architecture of miocene overbank deposits in northern pakistan abstract understanding the geometry of sediment bodies and patterns of sediment disruption in ancient fluvial overbank successions is fundamental to interpretations of overbank deposition overbank deposits of the fluvial chinji formation siwalik group can be divided into sequences 1 20 m thick defined by the alternation of stratified sediments and paleosols preservation of stratified sequences does not support a gradual vertical aggradation of overbank sediments over the extent of the floodplain but instead indicates periods of rapid sediment deposition followed by long hiatuses and soil development rapid deposition of overbank sequences is predicted by models that hypothesize 1 episodic vertical aggradation of the entire floodplain 2 deposition controlled by the growth of alluvial ridges 3 incision and filling of local valleys 4 rapid filling of localized low areas or 5 rapid deposition associated with river channel avulsion these models also have important implications for understanding fossil preservation and floodplain paleoecology paleosol bounded sequences in the chinji formation can pinch out laterally over kilometers indicating that episodes of rapid deposition were restricted to local areas on the floodplain there is no evidence that bases of sequences are erosional and they do not appear to record valley incision and filling sequences generally do not thin and fine systematically away from the terminating margins of major channel deposits suggesting they are not related to the growth of alluvial ridges along active channels lithologic variations and patterns of sediment disruption within sequences appear to reflect the rapid tilling of low areas on the floodplain both adjacent to and distal from the major river channel this infilling of floodplain topography may reflect a continuous process whereby local areas were always being filled somewhere on the floodplain by sediments from minor tributary or crevasse channels alternatively the filling of local flood plain topography may have been caused by widespread but short lived events associated with river channel avulsion

thinning factor distributions viewed through numerical models of continental extension a longstanding question surrounding rifted margins concerns how the observed fault restored extension in the upper crust is usually less than that calculated from subsidence models or from crustal thickness estimates the so called extension discrepancy here we revisit this issue drawing on recently completed numerical results we extract thinning profiles from four end member geodynamic model rifts with varying width and asymmetry and propose tectonic models that best explain those results we then relate the spatial and temporal evolution of upper to lower crustal thinning or crustal depth dependent thinning ddt and crustal thinning to mantle thinning or lithospheric ddt which are difficult to achieve in natural systems due to the lack of observations that constrain thinning at different stages between pre rift extension and lithospheric breakup our results support the hypothesis that crustal ddt cannot be the main cause of the extension discrepancy which may be overestimated because of the difficulty in recognizing distributed deformation and polyphase and detachment faulting in seismic data more importantly the results support that lithospheric ddt is likely to dominate at specific stages of rift evolution because crustal and mantle thinning distributions are not always spatially coincident and at times are not even balanced by an equal magnitude of thinning in two dimensions moreover either pure or simple shear models can apply at various points of time and space depending on the type of rift both ddt and pure simple shear variations across space and time can result in observed complex fault geometries uplift subsidence and thermal histories

facies and facies architecture of paleogene floodplain deposits willwood formation bighorn basin wyoming usa abstract paleogene deposits of the willwood formation were analyzed in two areas of the bighorn basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability despite similar facies and facies organization floodplain deposits in the two areas differ in grain size hydromorphy and maturity of the paleosols and scale and organization of ribbon sandstones in the avulsion deposits these differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas two kinds of cumulative floodplain paleosols red and purple paleosols formed on overbank deposits the red paleosols were better drained and formed on less clay rich parent material intense mottling and iron oxide nodules indicate that the purple paleosols which formed on more clay rich parent material were poorly drained the grain size differences reflect basin position in particular distance from a local sediment source avulsion deposits are volumetrically important comprising about half of the floodplain deposits in both areas the avulsion deposits resemble the stage iii splays described by smith et al 1989 smith n d cross ia dufficy j p clough s r 1989 anatomy of an avulsion sedimentology 36 1 24 in terms of characteristics of their ribbon sandstones and the presence of thin sheet sandstones or siltstones paleosols that formed on the fine grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas this study shows that avulsion belt paleosols provide a key to understanding avulsion rate in the stratigraphic record other differences between the willwood avulsion deposits in the two areas and between these ancient deposits and the modern avulsion deposits described by smith et al may reflect 1 problems in clearly recognizing ancient avulsion deposits in the field and 2 sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature at the same time because exposures are three dimensional the willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones

impact of india asia collision on se asia the record in borneo abstract borneo occupies a central position in the sundaland promontory of se asia it has a complex cenozoic geological history of sedimentation and deformation which began at about the same time that india is commonly suggested to have started to collide with asia some tectonic reconstructions of east and se asia interpret a large se asian block with borneo at its centre which has been rotated clockwise and displaced southwards along major strike slip faults during the cenozoic due to the indentation of asia by india however the geological history of borneo is not consistent with the island simply forming part of a large block extruded from asia the large clockwise rotations and displacements predicted by the indentor model for borneo are incompatible with palaeomagnetic evidence and there is no evidence that the major strike slip faults of the asian mainland reach borneo seismic tomography shows there is a deep high velocity anomaly in the lower mantle beneath se asia interpreted as subducted lithosphere but it can be explained just as well by alternative tectonic models as by the indentor model very great thicknesses of cenozoic sediments are present in borneo and circum borneo basins and large amounts of sediment were transported to the crocker turbidite fan of north borneo from the eocene to the early miocene but all evidence indicates that these sediments were derived from local sources and not from distant sources in asia elevated by india asia collision the cenozoic geological history of borneo records subduction of the proto south china sea and miocene collision after this ocean lithosphere was eliminated and a variety of effects resulting from long term subduction beneath se asia there is little to indicate that india asia collision has influenced the cenozoic geological record in borneo

late pleistocene alluvial plain sedimentation in lower narmada valley western india palaeoenvironmental implications abstract late pleistocene fluvial sediments that were deposited in a slowly sinking basin are now exposed as 30 50 m high incised vertical cliffs all along the lower narmada valley in western india the exposed fluvial deposits have been classified into two sediment packages alluvial fan sediments overlain by alluvial plain sediments the alluvial plain sequence has not been studied previously it consists mainly of sands and silts and is dominated by overbank deposits occurrence of large scale bedforms in the alluvial plain sequence points to the existence of a large sand bed river in an alluvial plain setting the major sedimentary facies in stratigraphic order include large channel fills giant epsilon cross bedded strata overbank fines occurring in horizontal massive and undulatory stratified forms associated with crevasse splay and backswamp deposits and a reddish brown palaeosol overlain by thinly stratified sands and silts at the top of the exposed sediment succession large sized channel fills occur at two stratigraphic levels which are morphologically similar and are indicative of high rates of deposition and avulsion the large channel fill structures and the giant epsilon cross bedded strata indicate a large single channel river that was consistently 10 15 m deep and about 70 80 m wide even during the dry seasons these dimensions are larger than those of the present day narmada river at low discharge levels the overbank sediments indicate rapid deposition through frequent overbank floods and floodplain aggradation by a laterally shifting river available chronologic data suggests that the reddish brown palaeosol correlates with a regional phase of pedogenesis in the alluvial plain of gujarat prior to the last glacial maximum lgm the thinly stratified sands and silts overlying the palaeosol were deposited by a considerably depleted but perennial river during the arid phase of the last glacial maximum overall the alluvial plain sediments of the lower narmada valley particularly those below the palaeosol have been attributed to a hyper avulsive large river with low sinuosity whose high discharge levels were determined primarily by a large catchment area further to the east and not by the semiarid climate prevailing in the gujarat alluvial plain during the upper part of the late pleistocene the study concludes that the narmada river has maintained a large catchment at least since the last 100 ka however the river was characterised by a much bigger channel during much of the late pleistocene with discharge levels higher than the present day

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

paleosols in the cretaceous goshoura and mifune groups sw japan and their paleoclimate implications abstract cretaceous sedimentary basins in kyushu island japan are developed as half grabens which accommodated very thick clastic wedges of non marine to shallow marine facies the albian to cenomanian goshoura group in western kyushu ca 1000 m thick consists of interbedded channel sandstone conglomerate and mudstone the cenomanian to turonian mifune group in central kyushu ca 2300 m thick comprises red and drab sandstone and intercalated siliceous ash fall tuff different types of paleosols occur in the goshoura and mifune groups paleosols in the goshoura group contain diagnostic vertic features such as subangular blocky peds irregularly directed slickensides and downward tapering fissures few obvious calcic horizons are developed except for the calcretes of rhizogenic origin paleosols in the upper formation of the mifune group are composed of cyclic acidic tuff vertic paleosol intervals with abundant pedogenic calcretes paleosols of the goshoura group belong to the order of vertic inceptisols and those of the mifune group can be classified as calcic vertisols in spite of diagenetic alteration of the paleosols paleo vertic soils of the goshoura and mifune groups show the exceptional preservation of a number of pedological features among various soil forming factors diversity in each paleosol is mainly attributed to differences in paleoclimatic conditions during the middle cretaceous our results show that the paleoclimatic conditions in the subtropical warm temperate region had changed from subhumid to semi arid with time

fossil andisols identified with mass balance geochemistry oligocene john day formation oregon u s a mass balance geochemistry of thick sequences of fossil soils oligocene john day formation central oregon show trends similar to those produced by modern andisol soil forming processes these andic trends include phosphorus accumulation potassium retention and alumina and iron depletion identification of these trends benefited from the geochemical comparison of numerous fossil soil horizons from two paleosol sequences one andic and one non andic the comparison of two paleosol sequences is justified because both sequences developed from the same type of parent material in nearly identical tectonic depositional settings the two sequences are 1 the early oligocene big basin member dominated by clayey alfisol like paleosols and 2 the mid oligocene turtle cove member dominated by ash rich andisol like paleosols with calcareous horizons the change in paleosol type occurred circa 30 ma throughout the john day basin and is most likely due to global cooling which changed climatic conditions in central oregon from humid to subhumid drier climate is interpreted to have caused a decrease in weathering intensity so that amorphous weathering products of volcanic glass accumulated in soils to form andisols prior to this circa 30 ma transition weathering produced crystalline clays from volcaniclastic detritus to form alfisols rather than amorphous weathering products

simplified process modeling of river avulsion and alluvial architecture connecting models and field data abstract modeling is an invaluable tool for studying sedimentary basin filling and for understanding depositional processes with long recurrence intervals including channel avulsion simplified modeling approaches such as cellular models and process analogue experiments are particularly useful for efficiently exploring alternative hypotheses and evaluating first order controls on river avulsion and alluvial architecture here we review the history and current state of the art in simplified avulsion and alluvial architecture models with a particular focus on how results and insights from these models can be incorporated into field and subsurface studies and vice versa simplified avulsion and alluvial architecture models have proliferated in the past decade providing a wide variety of models to serve as a basis for future coupled field modeling studies we compare features of leading models and discuss avenues for effectively pairing model capabilities with hypotheses and field data outstanding questions highlighted by recent modeling efforts include 1 what thresholds control avulsion initiation in different systems 2 how do floodplain processes and topography influence avulsion dynamics and alluvial architecture 3 what factors determine where avulsion channels stabilize answering these questions will require targeted modeling efforts coupled to data from ancient systems hence our model comparison emphasizes features that can be used to choose or design fit for purpose models and we outline how quantitative data useful for model selection and validation can be obtained from modern systems and ancient deposits matching model goals with targeted questions and model parameters and predictions with quantitative field data will help tighten communication between field and model oriented sedimentary geologists facilitating advances in our understanding of river avulsion and alluvial architecture

