Matthew P. Watkinson

Cretaceous tectonostratigraphy and the development of the Cauvery Basin, southeast India

The late Jurassic to early Cretaceous rifting between India/Australia and India/Antarctica resulted in the formation of a number of NE–SW-trending basins in the Indian Precambrian crystalline basement. The Cauvery Basin is the southernmost basin along the eastern margin of the Indian Sub-Continent, covering much of this part of India and extending a considerable distance offshore. The basin comprises several ‘depressions’, or sub-basins, with the Ariyalur–Pondicherry Depression in the north. The exposed successions are in the southern part of this sub-basin. The result of fieldwork (1994–8) has been a reassessment of the lithostratigraphy and the tectonostratigraphic history of the Ariyalur outcrop. Three major sedimentary groups were identified: the syn-rift Gondwana Group (of early Cretaceous age), the syn-rift Uttatur Group (of Albian to Coniacian age) and the post-rift Ariyalur Group (of Santonian to Maastrichtian age). Both microfaunal and macrofaunal information were used to develop a biostratigraphic framework for the basin and a new tectonostratigraphic model. This new model for the development of the basin is significantly different to that used by the Oil and Natural Gas Commission of India. Structures exposed onshore, which have been interpreted as Albian reefs, are interpreted here as irregularly shaped limestone olistoliths/olistostromes produced by intra-Cretaceous rifting and slumping within the basin. The paper discusses this model for the basin history which is calibrated by updated foraminiferal (and macrofossil) biostratigraphy.

Teredolites from the Garudamangalam sandstone formation (Late Turonian‐Coniacian), Cauvery basin, southeast India

Teredolites‐infested log‐grounds are abundant in the late Turonian‐Coniacian Garudamangalam Sandstone Formation in the Cauvery Basin, Southeast India. The morphological characteristics of the borings deviate from those of the ideal Teredolites clavatus Leymerie and Teredolites longissimus Kelly and Bromley, reflecting the response of the boring animals to substrate overcrowding and competition for space. Ichnological and sedimentological evidence indicate deposition of log‐bearing intervals in a delta‐plain or estuarine setting. Application of sequence stratigraphic principles indicates that log‐ground accumulation in the Cauvery Basin was closely linked to episodes of sea‐level rise (transgressive events).

Foraminiferal recovery after the mid-Cretaceous oceanic anoxic events (OAEs) in the Cauvery Basin, southeast India

Abstract The Cauvery Basin, SE India, is one of the best exposed late Mesozoic to Tertiary basins in India. The study of foraminiferal assemblages from the core samples obtained from two 120m deep wells in the basin records the occurrence of two mid-Cretaceous anoxic events in the basin. Abrupt increases in planktonic: benthonic ratios and reductions in benthonic diversity are recorded in the late Albian and in the late Cenomanian-early Turonian. These events coincide with worldwide oceanic anoxic events (OAEs). The OAEs had an impact on the microfauna of the basin, with approximately 35–45% of the benthonic species not surviving and major morphological changes occurring in the planktonic community. The late Cenomanian-early Turonian event was more significant than the late Albian event, with considerable readjustments occurring in the planktonic foraminifera. The genus Rotalipora disappeared and the genus Hedbergella was largely replaced by Whiteinella. Marginotruncana appeared for the first time and the Dicarinella population expanded considerably. Praeglobotruncana was the most tolerant genus, undergoing least change. The planktonic foraminifera evolved from small, weakly ornamented forms with poorly developed keels, into robust, well ornamented forms with well developed keels. The pattern of evolution of planktonic foraminifera suggests a recolonization of deeper water environments after the late Cenomanian-early Turonian anoxic event.

FORAMINIFERAL CHARACTERISATION OF MID-UPPER JURASSIC SEQUENCES IN THE WESSEX BASIN (UNITED KINGDOM)

The use of foraminifera in the characterisation of sequences (systems tracts, maximum flooding surfaces, etc.) has developed over the last decade. Much of this work has been based in the Cenozoic successions of the Gulf of Mexico, although there is a growing application of such data in the Middle East and the North Sea Basin. The easiest surface to characterise has been the maximum flooding surface with its high diversity and high(er) abundance faunas; the characterisation of individual systems tracts has been less successful. Using the well-known mid-Upper Jurassic successions of the Dorset coastal sections, we have investigated a number of high resolution (para)sequences for their foraminiferal content. Using data of foraminiferal diversity and standing crops from a range of modern substrates we have investigated the potential faunas available after deposition, taphonomy, compaction, groundwater dissolution and modern weathering. By understanding the processes involved we have identified the key foraminiferal features of typical mid-Upper Jurassic sequences and indicated how this work may help in the correlation of successions in North Dorset and Normandy.