Allogenic and autogenic processes combined in the formation of shallow-water carbonate sequences (Middle Berriasian,Swiss and French Jura Mountains)

Sediment production and accumulation on shallow carbonate platforms are controlled by allogenic, externally controlled processes (such as sea level, climate, and/or platform-wide subsidence patterns) as well as by autogenic factors that are inherent to the sedimentary system (such as lateral migration of sediment bodies). The challenge is to determine how and in which proportion these processes interacted to create the observed sedimentary record. Here, a case study of Middle Berriasian, shallow-marine carbonates of the Swiss and French Jura Mountains is presented. Based on vertical facies evolution and bedding surfaces, different orders of depositional sequences (elementary, small-scale, medium-scale) have been identified in the studied sections. The hierarchical stacking pattern of these sequences and the time span represented by the investigated interval imply that eustatic sea-level fluctuations in the Milankovitch frequency band were an important controlling factor. The small-scale and medium-scale sequences relate to the 100 and 400-kyr orbital eccentricity cycles, respectively. The elementary sequences are attributed to the 20-kyr precession cycle. Differential subsidence additionally produced accommodation changes. The present study focuses on one specific small-scale sequence situated at the base of the transgressive systems tract of large-scale sequence Be4, which is identified also in other European basins. This small-scale sequence has been logged in detail at eight different outcrops in the Jura Mountains. Detailed facies analysis reveals that different depositional environments (tidal flats, internal lagoons, open lagoons, carbonate sand shoals) were juxtaposed and evolved through time, often shifting position on the platform. The boundaries of the small-scale (100-kyr) sequence can be followed over the entire study area and thus must have formed through predominantly allogenic processes (eustatic sea-level fall, the effect of which was locally modified by differential subsidence). In two sections, five well-developed elementary sequences constitute the small-scale sequence. In the other sections, the identification of elementary sequences often is difficult because sedimentation was dominated by autogenic processes that overruled the influence of sea-level fluctuations. In low-energy, tidal-flat and internal-lagoonal settings, orbitally induced sea-level changes were recorded more faithfully, while high-energy shoals were mainly submitted to autogenic processes and the allogenic signal is masked. Consequently, the studied Jura platform experienced a combination of auto- and allogenic processes, which created a complex facies mosaic and a complex stacking of depositional sequences. Nevertheless, the 100-kyr orbital signal was strong enough to create correlatable sequence boundaries. Within a 100-kyr sequence, however, the unambiguous definition of sequences related to the 20-kyr orbital cycle is often difficult and the prediction of their lateral or vertical facies evolution impossible.     

History of the Middle Berriasian transgression on the Jura carbonate platform: revealed by high-resolution sequence- and cyclostratigraphy (Switzerland and France)

The Middle Berriasian deposits of the Jura platform in Switzerland and France have already been well studied in terms of high-resolution sequence stratigraphy and different orders of depositional sequences (large-, medium-, and small-scale) have been defined. The hierarchical stacking pattern of the sequences and the time span represented by the investigated interval imply that sea-level fluctuations in the Milankovitch frequency band as well as differential subsidence caused the observed changes of accommodation on the Jura platform. The present study focuses on three small-scale sequences within the transgressive interval of a large-scale sequence. The initial flooding of the platform is marked by a facies change from supra- and intertidal (Goldberg Formation) to shallow-marine subtidal deposits (Pierre Chatel Formation). Detailed logging and facies analysis of 11 sections allow recognizing small environmental changes that define elementary sequences within the well-established small-scale sequences and distinguishing between autocyclic and allocyclic processes in sequence formation. It is concluded that the small-scale sequences correspond to the 100-ka orbital eccentricity cycle, while allocyclic elementary sequences formed in tune with the 20-ka precession cycle. Based on the correlation of elementary and small-scale sequences it can be shown that the Jura platform has been flooded stepwise by repeated transgressive pulses. Differential subsidence and pre-existing platform morphology further controlled sediment accumulation and distribution during the transgression. The combination of high-resolution sequence stratigraphy and cyclostratigraphy then enables the reconstruction of hypothetical palaeogeographic maps in time increments of a few ten thousand years.     

