Clastic Sequence Research Articles

Initial pore pressures under the Lusi mud volcano, Indonesia

The Lusi mud volcano of East Java, Indonesia, remains one of the most unusual geologic disasters of modern times. Since its sudden birth in 2006, Lusi has erupted continuously, expelling more than 90 million cubic meters of mud that has displaced approximately 40,000 people. This study undertakes the first detailed analysis of the pore pressures immediately prior to the Lusi mud volcano eruption by compiling data from the adjacent (150 m away) Banjar Panji-1 wellbore and undertaking pore pressure prediction from carefully compiled petrophysical data. Wellbore fluid influxes indicate that sequences under Lusi are overpressured from only 350 m depth and follow an approximately lithostat-parallel pore pressure increase through Pleistocene clastic sequences (to 1870 m depth) with pore pressure gradients up to [Formula: see text]. Most unusually, fluid influxes, a major kick, connection gases, elevated background gases, and offset well data confirm that high-magnitude overpressures also exist in the Plio-Pleistocene volcanic sequences (1870 to approximately 2833 m depth) and Miocene (Tuban Formation) carbonates, with pore pressure gradients of [Formula: see text]. The varying geology under the Lusi mud volcano poses a number of challenges for determining overpressure origin and undertaking pore pressure prediction. Overpressures in the fine-grained and rapidly deposited Pleistocene clastics have a petrophysical signature typical of disequilibrium compaction and can be reliably predicted from sonic, resistivity, and drilling exponent data. However, it is difficult to establish the overpressure origin in the low-porosity volcanic sequences and Miocene carbonates. Similarly, the volcanics do not have any clear porosity anomaly, and thus pore pressures in these sequences are greatly underestimated by standard prediction methods. The analysis of preeruption pore pressures underneath the Lusi mud volcano is important for understanding the mechanics, triggering, and longevity of the eruption, as well as providing a valuable example of the unknowns and challenges associated with overpressures in nonclastic rocks.

Deformation within an exposed salt wall: Recumbent folding and extrusion of evaporites in the Dead Sea Basin

Despite the enormous global interest in salt tectonics, which is largely driven by its importance to hydrocarbon exploration, direct field-based studies of salt exposed at the Earth's surface are rare. However, Mount Sedom, located at the western side of the Dead Sea Basin, presents one such opportunity for detailed analysis of salt and the associated sedimentary and structural record of its movement. The Sedom salt wall is a 10 km × 1.5 km N–S trending ridge comprising a range of Late Miocene-Pliocene evaporites and clastics, which have penetrated the overlying Pleistocene clastic sequence. The salt wall displays a moderate-steep west dipping western margin and an overturned (west-dipping) eastern flank. The sedimentary record of passive wall growth includes sedimentary breccia horizons that locally truncate underlying beds and are interpreted to reflect sediments having been shed off the crest of the growing salt wall. Structurally, the overturned eastern flank is marked by upturn within the overburden, extending for >300 m from the salt wall. Deformation within the evaporites is characterised by ductile folding and boudinage, while a 200 m thick clastic unit within the salt wall forms a tight recumbent fold traceable for 5 km along strike and associated with a 500 m wide inverted limb. This overturned gently-dipping limb is marked by NE-directed folding and thrusting, sedimentary injections, and a remarkable attenuation of the underlying salt from ∼380 m to <20 m over just 200 m of strike length. The inverted limb is overlain by an undeformed anhydrite, gypsum and clastics caprock, thought to be the residue from a now dissolved salt sheet that extruded over the top of the fold. Expulsion of salt down the regional slope towards the NE, combined with subsequent dissolution of evaporites, may have resulted in local ‘pinching shut’ of the salt wall, leading to a distinctive hour-glass map pattern. This area also coincides with deposition of a thicker overlying clastic sequence, indicating continued subsidence of this part of the salt wall. Our detailed fieldwork forms the first direct observation and description of large recumbent folds within salt walls, and permits analysis below the limits of seismic resolution. It thereby allows more rigorous testing of salt tectonic models and mechanisms.

