Cenozoic Rocks Research Articles

Could the mantle have caused subsidence of the Congo Basin?

The Congo Basin is characterised by a near-circular shape, a pronounced negative free-air gravity anomaly, and a subsidence history that is slow and long-lived. The basin is often considered as an intracratonic basin, implying an unknown formation mechanism. However, the Congo Basin probably initiated by Precambrian rifting and the larger part of its older subsidence history could be explained by post-rift thermal relaxation. The uppermost layer of Mesozoic to Cenozoic sedimentary rocks in the basin appears discontinuous in its evolution and several studies have proposed that these rocks were deposited in response to a process in the mantle. We have examined gravity data and seismic tomographic models to evaluate the role of the sub-crustal mantle in the more recent evolution phase of the Congo Basin. Using seismic tomographic models of the upper mantle and lithospheric thickness models, we show that the Congo Basin is underlain by a thick lithosphere and that the basin boundary likely coincides with the boundary of the Congo Craton. We have reduced the EGM2008 free-air gravity field by correcting for topography and sediments. We find that the observed negative gravity anomaly is mainly due to the sedimentary units in the basin. The reduced gravity field has slightly negative to positive anomalies over the basin, depending on the densities assigned to the sedimentary rock package. We have analysed thirteen whole-mantle and five upper-mantle tomographic models and show that they do not provide supporting evidence that the sub-lithospheric mantle played a primary role in the more recent subsidence of the Congo Basin. We speculate that deposition of the Mesozoic–Cenozoic rocks could have raised the surface elevation of the Congo Basin to the present average level of ∼400 m above sea-level and that the last subsidence phase could be a consequence of the sediment load rather than the cause.

Two-way time–depth relationship for Waka Nui-1 well to aid geological interpretations of the offshore northwestern New Zealand

The Waka Nui-1 petroleum exploration well, about 90 km west of Northland, is the only offshore well north of Auckland and provides important lithologic and age control for interpretations of seismic reflection surveys in the petroleum-prospective Northland and Reinga basins. It is especially important because it intersects a nearly complete Cenozoic sequence and Mesozoic Murihiku Supergroup rocks, all of which may be part of the petroleum system in these basins. Correlation with seismic data is uncertain because neither sonic logs nor check shot data were acquired during drilling operations. This short communication documents the development of a two-way time– (TWT–) depth relationship derived by careful analysis of stacking velocities from nearby petroleum industry seismic data and logging-while-drilling logs from Waka Nui-1. Using this correlation, the age and lithology data from the well can be used to infer the age of unconformities and seismic sequences on seismic data in the Northland and Reinga basins. These interpretations provide control on the timing of structural events and the evolution of depositional processes in this region of extensive sedimentary accumulation and are vital for predicting the prospectivity of the basins.

Detrital U–Pb zircon analysis of an Eocene McMurdo Erratic sandstone, McMurdo Sound, Antarctica

Some of the most important Cenozoic sedimentary rocks in Antarctica are glacial erratics of fossiliferous Palaeogene to Neogene siliclastic rocks known as the McMurdo Erratics. Detrital zircon U–Pb isotopic data reported herein provide new information on the provenance of these siliciclastic rocks. The U–Pb age data from a sandstone glacial erratic that is likely of Eocene age show a dominant age cluster that ranges from 488 to 635 Ma, accompanied by subsidiary Neoproterozoic–Archaean peaks. The dominant Neoproterozoic–Ordovician age cluster, in conjunction with the arkosic lithologies of some erratics and the local presence of granitic and metasedimentary clasts, indicates that the Neoproterozoic–Ordovician basement complex of the Transantarctic Mountains likely served as a major source for the Eocene siliciclastic sediments. This is consistent with derivation of the McMurdo Erratics from locations proximal to the Transantarctic Mountains such as the Discovery Deep and/or major mountain outlet glacier areas.

