Carbon-isotope stratigraphy of the Toarcian in Kuwait

Abstract

The Early and Middle Jurassic Marrat Formation in Kuwait was deposited along the southwestern margin of the Neo-Tethys Ocean. The formation has been extensively cored and sampled in deep wells by Kuwait Oil Company for sedimentological and biostratigraphic analyses. Based on mainly long-ranging benthic species, the Middle Marrat Member is assigned in published articles to the Toarcian Stage. The formation has also been sampled for stable carbon and oxygen isotopic analyses in 19 wells across a depositional profile extending from sabkha to intrashelf basin. Stable carbon isotope analyses from these wells (∼2000 samples) reveal the signature of the negative Pliensbachian-Toarcian carbon-isotope excursion (Pl-T-CIE) in the uppermost part of the Lower Marrat Member. The younger early Toarcian carbon-isotope excursion (T-CIE) is recognized in the lower part of the Middle Marrat Member. A composite δ¹³C_carb curve spanning the Middle Marrat Member is constructed by combining overlapping δ¹³C segments from six wells guided by correlations of regional sequence boundaries and gamma-ray logs. Cyclostratigraphic analyses of gamma-ray logs in these six wells reveal a periodic signal formed by multiples of the 0.405-Ma long-eccentricity cycle. This result suggests the Middle Marrat Member in Kuwait may represent the complete Toarcian Stage consisting of depositional sequences controlled by sea-level cycles with durations ranging between 0.8 and 1.6 Ma. Correlations of the Toarcian succession between Kuwait, Saudi Arabia and Oman are hampered by imprecise biostratigraphic control. This limitation is bypassed by correlating the T-CIE in Kuwait to published δ¹³C records in the Marrat Formation in central Saudi Arabia and the Mafraq Formation in Wadi Sahtan in Oman. The correlations indicate that whereas the Toarcian Stage is apparently complete in Kuwait, most of the middle and upper Toarcian and the Aalenian are absent in Oman and Saudi Arabia.