Coarse Clastic Sediments Research Articles

Palaeoenvironment and provenance of the Early Eocene arenaceous sequence of Neyshaboor, Binalud region, Iran

The Early Eocene sequence of Neyshaboor, Binalud region of Iran is predominantly composed of arenaceous deposits. Two stratigraphically important sections from the Damanjan and Taghan areas have been investigated based on field work, petrographic and geochemical analyses. Eight lithofacies were identified and have been grouped in to conglomerate, sandstone, and mudstone facies association. Petrographic and geochemical data show that the Early Eocene sandstones are mainly composed of arkoses and litharenites classes. Provenance analysis indicates that sediments were supplied from a nearby andesitic–granitic source with minor contribution of metamorphic and sedimentary sources. The presence of predominance of monocrystalline over polycrystalline quartz and abundance of K-feldspars; Cu and Pb contents also support this interpretation. However, subordinate representation of polycrystalline quartz grains, chert, volcanic rock fragments, biotite, zircon, as well as higher percentage of MgO and Fe2O3, suggest some contribution from high-grade metamorphic gneissic rocks and, to a lesser degree, from intermediate to basic volcanics. Climate varied from humid in the beginning of the sedimentation to sub-humid and arid during the later phases. Sedimentation was also influenced by prominent tectonic activity in the source when coarser clastic sediments were deposited as multistoried conglomerates. Lithofacies characters of the rock succession suggest sedimentation took place in a piedmont fan environment, adjacent to a rising orogeny in an active foreland basin setting. Clay to sand and gravel-sized sediments were laid down by meandering and braided rivers and by debris flows under changing conditions of climate and tectonics.

Subduction, ophiolite genesis and collision history of Tethys adjacent to the Eurasian continental margin: new evidence from the Eastern Pontides, Turkey

This paper presents several types of new information including U–Pb radiometric dating of ophiolitic rocks and an intrusive granite, micropalaeontological dating of siliceous and calcareous sedimentary rocks, together with sedimentological, petrographic and structural data. The new information is synthesised with existing results from the study area and adjacent regions (Central Pontides and Lesser Caucasus) to produce a new tectonic model for the Mesozoic–Cenozoic tectonic development of this key Tethyan suture zone.The Tethyan suture zone in NE Turkey (Ankara–Erzincan–Kars suture zone) exemplifies stages in the subduction, suturing and post-collisional deformation of a Mesozoic ocean basin that existed between the Eurasian (Pontide) and Gondwanan (Tauride) continents. Ophiolitic rocks, both as intact and as dismembered sequences, together with an intrusive granite (tonalite), formed during the Early Jurassic in a supra-subduction zone (SSZ) setting within the İzmir–Ankara–Erzincan ocean. Basalts also occur as blocks and dismembered thrust sheets within Cretaceous accretionary melange. During the Early Jurassic, these basalts erupted in both a SSZ-type setting and in an intra-plate (seamount-type) setting. The volcanic-sedimentary melange accreted in an open-ocean setting in response to Cretaceous northward subduction beneath a backstop made up of Early Jurassic forearc ophiolitic crust. The Early Jurassic SSZ basalts in the melange were later detached from the overriding Early Jurassic ophiolitic crust.Sedimentary melange (debris-flow deposits) locally includes ophiolitic extrusive rocks of boninitic composition that were metamorphosed under high-pressure low-temperature conditions. Slices of mainly Cretaceous clastic sedimentary rocks within the suture zone are interpreted as a deformed forearc basin that bordered the Eurasian active margin. The basin received a copious supply of sediments derived from Late Cretaceous arc volcanism together with input of ophiolitic detritus from accreted oceanic crust.Accretionary melange was emplaced southwards onto the leading edge of the Tauride continent (Munzur Massif) during latest Cretaceous time. Accretionary melange was also emplaced northwards over the collapsed southern edge of the Eurasian continental margin (continental backstop) during the latest Cretaceous. Sedimentation persisted into the Early Eocene in more northerly areas of the Eurasian margin.Collision of the Tauride and Eurasian continents took place progressively during latest Late Palaeocene–Early Eocene. The Jurassic SSZ ophiolites and the Cretaceous accretionary melange finally docked with the Eurasian margin. Coarse clastic sediments were shed from the uplifted Eurasian margin and infilled a narrow peripheral basin. Gravity flows accumulated in thrust-top piggyback basins above accretionary melange and dismembered ophiolites and also in a post-collisional peripheral basin above Eurasian crust. Thickening of the accretionary wedge triggered large-scale out-of-sequence thrusting and re-thrusting of continental margin and ophiolitic units. Collision culminated in detachment and northward thrusting on a regional scale.Collisional deformation of the suture zone ended prior to the Mid-Eocene (~45 Ma) when the Eurasian margin was transgressed by non-marine and/or shallow-marine sediments. The foreland became volcanically active and subsided strongly during Mid-Eocene, possibly related to post-collisional slab rollback and/or delamination. The present structure and morphology of the suture zone was strongly influenced by several phases of mostly S-directed suture zone tightening (Late Eocene; pre-Pliocene), possible slab break-off and right-lateral strike-slip along the North Anatolian Transform Fault.In the wider regional context, a double subduction zone model is preferred, in which northward subduction was active during the Jurassic and Cretaceous, both within the Tethyan ocean and bordering the Eurasian continental margin.

