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Abstract:
Tufas provide virtually the only sedimentary and proxy-environmental records within karstic terrains. However, they are difficult to access. Shallow geophysical prospecting techniques, such as resistivity and shallow seismic reflection, fail to define the often complex internal bedform details in tufa deposits and many deposits appear too well lithified to auger-sample. Nevertheless, the application of ground penetrating radar (GPR) permits the recognition of up to five distinct types of radar reflectors that can be directly related to distinct lithologies commonly seen in tufa cores: (1) well-lithified phytoherms produce sharp, sinuous and often complexly truncated bright signals; (2) soft lime muds produce subhorizontal, laterally continuous lower contrast (dull) laminar bedform signals; (3) organic-rich deposits (sapropels and peats) produce poorly focused dull responses, often with internal noise'; (4) the tops of bladed and coarse-grained deposits, such as flint gravel, give a strong bright signal; and (5) the associated presence of clay-grade lime silts and muds within the top of gravel beds produces the same top-bed signal as 4, but internal details of the deposit are masked and a remarkably homogeneous dull signal response is typical throughout the lower parts of the deposit. From these GPR responses it is possible to make meaningful three-dimensional comparisons of the internal geometries of Holocene tufa deposits. Problematic tufa deposits in the valleys of the Derbyshire Wye and the Hampshire Test, UK, are presented to illustrate the universal value of GPR surveying for fresh-water carbonate recognition and for providing key information on valley-bottom resurgence locations