the yavapai mazatzal boundary a long lived tectonic element in the lithosphere of southwestern north america a seismic reflection profile crossing the jemez lineament in north central new mexico images oppositely dipping zones of reflections that converge in the deep crust we interpret these data as a paleoproterozoic bivergent orogen centered on the jemez lineament that formed during original proterozoic crustal assembly by collision of mazatzal island arcs with yavapai proto north american continent at ca 1 68 1 65 ga the two major sets of reflections within the yavapai mazatzal transition boundary dip at 15 20 and we interpret them as part of a south dipping thrust system and as a north dipping crustal scale duplex that formed synchronously with the thrust system the upper crust shows structures recording a succession of tectonic and magmatic events from the paleoproterozoic to the holocene notable among these structures is a system of nappes that formed during development of the bivergent orogen elements of the nappe system are exposed in rocky mountain uplifts and have been dated as having formed at 1 68 ga at depths of 10 km and at temperatures of 500 c we also see continuous bright reflections in the upper part of the middle crust that we associate with basaltic sills that postdate accretion the data show that the yavapai mazatzal suture is low angle 20 an observation that explains why the boundary between the provinces has previously been so hard to define in the surface geology the jemez lineament overlies the root of this bivergent orogen that we also suggest is a paleoproterozoic zone of weakness that has subsequently acted as a conduit for magmas and a locus of tectonism

sedimentology stratigraphy and geochronology of the proterozoic mazatzal group central arizona quartzite conglomerate and shale of the mazatzal group record the filling of a prot erozoic intra arc basin in central arizona u pb ages of zircons from rhyolite ash flow tuff indicate that deposition began at 1701 2 ma basal deposits of the newly defined pine creek conglomerate formed in an alluvial fan setting synchronous with the final phase of extrusive rhyolite volcanism and active faulting after volcanic activity ceased shallower slope braided stream environments developed in early deadman quartzite time subsequent marine transgression produced shallow subaqueous deposits of the upper part of the deadman quartzite these were overlain by prodelta sediments of the maverick shale indicating a phase of basin deepening finally the mazatzal peak quartzite was deposited as the basin shoaled again first to shallow marine and subsequently to fluvial conditions the quartzite is diagenetic quartz arenite originally deposited as lithic arenite and lithic arkose the provenance was initially restricted and earliest sediment was derived mainly from the subjacent red rock group when tectonic activity ceased how ever the surrounding highlands were planed down by erosion and detritus from a wider variety of source rocks was funneled into the basin this included contributions from arc related supracrustal rocks of the payson ophiolite and east verde river formation and finally a granitic basement input detrital quartz in the lower part of the mazatzal group is largely monocrystalline and volcanic in origin polycrystalline quartz associated with detrital feldspar and of probable continental affinity is concentrated in the uppermost parts of the sequence the life span of the intra arc basin was on the order of 30 m y from the formation of the payson ophiolite at 1 73 ga to the deposition of the upper mazatzal peak quartzite sometime after 1 70 ga the pre mazatzal red rock group represents the last stages of volcanic arc activity and the mazatzal group records the transition from orogenic to nonorogenic sedimentation in central arizona

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

diagenetic origin for quartz pebble conglomerates the occurrence of quartz pebble conglomerates qpc in the rock record increases backward through time from the tertiary through the precambrian the positive correlation between qpc abundance and age is valid both for numbers of reported qpc and for qpc as a percentage of all conglomerate and at both the era and the period level qpc are usually interpreted as being due to intense chemical weathering protracted transport or sediment recycling but none of these can account for the age distribution of qpc which is the opposite of the global mass age distribution for sedimentary rocks precambrian and tertiary conglomerates with similar sources and sedimentology have vastly different clast populations nonquartzose clasts being much more abundant in the younger rocks comparison of the petrology of qpc and polymict conglomerates shows that qpc have consistently higher proportions of diagenetic secondary matrix and pressure solved grain contacts we conclude that diagenetic factors play an important role in qpc formation by preferentially destroying less durable clasts

proterozoic history and crustal evolution in southwestern colorado insight from u pb and sm nd data abstract new u pb age constraints and sm nd isotope data further establish the timing of magmatic and tectonic events from 1 8 to 1 4 ga in southwestern colorado and give insight into the interaction of crust and mantle systems during the assembly and evolution of orogenic belts along the southern margin of laurentia proterozoic orogenic events in the needle mountains led to crustal assembly thickening and tectonic reworking that was punctuated by periods of relative crustal stability pre 1 75 ga arc successions in southwestern colorado were produced and assembled into juvenile crust in an orogenic period that culminated in regional deformation and metamorphism that accompanied intrusion of synorogenic plutonic rocks from 1 73 to 1 71 ga emplacement of post orogenic plutonic rocks at 1 70 ga was followed by uplift and erosion and the deposition and deformation of siliciclastic sedimentary sequences paleoproterozoic crust underwent local melting and thermal metamorphism during invasion of felsic to mafic magmas at 1 43 ga that coincided with a pulse of intracratonic deformation invasion and melting of first cycle 1 8 1 7 ga crust at 1 71 and 1 43 ga involved heat and volatiles from mantle melts that were a catalyst for crustal melting and magmatic recycling sm nd isotopic signatures of 1 8 1 4 ga volcanic and plutonic rocks in southwestern colorado lend evidence to the emplacement of melts from depleted mantle and juvenile crust in the production continental crust with minor if any contamination by crust older than 1 8 ga the proterozoic magmatic record in southwestern colorado is consistent with continental growth by the addition of juvenile crust extracted from the mantle followed by periodic intracrustal growth during orogenic pulses in which mantle magmas underplate and melt the crust with consequent thickening