the structure and rate of late miocene expansion of c4 plants evidence from lateral variation in stable isotopes in paleosols of the siwalik group northern pakistan this study uses stable isotope variation within individual mio pliocene paleosols to investigate subkilometer scale phytogeography of late miocene vegetation change in southeast asia between ca 8 1 and 5 ma a time interval that coincides with dramatic global vegetation change we examine trends through time in the distribution of low latitude grasses c4 plants and forest c3 plants on indo gangetic floodplains using carbon 13c and oxygen isotopic 18o values in buried soil carbonates in siwalik series sediments exposed in the rohtas anticline north central pakistan revised high resolution magnetostratigraphy and a new 40ar 39ar date provide improved age control for the 2020 m rohtas section carbon isotope results capture lateral variability of c3 versus c4 plants at five stratigraphic levels r11 8 0 ma r15 6 74 6 78 ma r23 5 78 ma r29 4 8 4 9 ma and upper boundary tuff ubt 2 4 ma using detailed sampling of paleosols traceable laterally over hundreds of meters paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments channel fill crevassesplay and floodplain environments 13c results show that near the beginning 8 0 ma and after 4 0 ma the period of major ecological change vegetation was homogeneously c3 or c4 respectively regardless of paleo landscape position in the intervening period there is a wide range of values overall with c4 grasses first invading the drier portions of the system floodplain surfaces and c3 plants persisting in moister settings such as topographically lower channel swales although abrupt on a geologic timescale changes in abundance of c4 plants are modest 2 per 100 000 yr compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the quaternary earlier research documented a sharply defined c3 to c4 transition in pakistan between 8 1 and 5 0 ma based on vertical sampling but this higher resolution study reveals a more gradual transition between 8 0 and 4 5 ma in which c3 and c4 plants occupied different subenvironments of the siwalik alluvial plain 18o values as well as 13c values of soil carbonate increase up section at rohtas similar to isotope trends in other paleosol records from the region spatially however there is no correlation between 13c and 18o values at most stratigraphic levels this implies that the changes in soil hydrology brought about by the shift from forest to grassland i e an increase in average soil evaporation did not produce the shift through time in 18o values we interpret the trend toward heavier soil carbonate 18o values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 ma

three dimensional dynamic response analyses of cogswell dam the recorded strong motions at cogswell dam during the 1987 whittier narrows and 1991 sierra madre earthquakes provide a valuable opportunity to evaluate the applicability of established engineering procedures for evaluating the dynamic response of dams with highly three dimensional geometries in addition these records provide an opportunity to back calculate the dynamic properties of rockfill materials which cannot be easily measured in the laboratory or in situ peak transverse crest accelerations of 0 15 g and 0 42 g were recorded during the whittier narrows and sierra madre earthquakes respectively combined with analyses of the trailing portions of these strong motion records the field data cover a wide range of induced shear strains two dimensional and three dimensional dynamic response analyses were performed using established engineering procedures and their results are compared with the recorded responses dam canyon interaction and topographic effects appear to have influenced the seismic

the continental extension discrepancy and anomalous subsidence pattern in the western qiongdongnan basin south china sea abstract passive margins are first order tectonic features on earth formed by polyphase extension and breakup of continental lithosphere it is accepted that mildly extended basins in the proximal margin form largely by uniform continental thinning but the continental extension in deepwater is much debated here we choose a line from the western qiongdongnan basin in the magma poor south china sea margin to further explore the style of continental extension and the corresponding subsidence patterns where high resolution seismic and heat flow data are available structural analysis reveals that the basin architecture is extremely asymmetric and mainly controlled by the se directed normal faults soling into a low angle detachment fault however fault timings nicely show that the faults form through polyphase rifting and from inside outwards and do not follow a simple sequential faulting like the west iberia margin where the upper and lower crustal deformation are strongly coupled perhaps the fault order here reflects a much weaker and decoupled lower crust then we divide the basin into five zones and calculate the average fault derived extension factors moreover the whole crustal extension factors are also estimated after assessing the initial crustal thickness on basis of the backstripped subsidence and the lithospheric extension factors are numerically modeled using a coupled lithosphere and basin scale model constrained by the heat flow and sedimentation data results show that along the profile the overall lithospheric extension is much less than that of the whole crust and only at the south end of the profile does lithospheric extension exceeds the whole crust showing lithospheric depth dependent thinning the whole crustal extension is roughly equal to the faults derived extension in the northern part displaying no extension discrepancy in the middle part the whole crustal extension is less than the faults derived extension denoting inverse extension discrepancy while the whole crustal extension is in far excess of the faults derived extension in the southern part showing obvious positive extension discrepancy we infer that the inverse and positive discrepancy of crustal extension was likely to be caused by landward directed lower crustal flow driven by the horizontal pressure arising from the combination of the differential extension in the faulted upper crust in the mantle lithosphere and the differential sedimentation during the syn rift period

high resolution sequence stratigraphy of fluvio deltaic systems prospects of system wide chronostratigraphic correlation a basin scale numerical model with a sub grid parameterization of fluvio deltaic processes and stratigraphy was used to study the relation between alluvial sedimentation and marine deltaic deposition under conditions of time invariant forcing the experiments show that delta evolution is governed by a robust morphodynamic feedback loop which provides a link between major avulsions delta lobe switches and sequestration of sediments on the delta plain major avulsions driven by local superelevation result in abandonment of delta lobes and initiation of new lobes progradation of the delta front lengthens the fluvial profile and reduces its gradient which induces aggradation upstream the aggradation in turn causes local superelevation of the channel belt each major avulsion causes a wave of incision to migrate upstream whereas downstream of the avulsion point the rate of aggradation temporarily increases until a new equilibrium situation has been established the feedback loop explains storage and release of fluvial sediments without the need to invoke changes in upstream or downstream controls and provides a plausible mechanism for the generation of high frequency incision aggradation cycles as the sole result of compensational stacking the stratigraphic expression of a depocentre shift is an essentially isochronous surface hence the stratigraphic record of fluvio deltaic systems may be subdivided into a series of units representing intervals during which a channel belt and delta lobe were forming at a fixed location in the basin so called chronosomes fluvio deltaic chronosomes are bounded by abandonment surfaces which are clearly expressed in the marine as well as the fluvial domain the surface marking the abandonment of a particular channel belt and delta lobe correlates with the surface at the base of a new delta lobe landward this surface forms the base of an aggradational package of fluvial sediments downstream of an avulsion site associated with a lobe switch upstream of this site the conformable surface passes into a surface of fluvial incision and terrace formation onlapped by aggradational channel belt deposits identification of the isochronous bounding surfaces of chronosomes in the field offers the attractive prospect of high resolution chronostratigraphic correlation throughout fluvio deltaic systems

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

palaeoenvironmental reconstruction of the early anisian from sedimentology and plant remains in the se iberian range e spain abstract the end of the permian period is characterized by the largest mass extinction ever recorded in marine and terrestrial sedimentary rocks the oldest plant macro remains after the p t boundary recorded in the castilian branch of the iberian range after this mass extinction belong to the eslida formation of anisian age these associations have been recovered at four localities and include sphenophytes equisetites perhaps neocalamites or schizoneura conifers volzia albertia pelourdea lycophytes pleuromeia and a possible seed fern peltaspermum facies associations and sedimentological analysis of the eslida formation have allowed for distinguishing four architectural elements ch channel deposits la lateral accretion cs crevasse splay and ff overbank fines the largest fossil remains are associated with ch and la architectural elements and high energy events and they form part of the basal lag or reactivation surfaces smaller and well preserved fragments are associated with the cs architectural elements a sedimentologic taphonomic and palaeoclimatic analysis reveals gradation from riparian hygrophytic vegetation to semi arid xerophitic vegetation across the alluvial complexes and a progressive aridification through time a comparison with other areas shows that this association is similar to coeval floras of iberia and of the gres a voltzia formation of northeastern france

luminescence chronology of river adjustment and incision of quaternary sediments in the alluvial plain of the sabarmati river north gujarat india river adjustment and incision in the sabarmati basin gujarat india have been examined at a site near mahudi towards this the morphostratigraphy and depositional chronometry of the middle alluvial plains were investigated the upper fluvial sequence along with the overlying aeolian sand and riverbed scroll plains provide clues to the evolution of the present sabarmati river sedimentological analyses of the upper fluvial sequence indicate its deposition by a meandering river system during what is believed to be a persistent wetter phase luminescence chronology bracketed this sequence to between 54 and 30 ka which corresponds to oxygen isotope stage 3 during which the sw monsoon was enhanced the overlying aeolian sand has been dated to 12 ka indicating that dune accretion occurred simultaneously with the strengthening of the sw monsoon during the early holocene adjustment of the sabarmati along a n s transect is placed around 12 ka and the incision is bracketed between 12 and 4 5 ka river adjustment could have been tectonic however the incision was facilitated by the availability of continuous flow in the river caused by the sw monsoon the basin experienced two tectonic events at about 3 and 0 3 ka as demonstrated by the morphology of the scroll plains

high frequency controls on alluvial successions an integrated sedimentological and palaeopedological approach to the plio pleistocene of argentina abstract plio pleistocene palaeosol bearing alluvial strata are continuously exposed along the marine cliffs of southeastern buenos aires province argentina the study interval was deposited by a mixed predominantly suspended load fluvial system outcrops are dominated by floodplain siltstones and mudstones that exhibit a cyclic alternation between calcic and vertic palaeosols this cyclicity is represented by meter scale alluvial cycles that show as a general trend a gradual increase in the proportion of channel fill deposits up section cyclic variations in palaeosols facies and architectural elements can be related to allogenic drivers such as climate and eustasy for the lower punta san andres alloformation the stratigraphic arrangement of the recognised palaeosols and the clay mineral distribution indicate that the palaeoclimate was subhumid average mean annual precipitation map 840 mm seasonal and temperate average mean annual temperature mat 7 85 c for at least 1 ma from ca 2 9 ma to ca 1 8 ma late pliocene early pleistocene with several intervals in which conditions became drier and probably colder even though climate was a major control on palaeosol development it is suggested here that it was not the main control on high frequency cycles variations in the orbital parameters more specifically in insolation were interpreted as having controlled the sedimentation and architectural evolution of the lower punta san andres alloformation depositional systems

mesozoic and cainozoic rocks dredged from the south china sea reed bank area and sulu sea and their significance for plate tectonic reconstructions abstract outcrops of pre quaternary rocks were sampled by dredging and coring during two cruises of rv sonne in the south china sea and sulu sea 1982 1983 late triassic deltaic sandstones wih plant remains are the oldest sediments recovered late jurassic early cretaceous regional metamorphism is indicated by gneiss and garnet mica schists exposed in the northwestern flanks of reed bank these rocks and occurrences of gabbro diorite dacite rhyolite and siliceous shale prove that the dangerous grounds and reed bank are underlain by a continental fragment during the period of seafloor spreading in the south china see a late oligocene to early miocene carbonate platform developed on this southwards drifting continental fragment which subsided in the middle miocene middle miocene andesite and early to middle miocene reefal carbonates were recovered from the cagayan ridge in the sulu sea the new data are in accord with the plate tectonic history developed by taylor and hayes 1980 1983 and holloway 1981

onland signatures of the palawan microcontinental block and philippine mobile belt collision and crustal growth process a review abstract the collision of the palawan microcontinental block with the philippine mobile belt had significantly influenced the geological evolution of the philippines multiple collisions involving several fragments through space and time resulted into the collage of terranes of varying origin exposed in this part of central philippines cusping of the overriding plate volcanic arc gap ophiolite emplacement incipient back arc rifting island rotation and tilting raised coastal terraces metamorphism intrusion of igneous rocks and steepened subducted slab as seen in focal mechanism solutions are some of the manifestations of this collision a late early miocene to early middle miocene age 20 16 ma is proposed for the major collision between the palawan indenter and the philippine mobile belt the collision boundary is located from the northern part of mindoro through the central mountain range swinging east of sibuyan island in the romblon island group and finally threading along the buruanga peninsula and eastern side of the antique ophiolite complex before exiting and connecting with the negros trench the collision through accretion and crustal thickening has contributed to the crustal growth of the philippine archipelago

