Rare-Earth Abundances in Basalts and Metabasalts from the Troodos Massif,Cyprus

Abundances of some rare-earth elements (REE) in twelve pillow lavas from the Upper Pillow Lavas and Axis Sequence of the Troodos Massif are reported. The samples consist of three fresh basalts and nine zeolite facies metabasalts, metamorphosed at temperatures between 0 and 200° C. All give similar light rare-earth element (LREE) depleted patterns, indicating that hydrothermal metamorphism within this temperature range does not appreciably affect these REE patterns. The LREE depletion is consistent with a petrogenetic model in which Troodos formed at a spreading axis. Variations in profile shape indicate that mantle melting beneath the axis may have taken place during a series of discrete episodes.     

Tectonics of the Troodos Massif (Cyprus): preliminary results

The core of the Troodos Massif consists of a major tectonitic harzburgite—dunite unit (Mt. Olympus Unit) overlain by three tectonic units which, in descending order, consist of dunites, wherlites and gabbros, respectively. Furthermore, the Mt. Olympus Unit is underlain by three other tectonic units with the same lithology as the upper ones. The thrusts show a north-to-south emplacement, probably of latest Cretaceous age.It is suggested that Troodos, Southern Turkey, Hatay and Oman ophiolites could belong to the same oceanic domain; they underwent different displacement toward the Arabian—African plate.     

Metabasalts from the Troodos Massif,Cyprus: Genetic implication deduced from petrography and trace element geochemistry

A wide variety of data support the contention that the Troodos massif, Cyprus is a fragment of oceanic lithosphere formed at a constructive margin some 85 m.y. ago. However, Troodos rocks differ significantly in major and trace element content and mineral parageneses from those formed at present day active constructive margins. It is here argued that the geochemical and petrographic characteristics of the massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.     

Foliations developed during slump deformation of Miocene marine sediments,Cyprus

Foliations within a Miocene slumped bioclastic sandstone unit of the Pakhna Formation, southern Cyprus, were investigated in order to assess the importance of slump strain, liquifaction and compaction in their generation. There are two approximately orthogonal sets of folds, F₁ and F₂. F₁ folds are upright to inclined slump folds formed during slope-failure translation of the sediment. The cores of upright F₁ folds have a steeply dipping macroscopic fabric defined by the axial surfaces of small tight folds in compositional layering. F₂ folds occur on steeply-dipping limbs of F₁ folds. F₂ folds are small and asymmetric with flat-lying axial surfaces, and are interpreted as compaction generated. A pervasive flat-lying microscopic fabric defined by grain and pore long axis orientation is found in both fold sets, and is probably a liquifaction fabric enhanced by compaction. A pervasive steeply-dipping microfabric parallel to the axial planes of slump folds is not present in any of the slumps investigated.     

Platinum-group minerals from the Troodos ophiolite,Cyprus

Summary Platinum-group minerals have been identified in chromitites from the Troodos ophiolitic complex of Cyprus. The host chromitites occur as podiform bodies within the basal harzburgite of the ophiolite, as occasional discontinuous layers in the dunites at the base of the overlying cumulate sequence and rarely as minor schlieren in clinopyroxene dunites at higher levels. Podiform chromitites are generally highly deformed and frequently brecciated while those from the cumulate sequence are well-preserved and display cumulate textures. Chromite grains from bodies at all levels host a broad and mineralogically complex assemblage of inclusions including silicates, platinum-group minerals, base metal sulphides and fluid inclusions, all of which have been studied in detail. The platinum-group minerals (PGM) and base metal sulphides (BMS), which are described here, are modally much less abundant than the silicate inclusions and generally small in size (< 50 µm). PGM comprise sulphides and alloys and are dominated by laurite (RuS₂). Other sulphides are Ru-poor. Alloys include iridosmine, osmian ruthenium and Ru-Fe alloys. Two generations of PGM are believed to be present, the first having been trapped during chromite formation, the second (including most of the alloys) having been formed during serpentinization. The base metal sulphides include common small Ni-Fe sulphides, sometimes associated with silicate inclusions, and larger Cu-rich inclusions, some of which are almost pure chalcopyrite. The origin of the latter is of genetic significance.

