AGE AND GROWTH RATE OF SPELEOTHEMS
Growth rates of Guadalupe speleothems have been measured, but these rates must be used very cautiously when estimating a speleothem's age. Water-seepage patterns change continually so that a speleothem which is actively growing for 10 years may be dormant for the next 100 or 1,000 years. Black (1954) arrived at a growth rate of 4.5 cm3/yr for Crystal Spring Dome, the largest active travertine formation in Carlsbad Cavern. Hill (1978b) reported 2 cm of cave-coral growth and 1 mm of flowstone growth on a metal drain pipe in Ogle Cave, placed there by guano miners some 40-60 years ago (Fig. 130).
Periods of activity and inactivity can be determined from an inspection of travertine material, and these can be important indicators of past climate. Columns in Ogle Cave record three periods of growth: they have an inner crystalline core indicating a wet period of uninterrupted growth, a continuous dark- and light-ringed shell around the core indicating a period of alternating wet and dry episodes, and a discontinuous light-colored outer rind indicating prolonged interrupted growth during a final dry period (Hill, 1978b; Fig. 131). This three-fold sequence of growth has also been noted in travertine deposits in Virgin Cave, Deep Cave, and Pink Panther Cave. Periodic growth of travertine may also be indicated by inclusion-defined growth surfaces. Kendall and Broughton (1977) noted that stalactites with inclusion-defined growth surfaces have been subjected to numerous interruptions in their deposition.
The only sure way of knowing the absolute age of a speleothem is to date it (Table 24). A number of dates have been obtained on Guadalupe speleothems by carbon-14, uranium-series, and Electron Spin Resonance (ESR) methods (see Part I of this paper for a discussion of these dating methods). Spar crystals have been dated at >350,000 years by the U-series method and 879,000 ± 124,000 years by the ESR method for spar collected at the Big Room level of Carlsbad Cavern. Type I cave rafts exposed in the wall have been dated from about 213,800 yrs to >350,000 yrs, but Type II rafts piled up into cones on the floor of the Balcony of the Lake of the Clouds have a U-series date of only 50,000 ybp. Various pieces of flowstone and dripstone material, from both Carlsbad Cavern and Ogle Cave, have absolute ages ranging from 20,000 to 600,000 ± 200,000 ybp. Popcorn has been dated from 33,000 to >350,000 ybp, and one moonmilk sample has a date of 18,000 ybp (Dunham, 1972).
Last Updated: 28-Jun-2007