Cenozoic Sediments Research Articles

Towards a clarification of the provenance of Cenozoic sediments in the northern Qaidam Basin

Towards a clarification of the provenance of Cenozoic sediments in the northern Qaidam Basin

PETROLEUM GEOLOGY OF THE NOGAL BASIN AND SURROUNDING AREA, NORTHERN SOMALIA: PART 1, STRATIGRAPHY AND TECTONIC EVOLUTION

In this study, 92 closely‐spaced reflection seismic profiles (∼4000 line‐km) were tied to biostratigraphic and lithological data from six deep exploration wells in the poorly‐known Nogal rift basin, northern Somalia, and were integrated with outcrop and aeromagnetic data to investigate the basin stratigraphy and tectonic evolution. Aeromagnetic data show NW‐SE trending magnetic anomalies which are interpreted as plutonic bodies intruded during the Early Cretaceous, probably contemporaneously with a pre‐Cenomanian uplift phase. The aeromagnetic data also suggest a change of basement type from Inda Ad Series metasediments in the SE of the study area to igneous and high‐grade metamorphic basement in the NW. Biostratigraphic data and seismic reflection profiles define the Nogal Basin as a WNW–ESE striking half‐graben, approximately 250 km long and 40 km wide, which formed as a result of mainly Cenomanian–Maastrichtian and Oligocene–Miocene intracontinental rifting. The depocentre contains at least 7000 m of Mesozoic and Cenozoic sediments and is located in the centre of the basin (east of well Nogal‐1), to the south of the Shileh Madu Range. To the north, the basin is bounded by a major border fault along which significant variations in the thickness of sedimentary units are observed, suggesting that the fault controlled basin architecture and patterns of sedimentation. Oligocene–Miocene normal faults which resulted in north‐tilted fault blocks are widespread within the main basin; smaller‐scale sub‐basins oriented NW‐SE to WNW‐ESE are observed to the NW of the basin and probably developed contemporaneously.The Late Jurassic rift phase which has been documented elsewhere in northern Somalia is either missing in the Nogal Basin or is preserved only in localised grabens in the western and central parts of the basin. This is probably due to the pre‐Cenomanian uplift and erosion which removed almost the entire Jurassic and Lower Cretaceous successions over a wide area referred to as the Nogal‐Erigavo Arch. A more pronounced rifting episode followed this erosional event in the Cenomanian–Maastrichtian and resulted in the deposition of well‐sorted fluvio‐deltaic sandstones (Gumburo and Jesomma Formations), more than 2000 m thick. In wells in the Nogal Basin, these formations are between two and three times thicker than in wells drilled in footwall locations, and include excellent reservoir rocks sealed by transgressive mudstones and carbonates. A final rifting event in the Oligocene–Miocene was related to the opening of the Gulf of Aden. A rift sag phase which accommodated the Early Oligocene continental sediments of the Nogal Group initially developed at the centre of the basin. This was followed by a period of strong rotational faulting and tilting, which reactivated the Cenomanian–Maastrichtian structures.

Application of Magnetic Resonance Data for Groundwater Prospectivity in The Fitzroy Basin, Western Australia

In the Fitzroy Basin of Western Australia, both ground and borehole Nuclear Magnetic Resonance (NMR) techniques have been applied during a groundwater prospectivity assessment of the Cenozoic sediments, and the Palaeozoic and Mesozoic sandstone aquifers. Ground magnetic resonance (GMR) data was acquired at ten sites from the within the basin and the data was inverted to obtain estimates of the one-dimensional water content for the top 100m. Where these sites were co-located with boreholes, the results were consistent with inverted borehole NMR data and other litho-stratigraphic information although with a reduced horizontal resolution. Both the GMR and borehole data suggest that the Palaeozoic Grant Group and Poole Sandstone have high water content and have the potential to be highly productive aquifers. Furthermore GMR can be used estimate the depth to water table for unconfined aquifer, using 5 volume % as the threshold for saturated zone.

