Circular geological structures outcropping in the sedimentary basins of Saudi Arabia

Abstract

Please cite this article as: Stewart, S.A., Circular geological structures outcropping in the sedimentary basins of ABSTRACT A number of circular geological structures outcrop in the sedimentary basins of Saudi Arabia, several are reviewed here to assess their mode of origin. They are unrelated to each other, are easily seen on aerial imagery, and their origins are assessed here on the basis of new fieldwork and reflection seismic data. The structures range in size from hundreds of meters to several kilometers in diameter. Jabal Rayah (28°39'N, 37°11'30"E) is 5 kilometers in diameter with ring faults and anomalously steep dips in Siluro-Devonian strata. Reflection seismic data demonstrate that underlying strata are structureless and Jabal Rayah is interpreted as a probable impact structure. Ash Shutbah (21°37'08"N, 45°39'21"E) is 2.3 kilometers in diameter and consists of a zone of concentric sub-horizontal folds in Mesozoic carbonates, surrounding a central area of disharmonic, steeply plunging folds and a mass of stratigraphically-anomalous sandstone. There are dissolution structures in the vicinity and although this is a viable explanation, the uniquely large and complex nature of Ash Shutbah suggests an alternative origin by impact. Reflection seismic data show that underlying strata in the vicinity of Ash Shutbah are structureless. A province of at least 100 circular structures occurs in northeast Saudi Arabia. Stewart : Exposed circular structures in Saudi Arabia Page 2Aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features. The Kidan crater (22°06'53"N, 53°26'38"E) is a subcircular feature approximately 100 m in diameter, exposed in Quaternary sabkha between large sand dunes in the Rub' Al-Khali. Field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies. 3D reflection seismic data shows a small fault underlying the crater, leading to a fluid escape interpretation for the depression.

3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of France

“…However, fossil-rich carbonate assemblages have always been considered as individual seep sites without comparing their distribution to the geometrical organization of modern clusters of seep sites. This lack of connection between isolated seep sites can be due 1) to poor outcrop conditions or stratigraphic uncertainty (Natalicchio et al, 2015), or 2) to the large size of modern pockmarks that are several hundred meters in diameter and a few meters only in depth (typically 500-800 m wide and 5-80 m deep) leading to difficult or incorrect correlations in the field (Gay, 2002;Stewart, 2015). To date, a fossil giant pockmark has never been identified nor characterized at outcrop, although some microbial assemblages have been reinterpreted in the light of examination of modern hot vents at mid-ocean ridges and cold seeps at continental margins (Whiticar, 1999;Gay et al, 2006).…”

Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt

“…The absence of vegetation and the reduced anthropogenic activity make desert regions ideal settings to study the interaction between geomorphological and geological processes. Although Aeolian deposits may hinder the study of localscale geomorphological structures, large-scale features may be recognized by satellite images (e.g., Parrot and Taud, 1992;Stewart, 2015). Using this approach, several studies were conducted in the Egyptian Western Desert (Barakat, 1994;Tewksbury et al, 2009Tewksbury et al, , 2012Tewksbury et al, , 2014.…”

Saqqar: A 34 km diameter impact structure in Saudi Arabia

“…() in Oman and Neville et al. () and Stewart (), other than Saqqar, remain to be substantiated by systematic studies on the occurrence of shock.…”

“…; Neville et al. ; Stewart ). Color shows the depth to base of the Phanerozoic as a guide to Phanerozoic basin thickness, simplified after Konert et al.…”