Circular geological structures outcropping in the sedimentary basins of Saudi Arabia

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 5km 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 2.3km 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. Aerial 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 100m 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.