• The eastern French Pyrenees: from mountain belt to foreland basin

      Satterfield, Dorothy; Rollinson, Hugh; Suthren, Roger; University of Derby (Wiley, 2019-12-09)
      The Pyrenees is a young mountain belt formed as part of the larger Alpine collision zone. This excursion explores the development of the Pyrenean Mountain Belt in southern France, from its early extensional phase in the mid‐Cretaceous and subsequent collisional phase, through its uplift and erosion in the Late Cretaceous and again in the Eocene, which led to the development of the Aquitaine‐Languedoc foreland basin. One of the complexities of the Pyrenean Belt is that thrusting, uplift and erosion during the Pyrenean orogeny exposed older Variscan basement rocks in the central core of the mountains, rocks which were metamorphosed during an earlier event in the late Carboniferous. Thus, this orogenic belt also tells the story of an earlier collision between Laurussia in the north and Gondwana in the south at c. 300 Ma, prior to the onset of the Pyrenean events at c. 100 Ma. Here we seek to unravel these two separate orogenic stories.
    • Micro-analytical perspectives on the Bishop Tuff and its magma chamber.

      Chamberlain, Katy J.; Wilson, Colin J. N.; Wallace, Paul J.; Millet, Marli Bryant; Victoria University of Wellington; Durham University; University of Oregon (Oxford Academic, 2015-04-23)
      New in situ major and trace element analytical data are presented for crystals (sanidine, plagioclase, biotite, orthopyroxene, clinopyroxene) and matrix glasses from juvenile materials representing the full Bishop Tuff sequence from the earliest fall unit (F1) to the latest ignimbrite package (Ig2Nc). These data are combined with published information to investigate the nature and zonation of the pre-eruptive Bishop magma chamber. Our data confirm that this magma chamber was a single unitary body that was thermally and compositionally zoned. The zonation was largely established prior to the growth of crystals, and also prior to mixing in the lower parts of the chamber induced by late-stage intrusion of a magma of contrasting composition and slightly higher temperature (the ‘bright-rim’ magma). Sparse mixed swirly and dacitic pumices show enrichments in Ba, Sr and Ti that identify these pumices as possible representatives of the ‘bright-rim’ magma. A model (revised from previously published work) for the pre-eruptive magma chamber comprises three main parts: (1) an upper, volumetrically dominant (∼2/3), relatively unzoned region that was the source of the earlier, eastern-erupted ignimbrite units and their coeval fall units; (2) a volumetrically minor transition zone that shows evidence for minor degrees of mixing and was the dominant source for the latest, eastern-erupted part of Ig1Eb (Sherwin subunit) and the earlier part of the northern-erupted ignimbrite (Ig 2Na); (3) a lower, volumetrically subordinate (∼1/3) region that was affected by mixing with the ‘bright-rim’ invasive magma in the lead-up to the eruption, and fed later northern-erupted units. Ingress of the ‘bright-rim’ magma introduced orthopyroxene and bright-rimmed zircon crystals, and induced partial resorption then overgrowth of rims enriched in Ti, Sr and Ba on sanidine and quartz, and development of zoning in clinopyroxene. Based on pumice proportions and associated crystal and glass chemistries through the eruptive sequence, we infer that the roof and floor of the magma chamber were stepped down to the north, such that the transition zone magma formed the floor of the southern part of the melt-dominant chamber and the roof of the northern part. Our data reinforce the previous concept of a single compositionally and thermally zoned Bishop magma chamber and additionally support a temporally constrained role for pre-eruptive magma mixing and the introduction of melts and minerals with contrasting compositions to the resident Bishop magma.
    • New perspectives on the Bishop Tuff from zircon textures, ages and trace elements.

      Chamberlain, Katy J.; Wilson, Colin J. N.; Wooden, Joseph L.; Charlier, Bruce L. A.; Ireland, Trevor R.; Victoria University of Wellington; Stanford University; The Open University; The Australian National University (Oxford Academic, 2013-12-19)
      We present zircon textural, trace element and U–Pb age data obtained by secondary ion mass spectrometry (SIMS) (SHRIMP-RG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)–(c) combined have a weighted mean age of 777·9 ± 2·2 ka (95% confidence) and a tail back to ∼845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766·6 ± 3·1 ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (767·1 ± 0·9 ka, 2σ) and 40Ar/39Ar (767·4 ± 2·2 ka, 2σ) techniques, the latter using the 28·172 Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862 ± 23 ka (95% confidence), significantly older than the currently accepted 790 ± 20 ka K–Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838·5 ± 8·8 ka (95% confidence), implying that the Bishop Tuff system was active for only ∼80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3× concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc + Y + REE3+)/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of the Ti-in-zircon thermometer, in addition to variations inherent to the 0·15–0·67 range in values proposed for aTiO2. The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ∼10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and 40Ar/39Ar techniques.
    • Origin and evolution of silicic magmas at ocean islands: Perspectives from a zoned fall deposit on Ascension Island, South Atlantic.

