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Logging Tasks:
At the given co-ordinates you will have a cliff face directly in front you and you will have to answer questions relating to this cliff face.
- Describe the colour and bedding of the lowermost three quarters of the cliff face?
- Describe the colour and bedding of the uppermost section of the cliff face?
- Describe in your own words why you think the lower and upper sections of this cliff face are so different?
- At the listed co-ordinates you will find a lamp post how many lights are attached to it?
Unconformities rank high among the pinnacles of geological experiences, especially when they are as well exposed as this one.
Unconformities are gaps in the geologic record that may indicate episodes of crustal deformation, erosion, and sea level variations. They are a feature of stratified rocks, and are therefore usually found in sediments (but may also occur in stratified volcanics). They are surfaces between two rock bodies that constitute a substantial break (hiatus) in the geologic record (sometimes people say inaccurately that "time" is missing). Unconformities represent times when deposition stopped, an interval of erosion removed some of the previously deposited rock, and finally deposition was resumed.
Commonly three types of unconformities are distinguished by geologists:
- ANGULAR UNCONFORMITIES
- DISCONFORMITIES
- NONCONFORMITIES
But as this is an angular unconformity we will only look at this aspect.
Angular Unconformities are those where an older package of sediments has been tilted, truncated by erosion, and then a younger package of sediments was deposited on this erosion surface. The sequence of events is summarized in the pictures at left. First: subsidence and sediment deposition occurs; Second: rocks are uplifted and tilted (deformation); Third: erosion removes the uplifted mountain range; Fourth: subsidence occurs, the sea covers the land surface, and new sediments deposition occurs on top the previous land surface. Then the cycle may repeat.
AT THIS SITE:
The lowermost three quarters of the face consists of well-bedded quartzite of the lower Witteberg Group. This was formed during the mid-Devonian, i.e. mid-Palaeozoic quartzite (380 million years old).
Geology of the Witteberg Group
The middle Devonian (380 million years ago) to middle Carboniferous (325 million years ago) strata of the Witteberg Group consist of approximately equal amounts of light grey to brownish quartzitic sandstones and micaceous mudrock. The overall depositional environment of the lower Witteberg sediments is believed to be near a shoreface, in a continuous inner shelf, along a storm dominated shallow marine coastal zone. An overall eustatic sea level change at this time resulted in a seaward progradation of fluviodeltaic sediments across a shallow marine shelf. The deposition of the Witteberg Group thus took place in a variety of environments from fluviodeltaic to shallow marine shoreface deposits. Sedimentary structures found in the Witteberg Group include trough and planar cross bedding, wave ripple structures, hummocky and swaley cross stratification, linsen, wavy and flaser bedding, and horizontal lamination. Although generally devoid of body fossils, the strata of the Witteberg Group contain abundant trace fossils.
Geology after the first deposition
Between the deposition of these two sequences of sandstone, a towering range of fold mountains was built and then removed by erosion and the whole Karoo Basin, covering more than half of South Africa was formed. After that, and still well before deposition of the overlying upper sandstone, the most dramatic chain of events in South Africa’s recent geological history took place: the break-up of Gondwana.
In the part of the face nearest the river you can see a basal conglomerate capping the lower beds, just below the upper sandstone. This is a sure sign that you are looking at the start of a new sedimentary cycle, where high-energy rivers flowed down steep gradients from freshly uplifted terrain, carrying coarser material than possible when flowing over gentler slopes.
The uppermost part of the exposure consists of softer beds of sandstone and is in direct contact with the quartzite. The sandstone overlying the quartzite is much younger and is of Late Miocene to Pliocence age (late Cenozoic, approximately 5-6 million years old).
Acknowledgements
Geology off the beaten track By Nick Norman
Geology of the Witteberg Group, Cape Supergroup By Wernich Corné Olivier visit link
Unconformities