evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

a detachment fold model for fault zones in the late archean abitibi greenstone belt abstract the abitibi belt is one of the largest and most extensively studied late archean greenstone belts the structural geology of the abitibi belt consists of one generation of upright to slightly overturned doubly plunging first order folds with half wavelengths of 20 60 km and e w striking steeply dipping fault zones that are parallel to the fold limbs two of the main fault zones are continuous for hundreds of kilometers previous tectonic models for the abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic sedimentary basin which were reactivated as thrusts during subsequent crustal shortening other models propose that the fault zones represent tectonic sutures implying that the abitibi belt is a collage of exotic terranes however distinct geological terranes have not been geologically demonstrated we propose a new detachment fold model for the deformational history of the southern abitibi belt in ontario that explains the formation of the fault zones during the single well documented folding event that deformed the entire region the internal structure of the fault zones documented here with emphasis on the porcupine destor fault zone consists of isoclinally folded strongly schistose highly metamorphosed rock cross cut by numerous fault segments we interpret that the upper crust greenstones was folded above a proposed detachment in the lower part of the volcanic stratigraphy the fault zones would be in essence highly evolved detachment anticlines ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines the numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones the detachment fold model is compared to the results of analogue experiments designed to investigate crustal scale folding using viscous and frictional materials detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues the experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern abitibi is mechanically viable

continental shelf topography key to understanding distribution of shelf sand ridge deposits from cretaceous western interior seaway a comparison of holocene shelf sand ridges on the georges bank nova scotia newfoundland labrador shelf system and the texas shelf with cretaceous shelf sand ridge deposits from the western interior seaway provides new insights into the possible role that cretaceous shelf topography may have had in controlling shelf sand ridge distribution as well as providing a possible sand source on an otherwise muddy shelf the holocene ridges are elongate asymmetric in cross section as much as 35 m 115 ft thick up to tens of kilometers in length and are spaced up to 15 km 9 3 mi apart in parallel sets they occur as much as 300 km 186 mi from the present shoreline and in different areas appear to be actively influenced by tidal storm and or oceanic currents the ridges enerally are part of the holocene transgressive sand sheet derived by reworking of underlying substrate while contemporaneous mud is deposited in adjacent low areas all of the holocene ridges are situated atop topographically high banks on the shelf surface the shelves are as much as 400 km 250 mi wide with water depth ranging from 20 to 200 m 66 656 ft often prolific hydrocarbon bearing cretaceous shelf sand ridge deposits e g shannon sussex hygiene viking cardium etc exhibit similar geometries and dimensions to these holocene ridges are generally encased in shale and are thought to have been deposited tens to hundreds of kilometers from the paleoshoreline in water depths comparable to those of the holocene ridges sediments for these sandstones are commonly thought to have been transported long distances across a flat muddy shelf the above comparisons suggest that the cretaceous shelf sand ridges may have been deposited upon topographic highs or banks similar to those upon which holocene ridges are situated such banks could have been developed on the cretaceous shelf surface by 1 recurrent folding or faulting 2 deposition of thick accumulations of sediment e g deltas 3 preservation of paleotopography from unconformities by draping and or compaction and 4 fluvial erosion and sculpturing during lowered sea level a sequence of events is proposed for the evolution of shelf sand ridge deposits that considers the role of shelf banks coupled with fluctuations in water depth in providing the source development and burial of the ridges the initial stage is development of the topographically high bank on the shelf by any of the above processes with shallowing of water sediment is formed on the bank by erosion and is then spread into a sand sheet the sand is then molded into ridges by shelf hydrodynamic processes with deepening of water by sea level rise and or basin subsidence the ridges stabilize then eventually become buried in mud repeated cycles of development of banks and changing water depth can give rise to a series of locally sourced shelf sand ridges associated with shale this s quence of events assumes that erosion and deposition can occur on different parts of a shelf bank that is the same general size as northwestern atlantic shelf banks upon which holocene ridges are situated

