Stromatolites: a guideline for development of a carbonate ramp,Basantpur Formation,Neoproterozoic Simla Group in Lesser Himalaya,India

Outcrop-based facies analysis of the Proterozoic Basantpur Formation, Simla Group in the Lesser Himalaya was combined with the stromatolites morphometry and sea-level fluctuation to delineate the stages of carbonate ramp development. On this basis, a vertical profile depositional model (Basantpur type) has been developed. Facies associations and variation in the patterns of microbial growth along with the sea-level fluctuations have contributed to the identification of the development of a tide-influenced carbonate ramp. Different stromatolitic structures (mega-, macro- and microstructures) are documented in the dolomudstones and dolosiltstones along with fenestral structures and their depositional facies together with evidences of marine transgression which leads to development of carbonate ramp where inner-mid-outer-ramp subenvironments are recognised. The “Basantpur”-type model is therefore unique in that it deals with lateral facies variation due to shift in shore line along with fluctuations in accommodation space on a carbonate ramp owing to fluctuations of sea level. This model will probably find its applicability in similar carbonate ramps.     

Biostratigraphy,sequence stratigraphy,and paleoecology of the Lower–Middle Miocene of Northern Bandar Abbas,Southeast Zagros basin in south of Iran

The Guri Member is a limestone interval at the base of the calcareous marls of the Mishan Formation. It is the youngest hydrocarbon reservoir of the southeast part of the Zagros sedimentary basin. This Member overlaid siliciclastic rocks of Razak Formation and is overlain by green and gray marls of the Mishan Formation. In order to consider the paleoecology and paleoenvironments of the Lower–Middle Miocene (Guri Member), we have studied biostratigraphy and sequence stratigraphy of the Guri Member based on foraminifer and microfacies in two stratigraphic sections including Dorahi–Homag and Chahestan. A total of 33 genera and 56 species of benthic and planktonic foraminifera were identified in two studied stratigraphic sections. Benthic and planktonic foraminifera demonstrate Aquitanian to Langhian age (Early–Middle Miocene) for this Member at the study area. Studied interval has deposited in four facies association including supratidal, lagoon, coral reef, and open sea on a carbonate ramp. Carbonate rocks of the Guri Member have precipitated in two and three depositional sequences at Chahestan and Dorahi–Homag sections, respectively. Sedimentation of marine carbonates of the Guri Member on siliciclastic deposits reflects a major transgression of sea level at Lower to Middle Miocene that led to creating a new sea in the Zagros basin at that age. Increasing siliciclastic influx along with a sea level fall finally caused burying of the carbonate ramp. Except for the beginning of sedimentation of carbonate at the base of both stratigraphic sections (depositional sequence 1), most of the system tracts are not matched to global sea level curve that reflect local effects of the basin. Distribution of foraminifera suggests precipitation in tropical to subtropical in mesotrophic to oligotrophic and eutrophic to oligotrophic conditions. Based on large benthic foraminifera (porcelaneous large benthic foraminifera and hyaline larger benthic foraminifera), water temperature average was determined between 25 and 30 °C that was confirmed by analyzing oxygen and carbon stable isotopes. Finally, we have utilized achieved data to reconstruction and modeling of paleoecology, paleoenvironments, and sea level changes in the southeast part of the Zagros basin.     

Lithofacies modelling and sequence stratigraphy in microtidal cool-water carbonates: a case study from the Pleistocene of Sicily, Italy

ABSTRACT Quaternary carbonates in SE Sicily were deposited in seamount and short ramp settings during glacio‐eustatically driven highstand conditions. They provide an excellent opportunity to investigate the depositional and erosional aspects of cool‐water carbonate sedimentation in a microtidal marine water body. The derived ramp facies model differs significantly from modern‐day, open‐ocean ramp scenarios in projected facies depth ranges and in the preservation of inshore facies. A sequence stratigraphic study of the carbonates has confirmed many established aspects of carbonate sedimentation (e.g. production usually only occurred during highstands). It has also revealed several new features peculiar to water bodies with little tidal influence, including ‘catch‐up’ surfaces taking the place of transgressive facies, second‐order sequence boundary events being most important as triggers for initiating resedimentation and a virtual absence of sediment shedding to the basin during the terminal lowstand. Production in the carbonate factory lasted for about 0·5 Myr. Despite this, carbonate production was considerable and included both bioconstructional and bioclastic‐dominated facies and the production of abundant lime muds. A model for eustatically controlled cool‐water carbonate production and resedimentation in microtidal marine water bodies is presented. This is considered to be more applicable to Neogene and Quaternary strata in the Mediterranean region than are current open‐ocean models.     

