A giant late Precambrian chert-bearing olistostrome discovered in the Bohemian Massif: A record of Ocean Plate Stratigraphy (OPS) disrupted by mass-wasting along an outer trench slope

An intriguing example of chert–graywacke olistostrome is exceptionally well preserved within the late Neoproterozoic to early Cambrian Blovice accretionary wedge, Bohemian Massif. The olistostrome exhibits a block-in-matrix fabric defined by chert blocks isolated within the graywacke matrix. The major and trace element composition indicates two distinct types of cherts that formed either in a hydrothermal pelagic or hemipelagic environment supplied with a distal terrigenous material. The former is documented by elevated contents of Fe, Co, Zn, Ni, and Ti whereas the latter by high Al₂O₃ contents, relatively lower La_N/Ce_N ratios, and higher Eu/Eu* and Ce/Ce* values. Based on these geochemical data integrated with field observations and detrital zircon U–Pb ages of the host graywackes (determined using laser ablation ICP-MS), a new model for the origin of chert–graywacke association is proposed. The cherts are interpreted as representing pelagic and hemipelagic members of the Ocean Plate Stratigraphy (OPS) that formed in a sedimentary basin, carried on top of a subducting plate towards the trench. While moving over the outer swell (rise), the chert basin was intensely fractured and disrupted into large blocks or slabs. Subsequent motion of the plate brought the blocks onto an outer trench slope where they became gravitationally unstable to slide down and mix in the trench with distal, ca. 580–570 Ma turbidites derived from the overriding plate. Finally, this chert–graywacke olistostrome was covered by younger, ca. 560–547 Ma trench-fill turbidites (devoid of chert blocks) and accreted to the accretionary wedge toe, deformed, buried, and exhumed back to the wedge surface. We propose that such an olistostrome composed of pelagic/hemipelagic chert blocks and terrigenous, arc-derived graywacke matrix represents a rarely documented case of submarine, outer trench slope mass-wasting deposits and may be considered a new type of subduction-related mélanges. We coin the term outer-trench-slope mélange.     

The sedimentological evolution of a Lower Palaeozoic accretionary fore-arc in the Southern Uplands of Scotland

Thick turbidites accumulated along the northern margin of the Iapetus Ocean in Britain from mid-Ordovician to late Silurian times. Recent plate tectonic reconstructions hold that, during subduction, packets of these sediments, together with the underlying pelagic facies and thin portions of the uppermost ocean crust, were stripped from the descending plate and accreted to the inner trench wall on the Laurentian (North American) continental margin. The resulting accretionary prism is represented today by the Ordovician and Silurian rocks of the Southern Uplands of Scotland and the Longford-Down massif of Ireland. In these areas major reverse faults separate tracts of steeply dipping greywackes and mudstones with minor amounts of cherts and basalts. These tracts are up to several kilometres wide; their constituent beds face predominantly to the northwest, away from the site of the ancient ocean, while becoming progressively younger in each major fault slice towards the Iapetus suture in the southeast. From the stratigraphic sequences in these fault slices the sedimentary history of a portion of the Iapetus Ocean, and the British sector of its northern margin, can be reconstructed. In the Southern Uplands the earliest turbidites (mid- and late-Ordovician) are preserved in the northernmost fault slices. Regional facies trends, and vertical facies analysis, suggest that they accumulated in a trench dominated by a series of relatively small lower trench slope-derived fans. Pelagic sediments of the same age are found in the fault slices to the south, suggesting that the Ordovician turbidites were confined to the trench. During the lower and middle Llandovery, volcaniclastic trench turbidites were separated from quartz-rich ocean-floor turbidites (represented in the southern fault slices) by an elongate rise, on which pelagic deposits accumulated. This is interpreted as the outer trench high. In late Llandovery times the rise was overwhelmed, and thick laterally derived quartzose turbidites blanketed both the trench and the ocean floor. Sedimentation was strongly influenced by the evolution of the accretionary prism. By Llandovery times a trench slope break had emerged, supplying sediment both south to the trench and north to an upper slope basin in the Midland Valley of Scotland. In this basin early Silurian turbidites were followed by shallow-water and terrestrial sediments. Most of the sediment was derived from the emergent trench slope break: the volcanic arc and the Grampian orogenic belt to the north provided little or no detritus. Throughout the Ordovician and Silurian, sediment in the trench and on the ocean floor was derived from the volcanic arc, from the lower trench slope/trench slope break, from a degrading plutonic/metamorphic terrain (the Grampian Orogen), and locally by a minor amount of submarine sliding from carbonate-capped volcanic seamounts. Progressive elevation of the trench slope break in Silurian (and perhaps late Ordovician) times indicates that sediment from the arc-orogen hinterland must have bypassed the upper slope in the unexposed section of the margin to the northeast of the Southern Uplands, and travelled into the area axially along the trench floor.     