depositional and statistical exploration models upper cretaceous offshore sandstone complex sussex member house creek field wyoming the upper cretaceous sussex sandstone reservoir at house creek field powder river basin wyoming is part of an offshore bar complex deposited many tens of miles from the contemporaneous shoreline and a few tens of miles from the shelf slope break deposition occurred in a narrow campanian seaway that extended from the arctic to the gulf of mexico the sandstone complex which overlies a thick progradational sequence of silt and clay may represent a delayed response to events at the shoreline that is sand escaped the littoral energy fence during initial phases of the rapid transgression that deposited the overlying shale and equivalents including the claggett shale of central montana the resulting shoreface retreat left residual sand sheets on the shelf where the sand was further transported structured and shaped into broad elongate sand complexes offshore bars by storm related and other marine currents within the study area at least sand trends are about n40 w a few tens of miles to the southeast the general trends of shelf slope breaks are n s to n45 e the sandstone complex at house creek is divided into six generally gradational facies ranging from sandstone through interbedded sandstone and claystone to silty claystone thickness and lateral facies distribution are asymmetrical and reflect relatively sustained higher depositional energies and a steeper slope along the northeast basinward flank of the complex the shoestring appearance of producing sandstone along the northeast flank does not reflect the overall geometry of the bar complex the complex was an extremely broad low relief feature with a few tens of feet of relief over a distance of tens of miles asymmetry of thickness and facies made possible an exploration development model that uses discriminant analysis with data taken from wireline logs of 78 wells dry holes with sandstone southwest and northeast of the productive trend were successfully reclassified with one exception into their proper categories by a discriminant function an additional model which used producing wells as a third category also yielded excellent results with the same variables discrimination in both models may be accomplished with significantly less than the full complement of variables used the general approach using discriminant function analysis with relatively standard mapping techniques should be considered more frequently by explorationists exploration and development of other sandstones even of the offshore bar type would probably require unique models

timing and kinematics of post timiskaming deformation within the larder lake cadillac deformation zone southwest abitibi greenstone belt ontario canada the larder lake cadillac deformation zone is one of several anastomosing zones of high strain within the abitibi greenstone belt in the kirkland lake area ontario the larder lake cadillac de

silurian k bentonites of the dnestr basin podolia ukraine the dnestr basin of podolia ukraine is an epicratonic basin consisting of neritic carbonate and calcareous mudstone facies including a nearly complete silurian sequence ranging from late llandovery to late pridoli in age the silurian section has served as a standard for regional and interregional studies as a consequence of its well documented macro and microfaunal assemblages approximately 24 mid to late silurian k bentonites are present in this succession and their lateral persistence has aided in establishing regional correlations the k bentonites range from 1 to 40 cm in thickness and occur in the bagovitsa late wenlock malinovtsy ludlow and skala pridoli formations discrimination diagrams based on immobile trace elements together with rare earth element data suggest the k bentonites had a volcanic origin in a collision margin setting related to subduction thickness and stratigraphic distribution considerations are consistent with a source area in the rheic ocean

bedform distribution and inferred sand transport on georges bank united states atlantic continental shelf four bedform provinces have been identified on georges bank using sidescan sonar and echo sounding techniques large sand waves superimposed on sand ridges small sand waves megaripples and featureless seafloor the large sand waves and sand ridges are found on the bank crest where the surface tidal currents are strongest areas of small sand waves and megaripples formed where tidal currents are moderate in strength border the area of large sand waves to the north and south featureless seafloor is found farthest from the bank crest where surface tidal currents are weakest sand wave asymmetry and surface sediment texture have been used to infer bedload transport paths on georges bank in the large sand wave area bedforms indicate a clockwise transport around each of the linear north west striking sand ridges with slight convergence of the sand waves on the ridge crests this transport pattern implies erosion from the troughs and accumulation on the sand ridges the asymmetry of the small sand waves along the south side of georges bank indicates that sand is also transported southward away from the linear sand ridges on top of the bank although the asymmetry of megaripples could not be determined the occurrence of megaripples between the small sand wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest this sand dispersal pattern is further supported by the surface sediments which become progessively finer to the north and sw away from the crest of georges bank

