Showing papers by "Malcolm B. Hart published in 2006"

Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995-2003): What happens when pyroclastic flows enter the ocean?

Abstract: The Soufriere Hills volcano, Montserrat, West Indies, has undergone a series of dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the ocean. Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow over distances >30 km from the source. The total volume of pyroclastic material off the east coast of Montserrat exceeds 280 × 106 m3, with 65% deposited in proximal lobes and 35% deposited as distal turbidites.

The Global Boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic), Wine Haven, Yorkshire, UK.

Abstract: Following votes by the Pliensbachian Working Group, the Jurassic Subcommission and the International Commission on Stratigraphy, IUGS rati.ed the proposed Global Boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic) at the base of bed 73b in the Wine Haven section, Robin Hood's Bay, Yorkshire Coast, UK. This level contains the characteristic ammonite association Bifericeras donovani Dommergues and Meister and Apoderoceras sp. Complementary data include: a) Strontium-isotope stratigraphy, based on analysis of belemnites which yield a calcite 87Sr/86Sr ratio for the boundary level of 0.707425 and data supporting interpretation of continuous sedimentation; b) Belemnite oxygen-isotope data indicate a signi.cant temperature drop (~5 oC) across the boundary at this locality; c) A Transgressive Systems Tract (TST) initiated in the Aplanatum Subzone (uppermost Sinemurian) continues into the Lowermost Pliensbachian (Taylori Subzone); it forms part of a transgressive facies cycle sensu Graciansky et al. (1998); d) The Upper Sinemurian to lowermost Pliensbachian at Wine Haven section has a predominantly normal magnetic polarity, but two discrete reversed polarity magnetozones are present. The .rst spans much of the latest Sinemurian Aplanatum Subzone. It terminates <0.5m below the Sinemurian-Pliensbachian boundary and may prove a valuable chronostratigraphic marker. The second extends from the latest Oxynotum Subzone probably through to the lower part of the Raricostatum Subzone.

The origin and early evolution of planktic foraminifera

Abstract: In the 1960s and 1970s Oberhauser and Fuchs described a range of new species from Austria, Northern Italy and Poland that were described as the “earliest” planktic foraminifera. Since the publication of these papers there has been a debate as to the validity of both the species and the interpretation of them as the earliest part of the Mesozoic plankton. Some authors have dismissed them, almost out of hand, while others have defended them as the ancestral forms of the planktic foraminifera. In the last two years all the type specimens have been inspected and photographed in an environmental SEM. This has allowed the close matching of the original drawings (almost all of which are very accurate) with the new photographs. The evolutionary trends detected by Fuchs are almost certainly correct, although it is clear that the majority of the taxa are benthic in character. That said, it is also clear that Oberhauserella and Praegubkinella are possible ancestors of the Jurassic planktic fauna that appears at, or about, the level of the Toarcian oceanic anoxic event in Europe. Records of such taxa in other parts of the world (e.g., N.W.Australia – Apthorpe [pers. comm.]) have still to be fully assessed. It is quite clear that, by the Bajocian and Bathonian (in the Middle Jurassic) there was a relatively diverse fauna of Conoglobigerina (and possibly Globuligerina) present in oceanic sediments deposited over a wide area of the Tethyan Realm. In the mid-Upper Jurassic this fauna diversified and expanded its range as new ocean basins (e.g., the North Atlantic Ocean) developed. In places, such as the Carpathians of Southern Poland, some of these Jurassic limestones yield assemblages that can only be described as Jurassic “Globigerina ooze”, so abundant is the fauna. There are very rare occurrences in the Upper Jurassic, but across the Jurassic-Cretaceous boundary the record is very incomplete. This gap in our knowledge is quite critical as the Jurassic taxa are regarded as being aragonitic in test composition, while those in the Lower cretaceous are calcitic. While there are still many questions to resolve, new data from Jurassic sediments throughout Europe are providing answers to the evolution of the planktic foraminifera. Anuár io do Inst i tu to de Geociências UFRJ ISSN 0101-9759 Vol. 29 1 / 2006 p. 167

Foraminifera and the tephrochronology of marine sediments around the Island of Montserrat, Lesser Antilles volcanic arc

Abstract: The Soufriere Hills Volcano, Montserrat, has undergone cyclic dome growth and collapse events since the present phase of eruption began in 1995. Over 90% of the pyroclastic deposits avalanched down the volcano flanks and were deposited into the ocean. The coarse components (pebbles to boulders) are deposited in dense slurries to form steep-sided, near linear ridges proximally that intercalate and amalgamate to form a submarine fan. The finer ash grade components are mixed into the overlying water column to form turbidity currents that flow distances in excess of 30 km from source. The present on-going eruption began in 1995 but marine sediment coring has recovered a record of intermittent volcanic activity over the last ~350,000 years. Using foraminiferal biostratigraphy (Globorotalia menardii ratios, etc.) and stable isotope stratigraphy we have identified significant eruptive events at 330,000 years BP, 240,000 years BP, 182,000 years BP, 137,500-120,000 years BP, 77,500 years BP and 20,000 years BP. The most interesting record is the 17,500 years of eruption activity between 137,500 and 120,000 years BP which agrees with the date for the origin of the Soufriere Hills volcanic centre. Work on the marine cores is on-going and will provide further information on the distribution of both planktic and benthic foraminifera, the impact of the volcanic sedimentation on the benthic faunas and a detailed analysis of the pteropods distribution in the samples, which appears to be recording a distinct climatic signal as well as a record of the movement of the Aragonite Compensation Depth within the water column.

Armstrong, H. A. & Brasier, M. D. 2005. Microfossils, 2nd ed.viii + 296 pp. Malden, Oxford, Carlton: Blackwell Publishing. Price £32.99 (paperback). ISBN 0 632 05279 1.

Abstract: In the words of the authors of this new book ‘In the 25 years since the first, highly successful, edition of Microfossils was published there have been significant advances in all the areas of understanding of microscopic life and their fossil counterparts’. This is certainly true and, over much of that 25 years, Microfossils (Brasier, 1980, with reprints in 1983, with reprints in 1988, with reprints in 1992) has been the standard reference for most UK students of micropalaeontology. Throughout most of that time, however, Microfossils was (almost) the only viable text available as other volumes (Haq & Boersma, 1978; Tappan, 1980; Haynes, 1981; Bignot, 1982 (and 1985 reprint); Traverse, 1988; Murray, 1991; Lipps, 1993; Jenkins, 1993; Jones, 1996; Sen Gupta, 1999; Martin, 2000; Haslett, 2002) were either too expensive for the average student, were not readily available or were too limited in scope for the ‘general’ student reader looking for a book …