Mixed deltaic and carbonate deposition in a tectonically restricted basin: An example from the lower and middle members of the Eocene Musawa Formation, Abat Basin, south‐eastern Oman Mountains

Abstract

Mixed siliciclastic and carbonate sediments of the Eocene Musawa Formation were deposited in the Abat Basin in the south‐eastern part of the Oman Mountains when the rest of the Arabian Plate was experiencing extensive carbonate sedimentation. The Musawa Formation was deposited as mixed siliciclastic and carbonate sediments in the Abat Basin bounded by the Ja'alan and Qalhat strike‐slip faults. Movements along these bounding faults and extensive deformation in the source area provided an enormous amount of clastic sediments in a laterally restricted basin undergoing strong tectonic‐related subsidence and was fed by basin axis parallel drainage system. The lithofacies assemblage represents fluvial and wave‐dominated deltaic sediments that accumulated as a local siliciclastic feature in a carbonate‐dominated subtropical environment. The Musawa Formation is divided into three members, the lower, middle, and upper, based on lithological characteristics; however, this study focuses on the lower and middle members of the formation. The lower Musawa member is about 350 m thick, comprising laterally confined conglomerate (lithofacies GL) beds interbedded with Alveolina‐bearing sandstone in the lowermost part of the formation. These beds were deposited as fan‐delta on top of the underlying Ypresian limestone of the Abat Formation. The conglomerate beds grade up‐section into over 170 m thick medium‐ to thick‐bedded channelized sandstone lithofacies (lithofacies CSL), occasionally interbedded with mudstone and peat streaks. The sandstone sequence northwards of the study area passes into fine‐grain lithofacies of siltstone and mudstone (lithofacies ML). The sandstone (CSL) was deposited in a delta‐plain setting with coastline shifting northwards. The enormous thickness of the sandstone sequence is due to the location of the depocenter around Wadi Musawa area, from where the streams entered the basin and deposited their sediment load. The upper part of the lower Musawa member comprises interbedded clay, siltstone, and thin‐bedded fine‐grain sandstone (lithofacies association LFA3) deposited as mud‐flat accumulations. The middle Musawa member is about 650 m thick, comprising both carbonate (LFA4) and siliciclastic (LFA2 and LFA3) lithofacies associations. The carbonate lithofacies association (LFA4) was deposited during flooding events. The marl lithofacies (CM) comprising branching corals, gastropods, and oysters was deposited in a lagoonal setting, whereas thick‐bedded limestone lithofacies (CL) comprising Nummulites was deposited in open‐marine conditions. Occasional coarse siliciclastic influx resulted in the deposition of mouth bar and shoreface lithofacies association (LFA2), especially in the upper part of the middle Musawa member. The mouth bar and shoreface sandstone interbedded with occasional chert‐bearing conglomerate are laterally persistent across the Abat Basin outcrops. The uppermost part of the middle Musawa member is defined by ridge‐forming, thick‐bedded open‐marine limestone (CL) extending across the Abat Basin. The upper boundary of this limestone facies (i.e., CL) marks the boundary between the middle and upper members of the Musawa Formation. This study helped in understanding the mechanism of mixed siliciclastic–carbonate deposition in the Abat Basin.