Lithostratigraphy and limestone microfacies of the Oligocene lagoonal, coral patch reef-bearing Maʼahm Beds (South Mawaleh, Oman)

Abstract

The Oligocene of the eastern area of the Arabian Plate is worthwhile studying due to it being both poorly exposed and understood. This first bed-by-bed lithostratigraphic/microfacies study of the Oligocene coral-bearing Ma’ahm Beds elucidates their depositional environments by analyzing their standard microfacies (SMF). The Ma’ahm Beds are >112 m thick. The basal Unit 1 is ≧22 m thick, dominated by thick-bedded foraminferal wackestones, packstones, and grainstones. Unit 1 evolves from restricted lagoonal conditions (SMF 16) to a transition toward open marine lagoonal conditions. Coarsening-up and thickening-up trends in Unit 1 last into Unit 2, which is 65 m thick, mainly characterized by very thick-bedded floatstones and float- to rudstones. This unit is more coarsely grained and thicker bedded than Unit 1, with prevailing corals and red algae. Unit 2 represents open marine lagoonal conditions (SMF 8). Unit 3 is 25 m thick and defined by alternating of boundstones (SMF 7) and floatstones (SMF 8), the dominant bioclasts being corals. Unit 3 represents an open marine lagoon, and its top contains a coral patch reef complex. The limestones are typically pure as the influx of siliciclastic material was negligible. The corals indicate clean water, lacking high amounts of suspended fines, which is compatible with (1) the slow regional doming of the southerly located Jabal Akhdar and Saih Hatat domes after 30 Ma (low/moderate relief), (2) the absence of the South Asian Monsoon (limited fluvial discharge of fines) and (3) the long-term rise of the eustatic sea level during much of the Oligocene shifting the depocenter landward. The lagoon was likely protected by coral barrier reefs, separated by reef gaps, allowing for an efficient water exchange with the open ocean. The coral patch reefs formed within the lagoon. Based on the scattered outcrop pattern of the Ma’ahm Beds, we suggest that future facies maps of the easternmost part of the Arabian Plate should consider marly deposits as the most widespread Oligocene sediment, while pure limestones (patch reefs and their debris) should represent only small speckles on such maps.