Upslope flow of turbidity currents on the northwest flank of the Cearà Rise: western Equatorial Atlantic*

A piston core (RC16-57) raised from the northwestern flank of the Ceará Rise contained several turbidites up to 62 cm thick with grain sizes ranging from clay to coarse sand. These turbidites were similar in composition to terrigenous turbidites found throughout the Amazon Cone, continental rise and abyssal plains of the western Equatorial Atlantic. The core site (RC16-57) on the Ceará Rise, however, was 156 m above the level of the adjacent Amazon Cone (the source of the turbidites). Thus the turbidity currents which deposited these beds apparently had to flow upslope for 17 km to reach the core site. Sub-bottom reflectors observed on a 3.5 kHz echogram that extended from the Amazon Cone upslope past the core site suggested that these and deeper turbidites extended from the cone up the rise flank to distances of up to 40 km from the cone/rise boundary and to elevations up to 400 m above the level of the cone at the base of the rise. An equally plausible explanation could be that the turbidity currents that deposited these sediments were in excess of 400 m in thickness and thus would not require uphill flow to reach their observed location on the rise flank. The absence of terrigenous turbidites from the bases of topographic knolls on the continental rise and abyssal plains throughout the western Equatorial Atlantic indicated, however, that turbidity currents were normally less than 100 m thick and hence would seem to rule out this explanation. The average gradient of the rise flank in this region was about 1 : 1000 (\sim 0.5°).     

Earthquake‐induced turbidites triggered by sea level oscillations in the Upper Cretaceous and Paleocene of Italy

Earthquakes are widely recognized as triggers for turbidites, submarine debris flows and slumps. In tectonically active areas, surprisingly small changes in stress can trigger seismic events, implying that past sea level changes may be important in controlling the timing of seismicity and the occurrence of turbidites. We apply this idea in an analysis of turbidites from the upper Cretaceous–Paleocene Scaglia Rossa Formation of the Umbria‐Marches region of Italy. These turbidites are composed of resedimented foraminiferal tests derived from fluidizing deep‐water (∼1500 m), pelagic sediments; seismic triggering is the most likely triggering mechanism given this setting and composition. The timing of these turbidites (and associated synsedimentary slumps), constrained by biostratigraphy and magnetostratigraphy, reveals an unusual, non‐random temporal pattern that appears to correlate well with proposed eustatic fluctuations. This correlation between turbidites and eustatic fluctuations leads us to suggest that stress and pore fluid pressure changes associated with changing sea level may trigger periods of increased seismicity in the geological past.     

北山造山带古生代浊积岩构造环境探讨及其大地构造意义

北山造山带是研究中亚造山带增生造山的关键地区之一,浊积岩是增生造山带的重要组成部分。北山古生代浊积岩主要出露于营毛沱、柳园和黑山口地区。营毛沱浊积岩发育于下奥陶统,古水流方向由南向北,内部砂岩具中高等风化程度的长英质源区,构造背景为被动陆缘。早二叠世柳园浊积岩内部砂岩具低到中等风化程度的中基性源区,构造背景为大洋岛弧。早二叠世黑山口浊积岩中的砂岩源区具中等风化程度,环境相对柳园砂岩较为稳定,和长英质源区的沉积岩具相似性,构造环境可能为活动陆缘弧。对北山古生代浊积岩的解剖揭示北山古生代经历了复杂的俯冲增生过程。早古生代花牛山-火石山一带发育向北的俯冲,火石山南部被动陆缘形成营毛沱浊积岩,之后的俯冲带局部后撤形成泥盆纪墩墩山岛弧。柳园地区晚古生代洋壳向花牛山和石板山岛弧带俯冲分别形成了柳园和黑山口浊积岩。本研究支持北山增生时间持续到早二叠世的观点,对认识天山、索伦缝合带的衔接对比研究具有重要的意义。     

