Cyclic Facies Research Articles

Ramp Depositional Model in an Intracratonic Basin: The Cambrian Sedimentary Successions in Yanshan Area, North China

ABSTRACTThe mechanisms for the deposition of the Cambrian carbonate sedimentary successions remain controversial. The carbonate‐dominated successions (Stage 3 to Stage 10) are well developed in the North China Craton and they are commonly thought to represent a platform setting because of the flat top. In this study, we propose a new scenarioi, that the Cambrian successions in North China are more likely a carbonate ramp rather than a platform. Detailed sedimentological investigation for the Cambrian rocks in the Yanshan area (Stage 4 to Stage 10) reveals a total of 21 lithofacies which can be grouped into 7 facies associations, including sabkha and mixed tidal flat, shoal, carbonate tidal flat, tidal flat on shoal, protected lagoon, mid‐ramp and outer ramp to shelf basin. Sequence stratigraphy analysis of the Cambrian in the Yanshan area reveals nine third‐order depositional sequences, which shows an overall second‐order transgression. A number of sedimentary characteristics, including the gentle depositional slope, abundant open marine deposits, scattered shoals and microbial mounds, absence of slump or gravity flow sediment, domination of subtidal facies cycles and widespread deposits that formed below the storm wave‐base, indicate that the Cambrian sediments in the Yanshan area were deposited in an intracratonic carbonate ramp depositional system. This intracratonic carbonate ramp includes inner ramp, mid‐ramp, outer ramp and shelf basin facies belts, with the inner ramp exhibiting sub‐facies corresponding to tidal flat, protected lagoon and shoal sub‐facies. Sequence stratigraphy analysis indicates that relative sea‐level changes, corresponding to these nine third‐order sequences, show synchronous with the global sea‐level change across the studied sections. This consistency implies that eustatic sea‐level changes could have been a major factor for the sequence stacking patterns in the Yanshan Area.

Cretaceous cyclic peritidal carbonates of the Apulia Carbonate Platform (Apulia, southern Italy) in a hierarchical sequence‐stratigraphic perspective: A case study from the Murge area (the Giovinazzo sea‐cliff section)

AbstractCretaceous cyclic peritidal carbonates form the bulk of the Apulia Region in Italy and represent the vestiges of the Apulia Carbonate Platform. To show from a sequence stratigraphic perspective the architecture of peritidal carbonates, the 17 m thick Albian Giovinazzo sea‐cliff section was studied at a centimetre detail, aiming to: (i) describe cyclic facies organisation in beds and bedsets; (ii) reconstruct the relative sea‐level curve and its evolution over time; (iii) interpret the long‐term evolution of the accommodation space in terms of sequence stratigraphy. The hierarchical stacking pattern of facies in beds and bedsets reveals Milankovitch cyclicity. As a working hypothesis, elementary sequences are assumed to represent the precession cycle (ca 20 kyr) and small‐scale and medium‐scale sequences the short (ca 100 kyr) and long (ca 400 kyr) eccentricity cycles, respectively. Four different types of elementary sequences (condensed, catch‐down, catch‐up and give‐up) are recognised and interpreted in terms of relative sea‐level changes to reconstruct the relative sea‐level curve of the entire succession. The envelope of the reconstructed relative sea‐level curve is used to represent the long‐term accommodation change on the platform, which covers a time span of approximately 1.8 Myr. Most of this time was spent in subaerial exposure, as approximately 1.2 Myr was predicted to be condensed in a stratigraphic interval encompassing both the sequence‐boundary zone/falling stage deposits and the lowstand deposits. Moreover, it was interpreted that about two‐third of the total thickness of the succession was formed in only 280 kyr and consisted of both transgressive and maximum‐flooding deposits. The main implication of this study is that unconformities do not necessarily correspond to single surfaces but, rather, to very amalgamated intervals or unconformity zones. Moreover, based on biostratigraphic constraints, there is a correlation between the unconformity zone of the studied succession and the third‐order KAl4 sequence boundary of the Cretaceous eustatic cycle chart.

