Early Miocene Research Articles

Evaluation of thermal maturity of source rock based on geochemical method, a case study of Late Oligocene source rock in Hoa Tra field, Cuu Long basin, Vietnam

The  Hoa Tra field is located in the southeast of the Cuu Long basin which has great potential for oil and gas. The source rocks in this area are mainly fine-grained sediments from the late Oligocene and early Miocene, which are located in a continuous deposition zone and under anaerobic conditions. This paper focuses on studying the maturity level of late Oligocene source rock through geochemical analysis methods. Specifically, the paper uses indicators such as T­, R<sub>o</sub>, MPI-1, T<sub>s</sub>/(T<sub>s</sub>+T<sub>m</sub>) and C<sub>29 </sub>steran 20S/(20S+20R) from Rock-Eval Pyrolysis, Vitrinite reflectance and Gas chromatography-Mass spectrometry methods, respectively, to evaluate the level of thermal maturity of the source rock. The source rocks in the Hoa Tra field mainly contain types I and II, which are poor in vitrinite particles, leading to low R<sub>o</sub> measurement results. However, when R<sub>o</sub> is calculated from T<sub>max </sub>and (MPI-1), it indicates that the thermal maturity of source rock from the upper part of the late Oligocene is in the early mature stage (R<sub>o avg</sub>= 0.56%), while those from the lower part of late Oligocene are mature and in the phase of oil generation (R<sub>o avg</sub>= 0.65%). These results are similar to those obtained from assessing thermal maturity using T<sub>s</sub>/(T<sub>s</sub>+T<sub>m</sub> and C<sub>29 </sub>20S/(20S+20R) indicator (the average values for the respective parameters are 0.32 and 0.31 for the upper part, while those are 0.39 and 0.42 for the lower part of late Oligocene). The article also shows that the contribution to the  Hoa Tra field is not only from in-situ generated oil but also from migratory oil coming from nearby troughs.

Ultrahigh-temperature metamorphism revealed by felsic granulite xenoliths in southern Tibet

The Himalayan-Tibetan orogen experienced extensive thermal disturbance during the Indian-Asian collision, yet evidence for ultrahigh-temperature metamorphism (UHTM, >900 °C) remains scant. We report a UHTM event in the Lhasa Block, identified from felsic granulite xenoliths in Sangsang near the Tangra Yum Co Rift (TYR) of southern Tibet. These xenoliths exhibit a mineral assemblage of garnet + rutile + quartz + K-feldspar + plagioclase ± kyanite. Elemental mapping indicates garnet underwent high-temperature chemical homogenization. Employing Raman elastic geothermobarometry, Ti-in-quartz, and Zr-in-rutile geothermometers, we deduced pressure-temperature (P-T) conditions of 12−16 kbar and 870−975 °C, suggesting conditions were in the ultrahigh-temperature (UHT) range. The predicted P-T range is the most reliable estimation for Sangsang xenoliths to date, although the actual P-T conditions were likely higher due to viscous reset and chemical diffusion. An in situ garnet Lu-Hf date (ca. 26 ± 8 Ma) suggests that the UHT event occurred during the early Miocene, and in situ rutile U-Pb dates (ca. 12−11 Ma) indicate that the UHT conditions persisted until at least the timing of xenolith exhumation. Combined with the geological records in the Himalayan-Tibetan orogen, we suggest that the mantle upwelling contributed most to heating the Tibetan crust in the TYR and ultimately induced UHTM. Our study serves as an example of the UHTM formation mechanism within the active orogenic system.

