coarse clastic barrier environments evolution and implications for quaternary sea level interpretation abstract this paper reviews recent work particularly from eastern canada on the dynamics and evolution of coarse clastic barriers it is argued that an understanding of quaternary coastal changes on paraglacial glacially influenced coasts can only be achieved with a knowledge of gravel barrier morphodynamics and sedimentation gravel barriers are distinctive coastal forms in which morphological inheritance and sediment sorting provide major behavioral controls spatially adjacent gravel barriers may display different depositional tendencies prograde seawards migrate landward or aggrade vertically such inconsistent spatial behaviour must be considered in terms of an erosional front advancing landward under stationary or rising sea level redistribution of sediment across or along the erosional front supplies sediment for the development of gravel barriers which in turn provides environments for fine grained deposition which is favourable for further barrier migration an example of an erosional front is given from chezzetcook inlet nova scotia three stages are recognized in the development of gravel barriers starting with the initiation of small drift aligned forms at the leading edge of the erosional front as sea level rises this is followed by a period of consolidation and stabilization when drift aligned forms may become swash aligned conditions for this transition are discussed under rising sea levels it is difficult to explain the conditions leading to the formation of multiple parallel swash aligned ridges although three hypotheses are advanced all of which may operate in different locations the third stage is associated with the breakdown of barriers as sediment supply fails and or sea level rises drift aligned forms are inherently unstable and show signs of pre breakdown stress in the form of reworking and remobilization of proximal sediments into cells swash aligned barriers may be breached revert to drift alignment become segmented or be overstepped some gravel is re incoporated in barriers farther landward while other material is passed seaward of the erosional front and becomes abandoned on the shoreface gravel barriers may exert a major influence on estuary and back barrier environments especially where they control the depositional record the implications for the reconstruction and interpretation of quaternary environments are summarized

early oligocene stratigraphic turnover on the west africa continental margin a signature of the tertiary greenhouse to icehouse transition the post rift stratigraphy on the west african margin is characterized by aggradation of a carbonate ramp during late cretaceous to eocene epochs and progradation of a terrigenous wedge from oligocene to the present such first order structure has been attributed in the past to geodynamic forcing however comparison of the stratigraphic record of the margin with eustasy 18o and 87sr 86sr curves shows a close temporal relationship with the tertiary climate cooling an increase of continental weathering and a long term lowering of sea level we suggest that the transition from low amplitude high frequency sea level changes during the greenhouse period to high amplitude high frequency sea level changes during the icehouse period may account for i the switch from an aggrading carbonate ramp to a prograding clastic wedge and ii the enhanced continental weathering and increased terrigenous influx to the margin

re os and 40ar 39ar isotope measurements of inclusions in alluvial diamonds from the ural mountains constraints on diamond genesis and eruption ages the re os isotope data for 20 syngenetic sulphide inclusions recovered from 15 diamonds and the 40ar 39ar laser probe eruption ages of 7 syngenetic clinopyroxenes recovered from 5 diamonds all from alluvial placer deposits in the ural mountains have been determined six eclogitic sulphide inclusions two of which coexist in the same diamond yield an isochron age of 1280 310 ma 2 with an unusually high initial 187os 188os ratio of 2 10 0 58 2 the age is interpreted to date remobilisation of carbon and sulphur and homogenisation of os during rift related thermal magmatic events that affected the east european craton eec at ca 1 3 ga the high initial os ratio suggests re os evolution over a 100 to 500 ma period within previously metasomatised lithosphere most likely the eec five eclogitic clinopyroxenes recovered from four diamonds yielded similar 40ar 39ar ages averaging 472 28 ma which likely approximate the time of source kimberlite lamproite eruption this age suggests that the ural diamonds are not likely to have derived either from the well known diamond bearing kimberlites of the siberian craton nor from presently known russian and finnish kimberlite provinces on the eec the urals placer deposits are mainly confined to 397 407 ma sedimentary rocks along the western side of these mountains with sediment transportation at that time generally from the north west present evidence suggests the existence of an undiscovered kimberlite lamproite source probably on the volgo uralia crustal segment of the eec which gave rise to the urals diamond deposits