北部湾下第三系中的浊积岩

北部湾拗陷位于海南岛西面,目前为一半封闭浅海区,拗陷在前白垩系基底上发育起来,接受中、新生代沉积约3—4千米。笔者在拗陷的钻井岩芯中发现浊积岩。该凹陷长轴北东东向,南北缘有同生断层发育,钻井位于北部同生断层南面的断层上盘。岩芯表明,上第三系为浅海相沉积,下第三系缺少标准化石,按岩性分三个岩组:下部红色岩组,中部暗色岩组和上部杂色岩组。浊积岩系位于暗色岩组的下段(图1)。     

Results of research into Holocene sediments of the South and Central basins of Lake Baikal (BDP-97 and short cores)

Results of investigations of Baikal bottom sediments from a long core (BDP-97) and several short (0–1 m) cores are presented. It has been shown that the Holocene sediments in the Baikal basins consist of biogenic-terrigenous muds, accumulated under calm sedimentation conditions, and of turbidites, formed during catastrophic events. The turbidites can be distinguished from the host sediments by their enrichment in heavy minerals and thus their high magnetic susceptibility. Often, Pliocene and Pleistocene diatom species observed in the Holocene sediments (mainly in the turbidites) point to redeposition of ancient offshore sediments. Our results indicate that deltas, littoral zones, and continental slopes are the source areas of turbidites. The fact that the turbidites occur far from their sources confirms the existence of high-energy turbidity currents responsible for long-distance lateral-sediment transport to the deep basin planes of the lake.     

Timing and distribution of calciturbidites around a deeply submerged carbonate platform in a seismically active setting (Pedro Bank, Northern Nicaragua Rise, Caribbean Sea)

The sedimentological study of thirteen sediment cores from the periplatform setting surrounding Pedro Bank (Northern Nicaragua Rise, Caribbean Sea) shows that during the last 300 ka turbidite deposition is controlled by at least four factors: (1) late Quaternary sea level fluctuations, (2) prolific fine-grained sediment production and export resulting in oversteepening of the upper slope environment, (3) the proximity to the bank margin, and (4) local slope and seafloor morphology. The most intriguing finding of this study is the paucity of turbidites, with only 101 turbidites in 13 cores in this tectonically active setting near the Caribbean plate boundary. Throughout the last 300 ka, the frequency of turbidite input during interglacial stages is three times higher than during glacial stages. Also it is obvious that changes in sea level influence the timing of turbidite deposition. This is especially evident during the transgressions resulting in rapid renewed bank-top flooding, subsequent neritic sediment overproduction, and offbank export. The flooding event during each transgression is usually recorded by the onset of turbidite deposition at various sites along several platform-to-basin transects in down- and upcurrent slope settings. Overall, however, more turbidites are deposited during the regressive rather than the transgressive phases in sea level, probably as a result of sediment reorganisation on the slope resulting in slope failure. Five cores show "highstand bundling" of calciturbidites, i.e. higher number of turbidites during highstands than during lowstands in sea level.     

Variability over time in the sources of South Portuguese Zone turbidites: evidence of denudation of different crustal blocks during the assembly of Pangaea

This study combines geochemical and geochronological data in order to decipher the provenance of Carboniferous turbidites from the South Portuguese Zone (SW Iberia). Major and trace elements of 25 samples of graywackes and mudstones from the Mértola (Visean), Mira (Serpukhovian), and Brejeira (Moscovian) Formations were analyzed, and 363 U-Pb ages were obtained on detrital zircons from five samples of graywackes from the Mira and Brejeira Formations using LA-ICPMS. The results indicate that turbiditic sedimentation during the Carboniferous was marked by variability in the sources, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (Mértola Formation and the base of the Mira Formation) inherited from a terrane with intermediate to mafic source rocks. These source rocks were probably formed in relation to Devonian magmatic arcs poorly influenced by sedimentary recycling, as indicated by the almost total absence of pre-Devonian zircons typical of the Gondwana and/or Laurussia basements. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (Mira and Brejeira Formations) included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism. The abundance of Precambrian and Paleozoic zircons reveals strong recycling of the Gondwana and/or Laurussia basements. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (Cadomian-Avalonian and Pan-African zircon-forming events), Paleoproterozoic, and Archean ages. The presence of late Ordovician and Silurian detrital zircons in Brejeira turbidites, which have no correspondence in the Gondwana basement of SW Iberia, indicates Laurussia as their most probable source.     