Devonian diamond reservoir of the Urals

Research subject. The Takata formation of the Emsian tier of the Lower Devonian (D1tk) in the Urals is a reservoir of Devonian diamonds. Aim. Confirmation and determination of the diamond-bearing conditions of the reservoir. Materials and Methods. Generalization of primary and interpreted geological information. Lithofacial analysis. Historical and geological reconstruction. Cartographic analysis and geological and cartographic modeling in the ArcMapESRY GIS environment using the ModelBuilder module. Results. The Takata formation is part of the Emsian tier of the Lower Devonian. Tectonically, the Takata formation is located in the West Ural Folded Zone. The diamond content in the Urals is spatially related to the outputs of the Takata formation. The industrial diamond-bearing capacity and the highest density of diamond placers are located in the Vishersky, Chykmansky and Chusovskoy diamond-bearing regions of the Northern and Middle Urals within the Perm Krai. In the early Devonian, the Takata Sea existed in the Vishera diamond-bearing area; a large river with a delta flowed in the direction from northwest to southeast. The sediments of the formation comprise the facies cycle: flood-plain - channel alluvium - delta sediments - underwater-delta - marine sediments. Diamonds are concentrated in the thickness of coarse-grained alluvium, small-pebble basal conglomerates that formed on the land-sea boundary of the paleocontinent of the Russian Plate. Three facies types of the Takata formation section are distinguished: continental, coastal-marine and marine. Its diamond-bearing capacity is associated with the continental and coastal-marine sediments of the formation within the Perm Region. To the north and south of the diamond-bearing areas of the Perm Krai, the Takata formation is represented by marine sediments. According to the geodynamic model based on the geological section of the Northern Urals, the alluvial and coastal-marine Takata diamond placers were formed in the early Devonian as a result of the demolition of material from the Russian Plate. The sources of destruction were rocks of the Precambrian, Ordovician and Silurian. The main spatial-geomorphological criterion for searching for diamond-bearing conglomerates of the Takata formation is the maximum distance of demolition from these conglomerations to placers, equal to 500 m.

Shallow-water carbonates of the Coimbra Formation, Lusitanian Basin (Portugal): contributions to the integrated stratigraphic analysis of the Sinemurian sedimentary successions in the western Iberian Margin

An integrated stratigraphic analysis of the Coimbra Formation was performed in the S. Pedro de Moel outcrops of the Lusitanian Basin (Portugal). This unit is dated from the lower–upper (Oxynotum Chronozone) Sinemurian and is subdivided into eight informal subunits. Except for its base, consisting of dolostones and microbialites, much of the succession consists of bioclastic and bioturbated micritic centimetric–decimetric limestones (sometimes rich in benthic macrofauna) alternating with millimetric–centimetric marly layers, all deposited in shallow-water carbonate ramp environments. Organic-rich sediments occur throughout, with total organic carbon reaching up to 12 wt%. At a broader scale, the Coimbra Formation is transgressive and part of a long-lasting 2nd-order transgressive–regressive facies cycle ending around the Sinemurian–Pliensbachian boundary. The vertical variation of δ 13 C determined in bulk carbonate is characterized by relatively normal marine values (0–2.5‰); however, several negative shifts are associated with the organic-rich sediments, with a maximum amplitude of ∼8‰ in the Obtusum Chronozone. These shifts in bulk carbonate δ 13 C are interpreted to be of diagenetic origin and, therefore, of local significance. However, it cannot be discarded that a regional/global signal is imprinted on the observed trends.

Role of facies cycles on the mid-upper Cretaceous Bangestan Reservoir rock typing in the Zagros Basin

The principal objectives of this research were to examine the role of sedimentary cycles and tectonics in the Ahwaz Bangestan Reservoir quality, view the vertical and lateral changes of these sedimentary cycles, and identify rock typing units. There is an urgent need to understand how interpretive data are perceived among diagenetic events, sedimentary cycles, exposure surfaces, disconformities, carbonate fabric units, porosity-permeability relationship, wettability index, and relative permeability and capillary pressure vs. saturation curves. The Ahwaz Bangestan Sequence consists of Ilam and Sarvak Formations from the late Albian to Santonian Stages, in which disconformity surfaces separated the third-order sequences. Early diagenetic processes of the Bangestan sequence include meteoric dissolution and the deposit of marine cement. The meteoric diagenesis plays an essential role in the reservoir quality of these sequences and sedimentary cycles, and the burial cement process reduces the Bangestan Reservoir quality. The Bangestan sediments experience significant disconformity surfaces in the upper Cenomanian and Post-Turonian Stages. Meteoric waters affect these exposure surfaces to 50 m, increase porosity by about 15%, and have a permeability of 5 mD. With this, the results showed that there is a positive relationship between permeability and porosity data of the Bangestan sedimentary facies. In this regard, grainstone and packstone facies in the Ilam Formation demonstrated a higher reservoir quality than other facies, except for secondary chalky facies in Zone C1, where severe meteoric diagenesis occurred before lithification. Besides, the sedimentary model of the Bangestan Reservoir revealed six facies association, including tidal flat, lagoon, back shoal, rudist biostorm, shoal, and deep-marine with terrigenous layers. Given rock typing classification, four geological static rock typing (GSRT) were recognized with good correlation among lagoon, back shoal, rudist biostorm, and shoal facies association. The results demonstrated a positive relationship between permeability and porosity characteristics, leading to higher porosity and permeability in facies association of shoal and rudist biostorm with better GSRT. Besides, facies association of back shoal and lagoon with lower porosity and permeability represented worse GSRT. However, due to different diagenesis events, open-marine facies did not follow an adequate GSRT. Furthermore, three oil-wet geological dynamic rock typing (GDRT) and two water-wet GDRTs were identified, indicating the dynamic behavior of facies association. As a result, facies association with better reservoir quality (i.e., higher permeability and porosity) present higher GDRT, while facies association with worse reservoir quality (i.e., lower permeability and porosity) show lower GDRT.

