Cenozoic Sedimentary Sequences Research Articles

U-Pb ages of detrital zircon from Cenozoic sediments in the southwestern Tarim Basin, NW China: Implications for Eocene–Pliocene source-to-sink relations and new insights into Cretaceous–Paleogene magmatic sources

A detailed investigation of potential provenance is still lacking in the southwestern Tarim Basin, which restricts our complete understanding of Cenozoic source-to-sink relations between the basin interior and the Pamir salient - western Kunlun Mountain Range. Debate also exists concerning the potential sources of the Paleogene and Cretaceous igneous detritus present in the Cenozoic sedimentary sequences. Here, we present U-Pb (LA-ICP-MS) ages of detrital zircons from the continuous Eocene-Pliocene sediment series in the well-exposed Aertashi section to investigate changes in sediment provenance through time. The U-Pb detrital zircon ages range widely from 45 to 3204 Ma and can be divided into seven main groups: 45–65 Ma (sub-peak at 49 Ma), 67–103 Ma (sub-peak at 95 Ma), 196–251 Ma (sub-peak at 208 Ma), 252–416 Ma (sub-peak at 296 Ma), 417–540 Ma (sub-peak at 446 Ma), 550–1429 Ma (sub-peaks at 614 Ma, 828 Ma and 942 Ma) and 1345–3204 Ma (sub-peaks at 1773 Ma and 2480 Ma). These zircons were mainly derived from the western Kunlun Mountain Range and northern Pamir salient to the west and south. The evolution of the provenance and source-to-sink relationship patterns in the southwestern Tarim Basin can be divided into three stages: (1) The Middle Eocene to Lower Oligocene sediments display a wide variety of detrital zircon ages, suggesting that the source area was extensive. (2) A major change in provenance occurred during the Late Oligocene to Early Miocene and was characterized by an abrupt increase in the proportion of Triassic and Lower Paleozoic igneous components, implying a significant adjustment in topography induced by the initial uplift and exhumation of the western Kunlun Mountain Range and northern Pamir salient. (3) In the Late Miocene, the source-to-sink system transformed again, and contributions of Triassic to Lower Paleozoic material weakened substantially due to the sufficient indentation of the Pamir salient. Our integrated analyses of zircon geochronology indicate that the main source terranes of the Paleogene and Cretaceous igneous detritus are the central and southern Pamir salient, respectively, which are speculated to have been continuously connected to the study area during Eocene-Pliocene times, although such detritus is scarce in certain formations and has not yet been detected.

Petrography and geochemistry of Cenozoic sedimentary sequences of the southern Samar Island, Philippines: Clues to the unroofing history of an ancient subduction zone

The Cenozoic sedimentary sequences of southern Samar Island in eastern Philippines were examined to understand the unroofing history of an ancient arc terrane. Petrographic and geochemical data revealed varying degrees of inputs from the ophiolite basement and differences in modal compositions. The sedimentary units are mostly made up of lithic fragments. The Late Oligocene to Early Miocene Daram Formation contains more chert and volcanic fragments whereas the late Middle Miocene to Early Pliocene Catbalogan Formation is dominantly composed of ultramafic components. These variances are correspondingly reflected in the geochemical signatures of these two sedimentary formations. The Catbalogan Formation clastic rocks have higher volatile-free MgO and Fe2O3 values (average: 8.4% for both oxides) compared to the Daram Formation samples (average: 5.1 and 6.3%, respectively). Geochemical variations are also reflected in the Co, Cr and Ni values: the Catbalogan Formation samples reflect higher concentrations (Co: 15–57ppm; Cr: 231–1094ppm; Ni: 84–484ppm) compared to the Daram Formation samples (Co: 24–32ppm; Cr: 234–418ppm; Ni: 212–323ppm). These observations suggest that the Daram Formation eroded and transported more of the crustal portions of the ophiolite, while the younger Catbalogan Formation represents a later exhumation and subsequent erosion of the ultramafic section. An oceanic island arc (OIA) setting is proposed for the two formations based on several tectonic discrimination diagrams (e.g., Th-La-Sc, La vs. Th). The OIA signature is further supported by their smooth chondrite-normalized rare earth element (REE) patterns with no obvious Eu anomaly as well as LREE enrichment which are typical of sediments deposited in OIA setting. Based on the dominantly ophiolitic provenance of the Daram and Catbalogan formations, the post-emplacement history of the nearby Samar Ophiolite is constrained during the Late Oligocene to Early Pliocene period.

