living on the edge evaluating the impact of stress factors on animal sediment interactions in subenvironments of a shelf margin delta the mayaro formation trinidad abstract integrating sedimentological and ichnological characteristics of the gelasian mayaro formation along the se coast of trinidad allows recognition of a river dominated and wave influenced shelf edge deltaic succession of the paleo orinoco river developed under strong slope instability four main sedimentary settings and twelve subenvironments developed on and beyond the outer shelf i e at the shelf margin have been identified extreme paleoenvironmental conditions make the succession rarely and sporadically colonized ichnologic evidence suggests that shelf edge deltas are among the most stressful marine environments due to a combination of physicochemical factors in response to the intrinsic sedimentary processes and the relative hierarchy of their influences specific to every subenvironment within any particular subenvironment the relative dominance of the fluvial feeder system the action of waves and rare influence of tides and slope instability determine the combinations and ranking of stress factors river dominated shelf edge delta subenvironments demonstrate the extreme influence of stress factors related to channel activity e g salinity changes high sedimentation and erosion rates flocculation of mud seasonal variations of fluvial flux whereas subenvironments dominated by waves storms and oceanic swells away from the axial feeder s are mostly influenced by barform morphodynamics and its effects on local physiography the paleo orinoco delta is represented not only by the delta lobe s developed at the shelf edge but by delta lobe s formed on the outer shelf as well outer shelf deltaic subenvironments manifest typical ichno sedimentological signatures of a normal wave influenced in this case inner shelf delta being susceptible to extensive gravitational instabilities the delta system suffered from canyonization and smaller gullying and subsequent filling n b hereafter we use the term canyon for both canyons and smaller gullies the shelf edge delta system was also associated with shelf attached mass transport systems such depositional subenvironments dominated by slope instability and sediment gravity flows appear to be the most unconducive for benthic colonization resulting in almost complete absence of bioturbation characterization of the stress factors as the functions of parameters arising from specific sedimentary and or oceanographic processes chemical conditions and preservational constraints specific to each subenvironment leads to the construction of a comprehensive ichnological and depositional analog model for shelf edge deltas in general and for the accommodation driven low latitude shelf edge deltas at an active oblique foreland setting in particular

slope and deep shelf gully sandstones upper jurassic east greenland the upper jurassic hareelv formation of jameson land east greenland occurs in an area of 60 75 km it consists of 200 500 m of black shale with thick closely spaced sandstone bodies the sandstones fill deep steep walled gullies and elongate scours or form more regular laterally extensive parallel sided but erosive gully mouth or lobe deposits both types of sandstone bodies occur completely juxtaposed and systematic vertical or lateral trends in bed thickness or grain size have not been observed the sands were deposited in a deep water shelf basin by high density turbidity currents traveling from basin margin source areas a northeastern source area was represented by a short lived rapidly prograding delta whereas the main northwestern source area was a shallow sandy barrier occurring along the length of a major north northeast striking fault controlled slope voluminous turbidity currents probably were triggered by earthquakes in the fault zone the resulting slope and basinal sand bodies are up to 50 m thick and hundreds of meters wide and may be more than 5 km long in a downcurrent direction they occur in a thick rich oil prone source rock thus they may form potential stratigraphic reservoirs and help drain the source rock the hareelv formation shows important similarities to the ramp facies model for delta fed sand rich turbidite systems however the formation is mainly tectonically controlled and independent of eustatic sea level changes the hareelv formation may serve as a model for an unusual type of stratigraphic hydr carbon reservoir and at least one north sea oil field seems to have formed in an analogous setting

depositional architecture of growth fault related wave dominated shelf edge deltas of the oligocene frio formation in corpus christi bay texas abstract growth faults within the frio formation define six subbasins on the south texas shelf and add to the complexity of the sediment dispersal along the shelf margin these growth faults influenced sediment pathways controlled sediment partition and provided increased accommodation for deltaic depocenters rollover and thickening of sediments occur on the downthrown side of growth faults with offsets of at least 150 m and up to 750 m growth strata are indicated by intervals that thicken landward by several tens of meters from anticline crests individual 4th order 100 500 ky regressive cycles expand about five to seven times across growth faults expansion ratios as great as 10 are observed the shelf edge is associated with the largest displacement expansion ratios and thickness of prograding deltaic sequences sedimentary structures and trace fossil associations identified in cores from nueces encinal and red fish bay subbasins indicate deposition on a wave dominated deltaic shoreline the cores exhibit highly bioturbated muddy sandstones alternating with decimeter thick clean sandstones with hummocky cross stratification alternation of fair weather wave deposits with sandy storm beds in two successive fault zones suggests that all subbasins developed under similar conditions during growth fault development high sediment supply conditions favored accumulation of thicker sediments on the downthrown sides of the faults wave dominance of deltas on the outer shelves in the growth compartments suggest that building of the upper slope rather lowers slope margin and decreases the likelihood of the presence of sandy deep water fans correlation of about 700 well logs integrated with seismic data provided the areal distribution and relative timing of sediment filling of the growth faulted compartments to unravel the evolution of higher order fourth and fifth sequences affected by these growth faults core data were used to recognize depositional environments to calibrate lithology to well logs and to characterize seismic amplitude anomalies within this shoreface succession the clean sandy storm beds will make attractive hydrocarbon reservoirs especially on the crest of rollover anticlines in downthrown compartments these sandstone bodies pinch out seaward into open marine shelf mudstones and are sealed by overlying shelf mudstones deposited during transgressive episodes

the thermal conductivity for granite with various water contents the thermal conductivities of granite were measured under different conditions of porosity and water content to investigate the effects of the porosity and water content on the thermal conductivity for the dry samples the thermal conductivities range from 2 12 w mk for the rocks with a high porosity to 3 12 w mk for the ones with a low porosity water sorbed samples have greater thermal conductivities than dry samples of the same granite the thermal conductivities range from 2 99 w mk for granites with a high porosity to 3 62 w mk for ones with a low porosity under saturated condition

transition from storm wave dominated outer shelf to gullied upper slope the mid pliocene orinoco shelf margin south trinidad shelf edge deltas are a key depositional environment for accreting sediment onto shelf margin clinoforms the moruga formation part of the palaeo orinoco shelf margin sedimentary prism of south east trinidad provides new insight into the incremental growth of a pliocene storm wave dominated shelf margin relatively little is known about the mechanisms of sand bypass from the shelf break area of margins and in particular from storm wave dominated margins which are generally characterized by drifting of sand along strike until meeting a canyon or channel the studied st hilaire siltstone and trinity hills sandstone succession is 260 m thick and demonstrates a continuous transition from gullied with turbidites uppermost slope upward to storm wave dominated delta front on the outermost shelf the basal upper slope deposits are dominantly mass transport deposited blocks as well as associated turbidites and debrites with common soft sediment deformed strata the overlying uppermost slope succession exhibits a spectacular set of gullies which are separated by abundant slump scar unconformities tops of rotational slides then filled with debris flow conglomerates and sandy turbidite beds with interbedded mudstones the top of the study succession on the outer shelf area contains repeated upward coarsening sandstone rich parasequences 2 to 15 m thick with abundant hummocky and swaley cross stratification clear evidence of storm swell and storm wave dominated conditions the observations suggest reconstruction of the unstable shelf margin as follows i the aggradational storm wave dominated shelf edge delta front became unstable and collapsed down the slope ii the excavated scars of the shelf margin became gullied but gradually healed aggraded by repeated infilling by debris flows and turbidites and then new gullying and further infilling and iii renewed storm wave dominated delta front prograded out across the healed outer shelf re establishing the newly stabilized shelf margin the moruga formation study along with only a few others in the literature confirms the sediment bypass ability of storm wave dominated reaches of shelf edges despite river dominated deltas being by far the most efficient shelf edge regime for sediment bypass at the shelf break this article is protected by copyright all rights reserved

evolution and stratigraphic architecture of marine slope gully complexes monterey formation miocene gaviota beach california abstract three small headlands in the sea cliffs west of gaviota beach california are the remnant fill of three discrete submarine gullies incised into the late miocene submarine slope environment these promontories provide excellent three dimensional exposure of the gully fill in outcrop permitting documentation of their complex internal stratigraphic architecture detailed study of these exposures elucidates the sedimentologic processes that occur in the filling of slope gullies guides interpretation of the acoustic records of otherwise unsampled modern gully systems on continental slopes and provides insight into the heterogeneity that may characterize slope gully petroleum reservoirs we develop a comprehensive facies scheme to describe the variability within these intercalated coarse and fine grained deposits and use two dimensional photopans to interpret the overall depositional system defined by internal bedforms sedimentation units and sediment size each facies records sedimentation under different hydrodynamic conditions and can be genetically related to a discrete depositional mechanism we differentiate channel axis margin and overbank sub environments within the gully fill which together define overall crudely braided low sinuosity channels within the gullies like many modern gully systems the gaviota gullies probably initiated through local slope oversteepening that led to slope failure slumping and initiation of sediment flows erosion and sedimentation from these high density turbidity currents formed the primary depositional process for the gaviota conglomerate units once initiated the gullies acted as sediment conduits from the shelf and possibly onshore regions despite their prevalence on modern upper slopes and their pivotal role in shaping shelf margins and transporting sediment to deeper water the details of submarine slope gully formation and filling remain obscure the gaviota gully complexes provide valuable insights into long term gully fill attributes not easily obtained from modern slope gully systems and rarely preserved in the rock record

rapid subsidence and preservation of fluvial signals in an otherwise wave reworked delta front succession early mid pliocene orinoco continental margin growth se trinidad abstract outcrops of the early mid pliocene orinoco delta succession on the atlantic facing trinidad continental margin demonstrate facies and stratigraphic architecture of a shelf margin developed in a setting of high sediment supply orinoco river and unusually rapid sediment accumulation subsidence averaging 1 km my the studied moruga clastic wedge of the orinoco margin consists of a series of topset to deepwater clinoforms with channelized sandstones river and mixed process delta front wave dominated delta front shoreface prodelta offshore transition shelf upper slope turbidite channels and mass transport deposits the succession is sand prone and generally consists of coarsening and thickening upward delta front lobes with hummocky swaley and wave ripple stratified sandstones indicating dominant storm wave reworking of the delta particularly when the delta lobes had reached outer shelf sites after long shelf transits the lower parts of distributary channels are well preserved and characterized by cross bedded to parallel laminated sandstones with mud clast lags the upper parts of channel fills were wave reworked during transgressions creating hummocky swaley cross stratified strata river dominated and mixed process wave and tide delta front deposits occur commonly within stacked coarsening upward units the river dominated delta front deposits exhibit parallel laminated to current ripple laminated intervals with normally graded thin beds changing upward to parallel and low angle cross lamination but mixed process delta front deposits also contain wave and bidirectional current ripples as well as discrete intervals of hummocky swaley cross stratification the preservation of the channel base as well as river and mixed process delta front deposits in the otherwise storm wave dominated atlantic facing setting suggests some sheltering of the coastal morphology and possibly low frequency of large storms but it is more likely that the unusually rapid subsidence at rates competing well with icehouse rates of eustatic sea level change and burial of deposits may have prevented the fluvial and tidal signals from being completely reworked by the storm waves in the studied outcrop area this study demonstrates an example of how the interactions of river tide and wave generated facies impacted stratigraphic architecture of a rapid subsiding shelf margin built by shelf edge deltas with high sediment supply

