Turbidite Fan Research Articles

Features of Sandy Debris Flows of the Yanchang Formation in the Ordos Basin and Its Oil and Gas Exploration Significance

Abstract:Sandy debris flow is a new genetic type of sand bodies, which has gained much attention in recent years and its corresponding theory is proved to be a significant improvement and even partial denial to the “Bouma Sequence” and “turbidite fan” deep‐water sedimentary theories to some point. Oil exploration researchers are highly concerned with sandy debris flows for its key role in controlling oil and gas accumulation processes. In this article, by applying sandy debris flows theory and combining a lot work of core, outcrop observation and analysis plus seismic profile interpretation, we recognized three types of sedimentary gravity flows that are sandy debris flows, classic turbidites and slumping rocks in chang‐6 member of Yanchang Formation in the deep‐water area of central Ordos Basin. Among the three types, the sandy debris flows are the most prominent and possesses the best oil bearing conditions. On the contrary, the classic turbidites formed by turbidity currents are limited in distribution; therefore, previous Yanchang Formation deep‐water sedimentary studies have exaggerated the importance of turbidite currents deposition. Further study showed that the area distribution of deep water gravity flow sand bodies in Yanchang Formation were controlled by the slope of the deep‐water deposits and the flows had vast distribution, huge depth and prevalent advantages for oil forming, which make it one of the most favorable new areas for Ordos Basin prospecting.

Multidiscipline Approach, Advanced LWD Boost Multistage Fracturing in Romania

Technology Update Since the start of production from the Lebada fields offshore Romania in 1987, operator Petrom tested various stimulation and completion measures to improve production from the fields’ low-permeability formations. However, by 2008, the company had not been able to achieve production of more than approximately 115 BOEPD per well. A new multidisciplinary approach to well design, combined with the implementation of advanced directional drilling, logging-while-drilling (LWD), and completion technologies, has now increased production per well to a peak of more than 1,000 BOEPD and to stabilized levels of up to 750 BOEPD. Petrom is the largest oil and gas producer in southeastern Europe. It was Romania’s national oil company until privatized in 2004, when it became a member of the OMV Group. Romania has been producing oil since 1856, mostly from onshore reservoirs. Petrom currently produces approximately 32,000 BOEPD from its five commercial offshore fields in the Black Sea, approximately 18% of the company’s annual 66 million BOE production in Romania (2009 figures). The Lebada Oil Fields Petrom’s Lebada Est (East) and Lebada Vest (West) fields are in block Histria XVIII in the Black Sea. Lebada Est was discovered in 1978 and represented Romania’s first offshore production when it came onstream in 1987. It produces oil from Upper Cretaceous carbonate shelf deposits—micritic, sandy and chalky limestones—overlain by overpressured Oligocene shales. Average porosity is 15% to 20% and average permeability is 0.8 md. Microfractures and intergranular porosity connect zones with good porosity in mainly calcareous reservoir forma-tions. Lebada Vest was discovered in 1985, with first production occurring in 1993. It is characterized by the same calcareous formations as Lebada Est in the top part and more clastic turbiditic fan deposits toward the bottom. A gas cap is often present. In 2008, Petrom reviewed technical proposals to increase production rates significantly, having achieved only very limited success until then with measures such as acid fracturing and hydraulic fracturing of older producers completed with cemented and perforated liners. It was concluded that high production would require long horizontal drain sections with good quality borehole to be drilled. Because the hydrocarbons lay within shelf deposits, horizontal continuation of reservoir properties could not be guaranteed. Thus, real-time petrophysical evaluation and well placement would be necessary. Synthetic oil-based mud (SOBM) would be beneficial to handle the Oligocene overpressured shale overburden, provide a better quality hole, and enable safe drilling, helping to ensure efficient, effective completions run along the reservoir section. However, this could limit the choice of formation evaluation logging tools. The low-permeability reservoir rock required hydraulic fracturing and fast, efficient, postdrilling reservoir characterization and natural open fracture detection to select the best locations for placing completion packers and ports.