three proterozoic orogenic episodes and an intervening exhumation event in the black canyon of the gunnison region colorado abstract u pb zircon titanite geochronology in situ monazite geochronology and 40ar 39ar thermochronology provide an unusually complete data set for reconstructing the tectonic history of proterozoic rocks exposed in the black canyon gunnison colorado these new geochronologic data record three protracted orogenic episodes and an exhumation event between orogenic pulses 1 yavapai orogeny 1741 1689 ma 2 exhumation marked by an angular unconformity beneath post yavapai pre mazatzal quartzites 3 mazatzal orogeny postquartzite deposition and 4 1434 1403 ma intracratonic tectonism supracrustal rocks of the black canyon succession were deposited or crystallized at or prior to documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wn

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

the pinal schist of southern arizona a paleoproterozoic forearc complex with evidence of spreading ridge trench interaction at ca 1 65 ga and a proterozoic arc obduction event the pinal terrane of southern arizona is composed mostly of quartz sericite schists meta wackes metavolcanic rocks and granitoid and gneissic rocks with minor meta chert meta conglomerate and carbonate rocks various tectonic settings have been suggested for the origin of this terrane the existence of tectonic melanges within the terrane the presence of granitic rocks in close proximity to the melanges the presence of both oceanic and arc like metavolcanic rocks within the terrane the age distributions of igneous and meta igneous rocks within the terrane the deformational patterns within the terrane and the width of the terrane all support a model in which the terrane was formed as a forearc subduction complex that experienced a ridge subduction event at ca 1 65 ga the pinal terrane was likely the forearc subduction complex to the mazatzal arc terrane which lies immediately to the north regional variations in the mineralogy of metavolcanic and metasedimentary rocks exposed within the terrane together with structural data suggest the terrane may have experienced an arc obduction event in the proterozoic after the ridge subduction event if it did most of this arc terrane was eroded off the pinal terrane in the 300 m y following the obduction event and prior to deposition of the apache group at ca 1 32 ga there is a paleoproterozoic arc terrane exposed in northern mexico that may represent remnants of an obducted arc terrane high pressure high temperature metamorphic rocks e g blueschists have not been identified within the pinal terrane

allogenic controls on the evolution of storm to tidal shelf sequences in the early proterozoic uncompahgre group southwest colorado usa dominantly coarse grained shallow marine metasedimentary rocks of the early proterozoic uncompahgre group ug record periods of shoaling and drowning on different temporal scales that are attributed to episodic long term oscillations in relative sea level with superimposed shorter duration excursions in relative sea level long term events are probably tectonic whereas short term events are eustatic the 2 5 km thick uncompahgre group consists of 250 600 m thick dominantly coarse grained quartzite units q1 q4 and 200 300 m thick mudstone pelite units p1 p5 five depositional systems comprise the uncompahgre group the outer shelf system oss is composed of bouma type beds and intercalated mudstones that are transitional vertically to parallel laminated to wave rippled sandstones and hummocky cross stratified sandstones of the inner shelf system iss trough cross stratified sandstones comprise the shoreface system shs the tidal inner shelf shoreface system tis shs consists of a complex interlayering of cross bedded sandstones thin bedded conglomerates mudstones and rippled sandstones trough cross bedded pebbly sandstones and thin to thick bedded conglomerates represent the alluvial system alls depositional systems in the ug are associated in transgressive and highstand systems tracts that make up four sequences 1 to 4 sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities the basal q1 p1 unit lower sequence 1 consists of alls to tis shs to iss comprising a transgressive systems tract a maximum marine incursion is reflected by deposition of oss facies in stratigraphic units p1 p2 shoaling in the transition from p2 to the uppermedial portion of q2 oss iss shs to a thick tis shs alls records the highstand systems tract of upper sequence 1 a subtle disconformity paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract the drowning to shoaling pattern is replicated in sequence 2 upper q2 to p3 to upper medial q3 sequence 3 upper q3 to p4 to upper medial q4 and an incomplete sequence 4 upper q4 through p5 thinner shoaling intervals of oss iss shs in p3 and in lower q2 q3 and q4 represent parasequences sequences of 107 years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin parasequences record shorter duration temporal controls of c 104 to 105 years related to eustatic oscillations as a consequence of shoaling and aggradation progradation in the highstand systems tract tis shs and alls overlie and are temporally separated from oss to iss to shs this transition records filling of the basin to sea level leading to a shelf geometry that was conducive to tidal amplification a composite relative sea level curve integrating long term pulsatory subsidence and short term eustasy best explains the stratigraphic evolution of the uncompahgre group

