*University of Hamburg GPIuM, presently University of Dar es Salaam, Geology Dept., Box 35052, Dar es Salaam, Tanzania, e-mail: firstname.lastname@example.org, email@example.com
**Makerere University, Dept. of Geology, P.O. Box 7062, Kampala, Uganda, e-mail: firstname.lastname@example.org
Cave Hill got its name because of the caves. They have been formed due to differences in the parent rock petrography. The fine grained carbonatite which contains layers of mafic minerals, responds much easier to weathering than the surrounding coarser grained, more homogeneous variety. Only where secondary calcite coatings have covered and hardened the finer grained carbonatite has it remained prominent. At Cave Hill up to six generations of calcite formation have been observed. The first and oldest is the calcite of the carbonatite itself, followed by hydrothermally precipitated calcite in veins. In some of these veins a third generation of calcite is found, formed by the precipitation of calcite from weathering solutions. In the field, the hydrothermal calcite is not easy to distinguish from the travertine formed by the weathering solutions. One criterion for distinguishing the travertine from hydrothermal calcite may be rhythmic layering of the calcite with laterite and the size and clarity of the crystals. The hydrothermal calcite is often white to colourless with well developed crystals whereas the travertine is mostly yellowish and milky with layers of laterite and shows smaller, less developed sub- to anhedral shapes. A fourth generation of calcite cemented the lateritic filling of fractures and a fifth is represented by very small calcite crystals in the cemented laterites. It ought to be noted that the cementation status is not uniform. There are non to very weakly cemented laterites (soft to moderately hard) and intensively cemented laterites (very hard). The different cementation status of the lateritic fracture fillings might itself indicate several generations of preferred calcite dissolution and precipitation. The sixth generation of calcite is represented by very recent thin travertine coatings found on laterites and on the parent rocks. These travertine coatings show sometimes well distinguished dripstones and even small stalactites.
One of the key localities is exposed on top of Cave Hill. In up to 3 m deep and 2 m wide karst caves - which developed along fractures - three generations of laterite and five generations of calcite can be distinguished (Fig.2). The different periods of laterite deposition may have been coinciding with repeated opening of the fracture.
In the medium intensively cemented laterite filling small rootlets are found, now hardened by iron crusts. This laterite is fossil bearing (Fig.3). According to a preliminary determination the fossil gastropod could be a Limicolaria of Late Pleistocene age (Pickford, 2001; pers. communication). The filling of the gastropod shell is the same as the matrix surrounding it. In addition to fossils the lateritic filling also contains subangular clasts of the carbonatite. This may indicate high rainfall conditions during the deposition of the lateritic filling and a rapid filling of the fracture.
Some fractures also contain manganese-iron crusts, often related to the weathering of mafic dykes (glimmerites). Iron-manganese precipitation sometimes also occurs in the form of small pisoliths, cemented by calcite.
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Paper first published in GSU Newsletter 1 (1) April 2001, Kampala, Uganda
© 2001 Gunnar Ries