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A fossil-bearing, tar-impregnated sandstone representing the deposit of a Miocene-Pliocene oil seep occurs near the top of the Agha Jari Formation in southwestern Iran. The sandstone is interbedded with typical Agha Jari clastics: red sandstone, siltstone, and marly mudstone. Sedimentologic and stratigraphic characteristics of the unit indicate that the tar was deposited contemporaneously with the sediment. The tar is concentrated in the 3-m-thick sandstone which is exposed laterally for 400 m, and ranges from cross-bedded medium-grained sandstone to interbedded sandstone and poorly sorted gravel. Tar is interspersed between grains and as detrital clasts. Mineralogically, the sandstone consists of calcium carbonate and tar-cemented litharenites. Small vertebrate fossils a d some freshwater gastropods are concentrated in the gravel lenses. Other characteristics, such as reworked levee deposits, lenticular gravel beds, and fining-upward sequences, indicate a fluvial origin for the unit. The source of the tar was probably an oil seep which emptied into a small stream or river. Recent oil seeps in Iran serve as analogs and illustrate how such a deposit may have formed.
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