Flood Basalt Sequence Research Articles

Flood basalt structures and textures as guides to cooling histories and palaeoclimates: the Deccan Traps of Saurashtra, western India

AbstractThe primary features (morphologies, structures, textures) of volcanic lava flows are determined by parameters such as composition, temperature, crystallinity, viscosity, flow velocity, strain rate and cooling rate. However, lava flows are open systems, and their primary features are strongly influenced by their emplacement environment. Among subaerial lava flows, those that solidify in a wet environment with rainfall develop very different internal structures (e.g. jointing patterns) and textures from those in a dry environment. Thus, the outcrop structures and textures of ancient lava flows, such as those forming continental flood basalt sequences thousands of metres thick, provide clues to their cooling histories and the palaeoclimates. Here we provide field, petrographic and geochemical data on large tholeiitic lava flows (sheet lobes) and associated dykes of the Saurashtra region in the northwestern Deccan Traps continental flood basalt province (India). The sheet lobes are dominantly pāhoehoe and rubbly pāhoehoe, and occasionally ‘a‘ā, with colonnade and entablature tiers. We show that the jointing patterns in the entablatures (irregular, chevron, rosette and skeleton jointing), and the textures of the sheet lobes and even some dykes (abundant glass, and quench crystals of plagioclase and Fe–Ti oxides) reflect convective heat removal, owing to widespread interaction with meteoric waters (rainfall) during solidification. The observations thus provide evidence for a wet climate in western India 65.5 million years ago.

Mafic subvolcanic intrusions from the southern Paraná‐Etendeka Large Igneous Province, Brazil: Insights from geochemistry and Sr–Nd–Pb isotopes

The Early Cretaceous Paraná‐Etendeka Igneous Province is a predominantly tholeiitic sequence of continental flood basalts and silicic magmatism that is intimately associated with the rupture of the western Gondwana. Intrusions of tholeiitic compositions are very common within the province and represent the magmatic plumbing system that fed the lavas at the surface. The investigated dikes are in the southeast of the Paraná Basin, southermost Brazil. The Silica Saturated Olivine Tholeiite (SSOT) dikes show restricted isotopic ranges (87Sr/86Sri = 0.70570 to 0.70585; ɛNdi = −1.01 to −4.49; 206Pb/204Pb = 18.031), with no equivalent low‐Ti magma type in the Paraná Province, but are close to the isotopic ratios of low‐Ti magma types with MgO > 7 wt%, such as Nil Desperandum in the Etendeka Province. The Silica Supersaturated Tholeiite dikes show broader isotopic ranges (87Sr/86Sri = 0.70787 to 0.71336; ɛNdi = −2.51 to −8.65; 206Pb/204Pb = 18.578–19.049), similar to the Gramado magma‐type and Tafelberg magma‐type in the Paraná‐Etendeka Province. This variation may have occurred due to the contamination at the time of magma rise and extrusion. Isotopic modelling showed that the fractional crystallization process and crustal assimilation played an important role from the magma rise through the conduits to the extrusion of the lavas. For the most primitive SSOT dikes, fractional crystallization was the most important factor for the magma evolution. The behaviour of the trace elements revealed that the assimilation of crust rocks with both Palaeoproterozoic and Neoproterozoic ages can be considered.

Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator

The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial εNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of magma through shallower fractures next to colder wallrock.

Quantitative geothermal interpretation of electrical resistivity models of the Rathlin Basin, Northern Ireland

In the evaluation of low- to medium-enthalpy geothermal resources on the island of Ireland, some of the most interesting targets are the deep sedimentary basins of Northern Ireland. The deepest of these is the Rathlin Basin, where Permian and Triassic reservoir sediments are known to exist to at least 2300 m depth. Two deep boreholes within the basin provide evidence of elevated temperatures at depth that are atypical within Ireland, prompting further geophysical exploration of the basin as one component of the IRETHERM project. The magnetotelluric (MT) method was selected as the investigative geophysical tool as it is capable of sensing and defining electrically conductive porous sediments beneath overlying resistive strata, in this case flood basalt sequences. MT data were acquired on a rectangular grid of 39 sites across almost half of the onshore basin to investigate the composition and spatial variation of the basin's formations.One-dimensional stochastic inverse modelling of the observed MT data was with a reversible-jump Markov chain Monte Carlo 1D inversion code, resulting in ensembles of models for each site. The use of model ensembles rather than single models avoids the pitfall of over-reliant interpretation on non-unique resistivity models, increasing the robustness of the interpretation. Interpreted models compare very favourably with nearby deep borehole records, and interpolation of the complete set of ensemble interpretations results in a conservative reservoir volume of approx. 32 km3 of combined Permian and Triassic sandstones beneath the MT survey.Based upon new, high quality temperature data available in the Ballinlea 1 borehole, an approximate estimation of thermal energy in place as a function of final reservoir temperature has been performed for the interpreted MT resistivity model volume. A final minimum temperature of 25 °C (being the temperature that comparable estimates have been made for adjacent geothermal prospects) results in a minimum estimated Indicated Geothermal Reserve (IGR) of 2.9 × 1018 J beneath the MT survey area. The modelling results suggest that exploitation of the maximum volume of sediments would occur for a final temperature of ≈55 °C.