Registro da deformação pós-paleozóica na Bacia do Amazonas, região de Itaituba (PA)

The Amazon Basin has an area of approximately 500.000 km² covering part of the states of Pará and Amazonas. The region studied is located in the southwest portion of the basin (near of Itaituba - PA city) and contains sedimentary rocks of the Fanerozoic cover and the basement in the Amazon Central Province. Structural analysis consisted of multiscale analysis and integration of images analysis (recognition of morphotectonics features in images SRTM, using data from aeromagnetometria) and using data from the field to determine structural framework and define the stages of post-paleozoic strain that hit the area. From the data of failures and systems of joints, shear zones, slickenside and steps found in outcrops, were developed the structural and kinematic analysis. The application of the method to determin the paleostress could to set the position of stress. The results indicate at least 3 stages of strain active at the end of the Paleozoic. The first phase (F1) has dextral kinematic and structure frontal position N60-70E, for maximum stress (σ1), allows the compressive stresses, although in a transcurrent context, correlated to the orogeny responsible for the formation of Pangea at the end of the Permian. The second phase (F2) is characterized by the presence of normal faults of NE direction coincident with the positioned dikes in the region, may be related to magmatism Penatecaua. The third phase (F3) would be related to a system composed of the combined structures N50-70W (sinistral) and N60-70E (dextral) with direction of σ1 E-W. Only this phase appears to have affected the cenozoic rocks, and probably is related to the spread of strains of Andean subduction.

Sinopsis geológica de la Cuenca de Burgos, noreste de México: producción y recursos petroleros

The Burgos Basin is located at the Coastal Plain of the Gulf of Mexico. It consists of more than 5000 m of siliciclastic Cenozoic rocks and more than 3000 m of Mesozoic carbonate, evaporite and siliciclastic rocks. The basin contains stratigraphic sequences with transgressive and regressive cycles with unconformities limiting several of the sequences. This basin experienced several superimposed tectonic events. The complex basement contains both metamorphic and igneous rocks, affected by rifting during the opening of the Gulf of Mexico. As a result of crustal subsidence, marine pre-tectonic and syntectonic sediments accumulated from Late Jurassic to Eocene. The Laramide episode is represented by foreland deposits, deeply erosive surfaces, and broad folds. An upper detachment level, with syndepositional listric faults and roll-over structures, denotes tectonic extension during the Paleocene and Eocene. The post-tectonic stage is associated to basement uplift during the Oligocene, deep detachment on Jurassic evaporites, decoupling of sedimentary cover link to extension faults, and a main unconformity at 30 Ma. The early phase of the Perdido Fold Belt occurred at this stage. This event is linked to cortical uplift and may be related to the emplacement of the East Alkaline Magmatic Arc, during the Oligocene. Finally, from late Oligocene to Neogene thick siliciclastic prograding sequences accumulated towards the Gulf of Mexico. The late phase of deformation consists of growth faults associated to shale and Jurassic salt diapirs, and a late deformation of the Perdido Fold Belt. In

Late Cenozoic tectonic activity and its significance in the Northern Junggar Basin, Northwestern China

The Dingshan area located in the northern part of the Junggar Basin of northwestern China is a significant prospect area for sandstone-type uranium deposits in China, where mainly Cenozoic rocks were deposited. The Cenozoic strata can be divided into four units according to the prior data and our own field observation. Sedimentary studies indicate that most Cenozoic strata were deposited under a hot and arid climate in a continental environment. The sedimentary facies are alluvial-fan, meandering-fluvial, and fluvio-lacustrine. Field investigation and interpretation of satellite images suggest that Cenozoic tectonics in the area is characterized by reactivation of early deep-seated thrusts, resulting in extensional fractures and formation of many small depressions in the shallow crustal level. Measurement of joint orientations suggests that regional shortening direction trends in north–south in the middle Pleistocene as indicated by the ESR (Electronic Spin Resonance) age of 0.1–0.4 Ma obtained from fault gouge and gypsum deposits. A four-stage sedimentation-tectonic evolution model of the northern Junggar Basin during the Late Cenozoic can be established based on reconstruction of sedimentary filling processes and Cenozoic tectonic movements. We suggest that landform evolution and groundwater movement are controlled by active tectonics, indicating that Late Cenozoic tectonic activities may also play important roles in the formation of sandstone-type uranium deposits. Therefore, a new metallogenic model for sandstone-type uranium deposits is proposed.