贵州瓮安陡山沱组含磷岩系沉积序列及磷块岩的形成

贵州瓮安陡山沱组磷块岩为碳酸盐岩型含磷岩系,属于浅海碳酸盐岩台地沉积区。对瓮安磷矿大塘剖面进行沉积环境研究,将Marinoan冰期后沉积的陡山沱组地层分为6个向上变浅的沉积序列。瓮安含磷岩系大部分为多旋回的叠置体,其常见的交错层理、波状层理、透镜状层理等表明了磷块岩形成于水体较浅的高能环境。南沱冰期后,陡山沱期剧烈的海侵作用引发上升洋流,携带富磷海水涌入浅水区,在海平面间歇性波动的条件下,为磷块岩的形成提供了物源与动力条件。     

鄂尔多斯东北部太原组上部灰岩段高分辨层序地层分析

根据对露头、测井和岩心资料的垂向分析和横向对比,结合古生物化石资料,在识别关键性界面的基础上,对鄂尔多斯东北部下二叠统太原组上部灰岩段建立了高分辨层序地层格架,划分出5个三级层序。研究层段为有陆源碎屑混入的碳酸盐缓坡沉积,形成于华北晚古生代最大海侵阶段。各层序分别由低位体系域的下切谷充填沉积的砂岩、砂砾岩,海侵体系的灰岩、泥灰岩、泥岩、煤和高位体系域的碎屑岩和煤组成。区内厚达数十米的桥头砂岩主要由几个层序低位域下切河谷充填沉积叠置而成。通过编制的各层序海侵域灰岩的厚度和分布图,证实层序3、4沉积期海侵范围最大。早二叠世早期海侵来自东南和西南两个方向      

Sequence stratigraphic framework for mixed aeolian,peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

Due to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross‐bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. Highstand systems tract deposits are lacking, either due to non‐deposition or post‐depositional erosion. The magnitude of inferred relative sea‐level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments.     

Anatomy and origin of a Cretaceous phosphorite-greensand giant, Egypt

Late Cretaceous epicontinental phosphorites, porcelanites/cherts, dark-coloured shales, glauconitic sandstones and bioclastic and fine-grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon-rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea-level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution-collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron-oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters. This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters.     

Evolution of a Callovian-Oxfordian carbonate margin in the Neuquén Basin of west-central Argentina: facies, architecture, depositional sequences and global sea-level changes

This paper reviews a detailed stratigraphic and sedimentologic study of a carbonate complex developed on the foreland side of the Neuquén Embayment, a protected shallow-water epicratonic site behind the active edge of the South American plate. Superb outcrops at the core of basement-involved Andean structures expose the shelf-to-basin transition and reveal with clarity the external and the internal architecture of the depositional sequences and component system tracts. Platform carbonates are largely represented by ooid and mainly rhodoid grainstones, with associated patches of coral framestone. The deeper platform and slope facies are composed of oncoidal and skeletal micritic limestones with scattered coral-sponge-algal build-ups. The overall composition and facies pattern bears resemblance to other Late Jurassic carbonate complexes form Europe and with the Smackover Formation from the Gulf Coast Basin of North America.

Analysis based on mapping of the stratal patterns and facies associations in outcrops allowed the recognition of four depositional sequences. Timing provided by ammonite biochronology suggests that eustatic fluctuations were a major factor influencing the carbonate-margin architecture, and regulated episodes of condensed sedimentation, shifts of the depositional belts, and development of stratigraphic discontinuities. The onset and the end of carbonate sedimentation were associated with episodes of marine retreat and accumulation of evaporites and eolian-fluvial deposits at basin-centre locations. However, most of the marine fluctuations recorded within the carbonate complex were insufficient to expose the shelf break (Type 2), and accordingly lowstand system tracts are poorly represented. On the shelf the transgressive system tracts are represented by thin grain-supported carbonate blankets. These taper out downslope into omission surfaces or are replaced by patches of small sponge buildups. Highstand system-tract organization changes through time, reflecting changes in productivity and accomodation, presumably tied to second-order sea-level changes. Callovian highstand accumulation featured a catch-up carbonate system and produced a thin-aggradational ramp configuration, whereas conditions during middle-late Oxfordian allowed a keep-up system and produced outbuilding depositional geometries with steeper slopes.     

