Stratigraphic correlation and paleoenvironmental analysis of the hydrocarbon-bearing Early Miocene Euphrates and Jeribe formations in the Zagros folded-thrust belt

The Lower Miocene Euphrates and Jeribe formations are considered as the main targets of the Tertiary petroleum system in the western part of the Zagros Basin. The formations consist of carbonates with some evaporate intercalations of the Dhiban Formation. This study utilized data from a field investigation including newly described outcrop sections and newly discovered productive oil fields within the Kirkuk embayment zone of the Zagros fold and thrust belt such as Sarqala and Kurdamir wells. This work is the first to show a stratigraphic correlation and paleoenvironmental interpretation by investigating both well data and new outcrop data. Three depositional environments were identified, (1) an inner and outer ramp belts environment, (2) shoal environment, and (3) restricted lagoon environment. Within these 3 environments, 12 microfacies were identified, based on the distribution of fauna mainly benthonic foraminifera, rock textures, and sedimentary structures. The inferred shallow water depths and variable salinities in both the Euphrates Formation and Jeribe Formation carbonates are consistent with deposition on the inner ramp (restricted lagoon and shoal) environments. Those found in the Euphrates Formation constrained the depositional environment to the restricted lagoon and shoal environment, while the microfacies in the Jeribe Formation provided evidence for an inner ramp and middle to outer ramp belt environments. This study represents the first detailed research that focuses on the stratigraphic correlation and changes in carbonate facies with the main aim to provide a wider understanding of stratigraphy of these carbonate reservoirs throughout the northern part of Iraq.     

Litho- and biostratigraphy,facies patterns and depositional sequences of the Cenomanian-Turonian deposits in the Ksour Mountains (Saharan Atlas,Algeria)

The Cenomanian-Turonian deposits exposed in the Ksour Mountains, western part of the Saharan Atlas (Algeria), document marine shelf environments that had been thriving on the North African passive margin, connected northwards to the Tethys Ocean, and fringed southwards by the Saharan craton. Their lithological, palaeontological, and sedimentological characteristics have been investigated to provide new insights into the biostratigraphy, palaeo-environmental evolution and sea-level changes in this western part of the Saharan Atlas. Three formations are recognized, from base to top of the studied succession: 1. The El Rhelida Formation comprising two informal units: the mixed siliciclastic-carbonate unit deposited under different flow regime conditions, from shoreline to backshore environments, and the limestone–claystone unit including coastal mudflat deposits prone to storm events. The early Cenomanian age of the El Rhelida Formation is supported by vertebrate assemblages. 2. The Mdaouer Formation comprising two units: the evaporitic unit dominated by claystone and evaporite alternations deposited on a flat coastal sabkha with occasional storms, and the marlstone–limestone unit formed in a peritidal-lagoonal environment. The Mdaouer Formation is of early-middle Cenomanian age. 3. The Rhoundjaïa Formation comprising three units: the lower limestone unit consisting of relatively homogeneous fossiliferous limestones; the middle marly unit composed of marlstone and bioclastic limestones, and the upper limestone unit consisting of carbonates showing vertical variations in faunal content and stratonomy. The Rhoundjaïa Formation was deposited in homoclinal ramp setting. Ammonite data indicate an early late Cenomanian to early Turonian age for this interval. Within an overall transgressive trend, the Cenomanian-Turonian deposits of the Ksour Mountains record three third-order depositional sequences bounded each by regional discontinuities.     

Facies Analysis, Sequence Stratigraphy and Diagenetic Studies of the Jurassic Carbonates of the Kohat Basin, Northwest Pakistan: Reservoir Implications

