DIRECT PRECIPITATION OF DOLOMITE FROM SEAWATER IN THE MESOPROTEROZOIC LEBA FORMATION OF ANGOLA, AFRICA

The Leba Formation consists of 60 meters of dolomite which occurs in several isolated patches in southwestern part of Angola, Africa. It is black in color, laminated, sparsely cross bedded, and contains abundant stromatolites. Grains of the Leba Formation include micrite, peloid, intraclast, oncoid, and ooid. Of particular importance is the growth of synsedimentary cement on the seafloor. Deposition of the Leba Formation began with mudstone and graded upward to packstone and grainstone. Lithologic characteristics and grain types indicate deposition in a shallow shelf environment influenced by storms. Average geochemical signatures of the Leba Formation are as follow: δ¹³C = +0.9‰ PDB, δ¹⁸O = -11.2‰ PDB, Sr = 41 ppm, Fe = 4269 ppm, and Mn = 310 ppm. Chemical composition of the Leba Formation is comparable with the geochemistry of other Mesoproterozoic carbonates.

Internal textures of all grains and cements including ooids, oncoids, and seafloor cements are perfectly preserved. If it were not for x-ray diffraction analysis, it would have been difficult to distinguish dolomite ooids of Leba Formation from calcite ooids of the Phanerozoic. Such a perfect and complete preservation of textures of grains throughout the Leba Formation suggest that dolomite precipitated directly from seawater and did not form by replacement of a precursor limestone.

Possible causes of the direct precipitation of dolomite from seawater were high temperatures and high CO₂ content of Mesoproterozoic atmosphere. These conditions caused undersaturation of seawater relative to aragonite and calcite. Therefore, dolomite was the primary carbonate precipitate.