Geological Magazine

Published by Cambridge University Press (CUP)
Online ISSN: 1469-5081
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Article
Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykal Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.
 
Article
Siberia contains several key reference sections for studies of biological and environmental evolution across the Proterozoic-Phanerozoic transition. The Platonovskaya Formation, exposed in the Turukhansk region of western Siberia, is an uppermost Proterozoic to Cambrian succession whose trace and body fossils place broad limits on the age of deposition, but do not permit detailed correlation with boundary successions elsewhere. In contrast, a striking negative carbon isotopic excursion in the lower part of the Platonovskaya Formation permits precise chemostratigraphic correlation with upper-most Yudomian successions in Siberia, and possibly worldwide. In addition to providing a tool for correlation, the isotopic excursion preserved in the Platonovskaya and contemporaneous successions documents a major biogeochemical event, likely involving the world ocean. The excursion coincides with the palaeontological breakpoint between Ediacaran- and Cambrian-style assemblages, suggesting a role for biogeochemical change in evolutionary events near the Proterozoic Cambrian boundary.
 
Article
Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. Even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post-Sturtian successions in western North America and elsewhere. -from Authors
 
Article
Silicified flake conglomerates and in situ stratiform stromatolites of the Upper Proterozoic (c. 700-800 Ma) Limestone-Dolomite 'Series', central East Greenland, contain well preserved microfossils. Five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages, providing a broad palaeontological representation of late Proterozoic peritidal mat communities. Comparison of assemblages demonstrates that the taxonomy and diversity of mat builder, dweller, and allochthonous populations all vary considerably within and among horizons. The primary mat builder in most assemblages is Siphonophycus inornatum, a sheath-forming prokaryote of probable but not unequivocally established cyanobacterial affinities. An unusual low diversity unit in Bed 17 is dominated by a different builder, Tenuofilum septatum, while a thin cryptalgal horizon in Bed 18 is built almost exclusively by Siphonophycus kestron. Although variable taphonomic histories contribute to observed assemblage variation, most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments. Twenty-two taxa are recognized, of which two are described as new: Myxococcoides stragulescens n.sp. and Scissilisphaera gradata n. sp.
 
Article
A submarine fissure eruption of Upper Miocene age produced a modest volume of alkaline basalt at Low Layton, on the north coast of Jamaica. The eruption occurred in no more than a few hundred meters of water and produced a series of hyaloclastites, pillow breccias and pillow lavas, massive lavas, and dikes with an ENE en echelon structure. The volcano lies on the trend of one of the island's major E-W strike-slip fault zones; the Dunavale Fault Zone. The K-Ar age of the eruption of 9.5 plus or minus 0.5 Ma. B.P. corresponds to an extension of the Mid-Cayman Rise spreading center inferred from magnetic anomalies and bathymetry of the Cayman Trough to the north and west of Jamaica. The Low Layton eruption was part of the response of the strike-slip fault systems adjacent to this spreading center during this brief episode of tectonic readjustment.
 
Article
During the last few months the preparation of a descriptive catalogue of the magnificent collection of reptilian remains obtained by Mr. A. N. Leeds in the Oxford Clay of Peterborough, has rendered it necessary to examine in some detail the fine series of more or less complete skeletons of Ophlhalmosurus preserved in the British Museum. In the course of this examination a certain number of new facts have come to light, and since it will be some time before the detailed description can appear it seems desirable to give a brief account of some of the more interesting points.
 
Article
A graphical method for the analysis of the orientation of planar and linear structural elements in drill core is presented. Simple computation and projection operations applied to data taken from elements traced on the cylindrical surface of the core are required. To know a structural element(s) of reference, the method requires detailed structural analysis of the surface geology around the drill hole.
 
Article
In this paper the author deals with some of the results obtained in the course of a survey of Eastern Sinai during the season of 1898–99, his remarks being based on a map carefully prepared by his colleague, Mr. H. G. Skill, F.R.G.S., and on his own topographical and geological observations.
 
Distances of the successive islands in the Canarian and Hawaiian archipelagoes from their respective active end of chain plotted against the oldest subaerial published ages of the different volcanoes. The Hawaiian volcanoes fit in a straight distance vs. time line corresponding to a plate velocity of about 10 cm/a. The Canaries fit in a similar line corresponding to a plate velocity of about 1.9 cm/a, with the exception of La Gomera and Lanzarote, as explained in the text.
Distribution of emission vents in island-volcanoes. Numbers indicate concentration of eruptive vents (vents/km 2 ). The islands of Tenerife and El Hierro, and the Mauna Kea Volcano evolved unbuttressed, and the rifts have preserved the regular, three-armed geometry. Many other Hawaiian volcanoes and the Cumbre Vieja Volcano in La Palma have evolved to asymmetric configurations as the rifts were constrained in their progress. The distribution of Mauna Kea vents is from Porter (1972).
Figure illustrating the origin of the Canaries in relation to a hotspot. HS: initial activity of the CHS, close to the continental-oceanic boundary. A: initial progression of volcanism along the continental-oceanic boundary (COB). B: general progression of volcanism induced by the displacement of the African plate. Ages in italics: the oldest dated subaerial volcanism of each island. The two scenarios (1 and 2) are explained in the text.
Article
The ideas presented in this paper have been greatly clarified and developed through our discussions with Uri ten Brink, Tony Watts, Tim Minshull, Robin Holcomb, Bruce Nelson and Hubert Staudigel. Stimulating comments from Hans Schmincke also helped us to focus our arguments. We thank all of them for their assistance in various stages of the preparation of this paper. The Canarian Archipelago is a group of volcanic islands on a slow-moving oceanic plate, close to a continental margin. The origins of the archipelago are controversial: a hotspot or mantle plume, a zone of lithospheric deformation, a region of compressional bock-faulting or a rupture propagating westwards, from the active Atlas Mountains fold belt have been proposed by different author. However, comparison of the Canarían Archipelago with the prototypical hotspot-related island group, the Hawaiian Archipelago, reveals that the differences between the two are not as great as had previously been supposed on the basis of older data. Quaternary igneous activity in the Canaries is concentrated at the western end of the archipelago, close to the present-day location of the inferred hotspot. This is the same relationship as seen in the Hawiian and Cape Verde islands. The latter archipelago, associated with a welldefined but slow-moving mantle plume, shows anomalies in a plot of island age against distance which are comparable to those seen in the Canary Islands: these anomalies cannot therefore be used to argue against a hotspot origin for the Canaries. Individual islands in both archipelagoes are characterized by initial rapid growth (the 'shieldbuilding' stages of activity), followed by a period of quiescence and deep erosion (erosion gap) which in turn is followed by a 'post-erosional' stage of activity. The absence of post-shield stage subsidence in the Canaries is in marked contrast with the major subsidence experienced by the Hawiian Islands, but is comparable with the lack of subsidence evident in other island group at slow-moving hotspots, such as the Cape Verdes. Comparison of the structure and structural evolution of the Canary Islands with other oceanic islands such as Hawii and Réunion reveals many similarities. These include the development af triple ('Mercedes Star`) rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones. The apparent absence of these features in the post-erosional islands may in part be a result of their- greater age and deeper- erosion, which has removed much of the evidence for their early volcanic architecture. We conclude that the many similarities between the Canary Islands and island groups whose hotspot origins are undisputed show that the Canaries have been produced in the same way. Peer reviewed
 
Article
Carbon stable-isotope variation through the Cenomanian13C values that provide a basis for high-resolution correlation. Positive and negative 13C excursions of up to +213C reference curve and an isotope event stratigraphy are constructed using data from the English Chalk. The isotope stratigraphy is applied to successions in Germany, France, Spain and Italy. Correlation with pelagic sections at Gubbio, central Italy, demonstrates general agreement between biostratigraphic and chemostratigraphic criteria in the CenomanianSantonian stages is less clear cut: magnetostratigraphic evidence for placing the base of Chron 33r near the base of the Upper Santonian is in good agreement with the carbon-iso-tope correlation, but generates significant anomalies regarding the placement of the Santonian and Campanian stage boundaries with respect to Tethyan planktonic foraminiferal and nannofossil zones. Isotope stratigraphy offers a more reliable criterion for detailed correlation of Cenomanian13C data from one of the English sections, a composite Cenomanian–Campanian age-calibrated reference curve is presented that can be utilized in future chemostratigraphic studies.
 