New interpretation of the Franciscan mélange at San Simeon coast,California: tectonic intrusion into an accretionary prism

Many concepts and interpretations on the formation of the Franciscan mélange have been proposed on the basis of exposures at San Simeon, California. In this paper, we show the distribution of chaotic rocks, their internal structures and textures, and the interrelationship between the chaotic rocks and the surrounding sandstones (turbidites). Mélange components, particularly blueschists, oceanic rocks, including greenstone, pillow lava, bedded chert, limestone, sandstone, and conglomerate, have all been brecciated by retrograde deformation. The Cambria Slab, long interpreted as a trench slope basin, is also strongly deformed by fluidization, brecciation, isoclinal folding, and thrusting, leading us to a new interpretation that turbiditic rocks (including the Cambria Slab) represent trench deposits rather than slope basin sediments. These rocks form an accretionary prism above mélanges that were diapirically emplaced into these rocks first along sinistral-thrust faults, and then along dextral-normal faults. Riedel shear systems are observed in several orders of scale in both stages. Although the exhumation of the blueschist blocks is still controversial, the common extensional fractures and brecciation in most of the blocks in the mélanges and further mixture of various lithologies into one block with mélange muddy matrix indicate that once deeply buried blocks were exhumed from considerable depths to the accretionary prism body, before being diapirically intruded with their host mélange along thrust and normal faults, during which retrograde deformation occurred together with retrograde metamorphism. Recent similar examples of high-pressure rock exhumation have been documented along the Sofugan Tectonic Line in the Izu forearc areas, in the Mineoka belt in the Boso Peninsula, and as part of accretionary prism development in the Nankai and Sagami troughs of Japan. These modern analogues provide actively forming examples of the lithological and deformational features that characterize the Franciscan mélange processes.     

The Misis–Andırın Complex: a Mid-Tertiary melange related to late-stage subduction of the Southern Neotethys in S Turkey

The Mid-Tertiary (Mid-Eocene to earliest Miocene) Misis–Andırın Complex documents tectonic-sedimentary processes affecting the northerly, active margin of the South Tethys (Neotethys) in the easternmost Mediterranean region. Each of three orogenic segments, Misis (in the SW), Andırın (central) and Engizek (in the NE) represent parts of an originally continuous active continental margin. A structurally lower Volcanic-Sedimentary Unit includes Late Cretaceous arc-related extrusives and their Lower Tertiary pelagic cover. This unit is interpreted as an Early Tertiary remnant of the Mesozoic South Tethys. The overlying melange unit is dominated by tectonically brecciated blocks (>100 m across) of Mesozoic neritic limestone that were derived from the Tauride carbonate platform to the north, together with accreted ophiolitic material. The melange matrix comprises polymict debris flows, high- to low-density turbidites and minor hemipelagic sediments.The Misis–Andırın Complex is interpreted as an accretionary prism related to the latest stages of northward subduction of the South Tethys and diachronous continental collision of the Tauride (Eurasian) and Arabian (African) plates during Mid-Eocene to earliest Miocene time. Slivers of Upper Cretaceous oceanic crust and its Early Tertiary pelagic cover were accreted, while blocks of Mesozoic platform carbonates slid from the overriding plate. Tectonic mixing and sedimentary recycling took place within a trench. Subduction culminated in large-scale collapse of the overriding (northern) margin and foundering of vast blocks of neritic carbonate into the trench. A possible cause was rapid roll back of dense downgoing Mesozoic oceanic crust, such that the accretionary wedge taper was extended leading to gravity collapse. Melange formation was terminated by underthrusting of the Arabian plate from the south during earliest Miocene time.Collision was diachronous. In the east (Engizek Range and SE Anatolia) collision generated a Lower Miocene flexural basin infilled with turbidites and a flexural bulge to the south. Miocene turbiditic sediments also covered the former accretionary prism. Further west (Misis Range) the easternmost Mediterranean remained in a pre-collisional setting with northward underthrusting (incipient subduction) along the Cyprus arc. The Lower Miocene basins to the north (Misis and Adana) indicate an extensional (to transtensional) setting. The NE–SW linking segment (Andırın) probably originated as a Mesozoic palaeogeographic offset of the Tauride margin. This was reactivated by strike-slip (and transtension) during Later Tertiary diachronous collision. Related to on-going plate convergence the former accretionary wedge (upper plate) was thrust over the Lower Miocene turbiditic basins in Mid–Late Miocene time. The Plio-Quaternary was dominated by left-lateral strike-slip along the East Anatolian transform fault and also along fault strands cutting the Misis–Andırın Complex.     

