Ooid diversity and Ooimmuration in the Neoproterozoic Kunihar Formation, Lesser Himalaya, India

摘要

Neoproterozoic Kunihar Formation is a mixed siliciclastic-carbonate succession where the carbonate units exhibit diverse types of microbial ooids and stromatolites. The ooids were classified based on: (i) ooid size (normal sized ooids and giant ooids), (ii) cortical microfabric (tangential and micritic ooids), (iii) ooid morphology (broken ooids, regenerated ooids, composite ooids, asymmetric and eccentric ooids) and (iv) ratio of cortex to nuclei (normal ooids and superficial ooids). In the Kunihar Formation, ooimmuration of stromatolitic and ooid clasts (regenerated and composite ooids) served as a mode for preservation of microbial imprints. SEM and petrographic analysis of the ooids indicate their primary generation from microbial activities. Ooimmuration of the studied ooids can be classed under bioimmuration by virtue of the biotic origin. Due to the absence of skeletal fossils, the dominant fossil imprints of the Precambrian constitute microbial stromatolites, ooids, oncoids, MISS, etc. Since ooids are widespread in the Proterozoic, ooimmuration can be considered as a potential mode of fossil preservation which will enhance our understanding of evolution of life, palaeoenviroment and sedimentary mechanisms.