Lithofacies and Depositional Environment of the Early Jurassic Ab-Haji Formation in Outcrop and Exploration Boreholes of Parvadeh Coal Mine, Tabas, East Central Iran

The Lower Jurassic Ab-Haji Formation consists of siliciclastic sediments, which are widely exposed in the Tabas Block. This formation varies in thickness from tens of meters to more than 500 m in northern Tabas Block. The Ab-Haji Formation with avearge thickness of 75 meters presents in the Tabas Coal Mine area and consists mainly of sandstone, siltstone, claystone with a few thin interbeds of coal seams. Two outcrops and three exploration boreholes were studied in order to identify lithofacies and to interpret the depositional environment. The lithofacies analysis in outcrops imply two siliciclastic coarse grained (Gcm, Gt), four medium grained (St, Sp, Sr, Sh), two fine grained (Fl, Fm) and one coal lithofacies. Identified lithofacies can be grouped into four architectural elements including channel, point bar, crevasse splay and coastal plain swamps. The electorofacies and log motif analysis led to recognition and differentiation of architectural elements in exploration boreholes. The lithofacies and architectural elements show that the siliciclastic sediments of the Ab-Haji Formation were deposited in the meandering river system within a coastal plain with widespread swamps. The sedimentological analysis led to reconstruction of paleogeography in eastern part of the Tabas Block, which shows deposition of nonmarine to coastal plain environment in this part of the block.