The Murdama group is in the upper part of the sequence of Proterozoic layered rocks in the Precambrian shield of Saudi Arabia. It consists of clastic sedimentary rocks and small amounts of limestone and volcanic rocks. The Murdama crops out in the eastern part of the exposed part of the shield, within 200 km of its margin. The principal outcrop area, herein designated the Afif belt, is 600 km long and has a maximum width of 80 km. Two other significant outcrop areas lying to the south are designated the Jabal Hadhah and the Mistahjed belts.

The Murdama group unconformably overlies older plutonic rocks that range in composition from diorite to granite and volcanic rocks ranging from basalt to rhyolite. It is unconformably overlain in the north part of the Afif belt by sedimentary and volcanic rocks of the Jurdhawiyah group, and in many areas is intruded by younger granite.

In the south part of the Afif belt the Murdama group lies in the Maslum synclinorium, a structure characterized by tight folding near its northwest margin and open folding elsewhere. To the north, the synclinorium is much broken up by faulting, particularly by left-lateral faults of the Najd system.

Sections on the west flank of the Mast urn synclinorium in the south part of the Afif belt have apparent thicknesses of about 5,300 m at Jabal Murdama, 5,700 m at Jabal Raqabah, and 1 ,400 and 4,000 m at As Sawadah. The strata in these sections consist mostly of greenish-gray, very fine to coarse-grained volcanic arenite that is poorly sorted and consists of angular grains. Basal conglomerates are thin and inconspicuous, but coarse arenite, locally pebbly, predominates in the jabal Raqabah section. A prominent air-fall ash tuff caps Jabal Murdama, and an ash-flow tuff caps Jabal Raqabah.

Sections on the east flank of the synclinorium have apparent thicknesses of about 2,500 m at Jabal Farida, 3,600 m at Jabal Damkh, 12,400 m at Jabal Zaydi, and 16,800 m, adjusted for folding, at Wadi Sirrah. Jabal Farida and Jabal Damkh are underlain principally by limestone of distinctive tan- and gray-weathering types, which has been named the Farida marble. Limestones of these and other colors occur in all the belts composing the Murdama and form a unifying element. The other sections consist mostly of sandstone similar to that at Jabal Murdama.

Sections in the north part of the Afif belt have apparent thickness of about 3,300 and 7,600 m at Wadi Jarir, 840 m at Jabal Ajam, 534 and 1,833 m at Jabal Muwasham, about 3,800 m adjusted for folding east of jabal Silsillah, 4,900 m at Jabal Khidar, and 8,400 m at jabal umm Sammah. The Wadi Jarir section consists of sandstone very similar to that at Jabal Murdama, with thin basal conglomerate and local ash tuff. The other sections consist of conglomerate, sandstone, and siltstone.

The predominance of silicic volcanic lithic grains in Murdama sandstones of the Afif belt indicates a source in a silicic volcanic belt, and discontinuous belts of such rocks lie to the southwest. Paleocurrent indicators also suggest a source from the southwest. In the north part of the belt, the lower part of the Murdama is probably a subaerial mass-movement or fluvial deposit. The strata in the rest of the belt were probably deposited at a prograding lacustrine or marine strandline. Near the northwest edge of the basin, particularly from Jabal Farida north, locally derived conglomerate was deposited and carbonate bank deposition prevailed for a time.

In the jabal Hadhah belt, limestone is identical to that at Jabal Farida. It is apparently overlain by conglomerate that fines upward to sandstone and may be part of the Murdama.

Structure in the Mistahjed belt is more complex and the stratigraphic sequence is less certain than in the Afif belt. Murdama rocks have been assigned to a number of formations, and traverses were made across most units. The rocks include sandstone, siltstone, limestone, conglomerate, and volcanic rock; much of the volcanic rock may not be part of the Murdama group.

The source of sedimentary materials in the Mistahjed belt, as in the Afif belt, appears to be a belt of volcanic rocks lying to the southwest.

The Junaynah belt of sedimentary rocks lies to the west of the Mistahjed belt. The rocks consist of basal cobble conglomerate grading up to coarse- and finegrained sandstone. These rocks have been assigned to the Murdama group in the past, but I conclude that they should be restricted from it, because their tectonic setting and age appear to be different.

Radiometric ages suggest an age range of 650 to 615 Ma for the Murdama group, the laterally equivalent Hibshi formation, and the overlying Jurdhawiyah group in the Afif belt. Ages from the Mistahjed belt suggest that the Murdama rocks there are younger than 650 Ma and ages Abstract from the junaynah area suggest that the sedimentary section is younger than 61 0 Ma.

The Murdama group probably was deposited in a back-arc basin on a continental platform bounded on the west by an active volcanic arc above an east-dipping subduction zone. The position of the subduction zone, which was active during most of the deposition in the Afif belt, is marked by a belt of gabbro and ultramafic rocks herein named the jabal Burqah belt. The subduction zone later stepped out to the southwest to the Nabitah belt, and Murdama strata were deposited in the Jabal Hadhah, Mistahjed, and smaller basins.