The geology of the western Windhoek and Rehoboth districts

Abstract

Abstract provided by author:

The granitic rocks are concordant, conformable, harmonic masses and bands that seem to be roofed and bottomed by metasediments near the rim of the anticlinorium. Their composition varies from quartz monzonite to typical granite, but highly enriched potash granite also occurs. The main granite mass in the core of the anticlinorium seems to be wholly reconstituted by ultrametamorphism. Two types of granodiorite are also identified. The relative ages of the granitic rocks cannot be determined in the field owing to the lack of critical outcrops

At least two periods of deformation seem to have affected the Abbabis Formations prior to overprinting by the Damara tectogenesis. A younger tectonic a direction is parallel to an older tectonic b direction and the combined effect is a ubiquitous down-dip lineation on the well developed northward dipping foliation of all these rocks

The Abbabis Formations underlying the mantled Hohewarte Gneiss Dome are reconstituted together with sediments of the Rehoboth Formation and the Damara System along the northern flank of this structure

A succession of clastic sediments overlain unconformably by the Damara System was hitherto called Nosib Formation. It is renamed Rehoboth Formation in agreement with the "Code of Stratigraphic Nomenclature". The Rehoboth Formation consists of a mainly psammitic Kamtsas Facies in the east, and a mainly pelitic Duruchaus Facies in the west. Lithosomes of the one facies occur as intercalations in the other. Typical sections and boundary relations are described. Facies distribution, lithology and thickness variations are treated in conjunction with paleo-current flow data and heavy mineral analysis to trace the origin and history of deposition of the sediments of the Rehoboth Formation. The Kamtsas Facies was derived mainly from the Dordabis System which enveloped the eastern part of the Rehoboth Anticlinorium. The Duruchaus Facies probably derived directly from rocks of the Abbabis System already exposed in the western parts of the Rehoboth Anticlinorium

The Damara System is defined as excluding the Rehoboth and Nosib Formations in order to have a common lower time boundary for both the Swakop and the Outjo Facies. As regards further subdivision of the System into two series it is argued that the intraformational tillite is the only time marker traceable throughout both facies and therefore the boundary between the series should be at the base of the tillite

Along the southern margin of the geosyncline the Rehoboth Formation filled troughs and was folded, uplifted and deeply eroded at least in the east, before a wide geosynclinal depression developed into which calcareous sediments were deposited during the Damara cycle

Post-Damara volcanicity in the Windhoek and the Rehoboth Districts must be of at least Jurassic age. The faults of the Windhoek area that originated during the Damara tecto-genesis, were reactivated during this volcanic activity

Structural Analysis: Megascopic and macroscopic structural analysis was applied to unravel the complex architecture of the southern marginal zone of the Damara geosyncline

The rocks near the base of the system underwent two phases of deformation resulting in a B1

Wrench faults and high angle thrusts mark a final surge of the Damara tectogenesis north of the Hohewarte Gneiss Dome, whereas a third fold phase B3 (

Kinematic fault analysis shows that a possible stress configuration derived for the faulting also accounts for the B2 structures. The distribution of the B3 phase and the faults, however, is controlled by inhomogeneities in the basement

The first fold phase, expressed throughout the root and marginal areas of the geosyncline, is the result of initial tangential movements. Near the base of the system the effective stresses were deflected from their regional north-westerly trend because of irregularities in the floor. The second phase has maintained its expression even where it penetrates the basement, thus leading to B1

The following facts are explained by an Alpine type tectonite of the deeper marginal levels:1) Constantly northward inclined second cleavage (2) Intensification of B2 structures southwardsand away from the root areas (3) A southward inclined major stress axis during the B2 phase (4)Co-axiality of B1 and B2 structures (5) Regularity of strike of the B2 structures right into the basement (6) Indications of strong vertical movements along the border between marginal and root areas

At the present stage of erosion in the marginal areas the Damara geosyncline has been deprived of the higher levels with their southward overriding nappé structures and cascade folds

The three successive deformational phases, outlasted by biotite grade metamorphism, took place within a minimum time interval of 150 m.y. Even in this respect comparison with the Alpine orogeny is significant

Economic Aspects: Amongst the many breccia zones around Windhoek the major transcurrent faults represent the most prolific and reliable aquifers. Henceforth all efforts to develop groundwater should be concentrated on drilling the main wrench faults