Facies, cycles, and controls on the evolution of a keep-up carbonate platform (Kimmeridgian, Swiss Jura)

During the Late Jurassic, accelerated ocean-floor spreading and associated sea-level rise were responsible for a worldwide transgression, which reached its maximum in the Late Kimmeridgian. In many Western European basins, this major sea-level rise led to the formation of marly and condensed sections. In the Swiss Jura, however, a shallow carbonate platform kept growing and only subtle changes in the stratigraphic record suggest an increasingly open-marine influence. Field observations and thin-section analyses reveal that the central Swiss Jura was at that time occupied by tidal flats and by more or less open marine lagoons where shoals and bioherms developed. The evolution through time of sedimentary facies and bed thicknesses permits the definition of small-, medium-, and large-scale depositional sequences. The diagnostic features of these sequences are independent of scale and seem largely controlled by the Kimmeridgian second-order transgression. A high-resolution sequence-stratigraphic correlation with biostratigraphically well-dated hemipelagic and pelagic sections in the Vocontian Basin in France reveals that: (i) The most important increase in accommodation recorded in the Kimmeridgian of the central Swiss Jura occurs in the Eudoxus ammonite zone (Late Kimmeridgian) and corresponds to the second-order maximum flooding recognized in many sedimentary basins. (ii) The small- and medium-scale sequences have time durations corresponding to the first and second orbital eccentricity cycle (i.e. 100 and 400 ka, respectively), suggesting that sedimentation on the platform and in the basin was at least partly controlled by cyclic environmental changes induced by insolation variations in the Milankovitch frequency band. The comparison of the high-resolution temporal framework defined in the Swiss Jura and Vocontian Basin with the sequence-stratigraphic interpretation realized in other Western European basins shows that the large-scale sequence boundaries defined in the Kimmeridgian of the Swiss Jura appear in comparable biostratigraphic positions in most Western European basins. Discrepancies that occur are probably because of local or regional tectonics.     

Shallowing-upward sequences in Purbeckian peritidal carbonates (lowermost Cretaceous, Swiss and French Jura Mountains)

Purbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow-subtidal, intertidal, supratidal, low-energy, high-energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing-upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record. Six types of small-scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal-marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal-flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area. The Purbeckian shallowing-upward sequences were generated by climatically controlled sea-level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea-level affecting the entire platform. The small-scale sequences are most probably related to the 20 000-year cycle of the precession of the equinoxes. Larger sequences with usually well-developed emersion surfaces are attributed to the 100 000 and 400 000-year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian peritidal carbonate environments.     

Long-distance correlations by sequence stratigraphy and cyclostratigraphy: examples and implications (Oxfordian from the Swiss Jura, Spain, and Normandy)

The Oxfordian sedimentary successions studied in the Swiss Jura, in Normandy, and in the Soria and Cazorla regions of Spain display complex facies evolution and stacking patterns. Based on biostratigraphy and absolute age dating, it is suggested that the shallow-water depositional settings in the Jura, Normandy, and the Soria region as well as the deeper-water environments in the Cazorla region, recorded climatic and sea-level fluctuations in the Milankovitch frequency band. Beds and bedsets corresponding to 20-, 100-, and 400-ka cyclicities can be identified. Facies evolution inside such small-scale sequences and also in the larger sequences of million-year scale is interpreted in terms of sequence stratigraphy. Superposition of high-frequency cyclicity on a longer-term sea-level trend led to multiplication of diagnostic surfaces: sequence-boundary and maximum-flooding zones in the large-scale sequences can thus be defined. These zones are correlated between closely spaced sections, but also from the Swiss Jura to Normandy and to Spain. The narrow time lines given by Milankovitch cyclicity then allow comparison of facies evolution in the different regions on a scale of 100 ka or less. By filtering out local effects of differential subsidence and sediment supply, a long-term sea-level curve valid for the northwestern margin of the Tethys ocean can be reconstructed for the Middle to Late Oxfordian. Differential subsidence is implied from varying thicknesses of the sequences as well as from the distribution of siliciclastics which have been channelized through depressions. Tilted blocks, reduced sedimentation, or increased input of siliciclastics appearing at the same time in all study areas point to a widespread regional tectonic event. Distribution through the sequences of climate-dependent facies components such as corals, ooids, palynomorphs, and siliciclastics indicates that climate changes were dependent on atmospheric circulation patterns and thus on paleolatitude. Rainy periods and related increase of siliciclastics in the Swiss Jura were more abundant during low sea-level stands, whereas in the Soria region they coincided with sea-level highs. Through the combination of high-resolution sequence stratigraphy and cyclostratigraphy, and supported by biostratigraphy and absolute dating, it becomes possible to analyze paleoenvironmental changes in a very narrow time framework.     

