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

Journal article
(Original article)

Publication Details

Author(s): Stollhofen H, Gerschütz S, Stanistreet IG, Lorenz V
Journal: Palaeogeography, Palaeoclimatology, Palaeoecology
Publication year: 1998
Journal issue: 140
Pages range: 185-215
ISSN: 0031-0182
Language: English


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.

FAU Authors / FAU Editors

Stollhofen, Harald Prof. Dr.
Lehrstuhl für Geologie (Exogene Dynamik)

External institutions with authors

Julius-Maximilians-Universität Würzburg
The University of Liverpool

How to cite

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://dx.doi.org/10.1016/S0031-0182(98)00029-7

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.


Last updated on 2018-11-08 at 01:25