Structural setting of the Koum sedimentary basin (north Cameroon) derived from EGM2008 gravity field interpretation

Abstract This study is a contribution to the planning of hydrocarbon exploration program of the Koum sedimentary basin in North Cameroon. 3D modeling of WGM2012 gravity data derived from EGM2008 geopotential model in the Koum basin was used together with existing geological and spectral analysis information to give structural picture of the basin. The 3D model of the Koum basin confirms that the basin is developed as a half graben bounded by sub-vertical faults. The thickness of the Cenozoic sediments is about 1.5 km in the eastern part and reaches 4.5 km in the western part of the basin. Gravity lineaments computed by multi-scale analysis revealed structural trends in the E–W, NW–SE, NE–WS and N–S directions. The faults in the sedimentary terrain reach 6 km depth and have a predominant NW–SE trend with E–W trending faults along the contact between the sedimentary section and the basement complex in the northern edge.

Mode conversion noise attenuation, modelling and removal: case studies from Cyprus and Egypt

The wavefront from a source that strikes an acoustic impedance contrast separates into four variables: transmitted and reflected compressional waves (P-wave) and transmitted and reflected shear waves (S-wave). Converted waves are those whose mode changes at the interface and can be recorded in a marine environment in the presence of large velocity contrasts. The difference in acoustic velocities at the boundary means that in high contrast media the distinction between compressional and shear velocities across the boundary is small; shear waves are not as refracted as compressional waves and can be recorded as partially converted reflected energy. The eastern Mediterranean contains two major sedimentary basins, the Levantine and Herodotus (see Figure 1). They have shared depositional regimes and therefore contain similar sedimentary sequences. The Levantine Basin sits offshore Cyprus, Israel, Palestine and Lebanon, and was formed by prolonged phased rifting, with subsequent subsidence and loading by deltaic sediments (Gradmann et al., 2005; Gardosh and Druckman, 2006). It forms a northeast-oriented depression about 2 km deep and contains more than 14 km of Mesozoic and Cenozoic sequences that include up to 2 km of Messinian salt (Druckman et al., 1995; Ben-Avraham et al., 2002; Netzeband et al., 2006). The local reservoirs include both Cretaceous and Jurassic sandstones and limestones, with older potential reservoirs in the Triassic sandstones. Shales and marls of Paleogene, Neogene, Cretaceous and Jurassic, along with Messinian salt and Triassic evaporites all form local seals (Roberts and Peace, 2007) in plays of a both stratigraphic and structural nature. The Herodotus Basin is a northeast trending 3 km deep depression, and represents a slab of the Early Mesozoic Neo-Tethys Ocean (see Figures 1 and 2). The basin is bounded on the north by the Mediterranean Ridge; on the northeast by the west portion of the Cyprus Arc and on the south by the Nile Delta cone. The Eratosthenes Seamount (ESM), a fragment of continental crust (Robertson, 1998), forms the eastern limit of the Herodotus Basin, and additionally delineates the boundary between the cone of the Nile Delta, the Herodotus Basin, and the Levant Basin. The Herodotus Basin itself contains up to 7 km of Mesozoic–Cenozoic sediment (Voogd and Truffert, 1992; Garfunkel, 1998), and is overlain by up to 3 km of Messinian salt (Garfunkel, 1998; Aal et al., 2000; Loncke et al., 2006).

Crustal structure of the northern Harrat Rahat volcanic field (Saudi Arabia) from gravity and aeromagnetic data

New gravity data reveal a prominent negative anomaly along the main vent axis of the northern Harrat Rahat volcanic field in Saudi Arabia. The low continues north of the volcanic field onto exposures of Proterozoic rocks, indicating that the low is caused not only by the volcanic field (and possibly also underlying Cenozoic sediments), but also the underlying Proterozoic basement. An inversion of the gravity field guided by analysis of aeromagnetic data indicates (1) a broad depression of the basement surface that is deeper along the main vent axis in the eastern part of the harrat and in the southwest part of the volcanic field and (2) less dense basement beneath the vent axis. Low densities within the basement most likely arise from lithologic variations in the basement, predating Cenozoic volcanism, although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth.

