Lower Miocene Research Articles

Fossil Samaras of Acer in the Lower Miocene of Central Inner Mongolia, China, and Their Phytogeographical Implications

Fossil Samaras of Acer in the Lower Miocene of Central Inner Mongolia, China, and Their Phytogeographical Implications

Surviving a Dark Age: The Oldest Baleen-Bearing Whales (Cetacea: Chaeomysticeti) of Pacific South America (Lower Miocene, Peru).

The evolution of baleen whales (Mysticeti) comprises two main phases, namely, (i) a Paleogene phase, which saw the diversification of stem lineages, and (ii) a Neogene phase, dominated by modern-looking, toothless, baleen-bearing forms in the monophyletic group Chaeomysticeti. These two phases are separated by a global turnover event coinciding with a gap-or "dark age"-in the mysticete fossil record. This dark age occurred between 23 and ~18 Ma and is apparently detected worldwide, except in Zealandia. Here, we report on a new mysticete fossil from the Lower Miocene (Burdigalian: ~19.2 Ma) strata of the Chilcatay Formation cropping out at the newly discovered locality of Cerro Tiza (East Pisco Basin, Peru), which represents a limited but precious testament from the last phase of the baleen whale dark age. Two previously mentioned, slightly geologically younger fossils from the same formation are also reappraised herein, revealing the occurrence of at least another baleen whale taxon in the upper Chilcatay strata-one that belongs in the mysticete crown group. Although the Early Miocene remains a problematic time interval for the fossil record of baleen whales, our new results encourage the search for mysticete fossils in the Lower Miocene strata of the East Pisco Basin, whose basin fill preserves a cornucopia of extraordinarily informative marine vertebrate fossils of the Cenozoic age, as well as in coeval deposits worldwide.

Lithofacies, architectural elements and depositional system of the Los Colorados Formation (Eocene - Lower Miocene), Tonco Valley, NW Argentina

Lithofacies, architectural elements and depositional system of the Los Colorados Formation (Eocene - Lower Miocene), Tonco Valley, NW Argentina

New observations on extinct Malvaceae from the lower Miocene of Japan: Fruit morphology, taphonomy, biogeography and paleocology of Banisteriaecarpum and associated foliage, pollen and wood

New observations on extinct Malvaceae from the lower Miocene of Japan: Fruit morphology, taphonomy, biogeography and paleocology of Banisteriaecarpum and associated foliage, pollen and wood

The Lower Miocene Askazansor Formation in Central Kazakhstan: Paleontological characteristics, biostratigraphy and paleogeographical conditions

The Lower Miocene Askazansor Formation in Central Kazakhstan: Paleontological characteristics, biostratigraphy and paleogeographical conditions

Upper Oligocene to Pleistocene planktonic foraminifera stratigraphy at North Atlantic DSDP Site 407, Reykjanes Ridge: diversity trends and biozonation using modern Neogene taxonomic concepts

