Shallowing-upward Trend Research Articles

An evaluation of morphometrical trends in a population of Woodringina hornerstownensis Olsson, 1960 (Foraminifera) reveals the role of intraspecific variations for paleoceanographical studies

The foraminiferal species Woodringina hornerstownensis is stratigraphically limited between the stages Danian and Selandian and was adapted to inhabit the mixed layer water column. We present a case study based on morphometry of W. hornerstownensis recovered in upper Danian strata (biozone P2) in the Pernambuco-Paraiba Basin at the Poty quarry. The morphometric data, associated with abundance and geochemical indicators, point to (i) an increase in size towards the top of the studied interval, associated with the shallowing-upward trend of the Maria Farinha Formation, and (ii) an increase in size directly related to the increase in abundance. By integrating and incorporating a specific variation of W. hornerstownensis into abundance parameters, we conclude that the shell increase appears to be consistent. Our results suggest that the increase in size responds to environmental optimum conditions for the species W. hornerstownensis. In a broader context, our work demonstrates the usefulness of morphometric indicators and the significance of intraspecific variation when interpreting macroecological patterns. Keywords: planktic foraminifera, paleoceanography, intraspecific variation, upper Danian, South Atlantic Ocean.

Linking high-frequency lacustrine sequences to orbitally-induced cyclicity (Lower Cretaceous, Iberian Basin)

The lacustrine El Castellar Formation in the Castillo de Aliaga section (Early Cretaceous Galve sub-basin, eastern Spain) features two carbonate successions of marl and limestone, separated by a mudstone and marl interval. Sequence analysis revealed small-scale (44 elementary sequences), medium-scale (ten complete and two incomplete parasequences), and large-scale (three sets of parasequences, one incomplete), high-frequency lacustrine sequences, with an average thickness of 2.9, 12.4 and 54m, respectively. These sequences start with a sudden facies change (deepening) and exhibit a shallowing-upward trend with features of subaerial exposure (bioturbation, oxidation, or brecciation) at the top, indicating phases of climate-modulated lake expansion and retraction. The temporal framework of the lacustrine sequences is further characterized by the correlation of these sequences with sedimentary-cycle periodicities of 3.3, 13.2 and 57.3m, attributed, respectively, to the long precession cycle (22.4kyr) and the short (95kyr) and long (405kyr) eccentricity cycles of the Earth’s orbit. The hierarchical stacking of sequences aligns with orbitally driven cyclicity, with thickness variations interpreted as tectonic subsidence effects (accommodation) resulting from normal fault slip in a rift system. The three sets of parasequences (SPS-1 to SPS-3) align with stages of lake system evolution. SPS-1 represents deposition in a lowenergy shallow carbonate lake. SPS-2 indicates a significant lake expansion and deepening, linked with clastic input and a mixed lake system, and correlates with a major increase in accommodation, over ~200kyr, involving fault-induced local tilting. SPS-3 represents deposition in a high-energy carbonate lake. The parasequences identified show variations in cyclic thickness tied to a >700kyr tectonic cycle. The elementary sequences, mainly corresponding to marl-limestone bundles, exhibit thickness changes probably due to shorter-term tectonic pulses. Accelerated tectonic activity resulted in increased accommodation in shorter (40-50kyr) periods, followed by longer (>100kyr) periods of progressive deceleration.

Evolution of a low-energy carbonate ramp, Lower Mississippian Pekisko Formation, northwestern Alberta, Canada

