Berthierine authigenesis as grain pseudomorph? A new insight from the early Maastrichtian Kallankurichchi Formation, India

Abstract

This study presents detailed sedimentological, geochemical, and mineralogical evidence of green clay formation at the basal part of the early Maastrichtian Kallankurichchi Formation, Cauvery Basin, India. Immediately overlying a basal scree conglomerate, the highly fossiliferous Kallankurichchi Formation hosted a ∼ 3.5 m thick dark grey limestone containing abundant green clay minerals formed by the alteration of metastable framework grains. Abundant fresh to partially palagonitized sideromelane glass and occasional preservation of gas bubble structures in green clays strongly suggest a volcaniclastic source. Petrographic observations manifested primary occurrences of the green grain pseudomorphs mimicking the shape of the precursor volcanic grains. The X-Ray Diffraction analysis of the randomly oriented bulk sample and oriented, smear-mounted green clay separates identified the berthierine by the strong ∼7 Å basal spacing. The mineral chemical data of the berthierine grain pseudomorphs recorded substantial compositional variation from Mg-, Si- and Al-rich core and Fe-rich rim, characterizing the maturation of berthierine from volcaniclastic substrate through ‘proto’-berthierine. Tiny berthierine flakes (1–5 μm long) grew vertically from the mineral grain boundary and feldspar cleavages, suggesting its authigenic origin. Contemporaneous tectonic events, marked by abundant slide planes, suggested continuous sagging of the basin, causing oxygen-depleted bottom water conditions. The unusual appearance of berthierine as grain pseudomorphs, rarely reported from the rock record, revealed the dissolution of metastable volcaniclastic grains in a slightly acidic, oxygen-depleted depositional environment as the primary mechanism of authigenesis. The early diagenetic calcite cementation ceased the berthierine formation, preventing compaction and transformation.