The Geology of the Jabal Al- Qunnah Area, Saudi Arabia PDF

Keywords

geology, Saudi Arabia, granites

Abstract

This paper describes the field geology of the area to the north of the Al-Jibub which is bounded by 41 degrees 05' E- 41 degrees 10' E longitude and 21 degree 00' N- 21 degree 05' N latitude. The Jabal Al- Qunnah is a elliptical body measuring 10 by 6 km with its major axis trending 032 degrees. The area has been studied topographically using aerial photographs, spot heights and contour lines, and has been divided into four landform categories. The highest point is 1620 m while the lowest elevation is 1400 m above sea level. This area broadly consists of hills of moderate relief cut by narrow and broad wadis (mostly fault controlled) in which the more common sediments are generally coarse gravels with some sand. Wind blown sand forms local small dunes. Sand and gravels derived from the local outcrops occur in the nearby wadis and samples from the alluvium wadi deposits have been studied geochemically. The solid geology consists of Bahah group metamorphics which are intruded by a large gabbro and peripheral ring granites. There are also numerous dykes, some of which are possibly of Tertiary age. To the west is a large granitic pluton that makes relatively low ground with a sand and gravel blanket through which small masses of weathered rocks arise. This granite cuts the gabbro and has a shear margin against it. The gabbro has been altered along shear zones and faults, and generally has developed a patchy greenschist facies metamorphic assemblage. The gabbroic body shows relict layering on various scales including megalayers some 300 m thick. The structure of this layered, inverted cone intrusion has been studied in detail by making several traverses, as a result of which, the igneous pluton has been petrographically divided into six different lithological units, clinopyroxene gabbro An 65, hornblende- clinopyroxene gabbro An55, hornblende gabbro (with kaersutite and An48-52), olivine gabbro, leucogabbro An55, and gabbro pegmatite with dark prismatic amphibole. There is a general reduction in colour index upward within each layer and from layer to another, the igneous rocks are probably of late Precambrian age and the metamorphics were formed about 1100 m.y. age. A development model has been postulated for the Al-Taif region which is applicable to the present area.