Submarine mass-transport deposits in the Semantan Formation (Middle-Upper Triassic), central Peninsular Malaysia

Relatively fresh exposures of the Semantan Formation along the East-Coast Highway (Lebuhraya Pantai Timur - Fasa 1) between Karak and Kuantan, central Peninsular Malaysia, have given new insights into the sedimentary processes in the Triassic flysch basin that once separated west and east Malaya. An eastward change from distal to proximal facies between Karak and Maran indicates a west-facing, active continental shelf to slope sedimentation. Outcrops between Karak and Temerloh, east of the Late Triassic-Early Jurassic Bentong-Raub collisional suture, are generally characterized by classical flysch-like, thinly-bedded sandstone-mudstone facies. Further east of Temerloh towards Maran, and nearer to the paleo-shelf and slope, more sandy and thick-bedded turbidite facies occur. A proximal deep- marine facies association in the Semantan Formation is exposed at the Chenor Junction (Exit 821), kilometre 139 along the highway. South- and north-facing cuts on either sides of the highway reveal large gravity-slide blocks (megaclasts), slumps, debris flow deposits, and associated syn-sedimentary thrust faults and glide surfaces. These features are strongly indicative of large-scale submarine mass-transport processes on the palaeo-slope of the Triassic active margin. The Chenor mass-transport complex is made up of zones of incoherent slump deposits intercalated with well-bedded turbidite/debrite facies. In the lower part of the succession, there are megaclasts of sandstone-mudstone facies, measuring several metres in size, encased in a plastically deformed silty matrix. The megaclasts are highly deformed internally by numerous meso- scale normal faults, probably due to gravitationally-induced extension. Along with other smaller sandstone blocks, these megaclasts are interpreted as slide blocks due to slope failure up-dip. There are other gravity-induced structural features such as rotational slumps, glide surfaces, thrust faults and associated soft-sediment folds. The slump folds and thrusts show vergence to the west, as opposed to the generally eastward tectonic vergence. A few of the well-stratified units show strongly inclined stratal surfaces which may be attributed to lateral accretion of turbidite fan lobes. Several sets of these inclined surfaces are bounded by erosional surfaces which could have resulted from different episodes of turbidity flow. The association of incoherent mass-flow units with the more well-stratified deposits reflects the close spatial and temporal relationship between submarine mass-transport events and turbidity flows on the Triassic active slope and basin plain.

STRUCTURAL GEOLOGY OF ROBBEN ISLAND: IMPLICATIONS FOR THE TECTONIC ENVIRONMENT OF SALDANIAN DEFORMATION

We present a detailed structural and lithologic map of Robben Island, offshore Cape Town, South Africa. Robben Island is underlain by the Tygerberg Formation, part of the Neoproterozoic to early Cambrian Malmesbury Group of the Saldania Belt. The depositional setting and structural history of the Tygerberg Formation are poorly constrained due to limited outcrop and lack of previous structural studies. Sedimentary structures are indicative of deposition at relatively high rates in a high energy environment and we concur with previous workers that deposition occurred on turbidite fan systems in a tectonically deepening basin. By comparison with active and ancient examples, we suggest that a forearc or trench slope, supra-subduction zone basin is a possible match to the setting of the Tygerberg Formation. However, limits on preservation and insufficient age data prevent comparisons in basin geometry and deposition rates which could be used to test depositional setting with more certainty. Northwest-southeast striking subvertical pressure solution cleavage is pervasive throughout the exposures. Upright folds, with axial planes parallel to the cleavage, plunge 10 to 15° to the northwest or southeast with approximately 20° variation in trend azimuth. The folds are limited in along-axis extent and often occur in asymmetric pairs. Subtle bedding-parallel shear zones divide folds of different plunge directions. This pattern of folds is consistent with experiments and observations of en echelon folding during distributed strain associated with oblique transpression. This finding is consistent with previous studies of parallel, slightly earlier orogenic belts to the north (Gariep and Kaoko Belts) although our observations do not allow us to distinguish whether transpressional strain was sinistral or dextral. Sinistral transpression is considered more likely given the dominantly sinistral strike-slip history on the nearby Colenso Fault and the southward migration of collision along the western margin of Africa during the late Neoproterozoic to early Cambrian.