Article
We report on the investigation of contact metamorphism provoked by the emplacement of a shallow magma chamber during the Timanfaya eruption of Lanzarote from 1730 to 1736 AD. The study was carried out on metamorphic xenoliths from basaltic Timanfaya lavas, and shows how the primary basanitic magma was contaminated by sedimentary and metamorphic rocks. Mineralogical and chemical studies allowed the definition of several xenolith types. Silica xenoliths (quartz, tridymite, cristobalite or a mixture of these, constituting more than 50% of the xenolith) and calc-silicate xenoliths (wollastonite, sometimes the 2M type, diopside, forsterite or mixture of these, constituting more than 50% of the xenolith) are the most frequent. Other minerals recognized were calcite, dolomite, augite, enstatite, hypersthene, spinet and scapolite. The mineralogy and some textures of the metamorphic forsteritic xenoliths are identical to those found in ultrabasic xenoliths (dunites) and point to a possible metamorphic origin for some of them. Major and trace elements showed a diversity of composition, controlled by the mineralogy. The REE composition of the metamorphic xenoliths is high, compared with the sedimentary xenoliths not affected by metamorphism. The mineral assemblages define metamorphic facies of low, medium and high grade, depending on the distance of the sedimentary rocks from the magma chamber border. The IGPETWIN-MIXING program was used to verify the contamination process, taking the xenoliths as representative of the sedimentary/metamorphic rocks that were melted. The results indicated that sedimentary/metamorphic rock contamination of a basanitic magma can produce tholeiitic compositions.
 
(a) The zones of NW Hellenides; (b) location of the study area; (c) simplified geological map of the study area (modified after IGRS-IFP, 1966).  
Representative stratigraphic column of the Ionian Zone (after Karakitsios, 1995).
Summary of biostratigraphic information for the Gotzikas section, based on observed distributions of calcareous nannofossils and planktonic foraminifera. Whilst 36 samples were investigated for nannofossils and 63 for planktonic foraminifera, only those samples yielding age-diagnostic taxa are represented here. (UBS – uppermost black shale).  
Article
We present new stable (C, O) isotopic, biostratigraphic and organic geochemical data for the Vigla Shale Member of the Ionian Zone in NW Greece, in order to characterize organic carbon-rich strata that potentially record the impact of Cretaceous Oceanic Anoxic Events (OAEs). In a section exposed near Gotzikas (NW Epirus), we sampled a number of decimetre-thick, organic carbon-rich units enclosed within marly, locally silicified, Vigla Limestone (Berriasian Turonian). All these units are characterized by largely comparable bulk geochemical characteristics, indicating a common marine origin and low thermal maturity. However, the stratigraphically highest of these black shales is further distinguished by its much higher total organic-carbon (TOC) content (28.9 wt%) and Hydrogen Index (HI) (529), and much enriched [delta]13Corg value ([minus sign]22.1% ). Planktonic foraminiferal and calcareous nannofossil biostratigraphy indicate a lower to middle Albian age for the strata immediately above, and a lower Aptian age for the strata below, the uppermost black shale. In terms of molecular organic geochemistry, the latter black shale is also relatively enriched in specific isoprenoidal compounds (especially monocyclic isoprenoids), whose isotopic values are as high as [minus sign]15 , indicating a substantial archaeal contribution to the organic matter. The striking similarities between the molecular signatures of the uppermost Vigla black shale and coeval organic-rich strata from SE France and the North Atlantic (ODP Site 1049C) indicate that this level constitutes a record of the Paquier Event (OAE 1b).
 
Article
A number of intra-continental alkaline volcanic sequences in NW Turkey were emplaced along localized extensional gaps within dextral strike-slip fault zones prior to the initiation of the North Anatolian Fault Zone. This study presents new palaeomagnetic and 40ArQuaternary tectonic evolution of the region and possible roles of block rotations in the kinematic history of the region. 40Ar–39Ar analyses of basalt groundmass indicate that the major volume of alkaline lavas of NW Turkey spans about 4 million years of episodic volcanic activity. Palaeomagnetic results reveal clockwise rotations as high as 73° in Thrace and 33° anticlockwise rotations in the Biga Peninsula. Movement of some of the faults delimiting the areas of lava flows and the timing of volcanic eruptions are both older than the initiation age of the North Anatolian Fault Zone, implying that the region experienced transcurrent tectonics during Late Miocene to Pliocene times and that some of the presently active faults in the region are reactivated pre-existing structures.
 
Isotopic and elemental compositions, basinal coniform microbialites (Measured Section 4)
δ 18 O values plotted v. Sr (left) and Mn (right) concentrations for samples from September Lake Sections 1-3 (top), Dismal Lake Sections 4-5 (middle) and Platy Microbialites at Section 4 (bottom). Symbols indicate petrographic components belonging to Group 1 (micritic and microsparitic components), Group 2 (synsedimentary precipitates) and Group 3 (secondary cements).
Stratigraphic and chemostratigraphic profiles from Dismal Lakes Group Sections 1 through 5. Inset map shows locality of measured sections, and symbols indicate petrographic components belonging to Group 1 (micritic and microsparitic components), Group 2 (synsedimentary precipitates) and Group 3 (secondary cements). For legends see Figures 2 and 3.  
Article
Mesoproterozoic marine successions worldwide record a shift in average delta(13)C values from 0 to +3.5parts per thousand, with the latter value evident in successions younger than 1250 Ma. New carbon isotope data from the similar to 1300 to 1270 Ma Dismal Lakes Group, Arctic Canada, provide further insight into this fundamental transition. Data reveal that the shift to higher VC values was gradual and marked by occasional excursions to values less than 0 parts per thousand. When compared to records from older and younger marine successions, it is evident that the difference between isotopic minima and maxima increased with time, indicating that the marine system evolved to become isotopically more variable. We interpret these patterns to record an increase in the crustal inventory of organic carbon, reflecting eukaryotic diversification and a change in the locus of organic carbon burial to include anoxic deep marine sites where preservation potential was high. We speculate that the release of O-2 to Earth's surface environments associated with increased organic carbon storage induced irreversible changes in the Mesoproterozoic biosphere, presaging the more extreme environmental and evolutionary developments of the Neoproterozoic.
 