hummocky cross stratification like structures in deep sea turbidites upper cretaceous basque basins western pyrenees france hummocky cross stratification is a sedimentary structure which is widely interpreted as the sedimentary record of an oscillatory current generated by energetic storm waves remobilizing surface sediment on the continental shelf sedimentary structures named hummocky cross stratification like structures similar to true hummocky cross stratification have been observed in the turonian senonian basque flysch basin south west france the bathymetry 1000 to 1500 m suggests that the observed sedimentary structures do not result from a hydrodynamic process similar to those acting on a continental shelf the morphology of these three dimensional structures shares similarities with the morphology of hummocky cross stratification despite a smaller size the lateral extent of these structures ranges from a few decimetres to many decimetres they consist of convex up domes hummock and concave up swales with a non erosive base four types of hummocky cross stratification like geometries are described they occur in association with structures such as climbing current ripple lamination and synsedimentary deformations in the basque flysch hummocky cross stratification like structures are only found in the tc interval of the bouma sequence hummocky cross stratification like structures are sporadic in the stratigraphic series and observed only in few turbidite beds or bed packages this observation suggests that hummocky cross stratification like structures are linked genetically to the turbidity current but form under a very restricted range of parameters these structures sometimes show an up current upslope migration trend antidunes in the described examples they could result from standing waves forming at the upper flow interface because of kelvin helmholtz instability

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

genesis and diagnostic value of soft sediment deformation structures a review abstract soft sediment deformation structures are recognized as important diagnostic features in the rock record for determination of depositional environments and slope processes the diagnostic value of these structures is reevaluated by analysis of the parameters controlling sediment deformation soft sediment deformation is contemporaneous with deposition and occurs dominantly in course silt to fine sand the high depositional rate low permeability and low shear strength of grains within this sediment range maximize the occurrence of deformation the dominant mechanisms responsible for sediment deformation include 1 liquefaction or fluidization 2 reverse density gradation 3 slumping or slope failure and 4 shear stress in most cases a combination of these mechanisms occurs the processes function in a continuum producing features that are microscopic to megascopic in scale it is shown that the processes and thus the structures are not environment specific the true diagnostic value of the structures may be in defining hydrodynamic conditions and in interpreting paleocurrents and paleoclimatic and paleoseismic events ultimately for the best diagnostic results soft sediment deformation structures should be studied in association with all other available lithologic structural and paleontological information

contourite drift construction influenced by capture of mediterranean outflow water deep sea current by the portimao submarine canyon gulf of cadiz south portugal abstract the margin of the gulf of cadiz is swept by the deep current formed by the mediterranean outflow water mow flowing from the mediterranean to the atlantic on the northern margin of the gulf algarve margin south portugal the mow intensity is low and fine grained contourite drifts are built up with an alongslope development from new sedimentological data this study emphasizes the presence of two types of contourite drifts separated only by a deep submarine canyon incising the slope with a north south orientation portimao canyon high resolution seismic and bathymetry interpretation shows that on the eastern side of the canyon the mow forms a thick and large detached drift albufeira drift prograding toward both north and west as shown in seismic profiles with a high sedimentation rate on this side of the canyon the mow intensity is high enough to erode the slope forming a moat channel alvarez cabral on the western side of portimao canyon the mow energy is lower preventing moat channel erosion only flat and thin drift develops portimao and lagos drifts with slow aggradation and a low sedimentation rate this difference in drift development is due to the presence of the canyon which generates an important change in hydrodynamic of the mow confirmed by temperature density measurements showing that mow flows down portimao canyon the canyon is responsible for the deviation of the direction of the mow as it partly catches the deep sea current flowing westward i e capture phenomenon it creates thus a decrease of the flow energy competency and capacity between the east and west sides of the canyon through this phenomenon of mow deep sea current capture the canyon constitutes a morphologic feature generating an important change in the contourite deposition pattern in addition to already known climatic and oceanographic influences our results show the role of canyons on contourite drift building this study provides new elements on autocyclic factors influencing the contourite sedimentation which are important to consider in future sedimentary paleo reconstruction interpretations

upper slope to shelf edge delta architecture miocene cruse formation orinoco shelf margin trinidad trinidad is an uplifted segment of the neogene orinoco shelf created largely by the paleo orinoco river and delta the atlantic facing shelf margin sediment prism has an internal clinoform architecture with both sandy marine and nonmarine topsets and muddy deepwater slope deposits that contain turbidite channels and collapsed shelf edge blocks the shelf prism is 10 km thick and 200 km wide at present built from late miocene to present the shelf margin had an irregularly rising trajectory towards the atlantic with very thick topset aggradation and rapid progradation of the fronting deepwater slope the 18 33 km my progradation rate for the shelf margin with exceptionally high shelf subsidence rates up to 1000 m my documents a high sediment supply despite very high shelf subsidence and strongly suggests prominent sand bypass from shelf into deepwater areas for much of the neogene interval the early proximal onshore south trinidad part of this sediment prism the late miocene and early pliocene cruse formation irregularly outcrops for tens of kilometers along southern trinidad in a disrupted but near downdip shelf to basin direction from west to east this distribution allows the late miocene shelf break position in the paleo columbus channel to be identified in the outcrops separating a western shelf and shelf edge delta segment from an eastern highly deformed segment with very large house size blocks of shallow water facies collapsed from the shelf edge that are disoriented and embedded in deformed slope mudstones these eastward distorted outcrops are interpreted as within the headward reaches of a columbus canyon system otherwise known from seismic interpretation offshore in the columbus channel in this eastward outcrop area below the shelf edge the facies are mainly large shelf edge collapse mass transport blocks as well as in situ turbidite filled slope channels thin bedded turbidites debrites and abundant slope mudstones most of the sandstone blocks and associated chaotic beds contain highly deformed parallel laminated and hummocky cross strata betraying their former shelf location landward of the identified shelf edge location there are stacked parasequences each 3 15 m thick of undeformed upward coarsening shelf edge delta deposits in places sharply truncating erosional truncation the slope mudstones and mass transport deposits the significant downcutting of most of the topset orinoco channels their position so close to the shelf edge as well as the generally erosional contact between base of topsets and underlying slope mudstones suggests that the topset deltas were forced regressive probably driven across the shelf by falling relative sea level despite high subsidence rates at this time the outcropping facies architectures both vertically and laterally from shelf to deepwater slope generate a hypothesis that the early paleo orinoco shelf margin grew by an alternation of upward and basinward growing clinoforms particularly marked is the earliest pliocene period of marked forced regression of the orinoco delta possibly when short periods of icehouse eustatic sea level fall outpaced subsidence rate this lesson from the outcrop area is used to further understand the flat rise flat rise shelf edge trajectory pattern that persisted throughout the neogene margin development as seen on seismic lines across the columbus basin

triassic jurassic silica undersaturated and silica saturated alkalic intrusions in the cordillera of british columbia implications for arc magmatism alkalic igneous rocks of early mesozoic age are found in both the quesnel and stikine terranes in the canadian cordillera and include both silica undersaturated and silica saturated types the saturated complexes are most abundant in quesnellia and are multiphase complexes dominated by monzonite to diorite intrusions undersaturated complexes are distributed through both terranes are dominated by syenite with lesser monzonite and pyroxenite and when present as a single intrusion are characterized by concentric zoning igneous layering and planar mineral fabrics both types of complex are associated with cu au mineralization accompanied by potassic and distinctive sodic and calc potassic alteration assemblages although undersaturated and saturated alkalic intrusions are petrographically distinct a petrogenetic association is suggested by their spatial coincidence in some districts and similarities in their tectonic environment and associated alteration the undersaturated complexes represent a distinctive suite of alkalic intrusion with magmatic arc affinities and their emplacement into both stikinia and quesnellia between 210 and 200 ma suggests that these terranes were either linked at that time or have shared unusual but similar magma generating tectonic events at identical times

morphological changes and sedimentary processes induced by the december 2003 flood event at the present mouth of the grand rhone river southern france the study which is based on repetitive bathymetric surveys assesses changes and effects of one of the most important floods recorded in the rhone delta area bottom morphology and sediment distribution in the rhone outlet after the december 2003 flood are discussed by comparison between digital terrain maps dtms of november 2003 and january 2004 the post flood morphology shows that the whole of the system is active mainly in the east the channel of the rhone has been hollowed out on the left bank by more than 5 m the eastern coast of the mouth has retreated 400 m the mouth bar has prograded 200 m and the slope of the delta front has increased by 0 19 the overlay of pre and post flood dtms makes it possible to estimate the total volume deposited in the 0 to 20 m zone as 7 8 10 the acquisition of bathymetric measurements immediately before and after a major flood allows more than a simple morphological description in addition an analysis of the solid load transfer processes towards the prodelta is achievable three mechanisms are highlighted most of the material eroded in the channel and supplied to the delta front corresponds to fine sediment that drape homogeneously over the pre flood morphology of the delta front the transit of the coarsest sediment primarily sand is slowed down in the channel of the rhone this sediment builds up in relatively small areas leading to the formation of gullies on the prodelta slope the gullies have fixed positions and disappear gradually towards the west following the progressive migration of the active band towards the east these old features are not reactivated by floods occurring after their formation during the flood only a small volume of coarse sediment bypasses through the gullies and the fine deposits are remobilised rapidly or compacted flood input concerns essentially the delta front which traps 90 of the fluvial solid discharge between 0 and 20 m depth the coarse prodelta supply is then due mainly to mass movements of unconsolidated material deposited beforehand on the top of the delta front consequently the main nourishment of the prodelta does not occur directly during and or immediately after a flood event but is delayed by sediment being temporarily trapped on the delta front