alterations in bottom sediment physical and chemical characteristics at the terra nova offshore oil development over ten years of drilling on the grand banks of newfoundland canada abstract this paper describes sediment composition at the terra nova offshore oil development the terra nova field is located on the grand banks approximately 350 km southeast of newfoundland canada at an approximate water depth of 100 m surface sediment samples upper 3 cm were collected for chemical and particle size analyses at the site pre development 1997 and in 2000 2002 2004 2006 2008 and 2010 approximately 50 stations have been sampled in each program year with stations extending from less than 1 km to a maximum of 20 km from source drill centres along five gradients extending to the southeast southwest northeast northwest and east of terra nova results show that terra nova sediments were contaminated with c 10 c 21 hydrocarbons and barium the two main constituents of synthetic based drilling muds used at the site highest levels of contamination occurred within 1 to 2 km from source consistent with predictions from drill cuttings dispersion modelling the strength of distance gradients for c 10 c 21 hydrocarbons and barium and overall levels generally increased as drilling progressed but decreased from 2006 to 2010 coincident with a reduction in drilling as seen at other offshore oil development sites metals other than barium sulphur and sulphide levels were elevated and sediment fines content was higher in the immediate vicinity less than 0 5 km of drill centres in some sampling years but there was no strong evidence of project related alterations of these variables overall sediment contamination at terra nova was spatially limited and only the two major constituents of synthetic based drilling muds used at the site c 10 c 21 hydrocarbons and barium showed clear evidence of project related alternations

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

proterozoic cuspate basement cover structure needle mountains colorado deformation zones along basement cover contacts which are commonly loci of significant displacement may also be zones of limited movement when cover and basement are folded together proterozoic basement gneisses and siliciclastic cover rocks of the uncompahgre group in the needle mountains colorado are in contact along steeply dipping deformation zones analysis of structures in the basement and cover and along their contacts provides evidence for 1 deformation of the basement prior to deposition of the uncompahgre group 2 north directed thin skinned thrusting and 3 cuspate infolding of cover into basement accompanied by upward and outward movement of the cover opposed senses of movement along basement cover contacts on opposite limbs of the synclinorium generated during cuspate infolding imply that the uncompahgre group is parautochthonous on basement conjugate strike slip shear zones in both units accommodated north northwest shortening deformation of basement and cover into cuspate structures may explain the presence of some cover sequences as isolated remnants within gneissic basement

evidence of tide storm and wave interaction on a precambrian siliciclastic shelf the 1 700 m y ortega group new mexico abstract the 1 700 m y ortega group in northern new mexico is a sequence of quartz arenites and subordinate mudstones in excess of 1 000 m sedimentation took place in diverse shallow shelf environments under the influence of tidal wave and storm processes time velocity asymmetry of tidal flow on the inner shelf produced large scale trough cross beds and tabular planar cross beds with multiple reactivation surfaces in contrast tabular cosets structured internally by small scale trough cross beds and herringbone cross beds resulted from symmetrical tidal flow on the inner shelf fair weather waves reworked the upper surface of tidal sand bodies on the inner shelf during quiet periods within the tidal rhythm during storms sand was entrained into the water column on the proximal inner she f leaving winnowed pebble lags at the top of tidal sand bodies sand transported by storm induced gradient currents was deposited on the outer shelf as discrete lobes progradation of sand lobes produced 1 11 m thick genetic packages which display an upward thickening of sandstone beds and are capped by cosets of tabular planar laminated sandstones storm sands were reworked by wave processes on the proximal outer shelf whereas storm deposited sands on the distal outer shelf remained unmodified no evidence of tidal processes exists on the outer shelf distribution of facies and paleocurrent patterns indicates that the shelf sloped to the south and experienced an overall transgression which culminated in the drowning of the outer shelf with onlap of black basinal muds absence of shallow water turbidites below storm wave base reflects low gradients on the shelf vertical transition from tide to storm dominated sedimentation in the ortega group does not represent a change in depositional style with time on the shelf rather all facies coexisted across the shelf with tidal processes prevailing on the shallow inner shelf and storm processes on the outer shelf the german bight in the north sea and parts of the bering sea off alaska are holocene counterparts for the depositional framework envisaged for the ortega group in both pro imal tide dominated environments pass distally into deeper water storm dominated environments

xenolithic evidence for proterozoic crustal evolution beneath the colorado plateau tertiary diatremes of the navajo volcanic field brought a wide variety of proterozoic xenoliths to the surface of the colorado plateau examination of crustal xenoliths from the navajo volcanic field diatremes permits reconstruction of proterozoic pressure temperature p t histories beneath the colorado plateau diatremes from the northwest part of the navajo volcanic field carry the greatest variety of xenoliths including metasedimentary rocks amphibolites felsic gneisses mafic granulites and crustally derived eclogites these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories in contrast diatremes from the southeast part of the navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration metasedimentary and eclogitic xenoliths are absent in these diatremes the p t paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 c and pressures equal to or greater than 10 kbar later hydrous alteration occurred around 500 c and 8 12 kbar xenoliths from the southeast diatremes preserve little evidence of their p t evolution but paths involve heating and or decompression the differing rock types p t paths and alteration histories of the northwest and southeast populations suggest that proterozoic tectonism juxtaposed two distinct crustal blocks beneath the colorado plateau previous workers have postulated that the boundary between the proterozoic yavapai and mazatzal provinces occurs in the region our data support the existence of a northeast trending boundary beneath the four corners area if the eclogitic metamorphism and hydrous alteration are proterozoic in age their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during proterozoic accretion of the mazatzal province onto north america