archean terrane docking upper crust collision tectonics abitibi greenstone belt quebec canada abstract the northern nvz and southern volcanic zones svz of the abitibi greenstone belt are separated by the major e trending destor porcupine manneville fault zone dpmfz the dpmfz is interpreted to be the locus of archean terrane docking between the older diffuse volcanic arc of the nvz 2730 2710 ma and the younger arc segments of the svz 2705 2698 ma two distinct evolutionary phases can be documented along the dpmfz of the abitibi greenstone belt and include 1 arc arc collision occurring between 2697 and 2690 ma and 2 arc fragmentation between 2689 and 2680 ma identification of these two events along the dpmfz is based on detailed structural studies sedimentary basin analysis and precise u pb age determinations the thrusting event representative of the arc arc collision phase is characterized by shallow north dipping foliations 20 40 and dip parallel stretching lineations in the eastern manneville segment of the dpmfz local overturned mafic pillowed units suggest recumbent folding late strike slip or transcurrent movement displayed in the late orogenic sedimentary duparquet basin records the arc fragmentation phase basin geometry e trending en echelon folds shallow e plunging stretching lineations and a late ne striking cleavage cross cutting the folds support a dextral shear sense along the western destor porcupine segment of the dpmfz the sedimentary facies observed in the basin are consistent with those of modern strike slip basins located along the east anatolian fault turkey hazar lake and the hope fault new zealand hanmer basin precise u pb zircon age determinations from porphyry stocks located at the northern and southern limits of the duparquet basin yielded 2681 1 ma and 2689 3 2 2 9 ma respectively these ages constrain the rapid change from thrusting to transcurrent movement it is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike slip displacement modern fold and thrust belts commonly show this evolution the deformation pattern is the result of oblique convergence the abitibi greenstone belt is considered to be an archean analogue of modern subducting oceanic plates such as those found in the western pacific

seismic geomorphology architecture and genesis of miocene shelf sand ridges in the pearl river mouth basin northern south china sea abstract multiple stages of large scale shelf sand ridges including the shoreface attached and the offshore types have developed in the miocene successions on the mid shelf region of the pear river mouth basin northern south china sea utilizing a high quality 3d seismic data set accompanying 2d seismic profiles and well logs the morphology architecture and genesis of these shelf sand ridges have been systematically investigated in this study the ridges are of very large scale with the largest one having a maximum height of 64 m a width of more than 20 km and a length of 37 km within the 3d survey area being mound shaped they also display obvious asymmetry character with the ridge crest preferentially located on the se side three main internal components including the ridge front central ridge and the ridge tail have been recognized through careful anatomy analysis of the two most well imaged ridges each displaying distinct expressions on seismic amplitudes and geometries in the plan view most of the shelf sand ridges are generally ne sw oriented and widening to the sw direction scouring features can also be clearly observed along the sw direction including scour depressions and linear sandy remnants on well logs the shelf sand ridges are represented by an overall coarsening upward pattern intervals with blocky sandstones are preferentially present on higher locations due to a differential winnowing process controlled by shelf topography plenty of evidence indicates that these ridges were primarily formed by the reworking of forced regressive or lowstand deltaic deposits under a persistent southwesterly flowing current during the subsequent transgression this very current is a composite one which is speculated to consist of winter oceanic current scsbk south china sea branch of kuroshio intrusion onto the shelf and internal waves propagating from the luzon strait tidal currents might have contributed to the se growth of the ridge in response to the reglaciation of antarctic ice sheet and the closure of pacific indian ocean seaway in the middle miocene the intensification of the north pacific western boundary current was considered to have potential links to the initiation of the shelf sand ridges at 12 ma the development of shelf ridges was terminated and replaced by rapid deltaic progradation at 5 5 ma

kinetics of the smectite to illite transformation in the denver basin clay mineral k ar data and mathematical model results clay mineral data potassium argon k ar ages and mathematical model simulations indicate the important kinetic factors controlling the smectite to illite transformation illitization in upper cretaceous mowry and niobrara core and outcrop bentonites from the denver basin the data also provide additional information on the thermal history of the basin data show that both the percentages of illite layers and the k ar ages of mixed layer illite smectite i s increase with increasing depths of burial and this trend was simulated mathematically employing a very simplified burial model with a fifth order overall kinetic expression for the formation of illite for most of the basin the data is compatible with i s having been formed in response to increased temperature from progressive burial because the oldest k ar ages of i s are from the deepest buried i s along the basin axis in addition these data qualitatively suggest that i s even as small as 0 1 micrometer retains radiogenic argon under denver basin burial temperatures i s separated from drill core bentonites from the wattenberg gas field astride the basin axis and known to have been subjected to an anomalously high temperature history are the most illitic and among the oldest measured from core in this study the ages of illitization about 60 ma are coincident with the laramide time of structural development of the basin and uplift of the front range the reconstructions of the burial history and the modelling of the timing and extent of illitization suggest that the area of the wattenberg gas field was an upper cretaceous depocenter an area of relatively deeper burial along the basin axis however the data can not rule out the possibility that these sediments at and near the wattenberg gas field were heated not only by burial but also by i neous and related hydrothermal activity associated with the colorado mineral belt during the laramide uplift 70 45 ma of the front range likewise the anomalously illitic i s in outcrop bentonites along the front range adjacent to the wattenberg gas field may have been formed in response either to deep burial and or by laramide age secondary heating superimposed on burial

