The ichnogenus Tremichnus Brett was established for small round pits in echinoderm endoskeletons, principally crinoids. It is morphologically identical to Oichnus Bromley. It is significant that infestation of the echinoderm test is an integral part of any diagnosis of Tremichnus. This is an unacceptable ichnotaxobase; the diagnosis should only define morphology and, with reservation, substrate, such as unlithified (= soft), lithic or wood. From a recent revival of Tremichnus, Tremichnus paraboloides Brett is a junior homonym of Oichnus paraboloides Bromley; both share a similar morphology. Tremichnus puteolus Brett is more correctly referred to Lacrimichnus Santos et al. Tremichnus excavatus (Donovan and Jagt) could only be force‐fitted into Tremichnus following a drastic revision of the original diagnosis and is here reassigned to Oichnus, as originally described. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Detailed facies architecture study of the Late Cretaceous (Cenomanian) Nimar Sandstone (Bagh Group), from Son‐Narmada rift valley, Central India, reveals deposition in a fluvial–marine environment. The lower part of the Nimar Sandstone succession depicts deposition in predominantly fluvial setting, whereas the middle–upper part of the succession manifests sedimentation under a tide‐dominated estuarine to a tide–wave influenced shoreface environment. Retrogradational nature of the successions in the upper part and lateral facies change from west to east signifies an eastwardly decreasing marine influence. This attests to a sustained event of marine transgression from the west affecting the Son‐Narmada rift valley during the Late Cretaceous time. Beds with various soft‐sediment deformation structures (SSDS), viz., convolute laminae, load and flame structures, pseudonodules, contorted beddings, syn‐sedimentary faults, and sand/silt dykes, occur in distinct stratigraphic levels within the middle part of the succession. These beds are separated by thick successions of undeformed beds. This paper reports these beds as seismites for the first time from the Son‐Narmada rift valley. The seismites signify instantaneous liquefaction and fluidization of unconsolidated sediments triggered by passage of repetitive earthquake shock waves during sedimentation in fault‐bound rift basin. These seismites are important as they mark a new phase of reactivation of the Son‐Narmada South Fault within the Son‐Narmada rift valley during the Cenomanian time. This reactivation of the Son‐Narmada South Fault led to basinal subsidence under prevalent extensional tectonism, which controlled the sedimentation during the Late Cretaceous time. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Detailed facies architecture study of the Late Cretaceous (Cenomanian) Nimar Sandstone (Bagh Group), from Son-Narmada rift valley, Central India, reveals deposition in a fluvial–marine environment. The lower part of the Nimar Sandstone succession depicts deposition in predominantly fluvial setting, whereas the middle–upper part of the succession manifests sedimentation under a tide-dominated estuarine to a tide–wave influenced shoreface environment. Retrogradational nature of the successions in the upper part and lateral facies change from west to east signifies an eastwardly decreasing marine influence. This attests to a sustained event of marine transgression from the west affecting the Son-Narmada rift valley during the Late Cretaceous time. Beds with various soft-sediment deformation structures (SSDS), viz., convolute laminae, load and flame structures, pseudonodules, contorted beddings, syn-sedimentary faults, and sand/silt dykes, occur in distinct stratigraphic levels within the middle part of the succession. These beds are separated by thick successions of undeformed beds. This paper reports these beds as seismites for the first time from the Son-Narmada rift valley. The seismites signify instantaneous liquefaction and fluidization of unconsolidated sediments triggered by passage of repetitive earthquake shock waves during sedimentation in fault-bound rift basin. These seismites are important as they mark a new phase of reactivation of the Son-Narmada South Fault within the Son-Narmada rift valley during the Cenomanian time. This reactivation of the Son-Narmada South Fault led to basinal subsidence under prevalent extensional tectonism, which controlled the sedimentation during the Late Cretaceous time. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The East Sarmatian Orogen was built through successive subduction and collision processes at the junction of the Sarmatia and Volga‐Uralia segments of the East European Craton. In this paper we present petrological and geochemical characteristics of diopsidites and host amphibolites from the central part of this orogen. We report a new occurrence of metabasites (meta‐gabbros and amphibolites) and anorthite–bytownite‐bearing diopsidites from the Palaeoproterozoic Vorontsovka terrane of the East Sarmatian Orogen. The diopsidites occur as networks of veinlets and dykes within the metabasite layers (thickness up to 100 m) and are composed predominantly of high‐Mg diopside (Mg# up to 0.87) and calcic plagioclase (An99–73) with traces of K‐feldspar, titanite, calcite, quartz with retrograde chlorite and epidote. The diopsidites are low in Ti and K and high in Mg (Mg# greater than 0.82) rocks. They have low REE; positive Ce, U and Sr anomalies; and negative Th, Nb and Ti anomalies. The meta‐gabbros and amphibolites are composed of calcic amphibole, albite, quartz, biotite, zoisite, prehnite and ore minerals (sulphides + ilmenite). The amphibolites are low Al, low K, magnesian–ferroan (Mg# = 0.46–0.75) and medium Ti (TiO2 0.75–1.37 wt.%) (SiO2 40.1–47.3 wt.%). The meta‐gabbros exhibit slight LREE and LILE enrichment, particularly in Rb, Ba and Sr. The amphibolites are tholeiitic in composition and have N‐MORB‐like REE and HFSE features, with high 143Nd/144Nd (0.512736) and positive εNd(T) (6.0).
