sandstone petrology and geochemistry of the turpan basin nw china implications for the tectonic evolution of a continental basin the evolution of the turpan basin nw china from the permian to the tertiary was reconstructed from sandstone petrology and geochemistry the sandstones are mostly feldspathic litharenite and litharenite and were deposited mainly in fluvial and lacustrine environments sandstone samples show large variations in the contents of quartz and lithic fragments a general increase in compositional maturity is observed from the permian to the tertiary in particular quartz content is only 11 in the permian increases to 27 in the triassic 35 in the lower jurassic 36 in the upper jurassic and more than 47 in the cretaceous and tertiary the geochemistry of sandstones also varies with time late permian sandstones have lower sio2 67 48 and higher fe2o3 4 87 and mgo 1 75 values whereas triassic and jurassic samples show high sio2 74 15 77 5 respectively and low fe2o3 4 06 2 8 and mgo 0 95 0 79 contents in the cretaceous and tertiary fe2o3 and mgo contents increased again k2o na2o and al2o3 also vary considerably within the study site the tectonic evolution of the basin can be subdivided into three stages from petrologic and geochemical data the first stage covers the permian the second stage the triassic and jurassic and the third stage the cretaceous and tertiary there are two large discontinuities in sandstone composition among these three stages these discontinuities signify the influence of collision and convergence of the tarim block at the end of the permian and the gangdise block between the jurassic and the cretaceous with the eurasian plate the source rock regions of the basin varied with time in permian triassic and jurassic time the paleo jueluotage shan mountains were the dominant source region for the turpan basin while in the cretaceous and tertiary the paleo bogda shan mountains were an important source region in the tertiary and later the bogda shan mountains continued to be strongly uplifted during the collision of the indian plate with the eurasian plate and were the dominant source region the trace element ratios eu eu gd n yb n la yb and la th subscript n refers to chandrite normalized values show that the source rocks of the sedimentary rocks were derived from upper continental crust the data from the turpan basin show that combined petrologic and geochemical analysis of sandstone suites can be used to track changes in the sediment supply from adjacent areas if 1 a long term record of the basin fill is available 2 the source signal is preserved by proximal depositional conditions and 3 diagenetic alteration of sediments is limited provenance derived variations in sandstone compositions are therefore a key in unraveling regional tectonic histories

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

analysis of sedimentary processes on the las canteras beach las palmas spain for its planning and management abstract the sedimentary dynamics of a sandy beach located on the north coast of the island of gran canaria has been studied in relation to physical processes the morphodynamic behaviour of the beach has been established and interpreted special emphasis has been placed on the characterization of erosive and accretionary processes by means of sediment transport calculations two sedimentary indices have been used to define these processes qualitatively and quantitatively these indices are of importance for management

formation of beach rock at siesta key florida and its influence on barrier island development seaward dipping strata of carbonate cemented shell debris located along the coast of siesta key on the gulf coast of the florida peninsula have long been interpreted to be beachrock equivalent in age to the pleistocene anastasia formation stage 5e of the east coast of florida detailed examination of thin sections along with radiometric dating and isotopic analyses demonstrates clearly that this is a holocene deposit that is not beachrock but was lithified in a meteoric environment whole rock dates dates from shells only and from cement only demonstrate that these beach deposits were in place by at least 1800 yr bp and might have been there as long ago as 4300 yr bp this means that some type of barrier island was in place at that time previous investigations have depicted siesta key as having a maximum age of 3000 yr with these deposits being located about 2 km landward of the beach deposits this suggests that the beach deposits might have been the site of the original position of siesta key these data also indicate that sea level must have been near its present position at the time that these foreshore beach deposits were deposited sometime between 1800 and 4300 yr ago this scenario indicates that sea level along this coastal reach probably reached its present level at least about 2000 yr ago

