sedimentology of the archean dore metasediments arliss lake area southern michipicoten greenstone belt superior province dore metasediments in the arliss lake area of the michipicoten greenstone belt predominantly represent submarine fan sedimentation in mid to outer fan settings subaerial deposition was rare occurring in a sandy braided stream environment and as mud poor debris flows in a probable fan delta setting the arliss lake subbasin was likely small and of an irregular configuration which restricted down fan sediment distribution promoting aggradational lobe development it existed on a convergent plate margin felsic pyroclastic volcanism was periodically synchronous with epiclastic sedimentation but only rarely dominated paleogeographic development addressed by comparing juxtaposed thrust fault bounded lithological packages of similar facing suggests that the subbasin experienced several episodes of instability which caused dramatic changes in relative sea level

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

structural variety and tectonic evolution of strike slip basins related to the philippine fault system northern luzon philippines the northern part of the philippine fault zone in luzon corresponds to a complex braided system of left lateral strike slip faults the nw oriented main active branch which emerges from the philippine sea splits into an array of north striking splays responsible for the tectonic evolution of the central cordillera this complex fault pattern has favored local stress field variations strike slip basins have evolved in this framework along or in the vicinity of the main splays of the philippine fault apart from the classical pull apart tectonics on a releasing fault termination overlap or bend we describe other mechanisms such as the strike slip tilting or the warping of a strip limited by two strike slip faults the strike slip basins are good recorders of the evolution of the philippine fault system those located along the north striking cordilleran faults individualized in late early pliocene to pleistocene time when the present day philippine fault initiated but their main control is the fault shape acquired in upper middle miocene time the recent tectonic evolution of the fault system is best recorded in central luzon where the active basins trace an asymmetrical v shape with the longest branch trending ne parallel to the east luzon trough and the shortest one trending nw related to the philippine fault both the fault pattern and the basin distribution demonstrate the influence of the benham rise in the tectonic evolution of luzon the structural setting is interpreted as the result of an early miocene collision event between the benham rise and the eastern margin of luzon and subsequent inception of the nw striking strand of the philippine fault the present locations of the basins result from the interaction between the structural heritage and the present day regional plate motion

subaqueous rhyolite block lavas in the miocene ushikiri formation shimane peninsula sw japan abstract a rhyolite mass of the miocene ushikiri formation in the western part of the shimane peninsula sw japan is a small subaqueous edifice about 600 m high and 4 km wide formed at water depths between 200 and 1000 m it consists mainly of three relatively flat lava flow units 50 300 m in maximum thickness each of which includes lobes and their polyhedral fragments the lava lobes are poorly to well vesiculated glassy to microcrystalline and flow banded and folded compared with mafic pillows they are large having thick quenched and brecciated glassy crusts because of their high viscosity surface tension and thermal conductivity their surfaces disintegrate into polyhedral fragments and grade into massive volcanic breccia the massive volcanic breccia composed of the lobe fragments is poorly sorted and covered with stratified volcanic breccia of the same rock type the rhyolite lavas commonly bifurcate in a manner similar to mafic pillow lavas however they are highly silicic with 1 5 vol phenocrysts and have elongated vesicles and flow folds implying that they were visco plastic during flowage their surface features are similar to those of subaerial block lava with respect to rheological and morphological features they are subaqueous equivalents of block lava

a comparison of miocene and archean rhyolite hyaloclastites evidence for a hot and fluid rhyolite lava abstract in this paper we compare the geology and petrography of miocene and archean submarine rhyolite hyaloclastites the hyaloclastites are sparsely 10 or less plagioclase quartz and pyroxene phyric the hyaloclastites consist of a feeder dyke from which branch lava lobes and irregularly shaped lava pods the lava bodies consist of a holocrystalline core with microlitic texture grading outward into a flow layered rim zone and finally into obsidian the proportion of plagioclase and pyroxene microlites decreases outward some layers of the rim zone may be pumiceous vesicularity up to 50 vesicle size 1 mm or less but most of the lava has less than 5 vesicles one or a few cm long the obsidian shows perlitic fracture patterns the lava bodies grade through an in situ breccia into a hyaloclastite composed of angular obsidian granules and in many cases of fragments of lava lobes evidence for alteration at high temperature is as follows in the archean rhyolite hyaloclastites plagioclase microlites are overgrown by quartz albite spherulites furthermore parts of the miocene and archean hyaloclastite have been cemented and granules have been marginally replaced by quartz and albite hyaloclastite cemented at high temperature locally shows columnar joints at low temperatures obsidian has been hydrated and or has been replaced by clay minerals zeolites chlorite or prehnite chess board albite and fibroradial prehnite in archean hyaloclastite is possibly a pseudomorph after zeolites the sparsely porphyritic nature of the lava and the absence of microlites from the quenched glass suggests that the thyolite hyaloclastites extruded at high near liquidus temperature furthermore pumice is present only locally in the flow layered rim zone and in fragments derived from that zone these features suggest that vesiculation was inhibited by the weight of the water column high temperature and possibly the volatile h 2 o content explain the relatively low viscosity and shear strength of the lava and resulted in the flow morphology particular to this type of hyaloclastic rhyolite flows

