ABSTRACT

Shallow-marine, coastal-plain, and fluvial sediments of the Lower Permian Gharif Formation in the Interior Oman Sedimentary Basin presently range in depth from outcrop in the Huqf-Haushi area to almost 5000 m in the northwest. This large depth range results from a varied subsidence history with only minor uplift during the Late Triassic to Early Jurassic.

The degree of diagenetic overprint varies, depending on burial history. Sandstone at shallow depth (<2600 m) shows intense dissolution of aluminosilicate and carbonate minerals, combined with minor kaolinite precipitation and compaction, whereas deeply buried sandstone (>2600 m) is highly compacted and tightly cemented by quartz, carbonate, and sulfate minerals. This varied diagenesis is caused by increasing temperature with burial, by surface-water infiltration during the Late Triassic to Early Jurassic, basin inversion and uplift in the Late Cretaceous along the southeastern basin margin, and obduction of the Oman Mountains in the Late Cretaceous.

Depositional pore water was initially of low salinity (<50 g/l) with ¹⁸O_SMOW values of -0.5 to -1.5o/oo in the marine Lower Gharif and 2o/oo in the arid Middle and Upper Gharif. During shallow burial (Late Triassic to Early Jurassic), pore water in the three Gharif members changed to high-salinity brines (180 g/l) with ¹⁸O_SMOW values of -4 to -7o/oo. Highly saline, deep-burial pore water with a heavy ¹⁸O signature (0.5-5o/oo) is the result of rock-water interactions in a rock-dominated system. Present pore-water compositions mainly reflect subsurface dissolution of Ara Group evaporites, precipitation of late diagenetic carbonates and sulfates, and recharge of surface water along the uplifted southeastern basin margin.