Tectonically controlled multiple stages of deformation along the Yangsan Fault Zone, SE Korea, since Late Cretaceous

We describe the structural characteristics and tectonic evolution of the NNE–SSW-striking Yangsan Fault, Korea. The surface trace of the fault extends for over 170 km on land, displaying ∼20–30 km of dextral offset. It transects mainly Mesozoic–Cenozoic sedimentary and igneous rocks. Our field observations suggest that the style of deformation within the fault core is controlled by the ductility of the protoliths. Where sedimentary rocks have been faulted, fault cores tend to be relatively wide, having hosted multiple events. Internal fault core structures are characterized by networks of multiple anastomosing strands of fault gouge and breccia, which enclose lenses of fractured protolith. However, where igneous (crystalline) rocks have been faulted, slip tends to be much more localized within narrow fault cores comprising cataclastic rocks. Combining our new data with previous research results, we infer that four main movement phases have occurred on the fault: (1) Late Cretaceous sinistral slip faulting with a component of extensional deformation, under a regime of NW–SE compression; (2) late Paleogene intense dextral slip faulting, which occurred under a regional NE–SW compressional regime; (3) middle Miocene weak sinistral slip faulting, during NNW–SSE compression; and (4) local Quaternary dextral slip faulting with a reverse component, under ENE–WSW to E-W compression. The most intense phase of deformation appears to have occurred during the late Paleogene due to a major reorganization of plate motion in and around the NW Pacific area. Our findings highlight that the fault has been a preferred zone of weakness for episodic reactivations having significant implications for the changing tectonic environments of East Asia from Late Cretaceous to Quaternary, and that its segments underwent distinguishable deformations at each phase probably due to their geometrical features and fault rock properties.

German military geology and military mining on the Eastern Front in World War I

Abstract At its peak, the Eastern Front encompassed the entire frontier between the Russian Empire and Romania on the Allied side and the Austro-Hungarian Empire, Bulgaria, the (Turkish) Ottoman Empire and the German Empire on the other: a distance of c. 1500–1700 km. Mobile warfare alternated with periods of static trench and siege warfare, when ‘mines’ (tunnels charged with explosives detonated to breach the overhead fortifications) and ‘dugouts’ (to protect troops from artillery or aerial bombardment) were constructed in at least 50 localities between 1915 and 1917, from Mitau in present-day Latvia to Rarancze in Moldova. Terrain ranged from plateaux floored by Cenozoic sediments in the north to mountains with more complex, older, stronger rocks in the south. Despite lessons learnt from the Russo-Japanese war of 1904–05, both sides began the conflict unprepared for tunnelling warfare. However, German and Austro-Hungarian forces soon developed units of military geologists whose duties included guidance of tunnelling projects. Eight teams of German military geologists and four of Austro-Hungarian, in total over 60 men who can be named, are known to have served on the Front, as did newly formed and equipped specialist engineer mining battalions from Prussia, Bavaria and Austria-Hungary.

Tectono-Thermal Evolution and Morphodynamics of the Central Dronning Maud Land Mountains, East Antarctica, Based on New Thermochronological Data

The lack of preserved Mesozoic–Cenozoic sediments and structures in central Dronning Maud Land has so far limited our understanding of the post-Pan-African evolution of this important part of East Antarctica. In order to investigate the thermal evolution of the basement rocks and place constraints on landscape evolution, we present new low-temperature thermochronological data from 34 samples. Apatite fission track ages range from 280–85 Ma, while single-grain (U-Th)/He ages from apatite and zircon range from 305–15 and 420–340 Ma, respectively. Our preferred thermal history models suggest late Paleozoic–early Mesozoic peneplanation and subsequent burial by 3–6 km of Beacon sediments. The samples experienced no additional burial in the Jurassic, thus the once voluminous continental flood basalts of western Dronning Maud Land did not reach central Dronning Maud Land. Mesozoic–early Cenozoic cooling of the samples was slow. Contrary to western Dronning Maud Land, central Dronning Maud Land lacks a mid-Cretaceous cooling phase. We therefore suggest that the mid-Cretaceous cooling of western Dronning Maud Land should be attributed to the proximity to the collapse of the orogenic plateau at the Panthalassic margin of Gondwana. Cooling rates accelerated considerably with the onset of glaciation at 34 Ma, due to climate deterioration and glacial denudation of up to 2 km.