Jurassic Subduction of the Paleo-Pacific Ocean in NE China: Zircon U–Pb and Sr-Nd-Hf isotopic Evidence from the Huashan and Taili Monzogranites

The Qilian orogen along the NE edge of the Tibet‐Qinghai Plateau records the evolution of Proto‐Tethyan Ocean that closed through subduction along the southern margin of the North China block during the Early Paleozoic. The South Qilian belt is the southern unit of this orogen and dominated by Cambrian‐Ordovician volcano‐sedimentary rocks and Neoproteozoic Hualong complex that contains similar rock assemblages of the Central Qilian block. Our recent geological mapping and petrologic results demonstrate that volcano‐sedimentary rocks show typical rock assembles of a Cambrian‐early Ordovician arc‐trench system in Lajishan Mts. along the northern margin of the Hualong Complex. Island arc rocks including basalt, andesite, dacite, rhyolite, and breccia is in fault contact with ophiolite complex consisting of mantle peridotite, serpentinite, gabbro, dolerite, plagiogranite, and basalt. Accretionary complexes are tectonically separated from the ophiolite‐arc rocks, with various rock assemblages spatially. They consist of pillow basalt, basalt breccia, tuff, chert, and limestone blocks with a seamount origin within the scaly shale in Dingmaoshan and Donggoumeikuang areas, and basalt, chert, and sandstone blocks within muddy shale matrix and mélange at Lajishankou area. Abundant radiolarians occur in red chert, and trilobite, brachiopod, and coral fossils occur within Dingmaoshan limestone blocks. Although partial basalt or chert blocks are highly disrupted, duplex, thrust fault, rootless intrafolial fold, tight fold, and penetrative foliation are well‐developed at Donggoumeikuang area. Spatially, accretionary complexes lie structurally beneath ophiolite complex and above the turbidites of the Central Qilian block. Ophiolite and accretionary complexes are also overlapped by late Ordovician molasse deposits sourced from Cambrian arc‐trench system and the Central Qilian block. These observations demonstrate that a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt formed during the early Paleozoic southward subduction of the South Qilian Ocean collided with the Central Qilian block prior to the late Ordovician.     

A giant catastrophic mud‐and‐debris flow in the Miocene Makran

We challenge the former interpretation of the ‘sedimentary mélange’ of the Makran accretionary complex as a tectonic mélange diapirically emplaced from below and provide evidence for its sedimentary gravitational emplacement from the north during Tortonian–Messinian times (between 11.8 and 5.8 Ma). It is an olistostrome that includes blocks of ophiolites and oceanic sediments derived from the ‘coloured mélange’ to the north, and reworked chunks of the turbidites on which it rests with an erosional truncation. The chaotic scattering of blocks of any size and lithology and the weak, soft‐sediment deformation of the matrix argue against a tectonic emplacement of the chaotic formation. Its size and internal structure, together with the size of the individual blocks, make the olistostrome a fossil equivalent of the large, gravitationally emplaced debris flows observed along present‐day continental margins and unstable volcanic edifices.     