The present study deals with the depositional facies, diagenetic processes and sequence stratigraphy of the shallow marine carbonates of the Samana Suk Formation, Kohat Basin, in order to elucidate its reservoir quality. The Samana Suk Formation consists of thin to thick-bedded, oolitic, bioclastic, dolomitic and fractured limestone. Based on the integration of outcrop, petrographic and biofacies analyses, the unit is thought to have been deposited on a gentle homoclinal ramp in peritidal, lagoonal and carbonate shoal settings. Frequent variations in microfacies based sea-level curve have revealed seven Transgressive Systems Tracts (TSTs) and six Regressive Systems Tracts (RSTs). The unit has undergone various stages of diagenetic processes, including mechanical and chemical compaction, cementation, micritization, dissolution and dolomitization. The petrographic analyses show the evolution of porosity in various depositional and diagenetic phases. The fenestral porosity was mainly developed in peritidal carbonates during deposition, while the burial dissolution and diagenetic dolomitization have greatly enhanced the reservoir potential of the rock unit, as is further confirmed by the plug porosity and permeability analyses. The porosities and permeabilities were higher in shoal facies deposited in TSTs, as compared to lagoonal and peritidal facies, except for the dolomite in mudstone, deposited during RSTs. Hence good, moderate and poor reservoir potential is suggested for shoal, lagoonal and peritidal facies, respectively.     

Siliciclastic deposit of the Nahr Umr Formation, sedimentological and depositional environment studies, central and southern Iraq

The depositional facies and environments were unraveling by studying 21 subsurface sections from ten oilfields in the central and southern Iraq and a large number of thin sections of the Nahr Umr (siliciclastic deposit) Formation (Albian). This formation is mainly composed of sandstone interlaminated with minor siltstone and shale, with occurrence of thin limestone beds. Nahr Umr Formation is subdivided into three lithostratigraphic units of variable thicknesses on the basis of lithological variations and log characters. Mineralogically and texturally, mature quartz arenite and sandstones are the common type of the Nahr Umr Formation. The sandstones are cemented by silica and calcite material and have had a complex digenetic history. Compaction, dissolution, and replacements are the main diagenetic processes. Prodelta, distal bar, distributary mouth bar, distributary channel, over bank, and tidal channel are the main depositional environments recognized for the Nahr Umr Formation, within the studied wells. This formation was deposited in shallow marine and fluvial–deltaic environments and exhibit progradational succession of facies. Eight sedimentary facies that have been identified in the Nahr Umr Formation include claystone lithofacies, claystone siltstone lithofacies, lenticular-bedded sandstone–mudstone lithofacies, wavy-bedded sandstone–mudstone lithofacies, flaser-bedded sandstone–mudstone lithofacies, parallel and cross lamination sandstone lithofacies, trough cross-bedded sandstone lithofacies, and planar cross-bedded sandstone lithofacies. The depositional model of the Nahr Umr Formation environment was built based on the lithofacies association concepts.     

Sedimentary facies and diagenetic features of the Early Cretaceous Fahliyan Formation in the Zagros Fold-Thrust Belt,Iran

The Early Cretaceous Fahliyan Formation (middle part of the Khami Group), is one of the important reservoir rocks in the Zagros Fold-Thrust Belt. The Zagros Fold-Thrust Belt is located on the boundary between the Arabian and Eurasian lithospheric plates and formed from collision between Eurasia and advancing Arabia during the Cenozoic. In this study area, the Fahliyan Formation with a thickness of 325 m, consists of carbonate rocks (limestone and dolomite). This formation overlies the Late Jurassic Surmeh Formation unconformably and underlies the Early Cretaceous Gadvan Formation conformably at Gadvan Anticline. The formation was investigated by a detailed petrographic analysis to clarify the depositional facies, sedimentary environments and diagenetic features in the Gadvan Anticline. Petrographic studies led to recognition of the 12 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environments. The absence of turbidite deposits, reefal facies, and gradual facies changes show that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. The diagenetic settings favored productioning a variety of features which include cements from early to late marine cements, micritization, dolomitization, compaction features, dissolution fabric, and pores. The diagenetic sequence can be roughly divided into three stages: (1) eugenic stage: marine diagenetic environment, (2) mesogenic stage: burial environment, and (3) telogenic stage: meteoric diagenetic environment.     