Sedimentation of deep-water turbidites in the SW part of the Pannonian Basin

Sedimentation of deep-water turbidites in the SW part of the Pannonian BasinThe Sava Depression and the Bjelovar Subdepression belong to the SW margin of the Pannonian Basin System, which was part of the Central Paratethys during the Pannonian period. Upper Pannonian deposits of the Ivanic-Grad Formation in the Sava Depression include several lithostratigraphic members such as Iva and Okoli Sandstone Member or their lateral equivalents, the Zagreb Member and Lipovac Marlstone Member. Their total thickness in the deepest part of the Sava Depression reaches up to 800 meters, while it is 100-200 meters in the margins of the depression. Deposits in the depression are composed of 4 facies. In the period of turbiditic activities these facies are primarily sedimented as different sandstone bodies. In the Bjelovar Subdepression, two lithostratigraphic members (lateral equivalent) were analysed, the Zagreb Member and Okoli Sandstone Member. The thickness of the Bjelovar Subdepression ranges from 50 meters along the S and SE margins to more than 350 meters along the E margin. Generally, detritus in the north-west part of the analysed area originated from a single source, the Eastern Alps, as demonstrated by sedimentological and physical properties, the geometry of the sandstone body and the fossil content. This clastic material was found to be dispersed throughout the elongated and relatively narrow Sava Depression and in the smaller Bjelovar Subdepression. Sedimentation primarily occurred in up to 200 meters water depth and was strongly influenced by the sub-aqueous paleorelief, which determined the direction of the flow of turbidity currents and sandstone body geometries. The main stream with medium- and fine-grained material was separated by two independent turbiditic flows from N-NW to the SE-E. Variability in the thickness of sandstone bodies is the result of differences in subsidence and cycles of progradation and retrogradation of turbidite fans.

Glaciation and deglaciation of the Libyan Desert: The Late Ordovician record

Detailed outcrop studies at the flanks of Al Kufrah Basin, Libya, reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Azbah), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW–SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal az-Zalmah) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed.

Omanichnus and Vitichnus—Two New Graphoglyptid Ichnogenera from Upper Triassic Deep-Sea Fan Deposits in Oman

Two new graphoglyptid ichnogenera were found in the Upper Triassic turbiditic deep-sea fan deposits in Oman. Omanichnus virgatus igen. nov. et isp. nov. represents a burrow having a central axis from which branches deviate alternating to each side, the axis being at the same level of branching or very close to it. Vitichnus nizwaensis igen. nov. et isp. nov. represents a V-shaped structure composed of narrow-spaced, biserially arranged tubes alternating to each other side forming a plait-like pattern. Behaviorally it contains elements of Saerichnites beskidensis (formerly Hormosiroidea beskidensis) such as the plait-like arranged elements, that is, narrow-spaced, biserially arranged tubes directed alternating to each other side, but differs in the overall V-shape. These new trace fossils represent some of the earliest uniramous graphoglyptids in deep-marine deposits, which became diverse since the Late Cretaceous.

