Flat pebbles and their edge‐wise fabric in relation to 2‐D microbial mat

Abstract

Flat pebbles and edge‐wise fabric in the Mesoproterozoic Rohtas Limestone Formation (RLF) and the Chikkshelikere Limestone Member (CLM), India, generate a new insight into their formation. Simple stacks of alternating dark micritic and light‐coloured microsparite planar layers in the targeted sedimentary successions manifest a low‐energy depositional background, disturbed only occasionally by weak storms. While the dark layers are homogeneous, light‐coloured layers are generally planar, wavy ripple laminated, and bear frequent load casts. The dark layers often display varieties of 2‐D microbial mat structures little known from carbonate formations. In compliance with their identity, they are comparatively enriched in carbon, a large part of which is organic and kerogen. The said structures with their intrinsic deformations indicate delayed cementation. Notwithstanding delayed cementation, the flat clasts in these formations are readily traceable to the dark laminae possibly because of early acquisition of their selective cohesiveness. Resultant stress led to brittle deformation in the mat layers, while the light‐coloured layers underwent ductile deformation. All the studied breccias bodies are in situ, but this fact does not preclude either the possibility of reworking of the clasts while exposed to high‐energy current or generation of intraclasts by high‐energy currents eroding the depositional surface.