rapid transition from continental breakup to igneous oceanic crust in the south china sea continental breakup represents the successful process of rifting and thinning of the continental lithosphere leading to plate rupture and initiation of oceanic crust formation magmatism during breakup seems to follow a path of either excessive transient magmatism magma rich margins or of igneous starvation magma poor margins the latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation discovery of magma poor margins has raised fundamental questions about the onset of ocean floor type magmatism and has guided interpretation of seismic data across many rifted margins including the highly extended northern south china sea margin here we report international ocean discovery program drilling data from the northern south china sea margin testing the magma poor margin model outside the north atlantic contrary to expectations results show initiation of mid ocean ridge basalt type magmatism during breakup with a narrow and rapid transition into igneous oceanic crust coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers asthenospheric upwelling yielding mid ocean ridge basalt type magmatism from normal temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre rift lithosphere

the fluvial evolution of the holocene nile delta abstract the evolution of the nile delta the largest delta system in the mediterranean sea has both high palaeoenvironmental and archaeological significance a dynamic model of the landscape evolution of this delta system is presented for the period c 8000 4500 cal bp analysis of sedimentary data and chronostratigraphic information contained within 1640 borehole records has allowed for a redefinition of the internal stratigraphy of the holocene delta and the construction of a four dimensional landscape model for the delta s evolution through time the mid holocene environmental evolution is characterised by a transition from an earlier set of spatially varied landscapes dominated by swampy marshland to better drained more uniform floodplain environments archaeologically important pleistocene inliers in the form of sandy hills protruding above the delta plain surface known as turtlebacks also became smaller as the delta plain continued to aggrade while the shoreline and coastal zone prograded north these changes were forced by a decrease in the rate of relative sea level rise under high rates of sediment supply this dynamic environmental evolution needs to be integrated within any discussion of the contemporary developments in the social sphere which culminated in the emergence of the ancient egyptian state c 5050 cal bp

tectonic stratigraphic division and blind fold structures in nansha waters south china sea extensive multiple channel reflection seismic data have been collected in the nansha spratley islands waters the southern margin of the south china sea stratigraphic correlation is shown with focus on a comprehensive geophysixcal survey line run front offshore nw sabah to offshore se vietnam according to the varying tectono stratigraphy from southeast to northwest five tectonic belts can be determined i e the palawan borneo nappe nansha trough nanwei liyue compressive belt zheng he extensional belt and the circum southwest subbasin belt in the palawan borneo nappe neogene quaternary deposits were highly upthrust northwestwards resulting in a series of moderately to tightly folded anticlines separated by open synclines the nansha trough was a narrow deep water belt filled with thick undisturbed neogene quaternary deposits the nanwei liyue compressive bell was dominated by strongly folded paleo anticlines overlain by an undeformed sedimentary cap with a pronounced hiatus of paleogene sediments the zheng he extensional belt consisted of a rugged topography and paleogene half grabens bounded by listic faults the major extensional faults were reactivated to cut through the overlying neogene quaternary deposits over the circum southwest subbasin belt neogene quaternary deposits draped on the largely subsided fault blocks related to the late oligocene mid miocene seafloor spreading based on the regional stratigraphic correlation the prominent paleo anticlines found within the nanwei liyue compressive belt are deduced to consist of mainly mesozoic marine sediments that were compressed in the late mesozoic era therefore the nansha microcontinent block is shown to be a collision complex assembled during late mesozoic era c 2004 elsevier ltd all rights reserved

siwalik mammals of the jammu sub himalaya india an appraisal of their diversity and habitats abstract the red bed sequence of the lower siwaliks around ramnagar and the nagrota formation of the upper siwaliks in the uttarbeni parmandal section are the two highly fossiliferous intervals of the siwaliks of jammu sub himalaya india the lower siwalik mammalian fauna of ramnagar is best represented in its upper 350 m interval small and medium sized forms mostly belonging to browsing species dominate the mammalian assemblage the ramnagar fauna represents the chinji mammalian fauna middle miocene of the potwar plateau pakistan the assemblage indicates a woodland type ecosystem having substantial forest component and subordinately developed grassy subareas the mammalian fauna of the nagrota formation in the uttarbeni parmandal section in contrast exhibits relatively low species diversity but considerable increase in the mean body size and progressive hypsodonty of the cheek teeth in the herbivores this becomes more pronounced above the 2 5 ma old volcanic tuff layers occurring at the middle of the formation broadly a savanna type ecosystem is indicated by the mammalian communities of nagrota formation which appears to have been dominated by grassland communities above the tuff zone the above tuff faunal zone of the nagrota formation can be correlated with the pinjor faunal zone plio pleistocene of the siwalik hills punjab

marginal basin evolution the southern south china sea abstract the southern south china sea is divided into contrasting morphology by the west baram line to the west is the sundaland extinct passive margin in which rifting began in the eocene 46 ma and ceased at 19 21 ma anomaly 6 where sea floor spreading began much later than along the shelf of china the break up hiatus lasted 3 5 ma marked by the mid miocene unconformity also preserved on land sarawak the post rift strata date from 16 ma and drape over the rifted topography to the east is a convergent margin that became a collision zone in the middle miocene the sunda shelf is of uniform 30 km thickness except in localised deep basins and extends to a water depth of 200 m the continental slope is narrow the continental rise dangerous grounds is covered by water ranging from 500 m to 3 5 km depth at the continent ocean transition its width ranges from 170 to 330 km the rajang delta extends over the shelf and continental slope its post rift sediments drape over the rifted proximal topography of the dangerous grounds to the east the drape is thinner and has not completely buried the rifted topography the cuestas appear to have supported the carbonate build up infrastructures of the spratly islands whose slopes rise abruptly from a sea floor of 2 3 km depth the sabah and brunei margin is a collision zone that was formerly convergent the on land geology indicates a mesozoic ophiolitic basement the main collision feature is the western cordillera constructed mainly of sandy eocene to lower miocene turbidites predominantly oligocene to lower miocene in the west crocker 32 18 ma uplifted episodically throughout the upper miocene and pliocene 14 8 ma the 2 km deep northwest borneo trough may be a relict of the convergence phase but could also be a collisional foredeep the oil prolific baram delta resulting from uplift and erosion of the western cordillera has built out as far as the northwest borneo trough it is suggested that the passive margin continental rise dangerous grounds has been underthrust beneath sabah to cause uplift of the western cordillera the west baram line accordingly abruptly separates the collision zone from the western extinct passive margin and is a now extinct major right lateral transform fault

lower eocene alluvial paleosols pedogenic development stratigraphic relationships and paleosol landscape associations abstract two kinds of cumulative floodplain paleosols red and grey paleosols formed on overbank deposits of the willwood formation in the sand coulee area of the bighorn basin wyoming although both kinds experienced down profile iron movement due to seasonal saturation of a clay rich a horizon the red paleosols were better drained and their b horizons were rubified various redoximorphic features indicate that the grey paleosols were poorly drained the red paleosols show progressive increases in the degree of profile development away from an associated channel sandstone a paleosol landscape relationship termed pedofacies by bown and kraus 1987 although the grey paleosols show relatively systematic changes in hydromorphy consistent pedofacies changes were not recognized furthermore no lateral relationship between the red and grey paleosols was observed these features suggest that because of the retarding effects of poor drainage on soil weathering poorly drained soils are not amenable to pedofacies modelling and that the landscape associations of well drained and poorly drained soils may be difficult to document without unusually extensive exposures results of this study also show that the pedofacies model is limited by sediment accumulation rates and the kind of fine grained facies on which paleosols developed although readily observable in sand coulee pedofacies are difficult to recognize in the elk creek area where accumulation rates were approximately half as rapid as in sand coulee the attainment of steady state conditions is believed to have obscured pedofacies in the elk creek area finally pedofacies are only developed on true overbank deposits which in the willwood formation and probably many other ancient alluvial sequences make up only a fraction of the fine grained deposits only immature soils formed on the remainder of the fine grained facies because they were deposited very rapidly as a result of channel avulsion

pedosedimentary history and floodplain dynamics of the lower cretaceous upper blairmore group southwestern alberta canada thick alluvial successions in the upper blairmore group in the foothills of southwestern alberta contain few well developed paleosol profiles but abundant evidence of pedogenic modification the pedogenically modified successions are characterized by vertical root traces compound illuvial clay coatings and ferruginous coatings and nodules five representative paleosol successions composed of nine microfacies are analyzed the paleosol successions indicate soil development on more or less continuously aggrading floodplains however locally rates of sedimentation and pedogenesis were highly variable the paleosol successions contain features similar to modern alluvial soils brunisols and luvisols vertical trends within individual paleosol successions preserve the changing paleoenvironmental record at each site the dynamic nature of the floodplain topography is reflected in alternating drainage conditions represented by quasi regular colour banding compound clay coatings and the type and degree of pedogenic development upwards within the paleosol successions overall the paleosols indicate soil development under a warm temperate paleoclimate with seasonal precipitation although representing minor diastems the paleosols formed contemporaneously with sedimentation and are an integral part of the alluvial successions local variations in paleosol development are attributed to variations in sediment supply and water table conditions related to the overall floodplain geomorphology

multistep modelling of teleseismic receiver functions combined with constraints from seismic tomography crustal structure beneath southeast china summary with a growing number of modern broad band seismographic stations in asia the conditions have improved to allow higher resolution structural studies on regional scales here we perform a receiver based study of the lithosphere of southeast china using waveform records ofexcellentqualityfrom14chinesenationaldigitalseismicnetworkandfourglobalseismic network stations calculating the theoretical receiver functions rfs that match the observed rfs from teleseismic waveforms is an established technique for retrieving information about crustalanduppermantlestructurebeneathaseismicreceiver rfs however arepredominantly sensitivetothegradientsinthelithosphericelasticparameters anditisimpossibletodetermine a non unique distribution of seismic parameters such as absolute shear wave speeds as a function of depth unless other geophysical data are combined with rfs thus we combine rfswithindependentinformationfromshearandcompressionalwavespeedsaboveandbelow the mohoroviy discontinuity available from the existing tomographic studies we introduce a statistical approach for automatically selecting only mutually coherent rfs from a large set of observed waveforms furthermore an interactive forward modelling software is introduced and applied to observed rfs to define a prior physically acceptable range of elastic parameters in the lithosphere this is followed by a grid search for a simple crustal structure an initial model for a linearized iterative inversion is constructed from multiple constraints including results from the grid search for shear wave speed the moho depth versus vp vs ratio domain search and tomography the thickness of the crust constrained by our multistep approach appears to be more variable in comparison with tomographic studies with the crust thinning significantly towards the east we observe low values of vp vs ratios across the entire region which indicates the presence of a very silicic crust we do not observe any correlation between the crustal thickness or age of the crust with vp vs ratios which argues against a notion that there is a simple relationship between mineralogical composition and crustal thickness and age on a global scale

geochemistry of the floodplain sediments of the kaveri river southern india abstract the floodplain sediments of the kaveri river southern india derived from archean gneissic and charnockitic source regions show interbedding of silty 4 4 7 and sandy units 1 4 3 7 the geochemistry of silty beds is remarkably uniform at a given location and over a lateral distance of nearly 250 km the sandy beds have more variable chemical compositions yet are comparable to those of silty beds except for the diluting effect of quartz silty sediments retain the geochemical signature of prominently exposed source rocks for almost all elements and provide evidence of tectonic instability in the source region the floodplain sediments contain all size grades sand silt and clay which may have resulted in minimizing the biases imposed on suspended and bedload sediments due to sorting the low chemical index of alteration cia the dominance of unweathered primary minerals and the minor proportion of smectitic clay all suggest that the region has been subjected to little chemical weathering this is possible if the region has undergone recent uplift exposing fresh archean rock to surface denudation the formation of fertile farmland along the kaveri river course and its delta is related to these recent geological processes