evidence for widespread approximately 1760 ma metamorphism and rapid crustal stabilization of the early proterozoic 1870 1820 ma penokean orogen minnesota

a proterozoic volcano plutonic terrane gunnison and salida areas colorado early proterozoic supracrustal rocks near gunnison and salida colorado include tholeiitic basalt dacite to rhyolite and intercalated sedimentary rocks these were intruded by essentially synchronous gabbroic sheets folded and intruded by major plutons ranging from quartz diorite to granite preservation of primary volcanic and sedimentary features is good to excellent sedimentary rocks in the gunnison area are mostly turbidite wackes whereas near salida they are mostly near source volcaniclastic rocks u pb age measurements on zircons indicate that the earliest bimodal volcanic rocks erupted between 1770 and 1760 m y ago and were intruded by granitic plutons between 1760 and 1750 m y ago renewed bimodal volcanism occurred between 1740 and 1730 m y ago and was followed by a period of deformation producing two fold sets before 1725 m y ago when the first of a series of younger plutons was emplaced the compositions and features of the sedimentary rocks and their association with volcanics are con

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

determining the extent and nature of mazatzal related overprinting of the penokean orogenic belt in the southern lake superior region north central usa abstract twenty one hornblende and mica 40ar 39ar dates from central and northwest wisconsin usa provide important information on the timing spatial extent and intensity of mazatzal age metamorphism and deformation which overprinted the paleoproterozoic 1870 1820 ma penokean orogenic belt in the southern lake superior region 1760 1750 ma mica plateau ages from bedrock beneath undeformed 1750 1630 ma quartzites are interpreted as the time of lower temperature 300 350 c cooling and crustal stabilization after the penokean orogeny six mica ages from bedrock underlying deformed paleoproterozoic quartzites cluster around 1600 ma 1576 1614 ma the complete absence of 1760 1750 ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300 350 c during mazatzal related deformation and metamorphism the 1600 ma mica ages are interpreted to date the cooling phase of this metamorphism two anomalously young biotite dates are interpreted to indicate partial resetting associated with mesoproterozoic rifting at 1100 ma ten hornblende 40ar 39ar dates obtained in this study address the higher temperature overprinting effects in the southern lake superior region one latest archean age of 2503 18 ma and two ages of 1853 and 1830 ma are interpreted as remnant evidence of archean and penokean age amphibolite metamorphic events respectively the majority of hornblende ages are younger than the penokean orogeny scattering between 1796 and 1638 ma microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages this suggests that shearing and recrystallization did not play an important role in the retention or loss of argon the 1638 ma hornblende age is concordant with the mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid related resetting associated with that event six other post penokean ages scatter over a 70 million year interval 1796 1723 ma and probably reflect variable retention of radiogenic argon they are interpreted to indicate variable degrees of partial intermediate temperature 350 500 c resetting of hornblende argon systematics at 1650 1630 ma collectively these data suggest that the effects of the mazatzal orogeny in the southern lake superior region involved 350 500 c metamorphism and penetrative deformation

reconciliation of conflicting tectonic models for proterozoic rocks of northern new mexico two major proterozoic tectonic events are documented in the taos range of northern new mexico regional structures involving the tectonic interleaving of c 1 65 ga granitoids with supracrustal rocks are interpreted to have formed before 1 42 ga and probably during collisional assembly of island arc crust into new 1 7 1 6 ga continental lithosphere supracrustal rocks record 650 750 c 6 8 kbar metamorphism m2 these high temperatures may have been reached during sandwiching between c 1 65 ga granitoids however the early history has been obscured by renewed tectonism at c 1 4 ga that resulted in partial melting fabric reactivation and new mineral growth at 4 kbar m3 metamorphic temperature variations from uppermost amphibolite to amphibolite facies rocks may be associated with c 1 65 and or 1 4 ga plutonism but not to a 1 4 ga extensional shear zone as previously proposed syn and post 1 4 ga contraction is suggested by high and low temperature microstructures showing top to the south east thrusting this work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c 1 65 ga but that the present fabric also records 1 4 ga tectonism involving high t metamorphism and fabric reactivation

