Much of the Oman Mountains was formed from allochthonous thrust sheets of the Semail Ophiolite and Hawasina Series, remnants of a basin which existed north and east of the Arabian shield during most of Mesozoic time. Cherts and siliceous mudstones collected from the various formations of the mountains range from Late Triassic to Late Cretaceous in age. A 1,445-ft (440 m) measured section was made of sediments of the Halfa Formation, the most distal facies in the Hawasina Series. The lowest part of the section is no older than Middle Jurassic (Callovian), on the basis of presence of Archaeospongoprunum sp., and the highest well-preserved sample is Early Cretaceous (Valanginian) in age, on the basis of the presence of Cecrops septemporatus. Although Radiolaria are abundant hroughout the section, preservation is strongly related to the color of the cherts and mudstones in which they are found--the green cherts and mudstones showing very poor preservation whereas the red show fair to excellent preservation. Halfa samples at other localities yielded Radiolaria

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associated with pectinacids (Halobia sp.) of Late Triassic age. In addition, samples of sediments interbedded with pillow lavas at the top of the ophiolite yielded well-preserved Late Cretaceous (Cenomanian to Turonian) Radiolaria.

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