Turbidite stratigraphy in proglacial lakes: Deciphering trigger mechanisms using a statistical approach

Turbidites embedded in lacustrine sediment sequences are commonly used to reconstruct regional flood or earthquake histories. A critical step for this method to be successful is that turbidites and their trigger mechanisms are determined unambiguously. The latter is particularly challenging for prehistoric proglacial lake records in high-seismicity settings where both earthquake-generated and flood-generated turbidites interrupt the background varved sedimentation. This calls for a new method to allow efficient and objective identification and classification of turbidites. This study examined turbidites in five long (9 to 17 m) sediment cores from Eklutna Lake, a proglacial lake in south-central Alaska, using standard core logging and grain-size data. A novel statistical approach is presented, in which varve-thickness distributions were first analyzed to objectively identify the thickest turbidites and distinguish them from background sedimentation. For each turbidite, a selection of variables were then measured, including: basal grain-size, thickness, magnetic susceptibility and spectrophotometric variables. Triggering mechanisms were discriminated by a combination of principal component analysis and clustering, and by calibration with historical events. Using this approach, a 2250 year long lake-wide event stratigraphy was constructed, with 94 prehistoric events, including 24 earthquake and 70 flood events. Basal grain-size and thickness variables turn out to be the most effective proxies for discrimination. This statistical approach is a powerful and new method to identify turbidites and their triggering mechanisms in long prehistoric sediment records. It opens up new prospects for palaeoseismological, palaeohydrological and palaeoclimate studies in proglacial lakes worldwide.     

Turbidite Megabeds in an Oceanic Rift Valley Recording Jökulhlaups of Late Pleistocene Glacial Lakes of the Western United States

Escanaba Trough is the southernmost segment of the Gorda Ridge and is filled by sandy turbidites locally exceeding 500 m in thickness. New results from Ocean Drilling Program (ODP) Sites 1037 and 1038 that include accelerator mass spectrometry (AMS) 14C dates and revised petrographic evaluation of the sediment provenance, combined with high-resolution seismic-reflection profiles, provide a lithostratigraphic framework for the turbidite deposits. Three fining-upward units of sandy turbidites from the upper 365 m at ODP Site 1037 can be correlated with sediment recovered at ODP Site 1038 and Deep Sea Drilling Program (DSDP) Site 35. Six AMS 14C ages in the upper 317 m of the sequence at Site 1037 indicate that average deposition rates exceeded 10 m/k.yr. between 32 and 11 ka, with nearly instantaneous deposition of one approximately 60-m interval of sand. Petrography of the sand beds is consistent with a Columbia River source for the entire sedimentary sequence in Escanaba Trough. High-resolution acoustic stratigraphy shows that the turbidites in the upper 60 m at Site 1037 provide a characteristic sequence of key reflectors that occurs across the floor of the entire Escanaba Trough. Recent mapping of turbidite systems in the northeast Pacific Ocean suggests that the turbidity currents reached the Escanaba Trough along an 1100-km-long pathway from the Columbia River to the west flank of the Gorda Ridge. The age of the upper fining-upward unit of sandy turbidites appears to correspond to the latest Wisconsinan outburst of glacial Lake Missoula. Many of the outbursts, or j?kulhlaups, from the glacial lakes probably continued flowing as hyperpycnally generated turbidity currents on entering the sea at the mouth of the Columbia River.     

