Oligocene Succession Research Articles

Depositional architecture of the tide-dominated late Oligocene hydrocarbon reservoirs in the Tapti-Daman block, Mumbai Offshore basin, India

The intricate interaction between tidal and river currents across a gently sloping shelf area complicates the geological interpretation of sedimentary deposits in subsurface sections. This study comprehensively analyzes the depositional framework and facies architecture in the late Oligocene siliciclastic succession of the Daman Formation, Mumbai Offshore Basin, which shows subtle changes in depositional stacking pattern in response to shoreline shifts. The late Oligocene siliciclastic succession being a major hydrocarbon producer in the Mumbai Offshore Basin; this research also aims to enhance the optimization of reserve exploitation and prospectivity evaluation. A detailed analysis was conducted using well-logs and seismic data integrated with available geological data. The Oligo-Miocene sequence demonstrates a coarsening-upward facies trend until the late Oligocene, followed by a fining-upward facies trend during the early Miocene and characterized by numerous stacked sandstone layers. The facies distribution pattern, sedimentary characteristics, and progradational geometry suggest a deltaic system dominated by tides for the late Oligocene succession and the latter part of the early Oligocene succession. The early Miocene succession shows a reversal in depositional trend with a retrogradation pattern. The late Oligocene deltaic deposits show a very high sediment load deposited during the peak river discharge from the Narmada-Tapti River system. The gentle depositional slope allowed redistribution of sediments by tidal currents. The deltaic system was further resolved into subaqueous mouth bar, delta front, and prodelta facies associations based on facies modeling. The correlation of the facies model with the regional depositional setup indicates that the late Oligocene succession corresponds to a third-order highstand system tract (HST) cycle.

Paleocene – Late Oligocene dinoflagellate cysts biostratigraphy, onshore Niger Delta

A new biostratigraphic biozonation for the Paleocene to Late Oligocene succession of the onshore Niger Delta has been developed based on the identification of significant stratigraphic dinoflagellate cyst events. This study is aimed at applying rarely used dinoflagellate cysts events in dating, correlating and inferring the paleoenvironment of penetrated sedimentary successions in the studied area. Palynological processing and analysis were carried out on two hundred and twenty ditch cutting samples recovered from two exploration wells (BN-1 and FJ-1) located in the Greater Ughelli and Northern Depobelts of the onshore Niger Delta respectively. Abundant and diverse dinoflagellate cysts recovered included marker Peridinales species Apectodinium sp., Palaeocystodinium australinum, and Gonyaulacales such as Areosphaeridium arcuatum, Cordosphaeridium gracile, Tuberculodinium vancampoae, Polysphaeridium zoharyi and Hystrichokolpoma rigaudiae. Three dinoflagellate cysts Palynological Zones - DPZ 100 (Early to Late Paleocene), DPZ 200 (Early to Late Eocene) and DPZ 300 (Late Eocene to Late Oligocene) are proposed herein. The biozones were subdivided into ten subzones based on the Top occurrence, Quantitative Tops and Quantitative base as well as ACME events of dinoflagellate cysts marker taxa which are used to date the rock sequence to be Paleocene to Late Oligocene. Abundant and diverse shallow outer neritic to inner neritic dinoflagellate cysts are identified from thick grey soft shales at the lower section of the studied wells. The upper section show significant reduction in dinocysts assemblage dominated by inner (coastal) neritic taxa. This dinocysts occurrence and distribution suggests inner (coastal) neritic paleoenvironment at the upper sections which deepened into shallow outer neritic conditions at the lower intervals of the studied wells. The thick brown to grey shales at the lower section and the medium to fine grained, moderately well sorted sandstone interbeds at the middle section of the studied wells are interpreted to be potential source and reservoir rocks respectively. These datasets are useful for correlating possible hydrocarbon source rocks and potential reservoir carrier beds in the Gulf of Guinea.

