Clay Mineralogy of Ir-Bearing Anjar Intertrappeans, Kutch, Gujarat, India: Inferences on Palaeoenvironment

Clay mineral studies on the K/T boundary succession at Anjar, Kutch show smectite, sepiolite and palygorskite as its major clay mineral assemblage. There is a distinct variation in the clay mineralogy across the section with sepiolite and palygorskite dominating the lower and upper lithounits, respectively, in association with smectite. The KTB layer, marked by the Ir (1271 pg/g) and Os (1414 pg/g) anomaly contain sepiolite and smectite in subequal proportions. The clay stratigraphy of the succession reflects progressive increase in aridity and basicity in the depositional environment during the deposilion of Anjar KTB sediments. It appears that the sepiolite-palygorskite-smectite complexes in the sediments were developed in a lacustrine or peri-marine environment of high basicity under arid conditions induced and influenced by Deccan volcanism. In this environment, the Si, Mg and Al ions necessary for the formation of sepiolite and palygorskite could be supplied by the hydrothermal fluids associated with volcanism by the dissociation of silicates already available in the depositional environment or simultaneously by both the processes. Clay mineral associations in the Ir-bearing Anjar intertrappean sediments across its lithounits are more compatible with the depositionai environment influenced by the volcanism, than an asteroidal impact. In the upper part of the succession, the predominance of palygorskite and smectite suggests that the latter contributed the necessary Al, Si and Mg ions to result the former. The REE signatures indicate that sepiolite-smectite is most likely the carrier phase of Ir in Anjar intertrappeans.