Late Eocene Early Oligocene Research Articles

Palynostratigraphy and Palaeoenvironmental Interpretation of Outcrop Formations in Parts of Ini-Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria

The palynological study of the outcrop Formations in parts of Ini Local Government Area of Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria has contributed to unraveling the palynostratigraphy and paleoenvironments of the outcrops through the use of diagnostic palynomorphs. The section is made up of carbonaceous shales, sandstones, mudstones and claystones. The conventional maceration technique for recovering acid-insoluble organic-walled microfossils from sediments was used to prepare twenty outcrop samples for palynological studies. The samples produced poor to fair diversity assemblages of palynomorphs. A total of 176 palynomorphs species were recorded, with 48% pollen grains, 33% spores, 12% dinoflagellate cysts, 5% fungal spores, and 2% microforam test wall linings. Based on stratigraphically selected index taxa recovered,age determination and correlation wereestablished. The samples from Imo Formation were assigned Late Paleocene-Eocene based on Psilatricolporites Crassus, Retitricolporites irregularies, Psilatricolporites sp. and Leiotriletes adriennis with the presence of Paleocene dinoflagellate cysts such as Lejeunacysta beninensis and Selenopemphix nephroides. Ameki samples were dated Early-Middle Eocene based on Proxapertites operculatus, Retistephanocolpites williamsi, Mauritidites crassiexinius, Monocolpites marginatus and Longapertites marginatus. Ogwashi-Asaba shale samples were assigned Late Eocene-Early Oligocene due to the abundant presence of Verrucatosporites usmensis, Laevigatosporites discordatus and Retitricolporites irregularis while Eze-Aku samples were tentatively assigned Paleocene because of the presence of two pollen index fossils Proxapertites operculatus and Pachydermites diederixi. Inferred depositional conditions suggest that Imo Formation was deposited in a shallow marine or near shore brackish water estuarine environment. The Ameki Formation was accumulated in a marginal marine or near shore, brackish water-estuarine environment in the upper deltaic setting. The Ogwashi-Asaba Formation was deposited in a progradational shoreline moving between the brackish water and fresh water deltaic plain. The Eze-Aku Formation was deposited in an upper foreshore fresh water environment. The palynological study of the samples are Paleogene Formations while the paleoenvironment is in a costal deltaic to shallow-marine environment and are appropriate for hydrocarbon accumulations and exploration.

Structural Interpretation by Applied Seismic Attribute in ‘I’ Field, Northwestern of Malay Basin.

The structural pattern within the Malay Basin is divided into fold and fault systems. Malay Basin is a complex rift composed of numerous extensional graben formed during the late Eocene-early Oligocene and the underwent thermal subsidence and sedimentation during the early Miocene. 3D seismic data and well data are used to map fault surface and stratigraphic reference horizons in the research field. The 3D surfaces and stratigraphy can thus be used to better understand and evaluate fold, fault interaction and structural trap. The quality of the data was improved by applying the seismic attribute (volume attribute) to intensify the geologic features. This paper is focusing on identified fold and faults by applying several seismic attributes on the specified parameter. The objective of this paper is to answer the question regarding how the fault is growing and its propagation. Thus, it is will help with the exploration to produce accurate structural maps in discovery oil and gas.

Chemical composition of late Eocene–early Oligocene corals in reef buildups from the Thrace basin, Bulgaria–Greece: Paleoenvironmental implications

Coral whole-rock geochemistry and in situ LA-ICP-MS analyses of coral skeletons were performed on late Eocene–early Oligocene coral reef buildups from the Eastern Rhodope–Thrace region of Bulgaria and Greece. Coral reefs are locally associated with voluminous Oligocene volcanism in the region. The reefs are subdivided into (i) eruption-associated reefs (Krumovgrad); (ii) pre-eruption reefs (Ivaylovgrad); and (iii) Metaxades-Didymotycho reefs from field relations, trace element and rare-earth element (REE) abundances. Coral assemblages are dominated by Cladocora sp., which is accompanied by Porites sp., Colpophyllia sp., Favites sp. and Leptoseris sp. Eruption-associated reefs are characterized by their higher REE content than the lower in all REE contents of pre-eruption reefs showing negative Ce anomaly, and Metaxades-Didymotycho reefs that have lower middle-heavy REE contents compared to previous groups. Trace element and REE geochemistry of the coral skeletons indicates volcanic contribution to seawater, mostly evident in the eruption associated reefs, and contribution from terrestrial input in the site of coral buildup deposition. Contribution from a different source of prior diagenetic nature, along with subsequent diagenetic modification, is inferred. The increase in REE+Y (ΣREY) from pre-eruption to eruption-associated reefs is well correlated with elevated amounts of terrigenous elements like Al and Fe.

