abstract

The siliciclastic beds outcropping around the village of Kuhlan, northwest Yemen were originally designated the Kohlan Series. Later the sandstone part of the sequence was renamed the Kuhlan Formation by the Yemeni Stratigraphic Commission. The lower part of the Kuhlan Formation (Unit A) yielded diverse and well-preserved palynological assemblages. Their quantitative character and the presence of Anapiculatisporites concinnus, Brevitriletes cornutus, B. parmatus, Deusilities tentus, Dibolisporites disfacies, Microbaculispora tentula, Spelaeotriletes triangulus and Verrucosisporites andersonii, suggests a confident correlation to the 2165A to 2141A biozones of southern Oman, and to Arabian OSPZ2, indicating a Late Carboniferous age for the unit rather than the Late Triassic to Early Jurassic age given on the geological map of western Yemen. The date for the lower Kuhlan Formation indicates that the underlying Akbra Formation, which was previously dated as possibly Early Permian, and Late Carboniferous to Early Permian in age respectively, is probably not younger than the 2165A Biozone and therefore likely to be equivalent to the lower parts of the Al Khlata Formation of Oman.

INTRODUCTION

The siliciclastic beds outcropping around the village of Kuhlan, northwest Yemen, were originally designated the Kohlan Series by Lamare and Carpenter (1932). Later the sandstone part of the sequence was renamed the Kuhlan Formation by the Yemeni Stratigraphic Commission (Beydoun et al., 1998). Kuhlan village, where the type section and several typical outcrops of the Kuhlan Formation are situated, is located about 70 km northwest of Sana’a city (Figure 1). There the Formation has a thickness of about 200 m but also crops out in a narrow belt in the mountains of the high plateau of northwest Yemen. It wedges out southward having a thickness of 10 m at Jabal Bura’a and 14 m at Wade Maksab (Kruck and Thiele, 1983; Beydoun et al., 1998). The Formation extends subsurface east and northeastwards and is encountered in several oil wells in the Ma’rib-Jawf Graben and Rub’Al-Khali Basin (Diggens et al., 1988; Beydoun et al., 1998). The Kuhlan Formation overlies the dominantly argillacous Akbra Formation.

Lithologically, the Kuhlan Formation consists of yellowish brown, pinkish and red, massive, cross-bedded, medium to fine-grained sandstone, which is interbedded with thick, fissile and stratified siltstone/shale beds of grey to red colour (Figure 2; Kruck and Thiele, 1983; Diggens et al., 1988; Beydoun et al., 1998). Diggens et al. (1988) originally considered the Kuhlan Formation to represent shallow marine and fluvial environments. Al-Mashaikie (2005) described ten lithofacies types within the Kuhlan Formation. The lower part of the Formation (Unit A of Al-Mashaikie, 2005), from which the palynological samples of this study came, consists of a series of alternating sandstones and fissile mudstones, with occasional coarser-grained beds (Figure 2). Plate 1 shows a range of lithofacies in Unit A of Al-Mashaikie (2005), which include dropstones embedded within an argillaceous matrix, and diamictite facies. Unit B includes cycles of conglomerates, sandy conglomerates, pebbly coarsegrained sandstone, fine-grained sandstone, silty and argillaceous sandstone and fissile shale, interbedded with massive and stratified diamictite beds considered of glaciomarine origin by Al-Mashaikie (2005). Unit C consists of massive sandstone, coarse-grained sandstone and laminated siltstone/shale, which Al-Mashaikie (2005) considered to represent a shallow-marine shelf.

Unit A contains no macrofossils and overall the Kuhlan Formation lacks fossils apart from the uppermost 6–10 m, which are composed of sandstone with intercalations of red and gray shale, marl and calcareous sandstone. These upper beds contain mollusc shells and a thin calcareous layer with plant impressions (Kruck and Thiele, 1983; Kruck et al., 1991). On this basis, Diggens et al. (1988) considered the age of the upper beds of the Kuhlan Formation to be Late Triassic to Early Jurassic. The Kuhlan Formation as a whole is portrayed as Late Triassic to Early Jurassic on the geological map of western Yemen (Kruck et al., 1991).

Samples of argillaceous layers of Unit A of the Kuhlan Formation were collected by the second author in 2008 with the aim of dating the unit using palynological assemblages.

PREVIOUS PALYNOLOGICAL STUDIES

There have been two previous attempts to date rocks in the Kuhlan area using palynology. Kruck et al. (1983) collected samples of grey claystone lithologies of the Akbra Formation (which underlies the Kuhlan Formation) from unspecified exposures along the Kuhlan – Hajjah road (Figure 1; Neves inKruck and Thiele, 1983). The organic residues recovered by Neves contained abundant vitrinite or detrital wood fragments and a palynomorph population consisting of Apiculatisporis spp., A. aff. abditus, Acanthotriletes sp., Cordaitina sp., Kraeuselisporites apiculatus, K. punctatus, Leiosphaeridia sp., Potonieisporites novicus, Protohaploxypinus goraiensis, P. jacobii, Punctatisporites sp., Reticulatisporites sp., Tympanicysta sp. and Vestigisporites sp. The assemblages were interpreted as being of Permian, possibly Early Permian age.