progradational gravel beach sequences in a moderate to high energy microtidal marine environment the anatomy of progradational gravel beaches was analysed in two different tectonic settings the strongly subsiding messinian foreland basin of the eastern southern alps and the rising pleistocene marine terraces of the outer margin of the apenninic chain progradation took place near the mouths of fan delta or braid delta distributaries debouching in microtidal landlocked basins palaeo adriatic and palaeo ionian seas a wind wave climate comparable to that of the present day was characterized by periodic intense storm activity most of the beachface progradation is thought to have taken place during the post storm recovery and fair weather stages whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly sorted and coarse grained gravels on the shoreface by storm intensified seaward trending flows and behind the highest berms by washover processes analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here the characteristics of the apenninic sequences suggest a strongly variable wind wave climate with periodic changes from a relatively dissipative barred morphology during storms accompanied by strong longshore currents and rip development to a more reflective state typical of recovery and fair weather stages the southern alpine sequences on the other hand are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear and gravity sorting in the lower beachface gravels these features indicate a comparatively more reflective average state due to location of beaches along the deeply embayed head of the palaeo adriatic and the very narrow directional width of the incoming wave spectrum which was almost shore normal most of the time in addition to well developed shape zonation of gravels the common presence of wave generated gravel megaripples in the southern alpine sequences indicates the greater influence of long period waves due to the greater available fetch distances in both settings the cyclicity is thought to be genetically related to minor changes of sea level

scour downstream of grade control structures a theoretical investigation of local scour downstream of gradecontrol structures based on twodimensional jet diffusion and particle stability is experimentally verified turbulent jet diffusion red

development of cusp related gravel size and shape facies at malin head ireland samples of beach gravels were obtained from foreshore cusp deposits at portmore beach near malin head republic of ireland axial dimensions a b and c were measured and the maximum projection sphericity was derived for each of 4360 clasts each sample was also categorized according to depositional subenvironment principal components analysis and cluster analysis were used to identify potential facies zonation results indicate that reworking of the foreshore by incident and edge waves resulted in clast size and shape segregation both parallel and perpendicular to the beach patterns of zonation were distinctly different from those described in earlier facies descriptions showing a complex set of foreshore facies the distribution of these facies was controlled primarily by location within beach cusp systems primary and secondary facies demonstrate alongshore variability of a magnitude comparable to across shore differences the results have important implications for palaeoenvironmental reconstruction morphosedimentary description and research sampling design and provide the basis for a conceptual model of gravel beach cusp facies

tertiary sea level movements around southern africa sedimentological micropaleontological and seismic profiling data elucidate the history of tertiary sea level movements around southern africa these new data show that landward movement of the sea began in early paleocene time and continued into the early eocene the sea probably reached its maximum paleocene height during the early eocene and is today represented by outcrops up to at least 204 m and probably as high as 360 m above modern sea level a brief regressive pulse occurred during the middle eocene with renewed transgression in the late eocene a major regression followed probably spanning all of oligocene and early miocene times this regression exposed much of the continental shelf and is clearly represented on seismic reflection profiles by a widespread unconformity the major neogene transgression began in the middle miocene and probably reached its greatest extent in the late miocene or early pliocene the overall middle miocene to early pliocene transgression was interrupted by a brie

scour control downstream of block ramps scour control downstream of a hydraulic structure is an important topic in river engineering a block ramp is a structure that can be built with a low environmental impact stilling basin realized by means of a sill that controls the scour hole the scour that occurs downstream of block ramps is analyzed and the effect of rock sills on scour geometry is evaluated experiments have been carried out with ramp slopes varying between 1v 4h to 1v 12h three different sill types were tested for a total of 300 experiments among which about 140 tests involved rock sills useful equations for the design of the stilling basin are proposed

erosion by plane wall jets with minimum tailwater the results of an experimental study on the erosion of sand beds by plane water wall jets with minimum tailwater is presented a comparison is made with the erosion caused by the corresponding deeply submerged water wall jet

regressive and transgressive sequences in a raised holocene gravelly beach southwestern crete both a modern and a raised late holocene gravelly beach have been studied along the cliffed coast near the village of chora sfakion in southwestern crete greece the holocene beach shows the record of regional well documented relative sea level changes in the form of regressive and transgressive sequences the texture fabric and depositional architecture of these sequences are described and compared with those of the modern microtidal beach there the depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea level changes the regressive sequences of both the modern and the holocene beach resemble those of the mesotidal beaches in the sker district of southwest wales described by bluck 1967 the transgressive sequence of the holocene beach resembles the mesotidal beach sequence near newton southwest wales recognized by bluck as another type of progradational regressive beach therefore transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive the stratigraphic depositional patterns of gravelly beach systems are apparently still poorly understood