Slope and Submarine Fan Turbidite Facies of the Upper Devonian Imperial Formation, Northern Mackenzie Mountains, NWT

Research Article| June 01, 2009 Slope and Submarine Fan Turbidite Facies of the Upper Devonian Imperial Formation, Northern Mackenzie Mountains, NWT T. Hadlari; T. Hadlari Northwest Territories Geoscience Office, 4601-B, 52nd Avenue, Yellowknife, NWT, X1A 2R3 thomas_hadlari@gov.nt.ca 1Current Address: Geological Survey of Canada, 3303 - 33rd Street NW, Calgary, AB T2L 2A7, thomas.hadlari@nrcan-rncan.gc.ca Search for other works by this author on: GSW Google Scholar S.A. Tylosky; S.A. Tylosky University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Search for other works by this author on: GSW Google Scholar Y. Lemieux; Y. Lemieux Geological Survey of Canada, c/o Northwest Territories Geoscience Office, 4601-B, 52nd Avenue, Yellowknife, NWT X1A 2R3 Search for other works by this author on: GSW Google Scholar W.G. Zantvoort; W.G. Zantvoort Northwest Territories Geoscience Office, 4601-B, 52nd Avenue, Yellowknife, NWT X1A 2R3 Search for other works by this author on: GSW Google Scholar O. Catuneanu O. Catuneanu University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2009) 57 (2): 192–208. https://doi.org/10.2113/gscpgbull.57.2.192 Article history received: 18 Aug 2008 accepted: 30 Mar 2009 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. Hadlari, S.A. Tylosky, Y. Lemieux, W.G. Zantvoort, O. Catuneanu; Slope and Submarine Fan Turbidite Facies of the Upper Devonian Imperial Formation, Northern Mackenzie Mountains, NWT. Bulletin of Canadian Petroleum Geology 2009;; 57 (2): 192–208. doi: https://doi.org/10.2113/gscpgbull.57.2.192 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract At the front of the Mackenzie Mountains west of Norman Wells, the Upper Devonian Imperial Formation was deposited by a fan-slope complex that prograded southwest from an eastern basin margin. Laterally extensive submarine fan deposits are composed of intercalated sandstone lobe and lobe fringe deposits. Sandstone lobes are composed of thick-bedded turbidites reflecting high sediment fallout rates. Lobe fringe deposits are thin-bedded turbidites deposited under upper flow regime conditions. Slope facies are shale-dominant, but include turbidites deposited by both fine-grained dilute flows and thin-bedded facies deposited under upper flow regime conditions but low sediment fallout rates. Near the eastern edge of the study area, at Imperial River, shallow marine deposits are preserved above and below slope deposits, a relationship interpreted to record local shelf construction at the basin margin during initial stages of subsidence. Near the western edge of the study area, at Flyaway Creek, Imperial Formation is composed entirely of shale, marking the western extent of well developed base-of-slope submarine fan deposits. In the western area Imperial Formation is overlain by Tuttle Formation sandstone, which marks an increase in the grain size of sediment supplied to the southwestward prograding slope system. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A high-resolution 2-DH numerical scheme for process-based modeling of 3-D turbidite fan stratigraphy

A generic three-dimensional process-based model is presented, aimed at simulation of the construction of turbidite fan stratigraphy by low-density turbidity current events. It combines theoretical formulations on density flow and sediment transport of multiple grain sizes to simulate 2-DH turbidity current flow and sedimentation over arbitrary topography. The model is solved on a rectangular grid by means of a robust and efficient second-order finite-difference approximation. A high-resolution shock-capturing technique is employed to accurately model the speed and shape of the discontinuous flow front. In this paper, the implementation of such a high-resolution scheme is explained in detail. Efficiency and robustness of the numerical solution are tested by comparing modeled flow behavior and sedimentation to measurements from flume-tank experiments in which turbidity currents interacted with obstacles representative of a tectonically deformed basin floor. Results illustrate that the interaction of the flow with the obstacles is realistically simulated, and that the experimental deposit geometries are reasonably well reproduced. Stability of the model depends on the length of the computational time step and the properties of the adopted flux limiter function. Run time of the simulations is somewhat shorter than the real-time duration of the experiments, which is deemed acceptable considering the small computational time step which must be adopted to keep the model stable at this small scale. At the much larger scale of real-world turbidity currents a much larger computational time step can be adopted, which will speed up simulation considerably.

