crustal properties in the epicentral tract of the great 1897 assam earthquake northeastern india the velocity structure for the upper and middle crust in the epicentral tract of the great 1897 assam earthquake western half of the shillong massif was estimated using locally recorded microearthquake data the relatively homogeneous upper crustal layer has p and s wave velocities of 5 9 0 2 and 3 4 0 1 km s respectively with a thickness of 11 to 12 km the average p and s wave velocities in the middle part of the crust down to a depth of about 26 km were estimated to be 6 3 0 6 and 3 5 0 2 km s respectively the larger scatter in p velocity estimate of the mid crustal layer was investigated using synthetic examples our preference is for the model in which this crustal region is assumed to comprise of a number of thin layers with alternate low and high seismic velocities from theoretical considerations we confirm that it is possible to estimate the velocity structure for a horizontally layered model using the velocity estimation procedure a combination of the wadati riznichenko and bune method we have used we also carried out detailed model analysis to check how effective this velocity estimation procedure is for the type of array data we have combining the estimated velocity structure with our earlier work on seismicity of this area we propose that the upper homogeneous crustal layer in the shillong massif may be moving relatively southward across an intracrustal thrust zone we suggest that the great 1897 assam earthquake may represent the most recent episode of this relative southward movement of the massif

the posidonia shale lower toarcian of sw germany an oxygen depleted ecosystem controlled by sea level and palaeoclimate abstract the lower toarcian posidonia shale is famous for its excellently preserved fossils and its high amount of organic matter up to 16 both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions high resolution geochemical sedimentological and palaeoecological investigations of various sections of the posidonia shale in sw germany however indicate that oxygen availability was variable and ranged from short oxygenated periods to longer term anoxia the benthic macrofauna consists of nine fossil communities and was used in combination with geochemical data to reconstruct a time averaged oxygen curve anoxic conditions prevailed during the deposition of the toarcian black shales they were however punctuated by various short periods weeks to years with oxygenated bottom water conditions sedimentological e g distinctiveness of microlamination siliciclastic content and geochemical parameters e g organic matter content isotopic signatures 18o and 13c molecular redox parameters pristane phytane ratio arylisoprenoids exhibit a remarkable covariation and seem to be controlled by sea level fluctuations maximum oxygen depletion and an extreme negative shift of 13corg values 34 occurred during the early falciferum zone this is explained by the recycling of 12c enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia the subsequent transgression permitted enhanced water exchange with the tethyan ocean and caused improvement of living conditions at the end of the falciferum zone other important factors controlling the depositional environment are the overall palaeogeographic situation and climate the early jurassic is the latest period before break up of pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade and monsoon wind systems an estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti estuarine circulation and a negative water balance in the winter during the summer months a stratified water column with anoxic conditions below the halocline developed 18o data indicate low salinity in the surface water during the monsoon influenced summer high productivity was then located in the photic zone and the corresponding isotopically light 18o signal was fixed in the calcareous nannoplankton during the winter months a saline circulation system brought oxygen to the benthic environment favouring temporary benthic colonization especially during times of relative sea level high stand

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

clinoforms and clinoform systems review and dynamic classification scheme for shorelines subaqueous deltas shelf edges and continental margins abstract clinoforms are inclined and normally basinward dipping horizons developed over a range of spatial and temporal scales in both siliciclastic and carbonatic systems the study of clinoform successions underpins sequence stratigraphy and all efforts to reconstruct the relative partitioning of reservoir seal and source rocks along shoreline to basin floor profiles here we review clinoform research and propose a more systematic description and classification of clinoforms this is a crucial step to improve predictions of facies and lithology distribution within shoreline to continental shelf and abyssal plain successions together with the genesis drivers and dynamics of their constituent sedimentary units four basic clinoform types are here distinguished in delta shorelines lacustrine and marine environments on the basis of their overall spatial and temporal scale morphology outbuilding dynamic and geodynamic and depositional setting 1 2 delta scale clinoforms which in turns are sub divided into shoreline and delta scale subaqueous clinoforms 3 shelf edge clinoforms and 4 continental margin clinoforms delta scale clinoform sets are tens of metres high and typically represent 1 103 kyr with progradation rates ranging from 1 000 100 000 m kyr for shorelines and subaerial deltas to 100 20 000 m kyr for subaqueous deltas shelf edge clinoform sets are hundreds of metres high and are nucleated and accreted in 0 1 20 myr usual progradation rates of 1 100 m kyr by successive cross shelf transits of delta scale clinoforms continental margin clinoform sets are thousands of metres high hallmark key geodynamic crustal boundaries e g continent ocean transition and slowly prograde basinwards in ca 5 100 myr with typical rates of 0 1 10 m kyr as a consequence of the very different progradation rates and of the difficulty of large scale clinothems to backstep during transgressions shorelines are the most dynamic clinoforms with regards to position continental margins the least and shelf edges are intermediate shortly after a transgression therefore the four clinoform types may prograde synchronously along shoreline to abyssal plain transects forming compound clinoform systems during the subsequent regressive cycle however due to the dissimilarity in progradation rates different clinoform types will normally merge progressively with each other giving rise to hybrid clinoforms e g shelf edge deltas and fewer depositional breaks in slope are distinguished along a single shoreline to abyssal plain transect overall all clinoform systems are the result of the dynamic evolution of compound and hybrid clinoforms along a temporal and spatial continuum regulated by the cyclical backstepping of the smaller scale system within natural progradational retrogradational cycles of larger scale clinothem outbuilding all clinothem types may show either an accretionary active or draping passive style depending on the proximity to the sediment source draping clinothems are nearly always composed of condensed fine grained sediments while actively accreting clinothems can be composed of predominantly coarse grained i e reservoir forming or predominantly fine grained i e non reservoir lithotypes a novel hierarchical classification scheme for both recent and ancient clinoforms is here proposed consisting of 12 classes the four basic clinoform types delta scale shoreline delta scale subaqueous shelf edge and continental margin are sub divided into eight accretionary active and draping passive sub types 8 division each accretionary sub type is then sub divided into a sandstone prone and mudstone prone variant 12 division which can be at least tentatively predicted on the basis of the clinoform morphology even in the absence of direct stratigraphic logs

calcretes at sequence boundaries in upper carboniferous cyclothems of the sydney basin atlantic canada abstract nodular and associated groundwater calcretes in coal bearing cyclothems of the sydney basin mark the boundary between underlying marginal marine and overlying alluvial deposits they are inferred to represent lowstand surfaces sequence boundaries the host sediments include both sand siltstones and palustrine limestones microfabrics of calcretes with detrital hosts show evidence of replacement displacement and shrinkage alpha fabrics suggesting that the calcretes formed under relatively arid conditions in contrast microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics with root induced brecciation and subordinate shrinkage fabrics the calcretes and host limestones are inferred to have formed in topographic lows thick alluvial sediments with red calcic vertisols overlie the calcretes and formed within the transgressive systems tract under conditions of abundant sediment supply coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract the presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity

subaqueous delta of the ganges brahmaputra river system abstract the ganges brahmaputra is among the world s three largest river systems in terms of sediment load but until now no high resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers mouths the other two amazon huanghe discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy high resolution seismic reflection profiles of the bengal shelf reveal similar stratigraphy topset beds dip gently 0 036 and diverge offshore more steeply dipping foreset beds 0 190 converge farther seaward and relatively thin gently dipping bottomset beds 0 022 extend across the outer shelf overlying an erosional surface presumed to be of late pleistocene age sediment accumulation rates are highest in the foreset region 5 cm year and reduced in the bottomset region

moho geometry and upper mantle images of northeast india 1 images of the crust and mantle beneath northeast india obtained by 2d migration of 1000 broadband p receiver functions clearly trace a northward dipping moho from the himalayan foredeep reaching depths up to 50 km further north beneath the himalayan convergence zone also these images reveal presence of largely coherent 410 km and 660 km discontinuities that conform to the iasp91 model marginal variations in the depth of the 410 km interface are observed that appear region specific the thickness of the mantle transition zone does not deviate significantly from a global average of 250 km interestingly our results reveal consistent presence of a signal from an interface around 300 km origin of such a boundary known as x discontinuity and unrelated to the lehmann discontinuity is discussed possible presence of the x discontinuity from the indian region is reported here for the first time