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

crevasse splay processes and deposits in an ancient distributive fluvial system the lower beaufort group south africa abstract up to 12 of the mud prone ephemeral distributive fluvial system stratigraphy in the permo triassic lower beaufort group south africa comprises tabular fine grained sandstone to coarse grained siltstone bodies which are interpreted as proximal to distal crevasse splay deposits crevasse splay sandstones predominantly exhibit ripple to climbing ripple cross lamination with some structureless and planar laminated beds a hierarchical architectural scheme is adopted in which 1 m thick crevasse splay elements extend for tens to several hundreds of meters laterally and stack with other splay elements to form crevasse splay sets up to 4 m thick and several kilometers in width and length paleosols and nodular horizons developed during periods or in areas of reduced overbank flooding are used to subdivide the stratigraphy separating crevasse splay sets deposits from crevasse splays differ from frontal splays as their proximal deposits are much thinner and narrower with paleocurrents oblique to the main paleochannel in order for crevasse splay sets to develop the parent channel belt and the location where crevasse splays form must stay relatively fixed during a period of multiple flood events beaufort group splays have similar geometries to those of contemporary perennial rivers but exhibit more lateral variability in facies which is interpreted to be the result of more extreme fluctuations in discharge regime sharp based crevasse splay packages are associated with channel avulsion but most are characterized by a gradual coarsening upward interpreted to represent progradation the dominance of progradational splays beneath channel belt deposits may be more characteristic of progradational stratigraphy in a distributive fluvial system rather than dominated by avulsion processes in a trunk river system this stratigraphic motif may therefore be an additional criterion for recognition of distributive fluvial systems in the ancient record

different expressions of rifting on the south china sea margins abstract rifting of continental margins is generally diachronous along the zones where continents break due to various factors including the boundary conditions which trigger the extensional forces but also the internal physical boundaries which are inherent to the composition and thus the geological history of the continental margin being opened quite recently in the tertiary in a scissor shape manner the south china sea scs offers an image of the rifting structures which varies along strike the basin margins the scs has a long history of extension which dates back from the late cretaceous and allows us to observe an early stretching on the northern margin onshore and offshore south china with large low angle faults which detach the mesozoic sediments either over triassic to early cretaceous granites or along the short limbs of broad folds affecting palaeozoic to early cretaceous series these early faults create narrow troughs filled with coarse polygenic conglomerate grading upward to coarse sandstone because these low angle faults reactivate older trends they vary in geometry according to the direction of the folds or the granite boundaries a later set of faults characterized by generally e w low and high angle normal faults was dominant during the eocene associated half graben basement deepened as the basins were filling with continental or very shallow marine sediments this subsequent direction is well expressed both in the north and the sw of the south china sea and often reactivated earlier detachments at places the intersection of these two fault sets resulting in extreme stretching with crustal boudinage and mantle exhumation such as in the phu khanh basin east of the vietnam fault a third direction of faults which rarely reactivates the detachments is ne sw and well developed near the oceanic crust in the southern and southwestern part of the basin this direction which intersects the previous ones was active although sea floor spreading was largely developed in the northern part and ended by the late miocene after the onset of the regional mid miocene unconformity known as mmu and dated around 15 5 ma latest miocene is marked by a regional basement drop and localized normal faults on the shelf closer to the coast the se margin of the south china sea does not show the extensional features as well as the northern margin detachments are common in the dangerous grounds and reed bank area and may occasionally lead to mantle exhumation the sedimentary environment on the extended crust remained shallow all along the rifting and a large part of the spreading until the late miocene when it suddenly deepened this period also corresponds to the cessation of the shortening of the nw borneo wedge in palawan sabah and sarawak we correlate the variation of margin structure and composition of the margin mainly the occurrence of granitic batholiths and mesozoic broad folds with the location of the detachments and major normal faults which condition the style of rifting the crustal boudinage and therefore the crustal thickness

evolution of miocene fluvial environments eastern potwar plateau northern pakistan the miocene pliocene siwalik group records changing fluvial environments in the himalayan foreland basin the nagri and dhok pathan formations of this group in the eastern potwar plateau northern pakistan comprise relatively thick tens of metres sandstone bodies and mudstones that contain thinner sandstone bodies metres thick and palaeosols thick sandstone bodies extend for kilometres normal to palaeoflow and are composed of large scale stratasets storeys stacked laterally and vertically adjacent to each other sandstone bodies represent single or superimposed braided channel belts and large scale stratasets represent channel bars and fills channel belts had widths of km bankfull discharges on the order of 103 cumecs and braiding parameter up to about 3 individual channel segments had bankfull widths maximum depths and slopes on the order of 102 m 101 m and 10 4 respectively and sinuosities around 1 1 these rivers are comparable to many of those flowing over the megafans of the modern indo gangetic basin and a similar depositional setting is likely thin sandstone bodies within mudstone sequences extend laterally for on the order of 102 m and have lobe wedge sheet and channel form geometries they represent crevasse splays levees and floodplain channels mudstones are relatively bioturbated disrupted and represent mainly floodbasin and lacustrine deposition mudstones and sandstones are extremely disrupted in places showing evidence of prolonged pedogenesis these mature palaeosols are m thick and extend laterally for km lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate the 500 m thick nagri formation has a greater proportion and thicker sandstone bodies than the overlying 700 m thick dhok pathan formation the thick sandstone bodies and their large scale stratasets thicken and coarsen through the nagri formation then thin and fine at the base of the dhok pathan formation compacted deposition rates increase with sandstone proportion 0 53 mm year for nagri 0 24 mm year for dhok pathan and palaeosols are not as well developed where deposition rates are high within both formations there are 100 m scale variations representing on the order of 105 years in the proportion and thickness of thick sandstone bodies and tens of m scale alternations of thick sandstone bodies and mudstone sandstone strata that represent on the order of 104 years formation scale stratal variations extend across the potwar plateau for at least 100 km although they may be diachronous however 100 m and smaller scale variations can only be traced laterally for up to tens of km alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel belt sizes mainly average deposition rate and avulsion frequency on a megafan comparable in size to modern examples 100 m scale variations in thick sandstone body proportion and thickness could result from regional shifts in the position of major channels possibly associated with fan lobes on a single megafan or with separate megafans however such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system formation scale and 100 m scale stratal variations are probably associated with interelated changes in tectonic uplift sediment supply and basin subsidence increased rates of hinterland uplift sediment supply and basin subsidence recorded by the nagri formation may have resulted in diversion of a relatively large river to the area alternatively changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland possibly linked to tectonic uplift climate during deposition of the siwalik group was monsoonal although the deposits contain no direct evidence for climate change independent evidence indicates global cooling throughout the miocene and the possibility of glacial periods e g around 10 8 ma corresponding to base of nagri formation if the higher himalayas were periodically glaciated a mechanism would exist for varying sediment supply to megafans on time scales of 104 105 years although eustatic sea level changes are related to global climatic change they are not directly related to siwalik stratigraphic changes because the shoreline was many 100 km away during the miocene

sedimentology geochemistry and osl dating of the alluvial succession in the northern gujarat alluvial plain western india a record to evaluate the sensitivity of a semiarid fluvial system to the climatic and tectonic forcing since the late marine isotopic stage 3 abstract the alluvial successions in the northern gujarat alluvial plain western india have been investigated for reconstructing the climatic fluctuations during the last 40 ka alluvial architecture and geochemical proxies indicate prevalence of a strengthened indian summer monsoon ism with fluctuations between the late marine isotopic stage 3 mis 3 37 ka to the early mis 2 27 ka a gradual onset of aridity declining ism after 27 ka with peak aridity at 22 ka is observed a gradual strengthening of ism at around 18 and 12 ka followed by a short reversal in ism intensity between 12 and 11 ka is attributed to the younger dryas yd cooling event the aeolian sand sheet dated to 6 and 3 5 ka represents the onset of regional aridity following this a short lived humid phase was observed after 2 ka which includes the medieval warm period mwp the study suggests that the variability in the ism to the latitudinal migration of the inter tropical convergence zone was caused by insolation driven cooling and warming events in the north atlantic the incision of the valley fill alluvium occurred in two distinct phases the older incision phase occurred after 11 ka and before 6 ka whereas the younger incision phase that led to the development of present day topography is bracketed between 3 5 ka and before 1 ka the older incision phase is ascribed to the early to mid holocene enhanced ism climatically driven whereas the younger incision seems to be modulated by the activation of basement faults tectonically driven

seismic evidence for the crustal deformation and kinematic evolution of the nansha block south china sea abstract to understand the crustal deformation of the nansha block nb this study presents an interpretation of a recently acquired multichannel seismic profile that traverses the southwest subbasin swsb the nb and the nansha trough nt a total of six cenozoic unconformities and three stratigraphic sequences are identified on the basis of tectonic structural and stratigraphic features the nb is divided from northwest to southeast into two structural domains namely the hyper stretched and forebulge domains based on the interpretation of the seismic profile the fault derived and whole crustal extension factors f and c respectively for the nb are calculated the calculation results show that c is greater than f in a limited space of appr 200 km where the crust underwent depth dependent extension we suggest that the crustal extensional pattern of the nb may be a combination of distal limited depth dependent extension and proximal uniform extension the interpretation of the unconformities is also used to calculated the fault derived crustal extension at various stages the two main extension stages the syn rift and syn drift account for appr 70 and 25 of the total extension respectively a comparison with the seismic profiles of the eastern nb shows that the crustal extension along the continental margin of the nb is heterogeneous with more significant crustal deformation on the west side of the nb the heterogeneity of the crustal extension along the depth and margin may be related to the thermal condition and rheological structure of the crust or lithosphere

seismic stratigraphy and tectonic structure from a composite multi channel seismic profile across the entire dangerous grounds south china sea abstract we interpret a more than 500 km long composite multi channel seismic reflection profile across the entire dangerous grounds the south china sea five tectonostratigraphic units are determined together with seven sequence boundaries detailed analysis of extensional features based on measurements of fault heaves revealed two major phases of extension separated by a distinct unconformity which likely correspond with the beginning of sea floor spreading in the south china sea early extension occurred during continental rifting late cretaceous early oligocene and resulted in formation of half grabens and rotated blocks controlled by a deeply rooted detachment system extension continued in our study area during the drifting phase of the east subbasin of the south china sea late oligocene early miocene until the subsequent opening of the southwest subbasin at about 25 ma but its intensity decreased markedly thereafter deeply rooted detachment systems evolved possibly during this second phase of extension at the continent ocean transition area from a coincidence of the lowermost reflections and moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust mantle boundary it is suggested that the dangerous grounds reflect a magma poor rift system at the initial stage of mantle unroofing we suggest that a widespread carbonate platform developed across the dangerous grounds concurrent with the period of seafloor spreading in the southwest subbasin of the south china sea fault related stretching factors f along the line show discrepancies with the whole crustal stretching factors c the continental crust of the dangerous grounds may have experienced nonuniform thinning

