gems, analytical, geology, Red Sea, Saudi Arabia, olivine basalts, Harrat Kishb

Caves are known to occur in the Phanerozoic limestones of northern and eastern Saudi Arabia. They formed in past intervals of moist climate rather than the present hyperarid one. Few caves are known in the west because the predominant rocks are metamorphosed, tectonized and crystalline, comprising the Precambrian shield. There are however two geological units younger than the crystalline basement in western Saudi Arabia, where caves might be found. These are: (1) raised Quaternary coral limestones along the Red Sea coastline; (2) the extensive Cenozoic basalt lava fields. The present report describes the first and successful search for lava-tube caves in the Cenozoic basaltic lava fields of Harrat Kishb (area 5,892 km²). Cenozoic lava fields, or harrats, occupy 80,000 km² of western Saudi Arabia, where they mainly occur as 12 large lava fields in a chain extending from Syria in the north, across easternmost Jordan, southward for the entire length of western Saudi Arabia; and into Yemen in the south. The investigation reported here was largely confined to an area of central Harrat Kishb. Here, six lava-tube caves were located in three closely spaced areas in central Harrat Kishb in basalt lavas of four different stratigraphic units. All occur in lavas of the Cenozoic Hil basalt, which is less than one million years old. The Hil basalt is subdivided into six subunits.