Links between anhydrite precipitation and dolomitization during cycles of sea level change: Insights from the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates

Abstract

Despite extensive research in the field of evaporative dolomitization, the close link between anhydrite precipitation and dolomitization is still poorly explored within the context of sequence stratigraphy. This comprehensive diagenetic and sequence stratigraphic study of the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates, provides important insights into the relationship between anhydrite precipitation and dolomitization within the 2^nd, 3^rd, and 4^th orders of relative sea-level cycles. Initial good connectivity between the inner ramp and open sea during 2^nd order late transgressive-early regression cycles accounts for the limestone-dominated lithology with limited anhydrite formation. Conversely, the following progressive 2^nd order fall in the relative periodic restriction of the inner platform during 2^nd order early regressive cycles (early highstand systems tracts, HST) caused seepage reflux dolomitization and the precipitation of scattered to pervasive anhydrite cement in the dolostones. Variations in the extent of dolomitization were controlled by the permeability and reactivity of the precursor limestones, resulting in the formation of what is known as dolomite fingers. The most laterally extensive dolomitization during late 2^nd order regression resulted in the formation of microcrystalline dolostones with nodular and chickenwire anhydrite by a combination of evaporative sabkha pumping and the seepage reflux of lagoon brines. The δ¹³C and δ¹⁸O of calcite and dolomite reveal the influence of degree of restriction of the inner ramp, and related extent of seawater evaporation. The lower limestone-dominated interval (3^rd order HST) is characterized by lower δ¹³C values (+2.0‰ to +2.5‰) owing to periodic restriction of seawater circulation, which resulted in oxidation of organic matter during aging of the seawater. This study approach provides important insights into the genetic links between interbedded platform limestones, dolostones, and anhydrites, and hence a better understanding and prediction of reservoir quality distribution and compartmentalization.