critical storm thresholds for the generation of significant dune erosion at dziwnow spit poland abstract storm influence on the southern baltic coast was examined for a 14 km long section of the polish coast along the dziwnow spit based on 30 years of post storm dune erosion observations in this study we assumed that a significant storm causes noticeable sand dune erosion significant changes of the coast after each major storm were analysed based on reports from 1978 to 2008 provided by the maritime office szczecin thirty two storms that caused dune losses were chosen for analysis correlation and hierarchical cluster analysis allowed us to distinguish three groups of storms that caused varying amounts of dune erosion g1 small g2 medium and g3 large sea level is the main parameter that influences the amount of dune erosion caused by storms on the southern baltic coast factors such as maximum significant wave height mean wave direction storm energy and duration can also but less significantly influence dune erosion the study area was divided into two sections dziwnow where there are many protection measures in place and miedzywodzie which has no protection measures in place in both areas we observed differences in the impact of sea level and significant wave height and storm groupings approximate minimum sea level and significant wave height thresholds were defined for particular storm groups

tectonic implications of finite strain variations in baraboo interval quartzites ca 1700 ma mazatzal orogen wisconsin and minnesota usa abstract variably deformed baraboo interval quartzites 1750 1630 ma are now correlated by detrital zircon ages holm d schneider d coath c d 1998 age and deformation of early proterozoic quartzites in the southern lake superior region implications for extent of foreland deformation during final assembly of laurentia geology 26 907 910 and were analyzed using finite strain techniques which document a se nw tectonic shortening orientation from the baraboo syncline south to the flambeau syncline north fabric interpretations using bedding and the principal strain axis orientations indicate there was a mix of layer parallel and layer normal synfolding shortening in the plane of tectonic transport se nw in the thrust belt finite strain magnitudes increase from the foreland barron sioux quartzites south toward the baraboo syncline electron backscatter device ebsd fabric diagrams pole figures of quartz optic axes also demonstrate an increasingly penetrative strain to the south anisotropy of low field ac magnetic susceptibility ams as a strain proxy preserve kmin measurements in the baraboo syncline quartzites that are sub horizontal e w and intersect bedding ams measurements in underlying rhyolite on opposite limbs of the baraboo syncline are clean appear to be primary magmatic as kmax intersects bedding but require a complex folding interpretation that is inconsistent with field kinematic indicators the flat lying foreland barron and sioux quartzites preserve a mix of results quartzites with no strain finite strain and ebsd measurements or small strains that preserve vertical shortening ams measurements in the foreland preserve kmin axes that are sub horizontal and parallel to the regional shortening orientation in the adjacent thrust belt the mazatzal orogen produced a thrust belt that is orders of magnitude wider 500 km and of higher metamorphic grade than most thrust belts where both the thrust belt and foreland are quartzites

post depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east atlantic abstract two cores from a ne atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age thickness and composition diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes the diagenetic process is thought to be driven by bacterially mediated organic oxidation and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process in one core a long turbidite section emplaced 330 000 years ago is overlain by clay organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a u concentration peak below the oxidation front marked by a colour change the u source is believed to be u initially associated with the organic matter in the oxidised section of the turbidite vanadium and cu behave similarly but upward migration of the redox sensitive metals mn and ni is also seen in the second core an 8 cm turbidite section was emplaced about 170 000 years ago in a clay column in this case organic consumption now appears complete but evidence for diagenetic effects is found in a 16 cm compositional alteration halo of the underlying pelagic clay fe ii has been enriched and the hydrogenous component of mn co ni and cu removed from the pelagic clay to form the halo