Short Communication: Calcareous nannofossil assemblages and age determination in Leuwi Kenit, Ciletuh Palabuhanratu Geopark, Indonesia

Abstract. Pratiwi SD, Nurdrajat, Pratiwy FM, Chiyonobu S. 2024. Short Communication: Calcareous nannofossil assemblages and age determination in Leuwi Kenit, Ciletuh Palabuhanratu Geopark, Indonesia. Biodiversitas 25: 3200-3207. Our meticulous and comprehensive investigation of nannofossil diversity in Leuwi Kenit, Sukabumi District, West Java Province, Indonesia, a part of the Ciletuh Palabuhanratu UGG, where the study of nannofossils is limited, is of significant importance their implications for age determination and clarifying the geological history of the region. The initial documentation of calcareous nannofossils in the Leuwi Kenit traverse of the Cikarang Member within the Jampang Formation resulted in the identification of thirty-three species from 62 samples. The studied section reveals seven nannofossil biozones, arranged from oldest to youngest as follows: LO Sphenolithus ciperoensis; LO Cyclicargolithus abisectus; FO Sp. disbelemnos; FCO Helicosphaera carteri; FO Sp. belemnos; LCO Sp. belemnos; LO Sp. conicus and FCO Sp. heteromorphus. Notably, the abundance of Coccolithus pelagicus, Cyclicargolithus floridanus, Reticulofenestra spp., and Sphenolithus spp. characterized their occurrence throughout the section. The diversity and preservation of nannofossils were found to be good, and based on nannofossil biohorizons, the age of Leuwi Kenit spans from 24.3 million years ago to around 17.7 million years ago at Paleogene (Oligocene age) to Neogen Periode (Early Miocene age). Based on the nannofossils assemblages, results show that the area has many important indicator species, making it essential for future research on ancient ocean conditions and past climate changes within this geopark.

Resolving the conflictive phylogenetic relationships of Oceanites (Oceanitidae: Procellariiformes) with the description of a new species

The family Oceanitidae, formerly considered a subfamily of Hydrobatidae, includes all the small storm-petrels of the southern hemisphere. The ancestor-descendent relationships and evolutionary history of one of its genera, Oceanites, have been partially studied, yielding contrasting results. We revised the phylogenetic relationships of this group using Bayesian inference (BI) based on new sequence data of the mitochondrial gene Cytb and linear morphological measurements of all species and five subspecies-level taxa in Oceanites, including a new taxon from the Chilean Andes. Our BI results show that the Oceanites genus is monophyletic and composed of four well-supported clades (posterior probability > 0.95): (1) chilensis; (2) exasperatus; (3) gracilis, pincoyae, and barrosi sp. nov.; and (4) oceanicus and galapagoensis. The species O. chilensis is a basal clade within Oceanites. According to our time-calibrated tree, the split between Oceanites and the other genera in Family Oceanitidae is estimated to be ~35.9 Mya, and the oldest divergence within Oceanites (the split between O. chilensis and other Oceanites) was dated to the early Miocene, around c. 21.3 Mya. The most probable geographic origin of Oceanites is the Southern Ocean. The morphological data suggest continuous size variation between Oceanites taxa, ranging from smallest in gracilis to largest in exasperatus. Based on our phylogenetic hypothesis, and morphological analyses, we suggest elevating to species status the taxa galapagoensis, chilensis, and exasperatus, and we describe a new taxon barrosi sp. nov., thus recognizing a total of seven species within the genus Oceanites.

Depositional architecture of early rift non-marine systems and implications for reservoir development: Oligocene to Miocene Al Wajh Formation, Midyan Basin of Red Sea, Saudi Arabia

