Evolution of a Paleogene shallow-marine platform in the northern sector of the Ibra Basin (NE Oman)

Abstract

Studies integrating sedimentology and biostratigraphy are key to understand the spatial-temporal evolution of ancient shallow-marine carbonate platform systems. In this study we provide an outcrop example from the Ibra Basin NE Oman, displaying a 420 m-thick succession. The studied section starts with red beds of the Qalah Formation (Upper Cretaceous) which lay unconformably over irregular topography of the Semail Ophiolite. These are abruptly overlain by a 150 m-thick package of limestones rich in mircrofauna, and previously mapped as the Jafnayn Formation (Paleocene). These inner platform carbonates (lower Jafnayn Fm) grade vertically into a ca. 100 m-thick unit of open shelf nodular marly limestones and tidally-influenced calcarenites (upper Jafnayn Fm), and conform the main objective of this study. The mixed carbonate-siliciclastic marine environment is abruptly overlain, through a shallow angular unconformity, by 300 m of a siliciclastic-dominated shallow-marine to coastal succession present in a growth section, previously unmapped in the area. Vertical changes in the ecosystem and depositional conditions are reconstructed through a detailed analysis of sedimentological features and the larger benthic foraminifera. Three assemblages are identified ranging from SBZ 3 to SBZ 5. The assemblage found in the lower Jafnayn Fm contains the biomarkers Fallotella alavensis, Glomalveolina primaeva, Miscellanea juliettae and M. yvettae. This assemblage is ascribed to the SBZ 3, Selandian-early Thanetian in age. The upper Jafnayn Fm assemblage contains Miscellanea miscella, Miscellanites meandrinus, identified as SBZ 4, upper Thanetian in age. Finally, the siliciclastic-dominated deposits overlying the Jafnayn Fm are dated by the presence of Orbitolites sp. found in an intercalated carbonate level indicating at least the SBZ 5, Ypresian in age. These findings suggest the equivalence of these uppermost deposits to the Seeb Formation, which crops out in other localities within the Ibra basin, but their presence in the northern part of the Ibra Basin was not previously documented or dated.