clinoform deposition across a boundary between orogenic front and foredeep an example from the lower cretaceous in arctic alaska the lower cretaceous fortress mountain formation occupies a spatial and temporal niche between syntectonic deposits at the brooks range orogenic front and post tectonic strata in the colville foreland basin the formation includes basin floor fan marine slope and fan delta facies that define a clinoform depositional profile texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge front that included older turbidites and melange these new interpretations based entirely on outcrop study suggest that the fortress mountain formation spans the boundary between orogenic wedge and foredeep with proximal strata onlapping the tectonic wedge front and distal strata downlapping the floor of the foreland basin our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load induced flexural subsidence of the foreland basin

carbonate platform evolution from a bioconstructed platform margin to a sand shoal system devonian guilin south china the depositional organization and architecture of the middle late devonian yangdi rimmed carbonate platform margin in the guilin area of south china were related to oblique extensional faulting in a strike slip setting the platform margin shows two main stages of construction in the late givetian to frasnian with a bioconstructed margin evolving into a sand shoal system in the late givetian the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies mostly renalcis and epiphyton these grew upwards and produced an aggradational locally slightly retrogradational architecture with steep foreslope clinoforms three depositional sequences s3 s5 are recognized in the upper givetian strata which are dominated by extensive microbialites metre scale depositional cyclicity occurs in most facies associations except in the platform margin buildups and upper foreslope facies in the latest givetian at the top of sequence s5 relative platform uplift subaerial exposure and associated rapid basin subsidence probably a block tilting effect caused large scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups subsequently sand shoals and banks composed of ooids and peloids and a little later stromatoporoid buildups on the palaeohighs developed along the platform margin from which abundant loose sediment was transported downslope to form gravity flow deposits another strong tectonic episode caused further platform collapse in the early frasnian at the top of sequence s6 leading to large scale breccia release and the death of the stromatoporoid buildups siliceous facies banded cherts and siliceous shales were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids the platform margin sand shoal bank system possibly with gullies on the slope persisted into the latest frasnian until the restoration of microbial buildups four sequences s6 s9 characterized by abundant sand shoal deposits on the margin and gravity flow and hemipelagic deposits on the slope are distinguished in the frasnian strata smaller scale depositional cyclicity is evident in all facies associations across the platform slope basin transect the distinctive depositional architecture and evolution of this yangdi platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy environmental factors oceanographic setting biotic and sedimentary fabrics

structural geological and metamorphic significance of the kaapvaal craton limpopo belt contact abstract the hout river shear zone forms the terrane boundary between the granite greenstone terrane of the kaapvaal craton cratonic terrane and the high grade granulite terrane high grade gneiss province of the limpopo belt this structure is superimposed onto a thin skinned northward verging thrust system which exercises a significant control on the distribution of the ene wsw trending greenstone belts the hout river shear zone however dips steeply to the north and flattens out toward the south this structure was responsible for the emplacement of hot granulites against and over the cooler granite greenstone terrane and strongly influenced the metamorphic evolution of the area the shear zone acted as a conduit for infiltrating co2 rich h2o bearing fluids into still hot crust producing a retrograde isograd and associated zone of rehydration in the granulitic hanging wall of the structure retrogressed granulites in the hanging wall are characterized by fine grained kyanite as part of the retrograde assemblages while the greenstones in the footwall are characterized by coarse grained prograde kyanite the terrane boundary appears to be in thermal equlibrium with the lithologies in both the footwall and hanging wall so that tectonically mixed prograde greenstones and retrogressed granulites in and adjacent to the shear zone are apparently at a similar metamorphic grade the granulites appear to have had a hot iron effect on the cooler footwall rocks over which they have been displaced

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

mesoarchean ttg magmatism from the northeastern margin of the kaapvaal craton southern africa arguments for an exotic terrane remnant of pilbara craton abstract high grade terranes along archean cratonic margins either represent remobilized equivalents of the adjacent low grade domain or an exotic entity which accreted to the developing craton this study addresses this debate for the southern marginal zone high grade terrane along the northeastern margin of the kaapvaal craton the hout river shear zone separates the high grade terrane from the adjacent low grade pietersburg block of the kaapvaal craton tonalite trondhjemites of the baviaanskloof gneiss unit constitute the dominant rock in the southern marginal zone both pyroxene bearing and pyroxene absent variants occur locally the granitoids are affected by anatectic and metasomatic overprints magmatic textures are better preserved in the massive granitoids u pb zircon analyses of two orthopyroxene bearing granitoids gave mesoarchean crystallization ages 3088 11 ma massive 3121 9 ma banded however overprint ages differ with 2706 50 34 ma from the massive variety and 1925 19 ma 1976 11 ma from the banded variety the syn tectonic 2 71 ga is interpreted as metamorphic overprint age while the post tectonic 2 ga represent age of local overprint available geochemical data on the baviaanskloof granitoids are similar to archean ttgs a possible spatial temporal transition from 3 13 ga low pressure through 3 12 2 98 ga medium pressure to 2 92 ga high pressure ttg is delineated from the northwest towards the southeastern southern marginal zone this trend is compatible with the southward directed polarity of accretion events recognised in the adjacent low grade pietersburg block we argue that the northeastern margin of the kaapvaal craton north of the hout river shear zone is an exotic terrane because 1 the baviaanskloof granitoid event 3 1 2 9 ga with peak at 3 1 ga is older than the accretion episodes 2 97 2 92 ga with peak at 2 95 ga and 2 87 2 84 ga with peak at 2 86 ga in the adjacent pietersburg block 2 the younger 2 86 ga event occurs closer to the hout river shear zone while the older 2 95 ga event was away from it and 3 contemporaneous 3 1 3 0 ga detrital zircons are absent in greenstone belts of the pietersburg block that are proximal to the hout river shear zone possible connection to rocks of similar age from the pilbara craton as part of the vaalbara supercraton is explored

