Hostname: page-component-78c5997874-xbtfd Total loading time: 0 Render date: 2024-11-20T04:17:11.480Z Has data issue: false hasContentIssue false
  • English
  • Français

Possible Late Palaeozoic Glaciation in the Central Parts of the Yemen Arab Republic

Published online by Cambridge University Press:  20 January 2017

Hamed A. El-Nakhal
Hamed A. El-Nakhal*
Affiliation:
Department of Geology, University of Sana’a, Sana’a, Yemen Arab Republic
Rights & Permissions [Opens in a new window]

Abstract

Occasional outsize clasts, some of which are striated, and laminae bent against the enclosed clasts have been recorded throughout the shales and siltstones forming the lower part of the Kohlan Group in the central parts of the Yemen Arab Republic. Such features are interpreted as an indication of continental glaciation. On stratigraphical grounds, these shales and siltstones are assigned to the Permian.

Résumé

Résumé

Des inclusions de gros blocs, certains striés, et des lamelles incurvées autour des blocs inclus ont été repérés dans les schistes et les grès formant l’assise inférieure du Groupe de Kohlan dans le Centre de la République arabe du Yemen. On interprète ces faits comme l’indice d’une glaciation continentale. Du point de vue stratigraphique ces schistes et grès sont attribués au Permien.

Zusammenfassung

Zusammenfassung

In den Schiefer- und Schlammlagen, die den unteren Teil der Kohlan-Gruppe in den zentralen Teilen der Arabischen Republik Yemen bilden, wurden gelegentlich grössere, manchmal gekritzte Gesteinsbrocken und Lamellen, die gegen die eingeschlossenen Bruchstücke gebogen sind, festgestellt. Solche Erscheinungen werden als Anzeichen einer festländischen Vereisung gedeutet. Aus stratigraphischen Gründen werden die Schiefer- und Schlammschichten dem Perm zugeordnet.

Type
Short Notes
Information
Journal of Glaciology , Volume 30 , Issue 104 , 1984 , pp. 126 - 128
Copyright
Copyright © International Glaciological Society 1984

Introduction

In the course of a recent investigation of the stratigraphy and sedimentology of the Kohlan Group, exposures were studied in the outcrop area bordering Kohlan town in the central part of the Yemen Arab Republic, about 65 km north-west of the city of Sana’a (Fig. 1). In this region the succession is well exposed in both the Khalaqah district and at Beit al-Kooli village which lie about 2 km north and west of Kohlan town, respectively (Fig. 1), forming an area of about 10 km.2 Here, the lower part of the Kohlan Group consists of a thick succession of rhythmically laminated silt-shale and banded sandy silt-stone interbeds with some features indicative of a glacial origin, such as outsize clasts (heterogeneous clasts which are clearly distinguishable and of a much larger size than the grains of the host rock), striated boulders and laminae bent against the enclosed clasts. This study is devoted mainly to the description and discussion of these features and their significance.

Fig. 1. Location map showing the area with glacial deposits in the central parts of the Yemen Arab Republic. The inset indicates the position of the Yemen Arab Republic in relation to the Arabian Peninsula.

Historical review

Late Palaeozoic glaciation in the southern parts of the Arabian Peninsula was first recorded by Reference HudsonHudson (1958) and Reference MortonMorton ([1959]), who described glacial deposits of a late Carboniferous–early Permian age from Oman. This record has recently been confirmed by Reference Braakman, Braakman, Levell, Martin, Potter and van VlietBraakman and others (1982). Similar deposits in the upper part of the Wajid Sandstone (Permian or older?) in the Bani Khatmah area, south-western Saudi Arabia, were recorded by Reference HelalHelal (1963, Reference Helal1965) and McClure (in Reference Braakman, Braakman, Levell, Martin, Potter and van VlietBraakman and others, 1982). Reference RolandRoland (1978) described the occurrence of striated erratic boulders in the lower shales of the Kohlan Group at Wadi Akbra near the Sa′dah area, northern Yemen Arab Republic. He explained these boulders as ice-rafted glacial material and he considered the shales of Wadi Akbra as late Palaeozoic in age.