Evolution of Calc-Alkaline Volcanism and Associated Hydrothermal Gold Deposits at Yanacocha, Peru

Clusters of high-sulfidation epithermal deposits containing more than 50 Moz of gold are hosted by advanced argillic-altered Miocene volcanic rocks in the Yanacocha district, northern Peru (lat. 6°59'30 S, long. 78°30'45 W). We describe the nature of the volcanism and its relation to the gold ores on the basis of new district-scale geologic mapping, 69 40Ar/39Ar ages on igneous rocks and hydrothermal alunite, and petrologic and geochemical investigations. Volcanic rocks of the Calipuy Group are the oldest Cenozoic rocks at Yanacocha, and include the Huambo Cancha andesite, andesitic lahars of Tual and Chaupiloma (19.5–15.9 Ma), and the dacitic Cerro Fraile pyroclastics (15.5–15.1 Ma). The younger Yanacocha Volcanics (14.5–8.4 Ma) form a cogenetic series of lavas and pyroclastic rocks with a cumulative volume of ~88 km3 that represents eruption from a moderate-size volcanic center. Early pyroxene>hornblende-bearing lavas of the Atazaico Andesite (14.5–13.3 Ma) erupted from small stratovolcanoes progressively younger from southwest to northeast. Dacitic dikes followed that are spatially associated with gold deposits at Quilish and Cerro Negro (~7 Moz Au) and stage 1 alunite (13.6–12.6 Ma). The Colorado Pyroclastics erupted in the center of the district and include the hornblende- and biotite-bearing andesitic to dacitic Cori Coshpa (12.6 Ma) and Maqui Maqui (12.5–12.4 Ma) ignimbrites. The Colorado Pyroclastics are overlain by hornblende>pyroxene-bearing andesitic to dacitic lavas, flow-domes, and minor pyroclastic rocks of the Azufre Andesite (12.1–11.6 Ma). Widespread stage 2 alunite (11.5 Ma) and minor gold deposition (~0.5 Moz) closely follow. The San Jose Ignimbrite (11.5–11.2 Ma) overlies the Azufre Andesite and stage 2 alunite and includes three members of hornblende-pyroxene (biotite) dacite and andesite that erupted in the center of the district and flowed southward. Mineralogically similar domes were emplaced into the inferred vents. The Coriwachay Dacite (10.8–8.4 Ma) forms the youngest and most silica rich igneous rocks in the district, and includes intrusions and flow domes of dacite to rhyolite at Corimayo (10.8 Ma), Cerro Yanacocha (9.9 Ma), and Yanacocha Lake (8.4 Ma). Most of the gold (>47 Moz) was deposited at Yanacocha during intrusion of the late Coriwachay Dacite. These late dacites are volumetrically smallest (~2% of the total volume of erupted magma) and are temporally associated with stage 3 to 5 alunites. Stage 3 alunite (11.0–10.7 Ma) developed along a northeast trend for 9 km that includes the gold deposits of Corimayo, San Jose, Carachugo, and Maqui Maqui. The deeper Kupfertal Cu-Au porphyry has an age of 10.7 Ma on hydrothermal biotite and underlies zones of stage 4 and 5 quartz-alunite alternation that are 0.8 and 1.5 m.y. younger, respectively. Stage 4 alunite ranges in age from 10.2 to 10.3 Ma at the Tapado and Chaquicocha Sur gold deposits to Cerro Sugares east of the Maqui Maqui deposit. Stage 4 also includes a younger alunite age of 9.9 Ma from the San Jose gold deposit. Stage 5 alunite ranges from ~9.3 to ~8.2 Ma at Cerro Yanacocha, the largest gold deposit in the district. All these deposits contain massive and vuggy quartz, quartz-alunite, and quartz-pyrophyllite alteration associated with pyrite±enargite-tennantite-covellite. Magmatism in the Yanacocha district lasted for ~11 m.y. The Yanacocha Volcanics spanned the last ~6 m.y. of this period and were associated with long-lived magmatic-hydrothermal activity and episodic gold mineralization. The Yanacocha calc-alkaline suite was oxidized, water and sulfate rich, and evolved from early pyroxene>hornblende andesite to late titanite-bearing dacite and minor rhyolite. Several dacites contain populations of both high- and low-aluminum hornblendes that crystallized in the middle and upper crust, respectively. The variation of Mg, Ti, P, Sr, and Ba contents in these rocks is consistent with a complex magmatic origin via both cooling, fractional crystallization, periodic recharge of deeply derived hydrous basaltic or andesitic melts, and mixing with silicic melts derived by crustal melting. Low eruption rates, high phenocryst contents of the volcanic rocks, and widespread hydrothermal alteration are consistent with the hypothesis that most of the magmas at Yanacocha crystallized in shallow chambers as granitoids that passively degassed ore fluids. The compositional diversity of the volcanic rocks together with an extended magmatic-hydrothermal history reflect complex magmatic processes that were optimum for producing the world-class gold deposits at Yanacocha.