δ18O and δ13C records from a Cenozoic sedimentary sequence in the Lanzhou Basin, Northwestern China: Implications for palaeoenvironmental and palaeoecological changes

In northwestern China, carbonate δ18O variation has been closely associated with evaporation and precipitation, whereas the variation of carbonate δ13C generally reflects patterns of palaeovegetation. Located within the transitional zone between the Chinese Loess Plateau and the Tibetan Plateau, the Lanzhou Basin has developed a continuous sequence of Cenozoic sediments which have been subjected to detailed sedimentological and high-resolution magnetostratigraphic analyses. In the present study, pedogenic carbonate O and C isotopic analyses were obtained throughout the entire Cenozoic sequence. The δ18O record exhibits a general positive trend with several abrupt changes. A dramatic positive shift in the δ18O record at ∼33Ma indicates the initiation of the aridification process within the basin, which was likely associated with the late Eocene westward retreat of the Tethys Sea and global cooling. Two significant positive shifts in the δ18O record at ∼22Ma and ∼3.5Ma are synchronous with major increases in aeolian dust deposition on the Chinese Loess Plateau and in the North Pacific Ocean, suggesting the intensified aridity of the Asian interior, which is likely related to the stepwise uplift of the Tibetan Plateau via the blocking of water vapour pathways. The δ13C values exhibit a weak positive trend with a remarkable shift at ∼3.5Ma. This trend is likely related to a decrease in vegetation density in response to the ongoing Cenozoic aridification, whereas the shift at ∼3.5Ma may reflect the large-scale expansion of C4 plants.

Geology and Hydrochemistry of the Deep Sandstone Aquifers of Jordan

The deep sandstone aquifer complex of Jordan consists of hundreds of meters of Paleozoic to Lower Cretaceous sandstones that extend from the Saudi Arabian border in the south up to northern Jordan. In the south, this aquifer is known as the Disi Aquifer and is near the surface, well understood and heavily exploited. Toward the north, thick accumulations of later Mesozoic and Cenozoic sedimentary sequences cover the complex. The depth of the aquifer in this area makes it less viable as a water resource. Thus, little is known about its origin, movement and chemical evolution. A number of exploration and production wells have penetrated this aquifer throughout central and northern Jordan. The data from these wells can help to draw a reasonable understanding about this aquifer and its water. Most of the aquifer is under artesian pressure, and the piezometric head data point to a general flow north with drainage of the aquifer into the Dead Sea Rift Basin. Stable isotopes show that the water differs from modern meteoric water in the region and thus is possibly Late Pleistocene in age. The water is slightly brackish, and according to Jordanian Standards, it can be used for drinking under certain conditions. Geochemical modeling shows that here is no evidence that the salinity is primarily the result of prolonged water–rock interactions, but more likely the result of mixing with possibly trapped connate water throughout the complex.

Detrital zircon provenance analysis in the Zagros Orogen, SW Iran: implications for the amalgamation history of the Neo-Tethys

The Zagros Orogen developed as a result of Arabia–Eurasia collision. New in situ detrital zircon U–Pb and Hf isotopic analyses from a Cenozoic sedimentary sequence in SW Iran are used to unravel the amalgamation history of Neo-Tethys. Data indicate that: (1) Paleocene and Eocene strata (58 and 45 Ma, respectively) were sourced from obducted ophiolite and Triassic volcanics, (2) Lower Miocene (~18 Ma) strata indicate mixed provenance from obducted ophiolite and Iranian magmatic rocks, (3) Mid to Upper Miocene sediments (~14 to 11.2 Ma) were mainly sourced from Sanandaj–Sirjan zone granitoids to the north, and (4) Lower Pliocene (~5 Ma) sediments mainly show Arabian age characteristics, with a minor Eurasian affinity component. Two hypotheses are outlined to highlight the key events: Hypothesis A, previously published by several workers, suggests that the sequence studied lay on the Arabia passive margin and that initial collision occurred prior to 18 Ma; Hypothesis B, modified from the Makran model, which is here preferred, suggests that Paleogene to Upper Miocene sediments were sourced from the northern Neo-Tethyan accretionary complex or Eurasia, and carry no input from Arabia, whereas the Lower Pliocene sample shows a mixed provenance from both Arabia and Eurasia, suggesting that collision occurred between ~11.2 and 5 Ma.

