Facies architecture and depositional model for a fine-grained hybrid-energy delta: An example from the Upper Cambrian to Lower Ordovician Barik Formation, Central Oman

Abstract

Recognizing ancient deltas' depositional architecture is challenging due to the complex interplay of the fluvial, waves, and tidal processes. The integration of field-based sedimentological and trace fossils studies of the Late Cambrian-Early Ordovician Barik Formation in Central Oman allowed better identification of different lithofacies architectures and depositional process signatures. The Barik Formation is a major tight-gas reservoir in the Oman Sedimentary Basins, where it is over 800-m thick. The only outcrop analogue is in the Qarn Mahatta Humaid area, where ca. 50-m thickness of the formation is exposed and extends laterally over 5 km. This study revealed the presence of four lithofacies associations, including (a) mouth bar/shoreface, (b) tidal flat, (c) tidal channel, and (d) delta distributary channels using 12 logged sections. The depositional features identified in the field include tidal, wave/storm, and fluvial. The tidal signatures are represented by mud-drapes, large-scale reactivation surfaces, sigmoidal cross-bedding, and climbing and flaser bedding, predominantly in the mixed mud-sand tidal flats of the lower delta plain and delta distributary channels of the upper to lower delta plains. The wave/storm signatures are represented by wave ripples and small-scale hummocky cross-stratification in the mouth bar/shoreface of the delta front platform. The fluvial and subaerial signatures are represented by the occurrence of the scour surfaces with lag deposits of channel-fill and the development of desiccation mud-cracks. The Barik Formation delta sandstones form amalgamated sheet-like bodies prograding to the north-northeast and deposited along a ramp-type low relief basin. This prograding delta represents a highstand system tract. This study presents an example of the ancient fine-grained hybrid-energy prograding delta, where fluvial processes operated beside the tidal and wave/storm processes over a broad and low relief shelf in a vegetation-free system.