synchronous detachment folds and coeval sedimentation in the prepyrenean external sierras spain a case study for a tectonic origin of sequences and systems tracts this paper reports on the structural and sedimentary evolution of the middle to late eocene of the prepyrenean external sierras southern pyrenees spain the initiation duration and kinematics of a set of growth structures that developed in a shallow marine depositional setting is documented the detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland the sedimentary units coevally deposited with these growth structures are arranged in four depositional sequences their boundaries correspond to flooding surfaces which grade basinwards into correlative conformities they are also indicated by the presence of both angular unconformities and onlap geometries each depositional sequence generally consists of two systems tracts the lower one or transgressive systems tract is formed by up to 400 m of azoic marls deposited in outer ramp areas the upper one or highstand systems tract mainly consists of shallow siliciclastic and carbonate facies up to 200 m thick deposited in middle to inner ramp areas these depositional sequences are interpreted to be controlled by regional tectonic pulses an increase of tectonic activity resulted in the flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue marls i e transgressive systems tract the overlying highstand systems tract developed following periods of diminished tectonism with the consequent growth and progradation of shallow carbonate platform facies

carbonate ramp drowning caused by flexural subsidence the south pyrenean middle eocene foreland basin abstract a detailed sedimentological and magnetostratigraphic analysis of three selected sections of the eocene carbonate platforms of the south pyrenean jaca pamplona foreland basin has been carried out in the aragonese external sierras huesca spain the stratigraphic record spans from the middle late lutetian chron 20r sbz15 to the bartonian chron 18r sbz17 the lutetian bartonian transition was characterized by a major paleogeographic change from a turbiditic foredeep with associated peripheral carbonate ramps to deltaic sedimentation in most of the outcrops of the external sierras the peripheral carbonate platforms guara formation are overlained by prodelta and outer ramp marls arguis formation the drowning unconformity between these two formations in the outcrops located in the westernmost tip of the external sierras is documented herein for the first time in this area the guara formation is overlain by glauconitic limestones deposited in a carbonate ramp setting between these units a characteristic unconformity is observed with hard ground formation and local evidence of subaerial exposure in the shallowest domains of the study area the abundant glauconite found on top of this unconformity records a starvation period in the basin followed by the development of a new carbonate ramp system during bartonian times santo domingo member of the arguis formation magnetostratigraphic and sedimentological data show that tectonic tilting occurred in latest lutetian times prior to the drowning process this suggests the influence of the flexural subsidence in the cratonic margin of the foreland as a result of the advance of a basement thrust in the hinterland nevertheless the correlation of the uppermost lutetian drowning unconformity with a basinwide regional sea level rise which may have further been global suggests an interplay between tectonic and eustatic processes in the drowning of the late lutetian south pyrenean carbonate ramps in this work we point out a narrowing of the carbonate ramp together with a relative sea level rise as the processes responsible for the drowning of the upper guara mb carbonate ramp these processes could be responsible for the development of drowning unconformities in cratonic margins of other foreland basins as carbonate ramps have a high sediment production potential the narrowing of the carbonate factory could provide the conditions for the drowning of the carbonate ramp system in a subsequent sea level rise

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

factors controlling prolific gas production from low permeability sandstone reservoirs implications for resource assessment prospect development and risk analysis low permeability reservoirs from the greater green river basin of southwest wyoming are not part of a continuous type gas accumulation or a basin center gas system in which productivity is dependent on the development of enigmatic sweet spots instead gas fields in this basin occur in low permeability poor quality reservoir rocks in conventional traps we examined all significant gas fields in the greater green river basin and conclude that they all occur in conventional structural stratigraphic or combination traps we illustrate this by examining several large gas fields in the greater green river basin and suggest that observations derived from the greater green river basin provide insight to low permeability gas charged sandstones in other basins we present evidence that the basin is neither regionally gas saturated nor is it near irreducible water saturation water production is both common and widespread low permeability reservoirs have unique petrophysical properties and failure to fully understand these attributes has led to a misunderstanding of fluid distributions in the subsurface an understanding of multiphase effective permeability to gas as a function of both varying water saturation and overburden stress is required to fully appreciate the controls on gas field distribution as well as the controls on individual well and reservoir performance low permeability gas systems such as those found in the greater green river basin do not require a paradigm shift in terms of hydrocarbon systems as some have advocated we conclude that low permeability gas systems similar to those found in the greater green river basin should be evaluated in a manner similar to and consistent with conventional hydrocarbon systems to date resource assessments in the greater green river basin have assumed a widespread continuous type resource distribution failure to recognize some of the fundamental elements of low permeability reservoirs has led to an underappreciation of the risks associated with exploration and development investment decisions in these settings and likely a significant overestimation of available resource levels

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

predicting flow properties in diagenetically altered media with multi scale process based modeling a wilcox formation case study abstract two approaches to capture the complex texture of wilcox sandstone samples in a pore scale model are employed a process based method and an image based method in the process based method petrographic analyses from transmitted light microscopy are used to infer the diagenetic history of the rock key steps in the diagenesis are quantitatively followed using a multi scale pore network modeling method to reconstruct the porous medium at different times in its history including the final state further information about pore sizes is gathered from scanning electron microscopy and nitrogen adsorption hysteresis measurements the ability to incorporate texture information of each diagenetic stage of a tight gas sandstone into a predictive multi scale pore network model is a novel aspect of this work in the image based method a micro ct image of the sample is acquired and a pore network model is extracted based on skeletonizing the resolvable pore space and mapping clusters of micropores to the unresolved porous areas the paragenesis of the formation was not captured using the image based method since sufficient details of its history could not be extracted the process based and image based methods produce comparable drainage capillary pressure curves while the drainage relative permeability curves in both methods display permeability jail effect they considerably differ from each other quantitatively the process based method produced a higher porosity and permeability compared to the image based method which is potentially due to the lower degree of its disorder high uncertainties in the petrophysical properties of tight gas sandstones is a costly challenge in their reservoir development our work provides insights into the effect of pore texture from different scales on rock fluid properties furthermore the methods demonstrated in this work can be pursued to extrapolate tight gas sandstone petrophysical properties between wells and throughout paragenetic sequences