Glaciogenic and related sedimentary rocks: Main lithochemical features. Communication 2. The Paleozoic and Cenozoic

Lithochemical features of the Early-Late Paleozoic and Cenozoic glaciogenic and related sedimentary rocks around the world are considered. Comparison of the bulk chemical composition of diamictites with the average Post-Archean Australian Shale (PAAS) revealed that the diamictites do not show any lithochemical characteristics unambiguously indicating their formation under cool climatic conditions. The chemical index of alteration (CIA) often used in the paleoclimatic reconstructions should be considered as additional (though very important) tool, because it is controlled by local factors. Taking into account these facts, there are likely no grounds to expect that the amount of the redeposited sedimentary material in the Late Paleozoic and Cenozoic glaciogenic rocks is more than in their Early Proterozoic and Late Riphean-Vendian analogues.

Paleomagnetism of Late Paleozoic, Mesozoic, and Cenozoic rocks in Mongolia

Rock complexes in Mongolia experienced two remagnetization events. Almost all secondary remanence components of normal polarity were acquired apparently in the Cenozoic, after major deformation events, and those of reverse polarity were associated with intrusion of bimodal magmas during the Late Carboniferous–Permian reverse superchron. Active continental-margin sequences in some areas of Mongolia were folded prior to the Late Carboniferous–Permian magnetic event. The primary origin of magnetization in Late Paleozoic and Mesozoic rocks has been inferred to different degrees of reliability. According to paleolatitudes derived from most reliable paleomagnetic data, the analyzed rocks were located far north of the North China block throughout the Late Paleozoic and Early Mesozoic. Mongolia, as well as Siberia, moved from the south to the north in the Paleozoic, back from the north to the south between the latest Triassic and the latest Jurassic, and remained almost within the same latitudes in Cretaceous and Cenozoic time. These paleolatitudes show no statistical difference from those for the Siberian craton at least since the latest Permian (275–250 Ma). Older Mongolian complexes (with ages of 290, 316, and 330 Ma) likewise may have formed within the Siberian continent, which makes their paleomagnetic determinations applicable to calculate the polar wander path for Siberia. The paleolatitudes of Early Carboniferous sediments in Mongolia differ significantly from those of Siberia, either because of overprints from the reverse superchron or because they were deposited away from the Siberian margin.

10.1007/s11491-008-1015-5

The Californian borderland is the largest oil and gas region of the Pacific coast of the United States. Here, a series of large and minor sedimentary basins with established or inferred oil and gas bearing properties are recognized. The Californian borderland is located on a Nevadan-type active continental margin. The bodies of such margins are composed of giant accretion formations and occupy not only their underwater parts but also vast land areas. The accretion formations are dominated by rocks of a deep-water origin. The Californian borderland represents a system of basins and ranges that compose the underwater margin and the Coastal Ranges of California and the basement within the Great Valley. In the depressions of the accretion orogen, which are small in size but feature high subsidence rates, significant thicknesses of deposits are accumulated (up to 6−8 km). In the depressions of the Californian borderland, they are represented by young Neogene (rarer Eocene-Oligocene) formations of mainly terrigenous or siliceous-terrigenous compositions. The active tectonic regime resulted in a sharp reduction of the age range of the oil and gas bearing deposits, which are represented only by Late Cretaceous and Cenozoic rocks. Full-scale oil and gas production here is performed in the Santa Barbara-Ventura and Santa Maria basins. The principal oil and gas bearing unit is the Monterrey Formation of the Middle Miocene. By January 2006, the production on the Californian borderland comprised about 17.8 million tons of oil and over 1 billion m3 of gas. Up to 1.4 billion tons of oil and 200 billion m3 of gas reserves are regarded to be not recovered on the Pacific coast of the United States in the region of the Californian borderland.

