Cycle Stacking Patterns Research Articles

Facies and geochemical characteristics of the Middle-Lower Ordovician Yingshan Formation in the Tarim Basin, NW China: Implications for the high-frequency sequence stratigraphy in shallow-water carbonate platform

Shallow-water carbonates from the Yingshan Formation are important target of hydrocarbon exploration in Tarim Basin. Detailed descriptions of outcrop, core, and thin sections, isotopic composition analysis, and trace element analysis were conducted. Seven lithofacies which consists of two facies belt groups were identified and used to interpret deposition environments of Yingshan Formation: (1) peritidal carbonate, with relatively lower δ13C values from −4.2% to −1.9%, mainly represented by intertidal to supratidal facies and restricted subtidal facies and (2) open-marine subtidal carbonate, with higher δ13C values (−1.5% to −0.3%), mainly consists of shoal facies and interbank sea facies. On the basis of the lithology sets, four types of meter-scale cycle model (types A–D) were grouped into peritidal sequences and subtidal sequences. On the basis of vertical lithofacies, cycle stacking patterns, and accommodation variations in Fischer plots, two third-order depositional sequences (SQ1–SQ2) were recognized. The sequence boundary between SQ1 and SQ2 is not only a surface of a positive shift of δ13C values, but also the transitional zone of cycle stacking patterns. The sequences can be further divided into four fourth-order sequence sets: Sq1–Sq4. The lower sequences (Sq1–Sq2) are dominated by peritidal facies and characterized by a progressive decreasing in accommodation space probably indicates a longer-term fall in sea level. The upper sequences (Sq3–Sq4) are mainly dominated by subtidal facies and characterized by the accommodation space with a progressive increasement, likely indicates a rising sea level with a longer term. The facies-controlled reservoirs are mainly present in the Sq3, related to the sequence boundary between SQ1 and SQ2.

Facies, cycle stratigraphy, and heterogeneity of the Clear Fork carbonates in the Eastern Shelf of the Permian Basin, Texas

The Clear Fork carbonate reservoirs at Iatan East Howard field are highly heterogeneous. Facies analysis and cycle stratigraphy are employed to characterize the Clear Fork Formation and reservoir heterogeneity at the field. One-dimensional (1-D), vertical stratigraphic analyses started from meter-scale cycles to cycle sets, system tracts, and then depositional sequences. A reverse process was followed to construct a 2-D framework from a 1-D stratigraphy. In the downdip areas, the Clear Fork Formation is dominated by subtidal facies; toward the updip, peritidal facies become more abundant and depositional facies may alternate between shallow subtidal and tidal-flat facies. Approximately 56 well-developed, meter-scale, upward-shallowing cycles were defined in the Clear Fork cores facilitated by wireline logs. Most of them have bioclast wackestone and packstone in the lower part and are capped by fenestral peloid packstone, silty to sandy dolostone, and/or dark mudrocks. Six cycle sets (high-frequency sequences) were identified, and two depositional sequences were recognized. Sequences are correlatable, but it is challenging to correlate meter-scale cycles. The Clear Fork cycles display relatively consistent thickness and facies stacking pattern plus evidence of subaerial exposure and erosion of subtidal facies, suggesting an allocyclic control and having been the result of eustatic sea-level fluctuations. Reservoir pores (porosity >4%) commonly occur in the transgressive system tracts and lower highstand system tracts (HSTs) within individual high-frequency sequences; the upper HSTs commonly consist of amalgamated peritidal deposits and are generally tight because pores (mostly fenestral and intergranular pores) have been occluded by anhydrite and carbonate cements. These new findings contrast the previous studies showing that porosity is typically higher in the cycle-set-top or cycle-top facies (mainly tidal-flat deposits) in the formation. Reservoirs are areally discontinuous and vertically compartmentalized by multiple nonporous zones. Reservoir characterization is critical to improving recovery efficiency. This case study provides an insight into complex facies and cycle stacking patterns as well as stratigraphic architecture on a carbonate platform during a waning icehouse phase and can help to better understand the controls on the distribution and quality of such carbonate reservoirs.

Tectono-depositional pattern and evolution of the middle Yangtze Platform (South China) during the late Ediacaran