The Paleogene continental basins from SE France: New geographic and climatic insights from an integrated approach

New insights into the geodynamic evolution, the paleogeographic framework and the paleoclimatic setting of the Upper Eocene-Oligocene lacustrine basins from South-East France have been provided by an integrative approach combining geochronology, micropaleontology and stable isotope analyses. UPb dating on calcite, pollen-based climatostratigraphy and strontium and sulfur isotope signatures of sulfates provided a robust chronostratigraphic framework for the Upper Eocene and Oligocene succession from the Vistrenque, Alès and Roussillon basins. The thick Priabonian- lower Rupelian lacustrine succession from the Vistrenque Basin is interpreted to be deposited in a strike-slip basin associated with the latest stages of pyrenean shortening. Floral assemblages are used to reconstruct the vegetation cover in the lacustrine systems and their neighbouring lowlands and highlands. Composition of saltwater inferred from strontium and sulfur isotope signatures of sulfates, the occurrence of calcareous nannofossils, dinoflagellate cysts and pollen grains of mangrove-related plants provided strong evidence for marine intrusions into the Alès Basin during the Priabonian and the Vistrenque Basin during the Rupelian and Chattian. Paleogeographic reconstructions of Southeast France during the Priabonian and late Rupelian strongly suggest marine incursions from the Alpine Sea via two potential pathways: upper Rhône Valley via the Crest Sill and lower Rhône Valley via the Haut-Var Pyrenean synclines. On the other hand, Upper Oligocene evaporite basins in the Camargue area have been subjected to marine intrusions from the Western Mediterranean Sea following the collapse of the eastern pyrenean relief during the Liguro-Provençal rifting stage. Finally, pollen analysis and Climate Amplitude Method revealed a brief but significant stage of cooling and aridification at the very base of the Rupelian which could represent the regional and terrestrial expression of the global climate cooling following the Eocene-Oligocene Transition.

Microfacies and depositional environment of Bajawan and Baba Formations in Kirkuk Oil fields north Iraq

The study of six selected wells from Kirkuk oil field petrographic and physical properties ; investigation while leads to interpret the microfacies and environment for Bajawan and Baba Formations. Five carbonate microfacies distingished from Bajawan Formation ( Lime Mudstone Microfacies, Fossiliferous Lime Wackstone Microfacies, Miliolid Packstone Microfacies, Miliolid Grainstone Microfacies and Coral, Boundstone microfacies ), while Two microfacies described from Baba Formation (Limestone bearing Larger foraminifera (Lepidocylina- Nummulites) packstone-wackstone microfacies and Fossiliferous Floatstone bearing Larger Foraminifera microfacies ).
 According to different types of microfacies that described from Bajwan and Baba Formations , the depositional environment of Bajawan Formation is backreef/reef or restricted lagoon (inner ramp), but for Baba Formation is shallow water forereef (middle ramp). The facies model were constructed on basis of stratigraphy, lateral and vertical facies change and depositional environment of Oligocene succession.

Stratigraphy of the Cenozoic succession in eastern Azerbaijan: Implications for petroleum systems and paleogeography in the Caspian basin

The Cenozoic succession in the Lower Kura Basin includes largely uniform, often carbonate-free mudstones. Because age dating of these sediments has proved difficult, the stratigraphy of the Cenozoic succession and the distribution of organic-rich strata are poorly known. A better understanding of the Cenozoic succession is not only important for petroleum-systems-analysis, but also for the understanding of the Cenozoic evolution of the Caucasus region. Therefore, bulk geochemical data (carbonate, TOC, sulphur, Rock-Eval) of 885 samples and biostratigraphic data were collected along seven outcrop profiles. This enabled the establishment of a continuous stratigraphic record from the Middle Eocene to the Late Miocene (Pontian). The study results show that potential source rocks are present in three stratigraphic units: the Middle Koun (Middle Eocene), Maikop Group (Oligo-Miocene) and Diatom Formation (Upper Miocene). The Middle Koun is about 100 m thick near the Caspian Sea and contains highly oil-prone sediments (2–24 wt% TOC; HI 300–577 mgHC/gTOC), which may generate 1.5 tHC/m2. The Maikop Group contains on average 1.8 wt% TOC, but is often gas-prone. Highly oil prone layers (2–5 wt% TOC; HI 300–450 mgHC/gTOC) are rare. Nevertheless, the Upper Maikop Formation may generate ∼2 tHC/m2. The Diatom Formation contains paper shales with high TOC contents (3–22 wt%) and HI values (350–770 mgHC/gTOC). The paper shale unit is more than 60 m thick near the Caspian Sea and can generate 3.8 tHC/m2. Previously it was thought that the Maikop Group had the highest petroleum potential, whilst Upper Miocene and Middle Eocene source intervals were overlooked. The Middle Eocene to Lower Oligocene succession in the Kura Basin is largely carbonate-free. In contrast, coeval successions elsewhere in the Caucasus region contain sediments with varying, but often high carbonate contents. Differences in carbonate content imply greater water depth during deposition in the Kura Basin.

