Gharif Fm

Type and reference sections are the Wadi Gharif area in western Al Huqf and outcrops in Haushi area, northern Al Huqf. Named by Hughes Clarke (1988). The outcrop sections are much thinner than those recorded in the subsurface. The principal subsurface reference section is Safiq South-1 in Southern Oman. Column: Oman Subsurface, Oman Outcrop

Synonym: The ‘Saiwan Formation’ can be considered to be a local, outcrop term for only the Haushi Limestone unit of the Lower Gharif Member.

"The Gharif Formation is a sequence of sands and shales. Over most of the area, the Formation shows some correlative cyclicity of sands and shales and contains marine fossils in the lower part. The sands are less lithic than the Al Khlata, but are still relatively arkosic, although becoming more mature and quartzose to the north. The sedimentologically immature composition of the Gharif clastics show that they are most probably derived from the varied material supplied by the Al Khlata glacial phase. The fluviomarine reworking of this material allows the clastics to become more mature from south to north. The overall thickness of the Gharif and its individual members is remarkably constant (some 230 m over Central Oman).

"An informal subdivision into Lower, Middle and Upper members is applied. The Lower Gharif can be further subdivided into the Haushi Limestone and Basal Sands units. Shales/claystones of the Upper and Middle Gharif members are dominantly red and purple, whilst those of the Lower Gharif Member are usually grey. There are no differences between the sandstones of the Upper, Middle and Lower Gharif as they are subfeldspathic throughout, except those of the Basal Sands, which can be more quartzitic. The Upper and Middle Gharif are everywhere characterized by a clastic sequence. Thin calcrete horizons occur locally within the claystones and siltstones. Most Upper and Middle Gharif sands are only a few meters thick and lenticular, unless stacked vertically or laterally. The upper part of the Middle Gharif is characterized by the presence of a continuous red-brown claystone, known as the Middle Gharif Shale. In most of North Oman the Lower Gharif Haushi Limestone Bed (wackestone) is capped by a thin dark grey shale, which separates it from the subfeldspathic sandstones of the overlying Middle Gharif. The upper boundary of the Middle Gharif is taken at the top of the red brown ‘Playa Shales’ unit, underlying the sands of the Upper Gharif. In North Oman (Greater Saih Rawl area) the boundary is tentatively picked either below very thin sands or below silty sands which are overlain by thick high Gamma ray claystones of the Upper Gharif (Saih Rawl-2) (Kharusi et al., 1995). … This base of the Upper Gharif has been interpreted to be erosional or unconformable, but in his report on Central Oman, Heward (2004) classifies the base Upper Gharif as a rejuvenation of sand supply without either significant erosion or unconformity. … The top of the Lower Gharif is defined by and taken at the top of the first marine grey shale or dark grey, micaceous lagoonal shale below Middle Gharif red sediments."

Paleobotany and climate: Five Palynozones with 4 Palyno-subzones are recognized. Both zonal and subzonal identification rely on the recognition of quantitative changes in miospore assemblages. Unlike the previous zonal definitions no species names are applied to the Palyno-subzones. … Spore and pollen assemblage changes through the Lower Gharif indicate a rapid warming of climate following the melting of the last Al Khlata ice sheet (Stephenson and Osterloff, 2002). This warming and associated sea-level rise culminated in the deposition of the Haushi Limestone within a temperate climatic zone (Angiolini et al., 2003; Stephenson et al., 2008). Cold- climate lowland ferns of the Al Khlata were replaced by Glossopterid-type floras producing bisaccate pollen (gymnosperms or seed plants). Gymnosperms, possibly primitive conifers, dominated the Middle Gharif, even though the soil types encountered suggest that this seems to be the most arid climatic period of Gharif deposition. A change in the vegetation occurred in the Upper Gharif probably associated with a seasonally wetter (?monsoonal) climate. Lowland vegetation seems to have been dominated by pteridophytes, with some lycopsids, and conifers grew on the better-drained uplands. Plant beds are known from two levels in the Upper Gharif of the outcrops in the northern Al Huqf (Broutin et al., 1995; Berthelin et al., 2003, 2006) and are significant in containing a mix of Gondwanan (Glossopteris), Euramerian (conifer and ginkgo) and Cathaysian floras (Tingia). Silicified wood occurs widely in Gharif outcrops in the Al Huqf area in channel sands and is thought to be from large conifers (Broutin et al., 1995; Angiolini et al., 2001). Silicified wood also occurs quite abundantly as smaller, disseminated pieces in the transgressive and shallow marine Lower Gharif sands.

