Cenomanian - Turonian transition in the Cretaceous of southern India K.AYYASAMI AND RANJIT K. BANERJI Ayyasami, K. & Banerji, Ranjit K.: Cenomanian - Turonian transition in the Cretaceous of southern India. Bull. geol. Soc. Denmark, vol. 33 pp. 21-30, Copenhagen, September, 10th, 1984. The Cretaceous formations of southern India range from Upper Albian to Maastrichtian in age on the basis of their macro- and microfauna. A considerable number of faunal elements are either identical or closely related to forms known from the European Cretaceous, and a precise correlation between the two areas is possible. In this paper the contact between the Cenomanian and Turonian in southern India is reviewed. Rocks of the Upper Cenomanian age consist of medium to coarse grained calcareous sandsto­ nes and limestone bands, frequently crowded with fossils. They are interbedded with brown shales. These are overlain by Turonian limestone full of Exogyra, in turn overlain by fossiliferous calcareous concretions embedded in brown shales. Lower Turonian beds may overlap the uppermost Cenomanian in the northern part of the study area. The uppermost Cenomanian yielded Eucalycoceraspentagonum and Pseudocalyco- ceras harpex associated with Inoceramus pictus. The Lower Turonian yields Pseudaspidoceras conciliantm, Neoptychites cephalotus and Mytiloides mytiloides. Foraminifera are rare in the Turonian succession. K. Ayyasami, 5-5-449, Mukram Jahi Road, Hyderabad-500001, India. Ranjit K. Banerji, Department of Earth Sciences,Indian Institute of Technology, Bombay-400076, India. March 4th, 1984. Introduction with the shell limestones forming high ground and the streams of the area have their source The Cretaceous formations of southern India oc­ among these. The stream discharge is seasonal cur in a series of isolated outcrops in the and the long dry season affords easy extraction of Tiruchirappalli district, Tanjore and Vridhackal- the fauna. However, as Blanford earlier ob­ lam i Tamil Nadu (formerly Madras state) and in served, the separation of fossils from nodules is Pondicherry separated by younger sediments. very difficult. The outcrops in the Tiruchirappalli district are In the exposed sections, the beds dip approx­ the largest and are dated as Middle Albian to imately 8° due east. The description of the lithol- Maastrichtian, whereas the rocks of the other ogy along the stream sections are as follows: three outcrop areas are dated as Campanian to Maastrichtian in age. The present study of the Stream section north of Adanur village Cenomanian - Turonian boundary in southern (N11°11':E 79°00'). India is thus limited to outcrops in the This section exposes the rock of the upper Tiruchirappalli district (Fig. 1). Uttattur beds for a distance of about half a mile. Many Lower Turonian fossils were collected from the limestone nodules in the shale unit from this Lithostratigraphy stream which is being brought under dry crop cultivation. The lower reaches of the stream are The present work is confined to the upper beds of concealed under arable lands, though in places, the Uttattur Group (Ootatoor group) as defined unfossiliferous calcareous sandstones are seen in by Blanford (1862) who gave a detailed account dug well debris and thus other lithological units of lithological variations in this group. Indeed, are not available for correlation. Blanford's observations on the upper beds of the Uttattur Group (1862, p. 93) form the basis of Stream section due east of Odiyam village. our study. (N11°13':E 78°59'30"). The topography of the area is much dissected THis section is one of the best developed and Ayyasami & Banerji: Cenomanian-Turonian in India Cenomanian-Turonian transition beds in the Cretaceous of southern India SCALE (Tiruchirapalli District outcrop area) I NDEX Ariyalur Group of rocks Trichinopoly Group of rocks 006 Limestone concretions in shales Calcareous sandstone Predominently limestones frequently crowded with fossils r* Uttattur Group //////// Gypseous shales with interbedded V//////A biostromal limestones + + + + + + Archean gneiss Fig. 1: Map showing the lithological units at the Cenomanian-Turonian contact in southern India. the rocks are exposed for nearly a mile. Numer­ Stream section