Potential and limits of combining studies of coarse- and fine-grained sediments for the coastal event history of a Caribbean carbonate environment

Abstract The coastal deposits of Bonaire, Leeward Antilles, are among the most studied archives for extreme-wave events (EWEs) in the Caribbean. Here we present more than 400 electron spin resonance (ESR) and radiocarbon data on coarse-clast deposits from Bonaire’s eastern and western coasts. The chronological data are compared to the occurrence and age of fine-grained extreme-wave deposits detected in lagoons and floodplains. Both approaches are aimed at the identification of EWEs, the differentiation between extraordinary storms and tsunamis, improving reconstructions of the coastal evolution, and establishing a geochronological framework for the events. Although the combination of different methods and archives contributes to a better understanding of the interplay of coastal and archive-related processes, insufficient separation, superimposition or burying of coarse-clast deposits and restricted dating accuracy limit the use of both fine-grained and coarse-clast geoarchives to unravel decadal- to centennial-scale events. At several locations, distinct landforms are attributed to different coastal flooding events interpreted to be of tsunamigenic origin. Coastal landforms on the western coast have significantly been influenced by (sub)-recent hurricanes, indicating that formation of the coarse-clast deposits on the eastern coast is likely to be related to past events of higher energy. Supplementary material: The entire dataset of ESR and 14 C dating results used in this paper is available at http://www.geolsoc.org.uk/SUP18637 .

Weathering rinds as mirror images of palaeosols: examples from the Western Alps with correlation to Antarctica and Mars

Weathering rinds have been used for decades as relative age indicators to differentiate glacial deposits in long Quaternary sequences, but only recently has it been shown that rinds contain long and extensive palaeoenvironmental records that often extend far beyond mere repositories of chemical weathering on both Earth and Mars. When compared with associated palaeosols in deposits of the same age, rinds often carry a zonal weathering record that can be correlated with palaeosol horizon characteristics, with respect to both abiotic and biotic parameters. As demonstrated with examples from the French and Italian Alps, rinds in coarse clastic sediment contain weathering zones that correlate closely with horizon development in associated palaeosols of presumed Late Glacial age. In addition to weathering histories in both rinds and palaeosols, considerable evidence exists to indicate that the black mat impact (12.8 ka) reached the European Alps, a connection with the Younger Dryas readvance supported by both mineral and chemical composition. Preliminary metagenomic microbial analysis using density gradient gel electrophoresis suggests that the eubacterial microbial population found in at least one Ah palaeosol horizon associated with a rind impact site is different from that in other Late Glacial and Younger Dryas surface palaeosol horizons.

A kinematic model for the formation of the Siletz‐Crescent forearc terrane by capture of coherent fragments of the Farallon and Resurrection plates

AbstractThe volcanic basement of the Oregon and Washington Coast ranges has been proposed to represent a pair of tracks of the Yellowstone hotspot formed at a mid‐ocean ridge during the early Cenozoic. This interpretation has been questioned on many grounds, especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and North America. Updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised. Forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55–49 Ma, and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments. This kinematic model indicates that there is no firm reason to reject the near‐ridge hotspot hypothesis on the basis of plate motions. A novel element of the model is the Resurrection plate, previously proposed to exist between the Farallon and Kula plates. By including the defunct Resurrection plate in our reconstruction, we are able to model the Farallon hotspot track as docking against the Oregon subduction margin starting about 53 Ma, followed by docking of the Resurrection track to the north starting about 48 Ma. Accretion of the Farallon plate fragment and partial subduction of the Resurrection fragment complicates the three‐dimensional structure of the modern Cascadia forearc. We interpret the so‐called “E” layer beneath Vancouver Island to be part of the Resurrection fragment. Our new kinematic model of mobile terranes within the Paleogene North American plate boundary allows reinterpretation of the three‐dimensional structure of the Cascadia forearc and its relationship to ongoing seismotectonic processes.