an integrated model for the tectonic development of the frontal brooks range and colville basin 250 km west of the trans alaska crustal transect we present a kinematic model for the sequence of deformation and sedimentation in the frontal brooks range and adjacent colville basin in the etivluk river region 250 km west of the trans alaska crustal transect tact the model is based on a tectonic subsidence analysis of the foreland basin combined with structural stratigraphic and thermal studies of the northern edge of the brooks range thrust belt we interpret six discrete tectonic events that led to the present day configuration of the thrust belt in this area 1 emplacement of ophiolitic allochthons over the distal continental margin rocks in valanginian time hundreds of kilometers south of this study 2 hauterivian uplift of the barrow arch rift margin affecting the northern part of the colville basin 3 barremian contraction involving emplacement of distal continental margin and ophiolitic allochthons onto the endicott mountains allochthon and creation of a southward dipping flexural basin on the north slope autochthon 4 mid cretaceous exhumation of imbricated rocks in the brooks range during northward propagation of the thrust front into the foreland 5 minor thrusting in late cretaceous paleocene in the northern foreland to the northern limit of contractional structures and 6 regional exhumation of the orogen and the foreland in paleocene eocene time this sequence of deformation agrees well with a simple model of a forward propagating thrust system

tectonic control on peritidal carbonate parasequence formation an investigation using forward tectono stratigraphic modelling studies of quaternary extensional faults indicate that they have instantaneous amounts of throw 0 4 to 4 m average slip rates 0 05 to 2 8 m kyr 1 and frequency of recurrence 40 000 years accounting for the accommodation space required for the accumulation of peritidal carbonate parasequences pcps hangingwall sites and graben are characterized by fault down dropping together with regional subsidence and footwall sites and horsts by fault related uplift alternating with periods of regional subsidence the relative sea level curves generated by these processes operating in a maritime rift setting are used as inputs to a forward stratigraphic modelling program sedtec2000 to simulate how fault related changes in accommodation space can account for high frequency pcp formation each instantaneous fault slip generates a flooding surface or aggradation in hangingwall and graben settings high frequency cycles in hangingwall sites are either symmetric deepening then shallowing upward or asymmetric shallowing upward the major factor controlling cycle types is the balance between rates of carbonate accumulation and generation of accomodation space high frequency cycles in footwall sites and horsts comprise shallow subtidal facies with no distinctive bathymetric trends capped by erosional boundaries generated by footwall uplift the modelled cycles are of the same thickness with bathymetric trends and frequency to cycles commonly interpreted to be due to orbitally driven eustatic sea level changes or autocyclic processes these numerical experiments demonstrate that high frequency pcps can be generated by tectonic fault related processes a hypothesis that is frequently discounted

architecture and depositional sequences of tertiary fault block carbonate platforms an analysis from outcrop miocene gulf of suez and computer modelling abstract this study combines outcrop with seismic data and tectonostratigraphic modelling to characterise the stratigraphy of tertiary fault block carbonate platforms seismic sections from the middle east and southeast asia indicate that carbonate platforms commonly develop on fault blocks in the late syn rift to post rift stage of basin evolution they are characterised by retrogradational or drowning morphologies with deep water clastics evaporites sealing the platforms a new tectono stratigraphic modelling program simulates the development of carbonate platforms on domino style fault blocks and predicts a distinctive tectonic control on the evolving syn rift stratigraphy detailed field work from a three dimensionally exposed miocene syn rift platform from the gulf of suez gebel abu shaar egypt shows the existence of unconformity bound depositional sequences which are predicted by the program and are interpreted to have formed in response to fault block rotation the sequences are characterised by synchronous hangingwall subsidence and footwall uplift and erosion the seismic images the modelling and the outcrop study provide different levels of information on fault block platforms and their characteristic stratigraphy the scale of the modelling is of particular value as it provides a link between the seismic scale imaging and outcrop data

tectonic controls on the development of unconformities the north slope alaska the geology of northern alaska is defined by two complementary synchronous tectonic events opening of the canada basin breached the south facing ellesmerian passive margin in the late jurassic cutting off northerly derived sediment and initiating thermal subsidence of the north facing beaufort passive margin uplift of the brooks range on thrust faults loaded the lithosphere creating the colville trough a foreland basin more than 10 km deep and providing the bulk of the sediments that filled it the northerly advance of the brooks range caused the two independent loads to flexurally interfere the beaufort passive margin is separated from the foreland by the barrow arch which is in part the flexural bulge due to the interaction of these two loading events nunn et al 1987 a record of this interaction is preserved in the sediments north and south of the barrow arch we have used this stratigraphy coupled with simple basin modeling to consider the flexural and stratigraphic effects of the evolving thrust wedge and basin filling using a simple sedimentation model and loading on an elastic lithosphere we have produced two unconformity bounded sequences without sea level change the unconformities are produced as a consequence of sedimentation and two increments in the thrust load that cause the flexural bulge to translate laterally rise and fall the generation of unconformities over a flexural bulge by mass redistribution on a purely elastic lithosphere calls into question the use of these unconformities in other yoked systems to document viscoelastic lithospheric rheologies even if the lithospheric strength decays significantly on geologic time scales we may not observe this decay in stratigraphy if the basin filling time is a significant fraction of the relaxation time the beaufort passive margin and the thrust belt strike at an oblique angle to each other causing the independent bulges to constructively interfere in the west and destructively interfere in the east the advance of the thrust related bulge beyond the eastern beaufort shelf has by flattening the basal decollement exerted significant control on the thrust wedge of the brooks range allowing the thrust wedge tip to advance beyond the coastline

episodicity of mesozoic terrane accretion along the pacific margin of gondwana implications for superplume plate interactions a review of evidence for deformation and terrane accretion on the late triassic early jurassic margins of pangea and the mid cretaceous margins of the palaeo pacific ocean shows that deformation was global and synchronous with probable superplume events late triassic early jurassic deformation appears to be concentrated in the period 202 197 ma and was coeval with eruption of the central atlantic magmatic province onset of pangaea break up a period of extended normal magnetic polarity and a major mass extinction event all possible expressions of a superplume event mid cretaceous deformation occurred in two brief periods the first from approximately 116 to 110 ma in the west palaeo pacific and the second from roughly 105 to 99 ma in the east palaeo pacific with both events possibly represented in northeast siberia this deformation was coeval with eruption of major oceanic plateaus core complex formation and rifting of new zealand from gondwana the cretaceous normal polarity epoch and a major radiation of flowering plants and several animal groups all linked with the mid cretaceous superplume event a simple unifying mechanism is presented suggesting that large continental or oceanic plates when impacted by a superplume tend to break up reorganize associated with gravitational spreading away from a broad thermally generated topographic high and with a resulting short lived pulse of plate marginal deformation and terrane accretion