Article
Quartz-rich xenoliths hosted in the basaltic13.5%, CaO=0.2Peloritani basement and generated a quartz-bearing residue. The fluids trapped inside these inclusions are rarely composed of high-density CO2, with a homogenization temperature to liquid phase (ThL) within the range 26.231.0°C, corresponding to density values of 150 to 460 kg/m3. It is proposed that, during ascent, batches of magma rising from the mantle sampled various fragments of a residual rock from a depleted metamorphic basal layer and carried them to the surface. Magmas probably stopped inside at least two distinct reservoirs located at different depths ([less-than-or-equal]12.7 km and between 1.7 and 4.7 km), in which they underwent fractional crystallization. The ascent speed of the xenolith-bearing magmas from the deeper storage area to the shallower reservoir was fairly high, as confirmed by the low number of high-density fluid inclusions preserved that show re-equilibration features, and by the contemporaneous presence of denser cumulitic Ol+Cpx±Opx xenoliths in the host lava.
 
Article
Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician-Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered Sr-87/Sr-86 ratios ranging between 0.7062 and 0.7068, and the best preserved delta(13)C values failing between +5.0 and +6.5 % obtained from two marble formations, are consistent with a seawater composition in the time interval 700-600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician-Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese-iron ores should be restricted to 700-600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides.
 
Article
Organic petrography and geochemical analyses have been carried out on the shales, carbonaceous shales and coals of the Shemshak Group (Upper Triassic-Middle Jurassic) from fifteen localities along the Alborz Range of Northern Iran. Organic matter (OM) has been investigated using Rock-Eval pyrolysis, elemental analysis of kerogen, vitrinite reflectance (VRr) and Thermal Alteration Index (TAI). Reflectance of autochthonous vitrinite varies from 0.6 to 2.2 % indicating thermally early mature to over mature OM in the Shemshak Group, in agreement with other maturity parameters used. The shales of the Shemshak Group are characterized by poor to moderate residual organic carbon contents (0.25 to 8.5 %) and the presence of hydrogen-depleted OM, predominantly as a consequence of petroleum generation and of oxidation of OM. According to light-reflected microscopy results vitrinite/vitrinite-like macerals are dominant in the kerogens concentrated from the shaly facies. The coals and carbonaceous shales of the Shemshak Group show a wide range in organic carbon concentration (3.5 to 88.6 %) and composition (inertinite- and vitrinite-rich types), and thereby different petroleum potentials. Among the studied sections only the coals and carbonaceous shales of the Hive locality show good residual petroleum potential and may still generate oil. Thermal modelling results suggest that low to moderate paleo-heat flow, ranging from 47 to 79 mW.m-2 (57 mW.m-2 on average), affected the Central-Eastern Alborz. The maximum temperature which induced OM maturation of the Shemshak Group seems to be related to its deep burial rather than to a very strong heat flow related to an uppermost Triassic-Liassic rifting. The interval of petroleum generation in the most deeply buried part of the Shemshak Group (i.e., Tazareh section) corresponds to Late Jurassic-Early Cretaceous times. Exhumation of the Alborz Range during Late Neogene time, especially along the axis of the Central-Eastern Alborz, where maxima of VRr values are recorded, probably destroyed possible petroleum accumulations. However on the northern flank of the Central-Eastern Alborz, preservation of petroleum accumulations may be better. The northern part of the basin therefore seems the best target for petroleum exploration.
 
Article
The tectonic origin of pre-Devonian rocks of Svalbard has long been a matter of debate. In particular, the origin and assemblage of pre-Devonian rocks of western Spitsbergen, including a blueschist-eclogite complex in Oscar II Land, are enigmatic. We present detrital zircon U-Pb LA-ICP-MS data from six Mesoproterozoic to Carboniferous samples and one U-Pb TIMS zircon age from an orthogneiss from Oscar II Land in order to discuss tectonic models for this region. Variable proportions of Palaeo- to Neoproterozoic detritus dominate the metasedimentary samples. The orthogneiss has an intrusion age of 927 +/- 3 Ma. Comparison with detrital zircon age spectra from other units of similar depositional age within the North Atlantic region indicates that Oscar II Land experienced the following tectonic history: (1) the latest Mesoproterozoic sequence was part of a successor basin which originated close to the Grenvillian-Sveconorwegian orogen, and which was intruded by c. 980-920 Ma plutons; (2) the Neoproterozoic sediments were deposited in a large-scale basin which stretched along the Baltoscandian margin; (3) the eclogite-blueschist complex and the overlying Ordovician-Silurian sediments probably formed to the north of the Grampian/Taconian arc; (4) strike-slip movements assembled the western coast of Spitsbergen outside of, and prior to, the main Scandian collision; and (5) the remaining parts of Svalbard were assembled by strike-slip movements during the Devonian. Our study confirms previous models of complex Caledonian terrane amalgamation with contrasting tectonic histories for the different pre-Devonian terranes of Svalbard and particularly highlights the non-Laurentian origin of Oscar II Land.
 
Collector curves to show increasing numbers of known 
Maps showing the locations of El Brete (Argentina) and Massecaps (France). Regions of countries are shown as 
Map to show the known geographic distribution and relative ages of Enantiornithes alongside the other main lineages of Mesozoic birds (re-drawn from Chiappe & Dyke, 2002). 
Humeri referred to Martinavis (see text for details): ACAP-M 1957, complete right humerus (holotype of M. cruzyensis ) in left lateral (a), caudal (b), right lateral (c) and cranial (d) views; PVL 4054, portions of complete left humerus in caudal (e) and cranial (f) views; KU-NM-37, proximal end of left humerus in caudal (g) and proximal (h) views. For measurements see Table 1; scale bars are 10 mm. 
Some of the skeletal elements (parts of PVL 4035 and 4020) previously referred to the El Brete euenantiornithine Enantiornis leali (Walker, 1981; Chiappe & Walker, 2002): proximal end of left humerus (PVL 4035) in cranial (a) and caudal (b) views; reconstructed complete left humerus of Enantiornis leali (PVL 4035 and 4020) in cranial (c), caudal (d), and right lateral (e) views. Scale bars are 10 mm. 
Article
We review historical approaches to the systematics of Enantiornithes, the dominant birds of the second half of the Mesozoic, and describe the forelimb remains of a new Cretaceous euenantiornithine. This taxon is known on the basis of fossil specimens collected from southern France, Argentina and the United States; such a wide geographical distribution is uncharacteristic for Enantiornithes as most taxa are known from single localities. Fossils from the Massecaps locality close to the village of Cruzy (Hérault, southern France), in combination with elements from New Mexico (USA) and from the Argentine locality of El Brete (Salta Province) testify to the global distribution of large flighted euenantiornithine birds in the Late Cretaceous. We discuss the systematics and taxonomy of additional isolated bones of Enantiornithes that were collected from the Argentine El Brete locality in the 1970s; the presence of these flying birds in Cretaceous rocks on both sides of the equator, in both northern and southern hemispheres, further demonstrates the ubiquity of this avian lineage by the latter stages of the Mesozoic.
 