beach cement incipient caco 3 cemented beachrock development in the upper intertidal zone north uist scotland abstract beachrock has only rarely been observed previously along high latitude coasts on several beaches on north uist nw scotland at over 57 n there is evidence of small scale beach cementation an area of beach cement at the back of traigh ear as well as unconsolidated sands from the beach and the adjacent traigh hornais have been studied using sem xrd and petrographical thin sections to throw light on the nature and causes of cementation unconsolidated sands from both beaches are similar in grain sizes shape and mineralogy but traigh ear grains commonly possess abundant cover of high mg calcite the beach cement on traigh ear exhibits low porosity with abundant micritic and aragonitic cements showing no evidence of vadose cementation along with traces of microorganisms these findings contrast with the vadose low mg calcite cements commonly reported from temperate zone beachrocks

upper quaternary coastal palaeoenvironments and palaeosea levels in las canteras beach gran canaria canary islands spain abstract las canteras beach outcrops are vestiges of palaeoenvironments that represent changes in sea level and or climate conditions using broad sedimentological mineralogical geochemical and dating studies three different palaeoenvironmental facies were distinguished 1 the lower facies was defined as beachrock formed by decametric calcarenite layers dipping 8 5 seaward and composed mainly of sand grains and two generations of isopachous low magnesium calcite lmc cement in a context of lower holocene interglacial stage 6600 30 cal years bp rising sea level and coastal progradation 2 during the middle holocene about 6600 30 cal years bp a coastal progradation phase and or lower sea level might lead to land emersion and soil development with an abundance of terrestrial gastropods 3 the upper holocene

hotspot volcanism close to a passive continental margin the canary islands the canarian archipelago is a group of volcanic islands on a slow moving oceanic plate close to a continental margin the origins of the archipelago are controversial a hotspot or mantle plume a zone of lithospheric deformation a region of compressional block faulting or a rupture propagating westwards from the active atlas mountains fold belt have been proposed by different authors however comparison of the canarian archipelago with the prototypical hotspot related island group the hawaiian archipelago reveals that the differences between the two are not as great as had previously been supposed on the basis of older data quaternary igneous activity in the canaries is concentrated at the western end of the archipelago close to the present day location of the inferred hotspot this is the same relationship as seen in the hawaiian and cape verde islands the latter archipelago associated with a well defined but slow moving mantle plume shows anomalies in a plot of island age against distance which are comparable to those seen in the canary islands these anomalies cannot therefore be used to argue against a hotspot origin for the canaries individual islands in both archipelagoes are characterized by initial rapid growth the shield building stages of activity followed by a period of quiescence and deep erosion erosion gap which in turn is followed by a post erosional stage of activity the absence of post shield stage subsidence in the canaries is in marked contrast with the major subsidence experienced by the hawaiian islands but is comparable with the lack of subsidence evident in other island groups at slow moving hotspots such as the cape verdes comparison of the structure and structural evolution of the canary islands with other oceanic islands such as hawaii and reunion reveals many similarities these include the development of triple mercedes star rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones the apparent absence of these features in the post erosional islands may in part be a result of their greater age and deeper erosion which has removed much of the evidence for their early volcanic architecture we conclude that the many similarities between the canary islands and island groups whose hotspot origins are undisputed show that the canaries have been produced in the same way

formation of holocene limestone sequences by progradation cementation and erosion two examples from the bahamas abstract holocene limestones on north bimini and joulter cays bahamas reveal sedimentary structures that are directly comparable to structures in the modern beach environment a complete sequence comprises shallow subtidal intertidal and supratidal deposits holocene and equivalent modern sedimentary features are found on more or less the same topographic levels freshwater cementation phreatic and vadose takes place after accretion and progradation of the shore subsequent erosion exposes the consolidated sediments secondary cementation in the intertidal zone leads to beachrock formation continued erosion may break off blocks of beachrock and cliffs which will be incorporated into the basal part of the sequence during a new phase of progradation a freshwater cemented sequence going from shallow subtidal to supratidal and eolian deposits does not therefore necessarily imply sea level changes but can be created by a cycle of progradation cementation and erosion

