Late Early Oligocene Research Articles

Early Eocene Metasequoia and Taxodium (Cupressaceae) from Lingbao, Henan Province and their implication for a perhumid climate in central China

In this study, fossil leaves of Metasequoia Hu and Cheng, along with leaves and cones of Taxodium Richard, from the lower Eocene of the Xiangcheng Group in Lingbao, Henan Province, central China, were examined. These fossils have been taxonomically assigned to Metasequoia sp. (cf. M. glyptostroboides Hu and Cheng) and Taxodium dubium (Sternberg) Heer. The closest modern relatives are Metasequoia glyptostroboides and Taxodium distichum (Linnaeus) Richard, which typically thrive in humid environments, indicating that similar conditions may have prevailed in early Eocene. Through the Species Distribution Model (SDM) experiment, it was suggested that T. distichum is significantly influenced by precipitation levels, highlighting the importance of moisture in their historical distribution. Moreover, the analysis of the survival environments of native Metasequoia and Taxodium indicates that the annual precipitation in Lingbao during the early Eocene exceeded 1000 mm, which is roughly double the current annual precipitation in the area. This precipitation level is comparable to the present-day subtropical climate zone. During the late Eocene and early Oligocene, the seasonal variation in precipitation was intensified due to the strengthening of the East Asian monsoon, which constrained the seed germination and seedling survival. This may have contributed to the decline in the populations of Metasequoia and Taxodium in East Asia during that period.

Fish otoliths from the basal Oligocene Viborg Formation in Denmark

The Viborg Formation is the lowest part of the early Oligocene in Denmark and represents the upper part of nannoplankton zone NP21. Here we describe a small assemblage of otoliths from the Viborg Formation obtained from five localities that yielded 88 specimens of nine species. The composition of the otolith-based f ish species is transitional between the terminal Eocene (Priabonian) faunas from the Latdorf Formation of northern Germany and its equivalents and the middle and upper Rupelian. The Viborg otolith association witnessed the first arrivals of the subsequently dominant Gadidae (Trisopterus elegans) and the migration of certain species of the merlucciid genus Palaeogadus from the Eastern Paratethys through the Polish gateway. Furthermore, the otoliths from the Viborg Formation f ill an important gap in the previous knowledge of late Eocene and early Oligocene otolith associations in the North Sea Basin and thus facilitate some refinement of their biostratigraphic usage.

An updated chronostratigraphic framework for the Cenozoic sediments of southeast margin of the Tibetan Plateau: Implications for regional tectonics

As one of the most important accommodation zones during the India-Asia collision, the southeast margin of the Tibetan Plateau (SEMTP) is characterized by many large-scale strike-slip faults, a regional low-relief relict that has been deeply incised by several continental scale rivers that originate from central Tibet, and numerous plant and vertebrate fossil biotas residing in Cenozoic sedimentary basins. Therefore, the SEMTP not only provides a meaningful constraint on the geodynamic evolution of the Tibetan Plateau, but also serves as an exemplar to understand the various interactions among surface uplift, drainage network reorganization, climate change, and biodiversity. Precise dating of Cenozoic sediments in the basins of the SEMTP is crucial for understanding these processes. Previously, the geochronologic framework of Cenozoic sediments in the SEMTP largely relied on fossils and regional stratigraphic correlations, which often involved circular reasoning and large uncertainties. In the past two decades, high-resolution magnetostratigraphy, detrital zircon geochronology, and isotopic dating of interbedded tephra layers have been carried out on SEMTP Cenozoic sedimentary basins in Yunnan and eastern Xizang provinces, SW China. Using these new data, we build a revised chronostratigraphic framework for Cenozoic sediments in SEMTP, and use this updated temporal framework to contextualize regional tectonics and climate change. The new chronostratigraphic framework shows that (1) many of the “Neogene” sedimentary basins based on plant fossils and regional lithostratigraphic correlation in the SEMTP were actually formed in the late Eocene and early Oligocene; (2) most of the Paleogene sedimentary successions in the SEMTP ended at the late Eocene-early Oligocene and were unconformably overlaid by Middle-Late Miocene sediments. Oligocene-Early Miocene sediments are often regionally absent. Sedimentary structures within the basin successions show that Paleogene basins were generally formed in compressional settings while the Late Miocene basins were mostly developed in extensional settings. The new chronostratigraphic framework, together with analyses of sedimentary basin structures, suggests that the SEMTP began to experience crustal shortening early in the India-Asia collision (∼50 Ma), producing Paleogene sedimentary basins and giving rise to the subsequent widespread surface uplift and regional erosion in the Oligocene-Middle Miocene. Tectonic inversion from compression to extension since the Middle Miocene precludes significant uplift of the SEMTP after this time, which may be related to geodynamic changes in the evolution of the Tibetan Plateau.