P–T grid displaying P–T estimates with fixed uncertainties of ± 50 ◦ C and ± 1 kbar superimposed on petrogenetic grid for basaltic compositions of Oh & Liou (1998), with divisions between metamorphic facies shown as hatched lines. Large arrows display possible trajectories for decompression path from eclogite facies to amphibolite facies. 
Article
Peak and retrograde P-T conditions of Grenville-age eclogites from the Glenelg-Attadale Inlier of the northwest Highlands of Scotland are presented. Peak conditions are estimated as c. 20 kbar and 750-780 degrees C, in broad agreement with previous work. The eclogites subsequently followed a steep decompression path to c. 13 kbar and 650-700 degrees C during amphibolite facies retrogression. Peak eclogite facies metamorphism occurred > 1080 Ma and retrogression at c. 995 Ma, suggesting fairly sluggish uplift rates of < 0.3 km/Ma and cooling rates of < 1.25 degrees C/Ma, when compared with other parts of the Grenville orogeny and/or modem orogens. However, current poor constraints on the timing of peak metamorphism mean that these rates cannot be used to interpret the geodynamic evolution of this part of the orogen. The P-T-t data, together with petrology and the field relationships between the basement rocks of the Glenelg-Attadale Inlier and the overlying Moine Supergroup, mean that it is difficult to support the currently held view that an unconformable relationship exists between the two. It is suggested that more data are required in order to re-interpret the Neoproterozic tectonic evolution of the northwest Highlands of Scotland.
 
Article
Flood basalts in associated volcanic rifled margins, such as the North Atlantic Igneous Province, have a significant component of lavas which are preserved in the present clay 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 oil the basis of a hyaloclastite front/apron consisting of a series of clinoforms prograding towards the eastern part of the basin. This body was > 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.
 
Article
Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to 87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustalmelts and and primarymantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place.
 
Article
The intensity and extent of anoxia during the two Kellwasser anoxic events has been investigated in a range of European localities using amultidisciplinary approach (pyrite framboid assay, gamma-ray spectrometry and sediment fabric analysis). The results reveal that the development of the Lower Kellwasser Horizon in the early Late rhenana Zone (Frasnian Stage) in German type sections does not always coincide with anoxic events elsewhere in Europe and, in some locations, seafloor oxygenation improves during this interval. Thus, this anoxic event is not universally developed. In contrast, the Upper Kellwasser Horizon, developed in the Late linguiformis Zone (Frasnian Stage) in Germany correlates with a European-wide anoxic event that is manifest as an intensification of anoxia in basinal locations to the point that stable euxinic conditionswere developed (for example, in the basins of the Holy Cross Mountains, Poland). The interval also saw the spread of dysoxic waters into very shallow water (for instance, reefal) locations, and it seems reasonable to link the contemporaneous demise of many marine taxa to this phase of intense and widespread anoxia. In basinal locations, euxinic conditions persisted into the earliest Famennian with little change of depositional conditions. Only in the continental margin location of Austria was anoxia not developed at any time in the Late Devonian. Consequently it appears that the Upper Kellwasser anoxic event was an epicontinental seaway phenomenon, caused by the upward expansion of anoxia from deep basinal locales rather than an ‘oceanic’ anoxic event that has spilled laterally into epicontinental settings.
 
Palaeogeography in southern Italy during Jurassic–Cretaceous times showing the Gargano Promontory divided between the Apulia Platform in the southwest and the Ionian Basin in the northeast (modified after Zappaterra, 1990). Shallow-water carbonate-platform facies indicated by brick ornament; deeper-water pelagic facies by grey shading.  
Stratigraphic column of the Coppitella section showing the sample numbers and the calcareous nannofossil and planktonic foraminiferal biostratigraphy. The main biotic events for the two groups are indicated by arrows. The close-up column shows the very closely spaced sampling below the upper black shale and the interval with very rare or absent Globigerinelloides ('Globigerinelloides eclipse').  
Chronostratigraphic framework and nomenclature for the Cretaceous slope-and-basin deposits of the northern Gargano area (modified after Cobianchi, Luciani & Bosellini, 1997). The vertical solid line indicates the stratigraphic interval investigated in the Coppitella section. Time scale follows Gradstein et al. (1994).  
Article
Microfossil distribution patterns and high-resolution 18O curves, calibrated against planktonic foraminiferal and calcareous nannofossil data, are provided for the Aptian pelagic Coppitella section of the Gargano Promontory (southern Italy). The succession consists of cyclically arranged couplets of bioturbated grey marlstones and off-white marly limestones, referable to the Marne a Fucoidi. In the lower portion of the section, two thin black shales were recognized. The high-resolution Tethyan sections, albeit with lower absolute values. The onset of deposition of organic-rich sediments falls at the top of the interval of unchanging carbon-isotope values, whereas the upper black shale is documented from the interval of the main Aptian positive nannoconid crisisGlobigerinelloides eclipse’ is recorded below and within the upper black shale. The distribution of meso-eutrophic indices (Zygodiscus spp., radiolaria) vs. moderate-fertility indices (Rhagodiscusasper and Lithraphiditescarniolensis) testifies to a modest increase of surface-water fertility only throughout the stratigraphically higher black shale. The occurrence of a benthic foraminiferal fauna, albeit impoverished, in both the basal and upper black horizons clearly documents dysaerobic rather than completely anoxic conditions on the sea floor. Relative sea-level rise at the time of the Selli Event in the Gargano Promontory is documented by drowning and foundering of the Apulia platform margin, situated adjacent to the basin in which the Marne a Fucoidi accumulated.
 
Article
The Waipounamu Erosion Surface is a time-transgressive, nearly planar, wave-cut surface. It is not a peneplain. Formation of the Waipounamu Erosion Surface began in Late Cretaceous time following break-up of Gondwanaland, and continued until earliest Miocene time, during a 60 million year period of widespread tectonic quiescence, thermal subsidence and marine transgression. Sedimentary facies and geomorphological evidence suggest that the erosion surface may have eventually covered the New Zealand subcontinent (Zealandia). We can find no geological evidence to indicate that land areas were continuously present throughout the middle Cenozoic. Important implications of this conclusion are: (1) the New Zealand subcontinent was largely, or entirely, submerged and (2) New Zealand's present terrestrial fauna and flora evolved largely from fortuitous arrivals during the past 22 million years. Thus the modern terrestrial biota may not be descended from archaic ancestors residing on Zealandia when it broke away from Gondwanaland in the Cretaceous, since the terrestrial biota would have been extinguished if this landmass was submerged in Oligocene–Early Miocene time. We conclude that there is insufficient geological basis for assuming that land was continuously present in the New Zealand region through Oligocene to Early Miocene time, and we therefore contemplate the alternative possibility, complete submergence of Zealandia.
 
Article
A volcano-plutonic complex in the Susa area, southwest Japan, consists of the Yamashima andesites, the Koyama gabbros and syn-plutonic porphyrite dykes derived from a common basaltic andesite magma. The complex is closely associated with middle Miocene turbidite deposits. The Yamashima andesites are composed mainly of basaltic andesite feeder dykes, massive submarine lavas with hyaloclastites, and their reworked deposits. The lavas and deposits immediately overlie turbidite deposits, indicating submarine volcanic activity. The Koyama gabbros formed hornfels by contact metamorphism of the surrounding turbidites and andesites. Highly purified clinopyroxene and plagioclase mineral separates from the Yamashima andesites were dated by a K) Ma with an initial 40Ar/36Ar ratio of 297.3±2.4 (1Ar. Most of the groundmass ages are considerably younger (12.1Ar ages, together with previous studies, show that a series of geological events took place in the Susa area between 16 and 13 Ma. Conglomerates and sandstones were deposited in the beginning of marine transgression. Subsequent abrupt deepening led to deposition of a thick black shale unit, turbidite deposits and large-scale submarine channel-fill deposits. Coeval igneous activity formed the volcano-plutonic complex. The magmato-tectonic event was synchronous with the opening of the Japan Sea and the associated clockwise rotation of the southwest Japan arc sliver, recording a unique tectonic setting.
 