Palaeogeographic reconstruction and sequence architecture of the middle‐upper Jurassic successions of Hawraman Basin (NW Iran): Implications for tectono‐depositional processes of the northeastern passive margin of the Arabian Plate

The middle‐upper Jurassic successions (Sargelu, Najmah, and Gotnia formations) of the NW sector of the Zagros Mountain belt are studied to generate a sequence stratigraphic model that helps to understand the Jurassic tectono‐depositional processes and palaeogeographic reconstructions of the northeastern passive margin of the Arabian Plate. The major geological structures observed in northwestern Iran indicate the predominance of an extensional deformation during the Toarcian to Aalenian owing to the eastern Mediterranean rifting. This extensional regime reached its maximum from the Early to Middle Jurassic, causing the fragmentation/drowning of the long‐lived Triassic to Early Jurassic carbonate platform and the development of the Hawraman intrashelf basin (HIB). Associated changes in marine current dynamics, trophic conditions, and degree of substrate oxygenation led to the bloom of oligotypical biota (Bositra limestones) creating unfavourable conditions for the development of other benthic communities (Sargelu Formation). Later, from the Callovian to Oxfordian, changes in carbonate production types (switch from the Bositra filament facies to the stromatolitic‐thrombolitic boundstones) remarkably decreased the dip angle of the depositional profile (Najmah Formation). A second‐order tectono‐eustatic cycle (a major long‐term transgressive/regressive [T/R] cycle) including six minor short‐term T/R facies cycles (TR1–TR6) is identified in the study area. The development of the T/R facies cycles during the Middle Jurassic (TR1–TR3; give‐up sequences; Sargelu Formation) as a result of combined global eustatic transgression and linear subsidence resulted in drowning and backstepping of the platform. In contrast, the short‐term cycles that developed in the upper Jurassic (TR4–TR6; keep‐up sequences; Najmah and Gotnia formations), following a sea‐level fall, resulted in progressive shallowing of the carbonate platform. This study suggests that extensional deformation of the area reached its peak in the transition of the Early to Middle Jurassic, leading to the creation of HIB, but either decreased or played a subordinate role to eustatic sea‐level variations during the deposition of the TR1–TR3 facies cycles (Bajocian‐Callovian). Changes in tectonic regime from an extensional deformation (Early Jurassic) to cooling subsidence (Middle Jurassic), accompanied by a long‐term transgressive sea‐level trend during the Middle Jurassic, were important factors in the development of anoxic trough environments with the deposition of organic‐rich intervals, represented by the TR1–TR3 facies cycles. Tectonic instability (tectonic uplift plus considerable differential subsidence due to the reactivation of faults) in the eastern margin of the Arabian platform during the Late Jurassic led to the development of the TR4–TR6 facies cycles.

