Quaternary phosphatized limestones with initial Fe-Mn mineralization from the Kammu Seamount (Milwaukee Seamounts, northern Pacific)

Phosphatized biogenic limestones and phosphorites with initial Fe-Mn mineralization dredged from the summit surface of the Kammu Seamount (Milwaukee Seamounts, northwestern Pacific) are studied. The rocks are largely composed of nannofossils and planktonic foraminifers with an admixture of benthic foraminifers, bryozoans, and other organic remains, presumably including bacterial ones. The nannofosssil and foraminiferal assemblages indicate the Quaternary age of the sediments, and their phosphatization is consistent with the phosphatization age determined previously based on nonequilibrium uranium (within the limits of 1 My). The age of phosphatization and the Fe-Mn mineralization in the sediments from Pacific seamounts that young implies the dependence of these ore-forming processes on oceanic environments favorable for ore accumulation rather than on their age.     

Phosphorite from the Continental Margin off Madras, Bay of Bengal

A recent survey reveals that phosphorites occur over an area of 800 km2 between 100 and 500 m depths on the outer continental shelf and upper slope ESE off Madras. Appreciable subsurface continuity of phosphorites is not indicated. They mostly occur over two terraces recorded between the 100 and 200, and 350 and 400m isobaths. Sampled phosphorites may be classified under four types: (1) dense conglomeratic phosphorites in the size range 2 5 cm, (2) dark gray phosphatic nodules in the size range 1 3 cm, (3) creamy white calcareous algal nodules with phosphatic laminations in the size range 0.5 3 cm, and (4) recent gastropod shells with phosphatic studs. Conglomeratic phosphorites confined mostly to the upper terrace were recov ered along with calcareous pelecypod and gastropod fossils belonging to Cretaceous to Oligocene age. Gray phosphatic nodules occur over upper as well as lower terraces. The algal nodules are mostly confined to the upper terrace and are less abundant. Acoustic profiles (3.5 kHz) and the recovery of fossils indicate that the upper terrace is a karstic surface of a carbonate platform, probably of Cretaceous to Oligocene age. Petrographic study of the conglomeratic phosphorite shows skeletal matter set in phosphatic matrix and what appears to be the phosphatized limestone. Selective phosphatization of foraminiferal tests is common. Dark gray phosphatic nodules show brownish-gray laminations containing phosphatized skeletal fragments set in phosphatic matrix. Creamy white calcareous algal nodules exhibit alternate gray phosphatic and white calcitic laminations. All the phosphorite types are composed of carbonate fluorapatite and calcite as major minerals, followed by quartz and aragonite. Phosphorous pentoxide content of conglomeratic type varies from 15% to 20%, dark gray nodules from 16% to 19.98%, and calcareous algal nodules from 8.10% to 17.54%. The significant enrichment of Mo and Pb in phosphorites is attributed to their fixation in organic matter under reduced condition. SEM studies indicate a microbial origin for the gray phosphatic and algal nodules. Studies suggest four episodes of phosphatization with intervening reworking and redoposition, probably commencing from the Eocene period. Phosphatization during the late Pleistocene and early Holocene period also can be ruled out.     

Geology of the Miami Terrace and its paleo-oceanographic implications

The Miami Terrace is a drowned early to middle Tertiary carbonate platform, the outer margin of which has been deeply incised by submarine erosion subsequent to deposition of the limestone terrace. This interpretation is based on the study of high-resolution seismic reflection profiles, rock dredge samples, observations from deep-diving submersibles, and correlation with drill-hole data. The karstic upper terrace (200–375 m) yields limestones and dolostones of early middle Miocene age that have shallow-shelf affinities. Truncated beds of an eastward-dipping deeper-water slope facies are exposed on the upper surface of the erosional lower terrace at 600–700 m. The upper and lower terraces are separated by a discontinuous ridge, probably a drowned Miocene, or post-Miocene bank margin complex. Both terraces and the ridge are capped by dense conglomeratic phosphorites and phosphatic limestones. Phosphorite nodules are more common on the lower terrace whereas the upper terrace yields large phosphorite slabs as well as shark's teeth and bones of marine mammals.