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Minerale der Platin-Gruppe aus denn Troodos-Ophiolith, Zypern

Zusammenfassung Minerale der Platin-Gruppe wurden in verschiedenen Chromititen des Troodos Ophiolithes, Zypern, nachgewiesen. Die Chromitite kommen als podiforme Körper in den basalen Harzburgiten, als gelegentlich unregelmäßige Lagen in den Duniten an der Basis der Kumulat-Abfolge, und selten als Schlieren in den Klinopyroxen-Duniten in höheren Bereichen vor. Podiforme Chromitite sind im allgemeinen intensiv deformiert und häufig brekziiert, während jene aus der Kumulat-Abfolge gut erhalten sind und Kumulat-Texturen zeigen. In Chromiten aus allen Niveaus des Ophiolithes kommt eine umfangreiche, und mineralogisch komplexe Assoziation von Einschlüssen vor; diese umfassen Silikate, Platingruppen-Minerale, Buntmetallsulfide und FlüssigkeitsEinschlüsse. Die Platingruppen-Minerale (PGM) und Buntmetallsulfide (BMS) die hier beschrieben werden, sind modal weniger verbreitet als die Silikateinschlüsse, und sind meist sehr feinkörnig ( < 50 µm). Die PGM umfassen Sulfide, einerseits dominiert von Laurit (RuS₂), aber auch Ruthenium-arme Sulfide, und Legierungen, bestehend aus Iridosmin, Osmium-führendes Ruthenium und Ruthenium-Eisenlegierungen. Die PGM können zwei Generationen zugeordnet werden. Die erste wurde während der Bildung der Chromite eingeschlossen, die zweite (und hierzu gehören die meisten Legierungen) wurden während der Serpentinisierung gebildet. Die Buntmetallsulfide unfassen die bekannten Nickel-Eisen-Sulfide, die manchmal mit Silikateinschlüssen vergesellschaftet sind, und größere Kupfer-reiche Einschlüsse, die zum Teil reiner Kupferkies sind. Die Entstehung dieser Einschlüsse ist von Bedeutung für die Genese.

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With 13 Figures     

Strontium isotopic contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos Massif,Cyprus

Twenty-six new high precision ${}^{87}\text{Sr}{}^{86}\text{Sr}$ratio determinations and existing analyses are used to discuss the strontium isotopic composition of the Upper Cretaceous ophiolitic rocks of the Troodos Massif, Cyprus. Relative to initial magmatic ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (0.70338 ± 0.00010 to 0.70365 ± 0.00005), the hydrothermally metamorphosed pillow lavas and dyke complex have been contaminated by isotopically heavier strontium.This observation confirms the hypothesis that hydrothermal metamorphism was a consequence of sea water-rock interaction, since sea water was the only readily accessible reservoir of isotopically heavier strontium. The fact that metagabbros and altered trondhjemites were also Sr isotopically contaminated shows that sea water penetrated approximately 2 km into the oceanic crust represented by the ophiolitic sequence.The amount of Sr isotopic contamination requires that the bulk sea water: rock ratio was at least ~15:1 and shows that water-rock interaction occurred in a flow system. The degree of oxidation decreases with increasing depth. This shows that the vertical component of fluid flow was downward. The absolute bulk water/rock ratio (for water at S.T.P.), as estimated from the oxidation profile, may have been as large as ~3 × 10³:1 —a large figure which independently confirms that rocks showing strong δ¹⁸O shifts have interacted with large volumes of water.The sites of discharge of the hot fluid, which must have come out of the system, are identified as the cupriferous pyrite ore deposits. This process of mass transfer corresponds to hydrothermal convection in a permeable medium with an open upper boundary surface.     

Petrological and Chemical Evolution of the Northeastern Troodos Extrusive Series, Cyprus

