Leaf-jams — A new and unique leaf deposit in the ephemeral Hoanib River, NW Namibia: Origin and plant taphonomic implications
This paper documents a previously unrecorded type of leaf deposit, comprising essentially monospecific linear accumulations of Colophospermum mopane leaves on a point bar of the ephemeral Hoanib River, NW Namibia. In these ‘leaf-jams’, leaf laminae stand on edge, orientated more-or-less normal to be...
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Published in | International journal of coal geology Vol. 83; no. 2; pp. 195 - 203 |
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Main Authors | , |
Format | Journal Article |
Language | English |
Published |
Oxford
Elsevier B.V
01.08.2010
Elsevier |
Subjects | |
Online Access | Get full text |
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Summary: | This paper documents a previously unrecorded type of leaf deposit, comprising essentially monospecific linear accumulations of
Colophospermum mopane leaves on a point bar of the ephemeral Hoanib River, NW Namibia. In these ‘leaf-jams’, leaf laminae stand on edge, orientated more-or-less normal to bedding. Leaf-jams, which formed upstream of cobbles, clumps of grass and sticks wedged against the former two, were orientated subparallel to the adjacent meandering river-bed, such that over the 40
m of their occurrence, their mean azimuth changed by 59° anticlockwise downstream. The longest leaf-jam was 50
cm and contained approximately 500 leaves, as well as grass culms, twigs (
C. mopane,
Tamarix usneoides and unidentified) and medium- to fine-grained sand and silt. Individual leaf-jams were partially buried in the point bar sediments up to a depth of 3
cm. Leaf-jam formation occurred in the austral summer of 2006, during the waning stage of a major flood caused by anomalous tropical to extra-tropical storms. Their monospecifity is due to the overwhelming preponderance of the zonal taxon
C. mopane in the catchment area, although the Khowarib Gorge contains a quite diverse azonal plant association due to the presence of a permanent water-seep. During leaf-jam formation, the water depth was less than the height of the cobbles (0.1
m), with stream flow-rates competent to transport medium-grained sand (velocity estimated at 0.5
m s
−
1
). Leaves must have been partially or fully waterlogged to inhibit buoyancy forces tending to lift them out of the developing leaf-jams, which propagated upstream in a manner comparable to longitudinal bars in a braided river. If fossilised, such deposits would probably lead to a very biased interpretation of the composition of the surrounding flora; the correct interpretation would be the one least favoured by palaeobotanists. |
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Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0166-5162 1872-7840 |
DOI: | 10.1016/j.coal.2009.09.005 |