The depositional environments and evolution of the Yangtze Block during the Ediacaran–Cambrian transition remain uncertain, preventing a thorough understanding of the coeval ocean stratification, redox heterogeneity and organism distribution. In this study, detailed facies and sequence stratigraphic analyses were carried out on the Dengying Formation and its correlatives to better constrain the tectono-depositional pattern and evolution of the middle Yangtze area in the late Ediacaran. Twenty-five lithofacies were identified in nineteen sections and grouped into six depositional facies: intrashelf basin, intrashelf shoal, restricted platform interior, platform margin, upper-middle slope, and lower siliceous slope-basin. The spatiotemporal distributions of these depositional facies reveal an isolated, rimmed middle Yangtze carbonate platform embayed with a westward-opening intrashelf basin. This intrashelf basin gradually became wider and deeper westwards, linking to a NNE-trending basinal seaway that separated the middle Yangtze Platform from its upper Yangtze counterpart. The enclosure of the marginal shoal-reef complex with an intrashelf shoal chain formed a large-scale restricted lagoon and associated tidal flat in the platform interior. Based on key sequence boundaries and cycle stacking patterns, two and a half transgressive-regressive sequences were distinguished in the Dengying Formation and its correlatives in the twelve representative sections. Furthermore, these sequences can be correlated well with those identified in other upper Ediacaran successions elsewhere in upper Yangtze, Namibia, India and Oman, indicating a eustatic origin for these sequences. However, the complex platform-basin configuration, with significant facies and thickness variations, reflects a transtensional tectonic regime in the middle Yangtze area during the late Ediacaran. The abrupt changes in lithofacies and thickness across the southern principal platform margin and those from sandy shoals/banks to storm-influenced shelf along the northern portion of the intrashelf basin in the Three Gorges (western Hubei) suggest synthetic extensional faulting across the margins during deposition. The two synthetic extensional fault zones might have been induced by NNE-trending transtensional faulting along the eastern margin of the upper Yangtze Platform. This structural pattern fundamentally controlled the depositional differentiation of the middle Yangtze Platform during the late Ediacaran and also laid the foundation for the subsequent facies distribution and depositional evolution during the early Cambrian Terreneuvian Epoch.

Depositional facies and stratal cyclicity of carbonate successions in the Yingshan and Yijianfang Group (Lower-Middle Ordovician) in Yuejin-Tuoputai Region, Tarim Basin, NW China

The Lower-Middle Ordovician Yingshan and Yijianfang Group in the Tarim Basin are overwhelmingly composed of cyclic carbonates. Based on microscopic observation and facies analysis from two borehole sections in Yuejin-Tuoputai area, two main types of facies are recognized: semi-restricted subtidal, open-marine subtidal, and these are further subdivided into six and nine lithofacies in the Yuejin and Tuoputai area, respectively. In general, these facies are vertically arranged into shallowing-upward, meter-scale cycles. These cycles are commonly composed of a thinner basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: semi-restricted subtidal and open subtidal, are further identified. The semi-restricted subtidal cycles, predominating over the middle-upper Yingshan Group and the lower Yijianfang Group, commence with algal bindstone in the bottom and are capped by pelletoid grainstone and bindstone with peloidal grains. In contrast, the open subtidal cycles, dominating the upper Yijianfang Group, are dominated by clastizoic grainstone or clastizoic Bindstone. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, nine larger scale third-order depositional sequences (Sq3-Sq11) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq3–Sq7, is at the rising–descending cycle in Fisher Curve plots. And this indicates the accommodation space keeps steady. The upper sequence set, including Sq8–Sq11, some features can be found that these Fisher Curves of sequences flunctuate strongly in the middle-lower Yijianafang Group while keep steady in the upper part. This indicates a phenomenon, where the accommodation space varies from violent to gentle.

Integration of static and dynamic data and high-resolution sequence stratigraphy to define reservoir architecture and flow units within a ‘super giant’ gas condensate and oil field, Kazakhstan

Abstract The Upper Serpukhovian (Mississippian) interval of the Karachaganak reservoir comprises stratified platform interior carbonates, which are characterised by relatively high porosity and matrix permeability (maximum 23.8% and 2198 mD, respectively), and comprise high frequency (fourth-order) glacio-eustatic cycles. Based on static (geological) and dynamic (well test and production) data, this cyclicity exerts a strong control on gas migration pathways, and reservoir quality is predictable within a third- and fourth-order sequence stratigraphic framework that defines two discrete flow units. The contribution to flow made by fourth-order cycles within flow units is dependent on their position within third-order transgressive or highstand systems tracts. An upper flow unit comprises thick (up to 40 m) fourth-order cycles within a third-order transgressive systems tract. These fourth-order cycles contain large scale, clinoformal bedforms, can be correlated for up to 6 km, and are the main pathways of injected gas through the Upper Serpukhovian reservoir. A lower flow unit comprises fourth-order cycles (within a third-order highstand systems tract) that are thinner, and less laterally continuous, than those in the upper flow unit, and contribute far less to horizontal gas flow. Pressure data indicate that vertical permeability baffles are developed at the third-order sequence boundary that separates the two flow units (linked to diagenetic alteration during prolonged subaerial exposure), and also within the low porosity, mud-rich, transgressive bases of thick fourth-order cycles of the upper flow unit, that formed during periods of increased accommodation space across the platform top during third-order transgression. The scale at which this data set is observed is critical to understanding the dynamic behaviour of the reservoir. Relatively small differences in the composition of microfacies of cycles within both flow units do not explain the variations in dynamic data. This variation is better explained within the broader context of the sequence stratigraphic framework as defined from analysis of cycle stacking patterns. Thicker high frequency cycles deposited in increased accommodation space on the platform top during third-order transgressive intervals are key to developing thick intervals of net pay.