U-Pb Geochronology and Geochemistry of the Povungnituk Group of the Cape Smith Belt: Part of a Craton-Scale Circa 2.0 Ga Minto-Povungnituk Large Igneous Province, Northern Superior Craton

Magmatism of the Povungnituk Group of the Cape Smith Belt, northern Superior craton, was formed in three stages: (i)early alkaline magmatism and associated carbonatites (undated), (ii) a main flood basalt sequence (Beauparlant Formation) (constrained between 2040 and 1991 Ma), and (iii) a late stage alkaline pulse (Cecilia Formation) (ca. 1959 Ma). We suggest that the main stage of magmatic activity (middle pulse) was of short duration.A new UPb baddeleyite age of 1998 ± 6 Ma is obtained from a dolerite sill intruding the uppermost section of the Beauparlant Formation. This age has regional significance because it matches the previously obtained 1998 ± 2 Ma age for the Watts Group (Purtuniq) ophiolite of the northern Cape Smith Belt and the 1998 ± 2 Ma U-Pb age of the Minto dykes intruding the craton to the south. These coeval units, along with additional units correlated on paleomagnetic grounds (Eskimo Formation), are interpreted to define a large igneous province (LIP), extending over an area of >400,000 km2, which we herein define as the Minto-Povungnituk LIP.Geochemical comparison between the Watts Group ophiolite, Minto dykes and the mafic Povungnituk Group shows significant differences allowing these data to be divided into two groups and domains within the LIP. A northern domain, comprising the Povungnituk and Watts groups, shows mixing between a depleted mantle source and a more enriched mantle plume-sourced melt. A southern domain comprising the Minto dykes and the paleomagnetically linked Eskimo Formation shows signs of an even more enriched source, while these magmas also show the effect of crustal contamination.Two distinct source mechanisms can be responsible for the observed geochemical differences between the two domains. First, a difference in lithospheric sources, where melting of different portions of Superior craton lithosphere caused the different melt signatures in the interior of the craton. In this case magmatism in the two domains is only related by having the same heat source (e.g.,a mantle plume) interpreted to be located on the northwestern side of the northern Superior craton. Second, two distinct deep mantle sources that remained separated within the ascending plume. This is analogous to some current hotspots interpreted to sample both large low shear velocity provinces (LLSVP) and adjacent ambient deep mantle. This latter interpretation would allow for the use of bilateral chemistry in LIPs as a potential tool for the recognition and mapping of the LLSVP boundaries throughout Earth's history.

Geochemistry of the Northern Paraná Continental Flood Basalt (PCFB) Province: implications for regional chemostratigraphy

The Parana Continental Flood Basalt (PCFB) province is one of the largest igneous provinces on Earth, but little is known about the architecture and geochemical characteristics of the lava flows. In this context, a set of borehole samples arranged in an NEE-SWW profile – more than 600 km long – located in the northern PCFB was used to define its chemostratigraphy. In this region, the outcrops are rare, and the volcanic piles are covered by hundreds of meters of sedimentary rocks from the Bauru Sub-Basin. This study aimed to explore the chemostratigraphic column from northern PCFB to improve the understanding of its internal architecture and to characterize the flood basalt sequences by using the petrography and elemental geochemistry of surface and borehole samples. The results showed the presence of the main four magma types, which were essentially distinguished by their P 2 O 5 , TiO 2 and Sr concentrations, as well as Zr/Y ratios, with excellent chemostratigraphy relations.