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

sequence stratigraphic analysis of the youngest nonmarine sequence in the cretaceous ferron notom delta south central utah u s a abstract this study evaluates the nature and internal complexity of high frequency fluvial aggradational cycles and linked paleosols that were previously not recognized in nonmarine sequences of the turonian ferron notom delta in central utah detailed facies analysis of the 33 meter thick youngest nonmarine sequence sequence 1 reveals 11 depositional facies that build 33 fluvial aggradational cycles 9 fluvial aggradation cycles sets and 3 sequences in what was previously classified as a single 100 ka 4th or 5th order sequence fluvial aggradation cycles in the succession are either simple floodplain bedsets or single channel stories that are bounded by paleosols or erosional surfaces they represent depositional events that likely do not span more than 2 ka depositional periods are followed by short hiati 10 to 400 a characterized by subaerial exposure and soil development possibly reflecting floodplain abandonment or short term river avulsions fluvial aggradational cycle sets show systematic stacking of fluvial aggradational cycles and are analogous to parasequence sets they are represented by either compound floodplain bedsets or multi story channel sandstones bounded by relatively mature paleosols or erosionally top truncated they represent multiple small scale depositional events that span not more than 14 ka followed by relatively long 100 a to 1 ka hiati characterized by subaerial exposure and soil development which suggest a relationship to longer term floodplain abandonment due to either longer term avulsion or fluvial entrenchment during base level falls fluvial aggradational cycle sets are often capped by coals and carbonaceous strata marine trace fossils and dinocysts above coal beds suggest marine transgression due to compactional subsidence or sea level rise the scale of cyclicity of the fluvial aggradation cycle sets 14 ka is similar to underlying marine parasequences 10 to 15 ka and suggests that nonmarine and marine cycles have a similar control fluvial sequences are aggregates of fluvial aggradational cycle sets that are bounded by unconformities or their correlative conformities that represent longer periods 1 to 5 ka of subaerial exposure and pedogenic development cyclicity on the sequence scale in this succession is attributed to high frequency milankovitch scale 20 ka 40 ka rises and falls in sea level

deep crustal structure of the conjugate margins of the sw south china sea from wide angle refraction seismic data abstract the south china sea is the largest marginal basin of se asia yet its mechanism of formation is still debated a 1000 km long wide angle refraction seismic profile was recently acquired along the conjugate margins of the sw sub basin of the south china sea over the longest extended continental crust a joint reflection and refraction seismic travel time inversion is performed to derive a 2 d velocity model of the crustal structure and upper mantle based on this new tomographic model northern and southern margins are genetically linked since they share common structural characteristics most of the continental crust deforms in a brittle manner two scales of deformation are imaged and correlate well with seismic reflection observations small scale normal faults grabens horsts and rotated faults blocks are often associated with a tilt of the velocity isocontours affecting the upper crust the mid crust shows high lateral velocity variation defining low velocity bodies bounded by large scale normal faults recognized in seismic reflection profiles major sedimentary basins are located above low velocity bodies interpreted as hanging wall blocks along the northern margin spacing between these velocity bodies decreases from 90 to 45 km as the total crust thins toward the continent ocean transition the continent ocean transitions are narrow and slightly asymmetric 60 km on the northern side and no more than 30 km on the southern side indicating little space for significant hyper stretched crust although we have no direct indication for mantle exhumation shallow high velocities are observed at the continent ocean transition the moho interface remains rather flat over the extended domain and remains undisturbed by the large scale normal faults the main decollement is thus within the ductile lower crust

evolution of an ancient coastal plain palaeosols interfluves and alluvial architecture in a sequence stratigraphic framework cenomanian dunvegan formation ne british columbia canada to date discussion of changes in alluvial style and in the character of palaeosols in relation to changes in accommodation and sediment supply on floodplains has primarily been from a conceptual standpoint few case studies are available against which to test ideas one hundred and thirty metres of non marine strata of the dunvegan formation were examined in 14 closely spaced sections in the canyon of the kiskatinaw river ne british columbia canada this site was located about 120 km inland from the transgressive limit of the contemporary marine shoreline and represents almost exclusively freshwater environments fluvial channels in the kiskatinaw river section are of two types small single storey very fine to fine grained sandstone ribbons with w t ratios 30 are interpreted as the deposits of meandering rivers filling incised valleys interchannel facies include the deposits of crevasse channels and splays lakes floodplains and palaeosols floodplain palaeosols consist of laterally heterogeneous simple palaeosol profiles and pedocomplexes similar to modern entisols inceptisols and hydromorphic soils interfluve sequence bounding palaeosols adjacent to incised valleys are laterally continuous up to 3 m thick and can be reliably identified using a combination of 1 stratigraphic position 2 field observations such as thickness structure colour degree of rooting and 3 micromorphological features such as evidence of bioturbation clay coatings ferruginous features and sphaerosiderite interfluve palaeosols are similar to modern alfisols and ultisols correlation of the local stratigraphic succession with the regional sequence stratigraphic framework based on 2340 well logs and 60 outcrop sections shows that the vertical changes in coastal plain character more coals and lakes vs more pedogenesis can be related to relatively high frequency base level cycles eustatic that are expressed as transgressive regressive marine cycles in downdip areas regional isopach maps show that these cycles were progressively overprinted and modified by an increasing rate of tectonic subsidence in the north and west the character of palaeosols developed on aggrading floodplains primarily reflects local sediment supply and drainage in contrast well developed interfluve palaeosols record pedogenesis during periods of reduced or negative accommodation base level fall vertical changes in floodplain palaeoenvironments and palaeosol types reflect changes in accommodation rate the detailed micromorphological analysis of interfluve palaeosols represents a powerful application of an under used technique for the recognition of key surfaces in the geological record this has broad implications for non marine sequence stratigraphy

micromorphology and sequence stratigraphy of the interfluve paleosols from the ganga plains a record of alluvial cyclicity and paleoclimate during the late quaternary

crustal character and thickness over the dangerous grounds and beneath the northwest borneo trough abstract new deep reflection seismic bathymetry gravity and magnetic data have been acquired in a marine geophysical survey of the southern south china sea including the dangerous grounds northwest borneo trough and the central luconia platform the seismic and bathymetry data map the topography of shallow density interfaces allowing the application of gravity modeling to delineate the thickness and composition of the deeper crustal layers many of the strongest gravity anomalies across the area are accounted for by the basement topography mapped in the seismic data with substantial basement relief associated with major rift development the total crustal thickness is however quite constant with variations only between 25 and 30 km across the central luconia platform and dangerous grounds the northwest borneo trough is underlain by thinned crust 25 20 km total crustal thickness consistent with the substantial water depths there is no evidence of any crustal suture associated with the trough nor any evidence of relict oceanic crust beneath the trough the crustal thinning also does not extend along the complete length of the trough with crustal thicknesses of 25 km and more modeled on the most easterly lines to cross the trough modeled magnetic field variations are also consistent with the study area being underlain by continental crust with the magnetic field variations well explained by irregular magnetisations consistent with inhomogeneous continental crust terminating at the basement unconformity as mapped from the seismic data

lacustrine records of holocene flood pulse dynamics in the upper paraguay river watershed pantanal wetlands brazil abstract the pantanal is the world s largest tropical wetland and a biodiversity hotspot yet its response to quaternary environmental change is unclear to address this problem sediment cores from shallow lakes connected to the upper paraguay river pr were analyzed and radiocarbon dated to track changes in sedimentary environments stratal relations detrital particle size multiple biogeochemical indicators and sponge spicules suggest fluctuating lake level lowstand conditions between 11 000 and 5300 cal yr bp punctuated by sporadic and in some cases erosive flood flows a hiatus has been recorded from 5300 to 2600 cal yr bp spurred by confinement of the pr within its channel during an episode of profound regional drought sustained pr flooding caused a transgression after 2600 cal yr bp with lake level highstand conditions appearing during the little ice age holocene pr flood pulse dynamics are best explained by variability in effective precipitation likely driven by insolation and tropical sea surface temperature gradients our results provide novel support for hypotheses on 1 stratigraphic discontinuity of floodplain sedimentary archives 2 late holocene methane flux from southern hemisphere wetlands and 3 pre colonial indigenous ceramics traditions in western brazil

characteristics of surface heat flow in the south china sea a total of 592 heat flow measurements ranging from 8 to 192 mw m 2 78 percent of which are between 50 and 100 mw m 2 have been collected in the south china sea scs to overcome shortcomings such as the uneven distribution of the measurements and the occurrence of abnormal heat flow values the tectonic evolution of different areas and their crustal thickness have been combined in analyzing geothermal characters the results show that the oceanic basins where they are floored by oceanic crust the western part of the southern margin and the western fault system have high heat flow values heat flow along the northern margin of the scs increases from 61 mw m 2 on the shelf to 73 80 mw m 2 in the slope area and in the xisha zhongsha area increases from about 70 mw m 2 to about 85 mw m 2 from the nw to the se heat flow in the nansha spratley islands is about 60 mw m 2 or even higher and decreases from the nw to se the average heat flow of the mekong basin is about 60 mw m 2 which is similar to that of the beibuwan basin on the northern margin heat flow on the eastern margin and on the eastern part of the southern margin is lower especially in the luzon trough where the average heat flow is lower than 40 mw m 2 observed heat flow values in the sw subbasin are generally lower than predicted theoretically from the age of the ocean floor unlike values in the eastern subbasin a high heat flow zone in the lower slope area of the northern margin is recognized for the first time c 2003 elsevier ltd all rights reserved

palaeogeography of neogene red bed sequences along the aksa ata river in the parkent nurekata intermontane basin tien shan mountains uzbekistan with special reference to the magnetic susceptibility of siliciclastic rocks abstract siliciclastic red beds formed by stream action in the parkent nurekata intermontane basin of the tien shan mountains e uzbekistan the sedimentary rocks were cross sectioned in the field and subject to heavy mineral clay mineral and thin section analyses in the laboratory in addition to these conventional methods a survey into the variation of the magnetic susceptibility was conducted during the paleogene calcareous rocks unit a developed in a subtidal through supratidal environment of deposition subsequently tectonic disturbances tilted the paleogene carbonate rocks and triggered the formation of an unconformity during the oligocene the miocene to pliocene red beds have the hallmarks of a delta like wedge actively prograding over near shore marine deposits two progradational stages units b and c were substituted for by a more regressive event unit d during deposition and burial the siliciclastics were percolated by strongly alkaline pore fluids under semi arid climatic conditions the joint action of rapid uplift in the hinterland and the afore mentioned hydrological processes were instrumental for heavy minerals such as epidote and staurolite to survive and some peculiar phyllosilicates such as smectite and palygorskite to precipitate debris from metamorphic and igneous rocks were delivered from the rising hinterland the petrographic and magnetic records of red beds are a mirror image of the unroofing of a magmatic edifice in the provenance area between 11 and 3 ma

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

extensional styles of the conjugate rifted margins of the south china sea abstract to elucidate continental rifting mechanism of the south china sea scs we characterize crustal structures of the two conjugate rifted margins and calibrate the continental ocean transition zone cot with multi channel reflection seismic gravimetric and magnetic data narrower than those found in many other magma poor margins the cot in the scs coincides well with a positive negative gravity boundary and shows moho reflection gaps basement highs and seaward normal faults post rifting spreading volcanic bodies and igneous intrusions often complicate the structures of the cot rifted basins here are often bounded by listric normal faults the majority of which terminate at the upper lower crust interface i e the brittle ductile transition boundary this observation suggests that the lower crust was weak and played a dominant role in the continental breakup we propose that the scs lower crust acted as a decouple layer during extension but the normal oceanic crust began to establish soon after crustal breakup we also show how thermal regime affected the extent and timing of continental rifting of the scs

continental record of the southwest monsoon during the last 130 ka evidence from the southern margin of the thar desert india abstract field stratigraphy sedimentology and luminescence dating of the exposed fluvial and aeolian sequences from the southern desert margin were used to discern the pattern of southwest monsoon variability during the last 130 ka the sedimentary architecture and textural attributes of the sediments belonging to the period 130 120 ka suggests the existence of a meandering river system consequent to enhanced southwest monsoon conditions a shift from a meandering to a braided pattern during 120 100 ka indicates a phase of reduced precipitation presence of flood plain fines and bedded calcretes during 100 70 ka suggest a period of enhanced monsoon with seasonality following this a phase of weak monsoon activity was observed during 70 60 ka regional flood plain aggradation and pedogenesis during the period termination of a regional fluvial phase around 30 ka and incision of dunes dated to 20 11 ka suggest that the present rivers occupied their course between 11 ka drowned valleys in the estuarine zones suggest that the incision began during the low sea stand around 20 ka when the rivers were graded to the exposed shelf the incision intensified with the re establishment of monsoon during the early holocene