migration direction of sand wave structures in pyroclastic surge deposits implications for depositional processes sand wave structures of both progressive downstream migrating and regressive upstream migrating types occur within the same pyroclastic surge deposit at both roccamonfina volcano southern italy and sugarloaf mountain arizona united states both wet and dry surge deposits are described that contain examples of regressive and progressive sandwave structures at sugarloaf mountain progressive and regressive sand waves occur within metres of each other within the same bedset indicating that penecontemporaneous deposition of the two different sand wave structures occurred in close proximity on the basis of this evidence it is suggested that the migration direction of sand waves in these pyroclastic surge deposits is controlled by the flow regime of the surge cloud rather than the temperature and or moisture content

stratigraphy and sedimentology of the paleoproterozoic thluicho lake group southwestern rae province canada alluvial basin development in the hinterland of the taltson orogen abstract the thluicho lake group is a middle paleoproterozoic succession of greenschist facies continental clastic rocks that was deposited on variably mylonitized crystalline basement rocks in the southwestern rae province of the canadian shield near the end of 2 02 1 92 ga thelon taltson orogenesis two informal formations are proposed for this fining upward succession the powder lake formation comprises predominantly coarse grained rocks including conglomerate gulo lake member pebbly sandstone camel lake member and sandstone wellington lake member whereas the overlying camsell portage formation consists of sandstone to siltstone falls member rhythmic sandstone and argillite waterloo lake member and argillite slate island member the two informal formations represent two principal facies associations a lower conglomerate sandstone facies association representing an alluvial fan to distal braided plain that underwent episodic flash flooding events and an upper sandstone siltstone muds

development of sedimentary basins in the archean abitibi belt canada an overview sedimentation in the archean abitibi greenstone belt occurred during four depositional episodes i sedimentary cycle 1 2730 2720 ma ii sedimentary cycle 2 2715 2705 ma iii sedimentary cycle 3 2700 2687 ma and iv sedimentary cycle 4 2685 2675 ma records of the first two sedimentary cycles are preserved in basins within the northern volcanic zone whereas basins formed during the latter two sedimentary cycles are located within the southern volcanic zone of the abitibi belt sedimentary cycles 1 and 3 represent deep water facies as indicated by turbidites resedimented conglomerates pelagic sediments and ubiquitous iron formations subaerial deposits have not been identified in contrast sedimentary cycles 2 and 4 show a prevalence of fluvial to shallow water marine and or lacustrine deposits tectono magmatic influence on sedimentation during cycles 2 and 4 is documented by i the presence of numerous unconformities underlain by plutonic and volcanic rocks ii locally voluminous sh

geochemistry of archean alkalic volcanic rocks from the crystal lake area east of kirkland lake ontario canada abstract twelve samples of timiskaming volcanics from the crystal lake area were analyzed for major and trace elements ta th zr hf v cr sc rare earth elements ree y etc by x ray fluorescence and instrumental neutron activation analysis techniques the samples which include mafic trachytes trachytes and leucite trachytes represent several batches of archean alkalic magmas differentiated at shallow depths the cr sc cr systematics and ree zr th and zr y characteristics show however that their primary magmas generated at great depths in the enriched mantle where garnet remained in the refractory residue of partial melt the chemical similarity between the timiskaming samples and modern alkalic rocks in mature volcanic arcs strongly implies that the timiskaming volcanism represents the last stage of a late archean island arc volcanism present in the southwestern abitibi belt superior province the occurrence of timiskaming type alkalic rocks is not rare in archean greenstone belts thereby suggesting that a tectonic setting similar to modern volcanic arc system might have been common in late archean time

sedimentation in an active strike slip basin southeastern turkey the modern structural drainage and depositional systems of the lake hazar basin in southeastern turkey exhibit an instantaneous picture of faulting and sedimentation within an actively evolving pull apart basin a general sedimentation model is proposed which consists of two types of drainage and depositional systems streams entering the basin laterally cross numerous oblique slip faults and are short steep associated with small drainage basins and characterized by profiles with many knickpoints they build alluvial fan deltas which are small steep and composed of coarse grained debris flow and fluvial deposits fluvial deposits consist of braided channel and bar sheetflood and overbank facies streams entering the basin longitudinally flow along the surface trace of single faults and are long gently sloping associated with large drainage basins and characterized by smooth profiles they build large gently sloping alluvial fan deltas composed of fluvial and lacustrine deposits lacustrine