early organic diagenesis the significance of progressive subsurface oxidation fronts in pelagic sediments abstract porewater and solid phase geochemical data at two contrasting ne atlantic stations are reported station 10552 on the cape verde abyssal plain is a site of slow pelagic accumulation ca 0 4 cm kyr 1 molecular oxygen is present in the sediment column to at least 2 m and probably much deeper labile organic carbon is almost totally consumed in the upper few centimetres of the sediment by contrast at station 10554 on the madeira abyssal plain the pelagic sequence has been interrupted by the occasional deposition of organic rich turbidites porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column this metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time this phenomenon exerts a controlling influence on the present nutrient profile and redox succession in a less extreme form substrate distributions of this latter type are not uncommon in atlantic sediments a model has been developed which is controlled by both oxygen and nitrate data this model permits a vertical profile of metabolic activity to be derived and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations about 30 of the total activity at station 10554 is located within the turbidite at the deepening reaction front this is a non steady state condition in fact it is found that the integrated metabolic activity at the two stations is not dissimilar ca 1 2 10 13 moles cm 2 sec 1 the striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column

new insights into the southern margin of the archean proterozoic boundary in the north central united states based on u pb sm nd and ar ar geochronology abstract new geophysical analysis of the precambrian basement in minnesota iowa wisconsin indicates that an archean proterozoic boundary spirit lake trend previously recognized in nw iowa can be continued eastward into central wisconsin and farther east as the spirit lake tectonic zone sltz to test the age of paleoproterozoic crust south of this structure several subsurface samples of precambrian basement from the north central united states have been analyzed or re examined using modern techniques of u pb sm nd and 40ar 39ar geochronology the results fill in a major data gap for the region and show that all u pb crystallization ages for samples south of the sltz are geon 17 1700 1800 ma bedrock core samples from eastern nebraska are ca 1760 1800 ma and two samples from se south dakota immediately south of the sltz yield ages of 1762 28 ma vermillion and 1733 2 ma elk point xenoliths from impact breccia in the manson structure in north central iowa yield a similar age of ca 1705 30 ma and metagabbro from se minnesota yields an age of 1760 9 ma farther to the northeast zircons from paleoproterozoic gneiss in the basement of manitoulin island only a few km south of the superior craton in ontario also yield a geon 17 age 1714 10 ma sm nd model ages tdm for samples immediately south of the sltz fall in the range 1 9 2 2 ga indicating limited involvement of archean crust in contrast sm nd tdm ages for samples north of the sltz typically range from 2 5 to 3 0 ga for both paleoproterozoic plutons and archean gneisses ion microprobe analyses of zircons from the elk point and manson samples also show the presence of geon 16 overgrowths indicating a strong regional thermal overprint during geon 16 accretion this is supported by mid geon 16 hornblende 40ar 39ar ages for samples from se south dakota and se minnesota although no u pb ages are available for juvenile basement beneath the ca 1760 ma granite rhyolite suite of southern wisconsin south of the sltz sm nd model ages are typically ca 1 9 2 0 ga consistent with basement to the rhyolites being geon 17 in age collectively the data require that most if not all of the paleoproterozoic crust immediately south of the sltz formed during geon 17 and probably represents eastward continuation from colorado through nebraska of the yavapai crustal province in the sw united states penokean geon 18 crustal rocks are limited mainly to northern and central wisconsin east central minnesota and northern michigan these results also show that medium grade 500 c tectonothermal effects of the subsequent geon 16 mazatzal orogeny to the south continue into the north central united states both terranes probably also continue eastward into ontario canada and farther east into protolith of the grenville province