Platform-to-basin correlation by high-resolution sequence stratigraphy and cyclostratigraphy (Berriasian, Switzerland and France)

Based on detailed analyses of facies evolution and stacking pattern of Berriasian carbonate-dominated sections in the Swiss Jura Mountains, the Swiss Ultrahelvetic and the French Vocontian Trough, a high-resolution platform-to-basin correlation is proposed. Biostratigraphical tie points are furnished by ammonites, dinoflagellates, calpionellids, and ostracod-charophyte assemblages. The hierarchical stacking of small-scale depositional sequences reflects Milankovitch cyclicity: sequences corresponding to the 20-, 100-, and, locally, 400-ka orbital cycles can be identified. Elementary (20 ka) sequences on the platform generally consist of one bed of shallow subtidal to intertidal, high- or low-energy carbonate facies, whereas on the slope and in the basin they are commonly developed as limestone-marl couplets. These elementary sequences group into small-scale composite sequences reflecting the first orbital eccentricity cycle (100 ka), which in turn build up large-scale (3rd-order) composite sequences. One 3rd-order sequence has been analysed in detail: according to the cyclostratigraphic interpretation, it took ?2 Myr to form, which is in accordance with the duration of the corresponding ammonite subzones. Sequence-stratigraphic and cyclostratigraphic platform-to-basin correlation shows that for about 900 ka the platform was exposed or only partly flooded, whereas on the slope and in the basin, lowstand deposits with channel fills and slumps accumulated. With rising sea level, accommodation space gradually increased on the platform and a thickening-upward sequential pattern with transgressive facies developed, while in the basin the facies still had lowstand characteristics with thick and nodular limestone beds. This situation lasted about 700 ka. The following 300 ka were characterized by sediment starvation and increased bioturbation on the platform, and by more marly, transgressive sediments on the slope and in the basin. The maximum-flooding phase is more or less isochronous on the platform and in the basin, although the surface with the best-developed maximum-flooding features may be displaced by one or two small-scale composite sequences because of superimposed high-frequency sea-level fluctuations, and/or local variations in substrate morphology and sediment distribution. Third-order highstand conditions prevailed for only about 100 ka. The combination of cyclostratigraphy and high-resolution sequence stratigraphy, constrained by good biostratigraphy, is thus a powerful tool for detailed stratigraphical correlation over long distances and from one sedimentary environment to another.     

Astronomical time scale for the Middle Oxfordian to Late Kimmeridgian in the Swiss and French Jura Mountains

Detailed investigation of facies and sedimentary structures reveals that, during the Middle Oxfordian to Late Kimmeridgian, the shallow carbonate platform of the Swiss and French Jura Mountains recorded high-frequency sea-level fluctuations quite faithfully. The cyclostratigraphic analysis within the established biostratigraphic and sequence-chronostratigraphic framework implies that the resulting hierarchically stacked depositional sequences formed in tune with the orbital cycles of precession (20 kyr) and eccentricity (100 and 400 kyr). The astronomical time scale presented here is based on the correlation of 19 platform sections and 4 hemipelagic sections from south-eastern France where good biostratigraphic control is available. The cyclostratigraphic interpretation suggests that the interval between sequence boundaries Ox4 and Kim1 (early Middle Oxfordian to earliest Kimmeridgian) lasted 3.2 myr and that the Kimmeridgian sensu gallico has a duration of 3.2 to 3.3 myr. The astronomical time scale proposed here is compared to time scales established by other authors in other regions and the discrepancies are discussed. Despite these discrepancies, there is a potential to estimate the durations of ammonite zones and depositional sequences more precisely and to better evaluate the rates of sedimentary, ecological and diagenetic processes. Editorial handling: Hanspeter Funk, Helmut Weissert, Stefan Bucher     

