R. Nagendra

Stratigraphy and paleobathymetric interpretation of the Cretaceous Karai shale formation of Uttatur Group, TamilNadu, India

The Karai shale Formation of the Uttatur Group is exposed in a bad land area at the western margin of the Cauvery Basin. This shale has been investigated based on foraminiferal fauna and clay minerals. The foraminiferal assemblages obtained contain predominantly calcareous benthic foraminifera, rare planktic and arenaceous foraminifera. The planktic foraminiferal index taxa Planomalina buxtorfi, Rotalipora reicheli, Praeglobotruncana stephani, and Hedbergella portsdownensis suggest the late Albian to middle Turonian age. The benthic assemblage dominated by Lenticulina, Gavelinella, Osangularia and Quadrimorphina, suggests an outer neritic (100–200 m) environment. The clay mineral content dominated by kaolinite-illite-montmorillonite indicates that the Karai shale was formed from weathering of igneous rocks.

Cretaceous Stratigraphy of Outcrop Sediments of the Ariyalur Area, Cauvery Basin, Southern India

During the Late Jurassic, a pericratonic basin with synrift and postrift sedimentation evolved in the Cauvery Basin in southern India. The sediments and associated fauna and flora form part of the Cretaceous stratigraphy in this part of the continent. This study presents an investigation of the lithostratigraphic and biostratigraphic architecture of these sediments, including the palaeoenvironmental conditions of sedimentation.

Spatial distribution of benthic foraminifera in the Palar estuary, Tamil Nadu

Spatial variability of estuarine benthic foraminiferal assemblages in the surface (0–1cm) sediments has been determined for the Palar river estuary, Tamil Nadu. Twelve bottom water and sediment grab samples were collected at different water depths in the estuarine zone to determine the ecological relationship of benthic foraminiferal assemblages with physicochemical characteristics. The water analyses revealed that hypo saline, high dissolved oxygen and low pH conditions were prevailing at water-substrate interface at the time of sampling. The study indicated that salinity, pH and substrate have a patchy linear relationship with living and total faunal frequency but individual species Ammonia beccarii, A. tepida and A. parkinsoniana showed positive linear correlation with salinity and dissolved oxygen values. The restricted connectivity of the estuary with Bay of Bengal and frequent anthropogenic activity appeared to have an adverse effect on the faunal abundance, diversity and spatial distribution. The Q mode cluster analysis based on single linkage approach was performed to determine if any groups of samples or species correlated with ambient environmental factors. The study identified a total of 36 benthic foraminifer species belonging to 3 sub-orders. The faunal assemblages were dominated by two species Ammonia parkinsoniana and A.beccarii nearly at all the sampling stations.

Stratigraphic status, age, and paleobathymetry of the Grey shale Member, Dalmiapuram Formation, Cauvery Basin, India

Grey shale Member of the Dalmiapuram Formation, Ariyalur Group, Cauvery Basin, India was studied for its stratigraphic position, age, and paleobathymetry with a re-look into the lithological relationship and foraminifer assemblages in the deepened limestone mine excavations at M/s Dalmia Cements, Dalmiapuram. Twenty grey shale samples from Kovandankurchchi (pit-4) and Kallakkudi mines yielded diversified calcareous, benthic, and rare index planktic foraminifera. The foraminiferal assemblages suggest a latest Albian age and middle neritic depositional conditions. The abundance of kaolinite and smectite clay minerals relate to warm/humid climate which corroborate with rising relative sea level during grey shale deposition. The grey shale occurs in patches within the marl bedded limestone member which exhibits cyclic deposition of limestone and marl. The limestone mine sections demonstrate that the grey shale forms part of basal marl bedded limestone, directly overlying the coral algal limestone. The present study demonstrates that the grey shale outcrops in Dalmiapuram Formation should be placed stratigraphically as part of marl bedded limestone. The member status for grey shale which is current usage stands discounted.

A revised geological range (Late Pliocene) for foraminifer taxon Bolliella adamsi Banner et Blow, 1959

The foraminifer taxon Bolliella adamsi Banner et Blow, 1959 is found to co-occur with index planktic foraminifer taxa of the Late Pliocene (Zone N21) in a core of 2.60 m at 1300 m water depth off Tuticorin, Bay of Bengal. This taxon has been previously known as a Holocene taxon of the Indo-Pacific province. This study significantly revises the known stratigraphic range of B. adamsi from the Late Pliocene (uppermost part of Zone N21) to the Holocene in the Bay of Bengal area.

Quartz surface microtextural studies of Cauvery River sediments, Tamil Nadu, India

The spatially distributed sand samples of the Cauvery River sediments in Tamil Nadu were investigated with respect to their surface microtextural features in north-east (NE) and south-west (SW) monsoon rainfall seasons. The quartz grains show distinct surface microtextures with unique mechanical, chemical and morphological features with respect to the sedimentation process. The mechanical features like conchoidal fractures with arcuate steps indicate that the sand grains were derived from crystalline rocks. The abundance of angular grains supports that the grains were transported long distance by fluvial process. The silica globule, overgrowth and precipitation marks on the grain surfaces decipher the chemical process in silica saturated environment. Similarly, the solution pits in sand grains near Poombugar indicate the influence of sea water. The chatter mark trails suggest the wet tropical climate. The abundance of mechanical features and smooth surface microtextures suggest that the grains were transported in high-energy conditions. The NE monsoon sediments are coarse grained to medium grained and SW monsoon sediments are medium to fine sand size, which are attributed to the high and moderate energy conditions, respectively.

Foraminiferal biostratigraphy and paleobathymetric trends across the Cretaceous-Tertiary boundary in Neyveli area, Cauvery Basin, India

Five planktic foraminiferal biozones, namely, Abathomphalus mayaroensis zone, Pseudotextularia deformis zone, Parvularugoglobigerina eugubina zone, Globoconusa daubjergensis zone and Praemurica inconstans-Subbotina triloculinoides zone, are identified in the subsurface lithosection NY-KT, Cauvery Basin. The succession of planktic zones P. deformis of the terminal Maastrichtian (zones CF1–CF2) and P. eugubina (zone Pα of (Caron, 1985) = P1a of (Keller, 1993)) of early Danian denotes that the Cretaceous-Tertiary transition is relatively discontinuous with no major hiatus. The occurrence of ecological index benthic foraminifera Nuttallides truempyi and Bulimina trinitatensis in part of the A. mayaroensis (=CF2) and M. inconstans-S. triloculinoides (=P2) zones suggests that the paleodepth was >500 m while their absence reveals that sea level dropped below 500 m through biozones CF1, Pα, and P1. This observation is in conformity with the regional paleobathymetric trends across the KTB in the Cauvery Basin.