Exhumed early rift alluvial fan and braided fluvial fan systems are rare and consequently understanding facies distribution, stratal architecture, reservoir connectivity, and controls on stratigraphic development of these systems is quite difficult. The outcrops of early Oligocene to early Miocene Al Wajh Formation in the Red Sea region of Saudi Arabia provide an opportunity to investigate the depositional architecture and reservoir potential of an early rift alluvial to fluvial system. Eleven lithofacies (F1-F11) and four facies associations (alluvial fan facies association (FA1), braided fluvial fan facies association (FA2), floodplain facies association (FA3) and delta front facies association (FA4)) are identified in four subbasins (Maqna 1–2, Sukk and Ifal), suggesting deposition in an alluvial-braided fluvial fan system. FA1 (36–100%) and FA4 (9–13%) have higher relative proportion and widely distributed in the northern subbasins (Sukk and Ifal subbasins) while the relative proportion of FA2 (50–82%) and FA3 (18%) increases towards southern subbasins (Maqna-1 and Maqna-2 subbasins). Distribution of facies associations, relative reduction in the clast diameter, and general southward paleoflow direction suggest that the northern subbasins (Sukk and Ifal) represent the proximal settings and southern subbasins represent medial (Maqna-1 subbasin) and distal settings (Maqna-2 subbasin) of a large alluvial-braided fluvial fan environment. Stratal architecture of proximal conglomeratic channel deposits highlight that despite reasonable thickness and spatial connectivity (several meters), these deposits may have marginal reservoir quality because of poor sorting and high matrix content. Sandstone beds occurring in medial settings have a higher potential to act as hydrocarbon reservoirs by displaying excellent reservoir properties on the account of their significant thickness (up to few meters), coarse grain sizes, better sorting, considerable spatial connectivity (10s of meters) and lower proportion of interbedded mudstone.

Unraveling the tectonic evolution of the Andean hinterland (Argentina and Chile, 30°S) using multi-sample thermal history models

The Andean hinterland of Chile and Argentina defines a region of voluminous magmatism, polyphase deformation, and high topography above a long-lived subduction zone. Construction of this high-elevation domain has been variably attributed to isostatic uplift during crustal thickening induced by internal hinterland shortening, underthrusting during growth of the external thrust belt, or lower crustal flow. Alternatively, uplift may be related to dynamic processes associated with lithospheric foundering or flat-slab subduction. This study integrates geo/thermochronological, structural, magmatic, and sedimentological datasets to reconstruct hinterland evolution and evaluate potential drivers of exhumation. Our thermal history modeling approach implements a new time-depth extension to HeFTy 2 software, which enables simultaneous inversion of multiple samples along a structural or topographic profile. This extension addresses transient effects such as isotherm deflection and the transition from geothermal to atmospheric gradients and permits changes in the relative position among samples (folding, tilting) within user-defined constraints. Single- and multi-sample modeling results based on published apatite (U-Th)/He and fission track data (AHe, AFT) along the western Andean hinterland confirm late Eocene (∼30–40 Ma) cooling below at least ∼120°C, whereas thermal histories derived from new AHe, AFT, and zircon He analyses from the eastern hinterland resolve Mesozoic cooling below ∼80–160°C, followed by protracted residence at ∼60–80°C and rapid exhumational cooling in the early Miocene. Multi-sample models further require ∼10° eastward tilting of eastern sample locations, compatible with hinterland uplift via underthrusting and development of a crustal-scale fault-bend fold that was geometrically and kinematically linked to shortening in the external (eastern) thrust belt. Results underscore the enhanced resolution of multi-sample thermal history models in testing structural hypotheses and deciphering the timing, magnitude, and mechanisms of exhumational cooling during changing geodynamic conditions.

MEDITERRANEAN IMPRINT ON CORAL DIVERSITY IN THE INCIPIENT RED SEA (BURDIGALIAN, SAUDI ARABIA)

Abstract Globally, scleractinian coral diversity peaked in the Early Miocene (Burdigalian) and declined afterwards. In contrast to this global trend, scleractinian coral diversity in the Lower to Middle Miocene was low in the Red Sea, which had begun to open in the Oligocene and experienced its first marine incursion in the Burdigalian. Here, we report on coral diversity of reefs assigned to the Burdigalian to Langhian Wadi Waqb Member (Jabal Kibrit Formation) from outcrops exposed in the foothills behind the Red Sea coastline near Umluj, Saudi Arabia. Compared to the global records from the Paleobiology Database and the literature, the fossil record from the Wadi Waqb member suggests a relation of the taxonomic spectrum to the Arabian Gulf and the Mediterranean. No clear relation to the taxonomic spectrum to the Indian Ocean is observed. These faunal differences are consistent with the hypothesis that the young Red Sea was connected to the Arabian Gulf via the Mediterranean through the Gulf of Suez, but there was no connection between the Red Sea and the Indian Ocean because of the Afar mantle plume that uplifted the southern area of the Red Sea rift and blocked direct exchange of marine biota.