Structural evidence for slip partitioning and inclined dextral transpression along the SE Sanandaj–Sirjan zone, Iran

The structural evolution of the Sanandaj–Sirjan zone is the result of the convergence of the Iranian microcontinent and the Afro-Arabian continent. The study area at Khabr in the SE Sanandaj–Sirjan zone, in the hinterland of the Zagros orogen, consists of Paleozoic, Mesozoic and Cenozoic rocks. In this area, deformation phases were distinguished in different rock units based on structural and stratigraphical evidence, and the deformational events are divided into two stages: (1) a Late Triassic event and (2) a Late Cretaceous to Miocene event. The Late Triassic deformation event caused regional metamorphism in the Paleozoic units. These units are overlain by unmetamorphosed Jurassic clastic sequences. Fabrics and structural evidence confirm that the F1 folding recumbent and refolded folds were synchronous with the metamorphism of the Paleozoic units and terminated in the Early Jurassic. The time table of the orogenic phases shows that this deformation event is related to the Cimmerian orogenic phase. From a geodynamic point of view, the early Cimmerian deformation in the southeastern Iranian margin suggests that the SE Sanandaj–Sirjan zone was an active margin at that time. The early Cimmerian discordance recorded the onset of a contractional component related to the oblique subduction of Neo-Tethys beneath the central Iranian microcontinent. Structures related to the Late Cretaceous to Miocene deformation phase are observed in Jurassic to Oligocene units, which contain moderately inclined and plunging folds. Comparing these folds with domains of deformation generated in models of transpression shows that the folding was caused by a combination of contractional and dip-slip components of movement, eventually resulting in the formation of a thrust system. The Khabr thrust systems consist of five sheets of oblique thrusts, duplex structures and shear zones. The shear zones generally strike E–W and dip moderately N (30°–40°). The occurrence of asymmetric folds with hinges that are either parallel to strike or plunge down dip demonstrates an oblique-slip component in these thrust shear zones. The stretching lineation in the mylonites within the shear zones is defined by the long axes of ellipsoidal grains of quartz, calcite, plagioclase and garnet. In general, stretching lineations trend from N40°W to N80°W with an intermediate (35°) plunge. The geometry of foliation and lineation within these shear zones shows the effect of dip- and oblique-slip shearing. Deformation continued with strike-slip faulting becoming important during the last stages of deformation from the Miocene to the present day. The results of this study demonstrate that the evolution of the SE Sanandaj–Sirjan zone, from Late Triassic to Miocene, is compatible with an inclined dextral transpression along this zone.

Oligocene lacustrine tuff facies, Abu Treifeya, Cairo-Suez Road, Egypt

Field investigations in the Abu Treifeya area, Cairo-Suez District, revealed the presence of Oligocene lacustrine volcaniclastic deposits of lacustrine sequences associated with an Oligocene rift regime. The present study represents a new record of lacustrine zeolite deposits associated with saponite clay minerals contained within reworked clastic vitric tuffs. The different lithofacies associations of these clastic sequences are identified and described: volcaniclastic sedimentary facies represent episodic volcaniclastic reworking, redistribution and redeposition in a lacustrine environment and these deposits are subdivided into proximal and medial facies. Zeolite and smectite minerals are mainly found as authigenic crystals formed in vugs or crusts due to the reaction of volcanic glasses with saline–alkaline water or as alteration products of feldspars. The presence of abundant smectite (saponite) may be attributed to a warm climate, with alternating humid and dry conditions characterised by the existence of kaolinite. Reddish iron-rich paleosols record periods of non-deposition intercalated with the volcaniclastic tuff sequence.