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

evolution of continental slope gullies on the northern california margin abstract a series of subparallel downslope trending gullies on the northern california continental slope is revealed on high resolution seismic reflection profiles imaging the uppermost 50 m of sediment the gullies are typically 0 m wide and have 1 to 3 m of relief they extend for 10 to 15 km down the slope and merge into larger channels that feed the trinity canyon in the lower half of the 50 m stratigraphic section the gullies increase in both relief and number up section to maxima at a surface 5 to 10 m below the last glacial maximum lowstand surface gully relief increased as interfluves aggraded more rapidly than thalwegs erosion is not evident in the gully bottoms therefore gully growth was probably due to reduced sediment deposition within the gullies relative to that on interfluves as the gullies increased in relief their heads extended upslope toward the shelfbreak at all times a minimum of 10 km of non gullied upper slope and shelf stretched between the heads of the gullies and the paleo shoreline the gullies did not connect with a subaerial drainage network at any time gully growth occurred when the gully heads were in relatively shallow water 200 m paleo water depth and were closest to potential sediment sources we suggest that prior to the last glacial maximum the mad river then within 10 km of the gully heads supplied sediment to the upper slope which fed downslope eroding sediment flows these flows removed sediment from nearly parallel gullies at a rate slightly slower than sediment accumulation from the eel river 40 km to the south the process or processes responsible for gully growth and maintenance prior to the last glacial maximum effectively ceased following the lowstand when sea level rose and gully heads lay in deeper water 300 m water depth farther from potential sediment sources during sea level highstand the mad river is separated from the gully heads by a shelf 30 km wide and no longer feeds sediment flows down the gullies which fill with sediment from the distal eel river approximately one half of the subsurface gullies have no expression on the seafloor because they have completely filled with sediment following the last glacial maximum lowstand of sea level

detrital zircon geochronology and regional correlation of metasedimentary rocks in the coast mountains southeastern alaska u pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the coast mountains in southeastern alaska these rocks belong to the port houghton assemblage which consists of upper paleozoic pelitic and psammitic schist metaconglomerate metabasalt and marble the port houghton assemblage rests unconformably on metamorphosed and deformed mid paleozoic arc type volcanics endicott arm assemblage which gradationally overlie upper proterozoic lower paleozoic continental margin strata tracy arm assemblage three main clusters of ages are present 330 365 ma 19 grains 1710 2000 ma 27 grains and 2450 2680 ma 6 grains additional grains are approximately 2334 2364 and 3324 ma comparison of these ages with detrital zircon ages in other cordilleran assemblages supports previous interpretations that metasedimentary rocks in the coast mountains i form a southwestern continuation of the yukon tanana terrane of eastern alaska and yukon

eocene clinoform growth in front of a storm wave dominated shelf central basin spitsbergen no significant sand delivery to deepwater areas abstract eocene shelf slope clinoforms on spitsbergen reflect asymmetric basin infill across the coastal plain shelf slope and deepwater segments of a foreland basin although four main types of clinoforms are recognized in the basin this study focuses on clinoforms generated by a storm wave regime on the shelf margin the key aspect of this type of clinoform type 3 is that only modest volumes of sand were delivered across the shelf edge onto the slope and little or no sand onto the basin floor despite the sand prone nature of the sediment delivery system right to the shelf edge the sedimentology and geometry of a series of such clinoforms are documented along a continuously exposed outcrop transect some 800 m high and 4 km long sandstone units of individual clinoforms are 2 12 m thick and are separated by thicker shale clinothems have subhorizontal topset and bottomset segments and steeper mid portions and are up to 150 m high nearly an order of magnitude higher than the deltas and shorefaces that constructed them this clinoformal configuration is thus interpreted to reflect shelf shelf edge slope and base of slope depositional environments and water depths at least as great as the decompacted clinoform amplitudes exposed shelf segments dip up to 0 4 degrees whereas slope segments some 2150 m wide dip up to 4 degrees the shelf and upper slope reaches of clinothems consist of hummocky and swaly cross stratified sandstone units without any significant interbedded shale on the middle slope and below these sandstone units become finer grained are ripple and plane parallel laminated and separated with interbedded shale individual topsets of clinothems are interpreted to have formed by strandplain or wave dominated delta progradation across the morphological shelf platform during short periods of sea level stillstand or slight fall the shale units immediately above sandy clinothems developed during longer intervals of relative sea level rise when the wave dominated shoreline retreated back across the shelf platform the long term trajectory of shelf margin growth shows a significant aggradational component during overall progradation compared to other clinoforms in this same basin where the sediment delivery system was fluvially driven and brought great volumes of sand onto the basin floor and slope respectively storm wave generated clinoforms brought only limited sand to the upper slope and little or no sand to the basin floor

the nicola group late triassic and early jurassic subduction related volcanism in british columbia mafic lavas of the nicola group are divided into three distinct petrographic and geochemical types type 1 lavas are strongly augite porphyritic picrites basalts and andesites that belong to a high potassium to shoshonitic rock series type 2 lavas are augite and plagioclase porphyritic basalts and andesites that belong to a low potassium calc alkaline series and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites low concentrations of ti zr y and nb and moderate to high concentrations of k rb ba and sr in type 1 and 2 lavas clearly indicate a subduction related tectonic setting of eruption type 3 lavas show chemical affinities intermediate between modern day island arc and intraplate volcanics type 1 shoshonitic lavas generally lie east of and are younger than type 2 calc alkaline lavas a relationship that implies an east dipping early mesozoic subduction zone beneath the nicola arc these interpretations resolve previous uncertainties regarding th

cache creek terrane entrapment oroclinal paradox within the canadian cordillera exotic and far traveled oceanic crustal rocks of the cache creek terrane cc are bordered by less exotic quesnel qn and stikine st arc terranes to the east north and west all of these terranes are enveloped by an arcuate belt of displaced continental margin rocks the kootenay ko nisling ns and parts of the yukon tanana ytt terranes that have indirect ties to ancestral north america na initial 87sr 86sr isopleths conform to this arcuate pattern such a pattern of concentric belts presents a geological conundrum how did the qn st and cc come to be virtually enveloped by terranes with ties to na past and current models that explain assembly of the canadian cordillera are deficient in their treatment of this problem we propose that early mesozoic qn and st were joined through their northern ends as two adjacent arc festoons that faced south toward the cache creek ocean panthalassa oceanic plateau remnants within the cc today were transported from the tethyan realm and collided with these arcs during subduction of the cache creek ocean counterclockwise oroclinal rotation of st and ns terranes in the late triassic to early jurassic caused enclosure of the cc rotation continued until these terranes collided with qn in the middle jurassic paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for permian to early jurassic sites in st but moderate clockwise rotations for sites in qn specific modern analogues for the cordilleran orocline include the yap trench where the caroline rise is colliding end on with the mariana arc and the banda arc located on the southeastern tail of the asian plate which is being deformed into a tight loop by interactions with the australian and pacific plates

ubiquity of wave dominated deltas in outer shelf growth faulted compartments abstract growth faults and deltaic coastlines are common features along basin margins with a high flux of sediment delivery it has been observed in previous studies that wave dominated deltas are the characteristic and long lived coastline type in such growth faulted extensional basins during deltaic progradation the structural and depositional systems migrate and adjust as sedimentation rates keep pace with fault displacement rates a numerical model indicates that increased subsidence along growth faults slows the progradation rate of deltas as new accommodation is being created the decrease in forward delta growth results in a longer period of wave reworking per unit of sediment due to the retention of large fetch and water depth since subsidence is maintained through time sediments tend to accumulate on delta topsets creating thick wave dominated delta front successions on the downthrown sides of faults the proposed model may guide subsurface interpretations where it is difficult to assess the shoreline type without cores it is proposed here to use values of expansion index derived from seismic reflection profiles or well log correlations as a proxy for the likelihood of wave dominance on the delta shoreline

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

experiments on bed configurations in fine sands under bidirectional purely oscillatory flow and the origin of hummocky cross stratification abstract we made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods t and maximum orbital velocities um in two closed flow ducts 0 1 m and 0 4 m wide the motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits the ripples produced were classified geometrically as two dimensional 2d or three dimensional 3d in plan view geometry and dynamically as oscillatory current ripples dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself and reversing current ripples dependent on he existence of the current in one direction or the other during the oscillation runs in the small duct with effectively 0 19 mm and 0 30 mm sand showed the familiar small 2d oscillatory current ripples at short t and low um passing into much larger ripples with spacing up to 2 m at periods of 6 20 s and velocities up to 1 m s with no apparent limit on size with increasing t and um even in such a narrow duct there was some evidence of 3d geometry of the large ripples runs in the large duct with effectively 0 11 mm sand at a single period of about 9 5 s showed a progression from small 2d oscillatory current ripples through small 3d reversing current ripples to large round crested 3d oscillatory current ripples with prominent superimposed 3d to 2d reversing current ripples the large 3d ripples which increased in spacing to 1 3 m at the h ghest attainable um of 1 m s shifted in position and changed in size with time seemingly at random causing substantial local and temporary erosion and deposition at a given point on the bed there was no net aggradation during the runs but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3d ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium and then plotting the profiles one above the previous with a constant upward increment the resulting stratification is strikingly similar to much hummocky cross stratification in ancient sandstones this suggests that some hummocky cross stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3d oscillatory current bed forms develop from a planar bed during strong but waning flow

the role of slump scars in slope channel initiation a case study from the miocene jatiluhur formation in the bogor trough west java abstract this paper investigates slope channel initiation by seabed irregularities that were initially formed by slump scars in the lower to middle jatiluhur formation part of the middle to late miocene successions in the bogor trough west java this miocene succession is up to 1000 m thick in the study area and is interpreted as a prograding slope shelf system that formed during a period of falling and lowstand stages in relative sea level the lower part of the formation is a siltstone dominated siliciclastic succession containing slump deposits slump scar fill deposits and minor channel fill deposits which formed in slope and shelf margin environments in contrast the middle part which gradationally overlies the lower part is characterized by shallow marine carbonates the slump scars fill deposits have an overall lenticular geometry and are 140 480 m wide and 0 4 1 6 m thick some have distinct erosional bases which cut into the underlying siltstones in association with medium to coarse grained sandstones with lateral accretion surfaces and tractional structures common in channel fill deposits the incident link of slump scar fill deposits and channel fill deposits in the prograding slope shelf succession of the lower to middle jatiluhur formation suggests that some slump scars formed incipient seabed irregularities that may have played an important role in the development of slope channels the present study provides one example of the various potential mechanisms that can result in channel formation in a slope setting