Stratigraphy

In the study area, the lower part of the Kohlan Group consists of about 110 m of silt-shale with silt-stone interbeds. Here, as elsewhere in the central parts of the Yemen Arab Republic, these shales and siltstones rest unconformably on the Precambrian basement rocks. In turn, this succession is unconformably overlain by a sequence of fine- to medium-grained light-coloured sandstones, which form the upper part of the Kohlan Group. The succession can be subdivided into two main units, which from the base to the top are as follows (Fig. 2):

  • (a) The lower unit consists of 2.5 m of alternating thin beds of light grey claystone, dark green siltstone and silt-shale, overlain by 5 m of dark green rhythmically laminated and highly weathered silt-shale, followed upwards by about 10 m of light green siltstone. Occasional outsize clasts, some of which are striated (Fig. 3), occur throughout this unit which is about 17.5 m thick.

  • (b) The upper unit consists of about 0.5 m of alternating thin beds of light grey claystone and dark green silt-shale with abundant outsize clasts (Fig. 4), overlain by a thick sequence of rhythmically laminated dark green to black, highly weathered silt-shale with numerous, intercalations of thin interbeds of light grey, compact banded sandy siltstone. The exposed lower horizons of this unit have three joint sets (Fig. 5). Occasional outsize clasts have been observed throughout the sequence. The uppermost 8 m of this unit contain abundant clasts. The total thickness of this unit is about 92.5 m.

Fig. 2. A measured section through the lower shaly and silty part of the Kohlan Group; Beit al-Kooli village, Yemen Arab Republic.

Fig. 3. Striated boulders: (a) From the Khalaqah district; (b) The lar gest striated boulder reaorded in the study area; its length is more than 5 m; Beit al-Kooli village.

Fig. 4. Exposure of a thin clast-rich bed; Beit al Kooli village.

Fig. 5. Three joint sets on a pavement of light grey claystone; Khalaqah district.

Features Indicative of Glaciation

The shales and siltstones of the Kohlan Group possess certain features which can be considered as indicative of glaciation:

  • (a) Outsize clasts. Occasional clasts of granitic and metamorphic basement rocks occur throughout the succession. These clasts have a wide range of particle size and morphology; many of them have a size much larger than that of the grains of the shales and siltstones containing them (Figs 3, 4, and 6). These are interpreted as glacial deposits.

  • (b) striated boulders. Some of the recorded clasts are striated (Fig. 3). The largest striated clast observed in this area is about 5 m long; it is an elongated boulder of coarse pink granite (Fig. 3b). These striations are attributed to a glacial origin.

  • (c) Bent laminae. In some cases, the laminae of the silt-shale are deformed and bent against the enclosed clasts (Fig. 6). This suggests that the clasts were rafted out by floating ice and dropped (cf. Reference LaheeLahee, 1961, p. 71); they are therefore interpreted as dropstones from an iceberg.

Fig. 6. Section through the rhythmically laminated dark green silt-shale, showing laminae bent against the enclosed clasts; Kohlan al-Soq.

Age

The clast-bearing shales and siltstones belong to and form the lower part of the Kohlan Group. These beds have yielded no fossils and therefore their age depends on their stratigraphical position. This unit is unconformably overlain by sandstones of the Kohlan Group from which Reference Carpentier and LamareCarpentier and Lamare (1932) recorded plant fossils of a Liassic age. In the northern part of the Yemen, the shales and siltstones of the Kohlan Group overlie the Wajid Sandstone. Both Reference HelalHelal (1963, Reference Helal1965) and McClure (in Reference Braakman, Braakman, Levell, Martin, Potter and van VlietBraakman and others, 1982) assigned the upper Wajid Formation to a late Carboniferous–early Permian age. Therefore, the shaly and silty sequence of the Kohlan Group can be bracketed between the early Permian and the Liassic, which means that this sequence is equivalent to the Gondwana glacial deposits of other regions. However, due to the fact that the shaly and sandy units of the Kohlan Group are separated by an unconformity, the hiatus of which cannot be estimated because the adjacent strata lack fossils, the Triassic may be either partly or completely absent from the Kohlan Group succession. Therefore, the clast-bearing shales and siltstones are tentatively assigned to the late Permian, whereas the overlying sandstones are considered to be Triassic (?)–Liassic in age.