The Variscan orogeny in Chios (Greece): Carboniferous accretion along a Palaeotethyan active margin

ABSTRACT New stratigraphic and structural data on the turbiditic succession of Chios (Volissos turbidites) suggest that this clastic wedge formed during the Early Carboniferous. These turbidites, fed long-distance by erosion of the Variscan orogen, were most likely deposited in a Palaeotethyan remnant-ocean basin. They were severely deformed and structurally thickened at anchi-metamorphic conditions before the close of the Palaeozoic. Both contractional and layer-parallel extensional structures, high strain rates, and broken-in-matrix fabrics along thick shear zones may suggest deformation at the toe of an accretionary wedge. Stratigraphic, petrographic and structural data indicate that Chios represents the westernmost part of Palaeotethys which escaped the Carboniferous collision between Gondwana and Laurasia, but received great volumes of terrigenous sediments from the Variscan belt, favouring the growth of a large accretionary prism.     

Recurrent uranium relocations in distal turbidites emplaced in pelagic conditions

The sediments of the Madeira Abyssal Plain, east of Great Meteor Seamount, are dominated by distal turbidite deposition. While the turbidites exhibit a wide compositional range (25–80% CaCO₃), individual examples can be correlated over a wide area and are relatively homogenous. Organic C oxidation, by bottom water oxygen, proceeds from the turbidite tops downwards after emplacement in pelagic conditions, and the progress of this oxidation front is marked by a sharp colour contrast in the sediments (Wilsonet al., 1985). In turbidites with C_org ? 0.5%, redistribution of authigenic U occurs to form a concentration peak (4–9 ppm U), just below the oxidation front or colour change. Several tens μg U/cm² may be mobilised, and in all examples studied ?60% of the remobilised U is relocated into the peak. Following burial by subsequent turbidites, such U concentration peaks are persistent as relict indicators of their extinct oxidation fronts for at least 2 × 10⁵ years. In the case of thin turbidites where labile C_org is almost exhausted, the U peaks may be located in underlying sedimentary units because of their relationship to the oxidation front. A redox mechanism for U peak formation is suggested from these data rather than a complexation with organic matter.     

The character of seismo-turbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece

The early Holocene S-1 sapropelic sequence in the northwest Hellenic Trench has been studied in six piston cores from the Zakinthos and Strofadhes basins. The S-1 sequence, 0.7-3.5 m thick, consists principally of silt to mud turbidites, with rare, thick, disorganized, sandy turbidites. These lithofacies are described and compared with fine-grained turbidites from the literature. Petrographical data, including the abundance of organic carbon and planktonic microfossils, indicate that the principal source of sediment to the turbidites was from the continental slope. On the basis of composition and texture, five turbidite units can be correlated between the two basins. These basins are fed by separate but adjacent drainage systems. The apparently synchronous occurrence of turbidites in the two drainage systems suggests that the turbidity currents were seismically triggered. Some of the turbidites show poorly organized beds which may reflect the slump origin and the short (30 km) distances of travel. Turbidites were deposited more frequently in the S-1 sapropelic interval than in the over- and underlying sediments. Application of slope stability analysis shows that on the 8° slopes above the basins, a 10-cm-thick sapropel would have a factor of safety of about 2, and would fail with earthquake accelerations in excess of 0.08 g. The frequency of earthquakes likely to produce such accelerations is similar to the observed frequency of turbidites. The low strength of the sapropelic sediment makes it particularly susceptible to such failure. Similar thin-skinned slumping may be an important process for the initiation of turbidity currents in other environments where there are steep slopes or high sedimentation rates.     

Flood-triggered versus earthquake-triggered turbidites: A sedimentological study in clastic lake sediments (Eklutna Lake,Alaska)

Lake sediments in Eklutna Lake, Alaska, reveal the presence of turbidites within varved sequences. These turbidites, which result from flood events and earthquakes, show a similar macroscopic appearance. In order to use turbidites to reconstruct flood variability and/or seismic history in the lake basin, it is crucial to determine the trigger of the turbidity currents. This study examined the turbidite caused by the ad 1964 Great Alaska earthquake as well as turbidites linked to historical flood events in order to differentiate between these earthquake-triggered and flood-triggered turbidites. In a suite of samples from throughout the lake, distinctive proxies are identified that can be associated with event-specific flow characteristics. The study presents straightforward discrimination methods related to the sedimentology and geochemical components of the turbidites. These methods are also applicable to other lakes, particularly proglacial lakes where the sediment composition of onshore and offshore sources is similar. Finally, the discrimination of the turbidite trigger can be used to reconstruct the palaeoflood and seismic history.     