the holocene volcanic history of gran canaria island implications for volcanic hazards previous published data combined with our results of 13 new radiocarbon ages and extensive geological fieldwork indicate that during the past 11 ka 24 monogenetic basaltic eruptions occurred in the north sector of gran canaria these eruptions can be grouped into three periods of eruptive activity 1900 3200 14c a bp 5700 6000 14c a bp and an older period represented by only one eruption el draguillo dated at 10 610 190 14c a bp archaeological studies have shown that the more recent eruptions affected prehistoric human settlements on the island field studies demonstrate that the eruptions typically built strombolian cones 30 250 m in height and associated relatively long lava flows 100 10 350 m in length a few eruptions also produced tephra fall deposits the total erupted volume of these eruptions is about 0 388 km3 46 1 as tephra fall 41 8 as cinder cone deposits and 12 1 as lava flows the relatively low eruption rate 0 04 km3 ka 1 during the past 11 ka is consistent with gran canaria s stage of evolution in the regional volcano tectonic setting of the canary archipelago the results of our study were used to construct a volcanic hazards map that clearly delimits two sectors in the ne sector of gran canaria where potential future eruptions would pose a substantial risk for densely populated areas copyright 2009 john wiley sons ltd

clay mineral diagenesis and quartz cementation in mudstones the effects of smectite to illite reaction on rock properties abstract the late cretaceous to early tertiary sequence of the voring and more basins from the norwegian sea has been examined with respect to mineralogy based on 319 cutting samples from five wells a clear relationship between mineralogy and well log data is demonstrated a significant change with respect to velocity porosity and density occurs within the depth interval corresponding to 80 90 c at shallow depths temperatures

diagenesis of quaternary bahamian beachrock petrographic and isotopic evidence abstract carbonate beachrock is a common feature on many beaches of san salvador island bahamas these well indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward constituent sediment grains are identical to those of adjacent unlithified beach sands beachrock exposed along the northern western and southern coastline of the island is commonly characterized by aragonite needle cement micritized grains and micritic envelopes which are features of early marine phreatic diagenesis these beachrocks have whole rock 13c values of 4 75 pdb and 18o values of 0 75 pdb which are identical to carbon and oxygen isotopic compositions of isolated aragonite needle cement and unlithified beach sand inclusion of recent artifacts in the rock indicates that these deposits are holocene beachrock from one locality on the western side of the island is covered with a calcrete crust indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic rich soils petrographic features include low mg calcite cement evidence of neomorphism of aragonitic cement and micritic rims to low mg calcite random needle fiber calcite calcified filaments and micritic networks whole rock 13c values of these beachrocks range from 3 66 to 6 85 and 18o from 2 16 to 3 68 the wide range of 13c values in these samples is probably a function of the relative proportions of meteoric vadose to meteoric and or marine phreatic cements in individual samples isolated meteoric phreatic cements are only slightly enriched in the light isotopes relative to marine cements whereas meteoric vadose cements are significantly lighter the diagenetic features suggest the beachrock passed through marine phreatic to meteoric phreatic and meteoric vadose diagenetic environments this diagenetic sequence commonly forms in response to sea level lowering evidence of deposition in a shoreline environment and prolonged residence in meteoric phreatic and meteoric vadose environments suggests that these beachrocks may be late pleistocene

petrogenesis of metaluminous a type rhyolites from the st francois mountains missouri and the mesoproterozoic evolution of the southern laurentian margin abstract predominantly acid volcanic and intrusive rocks of the approximately 1 48 ga eastern granite rhyolite province are exposed only in the st francois mountains inlier missouri combined nd isotopic major and trace element analyses are presented for the predominantly acid volcanic rocks of the st francois mountains they are metaluminous to mildly peraluminous a type compositions with positive initial e nd values of 3 1 5 0 they are interpreted to be the result of crystal fractionation of partially melted calc alkaline igneous rocks formed at a recently active subduction zone the likelihood of a juvenile calc alkaline subduction generated protolith for the st francois mountains rhyolites leads to a hypothesis in which dacitic parental magmas resulted from partial melting in a continental back arc setting basaltic magmatism due to decompression melting of the mantle resulted from lithospheric extension and thinning and provided the heat source for melting in the middle or lower crust penecontemporaneous magmatism elsewhere in laurentia included subduction related calc alkaline rocks in the pinware terrane of labrador some felsic igneous rocks of uncertain plate tectonic setting within the grenville province also have 1 5 1 6 ga t dm ages these are interpreted as magmatic products of approximately 1 5 ga northward dipping subduction beneath much of the length of the southern margin of laurentia the eastern granite rhyolite province is consequently seen as an integral inboard expression of this subduction related igneous activity petrogenetic models involving a mantle plume or the development of a continental rift are not required to explain this granite rhyolite terrane and evidence against their involvement is discussed