Petrographic study of dredged and submersible-sampled rocks shows that two or more episodes of phosphatization took place intermittent with erosion, grain rounding, reworking and redeposition. Early stages of phosphatization appear to be selective, replacing the more unstable carbonate minerals first, while later stages are nonselective. The origin of most of the phosphorites is the diagenetic replacement of calcium carbonate by francolite.

The karst surface of the upper terrace appears to have been produced by subaerial exposure in the middle to late Miocene as evidenced by shallow-water components in the rocks of the upper terrace plus considerations of local subsidence and eustatic sea-level curves. The erosional lower terrace, 200–300 m deeper than the upper terrace, would have been too deep to have been produced by subaerial or wave erosion. An erosional hiatus of middle Miocene age in JOIDES drill holes on the Blake Plateau indicates that bottom-current velocities in this area increased over this interval. The formation of the lower erosional terrace is related to increased flow of the Gulf Stream system (Florida Current) and bioerosion concurrent with the tectonic uplift and oceanographic closure of the western Caribbean in mid-Miocene time.

The bathymetry, petrology, structure and paleontology of the Miami Terrace, along with correlative data from adjacent areas, argue that a more active Gulf Stream system began as early as the middle Miocene (10.5–16 m.y.B.P.) rather than the early to middle Pliocene (3.5–4.0 m.y.B.P.) as previously suggested.     

Nannostructure and significance of fish coprolites in phosphorites

Phosphatic coprolites (0.5–2 cm long) occurring in Cretaceous-Eocene phosphorites of Tunisia, Morocco, Mauritania, Senegal, and Egypt were investigated for their petrology by using SEM. They exhibit a homogeneous porous apatite structure with a few fish remains. The nannostructures of the coprolites consist of inframicron-sized and botryoid-type apatite microparticles. Spherical cavities and cavity-infill cavity-infilled inframicron-sized apatite globules are typical in the coprolites and are apparently formed by extracellular precipitation of phosphate around microbial organisms. We presume that the coprolites studied here may belong to fishes, whose excrements contain abundant organic matter and phosphate. Phosphatization of excrements appears to be a microbial process controlled by the microenvironment.

The nannostructures observed in the coprolites investigated here differ significantly from the nannostructures of other studied phosphatic grains (pellets and coated grains) in the same samples which are mostly heterogeneous and consist of ovoid-type apatite particles. In the case they are not reworked, the centimetre to decimetre thick coprolite beds in phosphorites indicate a lack of detrital input and strong bottom water currents at the sediment-water interface during deposition and subsequent phosphatization of the excrements.     

On the origin of phosphorites from Christmas Island in the Indian Ocean

A detailed investigation of microstructures of the phosphorites from Christmas Island under a scanning electron microscope coupled with an analysis of their chemical composition revealed that both their structure and composition are quite similar to those of the granular and microgranular phosphorites on present-day continents, as well as those of the phosphorites on some of the Pacific guyots. Their composition, together with their geological position and interrelationships with the surrounding rocks, proves that the ornitogenic hypothesis based on the presumed guano accumulation followed by its transformation into phosphatic rock is not compatible with the field observations. Meanwhile, the problem of the source of the Fe and Mn impregnation in the phosphorites remains unresolved and needs further investigations.     

Phosphate inclusions in tuffites from the Krishtofovich Rise (Sea of Japan)

Tuffites of the core taken from the underwater Krishtofovich Rise in the southeastern Sea of Japan contain phosphate inclusions that were studied by scanning electron microscopy and chemical methods. The tuffites relatively enriched in phosphorus represent products of Miocene alkaline volcanism and are overlain by a thin layer of sediments partly transformed into smectites and containing microfossils presumably of the Late Miocene age. The phosphate inclusions variable in shapes and consistency are largely confined to the lower sedimentary layer, although occurring as single fragments of compact rock and nonlithified small inclusions in voids of altered tuffs as well. By their structure, microtextures, and main chemical composition, the phosphate inclusions are similar to the phosphorites from other areas of the ocean, which indicates their similar biogenic-diagenetic origin. Judging from the REE composition in the most phosphatized matter, they originate largely from seawater and, to a lesser extent, from tuffites. The redistribution of mobile phosphorus enclosed in the tuffites results in a small-scale phosphatization insufficient for ore formation.     