Depositional facies and cyclic patterns in a subtidal-dominated ramp during the Early-Middle Ordovician in the western Tarim Basin (NW China)

Due to a long-term transgression since the Early Cambrian, an extensive shallow-water carbonate platform was developed in the entire Tarim Basin (NW China). During the deposition of the Yingshan Formation (Early-Middle Ordovician), a carbonate ramp system was formed in the intrashelf basin in the Bachu-Keping area of the western basin. Four well-exposed outcrop sections were selected to investigate their depositional facies, cycles, and sequences, as well as the depositional evolution. Detailed facies analyses permit the recognition of three depositional facies associations, including peritidal, semi-restricted subtidal, and open-marine subtidal facies, and eleven types of lithofacies. These are vertically arranged into meter-scale, shallowing-upward peritidal, semi-restricted subtidal, and open-marine subtidal cycles, in the span of Milankovitch frequency bands, suggesting a dominant control of Earth’s orbital forcing on the cyclic sedimentation on the platform. On the basis of vertical facies (or lithofacies) and cycle stacking patterns, as well as accommodation changes illustrated graphically by Fischer plots at all studied sections, six third-order depositional sequences are recognized and consist of lower transgressive and upper regressive parts. In shallow depositional settings, the transgressive packages are dominated by thicker-than-average, shallow subtidal cycles, whereas the regressive parts are mainly represented by thinner-than-average, relatively shallow subtidal to peritidal cycles. In relatively deep environments, however, the transgressive and regressive successions display the opposite trends of cycle stacking patterns, i.e., thinner-than-average subtidal cycles of transgressive packages. Sequence boundaries are mainly characterized by laterally traceable, transitional zones without apparent subaerial exposure features. Good correlation of the long-term changes in accommodation space inferred from vertical facies and cycle stacking patterns with sea-level fluctuations elsewhere around the world suggests an overriding eustatic control on cycle origination, platform building-up and evolution during the Early-Middle Ordovician, although with localized influences of syndepositional faulting and depositional settings.

Late Pennsylvanian glaciation: Evidence of icehouse conditions from Canyon and cisco units, Midland Basin, Texas

Abstract The late Paleozoic has long been recognized a time of significant continental glaciation associated with high-frequency, high-amplitude glacioeustasy. Although, precise assessment of glacioeustatic amplitudes are difficult to extract from the stratigraphic record, best approximations may be possible by analyzing spatial and temporal distributions of depositional facies utilizing several wells across the study area. We used detailed sedimentologic description and depositional facies analysis from 12 wells and applied rigorous cycle stacking pattern analysis, exposure surfaces recognition and wireline logs to correlate timelines across the wells. On the basis of the high-resolution sequence stratigraphic framework built for the Scurry Area Canyon Reef Operators (SACROC) field, a highly-resolved glacioeustatic curve was generated for latest Desmoinesian to the latest Virgilian. Our eustatic curve suggests that the Missourian and Virgilian stages of the late Pennsylvanian are characterized by high frequency, high-amplitude eustatic fluctuations. The Missourian stage was a period dominated grainstone-capped cycles that are isopachous and conformable. These cycles reflect a eustatic amplitude of at least 37 m with an average duration of 100 ka suggesting the dominance of short-term eccentricity signal. The Virgilian stage is far more complicated and is characterized by highly shingled cycles compared to the Missourian stage. Deeper water phylloid boundstone and calichified shoal complex grainstones are juxtaposed next to each other which suggests a stronger eustatic amplitude (>40 m) compared to the Missourian stage. Although the eustatic amplitude of the Virgilian cycles is similar to those of the Missourian cycles, they have a longer frequency (343 ka) representative of long-term eccentricity. We conclude that the late Pennsylvanian stratigraphy of the SACROC field in the Midland Basin records extensive continental glaciation during the Missourian stage and increased and reached an acme during the Virgilian stage.

Depositional environments and cyclicity of the Early Ordovician carbonate ramp in the western Tarim Basin (NW China)