paleoproterozoic rocks of central colorado accreted arcs or extended older crust paleoproterozoic 1770 1735 ma bimodal volcanic rocks in central colorado have been considered to represent southward growth of laurentia by arc accretion although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs there has been little evidence for pre 1800 ma crust south of the wyoming craton other than the 1840 1 ma elves chasm pluton in the upper granite gorge of the grand canyon arizona we report shrimp u pb ages of inherited zircons from metarhyolites and plutons in central colorado that are latest archean earliest proterozoic 2520 2000 ma and trans hudson penokean 1878 1814 ma associated quartzites contain detrital zircons with mean ages of 1735 ma indicating only local derivation a meta arkose however contains detrital zircons of trans hudson penokean and archean ages we believe it likely that the 1900 1800 ma trans hudson penokean orogens including archean enclaves extended farther south and west than is currently thought and were the source of the bimodal volcanic rocks and associated plutons during the period 1770 1700 ma

precambrian tidalites from the baraboo quartzite wisconsin u s a abstract the baraboo quartzite is a 1 7 billion year old almost pure meta quartzarenite unit that is 1500 m thick and that crops out in a doubly plunging syncline in south central wisconsin u s a it is one of several similar quartzites that are exposed throughout the upper part of the midwestern united states two general depositional environments are represented by this well sorted quartzarenite a lower braided stream complex characterized by wedge sets of cross strata with pebble conglomerate at the base and small cut and fill structures and an upper tidally influenced environment characterized by small scale planar tabular cross strata separated by reactivation surfaces and scattered tidal bedding sequences the baraboo quartzite represents one of the oldest tidally influenced rock units known but has no direct modern analog

origin of thick first cycle quartz arenite successions evidence from the 1 7 ga ortega group northern new mexico abstract the 1700 ma old ortega group in northern new mexico represents a 1000 m thick succession of metamorphosed quartz arenites and subordinate pelites published ages of detrital zircon grains from the ortega group indicate that the sediments were derived mainly from 1760 1700 ma old volcano plutonic basement rocks and not from the archean wyoming province to the north evidence for recycling within the ortega group also is lacking consequently the ortega group quartz arenites are considered to be first cycle sediments the compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering abnormally high al contents of pelites within the ortega group imply intense chemical weathering in the provenance similar to present day low relief tropical regions quartz arenites in the ortega group were deposited exclusively within a shallow marine shelf environment influenced by tide storm and wave processes prolonged sediment transport especially on the tide dominated inner shelf in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains although the maturity of sediments in the ortega group can be accounted for the mode of origin of 850 m of quartz arenites remains enigmatic thick quartz arenite sequences of global distribution in the phanerozoic coincide with first order eustatic sea level rises world wide occurrences of 1700 ma old thick quartz arenite shelf successions suggest that the ortega group may have accumulated in response to a similar major eustatic sea level rise

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

effects of the terra nova offshore oil development on benthic macro invertebrates over 10 years of development drilling on the grand banks of newfoundland canada abstract this paper describes effects of drilling with water and synthetic based drilling muds on benthic macro invertebrates over 10 years at the terra nova offshore oil development as such the paper provides insight on the effects of relatively new synthetic based drilling muds sbms and makes an important contribution to our understanding of the long term chronic effects of drilling on benthic communities the terra nova field is located approximately 350 km offshore on the grand banks of newfoundland canada sediment and invertebrate samples were collected in 1997 baseline prior to drilling and subsequently in 2000 2001 2002 2004 2006 2008 and 2010 approximately 50 stations were sampled in each year at distances of less than 1 to approximately 20 km from drill centres summary benthic invertebrate community measures examined were total abundance biomass richness diversity and multivariate measures of community composition based on non metric dimensional scaling nmds decreases in abundance biomass and richness were noted at one station located nearest 0 14 km to a drill centre in some environmental effects monitoring eem years these decreases coincided with higher levels of tracers of drill muds in sediments barium and c 10 c 21 hydrocarbons abundances of selected individual taxa were also examined to help interpret responses when project related effects on summary measures occurred enrichment effects on some tolerant taxa e g the polychaete family phyllodocidae and the bivalve family tellinidae and decreased abundances of sensitive taxa e g the polychaete families orbiniidae and paraonidae were detected to within approximately 1 2 km from discharge source lagged responses three to five years after drilling started were noted for phyllodocidae and tellinidae suggesting chronic or indirect effects overall results of benthic community analyses at terra nova indicate that effects on summary measures of community composition were spatially limited but as seen elsewhere some taxa were more sensitive to drilling discharges

long term exhumation of an aegean metamorphic core complex granitoids in the northern menderes massif western turkey the egrigoz koyunoba and alacam plutons are located in the northern portion of the menderes massif a region characterized today by large scale extension to gain a better understanding of their exhumation history we acquired zircon ages geochemical analyses and cathodoluminescence cl images to search for evidence of micro to macro scales of deformation the combination is a powerful means to decipher their tectonomagmatic history in situ ion microprobe 238u 206pb zircon ages of the granitoids range from 30 0 3 9 ma to 14 7 2 6 ma 1 and indicate the plutons crystallized over 15 m y the dated zircons show cl zoning consistent with igneous crystallization and are only located adjacent to or as inclusions in biotite grains the youngest ages are dominated by a blue color in cl whereas a majority of older zircons are yellow higher th u contents are typically shown by zircons with black or dark green colors in cl most of the samples of these granitoids indicate they are magnesian calc alkalic and peraluminous granite to granodiorites but variations exist likely reflecting heterogeneity caused by magma mixing partial melting crustal contamination and post emplacement fluid interactions as evidenced by the cl images cl images show that all samples experienced magma mixing multiple episodes of brittle deformation and fluid mediated alteration sources for the generation of northern menderes massif granitoids include simple slab induced upwelling from the subduction of the african plate along the hellenic arc adiabatic decompression as the northern edge of turkey9s anatolide tauride block delaminates and upwelling asthenosphere as the continental lithosphere thins during extension we present a model in which western turkey is an amalgamation of stacked subduction zones that transition from north to south over time northern menderes massif granitoids document their complex geologic history in their ages chemistry and textures

