Dolomitization and chertification of the Early Eocene Rus Formation in Abu Dhabi, United Arab Emirates

Abstract

The Early Eocene carbonates of the Rus Formation in the Jabal Hafit area near Al Ain, United Arab Emirates, were sampled to study their diagenetic history. These carbonates were cyclically deposited in a shallow water environment, alternating with marls or marly limestones, and are characterized by extensive dolomitization and chertification (with associated glauconitization). The cyclic nature of sedimentation suggests that deposition was eustatically driven. The diagenetic history of the limestones proceeded through the following path: (1) marine cementation of allochems concomitant with formation of micrite envelopes; (2) micritization of peloids, ooids and skeletal grains; (3) neomorphism of formerly high Mg—calcite and aragonite grains to low Mg-calcite; (4) partial to complete dolomitization; (5) partial chertification of both calcite and dolomite; (6) glauconitization of chert; (7) fracturing during shallow burial; and (8) precipitation of late coarse calcite spar or mosaic and saddle (baroque) dolomite spar filling voids and fractures. Porosity is virtually nil in these rocks except for infrequent unfilled fractures, vugs and oomolds, due primarily to cementation and chertification.Biogenic grains are abundant in the limestones with Nummulites sp., Alveolinid and miliolid foraminifera and crinoid ossicles and other echinoderm fragments being most common. The tests of Nummulites sp. are commonly replaced by length-fast chalcedony fibers which are occasionally later replaced by glauconite. Spherulitic and zebraic chalcedony and, more rarely, megaquartz, occur as void-filling cements and fringing chalcedonic fibers frequently rim the tests of skeletal grains. All except the fringing chalcedonic fibers were subject to glauconitization.Prior to the chertification process is an extensive dolomitization period. Rhombic and sucrosic dolomite partially to completely replaced calcite at a relatively early diagenetic stage. Silicification caused the replacement of calcite by chert at a much more rapid rate than that of dolomite which was found “floating” within a chertified matrix. Fractures occurred both early and late in the Rus and are usually filled by a late coarse mosaic calcite spar (or coarse mosaic and saddle dolomite spar in completely dolomitized samples).