Rare Earth and Metal Geochemistry of Land and Submarine Phosphorites in the Baja California Peninsula,Mexico

The geochemical composition of phosphorites and phosphatic sediments in the Baja California peninsula is studied and used to assess the environment in which phosphogenesis took place. The deposits are classified in three groups: (1) stratified phosphorites, (2) phosphatic sandy sediments from beaches and dunes, and (3) submarine sediments. Some of the elements that might have substituted Ca and PO₄ during francolite mineralization were studied by means of ICP-AES. Significant differences are seen in the concentration of these metals (e.g., Cr = 72-406 μg g^?1 and V = 17-198 μg g^?1), indicating that their concentration is not only controlled by the P₂O₅ concentration, but also by paleo-environmental conditions existing during francolite precipitation. Shale normalized REE patterns suggest two main environments of formation: (1) a strong negative Ce anomaly (< ? 0.3) and La enrichment (La/Nd ≥ 1) enrichment, suggesting well oxygenated shelf environments and probably lower light REE weathering, and (2) a weak negative Ce anomaly (> ? 0.3) and La depletion (La/Nd ≤ 1) suggesting shallower waters or restricted circulation and probably LREE weathering.     

Origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in Dodan Field (SE-Turkey)

Gas occurrences consisting of carbon dioxide (CO₂), hydrogen sulfide (H₂S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO₂, with values of −1.5‰ and −2.8‰, suggested abiogenic origin from limestone. δ³⁴S values of H₂S ranged from +11.9 to +13.4‰. H₂S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120–145 °C. Basin modeling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulfate reducing bacteria were found in oil–water phase samples from Dodan Field. As a result, the H₂S in Dodan Field can be considered to have formed by BSR and TSR.As indicated by their isotopic composition, HC gases are of thermogenic origin and were generated within the Upper Permian Kas and Gomaniibrik Formations. As indicated by the heavier isotopic composition of methane and ethane, HC gases were later altered by TSR. Based on our results, the Dodan gas field may have formed as a result of the interaction of the following processes during the last 7–8 Ma: 1) thermogenic gas generation in Permian source rocks, 2) the formation of thrust faults, 3) the lateral-up dip migration of HC-gases due to thrust faults from the Kas Formation into the Bakuk Formation, 4) the formation of H₂S and CO₂ by TSR within the Bakuk Formation, 5) the vertical migration of gases into reservoirs through the thrust fault, and 6) lateral-up dip migration within reservoir rocks toward the Dodan structure.     

冲绳海槽中部岩心沉积物中浮岩的物理性质和地球化学特征差异及其对岩浆活动的指示

为进一步了解冲绳海槽浮岩的物理性质和地球化学特征差异,对冲绳海槽中部岩心沉积物S9中的浮岩进行了微观结构和地球化学组成分析。结果显示,冲绳海槽中部存在白色、灰白色及棕色3种浮岩,其中灰白色浮岩又可以根据构造特征分为气孔构造和流动构造浮岩两个亚类。浮岩的地球化学组成表明白色、灰白色及棕色浮岩都是由玄武质岩浆经过充分的分离结晶作用形成的流纹质或流纹英安质火山岩。玄武质岩浆在演化的过程中发生了斜长石、角闪石、辉石、Fe-Ti氧化物、磷灰石等矿物的结晶分离。结合有孔虫14C年龄,认为浮岩是冲绳海槽中部距今13.1 ka左右的长英质火山活动的产物。演化程度相对较低的棕色浮岩具有比白色浮岩高的TiO2, Al2O3, Fe2O3, MgO, CaO含量,且棕色浮岩具有相对低的稀土总量和轻稀土总量。根据浮岩的物理性质及地球化学组成差异推测,岩浆的黏度和压力是影响浮岩构造特征的主要因素。黏度大、连续减压的岩浆易于形成具有流动构造和密集气孔的浮岩,黏度小、阶段性减压的岩浆易于形成气孔大而疏松的浮岩。     

