rift propagation detachment faulting and associated magmatism in bransfield strait antarctic peninsula multichannel seismic data from bransfield strait a young 4 ma marginal basin located at the nw tip of the antarctic peninsula reveal a complex array of rifting styles short of clearly defined seafloor spreading the strait is undergoing basinwide extension while enriched mid ocean ridge basalts have been dredged from topographic highs in the ne part of the basin seismic evidence does not support the formation of oceanic crust there acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor their distribution and style support propagation of volcanism and associated extension from ne to sw active extensional structures along both margins control a strong physiographic asymmetry the south shetland islands margin is composed of a few closely spaced large offset normal faults the gentler antarctic peninsula margin exhibits broader distributed extension to the ne normal fault polarity reversal structures nfprs dominate listric faults sole into nw dipping low angle reflectors interpreted as detachments to the sw larger nfprs and convex up geometries up to 20 km across suggest broad uplift rather than detachment based extension such uplift may precedes seafloor volcanism and rapid associated deflation along detachments nw se trending accommodation zones also segment the basin along strike bransfield strait extension may result both from plate boundary tectonics at the south shetland trench and at the evolving antarctic scotia plate boundary extension was probably initiated by onset of subduction rollback at the trench as a slab window opened from the sw augmented extension in the ne part of the basin may be a response to left lateral transtension associated with reorganization of the triple junction at the shackleton fracture zone south scotia ridge intersection

late jurassic bimodal magmatism in the northern sea floor remnant of the rocas verdes basin southern patagonian andes magmatic and detrital zircon ages from the rocas verdes basin a tectonically juxtaposed remnant of sea floor in the magallanes fold and thrust belt southern patagonia south america indicates that a rifting phase of the rocas verdes basin occurred between 152 and 142 ma and was accompanied by bimodal magmatism a dacite dyke cross cutting pillow basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal sarmiento ophiolite complex contain 150 ma zircon crystals indicating that mafic submarine volcanism had started prior to or during the late jurassic 10 15 ma earlier than previously thought the silicic pyroclastic rocks of the tobifera formation with two samples dated at 148 and 142 ma were heralded by synrift sedimentation along fault bounded grabens within palaeozoic metasediments no evidence for an active volcanic arc during the early formation of the rocas verdes basin was detected in detrital zircon grains of the lower sedimentary member of the tobifera formation a minimum of 25 ma of continuous sedimentation in the rocas verdes basin is suggested by detrital zircon grains in the upper member of the zapata formation the rocas verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution

relationship between uplift of the metamorphic core of the southernmost andes and shortening in the magallanes foreland fold and thrust belt tierra del fuego chile rocks containing upper amphibolite facies assemblages reflecting mesozoic cenozoic metamorphism form the 5000 km2 core of the southernmost andes in cordillera darwin approximately 55 s latitude results from mapping along a 50 km continuous transect across the southernmost andes on the main island of tierra del fuego show structural relationships between the late cretaceous to early tertiary uplift of the metamorphic core in northern cordillera darwin and mid cretaceous to paleogene shortening in the adjacent magallanes foreland fold and thrust belt new data indicate that pre late jurassic basement schists exposed along the northeastern edge of cordillera darwin were uplifted along a discrete brittle continent vergent thrust fault this thrust overlies a series of thin skinned thrusts and back thrusts that affect mesozoic tertiary rocks of the magallanes foreland basin the thrust that uplifts basement schists is interpreted to have formed out of sequence with respect to the thin skinned structures because it placed the schists over previously shortened lower cretaceous sedimentary strata of the magallanes basin the geometry of thrusts observed across the transect and their inferred sequence of deformation indicate a thick skinned contractional mechanism for uplift of the pre late jurassic schists of northern cordillera darwin relative to the structural level of metamorphic basement in the magallanes foreland the relative time of out of sequence thrusting and basement uplift inferred from thrust geometries in this paper is consistent with the latest cretaceous to early tertiary episode of rapid exhumation and cooling of the metamorphic core of the southern andean orogen determined in previous structural metamorphic and geochronological studies

andean sinistral transpression and kinematic partitioning in south georgia the island of south georgia exposes remnants of a late jurassic to early cretaceous andean magmatic arc and marginal basin system that was compressively deformed during the mid cretaceous main andean orogeny forming widespread nw se trending folds and a coaxial penetrative cleavage displaying a predominantly ne sw stretching lineation detailed structural studies of the cooper bay to cape valisel area of south georgia reveal that intense mid cretaceous polyphase deformation was strongly influenced by sinistral strike slip shear parallel to the nw se regional structural grain and along a major pre existing fault which we interpret as the partitioned wrench component of bulk transpressional deformation the relationship between fold axial plane orientation and interlimb angle of widely distributed mesoscale folds is consistent with counter clockwise rotation and fold appression as a result of sinistral simple shear deformation suggesting kinematic strain partitioning of the wrench component was on the whole highly efficient locally the modification of steep tectonic anisotropies to shallow inclinations during d 2 deformation induced imperfect or inefficient partitioning with fold arrays exhibiting fold appression characteristic of a transpressional deformation path our partitioned transpression model for main andean deformation of south georgia fits well with tectonic interpretations of the cordillera darwin patagonia crown copyright c 2010 published by elsevier ltd all rights reserved

neolithic to copper age woodland impacts in northeast hungary evidence from the pollen and sediment chemistry records a 4 5 m sedimentary sequence collected from sirok n rjes to in northeast hungary reveals a record of environmental change from 10000 bp to the present day palaeoenvironmental analyses pollen charcoal geochemistry and physical properties of the sequence reveal an environment where parkland com posed of coniferous and broadleaved taxa existing at the start of the holocene developed rapidly into deciduous broadleaved forest from c 8200 cal bp corylus expanded rapidly and dominated the pollen assemblage until a series of fluctuations involving corylus and carpinus betulus which resulted in changes to the forest compo sition these changes occurred over timescales of 72 288 years and are attributed to neolithic and copper age forest use perhaps under a woodland management regime such as coppicing or pollarding sedimentolog ical and geochemical records for this period are stable suggesting that soil erosion was not occurring natural forest processes resumed following copper age settlement shifts bu

environmental changes during the holocene climatic optimum in central europe human impact and natural causes abstract the priority programme changes of the geo biosphere aimed to reconstruct the environmental history of central europe with emphasis on the time interval from 9000 to 5500 cal bp time slice ii coinciding with the holocene climatic optimum during this period the onset of human activities such as settlement agriculture and animal husbandry caused environmental changes studies of different landscape units in germany were carried out to identify these anthropogenically induced changes and to distinguish them from natural effects on the environmental system the investigated archives included laminated lake sediments fluvial sediments colluvia and soils speleothems peat and coastal sediments the different archives were examined using refined research methods including a variety of sedimentary and geochemical analyses together with pollen analysis and dating methods for the establishment of a reliable chronology the results of the various research groups are summarised and critically discussed based on these results the climatic optimum can be subdivided into three periods 1 the early atlantic from 9000 to 7500 cal bp with negligible human impact and stable environmental conditions 2 the late atlantic during early and middle neolithic from 7500 to 6300 cal bp with pollen evidence for vegetation changes but only negligible changes detectable in other proxy records and 3 the late atlantic during the younger neolithic jungneolithikum after 6300 cal bp with human impact observed in many archives and proxy records especially in the pollen record but also in lacustrine and fluvial sediments during the whole climatic optimum natural causes such as minor shifts of temperature did not induce substantial environmental changes though some changes such as temporary droughts may have facilitated and amplified the observed human impact

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

palynology and natural environment in the pannonian to holocene sediments of the early medieval centre pohansko near břeclav czech republic abstract pohansko czech republic is an important early medieval centre of the great moravian empire 9th century ad the locality has a settlement with archaeological findings from mesolithic to the modern times the studied sediments belong to the stream and flood deposits of the dyje river aeolian sands and buried soils the lower parts of some boreholes penetrated into the upper miocene sediments 172 samples were palynologically analysed the original forest with relative small human impact was observed on the base of holocene layers even sporadic cerealia pollen were found here a deforestation was visible in the cultural layer it is linked with existence of the great moravian agglomeration and its fortification later a partial forest reconstruction probably took place in the surroundings very abundant pollen of human exploited plants cereals herbs were discovered in the filling of archeological feature o1 the possible practical potential of rampart also as the flooding protection was indicated by the existence of marshy plants palynomorphs the existence of oxbow was proved in the excavated probe s3 the lowermost sample was dated by 14c as 7830 60 bp the findings of carpinus pollen in the layer dated as early atlantic support the earlier spreading of this genus in the czech republic i e south moravia

holocene environments of the central great plains multi proxy evidence from alluvial sequences southeastern nebraska abstract pollen plant macrofossils phytoliths carbon isotopes and alluvial history from sediments exposed along the south fork of the big nemaha river southeastern nebraska usa provide an integrated reconstruction of changes in holocene vegetation climate and fluvial activity from 9000 to 8500 uncalibrated 14 c yr bp climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly upland deciduous forest declined and prairie attained its holocene dominance from 8500 to 5800 yr bp upland forest elements disappeared and even riparian trees were sparse under dry climatic conditions alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr bp from 5800 to 3100 yr bp riparian forests returned to prominence and droughts were intermittent alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley aggradation on alluvial fans slowed and finally ceased near the end of this period during a short dry interval from 3100 to 2700 yr bp riparian trees except elm disappeared and prairie and weedy species became more abundant this interval is represented by the organic roberts creek member and the alluvial setting was a slightly incised meandering channel belt habitats became similar to presettlement conditions during the last 2700 yr bp weedy taxa dominate modern samples reflecting widespread disturbance alluvial fans and terrace surfaces were stable during the last 2500 years but episodes of floodplain aggradation were punctuated by incision of the main channel