toward a new 250 c pyrrhotite magnetite geothermometer for claystones abstract we investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation immature burial temperature 40 c of bure callovo oxfordian claystones in the basin of paris france early mature burial temperature 85 c of opalinus lower dogger claystones from the mont terri anticline in front of the jura fold belt switzerland and mature to overmature burial temperature 7 s 8 the p transition is also detected in early mature to mature opalinus and chartreuse claystones we conduct additional experimental heating of natural opalinus claystones one set of experiments is referred to as short term heating 1 h from 100 c to 200 c it is dedicated to an investigation of the effect of short lived heating processes in geology a second set of heating experiments is designed to approach burial conditions using a gold capsule in burial like experiments we heated opalinus claystones from 150 c to 250 c for several weeks under a pressure of 100 mpa in both experiments we observe a correlative diminution of the pyrrhotite signature at 35 k with increasing temperature we interpret this trend as the appearance of magnetite we derive a parameter pm from the warming curve of a saturated isothermal remanent magnetization acquired at 10 k zfc we report on a consistent evolution of pm with temperature in the range of 40 c to 250 c including natural samples heated samples at 95 c and burial like heated samples pm first increases between 40 c up to 85 c implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature for temperatures above 85 c pm decreases up to 250 c implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite pm values obtained from mature to overmature claystones from the chartreuse are lower than the pm values obtained from the burial like heated opalinus claystones suggesting that the formation of magnetite is driven by kinetics the continuous trend of the pm parameter suggests that the magnetic properties of pyrrhotite magnetite claystones can be used to infer paleo temperatures and we propose to name this geothermometer mageval

archean geodynamics and the abitibi pontiac collision implications for advection of fluids at transpressive collisional boundaries and the origin of giant quartz vein systems abstract giant quartz vein systems of all ages from the archean to cenozoic are characterised by similarities of geodynamic setting local structure paragenesis metal budget p t t paths and chemical fluid dynamic and isotopic properties that collectively imply a singular hydrothermal process such quartz vein provinces form in regional brittle ductile shear zones that define terrane boundaries including closure of back arc basins and arc continent or continent continent collisions examples include the svz of the archean abitibi belt the norseman wiluna belt yilgarn block kolar schist belt india foothills metamorphic belt california and coast ranges megalineament in the cordillera and the cenozoic monte rosa district insubric line evidence for an archean collisional environment comes from the abitibi and pontiac subprovince tectonic boundary which hosts giant quartz vein systems in this region the amphibolite facies lacorne tectonic block in the archean abitibi greenstone belt is anomalous with respect to the prevalent low grade supracrustal sequences in neighbouring blocks the lacorne block has mature clastic sediments with a zircon provenance age spectrum from 3040 2691 ma and two granitic magma series a late syntectonic monzodiorite monzonite granodiorite syenite series formed over 2685 2670 ma similar to phanerozoic volcanic arc granites and post tectonic garnet muscovite granites emplaced at 2650 2630 ma which compositionally resemble phanerozic collisional s type granites the pontiac subprovince to the south of the abitibi greenstone belt shares all the above features with the lacorne block and collided with and was locally thrust under the abitibi belt with differential uplift of the lacorne block following collision to generate a tectonic window following collision of allochthonous terranes the conjuction of large volumes of subcreted oceanic crust and sediments between accreted terranes displaced isotherms rise and metamorphic dehydration are all necessary conditions for forming giant quartz vein systems metamorphic fluids are expelled along the terrane boundary structures at deep levels and focussed into second and higher order splays at mid crustal levels where quartz and au precipitation occurs hydrothermal fluids that formed the giant quartz veins are remarkably uniform in h o c and sr isotopic compositions albeit with small provincial variations the veins and gold precipitated at 270 360 c from fluids with low salinity and moderate co2 at 2 3 kbar in the brittle ductile transition