Abstract. Deep Sea Drilling Project (DSDP) Site 407, located near the Reykjanes Ridge (southwest of Iceland) offers a rare and extensive record of Late Cenozoic planktonic foraminifera evolution spanning the Neogene and Quaternary periods. This ca. 300 m sequence provides a nearly continuous record of planktonic foraminifera with mostly good preservation quality, aiding the study of pelagic diversity changes over the past 25 million years as the modern North Atlantic Ocean system evolved. Initially investigated in 1979 by Poore, this study presents a taxonomic reassessment of upper Oligocene to Pleistocene planktonic foraminifera at Site 407, including species range documentation, assemblage analysis, biostratigraphic zonation, and age modelling based on planktonic foraminifera, calcareous nannofossils, and scanning electron microscopy. This study employs modern taxonomic perspectives that integrate morphological and stratophenetic frameworks for fossil species with genetic data for taxa having living representatives. Systematic species counts enable quantitative diversity analysis, with a particular focus on the genus Neogloboquadrina, which becomes increasingly prevalent at Site 407 from the late Neogene to Quaternary. The planktonic foraminifera assemblages at Site 407 exhibit a contraction in diversity and a shift in species dominance, notably around 160 m b.s.f. (metres below seafloor) (ca. 8.9–16.5 Ma) and 56 m b.s.f. (ca. 2–3.4 Ma). The upper Oligocene and lower Miocene include species belonging to the genera Catapsydrax, Globoturborotalita, Dentoglobigerina, and Paragloborotalia. An acme of “Ciperoella” pseudociperoensis (lower and middle Miocene), still of uncertain generic affiliation, may have biostratigraphic use. Well-preserved Turborotalita quinqueloba are relatively common throughout the sequence. In Oligocene and Miocene material, T. quinqueloba is accompanied by Tenuitella spp. From the upper Miocene onwards, neogloboquadrinids including Neogloboquadrina praeatlantica, N. atlantica, N. incompta, and N. pachyderma become increasingly common and dominate Pliocene assemblages, together with Globigerina bulloides. Assemblages with an increasingly high-latitude nature, i.e. where N. pachyderma dominates, take over in the lower Pleistocene. Multiple hiatuses are recorded, of which the largest is ca. 8 million years long, separating the middle and upper Miocene (8.9–16.5 Ma; 158.56–160.06 m b.s.f.). Continuous biozonation at Site 407 is challenged by limited species diversity and the absence of standard low-latitude biozone markers, rendering standard schemes ineffective. Recognizable biozones include the low-latitude O7 and M1 Zones in the late Oligocene and early Miocene, respectively; the high-latitude Neogloboquadrina atlantica sinistral Zone in the late Miocene and Pliocene; the Globoconella inflata Zone in the late Pliocene; and the Neogloboquadrina pachyderma Zone in the Pleistocene. The nannofossil biozonation faces similar challenges. A revised biostratigraphic age model integrates calibrated planktonic foraminifera and nannofossil events, incorporating abundant species like “C.” pseudociperoensis, N. atlantica dextral and sinistral, Globoconella puncticulata, G. inflata, and N. pachyderma. These findings are expected to contribute to the Neogene–Quaternary Middle Atlas of planktonic foraminifera and potentially improve the use of neogloboquadrinids in palaeoceanography and biostratigraphy.

The origin of island dolostones: Novel insights from in situ Mg isotope, major and minor elements analyses of Miocene dolostones on Shidao, the Xisha Islands

ABSTRACTZoned dolomite crystals, characterized by their dirty core and clear outer rim, are common in most island dolostones. The conditions under which these dolostones formed, however, remains controversial. To explain the origin of island dolostones, here, in situ determinations of Mg isotopic compositions, major (Ca, Mg) and trace (Fe, Mn, Sr, Na) elemental concentrations are carried out for the cores and rims of zoned dolomite crystals for dolostone samples from the Sanya Formation (Lower Miocene) and Meishan Formation (Middle Miocene) of well XK‐1 drilled on Shidao Island, the Xisha Islands. For all of the dolomite crystals, both cores and rims are formed of high‐Ca calcian dolomite, but the cores have higher %Ca, Sr and Na concentrations than the rims. Moreover, the cores (−3.85 to −2.95‰) have ca 0.2 to 0.5‰ lower δ26Mg values than the rims (−3.34 to −2.60‰). The difference in δ26Mg values between the dolomite crystal core and the rim cannot be explained by the presence of calcite inclusions or dolomite recrystallization, but rather reflect the nature of Mg isotopic fractionation due to the growth of the dolomite crystals during different stages of replacement. For zoned dolomite crystals, the progressive decrease in Ca and trace element concentrations but increase in δ26Mg values from dirty core to clear rim demonstrate that: (i) the dolomite crystal cores grow via a diffusion‐limited process; and (ii) the rims form through an incremental process (interface‐controlled) whereby the zone of dissolution/dolomite precipitation was very thin and simply repeated many times until it had fully developed. This growth model of zoned dolomite crystals may be applied to dolostones that share similar zoned patterns in petrography and geochemistry throughout the world.