the duparquet formation sedimentation in a late archean successor basin abitibi greenstone belt quebec canada in the south central part of the abitibi greenstone belt a succession of archean epiclastic sediments at least 2 km thick was deposited in the duparquet basin the destor porcupine fault and a secondary fault that splays off the major fault define the margins of this basin which is at least 15 km long and up to 2 5 km wide sedimentary evidence of tectonic influence includes i rapid vertical and lateral facies changes over tens of metres ii cyclic repetition of facies associations iii mixed fining upward coarsening upward fining upward sequences in conglomerate units iv local derivation of most clasts and v asymmetric distribution of fan deltas and braid deltas along the strike of the fault bounded basin three basic facies associations are recognized i conglomerate sandstone facies association csfa ii sandstone argillite facies association safa and iii argillite sandstone facies association asfa the csfa which predominates at the faulted basin margins constitutes 60

fan delta sequence in the archean fig tree group barberton greenstone belt south africa abstract the fig tree group of the barberton greenstone belt in south africa records the onset of tectonism and significant terrigenous and volcaniclastic sedimentation following a period of predominantly effusive volcanism in the southern part of the greenstone belt the lower fig tree conformably overlies the onverwacht group and can be grouped into either 1 volcaniclastic deposits consisting of pyroclastic and reworked volcanic detritus or 2 non volcanogenic terrigenous deposits of sandstone and conglomerate these petrologic groups are further divided into six lithofacies the lithofacies within the lower fig tree formation can be grouped into two facies associations 1 a subaqueous delta front association made up of fine grained airfall pyroclastic and slightly reworked pyroclastic debris along with resedimented land derived sandstone and conglomerate and 2 a subaerial association that is interpreted as forming from deposition of sandstone and conglomerate by flashy ephemeral stream processes on an alluvial fan surface the stratigraphic sequence coarsens upward from predominantly siltstone and fine grained sandstone at the base to pebble and cobble conglomerate at the top this sequence is interpreted as forming from the progradation of a fan delta into a body of relatively shallow quiet water composition of clasts in the conglomerates indicates that immediately underlying greenstone belt rocks or coeval volcanic rocks were the source for these coarse sedimentary accumulations erosion of uplifted greenstone belt terrain was apparently only to shallow levels both tectonic and volcanic activity played a role in producing several cycles of fan delta progradation

volcanism and related slope to shallow marine volcaniclastic sedimentation an archean example near chibougamau quebec canada abstract volcaniclastic sedimentary rocks and k rich lava flows of the archean hauy formation are discontinuously exposed in a 850 m thick sequence in the waconichi synclinorium northwest of chibougamau quebec the interstratified sedimentary rocks comprise two major sedimentary facies associations the graded bedded facies association wbfa indicative of deposition by sediment gravity flows and the wavy bedded facies association wbfa defined by wave induced structures the gbfa is near the base of the stratigraphic section and is interstratified up section with the wbfa in the gbfa the abundance of synsedimentary folding truncated slump folds load casts and low angle erosional surfaces as well as sediments deposited by turbidity currents and debris flows support designation as a volcanic slope deposit in the wbfa trough crossbeds wavy bedded gravels and plane bedded volcaniclastic sandstones are indicative of a nearshore setting the abundance of angular to subangular lithic volcanic fragments and broken and euhedral pyroxene crystals in the sediments is suggestive of a pyroclastic origin some beds may represent primary pyroclastic material but unequivocal petrographic recognition of pumice or glass shards is not possible because of metamorphic overprint composition suggests an initial magmatic or phreatomagmatic eruption subsequently reworked along the shoreline and transported downslope lava flows comprising massive to lobate andesites and massive to blocky basalts constitute most of the stratigraphic sequence pillowed flows are strikingly absent from the section both compositional types have an increase in vesicularity and change in flow form up section from massive to lobate or blocky suggesting progressively shallower water depths the composite character represented by hauy lava flows and associated volcaniclastic sediments in characteristic of stratovolcanoes the studied section is an integral part of an evolved arc with an adjacent backarc basin the volcano sedimentary facies indicate the development of a volcanic island in one of these basins

the secche di lazzaro pyroclastics of stromboli volcano a phreatomagmatic eruption related to the sciara del fuoco sector collapse widespread phreatomagmatic ash deposits secche di lazzaro pyroclastics overlying the lavas of the neostromboli period 13000 to ca 5000 years b p have been identified at stromboli volcano the deposits consist of accretionary lapilli rich ash fall layers and debris flows ash fall layers originated by phreatomagmatic explosive activity from a vent probably located in the uppermost part of the sector collapse scarp sciara del fuoco which formed at the end of neostromboli period stratigraphic position composition of fresh lapilli identical to the neostromboli lavas and an unusually large scale of explosions suggest that the secche di lazzaro pyroclastics were erupted in connection with the sciara del fuoco slope failure characteristics of the deposits are consistent with phreatomagmatic hydrothermal explosivity related to catastrophic decompression of the plumbing hydrothermal system occurrence of multiple ash fall episodes coupled with variation of nature of lithics lithology is suggestive of repeated explosions eventually triggered by multiple backward migrating block detachments