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

the tectonographic development of patagonia and its relevance to hydrocarbon exploration patagonia accreted successively from the southwest onto the southern margin of the proterozoic plata craton and brazilian guapore shield between the late proterozoic and early devonian the thrust like stacking of terranes onto the southern termination of the pelotas terrane is considered to have developed a pervasive northwest to north trending fabric during the permo triassic the northwest to north trending fabric of the patagonian plate was re activated by dextral strike slip movement causing extension the deformation was caused by oblique subduction and accretion of the madre dos dios to pichidangui terranes along its western margin to the northeast the more competent shield underwent compression ventania gond wanide folding and extension occurred parallel to the axis of the embryo south atlantic where a shallow sea transgressed ridge on its western side now preserved on the facing shelf margins of argentina and namibia in the late triassic lower jurassic the malvinas and microplate was situated south of the transkei south africa and an intracratonic basin separated it from two sutures formed at the margin of the argentine shelf and along the axis of the west malvinas basin subduction arc activity on the west flank of this intracratonic basin in association with trench pull is believed to have initiated late triassic earlymore jurassic strike slip extension and volcanicity in patagonia this exploited the pervasive northwest and north trending paleozoic fabric by the mid jurassic the malvinas microplate had docked with the eastern margin of the patagonian shelf and was undergoing clockwise rotation between the malvinas agulhas and burwood bank scotia ridge dextral strike slip systems rifting had now progressed southwestwards to the pacific and north eastwards to the colorado and outeniqua basins less

evolution of the holocene mississippi river floodplain ferriday louisiana insights on the origin of fine grained floodplains abstract the alluvial architecture and soil characteristics of holo cene mississippi river floodplain deposits in the southern lower mississippi valley provide evidence for significant changes in floodplain development in response to sea level rise floodplain cores acquired near ferriday louisiana show that holocene deposits consist of 15 30 m ave 20 m of sands silts and clays which overlie late wisconsin sands and gravels on the basis of differences in sediment grain size sediment body geometry and the abundance of soil features the holocene deposits are subdivided into lower and upper holocene units lower holocene deposits 5000 yr b p consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite pyrite and vivianite and show little evidence of soil formation muds encase crevasse splay and floodplain channel sand bodies 1 km wide and collectively these deposits represent a mosaic of shallow lakes poorly drained backswamps and multichannel streams similar to modern examples in the atchafalaya basin 100 km south of ferriday upper holocene deposits 5000 yr b p are represented by large mississippi river meander belt sand bodies that are up to 15 km wide and 30 m thick natural levee silts and sands and well drained backswamp muds are present between meander belt sands upper holocene deposits contain abundant soil features and sandy and silty soils are entisols inceptisols and alfisols whereas clayey soils are vertisols the presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that lower holocene sediments reflect a period of rapid floodplain aggradation during which crevassing lacustrine sedimentation and avulsion dominated floodplain construction no evidence of large meandering mississippi river channels represented by buried thick tabular sands exists near ferriday and discharge in lower mississippi valley flow was probably conveyed by a network of small multichannel floodplain streams upper holocene sediments record a dramatic change ca 5000 yr b p from rapid to slower floodplain aggradation which was accompanied by extensive lateral channel migration overbank deposition and soil formation on the basis of differences in meander belt dimensions and numbers of abandoned channels upper holocene meander belts are subdivided into simple and complex forms relative age relationships suggest that the smaller and older simple meander belts represent periods of divided mississippi river flow and early attempts to establish a large single channel meandering regime this type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of holocene sediment accumulation in the southern lower mississippi valley strongly suggest that decelerating holocene sea level rise in the gulf of mexico affected floodplain development at least 300 km inland from the present day coast the alluvial architecture of the lower holocene deposits and the absence of large meandering mississippi river channel deposits older than 5000 yr b p near ferriday indicates that most of the floodplain muds were deposited by avulsion related crevassing and lacustrine sedimentation rather than by overbank flooding of large mississippi river channels similarities between the floodplain history of the mississippi river and those of modern and ancient rivers elsewhere further suggest that avulsion rather than simple overbank deposition contributes to the construction of fine grained floodplains to a greater degree than generally recognized

holocene paleoenvironments in southeastern minnesota chasing the prairie forest ecotone this paper uses a multi proxy approach involving pollen plant macrofossils speleothem isotopes and alluvial history of streams to reconstruct the history of prairie expansion and contraction along the prairie forest border of southeastern minnesota usa early holocene forests were replaced by prairie along this border but eastward expansion of prairie stalled for 2000 yr when the prairie forest ecotone stabilized prairie invaded the area from the west 8000 9000 yr b p but mesic forest remained less than 100 km to the east until about 6000 yr b p changes in n 13 c values in speleothem calcite that reflect the rise of c4 grasses correlate well with the presence of c4 grass species identified in the plant macrofossil record periods of large floods correlate with speleothem evidence of dry summers increased cool season precipitation both resulting in less plant cover to absorb moisture and change to prairie vegetation fl 2002 elsevier science b v all rights reserved

late pleistocene evolution of the lower mississippi river valley southern missouri to arkansas the mississippi river served as the principal conduit for meltwaters discharging from the southern margins of north american ice sheets the northern half of the mississippi river alluvial valley in missouri and arkansas can be subdivided into two distinct channel courses the western and eastern lowlands which are separated by crowleys ridge the eastern lowlands contain two entry points the bell city oran and thebes gaps the latter is a narrow bedrock gorge that contains the modern river this paper summarizes a model for evolution of channel courses in the western and eastern lowlands during the illinoian and wisconsin glacial stages for the western lowlands stratigraphic relations between loess units and subjacent fluvial deposits show that major continuous terrace surfaces or valley trains are illinoian or late wisconsin in age investigations of tributary valley fills further show the western lowlands were deeply incised during the early to middle wisconsin stage then aggraded during the late wisconsin stage to such an extent that tributary valleys were backfilled with slackwater deposits moreover analysis of loess thinning and fining trends shows that early to middle wisconsin roxana silt and the late wisconsin peoria loess covers on western lowlands valley trains were derived from deflation of valley trains in the eastern lowlands we conclude that the western lowlands were the primary mississippi river course during illinoian deglaciation but were abandoned shortly thereafter then episodically reoccupied during the late wisconsin stage for the eastern lowlands course valley trains of early to middle wisconsin age are not preserved with the exception of a small remnant where the mississippi river broke through a gap in the southern margins of crowleys ridge and reentered the western lowlands all valley trains in the eastern lowlands are late wisconsin age a series of discontinuous terrace surfaces in the bell city oran gap represent dissected remnants of the primary late wisconsin full glacial and early late glacial course of the mississippi river or combined mississippi and ohio rivers whereas younger continuous braided stream surfaces in the bell city oran and thebes gap courses represent valley trains of latest wisconsin and earliest holocene age hence the bell city oran gap was the primary course during the entire wisconsin stage and the thebes gap was permanently occupied in earliest holocene time as much as 5 m difference in elevation between successive late wisconsin terrace surfaces of the eastern lowlands indicates that valley trains were rapidly constructed incised and abandoned in response to fluctuations in the delivery of meltwater and outwash from the former ice margin moreover a variety of data indicates that deglacial meltwater floods were commonly of sufficient magnitude to overflow from the primary course in the bell city oran gap throughout the late wisconsin substage floodwaters overflowed into the western lowlands constructed a series of secondary valley trains and filled tributary valleys with slackwater deposits during the latest wisconsin stage perhaps in association with the drainage of glacial lake agassiz floodwaters overflowed into the western lowlands and broke through the thebes gap the charleston fan represents the initial deposits of the thebes gap course following deposition of a large crevasse splay at the western lowlands entrance that course was no longer active but flow was routed through the thebes gap during periods of high magnitude flooding until the bell city oran gap was abandoned and the thebes gap course was permanently occupied during earliest holocene time

a 10 000 year record of dune activity dust storms and severe drought in the central great plains dune fi elds and loess deposits of the great plains of north america contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought we present a 10 000 yr reconstruction of dune activity and dust production in the central great plains region based on 95 optically stimulated luminescence ages the integration of data from both eolian sand and loess is an important new aspect of this record clusters of ages defi ne episodes of extensive eolian activity which we interpret as a response to frequent severe drought at 1 0 0 7 ka and 2 3 4 5 ka with peaks centered on 2 5 and 3 8 ka sustained eolian activity occurred from 9 6 to 6 5 ka parts of this record may be consistent with hypotheses linking holocene drought to sea surface temperature anomalies in the pacifi c or atlantic oceans or to the el nino southern oscillation phenomenon but the record as a whole is diffi cult to reconcile with any of these hypotheses