Maximizing oil production efficiency in mature field: a strategic approach to optimizing recovery and extending field life

This study develops a strategic approach to optimize oil production efficiency in Block X, a mature field on Vietnam’s southern continental shelf. Using comprehensive production data from the Lower Miocene, Oligocene, and Basement formations, the study identifies key challenges, including declining reservoir pressure, increased water cut, and reduced gas lift performance. Proposed solutions include hydraulic fracturing, acidization, gas lift optimization, and seawater injection, systematically tailored to Block X’s geological and reservoir conditions. A key contribution is the application of the Adaptive Neuro-Fuzzy Inference System (ANFIS) as a predictive tool for selecting hydraulic fracturing candidates. ANFIS demonstrated superior performance, achieving an R2 of 0.95 for training and 0.71 for testing datasets, highlighting its effectiveness in managing data uncertainties and optimizing well selection. The study presents a systematic methodology integrating advanced modeling with field-specific strategies, providing scientifically grounded solutions for improving production efficiency in mature oilfields.

Volcanic Evolution of the Southern Mountain Neogene Magmatic Belt in Baturagung Range Central Java, Indonesia

Various Neogene volcanic rocks associated with calcareous sediments compose the Southern Mountain located from Imogiri at Yogyakarta to Wonogiri at Central Java, Indonesia. The volcanic rocks are concentrated along the north side of the Mountains which was recognized as the Neogene magmatic arc of Java; Baturagung is part of them. The discussion focuses on the relationship between the volcanic features and its basin evolution. The methods were geological fieldwork including measuring sections and collecting samples, thin sections, XRF, AAS, and micropaleontology. More than 13 circular hogbacks, circular valleys, and domes are observed based on the SRTM image. The petrology observed pyroxene-rich basaltic volcanic rocks, dacitic volcanic rocks, andesitic volcanic rocks, coral-rich volcanic breccia. From the bottom to the top, its stratigraphy of the Kebo-Butak Formation that notes basaltic volcanic rocks, pumice-rich lapillistone, and tuff of Early Neogene (before P4-N6) with muddy sandstone above the basalts of N 5-6 (Lower Miocene). The Semilir Formation consists of pumice-rich and dacitic tuff. Andesitic volcanic rocks of the Nglanggeran Formation with inlayer of marl of N 13-14. In the volcanic rocks trace elements show REE-rich (84-140 ppm), higher mobile elements of Sr (~480-602 ppm), medium Rb (22.2-23.7 ppm), and medium La (12-21 ppm), and wide range of immmobile trace elements of Nb ~2.7-7.9 ppm, Zr ~53-171 ppm, P ~840-1300 ppm, higher Ti (4400-4900 ppm), and higher Vanadium (V) of 92-302 ppm. Plot TiO2 vs. Al2O3 into the volcanic rocks indicates those were volcanism within plate boundaries. Plot Ta/Th vs Th/Hf shows continental extensional volcanism to continental arc margin. Plot Nb/Zr vs. Th/Zr explains the transitional zone to continental arc volcanism. The spider diagram shows Rb, Ce, P, Ti, Sr, Zr, and Y strongly metasomatized as a result of the upper continental arc. The chondrite normalized REE shows negative trends for Tb, Tm, Lu, Sm, and Eu and positive trends for Ce, Gd, Zr, and Y, which indicate superimposed volcanism under a submarine environment during the Neogene Period. Stratigraphic sections inform were periodically active volcanism. The Kebo-Butak Formation was formed by multiple volcanoes, some of which were active simultaneously while others were not, though both fall within the Early Neogene age range. Similarly, the Semilir and Nglanggeran Formations were also formed by volcanic activity from several volcanoes that were active at different times and overlapped with each other.

The oldest swallow (Aves: Passeriformes: Hirundinidae)from the upper Lower Miocene of the southof Eastern Siberia

A new genus and species of fossil swallows, Miochelidon eschata gen. et sp. nov., are described from the Lower Miocene of the Baikal region. This is the only Miocene representative of the Hirundinidae and the oldest fossil record of the group. The coracoid of Miochelidon combines generalized and specialized features, suggesting a basal position in the family for the genus. The ulna has a more advanced morphology, like in modern swallow. The find indicates a widespread distribution of basal swallows in the Early Miocene.