the cretaceous tertiary boundary in the central alberta foothills i stratigraphy the cretaceous tertiary boundary in the rocky mountain foothills occurs within a continental sequence carrying economic coal deposits and known as the coalspur formation this formation occurs between the fluvial brazeau below and paskapoo formations the exact position of the boundary is located at the base of the lowest coal mynheer within the coalspur formation the initiation of coal deposition in the coalspur formation is not a unique event as it is consistent with the cyclic pattern established within the underlying brazeau formation however the magnitude and lateral continuity of coal zone development far exceed those found in the underlying formation the classic shift from a diverse late maastrichtian flora within which angiosperm and especially aquiloid pollen are prominent to a low diversity early paleocene flora dominated by gymnosperm pollen and miospores is documented deviations from most previously recognized floral events at the cretaceous tertiary boundary include 1 san angio

hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the limpopo belt south africa

chronology of ophiolite crystallization detachment and deeper parts of subduction zones emplacement evidence from the brooks range alaska sup 40 ar sup 39 ar data from early middle jurassic ophiolites 187 184 ma in the western brooks range alaska indicate that detachment related metamorphism occurred approximately 20 m y after crystallization and approximately 20 m y before emplacement onto the arctic alaska margin high temperature metamorphic rocks along the basal surfaces of many ophiolites have ages that are contemporaneous with ophiolite crystallization suggesting that detachment and thrust faulting occur while the lithosphere is young lt 10 ma and relatively hot from these relations it has been generally assumed that detachment and initial overthrusting of oceanic lithosphere occur near the site of generation such as a marginal basin or mid ocean ridge the new data from the brooks range ophiolites confirm previous indications that some ophiolites have much longer intervals between crystallization thrust related metamorphism and emplacement on the basis of these new results and data from other ophiolites the authors propose that ophiolites originating within large ocean basins will generally have longer crystallization to emplacement intervals than the more commonly recognized marginal basin type ophiolites

systems tracts and depositional sequences in a carbonate setting a study of continuous outcrops from platform to basin at the scale of seismic lines abstract the application of sequence stratigraphy concepts to carbonates is feasible and this paper presents an example of its successful application the continuity and lateral extent of outcropping sedimentary sections observed at the scale of a seismic line from platform to basin in the southern vercors plateau western alps france confirm the validity of sequence stratigraphy concepts when applied to carbonate depositional systems from the stratal patterns displayed in outcrop we determined that carbonate lowstand systems tracts are major components in the basin even in the deep pelagic realm transgressive and highstand systems tracts are very thin a response to starvation in contrast on the shelf transgressive and highstand systems tracts are the major components carbonate lowstand and shelf margin systems tracts pinch out in the vicinity of the offlap break it clearly appears that the stratal patterns and stratal termination patterns of carbonate systems tracts especially of carbonate lowstand systems tracts are basically similar to those of siliciclastic systems tracts this study documents that carbonate platforms may provide the basin with carbonate clastics during periods of relative sea level falls and lowstand this is one of the most controversial aspects of the application of sequence stratigraphy concepts in carbonate environments a second controversial aspect of the application of sequence stratigraphy concepts to carbonate rocks is the distinction between transgressive and regressive facies cycles and depositional sequences and systems tracts this paper demonstrates that the shoreline position for carbonates which indicates the facies transgression or regression is independent of prograding or retrograding shelf breaks the stratal pattern and facies within systems tracts vary depending on the overall evolution of the accommodation space i e on their position within the transgressive and regressive cycles

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

evolution of the northern high grade margin of the kaapvaal craton south africa the northern kaapvaal craton is divided into three distinctive crustal zones 1 the high grade southern marginal zone smz of the limpopo belt in the north 2 a transitional zone of technically stacked metamorphic rocks which includes the area from the sutherland belt in the north to the pietersburg and murchison belts in the south and 3 a low grade granite greenstone terrane farther to the south the change across the smz boundary is expressed by an overall increase in the grade of metamorphism a strip of retrogression related to a zone of thrusting abrupt changes in intensity of deformation rock densities magnetic lineament patterns and isotopic ages in compositionally similar rocks regional metamorphism in the northern kaapvaal craton is demonstrably not a continuum in the sense that the highest grade rocks underwent all earlier stages of metamorphism rather that rapid local tectonic processes must have occurred a proposed evolutionary path for the smz was initiated with crustal thicke

development and controlling factors of miocene carbonate platform in the nam con son basin southwestern south china sea miocene carbonate platforms are located on intrabasinal structural highs and along the southern margin of the nam con son basin also referred to as the wanan basin by chinese southwestern south china sea these isolated platforms together cover a gross area of over 25 000 km 2 we map the platforms reconstruct their evolution and evaluate the controls on platform growth and demise within the regional geological setting using available seismic and well data the platforms initiated in the late early miocene and reached their widest extent during the middle miocene the platform margins were controlled by synsedimentary faults associated with carbonate slope failure deposits and or reef talus the platforms formed as isolated platform platform growth history involves six phases start up second start up keep up exposure renewal and drowning phases quantitative analysis of the basin evolution indicates that tectonism eustasy the supply of both terrigenous sediment and nutrients were the dominant controls on platform development and demise c 2013 elsevier ltd all rights reserved

early tertiary marine fossils from northern alaska implications for arctic ocean paleogeography and faunal evolution marine mollusks and ostracodes indicate a post danian paleocene to early eocene thanetian to ypresian age for a fauna from the prince creek formation at ocean point northern alaska that also contains genera characteristic of the cretaceous and neogene quaternary the life association of heterochronous taxa at ocean point resulted from an unusual paleogeographic setting the nearly complete isolation of the arctic ocean from about the end of the cretaceous until sometime in the eocene in which relict cretaceous taxa survived into tertiary time while endemic taxa evolved in situ these later migrated to the northern mid latitudes paleobiogeographic affinities of the ocean point association with mild temperate faunas of the london basin england denmark and northern germany indicate that a shallow intermittent paleocene seaway extended through the norwegian greenland sea to the north sea basin early tertiary arctic ocean paleogeography deduced from faunal evidence agrees with that inferred from plate tectonic reconstructions