a contemporary appraisal of some key fiskian concepts with emphasis on holocene meander belt formation and morphology abstract looking back five decades and reflecting on the contributions of harold fisk to lower mississippi valley lmv geological knowledge it is apparent that he was highly successful in making engineers aware of the critical role of glacial advance and retreat sea level changes valley slope and river gradient and time in affecting geomorphic processes sedimentary patterns and river regimes perhaps his greatest contribution to river engineering was in recognizing and elaborating on the effects of bed and bank materials on stream meandering fisk however could only work within the prevailing geological theory and method of his time with regard to glacial chronology and stratigraphy and such matters as global structure and tectonics major advances in these areas with new concepts and techniques have shown his lmv glacial response model to be oversimplified and invalid in some respects more cycles of valley filling and cutting are now known to be represented in the alluvial sequence and his concept of deep valley entrenchment as a direct response to sea level changes is untenable in light of substantial new data the presence of a regional rectilinear fracture pattern with a corresponding network of major fault zones has not been substantiated by subsurface data the most widely known aspect of fisk s work is his detailed interpretation and colorful depiction of the complex pattern of overlapping cross cutting and discontinuous holocene meander belts and their hundreds of abandoned channels however it is now recognized that fisk s elaborate chronological reconstruction is largely invalid both because of some incorrect basic assumptions and his lack of radiocarbon and other numerical dating techniques for precise control a newly emerging model of meander belt formation is more generalized and relies heavily on archaeological evidence it places emphasis on episodes of multiple active river courses partial flow channels frequent formation of distributary channels and the significant influence of mississippi river tributaries and minor basin drainage interesting regional variations over time in meander belt configurations and morphology may reflect climate changes but at present broad scale variations in the lithology of the quaternary alluvial sequence appear to be the dominant control

loess stratigraphy of the lower mississippi valley abstract loesses of the lower mississippi valley lmv are world famous sir charles lyell 1847 hilgard 1860 stafford 1869 call 1891 and mabry 1898 thought the lmv loess was a single water deposit although double submergence was noted by call 1891 and salisbury 1891 shimek 1902 and emerson 1918 recognized lmv loess as a wind deposit which came from the valley although wind deposited loess gained wide acceptance russell 1944a published his controversial theory of loessification which entailed weathering of backswamp deposits downslope movement and recharge by carbonates to form loess wascher et al 1947 identified three lmv loesses mapped distributions and strongly supported eolian deposition leighton and willman 1950 identified four loesses and supported eolian deposition as did krinitzsky and turnbull 1967 and snowden and priddy 1968 but krinitzky and turnbull questioned the deepest loess daniels and young 1968 and touchet and daniels 1970 studied the distribution of loesses in south central louisiana west et al 1980 and rutledge et al 1985 studied the source areas and wind directions which deposited the loesses on and adjoining crowley s ridge b j miller and co workers miller et al 1985 1986 miller and alford 1985 proposed that the loveland silt was early wisconsin rather than illinoian age and advanced the name sicily island loess they proposed the underlying loess was illinoian and advanced the name crowley s ridge we termed the loesses from the surface downward peoria loess roxana silt loveland sicily island loess crowley s ridge loess and marianna loess researchers agree that the surfical peoria loess is late wisconsin and the roxana silt is late to middle wisconsin but little agreement exists on the age of the older loesses pye and johnson 1988 proposed early wisconsin for the loveland sicily island mckay and follmer 1985 suggested this loess correlated with a loess under illinoian till clark et al 1989 agreed on crowley s ridge but suggested the loveland sicily island loess on sicily island was older mirecki and miller 1994 and millard and maat 1994 suggested an illinoian age for the loveland sicily island loess miller and co workers suggested as did pye and johnson 1988 an illinoian age for the crowley s ridge loess mckay and follmer 1985 suggested it correlated with a loess under kansan till stratigraphy indicates the marianna is the older of the five loesses researchers identified loess on both the east and west side of the lmv as well as on higher terraces within the valley many researchers assumed unaltered loesses were commonly yellowish brown and silts or silt loams west et al 1980 miller et al 1986 the nonclay fraction of unweathered lmv loesses was dominated by quartz followed corresponding author by carbonates mainly dolomites followed by feldspars and micas clays were dominated by montmorillonite followed by micaceous minerals kaolinite and vermiculite miller et al 1986 soils in the crowley s ridge loess are most developed followed by the soils in the loveland sicily island which are more developed than the modern soils in the peoria loess soils in the roxana and marianna loesses are least developed and the farmdale soil of the roxana is the weaker of the two miller et al 1986 there is certainly overlapping range in the degree of soil development in the various loesses

loess record of dry climate and aeolian activity in the early to mid holocene central great plains north america thick holocene bignell loess sections provide new evidence for the timing of early to middle holocene aridity in the central great plains the immediate source of loess in these sections was dune fields just upwind based on thickness trends and grain size data thus periods of rapid loess accumulation indicate episodes of extensive dune activity under drier than present climate in typical bignell loess sections a thick zone of coarse textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid sustained holocene loess accumulation optical ages indicate that loess in this zone was deposited beginning about 9000 10000 years ago at five study sites accumulation ended shortly after 6500 years ago at three sites but possibly earlier at the other two in two well dated sections the average rate of loess accumulation between 9000 10000 and 6500 years ago was about 1 6 3 5 times greater than in the late holocene thus we infer that the most extensive sustained holocene

folding thrusting and development of push up structures during the miocene tectonic inversion of the austral basin southern patagonian andes 50 s abstract rift and post rift sections of the austral basin in the southern patagonian fold and thrust belt were examined in outcrop and in 2d seismic reflection sections to evaluate geometric and kinematic aspects of the extension and subsequent inversion the syn rift section is composed of dacitic lava flows of the el quemado complex jurassic the lowermost unit of the basin the coastal sandstones of the springhill formation tithonian berriasian and marine shales of the rio mayer formation berriasian albian are thought to be sag units however field data suggest that these units are also part of the syn rift successions as evidenced by extensional growth strata in the lower levels of the rio mayer formation a dacitic flow that is texturally and compositionally similar to those of the syn rift phase and that is termed here the rio guanaco dacite is interfingered with the basal strata of the rio mayer formation and has a u pb zircon crystallization age of c 141 ma berriasian the outcropping section was inverted by compression in the early miocene according to structural and isotopic data of adjacent areas producing a broad anticline with inverted and fossil extensional faults as well as newly generated thrusts at the seismic scale most of the extensional master faults present negligible reverse slip and shortening was accommodated in hanging wall push up structures this situation is interpreted as a result of the competent metamorphic basement and volcanic syn rift section inducing frictional lock up of the master faults

responses of floods to holocene climatic change in the upper mississippi valley abstract dimensions of holocene relict channels and sedimentological characteristics of point bars associated with these relict channels were used to reconstruct a holocene history of long term changes in magnitudes of 1 58 yr floods in upper mississippi valley watersheds of southwestern wisconsin the reconstructed record of floods shows relatively large and persistent nonrandom departures from contemporary long term average flood magnitudes the flood history indicates climatic changes that are broadly similar to climatic changes indicated from fossil pollen in the same region the holocene floods ranged from about 10 15 larger to 20 30 smaller than contemporary floods of the same recurrence frequency large floods were characteristic between about 6000 4500 and 3000 2000 yr b p and during a brief interval after 1200 yr b p small floods were common between about 8000 6500 4500 3000 and 2000 1200 yr b p these fluvial responses were found to be closely associated with a long term episodic mobility and storage of sediments in the wisconsin watersheds during periods of relatively large floods relatively rapid lateral channel migration either reworked or removed extensive tracts of valley bottom alluvium in contrast during periods of relatively small floods relatively slow lateral channel migration is apparent and the channel and floodplain system appear to have been relatively stable

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

along strike structural variations in the southern patagonian andes insights from physical modeling abstract the southern patagonian andes between 48 and 53 sl offers a unique opportunity to study the results of orogenic growth superimposed over a previously rifted region in this sector the northern austral or magallanes basin was affected by late to middle jurassic extension followed by late cretaceous to neogene tectonic compression the resultant fold thrust belt displays significant along strike variations in width and lateral position of the structural domains that may be reflecting a first order control of the jurassic extensional depocenters as proposed in previous works this hypothesis is tested using a series of scaled sandbox analog models involving the positive inversion of two adjacent depocenters with different extensional rates accommodated by a transfer fault the correlation between structures in the southern patagonian andes and the features in the model have important first order similarities supporting the theory of a positively inverted extensional basin with a previous history of southward increase in extension moreover structural cross sections analyzed from field and seismic data have a geometry that is consistent with serial cross sections resulting from the models

detrital zircon shrimp u pb age study of the cordillera darwin metamorphic complex of tierra del fuego sedimentary sources and implications for the evolution of the pacific margin of gondwana abstract the cordillera darwin metamorphic complex in the southernmost andes includes a basement of probable palaeozoic age a mid jurassic and younger volcano sedimentary cover and a suite of jurassic granites all of which were jointly metamorphosed during the cretaceous detrital zircon ages presented here show that some of the amphibolite facies metamorphic rocks previously mapped as basement have a jurassic protolith overall the detrital zircon age patterns for samples of the cordillera darwin basement differ from those of the madre de dios terrane of the western patagonian andes with which they had been correlated instead they are more comparable with those from the eastern andes metamorphic complex which apparently developed in a passive margin setting the paucity of cambrian detrital zircons indicates that the meta igneous basement of the magallanes foreland basin of central and northern tierra del fuego was not the main source of detritus for the protolith of the cordillera darwin metamorphic complex the possibility is envisaged that the magallanes fagnano transform fault boundary between the scotia and south america plates resulted from reactivation of an older pre jurassic suture zone between the basement terranes of north central tierra del fuego and cordillera darwin supplementary material geographical coordinates of analysed samples sensitive high resolution ion microprobe u th pb analytical data and representative cathodoluminescence images of zircon grains analysed are available at http www geolsoc org uk sup18397