Summary and Conclusions

The occurrence of outsize clasts, together with striated boulders and bent laminae throughout this succession, has led to the following conclusions:

  • 1. The occurrence of the above features indicates that the shales and siltstones of the lower part of the Kohlan Group of the Yemen were deposited in a glaciomarine or glaciolacustrine environment.

  • 2. The presence of possible glacial deposits in this succession is considered to be an indication that the late Palaeozoic Gondwana glaciation affected the southern Arabian Peninsula.

  • 3. On stratigraphical grounds, the lower shaly and silty unit of the Kohlan Group is tentatively assigned to the Permian, whereas the upper sandy unit is considered to be Triassic (?)–Liassic in age.

References

Braakman, J.H., and others. 1982. Late Palaeozoic Gondwana glaciation in Oman, by Braakman, J.H., Levell, B.K., Martin, J.H., Potter, T.L., and van Vliet, A.. Nature, Vol. 299, No. 5878, p. 48–50.Google Scholar
Carpentier, C.A. and Lamare, P. 1932. Vēgētaux fossiles du Yēmen. Bulletin de la Sociētē Gēologique de France, Sēr. 5, Tom. 2, p. 83–92.Google Scholar
Helal, A.H. 1963. Jungpaläozoische Glazialspuren aufdem Arabischen Schild. Eiszeitalter und Gegenwart, Bd. 14, p. 121–23.Google Scholar
Helal, A.H. 1965. On the occurrence and stratigraphic position of Permo-Carboniferous tillites in Saudi Arabia. Geologische Rundschau, Bd. 54, Ht. 1, p. 193–207.Google Scholar
Hudson, R.G.S. 1958. [Contribution to] Discussion. (In King, 1958, p. 70–71.)Google Scholar
King, L.C. 1958. Basic palaeogeography of Gondwanaland during the late Palaeozoic and Mesozoic eras. Quarterly Journal of the Geological Society of London, Vol. 114, Pt. 1, p. 47–77.Google Scholar
Lahee, F.H. 1961. Field geology. Sixth edition. New York, McGraw-Hill Book Co., Inc.Google Scholar
Morton, D.M. [1959.] The geology of Oman. Fifth World Petroleum Congress, New York, 1959. Proceedings, Sec. 1 [i.e. Vol. 1], p. 277–94.Google Scholar
Roland, N.W. 1978. Jungpaläozoische Glazialspuren aufdem Arabischen Schild. Eiszeitalter und Gegenwart, Bd. 28, p. 133–38.Google Scholar
Figure 0

Fig. 1. Location map showing the area with glacial deposits in the central parts of the Yemen Arab Republic. The inset indicates the position of the Yemen Arab Republic in relation to the Arabian Peninsula.

Figure 1

Fig. 2. A measured section through the lower shaly and silty part of the Kohlan Group; Beit al-Kooli village, Yemen Arab Republic.

Figure 2

Fig. 3. Striated boulders: (a) From the Khalaqah district; (b) The lar gest striated boulder reaorded in the study area; its length is more than 5 m; Beit al-Kooli village.

Figure 3

Fig. 4. Exposure of a thin clast-rich bed; Beit al Kooli village.

Figure 4

Fig. 5. Three joint sets on a pavement of light grey claystone; Khalaqah district.

Figure 5

Fig. 6. Section through the rhythmically laminated dark green silt-shale, showing laminae bent against the enclosed clasts; Kohlan al-Soq.