云南思茅地区上石炭统沉积特征及其构造背景

思茅地区位于东特提斯构造域的东段,晚古生代时期属扬子地台西缘的一部分。晚石炭世具有台盆相间的沉积格局,形成了三类不同的沉积：滨岸沉积、浅海台地沉积和深水浊流沉积。其中深水浊流沉积以火山源浊流沉积为特征,是在平缓的碳酸盐台地或陆棚之上通过断陷事件发展起来的,包括了５个沉积旋回,表现出强烈的火山活动期与火山休眠期交递进行的沉积旋回特征。自晚石炭世早期到晚期,火山活动期逐渐增长,休眠期逐渐缩短,反映出盆地性质自稳定向活动的转化过程。火山岩具有岛弧型火山岩特征,说明晚石炭世,思茅地区具有活动型大陆边缘沉积特征     

A comparison of the sedimentary records of the 1960 and 2010 great Chilean earthquakes in 17 lakes: Implications for quantitative lacustrine palaeoseismology

Seismically‐induced event deposits embedded in the sedimentary infill of lacustrine basins are highly useful for palaeoseismic reconstructions. Recent, well‐documented, great megathrust earthquakes provide an ideal opportunity to calibrate seismically‐induced event deposits for lakes with different characteristics and located in different settings. This study used 107 short sediment cores to investigate the sedimentary impact of the 1960 M_w 9·5 Valdivia and the 2010 M_w 8·8 Maule earthquakes in 17 lakes in South‐Central Chile (i.e. lakes Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). A combination of image analysis, magnetic susceptibility and grain‐size analysis allows identification of five types of seismically‐induced event deposits: (i) mass‐transport deposits; (ii) in situ deformations; (iii) lacustrine turbidites with a composition similar to the hemipelagic background sediments (lacustrine turbidites type 1); (iv) lacustrine turbidites with a composition different from the background sediments (lacustrine turbidites type 2) and (v) megaturbidites. These seismically‐induced event deposits were compared to local seismic intensities of the causative earthquakes, eyewitness reports, post‐earthquake observations, and vegetation and geomorphology of the catchment and the lake. Megaturbidites occur where lake seiches took place. Lacustrine turbidites type 2 can be the result of: (i) local near‐shore mass wasting; (ii) delta collapse; (iii) onshore landslides; (iv) debris flows or mudflows; or (v) fluvial reworking of landslide debris. On the contrary, lacustrine turbidites type 1 are the result of shallow mass wasting on sublacustrine slopes covered by hemipelagic sediments. Due to their more constrained origin, lacustrine turbidites type 1 are the most reliable type of seismically‐induced event deposits in quantitative palaeoseismology, because they are almost exclusively triggered by earthquake shaking. Moreover, they most sensitively record varying seismic shaking intensities. The number of lacustrine turbidites type 1 linearly increases with increasing seismic intensity, starting with no lacustrine turbidites type 1 at intensities between V½ and VI and reaching 100% when intensities are higher than VII½. Combining different types of seismically‐induced event deposits allows the reconstruction of the complete impact of an earthquake.     

