Microbialite grainstones in the Late Triassic fluvial deposit of a central Indian rift basin

This study discusses the occurrence and origin of grainstones (calcarenites) intimately associated with fluvial, mudstones and sandstones of a Late Triassic fluvial succession (Maleri Formation) deposited within an... [ view full abstract ]

This study discusses the occurrence and origin of grainstones (calcarenites) intimately associated with fluvial, mudstones and sandstones of a Late Triassic fluvial succession (Maleri Formation) deposited within an intra-cratonic rift basin of India. These grainstones represent a minor component of this siliciclastic succession (300-500m thick), yet they are ubiquitous at every stratigraphic level. They occur either as 30-70cm thick, lensoid and/or sheet-like bodies encased within thick, massive, vertisol bearing, red mudstones or constitute the basal part of the storeys within multistoreyed sheet-sandstone bodies deposited by ephemeral sheet-floods. The grainstones are typically cross-stratified and contain fragments of freshwater bivalve shells and bones of land vertebrates.

Coarse sand to granule-sized peloidal and micro-oncoidal carbonate grains are the dominant framework constituents. They are, at places, admixed with subordinate angular, fine to medium sand-sized siliciclastic grains, bioclasts, and mud-aggregates. The peloids dominate over the oncoids in the framework. The framework grains are cemented with multiple generations of coarse calcite spars.

These peloids are highly rounded, elliptical grains that are internally featureless and made up of a dense network of quant microspar, dotted with very small regions of iron stained micrite. These grains resemble fragments of subaerially modified carbonate mud in palustrine environment. The oncoid grains are semi-circular in outline having nucleus constituted of one or more peloids or even oncoid fragments at places. The surrounding cortex is thin and contains a few (<= 5) sparry calcite laminae separated by comparatively thinner micritic/microspar lamina. At places, very fine sand sized detrital gains are found to be trapped between adjacent laminae. The sparry laminae are typically irregular and non-isopatchous. The spars are fibrous, non-branching and organized in radially oriented brush-like clumps. They are similar to the organization produced by modern freshwater cyanobacteria (Dichothix) and filamentous algae (Vaucheria or Schizothrix). The intervening micrite/microspar laminae were possibly formed due to bacterial decomposition of sparry coats. These oncoids indicate limited and infrequent rolling of grains in sluggish channels or ponds and formation of microbialite.

The carbonate grains of the Maleri grainstones were likely to have formed in ponds and sluggish channels that bordered the rift basin margin and fed with Ca++ and HCO3- rich spring water draining thick Proterozoic limestones at the rift shoulder. The episodic erosion of the carbonates deposited in those peripheral areas possibly introduced the carbonate grains to the central part of the rift, were the deposition of siliciclastics prevailed.