Marsupials (Herpetotheriids) from the late Palaeogene of south-east Serbia

Three marsupial species are present in Palaeogene assemblages from south-eastern Serbia, Amphiperatherium minutum (Aymard, 1846), A. exile (Gervais, 1848–1852) and a species of Peratherium Aymard, 1850. These species are common in the late Eocene and early Oligocene of Western and Central Europe and their presence in South-Eastern Europe indicate that by the end of the Eocene, the eastwards dispersal of these marsupial species was already a fact. The presence of the same marsupial species in Serbia and in Western Europe is surprising, as the difference in composition between the Serbian and Western European rodent faunas indicates a limited faunal exchange between these areas in the late Eocene and early Oligocene. In marsupials, the stylar shelf of the upper molars shows a large morphological variation. A lesser-known variation is the presence of a transverse crista in the stylar shelf. These occur in various marsupial species, but so far were not known to be present in Amphiperatherium minutum. In our opinion, the large morphological variation of the stylar shelf is thus better not used to define a new species.

Northward expansion of Cenozoic Asian humid climate recorded by sporopollen

The distribution pattern of arid–humid climate in the modern northwest–southeast confrontation in Asia evolved from the zonal distribution pattern of the Paleogene. However, this evolutionary process remains poorly constrained by geological evidence. In this paper, we analyze the characteristics of sporopollen assemblages recorded in Late Eocene strata of the Jianchuan Basin in southwest China. We show that vegetation patterns exhibited a remarkable shift from tropical–subtropical sparse forest to subtropical–temperate mixed broad-leaved–coniferous forest at ∼40.6 Ma; this shift is further indicated by the rapid decrease in drought-tolerant plants, mostly Ephedra and Chenopodiaceae, evidence that the previous hot–dry climate was replaced by a warm–humid climate at this time. This evidence suggests that the Asian monsoon began to affect the study area at ∼40.6 Ma. We then comprehensively integrated sporopollen results from several basins across East Asia, revealing that the frontal edge of the humid monsoon climate moved intermittently to the northwest after the Late Eocene, and may have advanced to the Lunpola–Lanzhou line by the early Late Oligocene, this being similar to the position of the leading edge of the modern monsoon. Moreover, the frontal edge of the humid monsoon climate may have extended as far as the Qaidam Basin during the Middle Miocene, but then gradually retreated to the position of the modern monsoon front. The uplift of the Tibetan Plateau and global climate change may have played dominant roles in this evolution.

Phylogeny and paleobiogeography of the enigmatic North American primate Ekgmowechashala illuminated by new fossils from Nebraska (USA) and Guangxi Zhuang Autonomous Region (China)

Ekgmowechashala is a poorly documented but very distinctive primate known only from the late early Oligocene (early Arikareean) of western North America. Because of its highly autapomorphous dentition and spatiotemporal isolation, the phylogenetic and biogeographic affinities of Ekgmowechashala have long been debated. Here, we describe the oldest known fossils of Ekgmowechashala from the Brown Siltstone Beds of the Brule Formation, White River Group of western Nebraska. We also describe a new ekgmowechashaline taxon from the Nadu Formation (late Eocene) in the Baise Basin of Guangxi Zhuang Autonomous Region in southern China. Phylogenetic analysis suggests that North American Ekgmowechashala and the new Chinese taxon are sister taxa that are nested within a radiation of southern Asian adapiforms that also includes Gatanthropus, Muangthanhinius, and Bugtilemur. The new Chinese ekgmowechashaline helps fill the considerable disparity in dental morphology between Ekgmowechashala and more primitive ekgmowechashalids known from southern Asia. Our study underscores the fundamental role of southern Asia as a refugium for multiple primate clades during the cooler and drier climatic regime that prevailed after the Eocene–Oligocene transition. The colonization of North America by Ekgmowechashala helps define the beginning of the Arikareean Land Mammal Age and corresponds to an example of the Lazarus effect, whereby a taxon (in this case, the order Primates) reappears suddenly in the fossil record after a lengthy hiatus.