ice temperature and high geothermal flux at siple dome west antarctica from borehole measurements a verlical temperature profile through the west antarctic ice sheet wais at the summit of siple dome reveals an elevated geothermal flux this could be the root cause for the existence of a dynamic ice stream system in the wais siple dome is still frozen on its bed but adjacent ice streams have bed temperatures at the pressure melting point of ice although present day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice indirect ellbcts like a rapid disintegration of the floating ice shelves or additional melt water input at the surface could give rise to speed up of the ice streams with an ensuing rise in sea level ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration within a relatively short period of time of at least part of the wais

anatomy of collapsed and re established delta front in lower cretaceous of eastern spitsbergen gravitational sliding and sedimentation processes spectacular coastal cliff exposures in an early cretaceous deltaic succession near kvalvagen in eastern spitsbergen norwegian arctic show a variety of styles of gravity driven deformation and resedimentation associated with delta front progradation and instability the large scale collapse involved a distributary channel sand unit some 20 m thick and the underlying delta slope prodelta shaly heterolithics the deformation was essentially brittle whereby large blocks of the delta front succession slid and rotated its gross style involved two large cuspate tear away scars at least 1 1 5 km wide and 50 60 m deep separated by narrow remnant buttresses of in situ strata the basement in the region was tectonically active and the delta front failure was most likely sei mically triggered initial infill of the collapse scars involved slumps and debris flows derived from scar walls and locally from the sliding blocks themselves these deposits partly smoothed the local topography created by the slide blocks this early infill was followed by deposition of thin mainly muddy turbidites probably shed from the scar walls by small localized failure renewal of fluvial supply of sand led to the main infill of the scar depressions this stage involved effluent generated turbidites heterolithic and increasingly sandier turbidites and liquefied sandflows derived from unstable oversteepened distributary mouthbars the mouth bar systems eventually achieved stability and prograded across the infilled scars they were affected by periodic storm wave attack and sudden increa es of water depth the scar infill at that stage was subject to minor growth faulting possibly related to new seismic tremors

high frequency stacking pattern and stages of canyon gully evolution across a forced regressive shelf edge delta front abstract the succession of varying facies tracts filling within a canyon or a deep incised gully cutting across the delta front lobes at the shelf edge requires to be explained by high frequency allocyclic and or autocyclic events a fifth and sixth order sequence stratigraphic model for the shelf edge canyon fill or gully fill is here proposed for the first time the first gelasian cold phase and the renewal of transpressional uplift of basin margin occurring at the same time enforced regressive transit of the inner shelf paleo orinoco delta onto the paleo shelf edge its delta front developed along the shoulders of the major successive basin bounding growth faults the delivery of clastic sediments into the deeper slope and beyond in the columbus basin was through the gullies and canyons that incised and cannibalized across the shelf edge delta the mayaro formation of southeast trinidad furnishes the role of growth faulting during the first phase of delta front development at the paleo shelf edge thereby it serves as an example of several fifth order sequences within a fourth order equivalent episode which is contemporaneous to both the glacio eustatic episode the pretiglian and the renewal of transpressional uplift after a pause during the pliocene thus large incision and subsequent filling within the mayaro formation cover a passage of time fifth order equivalent that is unequivocally shorter than the span of the falling pretiglian eustatic sea level and second renewed phase of transpression tectonism the categorization of sedimentary facies of the canyon fill or gully fill and the encasing delta front outside the canyon gully cut into a series of sixth order genetic facies tracts shows variations of geomorphic stages of the canyon gully evolution from accommodation creation to the stages of filling these stages therefore should be attributed to the sixth order equivalent autogenic changes influenced by high frequency growth tectonics instead of assuming the stages to be related to the relatively lower frequency glacio eustatic allocyclicity of the earliest gelasian

chronostratigraphy and tectonostratigraphy of the columbus basin eastern offshore trinidad the columbus basin forming the easternmost part of the eastern venezuela basin is situated along the obliquely converging margins of the caribbean and south american plates the two primary structural elements that characterize the basin are 1 transpressional northeast southwest trending anticlines and 2 northwest southeast oriented down to the northeast extension normal faults the basin was filled throughout the pliocene and pleistocene by more than 40 000 ft 12 200 m of clastic sediment supplied primarily by the paleo orinoco delta system the delta prograded eastward over a storm influenced and current influenced shelf during the pliocene pleistocene depositing marine and terrestrial clastic megasequences as a series of prograding wedges atop a lower pliocene to pre pliocene mobile shale facies biostratigraphic and well log data from 41 wells were integrated with thousands of kilometers of interpreted two dimensional and three dimensional seismic data to construct a chronostratigraphic framework for the basin as a result several observations were made regarding the basin s geology that have a bearing on exploration risk and success 1 megasequences wedge bidirectionally 2 consideration of hydrocarbon system risk across any area requires looking at these sequences as complete paleofeatures 3 reservoir location is influenced by structural elements in the basin 4 the lower limit of a good quality reservoir in any megasequence deepens the closer it comes to the normal fault bounding the wedge in a proximal location 5 reservoir quality of deep marine strata is strongly influenced by both the type of shelf system developed bypass or aggradational and the location of both subaerial and submarine highs and 6 submarine surfaces of erosion partition the megasequences and influence hydrostatic pressure migration and trapping of hydrocarbons and the distribution of hydrocarbon type begin page 1906

thermal and depositional history of early permian rio bonito formation of southern parana basin brazil abstract the parana basin is a large volcano sedimentary basin with a complex depositional history from ordovician to the cretaceous period during permian significant amounts of organic rich beds accumulated within the rio bonito and irati formations representing deposition in coastal deltaic and barrier lagoon and shallow marine environments respectively in this work the thermal history of these beds is examined from 16 vitrinite reflectance analyses of coal beds rio bonito formation integrated with previous data on coal rank and 21 apatite fission track aft analyses collected from 17 well profiles in the south of the parana basin coal samples showed three different maturity levels two with reflectances of 0 4 0 6 and 0 6 0 8 rr formed by the natural burial history of the basin the latter is restricted to the torres syncline area in contrast the third group reached much higher values 1 0 5 0 rr related to intrusive igneous rocks the aft data show thermal consistency with the reflectance values the sandstone samples related to the two lower reflectance patterns were partially reset preserving aft ages older than the stratigraphic age of the bed otherwise the samples severely affected by the magmatism have cretaceous and early cenozoic ages but some were much younger than the last magmatic event implying deep burial before cooling onset the mean track length distribution of the partially reset samples is short due to the inheritance of tracks preserved from basement cooling history however the population of fully reset samples gives crucial information on the maximum temperature reached and the age when cooling started at these conditions around 1 0 to 3 0 km of the parana basin sequences would have been removed from the surface of the studied areas by erosion since cretaceous regional variations in the amounts of removed sections were controlled by the structural framework mostly regional nw se and ne sw oriented structures

on the frequency distribution of turbidite thickness the frequency distribution of turbidite thickness records information on flow hydrodynamics initial sediment volumes and source migration and is an important component of petroleum reservoir models however the nature of this thickness distribution is currently uncertain with log normal or negative exponential frequency distributions and power law cumulative frequency distributions having been proposed by different authors a detailed analysis of the miocene marnoso arenacea formation of the italian apennines shows that turbidite bed thickness and sand interval thickness within each bed have a frequency distribution comprising the sum of a series of log normal frequency distributions these strata were deposited predominantly in a basin plain setting and bed amalgamation is relatively rare beds or sand intervals truncated by erosion were excluded from this analysis each log normal frequency distribution characterizes bed or sand interval thickness for a given basal grain size or basal bouma division measurements from the silurian aberystwyth grits in wales the cretaceous great valley sequence in california and the permian karoo basin in south africa show that this conclusion holds for sequences of disparate age and variable location the median thickness of these log normal distributions is positively correlated with basal grain size the power law exponent relating the basal grain size and median thickness is different for turbidites with a basal a or b division and those with only c d and e divisions these two types of turbidite have been termed thin bedded and thick bedded by previous workers a change in the power law exponent is proposed to be related to i a transition from viscous to inertial settling of sediment grains and ii hindered settling at high sediment concentrations the bimodal thickness distribution of thin bedded and thick bedded turbidites noted by previous workers is explained as the result of a change in the power law exponent this analysis supports the view that a and b divisions were deposited from high concentration flow components and that distinct grain size modes undergo different depositional processes summation of log normal frequency distributions for thin and thick bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power law trend thus the occurrence of both log normal and segmented power law frequency distributions can be explained in a holistic fashion power law frequency distributions of turbidite thickness have previously been linked to power law distributions of earthquake magnitude or volumes of submarine slope failure the log normal distribution for a given grain size class observed in this study suggests an alternative view that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters in addition to the settling velocity of the grain sizes present

regional geologic and tectonic setting of the maracaibo supergiant basin western venezuela this special issue contains eight topical studies on the structure stratigraphy and petroleum system of the maracaibo basin a supergiant basin in western venezuela most of the work reported in this special issue is the product of thesis related research by master s and doctoral level students at the jackson school of geosciences of the university of texas at austin during a collaborative relationship with the venezuelan national oil company petrleos de venezuela s a that was initiated in the late 1980s this introductory article presents a regional overview of the tectonic setting and geology of the maracaibo basin with a cumulative oil production of more than 30 billion bbl since the first production well was drilled in 1914 and estimated ultimate oil reserves of more than 44 billion bbl the maracaibo basin is the most prolific hydrocarbon basin in the western hemisphere unlike the more extensive gulf of mexico giant hydrocarbon provinces the relatively small size 50 000 km2 19 305 mi2 relative simplicity in its structure and stratigraphy and wealth of surface and subsurface data make the maracaibo basin an attractive target for basinwide synthesis the objective of this article is to present a regional compilation of two dimensional 2 d and three dimensional 3 d seismic data wells and outcrop data at a basinwide scale to reveal the basin s 3 d structure and stratigraphy moreover we show regional tectonic reconstructions regional geologic maps and basin subsidence history to better constrain four major tectonic events that affected the basin and that are critical for understanding the timing and distribution of major unconformities and clastic wedges the distribution of the reservoir rocks the reactivation of older fault trends and the timing of maturation for underlying source rocks many of these topics are discussed in greater detail in the other eight articles in this special issue

