facies analysis of the lo valdes formation tithonian hauterivian of the high cordillera of central chile and the palaeogeographic evolution of the andean basin

the use of 7 lioh impregnated filters for the collection of acidic gases and analysis by instrumental neutron activation analysis a simple system was designed using7lioh impregnated filters to collect acidic gases from ambient air and from highly concentrated volcanic plumes or gas streams these filters were developed for analysis using instrumental neutron activation analysis but other analytical techniques could be used as well the sampling system was designed to use a series of 1 3 m7lioh impregnated filters to collect high concentrations of acidic gases found in gas plumes of active volcanoes the filters are quantitative for so2 and the halide acids when sufficient base is present to neutralize the acidic species extremely high concentrations of so2 may not be collected quantitatively since so2 is a relatively weak lewis acid compared to the halide acids the acidic oxides of sb as and se were also collected quantitatively a particle filter preceded the impregnated filters in order to remove particles from the fumes this system has proven effective under difficult sampling conditions and since it is portable and light weight it could be used for many volcanological applications where high levels of acidic gaseous phase species need to be collected

age and significance of sequence boundaries on jurassic and early cretaceous rifted continental margins the age and significance of sequence boundaries on jurassic to early cretaceous rifted continental margins in three ocean basins have been documented the margins are the santos basin in the south atlantic the grand banks in the north atlantic and the beaufort sea in the arctic ocean large industry data bases were used for the interpretation of each area megasequence boundaries separate the major phases of basin evolution for example syn rift and post rift boundaries developed with an average periodicity of 49 m y sequence boundaries define the component parts of each megasequence and developed with a modal periodicity of 10 15 m y out of 27 total boundary ages most 16 are developed on just one margin only two possible age ranges overlap on all three margins ighty percent of the megasequence boundaries and 50 of the sequence boundaries show a direct causal connection with coeval faulting and or folding the rest of the boundaries appear as unstructured surfaces separating transgressive and or regressive sedimentary wedges and are interpreted to result from changes in the rate of basin subsidence sediment input and long term eustatic sea level these data do not support theories advocating synchronous worldwide boundary development resulting from periodic short term falls in global eustatic sea level only in like basins of the same age with identical subsidence and sediment input rates are boundaries likely to develop synchronously hence the concept of global synchroneity of sequence boundary development may well be an illusion creat d by the similarity in age of the majority of basins studied as a result of this study it seems wise discard the global approach to basin analysis

stratigraphy and regional significance of the upper jurassic lower cretaceous byers group livingston island antarctica the byers group exposed on byers peninsula western livingston island antarctica comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore arc environment the basal 105 m thick anchorage formation consists of radiolarian mudstones and tuff rich interbeds of kimmeridgian tithonian age it correlates with upper jurassic organic rich mudstone units throughout the proto south atlantic region the succeeding 244 m thick devils point formation marks the first major pulse of coarse volcaniclastic material into the basin it is in turn followed by the extensive president beaches formation comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations molluscan and dinoflagellate cyst taxa indicate a berriasian age and comparatively nearshore depositional environment for this unit an unconformity of late berriasian or early valanginian age separates the three lowest formations from the chester cone formation the fine grained sealer hill member at the base of the latter is dated as valanginian and grades up into several hundred metres of pebbly sandstones and pebble granule conglomerates these mark the second major volcaniclastic pulse and may be of hauterivian or even younger age definition of this major new group will facilitate more precise upper jurassic lower cretaceous stratigraphical correlations within the southern south america scotia arc antarctic peninsula region it will also aid our understanding of the critical palaeogeographical transition in the northern antarctic peninsula from anoxic basin to active magmatic arc

multiple fault slip triggered above the 2016 mw 6 4 meinong earthquake in taiwan rapid shortening in convergent mountain belts is often accommodated by slip on faults at multiple levels in upper crust but no geodetic observation of slip at multiple levels within hours of a moderate earthquake has been shown before here we show clear evidence of fault slip within a shallower thrust at 5 10 km depth in sw taiwan triggered by the 2016 mw 6 4 meinong earthquake at 15 20 km depth we constrain the primary coseismic fault slip with kinematic modeling of seismic and geodetic measurements and constrain the triggered slip and fault geometry using synthetic aperture radar interferometry the shallower thrust coincides with a proposed duplex located in a region of high fluid pressure and high interseismic uplift rate and may be sensitive to stress perturbations our results imply that under tectonic conditions such as high background stress level and high fluid pressure a moderate lower crustal earthquake can trigger faults at shallower depth