Backbeach Deflation Aprons: Morphology and Sedimentology

Backbeach deflation aprons are heterogeneous, coarse clastic deposits associated with active transverse dune fields. The apron comprises variously angular, rounded, fractured, cracked, polished, and ventifacted clasts of bedrock (local and exotic), beachrock, and bioherm fragments (coral and worm reef). Terrestrial and anthropogenic clasts (from archeological sites, deflated dunes, flotsam, and shipwrecks) are also present. The unit forms as a lag deposit on top of a gently sloping (ca. 0.008), planar sandy deflation surface and is typically only one clast thick. It is up to 100 m wide and extends for tens of kilometers along the coast. It is replaced landward by a steep (ca. 0.1) deflated soil surface armored with a layer of iron concretions. Backbeach deflation aprons are compound deposits that result from marine storm, eolian, and human activity since the mid- Holocene. The depositional model proposed involves periodic storm deposition of coarse clasts (of beachrock, bedrock, and biogenic origin) and flotsam (shipwreck and other floating debris) on the backbeach or dunes. Some clasts are deposited directly into interdune depressions and enter the deflation plain immediately. Others are deposited initially on dune surfaces. Dune migration results in winnowing of the storm-deposited coarse clasts (together with rhizoliths and any other coarse material in the dune) until they come to rest on the deflation surface. The lag deposit is subsequently buried and uncovered multiple times by dune migration, during which time it is exposed to polishing and subaerial weathering. Consequently, the lag comprises a variety of materials that reflect both storm and eolian processes. The clasts exhibit variable degrees of rounding, polishing, and ventification according to how long they have been in the deposit.Backbeach deflation aprons are characteristic of sandy beaches along approximately 475 km (. 10% of the total length) of the South African east coast. They develop in association with large-scale transverse dunefields on the backbeach, which in turn depend on reversing shore-parallel winds and high sediment abundance. The variable degree of polishing and weathering, planar surface, low elevation, and back-beach location along extensive shoreline distances distinguish backbeach deflation aprons from tsunami or marine storm deposits.

Stratigraphy of the Gorges moraine system, Mount Kenya: palaeosol and palaeoclimate record

Moraines marking an Early Pleistocene glaciation on Mt. Kenya are known from several valleys along the eastern and southeastern flanks of the mountain. The most prominent group of end moraines delimiting the lowermost extent of the Early Pleistocene Gorges Glaciation (2900 m above sea level) overlie either older till or weathered bedrock, the latter composed of thin pedostratigraphic remnants covered with locally derived aeolian sediments truncated by the ingress of ice. The loess, tills and interbedded palaeosols of the Gorges Glaciation crop out adjacent to the Nithi River, which drains the Gorges Valley, and abut the upper timberline of montane forest. The lowermost palaeosols at these sites were formed either in till predating the Gorges Glaciation or in weathered bedrock of trachytic textured lavas, similar to the lithology forming the base of the Mt. Kenya volcanic series of Miocene or Pliocene age. Palaeomagnetism and weathering characteristics are here used to refine the age of sediments assigned to the Gorges Glaciation. These normally magnetized deposits carry a persistent reversed overprint, suggesting that they were deposited during one of the normal subchrons within the Matuyama Reversed Chron. They are underlain either by a reversely magnetized till, or by weathered bedrock and lower palaeosol, the latter exhibiting normal magnetization (Gauss?) with reversed overprint (Matuyama?). The sediments of the Gorges Glaciation are overlain at all three sites by normally magnetized loesses and palaeosols, presumably of Brunhes age but carrying a high percentage of well-weathered recycled grains. The normal magnetization and reversed overprint recorded in sediments of the Gorges Glaciation most probably span a considerable portion of the Olduvai subchron (1.78–1.950 Ma), which persisted for sufficient time to accommodate an extensive montane glaciation followed by a prolonged period of weathering and soil formation. The importance of these sediments in global palaeoclimate reconstruction and insolation changes is discussed. The stratigraphic evidence contradicts the cosmogenic exposure ages presented by previous workers attempting to obtain true ages of deposits using exhumed coarse clastic sediment without analysis of complex pedostratigraphic complexes. Supplementary materials: Tables 1S–5S are available at www.geolsoc.org.uk/SUP18592 .