Palaeozoic Lachlan orogen, Australia; accretion and construction of continental crust in a marginal ocean setting: isotopic evidence from Cambrian metavolcanic rocks

Abstract The Lachlan orogen developed as a classic accretionary orogen in an oceanic setting between the palaeo-Pacific subduction zone and the Australian craton. Direct evidence for the composition and age of the lower crust and the basement to the thick Palaeozoic turbidite fan of the Lachlan orogen is limited. Exposures of Cambrian metavolcanic rocks and geophysical data suggest that most of the basement is the mafic oceanic crust along with possible small fragments of older continental crust. The trace element compositions of Cambrian metavolcanic rocks in the western and central Lachlan orogen are similar to those of volcanic rocks formed in modern back-arc and forearc settings. Pb, Nd and Sr isotopic data from these Cambrian rocks suggest a supra-subduction zone setting with little or no influence of continental crust other than subducted sediment.

シーケンス層序学手法による東部南海トラフエリアのメタンハイドレート含有海底扇状地タービダイト砂岩分布解析およびＢＳＲ分布との関連の検討

Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the distributions and actual volume of methane hydrates in the eastern Nankai Trough area. This paper attempts to reconstruct depositional processes of submarine-fan turbidites, and examines the relationship between turbidite facies distributions and bottom simulating reflector (BSR) occurrence as a proxy of methane hydrate using sedimentologic and sequence stratigraphic methodology. First, 2D/3D seismic survey data and well data including cores and logs were used to identify turbidite facies, seismic facies, and depositional sequences. The targeted Plio-Pleistocene Kakegawa and Ogasa Groups can be divided into 17 depositional sequences, and include six seismic facies indicating submarine-fan elements and surrounding slope to basin-floor environments. Next, facies maps for each depositional sequence unit were created by plotting all information on seismic facies, 3D seismic geomorphology, and well facies data. The obtained facies maps reveal that 11 major submarine canyons functioned as positionally fixed sediment supply systems from main land Japan, along which submarine fans were formed in the forearc basins. Submarine-fan depositional styles changed through Plio-Pleistocene from a braided channel type, through small radial fan, trough-fill fan, and muddy sheet fan types, to a channel-levee system type. Finally, the facies maps of each depositional sequence were overlaid with the BSR distribution. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. Because the lower part of the Kakegawa Group is mainly composed of braided channel-type submarine fan turbidites, the lower Kakegawa horizon serves one of the major horizons bearing methane hydrates in the eastern Nankai Trough area.

Siliciclastic sedimentation in the interlude between two Neoproterozoic glaciations, Mirbat area, southern Oman: A missing link in the Huqf Supergroup?

ABSTRACT The Huqf Supergroup in Oman contains an exceptionally well-preserved and complete sedimentary record of the Late Neoproterozoic era, including the oldest components in some of Oman’s hydrocarbon plays. Outcrops of the Huqf Supergroup in northern and central Oman are now well-documented. However, a key succession in the Mirbat area of southern Oman, the Mirbat Group, which includes a stratigraphic interval missing elsewhere in the Arabian Peninsula, remains poorly understood. The <1.5 km-thick Cryogenian (850–635 Ma) Mirbat Group comprises an essentially continuous succession of little-deformed sedimentary rocks containing two glacial intervals separated by c. 1 km of non-glacial marine deposits. The lowermost glacial interval (Ayn Formation) occupies deep paleovalleys incised into crystalline basement. The overlying Arkahawl Formation records at its base a major post-glacial transgression over the previous basin margin and a 300 to 400 m-thick turbidite complex consisting of 1 to 5 km-wide, coarse-grained depositional lobes embedded vertically and laterally in fine-grained distal turbidite fan deposits. Ayn Formation paleovalleys continued to serve as sediment transport routes for the coarse-grained turbidite complexes of Arkahawl times. The turbidite complex deposits gradationally pass up into a c. 500 m-thick unit of distal-marine mudstone and siltstone. The overlying c. 100 m-thick Marsham Formation records highstand deposition and the pulsed progradation of shallow-marine and fluvial deposits over offshore mudstone and siltstone in the approach to a second glaciation, represented by the Shareef Formation. The sedimentary succession described in this paper is believed to largely fill the stratigraphic gap present between the Ghubrah and Fiq formations in the Al Jabal al-Akhdar in northern Oman represented by an unconformity.

