Sudwala rocks of life “Stromatolites”
If you continue north on the R539 up the Sudwala Pass towards
Rosehaugh, a few kilometers after crossing the Houtbosloop, you
will see excellent exposures of Malmami Dolomite in the road
cutting (see listed waypoint), including a well banded variety, and
giant domed fossil algal structures called stromatolites. The large
road exposure shows fresh, pale, with inter-bedded layers of black
chert. Note the striking uniformity of the dolomite. Not to be
missed are the mega-domal stromatolites, several meters
across.
More than just interesting for the evidence they present of
early Precambrian life, these stromatolitic dolomites represent a
mile stone in earth’s history that is as easily overlooked as it is
fundamentally important. The early life forms that helped build
these very dolomites also provided the earth’s first oxygen,
without which the higher forms of life would not have evolved.
From the co-ordinates of the EarthCache GC1TR1M,
you can see on top of the hill very good examples of stromatolitic
dolomites that stand out with the surrounding soil being weathered
away.
Malmami Dolomite; Earth’s first oxygen
generator
The Malmami dolomite (and its equivalent Campbell Rand |Dolomite
in the Northern Cape) is part of the Transvaal Super group that was
deposited in a vast inland sea on the Kaapvaal Craton. It makes up
a two thousand meter thick carbonate sequence over large parts of
central South Africa. Now occurring in two well defined structural
basins, these were connecting during the time of there formation
2600 to 2400 million years ago.
Dolomite is a fine grained, calcium-magnesium carbonate rock,
originally formed by the slow accumulation of microscopic grains of
calcium carbonate in a warm tropical sea. This process was
intimately involved with the production of layers of sticky algal
growth, called algal mats, in the very shallow intertidal to sub
tidal zone. If you look closely at the well edged, weathered
outcrops, you will see several diagnostic clues to the organic
origin of the dolomite. Notice a fine crinkly lamination that
reflects the original sticky algal mats or layers that formed on
the sea floor and trapped or precipitated fine carbonate material.
Look, on a larger scale, for small to large rounded or elongated
dome like structures, called stromatolites, which clustered in
coral like reefs in the shallow sea, and were earth’s prolific life
form at the time. These mound-like features are the fossilized
remains of large masses of algal mat build up that were shaped by
tidal currents. And if you could look through a high powered
microscope, you might also see traces of minute, single cell micro
organisms preserved in the rock. These are the remains of the 2.5
billion year old blue-green algae, not very different from those
existing today. Modern day algal mats are rare, but are known from
Shark Bay in Australia, where living stromatolites can still be
found in a strongly saline, shallow sea.
Photosynthesis process:
The rock was not the only thing that the blue-green algae, or
cyanobacteria, gave rise to. By the process of photosynthesis –
active even in matter as primitive as algae – the cyanobactria
where busy converting primeval carbon dioxide into oxygen on a vast
scale, and slowly releasing it into the sea. There the oxygen first
combines with iron to form vast sedimentary, layer formations rich
in iron; later, it was released into the atmosphere, hitherto
dominated by carbon dioxide and nitrogen. Photosynthesis continued
over a very long time, and oxygen to be “pumped” into the
atmosphere. Slowly, the protective ozone layer build up,
increasingly shielding the earth from deadly ultraviolet radiation.
This paves the way for the evolution of the multi cellular
organisms, and then the emergence of life from primeval oceans onto
the land.
(Acknowledgments: Geological Journeys by Nick Norman and Gavin
Whitfield).
To get credit for a find on this EarthCache you need to answer
the following questions in a mail to the cache owner.
- Take a picture at the listed waypoint with the stromatolitic
dolomites clearly visable and attach the photos in your log.
- Please indicate in the mail to the cache owner; how many
stromatolitic dolomites are next to each other over a distance of
20m?
- Estimate the overall width of a single stromatolitic
dolomite?
- Estimate the height of the stromatolitic dolomites from the
road surface?
- Explain the dolomite structure at the listed waypoint? (ie
colour, grain size)