During the Early Ordovician, the Tarim Basin (NW China) was mainly occupied by an extensive shallow-water carbonate platform, on which a carbonate ramp system was developed in the Bachu-Keping area of the western part of the basin. Three well-exposed typical outcrop sections of the Lower Ordovician Penglaiba Formation were investigated in order to identify the depositional facies and to clarify origins of meter-scale cycles and depositional sequences, thereby the platform evolution. Thirteen lithofacies are identified and further grouped into three depositional facies (associations): peritidal, restricted and open-marine subtidal facies. These lithofacies are vertically stacked into meter-scale, shallowing-upward peritidal and subtidal cycles. The peritidal cycles are mainly distributed in the lower and uppermost parts of the Penglaiba Formation deposited in the inner-middle ramp, and commonly start with shallow subtidal to intertidal facies followed by inter- to supratidal facies. In contrast, the subtidal cycles occur throughout the formation mostly in the middle-outer ramp and are dominated by shallow to relatively deep (i.e., intermediate) subtidal facies. The dominance of asymmetrical and incomplete cycles suggests a dominant control of Earth’s orbital forcing on the cyclic deposition on the platform. On the basis of vertical facies and cycle stacking patterns, and accommodation changes illustrated by the Fischer plots from all studied sections, five third-order depositional sequences are recognized in the Penglaiba Formation. Individual sequences comprise a lower transgressive part and an upper regressive one. In shallow-water depositional environments, the transgressive packages are dominated by thicker-than-average subtidal cycles, indicating an increase in accommodation space, whereas regressive parts are mainly represented by thinner-than-average peritidal and subtidal cycles, denoting a decrease in accommodation space. In contrast, in intermediate to deep subtidal environments, transgressive and regressive packages display an opposite trend in accommodation space changes. Sequence boundaries (except the basal and top boundaries of the Penglaiba Formation) are usually represented by laterally traceable, transitional boundary zones without apparent subaerial exposure features. Good correlation of the long-term changes in accommodation space (or sea-level) inferred from vertical stacking patterns of facies and cycles suggests an overriding eustatic control on the formation of meter-scale cycles and third-order depositional sequences as well as platform evolution superimposed with local and/or regional tectonic influence during the Early Ordovician. This study would help understand the controls on the tempo-spatial facies distribution, stratal cyclicity and carbonate platform evolution in the western Tarim Basin during the Early Ordovician, facilitating prediction for favorable subsurface carbonate reservoirs and future hydrocarbon exploration and production in the Penglaiba Formation.

Investigating the preservation of orbital forcing in peritidal carbonates

AbstractMetre‐scale cycles in ancient peritidal carbonate facies have long been thought to represent the product of shallow water carbonate accumulation under orbitally controlled sea‐level oscillations. The theory remains somewhat controversial, however, and a contrasting view is that these cycles are the product of intrinsic, and perhaps random, processes. Owing to this debate, it is important to understand the conditions that do, or do not, favour the preservation of orbital forcing, and the precise stratigraphic expression of that forcing. In this work, a one‐dimensional forward model of carbonate accumulation is used to test the ability of orbitally paced sea‐level changes to reconstruct cyclicities and cycle stacking patterns observed in greenhouse peritidal carbonate successions. Importantly, the modelling specifically tests insolation‐based sea‐level curves that probably best reflect the pattern and amplitude of sea‐level change in the absence of large‐scale glacioeustasy. This study found that such sea‐level histories can generate precession and eccentricity water depth/facies cycles in models, as well as eccentricity‐modulated cycles in precession cycle thicknesses (bundles). Nevertheless, preservation of orbital forcing is highly sensitive to carbonate production rates and amplitudes of sea‐level change, and the conditions best suited to preserving orbital cycles in facies/water depth are different to those best suited to preserving eccentricity‐scale bundling. In addition, it can be demonstrated that the preservation of orbital forcing is commonly associated with both stratigraphic incompleteness (missing cycles) and complex cycle thickness distributions (for example, exponential), with corresponding implications for the use of peritidal carbonate successions to build accurate astronomical timescales.

Sequence biostratigraphy and paleoenvironmental reconstruction of the Oligocene-early Miocene deposits of the Zagros Basin (Dehdasht area, South West Iran)

The Asmari succession is a predominantly carbonate-lithostratigraphic unit corresponding to an Oligocene-Lower Miocene shallow and homoclinal ramp system that developed in the southwest of the Zagros Basin. The sedimentation occurs within a transition from a mixed carbonate–siliciclastic to carbonate-dominated depositional environment. Carbonate production in this system is dominated by diverse benthic foraminifera, coralline algae, mollusks, bryozoans, echinoids, and corals. Based on fossil content and texture, 16 different facies types have been distinguished that accumulated in three interpreted paleoenvironments: The euphotic inner ramp is characterized by two facies zones: an inner and an outer shallow water zone. The inner shallow water zone is characterized by the occurrence of imperforated foraminifera together with mollusks in restricted parts (foramol assemblage). The outer shallow water zone is dominated by wackestones-packstones with benthic foraminifera and coralline red algae, minor mollusks, bryozoans, echinoids, and small epiphytic biota (LB-foralgal assemblage). The higher-energy parts on platform margin are represented by ooids, scattered corals, and robust foraminifera-rich grainstones (coralgal assemblage). The middle ramp is characterized by small coral patch reefs that developed in mesophotic conditions, and larger foraminiferal packstone facies often associated with coralline red algal debris (LB-foralgal assemblages) characterize the sediments of the deeper oligophotic zone, whereas the outer ramp facies–aphotic zone–are represented by bryozoans, mollusks, and echinoid association (bryomol assemblage). This facies passes basinward into wackestone, packstone, and rarely mudstone with pelagic and small benthic foraminifera, sponge spicule, and echinoid fragments (echinofor assemblages). Changes in the depositional facies and cycle stacking patterns indicate four transgressive–regressive sea-level cycles from the bottom to the top of the section. These depositional sequences have been separated by type 2 sequence boundaries. Sea-level fluctuations that cause changes in light penetration which may produce shifts from aphotic to oligophotic and euphotic conditions are an important factor in skeletal production and spatial distribution of carbonate factories. The presence of non-skeletal grains (ooids and ploeids), the occurrence of Porites patch reefs, and the existence of LBF such as Heterostegina, Operculina, Amphistegina, Archaias, and Lepidocyclina in the larger foraminifera associations together with Lithoporella mellobesioides and Spirolithon in coralline red algae assemblage all suggest that carbonate sedimentation took place in tropical waters under oligotrophic to slightly mesotrophic conditions.