autochthonous development of superior province greenstone belts abstract models for superior province greenstone belt development have changed over the last 30 years from ensimatic generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm models intermediate between these end members such as autochthonous terrane models have not been prominent within the superior province there is a secular variation from early ca 3 ga platformal assemblages with overlying rift related komatiite tholeiite units representing breakup of ca 3 ga continental fragments to late ca 2 7 ga pull apart basins autochthonous terranes should demonstrate some sort of logical order such as that described above whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order in the face of this situation this paper reviews evidence from throughout the superior province that suggests many but not all greenstone assemblages developed autochthonously or parautochthonously the earliest greenstone assemblages in the superior province are generally quartzite and carbonate bearing platform sequences in the north caribou terrane and the marmion terrane deposited unconformably upon granitoid or greenstone basement the evidence for autochthonous parautochthonous development includes the presence of unconformities between keewatin type greenstone units such as 1 basal unconformities between ca 3 ga granitoid or volcanic basement and slightly younger platformal assemblages 2 between ca 2 9 ga platformal assemblages and younger platformal and andean arc assemblages stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern central and southern superior province are considered suggestive of orderly autochthonous progression from platforms through rifting of continental fragments and late assembly during the kenoran orogeny petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the superior province suggests the presence of older basement implying autochthonous or parautochthonous development of many assemblages younger greenstone assemblages in the abitibi wawa subprovince and the north caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages this pattern has been interpreted to represent autochthonous greenstone development the current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect few actual unconformities between keewatin type greenstone assemblages have been recognized in the superior province to aid in future recognition of unconformities in keewatin type greenstone assemblages examples of unconformities of varying degrees of subtlety in archean sequences are reviewed archean unconformities ranging from sharp angular discordances in the northern superior province to subtle stratigraphic patterns to weathering related geochemical mobilization along inter formational contacts are described these styles of unconformity in archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between keewatin type assemblages in the superior province generally and the abitibi wawa subprovince specifically the most important disconformity suggested in the abitibi wawa subprovince is between the ca 2726 ma deloro and the ca 2708 ma tisdale assemblages the presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an autochthonous origin for many superior province greenstone belts this autochthonous origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place that is development of ca 3 ga platform sequences in selected locales followed by in place evolution of a succession of plume oceanic and arc assemblages culminating in late pull apart basin sequences implications of the autochthonous model for greenstone belts of the superior province particularly the abitibi wawa subprovince include the future ability to recognize unconformity bounded sequences potentially improving regional scale correlation

ductile structures and instabilities their implication for variscan tectonics in the ardennes abstract increasing consideration of instabilities may explain the development of structures at all scales in naturally deformed rocks this address is a review of geometric scale free concepts applied to the tectonic study of ductile structures the notion of instabilities is first given to introduce non equilibrium dynamics such as strain localisation and single deformation events producing superimposed structures these concepts lead to the idea that orogens are regions of accumulated strain in zones dominated by non coaxial deformation consequently stretching lineations depend on finite strain and do not represent the plate motions repeated phases of deformation represent local perturbations during a kinematic and dynamic event rather than being correlatable as regional phases the fold and thrust belt of the northern variscan belt in the ardennes is taken as a natural example where superposed structures represent the instable response to forces among which plate tectonic forces play a subsidiary role

a storm dominated offshore sandstone interval from the corallian group upper jurassic weald basin southern england abstract the upper jurassic corallian group of the subsurface of the weald basin southern england contains two stacked marine sand bodies they have an elongate geometry 9 25 m thick 100 km long 30 km wide and consist predominantly of cross bedded fine to medium grained glauconitic sandstones with bioturbated low angle cross laminated silty sandstones the sandbodies trend east west subparallel to the palaeoshoreline but oblique with respect to the south easterly palaeocurrent direction in general facies are stacked in progressively upward coarsening sequences with lower clays passing up into bioturbated siltstones followed by various types of cross bedded medium grained sandstones the characteristics and distribution of the lithofacies indicate that these sandbodies are not shoreline deposits however they are strikingly like offshore sand ridges widely documented from the cretaceous western interior seaway of north america the lack of any typical features of tidal sandbodies and the common occurrence of erosively based low angle cross lamination resembling hummocky cross stratification in the lower part of the coarsening upwards sequences indicate that storm currents may have been important in generating the corallian offshore sandbodies

crustal structure and kinematic framework of the northwestern pontiac subprovince quebec an integrated structural and geophysical study structural mapping gravity and magnetic modelling and interpretation of a deep seismic profile in the northwestern pontiac subprovince outline the crustal structure and early structural developme

