SEDIMENTOLOGY AND GEOCHEMISTRY OF THE PROTEROZOIC LEBA FORMATION, ANGOLA, AFRICA

Proterozoic sedimentary rocks of Angola, Africa, occur in several isolated localities in the southwestern part of the country. This succession was deposited on the southern edge of the Congo Craton and is historically referred to as the Chela Group. From bottom to top, it consists of the following five formations: the Tundavala Formation (sandstone and conglomerate), the Humpata Formation (volcanoclastics), the Bruco Formation (quartz-rich sandstones), the Cangalongue Formation (silty and sandy carbonates), and the Leba Formation (dolomite).

This investigation is concentrated on the Leba Formation in the Humpata Plateau locality of the Huila District near the town of Lubango. Although definitive age dates are not yet available, some estimates indicate that the Leba Formation was deposited during the Mesoproterozoic time interval.

In the study area, the Leba Formation consists of 60 meters of black-colored, laminated dolomite with abundant chert and stromatolite in some intervals. Laminae consist of synsedimentary cement. Ooids, oncoids, intraclasts, peloids, and carbonate mud are dominant allochems. Carbon isotope compositions of the dolomite range from +0.1 to +1.2 per mil PDB (average +0.9 per mil PDB). Oxygen isotope values vary from -10.1 to -12.5 per mil PDB (average -11.2 per mil PDB). Sr concentrations range from 24 to 115 ppm (average 41 ppm).

Abundant ooids and oncoids indicate that the Leba Formation was deposited in shallow water environments within a fair weather wave base. Dolomite is microcrystalline and microtextures in allochems and synsedimentary cements are well preserved. These observations suggest that dolomite may have been the primary carbonate precipitate at the time of the Leba Formation deposition. Carbon and oxygen isotope values vary little and show no major perturbations.