The Janban quadrangle, 20/44B, lies along the eastern margin of the Arabian Shield. The oldest rocks comprise the Precambrian Namtah Formation of the Halaban(?) group, and consist of basalt, dacite, siltstone, and calcareous greywacke metamorphosed in the lower amphibolite facies. Metamorphic rocks of uncertain Precambrian age in the northern part of the quadrangle are amphibolite, gneissic quartz diorite and metarhyolite that occur as roof pendants in plutonic rocks. Precambrian intrusive rocks, in order of decreasing age, are: biotite-hornblende tonalite; the locally gneissic Dawasir granodiorite and granite; the as Sabahah granite, coarse-grained alkaline granite; biotite-muscovite granite forming a small circular stock; the Janban granite, a coarse-grained non-foliated alkali-feldspar rapakiyl granite; aplitic granite; and fine-grained alkali granite that forms a stock, plugs, and dikes. Andesite dikes cut most of the plutonic rock units. Precambrian rocks are unconformably overlain by the Wajid Sandstone of Cambrian and Ordovician age, which covers the southeastern half of the quadrangle and consists of reddish-brown, medium-grained arkosic sandstone. Tertiary lake beds of gray marl, siltstone, and sandstone crop out locally, and lie unconformably on the Wajid. Most of the Wajid sandstone is mantled by an extensive blanket of Quaternary eolian sand and silt.