20 Myr of eccentricity paced lacustrine cycles in the Cenozoic Ebro Basin

Long-period orbital forcing is a crucial component of the major global climate shifts during the Cenozoic as revealed in marine pelagic records. A complementary regional perspective of climate change can be assessed from internally drained lake basins, which are directly affected by insolation and precipitation balance. The Ebro Basin in northeastern Iberia embraces a 20 Myr long continuous sedimentary record where recurrent expansions and retractions of the central lacustrine system suggest periodic shifts of water balance due to orbital oscillations. In order to test climatic (orbital) forcing a key-piece of the basin, the Los Monegros lacustrine system, has been analyzed in detail. The cyclostratigraphic analysis points to orbital eccentricity as pacemaker of short to long-term lacustrine sequences, and reveals a correlation of maxima of the 100-kyr, 400-kyr and 2.4-Myr eccentricity cycles with periods of lake expansion. A magnetostratigraphy-based chronostratigraphy of the complete continental record allows further assessing long-period orbital forcing at basin scale, a view that challenges alternate scenarios where the stratigraphic architecture in foreland systems is preferably associated to tectonic processes. We conclude that while the location of lacustrine depocenters reacted to the long-term tectonic-driven accommodation changes, shorter wavelenght oscillations of lake environments, still million-year scale, claims for a dominance of orbital forcing. We suggest a decoupling between (tectonic) supply-driven clastic sequences fed from basin margins and (climatic) base level-driven lacustrine sequences in active settings with medium to large sediment transfer systems.

Paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic-evaporite deposits in the western Qaidam Basin

Uranium (U) provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments. Its proxy potential under suboxic–oxic conditions associated with salinization of surface water in closed hydrologic systems, however, has not been adequately explored. This study examined the abundance and distribution of U in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality (938.5m-long) core drilled in the western Qaidam Basin, NE Tibetan Plateau. This study examined U concentrations in calcareous clastic sediments (including carbonates, acid insoluble fractions, as well as the amounts of iron and manganese oxides/oxyhydroxides) and salt layers. In the calcareous clastic sediments, variations in U concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations, providing a tool for tracing redox conditions in paleolake bottom water. Correlations between the variations in carbonate content with U and Sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the U distribution in carbonates. In the salt layers, U and Al concentrations correlate closely, implying that both derived mainly from the detrital fraction in the salt layers. However, enrichments of U and Mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine. These results indicate that during the process of carbonate precipitation, carbonate might effectively remove U from lake water. During the subsequent stage of salt mineral precipitation, U behavior is conservative, and U resides for a long period in dissolved forms within the saline water, giving rise to enhanced U enrichment in the more concentrated brines and the associated precipitated salt minerals. Our study suggests, however, that U-related redox proxies should be interpreted with caution in suboxic–oxic environments during the long-term paleolake evolution.

Grain size analysis of the Lower Cambrian–Lower Cretaceous clastic sequence of Jordan: Sedimentological and paleo-hydrodynamical implications

356 sand-sized samples ranging in age from Early Cambrian Epoch to Late Cretaceous Epoch of known depositional environments were analyzed mechanically to obtain the grain size parameters of mode, median, mean and sorting.The constructed grain size distribution curves for the various Paleozoic and Mesozoic formations were interpreted in terms of the paleo-hydrodynamic processes prevailing through transportation and deposition.The study proved that the grain size parameters of sand-sized sediments most sensitive to paleo-depositional environmental recognition were the graphic mean grain size and the inclusive graphic sorting.Mean grain size values below 1.77 ϕ of a sand-sized sample could indicate a fluvial channel paleo-depositional environment, whereas the mean grain size values above 2.8 ϕ are indicative to a shallow marine paleo-depositional environment. Sorting values above 1.1 ϕ may indicate a fluvial channel paleo-depositional environment.A bivariate discriminatory plot of mean grain size versus sorting yields two fields that can successfully discriminate between the fluvial channel paleo-depositional environment, and the shallow marine paleo-depositional environment.A new grain size parameter, termed the mean–sorting index, was introduced in this study and calculated by dividing the mean grain size value over the sorting value. It is recorded here that the mean–sorting index is more sensitive to paleo-depositional environment recognition than the individual mean grain size and sorting parameters.