The diopsidites reported in our study compare with convergent‐margin rocks formed via metasomatic processes associated with slab subduction. The metabasite protoliths represent oceanic crust with N‐MORB‐type features with the LILE enrichment accompanying the subduction zone process. Copyright © 2016 John Wiley & Sons, Ltd.
Book Review Authors Nigel Ainsworth PalaeoDate Ltd., 39 De Tany Court, St. Albans, AL1 1TU, UK Search for more papersRead more
This paper presents results of detailed facies and architectural element analyses of the sedimentary fill of the Ranibag valley in the Himalayan foothills of north‐western India. Forming a buffer zone between the source and sink of the Ganga foreland basin’s Gola alluvial fan, the Ranibag valley is filled with coarse gravels comprising five distinct facies. These are clast‐supported massive gravel, horizontal‐bedded gravel, crudely bedded gravel, sandy gravel and matrix‐supported disorganized gravel facies. The specific assemblages and sequences of these facies, defined by fifth‐ and fourth‐order bounding surfaces, belong to mutually associated channel (CH) and gravel bar and bedform (GB) architectural elements. Both the elements have accreted only during floods by combined fluvial and debris‐flow processes, but dominantly by the fluvial processes under conditions of high water‐to‐sediment ratio. Depositional processes in the valley are akin to those of the proximal alluvial fans of the Ganga Basin. Nonetheless, voluminous alluviation of coarser gravels in this extreme distal feeder channel valley has affected the fan processes downstream. It reduced the sediment budget and thus increased water‐to‐sediment ratio downstream to favour fluvial processes on the Gola fan. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The morphology of 19 adjacent westward‐flowing and four eastward‐flowing major catchments draining across the Western Ghat escarpment in southern Peninsular India was studied to examine how the channel patterns and their longitudinal profiles reflect the landscape evolution. Field surveys were complemented with the quantitative morphometric analysis of fluvial channel profiles using digital topographic data. Results show distinctive differences between eastward‐ and westward‐flowing drainage systems. The channel profiles of eight westward‐flowing drainage basins show an apparent morphological equilibrium characterized by concave upward shape, whereas most channels from the other westward‐flowing basins display knickpoint(s). All studied eastward‐flowing drainage basins display the morphological signature of disequilibrium in the form of knickpoints. The eastern and western margins of southern Peninsular India have experienced major tectonic events ~120 and ~65 Ma ago, respectively, and are since then tectonically quiescent. All studied westward‐flowing basins share the same Arabian Sea base level, flow over comparable lithologies and developed under similar climatic conditions. The studied eastward‐flowing basins have the Bay of Bengal as base level and similar climatic conditions. Therefore, the spatial variations in catchment morphometry is interpreted as a dynamic response to complex interactions and feedbacks between (i) pre‐existing topography along and across the escarpment margins, and (ii) vigorous drainage piracy in more recent times. We hypothesize that the studied drainage basins have experienced different forcing magnitudes that can be quantified to a first order using the present day topography. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Crinoids have the ability to regenerate their crowns when arms are autotomized or bitten off by predators. The evidence of predation is visible in many populations of crinoids, including the deep water stalked crinoid Democrinus off the coast of Roatán, Honduras. Throughout the videos taken in a submersible, many individuals displayed arms that are significantly shorter relative to their stalk lengths than the full‐grown adults in the area, indicating arm regeneration. Some individuals were missing crowns completely, strongly suggesting predation by fishes. Our calculated predation rate of 17.25% including individuals missing a crown or regenerating their arms in situ is significantly greater than that rates determined in previous studies of predation on Democrinus. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Book Review Authors Stephen K. Donovan Naturalis Biodiversity Center, Leiden, The Netherlands Search for more papers by this author NoRead more
Book Review Authors Paul Ensom Falmouth, Cornwall, UK Search for more papers by this author No abstract is available forRead more