Four magma series are distinguished in the northeastern TroodosExtrusive Series: (A) a Lower Low-Ti Series (Lo-LTS) of basalticandesites, (B) a High-Ti Series (HTS) of basaltic andesitesto rhyodacites, (C) a Low-Ti Series(DLTS), the last two beingof basaltic andesite. Trace-element characteristics vary systematicallyfrom Series A to D and are interpreted in terms of a variablecontribution of three major source components (SCs). LILE-enrichedwater-rich fluids (SCI) derived from dehydration of a subductedlithosphere slab were continuously added to the overlying mantlewedge. Increasing LILE/HFSE and LILE/REE ratios and decreasingabsolute HFSE and REE concentrations from Series A to Dindicateprogressive depletion of the actual mantle source (SCII). Anegative Ta anomaly in the lavas decreases from Series A toD and is interpreted to have resulted from partial melting ofthe lower crust (SCII) where Ta-Nb-Ti may be fractionated byTi-rich accessory phases. The contribution of SCIII decreaseswhen the eruptive sites successively move away from the centralaxial zone and the temperature of the lower crust decreases,preventing partial melting of the lower crust. Chemical compositionsof fresh glass separates and phenocrysts indicate a change ofmajor petrogenetic processes from series A to D. Lo-LTS andIITS lavas are intrepreted to be directly related by open-systemfractional crystallization in crustal magma chambers. Removalof observed phenocryst phases clinopyroxene, orthopyroxene,plagioclase, and magnetite, and repeated subsequent mixing ofdacitc to rhyodacitic magmas with batches of replenshing basalticandesites are the major processes, possibly induced by vesicleformation in the mafic layer after a period of some crystallization.LTS and DLTS magmas were directly fed to the surface withoutstagnating at crustal levels, with feeder dykes positioned marginalto the central rift zone and thus by-passing the central magmachambers. These magmas apparently experienced only limited fractionalcrystallization of 10–15 wt.% olivine+clinopyroxene+chromite,probably at the mantle-crust boundary.     

Petrology and origin of primitive lavas from the Troodos ophiolite,Cyprus

Parental magmas to the Troodos ophiolite are characterised by low TiO₂ and Al₂O₃ and high SiO₂. Extremely fresh and chemically primitive (high MgO) rocks are found within the Upper Pillow Lavas and along the Arakapas Fault Belt of Cyprus and contain forsteritic olivine±enstatite and groundmass clinopyroxene set in glass or plagioclase, with accessory magnesiochromite and sometimes hornblende. They are quartz-normative and may have originally contained up to 3 wt% H₂O. Geochemically, there are three distinct groups of primitive lavas, based on TiO₂ and Zr contents but also reflected by CaO, Na₂O and REE abundances. These groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region. The parental magma to the least depleted group (Group I) was that of the major portion of the Troodos plutonic complex and is similar to those postulated for other low-Ti ophiolites. Chemically it has close affinities with komatiitic basalts. The most depleted lavas (Group III) all have U-shaped REE profiles and variable ¹⁴³Nd/ ¹⁴⁴Nd ratios, interpreted in terms of metasomatism of the source region by an incompatible element-enriched component which was probably derived from a subducted slab. These lavas represent an intermediate step in the development of boninite series rocks.     

Natural radioelement concentration in the Troodos Ophiolite Complex of Cyprus

High-resolution γ-ray spectrometry was exploited to determine naturally occurring thorium (Th), uranium (U) and potassium (K) elemental concentrations in the whole area covered by the Troodos Ophiolite Complex of Cyprus. For that purpose, a total of 59 samples from surface soils and 10 from the main rock formations of the region of interest were analysed. Elemental concentrations were determined for Th (range from 2.5×10⁻³ to 2.0 ppm), U (from 8.1×10⁻⁴ to 0.6 ppm), and K (from 1.3×10⁻⁴% to 1.0%). The average values (A.M±S.D.) derived are (0.24±0.34) ppm, (0.10±0.10) ppm and (0.21±0.24)%, for Th, U, and K, respectively, in the soils, and (0.52±0.17) ppm, (0.17±0.11) ppm and (0.49±0.87)% in the rocks. From these values, a radioactivity (radioelement) loss of nearly 50% is estimated in the underlying surface soils due to leaching and eluviation during weathering of the rocks. The measured Th/U ratio exhibits values between 2 and 4, whereas the K/Th ratio is highly variable ranging between 1.5×10³ and 3.0×10⁴.     