evolution of the southern abitibi greenstone belt based on u pb geochronology autochthonous volcanic construction followed by plutonism regional deformation and sedimentation abstract new mapping and u pb zircon geochronological results in the southern abitibi greenstone belt sagb support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages rather than the collage of allochthonous terranes proposed in recent publications based on lithological and geochronological criteria these supracrustal assemblages also encompass the swayze shining tree and montcalm greenstone belts indicating that despite separation by large granitic intrusions they are in fact part of one extensive greenstone belt in conjunction with the mapping the geochronological results support coherent upward facing stratigraphic sections with about 20 of the samples containing inherited zircons with ages similar to those found in underlying assemblages the seven oldest assemblages represent semi continuous volcanism from 2750 to 2697 ma the volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc alkaline mafic to felsic lavas intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section regional fault control on the distribution of the volcanic assemblages provides evidence for early dip slip movement associated with volcanic extension dating back to at least 2725 ma the two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults the earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 ma and the youngest one of subaerial conglomerates fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 ma the sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons regional folding and reactivated movement on the regional faults probably related to accretion of the abitibi to the superior province craton autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic komatiitic assemblages and two dominantly calc alkaline assemblages as well as the extensive intermingling of these different magma clans in a number of the assemblages appears to be unique to the archean it suggests complex large scale and long lived interaction between mantle plumes and subduction zone magmas the nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with end 2 5 0 5 demonstrating a lack of contamination by ancient enriched components

paleotectonics and hydrocarbon accumulation powder river basin wyoming the belle fourche arch a subtle northeast trending paleoarch extends across the central part of the powder river basin wyoming to the black hills uplift the arch is the result of differential vertical uplift primarily during cretaceous time on numerous northeast trending structural lineaments lineament trends are identified from structural offsets in the black hills monocline offsets in well defined linear topographic escarpments and by linear drainages which connect trends between the monocline and the escarpments underlying basement zones of weakness are thought to be shear zones of precambrian age analogous to the mullen creek nash fork shear zone of southeast wyoming stratigraphic evidence suggests that the structural lineaments which form the belle fourch arch have rejuvenated periodically throughout the phanerozoic subtle movements along the lineaments have affected depositional environments and hydrocarbon accumulation in virtually all significant reservoirs in the northern two thirds of the basin evidence includes 1 localization of minnelusa formation permian hydrocarbon production along the crest of the arch 2 localization of dakota formation cretaceous alluvial point bar production on the crest of the arch 3 localization of lower muddy formation cretaceous channel deposits parallel with and on the downthrown sides of lineament trends 4 abrupt change in depositional strike of upper muddy formation cretaceous marine bars close to the arch 5 superposition of turner sandstone cretaceous channel deposits alon the trends of muddy channels and 6 localization of virtually all significant upper cretaceous shannon and sussex sandstone offshore marine bar production along the crest of the arch subtle uplift along the arch was persistent during at least lower muddy through sussex deposition a period of about 35 m y

a model for migrating shelf bar sandstones in upper mancos shale campanian northwestern colorado the duffy mountain sandstone was deposited by migrating submarine shelf bars that are now completely encased in marine mudstones of the mancos shale upper cretaceous the shelf bars were deposited at least 10 mi 16 km offshore and stood approximately 100 to 150 ft 30 to 46 m above a shallow shelf the bars were never emergent but were shallow enough for fair weather waves to form ripples along their crests these elongate sand bodies trend northeast southwest approximately parallel to the shoreline the sediment was probably derived from reworking of transgressed littoral deposits in southwestern wyoming four subenvironments are recognized 1 landward flank of the shelf bar the back bar 2 the axial crest of central bar 3 a seaward facing ramp subfacies and 4 the shelf upon which the shelf bars migrated two major processes were active during deposition 1 fair weather longshore southwesterly currents and 2 storm erosion and westerly across the shelf bar sediment transport during fair weather sand waves transported sediment along the central bar while the back bar and shelf were being bioturbated storms eroded the seaward face of the bar and portions of the central bar creating hummocky erosion surfaces eroded sediment transported across the bar was deposited in coarsening upward spillover lobes in the back bar after the storm waned sediments deposited in t e back bar were bioturbated and sand waves were deposited on the eroded central bar migration of sand waves onto the eroded face of the bar formed the ramp facies as the shelf bar migrated the central bar facies climbed stratigraphically over the back bar facies the direction of shelf bar migration was probably controlled by the interaction of fair weather and storm sediment transport