Parageneses of fractures in large fault zones of West Transbaikalia

Our study was focused on the parageneses of heterogeneous fractures in the large fault zones of West Transbaikalia,Russia. We reconstructed the latest deformation in the fault zones of Transbaikalia, within which paleoseismic dislocations are known and M 4.7 earthquakes take place. To obtain statistically justified solutions on the kinematic types of the largest faults ofWest Transbaikalia, we collected the required data and conducted the structural and paragenetic analysis of the fractures in the study area. In the Chikoi-Ingoda, Khilok, North Tugnui andNorth Zaganfault zones, we created a network of 54 observation points and measured more than 5500 details of local fractures and faults. Recorded were the observed slickensides, the displacements of markers, and other details of rock fracturing. Based on the analysis results, we calculated a ratio of heterochronous dynamic settings for formation of the observed fault group. It shows that NW-SE-trending extension and compression are dominant in the study region. The parageneses of E-NE-striking faults, i.e. regional faults longitudinal to the depressions ofWest Transbaikalia, are abundant in the studied fault zones and generally observed in heterochronous formations, including the Cenozoic sediments. This fact, along with the focal mechanisms of the recently recorded earthquakes, suggests that these faults are young. Besides, in the Tugnui basin and the area southeast of the Chikoy depression, the right-lateral strike-slip setting was reconstructed for E-NE-trending faults. Our study pioneers in the quantitative analysis of the fault parageneses ofWest Transbaikalia. Considering the development of the network of large faults in the study area, we reconstructed the main stages and the kinematic types of the second-order fractures that constitute the internal structure of the studied fault zones at each stage of their tectonic development.

Cenozoic Sediment Provenance in the Northern Great Plains Corresponds to Four Episodes of Tectonic and Magmatic Events in the Central North American Cordillera

AbstractDetrital zircon U‐Pb provenance in the northern Great Plains reveals histories of drainage reorganization of the paleo‐Missouri River in response to Cenozoic tectonic and magmatic processes in the central North American Cordillera. During the latest Cretaceous, sediment provenance was confined to the Cordilleran hinterland in central Idaho and southwestern Montana, probably due to the existence of a subtle paleohydraulic divide in northwestern Montana. During the early to middle Paleocene, the paleodrainage was expanded to cover the Belt Supergroup in northwestern Montana in response to the eastward propagation of the Cordilleran thrust belt. During the late Paleocene to early Eocene, the final movement of the thrust belt and the initial extension of the Cordilleran hinterland shifted the drainage divide to the orogenic front and caused focused erosion of the upper Lower to Upper Cretaceous rocks to the east. During the middle Eocene to early Oligocene, a significant increase of Archean grains suggests renewed exhumation of the Laramide province and sediment delivery by the paleo‐Yellowstone River originated in the central Rocky Mountains. The presence of middle Eocene to early Oligocene zircons during this stage also suggests headwater erosion of the paleo‐Upper Missouri River into the Dillion volcanic field in southwestern Montana. These interpretations indicate that a drainage pattern similar to that of the modern Missouri River was established during middle Eocene to early Oligocene time. The detrital zircon maximum depositional ages also improve the chronostratigraphic framework of the Paleogene strata. Our study demonstrates that orogenic processes can be well archived in sedimentary records far away from orogenic systems.

Biomass burning history in East Asia during the last 4 million years recorded in elemental carbon variability at IODP site U1423

The burning of trees and grasses produces charred particles, such as charcoal and soot, that can be transported over long distances via winds and rivers to coastal, deltaic, and ocean environments, where they are preserved in sediments. Charcoal contained in sediments has been widely used as a proxy for biomass burning and human activities as well as climate change. Charcoal and soot in Cenozoic marine sediments at Integrated Ocean Drilling Program (IODP) Expedition (Exp.) 346 Site U1423 were measured to examine the regional history of biomass burning in East Asia. Charcoal and soot were measured as elemental carbon (EC) in coarse (> 2 μm) and fine (< 2 μm) fractions using grain size separation by repeated settling followed by application of a thermal optical transmittance (TOT) method. Organic carbon (OC) was also quantified during the process. EC and OC in both coarse and fine fractions are higher from 0 to 1.8 Ma and lower from 1.8 to 4.3 Ma but have large variations, which suggest more frequent or intense biomass burning since 1.8 Ma. Terrestrial biomass and precipitation could be major controls on the EC supply. Fine EC varies independently from coarse EC, which suggests a remote origin of fine EC. Large increases in terrestrial vegetation cover have led to high-temperature burning, which is associated with interglacial stages.