Submarine fan facies of the Upper Cretaceous Great Valley sequence,northern and central California

The Upper Cretaceous part of the Great Valley Sequence provides a unique opportunity to study deep-marine sedimentation within an arc-trench gap. Facies analysis delineates submarine fan facies similar to those described from other ancient basins. Fan models and facies of Mutti and Ricci-Lucchi allow reconstruction of the following depositional environments: basin plain, outer fan, midfan, inner fan, and slope. Basin plain deposits are characterized by hemipelagic mudstone with randomly interbedded thin sandstone beds exhibiting distal turbidite characteristics. Outer fan deposits are characterized by regularly interbedded sandstone and mudstone, and commonly exhibit thickening-upward (negative) cycles that constitute depositional lobes. The sandstone occurs as proximal to distal turbidites without channeling. Midfan deposits are characterized by the predominance of coarse-grained, thick, channelized sandstone beds that commonly are amalgamated. Thinning-upward (positive) cycles and braided channelization also are common. Inner fan deposits are characterized by major channel-fill complexes (conglomerate, pebbly sandstone, and pebbly mudstone) enclosed in mudstone and siltstone. Positive cycles occur within these channel-fill complexes. Much of the fine-grained material consists of levee (overbank) deposits that are characterized by rhythmically interbedded thin mudstone and irregular sandstone beds with climbing and starved ripples. Slope deposits are characterized by mudstone with little interbedded sandstone; slumping and contortion of bedding is common. Progressions of fan facies associations can be described as retrogradational and progradational suites that correspond, respectively, to onlapping and offlapping relations in the basin. The paleoenvironments, fan facies associations, and tectonic setting of the Late Cretaceous fore-arc basin are similar to those of modern arc—trench systems.     

Matrix‐rich and associated matrix‐poor sandstones: Avulsion splays in slope and basin‐floor strata

A common facies observed in deep‐water slope and especially basin‐floor rocks of the Neoproterozoic Windermere Supergroup (British Columbia, Canada) is structureless, coarse‐tail graded, medium‐grained to coarse‐grained sandstone with from 30% to >50% mud matrix content (i.e. matrix‐rich). Bed contacts are commonly sharp, flat and loaded. Matrix‐rich sandstone beds typically form laterally continuous units that are up to several metres thick and several tens to hundreds of metres wide, and commonly adjacent to units of comparatively matrix‐poor, scour‐based sandstone beds with large tabular mudstone and sandstone clasts. Matrix‐rich units are common in proximal basin‐floor (Upper Kaza Group) deposits, but occur also in more distal basin‐floor (Middle Kaza Group) and slope (Isaac Formation) deposits. Regardless of stratigraphic setting, matrix‐rich units typically are directly and abruptly overlain by architectural elements comprising matrix‐poor coarse sandstone (i.e. channels and splays). Despite a number of similarities with previously described matrix‐rich beds in the literature, for example slurry beds, linked debrites and co‐genetic turbidites, a number of important differences exist, including the stratal make‐up of individual beds (for example, the lack of a clean sandstone turbidite base) and their stratigraphic occurrence (present throughout base of slope and basin‐floor strata, but most common in proximal lobe deposits) and accordingly suggest a different mode of emplacement. The matrix‐rich, poorly sorted nature of the beds and the abundance and size of tabular clasts in laterally equivalent sandstones imply intense upstream scouring, most probably related to significant erosion by an energetic plane‐wall jet or within a submerged hydraulic jump. Rapid energy loss coupled with rapid charging of the flow with fine‐grained sediment probably changed the rheology of the flow and promoted deposition along the margins of the jet. Moreover, these distinctive matrix‐rich strata are interpreted to represent the energetic initiation of the local sedimentary system, most probably caused by a local upflow avulsion.     