Constraints on the formation of geochemically variable plagiogranite intrusions in the Troodos Ophiolite,Cyprus

The geochemistry and petrology of tonalitic to trondhjemitic samples (n = 85) from eight different plagiogranite intrusions at the gabbro/sheeted dyke transition of the Troodos Ophiolite were studied in order to determine their petrogenetic relationship to the mafic plutonic section and the lava pile. The plagiogranitic rocks have higher SiO₂ contents than the majority of the glasses of the Troodos lava pile, but lie on a continuation of the chemical trends defined by the extrusive rocks, indicating that the shallow intrusions generally represent crystallised magmas. We define three different groups of plagiogranites in the Troodos Ophiolite based on different incompatible element contents and ratios. The first and most common plagiogranite group has geochemical similarities to the tholeiitic lavas forming the lavas and sheeted dyke complex in the Troodos crust, implying that these magmas formed at a spreading axis. The second plagiogranite group occurs in one intrusion that is chemically related to late-stage and off-axis boninitic lavas and dykes. One intrusion next to the Arakapas fault zone consists of incompatible element-enriched plagiogranites which are unrelated to any known mafic crustal rocks. The similarities of incompatible element ratios between plagiogranites, lavas and mafic plutonic rocks, the continuous chemical trends defined by plagiogranites and mafic rocks, as well as incompatible element modelling results, all suggest that shallow fractional crystallisation is the dominant process responsible for formation of the felsic magmas.     

Mass transfer during sub-seafloor alteration of the upper Troodos crust (Cyprus)

Five major alteration zones in the Extrusive Series and the Sheeted Dike Complex of the Troodos Ophiolite are each characterized by (a) distinct elemental changes compared to the original composition and (b) secondary mineralogy. The upper ca. 300 m of the extrusive crust, the highly oxidatedcold seawater alteration zone (CSA), is strongly enriched in K₂O and depleted in Na₂O. It is followed downwards by alow temperature alteration zone (<170° C) which is most widespread in the Troodos extrusives and where Na₂O and K₂O are enriched, the latter less strongly than in the CSA zone. Three types ofhigh temperature alteration zones (<440° C; HTA I–III) are found in the Sheeted Dike Complex. All are marked by thorough leaching of K₂O, while the behavior of Na₂O (e.g. unchanged in type III) and CaO (depleted in type I, enriched in types II, III) is variable. Mass budgets of elemental changes are quantified by calibration of whole rock analyses via systematic stable element variations of fresh glasses found throughout the extrusive section. The Troodos extrusive crust and upper Sheeted Dike Complex are a major sink for MgO, K₂O, and Na₂O, and a source for CaO; the overall scale of fluxes drastically exceeds estimates based on fresh basalt compositions from present ocean crust.     

Crystallization Orders and Phase Chemistry of Glassy Lavas from the Pillow Sequences, Troodos Ophiolite, Cyprus

Three genetically unrelated magma suites are found in the extrusivesequences of the Troodos ophiolite, Cyprus. A stratigraphicallylower pillow lava suite contains andesite and dacite glassesand shows the crystallization order plagioclase; augite, orthopyroxene;titanomagnetite (with the pyroxenes appearing almost simultaneously).These lavas can in part be correlated chemically and mineralogicallywith the sheeted dikes and the upper part of the gabbro complexof the ophiolite. The second magma suite is represented in astratigraphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton orderchromite; olivine; Ca-rich pyroxene; plagioclase. This magmasuite can be correlated chemically and mineralogically withparts of the ophiolitic ultramafic and mafic cumulate sequence,which has the crystallization order olivine; Ca-rich pyroxene;orthopyroxene; plagioclase. The third magma suite is representedby basaltic andesite lavas along the Arakapas fault zone andshows a boninitic crystallization order olivine; orthopyroxene;Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phaseequilibria experiments on a lava from the second suite indicateplagioclase crystallization from 1225?C, pigeonite from 1200?C,and augite from 1165?C. These experimental data contrast withthe crystallization order suggested by the lavas and the associatedcumulates. The observed crystallization orders and the presenceof magmatic water in the fresh glasses of all suites are consistentwith evolution under relatively high partial water pressures.In particular, high P_H2O (1–3 kb) can explain the lateappearances of plagioclase and Ca-poor pyroxene in the majorityof the basaltic andesite lavas as the effects of suppressedcrystallization temperatures and shifting of cotectic relations.The detailed crystallization orders are probably controlledby relatively minor differences in the normative compositionsof the parental magmas. The basaltic andesite lavas are likelyto reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relativelyless normative diopside and more quartz, reached the Ca-poorpyroxene-olivine reaction surface and crystallized Ca-poor pyroxeneafter olivine.     

