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<p>The Upper Cretaceous Shiranish Formation represents a major carbonate-clastic succession within the Zagros Folded Belt of the Kurdistan Region, northern Iraq. Although its stratigraphy and paleontology are well documented, basin-scale reconstructions integrating carbonate microfacies with inorganic geochemical proxies remain limited. This study presents an integrated microfacies-geochemical framework to constrain depositional environments, paleoredox conditions, paleosalinity, and paleoclimate of the Shiranish basin during the Late Cretaceous. Fifty-nine cutting samples from five wells (P-6, T-19, DD-2, TT-22, and CH-3) were analyzed using thin-section petrography, XRF, and ICP-MS. Microfacies analysis reveals lime mudstone, wackestone, and wackestone-packstone assemblages, indicating deposition along a deep shelf to a toe-of-slope carbonate environment. Planktic foraminifera constrain the age to Late Campanian-Early Maastrichtian. Facies distribution and elemental geochemistry show clear spatial differentiation: wells P-6, DD-2, and TT-22 record distal deep-shelf (FZ-2) conditions with minimal siliciclastic input (SiO₂: 1.2%), whereas T-19 and CH-3 reflect more proximal toe-of-slope (FZ-3) environments characterized by silty mudstones, glauconite, benthic foraminifera, and elevated silica contents (42.7%). Redox-sensitive proxies (U/Th, U<sub>auth</sub>, Ni/Co, U<sub>eff</sub>, Mo<sub>eff</sub>, δCe, and Ce<sub>anomaly</sub>) indicate a dynamic water column fluctuating between oxic, dysoxic, and anoxic states, with an overall shift toward oxygen-deficient conditions. Paleosalinity indices (Sr/Ba, Th/U, Rb/K₂O) suggest predominantly normal marine salinity with episodic brackish influence, while climate proxies (Sr/Cu, Rb/Sr, C-value) indicate generally dry, arid conditions. The REE distributions, together with petrographic evidence, demonstrate negligible diagenetic overprint, confirming preservation of primary geochemical signatures. This study provides the first basin-scale synthesis of carbonate microfacies and multi-element geochemistry for the Shiranish Formation, offering a robust depositional and paleoenvironmental model for Upper Cretaceous carbonate–clastic systems in tectonically active foreland basins and a transferable workflow for comparable successions.</p>

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Elemental geochemistry and facies architecture of the Shiranish Formation, Kurdistan Region, Iraq: implications for depositional environment, paleoredox conditions, paleoclimate, and paleosalinity in a Zagros foreland basin

  • 期刊论文

  • 发表日期：

    2026年7月6日

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  • Sardar S. Fatah,
  • Ibrahim M. J. Mohialdeen,
  • Khairul Azlan Mustapha

摘要

The Upper Cretaceous Shiranish Formation represents a major carbonate-clastic succession within the Zagros Folded Belt of the Kurdistan Region, northern Iraq. Although its stratigraphy and paleontology are well documented, basin-scale reconstructions integrating carbonate microfacies with inorganic geochemical proxies remain limited. This study presents an integrated microfacies-geochemical framework to constrain depositional environments, paleoredox conditions, paleosalinity, and paleoclimate of the Shiranish basin during the Late Cretaceous. Fifty-nine cutting samples from five wells (P-6, T-19, DD-2, TT-22, and CH-3) were analyzed using thin-section petrography, XRF, and ICP-MS. Microfacies analysis reveals lime mudstone, wackestone, and wackestone-packstone assemblages, indicating deposition along a deep shelf to a toe-of-slope carbonate environment. Planktic foraminifera constrain the age to Late Campanian-Early Maastrichtian. Facies distribution and elemental geochemistry show clear spatial differentiation: wells P-6, DD-2, and TT-22 record distal deep-shelf (FZ-2) conditions with minimal siliciclastic input (SiO₂: 1.2%), whereas T-19 and CH-3 reflect more proximal toe-of-slope (FZ-3) environments characterized by silty mudstones, glauconite, benthic foraminifera, and elevated silica contents (42.7%). Redox-sensitive proxies (U/Th, Uauth, Ni/Co, Ueff, Moeff, δCe, and Ceanomaly) indicate a dynamic water column fluctuating between oxic, dysoxic, and anoxic states, with an overall shift toward oxygen-deficient conditions. Paleosalinity indices (Sr/Ba, Th/U, Rb/K₂O) suggest predominantly normal marine salinity with episodic brackish influence, while climate proxies (Sr/Cu, Rb/Sr, C-value) indicate generally dry, arid conditions. The REE distributions, together with petrographic evidence, demonstrate negligible diagenetic overprint, confirming preservation of primary geochemical signatures. This study provides the first basin-scale synthesis of carbonate microfacies and multi-element geochemistry for the Shiranish Formation, offering a robust depositional and paleoenvironmental model for Upper Cretaceous carbonate–clastic systems in tectonically active foreland basins and a transferable workflow for comparable successions.