beaches and bedrock how geological framework controls coastal morphology and the relative grade of a southern namibian diamond placer deposit abstract the linear beaches of mining area 1 ma1 on namibia s south western coastline host the onshore portion of the world s largest diamond placer deposit these beaches have been actively mined for over 80 years but land based mining methods are close to reaching their practical limit on this vigorous wave dominated coast seismic data and sparse sampling data confirm that the linear beach deposits continue offshore onto the inner shelf which is characterised by a number of erosional nick points and wave cut platforms developed over the bedrock topography this environment constitutes a techno economic challenge and a target ranking methodology is required for economic development of the offshore extension of the deposit detailed analysis of the historical onshore mining dataset including footwall elevation survey data diamond grade information and detailed geological mapping and modelling was integrated with newly acquired geophysical data that spans the onshore and offshore sectors of the deposit this shows that relative diamond grade is controlled by the gradient of the bedrock and the wave cut platform width an inherited structural control on bedrock gradient is also shown the direct gradient grade and indirect platform width grade relationships can be extended to the offshore realm and used as target ranking criteria these results greatly aid risk mitigation of decision making against the backdrop of techno economic challenges associated with developing and mining the nearshore extension of the placer

ultra long distance littoral transport of orange sand and provenance of the skeleton coast erg namibia abstract quantitative provenance analysis based on high resolution bulk petrography and heavy mineral data on beach and dune sands integrated with detrital zircon geochronology and chemical analyses of pyroxene garnet and staurolite demonstrates that sand derived from the orange river is carried by powerful and persistent longshore currents as far as northern namibia and southern angola 1750 km north of its mouth this is the longest cell of littoral sand transport documented so far compositional forward modeling indicates that 80 of dune sand in the skeleton coast is orange derived the remaining 20 being supplied by slow erosion of the damara orogen chiefly via the swakop river a decrease in basaltic rock fragments and pyroxene with relative enrichment in garnet staurolite tourmaline and other metamorphic minerals north of walvis bay indicates that only one third of beach sand in the 350 km dune free gap between the namib and skeleton coast ergs is orange derived the remaining two thirds being supplied largely by the swakop river draining the damara orogen although volcanic gravel becomes dominant in beaches of the skeleton coast north of the uniab mouth detritus from cretaceous etendeka lavas accounts for only 4 of beach sand reflecting limited sand generation in the arid catchment contributions from the kunene river to either dune or beach sands is ruled out indicating that gem diamonds found in placer deposits along the coast of namibia are all derived from the orange river

particle size and shape in pebble beach sedimentation an in depth investigation was made of beach gravel fabrics at two beach sites located on the glamorgan heritage coast wales u k granulometric data from 31 080 pebbles were obtained and analysis confirmed the existence of a basic shape differentiation structure when the b axis was used at gileston beach a low energy location and nash beach a high energy location the use of the b axis in size and shape analysis seems to have led some workers to incorrect rejection of size as a factor in determining the transportational and depositional potential of a particle an inferred energy model can be constructed where the influence of particle size on sorting is predominant when energy conditions are high and particle shape factors predominate when energy conditions are low the c axis standard deviation value was the most effective discriminator of variation in shore parallel sedimentation structures at both beaches for shore normal structural discrimination the c axis mean and standard deviation values were the most effective

the transport and sorting of diamonds by fluvial and marine processes this paper describes the systematic variations in size and quality of diamonds that result from transport by fluvial and marine processes it is shown that diamonds originating in rather restricted source areas can be spread across many tens of thousands of square kilometers downstream or alongshore transport can be followed for several hundred kilometers with a systematic reduction in average diamond size accompanying such transport the diamond size distribution is progressively modified with increasing travel distance and the quality of the diamonds improves as inferior types are destroyed the accompanying variations in crystal form are outlined certain implications for exploration programs are mentioned