Seismic characterization analyses of the Bulgarian Black Sea shelf for assessment of reservoir properties

In the Bulgarian Black Sea shelf, several gas fields have been discovered in Maastrichtian–Paleocene sediments to date. In this study, amplitude versus offset analysis (AVO) was carried out, and the extended elastic impedance (EEI) was also used, to predict fluid and lithology properties. Thus, in these special cases, AVO (or reflection angle) can significantly de-risk the existing Paleocene prospects and allow to focus on the Miocene–Oligocene successions, where some prospects have also been identified. By examining the variations in angle amplitude (or offset), it is possible to unravel lithology and fluid effects on the top of a reservoir. Seismic modeling is used to determine what type of seismic response, in terms of AVO, to expect at the depth of interest in the study area. There are five classes or types of AVO responses, and in the study area classes 1 and 2 were defined in the Miocene–Oligocene prospects. Brine-filled reservoirs and conglomerates exhibit class 1 AVO response, whereas gas-filled reservoirs show class 2 AVO responses. EEI has the ability to give estimates for elastic parameters, such as Vp/Vs ratio, bulk modulus, shear modulus, Poisson’s ratio and other parameters. In the current study, simultaneous inversion testing was done and, analyzing all the wells together, Poisson’s ratio is the most promising fluid and lithology indicator, showing good separation between water and gas responses, although there is some overlap. Projection of EEI at an angle of 38° through brine points gives a useful separation of brine, 10% gas and 90% gas reservoirs. However, the amplitude of the resulting projection is still low, which may make it difficult to resolve fluid variation once seismic noise has been added.

Tectonic evolution of the Eocene–Oligocene Lushi Basin in the eastern Qinling belt, Central China: Insights from paleomagnetic constraints

Abstract The eastern Qinling orogenic belt, located in the conjoint region between the Neo-Tethys tectonic domain and the western Pacific tectonic domain, is key to understanding the tectono-sedimentary evolution of the East Asian interior. Extensive Late Cretaceous–Cenozoic fluvial and lacustrine deposits within the eastern Qinling orogen provide ideal archives of tectono-sedimentary evolution in Fenwei, the eastern Qinling orogen. We present an integrated rock magnetism and anisotropy of magnetic susceptibility (AMS) study of a Middle Eocene–Early Oligocene succession in the Lushi Basin, eastern Qinling region. Our rock magnetic results suggest that hematite and magnetite are the main magnetic carriers of remanence, while χ−T curves, hysteresis loops, and low values of bulk susceptibility indicate that paramagnetic minerals are major contributors to AMS in the Lushi Basin. Nevertheless, the anomalous magnetic fabrics in the Zhangjiacun and Lushi Formations may result from the contribution of the iron-bearing carbonates in deposits. The clustering of the minimum principal axes nearly vertical to the bedding plane and the well-defined NW–SE magnetic lineation almost parallel to the dip of the bounding fault suggest that the AMS of the Dayu Formation is not a sedimentary fabric but an incipient deformation magnetic fabric. The pronounced NW–SE magnetic lineation indicates the NW–SE stretching of the Lushi Basin during the Late Eocene–Earliest Oligocene, which may have been caused by the combined effects of the India–Eurasia collision and the subduction of the western Pacific plate.

Glauconite authigenesis during the warm climatic events of Paleogene: Case studies from shallow marine sections of Western India