The frequency and periodicity of preserved turbidites in submarine fans as a quantitative record of tectonic uplift in collision zones

The frequency and periodicity of preserved graded turbidite cycles in submarine fans in the Coral Sea and Sea of Japan are correlated to times of tectonic uplift in response to major collisions in the Owen-Stanley Range of Papua and the Hida Range of Japan, respectively. Large frequencies and shorter-term periodicities of turbidites at DSDP Site 210 were coeval with early Pliocene maximum tectonic-uplift rates which occurred in the Owen-Stanley Range in response to obduction. Similarly, large frequencies and shorter-term periodicities of turbidites at Site 299 (Toyama Submarine Fan) were coeval with the late Pleistocene uplift in the Hida Range; this uplift of 1000 to 1500 m occurred in response to collision tectonics. In both cases, trends of increasing frequencies and towards shorter-term periodicities of preserved turbidite depositional events correlate to trends of increasing rates of tectonic uplift.The role of sea-level fluctuations on changing denudation rates in these two collision zones is secondary. At Site 210, larger frequencies and short-term periodicities of preserved turbidites were coeval with early Pliocene high stands of sea level, whereas at Site 299, Pleistocene sea-level fluctuations are considered minor because at low stands of sea level, both relief and denudation rates were increased by about ten to 14%. At Site 286 (New Hebrides Basin), Eocene turbidite deposition is coeval with high stands of sea level, whereas at Site 297 (Northern Shikoku Basin), turbidite deposition was coeval with both rising and falling sea level.Analysis of both frequency and periodicity of turbidites by fan subenvironment at Site 299 indicates a record of continuous deposition, and maintainance of frequency and periodicity trends controlled by tectonic uplift. Late Pleistocene channel and overbank deposits showed periodicity differences of less than 28% of an order of magnitude, whereas Miocene-Pliocene overbank and distal turbidite periodicities differed by a 19% order of magnitude. Greater differences in magnitude occurred between distal turbidites or early Pleistocene age and Pliocene age than between Miocene-Pliocene overbank and distal turbidite deposition with a magnitude difference of 860%. These findings suggest that shifting depocenters and differences in sedimentation history in subenvironments of submarine fans are secondary to the role of tectonic uplift in this particular case.The minimal rate of tectonic uplift required to generate deep-sea fan turbidities appears to be approximately 400 m/million years. This figure is tentative and is based on very few observation points.Frequency and periodicity of preserved turbidite cycles in submarine fans in active continental margins and ancient counterparts should provide an independent measurement of rates and timing of tectonic uplift, particularly in collision terrains. Because this sediment parameter is a record of a single process from a single source and a record of “event stratigraphy”, its usage is preferable over standard and bulk sediment accumulation rates determined from age depth curves.     

辽河盆地大凌河油层湖底扇沉积特征

辽河盆地位于辽宁省南部,渤海湾盆地东北角,是继中生代发育起来的第三纪断陷盆地。分东部凹陷,西部凹陷和大民屯凹陷等二级构造单元(图1)。     

Turbidites in the Upper Carboniferous Ross Formation, western Ireland: reconstruction of a channel and spillover system

ABSTRACT The Upper Carboniferous deep‐water rocks of the Shannon Group were deposited in the extensional Shannon Basin of County Clare in western Ireland and are superbly exposed in sea cliffs along the Shannon estuary. Carboniferous limestone floors the basin, and the basin‐fill succession begins with the deep‐water Clare Shales. These shales are overlain by various turbidite facies of the Ross Formation (460 m thick). The type of turbidite system, scale of turbidite sandstone bodies and the overall character of the stratigraphic succession make the Ross Formation well suited as an analogue for sand‐rich turbidite plays in passive margin basins around the world. The lower 170 m of the Ross Formation contains tabular turbidites with no channels, with an overall tendency to become sandier upwards, although there are no small‐scale thickening‐ or thinning‐upward successions. The upper 290 m of the Ross Formation consists of turbidites, commonly arranged in thickening‐upward packages, and amalgamated turbidites that form channel fills that are individually up to 10 m thick. A few of the upper Ross channels have an initial lateral accretion phase with interbedded sandstone and mudstone deposits and a subsequent vertical aggradation phase with thick‐bedded amalgamated turbidites. This paper proposes that, as the channels filled, more and more turbidites spilled further and further overbank. Superb outcrops show that thickening‐upward packages developed when channels initially spilled muds and thin‐bedded turbidites up to 1 km overbank, followed by thick‐bedded amalgamated turbidites that spilled close to the channel margins. The palaeocurrent directions associated with the amalgamated channel fills suggest a low channel sinuosity. Stacks of channels and spillover packages 25–40 m thick may show significant palaeocurrent variability at the same stratigraphic interval but at different locations. This suggests that individual channels and spillover packages were stacked into channel‐spillover belts, and that the belts also followed a sinuous pattern. Reservoir elements of the Ross system include tabular turbidites, channel‐fill deposits, thickening‐upward packages that formed as spillover lobes and, on a larger scale, sinuous channel belts 2·5–5 km wide. The edges of the belts can be roughly defined where well‐packaged spillover deposits pass laterally into muddier, poorly packaged tabular turbidites. The low‐sinuosity channel belts are interpreted to pass downstream into unchannellized tabular turbidites, equivalent to lower Ross Formation facies.     