Sandstone Diagenesis in Sediment–lava Sequences: Exceptional Examples of Volcanically Driven Diagenetic Compartmentalization in Dune Valley, Huab Outliers, Nw Namibia

At the base of many flood basalt sequences and along volcanic rifted margins, volcanism can compete with the existing sedimentary environments, resulting in interbedded sequences of volcanic rocks and sediments. Here we report on sediment interlayers that are found in the lowermost volcanic units of the Etendeka flood basalts in NW Namibia (Twyfelfontein and Awahab formations), part of the much larger Parana–Etendeka Igneous Province. The sandstone bodies, predominantly eolian dunes, are isolated in a sequence of Lower Cretaceous (∼ 134 Ma) lava flows. The uppermost part or where sediment deposition and lava emplacement is observed to interact is characterized by barchanoid dunes, which were actively migrating during the emplacement of the lava flows. The fossil (isolated by lava) barchan dunes studied in Dune Valley show three characteristically different diagenetic styles. In Dune Valley, each dune body is completely encapsulated by lava, with additional igneous intrusions cutting through some bodies. We recognize three distinct styles of diagenesis: Type 1: fossilised dunes that are red in color and lack major authigenic mineralization, with grain compaction and subsequent porosity loss being the dominant diagenetic process. Type 2: dunes that have been bleached white, which have undergone a more complex diagenetic pathway. Type 2 dunes have abundant calcite, kaolinite, and bohmite as authigenic phases and lack hematite grain coatings. Detrital plagioclase is absent in white dunes (XRD analysis), with pseudomorphs of kaolinite common. This diagenetic assemblage results in the white dunes having lower porosity and permeability compared to the red dunes. The observations are probably due to a flux of carbon dioxide (CO2), hydrogen sulfide (H2S) and/or hydrogen (H2)–rich hydrothermal groundwater derived from igneous intrusions below. Type 3: “hot contact” effects at lava-flow contacts, where the unconsolidated dunes were rapidly indurated during lava emplacement (volcano-eogenesis). Type 3 diagenesis is restricted to << 1 m depth below lava contacts and common in dunes displaying both Type 1 and Type 2 diagenesis. The distribution of diagenetic Types 1 and 2 is dune specific, and throughout Dune Valley approximately half of the dunes have been bleached (e.g., Type 2 diagenesis), whereas diagenetic Type 3 is a hot contact phenomenon and is therefore found along all basal lava and dike contacts. This work has relevance to understanding the development of sediment–lava systems, to hydrocarbon exploration and development in preserved sediment–lava sequences, and the hydrothermal process described provides an example of natural CO2 sequestration.

Regional-scale Metasomatism in the Fortescue Group Volcanics, Hamersley Basin, Western Australia: Implications for Hydrothermal Ore Systems

Mafic to intermediate volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100 000 km2 Hamersley Basin on the southern margin of the Archaean Pilbara Craton, Western Australia. These represent one of the oldest (2·8–2·7 Ga) known examples of a continental flood basalt sequence. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to epidote–actinolite greenschist facies in the south and west. Superimposed on this metamorphic gradient, regional-scale metasomatism has affected extensive areas of the Fortescue Group. Metasomatized mafic lavas are characterized by well-developed assemblages dominated by pumpellyite–quartz or epidote–quartz associations. The mineral associations of metasomatic domain types broadly match the distribution of metamorphic isograd indicator minerals with a southward and westward increase in the proportion of epidote. A continuum exists between least altered rocks that preserve the regional metamorphic signature and the most intensely altered metasomatized rocks. Metasomatism is essentially continuous over a stratigraphic strike length of 100 km and across a strike width of 20 km. Regionally, metasomatically altered volcanic rocks occur widely across the Hamersley Basin and its outliers, over an area of some 450 km by 200 km. Metasomatic alteration is most conspicuous in the lower-grade metamorphic zones because pumpellyite- and epidote-rich rocks are green to yellow–green in outcrop. Whole-rock geochemical data indicate that metasomatism is associated with strong depletions in alkalis (Na, K, Li, Rb), alkali earths (Mg, Sr, Be, Ba) and heavy first transition series metals (Mn, Fe, Co, Ni, Cu, Zn), with a significant enrichment in Si. Calcium shows more variable mobility. Such geochemical trends, particularly depletions in Fe, Mn and base metals (Co, Ni, Cu, Zn), together with the metasomatic mineral assemblages, are comparable with those associated with the sub-sea-floor circulation of seawater, particularly in relation to the metal-depleted root zones of base metal deposits. Petrographic features indicate that the development of metasomatic mineral associations post-dates the formation of regional metamorphic assemblages. Consequently, it is interpreted that the hydrothermal fluid flowed through the buried pile after sufficient time to allow for the metamorphic mineral assemblages to approach equilibrium with ambient P–T conditions. The hydrothermal depletion in Fe observed across the Fortescue Group mafic lavas is intriguing given the abundance of iron ore in the overlying Hamersley Group, and a possible connection cannot be ignored. Furthermore, the scale of fluid flow observed in the Fortescue Group, which occurred through zones of inherent permeability, such as vesicular and brecciated lava flow tops, without the aid of major cross-cutting structures, has significant implications for the size of metasomatic systems in other mafic volcanic terranes, with potential consequences for the exploration for hydrothermal mineral deposits.