Seepage carbonate mounds in Cenozoic sedimentary sequences from the Las Minas Basin, SE Spain

A number of carbonate mounds composed of indurate, strongly folded and/or brecciated calcite and dolomite beds occur interstratified in Cenozoic sedimentary sequences from the Las Minas Basin. Part of the fabric of the rock forming the carbonate mounds is composed of laminated to banded dolostone similar to the host rock but showing contrasted lithification. Moreover, the carbonate deposits of the mounds display aggrading neomorphism of dolomite, partial replacement of dolomite by calcite, calcite cementation, and extensive silicification, locally resulting in box-work fabric. Eight main lithofacies were distinguished in the carbonate mound deposits. In some lithofacies, chert is present as both microcrystalline to fibro-radial quartz and opal, the latter occurring mainly as cement whereas the former replace the carbonate and infill voids. Yet one of the carbonate mounds shows distinctive petrography and geochemical features thus suggesting a distinctive growth pattern. The carbon isotope compositions of calcite from the mound samples range from −11.56 to −5.15 δ‰ whilst dolomite is depleted in 13C, with values of −12.38 to 3.02 δ‰. Oxygen isotopic compositions vary from −9.42 to −4.64 δ‰ for calcite and between −6.68 and 8.19 δ‰ for dolomite. Carbonate in the mounds shows significant enrichment in Co, Cr, Ni and Pb content, especially in the strongly deformed (F-2-2 lithofacies) and brecciated carbonate (F-4). The carbonate deposits show depletion in REE and Y in contrast to that determined in lutite. The formation of the carbonate mounds was related to local artesian seepage thermal water flows of moderate to relative high temperatures. Pressure differences between the low permeability host rock and the circulating fluids accounted for dilational fracturing and brecciation of the host sediment packages, which combined with precipitation of new carbonate and silica mineral phases. Locally, some carbonate mounds developed where groundwater intersected the lake floor, this resulting in bedded carbonate deposits composed of organic constituents.

40Ar/39Ar geochronology of supergene K-bearing sulfate minerals: Cenozoic continental weathering and its paleoclimatic significance in the Tu–Ha Basin, northwestern China

40Ar/39Ar incremental-heating analyses of supergene jarosite and yavapaiite were conducted for three weathering profiles at the Hongshan, Liuhuangshan, and Caihuagou deposits in the Tu–Ha Basin. The analyses provide numerical constraints on the timing and duration of weathering and supergene enrichment. Well-constrained plateau ages and best-fitting inverse isochrons yield 40Ar/39Ar ages ranging from 33.3±0.5Ma to 3.3±0.4Ma (1σ). Our 40Ar/39Ar ages, combined with the published ages, indicate that a protracted history of weathering and supergene enrichment and, by inference, arid–semiarid climate (with at least a moderate amount of precipitation (>10cm/y)) favorable to intense chemical weathering emerged at 33.3Ma, 27.7–23.3Ma, and 16.4–14.7Ma, and prevailed from 11 to 7.8Ma. Then, a progressive change from arid–semiarid toward hyperarid conditions and predominantly hyperarid conditions may have persisted since at least ca. 3.3Ma. The climatic implications inferred from the weathering geochronology are in agreement with the chemical parameters and isotopic compositions of the Cenozoic sedimentary sequence from the Lianmuqin section in the Tu–Ha Basin, attesting to the reliability of weathering geochronology by the 40Ar/39Ar method as an indicator of paleoclimate in arid areas. Our results suggest that the retreat of the Paratethys Sea, which would have reduced eastward water vapor transport by the westerlies to the Tu–Ha Basin, led to its aridification in the Oligocene and that increased rain shadow effects, resulting from uplift of the Tibetan Plateau and Tian Shan Mountains, played important roles in the aridification history of the Tu–Ha Basin since the late Miocene.

