Hm. Pedley et al., Three-dimensional modelling of a Holocene tufa system in the Lathkill Valley, north Derbyshire, using ground-penetrating radar, SEDIMENTOL, 47(3), 2000, pp. 721-737
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.