Palynology and clay mineralogy of the Deccan volcanic associated sediments of Saurashtra, Gujarat: Age and paleoenvironments

The intertrappean sediments associated with Deccan Continental Flood Basalt (DCFB) sequence at Ninama in Saurashtra, Gujarat yielded palynoassemblage comprising at least 12 genera and 14 species including Paleocene taxa such as Intrareticulites brevis, Neocouperipollis spp., Striacolporites striatus, Retitricolpites crassimarginatus and Rhombipollis sp. The lava flows of Saurashtra represent the northwestern most DCFB sequence in India. It is considered that the Saurashtra lava flows represent the earliest volcanic activity in the Late Cretaceous of the Reunion Mantle Plume on the northward migrating Indian Plate. The present finding of the Paleocene palynoflora from Ninama sediments indicate Paleocene age for the associated lava flows occurring above the intertrappean bed which suggests that the Saurashtra plateau witnessed eruption of Deccan lava flows even during Paleocene. The clay mineral investigation of the Ninama sediments which are carbonate dominated shows dominance of low charge smectite (LCS) along with the presence of mica and vermiculite. Based on the clay mineral assemblage it is interpreted that arid climatic conditions prevailed during the sedimentation. The smectite dominance recorded within these sediments is in agreement with global record of smectite peak close to the Maastrichtian–Paleocene transition and climatic aridity.

Mineralogy, geochemistry and petrogenesis of the Khopoli mafic intrusion, Deccan Traps, India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (eNdt = −9.0 to −12.7; 87Sr/86Sr = 0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm3, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.

Mahabaleshwar, Deccan Traps, India

The *1,500-km-long Western Ghats (Sahyadri) great escarpment of India is a mountainous region of great scenic beauty, as well as a biodiversity hotspot. In its northern part, the escarpment is cut across the Deccan Traps flood basalt lavas of Late Cretaceous (*65 Ma) age and forms the western precipitous edge of the Deccan Plateau. The town of Mahabaleshwar, situated here 1,436 m above sea level, is a popular tourist destination because of its scenic views and cool climate. The [1,200-m-thick Deccan flood basalt sequence exposed in the escarpment at Mahabaleshwar contains nearly fifty individual lava flows, with intervening red or green ‘‘bole’’ beds (too thin to be visible in this photograph). The sequence is capped by a regionally extensive ferricrete (‘‘laterite’’) blanket several tens of meters thick. This photograph, taken from the viewpoint named Arthur’s Seat (1,347 m) near Mahabaleshwar, shows a spectacular view of the sinuous and highly eroded Western Ghats escarpment, the thick, horizontally disposed lava sequence, and the flat and thickly forested ferricrete summits. The elevation difference between the summits and the lowest point in the valley in the foreground is *1,000 m. Photograph by Hetu Sheth, Indian Institute of Technology Bombay (hcsheth@iitb.ac.in). More information: Ollier and Sheth (J. Earth Syst. Sci., v. 117, pp. 537‐551, 2008) and references therein.

