Cretaceous chronostratigraphy, unconformities and eustatic sealevel changes in the sediments of Abu Dhabi, United Arab Emirates

Abstract

The Cretaceous of the United Arab Emirates is divided into three major lithostratigraphic units separated by three regional unconformities: Lower Cretaceous Thamama Group (Berriasian-mid-Aptian); Mid-Cretaceous Wasia Group (Albian-Cenomanian, possibly Early Turonian) and Upper Cretaceous Aruma Group (Coniacian-Maastrichtian). In the United Arab Emirates, because of the abundance of subsurface data, it has been possible to relate the character of the Mesozoic shelf carbonates and their associated minor clastics and evaporites to eustatic sea-level changes. The patterns of sedimentation were driven by gentle tectonic subsidence punctuated by eustatic sea-level variations. Most of the carbonates are sheets formed in response to deposition during sea-level highstands while some of the upper Lower, Middle, and Upper Cretaceous carbonates contain build-ups that caught up with the sea-level highstands following rapid marine transgressions that initially stressed deposition. Shale-rich units deposited during sea-level lowstands and transgressive phases are common in the Cretaceous sequences. They occur in the Lower Cretaceous Lekhwair Formation and Bab Member. The Middle Cretaceous includes the lowstand Nahr Umr Formation and the basinal Shilaif, while the Upper Cretaceous contains the transgressive Laffan shales. The timing of the older unconformity at the end of the Lower Cretaceous deposition coincides with a pronounced eustatic lowering of sea-level during the middle Aptian. From the Valanginian through the middle Aptian the stratigraphy of the United Arab Emirates was a time of relative crustal stability, and reflects a steady rising of sea-level with minor fluctuations. The sea-level was relatively higher during the Middle Cretaceous. Tectonism in the form of the collision of the Omani plate with the Arabian plate, played an even greater role in controlling deposition during the Late Cretaceous but the unconformity at the end of the Late Cretaceous correlates well with a major sea-level low.