Mesozoic-Cenozoic exhumation history and its implications for the uranium mineralization in the southern Junggar Basin, North China

The phases of tectonic activity and exhumation of the southern margin of the Junggar Basin during the Cretaceous through Cenozoic shows significant relationships with the timing of uranium mineralization episodes. To clarify the succession of metallogenetic stages that were controlled by different phases of uplift and exhumation, apatite fission track (AFT) thermochronology was performed on the Jurassic through Cenozoic sediments from the Manas section in the southern Junggar Basin. The mean ages of the AFT suites range from 63 Ma to 133 Ma, and are younger than the host sediment for the Mesozoic samples and older than the host for the Cenozoic samples. The mean track lengths (MTLs) are between 10 and 11 μm. These results indicate that most of the apatite grain assemblages in these samples experienced complicated annealing histories. AFT ages were combined with thermal history modeling to unravel the timing of the multi-phase regional uplift and exhumation history and to reveal the tectonic constraints on the uranium mineralization. The populations of AFT single-grain ages display four peaks, which are interpreted as regional uplift and cooling episodes: mid-Early Jurassic (189–180 Ma), late Jurassic–early Cretaceous (157–116 Ma), late Cretaceous–early Paleocene (88–59 Ma) and late Eocene–early Oligocene (37–28 Ma). Thermal history modeling indicates two episodes of rapid cooling of the basin sediment deposits during the late Cretaceous–Paleocene (80–50 Ma) and the Pliocene (5 Ma)–Present. A synthesis of the temporal relationship between AFT age peaks, of the modeled time–temperature paths, of a compilation of age data and regional geologic events from the Tianshan mountain belt, suggests a close relationship between tectonic evolution and uranium mineralization. Uranium metallogenesis episodes occurred in conjunction with tectonic uplift and exhumation episodes along the southern margin of the Junggar Basin during the late Cretaceous–early Paleogene and during the latest Miocene-Present. The earlier episode was the primary mineralization stage, and the later episode was a partial transformation stage superimposed on the earlier one.

APPLICATION OF OLEANANE AND STERANE INDEX FOR BIOSTRATIGRAPHIC AGE DETERMINATION: EXAMPLES FROM KANGEAN OILS, NORTHEAST JAVA BASIN

Northeast Java Basin is known as mature Cenozoic basin, yet this understanding override possibility ofsediment older than Cenozoic. This thoughthas brought current exploration strategy of this basin concerningwithin only Cenozoic sediments. Therefore, it is believed that the source rock in this basin was also derivedfrom Cenozoic sediments, especially the Ngimbang Formation, which was formed during Late Eoceneto Early Oligocene in the stage of Early Synrift. On the other hand, the occurrence of Alisporites sp haspointed Cretaceous sediments is a potential source rock. However, it is still debatable due to the presence ofAlisporites similis in Serawak Basin of Malaysia, which is present until Paleocene. Three crude oils from theKangean oil fi eldNortheast Java Basin, namely NEJB-748, NEJB-749 and NEJB-750have been investigatedusing gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS).Kangean oils areclassifi ed as mixed oil with organic matter originated from marine and terrestrial deposited under oxidizingand reducing conditions. Moreover, Kangean oils show very low oleanane and steraneindexthat may leadus to the conclusion that the oils were originated from Cretaceous source rock.

Origin and evolution of the Paleo‐Kuril arc inferred from detrital zircon U–Pb chronology in eastern Hokkaido, NE Asia

AbstractThe Nemuro and Saroma Groups and Yusenkyo Formation occur in eastern Hokkaido and are considered to be forearc or intra‐arc basin sediments of the Paleo‐Kuril arc (PKA) deposited during the Late Cretaceous to middle Eocene. To further clarify the origin of the PKA, we examined the U–Pb ages of detrital zircons within these sandstones and acidic tuff beds; based on our results, we drew the following conclusions. (1) The PKA originated from an oceanic island arc on the oceanic Izanagi Plate around 85 Ma, to which the Nikoro Group greenstone complex was accreted between 81–80 Ma; the Lowest Unit of the Saroma Group covered the surface of the Nikoro accretional greenstone complex. (2) The PKA then first collided with NE Asia around the beginning of the deposition of the Hamanaka Formation (~70 Ma) and transitioned to a continental arc adjacent to NE Asia. This collision generated giant slump beds during deposition of the Akkeshi Formation. During deposition of the Kiritappu Formation, the entire Nemuro Peninsula was uplifted, supplying large volumes of clastic sediments. (3) Although we do not have data directly bearing on why the North American Plate was established in the edge of NE Asia, we speculate that it separated from the Eurasian continent around ~70 Ma when NE Asia first collided with the PKA. Subsequently, the PKA has behaved as part of the North American Plate. The Paleo‐Japan arc (or East Sikhote–Alin arc) and the PKA became joined via the Hidaka Belt. Around 40 Ma, during the deposition of coarse conglomerate beds of the Urahoro Group, the PKA was uplifted and bent clockwise due to a second collision with NE Asia. (4) The modern Kuril arc was established around 36 Ma (late Eocene–early Oligocene), and the Kuril backarc basin opened into the present tectonic setting in the late Oligocene–early Miocene.