The Ordovician Mweelrea Formation is part of the basin fill of the South Mayo Trough in western Ireland and records progradation of alluvial facies northwestward across shallow‐ and marginal‐marine Mweelrea ‘passage beds’ to marine Glenummera Formation facies. Early in Mweelrea Formation sedimentation, the first of a series of ignimbrites was emplaced, causing a disruption in this progradation. The preserved thickness of the ignimbrite is up to ca. 12 m, and the sedimentary response varied depending on the environment that was affected. Eastern exposures of the Mweelrea Formation are coarse grained and represent fluvial facies. Emplacement of the ignimbrite caused overlying sediment to become richer in feldspar as well as coarser grained; heavy‐mineral laminations are common here and in many supra‐ignimbrite exposures, but the depositional style was unchanged. The ignimbrite overlies decreasing thickness of fluvial sandstone towards the west to where it directly overlies shoreline and shallow‐marine passage beds. Here, sandstone that overlies the ignimbrite is fluvial, indicating that emplacement of the ignimbrite brought about an abrupt progradation. At the westernmost outcrops of Mweelrea Formation, the ignimbrite lies at the contact with the underlying marine Glenummera Formation. Marine sedimentation continued after emplacement of the ignimbrite, although a coarse conglomerate horizon immediately overlying the ignimbrite together with development of a delta suggests that sediment input was vastly increased. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Significant progress has recently been made in tight oil exploration within the Permian Lucaogou (P2l) Formation of the Jimusar Sag. However, current tight oil exploration deployment of the P2l Formation is mainly based on reservoir prediction, which is high risk for tight oil exploration. In this study, the geological and geochemical characteristics of the P2l Formation source rocks, including the distribution, sedimentary environment, organic matter abundance, kerogen types and thermal maturity were investigated. Hydrocarbon generation and expulsion intensity were evaluated through an improved hydrocarbon generation potential methodology, and the significance of source rocks in tight oil source and occurrence was systematically investigated. Results indicate that P2l Formation source rocks with total organic carbon >1.0 wt% occur widely (an area up to 1500 km2), are thick (up to 160 m), were deposited in a lacustrine weakly reducing sedimentary environment with relatively low salinity, have a high total organic content with a mean value of 3.12 wt%, are dominated by type II kerogen and have reached the early mature to mature stage. Modelling results indicate that the source rocks reached the hydrocarbon generation threshold and hydrocarbon expulsion threshold at 0.48% and 0.86% vitrinite reflectance, respectively. The comprehensive hydrocarbon expulsion efficiency was approximately 30%, and the maxima of hydrocarbon generation and expulsion intensities for P2l Formation source rocks are 1200 × 104 and 425 × 104 t/km2. The tight oil is sourced from adjacent source rocks that are interbedded with, or are close to, the reservoirs. The migration of oil generated from the source rocks occurs over very short distances. The oil filling degree index (oil bearing thickness/P2l Formation thickness) is higher at a closer proximity to the source rocks, and where it is higher the hydrocarbon generation intensity of the source rocks is also elevated. In addition, the greater the hydrocarbon expulsion intensity of the source rocks, the higher the daily oil production values (ton/day) from prospect wells. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The Tibetan Plateau and the Himalayan region formed after 55–50 Ma, as a result of the intracontinental collision of the Indian and Eurasian plates, occupying the east–west trending, high‐altitude Himalaya and Karakorum ranges in the south and the vast Tibetan Plateau to the north of central Asia. The tectonic evolution of Tibet began between the late Palaeozoic and the Cenozoic, and the Himalayan mountain system evolved in a series of stages beginning 50–35 Ma and is still active.
Active tectonics significantly affect upheaval and the rate of erosion in the Himalaya. Therefore, different foreland basins of the Tibetan Plateau (e.g. the Lhasa terrane, the Hoh Xil Basin, the Qaidam Basin, and the Jiuquan Basin) and the Himalayan foreland basins (e.g. Gondwanaland Basin and the Siwalik and Quaternary basin) experience direct effects in terms of tectonic and sedimentary evolution. For the tectonic evolution and provenance analysis of foreland basins in the Tibetan Plateau and the Nepal Himalaya, researchers have adopted various techniques in past studies: This paper discusses petrography, U–Pb geochronology, and seismic reflection.
