Bio-lithofacies and diagenesis in the early-middle oligocene of Abu Dhabi, United Arab Emirates

Abstract

The Early to Middle Oligocene is missing throughout much of the Arabian Peninsula. Only in the Oman Mountains and related linear NW-SE trending mountains are these sediments found. In the United Arab Emirates shallow water carbonates of the Asmari Formation (Early to Middle Oligocene) range from 435–481 m (1427–1578 ft), cropping out at the Jabal Hafit area near Al Ain. Detailed measured sections, sampling and thin section analysis show these carbonates comprise seven distinct facies: 1) Nummulitic Packstone, 2) Foraminiferal Wackestone/Packstone (non-Nummulitic forams), 3) Echinoderm/Red Algal Packstone, 4) Coral Framestone, 5) Peloidal Packstone, 6) Mudstone/Wackestone, and 7) Dolomite. Diagenetic events which have affected the Oligocene section include early cementation, formation of micrite envelopes, inversion of original aragonite and high Mg-calcite fabrics (i.e., biogenic tests and early marine cements) to low Mg-calcite, leaching of tests, dolomitization, stylolitization and fracturing, late diagenetic coarse calcite spar cementation, emplacement of bitumen within stylolitic seams and finally, hematite and pyrite staining. A depositional model has been proposed based on the fossil and lithofacies assemblages which suggests that a deep, outer shelf, quiet water lagoon supplies peloids to the back-reef, reef and fore-reef facies as well as the shallow open marine facies of the inner shelf. A short-lived sea level low is believed to have occurred, where continental waters manifested their presence in rare chertification and partial to complete dolomitization of some samples.