БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2014“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2014”
Depositional and post-depositional textures of acid ignimbrites and vocaniclastic sediments from Ouarzazate Group, Anti-Atlas, Morocco Структури на отлагане и структури, последващи отлагането на кисели игнимбрити и вулканокластични седименти от групата Уарзазат, Анти-Атлас, Мароко Stoyan Georgiev1, Eleonora Balkanska2, Dimitar Kamburov3 Стоян Георгиев1, Елеонора Балканска2, Димитър Камбуров3 Geological Institute, Bulgarian Academy of Sciences; E-mail:
[email protected] Sofia University “St. Kliment Ohridski”, Dept. of Geology, Paleontology and Fossil Fuels 3 Kasbah Resources Limited, South Perth, Australia 1 2
Key words: Ediacaran acid ignimbrite, Anti-Atlas, Jebel Saghro.
The large acid pyroclastic eruptions represent one of the most catastrophic events in the Earth history. One of the oldest well preserved volcano-sedimentary successions of Moroccan Anti-Atlas gives good opportunity to study Ediacaran acid pyroclastic activity and post-depositional processes. The accretion of the Pan-African Anti-Atlas belt (790–540 Ma) to the West African supercontinent was a multi stage event, involving extension, subduction, moderate collision and post-collision extension (Gasquet et al., 2008 and reference therein). The rocks that represent the metacratonic (collision to post-collision) Pan-African stage (630–540 Ma; Ediacaran) are widespread in the Moroccan Anti-Atlas. The acid magmatic and metallogenic activity was greatest during the final post-collisional extensional stage, at the Precambrian–Cambrian boundary. It’s represented by the volcano-sedimentary succession of Ouarzazate Group that lays unconformably over the meta-turbidite sequence and cryogenian black shale deposits of the Saghro Group. The Ediacaran volcanic activity of Ouarzazate Group (580–540 Ma) is with intermediate at the bottom to acid (mostly ignimbrites) composition at the top, covered by sediment volcaniclastic deposits (Gasquet et al., 2008) and Carboniferous platform sediment series. The rocks studied are situated to the north of Nkob town at Jebel Saghro Mountain (Eastern Anti-Atlas). The acid ignimbrites, covered with erosional boundary by volcaniclastic sediments (Fig. 1) comprise the upper sections of the Ouarzazate Group. The rocks has not undergone metamorphic event. The ignimbrites
are presented by thick sequence of discrete ignimbrite sheets that probably represent successive large pyroclastic activity. At the upper parts of an ignimbrite sheet a colonnade of a well-developed prismatic jointing is observed while in its interior it transits to chaotic entablature to massive. The ignimbrites are moderately to densely welded. The crystalloclasts are presented mainly by quartz with well-expressed magmatic corrosion and feldspars. The groundmass is with vitriclastic texture formed by broken glass shards. The pumice is collapsed to fiamme, sometimes with lenticular flame outlines. Devitrification textures of quartz-feldspar spherulites are observed. Some of the gas cavities are filed with hydrothermal quartz and calcite. A well expressed reomorphyc post-depositional flowage is observed. It is expressed by drag to sheath fold formation and crystal tilting showing the flow direction along the paleo-slopes. The ignimbrites are overlain with erosion boundary by the volcaniclastic sediments. The clasts are presented mainly by ignimbrite fragments. It has massive structure, with clast to matrix supported texture in different parts of its interior. In some levels well expressed imbrications are observed. This volcaniclastic succession forms the Jebel Saghro needles near to Igli hamlet.
References Gasquet, D., N. Ennih, J.-P. Liégeois, A. Soulaimani, A. Michard. 2008. The Pan-African Belt. – In: Michard, A., O. Saddiqi, A. Chalouan, D. Frizon de Lamotte (Eds.). Continental Evolution: The Geology of Morocco. Springer Verlag, 33–64.
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Fig. 1. Depositional and post-depositional textures of acid ignimbrites and volcaniclastic sediments from the upper sections of Ouarzazate Group, Anti-Atlas a, general view; b, c, d, deposits of volcaniclastic sediments (Jebel Saghro needles); e, fiamme in acid ignimbrite; f, lenticular fiamme with flame outlines; g, devitrification quartz-feldspar spherulites; h, reomorphyc post-depositional flowage – drag fold formation and crystal tilting; i, vitiriclastic texture
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