Biogeographic history of pigeons and doves drives the origin and diversification of their parasitic body lice.

Despite their extensive diversity and ecological importance, the history of diversification for most groups of parasitic organisms remains relatively understudied. Elucidating broad macroevolutionary patterns of parasites is challenging, often limited by the availability of samples, genetic resources, and knowledge about ecological relationships with their hosts. In this study, we explore the macroevolutionary history of parasites by focusing on parasitic body lice from doves. Building on extensive knowledge of ecological relationships and previous phylogenomic studies of their avian hosts, we tested specific questions about the evolutionary origins of the body lice of doves, leveraging whole genome data sets for phylogenomics. Specifically, we sequenced whole genomes from 68 samples of dove body lice, including representatives of all body louse genera from 51 host taxa. From these data, we assembled >2,300 nuclear genes to estimate dated phylogenetic relationships among body lice and several outgroup taxa. The resulting phylogeny of body lice was well supported, although some branches had conflicting signal across the genome. We then reconstructed ancestral biogeographic ranges of body lice and compared the body louse phylogeny to phylogeny of doves, and also to a previously published phylogeny of the wing lice of doves. Divergence estimates placed the origin of body lice in the late Oligocene. Body lice likely originated in Australasia and dispersed with their hosts during the early Miocene, with subsequent codivergence and host switching throughout the world. Notably, this evolutionary history is very similar to that of dove wing lice, despite the stronger dispersal capabilities of wing lice compared to body lice. Our results highlight the central role of the biogeographic history of host organisms in driving the evolutionary history of their parasites across time and geographic space.

Variations in aridity across the Asia–Australia region during the Neogene and their impact on vegetation

Scientific drilling provides extended records of continental environmental conditions during the Neogene in Asia and northern Australia. Spectral data allow reconstruction of the environment using abundances of hematite and goethite. Hematite formation is favoured by dry or seasonal conditions. Hemipelagic sites show the most regular records. Monsoon strengthening started in the Early Miocene and peaked at 17–20 Ma in the Bay of Bengal and at 10–15 Ma in southern China before weakening after c. 12 Ma and c. 10 Ma, respectively. The Indus Basin dried after c. 8 Ma and again after 3 Ma, while aeolian sediment sources to the Sea of Japan show increased aridity after 5 and 3 Ma. The Mekong Fan indicates increasing aridity after 6 Ma, similar to Eastern Australia. In contrast, NW Australia shows a trend towards wetter conditions after 8 Ma, with a humid period at 4–6 Ma followed by drying. There is a link between drying and vegetation in the Mekong and Pearl River basins, as well as Eastern Australia. Monsoon strengthening is linked to topographic uplift in the Himalaya, together with Tethyan gateway closure. Long-term drying is likely driven by global cooling since the Middle Miocene.

Biogeographical patterns of the porcelaneous larger foraminifer Alveolinella quoyi through the integration of fossil data

In the present-day Indo-Pacific coral-reef settings two genera of alveolinoidean porcelaneous larger benthic foraminifera (LBF) occur, namely Alveolinella and Borelis. Alveolinella is represented by a single species, A. quoyi, whose northernmost record is in Okinawa-jima (central Ryukyu Islands, Japan). Although the Indo-Pacific area, and especially the Coral Triangle, is a biodiversity hotspot since the Early Miocene, in-depth investigation on fossil representatives of present-day LBF is limited to a few taxa. To help bridge this knowledge gap, the palaeobiogeographical dynamics of A. quoyi is assessed. Analysis of data from the palaeontological literature shows that its first appearance datum is from the Tortonian (Late Miocene) of East Kalimantan and Papua New Guinea. In the Pliocene–Pleistocene the Indonesian Throughflow constrained the species within the Central Indo-Pacific. Finally, during the Late Pliocene the northward migrants arrived in the shallow-water carbonate settings of Okinawa-jima where the species is still thriving.