Petrographic characteristics of the Proterozoic Vempalle carbonates,Cuddapah Basin,India and their implications

The Vempalle Formation of the Proterozoic Cuddapah basin has a well developed sequence of carbonate rocks, which are interbedded with shales, siltstones and chert. The stromatolitic carbonates are conspicuous at many places but the oolitic carbonates are less prominent and are present only in some areas. All the carbonates are pervasively dolomitized. Petrographic examination of these carbonates revealed that they are predominantly made up of fine grained micrite with patchy development of sparite and chert/quartz. The stromatolitic carbonates show distinct banding of alternate carbonate and cherty layers. The latter are rich in organic matter indicating prevalence of profuse biogenic activity. The oolitic carbonates comprise of ooids showing both concentric and radial patterns and made up of carbonate/chert and cemented by micro/mega quartz or carbonate itself. Diagenetic and post depositional features are reflected in cementation, recrystallization, compaction, stylolite formation and silicification processes. Various stages of cementing material are observed. Secondary vein fillings of carbonate or quartz traverse the carbonate/cherty groundmass. Intraclasts present suggest occasional erosional destruction of associated sediments, short lived transport and local redeposition. Accessory silicate minerals represent terrigenous influx during deposition. Dolomitization of the carbonates was fabric retentive and early diagenetic. The environmental conditions were characterised by low energy, within a shallow water zone, in occasional higher energy events and turbulence. The carbonates appear to have been deposited on a shallow water ramp within a tidal regime.     

Sedimentological and depositional environment studies of the Mauddud Formation, central and southern Iraq

Nineteen subsurface sections and a large number of thin sections of the Mauddud limestone (age Albian?CEarly Cenomanian) were studied to unravel the depositional facies and environments. The allochems in the Mauddud Formation are dominated by bioclasts (reach 23%) and peloids (reach 60%), whereas intraclasts are less abundant (reach 2.3%). The sedimentary microfacies of the Mauddud Formation includes lime mudstone, wackestone, wackestone?Cpackstone, packstone, packstone?Cgrainstone, in addition to dolostone lithofacies and green shale lithofacies. These microfacies have been deposited in shallow warm marine environment of varying salinities and energy levels. Cementation, neomorphism, dolomitization, compaction, and dissolution are observed affecting variably both ground mass and particles. The formation displays various extents of dolomitization and is cemented by calcite and dolomite. Dolomitization increases toward the north of the study area and exhibits different textures. Similarly cementation shows a variety of textures. In addition, authigenic minerals, such as glauconite and pyrite, are scattered within the groundmass and along solution surfaces. The formation has gradational contact with the underlying Nahr Umr Formation but is unconformably overlain by the Ahmadi Formation, despite local conformity. Thus, the vertical bioclast analysis indicates that the Mauddud Formation is characterized by four major depositional cycles, which control the distribution of reservoir quality as well as the patterns of calcite and dolomite, cement distribution. Petrographical study shows that the fossil represents the main Allchem, Peloids, whereas intraclasts come second in abundance. Calcite (more than 50%) and dolomite (as diagenetic products, range between 20% and 50%) are the predominant mineral components of Mauddud Formation. Fossils were studied as an environmental, age, and facial boundary indicators. Five major depositional microfacies are recognized. These depositional microfacies have been subdivided according to their primary and diagenetic constituents into ten submicrofacies. The determined four major depositional cycles were representing normal sequential regression from base upward. The lateral analysis shows the same regressive cycle and by using the lithofacies association concepts to build the depositional model of the Mauddud Formation environment.     

西天山早石炭世构造环境:大哈拉军山组底部沉积地层学证据

西天山伊犁地区广泛出露早石炭世大哈拉军山组火山-沉积岩系, 其内赋存了金、铁、铜等重要的金属矿床。但迄今为止, 关于大哈拉军山组形成构造环境还存在很大的争议。通过区域地层对比, 研究区大哈拉军山组与下伏地层之间存在区域性角度不整合接触关系。其底部自下而上依次发育砾岩、粗砂岩、砂岩、粉砂岩、泥岩、碳酸盐岩、安山岩。沉积学和层序序列分析显示, 该组底部的陆源碎屑岩建造组合形成于扇三角洲沉积环境, 进一步可划分出扇三角洲平原、扇三角洲前缘和前扇三角洲3个亚相, 向上出现的碳酸盐岩形成于浅海碳酸盐岩台地环境。这种沉积物由粗到细、水体由浅到深, 沉积环境由陆相变为海相的地层充填序列, 反映了一种裂陷拉伸的动力学背景。因此, 西天山早石炭世大哈拉军山组火山-沉积建造形成于后碰撞裂谷环境。     

