Khuff Fm
Type Locality and Naming
the Khuff Formation represents the product of a regional marine transgression over the margin of the Arabian continent. Column: Saudi Arabia Outcrop, Saudi Arabia Subsurface, NW Saudi Arabia, Bahrain, Kuwait, Qatar, United Arab Emirates, Oman Subsurface. Type section is near Ayn Khuff, Saudi Arabia (Powers, 1968). Divisions include Upper Khartam Mbr, Lower Khartam Mbr, Midhnab Mbr, Duhaysan Mbr, Huqayl Mbr, Ash Shiqqah Mbr, Basal Khuff Clastics.
Synonym:
Lithology and Thickness
The Khuff Fm is a sequence of dolomites and limestones with minor amounts of thin, shaly beds and one main interval (Middle/Top D Anhydrite elsewhere) with disseminated and nodular anyhydrite. Shelly fossils occur throughout the Middle to Lower carbonates (crinoids, brachiopods, corals, fusulinids, etc.), but are most common and best-preserved in the lower part over Central Oman. Towards the southeast, this virtually continuous carbonate sequence becomes interspersed with, and finally dominated by fine clastics, mainly in red-bed facies. In the extreme southeast truncation leaves only equivalents of the Lower Khuff present, developed in red-bed facies. In summary the Khuff Fm is predominantly represented by a continental red-bed claystone sequence (with occasional thin limestones) in South Oman, and a shallow-marine carbonate sequences in Central and North Oman.
In Saudi Arabia, it is further divided into Upper Khartam Mbr, Lower Khartam Mbr, Midhnab Mbr, Duhaysan Mbr, Huqayl Mbr, Ash Shiqqah Mbr, Basal Khuff Clastics. Alternating hard and soft, light-colored, chalky and dense limestones, commonly with thin hard shelly limestone layers. In some places considerable dolomitization is present. Cream-colored marls and red and green gypsiferous clays occur in moderate amounts in the upper half of the unit, and, in thin beds sporadically through the remainder. Generally, a basal 10 (or more) meter unit of sand and varicolored shale is present.
In Oman subsurface lithology column shown in "Middle East Geological Timescale 2008" by Al-Husseini et al., the Khuff Fm was divided into Limestone in Khuff Upper Member, Dolomitic limestone in Khuff Middle P40, Chalk in Khuff Middle P35, Clayey limestone in Khuff Middle P30, Evaporite and Limestone in Khuff Middle P27, Clayey limestone in Khuff Middle P23, Limestone in Khuff Middle P20, Dolomitic limestone in Khuff Lower P19, Sandstone in Khuff Lower P18, Claystone in Khuff Lower P17 whereas the description of lithology remain same. However, Forbes et al. (2010) only divide it into Lower Khuff, Middle Khuff and Upper Khuff, in which "the Permian – Triassic boundary is recorded at the boundary between the Middle and Upper Khuff (Saiq Fm to Mahil Fm transition in the outcrops of Al Jabal Al Akhdar in North Oman). …The top of the Middle Khuff coincides with the first downhole occurrence of Permian faunas (benthonic foraminifera, algae) and is routinely picked at the base of a grey-green shale unit, with a very distinctive log character. This shale is ubiquitous in the subsurface carbonate sections but cannot be correlated to outcrop sections in Al Jabal Al Akhdar, possibly due to facies variation or strong dolomitization. The Lower/Middle Khuff boundary in North Oman is extrapolated by log correlation from the South. It is not a clear boundary in the North. It is identified in the South by the first appearance of red/pale yellow/grey mottled claystones. … In the upper part of the Lower Khuff in South and Central Oman a ‘Khuff Marker Limestone’ (KML) bed is recognized. It loses its character northwards towards Lekhwair as the overall limestone development increases . . .Sequence stratigraphy: Sharland et al. (2001) place their Early Triassic MFS Tr10 within a clean carbonate, near the top Upper Khuff, as illustrated in well Hasirah-1 (their figure 4.29). Sharland et al. (2004) revised the age of this MFS to intra- Induan. They position their late Permian MFS P40, MFS P30 and MFS P20 surfaces within the Middle Khuff of the same well. Sharland et al. (2004) made minor revisions to the ages of the MFS P40 and P30 surfaces." (Forbes et al., 2010)
Relationships and Distribution
Lower contact
Regionally, the schematic strat column indicates the next older unit as Wajid Sandstone Fm. In Bahrain and Kuwait, it is Unayzah Gr. In Qatar, it is Haushi Fm.
