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EDUCATIONAL LOGGING REQUIREMENTS
In order to substantiate your visit and comply with the educational requirement for earthcaches you have to submit your answers to the following questions to the cache developer via his profile or e-mail (see hint blow) before you can claim it as a ”find”:
1) Order “Beaufort”, “Karoo” and “Katberg” in a (geo)logical sequence.
2) Describe in your own words what you think the environment around here looked like in “Katberg times”.
3) What is a calcareous concretion?
4) How do the rocks at the two listed sites differ from each other? You could, amongst others, mention colour, texture and the way in which the concretions are exposed in your answer.
5) OPTIONAL: Please share your experience with the caching community by uploading photos of the landscape, any geological features at the site you found interesting and/or your caching party with your log.
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OVERVIEW
This earthcache will bring you to a beautiful rocky beach where a pretty exclusive piece of Southern Africa’s geological history is exposed. The main goal of this earthcache is to introduce you to fascinating sedimentary rock bodies called concretions.
There is a striking difference between the rocks and the exposed concretions around the published coordinates (Location 1) and those at the second listed location ~250 meters to the south (Location 2; S 32° 50.773' E 28° 7.136'), we’d therefore recommend that you stroll down the beach and examine both areas.
Although the earth caching site is accessible 24/7, you’ll have access to more rocks during low tide.
ORIENTATE YOURSELF IN THE GEOLOGICAL LANDSCAPE
Sedimentary rocks originate from sediments that were laid down over millions of years in various basins and under varying conditions, which result in distinct, sequential layers where the top layers are younger than the ones below them (succession). Studying rock layers (strata) and the layering processes (stratification) is called stratigraphy and it is primarily used in the study of sedimentary and layered volcanic rocks.
Depending on their origin, age and physical characteristics sedimentary rock layers across a landscape are divided into hierarchical, increasingly smaller and more specific, lithostratigraphic units that are mappable and distinct from one another. The main lithostratigraphic ranks in this hierarchy are: Supergroup, Group, Formation, Member and Bed. The units are usually named after a geographical locality, typically the place where exposures were first described. The Karoo Supergroup, or more generally the Karoo Succession, is one of the dominant geological units in South Africa and also the geological basis for this earthcache (see fig below).
Two thirds of the present land surface of southern Africa is covered by rocks of the Karoo Supergroup, which is the most widespread stratigraphic unit in Africa south of the Sahara. This supergroup consists of a sequence of units, mostly of nonmarine origin, deposited over a period of about 120 million years, 290 to 180 million years ago (MYA) around the Karoo basin.
The Beaufort Group is the third of the five main subdivisions of the Karoo Supergroup strata. It follows conformably after the Ecca Group and consists essentially of sandstones and shales, deposited in the Karoo basin between 265 and 225 MYA. In turn the Katberg Formation is a sandstone-rich unit occurring stratigraphically three quarters of the way up in the Beaufort Group in the south-eastern part of the main Karoo basin. It is exposed in a relative small east to west extending area south of Queenstown. In addition, two downfaulted fragments of the formation have avoided denudation (long-term sum of all erosion) and are preserved along the coastal belt around Kidd’s Beach south of East London and at Cintsa (this earthcache site) north of the city. You’ll therefore be able to experience a pretty exclusive piece of South African geology when you visit this earthcache site (see maps below).
GET TO KNOW THE CINTSA ROCKS
Their origins
Katberg sedimentation took place under an arid climate which entrenched itself throughout Katberg times (~240 MYA) and extended regionally into the subsequent Burgersdorp sedimentation era. The virtual absence of fossils in Katberg sedimentation reflects a hostile, arid, climate with almost no source of standing fresh water. The Katberg streams that deposited these sediments were ephemeral and debouched into shallow lakes in the distal parts of the Beaufort basin which frequently dried out.
Most Katberg palaeostreams (ancient streams) are indicated to have had wide, shallow channels characterised by swift flow and mostly upper flow regime flatbed sedimentation. They also have the classic characteristics of modern braided streams of which constant channel switching, transverse bar formation and dissection and lack of ripple cross-lamination are amongst the main criteria.
Katberg concretions
The presence of numerous spherical, oval and irregular calcareous concretions is a distinctive feature of the Katberg Formation. The intense development of these calcareous concretions in parts of the Katberg Sandstone is indicative of penecontemporaneous formation (formed during or shortly after the formation of the containing rock stratum) and circulation of brines in an arid climate. The concretions tend to weather positively, giving the sandstone a characteristic knobby appearance in some outcrops (see foreground in photo below). The dimensions of these concretions generally range from 30 to 150 mm and oval concretions vary from slightly to highly elongated.
MORE ABOUT CONCRETIONS
Origins
Concretions are hard bodies that form in sediments before they become sedimentary rocks. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Slow chemical changes, e.g. increasing mineral concentrations due to evaporation, cause minerals to precipitate out of the groundwater and cement the sediment together. The cementation is often triggered by a central particle, also called a nucleus or seed, such as a fossil, that initiates crystallisation and cementation. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.
Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth. In concentric growth, the concretion grows as successive layers of mineral accrete to its surface. This process results in the radius of the concretion growing with time. In case of pervasive growth, cementation of the host sediments, by infilling of its pore space by precipitated minerals, occurs simultaneously throughout the volume of the area, which in time becomes a concretion.
Composition
Although the concretionary cement often consists of a single dominant mineral, more minerals can be present depending on the environmental conditions in which the concretions were created. Calcite (lime) is the single most abundant cementing mineral found in concretions – also here in the Katberg concretions, where many of the broken concretions can be seen to still contain calcite cores (see photo below).
Appearance
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several tons. They are usually similar in colour to the rock in which they are found and occur in a wide variety of shapes, including spheres, disks, tubes and soap bubble-like aggregates (see photos below).
Concretions should not be confused with nodules. Concretions are formed from mineral precipitation in the sediment during the process of diagenesis (rock formation), while nodules are replacement bodies, i.e. minerals precipitate in hollows in existing sedimentary rock. A concretion therefore consists of the same material as the rock around it, plus the cementing mineral, whereas a nodule is composed of different material.
Occurrence
Concretions are found in a variety of rocks, but are particularly common in sandstones, shales and siltstones. They often outwardly resemble fossils or rocks that look as if they do not belong to the stratum in which they were found. Occasionally, concretions contain a fossil, either as its nucleus or as a component that was incorporated during its growth but concretions are not fossils themselves. They appear in nodular patches, concentrated along bedding planes, protruding from weathered rock faces and perched on pedestals (see photo below).
Hope you enjoy exploring this fascinating area and the beautiful rocky beach. Happy earth caching!
REFERENCES
N Stavrakis (1979). Sedimentology of the Katberg sandstone in the Eastern Cape Province, MSc Thesis, Rhodes University.
MR Johnson (1989). Paleogeographic significance of oriented calcareous concretions in the Triassic Katberg Formation, South Africa. Journal of Sedimentary Petrology (59)6, 1008—1010.
http://en.wikipedia.org
http://geology.about.com
http://www.sahra.org.za/fossil-layers/katberg-formation
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