A sequential record of the Llandovery δ13Ccarb excursions paired with time-specific facies: Anticosti Island, eastern Canada

The Silurian climate was volatile and punctuated by multiple short-lived climate events, each associated with biotic crisis and perturbations to the global carbon cycle. In total, seven positive carbon isotopic excursions are globally recognized in the Silurian with the large excursions of the Wenlock-Ludlow interval in the Baltic succession garnering the most research; these excursions are recorded in association with cyclic facies changes and the enigmatic resurgence of microbial carbonates. Climate models, hypothesizing cycles of alternating humid and arid climate states, have been developed to explain these paired lithologic-isotopic fluctuations; however, little work has been done to test these models on lower Silurian strata. To test for the presence of δ13Ccarb excursions and the stratigraphic motif of proposed climate events, we developed a high-resolution integrated stratigraphic framework for the upper Hirnantian to mid-Telychian (Upper Ordovician to Llandovery) carbonate-dominated succession of Anticosti Island, by examining ~450 m of strata from a recent drill core, supplemented by ~120 m of outcrop. Four facies assemblages and three time-specific facies are identified and organized into three orders of superimposed transgressive-regressive cycles. High-resolution isotopic curves reveal the presence of four global positive carbon isotope excursions all sequentially recorded in the study interval; the HICE (peak of +5‰), Early Aeronian (peak of +2‰), Late Aeronian (peak of +6‰), and Valgu (peak of +3.5‰) excursions. Common cyclic stratigraphic trends are recognized with these δ13Ccarb excursions, where each excursion is associated with a shift from shale-rich to shale-poor facies, an intermediate-order transgression, and a minor δ18O excursion. These repetitive lithologic-isotopic patterns in the Llandovery succession of Anticosti Island are linked to cyclic fluctuations between low-latitude humid and arid climate states. In addition to these patterns, microbial-rich carbonates are associated with the Late Aeronian and Valgu δ13Ccarb excursions on Anticosti Island; this suggests a genetic link between the elevated δ13C values and the proliferation of calcified cyanobacteria and associated microfossils. This integrated stratigraphic model linking carbon excursions with climate and carbonate factory shifts provides an excellent framework for future studies, using a wide variety of geochemical systems, to better understand the processes that drove environmental and biodiversity changes through the Llandovery.

Orbital cycles, differential subsidence and internal factors controlling the high-frequency sequence architecture in a Sinemurian shallow carbonate platform (Mallorca island, Spain)

The ~125 m thick lower to lowermost upper Sinemurian peritidal to shallow subtidal platform carbonates in the Llevant Mountains of Mallorca (Spain) have been analysed from four stratigraphic sections, to provide new data on the hierarchical stacking pattern of high-frequency depositional sequences. Due to the fact that in shallow water environments the stacking of carbonate facies can be controlled by external (allocyclic) and internal (autocyclic) processes, deciphering the dominant controls on the high-frequency sequence architecture of these platforms is a challenge. The studied carbonates encompass a high variety of facies representative of open lagoon, internal bars, restricted lagoon and tidal flats with local beach sands. Based on a detailed analysis of vertical facies trends and bounding surfaces, large-, medium- and small-scale sequences have been identified within the long-term transgressive-regressive facies cycle defined by the entire succession. Large-scale sequences (~10–30 m thick) and medium-scale sequences (~1–10 m thick) are generally shallowing-upward sequences bounded by sharp facies changes to relatively deeper facies, and have been related to sea-level variations driven by long- (~400 kyr) and short- (~100 kyr) eccentricity cycles respectively. The overprinting of differential subsidence (probably related to extensional tectonic) and carbonate production and accumulation processes varied throughout time, controlling the lateral continuity and preservation potential of the ~100 kyr medium-scale sequences. Shallowing-upward, locally aggradational, small-scale sequences (~0.3 to 5 m thick) are very variable in number and thickness and cannot be correlated between sections, pointing out that their most important controlling factor was the internal processes (hiatuses and erosion related to subaerial exposure at the peritidal caps, lateral migration of internal bars, local wave and currents patterns, depositional and erosional processes related to spring tides and storms) in the frame of a complex mosaic of facies within the studied platform.

Geostatistical Earth modeling of cyclic depositional facies and diagenesis

In siliciclastic and carbonate reservoirs, depositional facies are often described as being organized in cyclic successions that are overprinted by diagenesis. Most reservoir modelling workflows are not able to reproduce stochastically such patterns. Herein, a novel geostatistical method is developed to model depositional facies architectures that are rhythmic and cyclic, together with superimposed diagenetic facies. The method uses truncated Pluri-Gaussian random functions constrained by transiograms. Cyclicity is defined as an asymmetric ordering between facies, and its direction is given by a three-dimensional vector, called shift. This method is illustrated on two case studies. Outcrop data of the Triassic Latemar carbonate platform, northern Italy, are used to model shallowing-upward facies cycles in the vertical direction. A satellite image of the modern Bermuda platform interior is used to model facies cycles in the windward-to-leeward lateral direction. As depositional facies architectures are modelled using two Gaussian random functions, a third Gaussian random function is added to model diagenesis. Thereby, depositional and diagenetic facies can exhibit spatial asymmetric relations. The method is applied in two regions of the Latemar carbonate platform that experience two different types of diagenetic transformation: syn-depositional dolomite formation, and post-depositional fracture-related diagenesis. The method can also incorporate proportion curves to model non-stationary facies proportions. This is illustrated in Cretaceous shallow-marine sandstones and mudstones, Book Cliffs, Utah, for which cyclic facies and diagenetic patterns are constrained by embedded transition probabilities.