晚三叠世龙门山前陆盆地早期(卡尼期)碳酸盐缓坡和海绵礁的淹没过程与动力机制

晚三叠世龙门山前陆盆地分布于扬子克拉通西缘,属于印支期造山楔构造负载驱动的挠曲型前渊凹陷.其中卡尼期马鞍塘组是分布于底部不整合面之上的第一套地层单元,记录了前缘隆起边缘碳酸盐缓坡和海绵礁的构建和淹没过程.据钻孔揭示马鞍塘组的最大厚度超过250m,显示为西北厚东南薄的楔形结构,从北西向南东依次分布了深水盆地、碳酸盐缓坡和海绵礁和浅水滨岸带等沉积物类型.其中碳酸盐缓坡和海绵礁分布于前陆盆地的远端,呈面向西的条带状展布,其走向线与龙门山冲断带的走向大致平行.碳酸盐缓坡和海绵礁的厚度介于30～100m之间,由北西向南东变薄.在垂向上,马鞍塘组由3部分构成,下部为鲕粒滩和生物碎屑滩,中部为海绵礁,上部为黑色页岩,显示为向上变细、变深的沉积序列.在Li et al.(2003)盆地模拟的基础上,本次对卡尼期前陆盆地的沉降速率、沉积速率、海绵礁生长速率、相对海平面上升速率进行了定量计算,其中沉降速率为0.10mm·a-1、沉积速率为0.04mm·a-1、海绵礁生长速率为0.03mm·a-1、相对海平面上升速率介于0.01mm·a-1～0.05mm · a-1之间.研究结果表明:在卡尼期早期,相对海平面处于初始上升阶段,相对海平面上升速率较小,盆地处于欠补偿状态,沉积了碳酸盐缓坡型鲕粒滩和生物碎屑滩;在卡尼期中期,相对海平面上升速率等于海绵礁生长速率,海绵礁持续保持垂直向上的生长状态,形成了高度达100余米的塔礁;在卡尼期晚期,相对海平面上升速率大于海绵礁生长速率,礁顶的水深逐步变大,导致礁体被淹溺致死,从而在卡尼期形成了鲕粒灰岩滩-生物碎屑滩-海绵礁灰岩-页岩的向上变细、变深的沉积序列,显示了前陆盆地早期碳酸盐缓坡和海绵礁生长并被淹没的特有模式.本次研究成果表明龙门山前陆盆地的底部不整合面和碳酸盐缓坡、海绵礁的淹没过程是扬子板块西缘印支期造山楔逆冲构造负载的挠曲变形的产物,显示了在卡尼期松潘-甘孜残留洋盆的迅速闭合和造山楔构造负载向扬子板块的推进过程.     

Sedimentology and lithostratigraphy of Upper Eocene sponge‐rich sediments,southern Western Australia

Late Eocene time in the Bremer and western Eucla Basins of southern Western Australia was a period of terrigenous clastic and abundant, unusual, biosiliceous sponge sedimentation. The Pallinup Formation (revised) consists of five units; 1 and 2 are basal sandstones, 3 and 4 are variably spiculitic mudstones, whilst the uppermost unit is spiculite and spongolite, and formalised as the Fitzgerald Member (new). The Pallinup Formation, plus coeval spiculites in palaeovalleys and carbonates in the western Eucla Basin, accumulated during one large‐scale, transgressive‐regressive relative sea‐level cycle. Drowned, low‐gradient rivers supplied mud but little sand. Instead, sand was locally sourced via transgressive shoreface erosion of deeply weathered regolith. Regression terminated shoreface erosion, eliminated the sand source, and resulted in a river‐supplied, clay‐dominated shallow‐marine depositional system. The unit 2–3 sandstone‐mudstone transition, which would normally be interpreted as transgressive drowning, is in this case the result of regressive cessation of sand supply. The peak relative sea‐level (highstand) horizon thus lies within unit 2 sandstones, a facies that would usually be considered wholly transgressive, and no highstand systems tract was deposited. The maximum flooding and downlap surfaces are the same horizon and cap the transgressive systems tract. They formed coincidentally or subsequent to peak relative sea‐level, but prior to initiation of unit 3 mudstone deposition. Upper unit 2 plus unit 3 represent a condensed section systems tract, and unit 4 plus the Fitzgerald Member comprise a regressive systems tract.     

