Description

Although the occurrence of petrified wood in rocks of the lower Karoo Sequence is not uncommon, the “Petrified Forest” 45 km W of Khorixas is the biggest accumulation of large petrified logs in southern Africa. The logs are in an excellent state of preservation and the Petrified Forest is a declared National Monument, and no samples may be taken. The logs occur at the 280 Ma old base of the Permian Ecca Group of the Karoo Sequence and have been deposited in an ancient river channel. The matrix carrying the logs is a brownish, cross-bedded sandstone.

 Recent erosion has exposed many of the logs and also several smaller pieces. The larger logs are up to 1.2 m in diameter and at least two trees are exposed with a full length of 45 m. Although the trunks are broken into sections 2 m and shorter, the individual segments are still in place. The trunks are straight and taper gradually. Several hundred different logs are partly or completely exposed, appearing as if they had drifted into their present position in the old river sediments. Such a drift, for example during a heavy flood event, and the associated rapid deposition and burial, provides a good explanation for the concentration of large amounts and good state of preservation of the fossilized wood.

The petrified wood pieces belong to seven different species of the collective type Dadoxylon arberi Seward  (Kräusel, 1928; 1956). The wood has been silicified and agatized except for some parts, which are filled with calcite. The colour varies from brown with white streaks to red with light coloured streaks, and some pieces are also white. Presence of annual growth rings of varying thicknesses indicates that the trees grew in a seasonal climate with pronounced rainfall variation. Cell structures are well preserved.

Dadoxylon arberi Sewardwas a conifer belonging to the now extinct order Cordaitales of the Gymnospermopsida class (Fig. 8.26.2). The woody plant formed a highly branched tree with simple, needle-like leaves. The simple pollen cones were not more than a few centimeters in length. The root system is believed to have been shallow and extending laterally for several meters (Stewart & Rothwell, 1983). 

Brief History

The trunks, dating back to the Permian Period 200-280 million years ago, were washed down prehistoric rivers and deposited in alluvial sands. In a diagenesis process under high pressure the sand turned into sandstone. The trunks silificated: a silicic acid solution went into the molecular structures of the wood and replaced the organic material over a long period of time. The product is called \\\"wooden opal\\\". From 1945 the SWA adminstration made southern Kaokeland available for settlement. The farmers AJ and JH Oberholzer discovered the trunks on the farm Rooiberg, No. 517. It was recorded scientifically by Dr Charles L Champ and RG Rodin in 1951.