Guest editorial

Surficial sediments rich in apatite pellets were recovered from an 80‐m‐deep area that runs parallel to the eastern coast of Vizcaino Bay, Baja California. This area is subject to intense seasonal upwelling, the sediments presenting elevated phosphate (P₂O₅), organic carbon (C_org), and carbonate concentrations. The pellets are moderately well sorted, concentrated mainly in the 3ø size class within a poorly sorted sediment matrix. Abundant nodular apatite and phosphatized bone fragments were found in only one of the pellet‐bearing samples. This sample has a P₂O₅concentration of over 20%, the ≥ — 2.5ø to 0.5ø grain size classes alone being formed of up to 32% P₂O₅. The high proportion of fragmented nodules and the presence of volcanic rock pebbles indicate that at least part of this deposit is allochthonous. The apatite pellets have a flat to concave‐downward heavy rare earth (HREE) shale normalized pattern and, except for La, are depleted in light rare earth elements (LREE). The close resemblance between the HREE patterns and the relatively high La concentration in the pellets of Vizcaino Bay, and the onland phosphorites of the southern Baja California Peninsula, suggest similar depositional histories for these deposits. No significant Ce anomalies were observed, but a negative Eu deviation is common in all of the pellet‐bearing sediments. The depletion of LREE in the pellets of Vizcaino Bay may indicate the preferential removal of these elements by weathering processes.     

Bicyclic alkanes in source rocks of the Triassic Yanchang Formation in the Ordos Basin and their inconsistency in oil-source correlation

The Triassic Yanchang Formation is the main source rocks for Mesozoic oil in Ordos Basin. The formation includes 10 oil-bearing beds (Ch 1–Ch 10), that each can be further divided into two to three intervals. Abundant C₁₂–C₁₄ and C₁₅–C₁₆ bicyclic alkanes have been detected in the formation in the Xifeng oilfield, Ordos Basin. The C₁₂–C₁₄ group is dominated by C₁₂ and C₁₃, and the C₁₅–C₁₆ group contains abundant C₁₅. The groups show three distribution patterns: A) the C₁₂–C₁₄ group is the major component in the non-source rocks of the Ch 7-1 and Ch 8-1 intervals; B) both groups are abundant and are common in source rocks of the Ch 7-3 interval; and C) the C₁₅–C₁₆ group is the major component in source rocks of the Ch 7-3 interval and also in sediments that contain type Ⅰ or partial sapropel type Ⅱ₁ organic matter (OM) in the Ch 7-2 and Ch 8-1 intervals. Although thermal maturities of the source rocks in the Ch 7 section are similar, they show significant differences with respect to the drimane isomerisation index, which indicates that the drimane rearrangement is controlled by thermal evolution of the sediments, but may also be closely related to the depositional environment. This study determined that reducing environments are more conducive to preservation of drimane than oxic environments. The drimane isomerisation index and the value of the hopane parameter Tm/Ts are positively correlated. The parameter Tm/Ts varies over a wide range within the sequence, and the large variations may be a result of terrigenous OM input by turbidity currents and/or gravity flows, mixed with the autochthonous sediments. Abundant homodrimane in both source rocks may reflect reducing environments in deep lakes and major input of higher plant OM. Organic-rich shale and oil shale in the Ch 7-3 interval of the Yanchang Formation are the primary sources of oil in reservoirs in the Xifeng area. The crude oil is rich in bicyclic alkanes that are dominated by C₁₅–C₁₆ as source rocks with pattern C for bicyclic alkanes, which indicates an origin mainly from the Ch 7-3 interval. The main peaks in all of the crude oils are associated with 8β(H)-drimane and lower abundance of rearranged drimane. However, most of the source rocks have a main peak associated with 8β(H)-homodrimane or rearranged drimane. Weak microbial action, selective degradation and water washing may be the cause of the significant difference in bicyclic sesquiterpane composition between the crude oil and the source rocks. The result suggests that oil-source correlations based on the bicyclic sesquiterpanes are questionable.     

Paleoenvironmental implications of the guano phosphatic cementation on Dongdao Island in the South China Sea

The phosphatic cement in the bioclastic sediment sequence on the northeastern shore of Dongdao Island in the South China Sea was studied and its paleoenvironmental implications discussed. Petrological characteristics and major, trace, REE element data unequivocally supported the notion that phosphatization was closely associated with avian guano decomposition and leaching, whereas carbon and oxygen isotope results further revealed that meteoric water were involved in these processes. AMS ¹⁴C dates on the brown phosphate cements indicate that they were formed around 5700, 5000–5100 and 2900 yr BP, respectively. The multi-episodes of phosphatization very likely correspond to intermittent seabird occupation on this island — possibly reflecting Holocene sea-level oscillation and/or long term climate changes in the South China Sea that have controlled seabird habitat. The phosphate cementation, which occurs widely in tropical islands, may be another useful monitor for sea-level and/or paleoclimate changes.     

