Glauconitization within the late Cretaceous Mahadek Formation and its stratigraphic implications.

This study explores sedimentological and stratigraphical implications of glauconites within the upper Cretaceous Mahadek Formation exposed at the Umsohryngkew River section in Meghalaya, India. The succession is overall fining upward, with sandstone dominating the lower part and calcareous shales and limestone bands constituting the upper part of the Mahadek Formation. Glauconite occurs within the impure limestone and shale beds of the upper part of the sequence. The glauconitic section gradationally passes upward to K/Pg boundary above the Mahadek Formation. Textural, mineralogical and chemical analyses were carried out to understand the origin of the glauconite. Petrographic investigation reveals three main types of glauconites, a) pellet b) vermiform c) infilling within bioclasts. The glauconite appears dark green and the long dimension of the glauconite grain averages 200 µm. Occasionally, the glauconite grains may be as long as 500 µm. The glauconite pellets show sharp boundaries and exhibit internal cracks. The content of glauconite grain  is nearly 15-20 % of the rock volume. These are sub-rounded to rounded in shape without showing evidence of transportation. X-Ray Diffraction on air-dried samples reveals very prominent (001) basal reflection at 10.23 Å, with reflections of (020), , , (003) and (060) planes at 4.52 Å, 4.23 Å, 3.66 Å, 3.27 Å, 5.51 Å respectively. On glycolation, the (001) peak shifts from 10.23 Å to 9.95 Å, while  and (003) reflections collapse into one single peak. The peaks get narrower, more symmetrical and sharper after heating the samples at 400^oC. Also, the  and (003) reflections show separations into 3.33 Å and 3.30 Å reflections respectively. The X-Ray diffraction characteristics confirm the pure nature of glauconite devoid of any interstratification. FEG-SEM imaging of glauconite grains shows a perfect lamellar structure and corroborates the X-Ray diffraction patterns of glauconites.  Further, the electron microprobe analyses data show average K₂O content of the glauconite is 6.5 %, suggesting an evolved variety. The Fe₂O_3(total) content varies from 2.9 % to 16.8 %, with an average of 9.9 %. While the average content of Al₂O₃ is 20.3%. The K₂O vs Fe₂O₃ plot (r²= 0.7) shows a strong positive correlation. The inverse correlation (r² = 0.8) between Fe₂O₃ and Al₂O₃ suggests the replacement of Al by Fe at octahedral sites. The averages of MgO, SiO₂ are 3.5% and 52.6% respectively. The evolved glauconite within the shelfal Mahadek Formation demarcates the stratigraphic condensation at the top of the transgressive deposits. The formation of glauconite in the late Cretaceous Mahadek Formation is significant because of its stratigraphic preference close to the K/Pg boundary.