problems in characterizing dynamics of mud shore profiles the relationship between profile shape and wave forcing at shores composed of soft mud is examined at a given still water level the modality of change in profile shape under wave action depends on the fluidization potential of mud which in turn is contingent on the wave characteristics and the dissipative properties of mud an important determinant of profile shape in areas where mud can be fluidized is the wave attenuation coefficient comparatively large values of this coefficient tend to correlate with erosional concave upward profiles and low values with accretionary convex upward shapes assuming that the rate of energy dissipation per unit area of nonbreaking normally incident waves is constant in the nearshore shallow zone an equation to simulate soft mud shore profiles has been developed this equation is shown to be applicable to profile data obtained from a variety of mud shore environments however it is noted that realistic simulations of the accretionary and erosional characteristics

reinterpretation of paleoproterozoic accretionary boundaries of the north central united states based on a new aeromagnetic geologic compilation abstract the paleoproterozoic crust in the north central u s represents intact juvenile terranes accreted to the rifted archean superior craton a new tectonic province map based on the interpretation of a new aeromagnetic compilation published geologic maps and recent geochronologic data shows progressive accretion of juvenile arc terranes from ca 1900 1600 ma contrary to earlier models geon 18 penokean interval crust is primarily confined to a 2100 ma tectonic embayment of the rifted superior craton the newly defined spirit lake tectonic zone characterized by a sharp magnetic discontinuity that marks the southern limit of archean and penokean interval rocks is here interpreted to represent an eastern analog of the cheyenne belt suture zone in southern wyoming south of this boundary geon 17 yavapai interval rocks form the basement upon which 1750 ma rhyolite and succeeding quartzite sequences were deposited substantial portions of the penokean and yavapai terranes were subsequently deformed during the 1650 1630 ma mazatzal orogeny the northern boundary of the mazatzal terrane is obscured by abundant 1470 1430 ma anorogenic plutons that stitched the suture with the older yavapai terrane rocks these data reveal a progressive tectonic younging to the south as the laurentian craton grew southward and stabilized during the proterozoic late mesoproterozoic rift magmatism produced pronounced geophysical anomalies indicating strong but localized crustal modification in comparison to the western u s little tectonism has occurred here in the last 1 billion years providing a uniquely preserved record of the precambrian evolution of the continental u s lithosphere

provenance of late paleozoic strata in the yili basin implications for tectonic evolution of the south tianshan orogenic belt this research was supported by the 973 program under grant no 2015cb453000 the national natural science foundation of china 41502109 the cultivating program of middle aged key teachers of chengdu university of technology kygg201718 innovation team of sedimentary geology of chengdu university of technology kytd201703 and by the china postdoctoral science foundation 2015m582528 cawood acknowledges support from the australian research council grant fl160100168

restoration of laramide right lateral strike slip in northern new mexico by using proterozoic piercing points tectonic implications from the proterozoic to the cenozoic early proterozoic rocks in northern new mexico can be used to document 100 170 km of right lateral slip along a network of north striking laramide late cretaceous early tertiary faults that formed the precursor of the rio grande rift piercing lines are defined by the intersection of a subborizontal 4 kbar isobaric surface with steeply dipping stratigraphic markers and regional structures restoration of slip provides new insight into the nature of proterozoic orogenic and crustal province boundaries the yavapai crustal province 1 75 1 72 ga juvenile volcanogenic rocks is imbricated with overlying hondo group sedimentary cover in an originally east trending ductile thrust belt that formed near the southern margin of the yavapai province a series of offset magnetic anomalies jemez lineament is interpreted to mark the thrust buried boundary between older yavapai province and younger mazatzal province crust our microplate tectonic model suggests that laramide strike slip movement new mexico was kinematically linked with coeval extensional collapse arizona and foreland shortening wyoming during decoupling and northward movement of the rigid colorado plateau relative to the undeformed midcontinent crustal scale strike slip faults in new mexico were reactivated during later tertiary extension and continue to localize high heat flow in the rio grande rift and northward into colorado

