Basement and the Cryogenian ice ages
Oman's basement is young, juvenile arc crust: zircon ages from the Mirbat, Huqf, Jebel Ja'alan and Al Jobah areas fall between 838 and 765 Ma, and their isotopes say the crust was new, not recycled. A pegmatite at Mirbat dates at 808.92 ± 0.33 Ma. On this crust rift basins opened and filled with the glacial rocks of the Abu Mahara Group, which record two of the great Cryogenian glaciations. A volcanic tuff in the Ghubrah Formation of Jebel Akhdar dates the older at 711.52 ± 0.20 Ma; the Fiq Formation, the younger, is younger than about 645 Ma and is usually matched with the Marinoan glaciation that ended about 635 Ma.
The Nafun Group and the Shuram excursion
A cap carbonate, the Hadash, sits on the Fiq glacial deposits. Above it the Nafun Group (about 635–548 Ma) is about 1 km of shelf sediment laid down as the basin cooled and subsided, with no major break. It forms two grand cycles, each a clastic unit passing up into a carbonate ramp: Masirah Bay to Khufai, then Shuram to Buah.
The Shuram holds a famous geochemical event. The carbon isotope ratio of the carbonate, δ¹³C, falls from about +4‰ at the top of the Khufai to −12‰ in the red Shuram siltstones and recovers only slowly, reaching about +2‰ at the top of the Buah. It is the largest negative carbon isotope excursion known in Earth history, and it is not glacial. Elsewhere it is bracketed between 574.0 ± 4.7 and 567.3 ± 3.0 Ma by rhenium–osmium dating; in Oman the base of the Shuram is not directly dated (about 560 Ma by extrapolation).
For petroleum the Nafun matters less than its age suggests. Nafun marls reach 5 wt% TOC with hydrogen index up to 700, but a 2009 biomarker study found no molecular evidence of a major Nafun contribution to Oman's oils. The Buah dolostone is vuggy (bitumen fills the vugs at Qarn Aswad) and is a deep-gas target; Khufai and Buah fields are reported south of the Huqf.
The Ara Group: salt cycles and the Cambrian boundary
From about 550–548 Ma the Nafun basin broke into blocks, and the restricted Ara basins began to fill with salt. The Ara is a stack of six or seven cycles, A0 to A6, each about 1.2–1.3 million years long: a carbonate platform, then halite and anhydrite as the basin dried. Ash beds date the cycles precisely, from 546.72 ± 0.21 Ma in A0 to 541.00 ± 0.13 Ma about 1 m above the base of A4. That last ash fixes the Precambrian–Cambrian boundary at the base of the A4 carbonate, where a negative carbon isotope excursion and the last of the shelly fossils Cloudina and Namacalathus coincide.
The modern scheme (Grotzinger and Al-Rawahi, 2014) groups the cycles into formations: the Birba (A0–A3), the 'U' Formation (A4), and the Al Noor and Dhahaban (A5–A6), the last with halite over 1,000 m thick and potash. In deep water between platforms the Athel succession was laid down: the Thuleilat Shale, the U Shale and the silicilyte, a microlaminated chert of about 85% quartz up to 400 m thick in the fault-bounded Athel trough. In the mountains the Fara Formation of Jebel Akhdar is the same age (547–542 Ma) as volcaniclastic rock.
The Huqf rocks also hold one of the oldest records of animals: biomarkers of demosponges (24-isopropylcholestanes) occur below the Marinoan cap carbonate, older than 635 Ma, and continue for about 100 million years.
The Angudan event and the Nimr wedge
The Huqf ends in deformation. Movement on the western margin of the South Oman Salt Basin uplifted a deformation front that shed a clastic wedge, the Nimr Group (Karim and Haradh formations, Early Cambrian), now placed at the top of the Huqf. Then the Angudan event, tied to the collision of East and West Gondwana, culminated in the regional Angudan unconformity (about 540–520 Ma) that separates the Huqf from the Haima.
Why engineers care: this one package, a few hundred metres to a few kilometres thick, holds the source of most of Oman's oil, the salt that seals it, and two of its most unusual reservoirs (carbonate stringers and silicilyte).