The Upper Jurassic to Lower Cretaceous, Oxfordian to Valanginian, lithostratigraphic succession of Saudi Arabia is divided into the Jurassic Shaqra Group and the Cretaceous Thamama Group, the boundary of which lies within the uppermost Tithonian. This interval includes at least five third-order sequence boundaries and maximum flooding zones. J60 and K40 are two regionally recognized sequence boundaries, and J50 and J100 represent two regionally established Jurassic maximum flooding surfaces. The Upper Jurassic succession includes shallow- to deep-marine carbonates of the Hanifa (Oxfordian) and Jubaila (Kimmeridgian) formations, and shallow-marine carbonates and evaporites of the Arab (Kimmeridgian) and Hith (Kimmeridgian to Tithonian) formations. The Upper Jurassic to Lower Cretaceous succession includes the shallow to moderately deep carbonates of the Sulaiy Formation (Tithonian to Berriasian) with the overlying Yamama Formation being of Cretaceous age (Valanginian). These formations were deposited in a period of increasing global temperature, during a transitional phase leading to greenhouse.

Carbonates of each formation host grainstones and packstones of reservoir quality, including the Hanifa (Hanifa Formation), Arab D (Jubaila-Arab formations), Arab C to A (Arab Formation), Rimthan, Hith stringers and Manifa (Hith Formation), Lower Ratawi (Sulaiy Formation) and Upper Ratawi (Yamama Formation). Seals to these reservoirs are provided by transgressive muds of overlying sequences or, in the case of the Arab and Hith formations, by interbedded evaporites.

Chronostratigraphic control of the Oxfordian to Valanginian succession is determined using a variety of methods of variable refinement. Of the non-biostratigraphic techniques, the Callovian to Aptian strontium-isotope character is featureless except for a gradual increase in the 87Sr/86Sr isotope ratio, and provides very limited stratigraphic control. Of the carbon and oxygen isotopes, δ16O displays a gradual decline from the Kimmeridgian to Middle Valanginian, but δ18C displays a clearly defined positive trend at the basal Berriasian. Although the Oxfordian palaeomagnetic character is highly variable, the Kimmeridgian to Valanginian displays palaeomagnetic reversals at a lower frequency and these can be used for regional correlation.

The succession is difficult to biostratigraphically date with precision, owing to the generally restricted palaeoenvironments and the mostly endemic nature of the regionally significant species. Type sections of Saudi Arabian formations within the age of interest have been dated at outcrop using ammonites and nautiloids, supplemented by foraminifera. Subsurface equivalents are easily recognized using biofacies, but are dated mostly on benthonic foraminifera and calcareous alga, as calpionellids have not yet been identified in the basal Cretaceous carbonates and planktonic foraminifera are rare and long ranging. The elusive Jurassic/Cretaceous boundary is based on calcareous nannofossil evidence. With cautious sensitivity to biofacies diachroneity, high-resolution intraformational stratigraphy is possible with establishment of local biofacies-based events of correlative value.

Adi Priyadi Kadar (Kuwait Oil Company <>), Stephen Crittenden (Addax Petroleum Services) and Khalaf Abdulaziz Karam (Kuwait Oil Company)

A new record of calcareous nannofossil datum markers from Middle Jurassic to Early Cretaceous (Bajocian to Valanginian) strata of onshore North Kuwait has been calibrated with nannofossil marker species of the global Jurassic to Early Cretaceous nannofossil biostratigraphy schemes of Bown and Cooper (1998), Bralower et al. (1989), Bown et al. (1998), Perch-Nielsen (1985), and compared with Lower to Middle Cretaceous calcareous nannofossil zones offshore Kuwait (Al-Fares et al., 1998)(Figures 1 and 2). The studied sections embrace, in ascending order, the Dhruma, Sargelu, Najmah, Gotnia, Hith, Makhul, Minagish and Ratawi formations, and comprise argillaceous limestones, grainstones, packstones, bituminous packstones, wackestones, dolomite, anhydrite, laminated bituminous calcareous mudstones and calcareous shales. These units represent a variety of environments from marginal marine (sabkha) and shallow hypersaline (salina), to fully marine mid-to outer-shelf settings (Neog et al., 2010; Crittenden et al., 2012).

The association of Assipetra infracretacea, Calcicalithina oblongata, Rucinolithus wisei and Tubodiscus verenae was identified in the Ratawi Limestone Member of the Ratawi Formation indicating the Lower Valanginian NK3a Subzone of Bralower et al. (1989). The nannofossils recorded in the Minagish Formation were few and include A. infracretacea, C. oblongata, R. wisei, Cyclagelosphaera margerelii, Watznaueria barnesae, and an influx of Nannoconus spp. suggesting a Late Berriasian age. The lower part of the Makhul Formation contains A. infracretacea, C. margerelii, Polycostella senaria, W. barnesae, W. britannica and W. manivitiae suggesting an age no older than the Early Berriasian.

Shale laminae in the Hith Formation contain the index fossil Polycostella beckmanii indicating a Late Jurassic (Tithonian) age. Nannoflora recovery in the lowest part of the non-evaporites portion of the Gotnia Formation (Neog et al., 2010), herein correlated with the calcareous bituminous mudstone of the Najmah 1 Member (Yousif and Nouman, 1997), is abundant and contains Watznaueria spp. but Lotharingius crucicentralis and Nannoconus spp. absence suggest that the sediments were deposited within Kimmeridgian to Tithonian interval in a marine inner to middle-shelf setting. The non-evaporitic portion of the Gotnia Formation overlies unconformably the Najmah Limestone and is overlain conformably by the Gotnia evaporites Formation. It might correspond with the Jubaila Formation that is sandwiched between the Najmah Limestone and Gotnia evaporites distributed in the Gotnia Basin of Kuwait (Al-Sahlan et al., 2011).

The Najmah Limestone is barren of nannofossils, yet the upper Najmah Shale contains the robust and dissolution resistant species W. barnesae and the high birefringence W. manivitiae, W. britannica and W. fossacincta indicating Oxfordian–Callovian lower NJ19 zone or older. The majority of the specimens are poorly preserved with the inner part of the coccolith covered by oil staining. The lower Najmah Shale contains common to few, moderate to poorly preserved W. britannica and W. manivitiae but C. magharensis absence, suggesting Early Callovian-Upper Bathonian 7NJ12-NJ11 zones. The Sargelu Formation is barren of nannoflora. Nannofossil assemblages in the Dhruma/Sargelu transition contain C. magharensis, Discorhabbus striatus and W. contractu suggesting the Bajocian NJ10 Zone of Bown et al. (1998).

The Dhruma Formation consists of calcareous bioclastic shale interbedded with wackestone and mudstone that yield common to abundant nannofossils. The NJ10 Zone of Bown et al. (1998) was recognized in this formation that suggests a correlation with the Bajocian. The upper NJ10 assemblages are characterized by the presence of L. crucicentralis, W. contractu and W. britannica, while the lower NJ10 assemblages are marked by the abundance of Schizosphaerella punctulata with D. criotus. Nannofossil assemblages in the samples indicate that the Dhruma Formation was deposited in a marine environment, more distal than the overlying Sargelu Formation, at mid to outer-shelf depositional environment.