marine evidence for episodic holocene megafloods in north america and the northern gulf of mexico hemipelagic muds deposited during the past 5 3 cal kyr in the northern gulf of mexico orca basin contain seven intervals punctuated by relatively coarse siliciclastic grain size peaks planktonic faunal turnovers and negative 13c excursions we believe these episodes represent megaflood deposits reflecting historically unprecedented outfall of north american floodwater and terrigenous mud plumes into the gulf resulting in collapse of the open ocean pelagic ecosystem the deposits record multidecadal episodes of high continental precipitation and large mississippi river floods at 4 7 3 5 3 0 2 5 2 0 1 2 and 0 3 cal ka 500 1200 year recurrence interval variations in tropical plankton frequencies define submillenial warming intervals that culminate in these fluvial episodes strengthened tropical currents in the gulf at these times appear to have increased sea surface temperatures and associated flow of moist gulf air to the midwest terrestrial paleohydrologic records support the marine evidence for millennial scale changes in recurrence of large midwest flood episodes

a preliminary assessment of the hydrocarbon potential of the larsen basin antarctica abstract the larsen basin on the northwest margin of the weddell sea formed as a mesozoic ensialic basin during gondwana breakup deposition was either in half grabens on the extending weddell sea margin or in a restricted back arc basin at the northern end of this basin 5 6 km of sedimentary rock crop out on james ross island exposing elements of a large potential hydrocarbon system aeromagnetic and outcrop data suggest that the basin structure inferred from james ross island can be recognised at least as far south as 70 s upper jurassic anoxic marine strata deposited prior to the main phase of arc development form a rich potential source t o c up to 3 5 with both marine and terrestrial kerogens arc derived volcaniclastic sediments of barremian oligocene age form a regressive megasequence basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across enveloped in slope apron mudstones late cretaceous fault reactivation and uplift led to dramatic shallowing of the basin with deposition of shelf facies although there are many potentially attractive reservoir targets there may be problems of pore occlusion due to the abundant labile volcanic grains however there is evidence of more quartzose sandstone towards the top of the section and inferentially toward the basin centre in the larsen basin there is moderate potential for oil generated from upper jurassic source rocks and reservoired in cretaceous and tertiary sandstones and conglomerates in large stratigraphic or structural traps caused by partial basin inversion during deposition

sedimentary evolution of an active margin during middle and upper cretaceous times the north peruvian margin from late aptian up to senonian the sedimentary evolution of the north peruvian margin during upper cretaceous can be summarized as follows

holocene geomorphic evolution and settlement distribution patterns in the mid lower fen river basins china abstract holocene geomorphic changes have fundamentally shaped the spatial temporal distributions of prehistoric and historical settlements in north china through intensive field surveys and careful field examination of typical sedimentary sequences we reconstructed the late pleistocene and holocene geomorphic history in the two major basins of the mid lower fen river central south shanxi china our first hand data provides crucial information for reconstructing the dynamic relationship between the characteristics of holocene geomorphic changes and settlement distribution patterns in the two basins from the neolithic period to the bronze age xia shang dynasties in the taiyuan basin due to river downcutting processes from the end of the late pleistocene to the early holocene edge of the basin emerged and evolved into tablelands the elevation of the flat lands atop the tablelands that was significantly above the level of floodwater provided an ideal environment for early settlements the holocene geomorphic changes are characterized by continous fluvio lacustrine aggradation and the central basin became void of human settlements due to uninhabitable hydrological and gemorphic conditions and especially due to frequent floods instead most settlements were located along the basin displaying a unique around basin distribution pattern in the linfen basin following large scale incision of the main channels and branches of the fen river during the late pleistocene platform type plain with deep incised valleys was formed similar to the surrounding loess tableland the central basin became an optimal environment for human activities and settlement construction forming a full basin like settlement distribution pattern that is distinctively different from the around basin distribution pattern in the taiyuan basin

the temperature of europe during the holocene reconstructed from pollen data we present the first area average time series reconstructions of warmest month coldest month and mean annual surface air temperatures across europe during the last 12 000 years these series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four dimensional gridding procedure this approach combines three dimensional spatial gridding with a fourth dimension represented by time allowing data from irregular time series to be focussed onto a regular time step we provide six regional reconstructed temperature time series as well as summary time series for the whole of europe the results suggest major spatial and seasonal differences in holocene temperature trends within a remarkably balanced regional and annual energy budget the traditional mid holocene thermal maximum is observed only over northern europe and principally during the summer this warming was balanced by a mid holocene cooling over southern europe whilst central europe occupied an intermediary position changes in annual mean temperatures for europe as a whole suggest an almost linear increase in thermal budget up to 7800 bp followed by stable conditions for the remainder of the holocene this early holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west which have continued to rise at a progressively decreasing rate up to the present day

age and paleotectonic setting of volcanogenic massive sulfide deposits in the guerrero terrane of central mexico constraints from u pb age and pb isotope studies the guerrero terrane of central and western mexico comprises volcanic and sedimentary strata of mainly mesozoic age with geochemical signatures indicating formation in juvenile back arc to slightly more evolved arc environments at least 60 volcanogenic massive sulfide vms deposits have been discovered thus far within the guerrero terrane they comprise two main belts the coastal belt mainly comprising deposits in the zihuataneo subterrane including the cuale bramador district and the central belt which includes deposits in the teloloapan subterrane campo morado tlanilpa azulaquez tizapa rey de plata etc the guanajuato subterrane el gordo las gavalinas and the zacatecas subterrane san nicolas and el salvador deposits the age of host rocks for the vms mineralization in the various subterranes was poorly constrained prior to this study eleven new u pb zircon ages for volcanic host rocks from the campo morado tlanilpa azulaquez leon guanajuato and san nicolas el salvador vms districts together with recently determined u pb ages from the cuale area demonstrate that vms deposits in the guerrero terrane range from latest middle jurassic to early early cretaceous callovian to valanginian in age the oldest ages were obtained from the cuale district total age range of 157 4 4 1 ma for three units and the youngest are from the tlanilpa azulaquez district total age range of 139 7 2 5 ma for two samples volcanogenic massive sulfide deposits in the central belt resemble those of the bimodal siliciclastic deposit type whereas those in the coastal belt are more similar to the bimodal felsic type published geochemical and radiogenic isotope data from the various subterranes suggest that vms deposits in the coastal belt and most of those in the central belt are hosted within juvenile to slightly evolved arc settings the san nicolas and el salvador deposits of the zacatecas subterrane are the only deposits that unequivocally formed in a back arc setting our new u pb dating results together with existing stratigraphic geochemical and radiogenic isotope data from throughout the guerrero terrane are consistent with the guerrero terrane having developed as a west facing continental margin arc that was built on mainly oceanic crust and overlying early mesozoic siliciclastic fans shed from the east along the western margin of nuclear mexico slab rollback in the middle jurassic led to the development of a rocas verdes type continental back arc inboard of the arc which is now preserved as the arperos basin in the central and southern guerrero terrane and the host rocks for the san nicolas and el salvador deposits farther to the north in southeastern zacatecas state we have determined pb isotope compositions of sulfides from vms deposits and several other styles of mineralization from throughout the guerrero terrane a compilation of these and previously published data shows that isotopic compositions from the various vms deposits fall in a relatively confined field that overlaps the average upper crustal growth curve this suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and or from the host volcanic rocks themselves or subvolcanic intrusive equivalents that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption lead isotope compositions from the francisco i madera deposit in zacatecas state indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely tertiary in age

contrasting styles of superposed deformation in the southernmost andes brittle fault arrays regional structural patterns and marine multichannel seismic reflection profiles across the magallanes basin near 53 s latitude show contrasting styles of superposed extensional contractional and transtensional deformation below the northeastern end of a ne sw transect of the basin seismic reflection profiles show deepening levels of pre late jurassic basement and upper jurassic to lower cretaceous volcanic horizons toward the southwest this trend is associated with apparent normal fault offsets that may be attributed to a widespread late jurassic early cretaceous extensional event contrastingly below the southwestern end of magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest integration of seismic reflection data with published seismic refraction data geologic mapping and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton vergent basement involved contractional deformation this style of shortening contrasts with thin skinned styles of thrusting in the northern and central parts of the basin on tierra del fuego structural relationships stratigraphic data and previously published metamorphic data indicate that basement involved contraction occurred during latest cretaceous to early tertiary time overprinting all contractional structures steep tertiary strike slip and oblique slip faults in the central and southern parts of the magallanes basin display oblique normal mostly down to the southwest offsets across wnw ese trending segments of the south american scotia transform plate boundary on the south american continent newly discovered features along the plate boundary segment include sediment filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the magallanes fault zone kinematic analyses of fault slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the straits of magellan and central tierra del fuego together these data indicate that 1 the southernmost andes were under compression during rapid cooling and exhumation of unique high pressure 8 kbars basement rocks now exposed in the darwin metamorphic complex dmc near 55 s latitude and 2 final exhumation of basement rocks in the dmc occurred during a shift away from compression toward transtension during late tertiary crustal relaxation we suggest that preexisting structural anisotropies formed during basement involved contractional deformation and the initiation of seafloor spreading south of south america during the oligocene controlled late tertiary sinistral transtensional patterns in the magallanes fault zone

evolution of deep water stratigraphic architecture magallanes basin chile abstract the 4000 m thick and 20 myr deep water sedimentary fill of the upper cretaceous magallanes basin was deposited in three major phases each with contrasting stratigraphic architecture 1 the oldest deep water formation punta barrosa formation comprises tabular to slightly lenticular packages of interbedded sandy turbidites slurry flow deposits and siltstone that are interpreted to record lobe deposition in an unconfined to weakly ponded setting 2 the overlying 2500 m thick and shale dominated cerro toro formation includes a succession of stacked conglomeratic and sandstone channel fill deposits with associated finer grained overbank deposits interpreted to record deposition in a foredeep axial channel levee system 3 the final phase of deep water sedimentation is characterized by sandstone rich successions of highly variable thickness and cross sectional geometry and mudstone rich mass transport deposits mtds that are interpreted to record deposition at the base of slope and lower slope segments of a prograding delta fed slope system the deep water formations are capped by shallow marine and deltaic deposits of the dorotea formation these architectural changes are associated with the combined influences of tectonically driven changes and intrinsic evolution including 1 the variability of amount and type of source material 2 variations in basin shape through time and 3 evolution of the fill as a function of prograding systems filling the deep water accommodation while the expression of these controls in the stratigraphic architecture of other deep water successions might differ in detail the controls themselves are common to all deep water basins information about source material and basin shape is contained within the detrital record and when integrated and analyzed within the context of stratigraphic patterns attains a more robust linkage of processes to products than stratigraphic characterization alone