鄂尔多斯盆地黄陵地区上三叠统延长组长7、长6油层组浊积岩沉积特征及地质意义*

利用岩心观察、薄片鉴定和粒度分析等方法,对鄂尔多斯盆地黄陵地区上三叠统延长组长7、长6油层组浊积岩沉积特征与油气地质意义进行了研究。研究结果表明,浊积岩主要为长石砂岩,以棱角状—次棱角状为主,粒度具有典型的浊流沉积特征。沉积构造可见泥底构造、同生变形构造、粒序递变层、鲍玛序列等。最常见的鲍玛序列有ABC型、AB型、ADE型、AE型、CDE型和A段叠置型,具备浊积岩的典型特点。识别出薄层浊积岩和中厚层浊积岩,其属于三角洲前缘滑塌成因,可分为中心微相和边缘微相。浊流砂体是半深湖—深湖区发育的良好储集体,其分布区可作为重要的油气勘探区。     

Franciscan melanges compared with olistostromes of Taiwan and Italy

Details of origin of Franciscan melanges are unknown, although subduction is accepted as the controlling process. Some melanges near plate boundaries in Taiwan and Italy are evidently olistostromes. How do Franciscan melanges compare with these? The Lichi melange and units of “argille scagliose” type in the Northern Apennines rest upon normal marine sediments. The time of accumulation was brief, as shown by limiting time brackets. These key types of evidence for olistostrome origin are rare or absent in the Franciscan, but the pervasive shearing would probably have obliterated such evidence.Similarities between the above-cited olistostromes and Franciscan melanges include the following: argillaceous matrix; large and small blocks of sedimentary rocks and ophiolites; phacoidal and joint-block shapes; soft-sediment deformation in some sandstone; rotation of blocks; extreme dispersal of distinctive rocks; reappearance of older rocks at younger levels. Collectively, these similarities suggest that Franciscan melanges were originally assembled by olistostrome accumulation.Differences between presumed olistostromes and the Franciscan include the following, in addition to stratigraphic relations mentioned above. The Lichi melange shows faint original gross layering where shearing is minimal. Franciscan melanges show various compositional units, but shearing allows tectonic explanations. Blueschist metamorphism is rare or absent in olistostromes of Taiwan and the Northern Apennines. It occurs in the Franciscan not only in random blocks, but also as extensive units of schist and phyllite near the structural top of the complex, toward the “hanging wall” (Great Valley sequence). In the structurally lowest levels, only zeolite facies metamorphism is prevalent. Similar generalities apply to ages of rocks at highest and lowest structural levels. The age distribution would be just the opposite if the entire Franciscan were simply an east-dipping pile of olistostromes.It is concluded that neither subduction alone nor olistostromes alone could have produced all the features of Franciscan melanges, but both played an important role. Critical original features of olistostromes have been modified or destroyed by recurrent underthrusting.     

Sedimentary serpentinite and chaotic units of the lower Great Valley Group forearc basin deposits,California: updates on distribution and characteristics

ABSTRACT

Sedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.     

Carbonate turbidite sequences deposited in rift-basins of the Jurassic Tethys Ocean (eastern Alps, Switzerland)

Syn-rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault-bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates. The most abundant redeposits are bio- and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high- and low-density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow-water carbonates and skeletal debris from nearby submarine highs. After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge-shaped with a horizontal upper boundary and consists of a thinning- and fining-upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin-bedded turbidites and hemipelagic limestones. The thick-bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick-bedded turbidites, thinning- and fining-upward cycles are common. The overlying thin-bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine-grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin-plain association developed comprising fine-grained and thin-bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.     

