2-PHASE DIAGENESIS OF QUATERNARY CARBONATES, ARABIAN GULF - INSIGHTS FROM DELTA-C-13 AND DELTA-O-18 DATA

Samples from a linear transect of borings in the shallow marine waters offshore of Al Jubayl, Saudi Arabia, represent dolomitic deposits (do lomitic facies) overlain by low-magnesium calcitic deposits (calcitic facies) and topped by a veneer of aragonitic sediments (aragonitic fac ies). The dolomitic facies, with its associated evaporites, accumulate d and was diagenetically altered as part of Pleistocene sabkha complex es. The calcitic facies is composed of a wide variety of shallow marin e Pleistocene carbonates and is petrographically indicative of diagene sis in a meteoric phreatic realm. The aragonitic sediments are Holocen e and have undergone only minor shallow marine diagenetic alteration. Whole-rock stable-isotope data (26 aragonitic and 112 calcitic samples ) show that the calcitic deposits have undergone two distinct episodes of diagenesis. (1) During Pleistocene sea-level lowstand(s), diagenes is in the meteoric vadose realm altered these originally aragonitic an d high-magnesium calcitic sediments, with estimated mean delta(18)O an d delta(13)C values of 0.0% and 3.3% PDB, respectively, to low-magnesi um calcites, with mean delta(18)O and delta(13)C values of -3.5% and 2 .0% PDB, respectively. (2) Under the present sedimentological regime, sabkha-derived evaporative waters, with relatively high delta(18)O val ues, descended from the modern coast, mixed with the regional groundwa ter, and flowed laterally gulfward within a confined phreatic aquifer (i.e., the calcitic facies) beneath the marine waters of the Arabian G ulf. Although the calcitic samples display diagenetic trends because o f the descent and concomitant lateral migration of waters beneath an e xposure surface, these trends differ from those recognized in similar studies. The subsequent diagenetic overprinting by the sabkha-derived surface waters has increased the delta(18)O values as much as 4%. Addi tionally, an overprint of the waters' path can be recognized to the se award end of the sample transect, almost 10 km from the shoreline, by a gradual decrease gulfward in the mean whole-rock delta(18)O values w ithin the calcitic facies. In contrast to analogous studies, diagenesi s of the Al Jubayl carbonates due to the sabkha-derived waters resulte d in delta(18)O values that are widely distributed. whereas the delta( 13)C values are narrowly distributed, and the sediments show the highe st delta(18)O values where the water had the greatest influence in the water-rock interaction; these trends are opposite to those of most ot her diagenetic sequences.