Shoreline changes and high-energy wave impacts at the leeward coast of Bonaire (Netherlands Antilles)

Supralittoral coarse-clast deposits along the shores of Bonaire (Netherlands Antilles) as well as increased hurricane frequency during the past decade testify to the major hazard of high-energy wave impacts in the southern Caribbean. Since deducing certain events from the subaerial coarse-clast record involves major uncertainties and historical reports are restricted to the past 500 years, we use a new set of vibracore and push core data (i) to contribute to a more reliable Holocene history of regional extreme-wave events and (ii) to evaluate their impact on shoreline evolution. Multi-proxy palaeoenvironmental analyses (XRF, XRD, grain size distribution, carbonate, LOI, microfossils) were carried out using nearshore sedimentary archives from the sheltered western (leeward) side of Bonaire and its small neighbour Klein Bonaire. In combination with 14C-AMS age estimates the stratigraphy reflects a long-term coastal evolution controlled by relative sea level rise, longshore sediment transport, and short-term morphodynamic impulses by extreme wave action, all three of which may have significantly influenced the development of polyhaline lagoons and the demise of mangrove populations. Extreme wave events may be categorized into major episodic incidents (c. 3.6 ka [?] BP; 3.2–3.0 ka BP; 2.0–1.8 ka BP; post-1.3 ka [?] BP), which may correspond to tsunamis and periodic events recurring on the order of decades to centuries, which we interpret as severe tropical cyclones. Extreme wave events seem to control to a certain extent the formation of coastal ridges on Bonaire and, thus, to cause abrupt shifts in the long-term morphodynamic and ecological boundary conditions of the circumlittoral inland bays.

Bonaire's boulder fields revisited: evidence for Holocene tsunami impact on the Leeward Antilles

Supralittoral boulders and blocks are prominent sedimentary features along rocky shorelines worldwide. In most cases, their deposition is attributed to high-energy wave events (tsunamis, severe storms). Even though tsunami waves are expected to have higher transport capacities compared to storm waves, megaclasts of up to 100 t were observed to have been moved laterally by the latter waveform. The deduction of certain extreme wave events (tsunamis, severe storms) from the boulder record thus remains a major challenge in palaeo-event research; the debate on their differentiation is ongoing. At the eastern coast of Bonaire (Leeward Antilles) in the southern Caribbean, numerous limestone blocks and boulders (up to c. 130 t) are distributed on top of a 3–6 m a.s.l. (above mean sea level) palaeo-reef platform. Disagreement exists among a number of authors concerning the transport processes involved in the formation of the boulder fields. In this paper, state-of-the-art modelling approaches of coastal boulder entrainment and transport were applied in order to provide new and more reliable data to support/challenge the working hypothesis of tsunami deposition. To improve the reliability of the boulder transport model, more realistic input parameters were provided by new DGPS measurements of the boulder dimensions and the calculation of bulk densities by taking into account the heterogeneity of the reef-rock boulders. Existing hydrodynamic equations were modified to allow for the irregular shape and real dimensions of the boulders. The results indicate that (i) boulder weight and dimension, and thus (ii) calculated wave energy and wave heights were overestimated in most of the previous studies, where calculations of boulder volume were based on multiplication of the main axes. The results of this study and wave heights observed during recent severe tropical cyclones seem to rule out storm-generated waves for the dislocation of the largest blocks. However, the majority of coarse-clast deposits may have been generated by periodic hurricane swells. The results underline the significance of more realistic field data in modelling boulder transports by waves.

Sedimentology, architecture, and depositional evolution of a coarse-grained submarine canyon fill from the Gelasian (early Pleistocene) of the Peri-Adriatic basin, Offida, central Italy