Article
The Sorachi Group, composed of Upper Jurassic ophiolite and Lower Cretaceous island-arc volcano-sedimentary cover, provides a record of Late Jurassic–Early Cretaceous sedimentation and volcanism in an island-arc setting off the eastern margin of the Asian continent. Stratigraphic changes in the nature and volume of the Sorachi Group volcanic and volcaniclastic rocks reveal four tectonic stages. These stages resulted from changes in the subduction direction of the Pacific oceanic plate. Stage I in the Late Jurassic was characterized by extensive submarine eruptions of tholeiitic basalt from the back-arc basin. Slab roll-back caused rifting and sea-floor spreading in the supra-subduction zone along the active Asian continental margin. Stage II corresponded to the Berriasian and featured localized trachyandesitic volcanism that formed volcanic islands with typical island-arc chemical compositions. At the beginning of this stage, movement of the Pacific oceanic plate shifted from northeastward to northwestward. During Stage III, in the Valanginian, submarine basaltic volcanism was followed by subsidence. The Pacific oceanic plate motion turned clockwise, and the plate boundary between the Asian continent and the Pacific oceanic plate changed from convergent to transform. During Stage IV in the Hauterivian–Barremian, in situ volcanism ceased in the Sorachi–Yezo basin, and the volcanic front migrated west of the Sorachi–Yezo basin.
 
(a) Felsite and granophyre in contact in hand sample (from McDonnell et al. 2004). (b) Thin-section photomicrograph of granophyre sample in PPL. (c) Thin-section photomicrograph of felsite sample in PPL. (d) Newry Granodiorite xenolith and xenocryst trails in a hand sample of felsite from Mullaghbawn Golf Course [IH 9880 1812]. (e) Newry Granodiorite xenolith and xenolith trail in a thin-section of felsite in PPL. (f) Thin-section photomicrograph of partial melting textures in a Newry Granodiorite xenolith in PPL.  
Article
The Palaeogene Slieve Gullion Igneous Centre in southern Armagh, Northern Ireland, consists of a layered central intrusive complex surrounded by a prominent and slightly older ring-dyke that intrudes both Lower Palaeozoic sedimentary rocks and the Caledonian Newry Granodiorite pluton (452 Ma). The ring-dyke comprises two major rock types: porphyritic felsite and porphyritic grano- phyre. We analysed both ring-dyke lithologies, both types of country rock, and a local Palaeogene basalt dyke sample for Sr and Nd isotopes. Trace element and whole rock data for this suite suggest that there are two distinct groups of both felsite and granophyre: one Si-rich and one Si-poor, most likely repres- enting two magmas from a zoned chamber and their mushy chamber wall equivalents (McDonnelletal. 2004). Isotope data show the low-Si rocks to be higher in radiogenic Sr than the high-Si rocks, which is inconsistent with a simple AFC-scenario of increasing sediment assimilation with higher degrees of differentiation. However, using MORB-type basalt as a starting composition, the low-Si ring-dyke rocks can be modelled through AFC with Lower Palaeozoic sedimentary rock as the contaminant. The decreasing 87Sr/86Sr trend from low-Si to high-Si dyke rocks, in turn, represents a second stage of con- tamination. Selective assimilation of the most fusible portions of Newry Granodiorite, which is lower in radiogenic Sr than the local sedimentary rocks, appears to be the most plausible solution. The Sr and Nd data are consistent with (a) at least a two-stage contamination history during upper crustal residence and storage, whereby fractionating magmas of basaltic and intermediate composition are contaminated by local sedimentary rocks, giving rise to rhyolite magmas that experience additional shallow contamin- ation by Newry Granodiorite, and (b) a zoned rhyolite magma chamber where high-Si magma is stored in the upper part of the chamber where crystallization and crustal contamination are most extensive.
 
Article
The Châteaupanne Unit belongs to the South Armorican domain of the Armorican Massif (France), which is part of the Variscan belt. This unit includes two Lower Devonian plant levels and one of them corresponds to the Basal Member of the Chalonnes Formation. A sedimentological and palaeontological analysis of these fossiliferous deposits from the Châteaupanne quarry (Montjean/Loire, Maine et Loire, France) is presented here for the first time. The age determination based on palynology indicates that the locality records the earliest occurrence of plant megafossils in the Armorican Massif. Their presence suggests an emergence event that has never been described before. Our study highlights the promising potential of the Basal Member of the Chalonnes Formation to aid in understanding these occurrences, and provides new insights into the history of the Variscan belt.
 
Article
The British Isles lay at a palaeolatitude of 4°S during the Early Carboniferous (Courceyan–Arundian) period. This paper examines fossil gymnosperm wood from ten localities in western Ireland and southern Scotland in order to analyse palaeoclimate. Fifty-two percent of the 77 fossil wood specimens studied exhibit growth rings that possess subtle, discontinuous ring boundaries and ring increments of narrow but variable width. These growth rings are qualitatively and quantitatively analysed, and are shown to bear a close similarity to growth rings in modern araucarian conifer woods; these araucarian growth rings are formed in response to tropical rainfall seasonality linked to monsoonal circulation. The findings of this study therefore support earlier palaeoclimatic interpretations, based on sedimentological evidence, which suggest that the British Isles experienced a monsoonal climate during the Early Carboniferous (Courceyan–Arundian) period.
 
Article
Sea surface temperature (SST) estimates using the C2An1r) are assessed for nine Atlantic Ocean sites. These are compared with SST estimates from fossil assemblages for the 3.29atmosphere General Circulation Model (GCM) for the same time interval. Most SST estimates derived from the 18O composition (18O data to be the product of: (a) calcite formed at a level deep within or below the ocean mixed-layer during the life-cycle of the foraminifera; (b) secondary calcite with higher 18Osw need to be refined.
 
Article
The study of volcanic rocks and igneous centres has long been a classic part of geological research. Despite the lack of active volcanism, the British Isles have been a key centre for the study of igneous rocks ever since ancient lava flows and excavated igneous centres were recognized there in the 18th century (Hutton, 1788). This led to some of the earliest detailed studies of petrology. The starting point for many of these studies was the British Palaeogene Igneous Province (BPIP; formerly known as the ‘British Tertiary’ (Judd, 1889), and still recognized by this name by many geologists around the globe). This collection of lavas, volcanic centres and sill/dyke swarms covers much of the west of Scotland and the Antrim plateau of Northern Ireland, and together with similar rocks in the Faroe Islands, Iceland and Greenland forms a world-class Large Igneous Province. This North Atlantic Igneous Province (NAIP) began to form through continental rifting above a mantle plume at c. 60 Ma, and subsequently evolved as North America separated from Europe, creating the North Atlantic Ocean.
 
