THE MESOZOIC SEQUENCE IN SOUTH‐WEST IRAN AND ADJACENT AREAS

Abstract

New outcrop and subsurface data from SW Iran have permitted a review of the stratigraphy of the area to the SW of the Zagros Crush Zone and a comparison with neighbouing areas. The Triassic sequence consists mainly of an evaporite and dolomite sequence in the coastal areas of the Persian Gulf which is the extension of the evaporite basin of Saudi Arabia and Iraq. Towards the high Zagros in the northeast, the evaporites are replaced by dolomites. Two unconformities are found at the base and top of the Triassic.The Jurassic in Fars and eastern Khuzestan consists of an argillaceous interval representing early Liassic time, overlain by a thick development of neritic carbonates of early to late Jurassic age. An evaporite unit developed in the upper Jurassic is present in coastal/subcoastal areas of Fars and eastern Khuzestan and is the north‐eastwards extension of Hith Anhydrite of Saudi Arabia. The end of the Jurassic was marked by uplift and erosion, giving rise to an unconformity over a large area. In western Khuzestan and Lurestan, the Lower Jurassic is a sequence of alternating evaporites and dolomites. The Middle Jurassic is represented by deeper water bituminous shales and argillaceous limestones of the Sargelu Formation, which is cut by a regional unconformity in this area. The Upper Jurassic is represented by the evaporites of the Gotnia Formation which is terminated by the possible Upper Jurassic unconformity. The Jurassic sequence of this area can be correlated well with that of eastern Iraq. In the high Zagros area to the south of the Crush Zone, the Jurassic consists of a thick development of shelf carbonates with no evaporites. The Cretaceous System in SW Iran is divided into Lower (Neocomian‐Aptian), Middle (Albian‐Turonian) and Upper (Coniacian‐Maastrichtian). The Lower Cretaceous is mainly made up of two shelf carbonate unit separated by shales in Fars and eastern Khuzestan. Towards Lurestan, the carbonates pass into deeper water black shales and limestones with radiolaria. The top of the Lower Cretaceous is marked by a regional unconformity in Fars and the Persian Gulf area. The Middle Cretaceous began with a transgression forming the shales and limestones of the Kazhdumi Formation which was followed by a shallowing of the sea and the deposition of Cenomanian and Turonian shelf carbonates over the entire area of Fars and Khuzestan. The Lurestan basin retreated northwards and northwestwards and covered only central Lurestan during Albian‐ Turonian time, with the deposition of dark grey to black shales and pelagic limestones of the Garau and the Oligostegina bearing limestones of the Sarvak Formation. At least two pronounced regional unconformities have been recognized, between the Cenomanian and Turonian and between the Turonian and Coniacian. The Upper Cretaceous is represented by limestones at the base and a transgressive shale unit at the top, which is terminated by a regional unconformity at the Cretaceous/Tertiary boundary. Isopach and lithofacies maps of various units and correlations of outcrop and subsurface sections indicate several important unconformities and facies changes in SW Iran during the course of the Mesozoic. The general stratigraphy of the region shows similarities to the Mesozoic sequence of Iraq and Saudi Arabia, with a gradual facies change from carbonates to sandstone towards Saudi Arabia. This change is most evident in the Upper Triassic and in the Barremian‐Cenomanian. The Upper Cretaceous sequence of SW Iran changes from mainly argillaceous sediments of deeper marine environment into carbonates of shallow water origin towards Saudi Arabia. The correlation of the Mesozoic sequence of SW Iran with those to the northeast of the Zagros Crush Zone indicates a rather abrupt change from the Upper Triassic onwards.