The Last Piece of the Puzzle? Phylogenetic Position and Natural History of the Monotypic Fungus-Farming Ant GenusParamycetophylax(Formicidae: Attini)

AbstractThe evolutionary history of fungus-farming ants has been the subject of multiple morphological, molecular phylogenetic, and phylogenomic studies. Due to its rarity, however, the phylogenetic position, natural history, and fungal associations of the monotypic genus Paramycetophylax Kusnezov have remained enigmatic. Here we report the first excavations of colonies of Paramycetophylax bruchi (Santschi) and describe its nest architecture and natural history. Utilizing specimens from these collections, we generated ultraconserved-element (UCE) data to determine the evolutionary position of Paramycetophylax within the fungus-farming ants and ribosomal ‘fungal barcoding’ ITS sequence data to identify the fungal cultivar. A maximum-likelihood phylogenomic analysis indicates that the genus Paramycetophylax is the sister group of the yeast-cultivating Cyphomyrmex rimosus group, an unexpected result that renders the genus Cyphomyrmex Mayr paraphyletic. A Bayesian divergence-dating analysis indicates that Paramycetophylax diverged from its sister group around 36 mya (30–42 mya, HPD) in the late Eocene-early Oligocene, a period of global cooling, expansion of grasslands, and large-scale extinction of tropical organisms. Bayesian analysis of the fungal cultivar ITS gene fragment indicates that P. bruchi practices lower agriculture and that the cultivar grown by P. bruchi belongs to the Clade 1 group of lower-attine fungi, a clade that, interestingly, also includes the C. rimosus-group yeast cultivars. Based on these results, we conclude that a better understanding of P. bruchi and its fungal cultivar, including whole-genome data, is critical for reconstructing the origin of yeast agriculture, a major transition in the evolution of fungus-farming ants.

Late Eocene–early Miocene evolution of the southern Australian subtropical front: a marine palynological approach

Abstract. Improvements in our capability to reconstruct ancient surface-ocean conditions based on organic-walled dinoflagellate cyst (dinocyst) assemblages from the Southern Ocean provide an opportunity to better establish past position, strength and oceanography of the subtropical front (STF). Here, we aim to reconstruct the late Eocene to early Miocene (37–20 Ma) depositional and palaeoceanographic history of the STF in the context of the evolving Tasmanian Gateway as well as the potential influence of Antarctic circumpolar flow and intense waxing and waning of ice. We approach this by combining information from seismic lines (revisiting existing data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A) in the western Tasmanian continental slope. We apply improved taxonomic insights and palaeoecological models to reconstruct the sea surface palaeoenvironmental evolution. Late Eocene–early Oligocene (37–30.5 Ma) assemblages show a progressive transition from dominant terrestrial palynomorphs and inner-neritic dinocyst taxa as well as cysts produced by heterotrophic dinoflagellates to predominantly outer-neritic/oceanic autotrophic taxa. This transition reflects the progressive deepening of the western Tasmanian continental margin, an interpretation supported by our new seismic investigations. The dominance of autotrophic species like Spiniferites spp. and Operculodinium spp. reflects relatively oligotrophic conditions, like those of regions north of the modern-day STF. The increased abundance in the earliest Miocene of Nematosphaeropsis labyrinthus, typical for modern subantarctic zone (frontal) conditions, indicates a cooling and/or closer proximity of the STF to the site . The absence of major shifts in dinocyst assemblages contrasts with other records in the region and suggests that small changes in surface oceanographic conditions occurred during the Oligocene. Despite the relatively southerly (63–55∘ S) location of Site 1168, the rather stable oceanographic conditions reflect the continued influence of the proto-Leeuwin Current along the southern Australian coast as Australia continued to drift northward. The relatively “warm” dinocyst assemblages at ODP Site 1168, compared with the cold assemblages at Antarctic Integrated Ocean Drilling Program (IODP) Site U1356, testify to the establishment of a pronounced latitudinal temperature gradient in the Oligocene Southern Ocean.

