Depositional conditions affecting proximal to distal variation in the crystallo-chemical properties of Paleogene glauconites: Example from the Mesopotamian basin, Iraq

The proximal to distal variation in glauconite composition across a basin, although rarely attempted, may provide crucial insights into the depositional conditions. The Paleogene glauconites from the transgressive and condensed deposits are tracked from the proximal to the distal parts of the Mesopotamian basin in Iraq, to delineate their compositional variation. They prevail within the Paleocene Upper Aaliji Formation and the Eocene basal Jaddala Formation. The Fe₂O₃(total) (av. 16.78 wt%) and K₂O (av. 7.04 wt%) contents, and the ordering of the crystal structure and maturity (decreasing d-spacing and FWHM of 001 basal reflection) of glauconites increase, while the Al₂O₃ (av. 9.82 wt%) and MgO (av. 5.24 wt%) contents decrease from the proximal to distal settings. Therefore, the Al-rich and less mature glauconite is attributed to the proximal setting, whereas Fe-rich and more mature glauconites are attributed to the central and distal settings. The proximal setting is enriched with detrital Al-rich clay minerals forming less mature, Al-rich glauconites in kaolinite and illite-smectite precursors within the prevailing gently suboxic conditions. Whereas the central and distal settings sustain a lower sedimentation rate and enhanced mobility of ferrous ions, forming more mature, Fe-rich glauconites within the prevailing oxygen-depleted conditions. Glauconites form within two warming intervals from Late Danian to Late Ypresian and Late Lutetian to Late Bartonian. Warm climatic conditions amplify elemental supply, promote transgressive conditions, intensify reaction kinetics, and produce redox conditions encouraging abundant growth of glauconite. The composition and abundance of glauconites corroborate their formation within warm climatic conditions.