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The Middle Eocene Gercus Formation was studied at four surface sections (Badi/Zawita, Dohuk Dam, Brifca, and Shaqlawa), in north and northeastern Iraq. It is dominated by distinct red beds of continental clastic sediments consisting of mudstones, sandstones, conglomerates, and minor carbonates and evaporites. The detailed sedimentological study reveals that these rocks accumulated in threemain distinct facies associations: aeolian, fluvial and lacustrine. The first consists of deflation lags/desert pavements, aeolian sandsheet, aeolian dune, aeolian bimodal, and interdune deposits. The second comprises pebble-sand and sand bedload stream, mud playa/ephemeral floodplain, gravel bedload stream, debris flow, sheet flood, hyperconcentrated flow, and intra-erg mass-flow deposits. The third consists predominantly of freshwater carbonates and playa gypsum. The overall characters of the facies associations indicate that the Gercus Formation was deposited in diverse suites of sedimentary environments. These are: arid to semiarid (dry) alluvial fans, ephemeral streams "wadis," aeolian dunes, interdunes, and lakes. The close association of these related environments throughout the studied sections indicates that the Gercus Formation represents the sediments of a former desert environment under arid to semiarid conditions. The distribution of these environments generally displays a coarsening upward sequence in which erg margin deposits occurs at the base of the succession and an alluvial fan system at the top. The erg system seems to reflect an expansion of central erg over the underlying erg margin. This may be attributed to an increase of aeolian supply to the desert over a long haul prior to basin-wide progradation of the overlying alluvial fan deposits. The development and the preservation of Gercus desert systems are associated with syndepositional tectonism in a rapidly subsiding foreland basin.