ABSTRACT

The Miocene-Pliocene Agha Jari and Bakhtyari Formations of southwestern Iran comprise a continuous coarsening-upward synorogenic molasse sequence deposited in a basin peripheral to the rising Zagros Mountains. The Agha Jari Formation consists of fining-upward point-bar and floodplain sequences deposited on an alluvial plain distal to a large alluvial fan complex. The Lahbari Member of the Agha Jari Formation consists of coarsening-upward cycles capped by coarse conglomerates separated by large thicknesses of flood-plain mudstone and siltstone. These cycles represent deposition in braided rivers that migrated across interfan areas composed predominantly of flood-plain deposits. The massive conglomerates of the Bakhtyari Formation were probably deposited in braided rivers on proximal alluvial fans or in a piedmont zone near the sediment source—Mesozoic and Tertiary units exposed on the northeast.

The sequence contains asphaltimpregnated units in which there are numerous clasts of asphaltlike material. In the Agha Jari Formation, one such unit is a point-bar sequence which was deposited near a Miocene-Pliocene oil seep. The clasts of asphaltlike material were eroded locally from hardened oil crusts while impregnating oil seeped into the sands soon after deposition. As with modem seeps in the same area, the degraded oil or asphalt trapped many small animals. Oil that fed this seep probably migrated upward along faults or fractures from the underlying petroliferous Asmari Formation. This paleoseep deposit not only records synsedimentary deformation of the underlying Asmari, Gachsaran, and Mishan Formations during molasse deposition, but also documents the effects of hydrocarbon migration during the Zagros orogenic event.

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