beachrocks from the island of la palma canary islands spain beachrocks on la palma island developed on platform forming lavas of the cumbre vieja volcano some of these lavas are related to the 1585 puerto naos 1677 and 1971 echentive eruptions radiocarbon dating of the charco verde beachrock gives a conventional age of 33 330 8 490 bp while that at playa chica beach gives a calibrated age of 14 940 8 525 bp the beachrocks up to 1 5 m thick and some tens of metres wide consist of several decimetre thick horizons dipping 2 15 seaward petrographically they can be classified as rudstones and arenites with volcanic clasts as their main component the original porosity of the beachrocks was intergranular and occasionally intragranular and was partially occluded by cementation and locally by internal sediments the main cements are fibrous aragonite and micrite high magnesium calcite hmc spar aragonite peloidal hmc and microbotryoidal hmc are scarce the elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters the evolution of beach deposits and especially the beachrocks in la palma island follows three stages 1 beach deposition 2 beachrock formation and 3 beach retrogradation and or erosion the studied beachrocks prompt us to make some important considerations 1 the mean tidal range in the canary islands has not varied over the last thousand years 2 the position of the beachrocks at the present day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years 3 volcanic activity supplies the sediment that forms the beaches 4 a dry warm climate with a very low rainfall below 250 mm year and a high insolation rate 6 11 h day favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches 5 the presence of beachrocks in the la palma beaches prevents the total disappearance of the beaches 2003 elsevier science b v all rights reserved

beachrock a tool for reconstructing relative sea level in the far field abstract today s understanding of sea level change developed through a combination of process based physical modelling and observational data observational data of sea level change derives from coral reefs in the far field of the former ice sheets where a geographically variable relative sea level signal is expected as a response of the earth to ocean loading given this variability and the limited geographical distribution of coral reefs there is a need to explore other non coral based sea level markers to further understand sea level change and for example to fingerprint melt water here we present beachrock as a coastal deposit suitable for relative sea level rsl observations in the far field beachrock is an intertidal deposit forming in the zone where carbonate saturated meteoric and marine water mix and p co 2 decreases we provide the conceptual framework for beachrock analysis and describe techniques suitable for analysing and dating the deposit the approach is standardised by outlining the sediment characteristics in terms of rsl indicative meaning and indicative range and is tested against published data a study conducted on coasts of the mediterranean sea exemplifies the utility of beachrock for rsl reconstruction it is shown that the precision of the reconstruction is derived from the combined uncertainty of age and tidal amplitude or tidal range the uncertainty can be reduced to half the tidal amplitude when a deposit can be ascribed to the upper or lower intertidal zone beachrock based data benefit from the lack of non quantifiable error terms such as post depositional compaction due to the instantaneous formation and high preservation potential of the deposit this underlines the high precision of beachrock based rsl reconstruction which is a prime requirement for testing and extending coral based records

climatic cycles inferred from the aminostratigraphy and aminochronology of quaternary dunes and palaeosols from the eastern islands of the canary archipelago aminochronological and aminostratigraphical methods have been used to study the quaternary a e o l i a n deposits f r om the islands located east of the canary archipelago fuerteventura and lanzarote islands and la graciosa montana clara and alegranza islets t h e extent of racemisation epimerisation of four amino acids isoleucine aspartic acid phenylalanine and glutamic acid was measured in land snail shells of the genus theba the age calculation algorithms of these amino acids h a v e b e e n determined to permit t he numerical dating of these deposits eight a m i n o z o n e s e a ch defining dune palaeosol formation episodes have been distinguished and dated at 4 8 6 6 4 42 5 6 0 37 8 4 6 29 4 4 8 22 4 4 5 14 9 3 6 11 0 4 0 and 5 4 1 1 ka b p t h e first f i ve of t h em defining c y c l e s of 5 7 ka the alternation of palaeosols and a e o l i a n deposits w h i c h are related to abrupt transitions from humid to arid conditions are t he reflection of globally induced changes in north africa palaeoenvironmental conditions linked to the effect of african palaeomonsoons on the trade w i n d s and the saharan air layer probably these a eolian cycles with a recurrence neriod of 5 7 ka are t he expression of multiples of the 2 4 ka solar cycle