1.3–0.9Ga Oaxaquia (Mexico): Remnant of an arc/backarc on the northern margin of Amazonia

Abstract Rocks with ages of ca. 1 Ga occur in central and southern Mexico as inliers surrounded by ubiquitous Mesozoic and Cenozoic rocks. They appear to share a common history consisting of: (i) ca. 1300–1200 Ma arc magmatism and deposition of sediments including evaporites; (ii) ca.1160–1100 Ma intrusion of syenite, granite and anorthosite, the later part of which is synchronous with migmatization; (iii) intrusion of a ca. 1035–1010 Ma anorthosite–gabbro–charnockite–granite (AMCG) suite; (iv) a 1000–980 Ma granulite facies tectonothermal event with a stretching axis parallel to the long axis of Oaxaquia; (v) gradual exhumation at 750 and/or 545 Ma; and (vi) 517 Ma intrusion of an isolated calcalkaline granitoid pluton. The common Precambrian geological record of these outcrops suggests that they belonged to a single terrane (Oaxaquia) and formed a juvenile arc/backarc bordering a continent that underwent collision with, and overthrusting of, the Avalonian arc at 1000–980 Ma. This buried Oaxaquia to 25–30 km and was followed by further supra-subduction zone magmatism at ca. 917 Ma. These Precambrian rocks are unconformably overlain by uppermost Cambrian and Silurian platform rocks containing Gondwanan fauna and ca. 1 detrital zircons of Oaxacan provenance. The neighbouring Mixteca terrane includes lower Paleozoic, rift-passive margin sedimentary rocks that also contain 900–750 Ma detrital zircons probably derived from the Goias arc in eastern Amazonia. The arc-backarc tectonic setting inferred for the 1300–900 Ma rocks also suggests that Oaxaquia lay on an active periphery of Amazonia until ca. 900 Ma, well after the amalgamation of Rodinia. This precludes a location for Oaxaquia off southern and western Amazonia that are inferred to have been juxtaposed against eastern Laurentia; contiguity with eastern Amazonia is also unlikely given the absence of the 900–750 Ma convergent tectonics in the Goias arc. This leaves northern Amazonia as the most likely position, a location that requires the least relative displacement between Oaxaquia and Amazonia. The inferred 750 and 545 Ma exhumation episodes of Oaxaquia correspond to two proposed breakup stages of Rodinia.

Miocene evolution of the western edge of the Nevadaplano in the central and northern Sierra Nevada: palaeocanyons, magmatism, and structure

The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young ( < 6 Ma); however, recent work has provided evidence that the range is very old (>80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ∼2–5 Myr of subduction-induced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11–10 Ma, records the birth of the ‘future plate boundary’ by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8–7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3–3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial- and mass-wasting deposits, and greater degree of encroachment by Walker Lane-related faults beginning at 10–11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events.

The Nordic Volcanological Institute: understanding volcanoes at spreading centres

The Nordic countries (known as ‘Norden’) are not immediately associated with volcanoes: Norway with folded mountains cut by fjords and its offshore oil and gas deposits, Sweden and Finland by the western part of the Baltic shield, a huge area of Precambrian rocks, of which gneisses form a large part, and Denmark, a country of Mesozoic and Cenozoic rocks, where glacial, superficial deposits are of major importance. But Norden also includes Iceland, where everyone immediately thinks of volcanoes and glaciers. Clearly volcanological research would be expected to be a major priority for the Icelandic nation. However, in the other Nordic countries old volcanic and other igneous rocks play a significant role, comprising a large part of the Precambrian and Caledonian terrains and being a key to many of the commercial mineral deposits which play a major role in the economies of Norway, Sweden and Finland. Even Denmark, a country of sedimentary rocks has an impressive sequence of Paleogene volcanic ashes and the Faeroe Islands, made up almost entirely of basalts, are part of Denmark.

Cenozoic sedimentary rocks on the northern slopes of the Kuril Deep-Water Basin (Sea of Okhotsk) and conditions of their formation

Sedimentary rocks from the northern slope of the Kuril Deep-Water Basin are examined. Four different-age lithological units are distinguished, and inferences about the probable conditions of their formation are made. The Paleogene-Lower Miocene deposits (lithological units I and II) are represented by the purest varieties of siliceous rocks, which implies their accumulation far away from the source areas under quiet hydrodynamical conditions of the waters. The composition of the microfossils suggests a relatively shallow-water marine basin. The Pliocene-Pleistocene deposits of lithological units III and IV were formed in an active hydrodynamical environment under conditions of synchronous explosive volcanism at bathyal depths. The presence of porcelanite outcrops on the continental slope of the basin, together with the micropaleontological data about the paleodepths of the sedimentation, allows us to assume that, in the region studied, the continental slope was formed as a result of vertical motions in the post-Middle Miocene time.

