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):
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(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.
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(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:
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(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.
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(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.
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(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:
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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.
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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.
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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.
