Pradip Samanta

Microbial mat-induced sedimentary structures in siliciclastic sediments : Examples from the 1.6 Ga Chorhat Sandstone, Vindhyan Supergroup, M.P., India

This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-old Chorhat Sandstone of the Semri Group — the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet. The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands.

Paleoenvironmental Context of Microbial Mat-Related Structures in Siliciclastic Rocks

The role of biological influences in forming carbonate rocks (e.g., Altermann et al., 2006) is almost universally accepted within geology. In contrast, many see clastic sedimentary rocks as being formed primarily through physical and chemical processes, with biological mediation of their genesis being considered as of relatively minor importance (Schieber et al., 2007a). While sedimentologists and most geologists are familiar with the importance of trace fossils within clastic deposits (cf., the seminal work of Seilacher (1964) and many others since), the role of microbial mats in terrigenous sediment accretion, and in the formation and preservation of a whole host of mat-induced (mi) and mat-related structures within clastic sedimentary rocks, is less well known.

Microbially Related Structures in Siliciclastic Sediment Resembling Ediacaran Fossils: Examples from India, Ancient and Modern

Ediacaran fossils represent a distinct group of large and structurally complex, enigmatic, soft-bodied organisms dominating the end-Precambrian (Ediacaran) oceans, with an age range from 630 to 542 Ma (Martin et al., 2000; Knoll et al., 2004, 2006). Microbes constituted the Precambrian biosphere almost entirely, and formed mats on wet sediment surfaces in the absence of grazers and burrowers. Evidence for microbial mats in the rock record dates back to 3.5 Ga (Altermann et al., 2006; Altermann, 2008) and mats continue to exist today, although largely confined to stressful environments due to metazoan activities.

Controls on Cyclic Sedimentation Within the Neoproterozoic Sirbu Shale, Vindhyan Basin, Central India

The present paper dwells upon high frequency lower orders cycles from the Neoproterozoic Sirbu Shale, Vindhyan Supergroup, central India, and aims to extract their causal factors. The Sirbu Shale, characterized by a transgressive lag at its base, is bounded between the coastal playa sediments of the underlying Lower Bhander Sandstone and the marginal marine to fluvial sediments of the overlying Upper Bhander Sandstone. The study focuses on the upper part of the Sirbu Shale that initiates with a thick pyrite rich shale, without bearing any wave features, representing the maximum marine flooding zone (MFZ). Lithofacies analysis suggests a storm dominated outer shelf to foreshore-beach setting. Lithofacies and lithofacies successions interpreted in terms of sequence srtartigraphic framework, suggests that the studied interval represents a shallowing upward prograding succession, designated as a Highstand Systems Tract (HST). Intrinsic studies unravel that the interval incorporates two different orders of high frequency cyclicities, in terms of parasequence and parasequence sets. The parasequences are genetically related shoaling-upward successions bounded by marine flooding surfaces and are mostly formed by autocyclic processes. Nonetheless, the parasequences towards the basal part of the interval shows evidences of geostrophic flows. The parasequence sets, encompassing two to five parasequences, are composed of relatively higher order genetically related shoaling-upward successions. The conspicuous existence of soft-sediment deformational structures at top of each parasequence sets are laterally correlatable. The role of tectonics might have been significant in creating the accommodation space and thereby controlling the cyclic sedimentation as exemplified from the studied interval of the Sirbu Shale.

Stratigraphic Evolution and Architecture of the Terrestrial Succession at the Base of the Neoproterozoic Badami Group, Karnataka, India

The multistoried siliciclastic succession at the base of basal Kerur Formation of the Neoproterozoic Badami Group shows ample variations in sequence building pattern within the ambit of the Precambrian fluvial sedimentation system. Detailed facies, architectural element, paleocurrent as well as stratigraphic architectural analysis invariably revealed a frequently avulsive braided pattern, with flashy discharges, for the paleoriver system; which is consistent with the basic tenet of the Precambrian alluvial sedimentation. Rare eolian features suggest seasonal flow fluctuations, referring to the semiperennial nature of the fluvial system. The studied interval represents a single valley fill, internally constituted by seven vertically juxtaposed channel belts. Each channel belt is fining upward along with the overall grain-size reduction up the succession. While the older channel belts inferred to be braided, channels possibly become more sinuous towards the top of the succession, as inferred from the appearance of bank-attached bars along with the omnipresent longitudidal bars. Flow durability within channel also increases with time, as the lower two belts appear to be ephemeral with highest energy flashy discharges, changing into semiperennial to perennial one upward. Bounded between an unconformity below and a thoroughly wave-featured limestone unit above, the coarse and poorly sorted clastic sedimentary rocks at the base of the basal Kerur Formation are interpreted as a base-level lowstand product, indicating gradual filling of the paleoriver valley under the backdrop of slow rise in base profile. Tectonics-related generation of accommodation space as well as the rejuvenation of slope along and across the basin-margin dictated the sediment distribution and sequence building pattern primarily. The increase in channel sinuosity up-the-succession is governed by the raised rate of base profile rise, which ultimately leads to termination of the terrestrial depositional system by complete marine inundation.