The lithofacies architecture and depositional evolution of the Lower Mississippian (Tournaisian) Pekisko Formation in the subsurface of the Hawk Hills area in northwestern Alberta have been established by integrating detailed core work and well log data. The formation is composed of skeletal-peloidal limestones and argillaceous limestones that were deposited along the northern flank of the Peace River Embayment, a semi-restricted and tectonically active oceanic re-entrant located along the western margin of Laurasia at low paleolatitude. Lithofacies associations recognized in the study area include the outer ramp to slope (LA 1), outer ramp (LA 2) and mid ramp (LA 3), which are stacked into three decameter-scale, deepening-upward and aggradational cycles that are of regional extent and have meter-scale deepening and shallowing-upward trends. A previously unrecognized paleosol horizon at the top of decameter-scale cycle 2, indicating widespread subaerial exposure of the ramp, is interpreted as a sequence boundary that divides the Pekisko and Shunda formations in the study area (and possibly elsewhere in the Peace River Embayment) into two third-order sequences, each consisting of transgressive and highstand systems tracts. The Pekisko Formation in the study area is interpreted to represent a low-energy, temperature-stratified ramp that was mainly homoclinal, but with transient, distal steepening occurring in the southern part of the study area. Ramp deposition was strongly affected by basement-fault reactivation causing differential subsidence and uplift in the Peace River Embayment. The paleogeography and paleoceanographic conditions of the embayment favored upwelling currents and development of a temperature-stratified ramp, as well as the formation of heterozoan carbonate deposits and mid ramp facies of predominantly packstones and wackestones. This depositional scenario is atypical, as most other documented examples of the Pekisko Formation and other Lower Mississippian ramp successions in western North America and western Europe are characterized by moderate to high-energy, mid to inner ramp facies deposited in open-ocean conditions. The results of this study contribute to an improved understanding of the range of depositional settings along the western margin of Laurasia during the Early Mississippian and demonstrate the applicability of the thermocline-stratified ramp model, with some modification, to ramps in semi-restricted embayments and other low energy settings.

Revisiting the stratigraphic development of the postglacial, transgressive paleo-Changjiang (Yangtze River) mouth: A journey from tide-dominated estuary to delta

Abstract The transition between a tide-dominated estuary and a tide-dominated delta, which is observed in modern settings and repeatedly throughout the geologic record, is a critical process for understanding sea-level and environmental changes. This transition remains poorly documented, however, leaving a knowledge gap about the sedimentary and stratigraphic processes involved. To help bridge the gap, we reexamined the postglacial transgressive system beneath the modern Changjiang (Yangtze River) Delta, built by one of the world’s largest rivers, using new chirp data supplemented by correlation with previously documented boreholes. Our results revealed a previously undocumented tide-dominated estuary-to-delta transition around the Younger Dryas, with the pretransition evolution dominated by the retrogradation of the tide-dominated estuary, which fills most of the accommodation space in the lowstand incised valley. This transition occurred when the middle and seaward parts of the estuary reached the subaqueous modern Changjiang Delta area and underwent extensive tidal erosion. Overlying the tidal ravinement, a series of ridge-and-swale structures formed due to tidal/fluviotidal channelization on the mud-dominated heterolithic substrate, primarily consisting of delta-front sediments characterized by flat, stratified reflections (bedding) with shallowing-upward trends. Ridge orientation rotates clockwise from north to south, with intervening swales bifurcating from a common apex point, and these features are flanked by inclined and sigmoidal levee-like channel fills dominated by homogeneous mud (acoustically transparent material). We reinterpret these features as tide-dominated delta islands and distributary channels, rather than the previously interpreted erosional mud ridges in the seaward part of the estuary, as they align with patterns observed in classic tide-dominated deltas. These patterns also suggest a shift from delta-front to delta-plain environments coinciding with shoreline advance ca. 10 k.y. B.P., but this was preceded by the evolution of a backstepping delta. Our study highlights the interplay between sediment supply and sea-level fluctuations in controlling the large-scale stratigraphic and environmental evolution of tidal-dominated river mouth systems. In the transgressive, paleo-Changjiang mouth, this interplay drove the system to evolve through stages of retrogradational estuary, backstepping delta, and progradational delta, and each environment differs significantly from the pattern observed in the modern Changjiang Delta. This study provides important insights for evaluating the evolution and sequence-stratigraphic reconstruction of other tidal depositional systems.

Anhydrite fabrics as an indicator for relative sea-level signatures in the sequence stratigraphic framework of a carbonate ramp

Evaporite deposits are increasingly recognized for their role in developing oil reservoirs. In the Shadegan Oilfield, anhydrite is an evaporite mineral identified in the Asmari Formation (Oligocene-Miocene) as clastic- and carbonate-evaporite assemblages. Our study found that diagenesis plays a critical role in controlling the distribution of different fabrics (texture and structure) in the area. The anhydrites resulted from two progressive processes: early diagenesis and formation of anhydrites related to supratidal to shallow settings, and late diagenesis and crystallization of anhydrites in deeper environments. The evaporite-bearing intervals provide an opportunity to investigate the interactions between sea-level fluctuations and the growth of anhydrites during the development of the carbonate ramp of the Asmari Formation. Despite previous studies on the Asmari Formation, little is known about the relationships between the occurrence of its anhydrite fabrics and sequence stratigraphic framework. In our study, the continuous shallowing-upward trends, exposure of sediments, and subsequent diagenesis were responsible for the deposition of anhydrite. Semi-arid to arid climates followed, resulting in stacking of anhydrite and dolomites, particularly in the upper parts of the Asmari, corresponding to the Burdigalian.

