Sedimentary phosphorite has been deposited during a transition from clastic to chemical sedimentation as a result of upwelling of nutrient-rich waters coinciding with a Late Cretaceous (?) transgression of the Tethys sea in a part of northwestern Saudi Arabia. A 3- to 5-m-thick zone of phosphorite, chert, mudstone, and magnesian limestone reflecting this transgression is exposed at West Thaniyat.The phosphorite consists of apatite, mostly in pellet form, and clastic quartz with minor montmorillonite or magnesian calcite, and was deposited on a shallow shelf having a gently undulating configuration of basins and arches with a north-northeast trend. Phosphorite deposited in basins is indurated by clay; on arches it is indurated by limestone.There is a constant positive correlation between conditions of moderate turbulence and phosphorite accumulation--even to the extent that inclusions in pellets are clasts of quartz and microsphorite. Thus, apatite is suggested to be precipitated directly from sediment pore waters under conditions of moderate turbulence. Cessation of turbulence and upwelling results in accumulation of montmorillonite mudstone in basins and calcareous oozes on arches. The culminating event of each of two phosphorite-mudstone-magnesian limestone deposition cycles during this transition is a chert layer. This might result from explosive growth of diatomaceous blooms after completion of phosphorite deposition and may depend on changes in the nature of nutrient supply.