Fracture stratigraphy in oil-rich shale of the Upper Xiaganchaigou Formation (upper Eocene), western Qaidam Basin

Natural fractures in wells from oil-rich shale of the Upper Xiaganchaigou Formation in Qaidam Basin, the largest Cenozoic sedimentary basin in the Tibetan Plateau, vary systematically by stratigraphic position, allowing reconstruction of fracture generation processes during regional tectonic deformation. Compacted fractures, layer-bounded fractures, and low-angle (or bed-parallel) fractures were widely identified in oil-rich shale cores obtained from vertical wells. The layer-bounded fractures can be subdivided into two sets: one prevalent in light-colored (low gamma-ray) stiff layers and arrested by dark-colored (high gamma-ray) compliant layers, the other set primarily confined to the dark-colored compliant layers. The strikes of layer-bounded fractures, revealed by horizontal well FMI logs, are dominantly N60°-80°E and N10°-30°E within the light and dark intervals respectively. These strikes are consistent with natural hydraulic fracturing of the light layers during the Oligocene and the dark layers during the Middle Miocene-Holocene, based on a regional anticlockwise rotation of the compressional strain orientation. The abutment of layer-bounded fractures against low-angle (or bed-parallel) fractures in dark layers implies that the low-angle fractures were hydraulically fractured under a vertical least compressive stress during the Early Miocene, which produced the most intense tectonic compression. Layer-bounded fractures generally remain unmineralized in the dark layers but were mineralized in the light layers, suggesting that fractures confined in light layers would not decrease the sealing capacity of the shale oil system and fractures confined in dark layers can increase the pore space of shale oil storage. The Oligocene and Early Miocene fractures represented mechanical flaws that affected the initiation of Middle Miocene-Holocene fractures, implying that all natural fractures may alter hydraulic fracture stimulations to some extent.

Fracability Modeling in Shale Hydrocarbon Development Based on Geomechanical Analysis of Well Log and Mineralogy Analysis of Drill Cuttings: A Case Study of the Lower Baong Fm and the Belumai Fm of the NSB-001 Well in the North Sumatra Basin, Indonesia

Abstract The North Sumatra Basin, Indonesia has a very large shale hydrocarbon potential, especially from the Lower Baong Formation and the Belumai Formation. The Belumai and Lower Baong Formations were deposited in the Early Miocene - Middle Miocene, in transgression conditions, when the North Sumatra Basin was in the regional sagging phase. These conditions greatly affect the facies and diagenetic changes of quartz, smectite and kaolinite minerals in rock fracability. The main problem in producing shale hydrocarbons is the very low permeability of unconventional-shale-reservoirs, so that at the production stage of shale hydrocarbon, a fracability model is needed as a basis for planning hydraulic fracking, thus the fractures are connected to each other and can flow the hydrocarbon fluid optimally. From the results of previous studies, it was shown that the shale hydrocarbon fracability value was controlled by lithofacies, and correlated with rock brittleness, brittle mineral content, clay content, and compressive strength. The fracability model is used to determine fracable zone intervals as a candidate for hydraulic fracking, by using geomechanical analysis of well log data and mineralogy analysis of drill cuttings data from the NSB-001 well as a case study. Based on the correlation results of lithology (Mud Log), mineralogy analysis, MBT, and TOC (Drill Cuttings), the sweetpot fracable window (fracable zone interval) can be determined, which is correlated with the presence of fracture barrier.