Depositional origin of subtidal meter-scale cyclic successions in the Cambrian Hwajeol Formation

Meter-scale cyclic successions of limestone and marl or mudstone commonly formed in shallow marine subtidal environments during the early Paleozoic. Such alternations have been believed to represent primary processes and/or post-depositional. The cyclic facies changes in association with primary processes have been interpreted as allocyclicity and autocyclicity. The Furongian Hwajeol Formation of the Taebaek Group in mid-eastern Korea is host to such early Paleozoic subtidal cyclic successions, showing limestone- mudstone alternations. Sedimentological and geochemical analyses including diagenetically inert element ratio analyses suggest that the cyclic successions were originated from sedimentary origin rather than differential diagenesis. Furthermore, trace elements analysis reveals that the mud fractions formed under suboxic or anoxic conditions, while the limestone facies accumulated under oxic conditions. The repetitions of oxic and oxygen-depleted conditions can be interpreted to be a result of the repeated deisolation and isolation of the water mass in the semi-closed epicontinental sea partly restricted from the open ocean. Repetitive opening and closing of the sea caused vitalization and deactivation of the carbonate factory in the platforms. Accordingly, this restriction of water-mass can deactivate the carbonate platform on the silled basin. This hypothesis provides a plausible explanation for the formative mechanism of the subtidal cyclic successions with meter-scale alternations of subtidal limestone and gradually upward thickening mudstone beds. The gradually thickening of mudstone beds would reflect an increasingly lengthened period of closure and deoxygenation of the semi-closed epicontinental sea due to gradual decrease in rising rate during low-order transgression and gradually extended duration of higher order relative sea-level stillstand or fall superimposed low-order transgressions.

Relative sea-level changes and sedimentary facies development of the lowermost Cretaceous (Berriasian–Valanginian) cycles in the north of Ar Riyad city, Saudi Arabia

A new high-resolution stratigraphic framework for the thick and well-exposed Lower Cretaceous rocks of Saudi Arabia has been provided. The studied successions consist mainly of carbonates with minor interbedded siliciclastics, forming the Silayy, Yamamah and Buwaib formations in the study area (north of Ar Riyad city). Based on the integrated sedimentary facies analysis, a relative sea-level curve for the Berriasian–Valanginian interval has been established, including stacking pattern of the different facies cycles. Two transgressive–regressive facies cycles have been identified and documented in the studied sections. The first cycle started during the middle Berriasian (∼143 Ma), comprising the Silayy and Yamamah formations. It terminated at the Yamamah–Buwaib formational contact during the early Valanginian (∼138 Ma), characterized by a friable brownish ferruginous sandstone bed at the base of the Buwaib Formation. The second transgressive–regressive cycle encompasses the entire lower Valanginian Buwaib Formation. A major unconformity surface formed during the early late Valanginian (∼136 Ma), marking the end of the second cycle and characterized by a distinctive hardground condensation. The correlation and dating of the recognized cycles, identified in central Saudi Arabia, shows a contemporaneity to equivalent surfaces globally and thus strongly supports an isochronous formation of Berriasian–Valanginian facies cycles by eustatic sea-level changes.

Architecture, growth patterns, and early diagenetic modifications of a greenhouse ramp to rimmed shelf system: Danian of the Urbasa–Andia plateau, western Pyrenees, North Spain

The Danian carbonates exposed in the Urbasa–Andia plateau (western Pyrenees, North Spain) provide the opportunity to examine at different scales the facies types and architecture of a greenhouse ramp to rimmed shelf from the slope to coralgal reefal margin through to the highly restricted evaporitic inner shelf areas, integrated with a range of porosity-forming, early diagenetic phases associated with third-order highstands and lowstands. The detailed nature of the stratal architecture of this greenhouse system provides an understanding of potential reservoir layering, lateral facies and facies cycle changes, and widespread early dolomitization but significantly allows a whole paleokarst macroporosity system related to a third-order lowstand to be accessed, illustrating how eogenetic karstic porosity can vary drastically from margins to shelf interiors. The outcrops serve as potential analogs for fields in Paleocene carbonates in North Africa and the Middle East and also offer valuable depositional and digenetic models for greenhouse platforms of other ages.