El-Nakhal et al. (2002) reported on six samples collected from the Akbra Formation at the Beit Al-Kooli section (2 km southwest of Kuhlan village; Figure 1). They used a different lithostratigraphic nomenclature to that adopted here (and by the Yemeni Stratigraphic Commission) referring to a lower Sharas Siltstone Member (of the Kooli Formation = Akbra Formation) and an upper Khalaqah Shale Member (Figure 3). The samples contained few palynomorphs and were dominated by dark brown and opaque wood fragments of probable land plant origin. Only two samples from the lower part of the Khalaqah Shale Member yielded palynomorphs including Alisporites cf. indarraensis, Brevitriletes cf. cornutus, Cristatisporites cf. crassilabratus, ?Diatomozonotriletes sp., Deusilites tentus, Leiosphaeridia sp., Leiotriletes cf. directus, Plicatipollenites malabarensis, Pteruchipollenites sp., Rugospora sp., Verrucosisporites sp., and indeterminate non-taeniate biasaccate pollen. These suggested only a tentative Late Carboniferous to Early Permian age for the lower part of the Khalaqah Shale Member.

MATERIALS AND METHODS

A number of samples were collected from the Kuhlan village type section (Figure 2) and of these two (AF-5 and AF-8; dark grey siltstones) yielded well-preserved and diverse assemblages allowing the application of palynological biozonal schemes of Oman and other parts of the Arabian Peninsula. The preparation of strew mounts for palynological analysis comprised crushing, followed by hydrochloric and hydrofluoric acid treatments (Wood et al., 1996). The post-hydrofluoric acid organic residues were oxidized using Schulze’s solution and dilute nitric acid. The slides are held in the Collection of the British Geological Survey, Keyworth, Nottingham, UK, NG12 5GG.

CHARACTER AND AGE OF THE PALYNOLOGICAL ASSEMBLAGES

The slides contain a rich organic residue consisting of brown well-preserved palynomorphs, woody and sheet cellular material, and rare amorphous organic matter (Figure 2). The full author citations of palynological taxa recorded are given in Appendix 1; selected taxa are shown in Plates 2 and 3.

The most common taxa are indeterminate monosaccate pollen (mainly radially-symmetrical forms, probably poorly preserved specimens of Cannanoropollis, Potonieisporites and Plicatipollenites), Cristatisporites spp., Cannanoropollis janakii, Deusilites tentus and Leiosphaeridia sp. Other common taxa include Brevitriletes cornutus, B. parmatus, Dibolisporites disfacies, Microbaculispora tentula and Verrucosisporites andersonii. There are no marked differences in the two samples, though the upper sample is slightly more diverse.

The quantitative character and the presence of B. cornutus, B. parmatus, D. tentus, D. disfacies, M. tentula and V. andersonii suggest close similarity with assemblages from the Al Khlata Formation of Oman. In particular, the presence of Anapiculatisporites concinnus and Spelaeotriletes triangulus suggests a confident correlation to the 2165A to 2141A biozones of South Oman (Penney et al., 2008) and to biozones B and C of the South Oman Mukhaizna field (Stephenson et al., 2008) which correspond to the lower part of the general Arabian OSPZ2 Biozone of Stephenson et al. (2003). The 2165A to 2141A biozones correspond to the middle part of the Al Khlata Formation (PDO Production Units lower P1 and P5) which are characterised by glacio-deltaic and glacio-lacustrine palaeoenvironments (Osterloff et al., 2004), similar to those suggested for units A and B of the Kuhlan Formation.

The 2165A to 2141A biozones, and biozones C and B of the Mukhaizna field were originally considered Early Permian (Asselian to Early Sakmarian) based on correlations with faunally-calibrated palynological biozones in Western Australia. For example Converrucosiporites confluens, the index fossil for the C. confluens Oppel Zone (see Stephenson, 2008, 2009) most common in 2141B (Penney et al., 2008), was originally considered Mid- to Late Asselian in age (Foster and Waterhouse, 1988). Recent work on radiometrically-dated sequences in Namibia (Stephenson, 2009) has shown that the range of Converrucosiporites confluens and the eponymous biozone probably extends lower than previously thought and therefore the 2165A to 2141A biozones (see Penney et al., 2008) may be slightly older than suggested by Penney et al. (2008). Thus the PDO 2165A to 2141A biozones and the biozones C and B of the Mukhaizna field may extend into the latest Carboniferous (Figure 4).

This new age for lower Kuhlan Formation indicates that the underlying Akbra Formation, which was previously dated by Neves inKruck and Thiele (1983) and El-Nakhal et al. (2002) as Permian, possibly Early Permian age and Late Carboniferous to Early Permian respectively, is probably not younger than the 2165A Biozone and therefore likely to be equivalent to the lower parts of the Al Khlata Formation of Oman.

