Title	Magnetostratigraphic, Biostratigraphic and Paleomagnetic investigations of Cauvery Basin sediments to constrain the drift history of the Indian subcontinent during Late Cretaceous period
Participants	R. Nagendra* and M. Venkateshwarlu *Anna University (Principal Investigator)
Sponsoring Agency	DST (No.SR/S4/ES-271/2007 dated 26-5-2008)

Work Description:

This studyis focused to re-look the onshore sediments in terms of Magnetostratigraphy and biostratigraphy to understand the paleogeography of Cauvery Basin during upper Cretaceous. This detailed paleontological and Palaeomagnetic study of these upper Cretaceous formations would result in scientific data with respect to the migration of the subcontinent with well constrained paleontological and paleomagnetic records. These integrated studies will complement and supplement each other.

Therefore, it was decided to take up a detailed study of the Cauvery Basin sediments to understand the drift history of the subcontinent with well constrained fossil evidence.

Objective

• 1.To re-look the biostratigraphy records of outcrop sediments of late Cretaceous formations of Cauvery Basin and to constrain the drift history by integration of fossils geology to the paleoclimate, depositional environment, chronology and paleobathymetry.
• 2. Construction of Magnetostratigraphy column of outcrop lithosections of late Cretaceous sediments and its correlation with standard GTS-2008 and global late Cretaceous sedimentary sections.
• 3. To measureNatural Remnant Magnetic (NRM) properties of late Cretaceous sediments of onshore sediments. Identify and isolate the Characteristic Remanent Magnetic (ChRM) directions in Cretaceous sediment samples by demagnetization studies using AF, Thermal methods and Interpretation of the isolated ChRM components in terms of drift history of the Indian subcontinent during the late Cretaceous.

The upper Cretaceous sediments exposed near Tiruchirapalli and Vridhachalam in the state of TamilNadu and in the union territory of Pondicherry have been extensively studied for biostratigraphy and mega faunal remains. The foraminiferal study of the complete succession of cauvery basin has enabled to infer their palaeoenviroment and to relate them to the sea level changes.

The Paleomagnetic results of different sedimentary lithosections of late Cretaceous formations and the overall mean is tabulated (Table-1). It is inferred that the Indian subcontinent during late cretaceous was placed just above the Deccan Traps at 19.3°S.

Fig 28.2 is the composite section of Cenomanian to Maastrichtianlithosections studied at 564 m thick sedimentary sections belonging to Kari, Garudamangalam, Sillakkudi, Kallankurichchi and Ottakkovil Formations of Uttatur, Trichinopoly and Ariyalur Groups of Cretaceous sequences of Cauvery Basin. The observed Magnetic Polarity Stratigraphy of these sequences is computed with 13 magnetozones, 6 normal and six reversal events. The observed Magnetic Polarity Stratigraphy is correlated with GTS-2008. The late Cretaceous sequences consists of Karai Formation which is developed during late Cenomanian to Turonian age (99.6-93.6Ma), Garudamangalam Formation is divided in to two members. The Kulakkalnattam sandstone formed during Coniacian to early Santonian age (88.6 – 85.8 Ma) and Alandipur limestone is of middle to late Santonian age (85.8 – 83.5). Sillakkudi Formation is developed during Campanian age (83.5– 70.6), Kallankurichchi Formation has developed during early-middle Maastrichtian (70.6-66.5) and the terminal Cretaceous are exposed in Ottakkovil sandstone of late Maastrichtian age (66.5–65.5 Ma). These chronology are based on biostratigraphy frame work (Govidan,1996, Raju, et.al, 1996, Chidambaram, 2000, Nagendra et.al 2011).

Up to Santonian, a long normal magnetic polarity is present in the Cauvery Basin (Fig. 28.2) which is very well correlated with the standard Geological Time Scale and this falls under the Chron C34n which is the longest Normal quite polarity zone in the entire Cretaceous period. Above this, in the Campanian, one reversal and one normal event is recorded in the Cauvery Basin and is- correlates very well with standard GTS of chron C33n. In Magneto Chron C32n, one normal and two reverse polarities were obtained in Cauvery Basin sequences and correlate well with GTS whereas one normal and one reverse polarity is missing in this magnetozone as compared to standard GTS. This is because lack of sampling in portion of the strata or missing in Kallankurichchi Formation. Above this, in the magnetozone C31n one normal and one reverse polarity is observed and correlated well with GTS. At the end, the magnetozone C30n where one reversal and one normal polarity is obtained and it correlated very well with the standard GTS. The magnetic polarity stratigraphy of the late Cretaceous successions in the Cauvery Basin is correlated successfully with the standard GTS from Magneto chrons C34n to C30n.

