Proterozoic Geology of Western Saudi Arabia, Northwestern Sheet, (Revised, Digital Edition), Notes on Proterozoic Stratigraphy
Peter R. Johnson
The Proterozoic geology of northwestern Saudi Arabia comprises volcanic, sedimentary, and plutonic rock units of Cryogenian and Neoproterozoic III ages. The Cryogenian layered assemblages (principally Zaam and Bayda groups) are metamorphosed to greenschist and locally amphibolite facies and are deformed by folding, faulting, and shearing. They include volcanic flow and pyroclastic rocks of basaltic to rhyolitic composition. The rocks are typical of the calc-alkaline volcanic-arc successions that make up large parts of the Arabian-Nubian shield, except for one unit of banded-iron formation (Silasia formation), which is perhaps contemporary with the Sturtian “snowball glaciation” and is unique to the far northwestern part of the Arabian shield and adjacent parts of the Nubian shield. An ophiolite complex is present in the southeast, representing a remnant of the juvenile Neoproterozoic ocean floor originally present in the region. Arc-related Cryogenian plutons emplaced in the layered assemblages range in composition from layered gabbro, norite, diorite, tonalite, trondhjemite, and granodiorite. They are variably deformed and metamorphosed and have discordant to structurally concordant contacts with the layered rocks. Younger layered and plutonic assemblages are mostly Neoproterozoic III. The plutonic rocks, chiefly monzogranite, syenogranite, and alkali granite, are a dominant feature of the regional geology and make up about 50 percent of the exposure in the area. Neoproterozoic III layered rocks lie unconformably on the Cryogenian rocks and consist of unmetamorphosed conglomerate, sandstone, siltstone, basalt, and rhyolite that were deposited in small, commonly fault-controlled basins. Two major brittle-ductile shear zones cross the region and affect Neoproterozoic rocks of all ages. Orthogneiss, paragneiss, and paraschist developed from a variety of protoliths along the shear zones and along smaller anastomosing shears that link the two main shears. The youngest reliably dated Neoproterozoic rock in the region is a pluton of nepheline syenite, emplaced at 553 Ma, shortly before the end of the Precambrian. Following a major period of end-Precambrian-early Cambrian erosion, Lower Paleozoic sandstones (the oldest of which dates from about the middle Cambrian) were deposited unconformably on the Neoproterozoic. Their deposition marked the cessation of orogeny in the northern Arabian shield, and the transformation of the accreted rocks of the shield into a stable, shallow-marine to fluviatile shelf.
Johnson, P.R., 2004, Proterozoic geology of western Saudi Arabia, Northwestern sheet, (revised, digital edition), notes on proterozoic stratigraphy: Saudi Geological Survey Open-File Report SGS-OF-2004-4, 29 p., 4 figs. 1 table, 1 plate.