gravelly spit deposits in a transgressive systems tract the pleistocene higashikanbe gravel central japan the pleistocene higashikanbe gravel which crops out along the pacific coast of the atsumi peninsula central japan consists of well sorted pebble to cobble size gravel beds with minor sand beds the gravel includes large scale foreset beds 5 10 m high and overlying subhorizontal beds 0 5 3 m thick showing foreset and topset structure from which the gravel has previously been interpreted as deposits of a gilbert type delta however 1 the gravel beds lack evidence of fluvial activity such as channels in the subhorizontal beds 2 the foresets incline palaeolandwards 3 the gravels fill a fluvially incised valley and 4 the gravels overlie low energy deposits of a restricted environment such as a bay or an estuary the foresets generally dip towards the inferred palaeoshoreline indicating landward accretion of gravel reconstruction of the palaeogeography of the peninsula indicates that the higashikanbe gravel was deposited as a spit similar to that developed at the western tip of the present atsumi peninsula rather than as a delta according to the new interpretation the large scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea level rise the subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars which are reworked sediments of the spit beach gravels during a transgression the lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion dome structure which is a cross sectional profile of a recurved gravel spit at its extreme point and sandy tidal channel deposits deposited between the small islets were also identified in the higashikanbe gravel the higashikanbe gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract suggesting that gravelly spits are likely to be well developed during transgressions the large scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas characteristic of highstand lowstand and shelf margin systems tracts

diamondiferous neoarchean fan delta deposits western superior province canada sedimentology and provenance abstract most fluvial diamond placers occur in non aggrading braided systems where the diamonds are preferentially trapped by very rough substrates the recognition that some of the neoarchean diamond bearing breccias near wawa ontario canada are actually fluvial conglomerates deposited by a mixture of traction current and mass flow processes provided the opportunity to add information on aggradational diamond placer systems the 454 m thick leadbetter conglomerate was studied to ascertain the depositional processes that led to the development and provenance of the placer the conglomerate is composed of a series of weakly sheared and highly viscous debris flows interbedded with boulder to pebble conglomerates representing longitudinal bars and channels pebble conglomerate and sandstone channels become more dominant upwards with a decrease in mass flow processes an abrupt marine transgression flooded this fan delta assemblage ending deposition of the coarse grained lithofacies the lowest 100 m of the conglomerate is composed of clasts derived solely from the underlying mafic volcanic assemblage above this zone the clast types become more diverse and felsic volcanic clasts become progressively more important upwards whole rock geochemistry performed on coarse grained sandstone samples indicates that major sediment contributions came from mafic rocks and nearby diamondiferous lamprophyres and related breccias though the possible existence of a localized kimberlitic source cannot be excluded geochemistry also indicates that minor enrichments of heavy minerals formed on the alluvial fan delta surface but the primary cause of the conglomerate s diamondiferous nature was a nearby diamond bearing source rock

sedimentology of gravelly lake lahontan highstand shoreline deposits churchill butte nevada usa abstract gravelly shoreline deposits of the latest pleistocene highstand of lake lahontan occur in pristine depositional morphology and are exposed in gravel pits along churchill butte in west central nevada four environments differentiated at this site are alluvial fan colluvium lakeshore barrier spit lake lower shoreface spit platform and lake bottom lakeshore deposits abut along erosional wave headcuts either unsorted muddy to bouldery colluvium fringing churchill butte bedrock or matrix supported cobbly and pebbly debris flow deposits of the silver springs fan the lakeshore barrier spit is dominated by granule pebble gravel concentrated by wave erosion of the colluvial and alluvial fan facies the lakeward side of the barrier consists of beachface deposits of well sorted granules or pebbles in broad planar beds 1 10 cm thick and sloping 10 15 they interfinger downslope with thicker 10 25 cm and less steep 5 10 lakeward dipping beds of fine to medium pebble gravel of the lake upper shoreface interstratified with the latter are 10 40 cm thick sets of high angle cross beds that dip southward alongshore higher angle 15 20 landward dipping foresets of similar texture but poorer sorting comprise the proximal backshore on the landward side of the barrier they were deposited during storm surges that overtopped the barrier berm gastropod rich sand and mud also deposited by storm induced washover are found landward of the gravel foresets in a 15 m wide backshore pond algal stromatolites ostracodes and diatoms accumulated in this pond between storm events the lake lower shoreface extending from water depths of 2 to 8 m consists of a southward prograding spit platform built by longshore drift the key component of this platform is large scale sandy pebble gravel in 16 southward dipping gilbert foresets that grade at a water depth of about 6 7 m to 4 dipping sandy toesets a shift from bioturbated lower shoreface sand and silt to flat and laminated lake bottom silt and mud occurs between water depths of 10 40 m and over a shore normal distance of 250 m this lake bottom mud facies unlike the others is areally expansive