Oceanography of the Eastern Equatorial Pacific Ocean Across the Oligocene‐Miocene Transition

AbstractThe functioning of the Pacific Ocean—the world's largest ocean—during a warmer‐than‐present paleoclimate state remains underexplored. We present planktonic and benthic foraminiferal stable oxygen (δ18O) and carbon (δ13C) isotope records from Integrated Ocean Drilling Program (IODP) Site U1334 that span the Oligocene‐Miocene Transition (OMT) interval, from 24.15 to 21.95 million years ago (Ma). We reconstruct (sub‐)surface and deep‐water conditions and provide better constraints on the physical and chemical oceanography of the eastern equatorial Pacific Ocean (EEP). Positive trends in planktonic and benthic foraminiferal δ18O values, mark a largely uniform imprint of increased land‐ice volume/global cooling on surface‐ and deep‐waters. We document a delayed planktonic foraminiferal δ18O increase across the OMT as well as an increase in the amplitude variability of planktonic foraminiferal δ18O values on eccentricity timescales during the early Miocene. We interpret this as an enhanced glacioeustatic sea‐level control on Atlantic‐Pacific salinity exchange through the Central American Seaway (CAS) or as the onset of more variable surface currents and oceanic fronts in the EEP. Positive trends in planktonic and benthic foraminiferal δ13C values characterize the whole‐ocean depletion in 12C linked to organic carbon burial during the Oligocene‐Miocene carbon maximum (CM‐OM). However, this depletion is more pronounced in the planktonic foraminiferal δ13C record, especially during ∼400 Kyr eccentricity minima, reflecting an increase in nutrient upwelling and the efficacy of the biological carbon pump (BCP) when global temperatures decreased across the OMT and during the early Miocene. Our study highlights the dynamic behavior of the EEP in a warmer‐than‐present unipolar icehouse state.

Stratigraphy of Salodik Formation in Batutambung section, Toili, Luwuk, Central Sulawesi

Abstract Salodik Formation is one of the formations representing a continental drift sequence prior to the collision of Banggai Basin, and it was widely cropped out in the East Arm of Sulawesi. The published geological maps of Luwuk dan Batui describe the Salodik Formation as massive and bedded limestone with marl intercalated and sandstone in the lower part, ranging from Eocene to Late Miocene. In Batutambung, Toili, the geological maps showing the Salodik Formation distributed in a NW-SE trend between two ophiolitic land masses, and suggesting intensive tectonic deformation along the trend. Preliminary geological investigations in this area show that Salodik rock sequences varied from other Salodik outcrops in different areas. Detailed stratigraphic measurements and rock sampling have been carried out along the road connecting Toili and Ampanan Tete., yielding a measured stratigraphy of 70 m vertically thick. Nine lithofacies were identified, i.e., coarse-grained sandstone (cS), micritic sandstone (mcS), fine-medium-grained sandstone (fmS), sandstone with coal spar (SMms), Mudrock (mM), micritic mudrock (mcM), Mudstone (MS), Wackestone (WS) and Packstone (PS). Facies association shows the rock sequence has shallowing upward or prograding pattern with depositional environment varies from an open marine, tidal channel, shoal, and restricted marine/ lagoon Based on foraminifera and nannofossil analysis, Salodik Formation in Batutambung section was deposited in Early Miocene (N6 or NN4-NN6), with paleobathymetry from middle to outer neritic. Based on its age and lithology character, the Salodik Formation in Batutambung is equivalent with Matindok Formation in Tomori PSC.

A new species of Pseudotsuga (Pinaceae) from the lower Miocene of Lesvos, Greece, and its palaeogeographical and palaeoclimatic implications

Pseudotsuga Carrière is commonly known as Douglas-fir, a minor genus attributed to Pinaceae. Four extant species are included in this genus, with dispersed distribution in East Asia and western North America. Pseudotsuga possesses a number of fossil records of needled leaves, seed cones, and seeds. Nevertheless, few reliable fossil wood of this genus has been specifically investigated. Here, we describe some anatomically well-preserved petrified fossil wood from the lower Miocene of Lesvos Island, Greece, as Pseudotsuga lesvosensis n. sp. The studied fossil wood remains are characterized by the presence of normal axial and radial resin canals, piceoid cross-field pitting, along with distinctive helical thickenings in cell walls consistent with Pseudotsuga. The present fossil conifer further enriches the species diversity of the Miocene Petrified Forest in Lesvos. The first report of Pseudotsuga lesvosensis from the southern Europe in this study extends the tempo-spatial distribution patterns of Pseudotsuga and contributes to further understanding of its origin and evolutionary history. The occurrence of P. lesvosensis fossil wood, as well as other palaeontological macrofloral composition evidence, indicates a humid warm temperate to subtropical climatic condition during the early Miocene within the Lesvos, which differs significantly from the current Mediterranean climate.