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

急速風化と成熟石英アレナイト堆積を支持する ウイスコンシン州 原生代初期珪岩の砕屑ジルコン年代

polyphase suprastructure deformation in metasedimentary rocks of the uncompahgre group remnant of an early proterozoic fold belt in southwest colorado isolated remnants of supracrustal rocks of the uncompahgre group in the needle mountains and uncompahgre gorge area have been subjected to polyphase deformation and greenschist facies metamorphism phase 1 structures recognized only in the west needle mountains include bedding parallel ductile shear zones f 1 mesoscopic folds and cleavage s 1 continued crustal shortening during phase 2 resulted in upright macroscopic folding f 2 and cleavage s 2 development in addition basement cover contacts were folded further shortening during phase 3 deformation resulted in conjugate shearing of cover rocks with the development of both a macroscopic f 3 fold and an s 3 crenulation cleavage in addition basement cover contact zones were activated as phyllonitic oblique slip reverse faults conjugate shearing resulted in lateral offset of stratigraphic units with loss of section conjugate reactivation of phase 1 ductile deformation zones was manifested by oblique dextral and sinistral slip the structural evolution of the uncompahgre group records the transition from a ductile north directed thin skinned fold thrust belt to the formation of a basement involved megamullion structure which was subjected to conjugate strike slip faulting to accommodate further shortening the results of this study indicate that the basal unit of the uncompahgre group was parautochthonous on basement a departure from some previous interpretations this implies that the metasedimentary cover rocks are younger than surrounding ca 1750 ma basement gneisses and ca 1690 ma granitoids the over all style of deformation may be analogous to the deep foreland suprastructure of an orogenic belt that developed between ca 1690 ma and 1600 ma in the southwestern united states

age provenance and tectonic setting of paleoproterozoic quartzite successions in the southwestern united states new field studies combined with u pb zircon geochronology constrain the ages of deposition and sedimentary provenance of paleoproterozoic quartzite successions exposed in the southwestern united states orthoquartzites were deposited in short lived basins at two times ca 1 70 and 1 65 ga during crustal assembly of southern laurentia the more voluminous ca 1 70 ga successions occur in southern colorado northern new mexico and central arizona and are interpreted here to be time correlative though not necessarily deposited in the same basins detrital zircon from quartzites and metaconglomerates exposed in southern colorado and northern new mexico is characterized by a single population with a relatively narrow range of ages 1 80 1 70 ga and minimal archean input 5 of grains analyzed peak detrital zircon ages 1 76 1 70 ga vary slightly from location to location and mimic the age of underlying basement unimodal detrital populations suggest local sources and a first cycle origin of the orthoquartzites within a short time interval 1 70 1 68 ga during unroofing of local underlying basement the maximum age of quartzite exposed at blue ridge colorado is constrained by the 1705 1698 ma coarse grained granitoid basement on which quartzite was deposited unconformably the minimum age of ortega formation quartzite new mexico is constrained by ca 1680 1670 ma metamorphic monazite overgrowths these dates agree with direct ages on the lower mazatzal group arizona and suggest that orthoquartzite deposition occurred over a wide region during and soon after the ca 1 70 ga yavapai orogeny regional structural arguments and the thrust style of quartzite deformation suggest that the metasedimentary successions were deformed during the ca 1 66 1 60 ga mazatzal orogeny thus making them important time markers separating the yavapai and mazatzal orogenic events our model for syntectonic deposition involves extensional basin development followed by thrust closure possibly due to opening and closing of slab rollback basins related to outboard subduction the first cycle origin of orthoquartzites near the end of the arc collisions of the yavapai orogeny seems to contrast sharply with their extreme compositional maturity this can be explained in terms of protracted extreme diagenesis and or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the proterozoic similarities among quartzites exposed throughout the southwestern united states and along the laurentian margin suggest that they represent a widespread regional and perhaps global episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions a combination that might make these units a signature lithology for paleoproterozoic time

late paleoproterozoic climate tectonics and metamorphism in the southern lake superior region and proto north america evidence from baraboo interval quartzites abstract red supermature quartzites of the baraboo interval of the lake superior region contain detrital zircon that ranges in age from 1782 to 1712 ma deposition clearly occurred after the geon 18 penokean orogeny these late paleoproterozoic sedimentary rocks consist largely of quartz kaolinite or pyrophyllite and hematite detrital feldspar and muscovite are rare or absent their chemical index of alteration ranges from 96 8 to 98 6 among the most chemically mature clastic sediments in the geological record the quartzites are underlain by mature feldspar free paleosols accounting for the absence of feldspar in the overlying sediments and indicating the presence of first cycle quartzose detritus such physical and chemical characteristics imply that late paleoproterozoic deposition in the lake superior region occurred in a stable tectonic setting with subdued topographic relief in a warm humid climate folding and low grade metamorphism of the quartzites is thought to reflect 1630 ma foreland