Formation of Hengchun Accretionary Prism Turbidites and Implications for Deep‐water Transport Processes in the Northern South China Sea

Located at the end of the northern Manila Trench,the Hengchun Peninsula is the latest exposed part of Taiwan Island,and preserves a complete sequence of accretionary deep-sea turbidite sandstones.Combined with extensive field observations,a’source-to-sink’approach was employed to systematically analyze the formation and evolutionary process of the accretionary prism turbidites on the Hengchun Peninsula.Lying at the base of the Hengchun turbidites are abundant mafic normal oceanic crust gravels with a certain degree of roundness.The gravels with U-Pb ages ranging from 25.4 to23.6 Ma are underlain by hundreds-of-meters thickness of younger deep-sea sandstone turbidites with interbedded gravels.This indicates that large amounts of terrigenous materials from both the’Kontum-Ying-Qiong’River of Indochina and the Pearl River of South China were transported into the deep-water areas of the northern South China Sea during the late Miocene and further eastward in the form of turbidity currents.The turbidity flow drastically eroded and snatched mafic materials from the normal South China Sea oceanic crust along the way,and subsequently unloaded large bodies of basic gravel-bearing sandstones to form turbidites near the northern Manila Trench.With the Philippine Sea Plate drifting clockwise to the northwest,these turbidite successions eventually migrated and,since the Middle Pleistocene,were exposed as an accretionary prism on the Hengchun Peninsula.     

Determination of turbidity current erosional characteristics from reworked coccolith assemblages, Canary Basin, north-east Atlantic

Turbidites contain mixtures of sediments of various ages. These sediments can include both material that was eroded to form the initial turbidity current plus additions derived from erosion of the sea floor during transport. It has been possible to interpret the age range of sediments incorporated into individual turbidites from the Madeira Abyssal Plain by examining the proportions of microfossil (coccolith) species that they contain. The pelagic record of coccoliths is well known for the Quaternary period and shows a succession of dominant species or acmes each lasting a few tens of thousand years. Hence, erosion of sediment representing more than a few tens of thousand years will produce coccolith mixtures not seen in the pelagic record, but dependent upon the age range of the sediments that were eroded. This age range can be estimated by comparison with synthetic ratios of coccolith species created by combining ratios of species from successively older layers in the pelagic record. These can then be compared with the ratios found in individual turbidites until a match is found. The results show age ranges of 54–500 kyr for the sediment mixture in seven turbidites from the Madeira Abyssal Plain. Since the volumes of these turbidites are also known, and accumulation rates in their source areas can be estimated, it is possible to determine both the thickness and the area of the eroded sediment mass that generated the turbidity current. Minimum depths of erosion on the north-west African continental margin vary from 8 to 50 m and minimum areas eroded from 1600 to 5800 km². None of the turbidites examined contains a significant excess of surface sediment, suggesting that, once formed, the turbidity currents that transported them were virtually non-erosional, and that they travelled several hundred kilometres in this state.