Application of fuzzy logic tools for the biogeochemical characterisation of (un)contaminated waters from Aljustrel mining area (South Portugal)

Aljustrel mining area (South Portugal) belongs to the Iberian Pyrite Belt (IPB). It is classified of high environmental risk due to its large tailings and to the Acid Mine Drainage (AMD) affected waters, generated by sulphides' oxidation. Integrating biological parameters (for the first time) in the input data matrix of the software PreFuRGe, allowed a better discrimination of the diatoms' responses to the stimuli caused by the hydrochemical changes imposed by the processes affecting water quality. Each hydrochemical scenario, was modeled by imposing maximum and minimum limits for each antecedent, according to the conditions imposed by the consequent, which in this case were the number of diatom species and pH. Thus, PreFuRGe evidenced some qualitative aspects that could not be achieved by classic statistics. pH appeared as the main discriminator of diversity and diatom species composition, nevertheless and due to the complex environment under study other chemical interactions must be considered: (a) AMD waters, with extremely low pH values, but also with extremely high hydrogeochemical complexity, represented by a mixture of metals, do not allow to associate, unequivocally, the reduction in diatom diversity to pH, but also to high metal (loid)s concentrations; (b) in the most alkaline waters, with higher abundance of diatom species, average to high concentrations of Na and Cl (due to Cenozoic sediments) do not seem to affect diatom diversity. This methodology proved to be an efficient tool to establish, for the first time, cause-effect relationships, improving the comprehension between biological (diatoms) and hydrochemical parameters.

Cenozoic Development of the Nonmarine Mula Basin in the Southern Yidun Terrane: Deposition and Deformation in the Eastern Tibetan Plateau Associated with the India‐Asia Collision

AbstractTo advance tectonic models of plateau growth in response to the India‐Asia collision, parameters such as the timing, mechanism (s), and extent of Cenozoic deposition and deformation throughout the eastern Tibetan Plateau need to be determined. To better understand these parameters, we examine the Mula basin, located in the southern Yidun terrane, using field data, detrital zircon geochronology, εHf zircon isotopic values, and thin section petrology. The Mula basin is a NW‐SE elongate (~28 km long and 5–8 km wide) exposure of nonmarine strata ~1,000 m thick, deposited in alluvial environments. The basin is bound to the northeast by a thrust fault (327, 34°NE) that places Triassic Daocheng Pluton and Triassic Yidun Group rocks on top of the nonmarine strata. The western boundary is defined by an unconformable contact between the overlying nonmarine strata and Triassic Daocheng Pluton and Yidun Group rock. An intrabasin thrust fault, parallel to the basin‐bounding fault, places older nonmarine strata on top of younger nonmarine strata, illustrating postdepositional deformation. Provenance results indicate that sediment influx originated primarily from localized drainage catchments and lacked major, well‐organized throughgoing systems. A maximum depositional age of 45.5 ± 0.5 Ma, based on the weighted mean average of the youngest detrital zircon population (s), demonstrates an Eocene or younger age for strata. We interpret the Mula basin to have developed in a contractional deformational regime driven by a far‐field, upper‐crustal response associated with the transition from an Andean style margin to a continent‐continent collisional margin as India impinged upon Eurasia.

Tectonic Control of a Change in Sedimentary Environment at ~10 Ma in the Northeastern Tibetan Plateau

AbstractTectonic uplift and climate change play a key role in the evolution of sedimentary systems in terrestrial basins. The Qaidam Basin in the northeastern Tibetan Plateau is well suited for studying these processes because of its location and its thick, continuous sequence of Cenozoic sediments. Unfortunately, however, there is a lack of continuous, well‐dated sedimentary records from the region. In the present study, we combined grain‐size analyses of sediment cores and outcrop samples with structural analysis of seismostratigraphic data from the western Qaidam Basin. The results reveal dramatic shifts in the depositional environment at ~11.5–10 Ma, coeval with the topographic development of the area. Together with paleoclimatic analysis and a conceptual model, we conclude that tectonic movements, rather than climatic changes, were the dominant forcing factor.