      Chamberlain, Katy J.; Barclay, Jenni; Preece, Katie; Brown, Richard J.; Davidson, Jon P.; University of Durham; University of East Anglia (2016-11-15)
      Ascension Island, in the south Atlantic is a composite ocean island volcano with a wide variety of eruptive styles and magmatic compositions evident in its ~ 1 million year subaerial history. In this paper, new observations of a unique zoned fall deposit on the island are presented; the deposit gradationally changes from trachytic pumice at the base, through to trachy-basaltic andesite scoria at the top of the deposit. The key features of the eruptive deposits are described and are coupled with whole rock XRF data, major and trace element analyses of phenocrysts, groundmass glass and melt inclusions from samples of the compositionally-zoned fall deposit to analyse the processes leading up to and driving the explosive eruption. Closed system crystal fractionation is the dominant control on compositional zonation, with the fractionating assemblage dominated by plagioclase feldspar and olivine. This fractionation from the trachy-basaltic andesite magma occurred at pressures of ~ 250 MPa. There is no evidence for multiple stages of evolution involving changing magmatic conditions or the addition of new magmatic pulses preserved within the crystal cargo. Volatile concentrations range from 0.5 to 4.0 wt.% H2O and progressively increase in the more-evolved units, suggesting crystal fractionation concentrated volatiles into the melt phase, eventually causing internal overpressure of the system and eruption of the single compositionally-zoned magma body. Melt inclusion data combined with Fe–Ti oxide modelling suggests that the oxygen fugacity of Ascension Island magmas is not affected by degree of evolution, which concentrates H2O into the liquid phase, and thus the two systems are decoupled on Ascension, similar to that observed in Iceland. This detailed study of the zoned fall deposit on Ascension Island highlights the relatively closed-system evolution of felsic magmas at Ascension Island, in contrast to many other ocean islands, such as Tenerife and Iceland.
    • Sedimentology of the Late Cretaceous in the Western Aude Valley, Southern France

      Satterfield, Dorothy; Suthren, Roger; University of Derby (British Sedimentological Research Group, 2014-12)
      The closure of the Tethys seaway between Iberian and European plates and early stage development of the Pyrenees produced continental sedimentation in the Western Aude Valley south of Carcassonne. Although timing is difficult in continental deposits, fluvial sedimentation occurred in much of Campanian time and changed gradually to lacustrine/ palustrine by Maastrichtian time. The aim of this poster is to assess sedimentary changes laterally and over time. Sedimentary logs over an area of 16 x 30 km have been measured to correlate main bodies of sandstone and initiation of carbonate sedimentation. Siliciclastic deposits can be grouped into 4 facies: trough cross-bedded conglomerate, massive conglomerate, trough cross-bedded sandstone and horizontally bedded sandstone. They suggest that they were formed in a high energy, low sinuosity braided river system with significant topography and coarse sediment supply. Breaks in coarse sedimentation, characterized by paleosols and fluvial stromatolites, suggest episodic high-energy sedimentation and are consistent with semi-arid climate hypothesized for the Campanian. Above coarse Campanian sediments are Maastrichtian-aged thick marls and several-metre sandstone beds. Marl deposits indicate generally lower-energy conditions and contain dinosaur bones whereas sandy beds can be rich in dinosaur egg fragments and Microcodium associated with vegetation. Fine-grained carbonates and marls continue to the end of the Cretaceous. The presence of charophyte stems and gyrogonites, lack of desiccation cracks and medium gray colour with some preserved organic matter suggest lacustrine carbonates. Above and often interbedded with gray carbonate is mottled pink, yellow and cream limestone with roots, nodules and desiccation cracks interpreted to be marginal lacustrine to palustrine. The interbedded nature of these deposits suggests fluctuating water level controlling lacustrine/palustrine cycles and episodic high-energy clastic input. Relatively thin gray carbonate may mean perennial lakes were shorter-lived whereas palustrine conditions persisted over a longer time. The Tethys seaway existed in the area until Santonian time, but was closed by the Campanian due to the IberiaEurope collision creating a continental basin. Early stage of Pyrenean uplift and moderate topographic relief promoted fluvial deposition in the basin. Palaeocurrents and clast types will be assessed to differentiate among potential source mountains to the north (Montagne Noire), east (Massif de Mouthoumet) and/or south (Pyrenees). Over time, sediment transport energy decreased suggesting lower topography and, possibly, sufficient uplift to the south to confine lacustrine deposits. Infill logging next field season will attempt to define Campanian and Maastrichtian sediment sources, lake boundaries, dinosaur nesting areas and the transition to early Paleocene deposits.
    • Teaching sedimentology: opportunities for interdisciplinary, variety, innovation and employability.