The early Pleistocene stratigraphic succession of the Peri-Adriatic basin, eastern central Italy, records the filling of an elongate, N–S oriented piggy-back basin located east of the growing Apennine fold-thrust belt. During the Gelasian (2.588–1.806 Ma), large volumes of gravel, sand and mud derived from the emergent Apennines were redistributed into the basin through a number of slope erosional fairways. These sediment conduits are preserved in the rock record as a series of coarse-grained canyon-fill successions that provide an opportunity for assessing, from an outcrop perspective, how this type of deep-water depositional systems evolves and fills. The present study uses measured stratigraphic sections, photopanels, paleocurrent data, careful lithological mapping, and well-log data from a nearby exploration well to constrain the internal organization of one of these canyon fills, referred to herein as the Offida Canyon. A detailed facies analysis suggests that a variety of gravity-driven subaqueous flows were involved in sediment transport and deposition within the submarine canyon, including slumps, cohesive debris flows, and high- and low-density turbidity currents. Five main lithofacies reflecting both canyon-bounding slope deposits and canyon-filling turbidite and debrite depositional elements have been identified within the exposed succession: (i) clast-supported conglomerates (gravel-rich channel complexes); (ii) medium- to thick-bedded sandstones (overbank lobe); (iii) medium- to very thin-bedded sandstones and mudstones (levee-overbank); (iv) pebbly mudstones and chaotic beds (mudstone-rich mass-transport deposits); and (v) massive mudstones (hemipelagic background deposits). These lithofacies are organized in recurring successions and define fining-upward packages that are regarded as the deep-water expression of high-frequency depositional sequences. Each sequence comprises the sedimentary record of major phases of canyon activity and comprises the following surfaces and systems tracts, in ascending stratigraphic order: (i) a pronounced surface of erosion (sequence boundary) generated by efficient turbidity currents during a period of erosion and complete bypass of sediment to more basinward settings; (ii) a lowstand systems tract composed of a diverse assemblage of genetically related lithofacies (channel-complex conglomerates, levee sandy heterolithics, and overbank lobe sheet-like sandstones) laid down by turbidity currents largely bypassing the area; and (iii) a transgressive to forced regressive systems tract comprising mass-transport deposits produced by instability of shelf-edge staging areas and/or failure of canyon walls when coastal sediment sources were far from the shelf edge. Correlation between sequences and oxygen isotope curve suggests that the recurring fluctuations in sedimentary activity of the submarine canyon are related to the switching on and off of coarse clastic sediments to the slope in response to obliquity-driven (41 ky duration) glacio-eustatic sea-level oscillations, which modulated timing of sediment storage on the shelf and its redistribution beyond the shelf edge.

Triassic platform-margin deltas in the western Barents Sea

The Early to Middle Triassic in the Barents Sea was dominated by prograding transgressive–regressive sequences. Internal clinoform geometries indicate that sediments were derived from the Baltic Shield in the south and the Uralian Mountains in the east and southeast. These systems were formed in a large, relatively shallow epicontinental basin, where modest variations in relative sea-level relocated the shoreline significantly. This study shows the development of strike elongated depositional wedges that thicken just basinward of the platform-edge. Seismic facies and time-thickness maps show the position and development of platform-margin delta complexes within each sequence. Seismic clinoforms and trajectory analysis show significant lateral variation from the axis of the delta complex to areas adjacent to the main delivery system. Frequent toplap geometries are observed in proximity to coarse-grained deposits, while aggradation of seismic clinoforms characterizes areas laterally to the platform-margin deltas. Complex shifts in depocenters are revealed by large-scale compensational stacking pattern and relict platform breaks. Locally, relict breaks are created due to pre-existing paleo-topography. Platform-margin deltas can be identified by careful mapping of clinoform geometries, clinoform angles and trajectories. However, seismic analysis of prograding clinoform units indicate that the shoreline and delta complexes commonly are positioned landward of the platform-edge. Deposition of platform-margin deltas is sometimes caused by locally increased sediment supply during slightly rising relative sea-level, and occasionally caused by a regional drop in relative sea-level with significant shelf bypass. Development, position, thickness and facies distribution of platform deltas and platform-margin deltas of very broad low-relief basins, like the Triassic of the epicontinental Barents Sea basin, are strongly sensitive to changes in relative sea-level due to rapid emergence and submergence of wide areas, and to changes in position of major rivers supplying sand to the delta systems. In this respect, the depositional model of the present study deviates from models of clinoform successions obtained from small and narrow basins or siliciclastic platforms with high coarse-clastic sediment supply.

Spatio-temporal framework of tectonic uplift stages of the Tibetan Plateau in Cenozoic

Four intensive uplift periods, i.e., 60-35, 25-17 and 12-8 Ma (but 18-13 Ma in the Himalayas of the southern Tibet), and since about 5 Ma, can be determined on the Tibetan Plateau by synthetical analysis of low-temperature thermo-chronology data, sedimentary deposit records, and structural deformation records of different areas. The strong tectonic uplift periods in different areas on the Tibetan Plateau are penecontemporaneous, except for the Himalayan area of the southern Tibet, where a rapid uplift and exhumation period, controlled by the activity of the South Tibetan Detachment System faults, occurred during 18-13 Ma. These strong uplift and exhumation periods correspond well to intensive deformation activity periods, suggesting tectonically-controlled uplift and exhumation. The deposit records, such as the distribution of coarse clastic sediments, the distribution of tectonically-controlled basins, stratigraphic discontinuousness or unconformity, and fault-controlled geomorphologic evolution, also match well with the strong uplift and exhumation periods. Expanding processes of the plateau are also discussed.

