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.

Revisiting the boundary between the Lower and Upper Vindhyan, Son valley, India

“…This tide-dominated environment intervened by storm is evidenced by the presence of hummocky cross-stratification, an amalgamation of sandstone beds and conglomerates/massive beds with sole features. The presence of numerous sandclasts within the Facies association II supports the influence of microbiota that turned sand into a cohesive sediment (Sarkar et al 2014(Sarkar et al , 2018. The vertical transition of the facies assemblages rules out any major palaeogeographic shift.…”

A review of biotic signatures within the Precambrian Vindhyan Supergroup: Implications on evolution of microbial and metazoan life on Earth

“…Although siliciclastic formations yielded MISS in the past, recent studies indicated the formation of similar features within ancient carbonate sediments in case of late cementation ( Fig. 6; Sarkar et al, 2016Sarkar et al, , 2018Choudhuri, 2019 (Banerjee et al, 2014). Many of the MISS structures, e.g., Manchuriophycus, have previously been thought of as metazoan burrows (Kulkarni and Borkar, 1996 and many others).…”

“…Recently Sarkar et al (2016Sarkar et al ( , 2018 expanded the list of two-dimensional carbonate MISS from the lower parts of the Rohtas Limestone and Bhander Limestone of the Vindhyan Supergroup (Figs. 5 and 6).…”

“…The activity of sulfate reducing bacteria (SRB) facilitates the CaCO 3 precipitation within a thin microbial mat (Sarkar et al, 2016). The delayed cementation of the sediments corresponds to the acidic composition of EPS secreted by the microbes (Sarkar et al, 2018 and references cited therein). The near-equatorial palaeolatitudinal position of India (Scotese, 2001;Pesonen et al, 2003;Evans and Mitchell, 2011;Zhang et al, 2012) during the Mesoproterozoic time might have restricted the growth of SRB, and the rate of sulfur reduction, resulting the delayed cementation and preservation of MISS in carbonates (Sarkar et al, 2016(Sarkar et al, , 2018Choudhuri, 2019).…”