Early diagenesis and its relationship to depositional environment and relative sea-level fluctuations (Upper Cretaceous Marshybank Formation, Alberta and British Columbia)

Early diagenesis of the Upper Cretaceous (late Coniacian to early Santonian) Marshybank Formation was controlled by depositional environment (composition of depositional water, Fe and organic content of the sediment, sedimentation rate, proximity to the shoreline) and influx of meteoric water related to relative sea-level fall. Five depositional environments, each characterized by a distinct early diagenetic mineral assemblage, have been recognized. Offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions, composed of ‘early’ calcite and siderite. Intense sulphate reduction, promoted by the dysaerobic depositional water, was the primary influence on early diagenesis. Offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions, composed of two generations of siderite (S1 and S2). In this environment, methanogenesis, rather than sulphate reduction, was more important. Early diagenesis of the inner shelf sands was generally limited. However, in sands deposited proximal to the shoreline, mixing of marine and meteoric waters promoted crystallization of Fe-rich chlorite and siderite. The shoreface was characterized by dissolution of detrital minerals in the upper portion, and precipitation of kaolinite or illite/smectite in the lower portion. In the coastal plain environment, brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite. Ankerite, rather than siderite, is the typical early diagenetic carbonate. The δ¹⁸O values of the earliest cements (i.e. ‘early’ calcite, siderite S1, inner shelf siderite) indicate crystallization from a low-¹⁸O, marine-derived porewater. Assuming crystallization at 25°C, a δ¹⁸O value of about ?7‰ (SMOW) can be estimated for the seaway during Marshybank Formation time. Similar calculations for the overlying Dowling Member (Puskwaskau Formation) suggest that the δ¹⁸O value of the seaway increased to about ?4% (SMOW), consistent with its transgressive nature. Very low δ¹⁸O values are exhibited by siderite S2. These results indicate crystallization during intermediate diagenesis (≥60°C) from meteoric water (≥? 15‰ SMOW) that entered the Marshybank Formation during sea-level lowstand.     

辽东大连金石滩新元古代碳酸盐岩臼齿构造形态及其沉积环境指示意义

辽东大连金石滩新元古代兴民村组上部灰岩段发育了一系列形态复杂多样的臼齿构造(Molar-tooth struc-ture,简称MT)。通过野外及室内综合研究鉴别出条带状、短直杆状、蠕虫状、细丝状、圆斑状(气泡状、瘤状)和碎屑状6种主要类型,其中以条带状最为发育。MT发育段主要由含砂屑或粉屑灰岩、泥晶灰岩、纹层状泥晶灰岩和含泥泥晶灰岩的韵律组成,系多个向上沉积动力减弱、水体变浅的潮下-潮间带沉积旋回序列。兴民村组MT发育的沉积环境具有一定水深和频繁波动的水动力特点,MT形态受宿主岩石性质的影响和控制。MT主要发育在浅潮下带-潮间带下部沉积微相。不同形态的MT及其组合分别宿主于不同的沉积岩相或沉积韵律,表现了MT形态的指相意义。     