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

short paper the significance of mid palaeozoic basement in graham land antarctic peninsula a mid palaeozoic basement to the antarctic peninsula is confirmed by a geochronological study of the gneissose country rock of the mesozoic magmatic arc of eastern graham land rb sr whole rock ages of 410 15 and 426 12 ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the antarctic peninsula during the silurian sm nd data on garnet whole rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late carboniferous times probably as a result of plutonism

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

continental rift to back arc basin jurassic cretaceous stratigraphical and structural evolution of the larsen basin antarctic peninsula the larsen basin developed in jurassic times as a result of continental rifting during the early stages of gondwana break up lower upper jurassic non marine sedimentary and volcanic rocks constitute a syn rift megasequence recording initial largely amagmatic extension and subsequent widespread extension related silicic volcanism a succeeding kimmeridgian early berriasian transgressive megasequence consisting largely of anoxic dysoxic hemipelagic mudstones is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak jurassic eustatic sea levels served to maximize sediment starvation the fragmentary record for late berriasian barremian times suggests that a regressive megasequence may have developed in the earlier part of this period recording increased sediment yield to the larsen basin from the increasingly emergent antarctic peninsula arc subsequently strata in the southern but not the northern part of the basin underwent relatively intense eastward verging deformation possibly during the formation of a retro arc fold thrust belt where exposed the lower part of the succeeding aptian eocene megasequence consists of a deep marine clastic wedge deposited along the fault bounded western basin margin during a phase of arc uplift and related differential subsidence following partial basin inversion in late cretaceous times regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward

coarse grained submarine fan and slope apron deposits in a cretaceous back arc basin antarctica the cretaceous of west james ross island antarctica represents the proximal fill of a late mesozoic back arc basin that was probably initiated by oblique extension during the early development of the weddell sea the succession records sedimentation in two contrasting depositional systems a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse grained submarine fans slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones mudstone association interspersed with non channelized chaotic boulder beds intraformational slump sheets and isolated exotic blocks representing a spectrum of mass flow processes from debris flow to submarine gliding localized sand rich sequences sandstone breccia association represent sandy debris lobes at the mouths of active slope chutes the submarine fan sediments conglomerate association are typified by coarse conglomerates and pebbly sandstones interpreted as the deposits of high density turbidity currents and non cohesive debris flows three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse grained submarine fans from major fan channels through ephemeral marginal channels or terraces to levee or interchannel environments the occurrence of both slope apron and submarine fan depositional systems during the early and mid cretaceous is attributed to localized input of coarse arc derived sediment along a tectonically active basin margin periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons slope apron sedimentation was controlled largely by intrabasinal tectonics local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults differential subsidence across the basin margin anchored the basin slope for at least 20 myr and precluded basinward progradation of shallow marine environments

review of the upper cretaceous lower tertiary sequence on seymour island antarctica the thick sequence of fossiliferous marine and non marine sandstones and siltstones on seymour i represents the most complete well exposed section of upper cretaceous to lower tertiary rocks known in the southern hemisphere the succession has been divided into the upper campanian to possibly lowermost tertiary marambio group comprised of the lopez de bertodano and sobral formations and the lower tertiary cross valley and la meseta formations of the seymour island group the lopez de bertodano formation consists of c 1067 m of medium grained to silty concretionary sandstones of middle campanian to possibly maestrichtian age and the sobral formation of c 210 m of medium to coarse grained glauconitic concretionary sandstones the precise age of the sobral formation is uncertain but may be maestrichtian to possibly danian the lower 110 m of the seymour island group consists of non marine gravelly to pebbly sandstones and tuffaceous plant bearing siltstones of the cross valley formation overlying which are 450 m of fine grained marine sandstones and siltstones with interbedded fossiliferous pebbly conglomerates recent studies of the diverse and abundant faunas and floras of the seymour island group have shown that they are older than previously thought the cross valley formation being palaeocene and the la meseta formation late eocene to possibly earliest oligocene