Palaeoenvironmental and palaeoecological change on the northern Tethyan carbonate platform during the Late Barremian to earliest Aptian

A major shift from Urgonian oligotrophic carbonate accumulation to orbitolinid‐rich mixed siliciclastic–carbonate deposition is observed near the Barremian–Aptian boundary in many sections both within and outside the shallow‐marine Tethyan Realm. This important facies change in the Swiss Helvetic Alps is documented here and interpreted in the context of general palaeoenvironmental change. To achieve this, a detailed micropalaeontological, sedimentological, mineralogical and geochemical study has been carried out on six sections across the upper part of the lower Schrattenkalk Member (Late Barremian), the Rawil Member (formerly ‘Lower Orbitolina Beds’, earliest Aptian) and the lowermost part of the upper Schrattenkalk Member (Early Aptian). The sediments of the Rawil Member exhibit inner‐platform facies with rudists, miliolids, orbitolinids and dasycladals to outer‐platform facies characterized by small benthic foraminifera, orbitolinids, crinoids and bryozoans. Stratigraphic trends in microfacies environments and the composition of microfossil assemblages, indicate that the Rawil Member includes a transgressive systems tract and the base of a highstand systems tract which are composed of an increasing number of parasequences in distal directions (five to nine in the sections studied here). The sea‐level rise discerned in the Rawil Member is coeval with increased detrital input and phosphorus burial, with maximum values up to 80 times and 21 times the background values in the subjacent part of the lower Schrattenkalk Member, respectively. Furthermore, the Rawil Member records the appearance of kaolinite, indicating a change towards tropical and more humid climate conditions. This change may have led to an increase in continental weathering rates and an associated increase in detrital and nutrient fluxes towards the ocean. The phase of climate change observed near the Barremian–Aptian boundary may have been triggered by a phase of intensified volcanic activity linked with the onset of the Ontong Java large igneous province and the Rawil Member may be the expression of a precursor episode to Oceanic Anoxic Event 1a in the shallow‐marine environment.     

Depositional architecture of a rimmed carbonate platform (Albian, Gorbea, western Pyrenees)

A Lower Cretaceous carbonate platform depositional system with a rimmed margin and an adjacent oversteepened slope was analysed in order to determine its depositional architecture and major depositional controls. The platform is made up of coral, rudist, orbitolinid and algal limestones and, in a 12-km dip transect, there is a gradation from lagoon to platform margin, slope and basin environments, each characterized by distinctive sedimentological features and facies associations. The rimmed platform is an aggradational system developed during approximately 4·2 million years of fluctuating relative sea-level rise, and it is bounded by unconformities at its base and top. Internal cyclicity in the construction of the system is evident, mainly in platform interior and slope settings. The seven recognized sequences average 0·6 million years in duration and are related to minor relative sea-level changes. Carbonate deposition occurred in shallow- and deep-water settings during periods of high relative sea level. Reduced rates of sea-level rise led to the development of shallowing upward sequences and, eventually, to the exposure of the shallowest parts of the platform during relative sea-level falls. During low relative sea level, erosion surfaces developed on the slope, and gravitational resedimentation occurred at the toe of slope. Basinwards, resedimented units pinch out over distances of a few hundred metres. Active faults controlled sedimentation at the platform margin, promoting the development of steep slopes (up to 35°) and preventing progradation of the shallow-water platform, despite high sediment production rates. The development of sequences is interpreted to be related to tectonic activity.     

Orbital forcing recorded in subtidal cycles from a Lower Miocene siliciclastic–carbonate ramp system (Central Italy)

Metre-scale siliciclastic–carbonate cycles are the basic depositional motif of the lower Miocene Guadagnolo Formation outcropping in the central Apennines. The mechanisms which formed the mixed-lithology cycles are still a matter of debate. The mixed siliciclastic–carbonate system discussed in this paper provides a new case study to illustrate the role of orbital forcing in controlling the facies evolution and cyclic stacking of small-scale sequences deposited on the outer sector of a ramp. Two sections are discussed that display mixed siliciclastics and carbonates arranged in upward-shallowing cycles. Each cycle shows an upward decrease in the terrigenous input and a parallel increase in benthic fauna. Time-series analyses indicate the cyclic carbonate-terrigenous pattern to be largely controlled by orbital forcing in the Milankovitch frequency band. Coupling of climate change and sea-level fluctuations in tune with orbital cycles are proposed as driving mechanisms.     

