Proterozoic sedimentary rocks of the Arabian Shield consist of three depositional phases. Phase I (the oldest) includes the Baish, Bahah, and Jiddah groups, which consist of an immature assemblage dominated by fine-grained metagraywacke, graphitic schist, chert, marble, and subordinate polymictic conglomerate and metasiltstone. Clastic rocks of Phase I are composed solely of volcanic material and do not contain plutonic or sialic detritus. Metasandstone, polymictic conglomerate, coarse-grained metagraywacke, and abundant marble (stromatolitic in places) constitute Phase II, the Ablah, Halaban, and Murdama groups. Plutonic and volcanic components are abundant in the clastic suite; K-feldspar detrital fragments are common in the Halaban and Murdama. The Shammar and Jubaylah groups form Phase III. Sedimentary rocks of this phase are fine- to coarse-grained terrigenous clastic rocks, boulder conglomerate, and stromatolitic limestone and dolomite. All phases include abundant volcanic rocks.
Sedimentary basins, excluding those containing Phase III rocks, generally trend north, especially in the central and southern parts of the shield. In the central and northern parts of the shield, north-trending basins and structures are truncated abruptly by parallel northwest-trending sedimentary troughs (Jubaylah basins) formed by sinistral wrench-fault tectonics.
Textural and compositional characteristics of the Proterozoic sedimentary rocks together with spatial relations of the depositional basins document a progressive change from oceanic to shallow marine and continental sedimentation with time, and a shift in the basin axes from west to east. Sedimentary parameters during this lengthy timespan show general increases in compositional and textural maturity, carbonate content, grain size, the ratio of sedimentary rocks to volcanic rocks, and plutonic and K-feldspar-bearing detritus. The sedimentary style of each phase was a function of tectonic conditions during deposition.