the opening of the magellan strait and its geodynamic implications terra nova 25 13 20 2013 abstract the existence of a neogene quaternary rift system responsible for the opening of the eastern magellan strait has been one of the more peculiar tectonic features of the southernmost andes since its discovery more than a decade ago however until now there has been no explanation pinpointing the specific tectonic factors that led to the development of the rift this work focuses on the interaction between the fault systems and the tectonic regimes that have affected tierra del fuego north of the south america scotia left lateral strike slip plate boundary due to a good match between a jurassic extensional fault array orientation and the principal stress directions related to the strike slip plate boundary we conclude that jurassic transfer faults reactivated extensionally generating the magellan rift system this process was enhanced by northward continental drift and has implications for the geodynamic behind the separation of south america from antarctica

neotectonics of the upper mississippi embayment abstract although the upper mississippi embayment is an area of low relief the region has been subjected to tectonic influence throughout its history and continues to be so today tectonic activity can be recognized through seismicity patterns and geological indicators of activity either those as a direct result of earthquakes or longer term geomorphic structural and sedimentological signatures the rate of seismic activity in the upper mississippi embayment is generally lower than at the margins of tectonic plates the embayment however is the most seismically active region east of the rocky mountains with activity concentrated in the new madrid seismic zone this zone produced the very large new madrid earthquakes of 1811 and 1812 geological and geophysical evidence of neotectonic activity in the upper mississippi embayment includes faulting in the benton hills and thebes gap in missouri paleoliquefaction in the western lowlands of missouri subsurface faulting beneath and tilting of crowley s ridge in northeastern arkansas and southeastern missouri subsurface faulting along the crittenden county fault zone near memphis tennessee faulting along the east flank of the tiptonville dome and numerous indicators of historic and prehistoric large earthquakes in the new madrid seismic zone paleoearthquake studies in the new madrid seismic zone have used trenching seismic reflection shallow coring pedology geomorphology archaeology and dendrochronology to identify and date faulting deposits of liquefied sand and areas of uplift and subsidence the cause of today s relatively high rate of tectonic activity in the mississippi embayment remains elusive it is also not clear whether this activity rate is a short term phenomenon or has been constant over millions of years ongoing geodetic and geological studies should provide more insight as to the precise manner in which crustal strain is accumulating and perhaps allow improved regional neotectonic models

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

zonal assessment of environmental driven settlement abandonment in the trans tisza region central europe during the early phase of the little ice age abstract this investigation focuses on the transformation of the settlement pattern of a lowland landscape as a social response to the hydrological challenges emerging in the late 13th century c overture of the little ice age lia results of the applied zonal analysis suggested a strong spatial connection between the geomorphological conditions the agro ecological suitability good excellent medium and low and the stability or instability of settlement patterns the elevation means of archaeological sites in the deserted zones proved significantly lower than those in zones with permanent settlement pattern brunner munzel test p 0 01 n 377 additionally the late medieval 14th mid 16th centuries site group was situated on average significantly higher than the high medieval late 10th 13th centuries site group within the permanent zones brunner munzel test p 0 01 n 219 these outcomes statistically confirm that not only did low lying inhabited areas shrink significantly but they also displaced vertically in the first phase of the lia when analysing the relation of settlement pattern to soil conditions the proportion of areas with good excellent agro ecological suitability proved 1 5 2 times higher in the permanent zones than in the deserted and uninhabited settlement suitability zones using the linear model different regression coefficients appeared between the extension of the high and medium agro ecological suitability zones and the number of high and late medieval settlements the different coefficients in the studied two periods suggest that the issue of agroecological suitability in the high middle ages did not bear such importance as in the late middle ages the findings of the paper may contribute to answering the question why the relatively dense settlement pattern of the deserted zones was abandoned almost completely by the end of the 13th c in areas where flood proneness and weak agro ecological suitability both meant a serious risk for human communities finally we presumed that if hydro climatic changes increased water levels they must have changed the plant composition of the studied landscape as well chi squared test of macrofossil plant remains narchaeological site 55 ntaxon 330 shows that the second part of the 13th c saw the ratio of species from humid habitat types grow 7 81 df 1 p 0 02 comparison of the two studied processes indicates a broad synchronism between the shrinkage of inhabited areas and the increasing proportion of plants with humid environment tolerance during the second part of the 13th c the reconstructed transformations in the composition of plant remains and settlement structure signal not a mere transitional change but a longue duree structural transformation of the landscape

tectonic implications of a paleomagnetic study of the sarmiento ophiolitic complex southern chile abstract a paleomagnetic study was carried out on the late jurassic sarmiento ophiolitic complex soc exposed in the magallanes fold and thrust belt in the southern patagonian andes southern chile this complex mainly consisting of a thick succession of pillow lavas sheeted dikes and gabbros is a seafloor remnant of the late jurassic to early cretaceous rocas verdes basin that developed along the south western margin of south america stepwise thermal and alternating field demagnetization permitted the isolation of a post folding characteristic remanence apparently carried by fine grain sd magnetite both in the pillow lavas and dikes the mean in situ direction for the soc is dec 286 9 inc 58 5 95 6 9 n 11 sites rock magnetic properties petrography and whole rock k ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the late cretaceous the mean remanence direction is anomalous relative to the expected late cretaceous direction from stable south america particularly a declination anomaly over 50 is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the jurassic el quemado fm located over 100 km north of the study area in argentina nevertheless a significant ccw rotation of the whole soc is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back arc basin sub parallel to the continental margin a rigid body 30 westward tilting of the soc block around a horizontal axis trending nnw is considered a much simpler explanation being consistent with geologic evidence this may have occurred as a consequence of inverse reactivation of old normal faults which limit both the soc exposures and the cordillera sarmiento to the east the age of tilting is unknown but it must postdate remanence acquisition in the late cretaceous two major orogenic events of the southern patagonian andes in the eocene ca 42 ma and middle miocene ca 12 ma respectively could have caused the proposed tilting

sources and paleoclimatic significance of holocene bignell loess central great plains usa abstract this paper reexamines the stratigraphy sources and paleoclimatic significance of holocene bignell loess in the central great plains a broadly similar sequence of loess depositional units and paleosols was observed in thick bignell loess sections up to 300 km apart suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis new optical ages together with previously reported radiocarbon ages indicate bignell loess deposition began 9000 11 000 yr ago and continued into the late holocene some bignell loess is

holocene mississippi river floods and geoarchaeological site formation processes in the sny bottom western illinois usa abstract holocene alluvial deposits are well represented in the sny bottom a reach of the mississippi river in western illinois characterized by the presence of a long anabranch channel the sny prior to its engineered confinement the flood stage mississippi functioned as an anastomosed system comprised of many secondary landscape elements hierarchically arranged to distribute water out across a broad floodplain radiocarbon dated stratigraphic sequences are used to reconstruct the geological history of this river reach as it metamorphosed from a sandy late pleistocene braided floodplain into a system dominated by mud by 7400 cal bp the mississippi migrated to its present position and began to build the natural levee it would use for the remainder of the holocene major subsequent geomorphological adjustments to the system resulted from large floods at ca 6900 4900 3300 and 2000 cal bp valley fill is a complex arrangement of individual sedimentary bodies that accumulated at highly variable rates vertical accretion by sedimentation and soil upbuilding acted together to preserve numerous buried archaeological sites of all ages

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

causes of river avulsion insights from the late holocene avulsion history of the mississippi river u s a abstract the emphasis on gradient advantages in studies of avulsion is misleading while gradient advantages are necessary for an avulsion to occur the late holocene avulsion history of the mississippi river in louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important cross valley to down valley slope ratios of the modern floodplain range from 16 to 110 and are typically 30 the slope ratio is 35 at the location of the mississippi atchafalaya diversion old river yet slope ratios are 83 to 110 immediately upvalley of old river all values of mississippi river floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models shallow floodplain cores 14c dating of organic remains and geologic mapping show that the mississippi river has avulsed only four times over the past 5 ky in the southern lower mississippi valley lmv gradient advantages are widespread yet avulsions are rare these observations indicate that factors other than gradient advantage control mississippi river avulsion several examples of mississippi and red river avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion incipient mississippi river avulsion and development of the atchafalaya river involved reoccupation of abandoned mississippi river channels and a red river crevasse splay complex the modern atchafalaya river also incises buried mississippi river channel belt sands abandoned channel belts and crevasse splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the mississippi river abandoned channels provide ready made conduits for mississippi river flow that can efficiently develop into avulsive channels multi storied sheet sandstones in ancient fluvial deposits may provide additional support for the idea that erodible substrates and floodplain channel distributions are critical influences on avulsion these features record episodic reoccupation of channel belts which at least in some cases may simply reflect successive avulsions rather than major changes in aggradation rate or extrabasinal factors such as climate

