Correlation of Sedimentary Rhythms on a Stormy Silurian Shelf Using Non-Biostratigraphical Techniques

Sarah Jane Veevers
University of Birmingham, Birmingham, United Kingdom

A thick succession of Silurian rocks is spectacularly exposed in a series of cliff sections in SW Wales. The late Llandovery - early Wenlock is represented by the Coralliferous Group, in which two facies associations occur. Facies Association A is the basal facies, dominated by pebbly granulestones. Above lie 130 m of rhythmically bedded sandstones and shelly sandstones (Facies Association B) with strongly bioturbated mudstone interbeds. Two key sections in F.A. B have been logged on a scale of 1:10. Although only 3 km apart and known biostratigraphically to be approximately coeval, detailed correlation is difficult. Integration of several “alternative” stratigraphical techniques provides a refined correlation. Over 2000 event beds have been identified in the two sections. Most are 0.5 to 1cm thick, with a maximum of 25 cm. Variations in the vertical pattern of distribution of event beds indicate that the position of wave base relative to the sea floor varied on a high frequency timescale. The correlations suggested by these sedimentological variations have been tested using chemostratigraphy. Mudstone samples collected at 1 m intervals through both sections have been analysed for major and trace elements using XRF, supported by XRF, ICP OES and MS analysis on interbedded tuffs. Systematic chemical variations have been identified that allow correlations between sections to be confirmed. The thin tuff beds also provide a potential tool for retrieving absolute ages. Samples from selected tuffs are currently being processed for high-resolution U-Pb zircon dating. Determining the time duration of the sequence, will help in identifying the forcing mechanisms underlying the sedimentary variations.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005