multistory duplexes with forward dipping roofs north central brooks range alaska the endicott mountains allochthon has been thrust far northward over the north slope parautochthon in the northern brooks range progressively younger units are exposed northward within the allochthon to the south the incompetent hunt fork shale has thickened internally by asymmetric folds and thrust faults northward the competent kanayut conglomerate forms a duplex between a floor thrust in hunt fork and a roof thrust in the kayak shale to the north the competent lisburne group forms a duplex between a floor thrust in kayak and a roof thrust in the siksikpuk formation both duplexes formed from north vergent detachment folds whose steep limbs were later truncated by south dipping thrust faults that only locally breach immediately overlying roof thrusts within the parautochthon the kayak lisburne and siksikpuk equivalent echooka formation form a duplex identical to that in the allochthon this duplex is succeeded abruptly northward by detachment folds in lisburne these folds are parasitic to an anticlinorium interpreted to reflect a fault bend folded horse in north slope basement with a roof thrust in kayak and a floor thrust at depth these structures constitute two northward tapered internally deformed wedges that are juxtaposed at the base of the allochthon within each wedge competent units have been shortened independently between detachments located mainly in incompetent units the basal detachment of each wedge cuts upsection forward northward to define a wedge geometry within which units dip regionally forward these dips reflect forward decrease in internal structural thickening by forward vergent folds and hindward dipping thrust faults

an unusual poriferan community in the albian of islares north spain palaeoenvironmental and tectonic implications abstract the discovery of a well preserved lower cretaceous siliceous sponge community in islares cantabria north spain is documented lower albian sponges crop out in great abundance as a rich necrocoenosis embedded in a marly matrix demosponges make up the bulk of an 11 m thick succession which is interpreted as having formed in a slope setting the sponges show body preservation and were buried in situ all types of growth forms are present they grew so tightly that they often fused secondarily shape polarity suggests steady low energy predominantly easterly palaeocurrents the main factor limiting the community seems to have been space competition a preliminary documentation of the systematic palaeontology is presented only taxa from the families tetracladinidae megamorinidae and corallistidae were found geometrical restoration of the platform basin transect allows estimation of a depth of approximately 40 m for the lithistid sponges sea level variations driven by block faulting controlled the available accommodation space sponges occur just below and above a sequence boundary and are associated with a major tectonic event linked with the opening of the bay of biscay

carbonate platform growth influenced by contemporaneous basaltic intrusion albian of larrano spain carbonate platform growth in active tectonic settings may be strongly influenced by the structural evolution of the basin including volcanic activity in this paper the sedimentary tectonic evolution of the duranguesado carbonate platform larrano in northern spain is described and an evolutionary depositional model is presented the albian duranguesado carbonate platform deposits are dominated by rudist and coral limestones with small intervening argillaceous limestone filled troughs 10 30 m deep the platform succession is divided into two parts the lower platform deposits were intruded by volcanics and tilted before the deposition of the upper platform succession a volcanic vent plug filling an upward flaring pipe occurs in the lower carbonate platform succession the timing of intrusion is well constrained to the early albian tectonic extension and active deep seated strike slip faults induced magma ascent at a fault intersection sedimentological analysis of these areas indicates that before volcanic intrusion they acted as weakened zones of extension and were slightly more subsident basinal areas these sites contain anomalous accumulations of siliceous sponge spicule deposits linked to the release of hydrothermal fluids on the sea floor after the phase of intrusion tilting and erosion of the platform and its associated volcanic products occurred as the intrusion cooled the platform downwarped leading to the formation of an overlying perched capping basin bordered by carbonate platforms over the volcanic plug carbonate mounds grew on local palaeohighs the presence of strike slip and magma emplacement in the duranguesado platform is related to movements between the iberian and european lithospheric plates which were accommodated by rotation and lateral movement of the crust along wrench faults

soil clay mineralogy as a key to understanding planation and formation of fluvial terraces in the south african lowveld abstract clay mineralogy of the fluvial plains reflects the complexity of geological setting material weathering and transport clay minerals produced in the weathering process are closely associated with the parent rock and serve as an indicator of the material sources soil clay mineralogy of south african river valleys is insufficiently studied and this research can make a meaningful contribution towards advancing the understanding the history of planation surfaces in africa and their evolution here we examine the valley of the letaba river one of the many limpopo tributaries originating in the drakensberg escarpment and flowing through the basement complex of the lowveld geomorphic province nine soil profiles were selected from a total of 37 profiles represented by ustifluvents haplustepts and haplustalfs for mineralogical study of the the upstream section of the study area is dominated by kaolinite associated with the general dominance of granite and granitic gneiss lithology in the upper reaches of the river the occurrence of mafic dolerite dikes along the river course immediately shows in soils as presence of smectite trail in the clay fraction as the river cuts through the greenstone belt the clay mineralogy of the fluvial terraces immediately reflects this change as an increase in talc and chlorite proportions and the appearance of vermiculite the traces of talc originating from greenstones are present throughout the valley within the middle reaches of the river and may be showing the contribution of small streams and tributaries transporting the material from the water divide to the valley of the letaba the changes in soil clay mineralogy of alluvial soils across the lithological boundaries indicate the prevalence of locally sources material over long distance transported sediment in aggradation of these terraces the analysis of clay mineralogy in fluvial environments may be a good indicator of local sediment source and short distance transport contribution towards formation of fluvial terraces in the valley of the letaba river system this contradicts the pediplane theory of lowveld planation and emphasizes the role of localized erosion sedimentation processes

new evidence for the age of the gubik formation alaskan north slope abstract at several alaskan north slope localities south of the shore of the arctic ocean the gubik formation herein regarded as latest pliocene and pleistocene in age contains a marine unit at its base near ocean point and near teshekpuk lake this basal unit or the lowest exposed marine unit of the gubik contains unusual relatively warm water marine mammals although these mammals have poorly known fossil histories consideration of what is known suggests that the basal marine unit near ocean point is of latest pliocene age between 2 2 and 1 7 my old and that the marine unit near teshekpuk lake is probably late pleistocene most likely correlating with the sangamon interglaciation and about 120 000 yr old

