JEDDAH: An international research team led by scientists from King Abdulaziz University has identified and documented the first marine sediments from the Oligocene Epoch in northwestern Saudi Arabia, a discovery that is reshaping understanding of the region’s geological history.
The newly recognized rock unit, named the Al-Qurayyat Formation, was discovered in Al-Qurayyat governorate in the Kingdom’s Jouf region and has been formally introduced to international scientific literature for the first time.
The formation consists of marine limestone deposits 15 to 26 meters thick. Researchers found fossil-rich gray limestone at its base, overlain by alternating layers of marl and limestone, and capped by coarse limestone containing fragments of sea urchins, sponge spicules and shellfish.
The sediments are exposed at Faydat Ar-Rishrashiyah, east of Al-Qurayyat, near the Saudi-Jordanian border.
One of the study’s key findings challenges previous geological interpretations of the area. Earlier studies suggested that the Eocene-age Ar-Rishrashiyah Formation directly overlies the younger Sirhan Formation. However, the new research identified locations where the newly defined Al-Qurayyat Formation occurs between the two units.
Researchers attribute these variations to tectonic faulting during deposition, which preserved the Al-Qurayyat Formation in some subsiding basins while preventing its preservation or removing it elsewhere.
The findings also redraw the stratigraphic framework of Paleogene marine deposits in northwestern Saudi Arabia. Rocks previously grouped within the Ar-Rishrashiyah Formation are now interpreted as two distinct geological units; an older Middle to Late Eocene sequence and the newly recognized Early Oligocene Al-Qurayyat Formation.
In an interview with Arab News, Dr. Mohammed Hamed Al-Jahdali, associate professor of paleo-marine environments and vice dean of the faculty of marine sciences at King Abdulaziz University, said that the team relied on microscopic marine fossils known as calcareous nannofossils to determine the age of the deposits.
“We identified 85 species belonging to 26 genera and placed them within the NP21 biozone, which corresponds to the earliest Oligocene, approximately 34 to 32 million years ago,” Al-Jahdali said.
The study identified the extinction of the species Discoaster saipanensis and Discoaster barbadiensis at the base of the formation, internationally recognized markers that occurred shortly before the Eocene-Oligocene boundary about 34.4 million years ago.
The team also detected a gap in sediment deposition at the base of the formation, indicated by the absence of the species Clausicoccus subdistichus, which would normally be expected in sediments of that age.
According to Al-Jahdali, the project aimed to rebuild the stratigraphic, biological and chronological framework of Paleogene deposits in northwestern Saudi Arabia using advanced microfossil analysis techniques that had not previously been applied in the region.
“For decades, geological interpretations relied largely on field observations alone,” he said. “This study integrates high-resolution fossil analysis with sedimentological investigations to provide a more precise geological timeline.”
The research also has practical implications beyond academic geology.
Al-Jahdali said that the findings helped to place the region within the broader geological evolution of the Arabian Plate, while creating a scientific database that could support future exploration of phosphate deposits, rare earth elements and hydrocarbon resources.
The Paleogene Period, which lasted from about 66 million to 23 million years ago, witnessed major climatic and tectonic changes, including the gradual collision of the Arabian and Eurasian plates and the closure of the ancient Neo-Tethys Ocean. About 34 million years ago, Antarctica became glaciated for the first time, causing global sea levels to fall and triggering major environmental changes worldwide.
At that time, much of the Arabian Peninsula formed part of a shallow marine margin along the southern edge of the Neo-Tethys Ocean.
Although the Al-Qurayyat region is now dominated by desert landscapes, the study presents multiple lines of evidence confirming that the rocks were deposited in a marine environment. These include abundant marine microfossils, limestone and marl deposited in offshore settings, trace fossils left by burrowing organisms on the seafloor, and fragments of sea urchins and marine shells preserved within the rocks.
According to the university, about 34-33 million years ago, the Al-Qurayyat area lay beneath a shallow sea connected to the Neo-Tethys Ocean. As the Arabian Plate gradually collided with Eurasia, the sea retreated and land emerged. While some low-lying basins remained submerged longer than others, the region eventually transformed into the landscape seen today.
“What we now see as desert was once an ancient seafloor,” Al-Jahdali said. “It was uplifted, deformed, buried, and later exposed through millions of years of tectonic activity and erosion.”
Researchers believe the discovery marks the beginning of a new phase of geological exploration in northwestern Saudi Arabia. Additional Oligocene deposits may remain preserved within down-faulted tectonic blocks that have yet to be studied.
The findings could also have economic significance.
“The close relationship between the newly documented strata and phosphate deposits in the Sirhan-Turaif Basin may help improve mineral exploration models and identify new areas of resource concentration,” Al-Jahdali said. “The revised geological framework may also contribute to reassessing the region’s hydrocarbon potential by providing new models for oil and gas trapping systems.”
The study was conducted by a research team led by Al-Jahdali, alongside Dr. Ibrahim Ghandour, Dr. Ramadan Al-Qahawi, Dr. Mahmoud Fares and Dr. Birgit Wade. Graduate researchers Mazen Al-Saddah and Khalid Al-Ruzaiqi also contributed to the project.
