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Stratigraphy and lithologic characterization of the Jurassic-Cretaceous boundary across the Arabian Plate
Fadhil Sadooni

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 03:10 PM – 03:30 PM
Last modified: 2018-11-24


The Jurassic-Cretaceous boundary in the Arabian Plate varies from conformable to unconformable at different parts of the basin. The boundary was the subject of early investigation that was associated with the exploration for hydrocarbons in the Middle East during the first half of the last century. Both Jurassic and Cretaceous strata were primary exploration targets and were subjected to extensive stratigraphic and paleontological studies in both outcrops and the subsurface. The first work on the Jurassic Cretaceous boundary was the identification of the Jurassic ammonites in the mountains of Kurdistan, northern Iraq which was adapted and used later at different parts of the world (Spath, 1950).  Presently, there are three stratigraphic configurations that describe the contact across the Arabian Plate that extends from northern Iraq to Oman. In the deeper parts of the basin such as northeastern Iraq, it is believed that the sedimentation was continues across the boundary as represented by the Chia Gara Formation (middle Tithonian- Berriasian). The thickness of this formation is around 232 m and it consists of radiolarian, ammonites-bearing, thin-bedded limestone and bituminous shale. Ammonite zonation was carried on the formation outcrops by the geologists of the oil companies working in the region at the middle of the last century. The major identified ammonites include Berriasella calisto Orbigny and Berriasella carpathica Zittel. In the subsurface, where it is difficult to get full ammonite samples, tintinnids and calpionellids were used for dating the encountered strata. Three zones have been recognized. These are, on the generic level, the Tithonian Calpionella zone; the lower Berriasian Tintinnopsella/ Calpionella zone and the upper Berriasian Tintinnopsella zone, which indicate that sedimentation was continuous in the subsurface also. This part of the basin is called the Permeant Basin, or Kirkuk Embayment in Iraq or Garau Basin in western Iran (Aqrawi et al., 2010).

On the shallower, shelfal areas of the Plate, the Gotnia Basin was the dominant sedimentation regime during the middle Tithonian.  At the center of the basin, sediments were of condensed, low-stand nature and consist of organic and evaporitic shales and thin-bedded micritic limestones. The Gotnia Basin eventually evolved into a full evaporite flat of the Hith Anhydrite Formation that marks the Upper Jurassic boundary in most of the southern parts of the Plate such as Saudi Arabia and United Arab Emirates (Fig. 1; Goff, 2005). At the flanks of the basin, normal, shallow water carbonates were forming such as the limestone and dolomite of the Najmah Formation and the Arab Formation to the west and the Surmeh Formation to the east. Although there are no specific foraminifera species that can be used to identify the Jurassic-Cretaceous boundary but there are certain foraminifera population that differentiate between the Jurassic and the Cretaceous sediments. These are formed of some large benthonic foraminifera. So, the Oxfordian-Kimmeridgian Najmah Formation is recognized by the presence of Valvulinella cf. jurassica Henson, Nautiloculina oolithica Mohler, Pfenderina spp., Trochammina spp., Haurania spp., while the Cretaceous Sulaiy Formation is characterized by the presence of Pseudocyclammina cf. lituus, Spirocyclina sp., and Ammobaculites. These Foraminifera assemblages were used for stratigraphic control because most of the studied samples were cores and cuttings derived from drilled oil wells. Some workers delineate the Jurassic-Cretaceous boundary in the lower part of the Sulaiy Formation based on the presence of Bramkampella sp. in the lower part of the Sulaiy and the Everticyclammina sp. Zone in the overlying Yamama Formation (Cretaceous), but these species have long range and cannot be used for this purpose.

In areas where the Hith Anhydrite Formation is prevalent, then this formation represents unconformity surface and considered as the top of the Jurassic and what covers it belongs to the Cretaceous. Although, this seems a reasonable assumption but some of these evaporites may be aqueous and do not necessarily indicate the presence of unconformity (Alsharhan & Kendall, 1994).

The third stratigraphic situation is found in the areas of western Iraq, Jordan and Syria. In these parts of the basin, the Late Jurassic-Early Cretaceous sediments are either missing or consist of clastic materials.