Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran

Abstract

ABSTRACT Here, a case example of a dolomitized Eocene ramp setting from the southeastern Zagros Basin is documented and discussed in the context of published work. This is of significance as well-documented case examples of Eocene dolomitized inner platforms are comparably rare. The same is true for detailed diagenetic studies from the Zagros Basin in general. Three measured field sections were combined with detailed petrographic and geochemical analyses and four main dolomite types were defined. The most significant dolomite type is present in the form of a volumetrically significant occurrence of meter-thick beds of strata-bound dolostones. These dolomites are characterized by near-stoichiometric composition, fabric-retentive and fabric-destructive textures, subhedral to anhedral in shape and most being in the tens-of-microns range. Dolomite δ18O (averaging -2.6‰) values are depleted relative to that expected for precipitation from Eocene seawater (averaging 0‰), while δ13C (averaging -0.1‰) values are within the range of Eocene seawater values (averaging 0.5‰). Dolomite Type II and III 87Sr/86S values from 0.7079 to 0.7086 are somewhat elevated with respect to Eocene seawater (0.7077 and 0.7078). Based on these data, it is suggested that moderately evaporated seawater, via shallow seepage reflux, acted as agent for the initial dolomitization process. Subsequently, early diagenetic dolomites were recrystallized during shallow burial to variable degrees. The absence of volumetrically significant evaporitic deposits indicates that the salinity of porewater during dolomitization was beneath the threshold limit for gypsum precipitation. In addition, ascending saline fluids from deep-seated salt diapirs might have affected dolomitizing fluids.