Fazies der Karoosedimente in der Huab-Region, Damaraland, NW-Namibia

Abstract

Abstract provided by author: The Karoo succession in the Huab Area reflects depositional conditions ranging from the Late Paleozoic glaciation to a Mesozoic desert environment. The Huab Basin was part of the Parana Basin during the Late Paleozoic. Characteristic lithologies of the formations define sedimentary periods and environments which are interpreted as marginal equivalents of the Parana Basin succession; -glacial and periglacial clastics of the Dwyka Formation (Permo-Carboniferous) - fluvial sandstones and carbonaceous shales of the Verbrande Berg Formation (Permian) - lacustrine, deltaic and fluvial clastics of the Tsarabis Formation (Permian) - the cyclic development of lacustrine clastics and autochthonous carbonates of the Huab Formation (Permian) -lacustrine to deltaic red beds of the Gai-as Formation (Permian). The alluvial and eolian clastics of the Etjo Sandstone Formation (probably uppermost Triassic) are deposited in an isolated palaeoerg

Deposition of the Dwyka Formation, which represents the oldest unit of the Karoo Supergroup, was influenced by the Permo-Carboniferous glaciation. During this stage the Huab Region acted as an elevated source area in relation to the depot centre in the Parana Basin. The basinward prograding inland-ice caused a pronounced glacier-controlled landscape as a basic relief for further Karoo sedimentation. An east-west trending U-shaped main valley and adjoining shallow channels and troughs characterize the relief shaped by the Dwyka glaciation. Whithin these morphologic depressions glacial lodgement tills and periglacial sediments are preserved as relics of restricted lateral extent. Partly fractionated and structured diamictites mark the influence of glacial outwash and massmovements in proximal positions. Sandstones document glacio-fluvial and glacio-deltaic environments, while argillaceous rhythmites represent distal deposits in meltwater lakes

During the second stage of deposition the Huab Area formed part of the enlarged Parana Basin. Deposition of the Verbrande Berg Formation, which reflects a swampy, fluvially controlled environment, developed as a consequence of large scale syneclitic subsidence, which had its maximum in the central Parana Basin. Isolated and solitary sandstone bodies set in a section of argillaceous overbank fines represent channel deposits of a meandering fluvial system. The depositional overbank-environments are indicated by limnic claystones and carbonaceous shales with small coalseams, different amounts of organic material combined with characteristic fossils and sedimentary structures. Compared to modern wetland classifications the overbank fines of the Verbrande Berg Formation have been deposited in shallow open waters, fens and woody swamps

The deposits of the Tsarabis Formation reflect the proximal position of the Huab Basin in a large cratonic inland sea, which resulted from further expansion of the Parana Basin. After an initial transgression, stable hydrologic conditions prevailed. During this stage, the Huab Basin was divided into a western pelagic to prodeltaic, an intermediate deltaic and an eastern fluvial environment. Accordingly, the exclusively clastic deposits are subdivided into silty to argillaceous shales of the western basin, intervening arenites of fluvially controlled fan deltas and coarse sandstones and gravels of a braided river system

The succession of the Huab Formation exhibits two distinct sedimentary cycles. At the beginning of each cycle, the sediments display all characteristics of a hydrologically closed, lacustrine system. During these stages autochthonous carbonate sedimentation prevailed in the predominantly shallow Huab Basin. Short-time transgressions led to stable conditions of hydrologically open systems dominated by clastic sedimentation in the upper parts of both cycles

To the east, the terrestric environment of the Huab Formation is subdivided into two members. The lower Probeer Member consists of hydromorphous carbonate soils that indicate a semiarid climate, while the sheetlike deltaic sandstone body of the upper Gudaus Member represents stages of high wave energy

The Gai-as Formation consists of continental red beds, which document a complete lacustrine succession. A"coarsening and shallowing upward" development is clearly indicated by upward increase in grain size andcharacteristic sedimentary structures. Deposition starts with argillaceous shales of a pelagic to hemipelagicenvironment, continues with alternations of prodeltaic clastics and precipitations from a saline carbonate lake andceases with coarse deposits of a partly vegetated delta plain. In the saline environment of the Gai-as Formation, material of four thin tuffaceous layers has been altered to authigenic analcime and potassiumfeldspar. The Permianphase of sedimentation ceased with coarse terrestrial red beds, deposited under subtropic to arid conditions

Eolian and alluvial deposits of the Etjo-Sandstone Formation represent the Mesozoic stage of sedimentation in the Huab Area. Morphological changes, caused by a slight uplift of the western Huab Area and erosion in the eastern parts predated the Mesozoic setting. The Etjo Sandstone Formation is divided into the uniform eolian Etjo Sandstone and the coarse alluvial clastics of the Krone Member, which are restricted to the lowermost part of the formation in the eastern Huab Area. The uniform dip direction of the large scale crossbedding suggests a permanent eolian transport from southwest, which indicates a close relationship to the Botucatu Sandstone desert in Brazil where corresponding measurements have been reported. Within a shallow depression in the eastern Huab Area, the Etjo Sandstone is made up of thick transverse dunes of an expanding dunefield. Smaller barchanoid dunes with restricted lateral extent suggest higher migration and smaller accumulation rates in the western Huab Area. This situation has been preserved by the sheet basalts of the Etendeka Formation

During the Permian, basin development in the Huab Area was closely related to the slowly subsiding Parana Basin. Extensional tectonic events, that may have induced erosion of the Top Permian sediments in the eastern Huab Area are not recognizeable until the uppermost Triassic. Subsequently the Karoo sediments were affected by normal faults and partly tilted by antithetic faults which are both related to rifting. These faults also affected the Etendeka basalt sheets (up to 800 m) and thus postdate the main phase of rift-volcanism

The wide range in maturation stages of organic material proves a slight influence of burial diagenesis and a strong effect of doleritic intrusions