formation exposure and evolution of a high latitude beachrock in the intertidal zone of the corrubedo complex ria de arousa galicia nw spain abstract the presence of beachrocks is noted on the galician shoreline at 41 n a latitude at which this phenomenon is rarely found the present study focuses on the petrological characterization and sedimentological evolution of a beachrock that was exhumed during an exceptional storm in the winter of 2001 in the intertidal zone of a high energy beach the cemented material comprises medium to coarse mixed siliciclastic carbonate sands unlike tropical beachrocks the cement consists of epitaxial low mg calcite presenting varied morphologies and textures acicular and bladed coatings and pore linings pore fillings of sparitic calcite and meniscus style cements seawater dominantly wave action plus marine and meteoric mixed waters are invoked as the main generative fluids beachrock formation and evolution are approached from a morphodynamic viewpoint considering four progressive stages 1 initial cementation in the intertidal zone 2 exposure and modelling by wave action 3 colonization and hardening and 4 disintegration preservation and burial finally the morphodynamic significance climatic implications and occurrence of this type of cementation along the north and northwest shoreline of spain are discussed

fluvial dissection isostatic uplift and geomorphological evolution of volcanic islands gran canaria canary islands spain abstract digital analysis of torrential gullies barrancos deeply incised into the volcanic island of gran canaria canary islands allows us to extract the longitudinal profiles and pre incision surfaces for individual basins from which morphometric parameters length elevation area slope have been calculated other derived parameters such as ridgeline profiles maximum incision values volume removed by fluvial erosion geophysical relief and isostatic uplift have also been computed based on k ar ages for the island well constrained incision uplift rates have been calculated by means of the combination of different methodological approaches commonly used in orogens and large mountain ranges the geomorphological and morphometric analyses reveal that the island is clearly divided into four environmental quadrants determined by the combination of a couple of key factors the age of the volcanic surfaces and the climatic conditions these factors determine a young sector covered with plio quaternary platform forming lavas finished at 1 9 1 5 ma evolving under contrasting wet ne to dry se climates and an older sector conserving the residual surfaces of the miocene shield building 14 5 8 7 ma at the ridgelines also subjected to wet nw and dry sw climates incision is related to the age zonation of the island maximum incisions

the dir utror metavolcanic sequence kohistan arc terrane northern pakistan the dir utror volcanic series forms a ne sw trending belt within the northwestern portion of the kohistan island arc terrane in the western himalayas of northern pakistan the kohistan arc terrane comprises a diverse suite of volcanic plutonic and subordinate sedimentary rocks of late mesozoic to tertiary age developed prior to and after suturing of the indo pakistan and asiatic continental blocks the dir utror volcanic series near dir is dominated by basaltic andesite and andesite with subordinate basalt high mgo basalt dacite and rhyolite porphyritic textures are dominant with less common aphyric and seriate textures plagioclase is the dominant phenocryst in mafic to intermediate rocks k feldspar and quartz phenocrysts predominate in the dacites and rhyolites chlorite epidote albite and actinolite are the most common metamorphic phases blue green amphibole andesine muscovite biotite kaolinite sericite carbonate and opaques are widespread but less abundant phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote amphibolite facies conditions attained locally whole rock major element compositions define a calc alkaline trend cao feo mgo tio2 a l2o3 v cr ni and sc all decrease with increasing silica whereas alkalis rb ba and y increase morb normalized trace element concentrations show enrichment of the low field strength incompatible elements ce la ba rb k and deep negative nb p and ti anomalies patterns typical of subduction related magmas mafic volcanic rocks plot in fields for calc alkaline volcanics on trace element discrimination diagrams showing that pre existing oceanic crust is not preserved here all rocks are lree enriched with la 16 112 chondrite la lu 2 6 9 8 chondrite and eu eu 0 5 0 9 dacites and rhyolites have the lowest la lu and eu eu ratios reflecting the dominant role of plagioclase fractionation in their formation some andesites have la lu ratios which are too high to result from fractionation of the more mafic lavas chondrite normalized ree patterns for these andesites cross those of the basaltic andesites indicating that these lavas cannot be related to a common parent the high proportion of mafic lavas rules out older continental crust as the main source of the volcanic rocks the scarcity of more evolved felsic volcanics dacite rhyolite can be explained by the nature of the underlying crust which consists of accreted intra oceanic arc volcanic and plutonic rocks and is mafic relative to normal continental margins andesites with high la la lu k2o and rb may be crustal melts we suggest that garnet rich high pressure granulites similar to those exposed in the jijal complex may be restites formed during partial melting of the crust 1999 elsevier science ltd all rights reserved