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

late mesozoic sedimentation in the northern antarctic peninsula and its relationship to the southern andes two distinct phases of island are evolution are recognized in late mesozoic sedimentary rocks of the northern antarctic peninsula during late jurassic kimmeridgian tithonian times alternating radiolaria rich mudstones and ash fall tuffs were deposited under shallow marine euxinic conditions in association with restricted volcanic centres the name nordenskjold formation is proposed for this lithostratigraphic unit subsequently during the early cretaceous a major episode of volcanism and uplift led to the construction of an emergent are terrane simultaneous development of a retro arc basin resulted in the accumulation of coarse volcaniclastic detritus along the eastern side of the arc retro are sedimentation and intermittent volcanism continued into the late cretaceous and early tertiary although the pattern of sedimentation in the northern antarctic peninsula is broadly comparable to that of the southern andes an active marginal basin has not been recognized in the former area interbedded mudstones and tuffs identical to those of the nordenskjold formation were deposited in the southern andes and south georgia during the late jurassic early cretaceous

creation and destruction of weddell sea floor in the jurassic new paleomagnetic data invite reassessment of the breakup history of parts of the pacific margin of gondwana due to the mesozoic opening of the southern ocean basins these data indicate clockwise rotation of the antarctic peninsula relative to east antarctica between 175 and 155 ma that would have created as much as 1000 km of pre anomaly m25 ocean floor in the weddell sea basin the paleomagnetic data also suggest that between 155 and 130 ma the antarctic peninsula rotated counterclockwise relative to east antarctica rotation that along with the southward motion of east antarctica would have caused subduction of the newly created weddell sea ocean floor beneath the southern antarctic peninsula east vergent folding and thrusting of jurassic back arc basin rocks along the southeastern antarctic peninsula known as the palmer land event probably resulted from deformation along this newly created convergent margin the presence of jurassic weddell sea floor would have isolated eastern gondwana east antarctica australia and india from the antarctic peninsula and western gondwana africa and south america encouraging increased marine faunal connections between the pacific and weddell sea mozambique somali ocean basins during the 175 to 130 ma interval after 130 ma the antarctic peninsula would have been connected to east antarctica via the other west antarctic terranes thereby facilitating land migrations between eastern and western gondwana

interpretation of neovolcanic versus palaeovolcanic sand grains an example from miocene deep marine sandstone of the topanga group southern california despite abundant data on volcaniclastic sand stone the compositional spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented one of the most intricate tasks in optical analysis of sand stone containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks i e palaeovolcanic noncoeval grains from grains generated by active volcanism subaqueous and or subaerial during sedimentation neovolcanic coeval grains deep marine volcaniclastic sandstones of the middle topanga group of southern california are interstratified with 3000 m thick volcanic deposits both subaqueous and subaerial lava and pyroclastic rocks ranging from basalt andesite to dacite these rocks overlie quartzofeldspathic sandstones petrofacies 1 of the lower topanga group derived from deep erosion of a mesozoic magmatic arc changes in sandstone composition in the middle topanga group provide an example of the influence of coeval volcanism on deep marine sedimentation volcaniclastic strata were deposited in deep marine portions of a turbidite complex volcaniclastic apron built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks distinctive petrofacies 2 5 directly overlying basalt and basaltic andesite lava flows petrofacies 2 is a pure volcanolithic sandstone including vitric microlitic and lathwork volcanic grains and neovolcanic crystals plagioclase pyroxene and olivine the abundance of quenched glass palagonite fragments suggests a subaqueous neovolcanic provenance whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance this petrofacies probably was deposited during syneruptive periods testifying to provenance from both intrabasinal and extrabasinal volcanic events deposited during intereruptive periods impure volcanolithic petrofacies 3 includes both neovolcanic 85 and older detritus derived from plutonic metamorphic and palaeovolcanic rocks during post eruptive periods the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus 48 14 and increase of plutonic metamorphic and palaeovolcanic detritus the upper topanga group calabasas formation conformably overlying the middle unit has dominantly plutoniclastic sandstone petrofacies 6 neovolcanic detritus is drastically reduced 4 whereas palaeovolcanic detritus is similar to percentages of the lower topanga group petrofacies 1 the middle topanga group sandstones are vertically organized into four