Dynamics of cyclic carbonate deposition and biotic recovery on platforms during the Famennian of Late Devonian in Guangxi, South China: Constraints from high-resolution cycle and sequence stratigraphy

To better understand the dynamics of platform carbonate deposition, evolution and biotic recovery during the Famennian, high-resolution cycle and sequence stratigraphic analyses were carried out upon three platform sections (one in ramp, and two in rimmed platforms), along with a biostratigraphically well-constrained basinal section, in Guangxi, South China. Four main facies and fourteen lithofacies are identified in the three isolated platforms; these are vertically arranged into decimetre- to metre-scale, shallowing-upward peritidal and subtidal cycles (or parasequces), in which the peritidal cycles predominate over the lower-middle Famennian. Six third-order sequences (S1 to S6) are identified based on the cycle stacking patterns as illustrated by Fischer-plots and vertical facies changes; these sequences are temporally calibrated through correlation with basinal sequences constrained with conodont zonation, and match well the six main Famennian transgressive–regressive cycles (T1 to T6) in Euramerica and elsewhere, indicating a eustatic control. However, an opposite longer-term trend in accommodation change (S3 to S4) between platform interior and deeper open shelf-to-basin settings during the middle Famennian suggests a differential subsidence across the platform–basin section, likely driven by block tilting. Thus, both eustatic fluctuations and local tectonism had played an important role in platform development and evolution. After the Frasnian–Famennian biotic crisis, main marine benthos preferentially recovered in the normal marine niches on the nearshore open shelves, and radiated gradually into offshore restricted platforms. Brachiopods, notably rhychonellides, were the taxa recovering the earliest (S1) in the normal (or near normal) marine conditions, then echinoderms (S3), stromatoporoids (upper S3) and corals (S5) stepped in episodically. However, these benthos generally appeared later in the restricted platform interiors only as if the ecological conditions were improved well enough for their colonization. Therefore, the biotic recovery processes were diachronous: both taxonomically- and ecologically-specified.

Milankovitch-driven cycles in the Precambrian of China: The Wumishan Formation

Carbonate strata of the Mesoproterozoic Wumishan Formation in the Jixian area near Tianjin are ~3300m thick and were deposited over some 100million years (from ~1310±20 Ma to ~1207±10 Ma). Metre-scale cycles (parasequences) dominate the succession. They are generally of the peritidal carbonate type, and mostly show an approximately symmetrical lithofacies succession with thick stromatolite biostromes and small thromboliteoncolite bioherms constituting the central part and tidal-flat dolomites forming the upper and lower parts. Lagoonal-supratidal dolomitic shales with palaeosol caps make up the topmost layers. The boundaries of the Wumishan cycles are typically exposure surfaces, and there is abundant evidence for fresh-water diagenesis.Widespread 1:4 stacking patterns indicate that the individual Wumishan cycles are sixth-order parasequences, with 4 parasequences constituting one fifth-order parasequence set. Locally, 5–8 beds or couplets, can be discerned in some of the cycles. The regular vertical stacking pattern of beds within the sixth-order parasequences, forming the fifth-order parasequence sets, are interpreted as the result of environmental fluctuations controlled by Milankovitch rhythms, namely the superimposition of precession, and short and long-eccentricity. The widespread 1:4 stacking pattern in the cyclic succession, as well as the local 1:5–8 stacking patterns of the beds within the cycles, suggest that the Milankovitch rhythms had similar ratios in the Mesoproterozoic as in the Phanerozoic. Based on the cycle stacking patterns, 26 third-order sequences can be distinguished and these group into 6 second-order, transgressive-regressive megasequences (or sequence sets), all reflecting a composite, hierarchical succession of relative sea-level changes.