CONCLUSIONS

Two samples from the lower part of the Kuhlan Formation in northwest Yemen yielded diverse and well preserved palynological assemblages. Their quantitative character and the presence of Anapiculatisporites concinnus, Brevitriletes cornutus, B. parmatus, Deusilities tentus, Dibolisporites disfacies, Microbaculispora tentula, Spelaeotriletes triangulus and Verrucosisporites andersonii suggests a confident correlation to the upper 2165B to lower 2141A biozones of Oman (Penney et al., 2008). This suggests a Late Carboniferous age for the unit.

ACKNOWLEDGMENTS

Mike Stephenson publishes with the permission of the Executive Director of the British Geological Survey (NERC). Jane Flint (BGS) processed the palynology samples. We acknowledge the Geological Survey and Mineral Resources Board of Yemen for help in producing maps and for sending samples. Mr. Radwan Al-Ashwal is acknowledged for drawing the final version of the map. The authors thank two anonymous referees for their helpful comments and GeoArabia’s designer Arnold Egdane for designing the manuscript for press.

APPENDIX – AUTHOR CITATIONS OF TAXA RECORDED

Alisporites cf. indarraensis Segroves, 1969

Anapiculatisporites concinnus Playford, 1962

Apiculatisporis aff. abditus (Loose) Potonié and Kremp, 1955

Brevitriletes cornutus (Balme and Hennelly) Backhouse, 1991

Brevitriletes parmatus (Balme and Hennelly) Backhouse, 1991

Cannanoropollis janakii Potonié and Sah, 1960

Converrucosisporites confluens (Archangelsky and Gamerro) Playford and Dino, 2002

Cristatisporites cf. crassilabratus Archangelsky and Gamerro, 1979

Deusilites tentus Hemer and Nygreen, 1967

Dibolisporites disfacies Jones and Truswell, 1992

Horriditriletes ramosus (Balme and Hennelly) Bharadwaj and Salujah, 1964

Horriditriletes uruguaiensis (Marques-Toigo) Archangelsky and Gamerro, 1979

Kraeuselisporites apiculatus Jansonius, 1962

Kraeuselisporites punctatus Jansonius, 1962

Leiotriletes cf. directus Balme and Hennelly, 1956

Lophotriletes sparsus Singh, 1964

Lundbladispora braziliensis (Pant and Srivastava) Marques-Toigo and Pons, 1976

Microbaculispora tentula Tiwari, 1965

Plicatipollenites malabarensis (Potonié and Sah) Foster, 1975

Potoniéisporites novicus Bhardwaj, 1954 emend. Poort and Veld, 1997

Protohaploxypinus amplus (Balme and Hennelly) Hart, 1964

Protohaploxypinus goraiensis. (Potonié and Lele) Hart, 1964,

Protohaploxypinus jacobii (Jansonius) Hart,1964

Punctatisporites gretensis forma minor Hart, 1965

Spelaeotriletes triangulus Neves and Owens, 1966

Vallatisporites arcuatus (Marques-Toigo) Archangelsky and Gamerro, 1979

Verrucosisporites andersonii Backhouse, 1988

ABOUT THE AUTHORS

Mike H. Stephenson is Head of Science (Energy) at the British Geological Survey (BGS), Nottingham, United Kingdom. His education has included a BSc, MSc and PhD from Imperial College and University of Sheffield, and various postgraduate teaching qualifications. Mike is an expert on the stratigraphy of the Middle East, and he has published around 30 papers on this region as well as working extensively as a consultant for oil companies in the area. He is a Fellow of the Geological Society, sits on the Petroleum Group Committee of the Geological Society and is a member of the Petroleum Exploration Society of Great Britain (PESGB). He was Secretary-General of the Commission Internationale de Microflore du Paléozoique (CIMP) between 2002 and 2008, and is presently Editor-in-Chief of the Elsevier science journal Review of Palaeobotany and Palynology.

mhste@bgs.ac.uk

Sa’ad Zeki A. Kader Al-Mashaikie was awarded an MSc in 1979 from Baghdad University for a study of the Paleocene Kolosh Formation of North and North East Iraq. He first worked as Assistant teacher in the Deptartment of Geology, College of Sciences, Baghdad University. He was also awarded a PhD in 2003 from Sana’a University in Yemen for a study of the stratigraphy, geochemistry and basin analysis of the glacio-turbidite Akbra Formation of Carboniferous – Permian age. He worked as an Assistant Professor from 2003 to 2005 in the Department of Marine Geology in the Faculty of Marine and Environmental Sciences, Al-Hodiedah University, and from 2005 in the Department of Geology and Environmental Sciences, Faculty of Applied Sciences, Dhamar University. Since 2005 he has been a Consultant in the Ministry of Oil and Minerals, Geological Survey and Mineral Resources Board, Yemen. He is interested in oil and gas prospectivity in the Palaeozoic rocks of the Rub’Al-Khali Basin, as well as palynology and facies in Palaeozoic successions in Yemen and adjacent countries.

geozakee@yahoo.com