Late Cenozoic Sedimentary Evolution of Pagri‐Duoqing Co graben, Southern End of Yadong‐Gulu Rift, Southern Tibet

The north trending rifts in southern Tibet represent the E–W extension of the plateau and confirming the initial rifting age is key to the study of mechanics of these rifts. Pagri–Duoqing Co graben is located at southern end of Yadong–Gulu rift, where the late Cenozoic sediments is predominately composed of fluvio‐lacustrine and moraine. Based on the sedimentary composition and structures, the fluvio‐lacustrine could be divided into three facies, namely, lacustrine, lacustrine fan delta and alluvial fan. The presence of paleo‐currents and conglomerate components and the provenance of the strata around the graben indicate that it was Tethys Himalaya and High Himalaya. Electron spin resonance (ESR) dating and paleo‐magnetic dating suggest that the age of the strata ranges from ca. 1.2 Ma to ca. 8 Ma. Optically stimulated luminescence (OSL) dating showed that moraine in the graben mainly developed from around 181–109 ka (late Middle Pleistocene). Combining previous data about the Late Cenozoic strata in other basins, it is suggested that 8–15 Ma may be the initial rifting time. Together with sediment distribution and drainage system, the sedimentary evolution of Pagri could be divided into four stages. The graben rifted at around 15–8 Ma due to the eastern graben‐boundary fault resulting in the appearance of a paleolake. Following by a geologically quiet period about 8–2.5 Ma, the paleolake expanded from east to west at around 8–6 Ma reaching its maximum at ca. 6 Ma. Then, the graben was broken at about 2.5 Ma. At last, the development of the glacier separated the graben into two parts that were Pagri and Duoqing Co since the later stages of the Middle Pleistocene. The evolution process suggested that the former three stages were related to the tectonic movement, which determined the basement of the graben, while the last stage may have been influenced by glacial activity caused by climate change.

Applied geophysics for cover thickness mapping in the southern Thomson Orogen

Potentially mineralised Paleozoic basement rocks in the southern Thomson Orogen region of southern Queensland and northern New South Wales are covered by varying thicknesses of Mesozoic to Cenozoic sediments. To assess cover thickness and methods for estimating depth to basement, we collected new airborne electromagnetic (AEM), seismic refraction, seismic reflection and audio-frequency magnetotelluric data and combined these with new depth to magnetic basement models from airborne magnetic line data and ground gravity data along selected transects. The results of these investigations over two borehole sites, GSQ Eulo 1 and GSQ Eulo 2, show that cover thickness can be reliably assessed to within the confidence limits of the various techniques, but that caveats exist regarding the application of each of the disciplines. These techniques are part of a rapid-deployment explorers’ toolbox of geophysical techniques that have been tested at two sites in Australia, the Stavely region of western Victoria, and now the southern Thomson Orogen in northern New South Wales and southern Queensland. The results shown here demonstrate that AEM and ground geophysics, and to a lesser extent depth to magnetic source modelling, can produce reliable results when applied to the common exploration problem of determining cover thickness. The results demonstrate that portable seismic systems, designed for geotechnical site investigations, are capable of imaging basement below 300 m of unlithified Eromanga Basin cover as refraction and reflection data. The results of all methods provide much information about the nature of the basement–cover interface and basement at borehole sites in the southern Thomson Orogen, in that the basement is usually weathered, the interface has paleotopography, and it can be recognised by its density, natural gamma, magnetic susceptibility and electrical conductivity contrasts.

The mafic alkaline volcanism of SW Madagascar (Ankililoaka, Tulear region): 40 Ar/ 39 Ar ages, geochemistry and tectonic setting