a dominant tectonic signal in high frequency peritidal carbonate cycles a regional analysis of liassic platforms from western tethys abstract meter scale peritidal carbonate cycles are a common feature of the geological record but debate continues about what processes lead to their formation three conceptual models or a combination thereof are commonly invoked to explain cycle formation eustasy tectonics or autocyclicity these three models are tested with a large new dataset from different early jurassic plate margins from western tethys study of seven logged sections from spain italy greece tunisia morocco and gibraltar enables an analysis of the possible roles of local versus regional patterns and controls on cyclicity within a sinemurian time slice cycle types are diverse and include shallowing upward cycles parasequences but also deepening upward and diagenetic cycles high frequency sequences and subtidal cycles numbers of cycles per section and cycle stacking patterns within this time slice vary from section to section statistical tests runs tests time series and bundling all indicate random stacking of cycles within sections and an absence of any bundling of thicknesses or of facies trends assessment of cycle types by their occurrence and stacking patterns indicates little support for either eustasy or autocyclicity being the dominant cycle forming mechanism however the variability in numbers of cycles per section thickness variations of the sections cycle type variability and randomness of stacking patterns all favor a pulsed tectonic control for the creation and filling of accommodation space this conclusion is further supported by evidence that has largely arisen during the course of this study of syndepositional extensional tectonics in the sinemurian on these rifted tethyan margins although tectonics appears to be the dominant control superimposed eustasy and or autocyclic processes cannot be discounted

growth of syndepositional faults in carbonate strata upper permian capitan platform new mexico usa abstract the upper permian capitan platform new mexico is cut by syndepositional faults and fractures these are parallel to platform margin and cluster within strata that steepen and thicken abruptly into the basin faults are steep dip slip have measurable displacements of up to 24 m are typically associated with growth strata and mostly tip out blindly within the platform basinward throwing faults are typically reverse and are steeper than shelfward throwing faults which have normal geometry the sense of displacement across some initially basinward throwing faults changed as they developed these patterns indicate that faults were rotated as they grew synkinematic rotation of faults is interpreted to have occurred due to syndepositional compaction driven down to basin tilting of the capitan platform faults grew through interaction with inherited fractures and by segment linkage vertically separated fault segments had a tendency to grow downward to link with underlying segments thus promoting accumulation of displacement across faults without vertical propagation of fault tips four fault types are distinguished on the basis of structures associated with their upper terminations i faults breaking free surface ii faults tipping out within non folded growth strata iii faults tipping out below growth monoclines and iv buried faults with no expression on free surface fault tips evolved from one type into another in response to variations in the rates of fault growth and deposition fault tip structures also vary laterally over distance of 750 1400 m inferably as a result of variable distances from fault centres

depositional architecture of a rimmed carbonate platform albian gorbea western pyrenees a lower cretaceous carbonate platform depositional system with a rimmed margin and an adjacent oversteepened slope was analysed in order to determine its depositional architecture and major depositional controls the platform is made up of coral rudist orbitolinid and algal limestones and in a 12 km dip transect there is a gradation from lagoon to platform margin slope and basin environments each characterized by distinctive sedimentological features and facies associations the rimmed platform is an aggradational system developed during approximately 4 2 million years of fluctuating relative sea level rise and it is bounded by unconformities at its base and top internal cyclicity in the construction of the system is evident mainly in platform interior and slope settings the seven recognized sequences average 0 6 million years in duration and are related to minor relative sea level changes carbonate deposition occurred in shallow and deep water settings during periods of high relative sea level reduced rates of sea level rise led to the development of shallowing upward sequences and eventually to the exposure of the shallowest parts of the platform during relative sea level falls during low relative sea level erosion surfaces developed on the slope and gravitational resedimentation occurred at the toe of slope basinwards resedimented units pinch out over distances of a few hundred metres active faults controlled sedimentation at the platform margin promoting the development of steep slopes up to 35 and preventing progradation of the shallow water platform despite high sediment production rates the development of sequences is interpreted to be related to tectonic activity

constraints on the structure of brooks range and colville basin northern alaska from flexure and gravity analysis tectonic thickening of the crust within the adjacent brooks range fold thrust mountain belt has been suggested as a probable cause for rapid subsidence of the colville basin in the early cretaceous a simple mechanical model in which the response of the lithosphere to loading was approximated by a semi infinite broken elastic plate overlying a weak fluid was used to analyze deflection of the basement beneath the colville basin comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the lithosphere during mountain building cannot produce a load sufficient to create the observed basin geometry and observed topography regardless of the position of the slab end or flexural rigidity assumed for the lithosphere an additional subsurface load capable of producing a deflection of the foreland basin roughly equal to the depression caused by the topographic load of the brooks range is required bouguer gravity data from the region are characterized by an asymmetric gravity low which spans the brooks range and colville basin and a gravity high to the south gravity modeling is consistent with a subsurface load with a mass of approximately 2 5 1013 g cm located beneath the southern brooks and the yukon koyukuk province to the south the origin of the subsurface load is interpreted as obduction of crustal blocks onto an old rifted continental margin during attempted continental a type subduction the colville basin was also influenced by the contemporaneous development of the northern arctic continental margin loading by subsurface and topographic loads to the south and loading on the continental margin to the north resulted in the formation of the barrow arch just offshore of northern alaska a best fitting value for mechanical thickness of the lithosphere beneath the colville basin is approximately 65 km

palynological and iridium anomalies at cretaceous tertiary boundary south central saskatchewan the cretaceous tertiary boundary in south central saskatchewan is marked by coincident anomalies in abundance of iridium and fern spores at the extinction level of a suite of cretaceous pollen taxa evidence of disruption of the terrestrial flora includes the fern spore abundance anomaly and local extinction of as much as 30 percent of angiosperm species the reorganized earliest tertiary flora is made up largely of surviving species that assumed new roles of dominance persistence of climatically sensitive taxa across the boundary indicates that if paleoclimate was altered by the terminal cretaceous event it returned quickly to the pre event condition

structures in granitoid gneisses and associated migmatites close to the granulite boundary of the limpopo belt south africa abstract river platform exposures of quartzofeldspathic gneiss and associated migmatites occur close to the boundary of the southern marginal zone of the limpopo belt with the adjacent low grade granite greenstone terrane of the kaapvaal craton the complex gneisses and migmatites are described and are considered to represent the tectono thermally reworked equivalents of the pietersburg sutherland greenstone belts and early tonalitic and trondhjemitic intrusive granitoids structural features associated with the hout river shear zone and the limpopo orogeny 2700 2650 ma old and which include subhorizontal overthrust fault slices produced as a result of lateral extrusion of rock material following transpressional displacement along a strike slip fault or thrust zone support structural metamorphic and geophysical evidence suggesting southwestwards overthrusting of the high grade southern marginal zone rocks onto lower grade rocks of the kaapvaal craton