Upper contact
The top of the Upper Khuff is conformable with the overlying Sudair Fm and is defined at the base of a persistent red shale unit above the Khuff carbonates. In Qatar, it is overlain by the Gulailah Fm;
"In Interior Oman, the Khuff Formation conformably overlies the Gharif Fm. The base of the Khuff Formation is taken everywhere at the base of the last recognisable limestone above the ‘Upper Gharif Shale’. Therefore situations occur where the base Khuff can be picked at the base of the Khuff Marker Limestone (KML, see Amal-9, Figure 9.8) and even at base Middle Khuff carbonates (Qaharir-4. This ‘Upper Gharif Shale’, below the base Khuff limestones and above the top ‘Upper Gharif Sandstone’ has also been called Lower Khuff Red Beds in South Oman and the Eastern Flank. Heward (2004) indicates that the previous use of the term Lower Khuff Red Beds is inappropriate in Central Oman, i.e. away from the Eastern Flank area. This shale unit was previously placed in the Khuff Fm and the top Gharif Fm was then picked at the first sand encountered downhole. Osterloff et al. (2004a) discuss this transition further. They suggest a cycle-based log correlation strategy and thereby illustrate the diachronous nature of the base of the Khuff Fm carbonates. The carbonate pick gets younger moving southwards (see their figures 27 and 28). Biostratigraphic control is unfortunately lacking at this time and it is not possible to fully confirm the time equivalences inferred. The reader needs to be aware of the dual terminology applied and the difficulties of correlation in this diachronous setting. A strict lithostratigraphic interpretation is applied herein (i.e. Khuff limestones versus Gharif Fm shales). In South Oman, the top Khuff/Upper Gharif redbed facies boundary is usually abrupt and formed by an unconformity overlain by Jurassic or younger units." (Forbes et al., 2010)
Regional extent
Saudi Arabia Outcrop, Saudi Arabia Subsurface, NW Saudi Arabia, Bahrain, Kuwait, Qatar, United Arab Emirates, Oman Subsurface. "The Formation is characterized by layer-cake type epeiric carbonate-evaporite platform deposition. Osterloff et al. (2004a) recognized 11 depositional sequences representing transgressive-regressive cycles. Markers can be correlated over several 100s of kilometers and the unit extends over more than 2,500 kilometers in a southeast-northwest strike direction and more than 1,500 kilometers in a southwest- northeast dip direction (Al-Jallal, 1995). The correlative succession on the Iran side of the rift is well described by Insalaco et al. (2006). In outcrops of Al Jabal Al Akhdar in North Oman of the Upper Saiq Fm and overlying Mahil Fm (lower part) are time equivalent to respectively the Lower/Middle and the Upper Khuff Fm. These outcrops are described by Glennie et al. (1974), Montenat et al. (1976), Rabu et al. (1986), Baud et al. (2001), Weidlich and Bernecker (2003) and Koehrer et al. (2010). The Lower Khuff Fm is well exposed in the Al Huqf area of Central Oman (Dubreuilh et al., 1992a; Angiolini et al., 1998, 2004). . . . At outcrop in the Al Hajar Mountains, the Saiq Fm (Glennie et al., 1974) is the lateral equivalent of the Permian part of the Khuff Fm (specifically the Lower and Middle Khuff). It differs in being a more uniformly marine facies (more diverse fauna, no shales or evaporites) and generally lies with angular unconformity on Lower Palaeozoic and Proterozoic sediments and basement. The Saiq Fm passes upwards conformably into Triassic Mahil dolomites, locally with a conglomerate marking the boundary (see Glennie et al., 1974). The Upper Khuff Fm is time equivalent to the lower part of the Mahil Fm. … During Lower Khuff times facies varied from fully carbonate in the North, via a full range of mixing, to fully clastic in the very south, where in strict lithostratigraphic terms an Upper Gharif Member designation is applied." (Forbes et al., 2010)
GeoJSON
Fossils
In Oman: "The Early Triassic Palynozone 2351 (Densoisporites nejburgii) with marine Palyno- subzone 1095 (Veryhachium spp.) is recorded in the Upper Khuff, whilst late – middle Permian Palynozone 2103 (Lueckisporites virkkiae) occurs in the Middle – Lower Khuff. Palynozone 2252 (Hamiapollenites spp.) may also be recorded in Khuff sections, although Osterloff et al. (2004a), would restrict its occurrence to the Gharif Formation. Consensus on how to consistently differentiate Palynozone 2252 from 2103 and their spatial and temporal relationships has yet to be resolved (see Osterloff et al., 2004a).
