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Tectonic and volcanic controls on Early Jurassic rift-valley lake deposits interleaved with Karoo flood basalts, Southern Namibia.

Stollhofen H, Gerschütz S, Stanistreet IG, Lorenz V (1998)

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Publication Language: English

Publication Type: Journal article, Original article

Publication year: 1998

Journal

Pages Range: 185-215

Journal Issue: 140

DOI: 10.1016/S0031-0182(98)00029-7

Abstract

The Karoo Igneous Province of southern Africa is one of the classic Mesozoic flood basalt provinces of the world. In the
case of the early Jurassic Kalkrand Formation of Namibia the succession comprises three major flood basalt units that are
separated by two stratigraphically important fluvio-lacustrine interlayers. These horizons preserve a record of the complex
interplay between sedimentation, effusion of Karoo flood basalts and extensional tectonics that predated and accompanied
the break-up of Gondwanaland. Both sediment layers start with the dominantly local derivation of weathered and eroded
lava debris, followed by the emplacement of subaqueous mass flows and subsequent deposition of chemical sediments. The
latter are characterised by interbedded stromatolitic carbonates, grass-like structured gypsum, and plane-bedded sandstones
and mudstones containing euhedral displacive gypsum crystals that grew in the subsurface as well as rosettes which
nucleated on the sediment surface. The central parts of the lacustrine units are overlain by thin deltaic sandstones showing
bottomset, foreset, and topset layering and, finally, braided fluvial, trough cross-bedded sandstones. Evidence of subsidence
synchronous with the formation of lake bodies can be explained by two principal mechanisms. The first acted in localised
areas only and is reflected by the development of small, centrally subsiding basins. From the repeated occurrence of
onlapping geometries within such a pool, a multiphase history of sagging is deduced, being most likely related to periodic
magma withdrawal and reduction in magmatic pressures in subsurface lava feeders beneath the basin floor. Abundant
syn-subsident fracture features at lava–sediment contacts, such as sediment-, hydrothermal calcite-, and sometimes basic
lava-filled fissure systems indicate the pronounced interaction between the underlying volcanics and these small areas of
pronounced subsidence. Fluids passing through the volcanic pile exhaled into the lake, giving it the characteristics of
alkaline lake systems described from more recent flood basalt areas associated with the modern African Rift System.
On the regional scale, however, northerly trending extensional fault systems controlled half-graben basin geometries and
both facies and thickness variations across faults indicate that tectonism operated contemporaneously with volcanism and
lacustrine sedimentation. The analysis of faults and associated structures, such as regularly aligned sediment-filled fissures,
sets of micro-faults, folds and basaltic dykes constrains the extensional opening direction for the Karoo graben structures
in this area that heralded the opening of the South Atlantic and thus provides a basis to discuss the extensional history of
the Namibian coastal margin within the regional tectonic framework.

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APA:

Stollhofen, H., Gerschütz, S., Stanistreet, I.G., & Lorenz, V. (1998). Tectonic and volcanic controls on Early Jurassic rift-valley lake deposits interleaved with Karoo flood basalts, Southern Namibia. Palaeogeography, Palaeoclimatology, Palaeoecology, 140, 185-215. https://doi.org/10.1016/S0031-0182(98)00029-7

MLA:

Stollhofen, Harald, et al. "Tectonic and volcanic controls on Early Jurassic rift-valley lake deposits interleaved with Karoo flood basalts, Southern Namibia." Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998): 185-215.

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