Palaeogene hardgrounds and associated intraclast lag deposits as the substrates of ferromanganese crusts and nuclei of nodules: Inferences of abyssal current in the Central Indian Ocean Basin

Abstract

The Palaeogene substrates of ferromanganese crusts and nuclei of nodules collected from the Central Indian Basin chiefly consist of incipient to younger pelagic hard grounds and associated intraclast lag deposits. The clayey lumps, ferromanganese micronodules, crystalline aggregates of phillipsite, relic volcanic glass shards, and ichthyoliths (the phosphatic micro-remains of fishes) constitute the coarse fractions of these hard substrates and the nuclei. Alteration of volcanic material into palagonite and its later diagenesis led to the synsedimentary lithification of pelagic zeolitic clays into the hardgrounds. The pre-lithification burrows and post-lithification borings in these clayclast nuclei and substrates are also reported. Trypanites type borings and Thalassinoides and Chondrites type burrows are either filled with ferromanganese oxide or with younger sediment infillings suggesting an omission-suite of a synsedimentary lithified hardground. Hummocky-relief over the hardgrounds and associated intraclast lag deposits are the result of scouring and exhuming by an abyssal current, probably the Antarctic Bottom Water Current, in the basin. The palaeoenvironment and sequence of events of accretion of nodules and crusts are discussed.