New evidence for late Eocene-early Oligocene uplift of Aves Ridge and paleogeography of GAARlandia

The GAARlandia hypothesis has produced vigorous debate among biologists regarding whether now-submerged landforms that existed in the Caribbean region during the late Paleogene might have acted as a barrier for marine organisms and as a bridge for terrestrial biotas migrating from South America into the Greater Antilles. This concept derived from the hypothesized emergence history of the Aves Ridge. In the quarter century since GAARlandia was first proposed, new paleontological, geological and geophysical information has greatly extended the database available. Here we reaffirm that GAARlandia was a positive topographic feature from middle Eocene, and was exposed above sea level between late Eocene and early Oligocene when it facilitated biotic colonization of the northern Greater Antilles and their satellite islands, whether as a series of closely spaced islands or as a continuous peninsula projecting from northeastern South America along the crown of the rise.

Multiple phases of deformation that shaped the southern Central Andes (36.5°S) from crosscutting relationships with Cenozoic retroarc magmatism

The Southern Central Andes developed though a complex succession of magmatic and deformational episodes, which after more than a century of studies, are still the subject of intense debate. One of the main controversies lies in the ambiguity regarding whether there was a single contractional phase in the Neogene or whether there were multiple contractional phases distributed over the last ∼110 My. We present 33 new K/Ar ages obtained from retroarc subvolcanic intrusives and lava flows and document their crosscutting relations with the host rock strata. These data enabled us to constrain the deformation's timing in several contractional structures along the eastern slope of the southern Central Andes ∼36.5°S. In the hinterland, the timing of compressional deformation has been constrained to the following: between Late Cretaceous and late Miocene age determined by Neogene dikes crosscutting Mesozoic folded strata; pre to syn-late Miocene determined by dikes intruded in the axial surfaces of small anticlines; and post-early Oligocene determined by folded sills. In the foreland, the timing of compressional deformation has been constrained to between the Late Cretaceous and early Oligocene by dikes crosscutting pre-deformed strata; to the post-Oligocene by folded sills; to the pre- and post-middle Eocene by Eocene to Miocene dikes crosscutting older folded strata; and to the post to syn-middle Miocene by folded lava flows. We conclude that two pre-Neogene and one Neogene contractional phases, and various retroarc magmatic events have affected this segment of the retroarc. We discuss our observations in relation to previous proposals, separating the tectonic evolution of the area into six tectonic scenarios from the late Early Cretaceous to the present.

Middle Cenozoic micromammals from Linxia Basin, Gansu Province, China, and their implications for biostratigraphy and palaeoecology

We report late Oligocene to late Pliocene micromammal assemblages from 19 localities of the Linxia Basin of Gansu Province, China. Abundant material (> 7000 specimens) includes members of Eulipotyphla, Chiroptera, Rodentia and Lagomorpha, and comprises 121 species arranged in 24 families. The Yagou assemblage (basal part of the Jiaozigou Formation) is one of the few early Late Oligocene micromammalian faunas known from China. The Galijia (Dongxiang Formation) and Yihachi (“Liushu” Formation) assemblages represent the richest and most productive middle and late Miocene micromammal faunas known from northwestern China. These middle Cenozoic faunas show a closely similar community structure and taxonomic composition, and a similar pattern of faunal turnover, with contemporary faunas throughout the Mongol-Xinjiang Region, implying the existence of a unified biogeographic zone. The discoveries contribute to the temporal ordering of individual fossil assemblages of northern China and help develop a framework for Cenozoic biostratigraphy and biochronology in central Asia. Our palaeoenvironmental analysis suggests change towards warmer and moister equable habitats of the late Oligocene to early and middle Miocene, followed by the onset of drier temperate shrub-steppe environments. The composition of extant families of micromammals that developed during the Neogene reflects a faunal distribution analogous to the Holarctic Region today and suggests a shared ecology.