Geohistory analysis and basin development of the Neogene succession, NE Iraq

The Neogene succession in the studied area is represented by seven formations (Serikagni, Euphrates, Dhiban, Jeribe, Fatha, Injana, and Muqdadiya formations). The area of study is located in the Unstable Shelf within the Low Folded Zone and the north part of the Stable Shelf (Mesopotamian Zone). This study included the geohistory analysis of the Neogene succession and interpretation the changes of the accumulation and subsidence rates and compared them with the space available to explanation the basin development. At The Early Miocene (Aquitanian age), the Sirekagni and Euphrates formations was deposited during a major transgression with high rates of subsidence and accumulation in the Himreen, Makhul, and north of Tigris subzones, while the Chemchemal–Arbil and Butmah–Mosul subzones were positive areas. This period ended with a sea withdrawal to the southeast to generate the Dhiban lagoonal basin which was characterized by low accumulation and subsidence rates. During the Early Burdigalian, the Jeribe Formation was deposited during another sea level rise that covered the area except the Chemchemal–Arbil and Butmah–Mosul subzones representing the uplifted positive area. The sea level rise continued to the early Langhian age where the transition beds for the Fatha Formation was deposited to mark the maximum flooding surface covering all the study area. The Fatha Formation was deposited at the Late Langhian to the Early Serravallian during sea level stillstand with high accumulation and subsidence rates in the Himreen subzone and the Chemchemal–Arbil subzone. This period ended without a clear tectonic activity. The period from Late Miocene to Pliocene was characterized by high tectonic activity and sea level fall where fluvial–lacustrine environment prevailed to deposit the Injana and Muqdadiya formations. The Injana Formation was deposited during the Late Serravallian–Tortonian in the Himreen and Chemchemal–Butmah subzones; in addition to the northern part of Tigris subzone. The areas of high rates of accumulation and subsidence were located near Jambour while the southwestern part was affected by an uplift generating the Himreen structure. The Chemchemal–Butmah subzones was characterized by a high uplift in the southeast part where Kirkuk and Chemchemal structures were forming, while The northeastern part (from Bi Hassan to the borehole Kirkuk-117) was with low accumulation and subsidence rates. The linear region between these parts (Khabaz oil field) showed an abnormal values for accumulation and subsidence rates (very high); this region corresponds to the location and direction of Anah–Fatha–Qalat Dizah Fault which suggest that the fault was active during that time. The tectonic activity continued to uplift all the north of the study area as well as the West and the East during the Late Tortonian to the Piacenzian where the Muqdadiya Formation was deposited in the area between Jambour to Khabaz oilfields. Then the succession was deformed and uplifted to approximately 800 m above the sea level as in the present day.     

Microfacies and Depositional Environments of Miocene Isolated Carbonate Platforms from Central Luconia, Offshore Sarawak, Malaysia

The Luconia Province – offshore Sarawak – is a key geological unit for understanding the distribution of hydrocarbon resources in Malaysia. Nevertheless, little effort has been made to address the palaeoenvironmental characteristics of the Tertiary carbonates in the key sector of Central Luconia. We study the sedimentology and petrography of core samples from a well in Central Luconia, for which thirteen microfacies have been identified reflecting different depositional settings. This is the first microfacies scheme elaborated for Luconian carbonates. Lithofacies and microfacies distribution are compatible with deposition in a reef complex, originating around a framework reef, within the euphotic zone. Sediments were deposited in environments of backreef, reef crest, and forereef. The fair weather wave base is marked by the presence of coralline red algae, foraminifera, decreasing degree of bioclast fragmentation and other microfacies features. As a result, a depositional-environmental model is constructed, depicting a reef complex built around a framework reef developed on the margin of an isolated platform. In addition, an innovative, preliminary time series analysis of facies, microfacies and depositional environment data reveal the existence of seasonal cycles in the stacking patterns of facies and microfacies.     

河南登封地区寒武系第三统张夏组米级旋回及其演化

河南登封地区寒武系第三统张夏组是一套167m厚的碳酸盐岩地层,出露连续且完整,发育以微生物岩主导和以后生动物扰动灰岩主导的两种米级旋回类型。通过对米级旋回的演化及其沉积学和古遗迹学特征分析,张夏组自下而上由以微生物岩主导的米级旋回逐渐让位于以后生动物扰动灰岩主导的米级旋回;沉积体系由无鲕粒滩的碳酸盐岩台地逐渐向发育厚层鲕粒滩的碳酸盐岩台地演化;沉积环境从潮下低能深水演变为开阔台地,并逐渐变浅形成鲕粒滩、局限台地;沉积岩类型从叠层石、凝块石灰岩等微生物岩逐渐变化为生物碎屑灰岩、生物扰动灰岩和含生物扰动鲕粒灰岩。     

