Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt

Adriano Mazzini,Matteo Lupi,Susanne T Schmidt,Alessandra Sciarra,Mohamed Hammed,Annette Suessenberger

doi:10.3389/feart.2019.00266

Abstract

Large-scale concentric structures are enigmatic geological features observed on the Earth surface and on other planetary bodies. Their formation has been attributed to several processes. Here we describe for the first time the results of mapping and characterization of ∼100 large concentric circular structures found in Early Cenomanian argillaceous strata of the Bahariya depression (Egyptian Western Desert). The geological processes that lead to the formation of these features have remained so far elusive. We investigate the concentric structures with a multidisciplinary approach combining field observations, statistical analysis, soil-flux gas measurements and laboratory analyses of rock samples. The whole depression is dissected by the ∼90 km long N60°E-striking dextral strike-slip Bahariya fault. Effusive lavas and shallow intrusions crop out in the down faulted blocks. The mapped circular structures increase in number approaching the fault zone. These features are up to 10 m high and 625 m wide, have a morphology similar in shape to impact craters with steeper external flanks and a gently dipping internal subsided zone. Halite-cemented brecciated sediments from different geological units have been sampled in the central part of the concentric circular structures implying a subsurface mechanism involved in their formation. Petrography analyses revealed also the presence of high- and low-temperature minerals (e.g., Ba-K-feldspars and ferroaluminoceladonite) suggesting former phases of hydrothermal circulation. Soil-gas flux profiles (CO₂ and CH₄) reveal a modest CO₂ increase when crossing the central part of the circular structures inferring enhanced permeability. Field and laboratory data are consistent with a scenario envisaging a diffused and vigorous hydrothermal venting. The proposed scenario includes multiple phases where several geological elements and processes interact. The Bahariya fault, which activity initiated during the Late Cretaceous, provided pathways for Miocene magma ascent toward the surface and for the development of a network of subsurface intrusions in the organic-rich sedimentary rocks of the Bahariya Formation. The interaction of the igneous intrusions with carbon-rich sedimentary deposits produced overpressured fluids, causing the formation of sparse hydrothermal vents at the surface. The elongation of the main axis of the vents and the deformed structures located within the strike-slip zone suggest that faulting controlled the emplacement and the final shape of some of the hydrothermal vents. We speculate that this system may represent a palaeo sediment-hosted hydrothermal system and could be related to the opening of the Red Sea.

Highlights

Circular structures are common geological features in nature, on planet Earth and on other solid planets and moons of our solar system
Similar morphologies are found in pull-apart basins (McDonnell et al, 2007), impact craters (Melosh and Ivanov, 1999), breccia pipes and hydrothermal vent complexes resulting from the eruption of fluids from sedimentary units affected by either igneous intrusions or hydrothermal fluids migration (e.g., Jamtveit et al, 2004; Svensen et al, 2006, 2007)
We propose that the circular structures (CS) mapped in the Bahariya Depression (BD) were the upper termination of paleo-hydrothermal vents triggered by subsurface magmatic intrusion intercepting C-rich sediments

Summary

Introduction

Circular structures are common geological features in nature, on planet Earth and on other solid planets and moons of our solar system. The mechanisms leading to the formation of these circular structures is often complex and elusive These geological features often represent the surface expression of complex phenomena occurring at greater depths. An insightful interpretation of the mechanisms leading to their formation may shed light on deep-rooted dynamics and on surface sedimentary and erosional processes. Such puzzling structures have been documented e.g., on Mars, Ceres, Europa and have been associated to cryovolcanism, karst dissolution, erosional processes of meteorite impact craters, dilational faulting, or magma rise-related (e.g., Fagents, 2003; Wyrick et al, 2004; Edgett, 2005; Levy et al, 2010; Sori et al, 2017). The surface eruptive expression of mud volcanoes may create circular structures (Mazzini and Etiope, 2017)

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