First record of contourites from Lower Devonian Liptrap Formation in southeast Australia

Typical contourite deposits associated with submarine turbidite fan deposits are recognized for the first time from the Lower Devonian Liptrap Formation at Cape Liatrap, Victoria in southeast Australia. The contourites are well integrated within the turbidite fan deposits and are characterized by thin (5–8 cm), lenticular, well-sorted coarse-grained siltstones to fine-grained sandstones with current-ripples and cross beddings. The palaeocurrent directions of the turbidite fan and contourites are perpendicular to each other, with the former directed generally westward while the latter varying from 165° to 190° southward. In view of the facies types and architecture, we suggest that the turbidite fan was developed at the base of a westward inclined palaeo-slope, at the front of which the contourites were deposited as a result of southward flowing deep-sea contour (geostrophic) currents. The depositional setting interpreted for the Liptrap Formation thus may provide a provisional model for the Lower Devonian continental slope and abyssal basin environment in the southeastern part of the Melbourne Trough.

Sequence stratigraphy and depositional systems in the Paleogene, Liaodong Bay

Based on drilling and seismic data, the Paleogene developed in Liaodong Bay can be divided into five third-order sequences bounded by six sequence boundaries. Through analyzing depositional systems in each sequence in detail, the Es-3 time was defined as fast rifting. During the deposition of the Es-3 member, the lake basin was continuous along the N-S direction but compartmentalized along the E-W direction by several N-S trending faults. Deposition was dominated by steep slope fans and fan deltas. The time of Es-2 and Es-1 was stable settling. The lake basin expanded substantially. The fan delta system and braided fluvial system were developed. Carbonate and clastic deposits were formed on the Liaoxi (west Liaohe) Rise. The time of Ed-3 time was again fast rifting. During this time, shale was deposited. In the time of Es-2, tectonic movement weakened. The basin was higher in the west and north, and lower in the east and south. A series of delta depositional systems were developed and small-scale slumping turbidite fans were present in semi-deep lake to deep lake. In the time of Ed-1, tectonic movement stopped. Flood plain deposition occurred. Finally the paper presents the characteristics of evolution and distribution of depositional systems both vertically and horizontally.

Impact of India–Asia collision on SE Asia: The record in Borneo

Borneo occupies a central position in the Sundaland promontory of SE Asia. It has a complex Cenozoic geological history of sedimentation and deformation which began at about the same time that India is commonly suggested to have started to collide with Asia. Some tectonic reconstructions of east and SE Asia interpret a large SE Asian block with Borneo at its centre which has been rotated clockwise and displaced southwards along major strike–slip faults during the Cenozoic due to the indentation of Asia by India. However, the geological history of Borneo is not consistent with the island simply forming part of a large block extruded from Asia. The large clockwise rotations and displacements predicted by the indentor model for Borneo are incompatible with palaeomagnetic evidence and there is no evidence that the major strike–slip faults of the Asian mainland reach Borneo. Seismic tomography shows there is a deep high velocity anomaly in the lower mantle beneath SE Asia interpreted as subducted lithosphere but it can be explained just as well by alternative tectonic models as by the indentor model. Very great thicknesses of Cenozoic sediments are present in Borneo and circum-Borneo basins, and large amounts of sediment were transported to the Crocker turbidite fan of north Borneo from the Eocene to the Early Miocene, but all evidence indicates that these sediments were derived from local sources and not from distant sources in Asia elevated by India–Asia collision. The Cenozoic geological history of Borneo records subduction of the proto-South China Sea and Miocene collision after this ocean lithosphere was eliminated, and a variety of effects resulting from long-term subduction beneath SE Asia. There is little to indicate that India–Asia collision has influenced the Cenozoic geological record in Borneo.