Alteration and mineralization features of the Akcal epithermal gold deposit, Biga Peninsula, Western Anatolia, Turkey: evidence for low-sulfidation Au–Ag–Sb mineralization

The Akcal epithermal Au–Ag deposit, located on the Biga Peninsula, southwestern Turkey, is estimated to contain 32.5 million tonnes of mineralization at an average grade of 1.03 g t −1 Au, equivalent to 1.2 million ounces of gold. Epigenetic mineralization is hosted by the lower Miocene Sapci volcanic suite, a calc-alkaline subduction-related volcanic arc consisting mainly of andesitic and basaltic andesitic rocks, while the coeval Soma Formation is composed of sandstone, claystone, conglomerate, marl-limestone, siltstone and tuffite. Near the base of the volcanic pile, the Karakaya Complex is represented by limestone blocks of various sizes (from a few millimetres to several kilometres) within the Sapci volcanic suite. Short-wave infrared and X-ray diffraction analyses have shown that kaolinite, illite–smectite, montmorillonite, illite, beidellite, gypsum, vermiculite, quartz and chlorite are the most common alteration minerals at Akcal. The alteration mineralogy indicates that the temperature of hydrothermal fluids broadly varied from 150° to 230°C, with a mildly acidic pH range of 5.0–6.5; these conditions accompanied by a lack of alunite suggest low-sulfidation epithermal conditions for the formation of the Akcal epithermal Au–Ag deposit. Petrographic examination and microanalysis by scanning electron microscopy indicate that mineralization at Akcal consists of pyrite and arsenopyrite with lesser hematite, ilmenite, rutile and apatite. The occurrence of pyrite and hematite on cleavage planes of hydrothermal biotite is evidence of replacement of an earlier potassic alteration phase. Further petrographic examination of the basal limestone reveals replacement by sulfides displaying complex colloform intergrowths of pyrite and marcasite. The lack of skarn alteration and marble supports a carbonate-replacement origin for sulfides occurring in the basal limestone and is likely related to epithermal mineralization at Akcal. Geochemical results show that an Au–Ag–As–Sb ± Pb ± Zn signature is dominant across the volcanic-hosted epithermal mineralization and carbonate-replacement occurrences in the basal limestone; the lack of Cu mineralization (<92 ppm) is typical of low-sulfidation epithermal systems. Strong positive correlations between Au and Ag ( r ′ = 0.77), As (0.80) and Sb (0.86) indicate that gold is likely hosted as a sub-microscopic phase within pyrite, arsenian pyrite and arsenopyrite; visible gold (or free gold) and (or) electrum were not identified at Akcal. A model of fluid mixing involving shallow circulating meteoric waters with deeper magmatic-hydrothermal fluids is proposed for the genesis of gold mineralization at Akcal; a positive Au correlation with Mo ( r ′ = 0.74) and Mo concentrations of up to 582 ppm support the involvement of magmatic-hydrothermal fluids in the epithermal system. Boiling as a mechanism for gold precipitation is unlikely given palaeodepths of greater than 250 m (3.0–5.0 MPa) and model temperatures of 150° to 230°C. Mineralogical and textural evidence of boiling, such as adularia, bladed calcite and lattice-texture quartz, have not been observed at Akcal. However, the presence of a silica-rich blanket with chalcedony (at the Akcal surface) and having formed near the palaeo water table may indicate some involvement of CO 2 -rich steam-heated waters, which could suggest some influence from boiling at depth, although evidence of boiling has not been confirmed in samples from Akcal. Precipitation of gold during the fluid mixing process would have occurred through a shift to higher f O 2 conditions and destabilization Au–thiosulfide complexes. This shift towards higher f O 2 is supported by the occurrence of gypsum and hematite with gold mineralization at depth.

Analysis of structural architecture in western Saudi Neom City area, northwestern Arabian Plate: Field investigation

Analysis of structural architecture in western Saudi Neom City area, northwestern Arabian Plate: Field investigation

Unveiling the paleosalinity constraints on southern peri-Pannonian Lower Miocene lacustrine systems in Serbia and Bosnia and Herzegovina: Lopare (Dinaride Lake System) versus Toplica basin (Serbian Lake System)

Unveiling the paleosalinity constraints on southern peri-Pannonian Lower Miocene lacustrine systems in Serbia and Bosnia and Herzegovina: Lopare (Dinaride Lake System) versus Toplica basin (Serbian Lake System)

From Circum Red Sea Sources to the Levant Basin Sink: An Integrated Provenance Study of Oligocene–Miocene Siliciclastic Sediments From Deep‐Sea Boreholes