Upper Palaeozoic subduction/accretion processes in the closure of Palaeotethys: Evidence from the Chios Melange (E Greece), the Karaburun Melange (W Turkey) and the Teke Dere Unit (SW Turkey)

During Late Palaeozoic time a wide ocean, known as Palaeotethys, separated the future Eurasian and African continents. This ocean closed in Europe in the west during the Variscan orogeny, whereas in Asia further east it remained open and evolved into the Mesozoic Tethys, only finally closing during Late Cretaceous–Early Cenozoic.Three Upper Palaeozoic lithological assemblages, the Chios Melange (on the Aegean Greek island), the Karaburun Melange (westernmost Aegean Turkey) and the Teke Dere Unit (Lycian Nappes, SW Turkey) provide critical information concerning sedimentary and tectonic processes during closure of Palaeotethys. The Chios and Karaburun melanges in the west are mainly terrigenous turbidites with blocks and dismembered sheets of Silurian–Upper Carboniferous platform carbonate rocks (shallow-water and slope facies) and poorly dated volcanic rocks. The Teke Dere Unit to the southeast begins with alkaline, within-plate-type volcanics, depositionally overlain by Upper Carboniferous shallow-water carbonates. This intact succession is overlain by a tectonic slice complex comprising sandstone turbidites that are intersliced with shallow-water, slope and deep-sea sediments (locally dated as Early Carboniferous). Sandstone petrography and published detrital mineral dating imply derivation from units affected by the Panafrican (Cadomian) and Variscan orogenies.All three units are interpreted as parts of subduction complexes in which pervasive shear zones separate component parts. Silurian–Lower Carboniferous black cherts (lydites) and slope carbonates accreted in a subduction trench where sandstone turbidites accumulated. Some blocks retain primary depositional contacts, showing that gravitational processes contributed to formation of the melange. Detached blocks of Upper Palaeozoic shallow-water carbonates (e.g. Chios) are commonly mantled by conglomerates, which include water-worn clasts of black chert. The carbonate blocks are restored as one, or several, carbonate platforms that collided with an active margin, fragmenting into elongate blocks that slid into a subduction trench. This material was tectonically accreted at shallow levels within a subduction complex, resulting in layer-parallel extension, shearing and slicing. The accretion mainly took place during Late Carboniferous time.Alternative sedimentary-tectonic models are considered in which the timing and extent of closure of Palaeotethys differ, and in which subduction was either northwards towards Eurasia, or southwards towards Gondwana (or both). Terrane displacement is also an option. A similar (but metamorphosed) accretionary unit, the Konya Complex, occurs hundreds of kilometres further east. All of these units appear to have been assembled along the northern margin of Gondwana by Permian time, followed by deposition of overlying Tauride-type carbonate platforms. Northward subduction of Palaeotethys beneath Eurasia is commonly proposed. However, the accretionary units studied here are more easily explained by southward subduction towards Gondwana. Palaeotethys was possibly consumed by long-lived (Late Palaeozoic) northward subduction beneath Eurasia, coupled with more short-lived (Late Carboniferous) southward subduction near Gondwana, during or soon after closure of Palaeotethys in the Balkan region to the west.     

鄂尔多斯盆地西南部延长组长7段浊积岩沉积特征

根据野外露头、岩心观察及粒度分析,对鄂尔多斯盆地西南部延长组长7段浊积岩的沉积特征进行了研究。结果表明,浊积岩主要为岩屑长石砂岩,以次棱角状为主,成分成熟度及结构成熟度较低;粒度分布上具有典型的浊流沉积的特征,发育粒序层理、包卷层理、沟模、槽模、重荷模、滑塌变形等沉积构造;常见的鲍马序列层序组合有AB型、ABC型、ADE型、AE型。研究区浊积扇可分为上扇、中扇、下扇亚相及主沟道、辫状沟道等相应的沉积微相,上扇、中扇分布范围较广,是浊积扇的主体。在古地理演化过程中,长73浊积岩体规模较小,长72—长71湖侵作用减弱,深湖线收缩,浊积岩逐渐发育,在华池—庆城一带连片展布;湖盆地形、物源供给及构造运动是影响浊积岩发育及分布的重要原因。     

NON-GREYWACKE "TURBIDITE" SANDSTONES IN THE WELSH GEOSYNCLINE

In contrast with the commonly accepted notion regarding ancient turbidites, non-greywacke sandstones are not uncommon in the typically graded turbidite facies of the Cambrian and Silurian sediments in north Wales. The sandstones are the arkosic and lithic types of PETTIJOHN (1957) or the feldspathic and lithic arenites of GILBERT (1954) and occur at the bottom of graded beds when the grain size tends to be above medium grade. Petrological features suggest that debris forming the sandstones in north Wales was not significantly modified during transportation and original provenance characters are well preserved. The occurrence of such sandstones implies that: (1) the current concept of ancient graded greywackes in the turbidite facies should be revised; (2) non-greywacke sandstones in ancient turbidites are comparable in petrological features to recent deep-sea sands; (3) these sandstones are important in connection with the origin of the clay matrix in greywacke.     