Authigenic seep-carbonates cementing coarse-grained deposits in a fan-delta depositional system (middle Miocene, Marnoso-arenacea Formation, central Italy)

Seep‐carbonates (¹³C‐depleted) are present at different levels within the Miocene terrigenous succession of Deruta (Marnoso‐arenacea Formation, central Italy); they are associated with pebbly sandstones and conglomerates in a tectonically active fan‐delta slope depositional system. Most of these seep‐carbonates are included in slide/slump horizons as scattered blocks. The occurrence of seep‐carbonates is clear evidence of the flow of methane‐rich fluids pervading the sediments. Fluids, probably of biogenic origin, may have reached the sea‐bottom through thrust faults and selectively infiltrated the more permeable coarse‐grained horizons deposited along the slope. Different stages of fluid emissions are documented: slow flux stage, corresponding to the development of large carbonate bodies and dense chemosynthetic communities; and fast fluid flow associated with intense carbonate brecciation, pipes and veins. Large amounts of authigenic carbonates are reworked by slope failures triggered by tectonics and fluids reducing sediment strength; in situ cementation of slide blocks may also have occurred due to remobilization of methane‐rich fluids by mass‐wasting processes.     

Facies analysis and sequence stratigraphy of Kometan Formation (Upper Cretaceous) in the imbricated zone,northeastern Iraq

Facies and sequence stratigraphic analysis of the Kometan Formation (Upper Cretaceous) were studied from Kometan village, Kurdistan region of northeastern Iraq. Lithologically, the formation consists of 44 m of white weathering, light grey, thin to medium-bedded highly fractured limestones with chert nodules. Petrographic study of the carbonates shows that both skeletal and non-skeletal grains were present. The skeletal grains include a variety of planktonic foraminifera (including Oligostegina), calcispheres, ostracods, pelecypods, larva ammonite, and echinoderms. Non-skeletal grains include peloids only. Three main microfacies types are distinguished in the studied formation. The results of stable carbon and oxygen isotopes of the studied carbonate samples show negative values of δ¹⁸O. These indicate that the seawater was warm with low salinity during precipitation of the carbonates in the Kometan Formation in northeastern Iraq. The positive δ¹³C values of carbonate samples, in the middle part of the formation, reflect the widespread deposition of organic-rich marine sediments during a transgression and deepening of the basin. Petrographic, facies and stable isotopic analyses revealed that the Kometan Formation was deposited in a warm, basinal, pelagic (open marine) environment with low salinity. The Kometan Formation consists of one complete third-order depositional sequence, separated by a sequence boundary (SB) of type 2. The third-order sequence is subdivided into a transgressive systems tract (TST) and highstand systems tract (HST). This reflects episodes of transgression and still stands of the relative sea level. The TSTs are topped by maximum flooding surface (MFS) characterized by deepening-/fining-upward parasequences implying a retrogradational stacking pattern. The HST is marked by shallowing-/coarsening-upward parasequences implying a progradational stacking pattern.     

The Triassic of the Thakkhola (Nepal). I: stratigraphy and paleoenvironment of a north-east Gondwanan rifted margin