ABSTRACTThe Levant Basin of the Eastern Mediterranean accumulated voluminous siliciclastic sediments during the Oligocene–Miocene. The deep‐sea section has attracted significant interest as it contains world‐class hydrocarbon reservoirs (‘Tamar Sands Play’). Our recent sandstone provenance study revealed that the hydrocarbon‐bearing, lower Miocene ‘Tamar Sands’ were recycled from older quartz‐rich sandstones that covered the Arabian flank of the Red Sea Rift. However, sandstones constitute just a third of the thickness of the Oligocene–Miocene siliciclastic section in the Levant Basin, with the rest being mainly composed of shales. Unravelling the provenance of the shale fraction is therefore essential for a comprehensive reconstruction of the Oligocene–Miocene source‐to‐sink system of the Levant Basin. In the present study, we examined the mineralogy and Sr‐Nd isotopes of clay samples retrieved from deep‐sea boreholes that penetrated the Oligocene–Miocene siliciclastic section. The isotopic composition of most clay fractions resembles that of Nile Delta sediments, indicating that unlike the ‘Tamar Sands’, their dominant provenance lay in NE Africa. Our investigations show that they were derived from Neoproterozoic basement rocks of the Arabian‐Nubian Shield and Tertiary continental flood basalts. The absence of chlorite and serpentine negates detrital contribution from the Arabia‐Eurasia suture in the north. Compilation of the available thermochronology data and major geologic events shows that the accumulation of the siliciclastic section in the Levant Basin coalesced with uplift of the continental areas around the Red Sea. The marked switch to shale deposition recognised in the basin during the late early Miocene signifies the downfall of the ‘quartzose’ Arabian sediment transport system, when it was partially captured by the evolving Dead Sea Transform valley. Our study highlights the strength of coupling sand and clay provenance investigations in source‐to‐sink studies of sedimentary basins.

Inferring the locomotor ecology of two of the oldest fossil squirrels: influence of operationalization, trait, body size and machine learning method.

Correlations between morphology and lifestyle of extant taxa are useful for predicting lifestyles of extinct relatives. Here, we infer the locomotor behaviour of Palaeosciurus goti from the middle Oligocene and Palaeosciurus feignouxi from the lower Miocene of France using their femoral morphology and different machine learning methods. We used two ways to operationalize morphology, in the form of a geometric morphometric shape dataset and a multivariate dataset of 11 femoral traits. The predictive models were built and tested using more than half (180) of the extant species of squirrel relatives. Both traditional models such as linear discriminant analysis and more sophisticated models like neural networks had the greatest predictive power. However, the predictive power also depended on the operationalization and the femoral traits used to build the model. We also found that predictive power tended to improve with increasing body size. Contrary to previous suggestions, the older species, P. goti, was most likely arboreal, whereas P. feignouxi was more likely terrestrial. This provides further evidence that arboreality was already the most common locomotor ecology among the earliest squirrels, while a predominantly terrestrial locomotor behaviour evolved shortly afterwards, before the vast establishment of grasslands in Europe.

Facies Analysis and Depositional Environment of the Euphrates Formation in Selected Sections, Southwestern Iraq

The current study aims to investigate the microfacies, diagenetic processes, and depositional environments of the twenty-three rock samples from the Lower Miocene Euphrates Formation of two outcrops collected at Bahar-Najaf in Najaf governorate and Al-Shinafiya in Diwaniyah governorate, southern Iraq. Thin sections of these samples were prepared; hence, the nature of microfacies and related depositional environments was established. The formation contains abundant benthonic foraminifers and other fossils like miliolids, echinoderm, coral, gastropods, and pelecypods. Seven distinct microfacies were identified: lime mudstone, miliolid packstone, Pelletal Grainstone, Lime Wackestone, Peloidal Grainstone, Oolitic grainstone Microfacies, and fossiliferous packstone packed with bioclasts. Diagenetic processes evidenced in this study are dissolution, neomorphism, cementation, micritization, and dolomitization. The Euphrates Formation was deposited in a shallow marine setting with fluctuating energy levels, ranging from low-energy lagoons to moderate and high-energy shoals and fore-reef environments. Various microfacies indicate a complex depositional environment on a carbonate platform influenced by biota and sedimentary processes. These observations support the inference that the Euphrates Formation accumulated in shallow open marine, inner barrier, and shoal settings, providing a detailed interpretation of its para-depositional and diagenetic development.

