686 SEDIMENTARY PROCESSES/Karst and Palaeokarst Figure 12 Heavily scalloped relict phreatic pressure tube entering a larger cave passage The strongly asymmetric scal lops provide clear evidence of upward flow, under pressure, into a larger water filled passage Deflection of these scallops above the pressure tube indicates flow from left to right in the main passage Ogof Llyn Du, North Wales Most caves form by an overall downward movement of water through the karst aquifer from sink to rising, but a few form by very different processes Some are created instead by hydrothermal springs or by the action of sulphuric acid formed from hydrogen sulphide rising from hydrocarbon deposits beneath Encompassing some of the largest and longest caves in the world, such as the Carlsbad and Lechaguilla caves in New Mexico, they contain some of the most extraordinary arrays of speleothems (cave mineral deposits) known, with gypsum a major component Over time, cave passages may collapse or be infilled by clastic sediment, transported in from the surface, or by speleothems (Figure 13) Clastic sediments provide important information on environmental histories and potentially can be dated using techniques such as optically stimulated luminescence Calcite and aragonite speleothems (stalactites, stalagmites, and flowstone) are the most common mineral deposits, precipitated in response to CO2 degassing of carbonate-saturated percolation water emerging into air-filled cavities and hence providing evidence of vadose conditions They are immensely important in karst research because the calcite can be dated radiometrically, particularly by uranium-series disequilibrium, and can provide information on palaeoclimate and hydrology Figure 13 Typical cave fills in an active vadose cave Spe leothems (stalactites) are deposited by carbonate saturated per colation water emerging along joints on roof Coarse clastic sediments are carried into the cave by fast flowing water Marble Arch Cave, Northern Ireland Photo by Tony Waltham Figure 14 Spectacular palaeokarst shaft developed in, and capped by, marine Carboniferous limestones The size of this pothole, filled with non marine clastics, provides evidence of the scale and duration of this intra Carboniferous emergent episode Cookstown, Northern Ireland Palaeokarst and Interstratal Karst Palaeokarst is defined as karst that has been buried by younger rocks It is a common, though often unrecognized, component of successions in which limestones are present (Figure 14), and serves as a clear indicator of terrestrial environments and, to some extent, duration of emergence Interpretation