鄂西—湘西北地区二叠纪栖霞期岩相古地理

对鄂西—湘西北地区多个沉积剖面的地层及沉积相进行了详细分析,结果表明,该区二叠纪栖霞期至茅口初期主要为内克拉通碳酸盐岩缓坡环境,发育内缓坡相、中缓坡相、外缓坡相和盆地相.内缓坡相以厚层至块状生物碎屑石灰岩为主,生物颗粒以绿藻和底栖有孔虫为主,缺乏高能沉积的生物颗粒.中缓坡相以中厚层含生物碎屑颗粒石灰岩以及厚层灰泥石灰岩...     

Paleoenvironments and origin of the sedimentary phosphorites of the Napo Formation (Late Cretaceous, Oriente Basin, Ecuador)

The Napo phosphorites were deposited at the edge of a stable marine shelf during the Upper Cretaceous (Coniacian) oceanic anoxic event (OAE 3) at the transition from bioclastic limestone to organic-rich shale facies. Phosphogenesis was triggered in the shelf margin environment by a number of factors including strong upwelling currents, high biological activity, plankton blooms, and large amounts of organic matter production and subsequent accumulation. Dissolved phosphate levels increased in the sediment from a combination of anoxic conditions and microbial activity. Once dissolved phosphate concentrations were high enough, apatite began to form around nucleic sites including mineral grains, shells, wood fragments, and foraminifera tests forming peloidal fluorine rich carbonate fluoroapatite (francolite). As the peloids formed, sedimentation continued and dissolved phosphate concentrations diminished. A period of minor winnowing ensued, and as dissolved phosphate concentrations remained low, shale layers were deposited separating the various phosphate layers.     

High-frequency cycles in the upper Aptian carbonates of the Organyà basin,NE spain

High-frequency cycles in Upper Aptian carbonates have been studied on the carbonate ramp of Organyà (southeastern Pyrenees). The depositional area comprises a shallow marine to deeper marine transect. A detailed facies model is developed subdividing the transect into an inner ramp area (above fairweather wave base), a mid-ramp area (between fairweather wave base and storm wave base) and an outer ramp area (below storm wave base). Based on microfacies analysis a cyclostratigraphic and sequence stratigraphic interpretation is established. Variations of the sedimentary patterns within different sections of the homoclinal ramp are due mainly to sea level changes. Sea level changes of third and fourth order are reflected by the shifting of the shallow subtidal facies belts up and down the ramp. The study of fifth-order sea level changes is based on statistical methods (quantitative facies analysis and principal component analysis). The ratio of the fourth- and fifth-order cycles is very similar to the well-known ratio of the eccentricity (100 ka) and the precession (18.6/22.5 ka). The absolute age values derived from the cyclostratigraphy fit into the biostratigraphic framework. Thus, a global eustatic control is assumed to be responsible for the cycles of higher frequency. The lower frequency third-order sequences, however, were considerably influenced by local tectonic processes.     

Study on Carbonate Sequence Stratigraphy of Ordovician in Northern Ordos Basin

INTRODUCTIONSeqUenCestratigraphyisderivedfromseismicstratigl'aphyanditSconCeptShaveholargelyappliedtosill~ticsetting(e.g.passivemargin,I'alnP,groWth--faulting,activemarginandnonmarinesettings).InrecentyearS,thestudyofseqUenCestratigraphyhasdlawnagreatattentionfromgeologistSovertheworld.Carbonaterocksareimportantcomponentsofthesedimentaryrecord,beingmajorresouras(e.g.salt,potash,~),containinghydrOCarbon"serves,andyieldingawealthofpaleoclilnaticandpaleo--enviromnentalinfonnation~r,1991…     

Sedimentation during carbonate ramp-to-slope evolution in a tectonically active area: Bowland Basin (Dinantian), northern England