holocene forest dynamics in northwestern romnania pollen analyses and ams radiocarbon measurements were carried out on peat deposits from the former crater lake preluca tiganului located in a mid altitude area of the gutaiului mountains nw romania in order to reconstruct the postglacial vegetation history at the lateglacial holocene transition the open vegetation was replaced by forest suggesting a fast response to climatic warming the rapid expansion of pinus betula and later picea may be due to their existence in the area during the lateglacial at approximately 11 250 cal yr bp the dense forest was composed primarily of ulmus and some picea betula and pinus a small population of ulmus might also have survived in the area during the lateglacial at 10750 cal yr bp quercus tilia and fraxinus followed by corylus at c 10200 cal yr bp expanded significantly in the forest the delay of approximately 550 years in their establishment as compared to the expansion of ulmus was probably due to their immigration from more distant localities

the falkland islands a key element in gondwana paleogeography stratigraphic and sedimentological fieldwork on the falkland islands confirms that they were originally part of south africa east and west falkland have contrasting geological and deformation histories and are separated by the falkland sound fault which marks a significant deformation edge to the cape fold belt the degree of structural deformation and metamorphism in west falkland is substantially less than that of east falkland adjustments to differential shortening between the two islands being reflected by structures developed along the falkland sound fault prior to fragmentation of gondwana the falkland platform and maurice ewing bank were positioned in the natal valley adjacent to the east coast of south africa this predrift reconstruction shows that the falkland islands have subsequently been rotated by 180 relative to south africa and complements recent paleomagnetic data there was a complete overlap of the maurice ewing bank and falkland platform to the exclusion of the falkland plateau basin which is a small oceanic basin formed during the jurassic fragmentation of gondwana extension in this basin was accommodated by movements along the aghulas falkland fracture zone and the gastre fault system predrift restoration of these faults shows that patagonia was positioned closer to both africa and antarctica the falkland plateau basin was an integral component of the west gondwana jurassic microplate system and provides an invaluable insight into the evolution of its western margin at some time prior to the main opening of the south atlantic the falkland platform was rotated clockwise by 120 against the agulhas falkland fracture zone such that the falkland islands were translated some 300 km to the south the driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the falkland plateau basin as the gastre fault system within continental crust became unable to accommodate the continuous dextral shear in the agulhas falkland fracture zone the falkland islands represent an outboard extension of the eastern cape and are significant for understanding the paleogeography of both the cape and karoo supergroups east falkland is closely similar to south africa in its geology while west falkland shows significant contrasts with both the devonian and permian successions being more proximal in character but still demonstrate the eastward continuation of the cape and karoo basins

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

holocene climate variability although the dramatic climate disruptions of the last glacial period have received considerable attention relatively little has been directed toward climate variability in the holocene 11 500 cal yr b p to the present examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000 8000 6000 5000 4200 3800 3500 2500 1200 1000 and 600 150 cal yr b p most of the climate change events in these globally distributed records are characterized by polar cooling tropical aridity and major atmospheric circulation changes although in the most recent interval 600 150 cal yr b p polar cooling was accompanied by increased moisture in some parts of the tropics several intervals coincide with major disruptions of civilization illustrating the human significance of holocene climate variability c 2004 university of washington all rights reserved

stratigraphic record across a retroarc basin inversion rocas verdes magallanes basin patagonian andes chile the mesozoic evolution of the andean cordillera of southern patagonia is recorded in two formations that are now part of the fold thrust belt the zapata formation and the punta barrosa formation extension in the ultima esperanza region began in the late jurassic with deposition of the marine volcaniclastic tobifera formation and eventually resulted in the full fledged oceanic rocas verdes basin the zapata formation was deposited over a time span of 50 m y in an irregular basin ultimately bordered by an early cretaceous arc to the west it is characterized by interbedded shale and siltstone mostly deposited in shallow water and in areas of oceanic crust emplacement also as deep water hemipelagic deposits sediments of the zapata formation were derived initially from local oceanic upwarps and tobifera highs and later from the ande sitic cover of the juvenile arc and or exhumed oceanic crust the paucity of sandstone in the zapata formation in the ultima esperanza region indicates a highly irregular basin partitioned by prominent horsts with sand deposition confined to sub basins closer to the arc changes in depositional regimes and sediment dispersal patterns related to the onset of andean contraction and formation of the magallanes foreland basin are recorded by sediments of the overlying punta barrosa formation this formation records the evolution of a fold thrust belt on the basis of the multimodal mineralogical and geochemical character of its sandstone and shale the presence of an arc is indicated but nearby andean metamorphic terranes are more significantly represented in punta barrosa sediments stratigraphic details across the zapata and punta barrosa formations indicate that deformation and development of a proto andean cordillera in southern south america was initiated in the late mesozoic and involved conspicuous crustal shortening

holocene flood plain soil formation in the southern lower mississippi valley implications for interpreting alluvial paleosols holocene mississippi river flood plain soils in central louisiana provide important insights on alluvial pedogenesis in an aggradational setting insights that are useful for interpreting alluvial paleosols the flood plain soils are younger than ca 5400 yr b p and are grouped into two categories 1 meander belt and 2 backswamp soils meander belt soils are entisols inceptisols and alfisols developed on point bar ridges and natural levee crests they consist of 3 5 m thick brown silty and sandy moderately well drained profiles with multiple parent materials common yellow brown and gray mottles and a few iron and calcite nodules backswamp soils are vertisols formed in flood plain depressions away from active and abandoned mississippi river channels and consist of up to 10 m thick gray clayey poorly drained profiles that have abundant yellow brown mottles iron and calcite nodules gypsum accumulations and slickensides the thickness of the soils and the presence of multiple parent materials show that as the flood plain aggrades concurrent or alternating episodes of sedimentation and pedogenesis produce cumulative profiles profile thickness is more closely related to sediment accumulation rates than to the depth of chemical weathering and the cumulative nature of the soils explains the difficulty of recognizing discrete paleosol profiles in many ancient alluvial deposits comparisons of soils formed on the three youngest holocene mississippi river meander belts saucier and snead 1989 show that the soils record episodes of meander belt growth abandonment and relocation meander belt 1 soils form as the active meander belt develops and consist of either vertically stacked entisols or a single inceptisol the soils contain few calcite nodules and form over intervals of a few hundred years meander belt 2 soils are alfisols lack free carbonates and form over intervals of a few thousand years following meander belt abandonment and soil leaching meander belt 3 soils are alfisols or vertisols that contain common calcite nodules and the profiles are developed in 1 to 2 m of backswamp mud that overlies meander belt silt and sand the mud inhibits profile leaching and leads to poor drainage and calcite precipitation this change in soil hydrology complicates interpretations of the effects of time on meander belt soil development backswamp soils are strongly influenced by seasonal episodes of soil wetting and drying as demonstrated by the gray colors and the abundance of mottles nodules and slickensides in the soils the distribution of calcite nodules in the backswamp and meander belt soils is more closely related to seasonal water table position than to mean annual precipitation which suggests that not all calcic paleosols in ancient alluvial deposits represent semiarid or arid climates slickensides attest to seasonal rainfall but the composition of the backswamp soils primarily reflects source area lithology and depositional influences rather than the effects of chemical weathering in general these soils provide an incomplete record of the humid subtropical climate of louisiana although short term sediment accumulation rates decrease away from mississippi river channels the backswamp soils are not better developed i e more mature than the meander belt soils because poor drainage and slow but continuous sedimentation inhibits weathering collectively these observations suggest that flood plain soils that developed in aggradational settings such as that of the mississippi river flood plain are best suited for interpreting depositional and hydrologic histories of ancient alluvial deposits rather than paleoclimates

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

foothills and intermountain basins does china s fertile arc have hilly flanks abstract in 2009 liu and colleagues considered the parallels in topographic context between early farming sites in southwest asia and those in north china associated with millet cultivation this paper extends the geography of this conceptual framework by moving south of the qinling mountains huai river divide incorporating sites in south china that are associated with the beginnings of rice exploitation we highlight the continuous mountain chains running from the greater khingan range in the northeast to the nanling mountains in the south that give form to china s fertile arc key sites in the northern part of the arc are situated along china s hilly flanks while southern sites are located in a diverse array of landforms including piedmont plains and intermountain basins a parallel can be drawn in the context of early land choice ecological opportunism between the fertile crescent and china s fertile arc

influence of attenuated lithosphere and sediment loading on flexure of the deep water magallanes retroarc foreland basin southern andes flexural subsidence in foreland basins is controlled by applied loads such as topography water sediment and subcrustal forces and the mechanical properties of the lithosphere we investigate the controls on subsidence observed within the upper cretaceous magallanes retroarc foreland basin of southern south america to evaluate the impact of lateral variations in flexural rigidity due to late jurassic extension conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep water cenomanian turonian basin strata however models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns results satisfy paleobathymetric observations in the magallanes basin and suggest that lithospheric thinning is necessary to produce both long wavelength and deep subsidence during late cretaceous basin evolution results indicate that elastic thickness decreases westward from 45 25 km in the distal foreland to 37 15 km beneath the foredeep these findings are consistent with a westward reduction in crustal thickness associated with the jurassic extensional history of the patagonian lithosphere our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift we propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time these findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories

