The Jabal Ishmas quadrangle (20/43A) is about 100 km northeast of Qal'at Bishah in the southern part of the Arabian Shield. Most of the terrane in the quadrangle slopes gently westward across broad expanses of pediment, from prominent mountainous north-south ridges in the east to the alluvial and eolian sand-covered plains of Wadi Bishah. Other than the widespread surficial deposits, the quadrangle is underlain by Precambrian metasedimentary, metavolcanic, and intrusive granitic to gabbroic rocks.
Paraschist, paragneiss, and amphibolite of the Hali Group, the oldest rocks of the quadrangle, were derived from pelitic sediments, quartzite, limestone, and mafic to felsic volcanic rocks. The rocks were intensely deformed and regionally metamorphosed to the almandine-amphibolite facies during a long and complex period of Precambrian geological events prior to deposition of the rocks of the Halaban Group. Granodiorite gneiss, the most abundant rock in the quadrangle, is intrusive into and tectonically deformed with the Hall rocks. Relatively late during Precambrian time sedimentary and volcanic rocks of the Halaban Group were unconformably deposited on the Hali and granodiorite gneiss. The Halaban Group consists of a lower metaclastic unit of conglomerate, graywacke, shale, limestone, and chert, and an upper metavolcanic unit that is predominantly andesitic flow rocks, pyroclastic deposits, tuff, and subordinate basaltic rocks.
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USGS Numbered Series
Geology of the Jabel Ishmas Quadrangle, Kingdom of Saudi Arabia