Proxy‐Model Comparison for the Eocene‐Oligocene Transition in Southern High Latitudes

AbstractThe Eocene‐Oligocene transition (EOT) marks the shift from greenhouse to icehouse conditions at 34 Ma, when a permanent ice sheet developed on Antarctica. Climate modeling studies have recently assessed the drivers of the transition globally. Here we revisit those experiments for a detailed study of the southern high latitudes in comparison to the growing number of mean annual sea surface temperature (SST) and mean air temperature (MAT) proxy reconstructions, allowing us to assess proxy‐model temperature agreement and refine estimates for the magnitude of the pCO2 forcing of the EOT. We compile and update published proxy temperature records on and around Antarctica for the late Eocene (38–34 Ma) and early Oligocene (34–30 Ma). Compiled SST proxies cool by up to 3°C and MAT by up to 4°C between the timeslices. Proxy data were compared to previous climate model simulations representing pre‐ and post‐EOT, typically forced with a halving of pCO2. We scaled the model outputs to identify the magnitude of pCO2 change needed to drive a commensurate change in temperature to best fit the temperature proxies. The multi‐model ensemble needs a 30 or 33% decrease in pCO2, to best fit MAT or SST proxies respectively. These proxy‐model intercomparisons identify declining pCO2 as the primary forcing of EOT cooling, with a magnitude (200 or 243 ppmv) approaching that of the pCO2 proxies (150 ppmv). However individual model estimates span a decrease of 66–375 ppmv, thus proxy‐model uncertainties are dominated by model divergence.

Magnetostratigraphy of the Oligocene and Miocene of the Linxia Basin, northwestern China

The Cenozoic biochronology framework for northern China has been based on few vertebrate fossil localities with unverified age constraints. The Linxia Basin, located on the northeastern margin of the Qinghai-Tibetan Plateau, northwestern China, preserves thick, successive terrestrial sequences with an enormous quantity of vertebrate fossils of ages spanning the Late Oligocene to Early Pleistocene. This record has bearing on the definition of Cenozoic mammalian biochronology of North China, and its better dating. However, our understanding of detailed mammalian evolution and infilling history of the Linxia Basin has been hampered by poor age constraints and disagreement with new paleontological findings. To clarify the age range of the fluvial-lacustrine deposits and their faunas in the lower part of the succession, we present here a revised magnetostratigraphy and its chronology obtained from three well-correlated and overlapping sections in the eastern Linxia Basin. Magnetite and hematite were identified as the main ChRM (characteristic remanent magnetization) carriers in the sediments. Reliable specimens from 1142 sampled layers representing a total thickness of 484.4 m from the three sections yielded 28 normal and 23 reversed polarities. With the aid of updated biochronological constraints, the fluvial-lacustrine sequence of five stratigraphic units (Tala, Jiaozigou, Shangzhuang, Dongxiang, and Liushu/Hujialiang formations), was successfully correlated to chrons of the GPTS (geomagnetic polarity timescale). These represent the top of Chron C13r (ca. 33.8 Ma) to lower Chron C5n.2n (ca. 10.8 Ma). The recalibrated basin-infilling history shows response to episodic growth of topographic highs during these time intervals. Moreover, this new age model yields an estimated age of ca. 27 Ma (within Chron C9n of early Late Oligocene) for the paracerathere fauna from the eastern Linxia Basin, which is in accordance with other biochronological findings. The new age estimate for the paracerathere fauna supports a proposal for relocating the basal boundary of the Chinese Tabenbulukian Land Mammal Stage/Age to Chron C9r.

Middle Miocene paleoenvironmental change and paleoelevation of the Lunpola Basin, Central Tibet

The Tibetan region underwent diachronous differential rises during the Cenozoic era. Its elevation history is critical for understanding the processes of mountain building, and for evaluating its climatic effects on atmospheric circulations. However, knowledge of paleoenvironment and paleoelevation in different geological domains of the Tibetan Plateau is still limited. Here we provide chronology of lacustrine deposits of the Upper Dingqing Formation in the Lunpola Basin in Central Tibet, which yields a middle Miocene age (15–12 Ma). Both stable isotope and palynological records indicate a climatic transition at ∼13.8 Ma marked by a change towards a cooler and drier climate after the Middle Miocene Climate Transition driven by global climatic cooling. Pollen and spore assemblages suggest that the vegetation was dominated by mixed coniferous–broadleaved forest with some grasslands. New palynological elevation reconstructions constrained by the co-existence approach yield a maximum paleo-elevation of 3100–3600 m (average 3350 m) of the Lunpola Basin, which was consistent with a maximum elevation of 3400 m based on floating-leaf plants. Our new data do not support previous views that the Lunpola Basin achieved its current elevation as early as the late Eocene or early Oligocene, but instead we suggest there has been a rise of up to 1200 m since 12 million years ago. Our results bring new light on both the paleoenvironmental reconstructions and the paleo-elevation estimates of the Lunpola Basin, Central Tibet.