seismotectonics in northeast india a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications summary in northeast india three major plates interact along two convergent boundaries the himalayas and the indo burma ranges which meet at the assam syntaxis to clarify this tectonic interaction and the underlying dynamics we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5 we then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements north south compression in a direction consistent with india eurasia convergence prevails in the whole area from the eastern himalayas to the bengal basin through the shillong mikir massif and the upper assam valley e w extension in tibet is related to this n s india eurasia convergence not only does the major n s compression affect the outer segments of the indo burma ranges it also extends into the descending slab of indian lithosphere below these ranges although stresses at depth are controlled by bending of the slab beneath the burmese arc the existence of widespread n s compression in the bengal basin far away from the himalayan front is compatible with the previously proposed convergence between a shillong mikir assam valley block and the indian craton e w compression inside this block supports the hypothesis of a component of eastward extrusion stress inversion of focal mechanism solutions in the indo burma ranges reveals a complex stress pattern the burmese arc and its underlying lithosphere experience nearly arcperpendicular extension with ese wnw trends in the northernmost ne trending segment and ene wsw trends in the main n s arc segment such extensional stress documented from many arcs is likely a response to pull from and bending of the subducting plate at the same time the indo burma ranges are under compression as a result of oblique convergence between the sunda and indian plates the maximum compressive stress rotates from ne sw across the inner and northern arc to e w near the bengal basin this rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors from a ssw directed sunda burma motion to a wsw directed burma india motion as a consequence of this partitioning the major belt parallel fault zones show a variety of movements across the main n s arc segment from right lateral slip in the inner ranges to oblique reverse dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt

crustal structure variations in northeast india from converted phases 1 teleseismic receiver functions from a ten station network deployed in northeast india region sampling the shillong plateau mikir hills himalayan foredeep and the himalayan convergence zone are analyzed to obtain the crustal structure in this seismically active but less studied region the shillong plateau and mikir hills away from the convergent margins reveal remarkably simple crust with thickness 35 km and poisson s ratio 0 25 akin to the indian shield values a surprisingly thin crust for the uplifted shillong plateau may be explained invoking presence of an uncompensated crust that popped up in response to tectonic forces in contrast crustal signatures from assam valley suggest a thicker crust and higher poisson s ratio with evidences for a dipping moho predictably the crust is much thicker and complicated in the eastern himalaya further north with values in excess of 50 km

evidence for pre folding vein development in the oligo miocene asmari formation in the central zagros fold belt iran in order to understand the interplay between vein development and folding in the carbonates of the oligo miocene asmari formation one of the main hydrocarbon reservoir rocks in iran several anticlines have been investigated in the central part of the zagros folded belt combining observations of relative chronology between veins based on calcite filling phases and crosscutting abutting relationships as well as aerial satellite image interpretation on several anticlines allowed proposing a tectonic model highlighting the widespread development of veins and other extensional micro meso structures in the central zagros folded belt our data suggest that most of the veins affecting the asmari formation predated the main miocene pliocene folding episode an early regional vein set striking n50 marked the onset of collisional stress build up in the region then n150 and n20 trending vein sets were initiated in response to local extension caused by large scale flexure drape folds above n s and n140 basement faults reactivated under the regional ne compression at the onset and during miocene pliocene folding of the sedimentary cover the early formed veins were reactivated reopened and or sheared while duplexes low angle reverse faults and thrusts formed beyond regional implications this study puts emphasis on the need of carefully considering regional local vein development predating folding as well as influence of underlying basement faults in models of folded fractured reservoirs in fold thrust belts

estimation of source parameters of local earthquakes originated in shillong mikir plateau and its adjoining region of northeastern india abstract we estimate the source parameters seismic moment source radius stress drop and source displacement and scaling laws for local earthquakes that occurred in the shillong mikir plateau assam valley and arunachal himalaya in northeast india during 2001 2008 the source parameters were determined using the spectral analysis of p waves from the vertical component seismograms after correction for attenuation seismic moments are observed within the range from 9 51 10 12 to stress drop ranges from 4 10 5 to 9 10 7 pa for the brune model and 7 10 5 to 1 10 8 pa for the madariaga model seismic events in this study are prominent with an average stress drop of 0 1 10 mpa the effect of site geology may be a contributing factor for such a variation in stress drop source dimensions are however found to be smaller within the major part of the plateau it is suggested that local earthquakes in the region are associated with a brittle shear failure mechanism on fault segments and or the presence of weakened zones and earthquakes are triggered by low deviatoric stress empirical relations between m w m l and m 0 m l are developed leading to the future prediction of calibration coefficients for the local earthquakes in the shillong mikir plateau and its adjoining region online material tables of source parameters

the late holocene gargano subaqueous delta adriatic shelf sediment pathways and supply fluctuations the gargano subaqueous delta formed on the eastern and southeastern sides of the gargano promontory in the western adriatic this subaqueous deposit represents the southernmost portion of the late holocene highstand systems tract hst growing along the western side of the adriatic as an extensive wedge of deltaic and shallow marine mud the late holocene hst rests above a regional downlap surface that marks the time of maximum landward shift of the shoreline attained around 5 5 cal kyr bp at the end of the late pleistocene holocene sea level rise high resolution seismic stratigraphic and tephra correlation indicate the presence of a thin basal unit recording condensed deposition between 5 5 and 3 7 cal kyr bp over much of the basin above this unit sediment accumulation rates increased to high values as much as 1 5 cm yr 1 reflecting the stabilisation of relative sea level and the forcing from high frequency climatic or anthropogenic changes affecting river dynamics the late holocene mud wedge of which the gargano subaqueous delta is a significant component reaches up to 35 m in thickness and has a volume of ca 180 km3 the shore parallel thickness distribution of the mud wedge reflects the dominant oceanographic regime of the basin and the asymmetric location of the mostly western sediment sources with a combined modern delivery of 51 7 106 t yr 1 of mean suspended load in sections perpendicular to the coast the late holocene mud wedge appears composed of forestepping clinoforms with gently dipping foresets typically 0 5 the gargano subaqueous delta is characterised by a submarine topset in water depths shallower than 25 28 m and accounts for about 1 7th of the total volume of the late holocene mud wedge despite the absence of direct river supply to the gargano area in the area of maximum interaction between shore parallel currents and basin morphology progradation occurs onto a flat and barren bedrock outcrop in about 50 80 m water depth the rapid transition from a thickness of 30 m of late holocene mud to nil is a good indication of the role of southward flowing bottom hugging shelf currents in causing the redistribution of sediment along the adriatic inner shelf additional evidence of this regime comes from 1 the most recent sigmoid defined at seismic stratigraphic scale deposited since the onset of the little ice age showing a shore parallel thickness distribution and a main depocentre to the southeast of the gargano promontory 2 the maximum values of sediment accumulation rates over the last century documented by 210pb measurements defining a narrow shore parallel belt immediately seaward of the depocentre of the most recent sigmoid the gargano subaqueous delta grows from the outbuilding of progressively younger progradational sigmoids that tend to parallel the previous ones the gargano subaqueous delta differs from other documented late holocene subaqueous deltas because its growth reflects 1 sediment transport dominated by bottom currents sub parallel to the strike of the composing clinoforms 2 a complex supply regime including the po delta 350 km to the north and several coalescing apennine rivers acting as line source 3 several alternating intervals of enhanced outbuilding and condensed deposition and 4 an in phase growth of the most recent sigmoid with the major progradation of the po delta during the little ice age