Suprasubduction ophiolite (SSZ) components in a middle to lower upper Jurassic Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel area)

The Northern Calcareous Alps in the Western Tethys realm were affected in Middle to Late Jurassic times by a mountain building process triggered by ophiolite obduction similar to that in the Inner Western Carpathians or Inner Dinarides. In contrast to these other mountain ranges, in the Northern Calcareous Alps the obducted ophiolites or ophiolite derived components in the Bathonian-Oxfordian mélanges are missing. Cr-spinels in Kimmeridgian basinal deposits are the oldest known relics of a Jurassic ophiolite obduction. This study reveals new data from a newly detected Hallstatt Mélange below the Late Jurassic Plassen Platform in the southeastern Northern Calcareous Alps (Raucherschober/Schafkogel area). Upper Triassic Hallstatt Limestone blocks from the former distal northwestern continental margin of the Neo-Tethys Ocean, as well as ophiolite and radiolarite blocks from the Neo-Tethys Ocean floor rest within an upper Middle to lower Upper Jurassic radiolaritic-argillaceous matrix. Ophiolitic blocks show calc-alkaline volcanic arc affinity, defining the rocks as the product of intra-oceanic subduction and the formation of an early arc during stacking of the oceanic crust. Resedimented ribbon radiolarite blocks deposited above the newly formed suprasubduction (SSZ) ophiolites in the Neo-Tethys Ocean east of the island arc have a Middle Jurassic age. Later, at a time of decreasing tectonic activity, the Hallstatt Mélange was sealed by the Kimmeridgian-Tithonian Plassen Carbonate Platform, showing a shallowing-upward trend.

Age and provenance of the Mio-Pleistocene sediments from the Sacaco area, Peruvian continental margin

Over the last decades, rocks from the East Pisco Basin (EPB), on the Central Peruvian coast (13°-16°S), have yielded an abundant and diverse collection of coastal-marine fossils, which are key for characterizing the onset and evolution of the modern Humboldt Current. Despite its paleontological richness, and after almost 40 years of study, the spatio-temporal context of the deposits of the southern part of the EPB (Sacaco area) remains only broadly constrained, being mostly tailored to particular vertebrate-rich levels occurring throughout the area.Here we build a composite stratigraphic section for the Sacaco area including three lithostratigraphic units (Pisco, Caracoles, and Pongo formations), which documents several discontinuities and intraformational unconformities. We infer depositional ages based on new radiometric (U–Pb), isotopic (Sr) and biostratigraphic data, synthesize previous litho-, chemo-, and bio-stratigraphic studies, and present a comprehensive chronostratigraphic review of the Mio-Pleistocene record for the Sacaco sub-basin that allows us to identify various local and basinal events. Our results indicate that, in the Sacaco area, the Pisco Formation ranges from ∼9.6 to 4.5 Ma, the overlying Caracoles Formation from 2.7 to ∼1.9 Ma, and the Pongo Formation accumulated from ∼1.9 up to at least 1.4 Ma. These sedimentary successions accumulated in a continually subsiding setting and show a shallowing-upwards trend. Zircon U–Pb provenance analysis mainly record Neoproterozoic, Cretaceous, and Mio-Pleistocene populations, revealing discrete up-section changes in source areas. Mio-Pleistocene and Cretaceous sources are continuously present, while older recycled-orogen sources vary through time in presence and abundance, indicating either paleogeographic changes or source exhaustion.

Variations in carbonate-evaporite shoreline and ramp facies trends: Middle Miocene Fatha Formation, Kurdistan Region, NE Iraq