Dynamic partial melting: its application to the petrogeneses of basalts erupted in Iceland,the Faeroe Islands,the Isle of Skye (Scotland) and the Troodos Massif (Cyprus)

The lava piles erupted in Iceland, the Faeroes, Skye, and Troodos are vertically compositionally zoned with the most Mg-rich and hygromagmatophile element (HE) depleted basalts concentrated towards the top of the lava pile in each case. The observed variations in the REE in the Mg-rich basalts from each lava pile cannot be explained in terms of batch partial melting of a homogeneous upper mantle source. Dynamic partial melting (Langmuiret al., 1977, Earth Planet. Sci. Lett.36, 133–156), in conjunction with, or in some cases as a possible alternative to postulating local inhomogeneities in the upper mantle source region, can explain the vertical chemical variations. In this model the presence and amount of liquid remaining in the residue has an important effect on the HE composition of subsequent melts, i.e. it introduces an additional variable to the incremental partial melting equation. The REE variation observed can be accurately reproduced by numerical models of dynamic partial melting using appropriate source compositions.The small but significant, radiogenic isotope variations in Icelandic basalts, whilst suggesting inhomogeneities in the source regions also lend some support to the dynamic melting process. In at least some of the HE-depleted basalts the Sm/Nd ratios are too high to explain the measured ¹⁴³Nd/¹⁴⁴Nd ratios for a single stage mantle evolution. It is necessary to infer a recent enrichment of the Sm/Nd ratios in their sources—an event predicted by the continuous melting process.The chemical inhomogeneities required in the mantle beneath Iceland are consistent with an HE depleted source region-veined to varying degrees by fluids or magmas related to previous events of melt transfer. Dynamic partial melting of such a mantle source would result in the vertical chemical zonation of a lava pile if the source was not continuously replenished at depth.     

The distribution of Hg, Ba, Cu and Zn in the vicinity of cupriferous sulfide deposits, Troodos complex, Cyprus

A geochemical rock- and soil-sampling program was carried out in the vicinity of eight concealed “Cyprus type” deposits, occurring in marginal mafic to intermediate metapillow lavas of the Troodos Ophiolite Complex. The mineralization of massive and stockwork sulfide ore is characterized by the predominance of pyrite, intergrown with less chalcopyrite and minor amounts of sphalerite.Background values of Hg are in the range of 8–12 ppb for soils and 3–6 ppb for surface rocks. Anomaly/background ratios of 10:1 (soils) and 5:1 (rocks) have been found only, where Hg migrated along channels formed by faults cutting shallow-seated mineralization. Here, Hg sometimes shows significant correlations with Cu, Zn, Ba and exceptionally with Co. However in one case an Hg anomaly in soils and surface rocks was detected directly over a deposit. The use of Hg as indicator element for these types of deposits is therefore limited. Buried mineralization may be delineated more distinctly by Cu, Zn and Ba.     

Long chain alkenones preserved in Miocene lake sediments

Over the last few decades, the occurrence of long chain alkenones (LCAs) in Quaternary lacustrine settings has been reported, the distributions being remarkably different from those of open ocean settings. We report here the occurrence of LCAs with modern lake-type composition from Middle Miocene sedimentary strata in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau. This is the first report of LCAs with preserved lake compositions in pre-Quaternary lacustrine sediments. The discovery implies that the producers of modern lacustrine LCAs could originate from similar lake species living at least as far back as the Miocene epoch. It also sheds new light on the potential of “alkenones” as palaeotemperature indicators in lacustrine systems back to at least 10.5 Ma.     

Lower Miocene sediments of central and western Ciscaucasia

The composition, structure, and geochemical properties of the thickest, relatively deep-water Lower Miocene sediments developed in western Ciscaucasia are considered. Of particular interest are materials from the Kuban superdeep borehole SG-12000 that recovered the uppermost layers of the Maikop Group at 3148–3961 m in the central Indol-Kuban Trough west of Krasnodar. However, the borehole did not penetrate the whole Lower Miocene section of the Maikop Group. Therefore, characteristics of the Maikop Group are supplemented with new materials from several other boreholes drilled in the eastern Kuban region. Thus, the typical (reference) Lower Miocene section of central and western Ciscaucasia has been sufficiently well described.     