how did thin submarine debris flows carry boulder sized intraclasts for remarkable distances across low gradients to the far reaches of the mississippi fan submarine density flows dominate sediment transport into many parts of the ocean and form submarine fans which are some of the largest sediment accumulations on earth previous studies often assumed that the distal fringes of submarine fans would be dominated by extensive sheet like deposits from dilute and expanded turbidity currents because only turbidity currents would transport large volumes of sediment for long distances across such low gradients deposits with a remarkable frondlike shape occur at the furthest fringe of the mississippi submarine fan understanding the emplacement process of these deposits is important because their frondlike shape suggests deposition from debris flows rather than turbidity currents previous analyses concluded that these deposits comprise a complex arrangement of thin interbedded turbidity current and debris flow deposits here we propose a different internal geometry for the frondlike deposits intervals previously described as in place turbidites are interpreted to be clasts within a single debrite interval that is 1 to 2 m thick this debrite interval is underlain by clean sand the debrite interval could have been emplaced by multiple debris flows or one event comprising multiple pulses clasts in the debrite interval are much bigger than previously thought including boulders with diameters that can exceed 50 cm we show that thin 2 m and extremely fluidal debris flows could have carried these boulders across gradients of only 0 06 to the fringes of one of the world s largest submarine fans without hydroplaning this long transport distance could occur if the density of the clasts was less than that of the surrounding debris flow emplacement of the basal clean sand layer appears closely linked to debris flow deposition because the clean sand layer pinches out in a location similar to that of the overlying debrite we therefore suggest that the basal clean sand most likely settled out from the overlying debris flows and that the clean sand is not a fore running turbidity current deposit shearing at the base of a mud rich debris flow would most likely produce a muddy basal sand interval rather than the clean sand that is observed this basal sand interval is therefore less likely to be a record of a lubricating layer with high pore fluid pressure on which the debris flows moved this study shows how thin and highly mobile debris flows can redistribute large volumes of sediment including bouldersized clasts long distances into the deep ocean across remarkably low gradients

effect of sea level variation on upper slope depositional processes offshore of tiber delta tyrrhenian sea italy abstract the upper slope and outer shelf in front of the tiber river mouth east central tyrrhenian sea exhibits a series of about 15 gullies that are not clearly related to present day erosional or depositional processes an extensive high resolution seismic reflection profiling survey of a 450 km 2 area in front of the tiber delta shows several generations of similar features within the older depositional sequences underlying the outer continental shelf the gully relief appears dominantly depositional probably developed during lowstand periods when the tiber river mouth was relatively near the shelf break the position of the gullied intervals including those on the modern slope relict from the last lowstand shows a successive northward shift with time this northward shift indicates continued tilting of this part of the eastern tyrrhenian continental margin probably resulting from a continued subsidence to the north

sm nd isotopic geochemistry of precambrian to paleozoic granitoid suites and the deep crustal structure of the southeast margin of the newfoundland appalachians in the eastern central mobile belt of the newfoundland appalachians late precambrian basement inliers have end from 3 to 2 but cambro ordovician metasedimentary rocks have initial end below 7

tectonics basin subsidence mechanisms and paleogeography of the caribbean south american plate boundary zone abstract using a mega regional dataset that includes over 20 000 km of on and offshore 2d seismic lines and 12 wells we illustrate three different stages of fault formation and basin evolution in the caribbean arc south american continent collisional zone transpressional deformation associated with oblique collision of the caribbean arc migrates diachronously over a distance of 1500 km from western venezuela in paleogene time 57 ma to a zone of active deformation in the eastern offshore trinidad area each diachronous stage of pre syn and post collisional basin formation is accompanied by distinct patterns of fault families we use subsidence histories from wells to link patterns of long term basinal subsidence to periods of activity of the fault families stage one of arc continent collision initial collision is characterized by overthrusting of the south and southeastward facing caribbean arc and forearc terranes onto the northward subducting mesozoic passive margin of northern south america northward flexure of the south american craton produces a foreland basin between the thrust front and the downward flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south as the collision extends eastward towards trinidad this same process continues with progressively younger foreland basins formed to the east on the overthrusting caribbean arc and forearc terranes north south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward thrusting arc terranes combined with slab pull of the underlying and subducting north dipping south american slab uplift of fold thrust belts arc continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins stage two this late stage of arc continent collision is characterized by termination of deformation in one segment of the fold thrust belt as convergent deformation shifts eastward rebound of the collisional belt is produced as the north dipping subducted oceanic crust breaks off from the passive margin inducing inversion of preexisting normal faults as arc continent convergence reaches a maximum strain partitioning also begins to play an important role as oblique convergence continues accommodating deformation by the formation of parallel strike slip fault zones and backthrusting southward subduction of the caribbean plate beneath the south caribbean deformed belt as subsidence slows in the foreland basins sedimentation transitions from a marine underfilled basin to an overfilled continental basin offshore sedimentation is mostly marine sourced by the collided caribbean terranes localized islands and carbonate deposition stage three this final stage of arc continent collision is characterized by 1 complete slab breakoff of the northward dipping south american slab 2 east west extension of the caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half grabens 3 continued strain partitioning strike slip faulting and folding the subsidence pattern in the caribbean basins is more complex than interpreted before showing a succession of extensional and inversion events the three tectonic stages closely control the structural styles and traps source rock distribution and stratigraphic traps for the abundant hydrocarbon resources of the on and offshore areas of venezuela and trinidad

cache creek ocean closure or enclosure exotic tethyan faunas within the cache creek terrane contrast markedly with faunas and lithologic associations in the adjacent quesnel and stikine terranes in northern british columbia and southeast yukon all three terranes are enveloped in the north by pericontinental rocks of the yukon tanana terrane a geometry that imposes severe constraints on terrane assembly models for the northern canadian cordillera our solution to the problem invokes a northern join between the stikinia and quesnellia arcs through the yukon tanana terrane forming an orocline that encloses the cache creek terrane this model involves 1 collision of a linear oceanic plateau at the cusp between quesnellia and stikinia 2 anticlockwise rotation of stikinia about an axis in the yukon tanana terrane 3 simultaneous enclosure of the cache creek ocean and 4 emplacement of quesnellia onto the margin of ancestral north america and the cache creek terrane onto stikinia during final closure of the orocline early mesozoic paleomagnetic declinations in stikinia are permissive of the large anticlockwise rotations predicted by the model similar large scale rotations and ocean basin enclosure are common features in the southwest pacific this model accounts for paleozoic and younger linkages between yukon tanana and both northern stikinia and quesnellia the striking similarity between triassic jurassic arcs east and west of the cache creek terrane and the profound early mesozoic deformational event in the yukon tanana terrane

facies in a shelf edge delta an example from the subsurface of the gulf of mexico middle pliocene mississippi canyon block 109 two vertically stacked shelf edge delta systems in the middle pliocene of mississippi canyon block 109 have been described using a three dimensional seismic survey 400 ft 122 m of conventional core and electric logs possible characteristic facies associations and features of these shelf edge deltas include extensive upper delta slope deformation locally ponded slump induced turbidites in the upper slope a complex association of gravity and tractional deposits relatively thin preserved mouth bar deposits and the absence of delta plain facies six facies have been identified in cores 1 structureless sands interpreted as high density turbidites 2 laminated sands 2 1 sharp based planar horizontal laminations interpreted as bouma b and bc division turbid tes 2 2 low angle discordant laminations interpreted as strong wave activity deposits 2 3 a complex mixture of lamination styles that may be a product of turbidity currents waves or possibly river fed underflows 3 thinly interbedded ripple laminated sands including 3 1 asymmetrical cross laminations interpreted as unidirectional traction current lamination and 3 2 c division turbidites or ripple lamination with mud drapes on foresets or with a symmetrical form interpreted as wave or possibly tidal current lamination 4 contorted muddy fine sands and silts interpreted as mud flows and slumps 5 laminated silts and muds interpreted as low energy wave deposits or possibly d division turbidites and 6 muds interpreted as quiet water deposits in the cored intervals o the deltas turbidite sands immediately overlie muddy slumps in the upper delta slope suggesting that retrogressive slope failure of developing slump structures tapped back into the mouth bar causing sandy turbidity currents to develop the upper delta is represented on seismic data as a series of prograding clinoforms approximately 400 ft 122 m thick electric logs indicate these clinoforms are composed of a lower mud dominated interval an upper delta slope dominated by slumps with areally restricted turbidite sands and coarsening and cleaning upward mouth bar deposits composed of ripple laminated and parallel laminated sands interbedded with muds and silts the lower delta contains similar vertical sequences but also contains a distinct log and seismic facies that may be either a major shelf edge failure or a later channel feature incised across the entire delta apart from this possible channel neither delta shows evidence of emergent delta plain facies

bed thickness and grain size of turbidites in measured sequences of limestone and greywacke turbidites the bed thickness is found to vary proportionally with the fall velocity of the maximum grain size found at the base of the bed a simple theoretical model based on the decay of isotropic turbulence suggests that bed thickness should be a function not only of this fall velocity but also of bottom slope flow depth and the concentration and grain size distribution of the sediment in the turbidity current the field data do show some influence of these additional factors nevertheless for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values thus grain size remains the primary variable and the modal regression curve of bed thickness on maximum grain size is well defined and resembles a fall velocity curve relatively steep basin floors near to source can theoretically cause these modal regressions for distal and very proximal parts of a turbidite to diverge introducing a crudely parabolic appearance in the form of the total regression curve the form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds such beds do occur they are different from the thin but relatively fine proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee forming processes

storm and tide dominated shorelines in cretaceous moosebar lower gates interval outcrop equivalents of deep basin gas trap in western canada the moosebar and gates formations crop out in the deformed foothills belt south of fort st john british columbia they permit examination of the depositional environments of sandstones and conglomerates that are gas bearing in the equivalent wilrich falher interval of the deep basin of alberta and british columbia the moosebar gates interval consists of several upward coarsening and marine to nonmarine sequences moosebar bioturbated shales pass upward into turbidites and offshore storm deposits flow directions indicate a north dipping paleoslope a second coarsening upward sequence at the top of the moosebar and base of the gates terminates in a lenticular fluvial conglomerate up to 30 m thick which is overlain by coals carbonaceous mudstones and sandstones above the carbonaceous zone a major lateral facies change is observed fluvial conglomerates in the south grade into beach conglomerates northward the upper part of the beach conglomerate consists of exposed three dimensional storm berms at least 500 m long up to 1 5 m high with wavelengths of 12 to 24 m the berms trend east west seaward of the conglomerate beach are herringbone cross bedded tidal sandstones with a marine trace fauna graded sharp based conglomerates also are present probably washed out to sea by storms farther north at the same stratigraphic level is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across the tops of these bars are covered with storm formed symmetric conglomeratic dunes details of the orientation and geometry of these conglomerate bodies along with their internal structure and pebble fabrics suggest that four major environments fluvial beach offshore conglomerate bars and offshore graded storm conglomerates should be identifiable and predictable in the subsurface

