The descriptive ecology of saxicolous lichens in an area of South Merionethshire, Wales

Saxicolous lichen vegetation on Ordovician rock at the mouth of the River Dovey, South Merionethshire, is examined in relation to aspect, slope angle, light intensity, rock porosity, rock microtopography and rock stability. A number of characteristic groups of species are recognized. The environmental factors measured are discussed in some detail. In addition, the wide tolerance of most saxicolous species is emphasized.

Palaeozoic volcanogenic mineral deposits at Parys Mountain, Avoca, and S.E. Canada: a comparative study. Available in 2 volumes.

The sulphide mineralisation at Avoca and Parys Mountain is intimately related to volcanism and is of volcanogenic sedimentary type. The associated volcanics are predominantly pyroclastics of rhyodacitic composition and of Upper Ordovician age. They were erupted from discrete small volcanic centres, products of single local volcanic events, whose spatial distribution was related to fractures in the sialic basement of the paratectonic Caledonides of the British Isles. These fractures resulted in linear controls on volcanic, plutonic and tectonic features; they are the result of predominantly strikeslip stresses generated in this part of the European plate during closure of the Iapetus ocean. The mineralisation, predominantly pyritic, consists of a siliceous footwall zone containing bedded and cross-cutting sulphides and an overlying non-siliceous zone of bedded sulphides which may show vertical zoning of metal ratios. The sulphides are associated with chert and iron formation and have been affected by slumping. Mineralisation developed near the vents during intense fumarolic activity accompanying strong volcanism; at Parys Mountain, fumarolic activity commenced prior to, and continued after, the rnain volcanic event. Comparison with similar deposits in Newfoundland and at Bathurst, in the Canadian Appalachians, shows that mineralisation can be associated with any discrete pulse of acid magmatism in shallow subaqueous conditions. Local features of the sulphides and associated sediments are similar, although in more distal deposits (with respect to a volcanic centre) footwall alteration and mineralisation are less well developed. The nature of the basement and the presence or absence of earlier volcanics are not critical, although establishment of a local tensional regime at the time of ore formation may be important. The volcanics hosting mineralisation are rhyodacitic pyroclastics, generally related to a small centre and representing a single episode of volcanism.

The relationship of mineralization to petrology at Parys Mountain, Anglesey

The Lower Palaeozoic succession at Parys Mountain overlies a Precambrian basement (the Iona Series). This succession consists of Ordovician slates, overlain by, and in part interbedded with, Ordovician dacitic and rhyolitic volcanics, which in turn are unconformably overlain by Silurian slates. Both basement and Palaeozoic rocks have been deformed during Caledonian and Variscan orogenies. The resultant structure of Parys Mountain is interpreted as an east-north-easterly trending, single syncline overturned to the north. Many primary extrusive characters are retained by the volcanic rocks, despite the high degree of deformation. The lithologies and textures allow subdivision and interpretation of these rocks as dacite, lithic tuff, siliceous sinter, rhyolitic tuff, rhyolitic ignimbrite, rhyolitic tuff-lava, and rhyolitic lava. The results of 61 bulk chemical analyses are interpreted to show that the volcanism was of the orogenic calc-alkaline type from a continental margin/island arc environment. The magmas probably result from either partial melting of the crustal part of the oceanic lithosphere on a Benioff zone, or partial melting of the mantle, above a Benioff zone, under high load pressures and high water pressures. The mineral deposits are largely confined within the volcanic succession though some occur in the Ordovician and Silurian slates near to their contacts with the volcanics. The majority of the deposits form conformable lenses and tabular bodies, with subordinate deposits as veins and stockworks. The ore mineral assemblages are of chalcopyrite, galena, sphalerite, and pyrite. The general paragenetic sequence (73 sections) is pyrite--chalcopyrite--galena-sphalerite. The main mineralization episode is interpreted to be syngenetic, genetically related to the volcanism. The veins and stockworks probably result from Caledonian and Variscan remobilization of the primary mineralization. Trace element analyses (Cu, Zn, Pb, Ni, Co, Cd, Cr, Hg, Ba, Sr), on 350 specimens, detected anomalous concentrations of these elements around the mineralized zones, though some occur where no mineralization was found. The analyses also indicate a close relationship between the mineralization and the volcanic horizons, especially the siliceous sinter.