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

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

evolution of a plio pleistocene volcanogenic alluvial fan the puye formation jemez mountains new mexico the plio pleistocene puye formation north central new mexico contains 15 km 3 of volcaniclastic alluvial sediments deposited in response to rift margin volcanism associated with the later evolution of the tschicoma volcanic center northeastern jemez mountains stratigraphic and sedimentologic evidence indicates that aggradation of puye deposits was nearly continuous and contemporaneous with graben development and establishment of the rio grande as the through flowing axial drainage system in this region much of the puye sediments debouched into an adjacent subsiding depocenter resulting in near complete preservation of this wedge shaped volcanogenic fan primary pyroclastic and reworked facies in the puye formation exhibit distinctive 5 to 30 m scale cyclicity directly related to volcanic activity in the northeastern jemez highlands reworked pyroclastic and conglomeratic sequences deposited by high energy braided streams sediment charged sheet floods sediment gravity flows and in shallow lakes reflect markedly increased sediment loads during and following explosive eruptions individual sequences change markedly in character with increased distance from the source area analysis of these abrupt lateral and vertical variations reveals that puye sedimentation was controlled by the influences of semiarid climate and syndepositional tectonism although the associations that most characterize the fan resulted largely from explosive volcanism cessation of fan growth is attributed to waning sediment supply as a result of volcanic quiescence and to the onset of basinwide pedimentation associated with downcutting of the ancestral rio grande the puye fan therefore provides a detailed record of rift basin alluvial sedimentation complete with closely spaced tephrostratigraphic control and it records the growth and partial denudation of high standing northeastern jemez mountains volcanoes

late archaean clastic sedimentary rocks shamvaian group of the zimbabwe craton first observations from the bindura shamva greenstone belt the 2 65 ga old shamvaian group sedimentary rocks occur as a folded succession in the central part of the bindura shamva greenstone belt of zimbabwe the strata comprise distinct shear zone bounded tectonostratigraphic units which may be stratigraphically arranged as follows the lower part of the succession is represented by a transgressive fining upward sequence of alluvial fan conglomerate overlain by fluvial braid plain pebbly sandstone and marine shoreface sandstone detritus was derived from a mid archaean granitoid gneiss terrain situated to the east sediment supply and subsidence rate must have been high shallow shelf sedimentation was followed by deep water sub wave base deposition by turbidity currents giving rise to a thick succession of fine to coarse clastic material the turbidite deposits were locally overlain by shallow marine sandstone and fluvial to alluvial fan conglomerate an upward increase in the abundance of intermediate and felsic volcanic clasts suggests an increase in the proximity of a volcanic terrain such as a volcanic arc deposition was followed by layer parallel shearing during thrust belt style tectonism major shear zones developed preferentially along the contact between shallow and deep marine facies associations basin initiation may have been related to extensional tectonics possibly on rifted continental crust whereas later stages of basin history were characterized by compression suggesting a foreland or fore arc basin setting sedimentary facies stratigraphy and facies distribution are remarkably similar to some late archaean sedimentary sequences of the superior province in canada

late orogenic basins in the archaean superior province canada characteristics and inferences abstract the late orogenic archaean duparquet kirkland and stormy basins of the canadian superior province are characterized by bounding crustal scale faults and abundant porphyry stock emplacement lava flows and pyroclastic deposits are restricted to the kirkland and stormy basins and coarse clastic detritus characterizes the duparquet basin seven distinct lithofacies are identified 1 mafic volcanic 2 felsic volcanic 3 pyroclastic 4 volcaniclastic 5 conglomerate sandstone 6 sandstone argillite conglomerate and 7 argillite sandstone tuffaceous sandstone the mafic and felsic volcanic lithofacies represent effusive lava flows the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris the conglomerate sandstone lithofacies is interpreted as alluvial fan fan delta or proximal braided stream deposits whereas the sandstone argillite lithofacies is consistent with sandy dominated flood or braidplain deposits a dominantly shallow water lacustrine setting is inferred for the argillite sandstone lithofacies these different lithofacies record the basin history and can be used to identify basin forming processes lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation volcanism can also influence basin evolution and the delicate balance between erosion sedimentation and prevalent transport processes is affected by volcanic input catastrophic influx of pyroclastic material facilitated mass wasting processes and formation of non confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift in addition climate which controls the weathering processes and vegetation which stabilizes unconsolidated material affects the transport and depositional process a co 2 rich aggressive weathering humid archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation although tectonism is the prevalent long term controlling factor in restricted basins the effects of volcanism climate and lack of vegetation can also be detected