The hydrocarbon source rocks of the Pliensbachian (Early Jurassic) in the Asturian Basin (Northern Spain): Their relationship with the palaeoclimatic oscillations and gamma-ray response.

Deposition of black shale facies, one of the main contributors for hydrocarbon production, is commonly assumed to be linked to Oceanic Anoxic Events (OAEs), which are supposed to be generated during warm palaeoclimats. This assumption could bias the exploration for hydrocarbon source rocks preferentially towards sediments deposited under warm palaoenvironments, as a preferential guide for hydrocarbon exploration. As a consequence, the establishment of the links between palaeotemperature and the formation of organic-rich deposits is of primary importance to find arguments on this subject. For this purpose, the Upper Sinemurian, Pliensbachian and Lower Toarcian (Lower Jurassic) deposits of the Asturian Basin in Northern Spain, including more than 100 m thick succession containing organic-rich and black shale deposits, has been studied. Correlation between palaeoclimatic data, previously obtained from oxygen isotopes, and the TOC content reveals that black shales and organic-rich sediments were deposited not only during warming intervals but also during a prominent cooling event that occurred at the Late Pliensbachian. This cooling interval has been pointed out as one of the main candidates to have developed ice caps in the poles during the Jurassic. On the contrary, no black shales were generated during the postulated Early Toarcian OAE, which coincides with a superwarming interval. Additionally, the study of the facies cycles and the measurement of gamma-ray in outcrops and its correlation with TOC content provides data on the use of natural radioactivity and sea level changes as a proxy for preliminary organic matter richness evaluation.

A facies distribution model controlled by a tectonically inherited sea bottom topography in the carbonate rimmed shelf of the Upper Tithonian–Valanginian Southern Tethyan continental margin (NW Sicily, Italy)

The Upper Tithonian–Valanginian shallow-water carbonates outcropping in the Palermo Mts (NW Sicily) consist of several facies associations reflecting different depositional environments of a carbonate rimmed shelf, pertaining to the Southern Tethyan continental margin. The reconstructed depositional model, based on the sedimentological features, cyclic facies arrangement and biota distribution, shows that a wide protected lagoon, dominated by algae, molluscs and scattered patch reefs, was bordered landward by a tidal flat, where stromatolitic algal mats were cyclically subaerial exposed, and seaward by a marine sand belt and reef complex. Oolitic packstone-grainstone lithofacies, cyclically subjected to subaerial exposure, suggests the occurrence of a barrier island, located nearly to the lagoonal carbonate shoreline, allowing the development of narrow embayments with restricted circulation. In the outer platform, the oolitic lithofacies of the marine sand belt pass landward into the protected lagoon, where washover oolite sands occur, and seaward into a high-energy zone (back-reef apron) gradually merging in the reef complex. In the latter, coral framestone occupied the inner sector (reef flat), while the facies association dominated by boundstone with Ellipsactinia sp. developed in the outer sectors (reef wall), adjacent to the fore-reef and upper slope environments. Stratigraphic evidence, associated with the recognized facies associations, helped to reconstruct the geo-tectonic setting of the carbonate platform, where the distribution of the depositional facies along the shelf and their extension were influenced by the tectonically-inherited sea bottom topography. In a regime of extensional tectonics, localized and thin succession of high-energy prograding oolite sand belt depositional facies occupied structural highs (footwall uplift), while the largely diffused and thick low energy aggrading peritidal-to-lagoonal depositional facies developed in subsiding hanging wall. The reef complex and the fore-reef depositional facies, exhibiting lateral migration and prograding geometries with clinoforms and downlap stratal terminations, developed on a subsident stepped margin, adjacent to a faulted shelf-break. This morphostructural setting can be comprised in a model, where the tilted and faulted-blocks (inner platform) and stepped margin (outer platform) paleo-topography, inherited from the rifting tectonics, was the most important factor controlling the geometry and depositional architecture of a carbonate platform growing on the Southern Tethyan passive margin.

