with stratigraphy and regional structure furnishes palinspastic reconstructions in map view these provide insights on the palaeogeography of the western alps from mid cretaceous to oligocene and give a picture of the evolution of the western mediterranean tethys during the same period the relationship between flysch deposition and alpine mountain building is confirmed while constraints on basin evolution are established

geothermal gradients in the hinton area of west central alberta temperature data from petroleum exploration well logs of 3360 wells in a region of west central alberta are used to estimate thermal gradients a relatively high geothermal gradient 36 c km of oblong shape located near hinton is observed the axis of the anomaly strikes approximately southwest northeast and passes through the miette hot springs area it appears that water is heated at depth in the rocky mountain disturbed region and travels eastward and toward the surface along fault planes

coalification shale diagenesis and thermal modelling in the alpine foreland basin the western molasse basin switzerland france abstract the analysis of coalification and shale diagenesis in oligocene and lower miocene sediments in the western part of the molasse basin western switzerland and haute savoie france shows a close relationship between the very low temperature metamorphism in the foreland and the tectonic history in the orogenic belt the degree of thermal maturation generally increases towards the alpine front isomaturity lines however run slightly oblique to today s alpine front this is probably due to the tectonic burying under the alpine nappes in early miocene time second order variations of the coalification pattern are explained as resulting from a regional discharge of warm fluids controlled by tectonic structures

warm fluid migration along tectonic melanges in the kodiak accretionary complex alaska analyses of methane water fluid inclusions from syntectonic quartz veins in melange boudins of the kodiak accretionary complex alaska indicate fluid temperatures ranging from 215 to 290 c at depths between 10 and 14 km these fluid temperatures are significantly higher than those predicted by conductive heat flow models of subduction zones given the available geological constraints this discrepancy is interpreted as the result of migration of warm fluids from deeper structural levels along active fault zones during subduction related underthrusting of these units this interpretation requires more careful consideration of how migrating fluids affect the general thermal structure of subduction zones

geothermal gradients hydrodynamics and hydrocarbon occurrences alberta canada using coal rank data and the demonstrated genetic link between the present hydrodynamic regime and geothermal gradient pattern it is possible to reconstruct the geothermal history of alberta over the past 60 m y palinspastic adjustment for tectonic compression caused by the major laramide thrusting shows that the predeformation isoreflectance lines increased logarithmically with depth in the late paleocene the geothermal gradient was about 23 c km 1 25 f 100 ft in the eastern alberta plains compared to about 30 c km 1 65 f 100 ft in the western alberta plains a regional trend opposite to the post laramide trends reconstruction of the early eocene surface indicates western uplands with geothermal gradients as low as 21 c km 1 15 f 00 ft and eastern lowlands with geothermal gradients of 27 c km 1 5 f 100 ft compared to the present situation this represents an enhanced topographic surface and a subdued geothermal gradient pattern the genetic relations of topography water table elevation hydrodynamic regime and geothermal gradient pattern in both the early eocene and the present conform to a model developed for any compacted sedimentary basin with subaerial relief in this model on a regional scale high topographic areas have high water table elevations with correspondingly high potentiometric surfaces these areas control the regional recharge of cold meteoric water and hence have low geothermal gradients areas of medium elevation exhibit regional lateral flow and intermediate geothermal gradients regional topographic lows correspond with low potentiometric surfaces and the regional discharge of warm formation waters from deep in the basin results in high geothermal gradients local topographic featur s cause perturbations in this picture study of both the regional pattern and fine detail of the early eocene and present topographic surfaces and geothermal gradient patterns shows the validity of the model the result of the second laramide orogenic pulse was therefore to reverse the geothermal gradient pattern resulting mainly from compaction flow and to impose a topographically controlled hydrodynamic regime with flow in the same direction but with a 9 c km 0 5 f 100 ft decreased geothermal gradient in western alberta and a 4 c km 0 2 f 100 ft increased geothermal gradient in eastern alberta as erosion proceeded during the tertiary the entire alberta basin warmed up until the present temperature regime was attained study of some major hydrocarbon occurrences shows a genetic link between the hydrocarbon occurrence and the present fluid regime which itself is controlled by the present topography for at least the significant upper cretaceous shallow gas reserves in southeastern alberta however their position may already have been determined as early as the beginning of the eocene by the topographic surface at that time therefore the extent to which the present topographic surface is responsible for hydrocarbon accumulation through gravity induced cross formational flow remains unknown because this process was operative as long ago as the early eocene what does not ap ear to be in doubt is the fact that gravity induced cross formational flow does control the hydrodynamic regime which in turn influences the geothermal gradient pattern and the accumulation of hydrocarbons