intraplate deformation and basin formation during the tertiary within the northern iberian plate origin and evolution of the almazan basin the tertiary almazan basin was a topographic high during the mesozoic rifting at the northern part of the iberian plate it was bordered by two mesozoic basins 1 the castilian basin with a maximum thickness of more than 2000 m to the south and 2 the cameros basin with a maximum thickness of 9000 m to the north to the east the almazan basin is bordered by paleozoic rocks of the nw se trending aragonian range which underwent little subsidence during the mesozoic the two mesozoic basins show an e w direction during the tertiary nne ssw compression mainly late eocene early miocene both mesozoic basins were inverted and uplifted and the almazan basin with an overall synclinal geometry was filled with syn tectonic terrestrial sediments reaching a maximum thickness of 3500 m deformation during the tertiary was strongly controlled by preexisting structures both mesozoic extensional faults and reactivated variscan folds the depocenter of the tertiary basin migrated southward during the paleogene this migration is consistent with a piggyback transport of the whole basin northward due to the movement on the underlying crustal scale north verging cameros thrust the geometry and sedimentary evolution of the almazan thick skinned piggyback basin was controlled by the uplift associated with the main crustal scale thrust which was the main source area and was separated from the foreland basin since the early stages of filling late eocene this makes the main difference with thin skinned piggyback basins controlled by the foreland thrust belt and separated from the foreland basin only at their late stages

pore types and pore network evolution in upper devonian lower mississippian woodford and mississippian barnett mudstones insights from laboratory thermal maturation and organic petrology abstract pore evolution models from immature organic matter om rich barnett 0 42 r o and woodford 0 49 r o mudstones were compared with models previously developed from low maturity om lean boquillas eagle ford equivalent mudstones to investigate whether 1 different mineralogy siliceous vs calcareous exerts different catalytic and sorption effects and influences om pore origin and evolution and 2 different types of macerals show different om pore evolution history laboratory gold tube pyrolysis scanning electron microscopy sem and thin section petrography organic petrography and geochemical characterization were used to investigate the role of bulk mineralogy maceral type and thermal maturation on om pore evolution results suggest that mineralogy has little impact on om pore development and evolution macerals identified using both sem platy om particulate om organic mineral admixtures tasmanites and organic petrology vitrinite inertinite amorphous organic matter aom bituminite telalginite leiosphaeridia tasmanites do affect the origin and evolution of om pores owing to differences in chemical compositions generation kinetics and activation energy distributions between tasmanites matrix bituminite and other types of macerals leiosphaeridia and tasmanites in woodford mudstone samples exhibit a delay in onset and a shorter period of petroleum generation and pore development compared to the matrix bituminite in the barnett and woodford mudstone samples pre oil solid bitumen was observed to have migrated into initial primary mineral pore networks at the bitumen generation stage in both barnett and woodford samples at higher levels of thermal maturation the volume of primary mineral pores decreases and the pore volume composed of modified mineral pores and om pores becomes greater pore evolution and pore type heterogeneity in these mudstones is a function of the initial mineral pore network types of kerogen and macerals and generation kinetics of individual macerals upon thermal maturation

pore network analysis of brae formation sandstone north sea abstract in this work we apply digital rock physics drp to characterize the pore networks of the brae formation sandstones from two different wells in the miller field area north sea uk using x ray micro ct scans we calculate the porosity and permeability and generate pore network models to assess pore shape characteristics the porous samples are marked by macroporosities ranging from 4 9 to 15 2 with the effective porosities varying from 0 to 14 8 the samples also contained some microporosity hosted in secondary and accessory mineral phases varying between 2 6 and 10 7 pore network model results for total porosity indicate that the samples have median pore and throat radii ranging from 5 5 m to 16 8 m and 6 4 m 12 9 m respectively the throat length of all samples has a median value ranging between 36 3 m and 82 4 m the ratio between effective porosity and total porosity varies with total porosity following the exponential relation 0 98 e 0 032 0 028 pore network connectivity is established at a porosity of 3 and full communication is achieved at porosities exceeding 10 permeability was found to vary with total porosity with an exponent of 3 67 based on these observations and the results from our models the connectivity of the pore network has important implications for predicting reservoir performance during large scale subsurface projects such as hydrocarbon production and co2 storage

formation of orogen perpendicular thrusts due to mechanical contrasts in the basal decollement in the central external sierras southern pyrenees spain two series of analogue models are used to explore the effect of ductile frictional contrasts of the basal decollement on the development of oblique and transverse structures during thin skinned shortening these models simulate the evolution of the central external sierras ces southern pyrenees spain which constitute the frontal emerging part of the southernmost pyrenean thrust the ces are characterised by the presence of n s to nw se anticlines perpendicular to the pyrenean structural trend and developed in the hanging wall of the thrust system they detach on unevenly distributed triassic materials evaporitic dolomitic interfingering the models simulated the effect of adjacent ductile versus frictional decollements during shortening model series a tests the thickness ratio between overburden and the ductile layer whereas model series b tests the width perpendicular to the shortening direction of frictional decollement model results confirms that deformation reaches further in areas detached on a ductile layer whereas above frictional decollement areas shortening is accommodated by additional uplift and penetrative strain this replicates the structural style of the ces higher structural relief of n s anticlines with regard to orogen parallel structures absence of a representative ductile decollement in the core plunging towards the hinterland and foreland side closure not thrusted by the south pyrenean thrust

the roles of complex mechanical stratigraphy and syn kinematic sedimentation in fold development insights from discrete element modelling and application to the pico del aguila anticline external sierras southern pyrenees

