UIS Commission on Karst Hydrogeology and Speleogenesis
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Tarhule-Lips, Rozemarijn Frederike Antoinette
Karst processes on cayman brac, a small oceanic carbonate island, phd thesis
1999

Cayman Brac is a good example of a small oceanic carbonate island which has undergone several periods of submergence and emergence since the Tertiary, resulting in the geological formations being well karstified. This study investigated several karst phenomena on the island including the occurrence and morphology of caves, the water chemistry and microclimate inside the caves, periods of speleothem growth and dissolution, and bell holes. Caves occur throughout the island at various elevations above sea level. Using elevation as a criterion, the caves were divided into Notch caves, located at, or one-two metres above, the Sangamon Notch, and Upper caves, located at varying elevations above the Notch. Analysis of the morphology, age and the relative abundance of speleothem in the caves further supports this division. The close proximity of the Notch and the Notch caves is coincidental: speleothem dating by U-series methods shows that the caves predate the Notch. They are believed to have formed between 1400 and 400 ka, whereas a late Tertiary to Early Quaternary age is assigned to the Upper caves. Speleothem on the island has suffered minor, moderate and major dissolution. Minor dissolution is due to a change in the degree of saturation of the drip water feeding the speleothem, whereas the last two are caused by flooding or condensation corrosion. Many of the speleothems in fact experienced several episodes of dissolution followed by regrowth. The latest episode appears to be caused by condensation corrosion rather than flooding. Eleven speleothems containing growth hiatuses were dated by U-series methods. The results indicate that growth cessation did not occur synchronously. Furthermore, the timing of the hiatuses during the Quaternary is not restricted to glacial or interglacial periods. Oxygen and carbon stable isotope analyses of seven of the samples reveal an apparent shift towards a drier and warmer climate around 120 ka. However, more data and further collaborative evidence is desirable. Of six samples with hiatuses, five show a bi-modal distribution of stable isotope values: before and after the hiatus. Oxygen isotope analyses of modern drip water found inter-sample variations of over 2[per thousand]. This is due to cave environmental factors such as evaporation, infiltration velocity and roof thickness. Inside the caves δ 18 O of drip water decreases with increasing distance from the entrance and thus decreasing external climatic influence. This distance-climatic effect is also reflected in the δ18 O calculated for modern calcite: -5.3, -6.5 and -7.6[per thousand] VPDB at 3, 10 and 20 m respectively. The morphology of bell holes, found only in certain Notch caves, was studied in detail. It is proposed that the bell holes are formed by condensation corrosion, probably enhanced by microbiological activity. The study represents a comprehensive and thorough analyses of karst features on a small oceanic island, and provides information useful for climatic reconstruction during the Quaternary

Cayman Brac, oceanic carbonate island, karst, speleothem dating, condensation corrosion, karst, Sangamon Notch
Tarhule-Lips, Rozemarijn Frederike Antoinette, 1999, Karst processes on cayman brac, a small oceanic carbonate island, phd thesis , PDF