A sedimentological investigation of Miocene deposits at the periphery of the Zagros foreland basin, Kurdistan Region, NE Iraq, reveals a cyclical arrangement of carbonate-evaporite ramp facies. The study aims to provide rare insight into the lateral variability of microfacies and environments of such systems' inner ramp and shoreline. Between 10 and 40 depositional cycles are preserved, separated by a flooding surface, and recording an overall basinward progradation. In each cycle, a shallowing-upward trend from a lower-energy calcareous mudstone and mudstone-wackestone carbonate microfacies at the base to a higher-energy packstone-grainstone-rudstone or low-energy algal mat/stromatolitic carbonate microfacies above is evident. Evaporite deposits of supratidal sabkha origin cap each carbonate deposit. Red clastic sediments that advanced south-westward into the basin from the adjacent Zagros hinterland overlie each evaporite deposit. A flooding surface and return to calcareous mudstones mark the start of the next cycle. Outcrop and thin-section analysis of the carbonate deposits of each cycle reveals a shoaling-up to inner-ramp facies with varied environmental settings that developed under a range of hydrodynamic conditions. Microfacies analysis indicates that these environments included normal marine salinity open lagoons, hypersaline lagoons, restricted and shallow lagoons, sand shoals, intertidal and supratidal flats, supratidal ponds, and a coastal alluvial plain that included channelised deposits and palaeosols. The cyclical facies trend changes toward the top of the succession in that the relative rate of siliciclastic supply markedly increased, whereas the rate of carbonate and evaporite production decreased. Progradation and shoreline migration through time caused lateral and vertical facies changes and thickness variations over the succession. Repeated relative sea-level changes, associated changes in climate, and variations in sediment supply from the hinterlands during the collision of the Arabian and Iranian plates, combined with a variable rate of accommodation generation, are inferred to have controlled the preserved cyclicity.

Paleoenvironmental conditions of the late Miocene “Entrerriense” epicontinental sea: A case study of the Camacho Formation, SW Uruguay

Marine sediments deposited during late Miocene transgressive events are recorded over large onshore areas in southeastern South America, characterizing the “Entrerriense Sea”. In Uruguay, these deposits crop out at the coast of the Department of Colonia, and are assigned to the Camacho Formation, which is well-known by its abundant fossil content. Nevertheless, high-resolution geochemical proxy records within the Camacho Formation succession are not yet available. Our goals are: (1) to assess in high-resolution the paleoenvironmental changes recorded by the marine and/or marine-influenced strata of the Camacho Formation in Uruguay; (2) to explore possible links between local responses and global climate shifts that occurred during the late Miocene. To achieve these goals, we applied a multi-proxy approach, in which sedimentary facies and trace fossil occurrences were compared with geochemical and physical properties records measured in high-resolution within the exposed succession of the Camacho Formation. Our results suggest that these strata record a shallowing-upward trend, also characterized by deposition in gradually more proximal settings [increasing-upward trends in log(Zr/Rb) and log(Ti/Rb)], coupled to increased influence of freshwater input in the upper half of the studied sedimentary succession (decreasing-upward Sr/Ba trend). Considering previously published ages for the Camacho Formation, we infer that this shallowing-upward trend was related to the high latitude cooling and/or ice volume expansion occurred between 7.2 and 6.9 Ma, and the associated long-term sea level fall from 7.2 to 6.5 Ma. These trends occurred in tandem with the onset of more humid climate conditions (recorded by the uppermost strata of the studied succession), which were likely related to the latitudinal migration of climatic belts.

Microencruster-microbial-cement framework of the Upper Jurassic reef developed on the slope of the intra-Tethyan carbonate platform (Bulgaria)

The carbonate succession in the Lyalintsi section of the western Moesian Platform (western Bulgaria) displays a shallowing-upward trend. Growth of the Tithonian–Valanginian coral biostromes and low-relief bioherms was preceded by Oxfordian–Kimmeridgian sedimentation of fine-grained peloidal-bioclastic limestones and Saccocoma-bearing limestones on the homoclinal ramp and the carbonate platform slope. In the late Kimmeridgian, boundstones with very rare corals, but with easily recognisable biohermal morphology, were developed. The main components of this reef are encrusting microorganisms, microbial crusts and synsedimentary cements. Microencrusters Labes atramentosa, Crescentiella morronensis, Perturbatacrusta leini and Radiomura cautica, as well as thin crusts of calcified sponges (sclerosponges), are the main biotic components. Corals (almost exclusively microsolenids) are sparse, whereas photophilic microencrusters (e.g., “Lithocodium–Bacinella”), are absent, although they are common in the overlying shallow-water part of the Lyalintsi sequence. Microbialites and synsedimentary cements provided additional support for the reef framework. The framework, especially the biotic components, and the reefal facies position within the sedimentary succession, implies that the high-energy upper slope of the carbonate platform was the depositional setting of the microencruster-microbial-cement reef studied. Encrusting microorganisms, except for C. morronensis and sponges, are only known from the intra-Tethyan platforms. This study supports conclusion of studies of coeval Alpine reefs that the presence of the microencruster-microbial-cement framework provides insight into the palaeobathymetry, palaeogeography and tectonic configuration of the intra-Tethyan carbonate platforms.