strain variations across the proterozoic penokean orogen usa and canada abstract strata in the huron 2 5 2 0 ga and animikie 2 2 1 85 ga basins were deposited on the southern margin of the archean superior province these rocks were deformed during the penokean orogeny 1850 ma followed by subsequent accretionary orogens to the south at 1750 ma yavapai and 1630 ma mazatzal strain patterns are unique to each orogenic belt with no far field effect archean wawa terrane rocks in the penokean foreland preserve deformation associated with archean accretion with no younger penokean yavapai or mazatzal strain overprint the penokean orogeny deformed huron animikie basin sediments into a north vergent fold and thrust belt with no yavapai or mazatzal strain overprint yavapai orogen strains sw ne margin parallel shortening are unique when compared to the younger mazatzal shortening n20 w shortening with no strain overprint penokean deformation is characterized by shortening from the south including uplifted archean gneisses and a northerly thin skinned fold and thrust belt with north vergent nappes and a gently dipping foreland our study of finite and calcite twinning strains n 60 along 1500 km and across 200 km the penokean belt indicate that this orogeny was collisional as layer parallel shortening axes are parallel across the belt or parallel to the tectonic transport direction n s penokean nappe burial near the margin resulted in vertical shortening strain overprints some of which are layer normal the sudbury impact layer 1850 ma is found across the animikie basin and provides a widespread deformation marker with many local unique strain observations we also report new geochronology u pb zircon and apatite for the gneiss mafic dike rocks at wissota chippewa falls wi and arbutus black river falls wi dams respectively which bears on penokean yavapai deformation in the archean marshfield terrane which was accreted during the penokean orogen pseudotachylite formation was common in the superior province archean basement rocks especially along terrane boundaries reactivated by contemporaneous penokean trans hudson cape smith and new quebec deformation in the hinterland south the younger yavapai orogen 1750 ma n 8 deformation is preserved as margin parallel horizontal shortening sw ne in yavapai crust and up to 200 km to the north in the penokean thrust belt as a strain and barrovian metamorphic overprint mazatzal deformation 1630 ma n 16 is preserved in quartzites on yavapai and penokean crust with layer parallel and layer normal shortening strains oriented n20 w

geometry and folding history of the baraboo syncline implications for the mazatzal orogeny in the north central u s abstract the baraboo syncline in south central wisconsin formed during the mazatzal orogeny it is a doubly plunging asymmetric syncline internal structures such as chocolate tablet style boudinage and tension gashes indicate stretching parallel to the fold axis and preclude folding by plane strain models both the fold geometry and the internal deformation within the fold limbs suggest a history of 1 buckling that varied along the fold axis resulting in doubly plunging folds and stretching along the fold axis followed by 2 continued buckling combined with top to the south simple shearing that may have varied along the fold axis the second stage of deformation could be the result of the onset of an overriding thrust package the deformation history is consistent with a typical south verging fold forming in a fold and thrust belt that inherited its geometry from a north dipping subduction zone

proterozoic metamorphism and cooling in the southern lake superior region north america and its bearing on crustal evolution abstract metamorphism along the southern margin of the archean superior province has been historically attributed to the penokean orogeny a narrow corridor of amphibolite facies rocks north of the main suture does record 1 83 1 80 ga metamorphic monazite u th pb ages that mark the culmination of arc accretion however subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest penokean crustal thickening the tectonically buried archean proterozoic continental margin in minnesota new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the 1 775 ga east central minnesota batholith at moderate depths in northern wisconsin and upper peninsula michigan metamorphic monazite growth at 1 78 1 745 ga and far from geon 17 intrusions reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca 1 76 ga yavapai interval accretion not solely penokean induced crustal collapse south of the penokean suture penokean terrane rocks were twice metamorphosed to upper greenschist facies first during yavapai accretion and again during geon 16 mazatzal accretion geon 16 overprinting also affected a small part of the continental margin in the northeast orogen the peavy metamorphic node south directed basement thrusts there likely accommodated substantial mazatzal foreland shortening suggesting thick skinned deformation mazatzal amphibolite facies metamorphism occurred throughout iowa and southernmost wisconsin south of the baraboo quartzite 40 ar 39 ar mineral cooling ages from eastern wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1 47 ga wolf river batholith in part reflecting its rapid emplacement at shallow crustal levels a local area of anomalously young 40 ar 39 ar mica ages away from known midcontinent rift exposures probably reflect shallow crustal level reheating during rifting not exhumation the pattern and degree of proterozoic metamorphism preserved across the northern interior of the north american midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1 87 and 1 47 ga collectively the geo and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of proterozoic metamorphism in this region these results demonstrate a changing tectonic environment from active margin to continental interior as laurentia grew southward and stabilized over a few hundred million years