upper pleistocene turbidite sand beds and chaotic silt beds in the channelized distal outer fan lobes of the mississippi fan cores from a mississippi outer fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized turbidite beds of graded sand and debris flow beds of chaotic silt off lobe basin plains lack siliciclastic coarse grained beds the basin plain mud facies exhibit low acoustic backscatter on seamarc ia sidescan sonar images whereas high acoustic backscatter characteristic of the lobe sand and silt facies the depth of the first sand silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes i e shallowest sand highest backscatter and youngest sublobe the high proportion 50 of chaotic silt compared to graded sand in the distal outer fan sublobes may be related to the unstable muddy canyon wall source areas of the extensive mississippi delta fed basin slope a predominace of chaotic silt in cores or outcrops from outer fan lobes thus may predict similar settings for ancient fans

late triassic arc related potassic igneous rocks in the north american cordillera igneous rocks of late triassic age are widespread in the cordillera of western north america and except in wrangellia consist of subduction related plutonic and volcanic suites many of these including those in the stikinia quesnellia rattlesnake creek and jackson terrenes and in southern california are clinopyroxene rich and belong to high potassium and shoshonitic rock series features that are generally absent from older and younger igneous rocks in the same terranes the late triassic subduction related rocks are exposed in two discontinuous belts that lie east and west of the cache creek terrane in canada and correlative melange terranes farther south stratigraphic and structural data suggest that these belts were spatially separate magmatic arcs in late triassic time tectonic implications of this analysis include an explanation of middle jurassic cordilleran deformation as the result of collision of the western with the eastern belt absence of late triassic links between stikinia and quesnellia disassociation of stikinia with terranes in northwestern nevada and tentative correlation of the wallowa seven devils terrane with stikinia rather than wrangellia

upper triassic takla group volcanic rocks stikine terrane north central british columbia geochemistry petrogenesis and tectonic implications the upper triassic takla group volcano sedimentary assemblage is part of the stikine terrane of the intermontane belt in the canadian cordillera and covers an area of more than 30 000 km2 in a belt up to 50 km wide and more than 800 km long in the mcconnell creek area of north central british columbia the assemblage consists of plagioclase clinopyroxene phyric dominantly basaltic to andesitic flows and pyroclastic rocks interlayered with volcanogenic sedimentary rocks compositionally the volcanic rocks are intermediate between tholeiitic and calc alkaline their mantle normalized trace element patterns are characterized by a moderate large ion lithophile element enrichment and nb and ti depletion suggesting that magmatism occurred in a volcanic arc setting flat heavy rare earth element chondrite normalized patterns with la yb n ratios from 2 to 4 5 suggest that the parent magma was produced by mantle melting in the spinel stability field the low sr isotopic ratios 87sr 86sri approximately equa

sediment mass flow processes on a depositional lobe outer mississippi fan abstract seamarc 1a sidescan sonar imagery and cores from the distal reaches of a depositional lobe on the mississippi fan show that channelized mass flow was the dominant mechanism for transport of silt and sand during the formation of this part of the fan sediments in these flows were rapidly deposited once outside of their confining channels the depositional lobe is formed of a series of long narrow sublobes composed of thin bedded turbidites normally graded siliciclastic sand and silt 20 cm thick on average debris flow deposits soft clay clasts up to 5 cm in diameter in a siliciclastic silt matrix 48 cm thick on average and background sedimentation hemipelagic muds the mass flows most likely originated from slope failure at the head of the mississippi canyon or on the outer ontinental shelf and flowed approximately 500 km to the distal reaches of the fan with debris flow being the dominant flow type an analysis that uses the geometry of the confining channels and strength properties of the debris flow material shows that these thin debris flows could have traveled hundreds of kilometers on extremely small sea floor slopes at low velocities if the flowing medium behaved as bingham fluids and were steady state phenomena

turbidite variability and architecture of sand prone deep water slopes eocene clinoforms in the central basin spitsbergen abstract the architecture and turbidite variability within six wedge shaped downslope thinning sand prone slope accumulations are documented from eocene shelf margins on spitsbergen the central basin formed as a small foreland or piggy back basin and the studied turbidites accumulated mainly on the slope portion of sand prone clinoforms that developed during depocenter migration and infilling of the basin the shelf margin clinoforms have amplitudes minimum water depths from 100 to 350 meters and their shelf slope and basin floor segments are well imaged and can be walked out along many of the mountainsides only a small percentage of the clinoforms are sand prone and these developed when sea level occasionally fell to or below the shelf edge of the sand prone clinoforms some had their sand budget partitioned mainly out onto the basin floor basin floor fans but most trapped the sand on the slope only the latter are now visible as downslope thinning wedges some 2 5 3 5 km in downdip extent the turbidites within this type of clinoform have been examined and classified the lower slope to base of slope segment of the studied clinoform complexes are dominated by lobes consisting of broad shallow channels and sheet like turbidites becoming heterolithic and muddy out on the basin floor beds on the lower slope vary from thick up to 4 5 meters ungraded or laminated sandstones to thinner ungraded sandstones with coarse cappings the middle slope segment of clinoform complexes is dominated by narrow channels chutes that feed downslope to progradational chute mouth lobes chutes contain ungraded and laminated sandstone beds up to 3 m thick whereas the chute mouth lobes show alternations of thinner ungraded to laminated or rippled sandstones these lobes become more heterolithic and muddy downslope the shelf edge to upper slope segment of clinoform complexes is dominated by upward coarsening and thickening sheetsands of steep fronted shelf edge deltas the sandsheets of the delta front can be traced updip into mouth bar and distributary channel sandstones it is argued that shelf margin accretion represented by the sand prone slope wedges was achieved mainly by sand laden currents that flooded from the shelf edge as hyperpycnal flows this hypothesis is supported by 1 the direct connection between channel and mouth bar systems at the shelf edge and the turbidites of the slope lobes 2 the systematic progradational character of the slope lobes 3 the absence of large scale slump scars gullies or canyons on the slope and 4 the dominance of a type of turbidite that implies deposition from sustained flow detailed examination of the architecture of one of the slope wedges shows that there are unconformities developed within the stratigraphy below the shelf edge and that these erosional terraces beheaded the deltas perched on the uppermost slope the erosion surfaces indicate fall of sea level to this position despite the magnitude of this fall up to 80 meters the lack of canyons on the slope prevented the construction of basin floor fans such falls of sea level on non canyoned slopes simply promote sand prone accretion of the shelf margin

deposition of ungraded muds from high density non turbulent turbidity currents turbidity current deposits of mud have recently been found up to 10m in thickness resulting from single depositional events1 2 turbidites 1 5 m thick from the madeira abyssal plain3 4 display long segments with little vertical change of grain size5 6 this suggests that the currents froze in some way making them unable to sort sediment here we suggest that a decline in flow speed of these currents led to a concentration gradient in silt close to the bed that was sufficiently steep to stratify the flow a concentration gradient of 18 kg m 3 over 1 5 m height gives a layer richardson number above the critical value of 0 25 for damping of turbulence we propose that with turbulence severely damped a dense suspension of fine material consolidates like a static suspension while continuing to flow downslope the current changes from a fully turbulent flow to a decelerating flow with suppressed turbulence and increasing density in the latter state interparticle forces prevent differential settling of the coarser grains

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

grain fabric in hummocky cross stratified storm beds genetic implications abstract a detailed analysis of grain fabric in samples from hummocky cross stratified hcs storm beds suggests that these shallow marine sandstones were deposited from waning combined oscillatory unidirectional flows particle a axis orientations in the deposits in plan view parallel to visible lamination vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators sole marks and subparallel to regional shoreline this alignment of a axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline imbrication of particles varies in both the horizontally laminated and hcs intervals of the hcs stor beds due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow in the basal horizontally laminated interval of an hcs storm bed grain imbrication angles vary about a mean of approximately 13 from the horizontal dipping into the paleoflow inferred from sole marks this preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow however quasi cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally laminated interval in hcs intervals imbrication angles vary about a mean that is parallel to visible laminae suggesting th t oscillatory flow had become predominant the sequence of structures and changing patterns of fabric through these hcs storm beds is consistent with deposition under waning combined flows erosional scour forming the sharp base and unidirectional sole marks occurred in response to combined flows with an effective unidirectional component the unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally laminated sands but waned and became negligible relative to a strong oscillatory component during formation of hcs

thermal conductivity of water saturated rocks from the ktb pilot hole at temperatures of 25 to 300 c the conductivitites of selected gneiss two and amphibolite one core samples have been measured under conditions of elevated temperature and pressure with a needle probe water saturated thermal conductivity measurements spanning temperatures from 25 to 300 c and hydrostatic pressures of 0 1 and 34 mpa confirm the general decrease in conductivity with increasing temperature but deviate significantly from results reported from measurements on dry samples over the same temperature range the thermal conductivity of water saturated amphibolite decreases with temperature at a rate approximately 40 less than the rate for dry amphibolite and the conductivity of water saturated gneiss decreases at a rate approximately 20 less than the rate for dry gneiss the available evidence points to thermal cracking as the primary cause of the more rapid decrease in dry thermal conductivity with temperature the effects of thermal cracking were also observed in the water saturated samples but resulted in a net decrease in room temperature conductivity of less than 3 these results highlight the importance of duplicating in situ conditions when determining thermal conductivity for the deep crust

aseismic controls on in situ soft sediment deformation processes and products in submarine slope deposits of the karoo basin south africa intervals of soft sediment deformation features including vertical fluid escape and load structures are common and well exposed in permian lower slope deposits of the tanqua depocentre karoo basin the structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones observed over a range of scales the presence of an upper structureless siltstone layer linked to the flames interpreted as a product of the debouching of fine grained material transported through the flame onto the palaeo seabed together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment rich fluids flames are oriented parallel to the deep water palaeoslope in lateral splay deposits between two major slope channel complexes statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism importantly deformation occurred in an instantaneous and synchronous manner liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north west facing slope seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep water succession the driving mechanisms that resulted in the final configuration of the soft sediment deformation structures involved a combination of vertical shear stress caused by fluidization development of an inverse density gradient and a downslope component of force associated with the local slope ground penetrating radar profiles confirm the overall north east orientation of the flame structures and provide a basis for recognition of potential larger scale examples of flames in seismic reflection data sets

mixed energy process interactions read from a compound clinoform delta paleo orinoco delta trinidad preservation of river and tide signals by mud induced wave damping

