Unusual Preservation of Sedimentary Structures in Sillimanite-Bearing Metaturbidites of the Damara Orogen, Namibia

Abstract

The Late Proterozoic Kuiseb Formation (Damara Sequence) of the Khomas Trough in Central Namibia consists of metapsammites (metagraywackes) and metapelites. Within the study area these rocks were metamorphosed under conditions above the first sillimanite isograd. The metamorphic grade is marked by assemblages containing sillimanite, staurolite, muscovite, and biotite in the pelitic schists, which indicate a temperature range of 600-660°C at 3-4 kbar water vapor pressure for the regional metamorphism. The structural setting of the investigated area is characterized by two phases of deformation with folds varying from open asymmetric through tight to isoclinal and with a heterogeneous distribution of strain. Notwithstanding strong deformation and metamorphic recrystallization delicate sedimentary structures have been discovered in several localities. These include cross-stratification, cross-lamination, load structures, ball-and-pillow structures, flame structures, scour surfaces, flute casts and rip-up clasts, associated with graded bedding. A high proportion of the sedimentary structures are arranged in Bouma sequences. The well-preserved sedimentary features, unusual in such high-grade metamorphic rocks, has allowed a detailed fades analysis within the Kuiseb schists, proving that the studied sequence has been deposited by turbidity currents. The organization of the Kuiseb schists into large-scale and small-scale cyclic successions of progradational and retrogradational character as well as the lateral extent of major sedimentary units indicate deposition within an elongate submarine fan system. Paleocurrents measured from flute casts and sets of cross-laminae indicate a sediment transport along the basin axis of the Khomas Trough from the ENE to the WSW. This supports the contention that sediment supply into the trough was derived from an easterly source.