Sedimentology and Stratigraphic Evolution of the Early Eocene Nammal Formation, Salt Range, Pakistan

The Early Eocene succession of the Salt Range from base to top comprises the Nammal Formation, Sakesar Limestone and Chor Gali Formation. The Nammal Formation of Ypresian age is well exposed throughout the Salt Range. Detailed sedimentological and palaeontological analyses of the Nammal Formation were carried out, based on six stratigraphically important measured sections in the Salt Range. Lithologically, the formation is predominantly composed of interbedded nodular limestone, marl and shale. Wackestone, packstone, wackestone to packstone, dolomitic limestone and grainstone facies dominate the Nammal Formation in a fine-grained bioclastic matrix with abundant grains of larger benthic foraminifera. The diagnostic larger benthic foraminifera are recorded, which includes Nummulites mamillatus, Assilina spinosa, Assilina subspinosa, Assilina granulosa, Assilina laminosa, Discocyclina dispansa, Alveolina dolioliformis, Alveolina pasticillata, Alveolina globula, Lockhartia tipperi, and Lockhartia conditi. Stable carbon and oxygen isotopic signatures of the Nammal Formation designate shallow marine depositional environment. During Eocene a carbonate sequence developed in the Salt Range, lower boundary of which is marked as SB-II at the base of the Nammal Formation overlying the Palaeocene Patala Formation. The Nammal Formation presents the retrogradational facies suggesting the transgressive system tracts. The Sakesar Limestone shows agradational to progradational pattern of facies, which developed in highstand system tracts. The Chor Gali Formation possesses shallowing-upward trend by forming progradational shift of facies, and represents the falling stage system tracts. Early Eocene carbonate sequence is terminated by a regional sub-aerial unconformity SB-I marked between marine carbonate sequence of the Chor Gali Formation and the overlying non-marine clastic Miocene Kamlial Formation. Overall, the Nammal Formation presents shallow water neritic carbonate deposits containing larger benthic foraminifers.

Bay-head deltas as hydrocarbon reservoirs: The Middle Jurassic Hugin Formation in Block 15/3 of the South Viking Graben, Norway

Bay-head deltas are important paralic sedimentary systems at the interplay between fluvial and marine processes. The resulting sand accumulation can be significant for the origin of hydrocarbon reservoirs with extraordinary properties and contribute with additional volumes in near-field situations or as secondary reservoirs. Nonetheless, this type of depositional system is barely captured in existing field descriptions and has received little attention from a sedimentological point of view, although our understanding of related sedimentological processes has increased recently by the investigation of comparable modern systems. The Middle Jurassic (Bathonian-Callovian) Hugin Formation in Block 15/3 (Gudrun Field area) of the Norwegian North Sea is up to 245 m thick and consists mainly of laminated mudstone deposited in a restricted (lagoonal) embayment. The succession contains numerous sand bodies with various thickness and restricted lateral continuity, each representing a shallowing-upward trend and facies types characteristic of prograding bay-head deltas. A stratigraphic analysis of five wells across a ca. 10 km long NW-SE transect shows the diachronous nature of the Hugin Formation, with early deposition close to the axis of the South Viking Graben (West) and gradually later deposition towards the graben shoulder (East). The bay-head deltas were fed by river systems (the laterally equivalent Sleipner Formation) from the Utsira High in the East and responded with back-stepping as a consequence of the overall transgression from the North (the overlying Heather Formation). Enhanced understanding of the depositional system of the Hugin Formation and its comparison with modern and outcrop analogues has increased the predictability of the heterogeneity and compartmentalisation in the Middle Jurassic reservoir of the Gudrun Field area.

