Programme Notes for Indoor Meetings 2005/2006
Notes from the lecture given on Wednesday 19th October 2005
The Ivrea-Verbano crustal section, NW Italy: a window into the lower continental crust
Dr Kate Brodie, School of Earth, Atmospheric and Environmental Sciences, University of Manchester.
Knowledge of the deformation behaviour of the rocks of the middle and lower continental crust is essential to the understanding of tectonic processes but we have to rely largely on remote imaging using geophysical techniques for much of our information. The Ivrea-Verbano (I-V) zone, some 100 km long and 15 km wide, situated in the inner arc of the western Alps, has long been considered to be an upended but relatively undisrupted cross-section through the lower and middle continental crust, providing one of a few areas where such rocks can be examined.
Detailed mapping, sampling and seismic modelling of this region has allowed information regarding the nature of the lower continental crust to be deduced and more recently the complex structure of the area to be interpreted. Metamorphic grade increases from the structurally higher part of the section, lying to the south-east, to the lower part, lying to the north-west. To the north-west the complex is bounded by the Insubric fault, and to the south-east by the Cossato - Mergozzo—Brissago (CMB) fault, that separates it from the structurally higher mid-crustal Serie dei Laghi. The lower part of the section is dominated by mafic and metasedimentary granulite facies rocks, with minor ultramafic bodies. It is intruded by a large basic igneous complex, layered in its lower part, and interpreted to represent mafic underplating during early Permian, east-west crustal extension that was accommodated by movement on high temperature shear zones. Subsequent large-scale folding occurred in association with the Alpine orogenesis and emplacement of the I-V zone into its present position, so that the present vertical attitude of the region is due to the formation of a large monocline, with its axial trace lying some 5km south-east of the CMB fault.
The model proposed here for the I-V zone structure explains the curious juxtaposition of four large antiformal structures in the 10 km long cross-section through the region that can be seen in the Valle d’Ossola (2 km of topographic relief), apparently without any intervening synforms!
***** Click here to see a block diagram which illustrates very well the complexity of Dr Brodie’s researches over the last 30 or so years. *****
Saturday 12th November 2005: “Evolution of the Cheshire Basin”
DUE TO CIRCUMSTANCES BEYOND OUR CONTROL, THIS SEMINAR HAS BEEN CANCELLED.
I HAVE RETAINED THE PROGRAMME NOTES.
The Cheshire Basin is part of a major rift system formed during an east-west extensional episode starting in Permian times and continuing to the early Cretaceous, with the main extension in the early Triassic. Sediment collecting in this half-graben formed, at its greatest thickness, a 6,000m sedimentary pile. During subsequent uplift most of the Jurassic marine sequence was removed leaving over 4,500m of Permo-Triassic red beds.
The lower part of the Triassic basin fill, known as the Sherwood Sandstones, consists of aeolian and fluvial sediments. The fluvial sediments, containing metamorphic, igneous and sedimentary clasts, formed in a northward-flowing river system draining the Armorican and Cornubian highlands to the south, with further input from tributaries on either side of the rift valley system (the present-day Welsh Borders and Pennines). Aeolian deposits were blown in from the Pennine area to the east.
The Mercia Mudstones overlie the Sherwood Sandstones, and consist of muds and evaporites formed in playas along the deeper axial parts of the basin. Three thick mudstone formations separate two economically important salt beds (halite) and many of lesser thickness, and also many thin beds of gypsum and anhydrite.
Pervasive barite cements and sulphide mineralisation formed in the Sherwood Sandstones during the late Triassic to early Jurassic when metal-rich brines descending from the Mercia Mudstones mixed with reducing fluids rising along fractures from the Carboniferous basement. The porosity of the Sherwood Sandstones makes them ideal aquifers and, together with the Carboniferous source rocks and Mercia Mudstone seals, potential hydrocarbon reservoirs.
Life, never abundant in arid and semi-arid environments, has even left its mark in the form of Rynchosaur and Cheirotherium footprints in the muds surrounding waterholes in Sherwood Sandstones times, and worm and other burrow traces in the Mercia Mudstones.
DUE TO CIRCUMSTANCES BEYOND OUR CONTROL, THE ABOVE SEMINAR HAS BEEN CANCELLED. I HAVE RETAINED THE PROGRAMME NOTES.
Saturday 10th December 2005: “Lead Mining in the Peak: Preserving the Heritage”
The Derbyshire Orefield, one of the largest and richest orefields in Britain, has played a major part in the local and national economy for many centuries. The limestone of the White Peak of Derbyshire formed as sediment in a tropical sea over 300 million years ago during the Carboniferous period, when Britain lay close to the equator. Various creatures, such as corals, brachiopods and crinoids, lived in the warm clear water, and their remains accumulated in the calcareous mud, which eventually hardened to form limestone.
Lead and other ores were deposited as a result of mineralising fluids migrating into faults and fissures within the limestone some 270 million years ago. Within the Peak District major veins, vertical ones being known as rakes, horizontal deposits, called flats, and irregular infillings of cavities, called pipes, host minerals such as galena, fluorspar, barytes and calcite, but with many others also occurring.