Spread and frequency of explosive silicic volcanism of the Carpathian-Pannonian Region during Early Miocene: Clues from the SW Pannonian Basin and the Dinarides

Spread and frequency of explosive silicic volcanism of the Carpathian-Pannonian Region during Early Miocene: Clues from the SW Pannonian Basin and the Dinarides

Evaluating offshore legacy wells for geologic carbon storage: A case study from the Galveston and Brazos areas in the Gulf of Mexico

Evaluating offshore legacy wells for geologic carbon storage: A case study from the Galveston and Brazos areas in the Gulf of Mexico

Stratigraphic overview of the Neogene Transylvanian Basin, Romania

Although the Transylvanian Basin is often referred to as part of the Miocene Pannonian Basin System, we point out important tectonic and sedimentary differences reflected in the stratigraphy. The Neogene of the Transylvanian Basin can be split into two tectonic megasequences: Lower Miocene and Middle to Upper Miocene. The Early Miocene onset of the back-arc extension in the Pannonian Basin was not observed in the Transylvanian Basin, where shallow to deep-marine sediments were deposited in a flexural basin. Diverse types of facies developed in the continental (e.g. Sânmihai) and marine (e.g. Coruș, Chechiș, Hida) Lower Miocene formations. Although these are poorly constrained to the standard biostratigraphy, recent 87 Sr– 86 Sr data support their poor correlation with the Burdigalian. The Middle Miocene extension in the Transylvanian Basin is very weak compared with that in the Pannonian Basin. As the high-resolution Middle Miocene regional biostratigraphy of the Badenian and Sarmatian was calibrated by magnetostratigraphy and radiometric dating of the volcanic tuffs up to the Serravallian/Tortonian boundary, the influence of the regional tectonics on the basin's evolution and stratigraphy can be better understood. The regional tectonics produced a progressive basin isolation starting with the late part of the Middle Miocene. The uplift of the Carpathians isolated the basin at the beginning of the Pannonian and initiated the fan delta, lacustrine and fluvial sedimentation. The basin was rapidly uplifted and eroded while post-extensional subsidence was still ongoing in the Pannonian Basin. Endemic fossil assemblages allow only regional-scale correlations for this interval. The uppermost part of the stratigraphic record is represented by the volcano-sedimentary formations generated by the late evolution of the Eastern Carpathians. The sedimentation and subsidence pattern of the Transylvanian Basin reflect the effect of retreating subduction zone in the Eastern Carpathians.

Calibrating lower-middle Miocene mammal faunas and unravelling climate change during the Miocene Climate Optimum: the Bardenas Reales de Navarra record (Ebro basin, NE Iberian Peninsula)

AbstractThe chronology of lower Miocene Iberian small mammal faunas is still poorly constrained given the scarcity of well dated sedimentary successions including small mammal fossil localities. Such scarcity has prevented also an accurate understanding of the response of European terrestrial ecosystem to global changes across the Miocene climate optimum (MCO), one of the best analogues of present-day global warming. Here we present an updated fossil small mammal record of the Bardenas Reales de Navarra (western Ebro basin, Spain), where an expanded lower to middle Miocene continental succession is superbly exposed. Previous and new magnetostratigraphic results from this succession have enabled us to propose, along with additional magnetostratigraphically-dated Iberian faunas, a new chronology for local zones Y to D (Mammal Neogene zones MN2 to MN5). In addition to that, the studied small mammal faunas point to a gradual increase in temperature and humidity conditions in SW Europe between 20 and 15.5 Ma, which appears to be coupled with the progressive shift towards warmer regional (Atlantic) and global conditions across the MCO, thereby pointing to gradual changes in oceanic circulation as the main driver of this period of global warmth. The evolution of sedimentary facies appears to indicate a threshold response of the Ebro basin hydrological balance to the MCO, whereas pedogenic formation of magnetic minerals seems to be linked to periods of enhanced climate variability. These results highlight the need of combining different paleoenvironmental indicators in order to obtain a reliable view of the response of continental ecosystems to global warming.