vacillatory turbidites barbados abstract a vacillatory turbidite is one containing bouma sequence divisions that repeat within a bed it presumably results from a single flow event or several closely related events for example retrogressive slope failure the most common vacillations or repetitions in such turbidites on barbados are between t b and t c intervals t bcbc for example many vacillations are accompanied by gradual to abrupt changes in the content of detrital organic matter other vacillations are clearly associated with flow surges as indicated by intra bed scours which die out laterally and separate divisions vacillatory turbidites may results from 1 multiple surging flows or 2 periodic deposition of concentrations of entrained detrital organic matter which modifies turbulent flow characteristics other features associated with vacillatory turbidites on barbados are fluidized beds syndepositional deformation features and hummocky cross beds vacillatory turbidites and related features on barbados occur principally in facies composed of fine grained sandstone beds with bedding thicknesses greater than 20 cm mostly in outer fan deposits but rarely in midfan deposits

convolute laminations and load structures in turbidites as indicators of flow reflections and decelerations against bounding slopes examples from the marnoso arenacea formation northern italy and annot sandstones south eastern france abstract this work discusses the significance of particular types of soft sediment deformations very common within turbidite deposits namely convolute laminations and load structures detailed facies analyses of the foredeep turbidites in the marnoso arenacea formation northern italy and annot sandstones south eastern france show that these deformational structures tend to increase near morphological obstacles concomitantly with contained reflected beds the lateral and vertical distribution of convolute laminae and load structures as well as their geometry has a well defined depositional logic related to flow decelerations and reflections against bounding slopes this evidence suggests an interaction between fine grained sediment and the presence of morphologic relief and impulsive and cyclic wave loadings which are produced by flow impacts or reflected bores and internal waves related to impinging bipartite turbidity currents

early jurassic evolution of the northern stikinian arc evidence from the laberge group northwestern british columbia this study resolves fundamental questions concerning the age provenance and depositional history of laberge group strata in the whitehorse trough the jurassic inklin formation straddles the stikine and cache creek terranes along much of the length of the whitehorse trough ammonite biochronology indicates an age range of early sinemurian to late pliensbachian and provides the temporal framework for interpreting basin history strong temporal trends in both paleocurrent patterns and sandstone conglomerate petrofacies allow definition of three discrete phases in basin fill history stable tectonics characterized by relative volcanic quiescence and low sedimentation rates prevailed during the sinemurian sinemurian sandstone conglomerate petrofacies record a transitional arc provenance derived from erosion of the upper triassic volcanic pile flanking coastal sediments and arc roots of stikinia to the southwest during the early pliensbachian arc dissection was interrupted by a major magmatic episode wi

the hilt bed an upper cretaceous compound basin plain seismoturbidite in the hornbrook forearc basin of southern oregon and northern california usa abstract the hilt bed is a thick laterally continuous marker bed of fine to medium grained sandstone that crops out for a nw se along strike distance of about 50 km on the southwestern flank of the cretaceous hornbrook forearc basin of northern california and southern oregon the hilt bed forms part of the blue gulch mudstone member of the hornbrook formation and is everywhere underlain and overlain by basin plain mudstone that contains thin interbeds of turbidite sandstone and siltstone the bed is as thick as 4 71 m in the central part of the outcrop belt and thins progressively to the nw and se it was derived from erosion of igneous and metamorphic basement rocks of the klamath mountains which flank the basin to the sw paleocurrents measured from flute casts at the base of the bed indicate sediment transport to the ne the hilt bed can be divided into a compound succession of nine bouma divisions all of which are generally present in outcrop the succession consists in ascending stratigraphic order of bouma abababcde divisions the basal a division is typically the thickest and coarsest layer with overlying divisions generally thinner and more variable in thickness the bed appears to have been deposited as a compound turbidite by at least three separate turbidity currents that appear to have been generated either by the same triggering event or by a rapid succession of related events only the uppermost part of the bed thus contains a complete bouma sequence the unidirectional paleocurrent indicators lateral continuity and extent of the three flows that make up the bed incompleteness of the two lower bouma sequences and absence of interbedded shale or bioturbated levels suggest that the three turbidity currents reached the basin floor at almost the same time to yield the bed the compound nature of the hilt bed suggests that a major seismic event triggered the three large flows which probably originated in either a single submarine canyon or three adjacent canyons

