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Logging Tasks:
1. What term is used to describe this prominent formation and why does it qualify for this name, (include the word joints as part of your answer)?
2. Describe this dolerite sill (see below) briefly summarising its size, colour and weathering (indicate what caused this) and what gives it the reddish colour?
3. At the waypoint S31 59.974 E25 59.050 describe the bedrock at the base of this mountain. Given that it is/was sedimentary why hasn’t it eroded faster than the dolerite, which is an igneous rock?
Geology
What we see in some places around here are the Triassic Tarkastad Subgroup sediments that are extensively intruded and thermally metamorphosed (baked) by subhorizontal sills and steeply inclined dykes of the Karoo Dolerite Suite. These Early Jurassic (c. 183 Ma) basic intrusions were emplaced during crustal doming and stretching that preceded the break-up of Gondwana.
The hot dolerite magma baked adjacent Beaufort Group mudrocks and sandstones to form dark grey, splintery hornfels and quartzites respectively.
Good cliff exposures of a thick dolerite sill are seen here when looking across the river from the published coordinates, showing the blocky jointing of the fresher dolerite beneath and corestones towards the surface. The underlying Katberg sandstones (now altered to metaquartzite) within the thermal aureole of the sill at Vlekpoort also form steep cliffs - see picture in the listing.
Note the blocky weathering of the dolerite sills. As dolerite cooled and solidified after its molten intrusion, joints formed, approximately at right angles to one another. When these rocks were exposed at the surface, more than a hundred million years later, the rainwater seeped down and along the joints, and the rocks began to weather inwards from the joints. As the process continues and material washes away, the formerly sharply angular block-edges become rounded.
Definitions:
Aureole - is the zone surrounding an intrusion.
Corestone - a rounded or broadly rectangular joint block formed as a result of subsurface.
Joint – fracture plane separating adjacent blocks of rock between which there has been no movement.
Metaquartzite - a quartzite in which all traces of the original grains and sedimentary structures are erased.
Triassic- a geologic period and system that extends from about 250 to 200 Ma. It is the first period of the Mesozoic Era.
Here is some more geological glossary:
Dyke– near vertical tabular intrusive body.
Sheet – in igneous geology, this refers to a shallow, more or less tabular intrusion that cuts through the intruded rock strata at a fairly shallow angle (see dyke); it may also refer to a layer of extrusive lava.
Sill – shallow, tabular intrusion that intrudes parallel to the bedding of the intruded sedimentary rock strata (see sheet).
Batholith - large body of igneous rock formed beneath the Earth’s surface by the intrusion and solidification of magma. It is commonly composed of coarse-grained rocks (e.g., granite or granodiorite) with a surface exposure of 100 square km or larger.
References:
PALAEONTOLOGICAL HERITAGE STUDY - John E. Almond PhD (Cantab.)
visit link
Sedimentary environments and provenance of the Balfour Formation (Beaufort Group) in the area between Bedford and Adelaide, Eastern Cape Province, South Africa - Monica E. Oghenekome
visit link
Geological Journeys - Nick Norman & Gavin Whitfield