Leaf-jams — A new and unique leaf deposit in the ephemeral Hoanib River, NW Namibia: Origin and plant taphonomic implications

• Published in: International journal of coal geology Vol. 83; no. 2; pp. 195 - 203
• Main Authors: Hofmann, Christa-Ch, Rice, A. Hugh N.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 01.08.2010; Elsevier
• Subjects: Earth sciences; Earth, ocean, space; Ephemeral river; Exact sciences and technology; Leaf deposits; Marine and continental quaternary; Paleobotany; Paleontology; Point bar; Surficial geology; Taphonomy; Southern Africa; Namibia; Africa; Point bar; Ephemeral river; Leaf deposits; Taphonomy; stream transport; floods; taphonomy; fine-grained materials; flora; vegetation; buoyancy; drainage basins; bedding; modern; flow; gorges; rivers; pebbles; streams; point bars; silt; transport; velocity; sand; interpretation; storms
• DOI: 10.1016/j.coal.2009.09.005
• ISSN: 0166-5162

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.