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

impact of synsedimentary metre scale normal fault scarps on sediment gravity flow dynamics an example from the gres d annot formation se france abstract synsedimentary faults result in the direct interaction between tectonic and sedimentary processes at similar spatio temporal scales sedimentological analysis of sediment laden gravity flows in the northern part of the gres d annot formation sanguiniere sub basin col de la moutiere tete ronde has revealed the presence of fault scarps of metre scale height these synsedimentary fault scarps were sufficient to disturb the sediment gravity flow dynamics resulting in i a strong variation of the erosive behaviour of a concentrated flow and ii the transformation of a strongly stratified laminar hyperconcentrated flow into a turbulent flow in short distance less than 500 m these disturbances develop without the flows being deviated by the fault scarps but produce great facies heterogeneity the least homogeneous facies mixing sand and clay being localized on the upstream obstacle side the most homogeneous massive sand downstream

quality of ptrm acquisition in pyrrhotite bearing contact metamorphic limestones possibility of a continuous record of earth magnetic field variations abstract the rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes magnetic susceptibility the pyrrhotite magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations a contact zone of a mixed magnetic assemblage and low susceptibilities a pyrrhotite bearing transitional zone where full thermoremanent magnetisations trms are acquired due to temperatures above the curie temperature of pyrrhotite t c po and a marginal zone containing pyrrhotite and magnetite generated at temperatures below t c po the fact that trms can consist of independent ptrms is successfully tested by modified thellier experiments it is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent ptrms multicomponent behaviour of the nrm residing in samples from the transitional zone can therefore be interpreted as a continuous record of earth magnetic field variations

reflected sediment gravity flows and their deposits in flysch of middle dalmatia yugoslavia the eocene flysch of middle dalmatia comprises several beds that are interpreted to have been deposited from reflected sediment gravity flows their compositions are similar and two bed types are differentiated complex beds that are debrite plus turbidite couplets and turbidites the sequence alternations in the turbidite part of the bed opposing ripples within the same bed and opposite flow directions indicated by flutes and ripples are indicative of flow reflections the influence of seiches is suggested by the occurrence of symmetrical oscillation ripples the palaeotransport directions of reflected flows show wide dispersal a geometry of small fault controlled sub basins with centripetal palaeotransport patterns is proposed

geophysical investigation of the beinn an dubhaich granite skye there has been a long standing controversy concerning the form of the beinn an dubhaich granite which intrudes cambro ordovician limestones and contains enclosures of limestone at outcrop we have undertaken a three part geophysical investigation with the aim of resolving some issues raised by previous workers our most significant finding results from the measurement of a gravity profile across beinn an dubhaich it reveals a negative anomaly of more than 2 mgal over the granite outcrop our preferred model to fit this anomaly is a steep sided granite stock extending down to about 1 km depth which implies that the limestone enclosures are roof pendants in a supplementary study we measured magnetic profiles across dykes in some of the limestone enclosures in order to determine the depth to the contact with the underlying granite however in the most clear cut cases we could only determine a minimum depth of 20 m and were unable to constrain the maximum depth in hindsight we suggest that microgravity surveying would be a better method for determining the thicknesses of the limestone roof pendants in the third part of the investigation we measured a tight grid of magnetic profiles to the northeast of the granite outcrop to test a previous suggestion that dyke like lobes of granite intrude pre existing fractures in the overlying limestones we located several large magnetic anomalies associated with outcropping doleritic dykes and concluded that these had not been correctly identified in the earlier work consequently the ground magnetic profiles acquired to date provide no valid evidence to support the idea of an interfingering contact between the limestones and the underlying granite

