In the recent GeoArabia, Haq and Al-Qahtani (2005) updated the chronostratigraphic Arabian Plate framework of Sharland et al. (2001). These studies cite the paper by Yousif and Nouman (1997) to represent the Jurassic type section of Kuwait. Yousif and Nouman published the composite log for the Minagish-27 well (see Figure on page 194) and depicted the Jurassic formations and stages, side-by-side, but only in a generalized manner.
In order to refine the ages for this section, I would like to share some preliminary unpublished biostratigraphic and Sr isotope data (see Table and Notes) from analyses by Varol Research (1997 unpublished report), ExxonMobil (1998 unpublished report) and Fugro-Robertson (2004 unpublished report). To convert Sr ages (Ma) to biostages, or biostages to ages, I have used the Geological Time Scale (GTS) 2004 (Gradstein et al., 2004). I thank G.W. Hughes, A. Lomando, M. Miller and O. Varol for their comments.
Notes on Ages
The top of the Lower Marrat is early Toarcian based on the common recovery of Nannoceratopsis triceras. However the presence of the common-abundant foraminifera Amijiella amiji of late Sinemurian-late Bajocian age (Whittaker et al., 1998) most frequently found in the late Sinemurian - Pliensbachian but ranges as young as Bathonian (Powers et al., 1966; Sartorio and Venturini, 1988), could shift the base of the Lower Marrat to late Sinemurian.
The Dhruma Formation is early Bajocian-early late Bajocian, based on the frequent occurrence of nannofossil Watznaueria (Ellipsagelosphaera) britannica and co-occurrences of the age diagnostic calcareous nannofossil Carinolithus superbus with the dinoflagellate Ctenidodinium sp. C of Colin et al. (1986) (no lower than late Bajocian) in the upper Dhruma. The Dhruma may be even older (?late Aalenian) based on the presence of (very rare-rare) dinoflagellates Dissiliodinium sp. and (very rare-fair) Dissiliodinium hyalinum (smooth) in the Lower Dhruma. These biostratigraphic data represent South Kuwait, supported by data from West Kuwait, that show the Dhruma is early to mid Bajocian. In North Kuwait, the Dhruma is not older than Bajocian (biostratigraphy from cutting samples). This suggests that Middle Jurassic paleolows may have preserved reworked rocks with a fossil record sourced from paleohighs; thus the ages of Lower Dhruma and Lower Marrat may be questionable.
The Base Sargelu is middle and late Bathonian based on numerous to common occurrence of Durotrigia filapicata and the presence of Gonyaulacysta pectinigera and Korystocysta gochtii/kettonensis.
The top Sargelu is Callovian or ?older based on the abundant presence of Dichadogonyaulax sellwoodii. According to the average ages in GTS 2004, the estimated age of the top Sargelu by Sr Isotope analyses (167.5 Ma = c. Bathonian/Bajocian Boundary) is significantly older than the estimate obtained by biostratigraphy (Callovian or ?older; i.e. greater than 161.2 Ma).
The Najmah Formation in North Kuwait is no older than Oxfordian based on the presence of the dinoflagellate Systematophora spp. and the nannofossil Stephanolithion bigoti maximum (earliest Oxfordian – latest Callovian) from the lower part of the formation.
The Hith Formation from Sr isotope analysis in West Kuwait, is assigned an age of about Tithonian/Kimmeridgian, which would confine the Hith-Gotnia section to Kimmeridgian.
In the offshore, for the Makhul Formation, the presence of the dinocyst Muderongia sp. cf. A Davy (1979) suggests middle and late Tithonian, and the occurrence of Phoberocysta neocomica indicates an age no older than Berriasian.