Age, petrogenesis and tectonic setting of the Thessalon volcanic rocks, Huronian Supergroup, Canada

The ∼2.5Ga Huronian Supergroup volcanic rocks in northern Ontario represent one of the better preserved Paleoproterozoic continental flood basalt sequences on Earth. The youngest age of Huronian Supergroup volcanism is refined to 2452.5±6.2Ma based on new U–Pb zircon analyses of the Copper Cliff rhyolite. Recent mapping and geochemical analyses of Thessalon Formation volcanic rocks define a well-developed chemostratigraphy; a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt–andesite volcanic series. These two mafic volcanic series are separated by felsic volcanic rocks (rhyolite and dacite). The mafic lavas are low-Ti tholeiites, dominated by basalt and andesite, and are characterized by enriched light rare earth element profiles and high Th contents (up to 12ppm). Most basalt–andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and, in some cases, limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle. All magma batches display prominent negative Nb–Ta and Ti anomalies on primitive mantle normalized multi-element diagrams, interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism. At least three distinct mantle source compositions are required to explain the range in the observed lava compositions. The most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high Th/Yb (∼10) and low Zr/Nb (∼11). This reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward Neoarchean subduction along the southern Superior craton margin.

Fault zone architecture and fluid flow in interlayered basaltic volcaniclastic-crystalline sequences

Faults in continental flood basalt sequences potentially control subsurface fluid flow. We present field and microstructural observations from fault zones cutting interlayered basaltic volcaniclastic-crystalline sequences within the North Atlantic Igneous Province. Fractures likely initiate within lava units, before linking through the volcaniclastic units. Through-going faults show refraction, with subvertical faults in the lavas joined to variably inclined faults in the volcaniclastic layers. At >1.0 m displacement, volcaniclastic units are progressively dragged into the fault plane forming a smear. Volcaniclastic sandstones deform by flow. Claystones fracture, and are incorporated into smears as breccia. Experimentally measured host and fault rock sample permeabilities, at aquifer to reservoir pressures (i.e., 10–90 MPa; ∼0.3–3.0 km depth) show fault rocks from low displacement faults have relatively low permeability (10−17–10−20 m2); fault rocks from higher displacement structures have comparatively high permeability (10−15–10−17 m2). Our observations suggest that permeability is determined by the opposing influences of clay mineralization, which decreases permeability, versus the development of interconnected, higher permeability zeolite veins. Brecciation and the formation of zeolite vein networks within claystone smears results in high permeability. Zeolite veins in volcaniclastic units form poorly-connected, spaced sets, parallel to the slip plane, hence sequence permeability remains low.

Geochemistry of the Palitana flood basalt sequence and the Eastern Saurashtra dykes, Deccan Traps: clues to petrogenesis, dyke–flow relationships, and regional lava stratigraphy

Recent studies of large mafic dyke swarms in the Deccan Traps flood basalt province, India, indicate that some of the correlative lava flows reached several hundred kilometers in length. Here we present field, petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data on the Palitana lava sequence and nearby dykes in the Saurashtra region of the northwestern Deccan Traps. These rocks are moderately evolved, many with low-Ti-Nb characteristics. We infer that most dykes are notably (and systematically) less contaminated by ancient continental crust than the Palitana flows, but four dykes are equally or significantly more contaminated, with some of the most extreme Sr-Nd isotopic compositions seen in the entire Deccan Traps (initial eNd is as low as −18.0). A Bhimashankar-type and a Poladpur-type dyke are present several hundred kilometers from the type section of these magma types in the Western Ghats escarpment. We find no geochemical correlations between the Palitana sequence and three subsurface sequences in NE Saurashtra containing abundant picritic rocks, surface lavas previously studied from Saurashtra, or the Western Ghats sequence. Intriguingly, the Eastern Saurashtra dykes cannot have been feeders to any of these lava sequences. Feeder dykes of these sequences may be located in southwestern or central Saurashtra, or in the Dhule-Nandurbar-Dediapada areas across the Gulf of Cambay, 200–300 km east of Palitana. Our results indicate polycentric flood basalt eruptions not only on the scale of the Deccan Traps province, but also within the Saurashtra region itself.

Detailed correlation and age of continental late Changhsingian and earliest Triassic beds: Implications for the role of the Siberian Trap in the Permian–Triassic biotic crisis

