The lithostratigraphy of area 2115 B and D in the central zone of the Damara Orogen, Namibia

Abstract

Abstract by author:

The sedimentary facies changes of particular lithostratigraphic units above the Chuos Formation were studied from south to north in four stratigraphic sections. These stratigraphic units are from the base upwards the Spes Bona Formation (schist), the Okawayo Formation (marble), the Oberwasser Formation (schist), the Daheim Formation (volcanics), the Karibib Formation (marble) and the Onguati Formation (marble/schist). The Spes Bona Formation consists of a sequence of graded rhythmites of calc-silicate rock, metagreywacke and schist exhibiting sedimentary structures such as flat-topped ripples and planar cross-bedding. This sequence is interpreted as representing a sub-tidal to intertidal environment on a siliciclastic-dominated shelf. Reduced influx of siliciclastic material initiated the first carbonate deposition (Okawayo Formation). The Okawayo Formation consists of interbedded calc-silicate rock and marble and synsedimentary carbonate breccias in the south. The sequence changes northwards into cross-bedded calc-silicate rock and marble rhythmites with rip-up clasts and to the far north into a marble unit with high proportions of schist. This change from south to north in the Okawayo Formation represents carbonate deposition on an upper slope through mid slope to a deep basin away from a carbonate platform situated to the south of the study area

A reverse back to siliciclastic deposition followed with the deposition of the Oberwasser Formation. The base of this sequence comprises relatively thick packages of graded metagrey-wackes and schist exhibiting partial Bouma sequences. This sequence is interpreted as representing mid to outer fan environment of a turbiditic fan system (from south to north). Towards the top of the Oberwasser Formation there is a calc-silicate rock unit characterised, by stacked wave ripple, oncolites and stromatolitic structures. The combination of these sedimentary structures indicates sedimentation in a shallow water wave-dominated sub-tidal environment for the final stages of the siliciclastic sedimentation over the whole of the study area

The Daheim Formation volcanics which occur only south of the Omaruru Lineament are stratigraphically situated in the upper parts of the Oberwasser Formation. The volcanic sequence consists of pillow lavas and lava breccias (extruded underwater) at the base overlain by fine and coarse pyroclastic material (scoria) that extruded sub-aerially. A return to carbonate deposition (Karibib Formation) over the whole of the Central Zone was still controlled by the same platform, slope and basin configuration from south to north. During the initial stages of carbonate deposition the carbonate platform was situated to the south of the Karibib-Usakos area. The platform edge was in the Karibib area terminating against a northwesterly inclined slope characterised by synsedimentary carbonate breccias. Mid slope carbonate facies lay to the north of Karibib, and open shelf facies (calc-silicate/marble rhythmites) to the far north. With the progradation of this carbonate platform and shelf to the north the upper slope carbonate breccias in the Karibib area were buried by intertidal to supratidal carbonates (oolites, fenestrai structures, conglomerates). During this stage the mid slope carbonate facies to the north were buried by outer reef carbonates (stromatolites?) and the outer ramp sediments to the far north were buried by well oxygenated shallow water fore-rest: carbonates (white graphitic marbles)

During the final stages of evolution of the basin the carbonate platform and shelf were terminally drowned and overlain by deep water siliclastic sediments with some carbonate detritus washed in from the surviving platform (Onguati Formation). With continual regional subsidence the whole area was overlain by deep water turbiditic sediments (Kuiseb Formation)