high resolution proxy record of holocene climate from a loess section in southwestern nebraska usa abstract multi proxy analysis was used to produce a high resolution paleoclimatic record from an exceptionally thick section of the holocene bignell loess near wauneta southwestern nebraska in the central great plains the wauneta section has excellent age control based on optically stimulated luminescence osl and radiocarbon dating and records multiple episodes of rapid loess deposition alternating with slower deposition and soil formation the lowermost and uppermost osl ages obtained from the bignell loess are 10 250 610 years 5 9 m depth and 100 10 years 0 1 m depth respectively as a result the holocene has been temporally confined stratigraphically the bignell loess overlies the late pleistocene peoria loess deposited 21 14 ka and the two units are separated by the brady soil which is distinguished by its color and other pedogenic features l a b color parameters and organic carbon content of bignell loess are sensitive proxies to differentiate drought induced aeolian sediment layers from the intercalated soil horizons soil organic carbon derived 13 c data suggest that the c 3 dominated floral environment during peoria loess deposition shifted dramatically to a c 4 dominated environment during brady soil formation in response to a warming trend even greater c 4 abundance characterized the late holocene high resolution 13 c data support the contention that c 3 vs c 4 vegetation change in the holocene reflects ecosystem response to frequent vegetation disturbance under arid conditions time series analysis reveals that 13 c and color parameters display high frequency variation with periodicities of 103 118 years and 103 years respectively similar periodicities were also reported in studies of north dakota lakes though the physical mechanism responsible is uncertain comparison of bignell loess color and tropical pacific sea surface temperatures ssts allows evaluation of a proposed teleconnection between drought in the great plains and la nina like conditions in the tropical pacific the loess color index and eastern tropical pacific sst display broad similarities through the late pleistocene and holocene that are consistent with this teleconnection on the other hand drought centered at 3800 years ago is not consistent with the teleconnection and the end of early holocene aridity at the wauneta section around 6500 years ago is much earlier than the corresponding rise in sst and increase in el nino frequency in the eastern tropical pacific

episodic holocene loess deposition in central nebraska abstract the mirdan canal section in central nebraska usa provides evidence for multiple episodes of holocene loess deposition in the central great plains this irrigation canal cut exposes more than 4 m of holocene sediment that overlies the late wisconsinan peoria loess the oldest holocene sediment is local slopewash that fills gullies but overlying beds are interpreted as holocene loess bignell loess three buried soil profiles occur within the holocene sediment and have yielded uncalibrated 14 c ages of 3010 yr bp uppermost buried soil and 8790 yr bp third buried soil from top a fourth buried soil yielded an age of 9330 yr bp and is correlated with the brady soil the bignell loess has a darker color slightly more organic matter more clay more coarse silt and sand less smectite and less volcanic glass than the underlying peoria loess the bignell loess is most plausibly interpreted as dust from local flood plains containing a mixture of reworked peoria loess and older silts with contrasting mineralogy episodes of loess deposition occurred in the early middle and late holocene probably during periods of dry climate but possibly also as a consequence of large erosive floods that made wind erodible sediment available on nearby flood plains

stratigraphy geochemistry and thermoluminescence ages of lower mississippi valley loess four periods of loess deposition in the lower mississippi valley can be identified on the basis of geochemical and mineralogical criteria radiocarbon dating and thermoluminescence dating these are designated loess units 1 2 3 and 4 in order of increasing age carbonate rich unit 1 loess comprises more than 70 per cent of the thickness of the loess profiles 14c and tl dates indicate this loess was deposited between 9 000 and 20 000 years ago a maximum sedimentation rate of 2 17 mm yr 1 has been recorded near vicksburg just after the last laurentide glacial maximum between 17 190 and 15 580 years ago the unit 2 loess which is thin and partly decalcified was deposited slowly between about 25 000 and 20 000 years ago the unit 3 and unit 4 loess formations which are both highly weathered have yielded tl ages of 76 000 85 000 years and 119 000 132 000 years suggesting they were deposited during the altonian substage of the early wisconsinan and the illinoian glacial stage respectively the four loess units are stratigraphically equivalent to the peoria farmdale roxana and loveland loess formations previously recognized in illinois the source of dust in both areas was glacial outwash in the mississippi valley during interglacials and interstadials when the supply of glacial debris was reduced and the mississippi river changed from a braided to a meandering regime dust sedimentation in southern mississippi virtually ceased allowing weathering and pedogenesis to proceed

alluvial pedogenesis in pleistocene and holocene mississippi river deposits effects of relative sea level change comparison of pleistocene and holocene soils developed in mississippi river meander belt deposits in louisiana illustrates the influence of relative sea level change on pedogenesis holocene and pleistocene meander belts have similar landscape morphologies lithofacies and soil characteristics and probably formed under similar hydrologic regimes the pleistocene avoyelles prairie meander belt is now terraced but it was originally graded to a lower relative sea level position than the holocene flood plain holocene alluvial soils are generally weakly expressed cumulative profiles with abundant redoximorphic features and formed on an aggrading flood plain with a shallow fluctuating water table the soil morphology varies with respect to landscape position and overbank lithofacies pleistocene alluvial paleosols show evidence for two stages of pedogenesis during the first stage of pedogenesis water table fluctuations and flood plain aggradation produced profiles with color mottles iron and carbonate nodules and weak b and c horizons similar to holocene flood plain soils the second stage of pedogenesis occurred after the avoyelles prairie was terraced and the regional water table fell a minimum of 30 m improved soil drainage created oxidizing and acidic conditions removed free carbonates dehydrated iron oxides and transformed clay minerals this stage produced pleistocene profiles with brighter matrix and mottle hues lower ph values larger and harder iron nodules and greater kaolinite contents than comparable holocene flood plain soils the primary influence on regional water table lowering in the southern lower mississippi valley was relative sea level fall mississippi river flood plain lowering adjacent to the avoyelles prairie most likely occurred between the end of oxygen isotope stage 3 and the maximum period of valley entrenchment during oxygen isotope stage 2 water table fall affected pedogenic processes a minimum of 400 km upvalley of the continental shelf margin pleistocene mississippi river paleosols represent excellent analogs for interpreting ancient alluvial paleosols formed during episodes of base level fall and valley incision as fluvial entrenchment terraced the meandering avoyelles prairie a pedogenic threshold was crossed that altered the pathway of chemical weathering the complexity of a multiple step pedogenic history also highlights the fact that interpretation of relative profile development as a function of time can be problematic

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

evidence of late glacial runoff in the lower mississippi valley abstract thousands of cubic kilometers of massive coarse grained glacial outwash underlie the alluvial plain of the lower mississippi valley between cairo illinois and the gulf of mexico however valley trains deposited by braided streams characterize less than one third of the valley area and those attributable to runoff from the laurentide ice sheet cover less than 15 000 km2 mostly in the st francis basin segment of the valley there they form a series of subdued terraces that reflect episodes of meltwater release and possibly catastrophic flood events radiocarbon dated sediment cores establish that the initial runoff entered the basin about 16 3 ka bp and continued without a significant lull for about 5000 years the distribution of archeological sites tends to support an effective brief cessation of runoff to the valley about 11 0 ka bp when meltwater is thought to have been diverted from the mississippi river valley to the st lawrence valley both radiocarbon dates and archeological evidence document a final pulse of outwash to the lower mississippi valley about 10 0 ka bp when the mississippi river occupied thebes gap near cairo and created the charleston fan all outwash deposition ended and the river adopted a meandering regime not later than 9 8 ka bp

perspectives on the geoarchaeology of the lower mississippi valley abstract the impact of harold n fisk s work on the archaeology and geoarchaeology of the lower mississippi valley lmv has been monumental as a result of his landmark publications on the geology of the alluvial valley of the mississippi river our comprehension of the interplay between geological geomorphic and human actions is relatively well developed however geologists and archaeologists still need to work together more closely to appreciate and understand the contributions each field has to offer examination of the interplay between geologists and archaeologists in the realm of dating and landscape evolution provides the basis for an investigation of the state of geoarchaeology in the lmv today integration of research demands an appreciation of scale which must be approached from an historical perspective humans have both in the past and the present impacted the natural environment of the mississippi river and its floodplain only when this fact is fully appreciated by archaeologists and geologists alike will it be possible to forge a new synthesis of the relationship between the dynamic alluvial valley and its human occupants

a chronological framework for the holocene vegetational history of central minnesota the steel lake pollen record abstract paleorecords from minnesota and adjacent areas have often been used to evaluate large scale climatic processes in the mid continent of north america however most of these records are compromised by chronological flaws making problematic any comparisons with climatic interpretations based on other records e g gisp2 in greenland we report here a high resolution pollen record with a secure chronology constrained by 26 14 c dates on terrestrial macrofossils from steel lake central minnesota about 11 2 ka cal bp 11 000 calibrated yr before present the late glacial picea forest near steel lake was succeeded abruptly by pinus banksiana and or resinosa the pinus forest began to open 9 4 ka cal bp with the expansion of prairie taxa and a pine parkland or savanna prevailed until about 8 ka cal bp when quercus replaced pinus to become the dominant tree in the prairie areas for 4500 yr the close chronological control permits the correlation of key vegetational changes with those at other reliably dated sites in the eastern dakotas and in minnesota suggesting that the abrupt decline of the spruce forest was time transgressive from southwest to northeast during 2000 yr and that the development of prairie was time transgressive in the same direction over 2600 yr correlation of key pollen horizons at steel lake with those in the high resolution pollen profiles of elk lake ca 50 km northwest of steel lake suggests that the well known elk lake varve chronology for the early holocene is about 1000 yr too young

