Dhruma Fm
Type Locality and Naming
Type section is the Tuwaiq Mountain scarp, Saudi Arabia (see Powers, 1968). Column: Saudi Arabia Outcrop, Saudi Arabia Subsurface, Kuwait, Oman Subsurface, Oman Outcrop, Bahrain, United Arab Emirates. Lower formation (2 of 5) in the Sahtan Gr of Oman subsurface. Local subdivisions (members)(: Upper Dhruma Fm, Middle Dhruma Fm, Lower Dhruma Fm, D7, D5+D6, D4, D3, D1+D2.
Synonym:
Lithology and Thickness
In Upper Dhruma Fm, Olive, calcareous, clay shale interbedded with white chalky limestone and Cream limestone with subordinate shale.
In Middle Dhruma Fm, Cream to tan limestone, mainly soft, with subordinate marl and shale, and, several prominent brown oolite beds. Cream soft limestone capped by a bed of hard oolite. Cream compact limestone with several thin calcarenite beds in upper part.
In Lower Dhruma Fm, Cream dense lithographic limestone, passing down into softer partly calcarenitic limestone with interbedded shale. Olive-green and golden-brown, slightly gypsiferous shale, with a few thin chalky limestone layers. Fossils scarce, none identified. Interbedded white and cream, mainly chalky, limestone and golden-brown, slightly gypsiferous clay shale; several thin hard limestone layers in lower part. Green to olive, in part varicolored clay shale, with several thin layers of calcaernite and fine sandstone in the upper part; in some places several meters of bedded gypsum at the base.
In Oman: Green to olive, in part varicolored clay shale, with several thin layers of calcaernite and fine sandstone in the upper part; in some places several meters of bedded gypsum at the base. In Oman Subsurface (reference well Yibal-85), the Dhruma is composed of mudstones and argillaceous limestones overlain by a slightly dolomitic, porous, grain-supported limestone. In Oman, the Dhruma Fm (and overlying Tuwaiq Mountain Limestone Fm shows a substantial and regular thickening (60 to 550 m) in a (north) westwards direction. The thickest development is in the Lekhwair area.
Relationships and Distribution
Lower contact
Regionally, the schematic strat column indicates the next older unit as Marrat Fm along with Pre-Dhruma unconformity. In UAE, it overlies the Izhara Fm. In Oman, the lower boundary is distinct and conformable, with limestones resting directly on clastics of the Upper Mafraq Fm; and in Oman, it is likely that the Mafraq-Dhruma transition is diachronous, as is the transition to the underlying Upper Mafraq Fm sandstones (Rousseau et al., 2006).
Upper contact
Regionally, the schematic strat column indicates the next younger unit as Tuwaiq Mountain Limestone Fm. In United Arab Emirates, it is Diyab Fm
Regional extent
Saudi Arabia Outcrop, Saudi Arabia Subsurface, Kuwait, Oman Subsurface, Oman Outcrop, Bahrain, United Arab Emirates. In Oman, the Dhruma Fm is confined to North and Central Oman and the northern part of South Oman; and this unit is seen at outcrop in the Al Huqf and the Al Jabal Al Akhdar. An equivalent sequence is present in the Musandam Limestone Fm in Ruus Al Jibal. It occurs in well-sections in interior Central Oman, but may be absent in South Oman by non- deposition or Cretaceous erosion over palaeohighs. The porous upper unit is probably continuous with the ‘Uwainat’ reservoir unit from Qatar and the Emirates (Sugden and Standring, 1975) and with the ‘Lower Fadhili’ reservoir unit in Saudi Arabia (Powers, 1968). Rousseau et al. (2006) correlate this porous Uwainat Member throughout Interior Oman.
GeoJSON
Fossils
In Oman, "Biozone F45 (Pfenderina salernitana, P. trochoidea). Palynozone 4009 (Ctenidodinium cf. ornatum) has been noted in the upper Dhruma but generally the Formation is characterized by Palynozone 4191 (Dichadogonyaulax spp.) and even down to 2262 (Sphaeropollenites scabratus). A Bathonian – Callovian age has been inferred from foraminiferal occurrences for the Dhruma of the Haushi area (Dubreuilh et al., 1992a), but Rousseau et al. (2006) argue that the assemblages can be reinterpreted as only Bajocian – Bathonian. They further suggest that the Dhruma-Tuwaiq Mountain boundary roughly approximates to the Bathonian – Callovian boundary. Rousseau et al. (2005) note a middle Dhruma brachiopod-rich bed of Early Bathonian – Middle Callovian age in Al Jabal Al Akhdar. Simon Petroleum Technology (1995) assigned Callovian ages to
the Dhruma Formation based on micropalaeontology (Trocholina palastiniensis, Posidonia spp. And associated taxa), and palynology (Dichadogonyaulax sellwoodii). In particular in two wells they interpret Early Callovian – Bathonian palynological associations (e.g. Korystocysta gochtii, Wanaea cf. acollaris). Jacovides et al. (1998) note that both D. sellwoodii and K. gochtii range up into the overlying Tuwaiq Mountain Formation (supporting a general Callovian age for that Formation) but that they are particularly characteristic of the Dhruma. They also recorded significant numbers of proximate dinocysts (Valensiella spp., Sentusidinium spp., Escharisphaeridia spp., Durotrigia spp., and Dissiliodinium spp.) and rarer ostracods and foraminifera as evidence to support a Bathonian – Late Bajocian age. Osterloff and Penney (1999) recognize similar gonyaulacid/ctenidodinoid dinocyst assemblages in sidewall cores from two Qarn Nihayda area wells. Woollam et al. (1999) similarly use dinocysts to indicate a Late Bajocian – Bathonian age for the Dhruma in well Lekhwair-319.
Age
Depositional setting
"The Dhruma carbonates in Oman represent a shallow-marine depositional environment. The mud-supported and somewhat argillaceous lower and middle units contain mollusc and echinoderm debris, and thin-shelled lamellibranchs of ‘pelagic’ aspect; so a sub-wave-base shelf setting is likely. The grainy textured sediments, particularly those in the upper porous unit, indicate shallow- shoal to intertidal settings. . . . [In Oman], "Rousseau et al. (2006) document depositional geometries evolving through time from a low-angle, homoclinal ramp dipping in a (north) westwards direction, to a pure aggradational, flat-topped platform (upper Dhruma and Tuwaiq Limestone Fm). This overall stratal pattern of the Dhruma-Tuwaiq Mountain carbonate platform is consistent with the general ‘shallowing-up’ trend inferred in the wells from logs. Aggradation continued in the overlying Tuwaiq Mountain Limestone Fm associated with a continued transgression of the shelf. Correlations by Rousseau et al. (2006) indicate a very low-relief system under permanent shallow-marine conditions without any further clastic input.
Additional Information