Paleomagnetic constraints on the tectonic evolution of the Costa Rican subduction zone: New results from sedimentary successions of IODP drill sites from the Cocos Ridge

AbstractThe near‐flat subduction of the Cocos Ridge (CR) along the Middle American Trench (MAT) plays a pivotal role in governing the geodynamic evolution of the central American convergent margin. Elucidating the onset of its subduction is essential to understand the tectonic evolution and seismogenesis of the Costa Rican convergent margin, a typical erosive convergent margin and modern example of a flat‐slab subduction. Initial subduction of the CR has been previously investigated by examining upper plate deformation that was inferred to have resulted from the initial CR subduction. However, little attention has been paid to the extensive sedimentary archives on the CR that could hold important clues to the initial CR subduction. Drilling on the CR during IODP Expedition 344 discovered a pronounced sedimentary hiatus at Site U1381. Here we present paleomagnetic and rock magnetic results of the Cenozoic sedimentary sequences at this site that bracket the hiatus between ca. 9.61 and 1.52 Ma. We also examine the areal extent, timing, and geologic significance of the hiatus by analyzing sedimentary records from five other ODP/IODP sites on CR and Cocos plate. The analyses show that the hiatus appears to be regional and the presence/absence of the sedimentary hiatus at different locations on CR implies a link to the onset of CR shallow subduction, as a result of either bottom current erosion or CR buckling upon its initial collision at the MAT. Records directly from CR thus provide a new window to unraveling the geodynamic evolution of the central American margin.

Middle Miocene to earliest Pliocene sedimentological and geochemical records of climate change in the western Qaidam Basin on the NE Tibetan Plateau

Global cooling, Tibetan Plateau uplift and the retreat of the Para-Tethys Sea have long been thought to be the key controls of the Cenozoic climate change in the Asian inland. However, due to the scarcity of long-term palaeoclimate records, it is not entirely clear which of these factors plays the major role. Here, we present new sedimentological and geochemical proxy records (TOC, carbonate, SO42− and Cl−) from a 1040m late Cenozoic sedimentary sequence (ca. 17 to 5.0Ma) from the western part of the Qaidam Basin, the largest inland basin on the NE Tibetan Plateau. Geochemical records reveal an evident two-stage change. From ca. 17Ma to 11.1Ma (Phase I), the proxies show an upward decrease of carbonate content and TOC concentration and a persistent increase in the concentration of the soluble anions SO42− and Cl−, even though superimposed high-frequency fluctuations are evident. From ca. 11.1Ma to 5.0Ma (Phase II), the average concentrations of SO42− and Cl− are higher than those from Phase I, and a sharp decrease in the carbonate content occurred.The sedimentological and geochemical proxy records demonstrate that the Qaidam Basin has experienced increased aridity since ca. 11.1Ma. We suggest that the Miocene global cooling exerted a significant influence on the drying of the Qaidam Basin by decreasing evaporation rates and water vapour concentrations in the atmosphere. The Mio-Pliocene episodic and persistent uplift of the NE Tibetan Plateau exerted an important influence superimposed upon this driving force, while during ca. 11.1 to 8Ma, the rapid uplift of the Altun Shan might have played a key role in climate change of the Qaidam Basin.

A high-resolution multi-proxy record of late Cenozoic environment change from central Taklimakan Desert, China

Abstract. The Taklimakan Desert in the Tarim Basin is the largest desert in Central Asia, and is regarded as one of the main dust sources to the Northern Hemisphere. Late Cenozoic sedimentary sequences with intercalated in-situ aeolian dune sands in this area preserve direct evidence for the Asian desertification. Herein, we report a high-resolution multi-proxy climatic record from the precise magnetostratigraphic dated Hongbaishan section in the central Taklimakan Desert. Our results show that a fundamental climate change, characterised by significant cooling, enhanced aridity, and intensified atmospheric circulation, occurred at 2.8 Ma. Good correlations between paleo-environmental records in the dust sources and downwind areas suggest a broadly consistent climate evolution of northwestern China during the late Cenozoic, which is probably driven by the uplift of the Tibet Plateau and the Northern Hemisphere glaciation.