Meter-scale cycles as a proxy for the evolution of the Apulian Carbonate Platform during the late Cretaceous (Llogara Pass, Albania)

Sedimentological investigations have revealed ten facies in Upper Cretaceous platform carbonates (1220 m thick) at Llogara Pass in southern Albania that can be grouped into four facies associations. These reflect specific environmental conditions, ranging from intertidal to subtidal. The facies form small-scale patterns attesting to high-frequency/low-amplitude cyclicity characteristic of relative sea-level fluctuations. Meter-scale cycles have been classified into six distinct types and characterized on the basis of diagnostic surfaces, diagenetic features, and reservoir properties. A closer look at diagenetic and petrophysical features revealed a drastic porosity reduction at each cycle top (i.e., cycle boundary). This underlines a strong interconnection of facies types and reservoir properties at the cycle scale, thus impacting the whole carbonate succession. The cycle stacking pattern is a reflection of the sedimentary evolution during the late Cretaceous in this southern part of the Apulian Platform. (1) Meter-scale peritidal cycles suggest that the Cenomanian was characterized by stable shallow-water conditions. (2) Thick stromatolitic-dominated cycles typify Turonian deposits. Although the nature of the deposits indicates inner platform conditions, the long-term hiatus documented from Italy (Turonian bauxites) is not evidenced in Llogara. (3) The thin (a few tens of meters) Coniacian–Santonian interval suggests the onset of a significant tectonic control over the sedimentary dynamics, likely accompanied by morphological changes on the Apulian Platform and an emergence of rudist-dominated cycles. (4) Peritidal-subtidal cycles persist during the Campanian, accompanied by a broad development of rudist organisms that evidence significant and repeated relative sea-level rise. These data allow refinement of the general sedimentological evolution during the late Cretaceous on the Apulian Platform.

Analyzing the Facies, Sedimentary Environment, Sequence Stratigraphy and Diagenetic Processes of Kangan Formation in Kish Gas Field (Kish Well#3)

Background/Objectives: Although many studies have investigated the reservoir formations in Dahrom group including Kangan formation, but most of these studies have been concerned to other hydrocarbon fields. Regarding the great reservoir importance of Kangan formation in Kish Gas field, therefore, it is important to examine these rich gas-condensate reservoirs. This study aims to restore the sedimentation condition and the conditions governing the sedimentary basins during that period (lower Triassic) and detect the factors affecting the reservoir quality. Methods: To determine facies, sedimentary environment, sequence stratigraphy and identify the most important diagenetic processes occurring in the evaporate-carbonate depositions of Kangan Formation in Kish Gas Field, a sample of 510 microscopic thin sections were collected out of the core extracted from Kish Well#3 and considered by polarizan microscope. Results: Detailed petrographic investigation and well log analysis of the Kangan Formation in Kish gas field led to the recognition of arid tidalflat (A), back barrier lagoon (B) and barrier/shoal (C) facies belts related to an inner part of a homoclinal ramp platform. The Kangan Formation is mainly composed of dolomite, dolomitic limestone with shale and anhydrite. This formation has been affected by several diagenetic processes which played a significant role in improving reservoir quality. Conclusions: Vertical facies changes and cycle stacking patterns demonstrate that the Kangan Formation consists of four depositional sequences of the Third-order in the Kish Well#3, each consisting of transgressive and highstand systems tracts.

Depositional facies and stratal cyclicity of dolomites in the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, NW China

The Upper Cambrian Lower Qiulitag Group in the Tarim Basin, NW China, is overwhelmingly composed of cyclic dolomites. Based on extensive field investigations and facies analysis from four outcrop sections in the Bachu-Keping area, northwestern Tarim Basin, four main types of facies are recognized: open-marine subtidal, restricted shallow subtidal, intertidal, and supratidal facies, and these are further subdivided into ten lithofacies. In general, these facies are vertically arranged into shallowing-upward, metre-scale cycles. These cycles are commonly composed of a thin basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: peritidal and shallow subtidal cycle, are further identified. The peritidal cycles, predominating over the lower-middle Lower Qiulitag Group, commence with shallow subtidal to lower intertidal facies and are capped by inter-supratidal facies. In contrast, the shallow subtidal cycles, dominating the upper Lower Qiulitag Group, are capped by shallow-subtidal facies. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, six larger-scale third-order depositional sequences (Sq1–Sq6) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker-than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average peritidal cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower-order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq1–Sq3, is characterized by peritidal facies-dominated sequences and a progressive decrease in accommodation space, indicating a longer-term fall in sea level. In contrast, the upper sequence set (Sq4–Sq6) is characterized by subtidal facies-dominated sequences and a progressive increase in accommodation space, indicating a longer-term rise in sea level.

