Faunal distribution and depositional environments in the Lower Bringewoodian (Ludlovian) of Wales and the Welsh Borders

• Main Author: Atkins, David Rohan
• Published: University of Glasgow 1979
• Subjects: 551.7; QE Geology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.349742

In this study a comprehensive quantitative analysis of the geographical and stratigraphical distribution of the faunas in the lower part of the Bringewoodian Stage (Ludlovian) in Wales and the Welsh Borderland was undertaken. Information and material was collected from 34 sections from 17 localities; over 1000 m of strata were examined and sampled using a bed-by-bed sampling technique to ensure that, as far as possible, a complete and representative picture of the faunal distribution was obtained. Over 91,000 fossil individuals were collected and identified. For each sample, relative abundance (% of each taxon), faunal diversity and faunal density calculations were made. This data together with lithological information for all the examined sections are presented in a series of charts and tables. The accumulated data show that the shelf fauna of the lower Bringewoodian is not as homogeneous as previous less-detailed studies have suggested. The lower Bringewoodian of the 'basin' and Llandovery-Landeilo areas was sampled for the first time in a palaeoecological investigation. Sedimentological studies were undertaken to establish the various environments represented by the strata examined during this study. The following environments are recognised in the lower Bringewoodian of the area examined: a) proximal (in the south-east inliers) to distal (around the shelf edge) shelf environments, with a decrease in the thickness and frequency of storm deposits in the quieter water, more offshore regions b) basinal environments either with uncirculated bottom waters of low oxygen content or with bottom currents and consequently more oxygenated waters; interbedded slumps and turbidites also occur c) outer delta platform, interdistributary bay and tidally influenced distributary channel environments in the Llandovery-Llandeilo region. The hypothesis of continuous regression during the Ludlovian is rejected from the evidence of published sedimentological work and supporting observations presented herein. The literature on the functional morphology of each individual lower Bringewoodian taxon is critically reviewed and on this, and the evidence from this study conclusions formulated regarding their autecology. By recognising and separating transported assemblages (e.g. storm deposits, slumps or turbidite faunas) from those that show little or no signs of transportation the 'original faunal distribution' is more accurately established. It proves possible to explain the absence, occurrence and abundance of a species by its degree of adaptation to the combined physical environmental parameters operating in each environment. These factors probably included variations in turbulence, wave buffeting, sediment reworking, sedimentation rate, substrate type, temperature fluctuations, degree of exposure and oxygenation of bottom waters with combinations of these factors operating in the various environments recognised herein. The distribution of the 9 lower Bringewoodian faunal assemblages recognised here can be similarly explained by variations in physical environmental factors. Changes in the physical environment produced changes in fauna whereby as conditions became limiting for individual species they were excluded or became rare, whilst other species better adapted for the new environment established themselves and proliferated. Species appear to have occurred together largely where their environmental tolerances overlap. A large degree of species independence seems to have existed. Changes in faunal density and diversity between different environments are related to the degree of stress exerted by each environment, with the highest stress conditions producing the lowest density and diversity values; the abruptness or gradation between faunal assemblages, which is controlled largely by environmental gradients, is also taken to indicate the strong influence of the physical environment on the lower Bringewoodian fauna.