Cenozoic volcanic rocks in the Pearl River Mouth and Southeast Hainan Basins of South China Sea and their implications for petroleum geology

Drilling, gravity, magnetic observation and seismic facies are combined to identify the Cenozoic volcanic rocks in the Pearl River Mouth Basin and the Southeast Hainan Basin in the northern South China Sea, and to analyze how their lithology, distribution characteristics and volcanic activity influence on hydrocarbon accumulation. As a result, more than 70 volcanic bodies are ascertained. They are divided into two types: (1) laminated lava and pyroclastic rocks in the Cenozoic sedimentary sequence; and (2) conoid, mounded or wedged basalt with rigid basement under the Cenozoic sedimentary sequence. In the early Cenozoic (E1-E2), the acidic, neutral and basic volcanic rocks developed, while in the mid-late Cenozoic (E3-Q), single basic rocks existed intensively. The volcanic lithofacies mainly include the lower and middle effusive subfacies and the pyroclastic flow deposit explosive subfacies. Spatially, from the Southeast Hainan Basin to the Pearl River Mouth Basin, the Cenozoic volcanic activity gradually enhances; from shallow water area to the Central Basin of the South China Sea, the scale of Cenozoic volcanic rocks increases gradually, and the age tends to be younger. These activities improve the formation temperature, reduce the hydrocarbon generation threshold, and promote the hydrocarbon accumulation. Volcanic rocks in Wenchang Formation (E2w) to Zhujiang Formation (N1z) provide favorable conditions for hydrocarbon accumulation, so they are important targets in the deep exploration.

Magnetostratigraphic results from sedimentary rocks of IODP’s Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 322

Abstract We conducted a palaeomagnetic study on the Cenozoic sedimentary sequences of the Nankai Trough, recovered by the Integrated Ocean Drilling Program Expedition 322 in SE Japan. Sedimentary sections of Late Miocene age from the two subduction input sites (sites C0011 and C0012) recorded a pattern of magnetic polarity reversals that correlates well with the known magnetic polarity time scale. The polarity of characteristic remanent magnetization could be identified throughout the majority of the recovered cores of the two sites, following removal of a low-stability drilling-induced remanence. Most of the observed magnetostratigraphy from the characteristic directions is in good agreement with that to be expected from the stratigraphic position of the sequence deduced from the biostratigraphic data. Palaeomagnetic data from both shipboard and shore-based studies indicate changes in the rate of sedimentation from 9.5 to 2.7 cm/kyr at about 11 Ma, suggesting that some fundamental palaeoenvironmental change in the Shikoku Basin and/or significant tectonic event may have occurred in Late Miocene.

Factors controlling selective abandonment and reactivation in thick-skin orogens: a case study in the Magdalena Valley, Colombia

Abstract The initial stages of tectonic inversion and the mechanisms of selective reactivation and abandonment of pre-existing normal faults during contractional orogenesis are explored in a partially buried Cenozoic thrust belt in the Andes of Colombia. A multidisciplinary approach that includes subsurface structural mapping, multimethod thermochronometry and detrital zircon U–Pb geochronology reveals the extent of a Palaeogene thrust belt buried underneath the Cenozoic strata of the Middle Magdalena Valley Basin. A less oblique orientation with respect to compressive stress and shorter traces in faults of the Middle Magdalena Valley Basin with respect to faults in the western part of the Eastern Cordillera, apparently acted as deformation inhibitors of the Magdalena faults in advanced Neogene stages of inversion. Protracted Cenozoic eastwards tilting of the Central Cordillera and the tectonic load from the uplifting Eastern Cordillera favoured the accumulation of a thick Cenozoic sedimentary sequence in an, at least episodically, closed basin. All the above-mentioned conditions helped to block deformation in the Magdalena Basin, favouring deformation to be taken up by structures in the western Eastern Cordillera. These relationships underscore the importance of buried structural records in elevated hinterland basins, in which the low-relief stratigraphic cover overlies a complex subsurface record, potentially including large magnitudes of deformation during early orogenesis. Supplementary material: Tables and figures on the laboratory methods for the thermochronometrical and geochronometrical analyses are available at http://www.geolsoc.org.uk/SUP18601 .

