Three-dimensional modelling of a Holocene tufa system in the Lathkill Valley, north Derbyshire, using ground-penetrating radar

Unlithified and partly lithified 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 global positioning system (GPS) methods. This methodology provides t he first clear information on the distribution and geometry of lithofacies within buried tufa complexes. The approach has been applied to a thick succ ession of Holocene tufas filling a gorge site along a 3.5-km length of the River Lathkill, north Derbyshire. Earlier studies have demonstrated the pre sence of up to 16 m of tufas and sapropels associated with two transverse t ufa dams (barrages). These strata have been accumulating throughout the Hol ocene, although tufa developments at present are of minor extent. Internal tufa morphologies are recorded by GPR as 'bright', laterally continuous ref lections for lithified, concretionary and lithoclast-rich horizons. The 'br ightest' reflectors occur within well-cemented barrages and delineate core areas and prograding buttress zones. In contrast, unlithified lime muds and sapropels produce low-contrast reflections. Lithostratigraphic control and depth calibration of the GPR profiles was provided by percussion augering at selected sites. Six distinct lithofacies and four secondary barrages are identified in the study. Constructional and destructional events can be id entified and correlated within the GPR profile network, and the internal gr owth morphologies of the barrages are apparent. GPR profiles also clearly d efine the evolution of the facies geometries. Three phases of tufa developm ent can be recognized within the GPR data and greatly extend our understand ing of Holocene tufa-forming processes in valley sites: (a) Early Holocene barrage build-ups but with limited paludal deposition; (b) Middle Holocene ponding and sapropel accumulation under 'warm' conditions; and (c) Late Hol ocene barrage termination and valley levelling, probably coincidental with anthropogenic activity. This type of multidisciplinary approach should be c onsidered as an essential prerequisite to all biostratigraphic and geochemi cal studies of Holocene freshwater carbonate sites.