Analyses of a large suite of intercalated carbonate and terrigenous samples from borings in the Arabian Gulf near A1 Jubayl, Saudi Arabia, reveal complex depositional and diagenetic patterns. The constituents composing the sediments and rocks are similar to those being deposited today in the Arabian Gulf. The most abundant allochems include molluscs, echinoderms, corals, red algae, pellets, ooids, and intraclasts. Terrigenous material ranges from trace amounts comprising the dominant framework constituent. The deposit is composed of three diagenetic facies. The lowermost part of the accumulation is a dolomite (at least 56 m thick) overlain by carbonates composed of low-magnesian calcite (5-23 m thick), while the uppermost portion is dominantly aragonitic in composition (averaging 5 m in thickness). The dolomite facies consists of sediments that accumulated in shallow-water to tidal-flat depositional environments. Dolomitization of the upper part of this facies took place as a result of evaporative processes during subaerial exposure. The nearly ideal stoichiometry, well-ordered structure, and relatively light delta 18 O composition indicates that the dolomites have undergone diagenesis since their formation. The low-magnesian calcite facies consists of shallow-marine deposits that have undergone diagenesis in a freshwater phreatic zone. These deposits underwent diagenesis (including dissolution of aragonitic allochems and cements, stabilization of high-magnesian calcite allochems, and precipitation of isopachous low-magnesian calcite cements) due to an extensive lens of freshwater that flowed through the sediments while they were beneath the marine waters of the Arabian Gulf. An aragonite-cemented crust acted as an effective upper seal for this freshwater lens. The common isopachous, fibrous aragonite cement indicates that the aragonite facies was deposited and has remained in the shallow-marine realm. Early freshwater phreatic diagenesis is generally interpreted to be associated with near-surface subaerial conditions. The presence of an extensive layer, extending at least 9 km seaward from the strandline, of shallow-marine sediments that have undergone freshwater diagenesis in the phreatic zone while under the marine waters of the Arabian Gulf suggests that previous interpretations of deposits exhibiting freshwater diagenesis may require reevaluation.