Abstract
This work addresses the characteristic sedimentary facies and stable isotopic (δ13C and δ18O) signature of the tufa unit that occurs in the uppermost part of the stratigraphic succession at North and South Gunna hills in the Farafra Depression (Western Desert, Egypt). This has been done to reconstruct the depositional setting and genesis of the tufa unit. The tufa unit occurs as chaotic dark fallen blocks and pile-like carbonate mound covering the slope of a marine Paleocene-Eocene substrate. In outcrop and at close-up view, the tufa unit displays three main lithofacies: laminated, massive and granular tufa, which can be interpreted as deposited in a fluvial setting. The microfacies analysis reveals the dominance of a variety of micro-textures including: a) crystalline laminated texture showing different shapes (bacterial-shrubs, crystal shrubs, and recrystallized robust palisades of leaf-like crystals), b) clotted-peloidal texture and c) clotted-oncoidal texture, all of them reflecting an influence of biotic processes (likely cyanobacterial activity) during tufa deposition. The moderate covariance (r = 0.53) between δ18O and δ13C suggests some evaporation effect during the tufa precipitation. Low δ13C values (−11.27‰ to −7.64‰ V-PDB) values indicate that soil CO2 is principally derived from C3 plants, pointing to flowing waters with low residence time. The recorded low δ18O values (−10.82‰ to −8.50‰ V-PDB) are also consistent with carbonates precipitating from meteoric waters related with pluvial episodes impoverished in 18O by continental effects. The pluvial period from which the tufa was formed could be related to air masses that crossed from the Atlantic-Mediterranean Sea during interglacial time period, bringing rain to the Sahara at least as far back as 450 000 yr (Late Pleistocene) before present.
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•Late Pleistocene tufa studied at North and south Gunna at Farafra Oasis, Egypt.•Three tufa lithofacies are recognized: massive, laminated and granular.•The microfacies analysis reveals the dominance of different micro-textures.•Tufa was deposited in fluvial setting under the control of microbial activity.