Depositional environment and geochemical characteristics of the Lower Carboniferous source rocks in the Marsel area,Chu-Sarysu Basin,Southern Kazakhstan

There are two sets of carbonate source rocks in the Lower Carboniferous layers in Marsel: the Visean (C₁v) and Serpukhovian (C₁sr). However, their geochemical and geological characteristics have not been studied systematically. To assess the source rocks and reveal the hydrocarbon generation potential, the depositional paleoenvironment and distribution of C₁v and C₁sr source rocks were studied using total organic carbon (TOC) content, Rock-Eval pyrolysis and vitrinite reflectance (Ro) data, stable carbon isotope data, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis data. The data were then compared with well logging data to understand the distribution of high-quality source rocks. The data were also incorporated into basin models to reveal the burial and thermal histories and timing of hydrocarbon generation. The results illustrated that the average residual TOC contents of C₁v and C₁sr were 0.79% and 0.5%, respectively, which were higher than the threshold of effective carbonate source rocks. Dominated by type-III kerogen, the C₁v and C₁sr source rocks tended to be gas-bearing. The two source rocks were generally mature to highly mature; the average Ro was 1.51% and 1.23% in C₁v and C₁sr, respectively. The source rocks were deposited in strongly reducing to weakly oxidizing marine–terrigenous environments, with most organic material originating from higher terrigenous plants and a few aquatic organisms. During the Permian, the deep burial depth and high heat flow caused a quick and high maturation of the source rocks, which were subsequently uplifted and eroded, stopping the generation and expulsion of hydrocarbons in the C₁v and C₁sr source rocks. The initial TOC fitted by the △logR method was recovered, and it suggests that high-quality source rocks (TOC ≥ 1%) are mainly distributed in the northern and central local structural belt.     

Geochemical characteristics of tight gas and gas-source correlation in the Daniudi gas field,the Ordos Basin,China

The molecular composition, stable carbon and hydrogen isotopes and light hydrocarbons of the Upper Paleozoic tight gas in the Daniudi gas field in the Ordos Basin were investigated to study the geochemical characteristics. Tight gas in the Daniudi gas field displays a dryness coefficient (C₁/C_1–5) of 0.845–0.977 with generally positive carbon and hydrogen isotopic series, and the C₇ and C_5–7 light hydrocarbons of tight gas are dominated by methylcyclohexane and iso-alkanes, respectively. The identification of gas origin and gas-source correlation indicate that tight gas is coal-type gas, and the gases reservoired in the Lower Permian Shanxi Fm. (P₁s) and Lower Shihezi Fm. (P₁x) had a good affinity and were derived from the P₁s coal-measure source rocks, whereas the gas reservoired in the Upper Carboniferous Taiyuan Fm. (C₃t) was derived from the C₃t coal-measure source rocks. The molecular and methane carbon isotopic fractionations of natural gas support that the P₁x gas was derived from the P₁s source rocks. The differences of geochemical characteristics of the C₃t gas from different areas in the field suggest the effect of maturity difference of the source rocks rather than the diffusive migration, and the large-scale lateral migration of the C₃t gas seems unlikely. Comparative study indicates that the differences of the geochemical characteristics of the P₁s gases from the Yulin and Daniudi gas fields originated likely from the maturity difference of the in-situ source rocks, rather than the effect of large-scale lateral migration of the P₁s gases.     

