The Ediacaran–Cambrian and Ordovician rocks of Al Qasim Province, Saudi Arabia: Facies, depositional history and regional correlation

• This paper presents detailed field investigation of the Cambro-Ordovician rocks.
• The study deals with the facies analysis based on field and petrographic investigation.
• The depositional history of these rocks have been discussed.
• Correlation with their equivalent rocks in the adjacent localities.

In the Al Qasim region, Saudi Arabia, Ediacaran to Lower Cambrian facies are represented by polymictic conglomerates that occur locally in channels formed by the Najd fault system. The Middle–Upper Cambrian–Ordovician succession is exposed along the western margin of the Arabian Basin in the Al Qasim region, and is represented by the Middle–Upper Cambrian Saq Formation and the Lower Ordovician Anz Formation. The Saq Formation consists of two lithologic units. The lower unit comprises massive, trough and low-angle cross-bedded sandstones and siltstone–mudstone channel layers. It is interpreted as channel facies, probably deposited in a fluvial or estuarine environment. The upper unit of the Saq Formation includes sandy siltstone and bioturbated thin-bedded and rippled sandstone with the ichnogenus Cruziana and vertical burrows such as Skolithos. The Anz Formation consists of thin-bedded, bioturbated and laminated sandstones intercalated with bedded siltstone to mudstone. Siliceous (silicified) lithoclastic sandstone occurs at the top. These facies indicate coastal and shallow intertidal environments. The Anz Formation contains different ichnospecies of Cruziana, e.g., Cruziana cf., C. furcifera; C. huberi and C. goldfussi. These fossils are indicative of Early Ordovician age and can be used for correlation with the adjacent countries. Both the Saq and the Anz Formations were eroded in some areas by the glacial movement of the overlying Lower Ordovician–Lower Silurian Zarqa and Sarah formations. Sedimentation occurred on a wide shelf on a passive margin with low accommodation.

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