facies architecture and geometry of a prograding carbonate ramp during the early stages of foreland basin evolution lower eocene sequences sierra del cadi se pyrenees spain this paper describes the early eocene sedimentological evolution of the sierra del cadi carbonate platform during the early development of the southeastern pyrenean foreland basin spain the platform developed as a south facing ramp system located either across the southern margin of a growth anticline within the newly formed basin or along the axial margin of the basin sequences i and ii both comprise basal transgressive systems tracts tsts dominated by a thick succession of outer ramp lime mudstones shallowing upwards into a highstand systems tract hst of mid and inner ramp bioclastic shoals sand banks and lagoonal deposits in sequence iii the tst comprises a thick wedge shaped unit of east to west backstepping inner and mid ramp nummulites shoals and lagoonal deposits progressively overlain by a thick succession of outer ramp lime mudstones and marls of the hst no lowstand systems tracts are recognized sequence boundaries correspond to flooding surfaces which grade basinwards into correlative conformities interpreted to represent the onset of high tectonic activity in the orogenic wedge and resulting high flexural subsidence across the foreland and drowning of inner ramp regions tsts represent continued high flexural subsidence creating accommodation space infilled by high rates of outer ramp sediment accumulation in sequences ii and iii tsts show a pronounced west to east thickening interpreted to represent the effect of variable subsidence across the margins of an uplifting growth anticline within the otherwise flexurally subsiding foreland prograda tional successions that characterize hsts developed during periods of reduced tectonic subsidence both within the foreland and in the orogenic belt allowing the growth and expansion of carbonate ramp facies belts

diagenetic controls on evolution of porosity and permeability in lower tertiary wilcox sandstones from shallow to ultradeep 200 6700 m burial gulf of mexico basin u s a abstract reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths 4 5 km analysis of paleogene wilcox sandstones on the upper texas gulf coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis reduction of porosity and permeability with burial in wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m at temperatures of 25 230 c diagenesis and petrophysical properties were interpreted from petrographic data and core analyses wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of q59f22r19 provenance did not change significantly during wilcox deposition in this area nor does average sandstone composition vary among lower middle and upper wilcox sandstones however composition does vary with sequence stratigraphic position lowstand slope fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts given observations from this onshore dataset wilcox sandstones deposited in deepwater environments in the gulf of mexico are likely to contain more rock fragments than their linked highstand equivalents pore types in wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones wilcox sandstones show steady porosity reduction from an average of 33 at 38 c to 12 at 132 c with little additional loss at higher temperatures by 132 c most primary pores have been lost by mechanical compaction or occluded by quartz cementation volume of secondary pores remains nearly constant despite dissolution of 3 5 average potassium feldspar during deep burial volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite albite illite and minor quartz in secondary pores the proportion of microporosity which occurs in authigenic clays altered grains and matrix increases in the deepest sandstones change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity permeability transforms because most deep to ultradeep wilcox sandstones are dominated by secondary pores and micropores porosity permeability transforms have lower slopes than those for shallower sandstones

pore types and pore size distributions across thermal maturity eagle ford formation southern texas pore types pore size and pore abundance vary systematically across thermal maturity in the eagle ford formation maverick basin southern texas scanning electron imaging of 20 samples from four wells is used to assess the complex response of pores to chemical and mechanical processes entailing both destruction of primary porosity and generation of secondary pores primary mineral associated pores are destroyed by compaction cementation and infill of secondary organic matter whereas secondary pores are generated within organic matter om destruction of primary pores during early burial to 0 5 occurs by compaction of ductile detrital om and clays and to a lesser degree as a result of cementation and infill of secondary om larger pores are associated with coccolith debris the dominant om is spatially isolated detrital om stringers porosity is volumetrically dominated average 6 2 by relatively large mostly interparticle mineral associated pores median size 51 6 nm 0 000002 in detection limit near 3 4 nm 0 00000012 0 00000015 in at low maturity porosity and pore size correlate directly with calcite abundance and inversely with om volumes at higher maturity further destruction of primary pores occurs through cementation secondary om infill and greater compaction mineral associated pores are present at high maturity 1 2 1 3 but are smaller median size 30 2 nm 0 0000011 in and less abundant average of 2 5 than at low maturity a large portion of om within high maturity samples is diagenetic in origin and has pervaded into primary pore space coating cement crystals and filling intraparticle pores substantial mineral associated porosity is locally present in samples where incursion of primary pore space by secondary om has not occurred abundant secondary porosity is generated as om matures into the wet gas window porosity in most high maturity samples is volumetrically dominated average of 1 3 by smaller om hosted pores median size 13 2 nm 0 00000051 in

folded onlap geometries implications for recognition of syn sedimentary folds growth strata are usually analysed from geological maps outcrop geometry or seismic sections many growth folds have been defined from geological maps especially in areas where plunging structures allow the syn tectonic sequence to be displayed at the surface the geometrical arrangement of these syn tectonic sequences can also be defined from the relationships between pre growth and growth strata as obtained from geological maps in this paper from theoretical models and natural examples from the southern pyrenees we argue that some cartographic patterns of strata associated with variably plunging folds traditionally ascribed to syn tectonic sedimentation with thinning of sedimentary units toward the anticlinal hinge zone can be explained as completely post sedimentary folds of sedimentary units lapping onto an inclined bedding surface and linked to basin margin or downlap geometries on the basin floor we conclude that 3 d analysis of syn tectonic structures combining data from independent sources i e geological maps and seismic reflection profiles is essential to determine relationships between sedimentary units and structures

evolution of a turbiditic foreland basin and analogy with an accretionary prism example of the eocene south pyrenean basin the south pyrenean tertiary basin was an east west trending foreland trough which developed from early eocene to miocene times in a north south trending compressive regime a turbiditic series of lower and middle eocene age hecho group enables us to define the typical features of the tectono sedimentary evolution of the basin 1 slope and shelf derived carbonate turbidites and megaturbidites bear witness to the seismic activity of the basin borders 2 the southern basin border evidenced by the onlap of the turbidites onto slope and shelf deposits migrated progressively southwards in time and 3 the deformation pattern is characterized in the meso cenozoic sediments by a complex thrust fold system of thin skinned tectonics style its specific geometry and kinematics were influenced by the pre deformation geometry onlap this thrust fold system is related to south directed basement nappes and its front advanced progressively southwards in time from this data we infer an evolutionary model for this highly mobile basin it was a flexural basin linked to the northward bending of the lithosphere in the foreland of a south directed crustal accretionary prism as the nappes advanced the basin migrated progressively southwards in the foreland direction as for the actual basin evolution many analogies with accretionary prism trench systems of active margins exist