The stratigraphy of the Fishguard volcanic complex and associated sediments, SW Dyfed, Wales

This thesis describes the geology of a Lower Palaeozoic terrain, situated west of the town of Fishguard, SW Dyfed, Wales. The area is dominated by the Fishguard Volcanic Complex (Upper Llanvirn), and sediments that range in age from the Middle Cambrian to the Lower Llandeilo. The successions represent an insight into sedimentation and volcanism for c. 100 Ma. along the south-western margin of the Lower Palaeozoic Welsh Basin. The stratigraphy of the sedimentary sequence has been completely revised and the existing volcanostratigraphy modified. The observed complexity of the stratigraphy is primarily the consequence of Caldedonide deformation which resulted in large scale repetition. Fold-thrust tectonics dominates the structural style of the area. Caledonide trending (NE-SW) cross-faults complicate preexisting structures. Middle Cambrian (?) sedimentation is documented by shallow marine clastics and red shales deposited within tidal - subtidal environments. Upper Cambrian sedimentation was dominated by shallow marine `storm' and `fair weather' sedimentation within a muddy shelf environment. Shallow marine conglomerates and heterolithic intertidal siliciclastics mark the onset of Ordovician sedimentation during the lower Arenig transgression. Mid-Arenig sediments reflect deposits influenced by storm, fair-weather and wave related processes in various shallow marine environments, including; shoreface, inner shelf, shoaling bar, and deltaic. Graptolitic marine shales were deposited from the upper mid-Arenig through to the lower Llandeilo; during which time sediments accumulated by pelagic processes and fine grained turbidites. The varied nature of sedimentation reflects both localised change within the depositional system and the influence of larger regional eustatic events. Ordovician subaqueous volcanic activity produced thick accumulations of lavas, pyroclastics, hydroclastics, and hyaloclastics. The majority of volcanism was effusive in nature, erupted below the Pressure Compensation Level. Basaltic volcanism was characterised by pillowed lavas and tube networks, whilst sheet-flow lavas, pillow breccias and minor hyaloclastites developed locally. Silicic volcanism was dominated by rhyolitic clastics of various affinities, although coherent silicic obsidian lavas, sheet-flow lavas and pyroclastics developed. Hypabyssal intrusives of variable composition and habit occur throughout the volcanic successions. Low-grade regional metamorphism has variably affected the area, conditions of the prehnite-pumpellyite and greenschist facies having been attained. Numerous secondary phases developed in response to the conditions imposed, which collectively indicate that P-T conditions were of low-pressure facies series in the range P= 1.2-2.0 kbars and T= 230-350oC, under an elevated geothermal gradient of 40-45oC km-1. Polymineralic cataclastites associated with Caledonide deformation indicate that tectonism and metamorphism were in part contemporaneous.