Provenance Response to Rifting and Separation at the Jan Mayen Microcontinent Margin

The Eocene-Miocene successions recovered at DSDP sites on the Jan Mayen Ridge (NE Atlantic) and on the adjacent East Greenland margin provide a sedimentary record of the rifting and separation of the Jan Mayen Microcontinent from East Greenland. A combination of palynology, conventional heavy mineral analysis, single-grain major and trace element geochemistry and radiometric dating of amphibole and zircon has revealed a major change in sediment provenance took place at the Early/Late Oligocene boundary corresponding to a prominent seismic reflector termed JA. During the Eocene and Early Oligocene, lateral variations in provenance character indicate multiple, small-scale transport systems. Site 349 and Kap Brewster were predominantly supplied from magmatic sources (Kap Brewster having a stronger subalkaline signature compared with Site 349), whereas Site 346 received almost exclusively metasedimentary detritus. By contrast, Late Oligocene provenance characteristics are closely comparable at the two Jan Mayen sites, the most distinctive feature being the abundance of reworked Carboniferous, Jurassic, Cretaceous and Eocene palynomorphs. The Site 349 succession documents an evolution in the nature of the magmatic provenance component. Supply from evolved alkaline magmatic rocks, such as syenites, was important in the Middle Eocene and lower part of the Early Oligocene, but was superseded in the later Early Oligocene by mafic magmatic sources. In the latest Early Oligocene, the presence of evolved clinopyroxenes provides evidence for prolonged magmatic fractionation. Initial low degrees of partial melting led to generation of alkaline (syenitic) magmas. The extent of partial melting increased during the Early Oligocene, generating basaltic rocks with both subalkaline and alkaline compositions. Towards the end of the Early Oligocene, the amount of partial melting and magma supply rates decreased. In the Late Oligocene, there is no evidence for contemporaneous igneous activity, with scarce magmatic indicator minerals. The provenance change suggests that the hiatus at the Early/Late Oligocene boundary represents the initiation of the proto-Kolbeinsey Ridge and separation of the Jan Mayen Microcontinent from East Greenland.

A new perspective on Late Eocene and Oligocene vegetation and paleoclimates of South-eastern Australia

We present a composite terrestrial pollen record of latest Eocene through Oligocene (35.5–23 Ma) vegetation and climate change from the Gippsland Basin of south-eastern Australia. Climates were overwhelmingly mesothermic through this time period, with mean annual temperature (MAT) varying between 13 and 18 °C, with an average of 16 °C. We provide evidence to support a cooling trend through the Eocene–Oligocene Transition (EOT), but also identify three subsequent warming cycles through the Oligocene, leading to more seasonal climates at the termination of the Epoch. One of the warming episodes in the Early Oligocene appears to have also occurred at two other southern hemisphere sites at the Drake Passage as well as off eastern Tasmania, based on recent research. Similarities with sea surface temperature records from modern high southern latitudes which also record similar cycles of warming and cooling, are presented and discussed. Annual precipitation varied between 1200 and 1700 mm/yr, with an average of 1470 mm/yr through the sequence. Notwithstanding the extinction of Nothofagus sg. Brassospora from Australia and some now microthermic humid restricted Podocarpaceae conifer taxa, the rainforest vegetation of lowland south-eastern Australia is reconstructed to have been similar to present day Australian Evergreen Notophyll Vine Forests existing under the sub-tropical Köppen-Geiger climate class Cfa (humid subtropical) for most of the sequence. Short periods of cooler climates, such as occurred through the EOT when MAT was ~ 13 °C, may have supported vegetation similar to modern day Evergreen Microphyll Fern Forest. Of potentially greater significance, however, was a warm period in the Early to early Late Oligocene (32–26 Ma) when MAT was 17–18 °C, accompanied by small but important increases in Araucariaceae pollen. At this time, Araucarian Notophyll/Microphyll Vine Forest likely occurred regionally.