Glauconite forms abundantly within the Paleogene warm climatic intervals. However, the role of warm climate on glauconitization is yet to be explored. Glauconitic shales are ubiquitous in transgressive shallow marine deposits of Cambay, Kutch, Jaisalmer, and Barmer basins at the western margin of India. Although the glauconite is most abundant in upper Paleocene-lower Eocene sedimentary deposits in these basins, it also occurs within the middle Eocene and upper Oligocene successions. High-resolution biostratigraphic data, integrated with carbon isotope signatures, demarcate the Paleogene hyperthermal events and reveal an exceptionally high abundance of glauconite corresponding to the warming events. Glauconite occurs as pellets and infillings within the pores of bioclasts, although they differ in chemical composition. The glauconite pellets show variable K2O content, ranging from 4 to 8 wt% with moderately high Fe2O3 (>20 wt%), representing the entire maturation spectrum. Glauconite marking the Paleocene-Eocene transitional sediments is distinctive by the high Al2O3 content (>10 wt%), while those within the middle Eocene and Late Oligocene show considerably low Al2O3 (<8 wt%). The glauconitic shales often show minor bioturbation and are rich in rectilinear benthic foraminifera, indicative of the oxygen-depleted bottom-water conditions. The unusual composition of upper Paleocene-lower Eocene glauconites relates to their formation within kaolinite substrates during the extremely warm climatic interval. Contrary to this, during the middle Eocene and Late Oligocene, the waning phase of Paleogene warm climatic conditions, glauconite formed by the initial authigenic precipitation of Fe-smectite/ Fe-Al-smectite and its subsequent maturation. The warm climatic condition enhanced the precipitation and runoff, which supplied enhanced nutrients including K, Fe, Al, Si, and Mg into the shallow marine environment, facilitating prolific organic growth and enriching the seawater with cations. The decomposition of organic matter might have resulted in an oxygen-depleted bottom water condition, which was suitable for the mobility and fixation of iron into the glauconite structure. The glauconite formed abundantly during hyperthermal events because of the convergence of favorable factors such as rapid transgression, reduced sedimentation rate, warm seawater condition, enhanced continental weathering, and enhanced supply of nutrients favoring dysoxic shallow shelves. However, rapid and extreme hyperthermal events such as PETM inhibits glauconite formation.

Digital geologic map, petrography and U-Pb geochronology of the Río Santiago Shear Zone, southernmost Sierra Madre Occidental, western Mexico

The boundary between the Sierra Madre Occidental (SMO) and the Trans-Mexican Volcanic Belt (TMVB) is well exposed along the Santiago River, on the border of the states of Nayarit and Jalisco. This region includes a polydeformed belt produced by the relative motion between the Jalisco Block and the SMO, here defined as the “Rio Santiago Shear Zone” (RSSZ). In this work, we present detailed geological cartography, petrographic descriptions, and new zircon U-Pb ages that allow us to establish the stratigraphy of the region surrounding the RSSZ, providing temporal constraints to its tectonic evolution. In the mapped area, the base of the SMO stratigraphic column consists of a succession of massive andesitic lava flows intruded by felsic dikes and a felsic subvolcanic body of 27.5 and 27.2 Ma, and it is covered by a series of ignimbrites of 26.8 and 25.3 Ma. A red beds succession separates a second package of ignimbrites of ∼20-19 Ma, in turn capped by rhyolitic lava domes of 19.7 and 18.6 Ma and a rhyodacitic lava flows of 15.6 Ma, this Miocene succession is also intruded by a felsic body of 18.3 Ma west of the La Yesca dam. The SMO succession is covered by rocks belonging to the bimodal volcanism of the TMVB, emplaced between the end of the Miocene and the Pleistocene. The RSSZ developed through several deformation phases that occurred since the end of the Oligocene. The Oligocene succession of the SMO is affected by NE-SW and WNW-ESE trending lateral faults, NW-SE open folds and WNW-ESE and E-W trending reverse oblique faults. Some of the reverse oblique faults were later reactivated as normal faults. WNW-ESE trending normal faults associated with the Tepic-Zacoalco Rift affected the area south of the Santiago River during the Pliocene and Pleistocene. Normal faulting produced an accumulated vertical offset up to 700 m between the northern and southern regions of the Santiago River.

Towards deciphering the Cenozoic evolution of the East Pisco Basin (southern Peru)

ABSTRACT The Cenozoic succession of the East Pisco Basin preserves the sedimentary record of several episodes of deformation of the forearc crust along the Peruvian margin. The 1:50,000 scale geological map presented here encompasses an area of about 1,000 km2 lying astride the Ica River, and contributes to our understanding of the timing and mode of basin filling and deformation. Our novel two-fold megasequence framework provides a sound basis for establishing a first-order tectono-stratigraphic setting of the mid-Eocene–upper Miocene succession exposed in the study area. We interpret that the mid-Eocene to lower Oligocene succession studied in this work (megasequence P) was deposited in a single forearc basin, which was dissected into the present-day West and East Pisco basins by a fault-bounded basement high during the late Oligocene, and subsequently overlain by the Miocene fill of the East Pisco basin (megasequence N).