Return to the Ross Ice Shelf Project (RISP), Site J-9 (1977–1979): perspectives of West Antarctic Ice Sheet history from Miocene and Holocene benthic foraminifera

Abstract. In 1977–1978 and 1978–1979, the Ross Ice Shelf Project (RISP) recovered sediments from beneath the largest ice shelf in Antarctica at Site J-9 (∼82° S, 168° W), ∼450 km from open marine waters at the calving front of the Ross Ice Shelf and 890 km from the South Pole, one of the southernmost sites for marine sediment recovery in Antarctica. One important finding was the discovery of an active macrofauna, including crustaceans and fish, sustained below the ice shelf far from open waters. The sediment has a thin, unconsolidated upper unit (up to 20 cm thick) and a texturally similar but compacted lower unit (>1 m thick) containing reworked early, middle, and late Miocene diatom and calcareous benthic foraminiferal assemblages. A probable post-Last Glacial Maximum (LGM) disconformity separates the upper unit containing a dominantly agglutinated foraminiferal assemblage, from the lower unit consisting mostly of reworked Miocene calcareous benthic species, including Trifarina fluens, Elphidium magellanicum, Globocassidulina subglobosa, Gyroidina sp., and Nonionella spp. The presence of the polar planktic foraminiferal species Neogloboquadrina pachyderma and the endemic Antarcticella antarctica supports the late Miocene diatom age for the matrix of the lower unit. The microfossil assemblages indicate periods of ice sheet collapse and open-water conditions south of Site J-9 during warm intervals of the early, middle, and late Miocene, including the Miocene Climatic Optimum (∼17–14.7 Ma), demonstrating the dynamic nature of the West Antarctic Ice Sheet (WAIS) and Ross Ice Shelf during the Neogene. The foraminiferal assemblage of the upper unit is unique to the Ross Sea and suggests the influence of a sub-ice-shelf water mass proximal to the retreating post-LGM grounding zone. This unique assemblage is strongly dominated by the bathyal, cold-water agglutinated genus Cyclammina.

Unlocking clues on the provenance of the Zigzag Formation in Mountain Province, Northern Luzon, Philippines using sedimentary geochemistry

Abstract Provenance analysis using whole-rock geochemical data was carried out on the Late Oligocene to Early Miocene Zigzag Formation in southwestern Mountain Province, North Luzon, Philippines. Major and trace element geochemistry of the siliciclastic samples from the Zigzag Formation revealed a first-cycle mafic source of the detrital materials. This is consistent with other discrimination diagrams such as those using La, Sc, and Th to determine the provenance. Tectonic discrimination diagrams suggest an oceanic island arc signature for the source rocks of the Zigzag Formation. The Cretaceous to Eocene Lepanto Metavolcanics is proposed to be the source of the detrital materials comprising this clastic sequence. The results present additional information on the current understanding of the evolution of the Central Cordillera.

New insights into the structural and stratigraphic evolution of the Malay Basin using 3D seismic data: Implications for regional carbon capture and storage potential