The deep-lake deposit in the Upper Triassic Yanchang Formation in Ordos Basin, China and its significance for oil-gas accumulation

The deep-lake facies of the Yanchang Formation represents a large outflowing lake basin in the Ordos area. Its deposition can be divided into four stages: lake genetic and expanding stage, peak stage, inversion stage and dying stage. All the stages are obviously consistent with the evolution of depositional environment and the paleoclimate in the region. The study indicates that the lake basin has evolution fluctuations from highstand to lowstand for four times in its evolution history, and the deposition center of the lake has not obviously moved, staying along the Huachi-Yijun belt. The deep lake sedimentary system mainly consists of deep water deltas and turbidite fans during the entire evolution course of the lake basin in the Late Triassic. The former mainly developed on the slope of steep shore of the delta in the early period of the deep-water expansion and gradually experienced a big shift from deep-water deltas to shallow-water platform delta. And the latter appeared almost in all the above stages and had two types of turbidite fans, slope-moving turbidite fans and slump turbidite fans. The slope-moving turbidite fans have relatively complete facies belts overlapping one another vertically and consist of the slope channel of inter fans, the turbidite channel, inter turbidite channel and turbidite channel front of middle fans and outer fans (or lakebottom plain). However, the slide-moving turbidity fans are formed in the deep lake with their microfacies difficult to be distinguished, and only the center microfacies and edge microfacies can be determined. The two types of the turbidity fans are similarly distributing in the near-root-slope and far-root-slope regions. The deep-lake deposition governs the distribution of the hydrocarbon and reservoir, while the slope-moving turbidite fans are excellent reservoirs for oil-gas exploration due to their great thickness, widespread distribution and accumulation properties.

Middle Jurassic trace fossils from the Ridang Formation in Sajia County, South Tibet, and their palaeoenvironmental significance

An ichnofauna consisting of 18 ichnospecies (one of them new) in 14 ichnogenera are described for the first time from the Middle Jurassic Ridang Formation in Sajia County, South Tibet. The ichnofauna can be subdivided into two ichnoassemblages in ascending stratigraphic order: the Palaeophycus–Megagrapton ichnoassemblage in the lower and middle parts of the Ridang Formation, followed by the Cosmorhaphe–Nereites–Paleodictyon ichnoassemblage in the upper Ridang Formation. Overall, the trace fossils occur in a middle–distal turbidite fan sequence, as evidenced by both sedimentological analysis and the composition of the trace fossils. Several subenvironments of the middle–distal fan system have been recognized on the basis of the spatial distribution of the trace fossils. Typically, the channel-fill deposits in the middle part of the turbidite fan lack trace fossils, the interchannel and upper channel-fill (levee) subenvironments of the middle turbidite fan contain abundant facies-crossing trace fossils, in contrast to the distal part of the turbidite fan where deep-water trace fossils are dominant. The ichnofauna is similar to typical flysch ichnofaunas from Europe and North America in characteristics, and is interpreted to represent a typical deep-sea Nereites ichnofacies. The presence of these deep-sea trace fossils therefore would suggest that a continental slope environment existed in southern Tibet during the Middle Jurassic and the study area was located in a slope-abyssal plain setting.