growth and propagation of buckle folds determined from syntectonic sediments the ubierna fold belt cantabrian mountains n spain abstract the geometry of syntectonic tertiary sediments in the ubierna fold belt can be explained using the buckling folding theory the geometric features of the fold train in the pre orogenic succession also agree with a buckling process each fold grew with progressive and coeval rotation of both limbs although rotation was asymmetric and greater on the forelimb than the backlimb the diachroneity of synrotational sequences progressive unconformities through different folds indicates that fold propagation migrated towards the foreland however the initial folds were tightened during a later deformation stage synchronous with the development of younger folds

carbonate platform flanks slope angle and sediment fabric more than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric plots of slope angle versus sediment fabric show that grainy non cohesive mud free sediments build steeper slopes than muddy cohesive sediments examples near the end members of grainy and muddy carbonate platform flanks are found in the triassic of the dolomites in northern italy and in the bahamas respectively they document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded the grainy flank sediments in the dolomites modified by shearing and avalanching produce straight slope profiles with declivities up to 35 whereas the muddy bahamian flank sediments modified by large scale creep and rotational to translational sliding and slumping produce a concave upwards slope profile inclined at less than 4 the comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field angle of shearing and mode of readjustment processes is linked to the composition of the slope sediment among the variables such as sea level subsidence climate plate motion and oceanographic setting windward leeward sediment fabric is suggested to be a major if not the major control on slope angle and slope curvature of carbonate platform flanks besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems furthermore it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling

oxygen isotopes of marine mollusc shells record eocene elevation change in the pyrenees abstract constraining paleoaltimetry of collisional orogens is critical to understand the dynamics of topographic evolution and climate tectonics retroactions here we use oxygen stable isotope record on oyster shells preserved in marine foreland deposits to examine the past elevation of the pyrenees during the eocene our approach is based on the comparison with the paris basin an intracratonic basin not influenced by orogenic growth the finding of a shift of 1 5 between 49 and 41 ma indicating more negative 18oc in the south pyrenean foreland is interpreted to reflect the inflow of river water sourced from higher elevation in the pyrenees to test this and provide paleoelevation estimate we adopt a morphologic hydrological model accounting for the hypsometry of drainage basin our best fitting model shows that the pyrenees rose up to 2000 m this indicates that the pyrenees reached high elevation in the eocene thus providing new critical constraints on their long term orogenic development 18o of marine mollusc shells are proved potentially attractive for paleoelevation studies especially for mountain belts where elevated continental surfaces have not been preserved

the qs problem sediment volumetric balance of proximal foreland basin systems although the stratigraphy of sedimentary basins depends on the balance between the magnitude and grain size characteristics of the sediment supply qs and the spatial distribution of tectonic subsidence generating accommodation x qs is problematical to measure in present day sediment routing systems and formidably difficult to predict in their ancient counterparts this challenge was tackled by treating the sediment discharge from the outlet of mountain catchments as the result of incision by a drainage network with a bulk diffusivity based on the length over which the mean annual rainfall is concentrated the size relief and slope of palaeo catchments acting as feeders for sediment routing systems are used to run simulations of sediment discharge and bulk diffusivity for a range of annual precipitation values a wide range of observable geological phenomena can be used to converge on the most likely solutions for qs including depositional volumes in the basin and bedrock thermochronology and detrital cosmogenic nuclide dating to constrain catchment erosion rates modelled sediment discharges can be checked with estimates derived from global regressions the sediment efflux of mountain catchments serves as a boundary condition for down system sediment transport and deposition variations in the volumetric ratio of sediment supply to available accommodation qs x determines patterns of transverse versus longitudinal axial sediment dispersal the volumetric ratio may change as a result of variations in climatic parameters tectonic uplift rate and catchment expansion an abrupt climate change to higher precipitation values promotes higher qs x but transient landscape response causes a return to values close to the baseline generating a distinctive down system extension of a gravel spike catchment expansion has a similar but more prolonged effect on gravel progradation in contrast a change in tectonic forcing such as an increase in slip rate on a border fault causes little change in qs x because increased subsidence compensates for the increased sediment supply studies of mid eocene oligocene sediment routing systems in the south central pyrenees allow the discrimination of different types of proximal wedge top sedimentary systems on the basis of the volumetric ratio of qs to accommodation x i small steep local fan systems in tectonically ponded underfilled basins supplied by low sediment discharges ii tectonically guided long range axial systems fed by large sediment discharges from widely spaced palaeovalleys and iii large shallow sloping transverse megafans burying underlying defunct or active tectonic structures supplied by high to very high sediment discharges understanding the role of variations in qs helps to explain the syntectonic evolution of proximal foreland basin systems the oligocene miocene north alpine foreland basin switzerland is qualitatively identified as a high qs example the miocene recent northern apennines of italy as a low qs example and the eocene oligocene southern pyrenees of spain as intermediate in character

geometries of syntectonic sediments associated with single layer detachment folds abstract three kinematic models have been proposed to account for the geometry and kinematics of detachment folds involving a homogeneous competent layer detached over a ductile unit model 1 variable limb dip constant limb length model 2 constant limb dip variable limb length and model 3 variable limb dip variable limb length because the same fold shape can be generated by any of the above mechanisms fold kinematics are best determined by the geometries of syntectonic sediments in single layer detachment folds growth strata patterns are controlled by axial surface activity limb rotation limb lengthening fold uplift rates sedimentation rates and deformation mechanisms of the syntectonic sediments asymmetric kink folds have been modelled and compared with natural examples from the southern spanish pyrenees under conditions of high sedimentation rates model 1 folds produce characteristic fanning growth stratal wedges that initially onlap and then progressively overlap the detachment anticline whereas at low sedimentation rates anticlines with fanning growth wedges on both limbs are formed model 2 folds under conditions of high syntectonic sedimentation rates form anticlines with growth strata largely parallel to the pre growth units and thinned over the fold crest in contrast at low sedimentation rates model 2 folds show offlapping growth strata onto both limbs model 3 growth folds under conditions of high syntectonic sedimentation rates form anticlines with intermediate features from model 1 and model 2 at low sedimentation rates anticlines are formed with offlapping growth structures