High-precision 40 Ar/ 39 Ar ages, major and trace element, and radiogenic isotope data are presented for the basanites and alkali basalts forming the southwesternmost monogenetic volcanic field in Madagascar. The volcanic rocks were erupted along fissure zones and aligned cones in a nearly flat area covered by the Cenozoic sediments of the Morondava basin. The high-precision 40 Ar/ 39 Ar ages constrain the beginning of the magmatism in the Ankililoaka area to about 12 Ma, significantly earlier than suggested by previously published K/Ar ages. The Ankililoaka basanites include primitive compositions (MgO &gt;10 wt%, Ni &gt;200 ppm and Cr &gt;400 ppm), whereas other basanites and alkali basalts experienced limited removal of olivine, chromiferous spinel and clinopyroxene. Initial Sr and Nd isotope ratios of the basanites are 0.70343–0.70445 and 0.51279–0.51282, respectively. The Pb isotope compositions are in the ranges 206 Pb/ 204 Pb = 19.08–19.38, 207 Pb/ 204 Pb = 15.61–15.64 and 208 Pb/ 204 Pb = 39.1–39.4. The alkali basalts have similar 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and 207 Pb/ 204 Pb, but slightly lower 206 Pb/ 204 Pb and 208 Pb/ 204 Pb than the basanites. The isotopic composition of the Ankililoaka rocks partially overlaps with that of the Cenozoic volcanic mafic rocks of northern Madagascar, and differs significantly from that of the mafic volcanic rocks of central Madagascar, which have lower 206 Pb/ 204 Pb and 207 Pb/ 204 Pb, and higher 87 Sr/ 86 Sr. Major and trace element systematics and geochemical modelling suggest that the Ankililoaka mafic alkaline rocks are low-degree melts of an incompatible element enriched peridotite source starting from depths where garnet is stable. Crustal contamination during ascent was insignificant. We argue that the genesis of the Ankililoaka alkaline magmas was triggered by melting an enriched, volatile-rich lithospheric mantle uplifted in the Cenozoic. Supplementary material: Analytical techniques, X-ray fluorescence whole-rock data, mineral compositions, 40 Ar/ 39 Ar dataset, mantle source modelling and figures showing volcanological features and thin sections are available at https://doi.org/10.6084/m9.figshare.c.4065743

U-PB Zircon Ages and Provenance of Upper Cenozoic Sediments from the Da Lat Zone, SE Vietnam: Implications For an Intra-Miocene Unconformity and Paleo-Drainage of the Proto–Mekong River

Abstract The present-day Mekong River is the twelfth longest river in the world. It drains the Tibetan Plateau and forms a large delta in south Vietnam. Remnants of upper Cenozoic fluvial to marginal marine proto-Mekong sediments are exposed in the Da Lat Zone in southeast Vietnam and are likely the on-land equivalents of large sediment packages offshore in the Cuu Long and Nam Con Son Basins. Provenance studies are used here to identify sources that allow reconstruction of sediment pathways. The Oligo-Miocene Di Linh Formation has a main Cretaceous zircon age population and subordinate Paleoproterozoic (c. 1.8-1.9 Ga) zircons, sourced mainly from Cretaceous plutons. In contrast, the early Pliocene to Pleistocene Song Luy Formation includes additional Permian–Triassic and Ordovician–Silurian age populations which are interpreted to be sourced from basement rocks in central and northern Vietnam. There is a significant change in provenance between these formations, interpreted as an intra-Miocene unconformity. The late Pliocene to Pleistocene Ba Mieu Formation shows a much greater abundance of Precambrian zircons, including a small age peak at c. 2.5 Ga, interpreted to be derived from old basement exposed in South China. Abundant Cretaceous zircons and a lack of Jurassic zircons in all three formations contrast with the present-day Mekong River delta. Therefore, the data are proposed to show a development of the proto–Mekong River progressing northward from the Da Lat Zone, to the Kontum Massif and northern Vietnam, to capturing of reversed rivers and/or reworking of sediments from old cratons in South China.

A case study on semiautomatic seismic interpretation of unconformities and faults in the southwestern Barents Sea

Recently, there has been a growing interest in automatic and semiautomatic seismic interpretation, and we have developed methods for extraction of 3D unconformities and faults from seismic data as alternatives to conventional and time-consuming manual interpretation. Our methods can be used separately or together, and they are time efficient and based on easily available 2D and 3D image-processing algorithms, such as morphological operations and image region property operations. The method for extraction of unconformities defines seismic sequences, based on their stratigraphic stacking patterns and seismic amplitudes, and extracts the boundaries between these sequences. The fault-extraction method extracts connected components from a coherence-based fault-likelihood cube where interfering objects are addressed prior to the extraction. We have used industry-based data acquired in a complex geological area and implemented our methods with a case study on the Polhem Subplatform, located in the southwestern Barents Sea north of Norway. For this case study, our methods result in the extraction of two unconformities and twenty-five faults. The unconformities are assumed to be the Base Pleistocene, which separates preglacial and postglacial Cenozoic sediments, and the Base Cretaceous, which separates the severely faulted Mesozoic strata from prograding Paleocene deposits. The faults are assumed to be mainly Jurassic normal faults, and they follow the trends of the eastern and southwestern boundaries of the Polhem Subplatform; the north–south-trending Jason Fault complex; and the northwest–southeast-trending Ringvassøy-Loppa Fault complex.