Conchostracan-rich beds between the Siberian Trap flood basalts and within the thick underlying Hungtukun tuffs of the Tunguska Basin can be closely correlated with conchostracan faunas of Dalongkou (NW China) and the Germanic Basin. The Germanic Basin faunas in turn can be closely correlated with the marine international stratigraphic time scale, and the accuracy of the biostratigraphic correlation of the Permian–Triassic boundary (PTB) is confirmed by a minimum in δ 13C carb values at this level. These high-resolution correlations demonstrate conclusively that the PTB is located within the temporally brief but thick Siberian Trap flood basalt sequence. The PTB lies slightly above the level of the main Permo-Triassic extinction event in low latitude marine beds, which occurred at the base of the C. meishanensis– H. praeparvus conodont zone and correlates with the beginning of the Siberian Trap flood basalt event. The main end-Permian continental extinction event was somewhat earlier, within the middle of the C. changxingensis– C. deflecta conodont zone. This horizon marks a mass extinction that devastated a diverse conchostracan fauna and left only low diversity faunas at low and high latitudes. This continental extinction event horizon lies within the middle of the Hungtukun tuffs of the Tunguska Basin and 107 m above the base of the Guodikeng Formation at Dalongkou (NW China). A “Triassic type” pioneer flora with numerous lycopod spores appears immediately above this level. Severe high northern and southern latitude marine extinctions occurred concurrently with this continental event, but low latitude marine biota was not then affected. This earlier event is best explained by global warming. The main low latitude extinction event in marine warm water faunas occurred somewhat later and left no signature in high latitude marine faunas or in continental faunas, but it does coincide with a rapid collapse of tropical rain forest environments (disappearance of the highly diverse Gigantopteris flora). This collapse likely was caused by global cooling due to a volcanic winter event.

Integrating streamer and ocean-bottom seismic data for sub-basalt imaging on the Atlantic Margin

ABSTRACT Stacked basalt flows cover much of the NW European continental margin, including potentially prospective sediments of the Faroes shelf. Such flows attenuate seismic energy, hindering sub-basalt structural imaging which is critical for both exploration and tectonic studies. Low-frequency, long-offset reflection surveys have yielded improved images below top basalt, while coincident ocean-bottom seismometer (OBS) data have mapped low velocity zones (LVZ) from tomographic inversion. Image correlation in a common depth domain is challenged by low spatial resolution of the tomographic image, the absence of turning rays in a LVZ and the lack of wide-angle arrivals in the reflection data. We integrate densely sampled reflection data with deep velocities from OBS data to give a common velocity model in a new, iterated, pre-stack depth-migration workflow using complementary constraints from the two datasets. The matched velocity model and depth image enable interpretation of (i) a uniform flood basalt sequence, 2–4 km thick beneath the Fugloy Ridge, with velocities correlating with those in the Lopra 1/1A borehole and (ii) a sub-basalt LVZ with chaotic reflections corresponding to sill-intruded, probably syn-rift sediments. Such a workflow could be extended to targets beneath high velocity salt or basalt and could provide constraints for 3D datasets. Supplementary material: Overlays of modelled arrival times on all the OBS gathers, similar to Figure 4, can be found at http://www.geolsoc.org.uk/SUP18433

The composition and structure of volcanic rifted continental margins in the North Atlantic: Further insight from shear waves

Imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non-volcanic rifted and convergent margins. Massive volcanism occurred during break-up at 70% of the passive margins bordering the Atlantic Ocean, the causes and dynamics of which are still debated. This paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four-component ocean bottom seismometers along two North Atlantic continental margin profiles. This traveltime tomography was performed using two different approaches. The first, a flexible layer-based parameterisation, enables the quality control of traveltime picks and investigation of the crustal structure. The second, with a regularised grid-based parameterisation, requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via Monte Carlo analyses. The velocity models indicate high lower-crustal velocities and sharp transitions in both velocity and Vp/Vs ratios across the continent–ocean transition. The velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins. Interpretation of the high quality seismic reflection profile on the Faroes margin confirms that this mixing is through crustal intrusion. Converted shear wave data also provide constraints on the sub-basalt lithology on the Faroes margin, which is interpreted as a pre-break-up Mesozoic to Paleocene sedimentary system intruded by sills.

Palynoflora from deccan volcano-sedimentary sequence (Cretaceous-Palaeogene transition) of central India: implications for spatio-temporal correlation

