Encountering desert deposits in Oman

Oman is a geologically fascinating country, where the bedrock beautifully exposes a one-billion-year history. I had the opportunity to explore this country in a group expedition, during which we pursued our own scientific studies from January to March 2014. My geological observations during the expedition were opportunistic and involved a variety of sights, having traversed from east to west from Muscat, across the dusty plains of the Empty Quarter (Rub’ Al Khali) desert to the Dhofar Mountains of Qamar.

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Fig. 1. Geode in the Empty Quarter desert.

Rub’ Al Khali: The Empty Quarter Desert

The Empty Quarter desert is the largest sand desert expanse in the world (Peter Vincent, 2008) and is considered to have great oil prosperity under the dunes. The desert may lack bedrock exposure, but it is home to some unexpected sedimentary deposits. We found the light golden sand to be littered with brown bubbly balls – geodes (Fig. 1). When broken open, the insides are glazed with white calcite crystals sparkling in the desert sun. These had formed when rock cavities filled with crystallised calcite. In time, these balls of calcite weathered out from the host rock, before being transported by water and deposited here on the desert plains.

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Fig. 2. Miscellaneous deposits showing evidence of human shaping, found in the Empty Quarter desert.

These were not the only interesting deposits found. Strangely shaped pebbles of flint and dark metallic-like forms also lay here (in an area previously documented to have archaeological interest). One can see how these appear to have been hand carved by humans thousands of years ago, to form mechanical cutting tools (Fig. 2). Furthermore, what initially looked like a carved archaeological object, transpired to be our first taste of the fossil rich landscape that is Oman (Fig. 3).

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Fig. 3. Mollusc fossil found in the Empty Quarter desert.

Location: Qamar terrain, Dhofar

The Dhofar Mountains are a relic of the Dhofar uplift event (Martin Pickford et al, 2014), when dominantly shallow marine environments became increasingly terrestrial (Oligocene-Miocene). They are made up of folded strata, steep slippery slopes, winding wadis (Fig. 4), giant boulders, and floors of crumbling rock deposits. Gypsum can be found in lumps, exposed against the slopes of the high ground.

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Fig. 4. Wadis – dried up fluvial channels in the Qamar terrain.

The finding of archaeological tools continued through this region, where a visibly crafted flint arrow head was accidentally found by torchlight in the pitch blackness of night (Fig. 5).

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Fig. 5. Flint arrow head, showing triangular form and hand cut grooves. Found in the Dhofar Mountains.

Descending into the canyons and dried up wadis, we encountered limestone walls of fossil crinoids, gastropods and bryozoa, and pavements of stromatolites. Hollow holes were numerous, which we suspected once had minerals inside that had since weathered out. Sharp rocks lay baking in the sun, looking almost like volcanic glass due to their glassy lustre and black colour. These are named ‘desert varnish’, which constitute a black layer as a result of the surrounding arid and exposed environment.

Among the loose rubble of the mountains, well-preserved fossils emerged, with the majority being gastropod molluscs cemented in calcium carbonate (Fig. 6A and B). These are early relatives of modern day snails. Sometimes, they were found in large clusters, in what could be flood deposits. Many of these finds are yet unclassified, although some species have been suggested with the help of Dr Eike Neubert (who specialises in terrestrial molluscs at Naturhistorisches Museum der Burgergemeinde in Bern). Among these are Ceristidae sp. (Fig. 8A), which has likewise been found in his own study at Wadi Darbat (Eike Neubert and Dirk van Damme, 2012). This is a land dwelling snail from the superfamily Enoidea. Prof Dirk Van Damme (Research Unit Palaeontology, Department of Geology, UGent) noticed a similarity of one mollusc (Fig. 7A) to the Cyclotopsis genus. We also found a well-preserved bivalve (Chlamys sp.) from the family Pectinidae (Fig. 7B), which is noticeably a relative of clams. This is distinguished from its close relative, Petena sp., due to the protrusion on one side of the hinge (Dr Neubert, Prof Walkden – personal correspondence) and reflects a once shallow marine environment. An unusual specimen (Fig. 8B) resembles a crinoid stem, although it could be a tube belonging to a sediment living bivalve named Kufus sp. (as suggested by Prof Walkden at the University of Aberdeen).

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Fig. 6A (top). Loose gastropod mollusc; and 6B (bottom): relatively large mollusc shown in situ (length approximately 10cm).

Dr Eike Neubert confirmed that the fossils discovered in the Dhofar Mountains were a variety of marine, fresh water and terrestrial molluscs, likely to be from the Zalumah Formation (Eocene-Oligocene). The dominance of gastropod molluscs discovered may reflect a swamp environment where other genera would struggle to survive. Calcium carbonate cement has successfully fossilised these specimens, possibly in lakes associated with the sea level rise of the early Oligocene (Eike Neubert and Dirk van Damme, 2012), coinciding with the continental opening of the Red Sea and the Gulf of Aden (Martin Pickford et al, 2014).

Prof Dirk Van Damme and Dr Eike Neubert published a study about the nearby location of Dhofar (Wadi Darbat) on Palaeogene continental molluscs of Oman (Eike Neubert and Dirk van Damme, 2012) and identified numerous new species of gastropod (9 out of 11 species were stated as new). There lies a level of mystery as to the true identification of many of the fossil specimens discovered on this excursion, for some of these could in fact be species new to science.

Oman has presented to us unique and interesting geological deposits: from the geodes and archaeological tools of the Empty Quarter desert, to the folded fossil rich beds of the mountains. It would certainly be scientifically fascinating for further geological studies to be carried out in great depth and detail across Oman, because of its wealth of undisclosed information and extensive bedrock exposure.

Acknowledgements

For sharing their palaeontological knowledge, thank you to: Professor Walkden (University of Aberdeen); Dr Eike Neubert (Naturhistorisches Museum der Burgergemeinde Bern); and Prof Dirk Van Damme (Research Unit Palaeontology, Department of Geology, UGent). Also thanks to Andrew Stoke-Rees (Fig. 6A, 7B), Rachel Fisher (Fig. 8A) and Conor Bolas (Fig. 6B) for providing images of the specimens they found.

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Fig. 7A. Loose gastropod mollusc; and 7B (bottom): relatively large mollusc shown in situ (length approximately 10cm).
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Fig. 8A. Loose gastropod mollusc; and 8B (bottom): Relatively large mollusc shown in situ (length approximately 10cm).

Availability of information

Further images of specimens and information can be provided on request to clarissa.wright.09@aberdeen.ac.uk.

References

Peter Vincent (2008). Saudi Arabia: an environmental overview. Taylor & Francis. p. 141. ISBN 978-0-415-41387-9. Retrieved 22 August 2010.

Eike Neubert, Dirk van Damme (2012). Palaeogene continental molluscs of Oman. No. 20: 1-28. 21 December.

Martin Pickford, Emmanuel Gheerbrant, Sevket Sen, Jack Roger and Zaher Sulaimani (2014). Palaeogene non-marine molluscs from Oman: implications for the timing of uplift of the Dhofar Plateau and the opening of the Red Sea and Gulf of Aden. Geological Society, London, Special Publications 2014, v. 392, p. 93-105.


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