characteristics of diamondiferous plio pleistocene littoral deposits within the palaeo orange river mouth namibia abstract the orange river the principal conduit transporting diamonds from hinterland sources to the namibian coast in post cretaceous times is characterised by an extreme wave dominated delta that has given rise to a progression of coarse rudaceous littoral deposits preserved onshore for 150 km north of the mouth under the long lived prevailing vigorous wave wind and northward longshore drift regimes the orange river outfall has been reworked into amongst others a series of economically viable diamondiferous plio pleistocene onshore gravel beach deposits these placers comprise spits and barrier beaches in the proximal reach within the palaeo orange river mouth that after ca 5 km northwards merge into extensive but narrow linear beaches that in turn after ca 70 km give way to pocket beaches gravel and diamond size decreases northwards away from the ancestral orange river mouth the linear and pocket beach types have considerably higher diamond content but lower average diamond stone size than the two proximal units that are characterised by low diamond grade but comparatively large average diamond size given the risk of delineating low grade alluvial diamond deposits accurately we present here sedimentological reconstructions of the subtidal intertidal and supra tidal facies that constitute the spit and barrier beach sequences based largely on face mapping of exploration trenches and open cast mine cuts as well as the results of large tonnage sampling campaigns diamond distribution is also linked convincingly to basic littoral processes that were operational within the palaeo orange river mouth during the complex transgression that gave rise to the 30 m package in plio pleistocene times in both the spit and barrier beach settings the intertidal deposits prove to be the most promising targets whereas the subtidal sediments are the least economic the constant raking associated with coarse cobble boulder sized gravel foreshore deposits in an energetic micro tidal wave regime increased the average diamond stone size in the intertidal deposits to 1 to 2 carats per stone cts stn but the lack of fixed trapsites no competent footwall within the palaeo orange river mouth at that level prohibited the accumulation of substantially enriched diamondiferous gravels consequently grades of only 1 5 to 6 carats per 100 tons cpht are realised the highest grades 2 to 6 cpht are found in the landward facing intertidal beach deposits on the spits where gentle reworking in that sheltered environment had somewhat enriched and preserved the diamond content significantly the low average stone size of ca 0 5 cts stn in this lower energy setting probably reflects that of the general diamond population available at that time in contrast the sand rich subtidal deposits in the spit sequence return the lowest grades 0 1 to 0 5 cpht similar to those in the slightly younger subtidal transgressive boulder lags of the barrier beaches however the stone size in the spit subtidal sediments is also low 0 1 to 0 5 cts stn due to the highly mobile fine grained character of those deposits whereas that in the subtidal transgressive lag is large 2 to 3 cts stn as a result of the local semi permanent turbulence associated with the boulder sized clasts in these gravel sheets diamond distribution is therefore also influenced by littoral facies and associated beach types in addition to the spatial and temporal parameters that have already been documented for the onshore marine placers of the southern namibian coast

a late pleistocene coarse grained spit platform sequence in northern jylland denmark several well preserved late pleistocene spit systems occur uplifted in northern jylland denmark their present day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits two characteristic vertical sequences are recognized in the systems the first sequence i consists of a giant scale cross bedded foreset unit overlain by topset and beach units while the second sequence ii consists of the foreset unit overlain by bar trough and beach units the two sequence types pass laterally into each other with a short overlap zone they can be interpreted in terms of meistrell s 1966 1972 model for spit platform growth based on scaled wave tank experiments the giant scale cross bedded unit corresponds to prograding of a coarse grained subaqueous spit platform while the topset bar trough and beach units reflect the growth of the subaerial spit the alternation between sequence i and ii reflects the inversely related growth of the spit and platform structures when the rate of subaqueous platform progradation declines the subaerial spit grows uniformly and when the platform progrades uniformly spit growth declines the model is probably only valid for relatively coarse grained systems because only these deposits would have a relatively steep front the water depth in which the spit system progrades and thus bottom topography determines the thickness of the giant scale cross bedded foreset unit because the water depth over the top of the platform is relatively constant if the water is less than a few metres deep the spit platform is not developed as seen where the late pleistocene spit systems prograded over elevations of the sea bottom conversely the correct recognition of spit platform sequences allows precise determination of sea level and water depth at the time of formation finally the model adds one further mode of formation of giant scale cross bedding to those already known from fluvial transverse lateral and point bars subtidal sand waves and gilbert deltas