the nadi basin neogene strike slip faulting and sedimentation in a fragmented arc western viti levu fiji the nadi basin in western viti levu is one of a number of sedimentary basins controlled by e w to ne sw faults that developed on the fiji platform in the late miocene the initial basin fill belonging to the upper miocene nadi sedimentary group consists of mainly basement derived breccias and sandstones deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided e w faults the main source area lay to the south across the actively down faulting basin margin in the latest miocene a change in provenance took place as debris spread southward from a newly developed e w chain of shoshonitic volcanoes koroimavua volcanic group some 12 km north of the southern basin margin continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side in the earliest pliocene the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre nadi sedimentary group basement as inversion proceeded the basin fill was folded and southward directed reverse displacement took place along the southern basin margin evidence from the nadi and other late miocene basins indicates that they developed as a result of broadly e w directed sinistral strike slip probably related to a shearing collision of the melanesian border plateau with the northern margin of the fiji platform during the breakup of the formerly continuous outer melanesian arc it is suggested that prior to the establishment of the fiji fracture zone at 7 6 ma much of the left lateral displacement currently taking place along that plate boundary was accommodated by strike slip deformation within the fiji platform

intra sill magmatic evolution in the cummings complex abitibi greenstone belt tholeiitic to calc alkaline magmatism recorded in an archaean subvolcanic conduit system abstract the stratigraphy of the abitibi greenstone belt in the chibougamau area southern superior province quebec is dominated by 2 cycles of mafic felsic metavolcanic and metasedimentary rocks constituting the roy group which is riddled by metagabbroic sills 25 the dore lake complex dlc 2728 ma is emplaced into the lower roy group the cummings complex sills 2717 ma were injected between the bruneau member and blondeau formations of the 2nd roy group cycle the sills of the cummings complex roberge ventures and bourbeau sills contain metaperidotite metapyroxenite metagabbro and metagranophyric facies the trace element contents of melts in equilibrium with these metacumulate rocks were calculated and are compared to roy group lavas to clarify the regional magmatic history many dlc model melts have fractionated trace element profiles with lile lree enrichment hree depletion and negative nb ta anomalies suggesting that the dlc formed largely from calc alkaline melts extracted from garnet bearing residues the dlc is coeval with and shows geochemical resemblances to waconichi formation tuffs the felsic cap of the 1st roy group cycle suggesting it could represent the waconichi s shallow magma chamber meta anorthosite rafts from the para autochtonous zone of the grenville province yield model melts closely resembling those of the dlc and are correlated on this basis most roy group sills yield model melts with trace element patterns typical of archaean tholeiites suggesting they fed the regionally dominant tholeiitic volcanic plain lavas of the roy group models for the cummings complex imply that it contained two types of magma model melts from the roberge sill have strongly fractionated calc alkaline like trace element patterns while those of the ventures and bourbeau sills are mostly flat n morb normalized tholeiitic like patterns that cannot be derived from the roberge sill melts by fractional crystallization the roberge sill must have a separate magmatic stem and represents the feeder system for a calc alkaline volcano possibly represented in part by blondeau formation rhyolites andesites and basalts if this is correct the roberge may be older than the ventures and bourbeau sills which would have fed the tholeiitic base of a third magmatic cycle since eroded

multiscale interactions in an idealized walker cell analysis with isentropic streamfunctions abstracta new approach for analyzing multiscale properties of the atmospheric flow is proposed in this study for that the recently introduced isentropic streamfunctions are employed here for scale decomposition with haar wavelets this method is applied subsequently to a cloud resolving simulation of a planetary walker cell characterized by pronounced multiscale flow the resulting set of isentropic streamfunctions obtained at the convective meso synoptic and planetary scales capture many important features of the across scale interactions within an idealized walker circulation the convective scale is associated with the shallow congestus and deep clouds which jointly dominate the upward mass flux in the lower troposphere the synoptic and planetary scales play important roles in extending mass transport to the upper troposphere where the corresponding streamfunctions mainly capture the first baroclinic mode associated with large scale overturning circulation the intermediate scale features of

