Sedimentology and evolution of a foreland desert basin, Middle Eocene Gercus Formation (North and Northeastern Iraq)
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ORIGINAL PAPER
Sedimentology and evolution of a foreland desert basin, Middle Eocene Gercus Formation (North and Northeastern Iraq) Salim H. Hussain & Thamer A. Aghwan
Received: 16 May 2013 / Accepted: 5 March 2014 # Saudi Society for Geosciences 2014
Abstract 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 three main 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 S. H. Hussain (*) : T. A. Aghwan Department of Geology, College of Science, University of Mosul, Mosul, Iraq e-mail: [email protected] T. A. Aghwan e-mail: [email protected]
overlying alluvial fan deposits. The development and the preservation of Gercus desert systems are associated with syndepositional tectonism in a rapidly subsiding foreland basin. Keywords Gercus Formation . North Iraq . Erg margin . Fluvial–aeolian interaction . Middle Eocene . Foreland basin
Introduction The rock successions of the Gercus Formation (Middle Eocene) crop out in north and northeastern Iraq (Fig. 1a). They are mainly dominated by distinct red beds of continental clastic sediments consisting of conglomerates, sandstones, and mudstones with some carbonates and evaporites. These rocks show a great lithological variation vertically and laterally from one place to another. They comprise a complex facies association (FA) which has not hitherto been described from the Gercus Formation. The overall aims of the paper are to: (i) describe and interpret the