Fluvial sedimentation and its reservoir potential at foreland basin margins: A case study of the Puig-reig anticline (South-eastern Pyrenees)

Fluvial fans represent one of the dominant sedimentary systems at the active margins of non-marine foreland basins. The Puig-reig anticline at the north-eastern margin of the Ebro Foreland Basin (SE Pyrenees, Spain) exposes continuous outcrops of Late Eocene-Early Oligocene fluvial deposits, from proximal to medial fluvial fan environments. The proximal deposits are found in the north limb of the anticline, especially in the northwest zone. These deposits are characterised by conglomerates with minor interbedded sandstones, with thick and wide sheet-like geometries with unscoured or variably scoured basal surfaces. These are interpreted to be the deposits of unconfined flash floods and wide-shallow channel streams. The medial deposits, covering the rest of the anticline, consist of interbedded conglomerates, sandstones and claystones. These are interpreted to have been deposited from braided to meandering channel streams and overbank areas. Distal deposits are found towards the south, beyond the anticline, and are characterised by sandstone and clay deposits of terminal lobes and lacustrine deltas. This study assesses the impact of the primary depositional characteristics, diagenesis and deformation of the most heterolithic portion of the system, with implications for increasing our understanding of folded fluvial reservoirs. Diagenetic processes, mainly mechanical compaction and calcite cementation, resulted in overall low intergranular porosity, with limited relatively high porosity developed in sandstone lithofacies in the medial deposits. Deformation associated with thrusting and fold growth resulted in the formation of abundant fractures, with relatively high fracture intensities observed in sandstone lithofacies in the anticline crest. This study shows that post-depositional processes can both improve and diminish the reservoir potential of basin proximal fluvial deposits, through the development of fracture networks and by compaction-cementation. The comparison of the Puig-reig anticline with other similar settings worldwide indicates that foreland basin margin locations may be potential areas for effective reservoirs, even in the case of low intergranular porosity.

Porphyry CU deposits in the Central Andes of Argentina: An overview

Porphyry Cu deposits are the world's most important source of Cu and the Central Andes of South America hold some of the largest Cu resources worldwide. In Argentina many porphyry-type deposits and prospects with different degrees of exploration straddle along ~1500 km of the Central Andes. The review and analysis of the tectono-magmatic, metallogenic and economic data of the better-known deposits and prospects allow to define the controls of porphyry-type mineralization and the metallogenic epochs and belts which in turn indicate the potential for porphyry-type mineralization in some relatively poorly explored segments of the Central Andes.Slab shallowing (particularly linked to the subduction of aseismic ridges) and associated eastward arc migration/expansion appears to be a favorable geodynamic scenario for Paleozoic and Cenozoic mineralizing magmatism which show variable adakitic signatures probably as a result of a progressive crustal thickening. In this context the V/Sc and especially the Sr/Y ratios are a useful tools for assessing magma fertility. Major WNW-trending deep crustal pre-Andean structures controlled the inland migration of the magmatism and are a first order control on porphyry emplacement during these epochs of magmatic activity.There are five favorable metallogenic epochs linked to the arc magmatism in Argentina: Permian, Upper Cretaceous-Eocene, late Eocene-early Oligocene, late Oligocene-early Miocene and Miocene-Pliocene. The Permian and Upper Cretaceous-Eocene metallogenic epochs are represented by a small number of deposits with low metal (Cu–Mo–Au) content; however, the extension of the magmatism of both epochs suggests they are favorable for further exploration. The late Eocene–early Oligocene magmatism involves few deposits but formed the second largest known copper resource in Argentina. The Miocene-Pliocene magmatism, particularly between ~11 and 6 Ma, generated the largest porphyry-type deposits with the greatest metal resources. Based on the available information ten metallogenic belts are outlined: 1) Permian Puna-Frontal Cordillera/Neuquén basement, 2) Upper Cretaceous–Eocene Neuquén, 3) late Eocene-early Oligocene, 4) late Oligocene-early Miocene, 5) middle Miocene Puna-Frontal Cordillera, 6) middle Miocene Precordillera, 7) late Miocene Cordillera, 8) late Miocene-Pliocene Cordillera, 9) late Miocene-Pliocene Northern Sierras Pampeanas, 10) late Miocene-Pliocene Famatina. These ten metallogenic belts accumulate 74.21 Mt Cu, 1.70 Mt Mo, 2160.66 t Au and 23,876.71 t Ag of known resources and approximately 239 Mt Cu, 6.4 Mt Mo, 4240 t Au and 77,900 t Ag contained in undiscovered deposits. The Miocene belts are the most endowed, confirming their economic importance as well as their prospectivity for exploration. The largest total known resources are hosted by the late Miocene belt in the Frontal Cordillera, followed by the late Eocene-Early Oligocene one and the Miocene-Pliocene belt in the Sierras Pampeanas. The continuation of the late Miocene-Pliocene Cordillera belt that extends from Los Bronces and El Teniente in Chile, further south into Argentina, is here considered the most prospective underexplored belt.