The Mesozoic sediments of Thakkhola (central Nepal) were deposited on a broad eastern north Gondwanan passive margin at mid-latitudes (28–41 °S) facing the Southern Tethys ocean to the north. The facies is strikingly similar over a distance of several thousand kilometres from Ladakh in the west to Tibet and to the paleogeographically adjacent north-west Australian margin (Exmouth Plateau, ODP Legs 122/123) and Timor in the east. Late Paleozoic rifting led to the opening of the Neo-Tethys ocean in Early Triassic times. An almost uninterrupted about 2 km thick sequence of syn-rift sediments was deposited on a slowly subsiding shelf and slope from Early Triassic to late Valanginian times when break-up between Gondwana (north-west Australia) and Greater India formed the proto-Indian Ocean. The sedimentation is controlled by (1) global events (eustasy; climatic/oceanographic changes due to latitudinal drift; plate reorganization leading to rift-type block-faulting) and (2) local factors, such as varying fluvio-deltaic sediment input, especially during Permian and late Norian times. Sea level was extremely low in Permian, high in Carnian and low again during Rhaeto-Liassic times. Third-order sea-level cycles may have occurred in the Early Triassic and late Norian to Rhaeto-Liassic. During the Permian pure quartz sand and gravel were deposited as shallowing upward series of submarine channel or barrier island sands. The high compositional maturity is typical of a stable craton-type hinterland, uplifted during a major rifting episode. During the early Triassic a 20–30 m thick condensed sequence of nodular ‘ammonitico rosso’-type marlstone with a ‘pelagic’ fauna was deposited (Tamba Kurkur Formation). This indicates tectonic subsidence and sediment starvation during the transgression of the Neo-Tethys ocean. During Carnian times a 400 m thick sequence of fining upward, filament-rich wackestone/shale cycles was deposited in a bathyal environment (Mukut Formation). This is overlain by about 300 m of sandy shale and siltstone intercalated with quartz-rich bioclastic grain- to rudstone (Tarap Shale Formation, late Carnian-Norian). The upper Norian to (?lower) Rhaetian Quartzite Formation consists of (sub)arkosic sandstones and pure quartz arenites, indicating different sediment sources. The fluvio-deltaic sandstones are intercalated with silty shale, coal and bioclastic limestone, as well as mixed siliciclastic-bioclastic rocks. The depositional environment was marginal marine to shallow subtidal. The fluvio-deltaic influence decreased towards the overlying carbonates of Rhaeto-Liassic (?) age (Jomosom Formation correlative with the Kioto Limestone), when the region entered tropical paleolatitudes resulting in platform carbonates.     

Depositional environments of marine-dominated bedded halite, Permian San Andres Formation, Texas

The interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology. Facies studies, supplemented by geochemical data, define an evaporite shelf depositional environment for the halite of the San Andres Formation of the Palo Duro Basin, Texas Panhandle, that is significantly different from modern and previously described ancient halite-precipitating environments such as playas, salinas, sabkhas, and barred basins. A coastal palaeoenvironmental setting for the San Andres halite is defined by its facies position between normal marine-shelf carbonates and aeolian deposits. The San Andres brine pool extended over more than 10 000 km² but was very shallow. Perennial brine-pool conditions (thick sequences of halite) alternated with ephemeral brine-pool conditions (halite, haloturbated mudstone-halite, mudstone interbeds, and microkarst). The depositional sequence in the perennial brine pool includes (1) flooding and minor dissolution of previously deposited halite, (2) precipitation of a thin lamina of gypsum, and (3) evaporation and precipitation of halite. Bottom-growth forms of halite, including chevrons and vertically elongated crystals, are well preserved. Cumulates of foundered, floating crystals and rafts were probably abundant but have been almost completely recrystallized. Mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine-pool deposition and the establishment of subaerial conditions. Vertically elongated karst pits originate at the palaeosurfaces, cross-cut primary brine-pool fabrics, and are filled with geopetal sediment and coarse halite cement. Mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity. The geochemistry of halite from both ephemeral and perennial brine-pool environments reflects the frequent influx of marine brine, indicating that communication with the marine environments to the south was ‘good’ considering that restriction was sufficient to precipitate halite.     

辽西义县组碳酸盐岩夹层及其环境意义

义县组是热河生物群最繁盛的层位,它承载了热河生物群演化的重要信息,开展义县组沉积环境的深入研究是认识和恢复热河生物群生活环境的有效途径.辽西地区义县组中夹有多层碳酸盐岩沉积层,所含沉积环境信息丰富.通过碳酸盐岩的研究,获取碎屑岩中丢失的环境信息,并与碎屑岩沉积层的研究相结合,为恢复辽西地区义县期热河生物群的古地理、古气...     