the deep structure of the limpopo belt from geophysical studies abstract geophysical studies in the limpopo belt and adjoining cratons contributed to the fundamental understanding of the deep structure in these tectonic provinces the seismic and geoelectrical data sets prove that the terrane boundaries of the limpopo belt dip in towards the belt to give it the appearance of a pop up structure the gravity data show that high grade metamorphic rock with densities that are high compared to the normal low grade upper crustal granitoids form the upper crust in the northern and southern marginal zones of the limpopo belt these rocks also show a low magnetic susceptibility the geophysical data clearly defined the position of the southern boundary of the limpopo belt the northern boundary of the central zone can be related to a southward dipping zone of seismic reflectors while the southern boundary of this terrane as formed by the palala shear zone seems to be near vertical the greenstone belts of the craton have a limited depth extent and are allochtonous the present position of these structures could be related to thin skinned tectonics that affected the upper crust on the kaapvaal craton and which were revealed by seismic reflection data

recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework sequence stratigraphy has been combined with total organic carbon toc analysis from well logs core and cuttings to develop a model of toc accumulation in marine source rocks routine well profiling of toc in marine source rocks has revealed that recurrent patterns in the vertical distribution of toc occur that can be explained with sequence stratigraphic concepts the model assumes that the principle controls on organic carbon accumulation are 1 the presence of low oxygen conditions at the sediment water interface and 2 sedimentation rate under these circumstances toc will vary inversely with terrigenous clastic influx if accommodation is high clastic influx is low in distal locations and the resulting weight percent toc in basinal sediments is increased th maximum toc in a vertical profile through a single marine sequence probably correlates to the maximum flooding surface toc decreases above this surface due to increasing sediment dilution during highstand progradation and decreases below it with the higher sedimentation rate of the older transgressive systems tract commensurate with toc increase towards the maximum flooding surface the organic matter type becomes more marine and therefore increasingly oil prone several marine source rock examples are presented as well as a lacustrine example which also conforms to the marine model

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

geochronology of the western and central brooks range alaska implications for the geologic evolution of the anarraaq and red dog zn pb ag deposits a compilation of published geochronology of rocks and minerals from the western and central brooks range provides a framework for understanding the complex history of the brooks range and northern alaska a simplified timeline of events comprises 1 devonian extension 2 mississippian extension and zn pb ag mineralization 3 a passive interval 4 pre brooks range orogeny rock formation and thermal event 5 inception of brooks range orogeny 6 exhumation and the end of main stage deformation and 7 subsequent episodic deformation this compilation is supplemented by new 40ar 39ar dates of white mica from the anarraaq and red dog zn pb ag barite deposits from the western brooks range the deposits are hosted in black shale and carbonate rocks of the late mississippian early pennsylvanian kuna formation quartz pyrite white mica grains in sedimentary rocks above the anarraaq deposit yield an age of 195 0 2 0 ma and paragenetically late quartz pyrite white mica from the main orebody at the red dog deposit has an age of 126 1 0 7 ma these white micas are much younger than the age of zn pb ag mineralization at red dog 338 5 8 ma re os age of pyrite the date for white mica from anarraaq 195 ma appears to be related to a large scale thermal event in the region immediately before the inception of the brooks range orogeny the white mica from the red dog deposit 126 ma correlates with the later stages of the orogeny a period of blueschist metamorphism extension and rapid exhumation which varied with geographic location these dates suggest that the red dog deposits underwent significant hydrothermal overprinting during multiple episodes of the brooks range orogeny

a review of the geochronological constraints on the evolution of the kaapvaal craton south africa abstract all the published u pb geochronological data from zircon titanite sphene and monazite and pb pb evaporation data applicable to the palaeoarchaean to neoarchaean geology of the kaapvaal craton are employed to reconstruct the history and crustal architecture of the ca 3600 2500 ma cratonic basement in south africa only data interpreted as representative of the age of crystallisation or the 207 pb 206 pb ages from zircon xenocrysts have been considered in constructing graphs of cumulative probability versus age of rock formation in this manner it has been possible to identify the main magmatic events that have contributed to the assembly of the craton for convenience of data handling the kaapvaal craton has been subdivided into eastern central northern and western geographic domains these domains are not intended as discrete geological terranes although each one has its own geological and chronological character although consanguinity of events is also evident the oldest rocks so far recognised are located in the swaziland barberton areas where ages 3600 ma have been recorded the early stages of shield development are best exposed in the barberton mountain land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks at barberton several diachronous blocks formed between 3600 and 3200 ma have been identified each of which represents a cycle of arc related magmatism and sedimentation this phase of crustal development was followed by a period of mesoarchaean cratonic magmatism particularly prevalent between approximately 3100 3000 ma and marked by the development of a major crescent shaped juvenile arc that was accreted onto the northern and western margins of the evolving kaapvaal shield cratonisation was accomplished by the emplacement of major granitoid batholiths which thickened and stabilised the continental crust during the early stages of this cycle subsequent evolution of the craton between 3000 and 2700 ma was associated with continent arc collision during which time the witwatersrand basin and its correlatives were deposited as foreland sequences followed by episodic extension and rifting when the gaborone kanye and ventersdorp sequences were developed

tertiary syntectonic carbonate platform development in indonesia cenozoic tropical carbonate sedimentation was strongly influenced by local and regional tectonics in se asia this paper outlines the evolution of the syntectonic eocene to middle miocene tonasa formation of south sulawesi evaluating controls on sedimentation facies distribution and sequence development development of a facies model for this cenozoic tropical carbonate platform provides a meaningful analogue for similar less well studied se asian carbonates which commonly comprise targets for hydrocarbon exploration this study also has considerable implications for the study of syntectonic carbonates controls on carbonate sedimentation carbonate platform development in backarc areas and se asian tectonics detailed facies mapping logging petrographic and biostratigraphic analyses indicate that the tonasa formation was deposited initially as part of a transgressive sequence in a backarc setting by late eocene times shallow water carbonates were being deposited over much of south sulawesi forming a widespread 100 km long platform area shallow water sedimentation continued unabated in some areas of the platform until the middle miocene elsewhere active normal faulting resulted in fault block platforms with local subaerial exposure of footwall blocks and the formation of basinal graben in adjacent hangingwall areas platform top facies were aggradational and dominated by larger benthic foraminifera low angle slopes particularly hangingwall dip slopes were characterized by the development of ramps faults controlled in part by pre existing structures were periodically active and formed steep escarpment margins variable regional subsidence strongly influenced the development of the tonasa carbonate platform whereas platform wide effects caused by regional eustacy have not been identified computer modelling of the tonasa platform confirms that the accommodation space and sedimentary geometries observed can be produced by block faulting and regional subsidence alone modelling also reveals that regional subsidence and extension oblique to the main stretching direction were low on the margins of the backarc basin shallow water accumulation rates for this foraminifera dominated tropical carbonate platform were an order of magnitude lower than those for modern warm water platforms dominated by corals or ooids