Geology, Geotechnical Engineering, and Natural Hazards of Memphis, Tennessee, USA

The generally flat landscape of the Memphis, TN, area hides a fascinating geologic history. Cambrian rifting resulted in the concealed faults of the Reelfoot rift, currently the active component of the New Madrid Seismic Zone, lying immediately northwest of the city. Memphis also sits within the Mississippi Embayment. Subsidence of the embayment brought about deposition of the thick Late Cretaceous and Cenozoic sedimentary sequence underlying the city. This sequence provides one of the finest groundwater aquifers in the world, the Memphis Sand. Pliocene ancestral Ohio River alluvium (Upland Complex) is an excellent source of sand and gravel for the region. Plio-Pleistocene fluvial degradation by the Mississippi River and its tributaries, in addition to deposition of Pleistocene loess sheets, have produced the subtle topography of Memphis and Shelby County. Memphis geology makes geotechnical engineering design challenging because loess is highly sensitive to disturbance. Exploratory sampling techniques tend to cause disturbance to loess during sampling and sample extraction. Thus, conventional sampling techniques yield variable engineering property test results (e.g., strength and compressibility). The city of Memphis and Shelby County have together become a major commodities distribution center for the United States, North America, and the global economy. The principal natural hazards are the frequent severe weather and the prospect of a repeat of the >7M 1811–1812 New Madrid earthquakes. These unavoidable exposures to severe weather, urban and riverine flooding, and earthquakes combined with social conditions tend to amplify the consequences of hazard events.

Magnetostratigraphic and paleoenvironmental records for a Late Cenozoic sedimentary sequence drilled from Lop Nor in the eastern Tarim Basin

The Tarim Basin, one of the largest inland basins in the world, is situated in the northwestern China and to the north of the Tibetan Plateau. Continuous Cenozoic deposits have accumulated in this basin, which are crucial for investigating the growth of the Tibetan Plateau and the paleoclimatic evolution in Asian interior. Here we report the magnetostratigraphic and paleoenvironmental records for a Late Cenozoic sedimentary sequence drilled from Lop Nor in the eastern Tarim Basin. Magnetostratigraphic results show that this core has recorded a magnetic polarity sequence from C3Bn to C1n, covering an age range from ca. 7 Ma to the present. Decreased magnetic susceptibility occurred after ca. 5.6–5.1 Ma, which was interpreted to indicate an enhancement of aridity in the Tarim Basin since this period. We attribute this aridification to the combined effect of global climate cooling and the uplift of the Northern Tibetan Plateau since the late Miocene.

The tectonic framework and hydrocarbon prospectivity of the western Yenisei–Khatanga regional trough

The western part of the Yenisei–Khatanga trough is one of the least explored regions, which are thought to possess an excellent hydrocarbon resource potential. This region is geographically located in the southern part of the Taimyr Peninsula, within the Krasnoyarsk Territory. Utilizing regional seismic data, structure and thickness maps for seismic sequences allowed the reconstruction of the structural framework and tectonic evolution of the region. The study reveals a number of tectonic processes in the region that took place in the Paleozoic, Mesozoic, and Cenozoic and establishes the main evolutionary stages of a series of large structures of different ranks. The study also provides some insights into the structural features and depositional environments of Paleozoic, Triassic–Jurassic, Neocomian, Aptian–Albian–Cenomanian, and Turonian–Cenozoic sedimentary sequences. Based on the relationship between tectonic activity and hydrocarbon generation, the conclusion was made about a high hydrocarbon resource potential of the Neocomian clinoform sequence in the Yenisei–Khatanga regional trough.

Magnetostratigraphy and palaeoenvironmental records for a Late Cenozoic sedimentary sequence from Lanzhou, Northeastern margin of the Tibetan Plateau

Geological studies indicate that several Tertiary fluvial–lacustrine basins existed in northwestern China. Progressive uplift of the Tibetan Plateau forced these Tertiary basins to dry, and eventually became accumulation areas for aeolian sediments. Continuous sedimentary sequences consisting of alluvial–fluvial sediments and aeolian Red Clay occurred in the Lanzhou area. We used palaeomagnetic and palaeoenvironmental techniques to investigate a late Tertiary section that has a Quaternary loess cover. Our results indicate that the area was a low altitude basin with a fluvial–lacustrine environment before ~ 7.2 Ma. Starting from ~ 7.2 Ma, the basin became a dry land and was exposed to aeolian accumulation. The presences of unconformities and sedimentary facies changes suggest that strong tectonic activity occurred during the interval of 3.5–1.8 Ma, which forced the Lanzhou Tertiary Basin to break up and became elevated to its present altitude. Gradual variations in sediment colour, grain-size and magnetic susceptibility imply that the Lanzhou area experienced progressive cooling and drying in the Late Cenozoic due to local tectonic uplift and global cooling.

