The sedimentology and sequence stratigraphy of contourites and associated rocks at the northeastern passive margin of Gondwana during the Early Triassic (Induan - Mirga Mir Formation), Iraqi Kurdistan

Abstract

Contourite deposits are recognized for the first time from the Lower Triassic Mirga Mir Formation in the Kurdistan region, Northern Iraq. The formation represents deposition on the southern passive margin of the Neo-Tethys during its early opening stages. The contourites are characterized by lenticular and sorted siltstones to fine-grained sandstones with current ripples, cross beddings and scour surfaces. These current structures are associated with thin-bedded (5–20 cm) limestones and shales. Petrographic investigations of the associated carbonate successions show both skeletal and non-skeletal grains such as peloids, ooids, intraclasts and extraclasts. Deposition occurred in a ramp setting at the southern passive margin of the Neo-Tethys and the skeletal grains reveal deep open marine and shallow warm water. Four different facies associations are recognized: Offshoal/basinal, foreshoal/slope, shoal and back shoal/lagoon. Field observations and thin section analysis revealed four main contourite facies in the studied outcrops, namely muddy contourites, silty contourites, sandy contourites and calcareous sandy contourites. The contourites are integrated with the basinal, offshoal, foreshoal and slope deposits. Stable isotope data from the carbonates of Mirga Mir Formation show that the δ18O (‰) values are negatives but the δ13C data shows negative and positive values. They show that had underwent meteoric diagenesis and that the δ13C values still preserving their primary depositional signal. The carbon isotopes data conforms to available data from the surrounding countries and by comparison with Early Triassic δ13C stratigraphic trends, it had captured at least two of the well-known positive spikes and at least one of the negative dips. Sequence stratigraphic analysis and stacking patterns revealed that the Mirga Mir Formation can be divided into two third-order depositional sequences. Each sequence is characterized by TST deposits followed by HST deposits and without LST.