Fluvial-shallow marine-glaciofluvial depositional environments of the Ordovician System in Jordan

The Ordovician System, cropping out in southern and west-central Jordan, consists entirely of a 750 m thick clastic sequence that can be subdivided into six formations. The lower Disi Formation starts conformably above the Late Cambrian Umm Ishrin Formation. According to Cruziana furcifera occurring in the upper third of the Disi Formation, an Early Ordovician age is confirmed. The Disi Formation, consisting mainly of downstream accretion (DA) fluvial architectural element, was deposited in a proximal braidplain flowing N–NE from the southerly-located Arabian–Nubian Shield towards the Tethys Seaway. The braidplain depositional environment evolved into a braidplain-dominated delta through the middle and upper parts of the Disi Formation and the lower part of the overlying Um Saham Formation. The delta was replaced by siliciclastic tidal flats, that in turn evolved into an upper to lower shoreface environment through the upper part of the Um Saham Formation. The depositional environment attained the maximum bathymetric depth during the deposition of the lower and central parts of the third unit, the Hiswa Formation, where offshore graptolite-rich mudstone with intercalated hummocky cross-stratified tempestites were deposited. The Tethys Seaway regressed back through the upper part of the Hiswa Formation promoting a resumption of the lower–upper shoreface sedimentation. Oscillation between the lower to upper shoreface depositional environment characterized the entire fourth unit, the Dubaydib Formation, as well as the Tubeiylliat Sandstone Member of the fifth unit, the Mudawwara Formation. The depositional history of the Ordovician sequence was terminated by a glaciofluvial regime that finally was gradually replaced by a shoreface depositional environment throughout the last unit, the Ammar Formation.     

Facies analysis and geochemistry of the Euphrates Formation in Central Iraq

The Euphrates Formation (Lower Miocene) in the Central Iraq consists mainly of shallow marine carbonates. Two hundred ten samples were collected from 21 wells (1E to 21ED) at Bahar Najaf area, and 18 samples were collected at Wadi Asadi in Baghdadi area, from Euphrates Formation. Four microfacies are identified, namely mudstone, wackestone, packstone, and rare grainstone with ten submicrofacies. The allochems in the Euphrates Formation are dominated by bioclasts. Peloids, ooids, and intraclasts are less abundant. The common fossils in the Euphrates Formation are miliolids, algae, ostracods, Miogypsina, and abundant shells of pelecypods and gastropods. Calcite and dolomite are the predominant mineral components of the Euphrates Formation. The carbonates of the Euphrates Formation have been affected by a variety of diagenetic processes such as micritization, dissolution, neomorphism, cementation, stylolitization, dolomitization, dedolomitization, and silicification. The Euphrates Formation was deposited in open to restricted platforms which indicated lagoonal environment with warm and restricted open circulation. In fact, prevalence and abundance of micrite provide an evidence of a shallow marine of low-energy environment and, in some places, may be approaching to be stagnant environment. The average of CaO in Najaf area (51.5 %) is slightly lower than that in Baghdadi area (53.3 %), which was reflected in calcite content found being 91 % in Najaf and 94 % in Baghdadi. Dolomite and gypsum appeared as minor minerals beside calcite, so low concentration of MgO (0.83 % in Najaf; 0.63 in Baghdadi) and SO₃ (0.55 % in Najaf; 0.53 % in Baghdadi) was reflected information of small amounts of dolomite (2 % in Najaf; 1.6 % in Baghdadi) and gypsum (0.7 % in Najaf and 0.6 in Baghdadi) in the Euphrates Formation. The insoluble residue in Najaf area (4.37 %) is relatively higher than that in Baghdadi area (1.9 %), indicating that the Euphrates Formation in Najaf Area has deposited in an environment closer to the shoreline. Concentrations of the trace elements Sr, Mn, and Fe which support the conclusion that reminds the Euphrates Formation had been deposited in a shallow marine environment of quiet energy, with the likelihood that the shoreline was the nearest to Najaf rather than to Baghdadi.