Coastal stratigraphies of eastern Bonaire (Netherlands Antilles): New insights into the palaeo-tsunami history of the southern Caribbean

A sediment record of three alluvial sites along the east- and northeast-oriented shore of Bonaire (Netherlands Antilles) provides evidence for the recurrence of several extraordinary wave impacts during the Holocene. The interpretation of onshore high-energy wave deposits is controversially discussed in recent sedimentary research. However, it represents a powerful tool to evaluate the hazard of tsunami and severe storms where historical documentation is short and/or fragmentary. A facies model was established based on sedimentary and geochemical characteristics as well as the assemblage and state of preservation of shells and shell fragments. Radiocarbon data and the comparison of the facies model with both recent local hurricane deposits and global “tsunami signature types” point to the occurrence of three major wave events around 3300, 2000–1700 and shortly before 500 BP. Since (i) the stratigraphically correlated sand layers fulfill several sedimentary characteristics commonly associated with tsunamis and (ii) modern strong hurricanes left only little or even no sediment in the study areas, they were interpreted as tsunamigenic. However, surges largely exceeding the energy of those accompanying modern hurricanes in the southern Caribbean cannot entirely be ruled out. The results are partially consistent with existing chronologies for Holocene extreme wave events deduced from supralittoral coarse-clast deposits on Aruba, Bonaire and Curaçao as well as overwash sediments from Cayo Sal, Venezuela.

Palaeogeomorphology and its control on the development of sequence stratigraphy and depositional systems of the Early Silurian in the Tarim Basin

The Silurian in the Tarim Basin was deposited on the basement deformed by the Caledonian tectonic movements at the end of the Late Ordovician. The development and distribution of sedimentary sequences of the Early Silurian have been clearly controlled by the palaeogeomorphology of the Late Ordovician. Based on unconformity characteristics and distribution of erosion, several zones can be differentiated including a high uplifted erosion zone, a transitional slope zone and a depression zone. The central and west Tabei Uplift zones show high angular unconformity and intense erosion. The Tarim Basin in the late Ordovician shows characteristics of higher in the west, lower in the east while higher in the south, lower in the north. The Early Silurian mainly developed transgressive and highstand systems tracts on the whole, while the lowstand systems tract only developed partly below the slope break. The palaeogeomorphology controlled the clastic source supply and deposit distribution. Braided delta system and tidal flat-estuary system were deposited. The duration of uplifting of the Tazhong paleouplift was longer than that of the Tabei paleo-uplift, and deposition was later. This led to the lower and middle members of the Kepingtage Formation missing in that area. As a large-scale transgression occurred during the deposition period of the upper member of the Kepingtage Formation, sediment from the west of the basin was transported and deposited by tides and waves, forming tidal-marine debris systems above the uplift. Proximal alluvial fan and fan delta coarse clastic deposits developed in proximal uplift zone in the east and southeast of the basin, and braided delta put forward to the transitional zone between the edge of uplift and the sea. Large-scale tidal channel, sub-distributary channel and mouth bar of the delta front can form favorable reservoirs, and they are primary targets for oil and gas exploration. This research on sequence-depositional systems development and distribution controlled by palaeogeomorphology is significant in guiding the prediction of reservoir sandstones.

Ash-flow tuffs in the Nine Hill, Nevada, paleovalley and implications for tectonism and volcanism of the western Great Basin, USA