Biotic community and landscape changes around the Eocene–Oligocene transition at Shapaja, Peruvian Amazonia: Regional or global drivers?

Since 2012, we have investigated a stratigraphic section encompassing the late Eocene–earliest Oligocene interval at Shapaja (Tarapoto area, Peruvian Amazonia, ca. 7°S), through paleontological and geological fieldwork. The measured sedimentary series (120 m-thick [West] plus 90 m-thick [East]), assigned to the upper member of the Pozo Formation, records fluvial micro-conglomeratic lenses intercalated with floodplain and evaporite-rich fine red deposits, estuarine/coastal-plain tidally-influenced fine sandstones, and oxbow lake nodule-rich blue clays. This sedimentary shift coincides locally with the demise of the large Eocene coastal-plain wetland known as Pozo System. The late Eocene–early Oligocene Shapaja section was extensively sampled for chemostratigraphy (δ 13 C on dispersed organic matter and pedogenic carbonate nodules), which in turn allowed for refining the location of the Eocene-Oligocene Transition (EOT) and other climatic events recognized at a global scale (i.e., Oi-1 and Oi-1a). The section has yielded nine fossil localities with plant remains (leaves, wood, charophytes, and palynomorphs), mollusks, decapods, and/or vertebrates (selachians, actinopterygians, lungfishes, amphibians, sauropsids, and mammals), documenting ~130 distinct taxa. Four localities of the upper member of the Pozo Formation at Shapaja predate the EOT, one is clearly within the EOT, while four are earliest/early Oligocene in age. The small leaf impressions found along the Shapaja section could be indicative of dry and/or seasonal conditions for this region throughout and after the EOT. Monkeys, indicative of tropical rainforest environments, are only recorded in a latest Eocene locality (TAR-21). Two biotic turnovers are perceptible in the selachian, metatherian, and rodent communities, well before the EOT [~35–36 Ma] and a few hundred thousand years after the EOT [~33 Ma]. The latter turnover seems to be primarily related to a global sea-level drop (ichthyofauna: marine-littoral elements replaced by obligate freshwater taxa) and/or the onset of a drier and more seasonal climate in early Oligocene times (terrestrial components). Changes in the structure of the Shapaja paleocommunities were mostly driven by the flexural subsidence during the late Eocene, and then globally driven by the earliest Oligocene climatic deterioration. • The Shapaja section spans the late Eocene–early Oligocene interval ( ca. 36–32.5 Ma). • Micro- and macroremains of 129 plant and animal fossil taxa were recognized. • Two faunal turnovers are recorded on both sides of the Eocene-Oligocene transition. • Shapaja records the earliest occurrence of South American primates and sloth. • Community changes are locally- then globally-driven (late Eocene/earliest Oligocene).

Continental palynomorphs from the Dabaa Formation, North-Western Desert, Egypt: a contribution to the reconstruction of the vegetation on the southern shores of the Tethys Ocean during the Early Oligocene

AbstractThe Eocene–Oligocene transition period was marked by one of the most abrupt and severe global environmental changes in the Cenozoic record, and this had a marked influence on the evolution of a number of animal and plant groups and entire ecosystems. This study documents continental palynomorphs recovered from the sedimentary rocks of the Dabaa Formation (Qattara area, North-Western Desert, Egypt) located on the southern shore of the Tethys Ocean and dated as Late Eocene–Early Oligocene. The botanical affinities, (phyto)ecology and distribution of the vegetation during the Eocene–Oligocene of the study area are discussed. The recorded assemblages are well preserved and comprise diverse lineages of algae, spores and pollen. They were identified, illustrated and assigned to 46 families encompassing chlorococcalean algae, lycopods, ferns, gymnosperms and angiosperms. The studied assemblages demonstrate the development of tropical vegetation, including tropical deciduous forest, grassland and (semi-)arid tropical shrubland, in which angiosperms were one of the main representatives; additionally open, drier habitats might have existed in the hinterland. Our data have been combined with previous megafossil and palynological evidence to assess and refine vegetation changes during the Early Oligocene time window in Egypt and across North Africa. Vegetation was a mosaic of different vegetation belts that ran more-or-less parallel to the coastline of the Tethys Ocean under the variable geographical influence of lagoons and streams. It is assumed that the belt of tropical forest along the coast of the Tethys Ocean narrowed during the Oligocene in parallel to climatic deterioration following the Eocene–Oligocene boundary, which may have also led to the fractionation of forest habitats.

