The Geology of the Middle Jurassic Dhruma Formation in Outcrops of the Oman Mountains (Sultanate of Oman)

Abstract

In outcrops of the Oman Mountains the Sahtan Group consists of the Early to Middle Jurassic Mafraq Formation and the Middle Jurassic Dhruma Formation. It is unconformably overlain by the Early Cretaceous Rayda Formation. Within an outcrop analog study, the Dhruma Formation was logged in several wadi sections of the central Oman Mountains, supplemented by spectral gamma-ray measurements and thin section analyses. A sequence stratigraphic framework and 2D correlations were established. In the type section in Wadi Sahtan, the Dhruma has a thickness of about 240 meters. Twelve facies types can be distinguished ranging from mud- to grainstones. They can be grouped into four lithofacies associations, representing depositional environments on a carbonate ramp. Environments range from a low-energy, mud-dominated lagoon to a high-energy, peloid and ooid dominated environment. While the lower part of the Dhruma is dominated by lagoonal mud- and wackestones, more and more peloidal and oolitic pack- and grainstones, approaching the high-energy environment, can be found towards the top. The Dhruma shows a pronounced cyclicity, recognizable in systematic, vertical lithofacies changes and characteristic gamma-ray patterns. The largest scale of cyclicity is tens of meters thick and can be correlated over a distance of 25 km between the studied sections.