Angiolini et al. (2004) describe an abundant and diverse Wordian marine fauna in the Lower Khuff Formation of the Al Huqf outcrop area, dominated by brachiopods, bivalves, gastropods and cephalopods. The macrofauna also includes bryozoans, echinoderms, trilobites, corals, conularids and vertebrate remains. Foraminifera, algae conodonts and ostracods are also present.
An ongoing micropaleontological study of several outcrop (Al Jabal Al Akhdar) and subsurface sections has already yielded significant improvements in the ability to subdivide the Khuff Fm. Previously, only one long-ranging Biozone (FZA, Hemigordius sp.-Schwagerina sp.) was recognized, covering the Middle – Lower Khuff members. Data and initial interpretations for both outcrop and subsurface sections have been supplied by Daniel Vachard (Université de Lille) and for outcrop only by Holger Forke (Naturkundemuseum Berlin). From this data set it is evident that several key events within often rich microfaunal, and macrofaunal, assemblages provide a good opportunity for Khuff Fm subdivision, biozonation and calibration. The results complement and build on the comprehensive work on the Khuff equivalent microfaunas of Saudi Arabia (Vaslet et al., 2005 and Vachard et al., 2005) and Iran (Insalaco et al., 2006). The recovery and interpretation of assemblages from the Saiq Fm and Mahil Fm (Khuff Fm equivalents) in the Al Jabal Al Akhdar outcrops is hampered by extensive dolomitization. To date some of the best recovery has come from the subsurface and in particular well Yibal-192 (Vachard and Forbes, 2009, see also Koehrer et al., 2010).
Koehrer et al. (2010) place the Permian – Triassic boundary towards the top of the basal Upper Khuff shale. However, Forbes et al. (2010) retain the top Permian for subsurface use at the base of this shale. Significantly the base of this shale exhibits an uphole decrease in uranium alongside decreases in thorium and potassium, as seen in the Spectral Gamma wireline log. All three shifts are quite marked and the ‘Uranium event’ in particular is a recognized worldwide, end Permian phenomenon (see Insalaco et al., 2006). If ever cored, this claystone (unfortunately not seen at outcrop in Oman) could add considerably to our knowledge of events at the end of the Permian.
The occurrence of Shanita amosi (Late Capitanian) prior to the Middle Anhydrite is again a well- documented, regionally recognizable correlative event, associated with the so-called Mid-Permian faunal crisis. In addition, Vachard (2007) working on outcrop from Al Jabal Al Akhdar, recognizes an Induan Biozone (sporadic Hemigordiellina sinensis in uppermost Khuff), an early Changhsingian Biozone (with Glomomidiellopsis uenoi), a general Wuchiapingian Biozone (Neodiscopsis ambiguus) with several potential subzones, and an early Capitanian Biozone (Sphaerionia sikuoides). Forke (2009) ranges G. uenoi up into the late Changhsingian.
No proof of Wordian-age carbonates was seen in Yibal-192, however, Koehrer et al. (2010) cite evidence for a lowermost Khuff (Saiq Fm) middle to late Wordian age at outcrop, noted as a zone with Neoschwagerina schuberti. Forke (2009) cites additional supporting evidence and also interprets a middle- late? Wordian assemblage in outcrop, recognized by a Chusenella-Schubertella assemblage. This suggests that a degree of base Khuff diachronism may also be present in North Oman." (Forbes et al., 2010)
Age
Depositional setting
" Deposition of the Khuff Formation followed the break up of the Permian supercontinent Pangaea (Konert et al., 2001). It was deposited as post-rift cover on a passive continental margin of the newly formed Neo-Tethys Ocean during a period of relative tectonic quiescence and steady subsidence (Stampfli, 2000). Thus the Khuff Fm represents the product of a regional marine transgression over the margin of the Arabian continent. This was caused partially by a major tectono-eustatic event related to the onset of rapid subsidence of the early Neo-Tethys passive margin and the drowning of its rift shoulders and partially in response to the eustatic sea-level rise caused by deglaciation after the Carboniferous – Permian glacial phase. This transgression was over a stable shelf of very low relief with minimal clastic supply. . . . The faunas and the sediments of the Khuff Fm indicate environments ranging from shallow-marine through tidal-flat to mature coastal-plain of very low relief, all within a generally arid climatic setting. The floral recovery, however, suggests affinity with Upper Gharif Fm vegetation and the potential for a wetter/seasonal climatic influence. " (Forbes et al., 2010)
Additional Information