Depositional age and sediment source of the auriferous Moeda Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil: New constraints from U–Pb–Hf isotopes in zircon and xenotime

The Moeda Formation is a metasedimentary clastic sequence of Witwatersrand-like metaconglomerate and quartzite at the base of the Minas Supergroup, Quadrilátero Ferrífero of Minas Gerais, Brazil, which is also known for its world-class iron-ore deposits. The Moeda Formation can be subdivided into three units (Units I to III from bottom to top) with each unit consisting of metaconglomerates at their base, mostly auriferous, grading into quartzite and locally into phyllite. This study presents the first combined U–Pb and Lu–Hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from Units II and III. These data indicate that deposition of sediments occurred ≤2.68Ga for Unit II and ≤2.62Ga for Unit III, as supported by the youngest magmatic zircon grains. The data further provide evidence that the source area of sediments was affected by crust re-working during two major magmatic-metamorphic events at 2.89–2.87 and 2.72–2.69Ga, and minor events between 3.3 and 3.0 and at 2.80 and 2.65Ga. This interpretation is supported by mainly subchondritic ɛHft between −10 and +1.4, and large ɛHft variations including all magmatic events. A few superchondritic ɛHft obtained for the magmatic events at 3.3, 2.9, and 2.7Ga, point to limited juvenile crust addition. The new dataset indicates that the clastic sediments of the auriferous Moeda Formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources, which are suggested for the Mesoarchaean auriferous sediments of the Witwatersrand Basin of South Africa, and those of the eastern Goldfield province of Australia.

British Pennsylvanian (Carboniferous) coal-bearing sequences: where is the time?

Abstract The Lower and Middle Coal Measures of Langsettian (Westphalian A) and Duckmantian (Westphalian B) age (together equals Bashkirian in part) in Britain comprise an alternation of clastic sediments and coal deposited on coastal and alluvial plains over a period of 2–3.5 million years, depending on which time scale is accepted. In both the Pennine and South Wales Basins there are no obvious unconformities. Many of the clastic sequences show evidence of rapid sedimentation (burial of trees, bivalve escape burrows) that may suggest that a significant amount of time is taken up during the peat-forming intervals. Thickness data from a range of boreholes that record continuous sections through this time period were collated. Coal and sediment thicknesses, as well as coal to sediment ratios, are compared both within and across the basins. Data on the coals has allowed consideration of the time taken to deposit the peats. This work considers the compaction of the peat to coal, as well as a range of peat accumulation rates. Assuming the largest de-compaction rates and slowest accumulation rates of the coal formation, less than 50% of the allocated time can be accounted for. In addition however, calculations suggest that peat formation accounts for less than 25% of the total time taken for sediment accumulation. It is suspected that there are major time gaps in the sequences, most probably occurring between seat-earths and coal and within the coals, and it is believed that this finding has significance for the debate over short-term climate changes in the Carboniferous and the causes of peat and sediment alternations. Supplementary material: Borehole thickness data and references are available at http://www.geolsoc.org.uk/SUP18788

Geology and genesis of the post-collisional porphyry–skarn deposit at Bangpu, Tibet