mineralogy porosity and fluid control on thermal conductivity of sedimentary rocks summary estimates of thermal conductivity of the main non clay and clay sedimentary minerals are presented especially for the case of argillaceous ones these estimates are averaged estimates based on the interpretation of laboratory conductivity porosity and mineralogy measurements performed on small volumes of the order of 100cm3 of water saturated moist and air saturated samples the sampling is representative of the main sedimentary rocks sandstones carbonates evaporites and shales and it is composed of samples considered as structurally isotropic material first we experimentally verify the first order control of mineralogy porosity and fluid content on bulk conductivity and we demonstrate that such influences may be predicted accurately using a geometric mean model as long as isotropic samples are used then we interpret the data using an inverse method in order to estimate the average mineral conductivities of the main non clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model the analysis is based on measurements on 82 non clay samples and 29 clay samples taken on oil well cores or cuttings on outcrops and on artificially recompacted samples estimates of average mineral conductivities returned by the inversion process for the main non clay minerals are similar to generally admitted values 7 7 w m k for quartz 3 3 w m i k for calcite 5 3 w m l k for dolomite and 6 3 w m k for anhydrite values obtained for the clay minerals are systematically lower than those obtained for non clay ones they are of the order of 1 9 w m k for illite and smectite 2 6 w m k for kaolinite and 3 3 w m l k for chlorite these estimates combined with porosity and mineralogy data are used with the geometric mean model in order to verify its first order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks the bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water saturated samples and of the order of f20 per cent for air saturated samples

effect of water saturation and temperature in the range of 193 to 373 k on the thermal conductivity of sandstone abstract the thermal conductivity of porous media is crucial for many geological and engineering projects although the thermal conductivity at low temperatures is often overlooked it is of great significance to calculate the amount of heat that the ice sheet absorbs from the earth in this study the thermal conductivity of dry and water saturated sandstone was measured in the temperature range of 193 to 373 k using a transient hot wire method all samples were collected from east china with a sample porosity distribution of 5 to 13 the effects of temperature water saturation and phase transition of fluid in the pores were investigated the results indicate that the thermal conductivity decreases with an increase in temperature and that the decrease is steeper when the temperature is below 273 k moreover it is found that the thermal conductivity of the saturated sandstone is larger than that of the dry sandstone with an increase in porosity the thermal conductivity significantly increases in saturated sandstone but remains almost the same or even decreases in dry sandstones finally it is revealed that the effect of phase transformation on the thermal conductivity is significant when the water in pores becomes ice the thermal conductivity increases significantly and increases further as the temperature decreases

lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows from cyclic steps to humpback dunes abstract the preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record although these bedforms are known from a variety of depositional environments this field based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps chutes and pools antidunes and humpback dunes from three dimensional outcrops the well exposed middle pleistocene successions from northern germany comprise glacilacustrine ice contact subaqueous fan and glacial lake outburst flood deposits the studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high energy depositional environments such as fluvial settings coarse grained deltas or turbidite systems deposits of cyclic steps occur within the glacial lake outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets cyclic steps formed due to acceleration and flow thinning when the glacial lake outburst flood spilled over a push moraine ridge these bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes and pools and antidunes the subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms which are interpreted as deposits of aggrading stationary antidunes which require quasi steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation humpback dunes are characterised by downflow divergent cross stratification displaying differentiation into topsets foresets and bottomsets and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common the absence of planar parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions the large scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows which was strongly affected by the occurrence of hydraulic jumps small scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge bed topography and pulsating unstable flows

geology and hydrocarbon accumulations columbus basin offshore trinidad the columbus basin situated on the eastern shelf of trinidad lies at the eastern extremity of a belt of severe deformation along the northern boundary of south america that has been affected by compressional and wrench tectonics in pliocene pleistocene time two major structural trends are present in the columbus basin a series of east northeast trending anticlines and north northwest oriented normal faults the basin was filled during late miocene to holocene time with sediments deposited by an ancestral orinoco river draining a hinterland to the southwest the pliocene pleistocene section which contains the hydrocarbon accumulations in the columbus basin was deposited in three coarsening upward sedimentary sequences followed by a late pleistocene transgressive sequ nce traps for hydrocarbon accumulation were formed by an easterly trending pliocene pleistocene wrench system with associated east northeast oriented anticlines combined with north northwest oriented normal faults oil was sourced in the late miocene lower cruse formation whereas gas was derived both from pliocene pleistocene pro delta shales and as a late high temperature phase of lower cruse hydrocarbon generation the north northwest faults formed migration conduits from the oil source rock to pliocene pleistocene reservoirs the temporal relationship of faulting to oil generation is a major factor affecting the distribution of oil and gas the size of hydrocarbon accumulations is limited to some extent by a lack of an effective hydrocarbon seal particularly in the western half of th basin

geochemistry of the neoproterozoic tilemsi belt of iforas mali sahara a crustal section of an oceanic island arc abstract the pre pan african neoproterozoic tilemsi belt of the western iforas mali sahara comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition the volcanics include island arc tholeiites and the calc alkali rocks rhyolites dacites and andesites the volcanic domain shows similarities to recent oceanic island arc systems as well as to archaean greenstone belts the tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a neoproterozoic intra oceanic island arc crust that is composed nearly exclusively of igneous material the middle crustal section is mainly of mafic composition while the upper crustal level contains more silica rich plutonic volcanic and volcaniclastic assemblages the mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites compositionally the arc changed through time from an immature to a relatively developed system the rocks became more silica and incompatible element rich with higher sr isotopic ratios this trend reflects the increasing growth and maturity of the arc

a model for cutting and healing of deltaic mouth bars at the shelf edge mechanism for basin margin accretion abstract the architecture and facies variability of shelf edge deltaic deposits from permian triassic strata of the karoo basin south africa are described the architecture of the deposits are used to propose a model of repetitive collapse and re establishment of mouth bar packages at the shelf edge and associated delivery of sand to the slope and basin floor accretion of basin margin sediment wedges is facilitated through deposition of sediment on the basin margin clinoform this sediment is delivered by deltas at the shelf edge in the form of turbidity currents delta collapse and other gravity driven processes a well exposed 3 km long transect that records the transition between the outer shelf and upper slope for one progradational unit is described the strata are defined by basinward dipping offlapping clinoform packages of soft sediment deformed and undeformed sandstone and mudstone undeformed sandstone units are characterized by meter thick beds of mouth bar sandstones with internal scour incisions 1 2 m deep sets of undeformed sandstone beds thicken in a basinward direction from less than 1 m to over 10 m and in multiple locations the bed sets are truncated by slump scar surfaces that are overlain by mass transport deposits soft sediment deformed strata are defined by packages of contorted broken and overturned beds the deformed deposits are interpreted as being the product of dewatering features as well as slumps slides and debrites all of which emanate from slump scar features at the shelf edge on the uppermost slope thin bedded prodelta turbidites are interbedded with mass transport deposits most of which pinch out within 2 km of the shelf edge the clinoformal packages presented record the basinward transition between a prograding mouth bar deposits on the delta topset b truncation of the mouth bar deposits by slump scar incisions 2 6 m deep c deposition of mass transport deposits on the delta front and prodelta and finally d re establishment of mouth bar deposition above and basinward of the slump scar and mass transport deposits the observed stratigraphic stacking patterns facies transitions and interpreted depositional systems form the basis of a model for cutting and healing of deltaic packages that describes the repetitive deltaic progradation delta collapse and subsequent re establishment of deltaic deposition this process of repetitive collapse and reestablishment records the depositional element scale progradation of the shelf edge and accretion of the basin margin within the same depositional sequence

distribution of soft sediment deformation structures in clinoform successions of the permian ecca group karoo basin south africa abstract the stratigraphic distribution of two groups of soft sediment deformation structures ssds detached dominated by lateral movement forming slides and slump deposits and debrites and in situ dominated by vertical movement forming flame structures pseudonodules and ball and pillow structures are compared in two permian successions in the tanqua and laingsburg depocentres of the karoo basin ssds occur in a series of well exposed 40 to 55 m thick sedimentary cycles cycle thicknesses decrease upwards and the proportion of deformed strata is around 30 in both successions each flooding surface bounded cycle is characterized by a lower unit dominated by thin bedded turbidites overlain by an intermediate section dominated by soft sediment deformed facies above which are undeformed thick bedded mouthbar and shoreface sandstones the upper part of each cycle fines upwards into thin bedded turbidites which are overlain by the cycle bounding flooding surface the cycles and the distribution of ssds are the response to delta outbuilding and retreat over a clinoform rollover and onto a clinoform slope repeatable trends in the distribution of ssds during the evolution of clinoforms in both depocentres indicate a level of stratigraphic organisation and predictability in the otherwise complicated record of processes around the clinoform rollover

the changing course of the orinoco river during the neogene a review abstract recent studies have revealed that the rich fresh water vertebrate fossil fauna of the urumaco formation in northwestern falcon of late miocene age belongs biogeographically to the orinoco river system this finding together with other evidence can be used to chart the changing course of the orinoco river during the neogene and thus date the uplift of various mountain ranges that affected the change in this course presence of the misoa delta in the middle eocene of the maracaibo basin has been suggested as evidence for a large river running in a south north direction and draining the central cordillera of colombia and the guayana highlands the late middle eocene uplifting of western venezuela changed the paleogeographic setting and a new delta building shifted to the south represented by the extensive carbonera formation of late eocene to oligocene age in the llanos basin of colombia and southwestern venezuela in the earliest miocene the falcon basin situated to the east of the maracaibo basin was primarily the site of marine shale sedimentation stratigraphic and microfaunal evidence suggests the possibility of the presence of the proto orinoco river in the western margin of the basin in any case from the mid early miocene until the end of the middle miocene a fluvio deltaic sequence was deposited in the northwestern falcon with a total thickness of 4 7 km the urumaco formation overlies this sequence and its vertebrate fauna indicates that the river which built the prior delta was definitely the proto orinoco the deformation and uplift of the eastern cordillera of colombia and of the southwestern end of the merida andes caused the distal course of the river to be deflected to a west east direction a late middle miocene age for this change of course is precisely dated on marine microfaunal evidence the late miocene sedimentary record of the orinoco river delta appears east of maturin in the eastern venezuela basin and pliocene and pleistocene deltaic sediments are especially evident in trinidad and the columbus basin