investigating the relative permeability behavior of microporosity rich carbonates and tight sandstones with multiscale pore network models the relative permeability behavior of rocks with wide ranges of pore sizes is in many cases still poorly understood and is difficult to model at the pore scale in this work we investigate the capillary pressure and relative permeability behavior of three outcrop carbonates and two tight reservoir sandstones with wide multi modal pore size distributions to examine how the drainage and imbibition properties of these complex rock types are influenced by the connectivity of macropores to each other and to zones with unresolved small scale porosity we apply a previously presented micro computed tomography based multi scale pore network model bultreys et al 2015b 2016b to these samples the sensitivity to the properties of the small scale porosity is studied by performing simulations with different artificial sphere packing based networks as a proxy for these pores finally the mixed wet water flooding behavior of the samples is investigated assuming different wettability distributions for the micro and macroporosity while this work is not an attempt to perform predictive modeling it seeks to qualitatively explain the behavior of the investigated samples and illustrates some of the most recent developments in multi scale pore network modeling

growth strata in foreland settings abstract the accurate analysis of growth strata has revealed their significance to unravel both fold kinematics and timing of deformation in both compressive and extensive settings the increasing acquisition of 3 d multichannel seismic lines with few tens of meters of resolution in complex tectonic areas reveals complex interplay between growing structures and deposition either in marine or continental environments interestingly growth strata reveal similar relationships in both compressive folds linked to thrusts at depth and folds related to propagating normal faults in extensive regimes to completely document the complete kinematic history of an individual fault or a group of faults it is necessary to combine studies at different scales to integrate the findings revealed by field work meters of resolution and multichannel seismic lines results the margins of the ebro basin to the south of the pyrenees have been the focus of abundant studies of syntectonic deposits this is because of its special long term evolution from a foreland to an intermontane basin and a final open basin to the mediterranean sea in neogene times the complete comprehension of growth strata in these natural laboratories is essential for our present needs in both petroleum exploration and earthquake prediction

mesophotic coral buildups in a prodelta setting late eocene southern pyrenees spain a mixed carbonate siliciclastic system lower priabonian coral bioherms and biostromes encased in prodelta marls clays occur in the ainsa jaca piggyback basin in the south central pyrenean zone detailed mapping of lithofacies and bounding surfaces onto photomosaics reveals the architecture of coral buildups coral lithosomes occur either isolated or amalgamated in larger buildups isolated lithosomes are 1 to 8 m thick and a few hundred metres wide clay content within coral colonies is significant stacked bioherms form low relief buildups commonly 20 to 30 m thick locally up to 50 m these bioherms are progressively younger to the west following progradation of the deltaic complex the lowermost skeletal rich beds consist of bryozoan floatstone with wackestone to packstone matrix in which planktonic foraminifera are abundant and light related organisms absent basal coral biostromes and the base of many bioherms consist of platy coral colonies floating in a fine grained matrix rich in branches of red algae corals with domal or massive shape locally mixed with branching corals and phaceloid coral colonies dominate buildup cores these corals are surrounded by matrix and lack organic framework the matrix consists of wackestone to packstone locally floatstone with conspicuous red algal and coral fragments along with bryozoans planktonic and benthonic foraminifera and locally sponges coral rudstone and skeletal packstone with wackestone to packstone matrix also occur as wedges abutting the buildup margins integrative analysis of rock textures skeletal components buildup anatomy and facies architecture clearly reveal that these coral buildups developed in a prodelta setting where shifting of delta lobes or rainfall cycles episodically resulted in water transparency that allowed zooxanthellate coral growth the bathymetric position of the buildups has been constrained from the light dependent communities and lithofacies distribution within the buildups the process product analysis used here reinforces the hypothesis that zooxanthellate corals thrived in mesophotic conditions at least during the late eocene and until the late miocene comparative analysis with some selected upper eocene coral buildups of the north mediterranean area show similarities in facies components and textures and suggest that they also grew in relatively low light mesophotic and low hydrodynamic conditions

the functioning of sediment routing systems using a mass balance approach example from the eocene of the southern pyrenees abstractsediment routing systems link source regions undergoing erosion with depositional sinks and involve a volumetric or mass budget understanding how these source to sink systems function is key to stratigraphic prediction but estimation of their surface sediment discharges and depositional fluxes on geological time scales is a challenging problem we recognize a paleosediment routing system from the geological record in the mid late eocene escanilla formation and time equivalents of the tectonically active wedge top region of the southern pyrenees by mapping the 200 km long fairway of the escanilla sediment routing system we obtain the sediment budget and grain size fractionation of three time intervals each of 2 5 2 6 m yr duration over the time period 41 6 33 9 ma four thousand cubic kilometers of sediment sourced principally from two feeder systems in the high pyrenees was deposited in a period of 7 7 m yr the positions of moving boundaries characterized by rapid reduction in the perc

application of mercury injection capillary pressure to mudrocks conformance and compression corrections abstract mercury injection capillary pressure micp is a commonly used technique for measurements of porosity pore throat size distribution and injection pressure vs mercury saturation for many types of rocks the latter two are correlated to and can be used to estimate permeability problems for micp application in mudrocks are associated with two types of system errors conformance and compression effects these two sources of error are well recognized but no standard procedures to correct them exist in this study a new method for conformance and compression corrections was developed and the method applied to five eagle ford shale samples conformance correction is based on comparison of mercury injection volume vs pressure curves between epoxy coated and uncoated samples compression correction is based on calculation of compressions before and after mercury intrusion in micp experiment different types of compressions are quantified and compression corrections on porosity pore throat size distribution and injection pressure vs mercury saturation are performed porosity and permeability were calculated based on corrected micp data results show that both conformance and compression corrections are important for accurate micp porosity calculation whereas conformance correction is more important than compression correction in permeability calculation

