Anastomosing fluvial sedimentation in a foreland basin: paleoclimatic and tectonic controls on the Middle Eocene Gercus Formation, Kurdistan Region, northeastern Iraq

Abstract

Integrated sedimentological, petrographic, mineralogical, and facies studies of the Gercus Formation reveal a tectonically influenced fluvial depositional environment responding to changing paleoclimate. Modal QFL analysis indicates a lithic-rich framework with subangular grains and low textural maturity, suggesting a proximal derivation from heterogeneous volcanic-plutonic-metamorphic-sedimentary source terrains, associated with limited transport reworking and reduced levels of chemical weathering consistent with foreland basin depositional settings. Polymictic assemblages of lithics suggest a provenance with a tectonically active and high rate of accumulation. XRD results exhibit palygorskite-dolomite patterns suggesting that the authigenic minerals present indicate a seasonal water-deficient environment with the existence of intermittent moisture. The accumulated clay mineral distributions document successive diagenetic modifications where shallow-buried sediments keep the primary illite-chlorite assemblages extracted from the physical weathering of metamorphic-igneous terranes, compared to deeply buried rock units that experienced a complete mineralogical conversion. Lateral mineralogical changes – hematite increase (northeastern Iraq – Khalakan) versus graphite accumulation (northeastern Iraq – Merade), indicate redox-limited facies successions from poorly drained floodbasin to well-drained interchannel settings. The micritic groundmass of calcareous palaeosol horizons shows local concentrations of microspar, indicating episodic water table depth (WTD) fluctuations and high evapotranspiration under a semi-arid climate with seasonally interrupted soil moisture. Six architectural elements (CH, SB, LV, CC, CS, and FF) define an anastomosing fluvial system with widespread floodplain aggradation and common crevasse channel-splay development. The existence of this architectural system, associated with calcareous pedogenic horizons, conclusively demonstrates low-gradient floodplain settings under climatic regimes varying seasonally from arid to semi-arid conditions.