unusual occurrence and stratigraphic significance of the glossifungites ichnofacies in a submarine paleo canyon example from a pliocene shelf edge delta southeast trinidad abstract sedimentary rocks belonging to the upper slope shelf margin delta of the paleo orinoco river are present at the southeast coastline of trinidad in the columbus basin the pliocene mayaro formation exposed as foreshore cliffs represents the wave influenced delta front and mouth bar of this system these deposits consist of thick to very thick hummocky cross stratified sandstone beds and thin bedded to laminated heterolithic sediments they also contain abundant soft sediment deformation structures and sparse well preserved softground burrows e g ophiomorpha nodosa towards the north central part of the outcrop the delta front deposits are cut across by a paleo canyon filled with younger mud dominated prodeltaic sediments the rare exposure of the canyon wall exhibits an unusual occurrence of glossifungites ichnofacies contrastingly distinct from archetypal examples this monospecific suite contains a low abundance of firmground thalassinoides filled with mud rather than sand the tracemakers burrowed into a firm medium grained sandy substrate of the delta front and the burrows were subsequently passively filled by the mud from the overlying prodeltaic sediments filling the canyon the deep tier firmground thalassinoides suite crosscuts the pre existing softground trace fossils integration of ichnologic sedimentologic and sequence stratigraphic datasets indicates that the older delta front sediments are separated from the prodeltaic deposits by distinct episodes of fluctuating relative sea level controlled by the basin bounding growth fault activities and the development of the canyon whereas the entire shelf margin megasequence might have been deposited through a regional scale sea level lowstand the local fluctuations in accommodation space resulting from the growth fault movements and the incision of the canyon were responsible for the shifting positions of the depositional architectural elements of the shelf edge delta

filter or conveyor establishing relationships between clinoform rollover trajectory sedimentary process regime and grain character within intrashelf clinothems offshore new jersey u s a abstract clinoform geometries and trajectories are widely used to predict the spatial and temporal evolution of sand distribution but most analytical approaches underplay the significance of topset and shelf process regime in determining how and when sediment is conveyed downdip or stored on the continental shelf we present an integrated study of clinoform rollover trajectory and detailed grain character analysis to assess the role of topset process regime in determining sand distribution and sediment character across clinothems this study targets the topset foreset and bottomset deposits of four successive miocene intrashelf clinothem sequences which represent deposition under either river dominated or wave dominated conditions seismic reflection data was combined with core analysis and grain character data derived from 664 samples collected from three cored research boreholes in river dominated clinothems the transfer of coarse grained sediment occurs under both rising and flat to falling clinoform rollover trajectories suggesting that process regime is more important in determining sediment delivery than clinoform trajectory river dominated systems are effective conveyors of sediment into deeper water wave dominated clinothems deposited exclusively under rising clinoform rollover trajectories largely retain sand within topset and foreset deposits wave dominated systems are effective sediment filters notably deposition under either river or wave dominated topset process regimes results in quantifiable differences in grain character attributes along clinoform profiles sediments in river dominated systems are coarser less well rounded and more poorly sorted and show greater intersequence and intrasequence variability than those in wave dominated systems prediction of sediment character is more challenging in river dominated systems this study highlights the need for caution when attempting to predict downdip sand distribution from clinoform trajectory alone and provides a novel perspective into downdip grain character profiles under end member topset process regime conditions the results of this study can be used to better constrain sediment grain size and grain shape distributions in process based forward models and have widespread applications in prediction of reservoir quality in both frontier and mature hydrocarbon basins

parallel lamination formed by high density turbidity currents experiments were conducted in order to investigate processes of sediment transport and deposition from sustained turbidity currents with initial volume fraction sediment concentration of 0 20 0 25 and 0 35 parallel lamination was formed at bed aggradation rates up to 4 mm s 1 and bedload layer sediment concentration up to 0 36 this observation contradicts the generally held assumption that parallel lamination forms only from low density flows and at low bed aggradation rates this misconception illustrates a poor understanding of threshold values of grain support mechanisms in the bedload layer the present results bring new insight into understanding sediment transport in the deep sea and interpreting the sedimentary record

shelf edge delta regime as a predictor of deep water deposition abstract shelf edge delta depositional style and particularly the presence of river dominated shelf edge deltas is a reliable indicator of deep water sand delivery as documented by a dataset of some 29 shelf margin clinoforms shelf edge delta regime defined as the sum of the effects of fluvial tidal wave and other oceanographic currents impacting deltas as they arrive at the shelf edge is a key predictor for likelihood of sand transport off the shelf edge to deep water conventional sequence stratigraphic concepts assume that delivery of sand to the deep water occurs primarily during falling and lowstand of relative sea level and that basinward transport is associated with shelf edge incision and sediment bypass recent analysis of outcrop and subsurface datasets has shown that sediment can also be transported to the deep water during rising and highstand of sea level provided sediment flux is high in addition some examples with documented shelf edge entrenchment associated with sea level fall did not deliver significant sand volumes to the deep water in contrast to the emphasis given to sea level and supply the role of the shelf edge delta itself and especially its process regime has received relatively little attention the data suggest that the behavior and process regime of the delta at the shelf edge is critical except where shelf width is minimal

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

a flume study on the development and equilibrium morphology of current ripples in very fine sand an empirical model is constructed for the development and equilibrium dimensions of small scale unidirectional bedforms in sand with a median grain size of 0 095 mm based on a series of steady flow experiments in a flume current ripples always attain a linguoid plan morphology with constant average height 13 1 mm and wavelength 115 7 mm provided that sufficient time is allowed for their formation the development pattern of these ripples on a flat bed is independent of flow velocity and involves four stages 1 incipient ripples 2 straight and sinuous ripples 3 non equilibrium linguoid ripples and 4 equilibrium linguoid ripples straight and sinuous ripples are non equilibrium bedforms at all flow velocities the time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours at flow velocities where washed ripples are stable the equilibrium wavelength is similar to that of equilibrium linguoid ripples but the equilibrium height rapidly decreases from 13 1 mm to zero towards upper stage plane bed conditions the results of the flume experiments correspond reasonably well with those of previous studies provided that various complicating factors such as different experimental methods different sediment characteristics shallow flow depths and non equilibrium runs are accounted for

river dominated shelf edge deltas delivery of sand across the shelf break in the absence of slope incision shelf edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and if un incised represent the main increment of shelf margin growth into the basin for that period the three dimensional complexity of shelf edge delta systems and along strike variability at the shelf edge in particular remains understudied the permian triassic kookfontein formation of the tanqua karoo basin south africa offers extensive three dimensional exposure 100 km2 and therefore a unique opportunity to evaluate shelf edge strata from an outcrop perspective analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following i in outer shelf areas parasequences are characterized by undeformed river dominated storm wave influenced delta mouth bar sandstones interbedded with packages showing evidence of syn depositional deformation the amount and intensity of soft sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth bar packages transport material down slope ii on the upper slope mouth bar and delta front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break iii further down the slope parasequences consist of finer grained turbidites characterized by interbedded thin tabular siltstones and sandstones the results highlight that river dominated shelf edge deltas transport large volumes of sand to the upper slope even when major shelf edge incisions are absent in this case transport to the upper slope through slumping debris flows and un channellized low density turbidites is distributed evenly along strike

wave enhanced sediment gravity flows and mud dispersal across continental shelves reappraising sediment transport processes operating in ancient mudstone successions recent studies of marine shelf sediment dispersal show that wave enhanced sediment gravity flows are widespread phenomena and can transport large volumes of fluid mud rapidly across low gradient shelves flow evolution is controlled by sediment supply seabed gradient and spatial distribution of wave energy at the seabed using existing flow models we predict that such flows in mud dominated sediments will develop a three part microstratigraphy produced by changing flow conditions beginning with wave induced turbulent resuspension then development of a wave enhanced sediment gravity flow prior to lutocline collapse and suspension settling petrographic examination of modern flow deposits collected from the eel shelf reveals that resultant beds possess a microstratigraphy consistent with our hypothesis a silt rich basal subunit with curved ripple laminae abruptly overlain by a subunit composed of continuous intercalated silt clay laminae and an upper clay rich drape analyses of beds from ancient mud rich outer shelf and basinal successions cleveland ironstone jurassic uk and mowry shale cretaceous united states show that they too contain beds with this three part organization suggesting that such flows were active in these ancient settings too recognition of these microstructures in these ancient mud dominated successions demonstrates that sediment in these settings was commonly reworked and transported advectively downslope by high energy events contrasting with previous interpretations of these units that deposition was dominated by quiescent suspension settling identification of these recognition criteria now allows the products of this newly recognized sediment dispersal mechanism to be identified in other shale dominated successions

thick turbidite successions from supply dominated shelves during sea level highstand emphasis on the association between relative sea level lowstand and the formation of sandy deep water fans has tended to downplay the significance of high sediment supply and its potential to create deep water fans even during sea level highstands the lance fox hills lewis shelf margin in southern wyoming suggests that high supply was critical in causing the accretion of this moderately wide maastrichtian shelf margin at a minimum rate of 47 8 km m y and the generation of large sand rich fans during every shoreline regression across the shelf it is surprising that fans developed from shelf margin clinoforms that show systematically rising shelf edge trajectories proxy for rising relative sea level as well as from those that show flat trajectories stable to falling relative sea level however the latter producing more sediment bypass resulted in bigger and thicker fans whereas the former produced somewhat smaller and thinner fans we term the former highstand fans and suggest caution in using the lowstand model for high supply systems

very high resolution seismics as a tool for sequence stratigraphy applied to outcrop scale examples from eastern tyrrhenian margin holocene pleistocene deposits three eastern tyrrhenian continental margin areas with very different depositional and tectonic settings were analyzed using single channel very high resolution reflection seismics coupled with sea floor sampling fourth and fifth order depositional sequences derived from quaternary glacio eustatic processes are now buried at shallow depths within the continental shelf the sequences have been reconstructed in detail with great lateral continuity the tectonic setting did not influence the development of high order sequences which are probably synchronous at a global scale within these sequences it has been possible to recognize all the systems tracts hypothesized by sequence stratigraphy models confirming that this method does not depend upon time or scale the geom try of depositional sequences is strongly dependent on sedimentation rate i e on the distance from sources local factors such as transport pattern variations caused by sea level changes paleotopography of the transgressed surface characteristics of feeding hydrographic nets and interactions with bioconstructions must be considered because they play an important role in the geometrical development of depositional sequences in high order depositional sequences lowstand systems tracts are volumetrically predominant highstand and transgressive systems tracts seem to be linked to point sources because they have a three dimensional 3 d geometry on the contrary lowstand systems tracts are related to linear sources parallel to the shelf break they cause the progradation of the whole margin so that they form the margin structure almost entirely