Provenance identification and sedimentary analysis of the beach and bar systems in the Palaeogene of the Enping Sag,Pearl River Mouth Basin,South China Sea

During deposition in the Palaeogene, the Pearl River Mouth Basin (Enping Sag) was filled by sediments derived from northern and southern source regions. The northern sources are the fold belt Caledonian and Hercynian metamorphic rocks that are widely exposed along the South China block and stacked with Mesozoic igneous rocks, and the southern sources are Mesozoic igneous rocks from the southern Panyu lower uplift. We analysed the characteristics of the provenance and sediment-dispersal patterns of the beach and bar systems in the Palaeogene third-order sequence framework on the basis of detritus, thin section, zircon and the geophysical response test data. The source and deposition areas were distinguished based on their palaeogeomorphology. The sedimentary material composition of the deposition area was determined based on the characteristics of the detrital assemblage and cathode luminescence, and the characteristics of the age distribution were determined in detail using detrital zircon U–Pb dating. This study suggests that the provenance system in the Palaeogene mainly consisted of Palaeozoic metamorphic rocks and Mesozoic igneous rocks, with a supplement of Proterozoic metamorphic rocks. The transport capacity of three large fault troughs that delivered the northern provenances gradually increased from the Wenchang Formation (E₂w) to the Enping Formation (E₃e), whereas the southern provenances were transported by four or five incised valleys in E₂w and then submerged in E₃e. In the 3D seismic area, the features of the beach and bar sedimentary systems were described and interpreted based an analysis of the well-based sedimentary facies, seismic reflection characteristics and multiple attribute clustering. The results show that the beach and bar systems by the short-axis braided deltas mainly developed in the upper member of E₂w and that the beach and bar systems on the subaqueous high in E₃e developed during the Palaeogene. In addition, the bedrock-beach bar sedimentary system in the lower member (E₃e^L) was transferred to the braided delta–beach bar sedimentary system in the upper member (E₃e^U).     

Characteristics and petroleum origin of the Carboniferous volcanic rock reservoirs in the Shixi Bulge of Junggar Basin,western China

Shixi Bulge of the central Junggar Basin in western China is a unique region that provides insight into the geological and geochemical characteristics of large-scale petroleum reservoirs in volcanic rocks of the western Central Asian Orogenic Belt. Carboniferous volcanic rocks in the Shixi Bulge mainly consist of striped lava and agglomerate, as well as breccia lava and tight tuff. Volcanic rocks differ in porosity and permeability. Striped lava exhibits the highest porosity (average: 14.2%) but the lowest permeability (average: 0.67 × 10⁻¹⁵ m) among the rock types. Primary gas pores are widely developed and mostly filled. Secondary dissolution pores and fractures are two major reservoir storage spaces. Capillary pressure curves suggest the existence of four pore structure types of reservoir rocks. Several factors, namely, lithology, pore structure, and various diagenesis, govern the physical properties of volcanic rocks. The oil is characterized by a high concentration of tricyclic terpane, a terpane distribution of C₂₃ < C₂₁ > C₂₀, and sterane distributions of C₂₇ < C₂₈ < C₂₉ and C₂₇ > C₂₈ < C₂₉. Oil and gas geochemistry revealed that the oil is a mixture derived primarily from P₂w source rock and secondarily from P₁j source rock in the sag west of Pen-1 Well. The gases are likely gas mixtures of humic and sapropelic organic origins, with the sapropelic gas type dominant in the mixture. The gas mixture is most likely cracked from kerogen rather than oils. The Carboniferous volcanic reservoirs in Shixi Bulge share some unique characteristics that may provide useful insights into the various roles of different volcanic reservoir types in old volcanic provinces. The presence of these reservoirs will undoubtedly encourage future petroleum exploration in volcanic rocks up to the deep parts of sedimentary basins.     

Spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the Yinggehai Basin,offshore South China Sea

About 120 gas seepage vents were documented along the west and southwest coast of the Hainan Island, South China Sea, in water depths usually less than 50 m. The principal seepage areas include the Lingtou Promontory, the Yinggehai Rivulet Mouth, Yazhou Bay, the Nanshan Promontory and the Tianya Promontory. They occur along three major zones, reflecting the control by faults and lateral conduits within the basement. It is estimated that the total gas emission from these seepage vents is 294–956 m³/year. The seepage gases are characterized by a high CH₄ content (76%), heavy δ¹³C₁ values (−38 to −33‰) and high C₁/C_1–5 ratios (0.95–1.0), resembling the thermogenic gases from the diapiric gas fields of the Yinggehai Basin. Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression. The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages. It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin. This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.     