Bangpu deposit in Tibet is a large but poorly studied Mo-rich (~0.089wt.%), and Cu-poor (~0.32wt.%) porphyry deposit that formed in a post-collisional tectonic setting. The deposit is located in the Gangdese porphyry copper belt (GPCB), and formed at the same time (~15.32Ma) as other deposits within the belt (12~18Ma), although it is located further to the north and has a different ore assemblage (Mo–Pb–Zn–Cu) compared to other porphyry deposits (Cu–Mo) in this belt. Two distinct mineralization events have been identified in the Bangpu deposit which are porphyry Mo–(Cu) and skarn Pb–Zn mineralization. Porphyry Mo–(Cu) mineralization in the deposit is generally associated with a mid-Miocene porphyritic monzogranite rock, whereas skarn Pb–Zn mineralization is hosted by lower Permian limestone–clastic sequences. Coprecipitated pyrite and sphalerite from the Bangpu skarn yield a Rb–Sr isochron age of 13.9±0.9Ma. In addition, the account of garnet decreases and the account of both calcite and other carbonate minerals increases with distance from the porphyritic monzogranite, suggesting that the two distinct phases of mineralization in this deposit are part of the same metallogenic event.Four main magmatic units are associated with the Bangpu deposit, namely a Paleogene biotite monzogranite, and Miocene porphyritic monzogranite, diabase, and fine-grained diorite units. These units have zircon U–Pb ages of 62.24±0.32, 14.63±0.25, 14.46±0.38, and 13.24±0.04Ma, respectively. Zircons from porphyritic monzogranite yield εHf(t) values of 2.2–8.7, with an average of 5.4, whereas the associated diabase has a similar εHf(t) value averaging at 4.7. The geochemistry of the Miocene intrusions at Bangpu suggests that they were derived from different sources. The porphyritic monzogranite has relatively higher heavy rare earth element (HREE) concentrations than do other ore-bearing porphyries in the GPCB and plots closer to the amphibolite lithofacies field in Y–Zr/Sm and Y–Sm/Yb diagrams. The Bangpu diabase contains high contents of MgO (>7.92wt.%), FeOt (>8.03wt.%) but low K2O (<0.22wt.%) contents and with little fractionation of the rare earth elements (REEs), yielding shallow slopes on chondrite-normalized variation diagrams. These data indicate that the mineralized porphyritic monzogranite was generated by partial melting of a thickened ancient lower crust with some mantle components, whereas the diabase intrusion was directly derived from melting of upwelling asthenospheric mantle. An ancient lower crustal source for ore-forming porphyritic monzogranite explains why the Bangpu deposit is Mo-rich and Cu-poor rather than the Cu–Mo association in other porphyry deposits in the GPCB because Mo is dominantly from the ancient crust.The Bangpu deposit has alteration zonation, ranging from an inner zone of biotite alteration through silicified and phyllic alteration zones to an outer propylitic alteration zone, similar to typical porphyry deposits. Some distinct differences are also present, for example, K-feldspar alteration at Bangpu is so dispersed that a distinct zone of K-feldspar alteration has not been identified. Hypogene mineralization at Bangpu is characterized by the early-stage precipitation of chalcopyrite during biotite alteration and the late-stage deposition of molybdenite during silicification. Fluid inclusion microthermometry indicates a change in ore-forming fluids from high-temperature (320°C–550°C) and high-salinity (17wt.%–67.2wt.%) fluids to low-temperature (213°C–450°C) and low-salinity (7.3wt.%–11.6wt.%) fluids. The deposit has lower δDV-SMOW (−107.1‰ to −185.8‰) values compared with other porphyry deposits in the GPCB, suggesting that the Bangpu deposit formed in a shallower setting and is associated with a more open system than is the case for other deposits in this belt. Sulfides at Bangpu yield δ34SV-CDT values of −2.3‰ to 0.3‰, indicative of mantle-derived S implying that coeval mantle-derived mafic magma (e.g., diabase) simultaneously supplied S and Cu to the porphyry system at Bangpu. In comparison, the Pb isotopic compositions (206Pb/204Pb=18.79–19.28, 207Pb/204Pb=15.64–15.93, 208Pb/204Pb=39.16–40.45) of sulfides show that other metals (e.g., Mo, Pb, Zn) were likely derived mainly from an ancient crustal source. Therefore, the formation of the Bangpu deposit can be explained by a two-stage model involving (1) the partial melting of an ancient lower crust triggered by invasion of asthenospheric mantle-derived mafic melts that provide heat and metal Cu and (2) the formation of the Bangpu porphyry Mo–Cu system, formed by magmatic differentiation in the overriding crust in a post-collisional setting.

