The broad lithologic, structural, and stratigraphic features of Neoproterozoic rocks exposed in northwestern Saudi Arabia have been characterized by reconnaissance geologic mapping, but precise information about the rocks is lacking.
Their detailed stratigraphic relations are unknown, their depositional and tectonic settings are ill determined, and their absolute ages are, in many cases, ambiguous.
Existing geologic maps of the region date from the 1970s and 1980s. This compilation is a revision of such map data, supplemented by interpretations of aeromagnetic data and Thematic Mapper imagery, and field observations, reinterpreted in terms of terrane concepts applied to western Saudi Arabia since the reconnaissance geologic mapping program, and stratigraphically constrained by newer geochronologic data. Neoproterozoic rocks underlie the northeastern two-thirds of the compilation area, partly exposed as the Arabian shield, and partly concealed beneath Paleozoic sandstone and siltstone, Cenozoic basalt, Tertiary sedimentary rocks, and tracts of alluvium, wind-blown sand, terrace gravel, and other types of unconsolidated Quaternary deposits.
The southwestern third of the compilation is occupied by Tertiary sedimentary rocks of the Red Sea basin superimposed on thinned and block-faulted Precambrian basement.
The bulk of the Neoproterozoic layered rocks comprise two volcanosedimentary assemblages mapped as the Birak (<870-?805 Ma) and Al ‘Ays (about 710-700 Ma) groups, and two predominantly sedimentary assemblages deposited on the volcanosedimentary groups mapped as the Hadiyah and Furayh groups.
The Zaam (~740 Ma) and Bayda (<775->725 Ma) groups underlie the far western part of the compilation area, the Mahd (about 775 Ma) and Ghamr (<760 Ma) groups occupy the southeastern corner, and volcanic and sedimentary rocks of the Banana formation (>740 Ma), Hadn formation (<630->610 Ma?), Shammar (±630 Ma) and Zarghat groups, and other rocks assigned to the Nuqrah (~840 Ma) and Hulayfah (>720 Ma) assemblages crop out in the east and northeast. Strongly deformed mafic-ultramafic rocks discontinuously exposed along several fault zones are among the oldest rocks in the compilation area.
They comprise the Wask-Ess complex (775-745 Ma), Bi’r Umq complex (±835 Ma), and the Bi’r Tuluhah complex (>840 Ma). Other igneous rocks, ranging in composition from gabbro to alkali granite and syenite, crop out as deformed batholiths, concordant with adjacent volcanosedimentary rocks, cross-cutting irregular to circular plutons, and dikes and sills.
The Rabigh suite (~805 Ma) is a gabbro-diorite-tonalite suite emplaced in the Birak group, as a probable arc-related batholith.
The Jar-Salajah batholith (±720 Ma) consists of granodiorite, tonalite, and subordinate trondhjemite and granite that, together with the Admar suite composed of granite, granodiorite, and diorite, intrude already folded volcanosedimentary rocks and sutures between various terranes in the compilation area, and are themselves locally deformed. Circular plutons of granite, monzodiorite, and syenite are among the youngest Neoproterozoic rocks in the compilation area. They represent igneous rocks that are coeval with some of the younger volcanic strata of the area and were emplaced in post-suturing tectonic settings.
The older, deformed and moderately metamorphosed (greenschist to lower amphibolite facies) layered rocks in the compilation area are believed to represent parts of the Midyan terrane in the west, the Hijaz terrane in the center, the composite Afif and the Ha’il terranes in the east and northeast, and the Jiddah terrane in the south. The younger, unmetamorphosed and relatively undeformed volcanic and sedimentary rocks are interpreted as post-amalgamation basins superimposed on these terranes following their collision. The zones of discontinuous mafic-ultramafic rock are interpreted as ultramafic-decorated sutures. Northwest-trending belts of orthogneiss, paragneiss, and high-grade schist (garnet-amphibole grade) in the compilation area resulted from ductile shearing, syntectonic granite intrusion, and amphibolite-grade metamorphism, and comprise a major element of deformation associated with the Najd fault system in northwestern Saudi Arabia.
Johnson, P.R., 2004, Proterozoic geology of western Saudi Arabia, North-Central sheet, (revised, digital edition), notes on proterozoic stratigraphy: Saudi Geological Survey Open-File Report SGS-OF-2004-5, 31 p., 6 figs., 1 plate.