"The mainly clastic sediments of the Gharif Formation were deposited during the early – middle Permian, covering a period in the region of 25 million years. This was a time of gentle subsidence in which the Arabian Plate rapidly drifted northwards to lower latitudes, moving from a cold to a tropical climate zone. Oman was situated on the eastern flank of a wider intra-continental (Rub’ Al-Khali Basin) at the edge of Neo-Tethys. … The Gharif Fm is dominantly continental with shallow- marine deposits in the Lower Gharif. The continental deposits represent river channel belts, paleosols and lakes, and the marine deposits, coasts, lagoons and offshore shallow- water shoals. The variety of marine characteristics decreases from north to south, and in the southeast the sequence seems entirely continental, being largely red-bed facies. The Lower Gharif Basal Sands comprise an irregular blanket of shallow-marine shoreface sands which may in places intercalate with possible pro/paraglacial deposits. A marine- flooding shale then follows in South Oman (Palynozone 1115). This is also called the Maximum Flooding Shale by Guit et al. (1995) and may correlate with a shale immediately below the Haushi Limestone in North and Central Oman, where the presence of Palynozone. 1115 has yet to be conclusively proven (Stephenson et al., 2006). Stephenson et al. (2006) suggest that the sands above the Maximum Flooding Shale in South Oman broadly correlate with the Haushi Limestone elsewhere. Sheetlike sands in the Middle Gharif of western Oman appear similar to the marine deposits of the Basal Sands. Above this the Middle Gharif sands are channel sands that may develop into multilateral stacks. Calcrete-bearing paleosols and reworked calcrete conglomerates are common. The Middle Gharif Shale is a distinctive thick sequence of weakly developed paleosols (interpreted as aridisols), locally containing substantial sand bodies. Multi-storey and multi-lateral channel belts occur in the Upper Gharif of northern Oman. The Upper Gharif shale consists mainly of paleosols (interpreted as red-grey gleyed vertisols) with channel sands and subaqueous shales. The latter, in particular are considered to represent the initial transgressive pulse, which ultimately leads to Khuff carbonate deposition. Ostracods and fish scales and teeth occur occasionally in lake deposits in the Upper Gharif (and even in the Middle Gharif; Heward, 2004). The change from aridisols in the Middle Gharif to gleyed vertisols in the Upper Gharif reflects a climatic change from arid to more tropical, seasonally wet.

Subdivision details: "The lithostratigraphic subdivision of the Gharif was first established by van Vliet (1983), see Hughes Clarke (1988). The Gharif Formation can be divided into three sedimentary sequences, which correspond to the Upper, Middle and Lower Gharif members. These together with the Haushi Limestone and Basal Sands units of the Lower Gharif represent the best workable subdivisions. Additional historical (internal PDO) units like the Fringe Clastics and the Amal Sand have not been widely applied and their inconsistent use tends to hinder regional understanding, especially if applied without any palynological control. In South Oman, the lower part of the Lower Gharif Member is a complex of fluvial and fluviodeltaic clastics succeeded by marginal-marine clastics toward the top; while in North Oman similar lower clastics (Basal Sands) give way to the bioclastic limestone, known as the Haushi Limestone. Guit et al. (1995) identified a Maximum Flooding Shale within the Lower Gharif of South Oman. Osterloff et al. (2004a), divided the Lower Gharif into three sub-members; two thicker sandy units, separated by the thin Maximum Flooding Shale and associated bioturbated sandstones (which corresponds to MFS P10 of Sharland et al., 2001). Penney et al. (2006) distinguish an upper marine-influenced section and a lower transitional sequence in the Lower Gharif of South Oman, which correspond to Palyno-subzones 2105B and 2105A (and top 2141C) respectively. The Maximum Flooding Shale falls within the 2105B Palyno-subzone and is delineated by the 1115 Palynozone when developed in marine facies. Their upper marine influenced section interdigitates and grades into the Haushi Limestone further north. The Middle Gharif Member is a sequence of marginal-marine clastics overlain by lacustrine and fluvial units, capped by stacked paleosols (the ‘Playa Shale’ sensu Guit et al., 1995), deposited in a semi-arid climate. The unit is barren of palynomorphs in its upper part (Osterloff et al., 2004a). The Upper Gharif Member, comprising a paleosol-dominated sequence (red-grey, gleyed vertisols) with variable scale subaqueous shales and channel sands, overlies the Middle Gharif Member. Osterloff et al. (2004a) proposed a finer subdivision of the Gharif into eight cycles which were considered to reflect periods of increased and then decreased sediment influx. Many of these picks are placed ‘sequence stratigraphically’ in the middle of shales and others at the bases of sandstones. This resulted in the cycles being of limited practical use and this stratigraphic scheme offers no advantage over the basic lithostratigraphy outlined above and has not been widely adopted.