Palaeontological and sedimentological characteristics of the Changhsingian (Late Permian) platform−margin sponge reef and intraplatform skeletal mound in NE Sichuan Basin, South China: their palaeoenvironmental implications

Well-preserved fossil reef/mound-building communities and shallow microfacies have been recovered from Changhsingian platform−margin sponge reef at the Panlongdong Section and intraplatform sponge skeletal mound at the Yanggudong Section in NE Sichuan Basin, South China. The present contribution focuses on the palaeoenvironmental implication of the differentiated development processes on the platform−margin reef and intraplatform skeletal mound based on the palaeontology and sedimentology. Precise field assessments and macroscopic/microscopic qualitative observations led to the identification of five reef/mound-building community and 14 reef/mound-related microfacies in the 2 sections of Changxing Formation. In the Panlongdong platform−margin sponge reef, reef-building communities are recognized into Amblysiphonella Community, Preperonidella−Amblysiphonella Community, and Preperonidella–Sollasia Community in ascending order. The reef-related microfacies are mainly classified into calcisphere−pelletoid wackestone to packstone, crinoidal wackestone to packstone of shallow water bank environment, hypercalcified sponge bafflestone, calcareous algae−hypercalcified sponge binding framestone, hypercalcifed sponge−calcareous algae bindstone, and calcareous algae−hypercalcifed sponge rudstone of reef environment, and benthic foraminifera−calcareous algae wackestone to packstone of shallower water lagoon environment. Two mound-building communities can only be found in the Yanggudong intraplatform skeletal mound, including Preperonidella−Intrasporeocoelia Community of lower part and Preperonidella−Sollasia Community of upper part. The mound-related microfacies include calcisphere−calcareous algae wackestone to packstone, crinoidal wackestone to packstone and crinoidal packstone to grainstone of shallow water bank environment, hypercalcified sponge bafflestone and cyanobacteria−hypercalcified sponge binding bafflestone of skeletal mound environment, benthic foraminifera−calcareous algae packstone grainstone of shallower water bank environment and benthic foraminifera−calcareous algae wackestone to packstone of shallower water lagoon environment. These palaeontological and sedimentological analysis results have facilitated the classification of sponge reef and skeletal mound sections corresponding to a solitary shallow benthic zone into multiple microfacies attributed to variable environmental depositional conditions. The differentiated evolutional process of multiple reef/mound-building stages at the Panlongdong and Yanggudong sections clearly demonstrate a general shallowing-upward trend from an inner shelf setting to a supratidal and low energy lagoon environment.

Eustatic and tectonic control on the evolution of the Jurassic North Qiangtang Basin, northern Tibet, China: Impact on the petroleum system

The Qiangtang Basin is the largest Mesozoic marine hydrocarbon-bearing basin in China. Investigating its tectonic and sedimentary evolution is therefore relevant for hydrocarbon exploration. Here we present a detailed stratigraphic and sedimentological analysis of five Jurassic stratigraphic sections in the North Qiangtang Basin. Nineteen facies associations were identified and allowed us to reconstruct changes of water-depth. The North Qiangtang Basin experienced two transgressive-regressive cycles during the Jurassic. The first cycle started in the mid-Jurassic (Bajocian-Bathonian), with braided-river and floodplain sediments passing up-section to tidal flat and eventually inner-to mid-ramp deposits. Tidal-flat deposition resumed during the Callovian. The second cycle began in the Late Jurassic (Oxfordian) with transgressive inner-to mid-ramp sediments followed by a shallowing-upward trend during the late Oxfordian, leading again to tidal-flat sedimentation in the Kimmeridgian-Tithonian. Petrographic analysis and detrital-zircon geochronology indicate provenance constantly from the Hoh Xil-Songpan Ganzi terrane, Kunlun arc, and central Qiangtang. Because Jurassic magmatic activity and active syn-depositional faults have not been reported, and steady detrital supply indicates a stable tectonic environment, eustatic change is inferred to represent the principal control on sedimentary evolution in the North Qiangtang Basin during the Jurassic. A comparison with the Amu Darya Basin in central Asia, characterized by similar sedimentological conditions and distribution of reservoir and source rocks, emphasizes the importance of eustatic control for the development of the source-reservoir-seal system of the North Qiangtang Basin.

Facies distribution, paleoenvironment and sequence stratigraphy model of the Oligo-Miocene Asmari Formation (Fars Province, south of Iran)