Timing of initiation of extension in the Tianshan,based on structural,geochemical and geochronological analyses of bimodal volcanism and olistostrome in the Bogda Shan (NW China)

This paper describes an olistostrome formation and accompanied bimodal volcanic rocks occurring in the Baiyanggou area, south of Bogda Shan. The main lithotectonic units consist of olistostrome, volcanic rocks and turbidite. The olistostrome is tectonically underlain by Upper Carboniferous limestone and sandstone along a NEE-trending detachment fault. Paleo-growth fault is locally observed. The olistostrome unit includes plenty of blocks of limestone, sandstone, rhyolite and volcaniclastic rocks, and a matrix of graywacke. Limestone blocks are dated as Pennsylvanian-Bashkirian in age by the coral and brachiopod fossils that are extensively recognized in the Upper Carboniferous strata. The volcanic unit consists of pillowed and massive basalt and rhyolite, the latter occur as an 8- to 10-meter-thick layer above the olistostrome unit. The turbidite unit is mainly composed of chert, siliceous mudstone and sandstone, within which the Bouma sequence can be locally recognized. Meter-wide gabbro and diabase dykes intrude these three units. Geochemically, rhyolites are characterized by high ACNK value of >1.1, depletion of Ba, Nb and Sm, and enrichment in Rb, Th and Zr. Basaltic rocks are rich in K₂O, they show a LREE-enriched pattern and depletion in Ba, Nb and Zr, and enrichment in Ti, Ce and Hf, similar to continental rift-type tholeiite series. A gabbro porphyrite intruding the olistostrome was dated at 288 ± 3 Ma by a sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb method, and a rhyolite at 297 ± 2 Ma by a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) zircon U–Pb method. The Baiyanggou olistostrome and accompanying bimodal volcanic series are linked to an extensional setting that developed in the south of the Bogda Shan. Several lines of evidence, e.g. occurrence of large-scale strike-slip shear zones, large number of mantle-derived magmatic rocks and available geochronological data, demonstrate a significant geodynamic change from convergence to extension in the Chinese Tianshan belt, even in the whole Central Asian Orogenic Belt. The extension in the Chinese Tianshan belt is initiated at ca. 300 Ma, i.e. around Carboniferous–Permian boundary times, and the peak period of intra-plate magmatism occurred in the interval of 300–250 Ma.     

藏北那曲盆地中—上侏罗统拉贡塘组浊流沉积特征及微量元素地球化学

藏北那曲地区中上侏罗统拉贡塘组发育典型的深水浊流沉积,根据浊积岩鲍马序列和粒度特征,可以进一步将其区分为近源砂质浊积岩和远源细屑浊积岩。微量及稀土元素地球化学研究表明,拉贡塘组浊积岩富集Cr、Co、Sc、Th等元素,无Ce负异常或负异常不明显,沉积物物源主要来自于变质岩区或再循环的沉积岩分布区。那曲盆地是冈底斯中生代多岛弧盆系统的组成部分之一,中上侏罗世深水浊流沉积形成于活动大陆边缘中的弧后盆地环境,该弧后盆地的形成与班公湖怒江中特提斯洋盆的洋壳向南的俯冲作用有关。     

Distinctive thin-bedded turbidite facies and related depositional environments in the Eocene Hecho Group (South-central Pyrenees, Spain)

The vertical and lateral stratigraphic relations of facies and facies associations, palaeocurrent directions, and geometry and internal organization of associated thick-bedded and coarse-grained bodies of sandstone provide the framework for distinguishing five thin-bedded turbidite facies in the Eocene Hecho Group, south-central Pyrenees, Spain. Each facies is characterized by a number of primary features which are palaeoenvironmental indicators by themselves. These features and their palaeoenvironmental significance are summarized below.

