Peter Homewood

Evolution of early planktic foraminifers

Abstract The evolution of Mesozoic species of planktic foraminifers, particularly the succession of their morphotypes and the variation in their diversity and their abundance, is shown to be related to the adaptation of evolutionary strategy to fluctuations in the oceanic environment. During periods of stress (“oligotaxic period”) the more primitive species tend to invade the oceanic surface waters by means of r-selection. During stable “polytaxic” conditions, the same species engage in an adaptive radiation colonizing progressively deeper water through K-selection. The succession of morphotypes from the middle Jurassic to the end of the Cretaceous is compared with living planktic foraminifers and related to the biogeographic (cold arctic, warm equatorial faunas) and bathymetric distribution. A general model for the evolution of early planktic foraminifers is made with reference to the preceding observations, and in relation to the pattern of variation of trophic resources.

SwissSed: past, present, and future trends in Swiss sedimentology

Sedimentology and Switzerland have had a long history together. Some of the names of the many precursors are well known and others less so, but it is certainly not the aim here to carry out any properly documented historical research on the subject. However, with regard as to how sedimentology has evolved since those early roots, it is appropriate to recall the work by Bernhard Studer (1827) linking sedimentology to alpine tectonics; by Amanz Gressly (1838) on outcrop-based facies sedimentology of reefal systems and regional palaeogeography; by FrançoisAlphonse Forel (1892, 1895, 1904) on depositional processes such as density currents, seiche tides and the organic carbon cycle in lacustrine settings; by Jean de Charpentier (1841) and Louis Agassiz (1840) on glaciation and glacial deposits; by Arnold Heim (1932) on rock falls and debris flows; by Arnold Bersier (1958) on fluvial and deltaic systems in the Molasse; and by Augustin Lombard (1956) on the grouping of facies successions into abstract models. These studies were based on meticulous examination and recording of sedimentary rocks in outcrops, as well as observations on modern depositional environments and processes, linking the present and the past while establishing the similarities but also the striking differences between them. They stand out as having forged major links between facies and depositional processes, establishing depositional models and characterising sedimentary architecture. But most of this was well before the development of sedimentology as a distinct branch of science. The founding of the International Association of Sedimentologists (IAS) in 1952 is perhaps the best marker of the moment in time when sedimentology was recognised as an independent scientific skills-set, standing on its own, apart from but related to palaeontology, mineralogy and stratigraphy. From this time on, and with the 5th International Congress of Sedimentology being held in Geneva in 1958 (Eclogae Geol. Helv. 1959), Sedimentology became a normal component of teaching at universities in Switzerland. The development over several decades of novel analytical techniques accompanied the slow but steady growth of student numbers. At the same time, advances in physics, chemistry, biology, engineering and ship-based drilling and exploration (sedimentology and industry have also maintained a strong relationship) brought laboratory studies to the fore in sedimentology. This was most evident where geochemistry and stable isotopes are involved. These laboratory-based studies then became a focus of research in their own right compared to the more classical outcrop and thin-section studies, rather than bringing just a complement to observations made in the field or under the microscope. And so it was that with the growing numbers of students and staff carrying out sedimentological research, the idea of SwissSed, a loosely structured and informal group of junior staff and graduate students in Switzerland, was mooted in the mid 1980s to share ideas and results between researchers, on their outcrops. However, this emphasis on field-based settings proved impractical, and the group finally consolidated around yearly in-house meetings at the University of Fribourg with presentations given as talks and posters, rather than getting together for field trips.

Internal Geometry of Sand Waves: A Comparison Between Modern and Fossil Examples: ABSTRACT

Recent developments in acquiring and processing very high-resolution geophysical data help us better understand large subtidal sand waves of the French continental shelf. They are compared with ancient analogs, especially from the Miocene Swiss Molasse. Internal structure, interpreted from seismic sections, vibracorings, and large outcrops, shows a hierarchy comparable to aeolian dunes. (1) Steep (25/degree/-30/degree/) reflectors, dipping leeward, are interpreted as foreset beds. Vibracoring shows that in modern cases they consist of alternating layers of medium- and coarse-grained sand, similar to those produced by sand avalanching. These deposits give the highest porosity values in the central body of the sand wave. They are comparable to the Miocene sand waves of the Swiss Molasse. (2) Erosional reflectors, dipping at lower angles cut across the foresets, are interpreted as reactivation surfaces created by high-energy events (equinox tides, added tidal and wave effects) rather than by the semidiurnal currents occasionally preserved in fossil sand waves. (3) Subhorizontal reflectors were probably created by truncation of sand waves during major storms. Fossil analogs more like larger present-day sand waves might be difficult to recognize due to the complex internal architecture of the sand body.