Provenance analyses have illustrated that the sediments of the Southern Tibetan foreland basin (i.e. the Lhasa terrane) derive from the Qiangtang, Tethys Himalaya, and southwest Australia. Similarly, the sediments of the Central Tibetan basin derive from the Qilian, Kunlun‐Qimantagh, and the Altyn Mountains; the sediments of northern side of the Tibetan foreland basin, from Qilian Shan Mountain; and the sediments of the Nepal Himalayan foreland basin, from the Tethys, Higher, and Lesser Himalaya. Copyright © 2016 John Wiley & Sons, Ltd.
Lower Cretaceous black shales in coastal southeastern China are of significance to the geological study of the Pacific tectonic domains. However, the stratigraphic correlations and occurrence patterns of the shales have not been well constrained so far. To address these limitations, 131 new zircon U–Pb dates were obtained from the tuff layers, which were interlayered with the shales from four outcrops. In combination with previously reported geochronological data, the stratigraphic correlations and occurrence patterns of the black shales were discussed. Results show that the black shales in the Shuidishan Formation (~145 Ma) in the Dayawan section, northern Guangdong, are older than those in the lower part of the Bantou Formation (~132 Ma) in the northward Yong’an section, southern Fujian, whereas they are roughly isochronous to the base of the Bantou Formation (~144 Ma). In contrast, the black shales in the upper part of the Bantou Formation (~117 Ma) from the Yong’an section in northwestern Fujian and the black shales in the Shixi Formation (~117 Ma) from the Yiyang section in northeastern Jiangxi can be isochronously correlated with the black shales in the Bantou Formation (~117 Ma) from the Chong’an section in northwestern Fujian, and they are slightly older than those in the Guantou Formation (~113 Ma) from the Shangzhang and Xiahuyuan sections in western Zhejiang. Black shales in Units I and II of the Shipu Group (~113–109 Ma) in northeastern Zhejiang can be isochronously correlated with those in the Guantou Formation (~113–106 Ma) in western Zhejiang. These new stratigraphic correlations indicate that the Lower Cretaceous black shales in coastal southeastern China can be divided into two regional‐scale sets. The first set was deposited during the early Early Cretaceous (Berriasian–Hauterivian) and is diachronous (i.e. 144 ± 2 Ma in the Dayawan section and 132 ± 2 Ma in the Yong’an section). The second set of black shales was deposited during the later Early Cretaceous and was roughly isochronous (~117 Ma). These results imply that the processes of the Pacific plate subduction in coastal southeastern China during the Early Cretaceous varied in two distinct stages that are identified by two volcanic–black shale cycles (i.e. ~145–120 and ~120–100 Ma). The distribution of the first set of black shales may extend to offshore basins in the South China Sea, whereas the second set may extend to the Taiwan Strait. Both sets are likely to be potential petroleum prospects. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The provenance of sands from the Manzanillo (MN) and El Carrizal (CAR) beach areas along the Mexican Pacific coast (southern Mexico) was investigated based on their modal composition, mineralogy and geochemical data. The average quartz, feldspar and lithic fragment (QFL) ratios revealed that the proportion of quartz is higher in CAR sands than in MN sands. In comparison with MN, the CAR sands are enriched in SiO2 content with higher SiO2/Al2O3 ratio indicating that sediment recycling is greatest for the CAR sands. The variation in K2O/Al2O3 and Na2O/K2O ratios between the MN and CAR sands specified a difference in the relative proportion of plagioclase. The chemical index of alteration (CIA) and Plagioclase Index of Alteration (PIA) values revealed a moderate weathering intensity in the source areas. The MN sands are characterized by a low degree of rare earth element (REE) fractionation (LaCN/YbCN = ~6–7), whereas the CAR sands define fractionated REE patterns (LaCN/YbCN = ~8.8–16.8). The fractionated REE pattern with elevated total low REE (∑LREE) content of the CAR sands suggested that sediment sorting is higher for the CAR than MN sands. Small negative and positive Eu anomalies (Eu/Eu* = ~0.91–1.23) of MN sands indicated the derivation of sediments from felsic‐intermediate source rocks. The CAR sands showed higher concentration of ∑LREE than total heavy REE (∑HREE) and characterized by a significant negative Eu anomaly (Eu/Eu* = ~0.46–0.78), suggested that the source rock is felsic type. The result obtained by the comparison of REE patterns of the beach sands to the probable source rocks is also consistent with this observation. The compositional differences identified between the MN and CAR beach areas suggested that longshore current in the mixing and homogenization of sands is not significant. The new tectonic discriminant‐function‐based multidimensional diagrams showed an arc setting (active) for the MN sands and a rift setting (passive) for the CAR sands, which is consistent with the regional geology of southern Mexico. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Acidic lava flows comprise a large proportion of the total volcanic succession in the rift basins of northeast China. The objective of this study was to explore the vesicle distribution in the acidic lava flows in those basins. Two representative areas with acidic lava flow outcrops in NE China were selected for study. These lava flows share similar magma sources and chemistries. The types and inner structures of the acidic lava flows were determined in detail by measuring the shape of the flows (thickness and lateral extent) and the vertical sequences in the outcrops and shallow coring well Y1D1 in the two outcrop areas. Furthermore, the interpretation of deep borehole data and 3D seismic data from the Changling Graben of the Songliao Basin provided additional thickness and geometry data as a robust complement to the outcrop data. This study revealed that the shapes of the acidic lava flows transition from dome to tabular to braided with decreasing size. Furthermore, the vertical sequences can be classified into three zones: the upper vesicle zone, the massive core zone and the basal zone. The borehole thickness data indicate a trend in the maximum proportions occupied by the upper vesicle zone: thick flows have a relatively low proportion occupied by upper vesicle zones, whereas thin flows display a wide range in the proportion represented by upper vesicle zones. Therefore, braided acidic lava flows are more likely to have a high proportion of upper vesicle zones. Upper vesicle zones usually have high porosity and permeability and represent the primary effective gas reservoir system. The greatest lateral extents of gas reservoirs are usually in the tabular acidic lava flows, but the largest proportions occupied by gas reservoirs are usually in stacked braided lava flows. This study can help us to better understand the distribution of volcanic petroleum reservoirs in acidic lava successions where hydrocarbons are encountered but borehole data are limited. Furthermore, this study is useful for interpreting acidic lava flows in logs and seismic data from basins. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The palaeosol–loess–aeolian sand sequence in the eastern Gonghe Basin, which is located at the convergence of the Asian summer monsoon, winter monsoon and the westerlies, reveals detailed moisture changes in the northeastern Qinghai–Tibetan Plateau (NETP) during the last 17 ka. Analysis of magnetic susceptibility (MS), total organic carbon, carbonate content, grain size and geochemical parameters indicate that the region was dominated by an extremely dry climate during ~17.0–15.82 ka, accompanying large‐scale desert expansion in the last glacial maximum (LGM). Subsequently, the climate became generally dry during 15.82–14.6 ka, an interval in which loess rapidly accumulated. Enhanced humidity occurred at 14.6 ka, probably associated with increased strength of the East Asian summer monsoon. The region experienced slightly decreased moisture at around 6.5 and 5.8 ka. After 2.7 ka, the climate became wetter. This interpretation correlates with climatic records from lacustrine and aeolian deposits in the NETP during the LGM, the last deglaciation and the early–middle Holocene. High climatic moisture in the Late Holocene facilitated the formation of a well‐developed palaeosol in the NETP. The effective moisture change in the Gonghe Basin is not simply ascribed to the influences of Asian summer monsoons; instead, it was possibly influenced by the interaction of evaporation and monsoonal precipitation forced by solar insolation variation. The balance between these variables was very influential on the effective moisture change in the closed inland basin of the NETP. Copyright © 2016 John Wiley & Sons, Ltd.Read more
By integrating diagenesis and sedimentary facies, the distribution of diagenetic alterations and their impacts on reservoir quality were investigated within a lacustrine fan delta depositional environment in the Lower Cretaceous of the Western sub‐sag in the Chagan Sag, Yin‐E Basin, northern China. Core observation and analyses of petrography and geochemistry of the sandstones revealed that the eogenetic alterations display spatial and temporal distribution patterns associated with sedimentary facies, including distributary channels of a fan delta front (DC), distributary bays (DB) and sheet sands (SS) and mouth bar (MB) (SS and MB are collectively referred to as S). Percolation of meteoric waters occurring in high permeability DC sandstones resulted in leaching of feldspar, the formation of kaolinite, as well as mechanical infiltrated clays around detrital grains. Conversely, the DB deposits containing abundant ductile lithic fragments were subjected to mechanical compaction and thus the development of a pseudomatrix. During a long burial residence time (the Early Cretaceous period) in a lacustrine environment, abundant early