time constraints on the evolution of southern palawan island philippines from onshore and offshore correlation of miocene limestones abstract the link between the deformation of southern and central palawan island philippines and the deformation of the adjacent offshore wedge is investigated the wedge is a continuation of the palawan fold and thrust belt and bounds the borneo palawan trough to the dangerous grounds and to palawan island key parameters for the understanding of the formation and development of this wedge are two limestone formations the oligocene to middle miocene nido limestone and the upper miocene to lower pliocene tabon limestone the initiation of the thrust belt formation is constrained by the nido limestone which was deposited from shortly before the breakup of the eastern south china sea 35 ma until the early miocene age data available from wells offshore central palawan gives an age of early miocene close to the base of the nido limestone while cropping out onshore north palawan these limestones were overthrusted by the wedge in southern and central palawan seismic images show gently east dipping carbonates below the wedge the seismic data show that these limestones are only mildly affected by the wedge formation the end of the wedge development can be constrained by the tabon limestone with an age of 9 to 4 ma this limestone sequence overlies unconformably the offshore wedge a detailed biostratigraphic correlation of the tabon limestone along the southwest palawan shelf using well data combined with multichannel seismic data and investigations onshore southern and central palawan shows a time and space transgressive development of these limestones they are progressively younging towards the west we propose that the formation of the tabon limestone is directly linked with the development of the wedge that tectonically controls the formation of this carbonate platform this constrains the time for the final phase of the formation of the palawan thrust belt after the final compressional phase and wedge formation in the lower early pliocene the wedge underwent a phase of subsidence based upon the detailed correlation of these limestones we propose that the wedge did not form in the southern palawan area prior to 18 ma using the sealing tabon limestone as time constraint we suggest that the development of the wedge in the south palawan area started in the lower middle miocene 15 ma and continued developing towards the west until the upper late miocene to early pliocene 5 ma after the wedge propagation stopped the wedge front collapsed in several places due to gravitational sliding southern and central palawan were uplifted above sea level during a second phase of compression in the late pliocene onshore outcrops give indications to a working spleothem since 1 2 ma

spatial and temporal distribution of sand containing basin fills in the holocene rhine meuse delta the netherlands the quantitative significance of coarse grained deposits in the overbank realm such as crevasse splay deposits has not been studied at the delta scale or at the holocene timescale such knowledge would be beneficial for understanding and explaining sediment distribution in delta plains this study addresses delta scale distribution of sand containing basin fills and their sand body proportion variability based on eight valleywide cross sections in the holocene rhine meuse delta in the netherlands we found that sand containing basin fills form 7 1 of the fluviodeltaic wedge of which splay deposits are most frequently observed midway between the delta apex and the coast organic clastic lake fills and bay head delta deposits in contrast are limited to the distal delta plain over four successive periods between 9000 and 800 cal yr bp the largest proportions of splay deposits remain at 50 150 km downstream of the upstream shifting delta apex we show that intermediate floodbasin widths between 3 1 and 3 6 km in the holocene rhine meuse delta yield the highest proportions of splay deposits high rates of base level rise and wide floodplains both facilitate the creation of accommodation which in turn provides conditions for peat forming wetlands in which organic clastic lake fills can develop the results show that sand bodies form 26 30 of sand containing basin fills this proportion is shown to be controlled by among other variables channel planform and superelevation of the trunk channel and substrate composition we conclude that potentially large volumes of nonchannel sand bodies exist in distal delta plains they constitute up to 39 of the reservoir volume in the distal rhine meuse delta and yield relatively high connectedness ratios 2011 by the university of chicago all rights reserved

climate changes during the early middle triassic transition in the e iberian plate and their palaeogeographic significance in the western tethys continental domain abstract until recently the climate of the early middle triassic at low latitudes was broadly considered as generally temperate warm with no major climate oscillations this work examines the climate of this period through a detailed study of the sedimentary plant soil and mineral records of continental rocks buntsandstein facies in eastern iberian basins our findings indicate temporal climate variations for these near equator 10 14 n regions and unveil the significance of such variations in the southern laurasian domain the climate of iberia s early triassic was mainly dominated by alternating brief although the beginning of the anisian aegean was represented by alternating arid and semi arid to subhumid intervals during the bithynian and pelsonian clearly wetter climates are recorded by the succession consisting of alternating semi arid to semi humid intervals this general tendency was interrupted by three short but marked intervals two humid intervals in the late bithynian and one arid period near the bithynian pelsonian boundary iberia was crossed by prominent irregular highs separating marked corridors or isolated areas this palaeogeography prevailing since variscan tectonics clearly conditioned dominant climates and their geographical distribution no clear climate belts developed in these conditions however isolated internal climate zones separated by elevated areas are identified this palaeogeographic configuration and the low latitudinal position of iberia determined central iberia highs in the southernmost border of laurasia beyond which more humid conditions clearly extended towards the equator reaching the present day moroccan meseta and argana basin

a hierarchical approach for evaluating fluvial systems architectural analysis and sequential evolution of the high net sand content middle wasatch formation uinta basin utah well exposed three dimensional fluvial outcrops of the high net sand content middle wasatch formation in three canyon uinta basin utah were used to create and develop a new methodology for describing the architecture of fluvial systems the methodology builds on the works of campbell jackson allen and miall and addresses sedimentary processes scale and temporal context for reservoir and non reservoir bodies the methodology developed herein is a three level hierarchical framework that classifies meso and macroscale architecture of fluvial systems the three level hierarchy contains from smallest to largest stories elements and archetypes eight story types provide the foundational building blocks of this framework and account for sedimentation in both channel belt and floodplain belt elements including 1 downstream accreting 2 laterally accreting 3 erosionally based fine grained fill 4 fine grained fill associated with laterally accreting 5 levee 6 splay 7 crevasse or overbank channels and 8 floodplain fines two types of elements are recognized 1 channel belt and 2 floodplain belt an archetype consists of a channel belt element and its genetically related floodplain belt elements two distinct upward stacking patterns differentiate braided and meandering archetypes in deconstructing the evolution of archetypes three distinct associations between channel belt elements and their adjacent splays are documented 1 unassociated splays 2 associated coeval splays and 3 associated non coeval splays width and thickness for stories channel belt elements and archetypes are documented providing dimensional constraints for analog high net sand content fluvial systems additionally this methodology provides object based models with shape defined reservoir and nonreservoir geobodies that realistically compare to fluvial systems

the structure and stratigraphy of deepwater sarawak malaysia implications for tectonic evolution abstract the structural stratigraphic history of the north luconia province sarawak deepwater area is related to the tectonic history of the south china sea the sarawak basin initiated as a foreland basin as a result of the collision of the luconia continental block with sarawak sarawak orogeny the foreland basin was later overridden by and buried under the prograding oligocene recent shelf slope system the basin had evolved through a deep foreland basin flysch phase during late eocene oligocene times followed by post oligocene molasse phase of shallow marine shelf progradation to present day seismic interpretation reveals a regional early miocene unconformity emu separating pre oligocene to miocene rifted basement from overlying undeformed upper miocene pliocene bathyal sediments seismic well data and subsidence analysis indicate that the emu was caused by relative uplift and predominantly submarine erosion between 19 and 17 ma ago the subsidence history suggests a rift like subsidence pattern probably with a foreland basin overprint during the last 10 ma modelling results indicate that the emu represents a major hiatus in the sedimentation history with an estimated 500 2600 m of missing section equivalent to a time gap of 8 10 ma the emu is known to extend over the entire nw borneo margin and is probably related to the sabah orogeny which marks the cessation of sea floor spreading in the south china sea and collision of dangerous grounds block with sabah gravity modelling indicates a thinned continental crust underneath the sarawak shelf and slope and supports the seismic and well data interpretation there is a probable presence of an overthrust wedge beneath the sarawak shelf which could be interpreted as a sliver of the rajang group accretionary prism alternatively magmatic underplating beneath the sarawak shelf could equally explain the free air gravity anomaly the sarawak basin was part of a remnant ocean basin that was closed by oblique collision along the nw borneo margin the closure started in the late eocene in sarawak and moved progressively northeastwards into sabah until the middle miocene the present day nw sabah margin may be a useful analogue for the oligocene miocene sarawak foreland basin

factors affecting the accumulation of organic rich deposits in a modern avulsive floodplain examples from the cumberland marshes saskatchewan canada since the 1870s the saskatchewan river at the cumberland marshes east central saskatchewan canada has been undergoing progradational avulsion resulting in development of a 500 km2 avulsion belt the avulsion belt consists of an unstable network of active and abandoned channels splays and floodbasins some containing small shallow lakes fine grained deposits of varying total organic content toc are currently accumulating within floodbasins inside adjacent to and outside the avulsion belt the toc of these deposits depends on factors that control the amount of siliciclastic sediment received by floodbasins five factors that affect toc content are identified distance from an active channel position relative to the avulsion belt presence of alluvial ridges changes in local channel activity and developmental stage of avulsion a channel to basin trend of increasing toc is present at many locations reflecting greater suspended sediment loads received by near channel areas and locations within the avulsion belt generally accumulate sediment with lower toc than those adjacent to or outside the belt alluvial ridges locally act as barriers to floodbasin deposition and channel abandonment reduces suspended sediment to adjacent floodbasins both of which increase toc of sediment finally as avulsion wanes less siliciclastic sediment is distributed throughout the avulsion belt resulting in increased toc in the geological record such fine grained avulsion deposits are likely to appear as intervals of mudrock between thin beds of coal or carbonaceous shale formed from organic rich sediment deposited between avulsion events a succession that is common in many ancient alluvial strata

decollements detachments and rafts in the extended crust of dangerous ground south china sea the role of inherited contacts we investigate the crustal structure of the dangerous ground south china sea through processing and interpretation of coincident wide angle reflection and refraction seismic data continental crust of dangerous ground has been moderately thinned down to 15 km so that most of the structures accompanying the early opening of the south china sea from cretaceous to miocene have been preserved subbasement reflectors as well as refraction velocities image an interpreted dismantled mesozoic metamorphic unit in the southernmost section of our study area a rollover structure indicates that the reflective base of the unit was used as a decollement where low angle normal faults root and blocks rafted the metamorphic unit is discontinued in a nearby basin located immediately to the north where the refraction velocity model shows thinning of the crust from 20 to 15 km with the presence of a 5 km high mantle dome in this deeper basin mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an similar to 15 km offset crustal detachment surface that we link down to the mantle dome we infer that the detachment reactivated an inherited low angle contact most probably related to the yanshanian belt in map view the reactivated structure forms a half graben basin oriented nne ssw oblique to the generally accepted direction of extension this orientation follows the general trend of a granitic belt that spanned the south china margin prior to extension related to the subduction of the paleo pacific

fluvial systems in the siwalik miocene and wyoming paleogene abstract the 3 km thick miocene siwalik group himalayan foredeep in northern pakistan and the 2 km thick paleogene fort union willwood formations bighorn basin in wyoming both preserve long records of fluvial deposition adjacent to rising mountain belts depositional environments and associated habitats change across large basins along with changing physiography and with the location of different river systems that may have varied greatly in patterns of channel deposition and the drainage of adjacent floodplain areas deposits exposed in these two basins provide very different records of shifting paleoenvironments and patterns of basin filling these differences reflect distinct patterns and scales of depositional environments the nature of the exposures and the types of sedimentologic studies that have been carried out in each basin the siwalik group fills a basin that extended at least 1000 km along its axis and 150 250 km away from the mountain front comparison of siwalik deposits and modern drainages in the himalayan foredeep suggests the ancient siwalik basin was filled by large rivers that deposited low gradient sediment fans covering areas on the order of 1000 km 2 and by smaller intrafan rivers with more poorly drained floodplains despite the scale of these river systems relative to siwalik exposures in pakistan transitions between different systems have been recognized deposits of coeval river systems in the siwalik basin show pronounced differences in alluvial architecture the character of overbank deposits and the abundance and taphonomy of organic remains in contrast the bighorn basin in wyoming is a relatively small intermontane foreland basin extending 200 km along its axis and about 80 km across bighorn basin strata were deposited by a river that flowed south to north along the basin axis and by smaller rivers that flowed transverse to the basin axis much of this basin is exposed and thus it is possible to reconstruct changing patterns of deposition and environments through time in more detail than in the siwalik basin these patterns indicate changes in basin wide drainage conditions and environments through time but there are also important differences among coeval strata upsection shifts in environments and vertebrate faunas within both the siwalik and bighorn basins may reflect tectonic or climatic forcing but this comparison emphasizes the importance of recognizing deposits from different contemporaneous river systems before inferring such large scale controls on paleoecological change through time