Facies analysis and sequence stratigraphy of the evaporite bearing Sachun Formation at the type locality, South East Zagros Basin, Iran

The Paleocene-Early Eocene Sachun Formation across the interior Fars in the SE of the Zagros Basin (Iran) consists of carbonates, evaporites and siliciclastic sediments. Lithostratigraphic evidence shows that the Sachun Formation can be divided into three units: lower evaporite (unit 1), middle limestone and marl (unit 2), and upper evaporite (unit 3). The lower and upper evaporate units are represented by gypsum-anhydrite layers that are interbedded with thin-bedded dolomite and red marl layers. In the evaporites of the Sachun Formation, brecciated gypsum, nodular, nodular-banded gypsum and laminated-banded gypsum with chicken-wire fabrics are identified. These associations indicate a sabkha or shallow-water setting, such as a lagoon. Petrographic studies led to the identification of twelve carbonate facies types that have been formed in four different types of depositional settings, across a ramp with a gentle dip, including; supratidal sabkha, shallow subtidal lagoons, barrier and shallow open-marine. The mixed carbonate and siliciclastic lithofacies are composed of red and gray marl which both are interbedded with carbonate and evaporite deposits. Three depositional sequences (A, B and C) are recognized from deepening to shallowing trends in the depositional facies, changes in cycle stacking patterns and sequence boundary features. Sequences A and C are evaporite-dominated shallowing-upward cycles and sequence B consists of carbonate-dominated shallowing-upward cycles.

A paleotropical carbonate-dominated archive of carboniferous icehouse dynamics, Bird Spring Fm., Southern Great Basin, USA

Much of our far-field knowledge of glaciation and climate during the late Paleozoic ice age is built on decades of study of mixed carbonate–siliciclastic cyclothemic successions from paleotropical Euramerica. Far fewer carbonate-dominated successions have been studied despite their high sensitivity to changes in accommodation space and environmental conditions. Here we present a sequence stratigraphic framework for the carbonate-dominated Bird Spring Formation, southern Great Basin, U.S.A. and infer from it a relative sea-level history for the latest Mississippian through latest Pennsylvanian. Correlation of four successions across the paleo-platform documents changes in the lithofacies composition of meter-scale cycles, their cycle stacking patterns, and cycle bounding surfaces that record substantial variation in relative sea-level at several temporal scales during the Pennsylvanian. Ten genetic sequences bounded by intervals of restricted peritidal lithofacies, karst, terra rossa, rooted caliche and/or Protosols are recognized. Within sequences, 126 parasequences stack into parasequence sets that in turn define lowstand, transgressive and highstand systems tracts. The stratigraphic cycle stacking patterns and across-platform architecture of systems tracts suggest a mid-Carboniferous lowstand in relative sea level followed by a middle Morrowan (middle Bashkirian) highstand as previously suggested (Bishop et al., 2010). Subsequent progradation of the platform occurred during a gradual long-term fall through the late Morrowan and Atokan (late Bashkirian–early Moscovian), upon which repeated higher frequency events were superimposed, culminating in widespread exposure across the platform proximal to the Atokan–Desmoinesian boundary. Long-term sea level increased into the mid-Desmoinesian (end-Moscovian) after which progradation (late Desmoinesian) and aggradation (Missourian–Virgilian) of shallow-water and peritidal carbonates occurred across the Bird Spring platform. The progradational-to-aggradational phase involved an abrupt shift from ramp to rimmed shelf geometry in the Missourian (Kasimovian) likely in response to progressively increasing sea level, dampened high-frequency sea level fluctuations and/or to regional faulting along the Bird Spring platform margin. Inferred magnitudes of short-term (105yr) fluctuations superimposed on the longer-term rises and falls range from 20 to >70m during the early to mid-Pennsylvanian and are substantially reduced (≤20m) during the late Pennsylvanian. The early to middle Pennsylvanian sea-level history inferred from the carbonate-dominated Bird Spring platform corresponds well with a recently published Pennsylvanian onlap–offlap history defined from the cyclothemic succession of the Donets Basin and with independent near-field reconstructions of glaciation during the Pennsylvanian.