paleoproterozoic crustal history of the southwestern grenville province evidence from nd isotopic mapping nd isotopic mapping in the north bay area of the central gneiss belt southwestern grenville province has revealed the precise trend of a tdm model age line developed between the uplifted southern margin of the archean superior craton tdm 2 7 ga and a paleoproterozoic allochthon tdm 1 9 ga separating these two crustal blocks is a narrow zone of gneisses with intermediate tdm ages these transitional gneisses are interpreted to reflect a remnant fault or ductile shear zone of uncertain age along which crustal material from both blocks mechanically mixed during their juxtaposition accordingly the nature of the tdm line in the north bay area is interpreted to be tectonic in the temiscaming area widespread exposures of mature metasedimentary gneisses are shown by their tdm ages to be dominantly of paleoproterozoic provenance these results are consistent with the existing detrital zircon geochronology inferring a maximum depositional age of 1 7 ga the anorogenic chemistry of the north bay o

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

u pb baddeleyite crystallization age for a corson diabase intrusion possible midcontinent rift magmatism in eastern south dakota the largely buried basement of the northern great plains includes suture zones and terrane boundaries that represent a significant part of the growth of laurentia in the proterozoic basement exposures in this region east of the black hills are rare in southeastern south dakota southwestern minnesota northeastern nebraska and northwestern iowa small outcrops of the proterozoic sioux quartzite occur in southeastern south dakota corson diabase sills or dykes have intruded the quartzite u pb id tims baddeleyite data from a corson diabase sample yield an upper intercept date of 1149 4 7 3 ma suggesting the diabase is related temporally to the midcontinent rift mcr the similarity in age of this diabase to the inspiration sill pigeon river kipling and abitibi dykes suggests that early midcontinent rift development was not localized within the nipigon embayment but extended along a roughly east northeast zone from the abitibi dykes to the corson diabase the presence of the corson intrusions 25

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

quartz arenites review and interpretation abstract quartz arenites orthoquartzites are exceptional sands pettijohn et al 1972 that in the past have formed extensive and commonly thick formations they are somewhat enigmatic in that they do not appear to be forming at the present time this paper examines the origin of quartz rich sands documents the range of environments in which they are found and suggests a model for their genesis quartz sand is most commonly formed by tropical weathering of coarse grained quartz bearing rocks such as granite it may be deposited as texturally immature fluvial sandstones generally under a humid climate with drying of the climate quartz rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts quartz sand when transported into shallow marine environments becomes very well rounded through reworking by tidal currents such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings quartz rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones precambrian orthoquartzites are commonly thick and widespread when compared to younger examples the thinness of post silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles there is an abundance of shallow marine and fluvial quartz arenites in the early paleozoic and the cretaceous this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents the shallow marine examples are commonly associated with bituminous sometimes phosphatic shale glauconite and possibly chamositic ironstone there is an abundance of eolian quartz arenites in the permo triassic this is related to a widespread dry continental climate during assembly of the pangean supercontinent

