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Characterization of chert in the Dammam Formation (Eocene), Kuwait: Clues to groundwater silicification processes

Abstract : Conspicuous chert horizons occur as discontinuous bands and isolated nodules in dolostones in theEocene Dammam Formation, which is exposed in the southeast of Kuwait. The Formation has never been deeplyburied, and so chert formation is likely to have resulted from silicification processes at or near the land surface.Erosional reworking of the chert constrains its formation to a time period between the late Eocene and the Mio-Pliocene. As there is no significant source of silica in the dolostones, the chert was formed from silica imported fromother sources. This process, together with the specificity of chert to particular non-bedding horizons, suggests thatsilicification is related to discrete locations of the groundwater table during landscape incision and resultantgroundwater discharge in the region. Detailed petrographical studies demonstrate that‘‘chertification’’was initiatedby precipitation of nanoglobules of silica (opal-A) from supersaturated groundwater solutions flowing through voidsformed concomitantly by dissolution of dolomite. Subsequently, silica was precipitated as more crystalline forms ofchalcedony, microquartz, and megaquartz from successively less saturated groundwater. The most likely mechanismfor triggering the precipitation of silica is considered to be significant cooling of the groundwater as it neared thelandsurface and came into contact with a cold regolith terrain. Precipitation of disordered forms of silica (opal-A)occurred at the cold front: progressively more crystalline phases formed as the host rock was warmed by the inflowinggroundwater and its degree of supersaturation diminished. If our interpretation is correct, this‘‘chertification’’process could have been initiated during global cooling related to one of the glaciations recorded during Oligocene andMiocene times.
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Submitted on : Saturday, December 12, 2020 - 11:26:10 AM
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Fikry Ibrahim Khalaf, Médard Thiry, Anthony Milnes, Rehab Alnaqi. Characterization of chert in the Dammam Formation (Eocene), Kuwait: Clues to groundwater silicification processes. Journal of Sedimentary Research, Society for Sedimentary Geology, 2020, 90 (3), pp.297-312. ⟨10.2110/jsr.2020.18⟩. ⟨hal-03059098⟩



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