oligocene miocene carbonates and their role for constraining the rifting and collision history of the dangerous grounds south china sea abstract the attenuated continental crust of the dangerous grounds is located in the southeastern part of the south china sea it was affected by unconformities as identified by several authors cullen et al 2010 hinz and schluter 1985 hutchison 2010 hutchison and vijayan 2010 in the northeastern dangerous grounds a prominent reflector in seismic data is associated with the top of a widespread oligocene to early miocene 18 20 ma carbonate platform this reflector and the underlying carbonates can be used to constrain the timing of the unconformities and the rifting history of the dangerous grounds by carefully interpreting seismic reflection lines we trace the platform carbonates based upon their appearance in the seismic image this platform is continuous in the palawan borneo trough and gets patchy toward the dangerous grounds in the dangerous grounds the image of this key reflector changes and here it merely forms the top of a clastic layer carbonates remain abundant but mainly as isolated reefs that grew on top of tilted fault blocks in the southwestern dangerous grounds the prominent unconformity sealing the tectonic activity is known as the middle miocene unconformity this in fact is an early miocene unconformity which represents a sequence boundary in the borneo palawan trough and in various parts of the dangerous grounds while in other parts of the dangerous grounds it represents a major angular unconformity the unconformity characteristics supplemented with tentative ages indicate that luconia and the southern dangerous grounds were sub aerial during the early miocene while the reed bank the northern dangerous grounds and parts of the central dangerous grounds were mostly submerged except for some islands concentrated on the western edge of the borneo palawan trough this trough is interpreted as a foreland basin where the flexural forebulge provided shallow marine conditions that promoted reef growth as the carbonate deposition migrated from the borneo palawan trough toward the dangerous grounds we suggest that the flexural forebulge provided shallow water conditions for further reef growth on the eastern dangerous grounds

the north west borneo trough abstract the north west borneo trough is bordered along its south east margin by a melange wedge that has been the subject of disagreement with insufficient discussion offshore palawan it has been interpreted as an accretionary prism that has been preserved in place when subduction ceased in the middle miocene it is unconformably overlain by undeformed upper miocene to holocene draping strata farther south west along the trough the seismically identical melange wedge has been named a major thrust sheet system which was assumed to have been thrust as a nappe north westwards over the autochthonous dangerous grounds terrane of attenuated continental crust of the south china sea passive margin the accretionary prism model is the simplest resulting in interpretation of the north west borneo trough as a fossil trench subduction was halted by the rifted and attenuated continental crust arriving at the benioff zone choking subduction and causing isostatic uplift of the western cordillera of sabah the subduction system therefore became a collision zone resulting in the term foredeep in which case the actual trench position remains obscure and somewhere towards the south east but shown in unrestrained geological cartoons enigmas remain in the palawan area of the philippines the trough position is bathymetrically obscure in places and its given position makes it impossible to derive the calamian micro continent from continental asia as required from its stratigraphy in the south west the trough terminates abruptly at the west baram line the trough contains several spectacular edifices formerly suggested to be volcanoes or mud volcanoes but herein shown to be carbonate build ups that were drowned indicating that the trough subsided too fast for build up continuation whereas post rift thermal subsidence of the dangerous grounds was slower allowing the spratly islands build ups to continue active

sedimentary products of avulsions in the holocene rhine meuse delta the netherlands abstract the main objective of this study was to describe the characteristics of deposits related to avulsion to determine if avulsion locations can be recognized by characteristic sediment architecture in the rhine meuse delta the majority of avulsion sites is accompanied by crevasse splay deposits with a relatively large areal extent these crevasse splay complexes occur near the base of the overbank deposits of the newly formed channel belt and are similar to avulsion belt deposits described by smith et al sedimentology 36 1989 1 can j earth sci 35 1998 453 the lithology of all investigated crevasse splay deposits ranges from coarse sand to silty clay sand is concentrated in crevasse splay channels and silty and sandy clay occurs as distal deposits and as levee deposits between the channels the total volume of sand is small relative to clay and silt of a total of 91 avulsion sites three sites were studied in detail one each representing the sedimentary architecture of a full nodal avulsion a failed avulsion and a crevasse splay complex that did not lead to avulsion it is concluded that dating evidence of the abandoned and newly formed paleo channel is always required to verify if avulsion really occurred although the presence of large scale sandy crevasse splay deposits in combination with two main meandering channels often indicates avulsion based on a 2 d outcrop or just one cross section it is impossible to determine whether a splay complex was associated with an avulsion or not because the architecture and lithology in both cases can be identical not all avulsion locations are characterized by crevasse splay deposits there are two possible explanations for this 1 splay deposits were eroded by the newly formed channel or 2 splay deposits were never formed this may be a result of reoccupation of an old channel in this case avulsive flow is immediately appropriated by the pre existing channel and no large scale splay formation occurs alternatively splay deposits were not formed because avulsion resulted from headward erosion

a miocene collisional belt in north borneo uplift mechanism and isostatic adjustment quantified by thermochronology subduction followed by underthrusting of continental lithosphere driven by oligocene early miocene spreading in the south china sea marginal basin account for the tectonic features of sabah isostatic rebound then caused late miocene uplift of the western cordillera the strata were buried under 4 8 km of overburden then rapidly exhumed and cooled at 10 c ma 1 a rate of exhumation of 0 5 0 7 mm a 1 is deduced from thermochronology the same order of uplift in the labuk highlands has exposed metamorphic rocks of the epidote glaucophane facies rapid erosion of the western cordillera supplied abundant clastic sediments to the miocene pliocene baram delta oil bearing basin and to the eastern lowlands and sulu sea the eastern lowlands were affected by miocene rifting of the sulu sea marginal basin in contrast to the western cordillera the strata contain apatite crystals whose fission track ages pre date the containing rocks indicating burial by only about 2 3 km of overburden the terrain has been isostatically stable some apatite and all zircon crystals extracted from tertiary strata yield cretaceous fission track provenance ages

reprint of nature and significance of the west baram and tinjar lines nw borneo abstract a key consideration in tectonic models for se asia and opening of the south china sea is the role that the west baram and tinjar lines of nw borneo may have played in accommodating the motion of crustal blocks displaced from asia following india s collision there are few studies that focus on these lines using onshore geological studies and offshore seismic data to address the origin and tectonic significance of these this paper concludes that rather than a major transform boundary between luconia and the dangerous grounds the west baram line marks the boundary between domains of continental crust that underwent differential extension in the eocene the baram basin is underlain by hyperextended continental crust on the ne side of the baram line the strong contrast in the geological features across the tinjar and west baram lines likely reflects ancient differences in crustal rheology with luconia being the more rigid block although lack of significant strike slip faulting along the west baram line poses problems for tectonic models in which a wide proto south china sea is subducted beneath nw borneo intra plate deformation such as partial inversion of the dangerous grounds rift offers a potential mechanism to mass balance blocks displaced from asia with the reduced strike slip motion along the west baram line

mid holocene extreme hydrological events in the eastern great rann of kachchh western india abstract the present study is an attempt to reconstruct the paleohydrological conditions during the mid holocene in the hyper arid terrain of the northern great rann of kachchh remote sensing detailed field geomorphology sedimentology geochemistry and optical chronology suggests that the fluvial sediments fringing the khadir island were transported locally and deposited during the extreme hydrological events corresponding to the mid to late holocene indian summer monsoon ism variability overall five extreme hydrological events are deciphered during the last 5 ka which broadly correlates with periods of strengthened ism our data indicate that the emergence and decline of the harappan civilization was unrelated to any major fluvial system in the region

middle to late cenozoic tectonic events in south and central palawan philippines and their implications to the evolution of the south eastern margin of south china sea evidence from onshore structural and offshore seismic data abstract using recently gathered onland structural and 2d 3d offshore seismic data in south and central palawan philippines this paper presents a new perspective in unraveling the cenozoic tectonic history of the southeastern margin of the south china sea south and central palawan are dominated by mesozoic ophiolites palawan ophiolite distinct from the primarily continental composition of the north these ophiolites are emplaced over syn rift eocene turbidites panas formation along thrust structures best preserved in the ophiolite turbidite contact as well as within the ophiolites thrusting is sealed by early miocene 20 ma sediments of the pagasa formation isugod formation onland constraining the younger limit of ophiolite emplacement at end late oligocene 23 ma the onset of ophiolite emplacement at end eocene is constrained by thrust related metamorphism of the eocene turbidites and post emplacement underthrusting of late oligocene early miocene nido limestone this carbonate underthrusting at end early miocene 16 ma is marked by the deformation of a seismic unit corresponding to the earliest members of the early middle miocene pagasa formation within this formation a tectonic wedge was built within middle miocene from 16 ma to 12 ma forming a thrust fold belt called the pagasa wedge wedge deformation is truncated by the regionally observed middle miocene unconformity mmu 12 ma a localized post kinematic extension affects thrust fold structures the mmu and late miocene to early pliocene carbonates e g tabon limestone this structural set up suggests a continuous convergent regime affecting the southeastern margin of the south china sea between end eocene to end middle miocene the ensuing structures including juxtaposed carbonates turbidites and shallow marine clastics within thrust fold belts have become ideal environments for hydrocarbon generation and accumulation best developed in the northwest borneo trough area the intensity of thrust fold deformation decreases towards the northeast into offshore southwest palawan

the continent ocean transition at the southeastern margin of the south china sea abstract the south china sea formed by magma poor or intermediate volcanic rifting in the paleogene we investigate the structure of the continent ocean transition cot at its southern margin off nw palawan between the continental blocks of reed bank and the islands of palawan and calamian several surveys recorded by the bgr from 1979 to 2008 established a comprehensive database of regional seismic lines accompanied with magnetic and gravity profiles we interpret two major rifted basins extending in the ne direction across the shelf and slope separated by a structural high of non volcanic origin the continent ocean transition is interpreted at the seaward limit of the continental crust when magnetic spreading anomalies terminate some 80 100 km farther north the area in between displays extensive volcanism as manifest by extrusions that occasionally reach and cut the seafloor by dykes and by presumed basaltic lava flows occurring after break up the cot is highly variable along the nw palawan slope one type shows a distinct outer ridge at the cot with a steep modern seafloor relief the other type is characterised by rotated fault blocks bounded by listric normal faults ramping down to a common detachment surface half grabens developed above a strongly eroded pre rift basement the seafloor relief is smooth across this other type of cot we suggest the pre rift lithospheric configuration had major influence on the formation of the cot besides transfer zones volcanic domains confined to the north of competent crustal blocks correlate with the style of the cot gravity modelling revealed an extremely thinned crust across the shelf we propose a depth dependent extension model with crust being decoupled from mantle lithosphere explaining the discrepancy of subsidence observed across the south china sea region

lower cretaceous paleo vertisols and sedimentary interrelationships in stacked alluvial sequences utah usa the yellow cat member of the cedar mountain formation in poison strip utah usa consists of stacked erosionally bounded alluvial sequences dominated by massive mudstones lithofacies fm with paleo vertisols sediment bodies within these sequences grade vertically and laterally into each other at pedogenic boundaries across which color texture and structures sedimentary vs pedogenic change slickensides unfilled sealed cracks carbonate filled cracks and deeper cracks filled with sandstone the latter features suggest thorough desiccation during aridification thin sandstones sms in some sequences typically as well as laminated to massive mudstones flm with which they are interbedded in some cases are interpreted as avulsion deposits the termini of many beds of these lithofacies curve upward parallel to nearby pedogenic slickensides as the features we call turnups turnups are overlain or surrounded by paleosols but strata sheltered underneath beds with turnups retain primary sedimentary fabrics turnups were produced by movement along slickensides during pedogenesis by differential compaction alongside pre existing gilgai microhighs or by a combination of both palustrine carbonates lithofacies c appear only in the highest or next highest alluvial sequences along with a deep paleo vertisol that exhibits partially preserved microrelief at the base of the overlying poison strip member the attributes of the yellow cat member suggest comparatively low accommodation slow accumulation long hiatuses in clastic sedimentation and substantial time intervals of subaerial exposure and pedogenesis it appears to be distinct among the members of the cedar mountain formation in these respects

the south china sea margins implications for rifting contrasts abstract implications regarding spatially complex continental rifting crustal extension and the subsequent evolution to seafloor spreading are re examined for the northern and southern rifted margins of the south china sea previous seismic studies have shown dramatic differences in the present day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south china although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment this difference was accommodated by the early formation of oceanic crust creating the present day south china sea basin adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west using the observed cross sectional areas of extended continental crust derived from deep penetration seismics two end member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin the two models examined predict that the oceanic crust of the south china sea basin toward the west did not begin forming until sometime between 6 12 my after its initial formation 32 ma toward the east these results are compatible with crustal age interpretations of marine magnetic anomalies assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the south china sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two we suggest the most likely causes of the rifting differences were east west variations in the rheology of the pre rift crust and associated east west variations in the thermal structure of the pre rift lithosphere the calculated widths of rifted continental crust for the northern and southern margins when combined with the differential widths of seafloor generated during the seafloor spreading phase indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within 10 for all three eastern central western segments examined