Article
An enigmatic buried structure, located in Mesozoic sedimentary rocks in the Perth Basin, Western Australia, was discovered in 1990 by Ampol Exploration. The basin-like Yallalie structure (centred on 30 26' 40.3'' S, 115 46' 16.4'' E) is circular in plan view and about 12 km in diameter. High resolution,seismic-reflection profiles across the structure show a basin-shaped area of chaotic reflections that extend to a depth of approximately 2 km below the surface. The structure has sharp boundaries with surrounding faulted, but otherwise relatively undisturbed, rocks. In the centre of the structure there is an uplifted area approximately 3?4 km across, similar to those described from complex meteorite impact structures. The seismically defined structure coincides with a circular topographic depression, and image processing of digital elevation data has allowed recognition of concentric and radial structures extending as far as 40 km from the centre of the depression. Gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu. Aeromagnetic surveys have defined annular anomalies associated with the central uplifted section, and possibly margins, of the structure. A search for siderophile element enrichments (by neutron activation analysis) in the rocks of the structure, which would indicate the presence of a meteorite component, proved negative. Quartz grains in cores that penetrate the structure show the development of prismatic cleavage fractures and irregular, slightly curved planes formed by brittle fracture. An allochthonous breccia of Late Cretaceous rocks occurs a few kilometres west of the western margin of the structure. Quartz grains from a thin veneer of Tertiary sediments that drape the structure are essentially undeformed. However, multiple sets of closely spaced planar deformation features in quartz, characteristic of highly shocked rocks, have yet to be observed in the rocks of the Yallalie structure and the allochthonous breccia. The morphology of the Yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin. Geological and geochemical evidence is equivocal, but is not inconsistent with this interpretation.
 
Article
Compositional zoning and dissolution in cement is a direct response to the fluctuation of pore water chemistry, the variation of which during burial can be controlled by many factors, including the interaction between pore water and rock-forming minerals and the mixing of fluids from different origins. This paper suggests that tectonic activity can, by altering the hydraulic gradient, also influence pore water chemistry and lead to dissolution of cement, made clear by zoning within siderite crystals. Three different stages of siderite cement have been described from the Tirrawarra Sandstone in the Moorari and Fly Lake fields of the southern Cooper Basin, here referred to as S1 (early), S2 (middle), and S3 (late). Ragged dissolution surfaces separate the main phases, occurring after precipitation of S1 and S2 with incipient dissolution suggested within S2. Back-scattered electron (BSE) images and electron microprobe analyses clearly differentiate each main phase of siderite. S1 is a homogeneous, iron-rich siderite whereas S2 displays patchy compositional zoning associated with several minor dissolution stages, and S3 commences with even compositional banding and grades into a thick homogeneous phase in the terms of composition. Isotope analyses and fluid inclusion studies indicate that S1 formed at a temperature around 30 °C, S2 precipitated at a minimum temperature of 68 °C, and S3 formed around 102 °C. The heterogeneous, pitted and zoned S2 is thought to have formed during a time of active tectonism in the Cooper Basin, whereas the evenly banded nature of S3 suggests that it precipitated during a quiet tectonic period when pore waters largely remained relatively constant. It appears that siderite cements in the Tirrawarra Sandstone record tectonic activity in the form of irregular growth and dissolution highlighted by compositional zoning with stages of strong dissolution recording particularly active times when pore waters changed composition dramatically. Some zoning could be related in part to tectonic pulses. The temperature recorded by each of the siderite stages allows their precipitation to be tied to a burial history curve, and by making some simple assumptions about that history, the timing of cementation can be estimated. This can be an additional tool for calibrating the thermal history of an area.
 
Article
An almost complete but predominantly disarticulated ptyctodont fish, Kimbryanodus williamburyensis n. gen., n.sp. from the Late Devonian Gneudna Formation, is described. The fossils occur as three-dimensionally preserved isolated plates, and this has allowed the reconstruction of the fish. A taxonomic revision of the ptyctodonts was undertaken based on recently described Australian taxa and new reconstructions of Australian, American and European specimens. The phylogenetic analysis supports a threefold division of the ptyctodonts, with Rhamphodopsis being the most basal taxon and the other ptyctodonts divided into those possessing a median dorsal spine, spinal plate and simple V-shaped overlap of the anterior lateral and anterior dorsolateral plates and those taxa which do not.
 
Article
Jaw fragments bearing teeth from the Barremian of Boca do Chapim (Lisboa e Setubal Province, Portugal), originally considered as crocodilian and identified as Suchosaurus girardi by Sauvage, are redescribed and referred to the spinosaurid dinosaur Baryonyx , on the basis of comparison with Baryonyx walkeri , from the Barremian of England. This extends the geographical distribution of this unusual theropod genus to Portugal. Baryonyx appears to have been a frequent component of Early Cretaceous dinosaur assemblages in the Iberian region, which may have formed a biogeographical ‘stepping-stone’ for baryonychine dispersal between Europe and Africa.
 
Article
A little before 4 Ma ago, deposition of Pliocene and Pleistocene strata described as the Omo Group began in the Turkana and Omo basins of northern Kenya and southern Ethiopia. Soon after, basaltic magma erupted as thin lava flows, and intruded as dykes into the oldest Pliocene strata of the basin. These flows and intrusions are similar petrographically and geochemically, and mark a basaltic magmatic event spanning latitudes from 2degrees45'N to 6degrees45'N at a longitude of about 36degrees E. By 3.94 Ma, this basaltic magmatic activity had ceased. Previous researchers used these lavas as an important seismic marker in their study of the southern part of the Turkana Basin. Subsequent volcanic eruptions formed North, Central and South islands in Lake Turkana, and the Korath Range in southern Ethiopia. Thus there was a hiatus in basaltic magmatic activity of nearly 4 Ma in the area presently occupied by Lake Turkana and the lower Omo Valley, although volcanism continued on the eastern margin of the basin. Here we review the field occurrences of these basalts, their distinctive petrography, composition, age and significance to Pliocene deposition in the basin.
 
Article
The Wuyi orogenic belt in South China is one of the early Palaeozoic orogenic belts in the world. To provide new perspectives on its deformation and denudation, seven sandstone samples were collected from the upper Ordovician to Silurian sequence in the adjacent lower Yangtze foreland basin for U-Pb dating of detrital zircon. All of the samples reveal a dominance of zircons with ages of 880-740 Ma that correspond to the Neoproterzoic basement of South China, indicating basement-involved deformation in the Wuyi orogenic belt. A fair number of detrital zircons at 1200-900 Ma represent material recycled from the preorogenic strata. A significant population of c. 458-425 Ma zircons in the Silurian samples recorded contemporaneous orogenic activity and suggest that the synorogenic magmatic and metamorphic rocks had been exhumed. The youngest zircons at 425 Ma provide a maximum depositional age for the top of the foreland basin sequence. This isotopic age, together with constraints from graptolite zones, suggests the tectonic activity of Wuyi orogeny took place from 455 to 425 Ma. Compared with the Appalachian-Caledonian orogenic belt in timing, deformation styles, magmatism and metamorphism, we propose that there is some link between the two orogenic belts.
 