linking the northern alps with their foreland the latest exhumation history resolved by low temperature thermochronology the evolution of the central alpine deformation front subalpine molasse and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the alps its latest exhumation history is poorly understood due to the lack of late miocene to pliocene sediments we constrain the late miocene to pliocene history of this transitional zone with apatite fission track and u th he data we used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data two investigated sections across tectonic slices show that the subalpine molasse was tectonically active after the onset of folding of the jura mountains this is much younger than hitherto assumed thrusting occurred at 10 8 6 5 ma and potentially thereafter this is contemporaneous with reported exhumation of the external crystalline massifs in the central alps the jura mountains and the subalpine molasse used the same detachments as the external crystalline massifs and are therefore kinematically coupled estimates on the amount of shortening and thrust displacement corroborate this idea we argue that the tectonic signal is related to active shortening during the late stage of orogenesis

archean mafic metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 1 mobility of the major elements reply in a suite of archean mafic pillows from the rouyn noranda region of quebec s abitibi greenstone belt including both tholeiitic and calc alkaline varieties spanning the prehnite pumpellyite to upper greenschist metamorphic facies three types of alteration can be defined i chlorite epidote actinolite ii chlorite epidote and iii chlorite sericite the number of mineral phases decreases as a result of progressive hydration from type i to type iii alteration albitization resulting from substitution of in calcic plagioclase is highly variable in type i alteration but in types ii and iii the plagioclase is totally albitized and in some cases silicified chloritization is closely linked to increasing hydration and ca leaching with mgo and feo substituting for cao in ferromagnesian minerals calcium was mobilized and carried by solutions as evidenced by the variable concentration of epidote at the margins of pillows this calcium leaching generated an excess of al2o3 with respect to the combine

early geochemical evolution of an oceanic island arc and backarc fiji and the south fiji basin the initial 25 to 35 ma of oceanic arc volcanism in fiji the early arc stage was dominated by eruption of lree depleted low k tholeiitic basalts and andesites the oldest volcanics include the highest hfse group morb like in composition plus high mg basalts and basic andesites with flat ree patterns as low as 4x chondritic concentrations these erupted either within the volcanic arc during its first 5 ma or in an adjacent backarc during its first ma of formation subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust most are intermediate between earlier extremes in hfse contents but at least one volcano produced strongly lree depleted island arc tholeiites in contrast medium k tholeiitic and calc alkaline andesites erupted during the mature arc stage parental basalts were enriched in lree but depleted in hree relative to earlier basalts due to a change in source composition counterflowing morb source asthenosphere within the mantle wedge dominated vol

the geology and geochemistry of an early proterozoic volcanic arc association at cartwright lake lynn lake greenstone belt northwestern manitoba the geology of the cartwright lake area consists of a 2 km thick conformable sequence of early proterozoic supracrustal rocks intruded by calc alkaline granitoid plutons the supracrustal successi

a review of the timing of coalification in the light of coal seam erosion clastic dykes and coal clasts abstract coal seam erosion clastic dykes penetrating coal seams and studies of coal clasts occurring in late paleozoic basins are used to demonstrate that coalification to the rank of subbituminous and bituminous coal occurred rapidly at shallow depth in some parts of the variscan orogen implying a precondition of high paleotemperatures for the coal basins the origin of clastic dykes particularly in the bolsovian westphalian c deposits of the plze and kladno rakovnik continental basins czech republic is shown to be a relevant factor in determining the timing of coalification pesek j 1978 erosion and clastic dikes in coal seams of the central bohemian basins and their significance for determination of plant substance coalification folia musei rerum naturalium bohemiae occidentalis geologica 12 1 34 is of the opinion that the 50 100 cm deep fractures filled with superposed clastics originated in fully ranked high volatile bituminous coal exposed during a hiatus between the early westphalian c early bolsovian and late westphalian d late asturian these dykes have either irregular shape or form conic bodies with sharp boundaries their filling does not show any signs of postsedimentary deformation coal clasts occurring in clastic sediments either close to or far above the roof of coal seams are a few millimetres up to several centimetres in size being subangular or angular however there also exist clasts of which one side is angular to subangular whereas the opposite side is markedly sharp in some basins coal clasts are embedded in units of the same age they are mostly of somewhat lower rank by 0 01 0 03 than the nearest coal seam whereas gayer r a pesek j s korova i valterova p 1996 coal clasts in the upper westphalian sequence of the south wales coal basin implication for the timing of maturation and fracture permeability in gayer r harris j eds coalbed methane and coal geology geological society special publication 109 pp 103 130 presumed in the south wales foreland basin a burial level of about 1 km with an elevated geothermal gradient 60 c km enhanced by hot fluids along thrusts resulting in rapid coalification to the bituminous coal rank no deep burial of organic matter could have occurred in the intramontane basins of the bohemian massif coalification of the late paleozoic seams must have proceeded at a very high rate the only logical reason leading to such rapid coalification of biomass could have been high value of geothermal gradient at least in the basins of the bohemian massif