The sedimentary beds associated with Deccan Continental Flood Basalt (DCFB) sequences exposed in the volcanic subprovinces of Jabalpur-Mandla-Chhindwara (JMC) regions of Madhya Pradesh and Nand-Dongargaon (N-D) basin and the adjoining areas to the west in Yeotmal-Nanded in Maharashtra were studied for their palynofloral analysis. The sediments were characterized palynologically and changes in the palynoflora are observed at different stratigraphic levels in a number of sections including several new intertrappean localities recorded in recent years. For the purpose of effective correlation of different subprovinces, palynofloras of some of the previously studied intertrappeans are also reviewed. Our studies suggest that before the start of the Deccan volcanic activity, the palynoflora found in the Lameta sediments, was dominated by gymnosperms-angiosperm association. The plant canopy consisted mainly of gymnosperms (Conifers and Podocarpaceae) whereas, the understory members were mostly of palms and herbs (Poaceae and Asteraceae). The eruption of Deccan volcanic flows severely affected the existing floral association and proved fatal for the well established plant community. The immediately overlying sediments associated with the earliest volcanic flows are dominated by pteridophytes and angiosperm taxa (Azolla cretacea, Aquilapollenites bengalensis, Ariadnaesporites sp., Gabonisporis vigourouxii and Triporoletes reticulatus). Higher up in the stratigraphic sequence, similar forms continued with simultaneous appearance of new taxa including Scabrastephanocolpites spp. At still higher stratigraphic levels, abundance of fungi especially the mycorrhizal fungi, concurrent with sharp decline in pollen/spore recovery was observed. In the culminating phase (i.e. Palaeocene) of Deccan volcanic history a new palynofloral assemblage of typical Palaeocene taxa (Dandotiaspora dilata, D. pseudoauriculata, D. plicata, Spinizonocolpites echinatus, Matanomadhiasulcites sp., and Lakiapollis ovatus) was encountered.

Similarities between mantle-derived A-type granites and voluminous rhyolites in continental flood basalt provinces

ABSTRACTMany continental flood basalt provinces contain rhyolites with ‘A-type’ compositions and many studies have concluded that these higher silica rocks are crustal melts from metapelitic or tonalitic country rock. However, although many of the low-Ti continental flood basalt sequences exhibit a marked a silica gap from ∼55–65 wt. SiO2, many incompatible element ratios, and the calculated eruption temperatures (950–1100°C) are strikingly similar between the rhyolites and associated basalts. Using experimental evidence, derivation of the low-Ti rhyolites from a basaltic parent is shown to be a viable alternative to local crustal melting. Comparison of liquid compositions from experimental melting of both crustal and mantle-derived (basaltic) source materials allows the two to be distinguished on the basis of Al2O3 and FeO content. The basalt experiments are reversible, such that the same melts can be produced by melting or crystallisation. The effect of increased water content in the source is also detectable in the liquid composition. The majority of rhyolites from continental flood basalt provinces fall along the experimental trend for basalt melting/ crystallisation at relatively low water content. The onset of the silica gap in the rhyolites is accompanied by an abrupt decrease in TiO2 and FeO*, marking the start of Fe–Ti oxide crystallisation. Differentiation from 55–65 wt. SiO2 requires ∼30 fractional crystallisation in which magnetite is an important phase, sometimes accompanied by limited crustal contamination. The rapid increase in silica occurs over a small temperature interval and for relatively small changes in the amount of fractional crystallisation, thus intermediate compositions are less likely to be sampled. It is argued that the presence of a silica gap is not diagnostic of a crustal melting origin for either A-type granites or rhyolites in continental flood basalt provinces. The volume of these rhyolites erupted over the Phanerozoic is significant and models for crustal growth should take this substantial contribution from the mantle into account.

Understanding the offshore flood basalt sequence using onshore volcanic facies analogues: an example from the Faroe–Shetland basin

AbstractFlood basalts in associated volcanic rifted margins, such as the North Atlantic Igneous Province, have a significant component of lavas which are preserved in the present day in an offshore setting. A close inspection of the internal facies architecture of flood basalts onshore provides a framework to interpret the offshore sequences imaged by remote techniques such as reflection seismology. A geological interpretation of the offshore lava sequences in the Faroe–Shetland Basin, using constraints from onshore analogues such as the Faroe Islands, allows for the identification of a series of lava sequences which have characteristic properties so that they can be grouped. These are tabular simple flows, compound-braided flows, and sub-aqueously deposited hyaloclastite facies. The succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m. Down to the top of the sub-volcanic sediments, the offshore volcanic succession has a thickness of about 2700 m where it can be clearly identified across much of the area, with a further 2700 m or more of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of the region. A large palaeo-waterbody is identified on the basis of a hyaloclastite front/apron consisting of a series of clinoforms prograding towards the eastern part of the basin. This body was &gt; 500 m deep, must have been present at the onset of volcanism into this region, and parts of the water body would have been present during the continued stages of volcanism as indicated by the distribution of the hyaloclastite apron.