Microfacies Analysis and Paleoecology of the Oligocene Succession in a Central Tethyan Carbonate Platform, Zagros Basin, SW Iran

AbstractTwo stratigraphic sections (Arjooieh and Firoozabad) of the Mymand anticline, located in the Interior Fars sub‐basin of the Zagros Mountains, were measured and sampled, in order to document sedimentological characteristics, microfacies types and paleo‐seagrasses indicators of the Oligocene succession (Asmari Fm.). Planktonic and benthic foraminifera and coralline red algae are the principal fossils from these strata. Foraminifera are represented by the following families: Soritidae, Peneroplidae, Austrotrillinidae, Alveolinidae, Planorbulinidae, Discorbidae, Lepidocyclinidae, Amphisteginidae, Rotaliidae, Nummulitidae and Globigerinidae. Nine microfacies types were recognized, namely planktonic foraminifera‐peloidal packstone (MF1), bioclast nummulitid/Nerorotalia/Amphistegina packstone‐grainstone‐rudstone‐floatstone (MF2), Neorotalia‐echinoid coralline red algae packstone‐grainstone (MF3), coral boundstone (MF4), coral/coralline red algae rudstone‐floatstone‐packstone‐grainstone (MF5), diverse imperforate foraminifera bioclast packstone‐grainstone (MF6), peloid wackestone‐packstone‐grainstone (MF7), fenestrated mudstone and microbial mats (MF8) and anhydrite (MF9). MF1 indicates an outer ramp, MFs 2‐4 represent a mid‐ramp and MFs 5–9 are interpreted as inner ramp environment. Paleo‐seagrass indicators consisting of foraminifera, hooked and tabular forms of coralline red algae and corals. They were identified in MFs 5 and 6, reflecting the presence of vegetated environments within the mid/inner ramp setting. The Mymand anticline was dominated by the outer ramp environment at the start of the Rupelian. Mid to inner ramp environments prevailed during the Rupelian. The Chattian corresponds to the spread of the inner ramp setting over the Mymand anticline.

Oligocene larger benthic foraminifera and sedimentation of the Burdur Basin, SW Anatolia, Turkey

This study focuses on the Oligocene successions that contain predominantly siliciclastic, limestones with larger benthic foraminifera in the Burdur Basin of southwest Anatolia, Turkey. The Burdur-1 and Burdur-2 sections in the Oligocene deposits are described sedimentologically and were sampled for lithology, biostratigraphy, microfacies analysis and fossil content. Larger benthic foraminifera were mainly recovered from limestones in the upper part of the Burdur-2 section. Thin-section analysis of the larger benthic foraminiferal assemblage reveals that the upper part of the Burdur-2 section includes Nummulites fichteliMichelotti, 1841, Operculina complanata (Defrance, 1822), Nephrolepidina praemarginata (Douvillé, 1908), Eulepidina sp., Planorbulina sp., Amphistegina sp., and Asterigerina sp. This foraminiferal association represents the Shallow Benthic Foraminifera Zone 22 (SBZ 22) of Rupelian-early Chattian age. Sedimentological analysis in the Burdur-1 and Burdur-2 sections has revealed an association of massive-unorganized conglomerate, parallel-bedded sandstone, massive mudstone, graded-massive conglomerate, graded-massive sandstone, well-sorted organized conglomerate, well-sorted parallel-bedded sandstone and microfossil-bearing parallel-bedded limestone facies. Alluvial fan /river, fan-delta, beach and shallow shelf carbonate facies communities were identified by lateral and vertical correlation of facies. It is concluded that the large Burdur Fan developed in a graben basin that opened and was flooded by a branch of the Tethyan Ocean that invaded southwest Anatolia during the Oligocene.