carbonate cements were precipitated, especially in the distal S sandstones. The sandstones lacking eogenetic cements were subjected to stronger mechanical compaction. Additionally, eogenetic alterations have an important impact on the distribution of the mesogenetic alterations. Sandstones containing few eogenetic cements or thin or discontinuous infiltrated clay rims around the detrital grains were subjected to quartz cementation. However, during the hydrocarbon generation stage, the most efficient percolation of meteoric waters and organic acid dissolved feldspar minerals and carbonate cements, resulted in DC sandstones having more intragranular and intergranular porosity. Owing to high matrix contents and early calcite cement contents, the DB and S sandstones had lower permeability and were rarely dissolved. During the late mesogenetic stage, late carbonate cements occurred in all sedimentary facies and iron and magnesium ions were released from the transformation from kaolinite to illite or to chlorite. The results from this study show possible diagenetic evolutionary pathways in the reservoir sandstones within the fan delta depositional environment, which in turn provides some insights into the controls on reservoir potential. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The Bujinhei region, situated in the southern reach of the Great Xing’an Range, has experienced multistage tectonic‐magmatic activities, which have led to the formation of different stages and types of magmatic rocks. LA‐ICP‐MS zircon U–Pb dating was performed, and geochemical data were obtained for magmatic rocks samples from the Bujinhei area to analyse the times of magmatic rocks and their tectonic setting. Zircon U–Pb dating indicates that Middle Permian (269.8 Ma) magmatic event formed two‐mica granite, and Middle Jurassic (158–170 Ma) magmatic events formed monzogranite, quartz monzonite‐porphyry and granite porphyry. Chemical analyses indicate that the Middle Permian two‐mica granite is S‐type granite, rich in SiO2, Al2O3 and K2O and low in Na2O, A/CNK > 1. Middle Jurassic monzogranite is a metaluminous I‐type granite, rich in SiO2, Al2O3 and K2O and low in Na2O and CaO. Quartz monzonite porphyry and granite porphyry are high in K2O and peraluminous and derived from a granitic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; they have high Zr saturation temperature and belong to A‐type granites. Geochemically, two‐mica granite formed in a compressional tectonic regime related to collision, whereas Middle Jurassic intrusive rocks have affinities with intrusive rocks from the extensional environment influenced by post‐collision. Therefore, the southern Great Xing’an Range has experienced the collisional orogenesis of the Central Asian Orogenic Belt in the Middle Permian, whereas in the Middle Jurassic, the southern Great Xing’an Range turned into a post‐collision extensional setting influenced by the Mongol–Okhotsk tectonic regime. Copyright © 2016 John Wiley & Sons, Ltd.Read more
An unnamed succession of volcaniclastic argillite, sandstone, and conglomerate (the ‘argillite unit’) is exposed in an inlier in southeastern Yukon (NTS 95C/5). These strata were hornfelsed during emplacement of the Pool Creek syenite (ca. 640–650 Ma) and are correlated with the late Cryogenian Hay Creek Group of the Mackenzie Mountains based on lithostratigraphic evidence and published constraints from detrital zircons. The unit preserves three argillite lithofacies, two sandstone lithofacies, and two conglomerate lithofacies. The argillite and sandstone can be grouped into three facies associations, reflecting differing volumes of sandstone and the presence or absence of chaotic bedding. Deposition was mainly from sediment‐gravity flows, particularly turbidity currents and debris flows. Post‐depositional slumping was common, and the succession is interpreted to have been deposited on a slope that trended NNW and dipped to the ENE. This orientation was subparallel to but facing towards the contemporaneous slope of the Hay Creek Group in the Mackenzie Mountains. Conglomeratic facies are dominated by angular to poorly rounded clasts of subalkali basalt that probably were entrained in debris flows during or soon after eruption and otherwise saw little transport or weathering. Geochemistry of the clasts is permissive of a rift‐related setting. Following deposition, but prior to deposition of overlying Cambro‐Ordovician strata, the argillite unit underwent compression that produced broad, open folds, consistent with recent proposals for late Neoproterozoic transtension–transpression on the present‐day northwest margin of Laurentia. The argillite unit provides a snapshot of the geological evolution of southeast Yukon during the late Cryogenian, providing a new data point for reconstructing the protracted and complex rifting history of Rodinia in western Canada. © 2016 Her Majesty the Queen in Right of Canada Geological Journal © 2016 John Wiley & Sons, Ltd.Read more
