Spheroidal weathering of basalt from Gebel Qatrani, Fayum Depression, Egypt

Essam El-Hinnawi,S D Abayazeed,A S Khalil

doi:10.1186/s42269-020-00453-2

Essam El-Hinnawi, S D Abayazeed + Show 1 more

Open Access

https://doi.org/10.1186/s42269-020-00453-2

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Abstract

BackgroundThree stages of basalt weathering are known: early or incipient weathering, intermediate weathering and advanced weathering. The Late Oligocene basalt of Gebel Qatrani in Fayum Depression, Egypt, shows signs of early weathering, particularly exhibited in basalt spheroids found at the top of the basalt flow. The present paper gives the results of detailed petrographical, mineralogical and geochemical study of the weathering of these basalt spheroids.ResultsThe core-stones of the basalt spheroids are composed of phenocrystals of plagioclase feldspars and clinopyroxenes set in a groundmass of tiny feldspars and pyroxenes, relatively altered olivine and opaque minerals. The basalt is subalkali (tholeiitic). The outer weathered shells surrounding the core-stones are composed of partly altered feldspars and pyroxenes. The calculated weathering indices show that there is marked weathering trend from the core-stones of the spheroids to the outer shells. The chemical mobility of the elements shows marked depletion of Mg, Ca, Na and K from the core-stones to the outer shells due to the weathering of olivine, pyroxene and feldspars. The trace elements Rb, Sr, Ni, V, Cr and Zn are also depleted.ConclusionsThe weathering of basalt spheroids from Gebel Qatrani, Fayum Depression, Egypt, is of the incipient type. The degree of weathering from the core-stones of the basalt spheroids to the corresponding weathered shells indicates that the weathering occurred under predominantly semiarid to arid conditions.

Highlights

The Fayum Depression has an area of about 12,000 k m2 and lies about 100 km south-west of Cairo (Fig. 1)
We studied the variability of the element concentrations in the shells of the basalt spheroids after normalizing them to T iO2 concentrations and compared them to the core-stones
El-Saadawi et al (2014) studied the silicified fossil wood found in the Early Miocene Khashab Formation and found that the woods are characterized by having few wide vessels which are indicative of warm humid climate

Summary

Introduction

The Fayum Depression has an area of about 12,000 k m2 and lies about 100 km south-west of Cairo (Fig. 1). The geology of the area was studied in detail by Beadnell (1905), and his work has remained the basis for subsequent studies (Said 1962; Vondra 1974; Bown and Vondra 1974 1974; Bown and Kraus 1988; Gingerich 1992, 1993, among others) These studies indicate that the sedimentary successions encountered in Fayum Depression are of middle and late Eocene and Oligocene age and can be divided into the following, formations, from base to top (Fig. 1): Wadi Rayan Formation This formation is of middle Eocene age and forms the base of the Fayum Depression and is exposed in its southern part, reaching a thickness of about 130 m. The present paper gives the results of detailed petrographical, mineralogical and geochemical study of the weathering of these basalt spheroids

Results

Discussion

Conclusion