Evolução Estratigráfica e paleogeográfica do grupo Santa Bárbara (Ediacarano) na sub-bacia Camaquã Central, RS

In the Central Camaquã Sub-basin, the Santa Bárbara Group (Ediacaran) is over 2,000 m thick, being well exposed in the Minas do Camaquã and Bom Jardim regions. The group is represented by a sucession of marine to coastal sandstones and rhythmites, and alluvial fan conglomerates, disposed in progradational-retrogrational cycles. Detailed stratigraphic studies of paleoenvironments, paleocurrents and provenance have lead to the subdivision of the Group in the central sub-basin into four formations: 1. Passo da Capela Formation: silty-sandy unit composed of outer- and inner- subaqueous fan turbidites and offshore tempestites; 2. Seival Formation: sandstones of coastal deposits, including tidal flat sistems; 3. Rincão dos Mouras Formation: alluvial fan and braided alluvial pebbly unit and 4. João Dias Formation: mainly sandstones interpreted as foreshore and shoreface coastal deposits. These studies have permitted the detailed correlation between the Minas do Camaquã and Bom Jardim regions, thus supporting the proposition of including in the Santa Bárbara Group as defined here, the deposits exposed in the Minas do Camaquã region, traditionally attributed to the Arroio dos Nobres Formation.

Sedimentary record of terminal Cretaceous accretions in Ecuador: The Yunguilla Group in the Cuenca area

A reappraisal of the “Late Cretaceous Yunguilla Formation” of the Cuenca area enables the definition of four distinct formations, correlatable with those of southwestern Ecuador. A mid- to late-Campanian marine transgression (Jadán Formation) is overlain by quartz-rich conglomerates of fan-delta to turbiditic fan environment (Quimas Formation) of latest Campanian–earliest Maastrichtian age, which are interpreted as evidence of the accretion of a first oceanic terrane (San Juan). Disconformable, arkosic turbidites and cherts (Tabacay Formation) of early Maastrichtian age are thought to represent the erosion of the newly accreted oceanic terrane. A major unconformity of late Maastrichtian age, caused by the accretion of a second oceanic terrane (Guaranda), is followed by the deposition of quartz-rich micaceous shelf sandstones (Saquisilí Formation) of Paleocene age. A third accretion event (late Paleocene) is recorded in coastal Ecuador. Each accretion event correlates with the uplift and erosion of the Eastern Cordillera and with a sedimentary hiatus in the eastern areas. In Ecuador, accretion of oceanic terranes contributed to the build up of the Andes through tectonic underplating of low-density material, and the eastern areas did not behave as flexural foreland basins during late Cretaceous–Paleogene times.

Barite-forming environments along a rifted continental margin, Southern California Borderland

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. δ34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have δ34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time of deposition of the host rocks for the PV samples. This indicates that the mineralizing fluids were not unaltered seawater. We develop a model in which the barites precipitated both below the sediment–water interface and at the seafloor from low-temperature fluids that circulated along faults. The isotopic, chemical, and textural data indicate that the barites formed by several processes. Mesozoic and Cenozoic basement rocks (crystalline and overlying sedimentary rocks), Quaternary basin fill, turbidite fans, and seawater provided various elements for the barites in different environments. The fluids had a deep-seated source and were hydrothermal in the deeper parts of the system for all the barite types, including the seafloor cold-seep deposit, based on Sr isotopes and regional geothermal gradients. These deep-seated fluids mixed with other fluids as they ascended, including seawater at and near the seafloor. The high δ34S values may have resulted from extreme Rayleigh fractionation during bacterially mediated (semi)closed-system sulfate reduction, probably driven by the flux of methane- and hydrocarbon-bearing fluids through basement rocks and basin sediments. Early diagenetic dolomite and pyrite in the host Monterey Formation in the PV Headland also formed in a zone of sulfate reduction, but within sediment buried only centimeters to a few meters and with a predominantly seawater source for the sulfur. Dolomite occurring with vein barite in the PV Headland formed at temperatures in the range of 40–90 °C. The cold-seep barites have δ34S values near that of modern seawater, although still somewhat fractionated. The barites that precipitated below the sediment–water interface have higher δ34S values, suggesting that the fluids were relatively reduced with molar dissolved barium in excess of dissolved sulfate. Those samples were exposed at the seafloor by uplift along faults and are composed predominantly of massive, brecciated, and vein barite.