an upper pennsylvanian desert paleosol in the d zone of the lansing kansas city groups hitchcock county nebraska abstract this study suggests that the upper shale regressive shale in the upper pennsylvanian missourian lansing kansas city groups d zone from hitchcock county nebraska is a paleosol pedogenesis of this unit occurred during prolonged subaerial exposure on a semiarid to arid alluvial flood plain pedogenesis occurred during maximum retreat of the late pennsylvanian sea during the d zone sedimentary cycle initially clays in this unit were deposited in marine environments but as eustatic sea level continued to drop deposition became dominated by continental alluvial processes characteristics which distinguish the upper shale as a paleosol are the presence of 1 a basal subsoil caliche c soil horizon characterized by weathered limestone regolith clasts soil nodules thin laminated calcrete beds with pseudo fenestral fabric pelleted and clotty calcareous and dolomitic caliche weathered clays circumgranular cracks and root casts and 2 an upper unit b soil horizon characterized by authigenic hematite intensely weathered clays soil nodules slickensides and gypsum root casts

anomalous recent growth of part of a large arctic ice cap austfonna svalbard 1 observations from repeat pass airborne laser altimetry acquired in 1996 and 2002 indicate an anomalous positive ice surface elevation change for the central accumulation area of the largest ice cap in the eurasian arctic austfonna eastern svalbard the increase is equivalent to 35 of the long term annual accumulation rate and coincides with the loss of perennial sea ice in the adjacent barents sea which we conclude is the most likely explanation for the increase extrapolation of the observed trends in sea ice decline over the next 50 years suggests large perturbations in the mass balance of other arctic ice masses may be expected

where does the time go assessing the chronostratigraphic fidelity of sedimentary geological outcrops in the pliocene pleistocene red crag formation eastern england it is widely understood that earth9s stratigraphic record is an incomplete record of time but the implications that this has for interpreting sedimentary outcrop have received little attention here we consider how time is preserved at outcrop using the neogene quaternary red crag formation england the red crag formation hosts sedimentological and ichnological proxies that can be used to assess the time taken to accumulate outcrop expressions of strata as ancient depositional environments fluctuated between states of deposition erosion and stasis we use these to estimate how much time is preserved at outcrop scale and find that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600 800 kyr of crag time much of the apparently missing time may be accounted for by the parts of the formation at subcrop rather than outcrop stratigraphic time has not been lost but is hidden the time completeness of the red crag formation at outcrop appears analogous to that recorded in much older rock units implying that direct comparison between strata of all ages is valid and that perceived stratigraphic incompleteness is an inconsequential barrier to viewing the outcrop sedimentary stratigraphic record as a truthful chronicle of earth history supplementary material further details of the regional geology and specific information on outcrops are available at https doi org 10 6084 m9 figshare c 4561001

are enigmatic erosional sandy wave like bedforms in jurassic bridport sands dorset due to standing waves a field of sandwave like bedforms produced by erosion rather than deposition with bedload traction is identified within the lower middle jurassic shallow marine bridport sands of the wessex basin which were previously interpreted as simple channel like erosional scours by davies in 1967 here this interpretation is refined by proposing that these sandy bedforms formed as near symmetrical scours into horizontally stratified and bioturbated very fine to fine grained sands in shallow water below standing waves possibly internal edgewaves these sandy bedforms and contemporaneous erosional surface which defines the base of the ham hill stone may represent different responses to a moorei subzone lowstand of sea level

a sequence stratigraphic model for an intensely bioturbated shallow marine sandstone the bridport sand formation wessex basin uk the bridport sand formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic carbonate shallow marine depositional system at outcrop and in subsurface cores conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces the geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3d seismic data facies analysis suggests that the bridport sand formation represents progradation of a low energy siliciclastic shoreface dominated by storm event beds reworked by bioturbation the shoreface sandstones form the upper part of a thick up to 200 m steep 2 3 mud dominated slope that extends into the underlying down cliff clay clinoform surfaces representing the shoreface slope system are grouped into progradational sets each set contains clinoform surfaces arranged in a downstepping offlapping manner that indicates forced regressive progradation which was punctuated by flooding surfaces that are expressed in core and well log data in proximal locations progradational shoreface sandstones corresponding to a clinoform set are truncated by conglomerate lags containing clasts of bored reworked shoreface sandstones which are interpreted as marking sequence boundaries in medial locations progradational clinoform sets are overlain across an erosion surface by thin 10 m and overlie conglomerate lags at sequence boundaries sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces to form composite erosion surfaces in distal locations oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces over most of the wessex basin clinoform sets corresponding to high frequency sequences are laterally offset thus defining a low frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation aggradational stacking of high frequency sequences occurs in fault bounded depocentres which had higher rates of localized tectonic subsidence

the carbonate flysch transition late maastrichtian late palaeocene in the arachova sequence of the parnassus ghiona zone central greece the transition from the carbonate to the flysch facies in the arachova sequence of the parnassus ghiona zone is represented by argillaceous limestone beds with flaser structures deposited during latest maastrichtian palaeocene time in a pelagic carbonate environment with a periodic clastic influx deposition was continuous except for a short interruption during the k t boundary interval and the earliest palaeocene when the area was subaerially exposed this interruption gave rise to the development of a brecciated carbonate horizon through soil forming processes the mineralogical composition of the clastic influx i e quartz feldspars clay minerals amorphous iron oxides amorphous phosphatic compounds in particular the clay mineral assemblages i e illite chlorite irregularly interstratified illite vermiculite shows that the clastic supply represents erosional material that originated from a tectonically active continental setting of both carbonate and clastic rocks presumably the pelagonian zone as for the flysch of the beotian and sub pelagonian zones the arrival of the first clastic material in the arachova area as early as latest maastrichtian time its pelagonian origin and the persistence of pelagic conditions of sedimentation throughout the palaeocene indicate that the arachova area was situated along the northeastern margin of the parnassus platform and that it subsided into the beotian basin while the central areas of the platform remained tectonically stable during middle palaeocene times and there was an extensive development of stromatolites the northeastern marginal areas transitional to the beotian basin continued to subside allowing pelagic carbonate sedimentation with periods of clastic influx the total collapse of the platform in the late palaeocene gave rise to the deposition of the flysch over the entire zone

pn velocity study using a temporary seismographic network in the shillong plateau northeast india abstract seismic p wave velocity pn of the uppermost mantle beneath the shillong plateau has been studied using a temporary seismographic network refracted pn arrivals of shallow distant earthquakes m 3 0 recorded at pairs of stations lying within 2 azimuthal differences with respect to a given epicenter are used in this study since the method is based on relative travel time differences the measurement is independent of origin time and location errors the average pn velocity is found to be 8 57 0 02 km sec which is higher than the average mantle velocity 8 13 km sec under other areas of the indian subcontinent the observed high pn velocity beneath the plateau may be attributed to the presence of high density material

isotopic composition of a calcite cemented layer in the lower jurassic birdport sands southern england implications for formation of laterally extensive calcite cemented layers abstract 18opdb and 13cpdb values have been measured on 107 calcite cement samples from a laterally extensive 3 km and continuous calcite cemented layer 0 5 m thick in the coastal exposures of the lower jurassic shallow marine bridport sands in dorset southern england the samples were taken from a two dimensional grid with 10 cm horizontal and vertical spacing between samples and along individual vertical lines across the calcite cemented layer 18opdb values vary between 4 8 and 9 2 and decrease radially outwards from points with lateral spacings on the order of 0 5 1 m in the middle of the calcite cemented layer the 18opdb values therefore indicate that the calcite cemented layer was formed by merging of concretions all 13cpdb values measured are in the narrow range 2 2 to 0 5 which suggests that the dominant source of calcite cement in the layer was biogenic carbonate