      Davies-Vollum, K. Siân; Satterfield, Dorothy; Suthren, Roger; Whiteley, Martin; University of Derby (British Sedimentological Research Group, 2015-12)
      The breadth of content and skills embodied by the subject of sedimentology provides the opportunity teach in multiple learning environments, engage in innovative teaching practice and embed employability skills. Field and practical-based work are essential components of sedimentology and provide opportunities to teach in different environments outside the normal classroom setting. This allows the inclusion of a variety of learning experiences, which can in turn address different student learning styles. Field-based studies in particular create learning environments that can contribute to transformative learning experiences. The emphasis on field and practical based learning experiences in sedimentology promotes experiential learning, founded on the tenets of Kolb’s learning cycle. For example field examination of clastic sequences can be used to determine their economic potential as oil, gas or water reservoirs, thus connecting experiential learning in the field with theoretical calculations. The use of a variety of teaching environments can also facilitate experimentation with innovative teaching practice. Teaching outdoors or in a laboratory or practical class setting opens up possibilities for using technology that may not be possible in a standard classroom setting. For example students can create mini documentaries in the field that focus on modern sedimentary environments and structures using simple equipment, multimedia presentation techniques and software. Sedimentology requires the development of a variety of field, practical, quantitative and problem solving skills. These skills are highly transferrable and can help build student employability. For example, students develop practical, geoscience specific skills in the study of an oil well, combining analysis and interpretations of thin sections, core and wireline data; in grain size analysis exercises they develop more generic statistical skills. Teaching sedimentology gives the instructor scope to create innovative, experiential learning exercises and assessments in which transferrable skills can be embedded across a variety of learning environments. The subject provides a rich learning experience for students and a stimulating teaching environment for instructors.
    • Timescales of mixing and mobilisation in the Bishop Tuff magma body: perspectives from diffusion chronometry.

      Chamberlain, Katy J.; Morgan, Daniel J.; Wilson, Colin J. N.; Victoria University of Wellington; University of Leeds (Springer, 2014-07-01)
      We present two-feldspar thermometry and diffusion chronometry from sanidine, orthopyroxene and quartz from multiple samples of the Bishop Tuff, California, to constrain the temperature stratification within the pre-eruptive magma body and the timescales of magma mixing prior to its evacuation. Two-feldspar thermometry yields estimates that agree well with previous Fe–Ti oxide thermometry and gives a ~80 °C temperature difference between the earlier- and later-erupted regions of the magma chamber. Using the thermometry results, we model diffusion of Ti in quartz, and Ba and Sr in sanidine as well as Fe–Mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these crystal phases. Diffusion profiles of Ti in quartz and Fe–Mg in orthopyroxene both yield timescales of <150 years for the formation of overgrowth rims. In contrast, both Ba and Sr diffusion in sanidine yield nominal timescales 1–2 orders of magnitude longer than these two methods. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. growth zoning exists). Utilising the divergent diffusion behaviour of Ba and Sr, we place constraints on the initial width of the interface and can refine our initial conditions considerably, bringing Ba and Sr data into alignment, and yielding timescales closer to 500 years, the majority of which are then within uncertainty of timescales modelled from Ti diffusion in quartz. Care must be thus taken when using Ba in sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling. Our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the Bishop Tuff magma chamber at least 500 years prior to eruption. Timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1–2 orders of magnitude, or that the minerals concerned did not experience a common history, despite being extracted from the same single pumice clasts. Introduction of the magma initiating crystallisation of the contrasting rims on sanidine, quartz, orthopyroxene and zircon was prolonged, and may be a marker of other processes that initiated the Bishop Tuff eruption rather than the trigger itself.