structural analysis and implicit 3d modelling of high grade host rocks to the venetia kimberlite diatremes central zone limpopo belt south africa abstract the beit bridge complex of the central zone cz of the limpopo belt hosts the 519 6 ma venetia kimberlite diatremes deformed shelf or platform type supracrustal sequences include the mount dowe malala drift and gumbu groups comprising quartzofeldspathic units biotite bearing gneiss quartzite metapelite metacalcsilicate and ortho and para amphibolite previous studies define tectonometamorphic events at 3 3 3 1 ga 2 7 2 5 ga and 2 04 ga detailed structural mapping over 10 years highlights four deformation events at venetia rules based implicit 3d modelling in leapfrog geo provides an unprecedented insight into cz ductile deformation and sheath folding d 1 juxtaposed gneisses against metasediments d 2 produced a pervasive axial planar foliation s 2 to isoclinal f 2 folds sheared lithological contacts and s 2 were refolded into regional open predominantly southward verging e w trending f 3 folds intrusion of a hornblendite protolith occurred at high angles to incipient s 2 constrictional prolate d 4 shows moderately ne plunging azimuths defined by elongated hornblendite lenses andalusite crystals in metapelite crenulations in fuchsitic quartzite and sheath folding d 4 overlaps with a 1 2 03 2 01 ga regional m 3 metamorphic overprint b transpressional deformation at 2 2 1 9 ga and c 2 03 ga transpressional dextral shearing and thrusting around the cz and d formation of the avoca bellavue and baklykraal sheath folds and parallel lineations

gravity signature of the limpopo kaapvaal fossil plate boundary in southern africa discussion discussion sur l article nld de emenike e a paru dans tectonophysics 1986 128 127 137

late mesozoic cenozoic exhumation history of northern svalbard and its regional significance constraints from apatite fission track analysis the late mesozoic cenozoic was a time of profound tectonic activity in the arctic with incipient spreading in the arctic ocean baffin bay labrador sea and north atlantic as well as the northward movement of the greenland microplate leading to collision and deformation in greenland arctic canada and svalbard eurekan orogeny it is however still unclear how northern svalbard situated at the northwestern edge of the barents shelf was affected by these processes furthermore northern svalbard has been proposed to have been a cretaceous cenozoic sediment source to surrounding regions because it lacks a post devonian sedimentary cover when erosion took place and how that related to the tectonic history of the arctic is yet unresolved in order to reconstruct the erosion history of northern svalbard we constrained its thermal evolution using apatite fission track aft thermochronology our data reveal aft ages between 62 5 and 214 10 ma recording late mesozoic early paleogene exhumation our data show that northern svalbard was emergent and experienced erosion from the early jurassic and presumably through the cenozoic although total exhumation was restricted to 6 km pronounced exhumation took place during jurassic cretaceous time probably linked to the extensional tectonics during the opening of the amerasian basin arctic ocean in contrast cenozoic ocean basin formation and the eurekan deformation did not cause significant erosion of northern svalbard nonetheless aft data show that late cretaceous early paleocene fault related exhumation affected some parts of northern svalbard fault zones were reactivated due to the reorganization of arctic landmasses during an early phase of the eurekan deformation which implies that this episode commenced 20 m y earlier in svalbard than previously understood

diagenesis and fracturing of a large scale syntectonic carbonate platform abstract the influence of coeval tectonics on carbonate platform development is widely documented yet the diagenesis of such syntectonic platforms is barely evaluated an outcrop petrographic and geochemical study details here for the first time the diagenesis of the tonasa limestone formation developed in an extensional regime in central indonesia this equatorial carbonate system was affected by block faulting tilt block rotation differential uplift and subsidence throughout its eocene to early miocene history wilson 1999 wilson et al 2000 the tonasa carbonate platform is dominated by alteration in shallow to deeper burial depths by fluids with predominantly marine precursor origins mechanical and chemical compaction features are common as are a range of mainly burial related granular mosaic blocky and equant calcite cements earlier marine cements and meteoric influences are rare being highly localised to block faulted highs and or bathymetrically upstanding platform margin areas early marine micritisation of allochems was common on the platform top tectonic uplift together with a major oceanic throughflow current are thought to be key influences on localised karstification meteoric diagenesis and marine cementation the distribution and orientation of faults fractures and calcite veins together with evidence for their relative timing are the strongest manifestation of tectonism coeval with diagenesis there is concordance in the orientation and timing of structures affecting the tonasa platform with those basin wide with the potential for reactivation of pre existing basement fabrics tectonic subsidence including fault associated differential subsidence controlled the degree of burial diagenesis impacting different areas of the platform a predominance of burial diagenetic features and dearth of earlier marine or meteoric cementation is seen in other tertiary equatorial platforms and is partly attributed to 1 predominance of non framework building larger benthic foraminifera and or algae that are prone to remobilisation have low production rates and limited potential to build to sea level and 2 high runoff due to the equatorial humid climate contributing to lowered marine salinities in se asia underlying tectonic reasons for the preponderance of a regional diagenetic signature over a syntectonic one fracturing excepted are 1 development on the flanks of a backarc basin not on typical continental crust 2 key platform influencing structures are oblique to the main extensional direction in the basin and 3 development in an overall subsiding tectonic regime post dating basin initiation the aim here is that this study will contribute to understanding diagenetic alteration of syntectonic carbonate platforms and those from equatorial regions