Braquiópodes (Rhynchonelliformea, Bouchardioidea) neógenos da bacia de Pelotas (RS) e seu significado paleoambiental

The occurrence of brachiopods in Cenozoic rocks of the Pelotas Basin is known since 1862. In spite of that, detailed systematic and taphonomic studies are still missing. Investigations made a half century ago, have suggested that these brachiopods could belong to Bouchardia cf. zitteli, a species found in the San Julian Formation, Late Oligocene, Argentina. Our data suggest that those brachiopods may resemble Bouchardia transplatina. In the Uruguayan portion of the Pelotas Basin B. transplatina is known in rocks of the Camacho Formation, Miocene. In addition, small recrystallized shells of brachiopods were also recovered from three Petrobras boreholes (2PJ-1-RS, 2PN-1-RS, and 2GA-1-RS) from the Pelotas Basin. Brachiopods come from the interval of 130 to 150 meters within the Miocene Henryhowella evax Zone. Despite the degree of taphonomic modification of those brachiopod shells they indubitably belong to Bouchardia sp. This is noteworthy for various reasons: 1- Bouchardia is a brachiopod with warm water affinities. Presently, extant members of this genus are unknown in latitudes up to 34ºS, with the main records at 23ºS. 2- Although occurring in depths down to 200 meters, the living member (Bouchardia rosea) of this genus is most abundant in shallow platformal, nutrient-rich waters. 3- The occurrence of Bouchardia in the Miocene of the Pelotas Basin indicates that, at least to the interval of Henryhowella evax Zone, warm waters of the Brazilian currents prevail. This interpretation is in strong accordance with other paleoeoceanographic and paleoclimatic data offered by various groups of co-occurring microfossils, such as ostracodes and foraminifers

Peculiarities of composition and possible source of PGE minerals from the placer gold deposit of the Taymen’ river (Western Priokhotsk Region, Russia)

Coarse platinoid-chromite intergrowths in association with native gold, as well as with Au‐(Ag)‐Cu and Au‐(Cu,Ag)‐Pd alloys, first discovered in the Taymen’ river alluvial sediments have been investigated. Identification of the essentially isoferroplatinum-ferrichromite composition of these intergrowths in the presence of native metal, intermetallic and chalcogenide phases of specific composition within the platinum matrix allowed the authors to connect genetically the origin of these intergrowths with platinum-bearing alkaline-ultrabasic massifs of the Chad-Konder type. Products of destroying similar rocks presumably accumulated in the Taymen’ river placer. Analogues of these circular intrusive rocks are now apparently overlapped by Mesozoic and Cenozoic rocks including Quaternary unconsolidated sediments. The revealed typomorphism of Pt‐Fe alloys, platinoid chalcogenides, and chromespinellides from the Taymen’ river gold placer allows one to prospect the Au‐(Cu)-Pt‐Cr mineralization occurrence formerly unknown in the subregion of the central part of Western Priokhotsk region. Special attention is focused on the peculiarities of the compositions of natural Pt‐Fe alloys, platinoid chalcogenides, and chrome-spinellides due to their well-known capabilities to indicate formation affiliation of the source rocks. In particular, this is important while typing the root supply sources of platinum-bearing placers, especially in regions with unknown endogenous platinoid sources. Such a situation appeared in connection with our discovery of large lumpy platinum-chromite intergrowths in the alluvial gold placer of the river Taymen’ (northeast of Khabarovsk krai). The Taymenskoye deposit of placer gold is situated to the southeast from the Lantar part of the Dzhugdzhur autonomous type anorthosite massif [1]. To the southwest, the deposit joins with the gold-bearing placer of the Lantar’ River valley. The Lantar’ and Taymen’ valleys are separated from the Sea of Okhotsk by the Pri

Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation*

The development and application of geochronological tools suitable for dating Cenozoic rocks and processes have been instrumental to our understanding of the modern history of Australia. Geochronology reveals a dynamic continent that traced a long and rapid trajectory from a position adjacent to Antarctica in the early Cenozoic to its present position near the tropics. The average travel velocity along this path is revealed by the age of hotspot volcanoes, derived by the K–Ar method, and is complemented by measured geomagnetic pole positions on dated igneous rocks and sedimentary deposits. K–Ar dating of volcanic rocks also provided constraints on rates of landscape evolution before and after volcanism and the timing and pattern of dispersion of life—including human inhabitation. K–Ar geochronological results reveal a history of faunal and floral evolution suggestive of a continent undergoing progressive cooling and dehydration with a few brief warm and humid excursions. In contrast, 40Ar/39Ar, SHRIMP U–Pb, fission-track thermochronology, luminescence techniques, and cosmogenic-isotope methods have played relatively minor roles in reconstructing the chronology of Cenozoic volcanism in Australia. Integrated application of these techniques will be critical to providing more precise constraints on the volcanic history of the continent and its climatic and biological evolution. While Cenozoic volcanism, uplift, and denudation were active along the eastern and southeastern margins, a significant part of Australia west of the Tasman Line remained relatively quiescent. The history of this part of the continent is marked by slow and subdued uplift and subsidence, with subtle displacements along major continental structures, and occasional invasion by shallow seas. Despite the general absence of Cenozoic igneous rocks west of the Tasman Line, Australia (east and west) is blanketed by Cenozoic sedimentary covers and weathering profiles. If we consider weathering as a fourth rock-forming process (in addition to igneous, metamorphic and sedimentary), Australia has one of the most complete Cenozoic rock covers of any continent. Retrieving information recorded in these weathering profiles is essential for unravelling its Cenozoic history. Paleomagnetic studies, calibrated δ18O curves, and weathering geochronology by K–Ar, 40Ar/39Ar, and (U–Th)/He provide insights into the imprint of climatic events and tectonic processes and illustrate the importance of erosion and weathering to the formation or enrichment of ore and mineral deposits. Except for diamondiferous lamproites of Western Australia and sapphire-rich volcanic rocks in eastern Australia, all other Cenozoic ore and mineral deposits in Australia are related to weathering and erosion. The widespread weathered blanket in Australia suggests low Cenozoic erosion rates. Numerical constraints on chronology and erosion rates are derived from the cooling and denudation histories retrieved from apatite and zircon fission-track and, more recently, (U–Th–Sm)/He thermochronology and cosmogenic isotope studies. Geochronological studies of veneers of sediments, lake and cave deposits, marine carbonates, organic matter and groundwaters provide information on sediment provenance, subtle tectonic movements, and the Australian Cenozoic climatic history. These studies reveal a continent sensitive to global climatic cycles and subject to active, but subtle, tectonism and erosion. This record shows that Australia suffered periods of extreme aridity during cyclical glaciation at high latitudes and precise dating of carbonate sediments and speleothems reveals the exact timing and duration of these glacial and interglacial periods. Cosmogenic isotopes also provide constraints on the age and migration paths of Australia's limited and finite groundwater resources. Lastly, age information extracted from surficial deposits reveals a protracted history of human occupation.

Trace-element and Sr–Nd isotopic evidence for the origin of the Sardinian fluorite mineralization (Italy)

Abstract The fluorite-bearing hydrothermal mineralization in Sardinia mainly occurs within Paleozoic volcanic and metasedimentary rocks. Only 3 occurrences are located in volcanic and siliciclastic Cenozoic rocks. Most Sardinian fluorites exhibit relatively high rare earth and Y (REY) contents, strong positive Y anomalies, slightly negative Ce and generally positive Eu anomalies. These features indicate that the REY were mobilized mainly from non-carbonate rocks. Neither Sr nor Nd isotopes can be used to date radiometrically the Sardinian fluorites. However, the measured Sr-isotope ratios of the fluorites hosted by Paleozoic rocks fit mixing lines in the 1000/Sr versus 87Sr/86Sr plot once recalculated at 280 Ma, suggesting that the age inferred for the correction probably represents that of the formation of the fluorite mineralization. Mixing likely occurred between diluted surficial waters and brines circulating mainly through the Lower Paleozoic metasedimentary basement. The Cenozoic fluorites exhibit chemical and isotopic features similar to those of the Paleozoic fluorites, except the Nuraghe Onigu fluorite displaying a possible contribution of Sr from Cenozoic magmatic rocks. The initial eNd values of the Paleozoic fluorites fit the age proposed for the formation of the deposits. Moreover, the values suggest that radiogenic Nd was provided to the fluids from the Ordovician siliciclastic basement, except for 3 deposits where the potential source rocks of Nd were mainly Ordovician acidic magmatic rocks. The initial eNd values of the Cenozoic fluorites suggest a provenance of Nd essentially from the leaching of Variscan granitoids.