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

bathymetry depth to magnetic basement and sediment thickness estimates from aerogeophysical data over the western weddell basin we estimated the bathymetry and sediment thickness of a remote and difficult to access portion of the antarctic continental margin using aerogeophysical surveying techniques the u s argentina chile aerogeophysical survey collected magnetic and gravity data over the basins surrounding the antarctic peninsula thirty seven of these flight lines were used to estimate bathymetry and depth to magnetic basement for the western weddell basin a wavenumber technique was applied to individual magnetic anomaly profiles in an automated fashion to obtain estimates of the depth to magnetic basement the bathymetric estimates were obtained by admittance inversions of the gravity field the results were then gridded at a 40 km interval for the region 64 w 44 w 73 s and 62 s bathymetric estimates and depth to magnetic basement estimates were differenced at each grid point to obtain a regional estimate of the thickness of nonmagnetic overburden assumed to be sediment subsequent spot measurements of topography in the estimated region of the continental margin generally agree to about 52 m the estimated magnetic basement deepens from the antarctic peninsula margin eastward to a maximum of 10 12 km near 54 w we also postulate the existence of two moderately large basins flanking the eastward continuation of the jason peninsula farther east the basement steps upward with a correspondent thinning of the sedimentary layer along the east coast of the peninsula results agree well with seismic studies on james ross island and magnetotelluric studies on marambio island and the larsen nunatak as well as the british antarctic survey basement estimates from aeromagnetic data this study further demonstrates the utility of combined application of airborne and satellite geophysical techniques in the study of structure and tectonic evolution of continental margins and marine basins

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

facies and depositional processes in an upper jurassic to lower cretaceous pelagic sedimentary sequence antarctica the nordenskjold formation oxfordian berriasian age is exposed on the east coast of the antarctic peninsula where it consists of interbedded ash layers and biosiliceous mudstones which accumulated under anaerobic to dysaerobic bottom waters the mudstones were deposited by pelagic settling and the ash layers by pelagic settling from suspension or as fallout from subaerial eruption columns the lower part of the succession accumulated in a basinal setting under anaerobic bottom waters and is characterized by parallel bedding mudstones deposited in this setting preserve abundant zooplankton faecel pellets compaction of these pellets has given rise to a bedding parallel fissility the upper part of the succession accumulated under dysaerobic bottom waters in a slope setting the sequence is wavy bedded and contains abundant evidence of post depositional sediment instability and resedimentation much of which was caused by tectonic activity discrete slide masses are absent from the slope sequence and it appears that slope processes were dominated by creep examination of the mudstones shows that as levels of dissolved oxygen in bottom waters increase pelleted mudstones give way to structureless mudstones before visible bioturbation is noted

the jurassic succession in west central argentina stratal patterns sequences and paleogeographic evolution abstract jurassic rocks in west central argentina are predominantly marine and marginal marine siliciclastics associated with prominent but volumetrically subordinate carbonates and evaporites facies developments were ruled by paleogeographic persistence within the southern infratropical latitude belt within 40 50 s and by siliciclastic derivation from the patagonian hinterland to the southwest minor volcanic and volcaniclastic supply arrived from the west out of a magmatic belt related to circum pacific convergence timing of the main marine flooding events and general correspondence with the high rank stratal packaging recorded in western north america suggest that global eustasy was also a factor in controlling the local stratigraphic record early jurassic sedimentation occurred within a series of semi isolated depocentres linked to fault bounded triassic troughs the sinemurian toarcian deposits record depocentre expansion and coalescence these trends were coeval with progressively more widespread marine invasions from the northwest and west leading to an elongate marine seaway which connected central patagonia with the pacific domain during the aalenian bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment bathonian and early callovian were times when coarse clastics prograded into the basin while the marine embayment shrank as a result of stepwise forced regressions during late callovian to oxfordian globally rising sea level the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance that promoted cratonward onlap and inception of widespread carbonate deposition ooidal coralline carbonate development was terminated after a relatively sudden messinian style event that desiccated a large tract of the andean basin and favored massive precipitation of anhydrite in the course of the kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat salina complex linked to a widespread erg and to an ephemeral drainage system by the tithonian at a period of peak oceanic stand marine connection was reinstated and recorded as the most widespread jurassic transgression across the neuquen aconcagua embayment shelfal deposition consisted of molluscs and ooid dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations like in other prolific petroleum provinces around the world the tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content