Microfacies Analysis and Paleoecology of the Oligocene Succession in a Central Tethyan Carbonate Platform, Zagros Basin, SW Iran

Microfacies Analysis and Paleoecology of the Oligocene Succession in a Central Tethyan Carbonate Platform, Zagros Basin, SW Iran

Sequence stratigraphic framework of the Upper Eocene–Oligocene succession, northwest Birket Qarun, Fayum-Egypt

The Upper Eocene–Oligocene sequence exposed northwest Birket Qarun; Fayum district covers two rock units, Qasr El-Sagha (Temple and Dir Abu Lifa members) and Gebel Qatrani formations were re-evaluated by sequence stratigraphy. Six depositional sequences were recognized based on facies geometry, and sedimentological aspects. The first sequence (Temple Member) was accumulated under marginal intertidal flat environment indicating highstand deposits. The second (the lower Dir Abu Lifa Member) displays a regressive sedimentary facies starting with progradational delta front environment reflecting lowstand deposits followed upward by a lagoonal environment indicating transgressive/highstand deposits. The third (the top of the Dir Abu Lifa Member) was accumulated under a low sinuosity stream reflecting lowstand deposits. The fourth which constitutes the topmost of Dir Abu Lifa Member initiating near-shore shallow marine points to transgressive sediments, followed upward by fluvial sediments reflecting highstand deposits. The fifth (the Lower of Gebel Qatrani Formation) accumulated as point-bar deposits demonstrating lowstand deposits. The last occupies the main of Gebel Qatrani Formation initiates with restricted riverine marking lowstand deposits, followed upward by several fining upward cycles that were accumulated in fluvial and flood plain conditions with evidence of tidal influence on sedimentation near its top denoting transgressive deposits. Several minor transgressive episodes accompanied by tidal influence on sedimentation are noticed near the top, which may reflect gradual subsidence of the Oligocene alluvial plain. The area is subjected to relative sea-level changes that show a similarity with the global sea level. The base-level fluctuations are mainly controlled by the regional tectonic setting.

A new stratigraphic framework for the Miocene – Lower Pliocene deposits offshore Scandinavia: A multiscale approach

Here, we present an updated stratigraphic subdivision of the Oligocene to Pleistocene succession (880–610 m) in the newly proposed type well for the Molo Formation, the industrial 6407/9‐5 well, located on the continental shelf in the eastern Norwegian Sea. Furthermore, new data from the Danish North Sea wells Nora‐1, Vagn‐2, and Tove‐1 are presented. The studied succession in the 6407/9‐5 well is composed of five sedimentary units separated by hiati. The dating of these five units is based on correlation to the stratigraphically more complete Neogene succession in the (Danish) central North Sea area. In this study, a robust stratigraphic framework of these five units, based on a combination of dinoflagellate cysts (dinocyst) stratigraphy and seismic data, is established. The Oligocene succession is referred to the NSO zonation of Van Simaeys et al., Review of Palaeobotany and Palynology, 2005, 134, 105–128, while the Miocene–Pliocene succession is referred to the dinocyst zonation of Dybkjær and Piasecki, Review of Palaeobotany and Palynology, 2010, 161, 1–29. In well 6407/9‐5 the two lowermost units, located below the Molo Formation, comprise a Rupelian (Lower Oligocene) succession up to 803 m, and an Aquitanian/Burdigalian (Lower Miocene) succession from 803 to 787 m, respectively. Both of these units are referred to the Brygge Formation. The Molo Formation is separated from the underlying Brygge Formation by an unconformity. Furthermore, and in contrast to previous studies, our study shows that the Molo Formation (787–703 m) has an unconformity within it. The lower part of the formation (787–724 m) is dated to late Tortonian (Late Miocene), and referred to the Hystrichosphaeropsis obscura dinocyst Zone. The upper part (724–703 m) is dated to Zanclean (Early Pliocene), and referred to the Melitasphaeridium choanophorum dinocyst Zone. The uppermost unit studied (703–670 m) in the well is referred to the Gelasian (Lower Pleistocene) and is included in the Naust Formation. The regional correlation of this stratigraphy with the complete succession in the Danish North Sea reveals that the hiati found in the Miocene succession on the Norwegian Sea shelf are controlled by tectonism, while the internal depositional patterns of the Molo Formation were controlled by eustatic sea‐level changes.