Application of airborne LiDAR data and airborne multispectral imagery to structural mapping of the upper section of the Troodos ophiolite, Cyprus

Structural maps are traditionally produced by mapping features such as faults, folds, fabrics, fractures and joints in the field. However, large map areas and the spatially limited ground perspective of the field geologist can potentially increase the likelihood that not all structural features will be identified within a given area. The ability to recognise and map both local and regional structural features using high-resolution remote sensing data provides an opportunity to complement field-based mapping to help generate more comprehensive structural maps. Nonetheless, vegetation cover can adversely affect the extraction of structural information from remotely sensed data as it can mask the appearance of subtle spectral and geomorphological features that correspond to geological structures. This study investigates the utility of airborne Light Detection And Ranging (LiDAR) data and airborne multispectral imagery for detailed structural mapping in vegetated ophiolitic rocks and sedimentary cover of a section of the northern Troodos ophiolite, Cyprus. Visual enhancement techniques were applied to a 4-m airborne LiDAR digital terrain model and 4-m airborne multispectral imagery to assist the generation of structural lineament maps. Despite widespread vegetation cover, dykes and faults were recognisable as lineaments in both data sets, and the predominant strike trends of lineaments in all resulting maps were found to be in agreement with field-based structural data. Interestingly, prior to fieldwork, most lineaments were assumed to be faults, but were ground-verified as dykes instead, emphasising the importance of ground-truthing. Dyke and fault trends documented in this study define a pervasive structural fabric in the upper Troodos ophiolite that reflects the original sea-floor spreading history in the Larnaca graben. This structural fabric has not previously been observed in such detail and is likely to be continuous in adjacent regions under sedimentary cover. This information may be useful to future exploration efforts in the region focused on identification of structurally controlled mineral and groundwater resources. Overall, our case study highlights the efficacy of airborne LiDAR data and airborne multispectral imagery for extracting detailed and accurate structural information in hard-rock terrain to help complement field-based mapping.     

Paleomagnetic ages of Miocene fluvio-lacustrine sediments in the Tianshui Basin,western China

Long and continuous terrestrial paleoclimate records are important in understanding forcing mechanisms of Cenozoic climate. However, such records are rare. Sediments in basins near the edges of the Tibetan Plateau are potential paleoclimate archives, but dating them is a challenge. Here we report a successful application of paleomagnetism to a set of fluvio-lacustrine sediments in the Tianshui Basin, NE Tibet, and found that this set of sediments spans ∼17.1–6.1 Ma. Preliminary biomarker studies found that this set of sediments has climatically important organic components and, thus, these sediments are perfect paleoclimatic archives. Generating high-resolution biomarker records will be able to cast light on climatic history in NE Tibet during the Miocene.     

Depositional history,processes and mechanism of early Miocene sediments of Upper Assam Basin

A detailed analysis of depositional history of Miocene sediments and various effects which are governed for creation of accommodation space as well as the processes of sedimentation inherent to the depositional system at that period is described in this work. The early Miocene clastic sediments are deposited in prograding environment where sediment supply exceeds the accommodation space available. The accommodation space created due to basin subsidence and source area upliftment due to local and regional tectonic activity in the basin. In the early Miocene time, the Assam shelf major transgression occurred and several minor transgression followed. There was wide spread deposition of the fluvial Tipam sandstones. In Miocene time due to thrust loading and flexure subsidence, accommodation space was created for deposition of the sediments. The Tipam Sandstone is deposited by cyclic deposition of fining upward sequence in a fluvial to brackish water environment of braided river processes. The mechanism of braided rivers is also discussed in which it laterally expanded, leaving sheet like or wedge — shaped deposits of channel and bar complexes preserving only minor amounts of flood plain material.     

塔里木盆地西南坳陷中新统安居安组辫状河三角洲沉积

在塔里木盆地西南坳陷中新统安居安组中发现一套辫状河进积到内陆湖泊中而形成的辫状河三角洲沉积体。三角洲3层结构清楚,可明显地分为辫状河三角洲平原亚相、辫状河三角洲前缘亚相及前三角洲亚相3部分,可识别出7种微相。辫状河三角洲特征显著,明显有别于扇三角洲和曲流河三角洲。辫状河三角洲的主体是由含砾砂岩及中、粗粒砂岩组成的辫状河道砂坝及水下分流河道砂坝,单一砂坝呈下粗上细的透镜状,透镜体最大厚度为0．7～3．5m不等。垂向上许多砂坝透镜体相互叠置而成厚的砂体。三角洲中交错层理丰富,其中尤以水道砂坝侧向迁移加积而形成的侧积交错层异常发育为标志。辫状河三角洲砂体是重要的油气储集场所。