Cenozoic ash-flow tuffs are key units for analyzing the tectonic and magmatic evolution of the Great Basin. The tuffs are commonly assumed to have spread nearly radially from source calderas and to have formed nearly continuous, flat-lying deposits. Based on detailed mapping and paleomagnetic investigations of the Nine Hill paleovalley in western Nevada and analysis of the regional distribution of the 27–23 Ma tuffs that crop out in the paleovalley, we find numerous features that differ markedly from these presumed characteristics. Many individual ash-flow tuffs can be correlated from their source calderas in the central Nevada caldera belt westward into the Sierra Nevada of eastern California. Present-day distances from source to distal, primary tuff deposits are as much as ∼295 km. Corrected for extension, original flow distances were as much as ∼200 km. The tuffs flowed, were deposited, and are preserved primarily in deep (as much as 1.2 km) but wide (8– 10 km) paleovalleys. The tuffs were able to flow these great distances because they were channelized, did not disperse, and did not mix with air as much as would tuffs that spread more radially. Tuffs probably spread more radially only within a few tens of kilometers of source. The paleovalleys held major rivers that drained from a “high” plateau in the present Basin and Range Province and Walker Lane and flowed across the Sierra Nevada to the Pacific Ocean in what is now the Great Valley. The Basin and Range–Sierra Nevada structural and topographic boundary did not exist before 23 Ma, the age of the youngest tuff in the Nine Hill paleovalley. Any faulting in western Nevada before 23 Ma was insufficient to disrupt the paleodrainages other than temporarily. Deposition of tuffs in paleovalleys produced several features relevant to interpreting the tectonic evolution of the region. Most tuffs show primary dips, commonly up to ∼20° and locally up to near vertical, because they compacted against gentle to steep paleovalley walls. Angular unconformities, unrelated to tectonism, are common where tuffs were deposited on eroded tuffs that had primary dips. The tuffs are commonly interbedded with coarse clastic deposits that originated either as channel deposits in high discharge, possibly high gradient rivers, or from floods induced by failure of dams, whereby a tuff temporarily blocked drainage. Tuffs show highly asymmetric, elliptical distributions, preferentially west of their source calderas, because they flowed in the westward-draining paleovalleys. Although deposited continuously along the paleovalleys, tuffs were subsequently eroded by the rivers, so that tuff distributions are highly discontinuous both between and along individual paleovalleys. Thicknesses of tuffs vary irregularly with distance from source, and some tuffs are as much as 300 m thick even 150 km (primary distance) from source. Thickness variations probably are in part due to variations in width and depth along paleovalleys. Estimates of tuff volumes should not assume radial distribution or continuous deposition between paleovalleys, which give unreasonably large volumes. In ideal cases where the source caldera is well understood, the best way to estimate erupted volume can be from the volume of caldera collapse (i.e., area within ring fracture × amount of subsidence).

Geological and mineralogical study of eclogite and glaucophane schists in the Naga Hills Ophiolite, Northeast India

AbstractA variety of low‐ to high‐pressure metamorphic assemblages occur in the metabasic rocks and metachert in the Upper Cretaceous–Eocene ophiolite belt of the central part of the Naga Hills, an area in the northern sector of the Indo–Myanmar Ranges in the Indo–Eurasian collision zone. The ophiolite suite includes peridotite tectonite containing garnet lherzolite xenoliths, layered ultramafic–mafic cumulates, metabasic rocks, basaltic lava, volcaniclastics, plagiogranite, and pelagic sediments emplaced as dismembered and imbricated bodies at thrust contacts between moderately metamorphosed accretionary rocks/basement (Nimi Formation/Naga Metamorphics) and marine sediments (Disang Flysch). It is overlain by coarse clastic Paleogene sediments of ophiolite‐derived rocks (Jopi/Phokphur Formation). The metabasic rocks, including high‐grade barroisite/glaucophane‐bearing epidote eclogite and glaucophane schist, and low‐grade greenschist and prehnite–clinochlore schist, are associated with lava flows and ultramafic cumulates at the western thrust contact. Chemically, the metabasites show a low‐K tholeiitic affinity that favors derivation from a depleted mantle source as in the case of mid‐ocean ridge basalt. Thermobarometry indicates peak P–T conditions of about 20 kb and 525°C. Retrogression related to uplift is marked by replacement of barroisite and omphacite by glaucophane followed by secondary actinolite, albite, and chlorite formation. A metabasic lens with an eclogite core surrounded by successive layers of glaucophane schist and greenschist provides field evidence of retrogression and uplift. Presence of S‐C mylonite in garnet lherzolite and ‘mica fish’ in glaucophane schist indicates ductile deformation in the shear zone along which the ophiolite was emplaced.

Large submarine slides on a steep continental margin (Camamu Basin, NE Brazil)

Abstract: We describe a set of thin-skinned structures from the offshore Camamu Basin. The margin is about 40 km wide and the sea floor has a slope of up to 10%. The sedimentary cover is up to 7 km thick. Neocomian strata accumulated during continental rifting. Aptian strata consist mainly of coarse clastic sediment and evaporite. Late Cretaceous strata are thin or absent and the Tertiary succession is about 1 km thick, above a prominent Eocene unconformity. The Neocomian and Aptian sequences contain abundant thin-skinned structures, which are extensional near the continental shelf and compressional toward the toe of the slope. The structures have detached on Aptian evaporite and on the base of Neocomian shale. One giant slide is about 5 km thick and 60 km wide. It displays mirror symmetry about a vertical plane perpendicular to the margin, as occurs in physical models. The main phase of sliding occurred between the Campanian and the middle Eocene. The trigger may have been a regional phase of uplift and exhumation, for which there is independent evidence on the shelf and inland.

