Depositional Environments of the Mamuniyat Formation in the Central Murzuq Basin, Libya

The depositional settings of the late Ordovician Mamuniyat Formation in the central part of the Murzuq basin, Libya were investigated using a combination of core (>2300’), wireline and image logs. The resulting interpretation was transformed for modelling purposes into a simplified semi-automated facies log and was then spatially rendered as gross depositional environment maps; both inputs for the resulting 3D facies model. Previous outcrop studies focussed on the Ghat and Gargaf areas (north and west of study area) differ in the interpretation of the Lower Mamuniyat. This study interprets the Lower Mamuniyat sandstones as sub-marine, deposited in a shallow marine shelf (lack of bioturbation and sharp juxtaposition of black marine shales with massive sandstones) in contrast to a deltaic deposition as interpreted in previous outcrop studies to the north. Facies on image logs and distinctive log responses enabled palaeogeographic reconstruction. Further up section, the Middle Mamuniyat was deposited in a deltaic setting emerging from the south, grading into a turbiditic setting towards the north, agreeing with the Ghat and Gargaf outcrop studies. The Upper Mamuniyat sediments are regarded as having formed on a braid delta with the ubiquitous truncated sediment packages reflecting the erosional nature of such a depositional system.