the uranium mineralization potential of the paleoproterozoic sioux basin and its relationship to other basins in the southern lake superior region abstract the paleoproterozoic sioux basin and the baraboo barron flambeau mccaslin and waterloo quartzites in northcentral usa are all supermature quartz arenites that are among the world s oldest redbeds these quartzites were deposited on a stable craton at ca 1750 ma following exhumation and uplift of penokean crust and major orogenic activity related to the amalgamation of archean microcontinents at ca 2 0 1 8 ga to form laurentia 40ar 39ar ages of 1615 1543 ma from diagenetic muscovite in the sioux basin provide the first geochronological evidence for mazatzal related deformation and low grade metamorphism of the quartzites during subsequent convergence on the southern margin of laurentia one muscovite sample from the sioux basin has a 40ar 39ar age of 1465 ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40ar 39ar ages for muscovite from the baraboo and waterloo quartzites thus the sioux basin and other quartzites in the region have experienced a similar tectonic and fluid history the sioux basin has similar sedimentologic characteristics as other paleoproterozoic basins in north america including the uranium rich athabasca basin canada that is host to some of the world s largest and highest grade unconformity type uranium deposits we compared key characteristics such as the stratigraphy character of diagenetic fluids paleoaquifer characteristics and composition of leachable pb in the sandstones from the sioux basin with the athabasca basin in order to evaluate the fluid history of the basin and the potential of the sioux basin to host unconformity type uranium mineralization based on the hydrogen and oxygen isotopic composition of muscovite m1 that formed during diagenesis at 150 200 c the pore fluids that were present during peak diagenesis in the sioux basin had 18o and d values of 0 5 to 5 2 and 32 to 46 respectively similar to those of the fluids associated with uranium mineralization and alteration in the athabasca basin however fluid flow in the sioux basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers in contrast to the extensive paleoaquifers in the athabasca basin that promoted large scale diagenetic reactions and alteration that led to the formation of unconformity type uranium mineralization in addition the non radiogenic isotopic composition of leachable pb in the sioux basin sandstones indicates that the majority of the pb is from a normal crustal source inconsistent with mobilization of radiogenic pb from a uranium rich source such as a uranium deposit although the sioux basin shares similar general characteristics to the athabasca basin the results of this study suggest that it has a lower potential than the athabasca basin to host significant high grade unconformity type deposits

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

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

middle proterozoic uplift events in the dunbar dome of northeastern wisconsin usa isotopic ages of granitic and metamorphic rocks exposed in the dunbar structural dome of northeastern wisconsin identify a protracted series of tectonic and hydrothermal events that culminated in major regional uplift during middle proterozoic keweenawan ca 1 100 ma continental rifting and volcanism the major rock forming events and the structural development of the dome occurred during the interval 1 862 4 ma to 1 836 6 ma whole rock rb sr ages are partly reset in response to a widely recognized but cryptic event in wisconsin and michigan at about 1 630 ma the scale and systematic character of the whole rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low grade metamorphic rocks the regional nature of the 1 630 ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south rb sr biotite ages for the dunbar dome this study the southern complex of the marquette district van schmus and woolsey 1975 and the felch trough area aldrich and others 1965 provide a remarkably coherent pattern that reflects multiple episodes of differential uplift younger events superimposed on a regional 1 630 ma imprint are recorded at 1 330 ma and 1 140 ma the 1 330 ma disturbance could reflect stabilization following intrusion of the wolf river batholith at 1 485 ma the 1 140 ma uplift event occurred during keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault bounded blocks with possible subcrustal influence the remarkably small variance in the 1 140 ma biotite age peak argues for rapid uplift and cooling and hence rapid erosion detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion

the behaviour of manganese in atlantic carbonate sediments abstract manganese porewater and solid phase data are considered from two different atlantic carbonate sediments both with good geochemical characterisation the first case is a continuously accumulated alternating pelagic marl carbonate ooze where there is a correlation in post oxic conditions between mn ii porewater levels and an acetate leachable mn fraction of the sediment this leachable mn appears to have an association with calcite and in this example the leached residue mn content approximates to that of shale in the second case a very recent compositionally homogeneous calcareous turbidite is underlain by mn oxyhydroxide at the former sediment surface which is now acting as a source of mn ii by reduction sorption of mn by the turbidite sediment and shale like leached residue mn levels in anoxic conditions are again observed as mn ii diffuses towards present oxic conditions although no unique equilibrium is achieved similar older buried turbidites have concentration decreases in solid phase mn content preserved from bottom to top as relict evidence that this process has occurred in these turbidites 54 64 of the total mn is leachable by the acetate method and therefore presumed to be sorbed by calcite the oldest example has persisted for 125 kyr it is suggested that the slow breakdown of metastable mn oxyhydroxide and sorption of mn ii to calcite surfaces are important controls on the behaviour of mn in pelagic carbonates in mildly reducing conditions where accumulation fluctuations occur these processes act together with accumulation to limit overall mn migration and to prevent the large mn enrichments seen near the sediment surface in more reducing environments