Two new dinoflagellate cyst species and their biostratigraphical application in the Eocene and Oligocene of the North Sea

The Cenozoic of the North Sea is among the best-documented stratigraphical successions in the world, and multiple palynological events have been recognised for chronostratigraphical control across the region. The ever-increasing number of wells studied for hydrocarbon exploration and production results in the generation of new biostratigraphical data that constantly increase our palynological knowledge of the area. Here we describe two new dinoflagellate cyst species from an Lower Eocene (Ypresian) to Lower Oligocene (Rupelian) succession in Gannet Field (UK Central North Sea). These are Reticulatosphaera valdereticulata sp. nov., a short-lived Rupelian index taxon, and Alisocysta heilmannii sp. nov., previously informally known as Alisocysta sp. 2, which is an Ypresian marker widely used by biostratigraphers working the North Sea region. The development of a dense network of trabeculae connecting the processes distally allows Reticulatosphaera valdereticulata sp. nov. to be clearly distinguished from the closely similar Reticulatosphaera actinocoronata. The main diagnostic feature in Alisocysta heilmannii sp. nov. is the development of delicate penitabular septa. Both species show widespread palaeogeographical distribution across the North Sea region. We also document the diverse palynofloras in which the two new species are encountered and discuss biostratigraphical application and palaeoenvironmental settings.

Slope destabilization provoked by dissociation of gas hydrates in the Outer Carpathian basin during the Oligocene: Sedimentological, petrographic, isotopic and biostratigraphic record

Sedimentological, biostratigraphic, petrographic, and stable C and O isotope study was carried out on chaotic complexes hosting carbonate concretions in the uppermost Oligocene successions of the Outer Carpathians. These chaotic complexes reveal a range of sedimentary features indicative of intrabasinal, submarine mass-wasting deposition, including soft-sediment deformation, brecciation, homogenization and fluidization of the parent material. Apart from carbonate concretions, they enclose blocks of rocks having lithologies typical for the underlying Oligocene strata. The concretions exhibit extremely depleted δ13C (by down to −51.3‰) indicating anaerobic oxidation of methane, whereas their δ18O values are strongly enriched relative to coeval brackish basin water indicating dissociation of gas hydrates as the main fluid source. These data and the geotectonic context imply that the chaotic complexes formed as submarine slumps on a N-inclined paleoslope of the basin in consequence to tectonically-induced dissociation of gas hydrates. Hydrates were destabilized within the sediments due to synorogenic uplift, related to northward progradation of the accretionary prism. These slumps and hosted concretions were dated by the combination of calcareous nannoplankton and organic-walled dinoflagellate cysts. The age of concretions indicated by dinoflagellate cyst assemblages always overlaps with that of the surrounding slump matrix determined by both micropaleontological groups. Together with sedimentological and petrographic evidence, this confirms that the concretions formed in situ within the sediment shortly after deposition and prior to slumping. Hydrate dissociation, seepage of methane-charged fluids and the resultant slope destabilization was not an isolated event, but a continuous process occurring diachronously in front of N-prograding accretionary prism during Oligocene, at least from early NP23 to late NP25 biozones.The stable C and O isotope analysis of carbonate concretions was crucial for this research, because the low δ13C and high δ18O values were the only preserved indicators of anaerobic oxidation of methane and dissociation of gas hydrates in the basin, respectively. Because, the concretions are composed of a mixture of dolomite and calcite, a semiautomatic method for measuring C and O isotope composition selectively for calcite and dolomite (Baudrand et al., 2012) was applied, which provided conclusive results. These measurements were combined with detailed petrographic investigations, which enabled assessment of isotopic composition for various carbonate constituents separately, e.g. sedimentary calcite (biogenic and detrital), early and late calcite cements, dolomite cement. They showed that the concretionary microcrystalline calcite was the main cement that precipitated due to hydrate dissociation and subsequent methane oxidation. This approach can provide valuable information when applied to other authigenic carbonates composed of a mixture of dolomite and calcite.

Depositional and erosional features of Oligocene paleokarsts in the Kirkuk Group, Kurdistan Region, Northeast Iraq

