|
|
|
|
The Makgabeng plateau, Northern Province:
|
 |
| Rolled-up mud laminations found in playa lake deposits on the Makgabeng Plateau, believed to be trace fossils of 2 billion year old cyanobacteria. |
The Makgabeng Plateau is located about 45km south-west of Vivo, at the western end of the Soutpansberg mountain range, and about 22km south-west of Blouberg mountain. Though it is not linked to the Soutpansberg mountains by topography, and is also comprised of different rocks, the Makgabeng Plateau remains as one of the most interesting geological phenomena in the Soutpansberg area. The Makgabeng plateau is underlain by characteristic fine- and medium-grained sandstone rocks. In places a younger rock, consisting of resistant coarse sandstone and conglomerate also outcrops on the Plateau, which is present as the steep-sided spires which tower over the northern end of the Makgabeng Plateau. The rocks which occur on the Makgabeng Plateau are thought to be almost 2 billion (i.e. 2 000 000 000 years old).
Sandstone consists of grains of sand, which have been cemented together during burial by the deposition of other minerals. Despite burial, cementation and some compaction, sandstones usually retain many of the structures that were present during the initial deposition of the sand. Examination of environments where sand is deposited around the world today, such as rivers, beaches and deserts, show that each depositional environment is characterised by specific structures, such as ripplemarks and dunes, which are recorded in the sand deposits. Recognition of these structures in ancient sandstone can therefore be used to determine the environment in which the original sand was deposited.
In the winter of 1998, a small team of geologists from South Africa and the U.S.A. examined the sandstone strata of the Makgabeng Plateau in an attempt to determine the source of the rocks. Initial work in the mid-1970's had proposed that the rocks of the Makgabeng Plateau may be the remains of a fossilised desert, and this idea was supported by the more recent work. Most sandstones around the world are believed to have been deposited in an aqueous environment (i.e. under water). The fact that the Makgabeng rocks appear to have been deposited under sub-aerial (i.e. wind-blown) conditions was indicated by the presence sedimentary structures, which are specific to wind-blown sand. One key indicator which can be used to discriminate between sub-aqueous and sub-aerial sand deposits concerns the angle of repose of a pile of sand. The transport of sand, whether by water or wind, tends to group sand into dunes. Wet sub-aqueous) sand is denser than dry sand, on account of all the water which is contained between the sand grains. As sand grains are washed or blown onto a dune, the lee (down-stream) side of the dune builds up as successive laminae of sand grains. Periodically the lee-face builds up to such a steep angle that it collapses. The maximum angle at which sand can rest (the angle of repose) is governed by the density of the sand. Wet, dense sand can only reach a maximum angle of repose of about 15°. Dry sand, however, only fails at angles exceeding c.30°. The sandstones of the Makgabeng Plateau contain laminations inclined up to about 25°, indicating that they were dry when deposited, and thus were wind-blown. A walk across the area today, especially through farms such as Montblanc 328LR (23º 14'S;28º 50'E) reveals the preserved lee side laminations in the plinths (lower parts) of barchan (crescent-shaped) and longitudinal desert dunes. Some of these lee side laminations still retain ripplemarks caused by sand migrating in wind storms, and small dimples caused by rain drops falling during showers 2 billion years ago. The tall spires of conglomerate and coarse sandstone which stand above the plateau are the remains of sheets of cobbles and sand which were deposited by rivers which flowed over the desert deposits as the climate became much wetter.
Deserts around the world today comprise not only sand dunes, but also other features, such as seasonal (ephemeral) rivers and lakes (otherwise known as playa lakes) amongst the sand dunes. Evidence for similar features can also be found within the Makgabeng strata. Of particular interest to the 1998 team of geologists were playa lake deposits, as it was in these strata that interesting trace fossils were found.
Life on the earth has gradually evolved over the last 3,5 billion years at least. Initially colonisation of earth took place by very simple prokaryotic cells, which are essentially microscopic bags of chemicals with the peculiar ability to reproduce by fission, thus leading to the spread of colonies. Such prokaryotic cells had their chance to reproduce enhanced by growing in colonies or even, ultimately, by growing inside other cells. Such traits lead to the evolution of multicellular organisms and, more importantly, to more complex eukaryotic cells with nuclei, which make up all higher species. As such, these simple monocellular prokaryotes can be seen as the ultimate human ancestors; they are still present as mitochondria enclosed within eukaryotic cells in our bodies. One particular type of prokaryotic cell which evolved early on in the history of the earth are known as cyanobacteria, or blue-green algae. They are still common today in a wide range of watery environments. The evidence for the growth of colonies of this organism is considerable. Limestones and cherts, which owe their deposition to biological action of these organisms, can be traced back to about 3,5 billion years ago. However, in terms of the environment in which these organisms grew, they are restricted to sub-aqueous conditions, such as shallow marine and fluvial (river) conditions. As organisms of this type photosynthesise and in turn produce oxygen at the expense of carbon dioxide, they are thought to be responsible for oxygenating the earth's atmosphere during the second quarter of the earth's history.
Amongst the playa lake deposits in the Makgabeng rocks the 1998 team of geologists discovered some structures which seemed rather enigmatic. As a lake dries up, eventually all water has evaporated and only mud is left on the surface. In turn this mud desiccates and cracks up into polygonal shapes. As desiccation continues, the mud polygons begin to curl up around their edges, often reaching a maximum curvature of about 90-180° (i.e. a 'U' shape) before the mud curl disintegrates. In the playa lake deposits of the Makgabeng strata, rolled-up curls of 1-2mm-thick mud laminations were found within the rock, in which up to 720° of curling had been accommodated (see above figure). These spirals of mudstone were about 7cm long, and have a diameter up to about 2cm, and resemble small hand-rolled cigars in their dimensions and internal structure. The behaviour of this desiccating mud appears to be very different from what could be expected of mud laminations. Why didn't the mud break apart during its desiccation? Clearly the mud possessed some property which allowed it to remain cohesive. In modern environments similar behaviour of desiccating mud has been reported, where a thin film of cyanobacteria grows across the surface of the mud. These jelly-like colonies growing in their muddy substrate provide sufficient cohesion to prevent fragmentation of mud curls on which they grow. It seems reasonable to believe that similar cyanobacterial colonies were growing amongst the playa lakes in the Makgabeng strata 2 billion years ago. The significant point about the Makgabeng cyano-bacteria is they appear to have grown in a considerably harsher environment than any earlier recorded bacterial colonies. The terrestrial setting of a desert, and the temporary nature of water bodies within a desert environment indicate that the bacteria growing in such an environment were considerably more robust than their relatives restricted to life in wholly aqueous conditions. The trace fossils in the Makgabeng rocks therefore record an important step in the evolution of life on the planet.
Eriksson, P.G., Simpson, E.L., Eriksson, K.A., Bumby, A.J., Steyn, G.L. and Sarkar, S. (2000): Muddy roll-up structures in siliciclastic interdune beds of the 1.8Ga Waterberg Group, South Africa. Palaios, 15. 177-183.
Simpson, E.L., Eriksson, K.A., Eriksson, P.G. and Bumby, A.J. (in press): Eolian dune degradation and generation of massive sandstones in the Palaeoproterozoic Makgabneg Formation, Waterberg Group, South Africa. Journal of Sedimentary Research.