Imaging deepwater carbonates offshore Brazil: Methodology and uncertainties

An anisotropic prestack depth-migration sequence was used in the imaging of a carbonate accumulation in deep water offshore Brazil. The reservoir is overlain by a complex sequence of evaporites, carbonates, and clastics. Although the latter display a modest magnitude of anisotropy, this parameter has to be taken into account for the depth accuracy required in reservoir evaluation. The seismic data input to the processing consist of two narrow-azimuth streamer surveys, acquired with distinct azimuths that overlap approximately at the center of the reservoir structure. The resulting velocity model is geologically plausible, exhibiting good accuracy when compared with average velocities calculated from check shots and vertical seismic profiles acquired at nearby wells. Such accuracy holds true to the base of the evaporitic sequence, which corresponds roughly to the top of reservoir, and makes this model adequate for depth positioning and hydrocarbon volume calculations. However, deep layers in the velocity models present larger errors and a spatial behavior that does not match geologic expectations. This is attributed to the lack of proper illumination that these conventional seismic acquisitions provide and arguably makes the case for a more modern full-azimuth seismic acquisition over the structure. This limitation is quantified by an illumination map simulated at the top of the reservoir from a waveform-modeling exercise that took into consideration the seismic-acquisition parameters over the field.

The Carboniferous geology of northern Donegal Bay

New geological mapping, along with sedimentological and microfaunal analysis, has enabled correlation among three disparate outcrops of early Carboniferous rocks on the north side of Donegal Bay. This area represents the northernmost exposures of the Carboniferous in north-west Ireland and is inferred to have lain close to the Viséan shoreline. A regionally traceable marine flooding event at the base of the Bundoran Shale approximately coincides with the base of the Arundian Stage. Below this, marginal-marine successions can all be assigned to the Lower Viséan based on microfaunas. These successions display a lower, non-marine clastic sequence that passes gradationally upward into a shallow-marine carbonate-dominant sequence. This sequence is succeeded by a regressive event that is regionally correlateable despite lateral variation in facies.

北辽河群浪子山组地层的重新厘定

古元古代北辽河群浪子山组变质沉积岩系主要分布在辽宁省鞍山市、海城市、辽阳市一带，其岩石组合以石英岩、片岩、千枚岩、变粒岩夹大理岩、透闪岩等为主，原岩恢复为砂岩、泥质砂岩、泥岩、泥灰岩等。前人将其置于北辽河群底部。通过对南北辽河群各组进行了详细的地质调查和分析对比工作，依据岩石组合特征和锆石年代学数据，笔者认为浪子山组应与南辽河群盖县组层位相当，故浪子山组不宜置于辽河群底部而应重新厘定为北辽河群顶部。The Paleoproterzoic clastic rocks are widely distributed in the southern portion of Liaoning prov-ince, grouped as the Langzishan Formation of the North Liaohe Group. The Langzishan Formation, previously defined as the bottom of the North Liaohe Group, mainly consists of sandstone, shaly sandstone, clay rock, siltstone interbedded with limestone, and argillaceous limestone. They were metamorphosed to be quartzite, schist, phyllite, marble, and tremolite rocks. Based on detailed field investigation, integrating with zircon U-Pb data, we proposed that the Langzishan Formation is equivalent to the Gaixian Formation (top of the South Liaohe Group). The Langzishan Formation should be grouped into the upper clastic sequence of the North Liaohe Group, but not the bottom as previous consideration.

The Carboniferous geology of northern Donegal Bay

New geological mapping, along with sedimentological and microfaunal analysis, has enabled correlation among three disparate outcrops of early Carboniferous rocks on the north side of Donegal Bay. This area represents the northernmost exposures of the Carboniferous in north-west Ireland and is inferred to have lain close to the Visean shoreline. A regionally traceable marine flooding event at the base of the Bundoran Shale approximately coincides with the base of the Arundian Stage. Below this, marginal-marine successions can all be assigned to the Lower Visean based on microfaunas. These successions display a lower, non-marine clastic sequence that passes gradationally upward into a shallow-marine carbonate-dominant sequence. This sequence is succeeded by a regressive event that is regionally correlateable despite lateral variation in facies.