New early oligocene zircon U-Pb dates for the ‘Miocene’ Wenshan Basin, Yunnan, China: Biodiversity and paleoenvironment

The sedimentary basins of Yunnan, Southwest China, record detailed histories of Cenozoic paleoenvironmental change. They track regional tectonic and palaeobiological evolution, both of which are critically important for the development of modern floral diversity in southwestern China and throughout Asia more generally. However, to be useful, the sedimentary archives within the basins have to be placed within a well-constrained timeframe independent of biostratigraphy. Using high resolution U-Pb dating, we redefine the age of fossil-bearing strata in the Wenshan Basin. Regarded as Miocene for the last half century, these basin sediments encompass 30±2 and 32±1 Ma early Oligocene tuffaceous horizons, thus indicating a significantly greater antiquity than previously recognized. Together with other regional age revisions our result points to widespread Yunnan basin and orographic development as largely having taken place by the end Paleogene. This age revision provides an important new perspective on the preserved biotas and their evolution in Yunnan, and especially our understanding of the origin of Asian biodiversity which, regionally, had a near-modern composition by the early Oligocene. Crucially, this revised age evidences late Eocene-early Oligocene regional tectonism, pointing to the rise of eastern Tibet and the Hengduan Mountains before the growth of the Himalaya, and that Asia's high plant diversity has a Paleogene origin.

Thermal maturation, hydrocarbon generation and expulsion modeling of the source rocks in the Baiyun Sag, Pearl River Mouth Basin, South China Sea

The Baiyun Sag plays a role as the most important, largest deepwater target in petroleum exploration across the northern side of the South China Sea. Three sets of primary source rocks are developed in the Baiyun Sag: early Eocene middle-deep lacustrine source rock (EMD), late Eocene-early Oligocene inshore-shallow lacustrine source rock (OIS), and late Oligocene transitional source rock (LOT). A quantitative 3D model of the Baiyun Sag was constructed by using a stratigraphic framework based on 3D seismic interpretations and lithology well logging data of 66 wells. The geothermal parameters of the 3D model were determined and calibrated based on the measurements of seafloor temperature, formation temperature, and vitrinite reflectance from the wells. The source rock property parameters of the EMD, OIS, and LOT in the 3D model were set according to the results of the total organic carbon test and Rock-Eval pyrolysis analysis of the well samples. The thermal maturation, hydrocarbon generation and expulsion histories of the EMD, OIS, and LOT were reconstructed to study how the evolution of source rocks influenced the current oil accumulation. Modeling results show significant differences among evolutions of the EMD, OIS, and LOT in both vertical and plane directions. The majority of the EMD reached the threshold of hydrocarbon generation (Ro = 0.5%) at about 45 Ma and kept in the oil window (Ro = 0.5–1.3%) from 45 to 15 Ma. The majority of the OIS and LOT was not capable of generating hydrocarbon until 35 Ma and 20 Ma, respectively, and have not entirely passed the oil window at present. The EMD, OIS, and LOT in the south and east region of the Baiyun Sag matured faster than the ones in the north and west region. The total oil generation quantities of the EMD, OIS, and LOT are 3772.51 × 108 t, 585.93 × 108 t, and 517.83 × 108 t, respectively; and the total oil expulsion quantities of the EMD, OIS, and LOT are 1246.04 × 108 t, 47.85 × 108 t, and 18.49 × 108 t, respectively. The oil generated and expelled in the Sub-sag B1 accounts for 82% of the total oil generation and 96% of the total oil expulsion in the study area, respectively. However, only 4% of the oil originated from the EMD, and 3% of the oil generated in the Sub-sag B1, was expelled after the deposition of primary reservoir rocks (23.8–0 Ma). On the contrary, 69% of the oil expulsion of the OIS and all of the oil expelled from the LOT took place after 23.8 Ma. The EMD in the Sub-sag B2, the OIS in the Sub-sag B1 and B4, and the LOT in the Sub-sag B1 are favorable to current oil accumulation due to their high proportions and large quantities of the oil expulsion after the deposition of primary reservoir rocks.