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

facies characteristics and cyclicity of lower siwalik sediments jammu area a new perspective the lower siwalik succession of the jammu area has been distinguished into three major lithofacies associations a sand dominant association a sandy mud dominant association and a siltyheterolithic association the sand dominant association is made up of three lithofacies cross bedded sandstone rippled silty sandstone and bioturbated sandy siltstone which are organized in multistoreyed sandbodies representing deposition in major river channels the sandy mud dominant association is made up of two lithofacies mottled clayey siltstone and interbedded sandstone siltstone and mudstone representing deposition in overbank areas of flood plain and natural levee crevasse splays the sand dominant association and sandy mud dominant association are grouped together as a channel related succession and are products of processes in the river channel the siltyheterolithic association consists of four lithofacies mottled siltstone mottled silty sandstone bedded calcrete and mottled mudstone they are considered to be deposits of doab upland interfluve areas operating independently of present day major river channels these deposits have been formed in minor channels sloping surfaces and lakes and ponds of the interfluve regions the cyclicity of both successions channel related and doab related has been determined using a partial independence statistical model

radiocarbon dating and sedimentation rates in the holocene alluvial sediments of the northern bihar plains india seven radtocarbon dates of carbonate shells and charcoal from the upper two metres ofsedrment in the indo gangetrc plarns of northem brhar eastern indra can be drvrded rnto three groups with the follow ing approxrmate ages 2400 45 a bp two samples ll00r45 a bp four samples and 765t45 a bp one sample thrs evtdence for at least three eprsodes of sedrmentatron rn the last 22100 a contrasts with evidence of greater ages from srmrlarly near surface sedrments rn the mrddle gangetic plains of uttar pradesh further west in these more westerly areas greater ages and well developed nver terraces polnt to much more restncted late flolocene sedtmentatron rates of net sedrment accumulatron calculated using our bihar ages spannrngapenodof theorderof l0 loaa aresrmr lartothosecalcul atedforperiodsof theorderof l0 106 a for the hrmalayan foreland basrn thrs uggests that in the whole basin case short period rates hrgher than the brhar rates have been compensated by longer than brhar periods of non deposrtion or eroston

tectono stratigraphic terranes and detachment faulting of the south china sea and sulu sea abstract geoscientific data acquired from the south china sea and sulu sea areas by bgr since 1977 are compiled and interpreted as comprehensive data sets together with available information of commercial wells and drillsites of the ocean drilling program odp generalized stratigraphy and lithology are discussed and correlated with the interpreted seismic profiles on the basis of the stratigraphic and structural analysis 5 main tectono stratigraphic terranes are defined for the south china sea and 4 main terranes for the sulu sea respectively the dangerous grounds reed bank palawan nw borneo trough and palawan island represent terranes of continental origin which developed on the proto china margin by uniform sense low angle simple shearing within the crust in late cretaceous to early paleocene this rift was abandoned and a new westward propagating rift system developed north of the dangerous grounds reed bank area from the middle eocene to the oligocene early miocene together with seafloor spreading in the south china sea basin the leading edge of the southward drifting continental terranes collided with the late cretaceous to early eocene subduction complex of the northernmost terrane of the proto sulu sea continuous convergence of these terranes back arc spreading creating the se sulu sea terrane and the assumed anti clockwise rotation of borneo are responsible for the complex collision compression structures of the sulu sea terranes including the formation of splinters of oceanic crust major right lateral wrench systems are nnw sse running up to 500 km long and cut across most of the identified terranes wrenching presumably ceased 12 to 16 myr ago

lateral evolution of the rift to drift transition in the south china sea evidence from multi channel seismic data and iodp expeditions 367 368 drilling results the south china sea margins represent a critical natural laboratory to study the processes and parameters controlling the rift to drift transition high quality seismic data previously suggested the occurrence of a relatively sharp transition between the continental and oceanic domains a hypothesis recently validated by the results of the iodp expedition 367 368 drilled in the northern south china sea distal margin drilling results in this transition cored mid ocean ridge type basaltic basement site u1502 while its lateral equivalent showed continental affinities site u1499 seismic data document the lateral evolution of this narrow transition zone 15 to 25 km wide and tectono magmatic context related to breakup a short period magmatic event occurred during the latest stage of continental rifting and intruded the edge of the thinned continental crust triggering crustal breakup and onset of steady state seafloor spreading the architecture of the transitional domain documented in this work is in marked contrast with those interpreted as either magma starved or magma rich at breakup time suggesting that the continental margin of the scs is intermediate between the classical end member magmatic archetypes we propose that the magmatic event triggering continental breakup is related to decompression melting linked to the ascending asthenosphere it is assumed that this event was rapid at geological time scale 10 ma and was favored by a high mantle temperature

allogenic controls on the fluvial architecture and fossil preservation of the upper triassic ischigualasto formation nw argentina abstract the upper triassic ischigualasto formation in nw argentina was deposited in a fluvial system during the synrift filling of the extensional ischigualasto villa union basin the expansive exposures of the fluvial architecture and paleosols provide a framework to reconstruct the paleoenvironmental evolution of this basin during the upper triassic using continental sequence stratigraphy the ischigualasto formation deposition can be divided into seven sequential sedimentary stages the 1 bypass stage 2 confined low accommodation stage 3 confined high accommodation stage 4 unstable accommodation stage 5 unconfined high accommodation stage 6 unconfined low accommodation stage and finally 7 unconfined high accommodation stage the sedimentary evolution of the ischigualasto formation was driven by different allogenic controls such as rises and falls in lake levels local tectonism subsidence volcanism and climate which also produced modifications of the equilibrium profile of the fluvial systems all of these factors result in different accommodations in central and flank areas of the basin which led to different architectural configurations of channels and floodplains allogenic processes affected not only the sequence stratigraphy of the basin but also the vertebrate and plant taphocenosis therefore the sequence stratigraphy can be used not only as a predictive tool related to fossil occurrence but also to understand the taphonomic history of the basin at each temporal interval

seismic images of a collision zone offshore nw sabah borneo multichannel reflection seismic data from the southern south china sea refraction and gravity modelling were used to investigate the compressional sedimentary structures of the collision prone continental margin off nw borneo an elongated imbricate deepwater fan the toe thrust zone bounds the northwest borneo trough to the southeast the faults separating the individual imbricates cut through post early miocene sediments and curve down to a carbonate platform at the top of the subsiding continental dangerous grounds platform that forms the major detachment surface the age of deformation migrates outward toward the front of the wedge we propose crustal shortening mechanisms as the main reason for the formation of the imbricate fan at the location of the in the past defined lower tertiary thrust sheet tectonostratigraphic province a high velocity body was found but with a much smaller extend than the previously defined structure the high velocity structure may be interpreted either as carbonates that limit the transfer of seismic energy into the sedimentary layers beneath or as paleogene crocker sediments dissected by remnants of a proto south china sea oceanic crust that were overthrust onto a southward migrating attenuated continental block of the dangerous grounds during plate convergence

necessary conditions for a meandering river avulsion there is as yet no rational basis for predicting the conditions leading to avulsion of a meandering river here we present a conceptual model and quantify it under simplifying assumptions as a first step toward the construction of a stability diagram for avulsion initiation it is assumed initially that a rectangular crevasse channel of arbitrary depth is cut into the levee of a meandering river because the water entering the crevasse channel is derived from relatively high in the main flow it contains low concentrations of suspended solids consequently the crevasse flow is under capacity and the entrance is eroded deepening of the entrance increases the crevasse channel discharge and the concentration of suspended solids supplied because more sediment laden waters are tapped from the deeper flow in the main channel the crevasse entrance is predicted to deepen until its sediment carrying capacity is satisfied by the suspended solids entering from the main channel whether an avulsion will occur for a particular combination of initial conditions depends upon whether there exists steady state hydraulic and sediment transport conditions for crevasse channel depths that are equal to or less than the main channel depth this conceptual model is quantified by writing the unsteady gradually varied one dimensional conservation of mass and momentum equations for water and sediment transport through a main and crevasse channel sediment transport is computed as the integral of a cross sectional mean velocity and a rouse concentration profile solutions show that stable crevasse channels exist only for particular combinations of the initial height of the crevasse bed relative to the water depth in the main channel and the ratio of initial crevasse bed slope to main channel slope

seismic stratigraphy of the central south china sea basin and implications for neotectonics coring logging data and physical property measurements from international ocean discovery program expedition 349 are integrated with and correlated to reflection seismic data to map seismic sequence boundaries and facies of the central basin and neighboring regions of the south china sea first order sequence boundaries are interpreted which are oligocene miocene middle miocene late miocene miocene pliocene and pliocene pleistocene boundaries a characteristic early pleistocene strong reflector is also identified which marks the top of extensive carbonate rich deposition in the southern east and southwest subbasins the fossil spreading ridge and the boundary between the east and southwest subbasins acted as major sedimentary barriers across which seismic facies changes sharply and cannot be easily correlated the sharp seismic facies change along the miocene pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5ma coeval with the onsets of uplift of taiwan and accelerated subsidence and transgression in the northern margin the depocenter or the area of the highest sedimentation rate switched from the northern east subbasin during the miocene to the southwest subbasin and the area close to the fossil ridge in the southern east subbasin in the pleistocene the most active faulting and vertical uplifting now occur in the southern east subbasin caused most likely by the active and fastest subduction obduction in the southern segment of the manila trench and the collision between the northeast palawan and the luzon arc timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities

formation of paleovalleys in the central himalaya during valley aggradation abstract the formation of paleovalleys in mountainous regions is considered to result from extreme events such as landslides and glacial or landslide lake outburst floods according to ipcc 2012 the extreme events are rarest of the rare weather climatic events when the climate weather variable is significantly above or below the defined threshold value the present study suggests that paleovalleys can also form during years long periods of valley aggradation a series of paleovalleys thus formed runs parallel to the present river course in this study we suggest that paleovalleys in the alaknanda valley of the central himalaya have formed in two ways 1 major valley aggradation and 2 local events of landsliding and lake breaching most of the paleovalleys in the alaknanda valley formed during a major valley aggradation phase between 15 and 8 ka paleovalleys formed due to local landsliding also formed around 8 ka local landslides were triggered due to high rainfall in lower valley reaches during unstable climatic conditions therefore the formation of paleovalleys both by regional and local mode within 15 8 ka indicates that the valley was receiving excess sediment from upper catchment as well as from lower reaches during this period this phase of excess sediment supply and valley aggradation coincides well with post glacial climatic amelioration therefore the study underlines the role of climate in the time scales of 10 3 years in shaping the landscape of an active mountain like the himalaya the role of other landscape changing agencies such as tectonics and erosion is not accounted in the present study