the fluvial and pyroclastic deposits of the cagayan basin northern luzon philippines an example of non marine volcaniclastic sedimentation in an interarc basin the cagayan basin of northern luzon an interarc basin 250 km long and 80 km wide contains a 900 m thick sequence of plio pleistocene fluvial and pyroclastic deposits these deposits are divided into two formations the ilagan and awidon mesa and three lithofacies associations the facies which are interpreted as meandering stream braided stream lahar and pyroclastic flow and fall deposits occur in a coarsening upward sequence meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits lahars and ignimbrites the coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent cordillera central volcanic arc uplift of the arc resulted in the progradation of coarser clastics further into the basin the development of an alluvial fan and migration of the basin depocentre away from the arc the coarsening of the pyroclastic deposits reflects the development of a more proximal calc alkaline volcanic belt in the maturing volcanic arc the cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins this is because the tectonic and volcanic processes which controlled sedimentation in the cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins

climatic and tectonic influences on fan deltas and wave to tide controlled shoreface deposits evidence from the archaean keskarrah formation slave province canada the 2 6 ga keskarrah formation located in the central slave province northwest territories canada is a late orogenic tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions the formation is adjacent to the crustal scale north trending beniah lake fault and overlies the 3 15 ga augustus granite the 2 69 2 7 ga mafic volcanic peltier formation and the turbiditic contwoyto formation unconformably principal lithofacies in the keskarrah formation include conglomerate sandstone and siltstone sandstone the conglomerate lithofacies represents coalescing gravelly streamflow dominated fan deltas adjacent to topographic highs up section quartz rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow water shoreface deposits influenced by wave action and tides the overlying feldspathic litharenites of the siltstone sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction tidal influence in both sandstone dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross beds as well as from abundant reactivation surfaces with local mudstone drapes intense chemical weathering during the archaean resulting from elevated atmospheric co2 levels higher temperatures and moist climatic conditions played an important role in the development of quartz rich arenites that appear to be first cycle deposits few lithic fragments and feldspar grains are preserved due to in situ host rock weathering chemical weathering during transport and wave and tide action hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment abundant conglomerate with up to 4 m large granitic boulders derived from the adjacent augustus granite and mafic clasts from the peltier formation indicate high relief and fault related uplift and subsidence the intimate association of fan deltas and wave and tide influenced shallow marine deposits in association with quartz rich sandstones forming in a high relief area make the keskarrah formation remarkable in the rock record

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

microthermometry of enargite hosted fluid inclusions from the lepanto philippines high sulfidation cuau deposit abstract the spatial relation between porphyry and high sulfidation epithermal deposits is particularly well revealed in the mankayan mineral district of northern luzon philippines where the lepanto high sulfidation cu au deposit lies over and adjacent to the far southeast fse porphyry cu au deposit consequently a study was undertaken to characterize the fluids responsible for epithermal mineralization in this environment the ore stage at lepanto consists of enargite luzonite cu 3 ass 4 pyrite tennantite tetrahedrite and chalcopyrite infrared petrography of the enargite reveals variable transparency with growth banding and twinning visible in euhedral specimens two phase liquid vapor fluid inclusions occur as primary and secondary types ranging from t h of fluid inclusions in enargite were measured from within the lateral 3 0 km and vertical 0 5 km extent of the enargite mineralization these values show a cooling trend toward the northwest away from the area over the porphyry deposit with average t h ranging from 285 c proximal to 166 c distal ice melting temperatures t m were measured using a cycling technique as ice was usually not visible in frozen inclusions apparent salinities range from 4 5 to 0 2 eq wt nacl with samples from the margins of the deposit showing a general decrease in apparent salinity with lower t h secondary fluid inclusions in quartz phenocrysts tend to have a higher average t h and lower apparent salinities compared to enargite hosted inclusion fluids from the same locations several samples of pyrite are also transparent to ir radiation and show internal features such as growth banding and in one instance a two phase liquid vapor fluid inclusion this inclusion yielded a salinity of 1 2 eq wt nacl there is a large discrepancy in t h and apparent salinities between the enargite mineralization and the subjacent porphyry deposit despite contemporaneous formation suggesting that the hot 500 c hypersaline magmatic brines associated with porphyry mineralization did not directly ascend to the elevation of the enargite deposit however absorbtion of magmatic vapors into overlying meteoric water may have created the mineralizing fluid of the lepanto deposit as this liquid moved to the northwest along the lepanto fault it was cooled and diluted by mixing with groundwater resulting in enargite deposition