The Oligocene (Rupelian-Chattian) to Miocene (Aquitanian)-aged Asmari Formation is considered one of the most significant petroliferous reservoirs in the Middle East. The Oligocene-Miocene Asmari Formation represents diverse facies patterns in different parts of Zagros, so analysis of its facies in different provinces of the Zagros Basin is necessary. In this study, the sedimentary depositional sequences of Asmari Formation in the Firuzabad (south of Shiraz) and Bastak (west of Bandar-Abbas) sections in the Zagros Basin (Fras Platform) have been analyzed for distinguishing facies characteristics, interpreting the paleoenvironment and determining dominant depositional sequences. Based on field investigations and laboratory studies, 13 microfacies have been determined; these are organized in 3 facies associations containing outer ramp (open marine), mid ramp, and inner ramp (shoal, lagoon, and tidal flat). Sedimentary characteristics, arrangement, and comparison of horizontal and vertical facies indicated that the successions of the Asmari Formation sediments were deposited in a homoclinal ramp-like platform with a gentle slope and the deeper part of the basin located in the Firuzabad section. Analysis of detailed facies frequency showed that the outer ramp is the most frequent facies association in the Firuzabad and the inner ramp (lagoon subenvironment) is the most frequent facies association in the Bastak. Sea level variations during Oligo-Miocene time resulted in creation of two third-order depositional sequences in both sections. The depositional sequences consist of transgressive systems tract and highstand systems tract. In all the identified sequences, transgressive systems tract (TST) demonstrate retrogradational stacking pattern with deepening-upward trend while highstand systems tract (HST) indicate progradational stacking pattern with shallowing-upward trend.

Ecostratigraphic implications of a Late Palaeocene shallow-marine benthic community from the Jaintia Hills, Meghalaya, NE India

Moderately preserved shallow-marine extinct, fossil benthic community has been recovered from a sub-surface Late Palaeocene limestone cave section near Lumshnong in the Jaintia Hills, Meghalaya, NE India. The present contribution focuses on the ecostratigraphic implications of the carbonate microbiofacies based on the evaluated facies gradients. Precise field assessments and microscopic observations led to the identification of three microbiofacies: benthic foraminiferal–algal grainstone, coralline algal framestone and oolitic grainstone–packstone. The microbiofacies distinguished in the study suggest a general shallowing-upward trend from an inner shelf setting to a lagoonal–shoal environment depicting the distinct changes in the benthic community. Presence of coralline alga Distichoplax biserialis and benthic foraminifera Idalina sinjarica, Daviesina khatiyahi, Miscellanea primitiva, Rotalia trochidiformis and Vania anatolica assign the studied carbonates to Early Thanetian (SBZ 3) corresponding to the lower part of the Lakadong Limestone. In this study, ecostratigraphy has facilitated the classification of a single carbonate section corresponding to a solitary shallow benthic zone into multiple microbiofacies attributed to variable environmental depositional conditions. This clearly demonstrates its potential in improving the applicability of biostratigraphy worldwide.

Environmental evolution and geological significance of the Miocene carbonates of the Eratosthenes Seamount (ODP Leg 160)

Miocene carbonates of the Eratosthenes Seamount, located offshore from Cyprus in the Eastern Mediterranean (ODP Leg 160, Site 966), have been re-analysed. The use of SEDEX sequential extraction to evaluate the nutrient concentrations and of CT-Scan for the identification of large benthic foraminifera has resulted in a more detailed palaeoenvironmental reconstruction and an improved stratigraphic framework. Three intervals are recognised in the succession and are attributed to the lower, middle and upper Miocene, respectively. The lower Miocene interval is dominated by large benthic foraminifera and echinoids. This association colonized the top of the seamount following its uplift from deep-water to the photic zone. The middle Miocene interval is characterised by rhodoliths and corals at its base and near the top, whereas seagrass/seaweed assemblages (with epiphytic foraminifera and hooked coralline-algal crusts) dominate its central part. The upper Miocene is represented by coral reef facies near its base and lagoonal facies toward its top. The succession was deposited in an oligotrophic environment and exhibits a general shallowing-upward trend that was mainly controlled by tectonic activity related to the northward subduction of the African plate. The succession of the Eratosthenes Seamount is characterised by a diverse large benthic foraminifera assemblage in its lower part and by corals in its uppermost part, whereas corallines characterise the central interval. This trend is common in Miocene Mediterranean carbonates and its occurrence on an isolated seamount confirms its connection with the general evolution of the basin. The decline of miogypsinids and lepidocyclinids after the early Miocene is notable because the Eratosthenes Seamount was geographically close to the Indo-Pacific Ocean where both groups also thrived during the middle Miocene. This decline was probably caused by the early Miocene closure of the deep-water connection between the Indo-Pacific and the Mediterranean.

