Ichnofossil insights into the Middle-Upper Ordovician Qasim Formation: implications for lithostratigraphy and paleoenvironmental interpretation in Saudi Arabia

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This study investigates the trace fossil assemblages and associated sedimentary facies of the Qasim Formation, aiming to reconstruct the depositional environments and ichnological signatures across a range of marine settings. Rather than attributing discrete environments to isolated ichnogenera, we interpret trace fossil occurrences within the framework of established ichnofacies models, including Skolithos ichnofacies (high-energy shoreface to foreshore), Cruziana ichnofacies (moderate-energy lower shoreface to offshore transition) and Zoophycos ichnofacies (low-energy distal offshore to deep marine settings). The assemblages reflect diverse behavioural modes: domichnia, fodinichnia and pascichnia expressed through vertical and horizontal burrow structures and varying bioturbation intensities. Ichnofabric analysis, including tiering and degree of bioturbation, in combination with sedimentological features (grain size, bedding structures and event beds), supports interpretations of high- to low-energy depositional settings ranging from shoreface to distal offshore environments. This integrated approach highlights the significance of trace fossil associations as environmental proxies when contextualised with physical sedimentary attributes, underscoring the value of ichnological-sedimentological integration in paleoenvironmental reconstructions. Importantly, the Qasim Formation spans the Middle to Upper Ordovician, a pivotal interval in early metazoan behavioural evolution and ichnofacies model refinement during the Ordovician biodiversification event.

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