On the coloured sandstones in Petra

J. Martín-Gil, M.C. Ramos-Sánchez and F.J. Martín-Gil

Introduction Petra is one of the world's most beautiful architectural sites carved from some of the oldest exposed sandstone on Earth: a Nubian Sandstone that is 450-550 million years old. Nubian Sandstone refers to a variety of sedimentary rocks deposited on the Precambrian basement in the eastern Sahara, north-east Africa and Arabia. It consists of continental sandstones with thin beds of marine limestones, and marls. Nubian sandstone was deposited between the Lower Paleozoic and Upper Cretaceous, with marine beds dating from the Carboniferous to Lower Cretaceous.[1] Kaolinite is the only clay mineral which is common to all the Nubian Sandstone parent materials[2] but in the semi-arid zones, as Petra, smectite (a probably pedogenic neoformation product) accompanies kaolinite. Some contamination of the clay fractions may have also occurred. [3]

By the Dead Sea, Nubian sandstone rests on Cambrian limestone: at Petra and other locations it rests unconformably on crystalline rocks. Nubian sandstone sometimes includes strata of clay and shale and thin seams of coal or lignite. Age calculation for Nubian sandstone is complicated in Western Sinai and the Dead Sea: since sandstone is assumed to form more rapidly than other rocks it is difficult to conceive that the 2,000 feet of sandstone in the southeastern Dead Sea was in the formative process from the Cambrian to the Cretaceous. [4] Nubian Sandstone is most commonly brown or reddish, but in places, as Petra, it shows a much wider variety of colours. The sandstone in Petra it is comprised of colourless quartz clasts (the particles in sandstone) attached by a coloured matrix binding that consist of clays with different composition both in mayor elements (silica, calcium, iron, aluminium, manganese) and minor elements (sodium, lithium, etc.). So the colours are a function of composition present in the matrix: browns from iron, yellows from Na and Fe, pinks from Na and Li, etc. These matrix elements (ie. Ca, Mg, Mn, Fe, Na, Li, etc) have mobilized and separated into what is commonly called 'Liesegang Banding' (banding in colour and composition of ores caused by diffusion) (Fig. 1). However, the banding has been erroneously attributed to its deposition, but in fact when examined the original bedding (from coastal fluvial and dunal deposition) is unrelated to these spectacular colours that are not to be seen in the same way anywhere else on Earth. In fact the colours are covered by a sort of 'drapery' given by recent and secondary deposition of simple calcium carbonates that are released from the matrix and then redeposited atop and below the sandstone. This drapery is not very durable and is easily broken-off or eroded. [5,6]

Fig. 1. Liesegang rings made of iron oxide in sandstone from a chamber in Petra, Jordan.

Results

The seven coloured earths of this study share 9 bands attributed to kaolinite: 3688, 3651 and 3618 cm-1 (OH stretching), 1114 cm-1, 1030 cm-1 (SiO deformation), 939 cm-1, 911 cm-1 (OH deformation), and 794 and 693 cm-1 (SiO stretching). The bands at 911 and 794 cm-1 also could be associated to goethite. The band at 3751 cm-1, assigned to quartz, is present in vermillion and Riotinto reds. Another quartz band, at 776 cm-1, is present in all the studied samples. Quartz interference also could occur at 1030 cm-1, in the region 785-820 cm-1, and at 693 cm-1. Another bands shared by all the samples are at 1610-1616 cm-1 and 1716-1722 cm-1, both assigned to oxalates. In the higher wavenumber region, whereas pale pink/vermillion red earths share two bands at 3956 and 3466 cm-1, garnet red/cinder grey earths share another two at 3778 an 3462 cm-1 (OH stretching, haematite). On the other hand, yellow/beige earths exhibit unique bands at 3994, 3232-3238 (OH stretching, goethite) and 2649 cm-1 (hydrogen bonding). So, these set of bands allowed the separate characterization of each pair of earths. In the central and lower wavenumber regions, whereas yellow/beige earths share a band at 606 cm-1 (PO4

3-?, clinoptilolite?), the tree red earths and the grey earth show a common and unique band in the region 1477-1508 cm-1 (COO, calcium oxalate) and another at 642-650 cm-1, presumably assigned to aluminous hematite. Finally, the comparison of the relative intensities of four main bands in the region 500-1500 cm-1 (Fig. 2) allowed to distinguish among red, yellow, beige and pink earths in Petra sandstones. Conclusion According to earlier results, the-following-relationship can be postulated: Purple/garnet red ……… Aluminous-HEMATITE + Fe-KAOLINITE Vermillion red ……… Quartz + Goethite + Al-HEMATITE + Fe-KAOLINITE Pink .......... Quartz + Goethite + Hematite + Kaolinite Yellow/Beige ……… Quartz + GOETHITE + KAOLINITE

W993300 (garnet red)

X993333 (vermillion red)

X663333 (Riotinto red)

Y996666 (cinder grey)

Z996600 (yellow)

XCC6633 (beige)

ZFF9966 (pale pink)

Fig. 2. ATR-FTIR spectra of coloured earths in Petra sandstones

References

1. B. Issawi, Review of Upper Cretaceous-Lower Tertiary Stratigraphy in central and southern Egypt. - American Association of Petrol. Geol. Bull., Vol. 56 No.8, 1973, p 1433

2. Qtaitat M.A. et al. Characterization of kaolinite of the Baten El-Ghoul region/south Jordan by infrared spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2005, 61, 1519-1523.

3. A. Singer, Characteristics of Nubian Sandstone - Derived Soils. European Journal of Soil Science 1974, p 310.

4. Tate, Ralph, On the Age of the Nubian Sandstone: Quarterly Journal of the Geological Society (1871); Vol. 27 No. 1-2, pp. 404-406

5. Paradise TR. Sandstone weathering thresholds in Petra, Jordan. Physical Geography, 1995, 16, 205-222

6. Paradise TR. Petra revisited: an examination of sandstone weathering research in Petra, Jordan, Geological Society of America, Special Paper 390, 2005, pp. 39-49.