Accumulation of uranium in sediments and phosphorites on the South West African shelf

Organic-rich sediments and coexisting phosphorites from the continental shelf off South West Africa have been analysed for uranium and thiorum by alpha-spectrometry. The uranium concentrations in the sediments range from 10 to 55 ppm, with an isotopic composition close to that of sea-water, indicating that uranium is passing into the sediments at the present time. The phosphorites occur in the sediments as thin unconsolidated laminae and as lithified nodules and pellets, with uranium contents ranging from 79 to 158 ppm. Based on the uranium isotopic composition, only the unconsolidated phosphorite laminae are recent, while the lithified nodules and pellets, with ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U ratios close to radio-active equilibrium, appear to be inherited from a previous period of phosphorite deposition. Deposition of uranium appears to take place predominantly by incorporation into carbonate fluorapatite growing authigenically within the sediment. Uranium accumulation rates, computed from ¹⁴C-dated sections of the sediment cores, and using only uranium values with modern isotopic composition, range from 232 to 765 μg/cm² per 1,000 years. These results stress the importance of organic-rich sediments containing authigenic phosphorite beneath areas of high organic productivity as a major sink for uranium in the ocean.     

中太平洋铁锰结壳碘分布及磷酸盐化对其影响

In the present paper, iodine(I), iron(Fe), manganese(Mn), cobalt(Co), phosphorus(P) and calcium(Ca)contents in three ferromanganese crusts from the Pacific Ocean are measured by spectrophotometric method and inductively coupled plasma atomic emission spectrometers(ICP-AES) to investigate the contents and distribution of iodine in ferromanganese crusts. The results show that iodine contents in three crusts vary between 27.1 and 836 mg/kg, with an average of 172 mg/kg, and the profile of iodine in the three crusts all exhibits a two-stage distribution zone: a young non-phosphatized zone and an old phosphatized zone that is rich in I, P and Ca. The iodine content ratios of old to young zone in MP5D44, CXD62–1 and CXD08–1 are 2.3, 3.4 and 13.7, respectively.The boundary depths of two-stage zone in MP5D44, CXD62–1 and CXD08–1 locate at 4.0 cm, 2.5 cm and 3.75 cm,respectively, and the time of iodine mutation in three crusts ranges from 17–37 Ma derived from 129 I dating and Co empirical formula, which is consistent with the times of Cenozoic phosphatization events. The present study shows that the intensity of phosphatization is the main responsible for the distribution pattern of iodine in the crusts on the basis of the correlation analysis. Consequently, iodine is a sensitive indicator for phosphatization.     

Upper Permian Main Dolomite microbial carbonates as potential source rocks for hydrocarbons (W Poland)

Various microbialite lithofacies were common in the shallow-water environments which flourished under highly saline and arid climate conditions in the Polish part of the Zechstein (Upper Permian) Main Dolomite (Ca2) carbonates. Data came from detailed analysis of 78 cores from wells located at the southern and northern margins of the Main Dolomite basin in west Poland. Microbialite lithofacies are represented by columnar, planar and domal stromatolites, clotted thrombolites and biolaminites developed in high-to-low energy environments within the upper slope, lower parts of oolitic barrier/shoal, restricted lagoon, and tidal flat and tidal channel zones. In barrier environments ooids were predominant whereas in lagoonal settings microbial oncoids and peloids were common. In basinal settings laminated dolomudstones formed which are regarded as hemipelagic and are partly pelagic microbial in origin. Microbial communities coexisted with algae and developed mainly within shallow marine environments. Microbialites are built of cyanobacterial filaments and organo-mineral coccoidal forms possibly of bacterial origin. Organic geochemistry data confirm that organic matter was transformed by cyanobacteria and bacteria which may have played a main role in hydrocarbon generation. This is confirmed by C₂₉ to C₃₅ hopanes, Pr/Ph ratio below 1.0, and the presence of gammacerane as typical for Ca2 profiles. The role and occurrence of algae, as confirmed by C₂₉-steranes, in hydrocarbon generation was possibly minor. Thus it is concluded that the Main Dolomite microbialites could be the likely source rocks for hydrocarbons, with calculated original TOC values from 0.8 to ca 2.0 wt. %. These units also form the reservoir rocks, with porosities reaching 20%.