Comparisons between the Urgonian platform carbonates from eastern Serbia (Carpatho-Balkanides) and northeast Iran (Kopet-Dagh Basin): Depositional facies,microfacies, biostratigraphy,palaeoenvironments and palaeoecology

In the Getic of the Carpatho-Balcanides (eastern Serbia) and the Tirgan Formation of the Kopet-Dagh Basin (northeast Iran), platform carbonates were deposited during the Barremian/Early Aptian in environments in the domain of the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, Urgonian carbonate platform deposits are discussed in detail with regard to depositional facies, microfacies, biostratigraphy, palaeoenvironments and palaeoecology. Detailed sedimentological and palaeontological investigations have been carried out on five sections in eastern Serbia and three sections in northeast Iran supported by an analysis of 392 thin-sections. Petrographic analysis of thin-sections led to the recognition of eight microfacies types grouped into four facies zones. A supratidal–intertidal (restricted)–intertidal (open-lagoon)–platform-margin sand-shoal transition was recorded in both areas. Supratidal facies are characterized by bioclastic mudstones and fenestral and peloidal wackestones and packstones; intertidal (restricted) facies are represented by bioclastic wackestones, whereas intertidal (open-lagoon) facies are indicated by bioclastic packstones/grainstones and oncoid grainstones. High-energy sand-shoal facies are dominated by ooid grainstones/rudstones followed by orbitolinid packstones. Benthic foraminifera are especially abundant and along with calcareous algae are the most important fossils used for age determination of shallow-marine carbonate deposits. Thirty-two benthic foraminiferal genera were identified from eastern Serbia with an additional 38 genera from northeast Iran dominated by agglutinated forms. Identified calcareous algae provide significant data for depositional environments and palaeoecology. The microfossil associations in the two regions are very similar and share a number of common characteristics, but also some differences and show a strong affinity to those of the northern margins of Tethys. In both study areas shallow-marine environments of the Barremian/Early Aptian were replaced by deep-marine conditions during the Late Cretaceous.     

蒸发背景沉积序列精细刻画及沉积学解译:以鄂尔多斯盆地中部中奥陶统马五6亚段为例*

云膏共生组合是受化学沉积分异作用控制而形成的岩石组合,对其沉积序列精细刻画,有利于揭示诸多蒸发环境下的沉积学信息。依据鄂尔多斯盆地中部奥陶系马家沟组五段6(简称“马五₆亚段”)亚段钻井、岩心资料,开展了岩石宏观和微观观察、典型沉积序列刻画等工作。研究结果表明: (1)鄂尔多斯盆地中部马五₆亚段普遍发育碳酸盐岩与蒸发岩共生组合,主要由10种岩石类型构成;(2)主要发育5种沉积序列,下云上膏的岩性组合和序列顶部多发育暴露面等特征表明单个序列具有向上变浅、变咸的演化特征,是蒸发台地叠合海水渐次补给作用的结果;(3)在局限—蒸发台地环境下,主要发育潟湖/滩间海、颗粒滩、灰泥丘、台坪4种沉积亚相及11种沉积微相,与经典的蒸发潮坪序列在岩性组合与沉积构造方面存在明显的差异;(4)马五₆亚段沉积期发育2次海平面升降,沉积环境可分别对应于云(膏)质潟湖—灰泥丘/颗粒滩—膏质潟湖—蒸干膏质潟湖和灰泥丘/颗粒滩—膏质潟湖—蒸干膏质潟湖—台坪。该研究结果可为鄂尔多斯盆地马五₆亚段沉积环境与海平面变化研究提供系统的岩石学证据。     

塔里木盆地西部寒武-奥陶系储层分布特征

塔里木盆地西部在寒武-奥陶纪大体经历了两次一定规模的海侵-海退沉积,并相应形成了由4个三级旋回层序和2个二级旋回层序构成的层序地层格架.它不仅控制着不同沉积相的平面展布特征,而且控制着生储盖组合的时空分布规律.下寒武统肖尔布拉克组与吾松格尔组的高水位体系域局限台地相沉积,早期普遍经历了白云岩化作用,及后期大气淡水淋溶的综合改造,形成了平面上展布非常稳定的优质针孔状碳酸盐岩储集层段.奥陶系储层主要形成于高水位体系域开阔台地相、台地边缘相和藻滩相中,由于受海水成岩和埋藏成岩环境影响,胶结作用发育,致使储层整体面貌呈Ⅳ类差储集层.     

