Early diagenetlc carbonate cements are a common feature of Quaternary alluvial conglomerates in Oman. Cements are formed in the vadose and, more commonly, phreatic zenes from near-surface groundwaters. In drainage areas underlain by the Senmil Ophiolite, groundwaters have Mg2+/Ca 2+ ratios greater than two, and cements are often dolomite or high-magnesium calcite in addition to low-magnesim calcite. In drainage areas underlain by limestone, groundwaters have Mg~+/Ca z+ ratios of around one or less and cement mineralogy is nearly always low-nagaesium calcite. The oxygen and carbon stable isotopic ratios of the cements vary widely, from - 10.6%c to + 3.0%o PDB and from - 10.0% to + 0.7% PDB, respectively. Cement 5'sO values principally reflect variation in rainfail 5~sO over a time scale of severai thoosund years. Rainfall and cement ~tsO values probably are inversely correlated with the mount of rainfall, which is related to the frequency and intensity of the Indian Ocean nonsoon. Thus, cement B~sO is potentially a proxy indicator of relative rainfail and monsoon activity. For each of three sampling areas, ~t~C is positively correlated to B~sO. Cement 5~3C vaiues are also related to rainfall mount because rainfall controls the plant population. Greater plant respiration of isotopically depleted CO2 to shallow groundwaters and burial of organic mterial in conglomerate deposits results in lower cement ~13C values compared to periods of lesser plant activity.

Early diagenetlc carbonate cements are a common feature of Quaternary alluvial conglomerates in Oman. Cements are formed in the vadose and, more commonly, phreatic zenes from near-surface groundwaters. In drainage areas underlain by the Senmil Ophiolite, groundwaters have Mg2+/Ca 2+ ratios great...