palaeomagnetic evidence for tertiary counterclockwise rotation of adria abstract with the aim of obtaining tertiary palaeomagnetic directions for the adriatic foreland of the dinaric nappe system we carried out a palaeomagnetic study on platform carbonates from stable istria from the northwestern and the central dalmatia segment of imbricated adria despite the weak to very weak natural remanences of these rocks we obtained tectonically useful palaeomagnetic directions for 25 sites from 20 localities all exhibit westerly declinations both before and after tilt correction concerning the age of the magnetizations we conclude that five subhorizontal and magnetite bearing eocene localities from stable istria are likely to carry primary remanence whereas three tilted and hematite bearing ones were remagnetized in the northwestern segment of imbricated adria the cluster of the mean directions improved after tectonic correction indicating pre tilting magnetization in contrast maastrichtian eocene platform carbonates from central dalmatian were remagnetized in connection with the late eocene oligocene deformation or miocene hydrocarbon migration based on the appropriate site locality means we calculate mean palaeomagnetic directions for the above three areas and suggest an alternative interpretation of the data of kissel et al j geophys res 100 1995 14999 for the flysch of central dalmatia the four area mean direction define a regional palaeomagnetic direction of dec 336 inc 52 k 107 95 9 from these data we conclude that stable istria in close coordination with imbricated adria must have rotated by 30 counterclockwise in the tertiary relative to africa and stable europe we suggest that the latest miocene early pliocene counterclockwise rotations observed in northwestern croatia and northeastern slovenia were driven by that of the adriatic foreland i e the rotation of the latter took place between 6 and 4 ma

experimental and numerical study of topographic effects on deposition from two dimensional particle driven density currents abstract topographic effects on deposition from particle driven density current were investigated the laboratory experiments were carried out on topography consisting of a ramp and a series of humps the results show a localized increase in deposit distribution downstream of the slope break and on the upslope of a hump a numerical model is developed to predict the topographic effects on deposit distribution the model based on layer averaged navier stokes equations is applied to the experiments and the process of the topographic influence is analyzed the preferential deposition downstream of the slope break is interpreted to result from deceleration of the flow which increases deposit distribution through loss of capacity of the flow and longer duration of the flow passage the former effect is more influential than the latter while additional effects of a hydraulic jump may be imposed increased deposit on the upslope of a hump is attributed to partial blocking of the flow at the hump crest as well as to the differential deposition due to deceleration on the upslope

reverse flow in turbidity currents the role of internal solitons pickering hiscott 1985 have demonstrated amply the presence of reverse flow units within the thick bedded calcareous wacke tcw beds of the turbiditic cloridorme formation middle ordovician gaspe peninsula quebec canada these reverse flow units are underlain and overlain by units which reveal flow in the primary obverse direction in this paper a model is proposed for this reverse flow based on the probable nature of the primary turbidity flow it appears that the initial flow was highly elongated thickness h length l with h 500 m velocity u 2 m s 1 and sediment concentration c 1 25 o the rate of momentum loss of the flow is estimated by means of a useful parameter which we call the drag distance symbol dd defined by where h and l are the thickness and length of the flow respectively ccd is a combined drag coefficient representing friction on the bottom and at the upper interface and fcd is a form drag coefficient related to the shape and size of the head dd is the distance travelled by a current of constant h and l flowing over a horizontal bottom and obeying a quadratic friction law for an e fold reduction in velocity simple considerations confirmed by our own experiments described in this paper show that such an elongated turbidity current cannot be reflected as a whole from an adverse slope when the nose of the current reaches the slope it forms a hump which surges backwards and sooner or later breaks up into a series of internal solitons the latter probably numbering 4 7 will cause reverse flow at a given point as they pass by provided that the residual velocity in the tail is not too great flow in the original obverse direction will be re established after the passage of the solitons quiescent periods in front of between and behind the solitons when soliton associated currents cancelled out the residual obverse flow would allow the deposition of thin mud drapes additional flow reversals observed in a few of the tcw beds cannot be explained readily by the re passage of solitons since wave breaking at the ends of the basin would cause massive energy loss internal seiches are the preferred explanation for these later reversals

seismic reflection test on the granite of the skye tertiary igneous centre we have conducted a seismic reflection test over a short profile on the granite of the skye tertiary central intrusive complex from previous gravity modelling work it had been inferred that the granite is approximately 1 5 km thick and overlies basic rocks the seismic data indicate that the granite is at least 2 km thick at the test location reflection events of alternating polarity between depths of 2 1 and 2 4 km suggest that basic and acidic sheets are interlayered at the base of the granitic mass