Exploitation of these ores began at least in the Bronze Age (with copper mining proven from Ecton near Warslow, ca. 2000 – 1500 BC), continued through Roman times (with several lead ingots, or pigs, found locally and as far away as Normandy), Saxon and Norman times (the Wirksworth mines were controlled from the Mercian abbey at Repton; the Domesday Book of 1086 records lead mining at Matlock, Wirksworth and Bakewell).
By the seventeenth century mines had become much larger and deeper; so much so that flooding became a major problem, to solve which drainage levels (or soughs) were driven. Various innovations such as pumping engines and the use of gunpowder led to a great increase in ore production during the seventeenth and eighteenth centuries, the heyday of the lead mining industry in Derbyshire. The industry went into terminal decline in the latter part of the nineteenth century as ores could be extracted more cheaply elsewhere.
The remains of this important industry are still to be seen in many parts of the Peak District today. However, within the last few years it has been realised that only twenty-five percent of these surface remains have survived, and these continue to disappear at an alarming rate.
In this seminar we will look at the geological history of the Derbyshire Orefield, view the deposits of various minerals to be seen underground, together with the surface remains, before finding out what the Peak District National Park Authority, in conjunction with English Heritage and English Nature, are doing to conserve this heritage.
Saturday 14th January 2006: “Exceptional Fossil Sites of Germany”
In their recent book, “Evolution of Fossil Ecosystems”, the authors, Paul Selden and John Nudds, describe a number of localities around the world where the exceptional state of preservation of fossils has enormously progressed our understanding of the evolution of life on Earth. In this afternoon seminar they will be reviewing four celebrated sites in Germany: -
The Hunsrück Slate – The slates of the Hunsrück Mountains are comprised of a thick sequence of early Devonian muddy marine sediments in which a slaty cleavage developed through low-grade metamorphism during Carboniferous times. The slates contain a rich fauna of fish, echinoderms and arthropods, often in an excellent state of preservation due to pyritisation, a circumstance allowing extremely clear radiographs of the fossils to be produced.
The Holzmaden Shale – The Posidonia shales around the village of Holzmaden, near Stuttgart, contain an abundant and sometimes completely preserved biota of early Jurassic marine reptiles (ichthyosaurs, plesiosaurs, pliosaurs and marine crocodiles), fish, and much rarer pterosaurs, often with the outline of the skin and soft body tissues clearly visible.
The Solnhofen Limestone – In the late Jurassic Solnhofen Limestone of Bavaria some spectacular fossils, though not common, have been found, including the delicate remains of plants, a wide range of marine and terrestrial invertebrates (for instance, preserving the fragile wings of dragonflies and the soft tentacles of squid), fish, marine reptiles, flying reptiles (sometimes complete with wing membranes), and, most famous of all, the only known examples of Archaeopteryx.
Grube Messel – Mammals are perhaps the best-known fossils from the Grube Messel (or Messel Pit), a disused open pit for brown coal near Darmstadt, and include early relatives of the hedgehog, as well as horses, primates, bats and ant-eaters. The avian fauna includes owls, swifts, rollers and woodpeckers. Beetles and other arthropods commonly show beautiful colours and patterns. In its entirety the fossils record life around an early Eocene lake.
Autographed copies of the book, “Evolution of Fossil Ecosystems” will be available for purchase on the day.
Look at the Photo Gallery to see some of these fossils.
Wednesday 12th April 2006: "Unravelling Folds at Rhoscolyn, Anglesey"
Dr. Sue Treagus, University of Manchester
This talk takes the audience to Holy Island, North West Anglesey, to the Rhoscolyn Anticline, one of the best areas in Britain to study major and minor folds in low-grade metamorphic rocks. We will traverse the Rhoscolyn Anticline to reveal the intriguing effects of two stages of folding in the beautifully layered rocks (greywackes and mudstones) and their quartz veins, and to determine the two-stage deformation history.
Wednesday 10th May 2006: "Fire and Ice: A Geologic and Social Perspective on Volcanism in Alaska"
Dr. Diana Roman, University of Leeds
Alaska's Aleutian arc is one of the most active volcanic areas in the world, with over 260 eruptions occurring in the area in the last two centuries. Although most of the volcanic centres are located on remote islands, they pose a significant hazard to over-flying commercial aircraft as well as to downwind communities. Because of this, careful monitoring of volcanic activity in the Aleutian arc is critical, and is carried out by the USGS Alaska Volcano Observatory. Volcano monitoring in the Aleutians includes seismic monitoring, satellite- and ground-based thermal and deformation monitoring, and routine gas monitoring. Recent notable eruptions in the Aleutians include the 1912 eruption of Katmai Volcano (the largest eruption in the 20th century), the 1989 eruption of Redoubt Volcano, the 1992 eruption of Crater Peak/Mt. Spurr, the 1986 eruption of Augustine Volcano (which is currently in eruption), and numerous eruptions at Okmok Volcano. The Aleutian arc is a fascinating volcanic region and has long been a natural laboratory for volcanologists working to understand volcanic activity and associated signs of unrest.
**************************************