rotation velocity of a thrust a paleomagnetic study in the external sierras southern pyrenees abstract an accurate calibration of chronologic and geologic data biostratigraphic and magnetostratigraphic with gpts has been performed for the middle and upper eocene syntectonic deposits in the surrounding areas of the pico del aguila anticline south pyrenean external sierras this calibration shows a variable sedimentation rate that changes from 4 8 cm ka at the bottom to 34 cm ka at the top with a maximum of 58 6 cm ka in the upper part of the studied stratigraphic section and gives a fine and continuous chronologic frame to understand the rotational kinematics of the infrajacent thrust system at the same time a detailed paleomagnetic study has been carried out 14 sites and a total of 157 thermally demagnetised samples to know the amount of rotation in the area taking advantage the tape recording effect of the syntectonic materials these samples display a chrm between 250 and 400 c both polarities that agree very well with the sequence of magnetostratigraphic zones and consistency with the magnetic reference due to the decreasing value of the magnetic declination from 46 to 3 of the primary components checked by fold and reversal test the calibration has been useful to assign an absolute age to every rotation value from 40 32 to 37 05 ma a time versus rotation curve does not show significant differences of rotation between the flanks of the pico del aguila anticline less than 8 thus the clockwise rotations detected in the study area result from the kinematics of the imbricate basal thrust system which is also responsible for the development of detachment folds e g pico del aguila anticline in a time against rotation graph the simplest fit for the overall rotation values is given by a linear segment rotation velocity of 10 ma however the best fit is given by a segment of a parabolic curve this pattern implies that there were changes of the rotation velocities between 21 ma and 5 3 ma that in this segment of the external sierras were caused by an acceleration of the rotational movement between 9 ma2 and 5 5 ma2 the thrust sheet shows maximum values of at least 40 of clockwise rotation

lutetian magnetostratigraphic calibration of larger foraminifera zonation sbz in the southern pyrenees the isuela section abstract in this paper we present a chronostratigraphic calibration of the shallow benthic zones sbzs of larger foraminifera during lutetian times based on new magnetostratigraphic data isuela section and two new densely sampled biostratigraphic sections isuela and gabardiella these sections are located in the external sierras southern pyrenees and represent an ideal location to avoid previous chronostratigraphic problems 1 they belong to a shallow carbonate shelf environment with abundant larger foraminifera which ensure the reliability of sbz data 2 the time span covers almost the entire lutetian interval 3 the profiles are sufficiently long to establish trustworthy local scales both the sbzs 520 m and 760 m in isuela and gabardiella respectively and the paleomagnetic local polarity sequence lps 520 m 4 the excellent outcropping conditions also permit a detailed paleomagnetic and biostratigraphic sampling to be performed the lps at the isuela section was built after sampling every 2 6 m in total more than 359 samples were thermal 244 and alternating field 115 demagnetized characteristic remanent magnetization chrm usually dropped at 560 c only 43 the samples 359 demagnetizations gave reliable directions high quality for the lps a further short section gas line was also cored 38 samples in 90 m to obtain more detailed data for the upper part of the section at the isuela section the results allow three pairs of trustable magnetozones to be assembled within the lutetian a further 255 samples were taken for biostratigraphic purposes 55 sites at the isuela and 52 at the gabardiella sections being determinant this density enabled a thorough study of changes in fossil associations to be performed assuming a constant accumulation rate within the chron boundaries absolute ages have been estimated for the biostratigraphic boundaries combining all these data allows the previous calibration of sbz during lutetian times in the geological time scale to be refined in particular with this new data we identify the sbz 13 sbz 14 boundary at 44 46 ma reaching c20r a boundary interval spanning from sbz 14 to sbz 15 41 36 and 42 45 ma is found within c20n and c19n and finally the top of sbz 15 can be accurately located at 41 12 ma c19r

high resolution rock magnetic cyclostratigraphy in an eocene flysch spanish pyrenees 1 a rock magnetic cyclostratigraphy based on anhysteretic remanent magnetization arm intensity variations was developed for the eocene arguis formation in the spanish pyrenees the arguis formation was sampled for arm cyclostratigraphy rock magnetic and paleomagnetic analyses rock magnetic measurements indicate that the dominant magnetic mineral controlling the arm cyclostratigraphy is depositional magnetite using thermal demagnetization a detailed magnetostratigraphy was developed for the arguis to provide an absolute time framework for the arm cyclostratigraphy the magnetostratigraphy is carried by a combination of depositional magnetite and secondary iron sulfides spectral analysis of the magnetostratigraphically scaled arm time series reveals the presence of significant milankovitch frequencies including eccentricity obliquity and precession the arm time series was tuned to the orbital eccentricity model for the eocene coherency analysis indicates that the eccentricity tuned arm is in phase with october november insolation for this site s paleolatitude varying amounts of terrigenous input delivered by a fluvial source at orbital time scales during the rainy season and diluted by a relatively constant input of marine carbonate is the most likely explanation for the cyclicity of the arm record the absolute age resolution of the arm chronostratigraphy is 1 or less based on an age offset with the reference chronology that was used for the eocene the offset in age between the two chronologies may be due in part to the uncertainty in the chron ages for this part of the eocene the relative age resolution within the arm chronostratigraphy is much better

