A new record of the earliest planktonic foraminifera Conoglobigerina sp. and Globuligerina sp. from the Middle Jurassic (Bajocian) D4 unit of the Dhruma Formation in Saudi Arabia

The Mesozoic Shaqra group has been the subject of numerous previous studies because of its 13 petroleum-producing reservoirs dispersed across different formations. Several geologists thoroughly examined the Dhruma Formation, the second formation of the Shaqra group, from a petrographic and sedimentological standpoint. However, other than the report by Kaminski et al. (2018), there are only a few micropaleontological investigations of the marly units within this formation, and no thorough evaluation of the maximum flood surfaces utilizing microfossil contents, such as planktonic foraminifera, has been carried out. The diversity, quantity, and preservation mode of planktonic foraminifera in older rocks could give insight into changes in past sea level or in other words the maximum flood surfaces (MFS). These free-floating foraminifera replace shallow-water species during transgressions as sea levels rise; their numbers peak at the maximum flood surface when sufficient water depths and nutrient-rich waters provide them with an ideal depositional environment. To provide new evidence for high-resolution sequence stratigraphy, we are attempting to trace the maximum flood surfaces within the carbonates and marl units of the Dhruma formation by examining their microfossil content. The material under consideration was collected from a marly D4 unit exposure (~15 m thick) located on Highway Road no. 5395, southwest Riyadh City, about 1 km from a Saudi poultry farm (N24°12'48'', E46° 14'5''E). The samples were obtained from the lower marly limestone part of the D4 Unit and examined using traditional micropaleontological procedures and the acetolysis technique. The main new finding of this study is that, as previously noted in Unit D5, the bottom part of Unit D4 contains the low-spired species Globuligerina sp. and the high-spired variant Conoglobigerina sp. Alongside the planktonic foraminifera, there is a benthic foraminiferal assemblage consisting of a mixture of smaller agglutinated species (Haplophragmoides and Nautiloculina) and calcareous species (polymorphinids and nodosariids). This assemblage suggests conditions on an open sea shelf and characteristics of a Middle Jurassic foraminiferal content from a marly carbonate unit. Considering their small size, calcareous benthic foraminifera indicate the presence of an oligotrophic depositional environment for Unit D4. The D4 Unit of the Dhruma Formation contains planktonic foraminifera, which provides a novel correlation tool for determining a new maximum flooding surface between the J20 and J30 maximum flooding surfaces in the Middle East.