The barite–fluorite metallogenic belt in southeastern Sichuan is one of the most important barite–fluorite ore‐concentrated areas in China. In order to find the ore‐forming source and the mineralization age of the Fengjia and Langxi barite–fluorite deposit, we systematically evaluated the rare earth element (REE) contents and isotopic characteristics of S, Sr, and Sm–Nd. The results show that the δ34S values of barites and pyrites are high and similar to S isotopic values of gypsum in the Cambrian Qingxudong Formation (∈1q), suggesting that the sulphur source was from Cambrian evaporite strata. The initial 87Sr/86Sr ratios of fluorites and barites range from 0.708800 to 0.712999, and these values are similar to Cambrian carbonate rocks and Lower Ordovician limestone. Except for black shales of the Lower Cambrian Niutitang Formation, the Ba content of other strata was generally low. The REE characteristics of barite and fluorite have close relationships, in regard to the source of ore‐forming materials, to the Lower Cambrian Niutitang Formation black shales. Hence, we have concluded that the Cambrian carbonate rocks and Lower Ordovician limestone provide the source of calcium for mineralization and that the source of barium comes from the Lower Cambrian Niutitang Formation black shales. The fluorine contents of the Upper Sinian Doushantuo Formation and the Lower Cambrian Mingxinshi Formation are much higher than other strata, and these formations could provide the source for fluorine. The Sm–Nd isochron age of fluorites is 104 ± 11 Ma, which demonstrates that the mineralization mainly occurred during the late Yanshanian. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The Nagaland–Manipur ophiolites (NMO) of Northeast India forms a part of the Tethyan ophiolites and comprises a suite of tectonite peridotites and cumulate mafic–ultramafic sequence with mafic extrusive–intrusive rocks, felsic intrusives and oceanic pelagic sediments along with minor podiform chromitites. However, sheeted dykes, which are considered as a significant component of ophiolites, are absent in the NMO. The tectonite peridotites are distinguished from the cumulate pyroxenites by the presence of pyroxene lineation, deformed bands and strained extinction in olivine, kink twin lamellae in pyroxene. Both the tectonite peridotites and cumulate pyroxenites contain aluminous spinel with Cr number [Cr# = Cr/(Cr + Al)] in the range of 0.14 to 0.29 and 0.27 to 0.48, respectively. Mg number [Mg# = Mg/(Mg + Fe2+)] in Cr‐spinel is higher in tectonite peridotites (0.71–0.76) than cumulate pyroxenites (0.44–0.53). Chondrite‐normalized rare earth elements (REE) patterns of cumulate pyroxenites exhibit depleted at light REEE (LREE) (LaN/SmN = 0.380–0.759) but flat middle REE (MREE) to heavy REE (HREE) (SmN/YbN = 0.622–0.756). However, the tectonite peridotites show gradual decrease in concentrations from HREE to MREE (SmN/YbN = 0.285–0.460) and slight increase in LREE (LaN/SmN = 0.721–2.201). The cumulate pyroxenites show strong enriched PPGE patterns and higher PGE concentrations (∑PGE = 85.8–163.5 ppb) compared with the tectonite peridotites (∑PGE = 34.8–113.0 ppb). The estimated equilibration temperature ranges from 890 to 931 °C for cumulate pyroxenites and 971 to 1156 °C for tectonite peridotites. The olivine–spinel equilibrium along with Cr‐spinel chemistry and PGE data suggests that the tectonite peridotites represent the residual mantle left after limited extraction of basaltic melts by low‐degree partial melting (<15%). Conversely, the presence of highly magnesian orthopyroxene and clinopyroxene in the cumulate pyroxenites in conjunction with their geothermometry suggests that they were formed at high pressure and temperature by magmatic fractionation from the basaltic melt. The geochemical data together with field and petrographical evidences indicate that both the tectonite peridotites and cumulate pyroxenites are essentially spinel‐bearing and devoid of plagioclase, suggesting their derivation in the mantle beyond the stability limit of plagioclase in a mid‐oceanic ridge tectonic setting. We conclude that the ultramafic sequence of the NMO was initially generated at a mid‐oceanic ridge tectonic setting close to the eastern boundary of the Indian passive margin and then thrust over the continental margin of the Indian Plate towards the west during its collisional and subduction process with/beneath the Myanmar Plate. Copyright © 2016 John Wiley & Sons, Ltd.Read more