Geological setting, sedimentology and biota of the estuarine late Oligocene Pomahaka Formation, New Zealand

ABSTRACTPomahaka Formation, a late Oligocene succession of interbedded mudstone, lignite, shell beds and thin sands, was studied in small exposures along the Pomahaka River near Tapanui, Otago and in four adjacent sections in Waikoikoi Creek where about 17 × 0.25–4.0 m thick facies cycles were exposed. At the base of an ‘ideal’ cycle, thinly laminated sandy mudstone overlying lignite of the previous cycle records transgressive flooding of forest swamp by fresh or estuarine waters. Laminated shelly mudstone containing venerid bivalves (Hinemoana acuminata) forming the middle of several cycles represents maximum marine flooding. Above this, trough cross-stratified beds and sand with mud flaser laminae record progradation of distributary bars and/or intertidal sands. Finally, deposition of root-burrowed mudstone and lignite mark the re-establishment of swamp forest. Predominantly muddy sediment texture, the association of lignite, distinctive fresh- and brackish-water faunas and thin depositional cycles indicate a microtidal estuarine paleo-environment in southern Zealandia during late Oligocene time.

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.

Deciphering the history of hydrologic and climatic changes on carbonate lowstand surfaces: calcrete and organic-matter/evaporite facies association on a palimpsest Middle Jurassic landscape from Portugal

The unusual occurrence of calcretes and prominent organic matter in the Middle Jurassic (Lower Bathonian, Serra de Aire Formation) of the Lusitanian Basin of western Portugal (Western Iberian Margin) revealed a complex palimpsest exposure record, here interpreted as reflecting hydrological changes caused by phases of emergence and immersion. It serves as a potential model for understanding stratigraphic development at lowstand surfaces in carbonate successions. The exposure-dominated facies association grades upwards into peritidal and lagoonal limestones, and the interval is assigned to the regressive peak of a Transgressive-Regressive Facies Cycle (2nd order) of the thick Middle Jurassic carbonate ramp succession.The Galinha Quarry, Fátima region, NE of Lisbon, a type section for this lowstand assemblage, exhibits varied calcretes, with black-clasts, interbedded with, and grading into: organic-rich marly/clayey seams and lenses, locally with carbonate nodules; carbonates with evaporite traces; microbial laminites; black-clast and fenestral limestones; some lithofacies are dolomitized. The palynofacies contains phytoclasts associated with less refractory, more prone to degradation components, which suggests natural combustion/pyrolysis (wild fires). The lowstand surface represents a low relief landscape with small depressions/ponds bordering a more distal marginal-littoral setting; the partly subaerial and partly subaqueous settings were subjected to lengthy exposure and to fluctuating, very shallow water bodies and water table. Coeval climatic regime was a seasonally dry/wet one, with dry/semi-arid phases dominating over the sub-humid, as shown by the combined record of intense calcrete development, rhizogenic structures, microbial mats, brecciation, desiccation, evaporites and wild fire evidence. However, sea level rise caused changes to shallow, sea-water influenced restricted lagoonal-peritidal settings.Comparisons and differences with modern and ancient coast marginal carbonates are made to provide a guide to the variability in such lowstand deposystems in the stratigraphic record.

Genesis of Upper Triassic peritidal dolomites in the Transdanubian Range, Hungary

In the Late Triassic, a 2–3-km-thick platform carbonate succession formed along the passive margin of the Tethys Ocean. Certain parts of the succession were affected by pervasive dolomitization whereas other parts are only partially dolomitized or non-dolomitized. In the Transdanubian Range, Hungary, the Upper Triassic platform carbonates are extensively distributed and numerous data are available for the space and time relations of the dolomitized and non-dolomitized units. This geological setting provides a unique opportunity for the study of paleogeographical and diagenetic controls of dolomitization of the whole platform complex. This paper presents the characteristic features of the dolomite types of the dolomite-bearing formations and lithofacies types, with a view to interpret the dolomite-forming processes and to determine the main controlling factors of the dolomite genesis. Petrographic features and stable isotope characteristics of the studied successions suggest the predominance of penecontemporaneous and early diagenetic dolomite genesis. Study of the transitional interval between the pervasively dolomitized and the non-dolomitized sequences revealed the general presence of microcrystalline dolomite in the peritidal microbial deposits and the characteristics of partial dolomitization both in the peritidal and subtidal facies. In the peritidal facies, microbially induced Ca–Mg carbonate precipitation is inferred, which was probably complemented by penecontemporaneous mimetic dolomitization of precursor carbonates due to evaporative pumping or seepage influx. Dolomitization of the subtidal facies took place via reflux of slightly evaporated seawater. Dolomitization of the previously deposited carbonate mud commenced during subsequent subaerial exposure but the process of early diagenetic dolomitization may have continued during later exposure events. Recurring subaerial exposure is one of the factors that determine the areal extent of the early dolomitization of the platform carbonates. However, climatic conditions were also crucial. Although the sea-level-controlled, unconformity-bound cyclic facies pattern did not change significantly in the internal platform belt during the nearly 20-My-long time range, a drier climate favored dolomite formation while increasing humidity led to a gradual decreasing intensity of early dolomitization.