relative sea level fall since the last interglacial stage are coasts uplifting worldwide the growing interest in quantification of vertical ground motion stems from the need to understand in detail how the earth s crust behaves for both scientific and social reasons however only recently has the refinement of dating techniques made possible the use of paleoshorelines as reliable tools for tectonic studies although there are many local studies of quaternary vertical motions of coastlines we know of no comprehensive worldwide synthesis here we provide a compilation of 890 records of paleoshoreline sequences with particular emphasis on the last interglacial stage marine isotopic stage mis 5e 122 ka the quality of dating mis 5e makes it a reliable marker to evaluate vertical ground motion rates during the late quaternary on a global scale the results show that most coastal segments have risen relative to sea level with a mean uplift rate higher than 0 2 mm yr i e more than four times faster than the estimated eustatic drop in sea level the results also reveal that the uplift rate is faster on average for active margins than for passive margins neither dynamic topography nor glacio hydro isostasy may explain sustained uplift of all continental margins as revealed by the wide distribution of uplifted sequences of paleoshorelines instead we suggest that only plate tectonic processes reconcile all observations of quaternary coastal uplift we propose that long term continental accretion has led to compression of continental plates and uplift of their margins therefore this study concludes that plate tectonics processes impact all margins and emphasizes the fact that the notion of a stable platform is unrealistic these results therefore seriously challenge the evaluation of past sea levels from the fossil shoreline record

tertiary age diamondiferous fluid deposits of the lower orange river valley southwest africa the orange river the principal conduit for transportation of diamonds from the southern african interior to the atlantic coast has within its lower valley two recognized suites of gravel terraces in which part of the passing diamond population has been trapped both suites lie on eroded bedrock and both are downstream thickening and fining coarse clastic wedges the older and higher terrace suite comprising the arries drift gravel formation is early mid miocene in age 19 17 ma and is referred to locally as the proto orange terrace the lower and younger terrace suite locally known as the meso orange terrace has not yet been dated but is considered to be plio pleistocene in age middle to late proterozoic bedrock underlying both suites of terrace deposits was deeply eroded during the incision of the orange river which began in late cretaceous times the proto orange terraces richer in diamonds than the younger mesozoic deposits form low grade but large average diamond size gem quality placers which are mainly discussed here diamonds are best concentrated in cobble boulder basal gravels on or close to the rough bedrock contact in three main trapsite settings scour pools hosting oversize obstacle clasts return grades of 10 to 40 carats per hundred ton cpht push bars where the grades range from 10 to 55 cpht and bedrock highs with their associated oversize clasts yielding grades of 6 to 12 cpht do not concentrate diamonds to the same extent as the two former trapsites but occur over a wider area and thus potentially enrich a greater volume of gravel the average diamond stone size in these three trapsites is also upgraded ranging from 1 5 to 2 8 carats per stone ct stn the key factors in concentrating diamonds at the bedrock gravel interface are turbulence scale and intensity created by the rough boundary conditions and the presence of fixed bedrock sites of turbulence which are stable enough to initiate and retain around them the slow and stable growth of gravel that hosts the concentrated diamonds the upper more mobile gravels have far lower concentrations of diamond than the basal gravels with grades dropping to an average of 0 3 cpht and stone size decreasing to 1 to 1 3 ct stn sites of higher concentration in the upper gravels of the proto terraces are bar platforms riffles 1 to 3 cpht and average stone sizes of 1 1 to 1 3 ct stn and bar heads 0 7 to 1 5 cpht and average stone sizes of 1 to 1 3 ct stn while bar tails are practically barren although the vertical decline in diamond concentration may be attributed to a decrease in diamond supply during the aggradation of the proto orange gravels the younger lower grade mesozoic gravels also display a similar vertical profile hence diamond concentration in the lower orange river deposits is influenced positively by the occurrence and nature of the localized fixed bedrock trapsites from an exploration point of view these fixed trapsites are more difficult to locate often requiring extensive drilling whereas the areally extensive but low grade upper gravels are relatively easy to delineate during exploitation the localized but higher grade fixed trapsites provide a reliable source of above average sized diamonds in an otherwise low grade