sediment routing in a small drainage basin in the blast zone at mount st helens washington u s a abstract a quantitative description of the routing of 400 10 3 t km 2 of sediment delivered to a 2 4 km 2 basin by the may 18 1980 eruption of mount st helens has been developed for the period may 1980 june 1981 on hillslopes this sediment resided in primary and secondary remobilized primary storage sites hillslope erosion processes redistributed sediment on hillslopes and delivered sediment to the channel system these processes included shallow sliding from slopes steeper than 0 70 sheet erosion and rill erosion the most important discrete sites of storage on hillslopes were colluvial wedges and sites upslope of logs or other organic debris most tephra delivered to the basin by the eruptions is likely to be preserved in the stratigraphic record twelve percent of the sediment delivered to the study area was transported to the channel system directly by the eruption and by hillslope erosion processes of this approximately half remained in storage in the channel system and the remainder was exported from the drainage basin important sites of alluvial sediment storage included alluvial fans sites related to logs channel beds and alluvial terraces

alluvial fan sedimentation of the cutler formation permo pennsylvanian near gateway colorado the permo pennsylvanian cutler formation near gateway colorado is the most complete 1 334 m proximal section of alluvial fan sediments deposited along the western flank of the ancestral uncompahgre uplift cutler facies can be correlated with depositional processes observed on modern dry alluvial fans proximal cutler facies include matrix supported bouldery debris flow and channel form streamflood conglomerates midfan sedimentation in the cutler is represented by trough cross bedded granular braided stream sandstones laterally continuous streamflood conglomerates and sheetflood deposits laterally continuous streamflood conglomerate was deposited at the mouth of large channels near the intersection point and consists of a cross bed set as much as 2 m thick with a basal boulder cobble lag rippled and laminated siltstone with gravel channels represents distal sheetflood sedimentation pedogenic features include rhizocretions and calcareous nodules vertical changes in facies and maximum clast size delineate three megasequences on the scale of hundreds of metres thick each mega sequence is composed of a coarsening upward sequence of proximal facies overlain by a fining upward sequence of more distal facies coarsening upward sequences record periods of tectonic uplift and fan progradation whereas fining upward sequences result from tectonic quiescence and weathering back of the mountain front small scale cycles on the scale of ten metres occur within the larger megasequences and represent changes inherent to the alluvial fan system autocyclic sedimentologic data on cutler alluvial fan sediments at gateway support previous interpretations of semiarid or arid paleoclimate during permian time along the western flank of uncompahgria and may act as a standard of comparison for tests of the role of tectonism on sedimentation trends in the paradox basin

the lower cretaceous way group of northern chile an alluvial fan fan delta complex abstract alluvial fan sediments of the lower cretaceous coloso basin in northern chile were deposited in a half graben and derived from andesitic volcanics of a former island arc transport directions were towards the east away from the present day peru chile trench grain flow density modified grain flow and sheetflow processes were responsible for most of the sediment deposition with cohesive debris flows playing only a minor part an early phase of conglomerate deposition coloso formation into a restricted basin records the transition from proximal fan facies with abundant grain flows and remobilized screes to mid fan facies dominated by sheetflows stratiform copper mineralization near the top of the lower conglomerates is related to the unroofing of the jurassic island arc this mineralization comprises copper sulphide cemented sands and gravels and formed by the reaction of mineralized detritus with diagenetic and hydrothermal solutions a later phase of deposition lombriz formation includes sandstones siltstones and conglomerates with a source area different from the coloso formation this change in source may be related to strike slip tectonics as the basin extended the lombriz conglomerates pass distally eastwards into red sandstones and purple siltstones with thin limestones deposited under marine conditions this sequence is interpreted as a major fan delta complex it passes conformably into marine carbonates of the tableado formation signifying the complete drowning of the basin in lower cretaceous times

the middle devonian shoreline in north devon england the hollowbrook formation is an early middle devonian regressive succession of sandstones and heterolithic facies marking a passage from shallow marine shelf muds through a sandy shoreface to continental ephemeral stream sheet sands ten facies are recognized arranged in two sequence types the shelf sequence consists of alternations of bioturbated muddy sandstone fair weather sediment and shelly concentrates or parallel laminated fine sandstone storm deposits the nearshore and foreshore sequences coarsen upwards and pass from bioturbated muddy sandstone shelf to lenticular beds inner shelf then either wavy and flaser beds fair weather lower shoreface or hummocky beds and multistorey parallel laminated sands with bioturbated tops lower shoreface storm shoals upper shoreface wave ripple cross laminated sandstones follow then parallel and planar cross laminated sandstones foreshore and parallel laminated lithic sandstone continental sheet flood the lower parts of the shoreface sequence and inner shelf are burrowed by skolithos arenicolites and chondrites the sequence differs from the high energy 9oregon coast9 shore in the absence of a megarippled facies and presence of much bioturbation but closely matches the sequence found in modern low energy wave influenced sandy shores such as sapelo island georgia

sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt ellensburg formation late miocene central washington late miocene volcaniclastics of the ellensburg formation washington northwestern usa are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a cascade range source area two facies associations are recognized the first is composed of laterally restricted well sorted polymictic conglomerates representing a gravel bedload regime during inter eruption periods the second comprises laterally extensive sheets bounded by deeply developed paleosols composed of monomictic pebbly pumiceous dacitic sandstones with intercalated debris flow and hyperconcentrated flood flow deposits these sheets aggraded in response to eruptive events that are sometimes recorded by air fall tephras at the bases of sequences debris flow and hyperconcentrated flood flow deposits occur as far as 120 km from source but are uncommon beyond 50 km hyperconcentrated flood flow deposits initially increase in abundance away from source as debris flow deposits diminish suggesting formation of hyperconcentrated flood flows by dilution of debris flows sandy facies form broad sheets dominated by scour fill bedding in proximal and medial settings and grade to narrower trough cross bedded sheets in distal settings suggesting moderation of flood discharges with distance base level changes associated with episodic sediment influx caused incision aggradation cycles in mainstream settings and episodic impoundment of tributaries to form lakes or rapidly aggrading poorly drained floodplains although volcanism was the primary control on depositional style concurrent development of the yakima fold belt produced a structure consequent drainage pattern that determined sediment dispersal and basin subsidence permitted preservation of both syn eruption and inter eruption facies detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics but dominates deposits of small tributary streams flowing toward rather than from the cascades

sedimentology of a lacustrine fan delta system miocene horse camp formation nevada usa a 1600 m thick succession of the miocene horse camp formation member 2 exposed in east central nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan delta environments and nearshore and offshore lacustrine environments these four depositional environments are distinguished by particular associations of individual facies 14 defined facies subaerial and subaqueous fan delta facies associations include ungraded matrix and clast supported conglomerate normally graded matrix and clast supported conglomerate ungraded and normally graded sandstone and massive to poorly laminated mudstone subaqueous fan delta deposits typically have dewatering structures distorted bedding and interbedded mudstone the subaerial fan delta environment was characterized by debris flows hyperconcentrated flows and minor sheetfloods the subaqueous fan delta environment by debris flows high and low density turbidity currents and suspension fallout the nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments high energy oscillatory wave currents probably related to a large fetch reworked grains as large as 2 cm into horizontally stratified sand and gravel offshore directed currents produced uncommonly large typically 1 2 m thick trough cross stratified sandstone in addition stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high energy conditions alternating with periods of carbonate precipitation under reduced energy conditions massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout a depositional model constructed for the horse camp formation member 2 precludes the existence of all four depositional environments at any particular time rather phases characterized by deposition in subaerial fan nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan delta subaqueous fan delta and offshore lacustrine deposition this model suggests that high energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge thus preventing development of a subaqueous fan delta environment and promoting development of a well defined nearshore lacustrine environment low energy nearshore currents induced by shallow water along the

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene

a terrestrial shallow marine transition in the archean opemisca group east of chapais quebec abstract the stella and hauy formations of the archean opemisca group in the eastern chapais syncline feature numerous fluvial to shallow marine transitions throughout the stratigraphic sequence during the first depositional phase stella formation relatively high gradient alluvial fans prograde into a shallow marine environment the coarse clastic sediments reflect a scott type depositional environment the initial depositional environment is envisaged to be a high energy wave and storm flood dominated coastline bordered by coastal alluvial fans shallow marine deposits consist of coarse to very coarse grained sandstones and intraclast and pebble to cobble conglomerates the prevalent shallow marine sediments are related to storm and or flood deposition terrigenous sediments of the donjek type depositional environment prevail during the second depositional phase hauy formation the shallow marine sediments although still wave and storm flood dominated are finergrained and when correlated with the terrigenous sediments imply a lower gradient and a maturing of the coastline furthermore contemporaneous volcanism of andesitic shoshonitic to basaltic composition occurs during this evolutionary stage the sedimentary basin gradually evolved during the two depositional phases the repetition of terrigenous and shallow marine sediments throughout the stratigraphic sequence coarsening and fining sequences in the conglomerates and the alluvial fan setting strongly suggest a fault bounded basin margin tectonic uplift of the hinterland initiates alluvial fan progradation onto the shelf whereas quiescent periods are documented by marine transgression the clast composition of the conglomerates and the alluvial fan environment suggest local derivation of clasts the synvolcanic chibougamau pluton and the lake dore complex acted as a stable basement during deposition of the opemisca group the small landmass served as a stable platform for shallow marine deposition even when tectonically influenced a late phase intraplutonic cratonic development of an initial backarc basin system internal zone of the abitibi greenstone belt is proposed as the overall depositional setting of the chapais basin