large camborygma isp in fluvial deposits of the lower permian asselian dunkard group southeastern ohio u s a abstract the lower washington formation of the lower permian asselian dunkard group in southeastern ohio contains large burrows exposed in cross section in a laterally discontinuous sandstone bed this study examined the morphology of the burrows and their associated lithofacies to interpret the environmental conditions under which they were produced as well as the behavior and identity of the trace maker mudstone facies below and above the burrow bearing sandstone consist of reddish brown to variegated paleosols containing rhizohaloes argillans and large scale slickensides as well as thinly laminated organic rich shales containing plant fossils the sandstone facies is 94 147 cm thick thinly bedded to massive and fine to medium grained the lithofacies are interpreted as deposits of crevasse splays abandoned channels and proximal to distal floodplains sixty burrows consist of vertical subvertical j shaped y shaped and complex networks of branching shafts and tunnels the burrows range from 5 to 180 cm in length and 0 8 3 6 cm in width the main shaft s angles are typically 90 although are rarely 30 70 branching angles of subvertical and y shaped burrows vary from 0 to 90 many burrows extend to the top of the sandstone bed but others turn into or out of the bed the morphology of the burrows is most similar to camborygma known from continental deposits of the permian to recent while camborygma has been previously reported from the permian this is the first occurrence in the late paleozoic appalachian basin extending its geographic range the dunkard burrows are similar to those of modern freshwater decapods particularly crayfish decapod body fossils are not known from the dunkard group yet these burrows are highly suggestive of their presence therefore these fossil burrows contribute to the paleoecological interpretation of the dunkard group helping to fill in gaps of the terrestrial fauna not otherwise preserved in the fossil record decapod burrow morphologies are highly variable depending on burrow function and environment the dunkard burrows are similar to secondary dwellings with few open water attachments multiple branches and shafts likely extending to the water table

new insights into path attenuation of ground motions in northeast india and northwest himalayas abstract we compile a data set of 585 weak broadband and strong ground motion recordings from 184 earthquakes covering hypocentral distances ranging from 9 to 436 km in northeast india and the northwest himalayas source normalized vertical component fourier amplitude s wave spectra are used to study path attenuation characteristics namely geometrical spreading and quality factor in the two regions a trilinear model of geometrical spreading of r 1 0 for hypocentral distance r r 0 0 for r between 125 km and 140 km and r 1 0 for r 140 km approximates the decay of low frequency 1 0 hz amplitudes in northeastern india better than the conventional theoretical bilinear geometrical spreading of r 1 0 for r r 0 5 otherwise in the northwestern himalayas the geometrical spreading is found to be r 1 0 which is the same as that of a bilinear model at hypocentral distances r f given by in the frequency range of 0 9 20 hz

the role of trace fossil ichnofabric analysis in the development of depositional models for the upper jurassic fulmar formation of the kittiwake field quadrant 21 ukcs

microearthquake activity in some parts of the himalaya and the tectonic model abstract microearthquake data from temporary permanent networks in different parts of the himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region the microearthquake activity in arunachal pradesh northeastern himalaya is found to be pronounced at the main boundary thrust mbt and to its south the subcrustal earthquakes depth 50 80 km occur much below the plane of detachment of the tectonic models proposed by seeber et al 1981 ni and barazangi 1984 the mbt is not the seismogenic fault the earthquakes are generated by strike slip movement on deep seated hidden faults transverse to the mbt the high seismic activity in the shillong plateau about 200 km south of the mbt in the northeast region is due to the influence of himalayan collision tectonics to the north and burmese arc subduction tectonics to the east the activity in the plateau is not directly related to the himalayan thrust belt or seismic belt these are mostly crustal earthquakes depth 10 30 km and are caused by local active faults lineaments in the eastern himalaya in the sikkim and darjeeling area the seismic activity is found to be clustered mostly to the north of the mbt the earthquakes occur at a depth range 0 50 km the majority of them occur below the detachment plane by thrust faulting in the central part in the nepal himalaya lateral variations of the seismic activity are observed which represent lateral segmentation of the mbt by transverse tectonic features in the western himalaya however the tectonic models fit well with the microearthquake data in the himachal pradesh of the western himalaya the microearthquakes are mostly recorded in the mbt zone and the hypocentres depth 0 20 km are confined above the plane of detachment or on the basement thrust the earthquakes mostly occur to the south of the main central thrust mct which suggests that the mct is not seismogenic it is rather a dormant fault no single tectonic model explains the himalayan earthquakes

3 d seismic structure of the northeast india region and its implications for local and regional tectonics abstract in this study we attempted to estimate 3 d p wave velocity structure of northeast india region using the first arrival data of local earthquakes that were recorded by about 77 temporary permanent local seismic stations the data set the published bulletins include 3494 p wave travel times and 3064 s p travel times from 980 local earthquakes that were located with a minimum of six observations the located earthquakes having a travel time root mean square rms residual 0 49 s and azimuthal gap 180 are selected to compute a 1 d velocity model for the region which is used as initial model for the subsequent 3 d inversions our results demonstrate that the computed 3 d velocity model has significantly improved hypocenter locations of the selected 980 earthquakes by reducing the rms error 0 06 by about 88 with respect to that by the 1 d velocity model the reconstructed p wave velocity v p structure with relocated events reveal strong heterogeneity in lateral as well as in vertical direction corresponding to the local and regional geology tectonics of the region high v p is mapped beneath the shillong plateau mikir hills and in the vicinity of indo burma ranges at shallower crust v p structure is imaged between the shillong plateau and mikir hills at 20 km depth which corresponds to the kopili fault the kopili fault system extends down to 30 km depth as evidenced by the low v p a high v p is imaged below the mikir hills at 40 km depth which is possibly the stress concentrator for high seismic activity along the kopili fault particularly at the fault end the bengal basin south of the shillong plateau is identified as a low v p zone extending down to a depth of about 20 km that indicates the thick alluvium sediments

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

facies characteristics and depositional models of highly bioturbated shallow marine siliciclastic strata an example from the fulmar formation late jurassic uk central graben abstract an extensive core database from the late jurassic fulmar formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated heterolithic shallow marine siliciclastic strata twelve major sedimentary facies 1 12 are defined on the basis of lithology grain size visually estimated sand silt content and the presence or absence of primary sedimentary structures ichnofabric characteristics are also defined and have proved to be not always facies specific for purposes of depositional modelling the 12 facies types are resolved into six facies associations a f each representative of distinct shoreline shallow marine environments from these associations three broad depositional models have been constructed storm influenced shoreface model 1 bioturbated shoreface model 2 and a speculative bioturbated shelf model model 3 stratigraphical and palaeontological criteria suggest that models 1 and 2 are shoreline attached sedimentological data indicate that they occur as end members to a spectrum of shoreline settings constrained by incident wave energy bands for shorefaces subject to significant storm influence well preserved event beds occur seaward of the upper shoreface in the bioturbated shoreface model however the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking the bioturbated shelf model displays no clear evidence of shoreline connection with the resultant facies often exhibiting highly abundant siliceous sponge spicules together with other open marine indicators such as belemnites and rare ammonites unlike the shoreface sand bodies which received sediment from basin margin fluvial systems shelf sand bodies in the fulmar formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone this sediment may have undergone a long transit residence period on the shelf prior to eventual burial in hanging wall depocentres strongly influenced by zechstein salt withdrawal this study demonstrates that an ichnofabric approach as opposed to one of simple ichnodiversity is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata in the fulmar formation ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels water depth sedimentation rates erosion rates substrate consistency and dissolved oxygen levels evidence of these fluctuations is often apparent at intra facies scale but would largely go unnoticed using a conventional sedimentological approach to data gathering based on ichnodiversity at best depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling

sedimentology and vertebrate taphonomy of the tritylodon acme zone a reworked palaeosol in the lower jurassic elliot formation karoo supergroup south africa abstract a semi continuous bed of pedogenic nodule conglomerates in the floodplain deposits of the lower jurassic elliot formation upper karoo supergroup of south africa is interpreted as a lag deposit that resulted from an extended period of floodplain degradation the large scale geometry of the conglomerate bed resembles a sheet with holes only 0 5 1 25 m thick yet it covers at least 11 000 km2 of the northwestern part of the elliot outcrop area it is commonly underlain by eroded remnants of a calcic palaeosol and grades upwards into a fully preserved calcic vertisol the conglomerate contains reworked pedogenic nodules mudrock pebbles and numerous fossils of the advanced cynodont tritylodon fossils of tritylodon occur in such abundance that this interval has been dubbed the tritylodon acme zone this study uses field observations of the sedimentology palaeopedology and vertebrate taphonomy of the tritylodon acme zone to reconstruct the geomorphological history and the time constraints involved in the formation of this lag conglomerate and associated palaeosols paleosols in the loessic siltstones below the tritylodon rich conglomerate are characterised by a rectilinear pattern of large sandstone filled shrinkage cracks which are commonly offset by small slickensided arcuate shear planes the fissures in the lower palaeosol are heavily calcretized and some contain vertically stacked calcareous nodules the upper palaeosol is characterised by similar vertical fissures except that they are filled with siltstone and mudstone and are rarely calcretized the upper palaeosol contains scattered pebble sized calcareous nodules along with vertical and horizontal rhizocretions and termitaria like burrowed sandstone pods both palaeosols are interpreted as calcic vertisols formed in clay rich alluvium on seasonally wet floodplains under warm semi arid climatic conditions the relatively high abundance of tritylodon fossils in the nodule conglomerate is attributed to regional denudation of an estimated 3 5 m of fossil bearing calcareous soil resulting in reworking of the resistant glaebules and partlymineralised bones into a lag deposit most of the bones occur as disarticulated and broken elements in multitaxa bonebed type accumulations within shallow depressions scoured into the underlying alluvium haematite perimineralization of bone is ubiquitous and its precipitation on the surfaces of brittle fractured bone surfaces is evidence that many of the bones were mechanically broken by reworking after they had been buried for long enough to become partially calcified a time restored stratigraphic column gives an estimated 50 000 yr for the accumulation of the 2 25 m of tritylodon acme zone strata using figures derived from studies of modern landscapes it is estimated that the tritylodon conglomerate itself records a period of floodplain incision in this part of basin during the earliest jurassic times which lasted for some 30 000 yr

moho depth variation in the central part of northeastern japan estimated from reflected and converted waves abstract we detected reflected p and s waves at the moho in seismograms of shallow crustal earthquakes in the central part of northeastern ne japan p to s and s to p mode converted waves at the moho are also detected in seismograms of intermediate depth earthquakes in the subducted pacific slab the depth distribution of the moho in the central part of ne japan is estimated by using travel time differences between these later phases and the first p and s arrivals we collected travel time data of 496 reflected waves from 67 shallow earthquakes and 200 converted waves from 65 intermediate depth earthquakes the depth to the moho estimated is deep beneath the land and becomes shallower toward the eastern and western coastlines the moho depth is about 34 km in the middle of the land area about 30 km under the coastline of the japan sea and 27 29 km under the coastline of the pacific ocean

sedimentologic character of ancient muddy subaqueous deltaic clinoforms down cliff clay member bridport sand formation wessex basin u k abstract muddy subaqueous clinoforms are a common feature of many modern deltas particularly those developed in basins with strong waves tides or oceanographic currents ancient examples have only rarely been reported implying that they are under recognized herein the sedimentological characteristics of muddy subaqueous deltaic clinoforms from the lower jurassic down cliff clay member of the bridport sand formation wessex basin uk are described and interpreted as a reference to re evaluate other ancient shallow marine mudstone successions deposits below the down cliff clay clinoforms consist of erosionally based bioclastic sandy limestone beds intercalated with upward thickening fossiliferous claystones and siltstones facies association a which record upward increasing water depth under conditions of minor input of clastic sediment and reworking by storm waves these deposits are downlapped by foreset to toeset deposits of the subaqueous deltaic clinoforms which comprise claystones and siltstones that contain calcareous nodules and are variably bioturbated by a low diversity trace fossil assemblages dominated by chondrites facies association b these foreset to toeset deposits record slow deposition undecompacted sediment accumulation rate of c 4 4 m myr from suspension fall out and distal sediment gravity flows under conditions of intermittent and or poor oxygenation of bottom waters clinoform foreset deposits dip paleoseaward at 2 and consist of siltstones and sandy siltstones that are moderately to completely bioturbated by a high diversity trace fossil assemblage facies association c thin

3 d seismic velocity structure of the crust and the uppermost mantle in the northeastern japan arc 3 d seismic velocity structure of the crust and the uppermost mantle beneath the northeastern japan arc is investigated by using arrival time data from local earthquakes we use a velocity model composed of three layers corresponding to the upper crust the lower crust and the uppermost mantle respectively taking into account the observed regional variation of pn velocity the uppermost mantle is further divided into three parts by two pn velocity boundaries near the coasts of the japan sea and the pacific ocean the velocities within the upper crust the lower crust and the three parts of the uppermost mantle are assumed to be constant locations of two pn velocity boundaries and depth distributions of the conrad and the moho discontinuities are expressed by continuous functions of unknown parameters station corrections and hypocenters are also introduced to be unknowns these unknown parameters are determined by inverting arrival time data of p and s wave first arrivals and clear later arrivals of pg and p waves the pn velocity boundary between the land and the pacific ocean is located approximately along the pacific coastline and that between the land and the japan sea is nearly along the japan sea coastline the conrad and the moho discontinuities are at depths ranging from 14 to 20 km and from 27 to 36 km respectively the conrad and the moho depths have similar spatial distributions they are deep beneath the land and become shallower toward the western and the eastern coastlines beneath the land they are shallow in the central part of the tohoku district and become deeper toward both the north and the south directions in the north they become shallow again

geological evidence for intra jurassic faulting in the wessex basin and its margins geological observations on jurassic outcrops close to major faults in the wessex basin mendip area reveal the local presence of ammonite and brachiopod bearing sediments penetrating underlying strata toarcian and bajocian neptunian dykes and particularly sills are associated with the eypemouth fault and bajocian sills with the bride fault and mere fault in the mendip area numerous neptunian dykes of hettangian sinemurian pliensbachian and bajocian ages cross cutting carboniferous limestone are recorded typically also associated with major basement faults e g cranmore and leighton faults these periods of assumed sediment injection are taken as indicating times of displacement along the faults in question variations in facies hettangian sinemurian toarcian bajocian uppermost oxfordian kimmeridgian spatially linked to faults are documented from some areas and boreholes reveal considerable fault controlled thickness changes in hettangian sinemurian bajocian and kimmeridgian sediments the timing of jurassic faulting in the wessex basin mendip area thus polarizes into two intervals hettangian bajocian and latest oxfordian onwards correlating with the early rifting phases of the central and north atlantic respectively

fluvial and marine controls on combined subaerial and subaqueous delta progradation morphodynamic modeling of compound clinoform development 1 fluviodeltaic systems commonly display a compound clinoform geometry that consists of a subaerial subaqueous delta couplet the extent of subaqueous delta development varies significantly and in modern systems is a function of fluvial input and basin hydrodynamics we present a model of fluviodeltaic progradation in which the repeated occurrence of characteristic terrestrial floods and large coastal storms drives fluvial and shallow marine morphodynamics respectively we couple fluvial and shallow marine sediment dynamics via the surf zone which we collapse to a shock condition and treat as a moving boundary with steady sediment supply and sea level and simple basin geometry our model naturally develops prograding deltas with compound clinoform geometries the subaerial delta grows via fluvial aggradation and shoreface progradation whereas the subaqueous delta expands through foreset progradation with only minor topset aggradation the interplay of fluvial input with the wave current field controls the basic partitioning of sediment between subaerial and subaqueous deltas and by extension the compound clinoform geometry increasing the frequency or magnitude of coastal storms decreasing flood frequency or discharge and reducing grain size all increase the fraction of sediment delivered to the shallow marine environment and the extent of subaqueous delta progradation relative to subaerial delta development our model which emphasizes the intrinsic coupling of fluvial and shallow marine sediment dynamics and downplays the importance of allogenic fluctuations can explain many of the first order morphologic features of natural delta systems including significant lateral separation of the shoreline and clinoform rollover and differing rates of subaerial and subaqueous delta progradation