Article
Middle Triassic radiolarian bedded cherts in the Mino Belt, central Japan, include a sequence showing an abrupt facies change from the lower to the upper, where grey–black bedded cherts enriched in carbonaceous matter and framboidal pyrite are overlain by brick-red hematitic bedded cherts. Brownish-yellow chert enriched in goethite and purple-red chert occur at the boundary between the grey–black bedded cherts and the brick-red bedded cherts. This facies change is in accordance with stratigraphic variations of geochemical characteristics; the lower section grey–black bedded cherts, compared with the upper section brick-red bedded cherts, are enriched in C_tot and S_ tot, and are characterized by lower MnO/TiO_2, higher FeO/Fe_2O_3* (total iron as Fe_2O_3) and more variable Fe_2O_3*/TiO_2 values. Some of the lower section samples, in addition, are characterized by an enrichment in some transition metals (Ni, Cu, and Zn). The covariation of mineralogical and geochemical characteristics indicates that sedimentary environments and diagenetic processes were different between the lower and the upper section bedded cherts. During the deposition of the lower section bedded cherts, the sedimentary environment was anoxic and bacterial sulphate reduction occurred during the early diagenetic stage. In contrast, the upper section bedded cherts were subjected to less reducing diagenetic processes; active sulphate reduction did not occur. The change of sedimentary environment and diagenetic process at the site of deposition is likely to be attributed to the fluctuated concentration of dissolved oxygen in the water mass of a semi-closed marginal ocean basin, which was potentially caused by sea-level change that occurred during Middle Triassic time. Geological Magazine . v.135, n.6, 1998, p.735-753
 
Article
The Miocene–Quaternary Trans-Mexican Volcanic arc is thought to have grown southwards (i.e. trenchward) since the Pliocene. This theory is mainly supported by roughly N–S-directed polygenetic volcanic ranges along which volcanic activity migrates southwards with time. We investigated the eruptive history of one of these ranges, the Sierra Nevada (east boundary of Mexico City basin), by compiling literature ages and providing new K–Ar dates. Our K–Ar ages are the first ones for the northernmost Tláloc and Telapón volcanoes and for the ancestral Popocatépetl (Nexpayantla). The obtained ages reveal that the four stratovolcanoes forming the range worked contemporaneously during most of the Middle to Late Pleistocene. However, taking into account the onset of the volcanic activity, a southward migration is evidenced along the Sierra Nevada: volcanism initiated at its northern tip at least 1.8 Ma ago at Tláloc volcano, extended southwards 1 Ma ago with Iztaccíhuatl and appeared at its southern end 329 ka ago with the Nexpayantla cone. Such a migration would be most probably primarily driven by Cocos slab roll-back and steepening rather than by regional crustal tectonics, which played a secondary role by controlling the apparent alignment of the volcanoes.
 
(a) Regional map with national grid markings of the Southern Uplands Terrane showing main tectonostratigraphic units and tract-bounding faults. The likely correlations between the Berwickshire outcrops and the main outcrop to the southwest are also shown. Box indicates location of Figure 1b. SUF-Southern Uplands Fault; OBF-Orlock Bridge Fault; LF-Laurieston Fault. (b) Simplified sketch geological map with national grid markings of southeastern Scotland between Siccar Point and Burnmouth (after BGS map). Box shows location of Figure 2a. FC-Fast Castle; PW-Pettico Wick; LS-Linkim Shore.
Folds and associated structures of the Fast Castle-Pettico Wick coastal section. (a) Upright, subhorizontally plunging anticline typical of northwest end of coastal section, just west of Souter (fold 4 in Fig. 2). View looking northeast, horizontal field of view 50 m. The oldest rocks exposed in the section are found in the core of this fold. (b) NW-verging, subhorizontally plunging syncline typical of southeast end of coastal section, Heathery Carr (fold 13 in Fig. 2). View looking south, with cliffs approximately 70 m in height. Dotted line corresponds to axial trace of synform in cliff. (c) Typical gently fanning and refracted nature of spaced solution cleavage in folded interlayered mudstones and greywacke sandstones; note how cleavage development picks out upwards fining nature of graded sandstones, indicating that the folds face steeply upwards. View looking southwest, with pencil 160 mm long, foreshore west of Fast Castle. (d) Upright fold and cleavage showing cleavage-bedding intersection (in line with pencil) parallel to the gently SW-plunging fold hinge. View looking southwest, with pencil 160 mm long, foreshore west of Fast Castle.
(a) Stereoplots of structural orientation data from the Fast Castle-Pettico Wick area shown in Figure 2a. (i) Poles to bedding, with best-fit great circle (146/86NE) and regional β axis (04/236). (ii) Poles to fold axial planes and fold hinges, with mean fold hinge (08/240), axial plane (solid, 059/75NW) and cleavage plane (dashed, 058/80NW) shown. (iii) Poles to cleavage and bedding-cleavage intersection lineations (BCIL), with mean BCIL (03/241) and cleavage plane shown. (iv) Slickenfibre lineations from bedding-parallel shears with best-fit great circle shown (141/89NE) (b) Facing data from the Fast Castle-Pettico Wick section. (i) Fold and cleavage facing lines and azimuths plotted using the construction method of Holdsworth (1988). (ii) Plots showing the pitch of fold and cleavage facing directions in planes parallel to the fold axial/cleavage planes in which they were measured. Note that facing data are omitted from the region of folded cleavage associated with the synform at Pettico Wick (fold 21 on Fig. 2). In addition, the scarcity of NW-facing fold data reflects the relatively inaccessible nature of the southeastern part of the cliff section where most NW-verging folds occur (see Fig. 2b).
(a) Diagrammatic sketch showing cleavage (S1; dashed)–bedding (So; solid curved line) relationships around minor synclines and anticlines seen in the larger synclinal fold hinge zone exposed along the foreshore at Pettico Wick (fold 21 in Fig. 2). The bedding is right way-up. Viewed in fold profile plane. It is suggested that the cleavage is locally coaxially refolded due to migration of the fold hinges to produce local downward facing relationships in S1; note also minor SE-vergent folding of cleavage in regions of hinge migration. (b) Stereoplots of structural orientation data from the syncline at Pettico Wick. (i) Poles to bedding, with best-fit great circle (153/84NE; dashed) and β axis (06/243). Mean fold hinge (02/240) and axial plane (061/73SE)  
Regional map of the Southern Uplands terrane showing general distribution of timing of sininstral shear relative to primary phase of contraction. This changes across the Laurieston Fault (LF) and a regionally recognized zone of NW-vergent backthrusting and backfolding (after Barnes, Anderson & McCurry, 1987, and McCurry & Anderson, 1989, in southwestern Scotland) is consistently located in Gala Group rocks immediately north of this boundary, including those in Berwickshire. OBF-Orlock Bridge Fault, SUF-Southern Uplands Fault.
Article
Structures deforming Llandovery turbidites of the Gala Group in the Southern Uplands terrane are spectacularly exposed in the Berwickshire coastal section, southeastern Scotland. The upward-facing, upright to NW-vergent folds and associated structures appear to record a single regional phase of subhorizontal NW-SE contractional deformation, with a steeply dipping direction of bulk finite extension. These structures are markedly different from those developed in rocks correlated with the Upper Llandovery Hawick Group exposed some 5 km to the south in the Eyemouth-Burnmouth coastal section. Here a highly domainal system of sinistral transpressional strain occurs, with zones of steeply plunging curvilinear folds, clockwise cleavage transection and bedding-parallel sinistral detachment faults. The markedly different bulk strain patterns in the Berwickshire coastal sections are thought to reflect the regionally diachronous nature of transpressional deformation in the Southern Uplands terrane. There are striking similarities in the structures recognized in the Berwickshire coastal sections and those developed in stratigraphically equivalent units along strike in southwestern Scotland and Northern Ireland. This confirms the lateral structural continuity and correlation of tracts and tract boundaries along the entire length of the Southern Uplands terrane. The regional structure suggests that a phase of top-to-the-NW backtbrusting and backfolding associated with the southern margin of the Gala Group outcrop marks the transition from orthogonal contraction to sinistral transpression in the Southern Upland thrust wedge during late Llandovery times.
 