identification of systems tracts in low angle carbonate ramps examples from the upper jurassic of the iberian chain spain abstract the upper jurassic of the iberian chain in northeast spain was selected for the study and identification of sequence boundaries and systems tracts in low relief carbonate ramp systems the study area provides a special opportunity for sequence stratigraphic analysis because the outcropping upper jurassic carbonate ramps are relatively undeformed well exposed from proximal to distal areas and have an established ammonite zonation two sequences are recognized sequence a uppermost callovian to uppermost oxfordian in age up to 60 m thick in the proximal areas and between 15 and 5 m thick in the middle and distal areas and sequence b kimmeridgian to lowermost tithonian from 80 to 160 m in thickness across the basin the evolution of the first sequence is related to regional relative sea level changes i e tectono eustatic changes superimposed on the sedimentary basin on the other hand the larger accommodation created during the second sequence is regarded as controlled mainly by steady subsidence sequence boundaries indicative of subaerial exposure show as unconformities with stratigraphic hiatuses in the proximal areas of the basin the lowstand systems tract in sequence a is represented by a decimetre thick reworked level consisting of several ammonite biozones in sequence b this systems tract consists of a thick wedge shaped marly unit in both sequences the transgressive surface overlaying these lowstand deposits is generally planar and marks a sharp boundary between the restricted and open marine fossils however in sequence b in the middle part of the basin the transgressive surface can be absent instead a transitional change to open marine facies exists the transgressive systems tracts show sedimentary condensation at least in the distal areas of the basin with mostly ammonites and benthic fossil wackestones the transgressive unit is generally overlain by a hard ground surface that corresponds to a maximum flooding surface finally in the upper part of both sequences highstand systems tracts are represented by progradational siliciclastic and shallow carbonate complexes in the proximal areas of the basin in sequence a sedimentation is condensed in the distal areas with glauconite marl levels present the descriptions of sedimentary features of the systems tracts of these two sequences developed in a low angle carbonate ramp are suggested to be used as tools for the recognition of systems tracts patterns within sequences in comparable depositional settings

representation of multiscale heterogeneity via multiscale pore networks 1 developing a better understanding of single multiphase flow through reservoir rocks largely relies on characterizing and modeling the pore system for simple homogeneous rock materials a complete description of the real pore structure can be obtained from the pore network extracted from a rock image at a single resolution and then an accurate prediction of fluid flow properties can be achieved by using network model however for complex rocks e g carbonates heterogeneous sandstones deformed rocks a comprehensive description of the real pore structure may involve several decades of length scales e g from submicron to centimeters which cannot be captured by a single resolution image due to the restriction of image size and resolution hence the reconstruction of a single 3 d multiple scale model of a porous medium is an important step in quantitatively characterizing such heterogeneous rocks and predicting their multiphase flow properties in this paper we present a novel methodology for the numerical construction of the multiscale pore structure of a complex rock from a number of ct images models of a carbonate sample at several length scales the success of this reconstruction relies heavily on image segmentation pore network extraction and stochastic network generation which are provided by our existing software system referred to as pore analysis tools pat specifically the statistical description of pore networks of 3 d rock images at multiple resolutions makes it possible for us to a construct an arbitrary sized network which is equivalent in a specified domain and b integrate multiple networks of different sizes into a single network incorporating all scales using multiscale networks of carbonate rocks generated in this manner two phase network modeling results are presented to show how the resulting flow properties are dependent on inclusion of information from multiple scales these outcomes reinforce the importance of capturing both geometry and topology in the hierarchical pore structure for such complex pore systems the example presented reveals that isolated large scale e g macro pores are mainly connected by small scale e g micro pores which in turn determines the combined effective petrophysical properties capillary pressure absolute and relative permeability it is also demonstrated that multi three scale networks reveal the effects of the interacting multiscale pore systems e g micropores macropores and vugs on bulk flow properties in terms of two phase flow properties

sedimentary basins formed and carried piggyback on active thrust sheets small sedimentary basins that formed on the apennine southern margin of the po basin complex northern italy have been studied using borehole and seismic reflection information and their evolution can be dated using the largely marine biostratigraphy of the miocene to quaternary sediments these studies show that the basins formed and were filled while being carried on moving thrust sheets and they are therefore named piggyback basins each of these basins corresponds to the active phase of a different thrust front ramp on the pyrenean northern margin of the ebro basin complex northern spain similar piggyback basins formed between eocene and mid miocene time they have since been uplifted and eroded and may therefore be studied in outcrop in northern spain much of the basin fill sediment was deposited in nonmarine environments

thrust sequences in the eastern spanish pyrenees detailed mapping of the south pyrenean zone in the eastern spanish pyrenees shows that a great number of alpine thrusts affect the basement and the cover the geometry of the thrust structures and their propagation sequence can be interpreted from balanced sections the folding dies out downwards from the higher thrust sheets to the lower ones and demonstrates a piggy back thrusting sequence these thrusts form a duplex the sole thrust of this duplex is the vallfogona thrust the roof thrust is rooted somewhere within the axial zone the thrusts climb southwards from the axial zone basement up into tertiary cover sediments these thrusts have been folded so that they now dip to the south and form the southern side of the axial zone antiformal stack locally smaller antiformal stacks developed and produced strong folds in the higher thrust sheets the south pyrenean zone studied is equivalent to the gavarnie nappe and the connection between them below the detached south pyrenean central unit may be the nogueres zone

unfolding an inverse approach to fold kinematics preservedfoldshapesusuallyreveallittleabouttheirkinematicevolution syntectonic strata preserved in growth synclines in contact with a fold however can permit unfolding a sequential reconstruction of fold growth backward through time from a geometry observed at present to an initial undeformed state such reconstructions can define the kinematics of fold growth growth strata associated with anticlinal forelimbs in the ebro basin exhibit depositional tapering of beds on fold flanks and progressive limb rotation unfoldingawell dateddetachmentfolddefinesitskinematicevolutionandcoevallyvarying rates of shortening forelimb uplift and forelimb rotation interplay of these rates with sedimentation rates controls onlap and offlap relations