Flexural subsidence modelling of post-rift paleobathymetry and sedimentary infill in the northern South China Sea margin

The South China Sea (SCS) is the largest marginal sea on the SE Asian continental margin, located at the junction of the Pacific, Indo-Australia and Eurasian plates, and has undergone the joint effect of the Tethys and Pacific tectonic domains. Currently, the SCS remains debatable for its evolution and origin. However, its northern margin preserves the Cenozoic tectonic and subsidence history, which allow us to restore the past continental margin (0–33.7 Ma), to estimate subsidence, thickness and sediment accumulation correlated with regional tectonics. This paper has employed a 2D flexural backstripping technique and spatio-temporal variations by enhancing the seismic profiles with flexurally interpolated modelled sections. According to our findings, the transition was mainly controlled by three structural stages. (1) Early-Late Oligocene: records the highest subsidence due to the initiation of seafloor spreading, isostatic response of continental breakup, associated uplifts therein, and retreat of the Pacific Plate. (2) Early-Middle Miocene: mature spreading attributed to a slow phase of subsidence during the Early Miocene and a rapid increase of subsidence associated with the closure of the SCS in the Middle Miocene. (3) Thermal subsidence: sediments shifted since the Late Miocene towards mechanically weak and stretched regions (central-southern and eastern) which were triggered by a gravitational pull force. A fault system and differential stretched crust relocated sediment to the regional stress directions of the Pacific Plate.

First Paleocene Lepidozona (Mollusca: Polyplacophora: Chitonida) from Ukraine

A new species Lepidozona luzanovkensis sp. nov. from Paleocene deposits of Ukraine is described. This find is much earlier than the known finds from the late Eocene or Early Oligocene, which increases the age of the genus Lepidozona Pilsbry, 1892 by several million years.

Dietary niches of creodonts and carnivorans of the late Eocene Cypress Hills Formation.

Modern North American carnivorous mammal assemblages consist of species from a single clade: the Carnivora. Carnivorans once coexisted with members of other meat-eating clades, including the creodonts (Hyaenodontida and Oxyaenida). Creodonts, however, went extinct in North America during the late Eocene and early Oligocene, potentially due to niche overlap and resource competition with contemporary carnivorans. In this study, we employ a community ecology approach to understand whether the dietary niches of coexisting creodonts and carnivorans overlapped during the late Eocene (Chadronian North American Land Mammal Age), a time when creodonts were dwindling and carnivorans were diversifying. We quantify niche overlap based on inferences of diet from carnassial tooth shape estimated using Orientation Patch Count, Dirichlet’s Normal Surface Energy, and linear dental measurements as well as from body mass for all species in the Calf Creek Local Fauna of Cypress Hills, Saskatchewan (Treaty 4 land). Although creodonts and carnivorans shared characteristics of their carnassial tooth shape, suggesting similar chewing mechanics and feeding habits, we find that marked differences in body size likely facilitated niche partitioning, at least between the largest creodonts and carnivorans. Calculations of prey focus masses and prey mass spectra indicate that only the smallest creodont may have experienced significant competition for prey with the coeval carnivorans. We suggest that the ultimate extinction of creodonts from North America during the late Eocene and Oligocene was unlikely to have been driven by factors related to niche overlap with carnivorans.

Post-compactional porosity in a hydrocarbon-bearing carbonate succession: Potential timing and processes controlling porosity creation in the Aptian Shuaiba Formation, Oman

