Micromorphology and genesis of Quaternary stromatolitic tufa crust within the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman.
The Al-Jabal Al-Akhdar is located in the central part of the Oman Mountains. It is an eroded antiform where approximately 2500 m thick succession of Precambrian to Cretaceous carbonates is exposed in its core (Haan et al., 1990). The study area is located in the nearby of Al-Hamra village within the Al-Jabal Al-Akhdar (Fig. 1) where a sequence of the Early Cretaceous latest Jurassic Rayda Formation is well exposed along a roadcut (Fig. 1). It occurs as thin bedded light grey hard porcellaneous limestone characterized by frequent occurrence of cephalopods fossils, mostly beleminites. Beds range in thickness between15 to 40 cm and vary in dip from nearly horizontal to less than 15o to the southeast. The exposed surface of the Rayda Formation displays a low step terrace morphology, where the surface is cut across in the dip direction. The thin bedding is clearly marked by reddish deposits along the bedding planes. Close inspection of these deposits revealed that they are calcareous tufa crusting narrow dissolution grooves along the bedding planes. They reach 30 cm in thickness and extend laterally along the bedding planes for several meters (Fig. 2Aand B). Twenty samples were collected from the tufa crust. The collected samples were slabbed and thin sec¬tioned. Hand specimens and slabs were macroscopically ex¬amined and photographed. Thin sections were petrographically examined where the structures and textures are described. Fifteen samples representing the various textures of the studied tufa were selected and examined using a Supra 50 LEO-141 variable pressure scanning electron microscopy (VPSEM) system equipped with an energy dispersive X-ray spectroscopy (EDS) detector. Seven samples were analysed by X Ray Diffraction and X Ray Fluorescence at the laboratories of Kuwait University. A set of 7 samples was selected for bulk sample analyses of mineral constituents, concentrations of major oxides and carbon and oxygen stable isotopes. The mineralogical composition of the samples was determined by X-ray powder diffraction (XRD) using CuKα1 radiation (λ= 1.5406 Å) and a Ni filter, at a voltage of 40 kV and a current of 40 mA. Semi-quantitative analysis of the major oxides was carried out using X-ray fluorescence (XRF). Fragments of millimeter size were selected from pristine parts of the columnar tufa stromatolites for stable isotope analysis that was performed at the stable isotope laboratory of the University of Arizona. The isotopes were measured using an automated carbonate preparation device (KIEL-III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252). Powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70°C. The isotope ratio measurement is calibrated and precision is ±0.10‰ for δ18O and ±0.08‰ for δ13C (1 sigma). The results are presented in δ‰ units relative to Vienna Pee Dee Belemnite (VPDB) standards (Table 1).