Molteno Formation
Molteno Formation | |
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Ma | |
Molteno, Eastern Cape | |
The Molteno Formation is a
Geology
The Molteno Formation is the lowermost geological formation of the Stormberg Group. The Molteno Formation overlies the Burgersdorp Formation of the Beaufort Group, and underlies the lower Elliot Formation (LEF).[3] The Molteno Formation contains alternating finely-grained, medium-grained, and coarse-grained sandstones, and greyish mudstone layers. The coarser sandstones exhibiting trough cross-bedding structures.[4][5][6] The mudstones also contain siltstones. The sandstones contain secondary quartz over growths and clasts, giving them a distinctive glittering appearance. Finer-grained sandstones occur throughout the lower deposits of the Molteno Formation, growing coarser toward the upper sections. Sporadic coal seams populate the Molteno Formation, and less common quartz pebble and cobble conglomerates occur in the lower sections.[7] These geological features form part of six stacked fining upward cycles where repeating patterns of the coarser-grained rocks (conglomerates, coarse-grained sandstones) grade upwards into finer-grained rocks (medium to fine-grained sandstone, mudrocks, and coal seams).[8] Sandstones are more common in the lower deposits while mudstones are more dominant in the upper deposits. However, the mudstones are part of repeating fining upward cycles with coarser-grained sandstones and thin coal seams.[9][10][11]
The Molteno Formation comprises five members, from oldest to youngest:
- Bamboesberg Member: (>130 metres (430 ft)) The lowermost member of the Molteno Formation, it is only found in southern/southeastern deposits where it rests conformably over rocks of the Beaufort Group. It is composed almost entirely of fine to medium-grained sandstone, which exhibit fining upward sequences. It also contains thin, lenticular mudstone and siltstone intercalations. In the uppermost sections, two thin coal seams (Indwe and Guba Seams) appear. The Guba Seam caps the Bamboesberg member at its top.
- Indwe Sandstone Member: (>60 metres (200 ft)) This member constitutes the base of the Molteno Formation in its northern localities, lying unconformably over Beaufort Group rocks. Composed primarily of sandstone, these fine upwards although are very coarse-grained in its lower sections. An extremely coarse bed of pebble and cobble conglomerates sits at the base of this member.
- Mayaputi Member: (>50 metres (160 ft)) This member is rich in argillaceous mudstone. A coal seam called the Cala Pass Seam caps this member.
- Qiba Member: (>60 metres (200 ft)) This member contains fine to medium-grained sandstone, thin mudstone layers, and a single coal seam called the Ulin Seam.
- Tsomo Member: (>300 metres (980 ft)) The thickest of the Molteno Formation members, the Tsomo Member exhibits repetitive successions of coarse-grained, pebbly sandstones that grade upwards into mudstone units. Thin, lenticular coal seams appear sporadically across the Tsomo Member. The thickest of the coal seams is the Offa Seam.
The sediments of the different Molteno Formation members preserve the different environments in which they were first deposited. The
These depositional environments each have associated fossil flora and fauna that are unique to them.[13] The fossil flora and fauna co-associations reveal different, distinctive habitats that were present during the time. While the different depositional environments were temperate overall, they experienced extreme seasonal differences. Winters were near-freezing with moderate rainfall, and midsummer temperatures were harsh. The Permo-Triassic extinction event occurred a few million years before the Molteno Formation rock sediments were deposited. However, its lingering effects continued to influence the stability of the Earth's climate when the Molteno Formation rocks were deposited.[14]
Paleontology
Local and international paleobotanists and entomologists revere the Molteno Formation for its richly diverse plant and insect fossils.[15][16] The Molteno Formation fossils include 204 plant species and 333 insect species, making it one of the richest Triassic-age plant and insect assemblages ever discovered. Entomologists consider the insect fauna particularly important, because well-preserved fossil remains of insects are very rare.[17]
The plant and insect fossils had unique ecological co-associations, and occupied distinct habitats. The dominant fossil flora is associated with seven recognized habitat types. The first two habitats include arboreal species of Dicroidium, an extinct genus of seed fern that grew in either lush, riparian forests or more temperate woodlands. Herbaceous forms of Dicroidium are also found but did not dominate. Nineteen species of Dicroidium alone have been recovered from the Molteno Formation.[18]
The next habitat is also
, comprising 21 species in five genera, and Ginkgophytopsis fern meadows.The seven habitat types contain various
The Molteno Formation's
Dinosaur
Correlation
The Molteno Formation is currently considered to correlate with parts of the Tuli Basin in the northern parts of South Africa, Botswana, and Zimbabwe.[22]
References
- ^ Br, Turner (2016-09-22). "The stratigraphy and sedimentary history of the Molteno formation in the main Karroo basin of South Africa and Lesotho".
{{cite journal}}
: Cite journal requires|journal=
(help) - ISSN 1464-343X.
- ISSN 1012-0750.
- ^ Christie, A.D.M. (1981). "Stratigraphy and sedimentology of the Molteno formation in the Elliot-Indwe area, Cape Province". Masters Dissertation.
- ISSN 1012-0750.
- ^ B, Cairncross (2016-09-22). "Depositional framework and control of coal distribution and quality, Van Dyks Drift area Northern Karoo basin".
{{cite journal}}
: Cite journal requires|journal=
(help) - doi:10.1016/0899-5362(93)90164-L.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - ISSN 0899-5362.
- ISSN 1012-0750.
- ^ B, Cairncross (2016-09-22). "Depositional framework and control of coal distribution and quality, Van Dyks Drift area Northern Karoo basin".
{{cite journal}}
: Cite journal requires|journal=
(help) - ISSN 0371-7208.
- S2CID 56420970.
- ^ Cairncross, B. and Anderson, J.M. (1995). "Palaeoecology of the Triassic Molteno formation, Karoo basin, south Africa-sedimentological and palaeontological evidence. South African Journal of Geology, 98(4), pp.452–478" (PDF).
{{cite web}}
: CS1 maint: multiple names: authors list (link) - ^ Eriksson, P.G. (1984). "A palaeoenvironmental analysis of the Molteno Formation in the Natal Drakensberg. South African Journal of Geology, 87(3), pp.237–244" (PDF).
- ^ Anderson, H.M. (1974). "A brief review of the flora of the Molteno Formation (Triassic), South Africa". ResearchGate. Retrieved 2019-02-07.
- ISBN 9789061912835.
- ISBN 9789061912835.
- ^ Anderson, H.M. and Anderson, J.M., 1997. Why not look for proangiosperms in the Molteno Formation. In Proceedings 4th European Palaeobotanical and Palynological Conference. Meded. Nederl. Inst. Toegep. Geowetens. TNO (Vol. 58, pp. 73–80).
- ISSN 1342-937X.
- S2CID 129630181.
- ISSN 0078-8554.
- ISSN 1012-0750.