The Biostratigraphy and Sedimentology of the Aruma Group: An Integrated Outcrop and Subsurface Study in Ras Al Khaimah, United Arab Emirates

Abstract The Santonian-Maastrichtian age Aruma Group was deposited in a foredeep basin created by stratigraphic loading due to compressional thrusting and obduction of the Hajar Mountains. The Aruma requires high resolution biostratigraphy with seismic guided sequence stratigraphy to identify mappable units and unconformities across the foredeep basin (Swire, 2022). Aruma Group sediments outcropping in Ras Al Khaimah, United Arab Emirates were reviewed for reservoir potential and have been correlated with the subsurface Juweiza-Qahlah reservoirs of Campanian age that outcrop in Sharjah and further to the south in Abu Dhabi along the thrust front. The British Geological Survey completed geological mapping of outcrops in the Northern Emirates in 2006. The outcrops originally assigned to the Pabdeh Group were mapped in the absence of biostratigraphic data and there was uncertainty on the assigned age. RAKGAS, PKN ORLEN and Viridien undertook sedimentological logging and biostratigraphic analysis of eight samples from Tawi al Harf and Shaam outcrops near to RAK City to determine their affinity, age and depositional environment. The Tawi al Harf and Shaam samples were reassigned as Aruma Group after biostratigraphic analysis. The Tawi al Harf outcrop is a series of cherty/siliceous calciturbidites, including grainstones, rudstones and conglomeratic traction flows (0.1-2m) with clasts reaching up to 10cm, separated by a series of red, purple and grey shale beds. Samples were dominated by siliceous sponge spicules and radiolaria and contained few age constrained marker taxa, with a range of Middle Campanian-Late Maastrichtian age. Deposition occurred in an outer ramp to upper bathyal setting in >200m water depth, dominantly in the deeper bathyal range. Siliceous forms were autochthonously deposited under reduced carbonate conditions, with subsequent carbonate deposition via turbidity currents. This is potentially representative of the Juweiza Formation. The Shaam outcrop is a series of sandy-silty calciturbidite beds (0.1-1m) with hummocky cross stratification and normal grading, protruding between calcareous mudstones. Samples contain taxa that range from Early Campanian-Early Maastrichtian age. The abundance of globular planktonic foraminifera and the scarcity of larger, carinate planktonic foraminifera suggested deposition within a middle to outer ramp setting, between 50-200m water depth. Although different than the Tawi Al Harf outcrop, it is thought that the Shaam outcrop could be assigned to a nearer shore facies of the Juweiza Formation. Eight wells penetrating the Aruma Group were tied across Ras Al Khaimah, integrating sedimentological and biostratigraphic data obtained from outcrops and geophysical logs and biostratigraphic data from wells. Mappable units were correlated between wells and interpreted on 3D PSDM seismic data, revising the structural model by demonstrating thrusts within the Aruma. Compressional fault duplexes developed in front of the Hagab thrust, which thicken the Aruma with repeated sections in the present-day onshore subsurface area.