basin scale reconstruction of the geological context of human settlement an example from the lower mississippi valley usa abstract large river valleys integrate hydroclimatic change differently than smaller ones whereas sedimentary deposits in smaller valleys serve to record temporally short lived and or low amplitude hydrological and geomorphic responses larger valleys are relatively insensitive to changes at these scales however because of their size larger valleys archive sedimentary records of longer term geographically extensive flooding and climate related landscape evolution recent work in the upper tensas basin of the lower mississippi river valley usa demonstrates the utility of analyzing sediments from a large geomorphically and fluvially complex river basin as a proxy for significant climatic and landscape changes and their relation to human history a 6000 year sedimentary archive is used to explore the interplay between climate change landscape evolution and human responses to changing environmental parameters once the mississippi river basin stabilized following glacial retreat and deceleration of sea level rise the history of the river and its inhabitants was dominated by long periods of landscape stability punctuated by episodes of significant landform alteration modifications of the landscape are correlated with intervals of rapid climate change recorded in globally distributed multiproxy data sets periods of significant landscape instability and fluvial re adjustment are recorded for the periods ca 4800 3800 3000 2500 and 1000 800 cal bp these punctuations in the history of the lower mississippi valley are related to periods of major cultural transformation and suggest climate change plays a significant role in the long term history of human occupation in this river valley this research demonstrates how geoarcheology can provide information on human environmental relationships and long term landscape histories at temporal and spatial scales relevant to understanding cultural responses to climate change

patterns of holocene environmental change in the midwestern united states abstract four pollen sequences along a transect from north central iowa to southeast wisconsin reveal the distribution of prairie and forest during the holocene and test the use of pollen isopolls in locating the holocene prairie forest border prairie was dominant in central iowa and climate was drier than present from about 8000 to 3000 yr b p during the driest part of this period in central iowa 6500 5500 yr b p mesic forest prevailed in eastern iowa and wisconsin suggesting conditions wetter than at present prairie replaced the mesic forest about 5400 yr b p in eastern iowa but did not extend much farther east mesic forests were replaced in southern wisconsin and northern illinois about 5400 yr b p by xeric oak forests this change from mesic to xeric conditions at 5400 yr b p was widespread and suggests that the intrusion of drier pacific air was blocked by maritime tropical air from the gulf of mexico until the late holocene in this area

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

petrogenesis of the puerto eden igneous and metamorphic complex magallanes chile late jurassic syn deformational anatexis of metapelites and granitoid magma genesis a suite of schists gneisses migmatites and biotite granitoids from the puerto eden igneous and metamorphic complex peimc and biotite hornblende granitoids of the south patagonian batholith southern chile has been studied for that purpose the chemistry of minerals and the bulk rock composition of major and trace elements including rb sr and sm nd isotopes were determined mineralogical observations and geothermobarometric calculations indicate high temperature and low pressure conditions ca 600 700 c and 3 to 4 5 kbar for an event of metamorphism and partial melting of metapelites in late jurassic times previously determined by shrimp u pb zircon ages structures in schists gneisses migmatites and mylonites indicate non coaxial deformation flow during and after peak metamorphic and anatectic conditions andalusite schists and sillimanite gneisses yield initial 87sr 86sr ratios of up to 0 7134 and end150 values as low as 7 6 contemporaneous biotite granitoids and a coarse grained orthogneiss have initial 87sr 86sr ratios between 0 7073 and 0 7089 and end150 values in the range 7 6 to 4 4 this indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source thus a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids the peimc is considered as a segment of an evolving kilometre sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in late jurassic times a biotite hornblende granodiorite and a muscovite garnet leucogranite show initial 87sr 86sr ratios of 0 7048 and 0 7061 and end100 values of 2 6 and 1 8 respectively and are thus probably related to early cretaceous magmas not involved in the anatexis of the metasedimentary rocks

volcanic stratigraphy and geochronology of the cretaceous lancones basin northwestern peru position and timing of giant vms deposits a 10 km thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the cretaceous lancones basin in northwestern peru and underlies part of the huancabamba deflection the marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin the earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar quartz porphyritic felsic volcanic rocks these volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide vms deposits which are inferred to have formed in a localized but relatively deep marine setting u pb zircon dating of felsic volcanic rocks associated with vms deposits at tambogrande indicates ages from 104 8 1 3 to 100 2 0 5 ma for the ore bearing volcanic sequence the timing of onset of rift related volcanism is not well constrained but is therefore of middle albian age or older subsequent latest albian to turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields u pb zircon ages of 99 3 0 3 to 91 1 1 0 ma these later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the lancones basin finally the basin was intruded by late cretaceous to tertiary granitoids of the coastal batholith the genesis of the cretaceous lancones basin and other equivalent volcanic rift related marginal basins in western south america including the western peruvian trough is related tectonically to the break up of gondwana early volcanism and associated vms deposits formed in the lancones basin during the albian coincided with the initial rifting stage prior to active oceanic spreading between south america and africa during this time the relatively stationary western margin of continental south america was undergoing extension and rifting due to a westward and oceanward retreating arc resembling a mariana arc type setting the mochica orogeny marks the termination of rifting subsidence and related volcanism along the western margin of south america this orogenic event also broadly coincides with the onset of spreading of the south atlantic and westward drift of the south american continent subsequent volcanism in the lancones basin was more continental arclike under an andean type scenario

constraints on deformation of the southern andes since the cretaceous from anisotropy of magnetic susceptibility the southernmost segment of the andean cordillera underwent a complex deformation history characterized by alternation of contractional extensional and strike slip tectonics key elements of southern andean deformation that remain poorly constrained include the origin of the orogenic bend known as the patagonian orocline here renamed as patagonian arc and the exhumation mechanism of an upper amphibolite facies metamorphic complex currently exposed in cordillera darwin here we present results of anisotropy of magnetic susceptibility ams from 22 sites in upper cretaceous to upper eocene sedimentary rocks within the internal structural domain of the magallanes fold and thrust belt in tierra del fuego argentina ams parameters from most sites reveal a weak tectonic overprint of the original magnetic fabric which was likely acquired upon layer parallel shortening soon after sedimentation magnetic lineation from 17 sites is interpreted to have formed during compressive tectonic phases associated to a continuous n s contraction our data combined with the existing ams database from adjacent areas show that the early cretaceous late oligocene tectonic phases in the southern andes yielded continuous contraction variable from e w in the patagonian andes to n s in the fuegian andes which defined a radial strain field a direct implication is that the exhumation of the cordillera darwin metamorphic complex occurred under compressive rather than extensional or strike slip tectonics as alternatively proposed if we agree with recent works considering the curved magallanes fold and thrust belt as a primary arc i e no relative vertical axis rotation of the limbs occurs during its formation then other mechanisms different from oroclinal bending should be invoked to explain the documented radial strain field we tentatively propose a kinematic model in which reactivation of variably oriented jurassic faults at the south american continental margin controlled the late cretaceous to cenozoic evolution of the magallanes fold and thrust belt yielding the observed deformation pattern

jurassic extensional tectono magmatism and associated mineralization of the el faldeo polymetallic district chilean patagonia geochemical and isotopic evidence of crustal contribution the el faldeo is the southernmost andean mineral district and consists of paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the ibanez formation in addition two epizonal intrusive units and related subvolcanic porphyries sills and dikes which intrude both the metamorphic basement and the ibanez formation are recognized the quebrada colorada granodiorite qcg and the cerro esmeralda tonalite cet the polymetallic mineralization was formed in four stages and includes pyrite arsenopyrite gold silver sphalerite galena and chalcopyrite hosted in felsic tuffs of the ibanez formation hydrothermal breccias and epizonal intrusions field observations and geochronological data u pb ar ar and fission track on cet rocks suggest 1 the development of subsiding pull apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid jurassic time 2 a tectonic exhumation of the rocks of the district probably triggered by the chile ridge trench interactions that started in late miocene time sr nd isotopic data from cet samples and the pb isotope characteristics of galenas from the second and third mineralization stages are indicative of crustal contribution to the origin of the magmas and galena mineralization rocks of the paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas

development of an osl chronology for late pleistocene channel belts in the lower mississippi valley usa abstract the broad alluvial valley of the lower mississippi river contains large channel belts for which age control has been limited by the lack of organic material for radiocarbon dating and deposit ages greater than the limit of radiocarbon in order to produce a detailed chronology of fluvial evolution samples for optical dating were collected from late pleistocene channel belt deposits quartz sand was analyzed using the coarse grained single aliquot regenerative dose sar technique samples showed little evidence of partial bleaching and optical ages are for the most part consistent with the available radiocarbon and relative age constraints optical ages range from 85 to 11 ka and along with geomorphic and stratigraphic relationships indicate that the mississippi river was meandering during the end of the last interglacial 85 83 ka and switched to a braided regime by 64 50 ka during initial marine oxygen isotope stage ois 4 3 glaciation of the upper mississippi drainage basin the mississippi river switched back to a meandering regime after formation of the youngest braid belt at 11 ka the 69 optical ages presented here provide the first detailed chronology of lower mississippi valley braid belt and pleistocene meander belt formation and provide an example of the utility of optical dating in fluvial settings where material for radiocarbon dating is scarce

40ar 39ar dating of the jurassic volcanic province of patagonia migrating magmatism related to gondwana break up and subduction abstract the mesozoic large igneous province lip of patagonia southern south america which is one of the largest silicic provinces on earth has been investigated by the 40ar 39ar method twenty seven ages considered as valid including twenty plateau ages show that the volcanic activity ranging from 187 to 144 ma occurred between and contemporaneously with the initial break up of gondwana starting with the karoo antarctic tasmanian kat flood basalt province in the east and a subduction in the west the data display a regular decreasing of ages from the ene 187 ma to the wsw 144 ma along about 650 km apparently related to the tectonic structure in half grabens oriented nnw sse the good fitting of this trend with the opening of the rocas verdes sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the ssw in a continental disruption behind the magmatic arc the observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction the spreading and thermal effect of the kat plume may have an additional effect and also could account for the unusually large volume of magma