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 the Eocene Dammam Formation, which is exposed in the southeast of Kuwait. The Formation has never been deeply buried, 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 from other sources. This process, together with the specificity of chert to particular non-bedding horizons, suggests that silicification is related to discrete locations of the groundwater table during landscape incision and resultant groundwater discharge in the region. Detailed petrographical studies demonstrate that ‘‘chertification’’ was initiated by precipitation of nanoglobules of silica (opal-A) from supersaturated groundwater solutions flowing through voids formed concomitantly by dissolution of dolomite. Subsequently, silica was precipitated as more crystalline forms of chalcedony, microquartz, and megaquartz from successively less saturated groundwater. The most likely mechanism for triggering the precipitation of silica is considered to be significant cooling of the groundwater as it neared the landsurface 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 inflowing groundwater 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 and Miocene times.