Structure and oil and gas perspectives of the Californian borderland

The Californian borderland is the largest oil and gas region of the Pacific coast of the United States. Here, a series of large and minor sedimentary basins with established or inferred oil and gas bearing properties are recognized. The Californian borderland is located on a Nevadan-type active continental margin. The bodies of such margins are composed of giant accretion formations and occupy not only their underwater parts but also vast land areas. The accretion formations are dominated by rocks of a deep-water origin. The Californian borderland represents a system of basins and ranges that compose the underwater margin and the Coastal Ranges of California and the basement within the Great Valley. In the depressions of the accretion orogen, which are small in size but feature high subsidence rates, significant thicknesses of deposits are accumulated (up to 6−8 km). In the depressions of the Californian borderland, they are represented by young Neogene (rarer Eocene-Oligocene) formations of mainly terrigenous or siliceous-terrigenous compositions. The active tectonic regime resulted in a sharp reduction of the age range of the oil and gas bearing deposits, which are represented only by Late Cretaceous and Cenozoic rocks. Full-scale oil and gas production here is performed in the Santa Barbara-Ventura and Santa Maria basins. The principal oil and gas bearing unit is the Monterrey Formation of the Middle Miocene. By January 2006, the production on the Californian borderland comprised about 17.8 million tons of oil and over 1 billion m3 of gas. Up to 1.4 billion tons of oil and 200 billion m3 of gas reserves are regarded to be not recovered on the Pacific coast of the United States in the region of the Californian borderland.

Large-magnitude Miocene extension of the Eocene Caetano caldera, Shoshone and Toiyabe Ranges, Nevada

Because major mineral deposits in north-central Nevada predate significant Basin and Range extension, a detailed understanding of the timing and kinematics of extensional faulting is necessary to place these deposits in their original structural context. The complexity of pre-Cenozoic deformation in northern Nevada makes restoring Basin and Range faulting difficult without locating well-dated, regionally extensive Cenozoic units that can be used to restore slip along normal faults. The goal of this study is to reconstruct extensional faulting in the Shoshone and northern Toiyabe Ranges by using Cenozoic rocks in and around the Caetano caldera, which formed ca. 33.8 Ma during eruption of the Caetano Tuff. The caldera filled with more than 4 km of intracaldera tuff during initial caldera-forming eruptions, and additional sedimentary and volcanic rocks subsequently filled the topographic depression left by the caldera collapse. These rocks are conformable over the interval 34–25 Ma, consistent with little, if any, extension during that time. The 34–25 Ma rocks were later cut by a set of closely spaced (1–3 km) normal faults that accommodated significant extension and foot-wall rotations of 40°–50°. Restored structural cross sections indicate that the present ∼42 km (east-west) width of the Caetano caldera has been extended 110%, resulting in 22 ± 3 km westward translation of the Fish Creek Mountains relative to the southern Cortez Range. Major normal faults mapped within the caldera continue south and north along strike into the surrounding Paleozoic basement rocks; therefore it is likely that parts of surrounding areas are also significantly extended. Miocene (16–12 Ma) sedimentary rocks in the hanging walls of major normal faults include both fluvial/lacustrine facies and coarser alluvial fan deposits. Where exposed, the bases of the Miocene sedimentary sections are in angular conformity with underlying ∼40°E tilted 34–25 Ma volcanic and sedimentary rocks. The distribution, composition, and geometry of these deposits are best explained by accumulation in a set of half-graben basins that formed in response to slip on basin-bounding faults. Extension thus appears to have taken place in the middle Miocene, beginning at or shortly after 16 Ma, and was mostly completed by 10–12 Ma. Fault blocks and basins formed during middle Miocene extension are cut by younger, more widely spaced, high-angle normal faults that began forming more recently than 10–12 Ma. These faults outline the modern basins and ranges in the study area and some have remained active into the Holocene.