Last Glacial Maximum age for the northwest Laurentide maximum from the Eagle River spillway and delta complex, northern Yukon

The Eagle River spillway and braid delta complex provide a record of the maximum extent of the northwest Laurentide Ice Sheet and diversion of meltwater from Bonnet Plume Basin into the interior basins of non-glaciated northern Yukon. Development of the spillway can be characterized in three distinct zones based on the distribution of erosion and deposition along each reach: erosion-dominated channel initiation and incision; followed by channelization and coarse clastic deposition along channel margins and into tributary valleys; and lastly, fine-grained deltaic and lacustrine sedimentation in the lower channel reach. Deltaic sedimentation within the spillway is crudely-coarsening upward from alternating beds of massive clay and silt to ripple-cross-bedded sand. All sediments occur in rapidly-aggrading forms with no evidence for a significant hiatus in deposition. Radiocarbon ages on woody plant macrofossils and spruce needles are non-finite, while radiocarbon ages on macrofossils from herbaceous plant taxa and insects with ‘steppe-tundra’ ecological affinity from the upper part of the delta range from 15 840 ± 90 to 21 600 ± 1300 14C yr BP. These ages, coupled with the rapidly-aggrading nature of the delta sediments and landform, suggest an age of ca 15–16 000 14C yr BP. Non-finite and mixed ages underscore the significant problem of reworked, well-preserved macrofossils in Arctic environments and the need for careful selection of both fragile and ecologically-representative macrofossils to establish reliable chronologies.

Las formaciones Plio-Cuaternarias de El Abalario, en el litoral de la provincia de Huelva

The study of new and ancient boreholes in El Abalario area (Lower Guadalquivir basin) permitted to identify four main plio-quaternary formations above the Miocene marls, namely: Huelva Sands, Bonares Sands, Almonte Sands and Gravels, and El Abalario Sands. Respectively, they are interpreted as shallow marine, deltaic, alluvial and eolian sediments, together forming a marine to continental sequence that reaches a thickness of more than 250 metres. All sediments are arranged as a gentle dipping and thickening structure to the southeast. Only a few boreholes of the western border of El Abalario reach the top of the Miocene marls. In the rest of the Abalario area the Miocene marls are confined to an unknown level bellow the deepest boreholes. Upward, the marls grade to the Huelva sands, without any appreciable break in the sediments. To the north, the upper part of the Huelva formation grades to the Bonares sands, which is mainly developed outside the study area. Both formations are covered by the coarse clastic deposits of the Almonte formation. At the top, El Abalario sands recover all previous deposits and configure the current relief. The study performs the early existing geology profile and the connection with the surrounding areas.

Middle Devonian olistostromes in the Rheno-Hercynian zone (Rheinisches Schiefergebirge) — An indication of back arc rifting on the southern shelf of Laurussia

The Rheinisches Schiefergebirge in the Rheno-Hercynian zone of the mid-European Variscides was geographically situated on Laurussia's southern shelf during the Devonian. In its eastern part (Lahn–Dill area), Givetian coarse clastic sediments, formerly misinterpreted as transgressional conglomerates, are recognised as olistostrome deposits intercalated in basinal sediments that document a phase of rifting on the southern shelf of Laurussia. The clastic components, redeposited as olistolithes and rounded boulders, are locally derived rocks of Emsian and Givetian age. The olistostromes show facies patterns comparable with other syn-rift deposits. The proximal facies is characterised by olistolithes and slideflow debrites, overlain by debrites and intercalated with turbidites. The distal facies is represented by intercalation of debrites and turbidites. Both grade upwards into basinal sedimentation. Based on conodont stratigraphy, the redeposition lasted from the Middle varcus Zone to the disparilis Zone. During the Middle varcus Zone footwall uplift raised the sedimentary column of the source area by about 500 m above, which brought it above sea level. Rifting of the Laurussian shelf as a result of hinterland transtension and extension caused by the oblique collision of Peri-Gondwana with Laurussia provides the most plausible explanation for the development of the olistostromes, olistolithes and slideflow debrites in the eastern Rheinisches Schiefergebirge.