Conclusions

The project is organized with objectives (1) to re-look the biostratigraphy records of outcrop sediments of late Cretaceous formations of Cauvery Basin and to constrain the drift history by integration of fossils geology to the depositional environment, chronology and paleobathymetry (2) Construction of magnetostratigraphy column of outcrop lithosections of late Cretaceous sediments and its correlation with standard GTS-2008 and global late Cretaceous sedimentary sections. (3) To measurement of NRM properties of late Cretaceous sediments of onshore sediments and (4) Identify and isolate the ChRM directions in Cretaceous sediment samples by demagnetization studies using AF, Thermal methods and Interpretation of the isolated ChRM components in terms of drift history of the Indian subcontinent during the late Cretaceous.

The study conducted in the Cenomanian-Maastrichtian sedimentary sections exposed in Cretaceous outcrop areas. The Paleomagnetism study qualified lithosections are identified in Karai, Garudamangalam, Kallankurichchi and Ottakkovil Formations of Uttatur, Trichinopoly and Ariyalur Groups. The studied outcrop site of Pondicherry and Vridhachalam areas samples are not yielded the paleomagnetic measurements records. However the biostratigraphy data were used in the comprehensive inferences. The identified lithosections were sampled for paleomagnetic and biostratigraphy studies. The oriented sample blocks were collected with attention to the suitability for Paleomagnetic measurements. The total of 193 blocks was cataloged for the studies. The limitations in sampling were notified, as the non-suitability and lack of outcrop lithosections. The macro and microfossil records were documented, in addition to the references of biostratigraphy framework of researcher’s published records. The magnetic measurements and its inferences were correlated with confidence of fossil records as it provides the physical evidences in terms to the prevailed geological conditions and its characteristics expresses to the dynamic conditions of the Cretaceous environment. The work outlined to bring out the drift history of the Indian subcontinent during upper Cretaceous as there is a data gap exists in the Latitudinal drift history of the Indian subcontinent during this period. This study has brought out very good magnetostratigraphy of the different litho sections in Cauvery Basin (Cenomanian-Maastrichtian) and was correlated very well with the standard GTS-2008 from Chron C34n to C30n and every magnetozone was matched very well. This correlation also supportive with the global upper Cretaceous sections from Russia, Europe and other upper Cretaceous sequences worldwide. The gap in the Latitudinal drift history of the Indian subcontinent has been filled with the Palaeolatitudinal position obtained from this study and placed Indian subcontinent at 20°S during entire upper Cretaceous period (65-100 Ma).

Fig.28.1:
Geological map of the study area

Fig.28.2:
Correlation of Magnetic Polarity Stratigraphy of Late Cretaccous with the standard geological Time Scale of Gradstein et al., 2004 and ogg et al., 2008

Table 1. Paleomagnetic results of the late Cretaceous sequences, Canvery Basin
Section	n	Dm	Im	a95	ΛP	LP	dp	dm	Λm
O	5	165	-20	23.29	-75.24	167.43	12.78	24.4	-10.2
K	23	334	-52	13.14	40.24	287.73	12.17	17.88	-32.6
S	5	323	-32	50.43	43.75	311.74	31.87	56.7	-17.4
A	4	345	-39	35.21	24.58	311.83	53.02	82.89	-22.0
KN	3	344	-27	41.05	60.32	291.46	24.16	44.54	-14.3
KA	4	240	-47	30.82	-31.7	355.59	25.81	39.89	-28.2
Mean	5	334	-35	28.16	47	305/56W	16.7	30.60	-19.3
O-Ottakovil, K-Kallankmuchi, S-Sillakndi, KN-Knlkamhattam, KA-Karai, Dm-mean Declimation, Im-mean inclimation, a95-Circle of confidence at 95% probability level, n-number of samples, Λp VGP Latitude, Lp-VGP Latitude, dp-ellipse of error, dm-perpendicular to ellipse and Λp-palaeolatitude.