experiments on oscillatory flow and combined flow bed forms implications for interpreting parts of the shallow marine sedimentary record abstract analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm like oscillatory and combined flow experiments were carried out in a large wave tunnel and used a range of sand sizes fine and very fine wave periods 10 5 and 8 s oscillatory velocities 0 to 125 cm s and unidirectional velocities 0 to 25 cm s at low unidirectional velocities 10 cm s addition of an increasing collinear oscillatory flow caused the bed to evolve from small scale wavelength 100 cm symmetric orbital ripples to plane bed at higher unidirectional velocities 10 cm s a similar trend was noted but ripples were more asymmetric three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions these diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent distinctive features of small scale asymmetric combined flow ripples generated are a 3d planform round crest and convex up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a boxy appearance similarly large scale asymmetric combined flow ripples had broad and round crests convex up stoss sides and compressed profiles due to scouring at the toe of the stoss side hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed form scale and symmetry hummocks were more prevalent at longer oscillatory periods and in finer grained sediment stratification produced by synthetically aggrading hummocky bed profiles closely resembles hummocky cross stratification with the introduction of only a small unidirectional flow component hummocks evolve into downstream migrating large scale asymmetric ripples and the resultant cross stratification becomes similar to that produced by unidirectional flow dunes accordingly these experiments suggest that much of the hummocky cross stratification observed in the stratigraphic record is produced by storm generated long period oscillatory dominant combined flows inasmuch as long period high energy waves require deep wide basins to form hummocky cross stratification may therefore serve as a useful indicator of deposition in unrestricted open water conditions

mechanisms for forming discontinuity surfaces within shoreface shelf parasequences sea level sediment supply or wave regime abstract a 2d process response model was used to evaluate potential mechanisms involved in the formation of small scale stratigraphic variability that is observed within progradational wave dominated shoreface shelf parasequences model experiments which are based on scenarios that consist of a 25 ky progradational phase suggest that discontinuity surfaces form when either sediment supply or sea level fall rapidly or when wave height regime increases over relatively short time periods 102 104 y during coastal progradation in these cases mean deposition rate near the lower shoreface drops which combined with coastal progradation results in a discontinuity surface both observed and simulated discontinuity surfaces form predominantly in lower shoreface deposits and bound meter scale stratigraphic cycles simulations show that wave regime variability affects the shoreface shelf to a much greater depth than minor sea level change therefore facies shifts along wave climate induced bounding surfaces occur over wider reaches of the shoreface shelf system than sea level induced facies shifts high frequency variability in sediment supply rate results in the formation of discontinuity surfaces across which there are no distinct facies shifts

crustal structure beneath northeast india inferred from receiver function modeling abstract we estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast india by using h vp vs stacking method and a non linear direct search approach neighbourhood algorithm na technique followed by joint inversion of rayleigh wave group velocity and receiver function calculated from teleseismic earthquakes data results show significant variations of thickness shear velocities vs and vp vs ratio in the crust of the study region the inverted shear wave velocity models show crustal thickness variations of 32 36 km in shillong plateau north 36 40 in assam valley and 44 km in lesser himalaya south average vp vs ratio in shillong plateau is less 1 73 1 77 compared to assam valley and lesser himalaya 1 80 average crustal shear velocity beneath the study region varies from 3 4 to 3 5 km s sediment structure beneath shillong plateau and assam valley shows 1 2 km thick sediment layer with low vs 2 5 2 9 km s and high vp vs ratio 1 8 2 1 while it is observed to be of greater thickness 4 km with similar vs and high vp vs 2 5 in rup lesser himalaya both shillong plateau and assam valley show thick upper and middle crust 10 20 km and thin 4 9 km lower crust average vp vs ratio in assam valley and shillong plateau suggest that the crust is felsic to intermediate and intermediate to mafic beneath shillong plateau and assam valley respectively results show that lower crust rocks beneath the shillong plateau and assam valley lies between mafic granulite and mafic garnet granulite

pop up tectonics of the shillong plateau in the great 1897 earthquake ms 8 7 insights from the gravity in conjunction with the recent seismological results 1 it was reported that a hidden 110 km long south dipping fault named oldham fault was responsible for the 1897 great earthquake ms 8 7 in the shillong plateau by reverse faulting due to pop up tectonics of the plateau in northeast india here we report the results of our geophysical investigations where we critically examine the crustal structure of the plateau on the basis of gravity modeling and attempt to shed light on the hitherto debated hidden oldham fault at the northern boundary of the shillong plateau our gravity model constrained by broadband seismological data suggests that the moho beneath the shillong plateau is at a shallower depth of about 35 km when compared to the bengal basin to the south and the assam valley to the north which is about 42 km thinning of the crust under the plateau may be a consequence of the pop up mechanism to examine the possible pop up of the plateau during the great 1897 earthquake we have estimated the energy released by this earthquake and compared it with the energy required for the pop up of the plateau using a simplistic rigid model it is found that the shillong plateau between the oldham fault and dauki dapsi thrust would require an energy of 4 5 1019 j which does match well with the energy released by the great earthquake of ms 8 7

clinoform development by advection diffusion of suspended sediment modeling and comparison to natural systems clinoforms are the building blocks of prograding stratigraphic sequences these sig moid shaped surfaces can be found forming today on modem deltas sedimentation rate profiles over the clinoform surface of these deltas show low rates of sediment accumulation on both topset and bottomset regions with a maximum accumulation rate on the upper foreset region we pres ent a model for the formation of clinoforms that relies on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river model clino form surfaces are generated using an equation for the conservation of suspended sediment concentration together with a conservation of fluid equation for simple time averaged flow velocity fields in the model suspended sediment is advected horizontally into a basin and gravitational settling of sediment particles is counteracted by vertical turbulent diffusion in shallow water shear stresses are too large to allow deposition and sediment bypasses the topset region with increasing water depth near bed shear stresses decrease and sediment is allowed to deposit at the foreset region with gradually decreasing rates toward deeper water this sedimentation pattern leads to progradation of the clinoform surfaces through time the clinoform surfaces produced by the model capture the fundamental morphological characteristics of natural clinoforms these include the gradual slope rollover at the topset and bottomset steeper foreset slopes with increased grain size and an increase in foreset slope through time as clinoforms prograde into deeper water because the parameters controlling the model clinoforms have a direct relation to physical quantities that can be measured in natural systems the model is an important step toward unraveling the physical processes associated with these deposits

the deltaic nature of amazon shelf sedimentation despite the annual discharge of more than a billion tons of sediment by the amazon river the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta the river mouth however is not an estuary either these observations raise a major question as to what type of sedimentary environment the amazon river mouth represents seismic stratigraphy has been examined on the continental shelf at the mouth of the amazon river using high frequency 3 5 khz seismic records from about 6 000 km of ship track these records demonstrate three regions 1 the amazon river has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth the feature is prograding seaward and accreting upward and it contains fine scale stratification typical of classic deltas the feature forming at the mouth of the amazon is a subaqueous delta it differs from classic deltas primarily in its lack of subaerial expression subaqueous deltas such as the amazon represent the general case of a major river entering an energetic oceanic regime

interpreting the style of faulting and paleoseismicity associated with the 1897 shillong northeast india earthquake implications for regional tectonism 1 the 1897 shillong assam northeast india earthquake is considered to be one of the largest in the modern history although oldham s 1899 classic memoir on this event opened new vistas in observational seismology many questions on its style of faulting remain unresolved most previous studies considered this as a detachment earthquake that occurred on a gently north dipping fault extending from the himalayan front a recent model proposed an alternate geometry governed by high angle faults to the north and south of the plateau and it suggested that the 1897 earthquake occurred on a south dipping reverse fault coinciding with the northern plateau margin in this paper we explore the available database together with the coseismic observations from the region to further understand the nature of faulting the geophysical and geological data examined in this paper conform to a south dipping structure but its location is inferred to be in the brahmaputra basin further north of the present plateau front our analyses of paleoseismic data suggest a 1200 year interval between the 1897 event and its predecessor and we identify the northern boundary fault as a major seismic source the shillong plateau bounded by major faults behaves as an independent tectonic entity with its own style of faulting seismic productivity and hazard potential distinct from the himalayan thrust front a point that provides fresh insight into the regional geodynamics