Paleokarst features are rarely preserved in geologic history due to terrestrial erosion and later burial diagenetic changes. In rare cases, where karstification was intense it can be preserved by burial in areas where erosion was slow. In Iraq, one of the ages that preserved clear features of the paleokarsts is the Oligocene. During this age, a thick succession of shallow (platform) carbonates were deposited and assigned stratigraphically as Kirkuk Group. During the late Oligocene, the inner shelf of the platform exposed for long time and an unconformity was developed and associated with well-developed epi- and endokarst features. The present study focused on the documentation of the erosional and depositional features of paleokarstification during the latter age for the first time in Iraq and Zagros Orogenic Belt. In the northeast Iraq, many Oligocene paleokarst features are discovered in the medium beds of Anah and Bajwan Formations (as two units of the Kirkuk Group) in the Sulaimani and Erbil Governorates. These features are such as vertical and inclined shafts filled with brown pebbly sandstone, horizontal cavities, large planoconcave cavities filled with conglomerates, solution breccias, underground solution networks, travertine, and palaeosoil. All these features are discussed to prove their attribution to the Oligocene by highlighting many field and petrographic evidences. The documented paleokarst can indicate accurately depositional extent, paleogeography, and paleoshoreline of Oligocene succession in Iraq and part of Western Zagros. Therefore, this study is significant for paleo-mapping of the northwestern margin of the Arabian plate and applicable for paleoenvironment concerns and oil explorations in the studied area.

Source rock potential and depositional environment of the Lower Oligocene İhsaniye Formation in NW Turkey (Thrace, Karaburun)

The Lower Oligocene succession of the Ihsaniye Formation, exposed at Karaburun on the Turkish (Thrace) coast of the Black Sea, provides insights into potential hydrocarbon source rock extent in the southwestern part of the Black Sea Basin. Presently, no detailed study of the Ihsaniye Formation exists with regards to organic matter content and type. We analyzed 78 fine-grained samples from a c. 70-m-thick section near Karaburun. Relatively high total organic carbon (TOC) contents (average 1.45 wt.%) are present in the lower part of the Ihsaniye Formation; the hydrogen index values reach a maximum of 252 mg HC/g TOC, indicating the presence of type III-II kerogen, and the organic matter is thermally immature (Tmax 418 °C). The presence of foraminifera and TOC/S ratios below 2.8 confirm a fully marine environment. Elevated TOC contents and very low pristane/phytane ratios indicate an oxygen-depleted environment during deposition of the lower part of the Ihsaniye Formation. The quartz/clay minerals ratio suggests a deepening trend in the lower part of the succession and a shallowing trend in the upper part. Peculiarly, the low salinity Solenovian Event (in nannofossil zone NP23) is not developed although Sr isotope ratios indicate that the basin was only connected to the Mediterranean for short periods during deposition of the Ihsaniye Formation.

Biostratigraphy and paleoenvironments of the Oligocene succession(İhsaniye Formation) at Karaburun (NW Turkey)

The biostratigraphy and paleoenvironments of the Ihsaniye Formation exposed at Karaburun in northwest Turkey is described based upon the study of abundant and well-preserved foraminifera, calcareous nannofossils and palynomorphs. The studied succession is Early Oligocene in age, with calcareous nannofossil zones upper NP21 (Subzone CNO1) to lower NP23 (Subzone CNO3) and planktonic foraminifera zones O1 (~P18) and O2 (~P19) represented, and palynological assemblages suggestive of zones D13 to D14a. Based on these new data, a revised interpretation of the stratigraphic succession is presented. Deposition was controlled by a now inverted normal fault, with deposition of older stratigraphy (upper NP21 to NP22) restricted to the original hanging wall. During NP23, deposition commenced on the footwall, resulting in progressive onlap of an exposed Eocene reefal limestone (Sogucak Formation). Three primary sedimentary facies are present: marls with thin calcareous siltstones, marls with synsedimentary slumps and debris flows, and coarse pebbly sandstones. The coarse pebbly sandstones were deposited in a fan-delta/shoreface paleoenvironment and represent the initial phase of onlap during biozone NP23 onto a rocky shoreline on the footwall side of the fault. The marl-dominated facies represent deposition in outer shelf?upper bathyal environments. The succession demonstrates evidence for a near-end Eocene relative sea-level fall. Changes in the abundance of planktonic foraminifera and the onlap onto the footwall demonstrate maximum subsequent transgression within NP23. This reflects eustasy rather than Paratethyan relative sea-level. No interpretation of sea-water salinity reduction can be made for the sediments deposited during biozone NP23 in the studied sections, although this is noted in coeval sediments in parts of Paratethys (the Solenovian Event). Together with the open marine nature of the diverse and abundant fossil assemblages, it is suggested that deposition of the Karaburun section was strongly influenced by a connection to the global ocean, via the Catalca Gap, as suggested in a recent study.