鄂尔多斯盆地奥陶系马家沟组四段沉积特征及其控储效应*

鄂尔多斯盆地奥陶系马家沟组四段(简称“马四段”)是下古生界天然气风险勘探的重点层位。文中基于岩心、薄片观察,结合地层对比、沉积结构描述等,并运用岩石结构组分测井解释和井震结合古地貌恢复等手段,对鄂尔多斯盆地马四段沉积时期古地貌、沉积特征和储集层展布进行了详细分析。结果表明: (1)受控于沉积期隆坳相间的古地貌格局,马四段发育大量台地内丘滩微相,其易于发生准同生溶蚀和白云石化作用,是储集层发育的有利相带;(2)马四段储集层以(藻)砂屑白云岩和粉—细晶白云岩为主,储集空间主要为粒间孔、晶间孔、溶蚀孔等;(3)纵向上,储集层主要分布于马四₂亚段和马四₁亚段;平面上,马四段储集层主要分布于台地内低凸起带和台缘带;(4)储集层的发育受控于3个因素,其中古隆起部位发育的有效丘滩体是储集层形成的物质基础,高频层序控制的准同生溶蚀作用是储集层形成的关键,白云石化是储集层后期保存的重要因素。马四段沉积时期碳酸盐岩台地内隆坳相间的古地貌控制了丘滩相带和白云岩储集层的展布,这一新认识为鄂尔多斯盆地下古生界天然气勘探提供了有力支撑,也为碳酸盐岩台地沉积储集层的研究提供了新思路。     

华北地台晚寒武世层序地层及其与北美地台海平面变化的对比

华北地台在晚寒武世为一个较为典型的缓坡型台地。在其北部地层中,以发育风暴砾屑灰岩及生物丘灰岩闻名,包括崮山组、长山组和凤山组。在该套地层中,深缓坡相泥岩和泥灰岩与浅缓坡相颗粒灰岩和泥粒灰岩一起构成若干潮下型碳酸盐米级旋回,这些米级旋回本身即为有序的异成因岩相序列而成为基本工作单元。在长周期三级层序中。米级旋回形成有序垂直叠加形式,反映了三级层序本身是一个与三级相对海平面变化相关的、环境加深和变浅过程中所形成的沉积相序列根据这些特征,在华北地台北部晚寒武世地层中可识别出4个三级层序。从北向南,相变为一套白云岩,其中三级层序的高水位体系域则发育较多的潮坪相白云岩以及由其构成的环潮坪型米级旋回。这些层序的典型特征是：层序界面为淹没不整合面,层序的相序序列以“深缓坡相—浅缓坡相”为特征,从而构成特殊的“CS(?) HST”序列;而在华北地台南部,层序界面多为暴露间断面,层序的相序组构多以“缓坡相—潮坪相”为特征。从北向南的变化,形成了一个富有规律性的层序地层格架。从旋回到层序、从岩相序列到沉积相序列的层序地层研究表明：作为基本工作单元,米级旋回实际上是由“间断—加积作用”旋回机制所形成的岩相序列,由米级旋回的有序叠加形式所构成的三级层序则是一个沉积相序列。由各剖面点的古水深变化曲线,可以定性地综合出华北地台晚寒武世的海平面变化曲线。与北美晚寒武世海平面变化曲线相对比,既存在相似性更存在差异性,说明了长周期海平面变化既受全球性海平面变化的控制,同时也受区域因素的影响。     