the transition between sheet like lobe and basin plain turbidites in the hecho basin south central pyrenees spain genetic facies analysis based on bed by bed correlations from sheet like lobes to the basin plain in some hecho turbidite systems demonstrates that at least 50 of the flows building the sheet like lobes kept moving downcurrent to the closely related basin plain and underwent flow transformations interpreted to have resulted from interaction with topography at the basin margin s that gave rise to specific facies these facies form a new facies tract that replaces the fine grained group of turbidite facies very fine sand to mud and characterizes the basin plain beds beds in the sheet like lobes evolve downcurrent in a way that is predictable by the existing turbidite facies tract models whereas 36 of basin plain beds which account for 78 of the basin plain volume do not the latter have deposits from high density turbidity currents at their bases and typically complete basin floor coverage the new facies tract developed when the flows obliquely encountered the southern foreland margin ramp at the ramp the lower sand laden and high density part of the larger flows was deflected evolving downcurrent along the ramp trend the upper part of the flow more dilute and thicker was reflected from the foreland margin as a train of declining undular bores moving hydraulic jumps subsequent reflections generated against the flanking margins in the closed basin led to ponding which resulted in an overall sheet like stacking pattern across sheet like lobes and the basin plain and is the diagnostic feature of the distal element in the lower sand rich stages of the turbidite systems calcilutites on top of beds interpreted until now as hemipelagites show field evidence of having a turbiditic origin hemiturbidites thus forming the facies capping the new facies tract

reflected turbidity currents on an ordovician basin floor canadian appalachians sedimentary structures associated with turbidity current deposits show either partial or complete bouma ta e sequences1 that suggest deposition of sediment from a waning decelerating unidirectional flow2 bathymetric highs and basin margins can cause turbidity currents to reflect back along their original flow paths a rare situation in both ancient3 4 and modern5 marine basins the internal structures of such contained or reflected turbidites have only been described in one case the contessa bed ref 4 pp 268 300 our data from 1 6 m thick graded mud dominated turbidites from the quebec appalachians using mainly sole marks megaripple ripple and grain fabric data suggest that during the deposition of these beds the turbidity currents were reflected several times from either internal basin highs or basin slopes the recognition of such beds not only provides an insight into the depositional environment deep water basin floor but also allows detailed bed by bed correlations over tens of kilometres of coastal sections since the beds are reliable chronostratigraphical datum lines

tectonic evolution of the northwestern internal dinarides as constrained by structures and rotation of medvednica mountains north croatia this paper attempts to explain the tectonic history and the possible reasons for the change of trend of the northwestern part of internal dinarides in a transitional area between the southern eastern alps central dinarides and tisia north of zagreb structural and palaeomagnetic data collected in pre neogene rocks at medvednica mts combined with palaeomagnetic data available form neogene rocks in the surrounding area point to the following conclusions 1 the reason for dramatic deflection in structural trend of the internal dinarides in the area north of zagreb is a 130o clockwise rotation and eastward escape of a tectonic block comprising medvednica mts and the surrounding inselbergs bounded to the north by the easternmost tip of the periadratic lineament in medvednica mts the main period of tectonic escape and associated clockwise rotation occurred in late palaeogene possibly in oligocene earliest miocene 2 when rotated into the original position the trend of observed pre neogene structures of medvednica mt becomes parallel to the major structural trend of the central dinarides in view of their original orientation these structures are interpreted in the following way a the first d1 deformational event is attributed to the aptian albian nappe stacking in the central northern dinarides that was accommodated by a top to the north directed shearing and northward propagation of already obducted ophiolites of the central dinaridic ophiolite zone this nappe stacking which resulted in a weak regional metamorphism in tectonic units underlying the ophiolites was orogen parallel or at a very acute angle to known structural and possibly palaeogeographic trends this implies a major left lateral shear component along the former adriatic margin and obducted dinaridic ophiolite zone b this was followed by early albian orogen perpendicular shortening d2 that was accommodated by folding and top to the west thrusting this deformation resulted in gradual cooling of the metamorphic stack and also in uplift and erosion of the higher structural units c d3 deformational event was driven by renewed e w shortening that took place after the paleocene most probably during the middle eocene oligocene i e synchronous with the main dinaridic tectonic phase of the external dinarides this shortening was probably triggered by collision and thrusting of tisia over the northern segment of the internal dinarides d this was finally followed by d4 pervasive right lateral n s shearing that is tentatively interpreted as being related to the right lateral shearing of the sava zone during the eocene oligocene e following the main period of tectonic escape and induced clockwise rotation in late palaeogene possibly in oligocene earliest miocene the medvednica mts and the surrounding area were affected by repeated extensions and inversions since the early miocene to recent times palaeomagnetic data suggest that in the early miocene but probably before the karpatian this area was part of a regional block that shifted northwards and rotated in counter clockwise sense a second episode of counter clockwise rotation occurred at the present latitude in post pontian times since ca 5 ma driven by the counter clockwise rotating adriatic plate

