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Sea Level and Current Dominated Diagenetic Regimes of the Kardiva Platform, Maldives

Abstract

Shallow carbonate platforms exhibiting concurrent or transitionary diagenetic influences of sea level and ocean currents have been described in the Grand Bahama Bank, Florida Straits, NE Australia and most recently Maldives. While sea level has often been invoked as a key control on carbonate diagenesis, less attention has been given to the role of bottom currents and fluid flow as they relate to diagenetic alteration. This is especially true for the shallow marine realm, as most studies of fluid flow have focused on diffusive processes in the deep burial realm leaving a gap in our understanding of shallow advective processes.Using bulk stable isotope analyses, strontium isotope dating, x-ray diffraction and petrographic methods, we sought to categorize the diagenetic signature of both sea level and increased fluid flow by investigating the variations in mineralogy, fluid chemistry, stable isotopes of C, and O at IODP Sites U1645, U1466, U1468, U1469, and U1470. Cement abundance generally increases within the platform while porosity decreases with depth, with some variability. δ18O and δ13C range from -7.0‰ to 3.2‰ and -7‰ to 2.5‰, respectively. Petrography and isotope values show evidence for subaerial exposure and alteration by meteoric fluids. The drift deposits overlying and adjacent to the platform show a consistent change in mineralogy characterized by the disappearance of aragonite and high magnesium calcite coupled with increased dolomitization. The interstitial water chemistry at U1465, U1469 and U1470 showed relatively constant values over the first 65 - 150 mbsf at these sites reflecting pore water with values similar to that of mean seawater. Prior to the drowning of the platform ~13 Ma, sea level was the major driver of diagenetic events. This is best demonstrated by the exposure of the platform between 14.3 - 14.7 Ma. Following drowning, bottom currents enhanced an advective flow regime, which appears to be linked to dolomitization events and possibly dissolution involving mean seawater. This multi-phase diagenesis of the Kardiva Platform suggests a complex paragenetic history that has direct implications for the development and quality of carbonate reservoirs during the Miocene in the Indo-Pacific region.