geochemistry and geochronology of meta igneous rocks from the tokat massif north central turkey implications for tethyan reconstructions located in the eastern pontides of the sakarya zone in north central turkey the tokat massif records the closure of both the paleo tethyan karakaya complex and neo tethyan ocean basins meta igneous samples collected from the region were studied to determine their sources and ages we find significant geochemical differences between metagabbros of the karakaya and neo tethyan units in terms of their trace elements neo tethyan rocks are consistent with generation in an island arc setting whereas karakaya assemblages were likely generated in an oceanic spreading center environment karakaya metagabbros also contain glaucophane consistent with subduction subsequent to formation small 2 50 m zircon and baddeleyite grains from four karakaya metagabbros were dated in thin section using an ion microprobe the results demonstrate the reliability of the method to directly constrain the tectonomagmatic history of these types of assemblages the rocks yield late permian early triassic 238u 206pb crystallization ages of 258 14 ma 1 zircon and 254 8 ma 1 baddeleyite and an early cretaceous minimum metamorphic age of 137 8 ma 1 zircon some zircon grains and baddeleyite grains with zircon overgrowths yield early to middle jurassic ages here we present a model in which metamorphism and deformation in this region occurred during northward subduction and closure of a paleo tethyan ocean basin and accretion of the karakaya units to the laurasian continental margin this was followed by the onset of closure of the neo tethys during the campanian paleocene and accretion of island arc units to the tokat region

heterogeneous deformation in a ductile fold thrust belt the proterozoic structural history of the tusas mountains new mexico the presence of widely variable deformational styles in the proterozoic terranes of northern new mexico has led to conflicting interpretations of the geologic history of the region proterozoic rocks of the tusas mountains display a range of deformational styles typical of northern new mexico and suggest a model for interpreting the heterogeneity three broad types of structural domains have been identified 1 regions characterized by complex polyphase fold patterns 2 regions dominated by a single intense schistosity and 3 regions with nearly pristine primary sedimentary and volcanic structures domain boundaries range from continuous deformational gradients to localized faults or shear zones structural and metamorphic evidence suggests that all domains were deformed in a single regional shortening event the deformation occurred during prograde metamorphism at upper greenschist and lower amphibolite facies conditions despite the metamorphic grade kilometer thick massive units were shortened by folding and thrusting producing geometries similar to those of thin skinned deformational belts regions located beneath the ramps of large scale ductile thrusts preserve primary structures and simple apparent deformational histories because much of the regional strain was partitioned to the overlying faults regions structurally above ramps display complex multiple fold patterns with strain histories that progress from subhorizontal shearing to upright folding and shortening regions away from ramps have an intense multi phase schistosity and a deformational history involving progressive simple shearing the large scale of the domains as much as 75 km 2 and the extreme structural heterogeneity from domain to domain may reflect at least in part the competence homogeneity and extreme thickness 1 km of the ortega orthoquartzite spectacularly refolded and multiply sheared regions in the tusas mountains reflect complex local strain environments that cannot be correlated from domain to domain the kinematic histories of these environments are locally important but are misleading on a regional scale the regional kinematic history was relatively simple involving a single shortening event deformational styles in other proterozoic terranes in northern new mexico are directly comparable to those of the tusas mountains and can be interpreted in terms of this large scale heterogeneous shortening model

environmental effects monitoring at the terra nova offshore oil development newfoundland canada program design and overview an environmental effects monitoring eem program was developed by suncor formerly petro canada in 1997 98 to assess effects of the terra nova offshore oil and gas development on the receiving environment the terra nova field is located on the grand banks approximately 350 km southeast of newfoundland canada at approximately 100 m water depth the eem program was developed with guidance from experts in government academia and elsewhere and with input from the public the eem program proposed by suncor was accepted by canadian regulatory agencies and the program was implemented in 2000 2001 2002 2004 2006 2008 and 2010 with pre development sampling in 1997 the program continues to be implemented every two years eem includes an assessment of alterations in sediment quality through examination of changes in sediment chemistry particle size toxicity and benthic invertebrate community structure a second component of the program examines potential effects on two species of commercial fishing interest iceland scallop chlamys islandica and american plaice hippoglossoides platessoides chemical body burden for these two species is examined and taste tests are performed to assess the presence of taint in edible tissues effects on american plaice bioindicators are also examined a final component of the program assesses potential effects of the terra nova development on water quality and examines water column chemistry chlorophyll concentration and physical properties the papers presented in this collection focus on effects of drill cuttings and drilling muds on the seafloor environment and as such report results on sediment quality and bioaccumulation of drilling mud components in iceland scallop and american plaice this paper provides information on drilling discharges an overview of the physical oceanography at the terra nova field and an overview of the field program designed to assess environmental effects of drilling at terra nova

detrital zircon ages from early proterozoic quartzites wisconsin support rapid weathering and deposition of mature quartz arenites abstract the ages of more than 200 detrital zircons from two proterozoic quartzites the baraboo and hamilton mounds quartzites from central wisconsin have been measured utilizing a laser ablation icpms technique both quartzites contain penokean 1 83 1 89 ga and archean components as well as a younger post penokean component the 69 detrital zircons from hamilton mounds contain major pre penokean age populations at 3 2 2 9 and 2 55 ga penokean ages at 1895 1853 and 1811 ma and a post penokean population at 1772 ma most detrital zircon ages are similar to previously measured ages of locally underlying gneisses and intrusive rocks of the marshfield terrane the 147 detrital zircons from two samples of the baraboo quartzite show a grossly similar age distribution with populations at 1754 and 1850 ma and 2 55 ga however the zircon populations in the baraboo quartzite differ from those of hamilton mounds in having a large population at 1920 ma a broad distribution of ages from 2 0 to 2 4 ga a