AGE AND MODE OF THE MIDDLE MIOCENE MARINE FLOODING OF THE PANNONIAN BASIN—CONSTRAINTS FROM CENTRAL SERBIA

The timing and mode of the marine flooding of the southern margin of the Pannonian basin in SE Europe is still a matter of debate. In central Serbia, integrated bio-magnetostratigraphic data and quantified high-resolution records are completely missing. Here, we provide paleoenvironmental and paleoecological constraints from the Slanci section located near Belgrade that has an excellent preservation of micro-fauna and flora, i.e., planktonic and benthic foraminifera and calcareous nannoplankton. We integrate their quantified records with sedimentological, natural gamma radioactivity and magnetic susceptibility logs and include the non-quantified records of mollusks, corals, and ostracods to reconstruct the regional depositional history. The section shows upper bathyal to outer shelf depositional settings and alternating nutrient bottom conditions. The shallowing upwards trend marked by increasing terrestrial input, attributes it to the early Highstand Systems Tract of the first marine third order sequence in this part of the Pannonian Basin. We infer that the marine flooding in central Serbia took place at an age of ∼ 14 Ma, slightly predating the Langhian/Serravallian boundary (13.82 Ma) and the Badenian Salinity Crisis (∼ 13.8–13.4 Ma) in the Central Paratethys. Our results support an eastwards directed, tectonically forced, flooding of the southern Pannonian Basin. This major paleoenvironmental turnover was forced by syn-rift tectonics in the Pannonian Basin system, which started in the late early Miocene in the Styrian Basin (SE Austria) and ended more than 3 Myr later in the late middle Miocene in the Morava depression (E Serbia).

The impact of eustatic sea-level fluctuations, temperature variations and nutrient-level changes since the Pliocene on tropical carbonate platform (Xisha Islands, South China Sea)

Sedimentological, paleontological, and geochemical data from core XK-1 in the Xisha Islands, South China Sea, allow a detailed documentation of the evolution of the reef-bank system from the early Pliocene to the late Pleistocene and of the aeolianite deposits from the late Pleistocene to the Holocene. A bank characterized by heterozoan carbonates occurred until 1.7 Ma. Between 1.7 and 0.2 Ma, the system turned into a reef dominated by photozoan carbonates. After 0.2 Ma, the aeolianite deposits, composed of heterozoan skeletal grains, occupied this area. A Pliocene to Pleistocene decrease in the abundance of planktonic foraminifera and some benthic foraminifer species generally associated with deep water settings indicates a long-term shallowing-upwards trend, resulting from the large-scale eustatic sea-level falls. Seven exposure horizons are recorded in core XK-1, due to high-frequency sea-level changes. The thickness of the reef body in the individual cycles increased after the Middle Pleistocene transition. This is interpreted as a consequence of the increase in duration of the short-term sea-level highstands. The formation of heterozoan aeolianite deposits after 0.2 Ma is probably caused by climate cooling, related with the intensification of East Asian winter monsoon. The rapid increase in coral abundance at ~1.7 Ma correlates well with a sharp decline in nutrient level, recorded by decreases in geochemical nutrient indexes values, declines in bryozoan and red algae (rhodolith) abundance, and decreases in the abundance of foraminifer species indicative of elevated nutrient-level. This correlation indicates that the carbonate factory turnovers were also affected by nutrient-level fluctuations, which could be linked to a paleoceanographic reorganization. These results demonstrate that together with sea-level fluctuations, also paleoceanographic changes had a significant effect on the Pliocene-Holocene tropical carbonate system.

Multi-proxy facies analysis of the Opalinus Clay and depositional implications (Mont Terri rock laboratory, Switzerland)

Located in NW Switzerland, the Mont Terri rock laboratory is a research facility primarily investigating the Opalinus Clay as potential host rock for deep geological disposal of radioactive waste. In the Mont Terri area, this Jurassic shale formation is characterized by three distinctive lithofacies: a shaly facies, a carbonate-rich sandy facies and a sandy facies. However, the lithological variability at dm- to cm-scale is not yet fully understood and a detailed lithofacies characterization is currently lacking. Within the present study, petrographic descriptions at micro- and macro-scale, geophysical core logging (P-wave velocity and gamma-ray density), geochemical core logging (X-ray fluorescence) and organic matter quantification (Rock-Eval pyrolysis) were combined on a 27.6 m long Opalinus Clay drillcore comprising the three major lithofacies. The high-resolution investigation of the core resulted into a refinement of the three-fold lithofacies classification, and revealed high intra-facies heterogeneity. Five subfacies were defined and linked to distinctive depositional regimes. The studied succession is interpreted as a shallowing-upward trend within a storm-wave-dominated epicontinental sea characterized by relative shallow water depths.