The Harrat Tuffil quadrangle, sheet 20/39 B, is located between lat 20°30’ and 21°00’ N. and long 39°30’ and 40°00’ E. about 50 km southeast of Jiddah. A western belt of Proterozoic metapelites and related rocks, defined as the Sa’diyah formation, structurally underlies and locally is interlayered with Proterozoic metabasaltic rocks of oceanic chemical affinity to the east. The Sa’diyah formation and the metabasaltic rocks are tentatively correlated with the Sabya formation and Baish group. These rocks may represent a miogeosynclinal tectonic setting in the 800 to 850 Ma period of development of the southwestern Arabian Shield. The presence of aluminous metasediments suggest a continental source area, probably west of oceanic and (or) immature island-arc rocks of the Baish(?) igneous suite.

The Sa’diyah formation and metabasaltic rocks are intruded by a granite gneiss batholith, probably about 600 Ma old, and by small plutons of granodiorite and syenogranite. Peraluminous monzogranite occurs within the western part of the Sa’diyah formation belt and may provide an alternate source for the aluminous politic rocks.

Tentative correlation of an isolated exposure of Jurassic(?) dolomite with the Hanifa Formation significantly extends the areal range of possible Jurassic sedimentary rocks in the Tihamat province.

An intense dike swarm intrudes all of the pre-Miocene rocks in the quadrangle. This Damm dike complex is named for Wadi ad Damm. Dike chemistry shows both subalkaline and alkaline bimodal suites. The Sita formation is broadly coeval with the Damm Dike complex and contains volcanic rocks with similar bimodal chemistry. The Sita formation conformably overlies and is locally interbedded with the Sumaysi formation, which is palynologically dated in the Makkah quadrangle as Eocene (~50 Ma).  Potassium-argon dating indicated intrusion and volcanism over the period ~50Ma to ~20 Ma ago. The Tertiary volcanic, hypabyssal, and plutonic rocks are all light rare-earth element enriched as would be expected in a rift-related tectonic setting. These rocks represent proto-Red Sea igneous activity and are correlated with the Jizan group. Shama rhyolite near Harrat Tuffil is unconformably overlain by a boulder conglomerate correlated with the Bathan formation.

Miocene alkali basalt forms a large paleovalley-fill lava flow at Harrat ad Damm and small flow remnants north of Jabal Sita. Erosional downcutting at Harrat ad Damm averaged ~10 m per MA over the past 11 Ma. Pliocene alkali basalt unconformably overlies Bathan formation and Shama rhyolite at Harrat Tuffil.

The uplift of Quaternary reef limestone along the Red Sea coast indicates recent faulting as shown in the adjoining Shu’ayba and Al Ghalah quadrangles.

A perlite deposit at Jabal Shama may be suitable as light aggregate for concrete. Sparse barite veins were discovered in the lower Shumaysi formation. Relatively small deposits of Sa’diyah formation marble may be of interest for local use in building or cement. The Jurassic dolomite is locally quite pure and may have economic applications.