The Bowland Basin (northern England) contains a series of carbonates and terrigenous mudstones deposited during the Ivorian to early Brigantian. Two regional depositional environments are indicated by facies and facies associations. Wackestone/packstone and calcarenite facies indicate deposition in a carbonate ramp environment, while lime mudstone/wackestone, calcarenite and limestone breccia/conglomerate facies, often extensively slumped, represent a carbonate slope environment. Stratigraphic relations suggest that the depositional environment evolved from a ramp into a slope through the Dinantian. Two main sediment sources are indicated by the sequence; an extra-basinal terrigenous mud source and a supply of carbonate from the margins of the basin. Deposition from suspension and from sediment gravity flows, in situ production and remobilization of sediment during sedimentary sliding were important processes operating within the basin. Periods of enhanced tectonic activity in the late Chadian to early Arundian and late Asbian to early Brigantian are indicated by basin-wide horizons of sedimentary slide and mass flow deposits. Both intervals were marked by a decline in carbonate production resulting from inundation and uplift/emergence. The first of these intervals separates deposition on a seafloor with gentle topography (carbonate ramp) from a situation where major lateral thickness and facies variations were present and deposition took place in a carbonate slope environment. The second interval marks the end of major carbonate deposition within the Bowland Basin and the onset of regional terrigenous sedimentation.     

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.     

Facies analysis and palaeoenvironmental interpretation of the Late Oligocene Attard Member (Lower Coralline Limestone Formation), Malta

The Oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale. So far, few studies have considered the impact that these early reefs had on ramp development. Consequently, this study aimed at presenting a high‐resolution analysis of the Attard Member of the Lower Coralline Limestone Formation (Late Oligocene, Malta) in order to decipher the internal and external factors controlling the architecture of a typical Late Oligocene platform. Excellent exposures of the Lower Coralline Limestone Formation occurring along continuous outcrops adjacent to the Victoria Lines Fault reveal in detail the three‐dimensional distribution of the reef‐associated facies. A total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp. The inner ramp was characterized by a very high‐energy, shallow‐water setting, influenced by tide and wave processes. This setting passed downslope into an inner‐ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals. The middle ramp lithofacies were deposited in the oligophotic zone, the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents. Compositional characteristics and facies distributions of the Attard ramp are more similar to the Miocene ramps than to those of the Eocene. An important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone. This expansion may have commenced in the Late Oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies. Stacking‐pattern analysis shows that the depositional units (parasequences) at the study section are arranged into transgressive–regressive facies cycles. This cyclicity is superimposed on the overall regressive phase recorded by the Attard succession. Furthermore, a minor highstand (correlated with the Ru4/Ch1 sequence) and subsequent minor lowstand (Ch2 sequence) have been recognized. The biota assemblages of the Attard Member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions. The apparent low capacity of corals to form wave‐resistant reef structures is considered to have been a significant factor affecting substrate stability at this time. The resulting lack of resistant mid‐ramp reef frameworks left this zone exposed to wave and storm activity, thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths.     

Facies mosaic and diagenetic patterns of the early Devonian (Late Pragian–Early Emsian) microbialite‐dominated carbonate sequences,Qasr Member,Jauf Formation,Saudi Arabia

The Al‐Jawf area of northern Saudi Arabia provides spectacular outcrops of Early Devonian carbonate bioherms in the Wadi Murayr and Dumat Al‐Jandal areas. These carbonate bioherms belong to the Qasr Member of the Late Pragian–Early Emsian Jauf Formation (~405 Ma) and are surrounded by a bioclastic carbonate succession. The Qasr Member is the first major carbonate unit of the Palaeozoic succession in Saudi Arabia that mainly consists of microbialite carbonates and metazoan reefs exhibiting distinct mound features. These bioherm complexes and their associated carbonate facies are pervasively dolomitized. Stratigraphic, petrographic and geochemical analyses were conducted to determine the facies distribution and interpret their depositional and diagenetic processes. A total of 11 facies are identified from a range of depositional environments within a carbonate platform system, ranging from tidal flats, lagoon, shoal, patch reefs to reef front. The main diagenetic processes are carbonate cementation and dolomitization. Dolomitization occurred as both fabric preserved (mostly in grain‐dominated facies) and fabric destructive (mud‐dominated facies). The microbialites and coralline sponges facies show poor reservoir with visual porosity less than 5%, but this succession may have a potential to serve as a good source for the underlying and overlying facies. Ooid and peloidal grainstone facies show fair to good visual porosity that locally exceeds 10% with intergranular porosity as the dominant type. However, in the most studied samples, vuggy and intraparticle porosities are observed as the dominant type. Copyright © 2015 John Wiley & Sons, Ltd.     