The evolution of Cenozoic explosive volcanism: The Iceland Plume

This paper reviews original and published data on the abundance and composition of pyroclastics due to explosive discharges by volcanoes on the Iceland Plume. The pyroclastics were deposited in the Cenozoic sediments in the North Atlantic Ocean and in the Norwegian-Greenland basin. The DSDP and ODP initial reports (70 deep wells), 100 geologic columns sampled during cruises of the R/Vs Akademik Kurchatov and Mikhail Lomonosov furnished the database from which we constructed stratigraphic and areal-maps of pyroclastics abundance and computed the distribution of the volumes and amounts of pyroclastic layers over the stratigraphic intervals of the Cenozoic sedimentary sequence. The distribution of these layers was found to be cyclic; the highest frequency occurred during the Quaternary. Basaltoid pyroclastics prevailed in the late Paleocene and Early Eocene. The Oligocene has typically subalkaline ankaramite pyroclastics. From the Miocene until the Quaternary the pyroclastics became bimodal (basalt-rhyolite) and high potassium rhyolite pyroclastics appeared. This evolution seems to have been caused by crystallization differentiation of basaltoid magmas in magma chambers that came into being in prespreading grabens where a thick (> 20 km) sequence of volcanic rocks accumulated to produce a dipping reflector.

Towards ground truthing exploration in the central Arctic Ocean: a Cenozoic compaction history from the Lomonosov Ridge

ABSTRACTThe Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), recovered the first Cenozoic sedimentary sequence from the central Arctic Ocean. ACEX provided ground truth for basin scale geophysical interpretations and for guiding future exploration targets in this largely unexplored ocean basin. Here, we present results from a series of consolidation tests used to characterize sediment compressibility and permeability and integrate these with high‐resolution measurements of bulk density, porosity and shear strength to investigate the stress history and the nature of prominent lithostratigraphic and seismostratigraphic boundaries in the ACEX record. Despite moderate sedimentation rates (10–30 m Myr−1) and high permeability values (10−15–10−18 m2), consolidation and shear strength measurements both suggest an overall state of underconsolidation or overpressure. One‐dimensional compaction modelling shows that to maintain such excess pore pressures, an in situ fluid source is required that exceeds the rate of fluid expulsion generated by mechanical compaction alone. Geochemical and sedimentological evidence is presented that identifies the Opal A–C/T transformation of biosiliceous rich sediments as a potential additional in situ fluid source. However, the combined rate of chemical and mechanical compaction remain too low to fully account for the observed pore pressure gradients, implying an additional diagenetic fluid source from within or below the recovered Cenozoic sediments from ACEX. Recognition of the Opal A–C/T reaction front in the ACEX record has broad reaching regional implications on slope stability and subsurface pressure evolution, and provides an important consideration for interpreting and correlating the spatially limited seismic data from the Arctic Ocean.

Geological, pedological and paleomagnetic study of the late Cenozoic sedimentary sequence in southwestern Uruguay, South America

The results of interdisciplinary studies carried out on the coastal cliffs of southern Uruguay are presented here. The Raigón Formation (late Pliocene–middle Pleistocene) has a cratonic provenance and comprises two sections limited by a regional discontinuity. Each section is composed of an association of lithofacies Ss, Sp, Sr, rare St, and Fm. The last one represents floodplain deposits and the others are interpreted as channel-filled deposits. In the upper part, a well-structured paleosol with clay coatings and abundant rhizoliths is observed in a sharp contact with silty deposits of the Libertad Fm. (early-middle Pleistocene); these sediments have a mixed provenance (magmatic arc and cratonic), sheet-like geometry, thick stratification and vertical walls, and are composed of lithofacies Fr. These characteristics suggest a loessic origin. A paleosol with illuvial B horizons and silty loam texture is recognized at the bottom of this unit. The recent soil (Argiudoll) is developed at the top of the Libertad Fm. The characteristics of the sequence indicate climatic fluctuations and several cycles of erosion, sedimentation and pedogenesis. According to magnetostratigraphy, the chronology is roughly compatible with that one based on the fossil contents and is in agreement with magnetic ages determined in Argentine fossiliferous sequences.