Sediment Composition and Provenance of the Pab Formation, Kirthar Fold Belt, Pakistan: Signatures of Hot Spot Volcanism, Source Area Weathering, and Paleogeography on the Western Passive Margin of the Indian Plate During the Late Cretaceous

Petrographic and geochemical data collected from the Pab Formation, a late Cretaceous clastic sequence exposed in the Kirthar Range of western Pakistan, yield important clues about the influence of the varied source regimes, transportation routes and volcanic input that have influenced the composition of sediments comprising this formation. Detrital compositional modes, petrotectonic discrimination diagrams and paleocurrent data all demonstrate that the Pab sands were ultimately derived from granitic–gneissic terranes forming the Indian Craton, exposed to the E and SE of the study area, with probable supplementary contribution from the mature, ancient sedimentary cover of the craton. These data also confirm that the two contemporaneous depositional systems operating during accumulation of the Pab Formation in this area were supplied from somewhat different sources and through different routes. Sediments deposited in the Central Kirthar sub-basin were derived from the East, while coeval deposits within the Southern Kirthar sub-basin were supplied from SSE and include relatively fresh volcanic detritus that is interpreted as the product of the major Deccan Trap volcanic episode, initiated during late Maastrichtian times. Moreover, geochemical indicators demonstrate that the ultimate composition of Pab sediments has been strongly influenced by intense chemical weathering, associated with warm/humid climatic conditions in the source areas, accompanied by significant diagenetic effects.

New data on the Upper Devonian biostratigraphy and sedimentology of Stolb Island (Lena River delta)

The obtained new data on conodonts, brachiopods, ammonoids, and ostracods from the Stolb Island section indicate the predominantly Early Famennian (triangularis–Lower rhomboidea Zones) age of sedimentation. The Upper Kellwasser global biotic event, which marks the Frasnian/Famennian boundary, has been distinguished for Arctic Siberia for the first time. It was found that carbonate-terrigenous sequences accumulated in basinal sedimentary environments. Fine-clastic material was supplied to the sediments from different sources. A flow of dolomite debris is associated with the most distant source—areas of evaporite sedimentation. The presence of calcareous fragments is due to destruction of skeletal material (close provenance areas). Siliciclastics, which make up a considerable part of the sediments, were produced by partial rewashing and eolian differentiation of felsic pyroclastics. The existence of organic-rich horizons and beds of well-washed and well-sorted clastic sequences suggests that the Upper Devonian sediments have a high general petroleum potential.

Provenance of Late Triassic sediments in central Lhasa terrane, Tibet and its implication

In southern Tibet, Late Triassic sequences are especially important to understanding the assembly of the Lhasa terrane prior to Indo-Asian collision. We report new data relevant to the provenance of a Late Triassic clastic sequence from the Mailonggang Formation in the central Lhasa terrane, Tibet. Petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance, including volcanic, sedimentary and some ultramafic and metamorphic rocks. In situ detrital zircon Hf and U–Pb isotope data are consistent with derivation of these rocks from nearby Triassic magmatic rocks and basement that comprise part of the newly recognized Late Permian–Triassic Sumdo–Cuoqen orogenic belt. The new data suggests correlation with the Upper Triassic Langjiexue Group which lies on the opposing (southern) side of Indus–Yarlung ophiolite. Sediments from both the Mailonggang Formation and Langjiexue Group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the Sumdo–Cuoqen belt.

Development of intra-basaltic lava-field drainage systems within the Faroe–Shetland Basin