carbonate depositional sequence development on active fault blocks the albian in the castro urdiales area northern spain the aptian to lowermost albian carbonate platform of castro urdiales cantabria northern spain was broken up by extensional tectonic movements shortly after the beginning of the albian block faulting characterized this rifting episode the effects of which waned during the albian in crestal locations tilting of the fault blocks caused the subaerial exposure of parts of the older platform resulting in intense karst diagenesis differential subsidence of the blocks controlled the development of a crestal residual carbonate platform arenillas which was surrounded by deeper water on both sides seven unconformities related to platform exposure and karstification are identified on the arenillas platform and form the basal boundaries of seven depositional sequences s1 s7 on the platform lowstand systems tract deposits consist of breccias filling caves and grainstones and debris flow deposits filling incised canyons transgressive plus highstand systems tracts consist of rare marls plus shallow water rudistid coral and chondrodontid limestones in the basin the unconformities are erosional surfaces at the base of resedimented limestones marls or sandy or silty siliciclastics lowstand systems tracts transgressive plus highstand systems tracts in this setting consist of marls and hemipelagic marly limestones tectonism is believed to be the main control on sequence formation and only a few sequence boundaries e g the base of s6 in the upper albian inflatum zone can be correlated with eustatic events

when was the limpopo orogeny abstract the term limpopo orogeny refers to the common high grade metamorphic and tectonic event that affected the archaean rocks of the central and southern marginal zones of the limpopo belt and its influence on the adjacent portions of the kaapvaal craton available geochronological data from muscovite kyanite prograde metamorphic assemblages zircon in igneous charnoenderbite and enderbite muscovite in syntectonic pegmatite bodies formed in shear zones partial melts and syn and post tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from 2700 ma ago to 2650 ma ago the retrograde phase of this orogeny from the onset of the response phase m 2 until the rocks were subjected to rehydration m 3 was very rapid probably lasting 7 ma from 2671 ma to 2664 ma ago similar but limited data from the northern marginal zone and adjacent portions of the zimbabwe craton suggest that rocks there were subjected to a more recent interval of high grade metamorphism and tectonism extending from 2600 ma until 2550 ma

geology and geochemistry of the reocin zinc lead deposit basque cantabrian basin northern spain the reocin zn pb deposit 30 km southwest of santander spain occurs within lower cretaceous dolomitized urgonian limestones on the southern flank of the santillana syncline the reocin deposit is one of the largest known strata bound carbonate hosted zinc lead deposits in europe the total metal endowment of the deposit including past production and remaining reserves is 62 mt of ore grading 8 7 percent zn and 1 0 percent pb the epigenetic mineralization consists of sphalerite and galena with lesser marcasite and trace pyrite with dolomite as gangue microprobe analyses of different generations of dolomite revealed nonstoichiometric compositions with various amounts of iron up to 14 mol of feco3 replacement of host dolomite open space filling of fractures and cementation of breccias derived from dissolution collapse are the principal types of ore occurrence detailed cross section mapping indicates a stratigraphic and structural control on the deposit a stratiform morphology is present in the western part of the orebody capa sur whereas mineralization in the eastern part is highly discordant but strata bound barrendera stratigraphic studies demonstrate that synsedimentary tectonic activity related to the rifting of the north atlantic bay of biscay was responsible for variation in sedimentation presence of unconformities including paleokarsts local platform emergence and dolomitization along the n60 fault trend in the reocin area two stages of dolomitization are recognized the first stage is a pervasive dolomitization of the limestone country rocks that was controlled by faulting and locally affected the upper part of the aptian and the complete albian sequence the second dolomitization event occurred after erosion and was controlled by karstic cavities this later dolomitization was accompanied by ore deposition and locally filling of dolomite sands and clastic sediments in karstic cavities the circulation of hydrothermal fluids responsible for sulfide deposition and the infilling of karst cavities were broadly contemporaneous indicating a post albian age vitrinite reflectance data are consistent with previously measured fluid inclusion temperatures and indicate temperatures of ore deposition that were less than 100 c carbon and oxygen isotopic data from samples of regional limestone host rock dolostone and ore stage dolomite suggest an early hydrothermal alteration of limestone to dolostone this initial dolomitization was followed by a second period of dolomite formation produced by the mixing of basinal metal rich fluids with local modified seawater both dolomitization events occurred under similar conditions from fluids exhibiting characteristics of basinal brines the 34s values of sulfides are between 1 8 and 8 5 per mil which is consistent with thermochemical sulfate reduction involving organic matter as the main source of reduced sulfur galena lead isotope compositions are among the most radiogenic values reported for zn pb occurrences in europe and they are distinct from values reported for galena from other basque cantabrian deposits this suggests that a significant part of the lead was scavenged from the local underlying asturian sediments the stratigraphic and structural setting timing of epigenetic mineralization mineralogy and isotopic geochemistry of sulfide and gangue minerals of the reocin deposit are consistent with the features of most of mississippi valley type ore deposits

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

direct current resistivity and lotem model for the deep structure of the northern edge of the kaapvaal craton south africa abstract the transition from the high grade metamorphic southern marginal zone smz of the limpopo belt to the low grade kaapvaal craton was studied by means of geoelectrical techniques the d c data showed conclusively that the high grade rocks were thrust over the low grade rocks this contradicts earlier geological ideas but agrees with recent seismic reflection data the area where high grade rocks occur above low grade strata was mapped out and the southern boundary of the smz was established the conductor that occurs at lower crustal depths in the kaapvaal craton was traced into the smz using the lotem method the conductor shallows from 30 to 22 km over 12 km on the surface but does not reflect the difference in metamorphic depth of 30 km seen at the surface this indicates that the conductor is a transient feature of the crust the conductor is not associated with a seismically reflective crust the cause of the conductor remains enigmatic

palynology of maastrichtian and paleocene rocks lower colville river region north slope of alaska outcrops of maastrichtian rocks are rare on the north slope of alaska and it is even more unusual to find outcrops of maastrichtian and paleocene age in the same vicinity in general late cretaceous and paleogene rocks have not been well dated in published papers pertaining to northern alaska in this article we describe palynomorph assemblages from 20 outcrop samples taken from nine localities along the lower colville river and nearby areas the latest cretaceous palynomorph assemblages that are from marine rocks contain late campanian or maastrichtian dinoflagellate cysts however these cysts and other marine fossils are absent from the tertiary samples indicating that the tertiary strata are nonmarine our latest cretaceous and early tertiary samples contain abundant spores and pollen grains and can be readily dated by reference to well known pollen assemblages from the northern great plains and northwestern canada by dating each sampled outcrop we can approximate the position of the cretaceous