Cenozoic Exhumation History of the Eastern Margin of the Northern Canadian Cordillera

AbstractNew low‐temperature thermochronology data from clastic sedimentary rocks in the northern Richardson Mountains, Canada, indicate significant exhumational cooling during late Eocene–early Oligocene time. Apatite (U‐Th‐Sm)/He (AHe) data were collected from 19 Proterozoic–Paleocene rocks across a 115 km transect. Eighty‐eight single‐grain AHe dates range from 16–300 Ma and are generally younger than stratigraphic ages, indicative of thermal resetting by burial. Additionally, zircon (U‐Th)/He (ZHe) dates from two Proterozoic–Cambrian rocks range from 49–123 Ma and suggest burial to >160°C. In contrast, ZHe dates from Jurassic sandstones are older than the stratigraphic age, which limits maximum burial to <160°C. Thermal history modeling reveals three phases of cooling, during the Paleocene–early Eocene (>65–50 Ma), late Eocene–early Oligocene (40–30 Ma), and late Oligocene–early Miocene (30–15 Ma). Most samples cooled during the first and second phases, whereas the third phase is less well constrained. In general, most rocks were below the sensitivity of AHe analysis since the early–middle Miocene. The results suggest a previously unrecognized phase of inferred deformation in the northern Richardson Mountains between 40–30 Ma. Our findings contribute to previous work that recognizes Late Cenozoic deformation along the eastern margin of the Northern Cordillera. We further investigated the potential mechanisms of this widespread deformation and suggest exhumation may relate to kinematic changes of the North American plate relative to structural trends along the margin of the Northern Cordillera.

Middle Eocene vertebrate fauna from the Aridal Formation, Sabkha of Gueran, southwestern Morocco

In the Sahara Desert of southwestern Morocco, the Aridal Formation of Gueran is known for the world's richest Bartonian archaic whale assemblage, which includes both protocetids and basilosaurids. Gueran has also yielded another rich and diverse vertebrate fauna described in detail herein —The chondrichthyan assemblage of twelve species is quite similar to that of the Midawara Formation (Egypt). Actinopterygians include Siluriformes, Percomorpha and rostra of Cylindracanthus Leidy, 1856. Turtles are attributed to at least three indetermined species: two marine cryptodires – a cheloniid and a dermochelyid, and a possible littoral pleurodire, as found in Ad-Dakhla (Morocco) and Fayum (Egypt). The crocodylians comprise at least two longirostrine taxa, including a gavialoid that resembles the late Eocene-early Oligocene Eogavialis africanumAndrews, 1901 from Egypt. The second form is too fragmentary to be identified more precisely than Crocodyliformes indet. Two snake vertebrae belong to Pterosphenus cf. schweinfurthiAndrews, 1901. Two other incomplete snake vertebrae probably belong to Paleophiidae as well. Seabird remains belong to a gigantic soaring pseudo-toothed bird (Pelagornithidae) and constitute the earliest occurrence of the genus Pelagornis sp.Lartet, 1857. This material extends the fossil record of Pelagornis back in time by at least 10 million years. Based on their size and enamel microstructure, mammal dental fragments are attributed to the proboscidean ?Barytherium sp. The Bartonian age of the fauna, initially based on an archaeocete cetacean assemblage, is also supported by chondrichthyans. Affinities of the Gueran faunal assemblage are analyzed in comparison with those from other middle and upper Eocene deposits of North Africa and elsewhere.

The Gondwanan heritage of the Eocene–Miocene Patagonian floras

The breakup of Gondwana and the associated climatic changes led to the fragmentation of floras that were once connected across the Southern lands. The diversity of the Gondwanan remnants has long been assumed to have fluctuated in Patagonia across the Cenozoic, although it has never been quantified so far. Here we address when the major floristic members of the Gondwanan legacy (e.g., southern beeches, proteas, podocarps, gumtrees) expanded, contracted, or became extinct during the Patagonian biogeographic isolation (Eocene–Miocene) on the basis of the re-assessment of the fossil record (i.e., woods, leaves, and spore-pollen grains). We found that the Patagonian floras experienced moderate to severe shifts in the diversity of the Gondwanan component —relative to the total flora— with the highest estimates in the late Eocene–early Oligocene (~50%) and the lowest estimates in the late Miocene (~20%) according to the fossil pollen record. The most important floristic changes include two major replacements: 1) tropical Gondwanan taxa (e.g., Akania, Eucalyptus, Gymnostoma) by typically cool-temperate taxa (e.g., Nothofagaceae) in the Eocene, and 2) humid taxa (e.g., Podocarpaceae) by arid-adapted floras, mostly of non-Gondwanan affinity, across the Miocene. The variation in diversity of the Gondwanan component from Patagonia shows a striking resemblance to that from Australia for the same period, probably indicating a global-scale driver of floristic turnover (e.g., global cooling conditions). Today, the Patagonian subantarctic forests harbor only about ~15% of the Gondwanan diversity, representing a three-fold decrease from its climax in the late Eocene–early Oligocene.