paleogene propagation of the southern pyrenean thrust wedge revealed by finite strain analysis in frontal thrust sheets implications for mountain building abstract to better evaluate the sequence of shortening and estimate shortening rates associated with the propagation of the southern pyrenean front we propose an original approach based on the reconstruction of finite displacement and finite vertical axis rotations this study is based on the quantitative analysis of published datasets including balanced cross sections paleomagnetic and thermochronologic data we distinguish three main episodes of shortening between 41 and 37 ma 37 and 30 ma and between 30 and 16 ma they are associated with a decrease of accretionary rates from 3 km ma to 0 39 0 27 km ma on the pro wedge side of the pyrenean orogenic wedge based on the comparison between frontal accretionary flux and outcoming fluxes in the axial zone of the pyrenees we show that the period ranging from 37 to 30 ma recorded a remarkable increase in erosion that is not compensated by accretion in the wedge in the context of the overall decrease in the iberia eurasia plate convergence we suggest that the main cause of the acceleration of erosion on top of the growing pyrenean orogen is related to climate changes that occurred at the eocene oligocene boundary

inclined shear restoration of growth folds we present a technique of progressively restoring seismic well and outcrop data as well as structural interpretations of contractional growth folds data restorations allow viewing of seismic well and outcrop information in the restored state providing a basis to evaluate whether structural interpretations are balanced and geologically reasonable validating structural interpretations helps to reduce the exploration risks associated with trap definition moreover sequential restorations reveal folding kinematics and define the geometry of structures at various stages in geologic history these sequential restorations may be used to define the timing of trap development relative to hydrocarbon migration and to determine how traps may have been modified by subsequent deformation to restore fold growth we use the approximation of inclined shear restoration vectors parallel to the axial surfaces of the folds and propagate them homogeneously downward in the section this choice of restoration vectors is commonly based on direct observation of axial surface orientations in data and generally satisfies appropriate balancing constraints inclined shear restoration works well for simple kink band migration folds and gives good approximations of restored geometries for folds developed by limb rotation and some more complex folding mechanisms we present examples from southern california the spanish pyrenees and angola

sea level cycles during the growth of atlantic type oceans abstract first order changes in global sea level are commonly thought to be the result of short term fluctuations in oceanic spreading rate however eustatic high stands have occurred only twice in phanerozoic time and in both cases have followed supercontinent break up by a lag period of 50 100 ma therefore if pulses in spreading rate were the primary control of eustasy then for some reason rate increases would have had to have occurred with predictable periodicity with respect to major rifting events alternatively we propose that the major control on first order sea level cycles is the formation of atlantic type oceans with its attendant changes in the area age distribution of the global ocean floor and the formation of extensional passive margins our calculations indicate that both the approximate magnitude and relative timing of sea level highstands relative to supercontinent break up are predicted by this mechanism thus the importance of short term spreading rate variations in eustatic cycles becomes secondary

large scale facies change in the middle eocene south pyrenean foreland basin the role of tectonics and prelude to cenozoic ice ages abstract the present study reports a sedimentological analysis of the guara limestone formation deposited during the lutetian in the sierras exteriores in the south pyrenean foreland basin we provide a detailed facies analysis of the carbonates to precise the paleoenvironmental context during their deposition we show that those limestones are mainly composed of shallow water foraminifers and were deposited in relative shallow water environments

reverse modelling of detachment folds application to the pico del aguila anticline in the south central pyrenees spain abstract a simple method to estimate fold amplification and thrust movement rates for detachment folds is documented and illustrated by its application to a symmetrical detachment fold in the southern pyrenees spain the technique provides a complete record of the kinematic evolution of detachment folds and is based on the application of equations for detachment folds involving limb rotation the method uses the stratal pattern of the syntectonic sediments and assumes that these growth strata were deposited horizontally that the folds involve a homogeneous competent unit detached over a ductile horizon and that the folds can be represented by chevronkink bands the procedure is applicable to any detachment fold with associated growth strata that display wedge geometries progressive unconformities indicating limb rotation through time this method can be used for both detachment folds formed with constant limb length or variable limb length and it can also accommodate undecompacted or decompacted growth strata

anisotropy of magnetic susceptibility ams records synsedimentary deformation kinematics at pico del aguila anticline pyrenees spain

formation of the mid fifteenth century kuwae caldera vanuatu by an initial hydroclastic and subsequent ignimbritic eruption in the mid fifteenth century one of the largest eruptions of the last 10 000 years occurred in the central new hebrides arc forming the kuwae caldera 12x6 km this eruption followed a late maar phase in the pre caldera edifice responsible for a series of alternating hydromagmatic deposits and airfall lapilli layers tuffs related to caldera formation 120 m of deposits on a composite section from the caldera wall were emitted during two main ignimbritic phases associated with two additional hydromagmatic episodes the lower hydromagmatic tuffs from the precaldera maar phase are mainly basaltic andesite in composition but clasts show compositions ranging from 48 to 60 sio2 the unwelded and welded ashflow deposits from the ignimbritic phases and the associated intermediate and upper hydromagmatic deposits also show a wide compositional range 60 73 sio2 but are dominantly dacitic this broad compositional range is thought to be due to crystal fractionation the striking evolution from one eruptive style hydromagmatic to the other magmatic with emission of a large volume of ignimbrites which occurred either over the tuff series as a whole or at the beginning of each ignimbritic phase is the most impressive characteristic of the caldera forming event this strongly suggests triggering of the main eruptive phases by magma water interaction a three step model of caldera formation is presented 1 moderate hydromagmatic sequences hd 1 4 and magmatic fallout deposits activity from a central vent probably over a period of months or years affected an area slightly wider than the present caldera at the end of this stage intense seismic activity and extrusion of differentiated magma outside the caldera area occurred 2 unhomogenized dacite was released during a hydromagmatic episode hd 5 this was immediately followed by two major pyroclastic flows pfd 1 and 2 the vents spread and intense magma water interaction at the beginning of this stage decreased rapidly as magma discharge increased subsequent collapse of the caldera probably commenced in the southeastern sector of the caldera 3 dacitic welded tuffs were emplaced during a second main phase wfd 1 5 at the beginning of this phase magma water interaction continued producing typical hydromagmatic deposits hd 6 caldera collapse extended to the northern part of the caldera previous c14 dates and records of explosive volcanism in ice from the south pole show that the climactic phase of this event occurred in 1452 a d