Porosity formed after compaction has been recognised for many years in carbonate rocks, but there is little consensus as to the origin of the fluids responsible for dissolution, and the processes by which pores are generated. In particular, it is often hard to identify the source of fluids that are below calcite saturation, and in sufficient volumes, to explain the apparent volume of porosity. This study directly addresses this challenge through analysis of core data from a number of oil fields across north Oman, which produce from the lower Cretaceous (Aptian) Shuaiba Formation. The Shuaiba Formation in all these fields exhibits high total porosity, comprising pores that range in size from less than a micron to centimetres in diameter. There is an increase in the effective porosity upwards within the succession; permeability is highest in the upper 15 m of the formation, beneath the top-Shuaiba unconformity. Since this unconformity represents a hiatus of over 5 million years (My), caused by platform emergence, previous studies often interpreted porosity modification to be due to dissolution by groundwater during penecontemporaneous exposure of the platform. Nevertheless, paragenetic relationships indicate that most of the secondary porosity that was formed early in the burial history was either occluded by drusy calcite cements, or destroyed by compaction. Furthermore, many of the drusy calcite cements are etched, indicating that dissolution took place after cementation. Consistent with this, many of the pores formed on the margins of tectonic fractures and stylolites, indicating that some pore-formation post-dates compaction.The top of the Shuaiba Formation in all fields is between 600 and 1600 m, following uplift in the late Cretaceous and early Oligocene by inversion along deep-seated, strike-slip faults. Possible causes of dissolution are cooling of upward-rising brines fluxed along faults and corrosion by organic acids prior to hydrocarbon emplacement. It is unclear, however, as to the extent to which these fluids could dissolve carbonate before reaching equilibrium. A third possible fluid is groundwater. Sulphur isotope data from paragenetically late pyrite, is consistent with bacterial sulphate reduction, indicative of relatively cool temperatures (<60-80 °C). Porosity enhancement might, therefore, have been facilitated by mixing of groundwater with highly saline reservoir fluids, resulting in undersaturation with respect to calcium carbonate. Overall, the data reveal a complex history of porosity modification, which relates to rock fabric, stratal architecture and tectonic history and suggests multiple controls on post-compactional porosity modification. Most significantly, the data strongly suggest that dissolution occurred during orogenic uplift and telogenesis, providing a new explanation for how porosity formation can occur after lithification, along stylolites and fractures, from a volumetrically significant source of carbonate-undersaturated fluid.

Reconstructing Cenozoic Patagonian biotas using multi-proxy fossil records

The fossil record from Cenozoic sediments provides a great deal of information that has direct bearing on the early assembling of modern Patagonian ecosystems. In this synthesis, we revise selected fossil marine and terrestrial records from the last 66 Ma with the aim of understanding major shifts of Patagonian biotas. From the Paleocene to the mid Eocene this region supported outstandingly diverse terrestrial assemblages that show strong connections to modern-day Australasia (e.g. gum trees, casuarinas, monotremes). Nearshore marine biotas confirm peak warmth conditions, with tropical species with Tethyan affinities. The late Eocene and early Oligocene marks the onset of a period of overall regional cooling, drying, and increasingly variable ecological conditions. The rise of palm-dominated flammable biomes in hinterlands and the prevalence of Gondwanan gallery forest (e.g. southern beeches and podocarps) along river-sides supported the existence of mosaic habitats maintained by edaphic and regional climatic conditions. This shift in landscapes reflects the evolution of a wide range of herbivorous mammals (e.g. Notoungulata, Litopterna, and Astrapotheria). The late Oligocene and early-to-mid Miocene witnessed a dramatic modification of landscapes including the incursion of high sea-level episodes, the emergence of specialized coastal (i.e. salt-marsh) plant taxa and the expansion of large herbivorous mammals with predominantly high-crowned teeth (e.g. Notoungulata: Hegetotheriidae, Interatheriidae, and Mesotheriidae). The cooling trend of this interval was interrupted by a mid-Miocene transient warming event, with the dispersion of terrestrial (e.g. platyrrhine monkeys, palms) and marine (e.g. Tuberculodinium vancampoae) elements with tropical affinity into southernmost South American regions. Seasonally-dry conditions increased towards the end of the Miocene, yet subtropical species persisted either in terrestrial (e.g. malpighs, passion vines, capybaras), and marine (e.g. Subtropical and Caribbean molluscs) environments. The increasing aridity caused by the Andean uplift wiped out most of the forest species and promoted the diversification of open-habitat species; the emergence of the current grass-dominated Patagonian Steppe occurred later on, probably during the Quaternary.

Geological structure and paleogeographic zoning of the Khadum formation in Pre‑Caucasus

Unconventional oil and gas resource development and exploration is the one of the most prospective concept in petroleum geology. High carbon Khadum facies are investigated as unconventional resources in the Precaucas basin and contain gas, gas condensate and oil accumulations. Oligocene and Lower Miocene structure, prospectivity and fields distribution are the one of the most relevant subjects nowadays. This study is focused on the main stages Late Eocene and Early Oligocene deposits formation and the paleogeography of the Khadum formation in the Precaucas oil and gas basin. The new prospective zones are developed in Khadum formation based on the complex analysis of seismic, paleogeography and lithofacies analysis.