late jurassic palaeogeography and anaerobic dysaerobic sedimentation in the northern antarctic peninsula region late jurassic anaerobic dysaerobic mudstones crop out on both the weddell sea back arc and pacific fore are margins of the northern antarctic peninsula the only known occurrence on the pacific margin of the antarctic peninsula is the anchorage formation of livingston island this mudstone dominated unit comprises interbedded volcaniclastic sandstones pyroclastic epiclastic tuffs and radiolarian mudstones the volcaniclastic sandstones are interpreted as representing deposition from turbidity currents the tuffs represent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents the radiolarian mudstones represent suspen sion sedimentation and reveal an upward increase in bioturbation with a transition from anaerobic dysaerobic conditions to dysaerobic aerobic conditions these facies and the observed vertical change in oxygenation conditions are similar to those seen in the nordenskjold formation on the weddell sea margin of the antarctic peninsula however biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late kimmeridgian early tithonian in the anchorage formation but late tithonian or early berriasian in the nordenskjold formation this diachroneity is related to the palaeogeographical development of the antarctic peninsula magmatic arc a wide epicontinental sea and subdued arc relief in the early kimmeridgian was followed in the tithonian by arc uplift increasing oxygenation in the fore are basin and the development of a restricted basin in the hack arc region in latest tithonian earliest berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore arc basin only then was the back arc basin undergoing the transition from dysaerobic to aerobic conditions anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back arc area formed by the emergent arc

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

late cretaceous extinction patterns in antarctica new correlations of marine clastic sedimentary rocks exposed within the james ross basin antarctica have shown that the mid to late cretaceous succession is in excess of 5 km thick plotting the ranges of the principal molluscan macrofossils against the revised stratigaphy indicates that inoceramid bivalves are totally absent and dimitobelid belemnites extremely rare throughout an extensive 1400 m thick maastrichtian succession these early extinction patterns are interpreted to be due to both a regional shallowing event and a pronounced phase of high latitude campanian maastrichtian cooling cool polar bottom waters may have been forming by as early as mid to late campanian times

weddell sea tectonics and gondwana break up an introduction the weddell sea part of the circumpolar southern ocean is probably the most remote least known and least accessible sea in the world difficult ice conditions have limited the acquisition of ship data although this has been partly offset in recent years by access to satellite radar altimetry data the weddell sea was originally defined by the admiralty hydrographic department in 1932 and redefined by the antarctic place names committee in 1976 hattersley smith 1991 it is bounded on the western side by the east coast of the antarctic peninsula on the southern side by the ronne and filchner ice fronts and on the southeastern side by the dronning maud land and coats land coasts of east antarctica fig 1 the south scotia ridge separates the weddell sea from the scotia sea to the north and a line joining southern thule in the south sandwich islands and kapp norvegia in dronning maud land separates it from the south atlantic ocean to the ne within this volume papers relate to the weddell sea as defined above together with part of the adjoining south atlantic ocean up to 50 e and to the geology of the once neighbouring continents of gondwana the term weddell sea embayment is also used informally throughout this volume to include the embayment area to the south of the weddell sea now covered by the ronne and filchner ice shelves including berkner island and the continental shelf north of the ronne and filchner ice fronts figs 1 2