<p><strong>A new genus and species of spoon-winged lacewing from the Cretaceous Crato Formation in Brazil (Neuroptera: Nemopteridae)</strong></p>

The lacewing family Nemopteridae Burmeister, 1839 is very poorly represented in the fossil record with three Early Cretaceous genera of uncertain affinities from the Brazilian Crato Formation, one ‘mid’-Cretaceous representative of the stem group of the Crocinae Navás, 1910 in the Burmese amber, and two Cenozoic nemopterine genera Marquettia Navás, 1913 (late Eocene-early Oligocene) and Paleonemia Claisse et al., 2019 (middle Oligocene). Also two undetermined Nemopterinae are recorded from the late Eocene and the Oligocene (Lu et al., 2019a: Table 1; Claisse et al., 2019).

Astronomically forced climate cooling across the Eocene–Oligocene transition in the Pearl River Mouth Basin, northern South China Sea

During the late Eocene–early Oligocene, major changes in the global climate and environment occurred, including decreases in global temperature and the appearance of Antarctic glaciation. These changes marked the transition from early Cenozoic greenhouse conditions to icehouse conditions. Research on this cooling trend in the Atlantic and high-latitude regions has increased, but few reports have studied low-latitude regions (e.g., the northern South China Sea), and it remains unclear whether sediment records in these areas preserve signals related to this global cooling event. In this study, we studied records from a lacustrine well in the Wenchang Sag, Pearl River Mouth Basin (PRMB), northern South China Sea. We conducted palynological and cyclostratigraphic analyses to reconstruct the depositional environment and assess the influence of orbital forcing. The cyclostratigraphic analysis showed that depositional cycles (including fining-upwards and coarsening-upwards sequences) were controlled mainly by long eccentricity, forcing the expansion and contraction of the lake system at ~1.2 Myr time intervals. In addition, changes in the pollen-spore assemblage before and after the Eocene–Oligocene (E-O) transition show a dramatic shift from a moderately deep lake to a shallow lake with delta facies, as recorded by the Wenchang to Enping Formations in the Wenchang Sag, PRMB. These findings suggest that the climate deteriorated in response to global cooling. The amplification of obliquity forcing and the coevolution between obliquity power and global pCO2 during the E-O transition suggest that the low-latitude sedimentary records from China contain a high-latitude signal. Furthermore, the low amplitudes of the precession, obliquity, and eccentricity cycles are coupled with the initial decreases in the global sea-level and pCO2 and a positive bias in the carbon‑oxygen isotopic compositions at ~34.3 Ma. These results suggest that orbital forcing played a role in elevating pCO2 across a critical threshold during the E-O transition (~33.9 Ma).

Provenance of the Paleogene sediments in the Pearl River Mouth Basin, northern South China Sea: Insights from zircon U-Pb and fission track double dating

Abstract The Paleogene provenance of the Pearl River Mouth Basin in the northern South China Sea is poorly understood. In this paper, we present the first batch of zircon U-Pb and fission track double-dating ages of the Wenchang, Enping, and Zhuhai Formations in this basin. Together with data from previous provenance studies, the new dataset has been used to document the provenance shift during the Paleogene period. Characterized by a rather limited spectrum of both U-Pb and fission-track ages, the Eocene Wenchang Formation was probably mainly derived from intra-basinal uplifts. Multimodal distribution and large range of zircon ages of the Late Eocene-Early Oligocene Enping Formation is compatible with the age signatures of modern drainage sediments from the Southeastern Cathaysia Block. Age signatures of the Zhuhai Formation (Late Oligocene) and Enping Formation are indistinguishable, suggesting that they may have similar source areas and sediment provenance did not change dramatically during the Oligocene. Although variations in age patterns from the Wenchang Formation to the Enping-Zhuhai Formations imply a dominant provenance shift from local uplifts to the regional Cathaysia Block, it is difficult to exclude the influence of the local uplifts on sediments in the Enping-Zhuhai Formations.