Journal of Conference Abstracts

Three Dimensional Modelling of a Holocene Tufa System in the Lathkill Valley, N. Derbyshire, Using Ground Penetrating Radar

Martyn Pedley (H.M.Pedley@geo.hull.ac.uk)¹, Ian Hill (iah@leicester.ac.uk)², Paul Denton (pd@leicester.ac.uk)² & James Brasington (J.Brasington@geo.hull.ac.uk)³

¹ Department of Geology, Leicester, and Research Institute of Environmental Science and Management, University of Hull. HU6 7RX, UK

² Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK

³ Research Institute of Environmental Science and Management, University of Hull, Hull, HU6 7RX., UK

Mixed lithified and unlithified carbonate sequences are ideally suited to the application of ground penetrating radar (GPR), augmented by percussion augering, and shallow seismic techniques, all tied to present day topography using GPS. This approach has been applied to a thick succession of Holocene tufas filling a gorge site along a 3 km length of the River Lathkill, Derbyshire. Earlier studies (Pedley, 1993; Andrews et al., 1994; Taylor et al., 1994) have demonstrated the presence of up to 16 m of tufas and sapropels associated with two transverse tufa dams (barrages). These strata span a time range of 9800 BP to historical times. The effective penetration depth of GPR into tufas in the Lathkill valley is about 8 metres. Internal morphologies are recorded by 'bright', laterally continuous reflectors for lithified, concretionary and lithoclast-rich horizons. The clearest reflectors occur within well-cemented barrages and delineate core areas and prograding buttress zones. In contrast, unlithified lime muds produce 'dull' reflectors. Sapropels also produce low sinuosity dull reflector signals. Their felted and spongy nature results in signal attenuation and also creates problems in percussion augering through them. Lithostratigraphic control on the GPR profiles was provided by percussion augering at selected sites. Shallow seismic provided information on substrate where tufa thicknesses exceeded 10 metres. The present study has given the first clear information on the distribution of tufa lithofacies within buried tufa complexes. It has revealed four secondary barrages in addition to the two principal barrages. Both constructional and destructional events can be identified in the GPR profiles and the internal growth morphologies of the barrages are apparent. GPR profiles also clearly show the dimensions and distribution of pool deposits. It is possible to follow reflector signals from barrage construction into pool deposit. Correlation between adjacent lithofacies both down and across valley can also be made. Lime mud events can be separated out from sapropel events and each can be directly correlated with barrage build-up or termination. Finally, the evolution of the site can be determined, when tied to existing radiocarbon dates, and permits the recognition of three phases of tufa development:

a) Early Holocene barrage build-ups but with limited pool sediment deposition.

b) Middle Holocene ponding and sapropel accumulation under 'warm' conditions.

c) Late Holocene barrage termination and valley levelling, probably coincidental with anthopogenic activity.

Andrews JE, Pedley, HM & Dennis P, The Holocene, 4, 349-355, (1994).

Pedley HM, Proceedings of the Yorkshire Geological Society, 49, 197-206, (1993).

Taylor, DM, Griffiths HI, Pedley HM & Prince I, The Holocene, 4, 356-364, (1994).

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