• 1 The impressive regularity and lateral persistence of bedding and depositional structures, combined with the association of thin hemipelagic intercalations are typical characteristics of the basin plain thin-bedded turbidites. Lateral variations in bed thickness, internal structures, grain size, sand: shale ratio, and amounts of hemipelagic intercalations are present in these sediments, but take place so gradually that they cannot generally be recognized at the scale of even very large exposures. The basin plain facies has a remarkable character of uniformity over great distances and considerable stratigraphic thicknesses.
• 2 Thickening-upward and/or symmetric cycles with individual thicknesses ranging from a few metres to a few tens of metres are typical of lobe-fringe thin-bedded turbidites. The sediments that comprise the cycles contain small but recognizable variations in bed thickness and sand: shale ratio. The diagnostic cyclic pattern can be detected in relatively small exposures. It should be noted that in absence of coarse-grained and thick-bedded sandstone of the depositional lobes the above cyclic pattern is diagnostic of fan-fringe areas.
• 3 An extremely irregular bedding pattern with lensing, wedding, and amalgamation of individual beds over very short distances, sharp rippled tops of many beds, and internal depositional structures indicative of mainly tractional processes without substantial fallout, are typical and exclusive characteristics of channelmouth thin-bedded turbidites.
• 4 Bundles of interbedded thin-bedded sandstone and mudstone as thick as a few metres that are separated in vertical sequences by mudstone units of roughly similar or greater thickness are typical of interchannel thin-bedded turbidites. The most diagnostic feature of this depositional environment is the presence of beds of sandstone filling broad shallow channels as probable crevasse-splays.
• 5 Thin, thoroughly rippled sandstone beds with marked divergence of the bedding attitude characterize the channel-margin facies. The divergence or expansion in thickness, is consistently toward the channel axis. Small and shallow channels filled with thin-bedded deposits, interpreted here as crevasses cut into channel edges or levees during period of severe overbanking are also characteristic.

     

南阳地区泌阳凹陷沉积相展布与油气勘探

利用地震、测井、岩心资料,根据沉积构造、岩性剖面结构、砂体厚度确定了泌阳凹陷毕店地区古近系核桃园组三段沉积相类型;借助新的地震反演方法追踪三维数据体中的薄储层,在解释追踪基础上,得到核三段等时单砂层(或小层)厚度变化图,并编制了沉积相分布图。结果表明这里以广泛发育的浊积扇、滑塌扇等浊流沉积为特色,呈现出"南北部浊积扇夹中部深湖相"的沉积格局;早期富浊积砂、晚期富深水泥的沉积演化反映了湖平面的上升过程。基于有利的沉积相带,预测并评价了隐蔽圈闭,为油气勘探提供了依据。     

Precession-punctuated growth of a late Miocene submarine-fan lobe on Gavdos (Greece)

Sedimentary cycles in an upper Miocene succession of hemipelagic sediments (marls) and laminites (sapropels) were deposited in an outerarc basin and are related to the astronomical cycles of precession and eccentricity. Individual marl-laminite couplets correspond with the cycle of precession which has a periodicity of about 22 kyr. The lower part of the succession contains a turbidite interval comprising a number of distinct turbidite sequences. The turbidite sequences occur within or substitute entirely the laminite beds, so that turbidite deposition is similarly precession punctuated. The turbidite facies is characteristic for small, prograding fan lobes fed by small-volume turbidites. The abundant plant remains, the local palaeogeographic setting and the association with laminites (related to wet climate) suggest a river-fed submarine fan-lobe, where the timing of sediment transport is largely controlled by river floods during periods of high precipitation and continental run-off. The onset and ending of the turbidite interval is most likely linked with either autocyclic processes or by tectonic steepening of the hinterland relief. Sea-level changes seem least important for the triggering of turbidites, which is in contrast with current beliefs.