Article
Research on the British Paleocene Igneous Province (BPIP) has historically focused on the emplacement, chemistry and chronology of its elaborate central intrusive complexes and lava fields. However, the BPIP has also been dramatically shaped by numerous erosion, sedimentation and volcano-tectonic events, the significance of which becomes ever clearer as localities in the BPIP are re-investigated and our understanding of volcano-sedimentary processes advances. The resultant deposits provide important palaeo-environmental, palaeo-geographical and stratigraphical information, and highlight the wide range of processes and events that occur in ancient volcanic settings such as the BPIP. In this paper we review the sedimentary and volcano-tectonic processes that can be distinguished in the BPIP, and conceptualize them within a generalized framework model. We identify, and describe, the sedimentary responses to four broadly chronological stages in the history of the BPIP volcanoes: (1) the development of the lava fields, (2) early intrusion-induced uplift, (3) caldera collapse and (4) post-volcano denudation and exhumation of central complexes. We highlight and illustrate the range of sedimentary processes that were active in the BPIP. These operated on and helped shape a dynamic landscape of uplands and lowlands, of alluvial fans, braided rivers, lakes and swamps, and of volcanoes torn apart by catastrophic mass wasting events and/or caldera collapse.
 
Article
A much-revised Quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the Bandas del Sur, in southern Tenerife. New Ar-41/Ar-39 data obtained for the Arico, Granadilla, Fasnia, Poris, La Caleta and Abrigo formations are presented, allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments. We also provide a minimum age of 287 +/- 7 ka for a major sector collapse event at the Gaimar valley. The Bandas del Sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics, including widespread basal Plinian layers, predominantly phonolite composition, ignimbrites with similar extensive geographic distributions, thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns, lateral gradations into localized massive facies within palaeo-wadis, and widespread lithic breccia layers that probably record caldera-forming eruptions. Each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean. The succession indicates that Las Canadas volcano underwent a series of major explosive eruptions, each starting with a Plinian phase followed by emplacement of ignimbrites and thin ash layers, some of coignimbrite origin. Several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption. Because passage of each pyroclastic density current was characterized by phases of non-deposition and erosion, the entire course of each eruption is incompletely recorded at any one location, accounting for some previously perceived differences between the units. Because each current passed into the ocean, estimating eruption volumes is virtually impossible. Nevertheless, the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the Bandas del Sur succession and probably formed a complex, nested collapse structure. Detailed field relationships show that extensive ignimbrite sheets (e.g. the Arico, Poris and La Caleta formations) relate to previously unrecognized caldera collapse events. We envisage that the evolution of the nested Las Cahadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite-related caldera collapse events, and large sector collapse events, interspersed with edifice-building phases.
 
Location of studied sections and of the Alamo impact site against the Late Devonian palaeogeography (from Golonka, 2000, modified). 
High-resolution carbon isotope record from the late Emsian to late Frasnian. Column 1 – palaeotemperatures calculated from the oxygen isotope record of conodont apatite from Joachimski et al. (2004); column 2 – absolute ages from (A) Kaufmann (2006), (B) Sandberg & Ziegler (1996); column 3 – stratigraphy; column 4 – standard conodont zonation from Klapper & Becker (1999), column 5 – Events from House (2002) except Alamo event (this study); events characterized by short-term dysoxic or anoxic facies in their name areas are shown as black rectangles; column 6 – M.T.P. ( = Major Transgressive Pulses) from Johnson, Klapper & Sandberg, 1985; Boulvain & Herbosch, 1996; Racki, 1997; Gouwy & Bultynck, 2000. Curve A from sampling in Belgium (190 samples). Curve B from sampling in Poland (67 samples). Less precisely dated data set from Poland is arranged in intra-zonal time intervals, because several single brachiopod samples have been taken from sections difficult to correlate and/or some sampled successions are not very accurately biostratigraphically constrained (see  the two graphs are presented to reflect mean and maximal values for the particular stratigraphic intervals. Grey lines are located at the base of each cycle in relation with sea-level changes. The main object of our study (grey-filled segment) is detailed in Figure 3. U – Upper, M – Middle, L – Lower; E.D. – Early Devonian; Fam. – Famennian; Ems. – Emsian; herm. – hermanni ; falsio . – falsiovalis . 
High-resolution carbon isotope record of the Early–Middle Frasnian (upper part of cycle 5 to lower part of cycle 7 in Fig. 2) in Belgium (this study), Poland (this study), China (Zheng & Liu, 1997) and Moravia (Hladikova, Hladil & Zuskowa, 1997, followed by Ger sl & Hladil, 2004). For explanation see Figure 2. 
Impact events plotted against percent extinctions of ‘well-preserved’ marine genera through Eifelian to Famennian stages (Devonian), from Sepkoski (1996, fig. 6), modified; with kind permission of Springer Science and Business Media. The major punctata isotopic event is definitely not related to a mass extinction, despite the large scale of this carbon cycling perturbation. 
Article
New carbon isotopic data from the Devonian of Ardennes (Belgium) and partly from the Holy Cross Mountains (Poland) highlight an abrupt and high-amplitude negative excursion in the punctata conodont Zone. Published information from Moravia and China suggests that this Middle Frasnian negative excursion, jointly with the preceding large-scale positive shift, should be used as a global chemostratigraphic marker. Causation scenarios for this negative 'punctata Event' are correlated neither with major biota turnover nor major sea-level changes, but may be related to: (1) the Alamo Impact Event, that led to (2) the massive dissociation of methane hydrates and (3) the rapid onset of global warming.
 
Article
Carbon- and oxygen-isotope ratios are commonly used to correlate shallow- and deep-marine successions. Carbon- and oxygen-isotope analyses were performed on bulk-carbonate samples from two Kimmeridgian sections of the Swiss Jura platform in order to correlate them with biostratigraphically well-dated coeval sections in the adjacent basin. On the platform, a general decrease in δ¹³C and δ¹⁸O values from the base to the top of the studied interval is measured, whereas time-equivalent pelagic–hemipelagic carbonates record an increase in carbon- and oxygen-isotope ratios. Moreover, the measured δ¹³C and δ¹⁸O values are generally lower than those indicated for the Kimmeridgian open ocean and show high-frequency variations superimposed on the general trend. Samples were screened for diagenetic alteration using optical and cathodoluminescence petrography and coupled carbon- and oxygen-isotope and trace-element analyses. Some observations favour a role for diagenetic alteration, but isotopic and elemental trends as well as sedimentological evidence suggest that the more negative values of δ¹³C and δ¹⁸O relative to Kimmeridgian seawater are also due to local environmental conditions. High-frequency changes in δ¹⁸O and δ¹³C values most likely result from variations in salinity and carbonate production and accumulation rates. These variations were produced by different water masses that were isolated from the open ocean and developed their own geochemical signatures. Repeated isolation was induced by high-frequency sea-level fluctuations and helped by irregular platform morphology. Consequently, carbon- and oxygen-isotope records in shallow-marine carbonates can be used for stratigraphic correlation only if their origin is well known.
 
Top-cited authors
Hugh C. Jenkyns
  • University of Oxford
Andy Gale
  • University of Portsmouth
Martin D Brasier
  • University of Oxford
Adrian W.A. Rushton
  • Natural History Museum, London
Simon Leigh Harley
  • The University of Edinburgh