Health and social factors associated with alternate routes of administration of prescription opioids among young adult multidrug users

The unusual occurrence of calcretes and prominent organic matter in the Middle Jurassic (Lower Bathonian, Serra de Aire Formation) of the Lusitanian Basin of western Portugal (Western Iberian Margin) revealed a complex palimpsest exposure record, here interpreted as reflecting hydrological changes caused by phases of emergence and immersion. It serves as a potential model for understanding stratigraphic development at lowstand surfaces in carbonate successions. The exposure-dominated facies association grades upwards into peritidal and lagoonal limestones, and the interval is assigned to the regressive peak of a Transgressive-Regressive Facies Cycle (2nd order) of the thick Middle Jurassic carbonate ramp succession.The Galinha Quarry, Fátima region, NE of Lisbon, a type section for this lowstand assemblage, exhibits varied calcretes, with black-clasts, interbedded with, and grading into: organic-rich marly/clayey seams and lenses, locally with carbonate nodules; carbonates with evaporite traces; microbial laminites; black-clast and fenestral limestones; some lithofacies are dolomitized. The palynofacies contains phytoclasts associated with less refractory, more prone to degradation components, which suggests natural combustion/pyrolysis (wild fires). The lowstand surface represents a low relief landscape with small depressions/ponds bordering a more distal marginal-littoral setting; the partly subaerial and partly subaqueous settings were subjected to lengthy exposure and to fluctuating, very shallow water bodies and water table. Coeval climatic regime was a seasonally dry/wet one, with dry/semi-arid phases dominating over the sub-humid, as shown by the combined record of intense calcrete development, rhizogenic structures, microbial mats, brecciation, desiccation, evaporites and wild fire evidence. However, sea level rise caused changes to shallow, sea-water influenced restricted lagoonal-peritidal settings.Comparisons and differences with modern and ancient coast marginal carbonates are made to provide a guide to the variability in such lowstand deposystems in the stratigraphic record.

Depositional systems of the Lower Ordovician Mungok Formation in Yeongwol, Korea: implications for the carbonate ramp facies development

Detailed study on the Lower Ordovician Mungok Formation in the mideastern part of the Korean peninsula has led to recognition of seven lithofacies which can be grouped into seven facies associations (FAs): FA1 (lagoon), FA2 (shoal), FA3 (shoreface), FA4 (inner to mid-ramp), FA5 (inner to outer ramp), FA6 (mid- to outer ramp), and FA7 (outer ramp). Spatio-temporal organization of the FAs represents homoclinal ramp environments with fringing ooidpeloid shoals. Correlation and lateral thickness variation of the FAs suggest that deep part of the basin might have been located toward the northern part of the study area. The relative sea-level curve inferred from the facies analysis suggests that the Mungok Formation evolved through three depositional stages in accordance with 3rd-order sealevel change. The organization of the facies succession indicates that the inner ramp facies associations (FA1 to 3) of the Mungok Formation are characterized by non-cyclic facies succession whereas the midand outer ramp facies associations (FA 4 to 7) have distinct cyclicity except non-cyclic FA 7. The difference in the facies successions between the inner ramp and the mid- to outer ramp area may result from the various depositional regime across a ramp. A non-cyclic facies successions of inner ramp facies (e.g., packstone to grainstone and lime mudstone) are suggestive of mosaic-type distribution that is controlled mostly by wave, storms, tides, currents, and local geomorphology. In contrast, cyclic facies successions (e.g., pebbly limestone conglomerate, limestone-shale alternation, and calcareous shale) represent a simple belt-type distribution of mid- to outer ramp facies that seems to reflect shifts of facies belt caused by sea-level changes. The partitioning of inner and mid- to outer ramp facies is appropriate for understanding the facies development of the Mungok Formation and similar ramp successions with distinct inner and mid- to outer ramp facies.