AbstractThe Malay Basin is a mature hydrocarbon province currently being re‐assessed for CO2 storage. Selecting an appropriate storage site requires a comprehensive understanding of the structural and stratigraphic history of the basin. However, previous studies have been limited to observations from either regional 2D seismic lines or individual 3D seismic volumes. In this study, we access and utilise a basin‐wide (ca. 36,000 km2) 3D seismic and well database to describe the structural and stratigraphic features of the basin, particularly those within the uppermost ca. 4 km (Oligocene to Recent) and gain new insights into the basin's evolution. E–W transtensional rift basins first developed due to sinistral shear across an NW‐SE strike‐slip zone. The NW‐SE basin morphology seen today was generated during the late Oligocene–early Miocene, during which time dextral motion across marginal hinge zones created en‐echelon antithetic, extensional faults and pull‐apart basins, especially well preserved along the western margin of the basin. Collisional forces to the southeast during the early to middle Miocene resulted in the shallowing of the basin, intermittent connection to the South China Sea and a cyclic depositional pattern. Around 8 Ma (late Miocene), a significant uplift of the basin resulted in a major unconformity with up to 4.2 km of erosion and exhumation in the southeast. In the centre and northwest of the basin, the inversion of deeper E–W rifts resulted in the folding of Miocene sequences and the formation of large anticlines parallel to the rift‐bounding faults. The Pliocene to Pleistocene history is more tectonically quiescent, but some extensional faulting continued to affect the northwest part of the basin. Larger glacio‐eustatic sea‐level fluctuations during this time resulted in major changes in sedimentation and erosion on the Sunda Shelf, including the formation of a middle‐Pliocene unconformity. These structural events have created a variety of hydrocarbon traps across the basin of different ages, including transpressional anticlines, rollover anticlines and tilted fault blocks. Each of these has discrete and distinct trap elements with important implications for their CO2 storage potential.

Mineralogical and ore-fluid characteristics of the Onto Porphyry-HSE transition in Hu’u district, West Nusa Tenggara, Indonesia

Abstract Onto prospect is located in the eastern part of Sumbawa Island in West Nusa Tenggara Province, Indonesia. The island is situated within the Sunda-Banda magmatic arc, which extends from west to east, consists of volcanic rocks from the early Miocene to the Holocene. Onto is one of the highly potential prospects and holds significant economic value among ore deposits in Indonesia. The aim of this research is to explain the geological setting and determine the genesis of mineralization based on petrographic, ore microscopic, and fluid inclusion analyses. Eight lithological units have been identified, namely early andesites, polymictic diatreme breccia, upper sedimentary package, caping andesite, quartz-phyric andesite sill, early porphyry, early intra-mineral porphyry, and late intra-mineral porphyry. Nine samples were analyzed for petrography to identify the alteration types consisting of illite-smectite, quartz-dickite, residual vuggy quartz, silicic, quartz-alunite, quartz-pyrophyllite±diaspore, potassic, phyllic-chloritic, and propylitic alterations. Additionally, pyrite, chalcopyrite, and covellite are predominantly present in the study area. Meanwhile, native gold is found as an inclusion in covellite and pyrite. From micro thermometric analysis of fluid inclusions in quartz veins associated with multiphase intrusive rocks, it is shown that the homogenization temperature of early porphyry ranges on average from 279°C to 377°C, early inter-mineral porphyry ranges from 263°C to 361°C, and late intra-mineral porphyry ranges from 275°C to 336°C. The salinity level of the measurements ranges from 5.71 to 10.27% by weight NaCl equivalent, with an average of approximately 8.15% by weight NaCl equivalent. The mineralization formation during the early or prograde phase is associated with the EDM vein system with a mineralization set of biotite-magnetite, digenite, chalcocite, and coevellite, type A veins with quartz-sulfides + anhydrite, M veins type associated with quartz-magnetite-chalcopyrite-bornite mineralization, AB veins type associated with quartz-sulfides + anhydrite textures, and B veins type with sulfide centerline characteristics. The mineralization system during the early stage is generally associated with potassic alteration (K-feldspar-magnetite-secondary biotite). Whereas the retrograde phase, the vein system is associated C veins type with sulfide mineralization groups consisting of massive chalcopyrite + quartz with phyllic alteration (quartz-muscovite-chlorite ± dickite), D veins type with quartz-pyrite+chalcopyrite associated with advanced argillic alteration (quartz-alunite-pyrophyllite ± diasporite), E veins representing the epithermal stage associated with minerals such as quartz-carbonates, galena, sphalerite, chalcopyrite, tetrahedrite-tennatite in the alteration stage of kaolinite-illite-smektite-sericite.

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

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