--> ABSTRACT: Petrographic and Chemical Characteristics of Altered Low-Mg Calcite from Submarine Artesian Aquifer Extending for 10 Km Beneath Arabian Gulf, by Patrick F. Rush, Henry S. Chafetz, and Nancy M. Utech; #91022 (1989)
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Petrographic and Chemical Characteristics of Altered Low-Mg Calcite from Submarine Artesian Aquifer Extending for 10 Km Beneath Arabian Gulf

Patrick F. Rush, Henry S. Chafetz, Previous HitNancyTop M. Utech

An unaltered veneer of Holocene carbonate sediments, composed primarily of aragonite diagenetic facies (ADF), overlies a 5-23-m thick sequence of Holocene limestone stabilized to a low-Mg calcite (LMC) diagenetic facies (DF). The calcite diagenetic facies (CDF) has undergone dissolution of aragonitic constituents, stabilization of high-Mg calcite (HMC) allochems, and precipitation of nonluminescent isopachous LMC cements.

Differences between the aragonite and calcite diagenetic facies (ADF vs. CDF) reflect alteration in the presence of fresh water. The mean stable isotopic values of bulk samples from ADF vs. CDF are ^dgr13C = 3.6^pmil vs. 2.05^pmil PDB and ^dgr18O = 0.05 ^pmil -2.4 ^pmil PDB, respectively. Mean bulk elemental analyses from these two facies show that Sr decreases from 7,300 ppm (ADF) to 820 ppm (CDF), Na decreases from 9,300 ppm (ADF) to 5,700 ppm (CDF), Mg increases from 7,700 ppm (ADF) to 9,900 ppm (CDF), and Mn, Fe, and K increase into the CDF.

Individual biogenic HMC allochems, however, deviate from these trends. Red algae and echinoids exhibit a positive shift in ^dgr13C from -1.1^pmil to +1.9^pmil and +2.1^pmil to +2.3^pmil (ADF vs. CDF), respectively. Magnesium abundances decrease dramatically from 42,500 ppm for red algae (ADF) to 4,700 ppm (CDF) and echinoids from 23,100 ppm (ADF) to 6,000 ppm (CDF). The ^dgr18O values, Sr, Na, Mn, and Fe all show trends similar to bulk sample analyses.

Downflow, through 10 km of the aquifer (CDF), bulk rock analyses show an increase in Mg (5,000 ppm), Fe (1,000 ppm), K (500 ppm), and Mn (120 ppm). Stable isotopic values become lighter by 1.0 and 2.0^pmil (^dgr13C and ^dgr18O), respectively. Echinoids show increased concentrations of Mg (2,500 ppm), Sr (150 ppm), Fe (200 ppm), and Mn (30 ppm) downflow, whereas red algae show increases in Sr (280 ppm), Fe (60 ppm), and Mn (250 ppm).

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.