Abstract

Samples from a linear transect of borings in the shallow marine waters offshore of Al Jubayl, Saudi Arabia, represent dolomitic deposits (dolomitic facies) overlain by low-magnesium calcitic deposits (calcitic facies) and topped by a veneer of aragonitic sediments (aragonitic facies). The dolomitic fades, with its associated evaporites, accumulated and was diagenetically altered as part of Pleistocene sabkha complexes. The calcitic facies is composed of a wide variety of shallow marine Pleistocene carbonates and is petrographically indicative of diagenesis in a meteoric phreatic realm. The aragonitic sediments are Holocene 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), diagenesis in the meteoric vadose realm altered these originally aragonitic and high-magnesium calcitic sediments, with estimated mean delta 18 O and delta 13 C values of 0.0 per thousand and 3.3 per thousand PDB, respectively, to low-magnesium calcites, with mean delta 18 O and delta 13 C values of -3.5 per thousand and 2.0 per thousand PDB, respectively. (2) Under the present sedimentological regime, sabkha-derived evaporative waters, with relatively high delta 18 O values, descended from the modern coast, mixed with the regional groundwater, and flowed laterally gulfward within a confined phreatic aquifer (i.e, the calcitic facies) beneath the marine waters of the Arabian Gulf. Although the calcitic samples display diagenetic trends because of the descent and concomitant lateral migration of waters beneath an exposure 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 per thousand . Additionally, an overprint of the waters' path can be recognized to the seaward 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 within the calcitic facies. In contrast to analogous studies, diagenesis of the Al Jubayl carbonates due to the sabkha-derived waters resulted in delta 18 O values that are widely distributed, whereas the delta 13 C values are narrowly distributed, and the sediments show the highest delta 18 O values where the water had the greatest influence in the water-rock interaction; these trends are opposite to those of most other diagenetic sequences.

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