Dynamic Carboniferous climate change, Arrow Canyon, Nevada

The Phanerozoic9s longest-lived and most widespread glaciation, the late Paleozoic ice age, is undergoing a resurgence in interest. Long-held models of the timing, duration, and magnitude of glaciation are being reevaluated due to emerging evidence from former high latitudes, evidence that the late Paleozoic ice age was punctuated by long-lived glacial minima or possibly ice-free times. The history of the late Paleozoic ice age is archived within the biostratigraphically well-constrained, carbonate-dominated succession of Arrow Canyon, Nevada, United States. In this paleo-tropical succession, the distribution of lithofacies, flooding surfaces, and subaerial exposure horizons and their stacking into meter-scale cycles record a detailed climate history. The onset of this phase of glaciation during the middle Mississippian was followed by a dynamic evolution of glacioeustasy through the late Mississippian to late Pennsylvanian. Moderate- to high-amplitude glacioeustasy was likely interrupted by an earliest Pennsylvanian short-lived glacial minimum, but otherwise appears to have persisted through the middle Pennsylvanian. Upper Pennsylvanian strata record low- to moderate-amplitude relative sea-level changes, suggesting a long-lived interval of diminished ice volume. This proposed glacial minimum is coincident with a notable minimum in glaciogenic sedimentation near the former southern pole, aridification across paleo-tropical Pangea, and significant floral and faunal turnover, suggesting a link between tropical environmental change and high-latitude glaciation. These conclusions, however, are at odds with those traditionally inferred from Euramerican cyclothems, i.e., persistent high-amplitude glacioeustasy driven by a single, large ice sheet. Rather, the Arrow Canyon archive of varying depositional facies and cycle stacking patterns records major changes in the magnitude of short-term glacioeustasy. This finding contributes to recent and growing near- and far-field evidence for a more dynamic glaciation history than previously inferred from the classic Euramerican cyclothems.

Carbon and oxygen isotope evidence for high-frequency (10 4–10 5 yr) and My-scale glacio-eustasy in Middle Pennsylvanian cyclic carbonates (Gray Mesa Formation), central New Mexico

We combine cyclo- and sequence stratigraphy along with whole rock δ 13C and conodont apatite δ 18O analysis to document high-frequency (10 4–10 5 yr) and My-scale sea-level changes for the Middle Pennsylvanian (Desmoinesian or Moscovian) Gray Mesa Formation of central New Mexico. Approximately 75 subtidal cycles (1–8 m) are grouped into 4 1/2 My-scale depositional sequences (40–80 m). About 50% of the cycles show evidence of prolonged subaerial exposure at cycle tops with the development of calcretes, diagenetic mottling, and regolith intraclasts. High-resolution δ 13C analysis of whole rock limestones across nine of the cycles indicates that the cycle tops were diagenetically altered by isotopically light, meteoric fluids during sea-level fall and lowstand. These δ 13C trends support the interpretation that high-frequency sea-level changes were responsible for cycle development. Conodont apatite δ 18O values from sampled cycles indicate that the high-frequency sea-level changes were driven by glacio-eustasy combined with changes in surface seawater temperature (SST). δ 18O values from conodont apatite, spanning parts of three depositional sequences indicate that My-scale glacio-eustasy and/or SST changes controlled sequence development. δ 18O shifts indicate that the magnitudes of 10 4–10 5 yr glacio-eustasy were between ∼ 55 and 170 + m combined with tropical SST changes of ∼ 1.5°–6 °C. Calculated My-scale glacio-eustatic oscillations were between ∼ 60 and 140 m with SST changes of < 3.5 °C. The most plausible driver for the My-scale paleoclimate changes is long-period obliquity (∼ 1.2 My) variations. These calculated high-frequency, glacio-eustatic values are similar or greater than Pleistocene values, and lie within the range estimated for other Middle Pennsylvanian successions using a variety of independent eustatic proxies. The similarity in range of magnitudes between high-frequency and My-scale sea-level changes combined with the large differences in magnitudes between individual high-frequency sea-level oscillations helps explain the lack of systematic cycle-stacking patterns within these Pennsylvanian icehouse sequences.

A Dominant Tectonic Signal in High-Frequency, Peritidal Carbonate Cycles? A Regional Analysis of Liassic Platforms from Western Tethys

Abstract Meter-scale, peritidal carbonate cycles are a common feature of the geological record but debate continues about what processes lead to their formation. Three conceptual models, or a combination thereof, are commonly invoked to explain cycle formation; eustasy, tectonics, or autocyclicity. These three models are tested with a large new dataset from different Early Jurassic plate margins from western Tethys. Study of seven logged sections from Spain, Italy, Greece, Tunisia, Morocco, and Gibraltar enables an analysis of the possible roles of local versus regional patterns and controls on cyclicity within a Sinemurian time slice. Cycle types are diverse and include shallowing-upward cycles (parasequences) but also deepening-upward and diagenetic cycles (high-frequency sequences) and subtidal cycles. Numbers of cycles per section and cycle stacking patterns within this time slice vary from section to section. Statistical tests (runs tests, time series, and bundling) all indicate random stacking of cycles within sections and an absence of any bundling of thicknesses or of facies trends. Assessment of cycle types by their occurrence and stacking patterns indicates little support for either eustasy or autocyclicity being the dominant cycle-forming mechanism. However, the variability in numbers of cycles per section, thickness variations of the sections, cycle type variability, and randomness of stacking patterns all favor a pulsed, tectonic control for the creation and filling of accommodation space. This conclusion is further supported by evidence that has largely arisen during the course of this study of syndepositional extensional tectonics in the Sinemurian on these rifted Tethyan margins. Although tectonics appears to be the dominant control, superimposed eustasy and/or autocyclic processes cannot be discounted.