Stromatolites from Terminal Proterozoic Krol - Tal Sequence of Nainital Syncline, Lesser Himalaya and their Environment Interpretations

Abstract

The terminal Proterozoic Krol-Tal succession disposed in a doubly plunging syncline of Nainital in the domain of Lesser Himalaya is an allochthon and forms the southernmost sub-basin of ca. 280 km linear belt comprising of six sub-basins. The mixed siliciclastic-carbonate sediments of the sequence show a wide spectrum of stromatolitic morphology, such as stratiform, laterally linked hemispheroids (LLH), columnar, domal and branching columnar, and other microbialites, such as thrombolites. These various morphological characteristics are result of a subtle balanced interaction among biological, hydrodynamic and sedimentation processes and have high potential for inferring different environments of their accretion. Among the five members of Krol Formation, viz. Krol A, Krol B, Krol C, Krol D and Krol E, the lowermost member, i.e. Krol A stromatolites represent a supratidal to intertidal environment, whereas Krol B stromatolites favour a supratidal zone of deposition. The stromatolite characteristics of intertidal environments marked with channels have been recorded from the Krol C member. A peritidal realm of deposition for Krol D member and a partly protected intertidal to upper subtidal zone for Krol E have been inferred. The lower Tal sediments were laid down in a supratidal to lower intertidal/upper subtidal environment. No stromatolite was recorded from the siliciclastic dominated upper Tal member that was deposited in supratidal to intertidal zones. The entire Krol-Tal stratigraphic succession is characterised by a repetitive shallowing-upward succession and the environmental interpretation based on the stromatolite morphometry is well in accordance with various observed sedimentary structures and sedimentological characteristics of the succession.