The Middle–Late Jurassic forced regression and disconformity in central Portugal: eustatic, tectonic and climatic effects on a carbonate ramp system

Abstract Successions across the Middle–Upper Jurassic disconformity in the Lusitanian Basin (west‐central Portugal) are highly varied, and were probably developed on a large westward‐inclined hangingwall of a half‐graben. The disconformity is preceded by a complex forced regression showing marked variations down the ramp, and provides an example of the effects of rapid, relative sea‐level falls on carbonate ramp systems. In the east, Middle Jurassic inner ramp carbonates (‘Candeeiros’ facies) are capped by a palaeokarstic surface veneered by ferruginous clays or thick calcretes. In the west, mid‐outer ramp marls and limestones (‘Brenha’ facies) are terminated by two contrasting successions: (1) a sharp‐based carbonate sandbody capped by a minor erosion surface, overlain by interbedded marine–lagoonal–deltaic deposits with further minor erosion/exposure surfaces; (2) a brachiopod‐rich limestone with a minor irregular surface, overlain by marls, lignitic marls with marine and reworked non‐marine fossils and charophytic limestones, with further minor irregular surfaces and capped by a higher relief ferruginous erosional surface. The age ranges from Late Bathonian in the east to Late Callovian in the west. This disconformity assemblage is succeeded by widespread lacustrine–lagoonal limestones with microbial laminites and evaporites (‘Cabaços’ facies), attributed to the Middle Oxfordian. Over the whole basin, increasingly marine facies were deposited afterwards. In Middle Jurassic inner‐ramp zones in the east, the overall regression is marked by a major exposure surface overlain by continental sediments. In Middle Jurassic outer‐ramp zones to the west, the regression is represented initially by open‐marine successions followed by either a sharp marine erosion surface overlain by a complex sandbody or minor discontinuities and marginal‐marine deposits, in both cases capped by the major lowstand surface. Reflooding led to a complex pattern of depositional conditions throughout the basin, from freshwater and brackish lagoonal to marginal‐ and shallow‐marine settings. Additional complications were produced by possible tilting of the hangingwall of the half‐graben, the input of siliciclastics from westerly sources and climate change from humid to more seasonally semi‐arid conditions. The Middle–Late Jurassic sea‐level fall in the Lusitanian Basin is also recorded elsewhere within the Iberian and other peri‐Atlantic regions and matches a transgressive to regressive change in eustatic sea‐level curves, indicating that it is related in part to a global event.     

Facies and sequence stratigraphic architecture of the Mural Limestone (Albian), Arizona: Carbonate response to global and local factors and implications for reservoir characterization

This study documents the detailed facies and sequence stratigraphic architecture of a multi-cyclic patch-reef and its associated ramp interior facies that formed during Oceanic Anoxic Event 1b in the Mural Limestone, Arizona, USA. Ramp interior facies are comprised of bedded wackestone/packstone, rudist build-up and coral–algal patch-reef facies located north of Bisbee, Arizona, at the Grassy Hill locality. The larger multi-cyclic patch-reef that developed coevally ca 5 km to the south of Grassy Hill consists of a high-angle windward margin with a narrow ca 70 m long reef frame containing vertically zonated Microsolena, Actinastrea, diverse branching coral and rudist assemblages, and an 870 m long low-angle leeward margin comprised of reef debris rudstone and grainstone shoal facies. Similar reef geomorphology and orientation is documented across the Gulf of Mexico and reflects the shelf-wide north to north-east-trending prevailing wind and current energies. Controls affecting reef formation and growth patterns include changes in accommodation space associated with low-amplitude global sea-level rise and regional thermotectonic subsidence, local accommodation space and nutrient fluctuations associated with the inner shelf depositional setting within a humid and siliciclastic-rich environment. Four aggradational to retrogradational high-frequency sequences are documented in Arizona: High-frequency sequences 1 and 2 represent the first pulse of patch-reef development in an overall second-order marine transgression over the Sonora/Bisbee Shelf. These sequences correlate to δ¹³C signatures associated with Oceanic Anoxic Event 1b across the Gulf of Mexico and suggest that carbonate reefs persisted on the ramp interior during this time. High-frequency sequences 3 and 4 record a second brief transgression and backstepping of reef facies followed by the final regression of shallow shelf carbonates that correlates to more robust patch-reef development in Sonora, Mexico. The patch-reef at Paul Spur is an excellent outcrop analogue for productive patch-reefs in the Maverick Basin (Comanche Shelf) of Texas. Detailed facies mapping of this outcrop analogue shows that the greatest reservoir potential is contained within the backreef grainstone shoals where primary porosity of up to 15% is observed.