The Faroe–Shetland Basin, located on the NW continental shelf of the UK, represents arguably the last frontier area of hydrocarbon exploration of the UK territorial waters. The basin contains an aerially extensive lava sequence, which forms part of the Palaeogene-aged North Atlantic Igneous Province (NAIP). In 2004, a major oil and gas discovery was made within the Palaeocene–Eocene lavas in the form of the Rosebank Field. Unusually, the reservoir intervals are a series of intra-basaltic fluvial clastic sequences separated by basalt lava flows, hyaloclastites and volcaniclastic sediments, giving rise to a new hydrocarbon play concept. The discovery has led to an increased emphasis on trying to understand the detailed stratigraphy of the offshore lava fields within the Faroe–Shetland Basin, as well as the nature of the regional intra-basaltic drainage systems and sediments that form the reservoir intervals in Rosebank. In this paper we show a regional integrated three-dimensional (3D) seismic and well analysis of the lava sequences within the area of the Corona Ridge, exploring the age relationships and spatial distribution of the sequence T40 and T45 Palaeocene lava field. In particular, we note the occurrence of an intra-basaltic (sequence T40, Colsay Member) drainage network that appears to represent a major drainage pathway within the Faroe–Shetland Basin lava fields. Few published studies have dealt with drainage system development on lava but our preliminary analysis appears to indicate that lava flow-field morphology plays a dominant role in controlling the development and evolution of a drainage system.

Origen y naturaleza de los minerales fosfato-sulfato alumínicos (APS) asociados a la mineralización de uranio en los red-beds triásicos (Cordillera Ibérica, España)

Este trabajo se centra en el estudio de los minerales fosfato-sulfato alumínicos (APS) que se producenen una secuencia clástica del Triásico (Buntsandstein) de la Cordillera Ibérica. El depósito está constituido por areniscas, lutitas y conglomerados con matriz arenosa, que fueron depositados en ambientes de fluviala marino somero. En la secuencia clástica además de los APS están presentes los siguientes minerales dia-genéticos: cuarzo,feldespato potásico, minerales del grupo de la caolinita, illita, óxidos-hidróxidos de Fe,cementos-reemplazamientos de carbonatos y sulfatos y minerales secundarios uraníferos. Los mineralesAPS fueron identificados y caracterizados por microscopía óptica, difracción de rayos X, microscopía elec-trónica de barrido, y microsonda electrónica. Los microcristales de APS reemplazan a minerales uraníferos,asociados con caolinita-dickita, mica y rellenando la porosidad de arcosas-subarcosas fluviales a marealesdistales. Dado su contenido en Ca, Sr y Ba, los minerales APS se pueden definir como una solución sólida de crandallita-goyacita-gorceixita (0,53 Ca, 0,46 Sr y 0,01 Ba). La baja concentración en LREE y elcontenido de Sr&gt;S, sugieren que los minerales APS se originaron durante la alteración supergénica de lasareniscas del Buntsandstein, debido a la presencia de fluidos mineralizantes que provocaron la formaciónde areniscas uraníferas en una zona de alteración distal. Además, la aparición de dickita asociada a losminerales APS, indica que éstos precipitaron a temperatura superior a 80ºC, relacionada con el levanta-miento que tuvo lugar durante la etapa post-rift del Cretácico Superior.

A technique for improving pseudo-synthetic seismograms generated from neutron logs in gas-saturated clastic rocks

In clastic and carbonate rock sequences, the neutron and sonic log curves usually deflect in a similar fashion. Moreover, in some cases the two curves can be overlain and they generally appear to mimic each other, with variations between them only in the amplitudes of the two curves. This descriptive correlation is the basis of direct cross-plot techniques used to convert a neutron log into a pseudo-sonic log, which can then be combined with a density log to create a pseudo-synthetic seismogram. Unfortunately, the seismograms produced in this way may not match the standard synthetic seismograms produced from the sonic and density logs if the ?gas effect? is not taken into account. In order to correct for the gas effect, the inter-log correlations between the compensated neutron log (CNL) and the borehole-compensated (BHC) sonic log curves from a well in Taiwan were carefully examined. Then, we developed a technique for transforming the CNL log into a pseudo-BHC log by splicing together several continuous sandstone intervals in which the gas effect could be identified from the scattered data on the cross-plot of neutron porosity versus sonic interval transit time. Based upon our results, application of the new composite transform method yields a pseudo-synthetic seismogram that better matches the standard synthetic seismogram (made from the sonic and density logs) according to frequency, amplitude and polarity. This gas correction technique may be particularly useful in oil and gas exploratory and development areas where neutron logs are more prevalent than sonic logs or where sonic logs are scarce.