The structure and dynamics of saxicolous lichen communities

The saxicolous lichen vegetation on Ordovician slate rock at the mouth of the River Dovey, South Merionethshire, Wales was described in relation to several environmental variables which include aspect, slope angle, light intensity, rock porosity, rock microtopography and rock stability. Each of the measured environmental variables was shown to influence the lichen vegetation. A number of groups of species which were characteristic of certain environments were described. The data from the saxicolous lichen communities were analysed using multivariate analysis. Qualitative and quantitative data were ordinated, the qualitative data being easier to interpret ecologically, and site number (which reflects distance from the sea and altitude), rock porosity and light intensity were shown to be important environmental variables. A classification of the data was also carried out. The results of the ordination and classification were combined together and a model constructed which describes saxicolous lichen vegetation. A method which uses the model as an aid to the design and interpretation of field experiments is described. The model is applied to an experiment which investigates the effect on growth of transplanting four saxicolous lichens to different aspects. Growth was inhibited in Physcia orbicularis and Parmelia conspersa on rock surfaces of northwest aspect compared with growth on rock surfaces of southeast aspect. Growth was inhibited in Parmelia glabratula ssp. fuliginosa on rock surfaces of southeast aspect compared with rock surfaces of northwesr aspect. The growth of Parmelia saxatilis was similar at both southeast and northwesr aspects. Growth inhibition or stimulation in thalli of Physcia orbicularis, Parmelia conspersa and Parmelia glabratula ssp. fuliginosa after transplantation was consistent with the predictions of the model while the results for Parmelia saxatilis were not as expected. There was evidence that the frequency of Parmelia conspersa and Parmelia glabratula at a site is related to an effect of the environment on the growth of the thalli. There was also evidence that the frequency of Physcia orbicularis at a site is related to an effect of the environment on the establishment phase of the thalli and for the competitive exclusion of Parmelia saxatilis thalli from southeast facing rock surfaces. The distribution of lichens in relation to height on nine rock surfaces was investigated. It was suggested that the distribution of the lichens was influenced by microclimatic factors which are related to height on the rock, environmental variables which are associated with the rock substratum (e.g. rock porosity and rock microtopography) and by historical factors. The pattern of one crustose and one foliose lichen on four rock surfaces of different aspect and slope was investigated. On the vertically inclined surface the density of small thalli of Buellia aethalea and Parmelia glabratula ssp fuliginosa was correlated with the microtopography of the surface in transects horizontally across the rock surface but not in transects vertically down the rock surface. there were consitent differences in the scale and intensity of pattern horizontally and vertically and also a decrease in the intensity of pattern vertically as the slope of the rock surface decreased. These results were consistent with the suggestion of a gradient of microclimatic factors up the rock. The differences in the scale and intensity of pattern in different size classes in the population were consistent with the changes in pattern with time which have been shown to occur during succession in sand dune and salt marsh vegetation. The relationship between thallus size and height on a rock surface and between the radial growth rate and location of a thallus on a rock surface were investigated. Thalli of Parmelia glabratula ssp. fuliginosa were larger at the top of the rock surface than at the bottom and the data were consistent with the suggestion that the colonisation of the rock surface began at the top and, in time, spread downwards. The radial growth rate of the thalli could not be related to variation in slope, porosity, microtopography or directly to height on the rock but could be related to the horizontal location of the thalli on the rock. These results were consistent with the suggestion that here is a gradient of microclimatic factors across the rock surface which is also modified by height on the rock surface. The succession of lichen communities was described by relating the vegetation to rock porosity, rock microtopography, species diversity and rock stability. An initial stage dominated by crustose lichens leads to communities dominated by crustose, foliose and fruticose species. In the late stages of the succession on some rock surfaces crustose species again become dominant. The occurrence of the climax state and cyclic vegetation change in lichen communities are discussed. A mthod of estimating the age structure of a lichen population by relating thallus size to growth rate is described. The sources of error in the method are discussed in some detail and several refinements suggested to increase the accuracy of the method. The population dynamics of Parmelia glabratula ssp. fuliginosa was investigated by applying life tables to the age structures of eight different populations. The data were consistent with a period of relatively constant recruitment of thalli into the populations. Mortality in lichen populations was divided into deaths which occur after fragmentation of the thallus and deaths which occur after catastrophic environmental events. THe data suggest that the rate of fragmenting death is dependent on the age of the thallus while the rate of catastrophic death is dependent on the number of thalli established in an age class. A comparison of the numbers of thalli in each age class in the eight populations suggested that population density is controlled firstly, by climate and secondly, by variables related to the local rock surface environment. The rate of fragmenting death is related to the diversity of the community and the influence of diversity together with environmental variables in fluctuating or cyclic changes in population number.