Biostratigraphy and sequence stratigraphy of the Minjur and Marrat Formations (Upper Triassic to Middle Jurassic) in Kuwait

Abstract

This paper summarizes sedimentological and biostratigraphic data for the Minjur and Marrat Formations in Kuwait and places them within an expanded sequence stratigraphic framework based on the letter and number scheme initiated by Sharland and others (2001). Only two sequences (Tr80 and J10) were identified in this interval in the original scheme, whereas, between Tr80 and the top of the Upper Marrat, 18 sequences are now recognized and their sequence boundaries and maximum flooding surfaces identified and illustrated herein. One new sequence (Tr90) was proposed in the underlying Jilh A Member. Three new sequences (Tr100, Tr105 and Tr110) are recognized in the Minjur Formation (= Lower Minjur in Saudi Arabia). Four sequences (J02, J04, J06 and J08) and part of a fifth (J10) are recognized in the Lower Marrat. Seven sequences (part of J10 and J11-J16) are recognized in the Middle Marrat. Three sequences (J17, J18 and J19) are recognized in the Upper Marrat. Biostratigraphic data are sparse for the Minjur and Marrat Formations. In the Marrat Formation, calcareous nannofossils are extremely rare, benthic foraminifers are relatively common but long-ranging and palynomorphs are sparse and most commonly non-age diagnostic. Published and unpublished proprietary data from 33 wells have been combined to determine the ages of the succession. Using palynology, the Minjur Lower Member was dated as Norian. An undifferentiated Norian-Rhaetian age range is assigned to the Middle and upper Members of the Minjur Formation. The Triassic/Jurassic boundary is placed just below the base of the Lower Member of the Marrat Formation, where cuttings samples from a thin interval of strata yielded long age-ranging palynomorphs. The Sinemurian/Pliensbachian boundary occurs in the Lower Marrat at the base of the S J06 transgressive systems tract. The Pliensbachian/Toarcian boundary, dated on the basis of Nannoceratopsis triceras and Lotharingius crucicentralis, is also in the Lower Marrat at the base of the S J10 transgressive systems tract. The contact of the Middle and Upper Marrat is unconformable and locally karsted but appears to correlate to the Toarcian/Aalenian boundary. The upper section of the Upper Marrat and the base of the overlying Dhruma Formation have not been cored but the Aalenian/Bajocian boundary is placed in the upper portion of Sequence J19. SB J20 is placed at the top of the Upper Marrat Member. Change in the age of the Late Triassic Baluti Formation in Iraq has resulted in changes in correlations of the Minjur and Marrat Formations to each other and to other formations in the region. The Lower Marrat is shown to be the shallow marine equivalent of the Upper Minjur siliciclastics in the subsurface Rub al Khali Basin and in the shallow subsurface Arabian Basin near the outcrop belt in Saudi Arabia. The Lower Marrat correlates to the Adaiyah Formation and the Upper Sarki Formation in Iraq. The Middle Marrat correlates to the Mus and Alan Formations in Iraq and to all of the Marrat Formation as defined in outcrop in Saudi Arabia. The Upper Marrat is largely represented by an Aalenian-age hiatus in the Saudi Arabian outcrop belt but a thin, condensed Aalenian section is present in the subsurface, basinward of the outcrop belt, and correlated with the Lower Dhruma (D1-D2). The Upper Marrat is correlated to the lower Sargelu Formation in Iraq. These new and revised correlations clarify the timing of the Late Triassic (Norian) uplift of the Qatar Arch.