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

calcareous nannofossil age constraints on miocene flysch sedimentation in the outer dinarides slovenia croatia bosnia herzegovina and montenegro flysch deposits are associated with the outer dinaride nappe front they overlie eocene platform carbonate to bathyal marl successions that subsequently cover cretaceous plat form carbonates of apulia and the dinaride nappes planktonic foraminifer biostratigraphy indi cates eocene age of flysch sedimentation new calcareous nannofossil data reveal that several assemblages are present besides the dominant mid eocene species cretaceous paleocene oligocene and miocene taxa were also identified throughout the entire flysch belt widespread occurrence of nannofossil species of zone nn4 6 indicates that flysch deposition lasted up to at least the mid miocene ubiquitous occurrence of various pre miocene taxa demonstrates that extensive possibly submarine sediment recycling has occurred in the cenozoic as flysch remnants are typically sandwiched between thrust sheets these new stratigraphic ages give a lower bracket on deformation age of the coastal range the data provide a link between cretaceous compression in the bosnian flysch and recent deformation in the adriatic offshore area

changes in flow direction at a point caused by obstacles during passage of a density current the flow pattern and sediment character of laboratory density currents partly blocked by bed topography is used to demonstrate how changes in paleocurrent direction through a turbidite at a single site can be explained these experiments confirm that a relatively small obstacle may affect bed thickness and paleocurrent patterns at considerable distances away from obstacles density currents generated by a lock exchange mechanism spread radially across the tank floor and were partially blocked by a wedge shaped obstacle with steep face scarp facing upstream when the body of the current impinged on the obstacle a thickened region of the flow developed upstream of the scarp with increased entrainment of ambient fluid above it the upflow edge of the thickened region was at a small angle to the obstacle front and propagated rapidly upflow and to the sides of the obstacle internal flow vectors observed with thread flags were radial from the lock before the head reached the obstacle after the head passed the obstacle flow vectors in the region of thickened flow upstream and to the sides of the obstacle were reoriented nearly parallel to the obstacle scarp thus flow vector at a point rotated by as much as 45 during passage of the current the deposits of sediment laden density currents thickened in the region of observed flow thickening because of an increased particle flux to the bed caused by velocity reduction the upflow limit of the influence of the obstacle on deposition is marked by an abrupt increase in the thickness of the deposits this area of thickening occurs in an area where the flow is accumulative

contained reflected turbidity currents from the middle ordovician cloridorme formation quebec canada an alternative to the antidune hypothesis the parautochthonous cloridorme formation is a syn orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle fan outer fan and basin floor deposits the basin floor deposits about 1 5 km thick are confined to members 1 2 and 1 and are characterized by graded thick 1 10 m mud rich calcareous greywacke beds previously interpreted as deposits of concentrated muddy unidirectional turbidity currents that locally generated backset antidune lamination in internally stratified flows the dominant flow directions were from east to west but west to east transport also occurred as seen in the orientation of ripples climbing ripples flutes consistently overturned flames and grain imbrication we believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180 one or more times during deposition of the lower sandy part of the beds flow reversals are consistent with the sharp grain size breaks and mud partings within sandy divisions measurement of grain fabric relative to stratification in the most celebrated antidune bedforms indicates flow from west to east thus the bedforms were produced by west to east migration of megaripples not by the upcurrent migration of antidunes the thick muddy beds were deposited by large volume muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor much like the contessa and similar beds of the italian apennines large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed because of their mode of emplacement we propose that these beds be called contained turbidites