Lithofacies Architecture of the Gercus Formation in Jabal Haibat Sultan, NE Iraq; New Concept of Lithostratigraphy and Depositional Environmental

Lithostratigraphic analysis of the Gercus Formation reveals four associations based on lithofacies types, petrography and sedimentary structures. The identified lithotypes are shale/claystone, sandstone, carbonate, conglomerate and debrite. Lithofacies association reveals four types arranged from bottom to top; FA1 proximal turbidite (sand dominated), FA2 distal turbidite (clay dominated), FA3 slop-apron turbidite (sand dominated) and FA4 distal deep turbidite (clay dominated). Lithofacies and associated sedimentary structures suggest sedimentation in marine turbidite regime followed the deep marine turbidites of Tanjero and Kolosh Formations in Tethyian foreland basin. The Middle-Late Eocene Gercus Flysch sediments composed of predominant litharenitic sandstones and interbedded mudstones, of turbiditic origin and mostly derived from a NE Arabian Plate margins. The sediments are well exposed in the NE side of Haibat Sultan Mountain, and provide excellent examples of fan sands associated with turbidites and related facies. The examined section indicates that the sandstones were deposited in larger channel complexes, which fed a mud-dominated slope. The more proximal facies have proportion of pebble conglomerates, shale-clast conglomerates and thick-bedded structureless pebbly sandstones, deposited by high-density turbidity currents, debris flows and slump. Other facies, either more distal or with a more uniform sand-rich are dominated by thick-bedded and amalgamated structureless sandstones. The massive sands are thought to originate from the gradual aggradations of sediment beneath or near-steady flows. In this paper, new sedimentological and lithofacies evidences prove for the first time that the Gercus Formation was deposited in gravity-flow regime in marine environment. The identified sedimentary structures display alternative graded and fining upward cycles, load and flute casts, submarine channels, sand and clay balls and pillow structures, convolute and slump beddings, of turbidity origin, which is supported by index glauconite mineral in some sandstone horizons. Keywords: Turbidites, Sedimentary structures, Gravity flow, Flysch, Gercus, Eocene, NE Iraq.