microseismicity and tectonics in northeast india abstract three microearthquake surveys were carried out in shillong plateau mikir hills and assam valley areas during 1984 to 1986 some 422 events are relocated by the homogeneous station method the microseismicity map reveals intense crustal 10 to 40 km activity beneath the tura area of shillong plateau the areas to the west of shillong and the area around mikir hills also show high activity the microseismicity map further shows major tectonic lineaments that are compatible with the landsat imagery lineaments and the major faults composite focal mechanisms of the microearthquakes show spatial variation of the tectonic stresses in the region an ene wsw horizontal compressive stress is dominant in the tura area whereas a se nw horizontal compressive stress is dominant in the shillong and mikir hills areas

seismic stratigraphic analysis of the middle jurassic krossfjord and fensfjord formations troll oil and gas field northern north sea abstract the syn rift middle jurassic krossfjord and fensfjord formations troll field northern north sea contain a complex distribution of wave and tide dominated deltaic shoreline and shelf depositional environments of varying reservoir potential uncertainty exists in depositional models used to explain the spatial and temporal distribution of these depositional environments and the absence of coeval coastal plain deposits to date the proposed influence of growing rift related structures on stratigraphic architectures and sedimentation patterns in the units has been poorly defined in this study 3d seismic data are integrated with core biostratigraphic and wireline log data to produce a consistent geological interpretation for the formations seismic analysis has identified nine parasequences containing nne ssw striking delta scale clinoforms that prograded westwards over much of the field quantitative analysis highlights an increase in height and dip of clinoforms from proximal to distal locations coincident with an increase in grain size clinoform geometry is sigmoidal with well developed topsets that based on core data lack subaerial deposits these geometrical and sedimentological characteristics suggest that a subaqueous delta depositional system deposited the krossfjord and fensfjord formations in the troll field in the northeast of the field clinoforms exhibit highly variable strike and oblique cross sectional geometries which suggest that sediment was supplied from here and then redistributed through southward directed wave and longshore current activity rift related faulting is recognised to have occurred in the western part of the troll field only during deposition of the youngest fensfjord formation parasequence thus challenging the notion that these units are syn rift seismically imaged clinoforms in the under explored area south of the troll field prograded southward and are interpreted to represent coeval formation of a southward prograding spit that developed landward of the subaqueous delta platform present in the troll field the interpreted long lived subaqueous delta in the troll field constitutes a novel type of shallow marine reservoir

birdseyes fenestrae shrinkage pores and loferites a reevaluation abstract birdseyes birdseye limestone fenestrae fenestral fabric shrinkage pores and loferites are considered similar if not synonymous when occurring in lime mudstone or syndepositional dolomite especially in association with mudcracks and stromatolites compaction experiments indicate however that in the absence of early cementation these rugs can be completely obliterated whereas mudcracks and stromatolites remain relatively unchanged lumping of burrows and root tubes under the general terms birdseyes or fenestrae such as for example tubular fenestrae burrow fenestrae or root tube fenestrae is discouraged they should be called burrows or root tubes because the birdseyes fenestrae and shrinkage pores are so intimately associated with tidal flats in the minds of most geologists submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the bahama banks has resulted in rugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and assoc ated sedimentary structures

distribution paleoenvironmental implications and stratigraphic architecture of paleosols in lower permian continental deposits of western kansas u s a abstract cumulic paleosols in core from the lower permian asselian sakmarian council grove group of western kansas contain a diverse suite of pedogenic features including carbonate nodules gleying fossil burrows rhizoliths peds and cutans abundant gleying burrowing by organisms with a high soil moisture requirement and extensive plant growth are evidence of at least intermittently high soil moisture during this time paleosol character was strongly influenced by regional topography and paleogeographic position as well as the climate changes accompanying hierarchical glacioeustatic cycles the distribution of pedogenic features across formations also indicates a shift toward more humid conditions during a larger 3rd order regression climatic variability during the interval of soil formation resulted in overprinting and close stratigraphic juxtaposition of both humid and arid climate indicators variability is likely the result of exposure to multiple 5th order eustatic and climatic cycles during the lowstands of 4th order cycles fifth order cycles are expressed as lithologic changes in stratigraphically equivalent outcrops across the state but lack of accommodation space prevented cycles from being expressed as differing lithologies in the study area biologic activity assisted in consolidation and stabilization of eolian and alluvially deposited sediments this study emphasizes that detailed quantitative study of pedogenic features can reveal important paleoclimatic and paleoenvironmental information in otherwise lithologically similar paleosols close juxtaposition of opposing climatic indicators over intervals of known sea level change adds evidence to the hypothesis of a link between sea level change and climate change in the early permian pedogenic features thus support a semiarid or wetter climate for the midcontinent during the early permian

estimation of crustal discontinuities from reflected seismic waves recorded at shillong and mikir hills plateau northeast india in this study an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the shillong mikir hills plateau in northeast india region the principle of the technique is to relate seismic travel times with crustal thickness above the conrad and moho discontinuities broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities the results show that the conrad discontinuity is at 18 20 0 5 km beneath the shillong mikir hills plateau and the moho discontinuity is at 30 1 0 km beneath the shillong plateau and at 35 1 0 km beneath the mikir hills

the origin and recognition of laterally continuous carbonate cemented horizons in the upper lias sands of southern england abstract the upper lias sands of southern england contain numerous laterally extensive carbonate cemented horizons petrographical analyses of samples from outcrop sections and the marchwood no 1 borehole indicate that these horizons result from preferential cementation of bioclast rich clay poor sediments by comparison with interbedded clay rich bioclast poor sediments the alternation of the two sediment types is attributed to the effects of respectively fairweather and storm processes on a submerged marine shoal petrographical and ichnological data indicate an early distinction of the strongly and weakly cemented horizons the widespread extent of the cemented horizons as indicated by outcrop studies on the dorset coast is considered to be a direct consequence of episodic storm activity on the low relief shoal sedimentological palynological and petrophysical criteria are presented to assist in recognition of similar extensive cements in subsurface reservoir horizons that do not outcrop

upper crustal structure of the rio grande rift near socorro new mexico from inversion of microearthquake s wave reflections abstract the existence of impulsive s to s wave s z s reflections off an extensive sillshaped magma body on many local microearthquake seismograms near socorro new mexico allowed a straight forward least squares inversion to determine an s wave velocity crustal model the data consisted of 163 microearthquakes recorded by a small aperture array during 316 recording days from may 1975 to january 1977 two least squares inversion models were attempted m1 was used to calculate the overall average s wave velocity for approximately 19 km of the crust and depths to 36 separate horizontal rectangular surfaces representing the upper surface of the magma body due to lack of resolution in the data the next model was simplified m2 had as its initial unknown parameters an average s wave velocity and depth to the reflector m2 was then used to calculate the velocity of a lower 9 2 km thick crustal layer starting just above the magma body the data was then separated into different groups to search for local heterogeneities and anisotropy in the lower half of the upper crust the best average s wave velocity for the rio grande rift near socorro was found to be 3 41 0 03 km sec depth to a horizontal magma body based on a velocity of 3 41 km sec was 19 2 0 6 km the data do allow a maximum northward dip of 2 which places the magma body parallel to the moho the lower 9 2 km of the crust above the magma body appears to have a 3 per cent higher velocity than the upper layer and on the average there are no differences between east west and north south s wave velocities one segment of the crust west of socorro appears to have an anomalously low velocity

three dimensional seismic structure beneath shillong plateau and assam valley northeast india abstract northeast india is bounded by the himalayan arc to the north and the burmese arc to the east the shillong plateau and assam valley lie at the boundary zone of the two arcs three microearthquake surveys were conducted in this area from 1984 to 1986 we have applied a tomographic method to about 2800 high quality p and s wave arrival times from 364 local earthquakes that were recorded in the magnitude range 2 0 to 4 0 by 22 temporary seismic stations during the surveys to determine the 3d velocity structure of the crust and upper mantle in this region the result reveals significant lateral heterogeneities in the study area the tomographic images obtained in this study are compatible with the major tectonic features such as active faults and seismicity trends

crustal p wave velocity and velocity ratio study in northeast india by a microearthquake survey the upper crustal 20 km p wave velocity beneath the shillong plateau and nowgong area has been studied by the time distance plot method thep arrival data of the shallow 20 km microearthquakes from three temporary networks are used and the average velocity is found to be 5 55 km s the velocity ratio v p v s for the upper crust 0 20 km as well as for the lower crust 21 40 km are determined by the wadati plot method and station by station method the average value obtained by the two methods is compatible thev p v s ranges between 1 74 to 1 76 a generalized seismic velocity model of the area is suggested by this study which has been very useful for microearthquake location