paleoshoreline record of relative holocene sea levels on pacific islands abstract understanding the history of relative holocene sea levels on pacific islands is important for constraining fundamental geodynamic theories interpreting the environments of early human occupation sites and forecasting future environmental conditions on the islands an observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels hence standing at higher elevations than modern counterparts emergent paleoshoreline notches in limestone seacliffs record paleo high tide levels and emergent paleoreef flats record paleo low tide levels whereas emergent paleobeachrock locally records paleo intertidal levels both paleonotches and paleoreefs occur along the coasts of high standing islands exposing volcanic bedrock and uplifted reef complexes but low lying coralline atolls lack sufficient relief to preserve paleonotches controls on relative holocene sea level include global eustatic and regional hydro isostatic changes in ambient sea level relative to island landmasses and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism by flexure of the lithosphere under the load of growing volcanic edifices hawaii samoa society islands by arching of the lithosphere over trench forebulges loyalty islands niue bellona rennell and by tectonism within forearc belts between active volcanic chains and trenches mariana islands tonga vanuatu the dominant pattern of relative sea level change where not overprinted by local tectonism or lithospheric flexure was a uniform early holocene rise in eustatic sea level followed by a regionally variable late holocene hydro isostatic drawdown in sea level the resultant was a mid holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical pacific ocean the earliest human migrations into intra oceanic island groups of both the northwestern and southwestern pacific ocean followed close on the heels of the mid holocene sea level highstand and took advantage of newly attractive coastal environments engendered by sea level drawdown the effects of the mid holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence

a coarse grained turbidite system with morphotectonic control middle albian ondarroa northern iberia the middle albian ondarroa turbidite system is a coarse grained deep water unit which outcrops in the north eastern part of the basque cantabrian region west of the pyrenees it is about 18 km long and 7 km wide and shows an unusual l shape resulting from both a direct morphotectonic confinement and the presence of nearby shallow water carbonate buildups eight main facies have been distinguished within this turbidite system 1 clast supported conglomerates 2 mud supported conglomerates 3 slump deposits 4 normally graded pebbly sandstones 5 cross stratified sandstones 6 interbedded graded sandstones and mudstones 7 interbedded non graded sandstones and mudstones and 8 mudstones inner system middle system outer system and basin plain divisions have been distinguished the inner turbidite system is characterized by stacked channel fill conglomerates and lesser sandy turbidites and mudstones the middle system consists of sandy and conglomeratic fining upwards sequences normally several metres thick the outer system has alternating non channellized sandstones and mudstones without any predictable vertical arrangement the basin plain is characterized by mudstone siltstone laminations and lesser randomly occurring thin bedded sandy turbidites three main channel fills make up the inner turbidite system although all of them can be compared with the valley channel fills of the modern mississippi fan and thus their bases can be interpreted as sequence boundaries only the lowermost and the uppermost channel bases are documented as allocyclic boundaries the ondarroa turbidite system was deposited in an immature passive margin subjected to transtensional movements it filled a composite pull apart depression with coarse clastics derived from a narrow platform to the north of the present outcrops which was invaded by fan deltas a major pattern of sinistral strike slip faulting linked to the opening of the bay of biscay is invoked to explain the ondarroa turbidite system appearance and its tectonic confinement