Articulated echinoids are rare in Palaeozoic strata. In order to gain a better understanding of palaeodiversity and community composition, it is more than useful to incorporate disarticulated specimens into estimates of such metrics. It has been demonstrated that disarticulated ossicles of echinoids from the post‐Palaeozoic can be diagnostic at the species level and can be used to bolster analyses of diversity. Although usually not identifiable to the species level, many families, and some genera, of Palaeozoic echinoids have diagnostic properties that can be recognized from disarticulated plates. Herein, it is demonstrated that such diagnostic properties exist for plates of the Palaechinidae, Archaeocidaridae and Proterocidaridae, and that the utility of using disarticulated echinoid ossicles to aid in palaeodiversity studies extends back into the Palaeozoic. Portions of the echinoid fauna from Tournai, Belgium, are revised and disarticulated plates of echinoids from two upper Tournaisian localities in Belgium, Pair and Petit‐Modave, are examined and described. The fauna of Pair yielded an indeterminate palaechinid and Hyattechinus sp., while the fauna from Petit‐Modave yielded only indeterminate palaechinid plates. Although no articulated specimens are known from these localities, when disarticulated plates are taken into account, a more accurate estimate of palaeodiversity becomes clear. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The Buqingshan Tectonic Mélange Belt in the south margin of the East Kunlun Orogen, located in west section of Buqingshan–A’nyemaqen Suture Zone, is one of the key areas to understand continental tectonics and continental dynamics of China. This paper reports zircon U–Pb dating results and geochemistry of the Manite granodiorite (rock mass) in the Buqingshan Tectonic Mélange Belt. Zircons from the granodiorite show oscillatory zoning structures and relatively high Th/U ratios, indicating that they are magmatic zircons. Zircon LA–ICP–MS U–Pb dating for the Manite granodiorite yields ages of 487 ± 11 Ma (MSWD = 2.3) and 479 ± 2 Ma (MSWD = 0.56), implying that the Manite granodiorite were formed in the Late Cambrian to Early Ordovician. Geochemical analyses show that the rocks have high contents of SiO2 (66.06 wt.%–68.60 wt.%) and Al2O3 (14.84 wt.%–16.54 wt.%), and low alkaline (6.17 wt.%–7.43 wt.%), belonging to the middle‐K calc‐alkaline series. The A/CNK (Al2O3/(CaO + Na2O + K2O)) ratios are 1.02–1.15, indicating that the granodiorite is weakly peraluminous. The contents of rare earth elements (REEs) are lower (89.64–130.41 ppm), with weakly negative to weakly positive Eu anomalies (δEu = 0.83–1.12). The primitive mantle‐normalized trace elements are characterized by evidently negative anomalies of Nb, Ta, P, Hf, Ti, etc., and positive anomalies of Th, La, Nd, Zr, Eu, etc. Moreover, the rocks show features of typical I‐type granite. This leads us to conclude that the Manite granodiorites is derived from the partial melting of crust and formed in an island‐arc environment. Combined with previous studies, we believe that the subduction of the Proto‐Tethyan Ocean in the Buqingshan area was ongoing during 487 to 479 Ma and formed island‐arc‐type granites represented by the Late Cambrian to Early Ordovician Manite granodiorite. Copyright © 2016 John Wiley & Sons, Ltd.Read more
The age, petrogenesis and geodynamic implication of Xiaomiaogou granite porphyry dykes in the northern margin of the North China Craton were investigated using geochronology, geochemistry and Sr–Nd–Pb–Hf isotopic data. LA‐MC‐ICP‐MS zircon U–Pb dating yielded the concordant 206Pb/238U weighted mean age of 350.4 ± 1.4 Ma for the dykes, suggesting they were the productions of the Early Carboniferous magmatism in the northern margin of the North China Craton. The granite porphyries display the pronounced A‐type affinities: apart from higher zircon saturation temperature, they are high‐K calc‐alkaline and metaluminous, with high concentrations of K2O + Na2O varying from 7.93 to 8.55 wt.%, incompatible elements (e.g. Zr, Y, Nb and Ce), REEs and 10 000 Ga/Al ratios but less obvious LREE and HREE fractionation. With the lower initial 87Sr/86Sr ratios (0.7055 to 0.7070), negative values of εNd(t) (−7.1 to −4.9) and εHf(t) (−11.2 to −8.0) and old Nd (1505 to 1681 Ma) and Hf (1861 to 2061 Ma) model ages, all the rocks exhibit the similar geochemical compositions to the melts obtained in experimental studies by dehydration melting of meta‐basalts, implying a lower continental crust origin. However, the high Mg# values (47.53 to 53.60) and subcontinental lithospheric mantle‐like εNd(t), εHf(t) and Pb isotopic compositions ((206Pb/204Pb)i = 17.296–17.766, (207Pb/204Pb)i = 15.460–15.490 and (208Pb/204Pb)i = 37.097–37.568) indicate the involvement of melts from subcontinental lithospheric mantle. Combined with the special features of mafic microgranular enclaves occurring within the granite porphyries, mixing of the lower crust and subcontinental lithospheric mantle of the North China Craton is proposed for the genesis of granite porphyry dykes in this study. During the hybrid magma ascending, pronounced fractional crystallization and crustal assimilation may have also occurred. Based on geochemical criteria, the Xiaomiaogou granite porphyries can be classified into A1 group. Characterized by the compositional features of anorogenic granites, an intraplate setting is envisaged for the formation of Xiaomiaogou granite porphyry dykes. Such a tectonic environment may be related to the northward drift of the North China Craton during rifting away from East Gondwana. Copyright © 2016 John Wiley & Sons, Ltd.Read more