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

phanerozoic ironstones an introduction and review s u m m a r y ironstones particularly ooidal ironstones have long fascinated sedimentary geologists and have generated an enormous variety of interpretations but have remained poorly understood a recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences the first part of this introduction examines recent advances and attempts to produce a synthesis of them with particular reference to the genesis of marine ooidal ironstones current models for the formation of marine oolitic ironstone formations are discussed and a working model based on various lines of recent research is proposed this model invokes the intrasedimentary formation of berthierine ooids in marine environments during postoxic diagenesis the ooids may be mechanically modified and mineralogically transformed by reworking as well as by later diagenesis goethite ooids may be oxidized berthierine ooids reworked lateritic ooids or primary grains the formation of oolitic ironstoneformations is favoured by a break in clastic sediment supply continued supply of iron and physical reworking many sedimentary environments may supply these parameters but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply in the second part of this introduction the terminology and classification are discussed and conventions proposed the term berthieroid is introduced as a non specific term for material of undetermined berthierine or chamosite composition the term ironstone is proposed as a petrological term and as an informal lithostratigraphic term a deposit composed of ironstones may be termed an ironstone formation it is recommended that a dunham style of classification is employed petrographically with ironstone replacing the stone of the original carbonate classification the nomenclature of allochems in ironstones is also discussed a glossary of ironstone terminology is provided

three dimensional modeling of a shoreface shelf parasequence reservoir analog part 2 geologic controls on fluid flow and hydrocarbon recovery wave dominated shoreface shelf parasequences are commonly modeled as simple layer cake reservoirs yet analysis of modern and ancient analogs demonstrates that these intervals contain a more complex physical stratigraphy we investigate the impact of depositional and diagenetic heterogeneity associated with gently dipping clinoform surfaces on fluid flow and recovery during water flooding using a three dimensional model reconstructed from a well exposed outcrop analog we demonstrate that the volume of oil in place is affected by variations in facies thickness associated with interfingering along clinoforms whereas waterflood sweep efficiency is affected by barriers to flow along clinoform surfaces such as calcite cemented layers mudstones and siltstones sweep efficiency is low when water flooding is down depositional dip because oil is bypassed at the toe of each clinothem as water flows preferentially through high quality sandstone facies in the upper part of the parasequence sweep efficiency is higher when water flooding is up depositional dip because the gravity driven downward flow of water sweeps poorer quality sandstone facies in the lower part of the parasequence in both cases injectors may offer limited pressure support to producers water flooding along depositional strike yields good pressure support but poor sweep because the gravity driven flow of water into the lower part of the parasequence is significantly reduced this yields highly variable fluid saturations but a uniform pressure gradient which is consistent with pressure and fluid saturation data from the mature rannoch formation reservoir brent field united kingdom north sea simple layer cake models fail to capture the range of flow behaviors described above and overpredict recovery by up to 20 as a result

quantification of macroscopic subaerial exposure features in carbonate rocks abstract the macroscopic features that characterize subaerial exposure surfaces in carbonates are well known but their significance has not been quantitatively evaluated this study presents such an analysis in the lower oligocene suwannee limestone of west central florida eleven cores were point counted on a foot by foot basis for the abundance of caliche rhizoliths karst breccia open vugs infiltrated sediment fractures and pedogenic alteration these features occur at and below intraformational exposure surfaces which represent hiatuses estimated at 104 to 105 years and an uppermost sequence bounding unconformity representing 0 5 myr as revealed by sr isotope data statistical analyses of the point count data reveal only a few significant relationships 1 the hierarchy of exposure surfaces and by inference duration of exposure is differentiated only at a marginally significant level by sediment filled vugs preferentially associated with the sequence boundary duration of exposure did not have a significant impact on the relative abundance of all other features 2 proximity 5 ft to both types of surfaces fractures open vugs and infiltrated sediment show no proximal or distal preference for either type of surface 3 depositional texture has no statistically significant affect on the presence or abundance of the exposure features with the exception that rhizoliths and open vugs are preferentially more abundant in packstones relative to grainstones this is interpreted to be the result of a soil moisture effect factor analysis defines four factors that explain 46 to 52 of the total variance in the abundance data relative to the sequence boundary and the intraformational surfaces respectively the loading of each exposure feature on each factor is the same with respect to both types of surfaces which is further evidence that the abundance of exposure features is independent of duration of exposure factor 1 is interpreted to be the amplitude of base level changes and controls the abundance of karst breccia factor 2 is interpreted to be abundance of vegetation and relates to the abundance of rhizoliths and fractures factor 3 is interpreted to be a combination of soil zone pco2 and the availability of water and affects the abundance of pedogenic overprinting caliche and open vugs factor 4 is stratigraphic proximity to the sequence boundary which controls the presence of sediment filled voids the amount of uncorrelated unique variance associated with infiltrated sediments pedogenic overprinting caliche and open vugs is large 60 meaning that feature abundance is also influenced by other unidentified site specific factors these results demonstrate that quantifying the abundance of macroscopic subaerial exposure features in limestones has the potential to yield more insight into the significance of those features than a mere qualitative assessment this is particularly true when assessing the potential role of the many variables that can affect the development of these features

paleosols and tidal flat lagoon sequences on a carboniferous carbonate shelf sedimentary associations of triple disconformities abstract carbonate shelf sequences in the lower carboniferous of southwest britain contain thin peritidal intercalations within open shelf facies analysis of one such unit the caswell bay mudstone at miskin south wales shows it to contain a lower paleosol and an upper tidal flat lagoon sequence these are separated and bounded by erosion surfaces directly related to the origin of the units they underlie the complete sequence using letters for rock units and numbers for disconformities with interpretation in brackets is a oolite inner shelf shoal 1 irregular disconformity subaerial subsoil surface b oolite breccia conglomerate in clay matrix regolith with clay introduced as windblown or overbank material c mottled micrites calcrete 2 planar disconformity back barrie shoreface d peloid micrites e laminated micrites f paper shales and thin limestones all shallow subtidal lagoon low intertidal flat sediments 3 planar disconformity barrier shoreface g bioclastic limestone inner shelf open marine sands and gravels units b f total 6 m thickness and constitute the caswell bay mudstone they comprise a couplet of terrestrial and back barrier sediments preserved during a transgression probably because of the resistance of the calcrete and a moderate subsidence rate breaks 1 3 represent a subaerial surface plus two barrier associated erosion surfaces these three breaks are here termed a triple disconformity the subaerial surface is the major break of the three and the back barrier shorezone surface marks the base of the transgressive sequence the uppermost break the barrier shoreface is a relatively minor intra sequential erosion plane recognition of this sequence of deposition and erosional events clarifies the origin of the units involved provides an environmental framework for further biofacies and lithofacies analysis and indicates the relative significance of breaks that may be used for stratigraphic subdivision paleosol tidal flat lagoon couplets and triple disconformities should be widespread features of transgressive barrier island carbonate shoreline sequences