层序地层中的混合沉积作用及其控制因素

简要介绍了硅质碎屑与碳酸盐混合沉积有关概念及混合沉积作用类型,分析了层序体系域中的混合沉积作用及其控制因素。总结认为,硅质碎屑与碳酸盐的混合沉积作用不仅发生在低水位体系域（LST）,而且其它各体系域中也都有不同程度的混合沉积现象,但混合沉积作用方式有差别：LST主要为源区混合,陆棚边缘体系域（SMT）和高水位体系域（HST）晚期以间断混合及相混合占优,海浸体系域（TST）早期以间断混合为主。综合研究显示,准层序一级的海平面变化型式差异可能是导致层序体系域混合沉积作用的主要因素,其中,LST、SMT、晚期HST的混合沉积作用分别与准层序一级海平面变化的幅度、速率、位置和持续时间有关,TST则受控于物源供给;先期地形、气候变化（含碳酸盐生产速率影响）或多或少对体系域的混合沉积作用有影响。此外,某些情况下层序体系域的混合沉积作用可能会受到局部构造活动影响,而层序组或超层序内层序体系域的混合沉积作用强弱及其旋回性变化可以提供脉动性区域构造活动信息。     

川中—川南过渡带嘉陵江组二段沉积相及其展布特征

川中—川南过渡带下三叠统嘉陵江组划分为局限台地相、蒸发台地相、开阔台地相3个相,8个亚相(开阔潮下、台内滩、台内雏滩、局限潟湖、台内洼地、局限潮坪、蒸发潟湖、蒸发潮坪)和18个微相。区域上嘉陵江组二段纵、横向沉积相对比结果表明:嘉陵江组二段沉积相横向上较均一、相变化较小;沉积相垂向上变化在较大范围内具有同步性。嘉二段共分5个作图单元,采取单因素法和优势相法相结合,并融合了层序地层学研究成果,较精确地展示了嘉陵江组二段不同层段的沉积相展布和演化。相对海平面升降变化和碳酸盐台地内隆坳相间古微地貌共同控制着沉积相带展布和演化。嘉陵江组二段以局限台地相和蒸发台地相为主,储集体有利发育相带是台内雏滩亚相和局限潮坪亚相,台内雏滩发育于潮坪与潟湖的过渡区附近相对高能带,局限潮坪则分布广泛。     

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin,China

Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.     

巫溪田坝长兴组沉积相及层序地层研究

摘要：巫溪田坝剖面长兴组发育齐全,出露连续完整。根据露头观察和室内镜下岩石薄片鉴定分析,结合岩石学、古生物等沉积相标志的研究,认为巫溪田坝长兴组属碳酸盐台地和斜坡沉积体系,可划分为开阔台地、台地边缘、斜坡相及若干亚相、微相。巫溪田坝长兴组可划分为两个三级层序,每个层序由海侵体系域和高位体系域构成,层序顶底界皆为Ⅱ型层序界面。     

Depositional systems and sequence stratigraphy analysis of the Upper Callovian to Tithonian sediments in the Central and Western Kopet‐Dagh Basin,Northeast Iran

Upper Callovian to Tithonian (late Jurassic) sediments represent an important hydrocarbon reservoir in the Kopet‐Dagh Basin, NE Iran. These deposits consist mainly of limestone, dolostone, and calcareous mudstone with subordinate siliciclastic interbeds. Detailed field surveys, lithofacies and facies analyses at three outcrop sections were used to investigate the depositional environments and sequence stratigraphy of the Middle to Upper Jurassic interval in the central and western areas of the basin. Vertical and lateral facies changes, sedimentary fabrics and structures, and geometry of carbonate bodies resulted in recognition of various carbonate facies related to tidal flats, back‐barrier lagoon, shelf‐margin/shelf‐margin reef, slope and deep‐marine facies belts. These facies were accompanied by interbedded beach and deep marine siliciclastic petrofacies. Field surveys, facies analysis, parasequences stacking patterns, discontinuity surfaces, and geometries coupled with relative depth variation, led to the recognition of six third‐order depositional sequences. The depositional history of the study areas can be divided into two main phases. These indicate platform evolution from a rimmed‐shelf to a carbonate ramp during the late Callovian–Oxfordian and Kimmeridgian–Tithonian intervals, respectively. Significant lateral and vertical facies and thickness changes, and results obtained from regional correlation of the depositional sequences, can be attributed to the combined effect of antecedent topography and differential subsidence related to local tectonics. Moreover, sea‐level changes must be regarded as a major factor during the late Callovian–Tithonian interval. Copyright © 2015 John Wiley & Sons, Ltd.