southwest of Kunnam village. ous fossils were collected from this locality. (N11C13'30":E 79°01'30"). Hence, it is designated as the type section. The At the confluence of the small stream flowing stratigraphic column prepared along this section west and joining the main stream southwest of is shown in Fig. 4. Kunnam village, the limestone with numerous At the confluence of this tributary stream and Inoceramus pictus (Woods) is exposed. This lime­ the main stream flowing southwards to the west stone is overlain by shales with limestone nod­ of it, the shell limestones with numerous mol- ules. The traverse upstream leads to two bands of luscan fossils are exposed followed by limestone shell limestone with abundant Exogyra subor­ nodules in a gypseous clay unit. Further up­ biculata (Stoliczka) intercalated by shales of stream, the shell limestones with numerous about 3 m. Overlying the second band of lime­ bivalve Exogyra suborbiculata (Stoliczka) cross stone are the shales with limestone nodules yield­ the stream. From then on, shell limestones and ing Pseudaspidoceras footeanum (Stoliczka). The limestone nodules in shales almost alternate for a whitish shales form the next overlying lithological short distance. The strike extension of these units unit, in turn overlain by the basal shell limestones may be explorable in the still smaller tributaries of the Trichinopoly Group. flowing into this stream from north and south directions. Non-gypseous whitish shale, forming Stream section west of Andur village. (N11°15':E the uppermost unit of the Uttattur Group, are 79°01'30"). exposed at the source of the stream, and are The stream flowing northwards and its small immediately overlain by shell limestones of the tributaries west of Andur village is one of the best Trichinopoly Group. Bulletin of the Geological Society of Denmark, vol. 33 1984 23 Cenomanian-Turonian transition beds in the Cretaceous of southern India (Trruchirapalli District outcrop area) INDEX _--•" Cenomanian-Turonian contact ® Romaniceros (Yubariceras) ornatissimum Zone ® Pseudaspidoceras conciliatum Zone © Eucalycoceras pentogonum Zone © Calycoceras newboldi Zone | ^^~\ Boundary between Uttattur & Trichinopoly beds J^>^JArchean gneiss-Uttattur beds contact G. S.I.S.R.D.O.NO.676/82 Hydir Fig. 2: Map showing the proposed biostratigraphic zones at the Cenomanian-Turonian contact on the toposheet. localities for fossil collection, as its course coin­ chinopoly Group (?) are visible where the stream cides with the general strike of the beds. A few turns abruptly eastward, north of Andur village. limestone bands yielding Calycoceras newboldi planicostatum (Kossmat) and Calycoceras Biostratigraphy stoliczkai (Collignon) are seen directly overlain by fossiliferous shell limestones yielding Exogyra There is a striking correspondence between suborbiculata (Stoliczka) shells. They are suc­ lithological boundaries and the biostratigraphic ceeded by a litho-unit of limestone nodules in zones proposed here (Fig. 2). The biozonation is shales. Numerous specimens of the giant am­ based on ammonoids, which are abundant and monites Pachydesmoceras denisonianum well preserved. Foraminifera, brachipods, corals, (Stoliczka) and P. pachydiscoide (Matsumoto) and echinoderms are rare, at least in the early were collected. The shell limestones of the Tri­ Turonian rocks. The biostratigraphic scheme is as follows: European stage Ammonite biostratigraphic zones Lower Turonian Pseudaspidoceras conciliatum Zone Upper Cenomanian Eucalycoceras pentagonum Zone Middle Cenomanian Calycoceras newboldi Zone Ayyasami & Banerji: Cenomanian-Turonian in India :usr Cenomanian-Turonian transition beds in the Cretaceous of southern India (Tiruchirapalli District outcrop area) INDEX _,' Cenomanian-Turonian contact A Inoceromus qeinitzianus . Inoceramus (Myt.) mytiloides x Inoceramus (Myt.) borkeri + Inoceramus (Myt.) labieformis Sphenoceramus diversus o B Inoceramus pictus I ^*^\ Boundary between Uttattur S Trichinopoly beds. yjZ^\ Archean gneiss-Uttattur beds contact G.S.I. S.R.D.O.NO.677/82 H^ " Fig. 3: Map showing the localities yielding species of inoceramids at the Cenomanian-Turonian contact. The Calycoceras newboldi Zone is one of the the northern part of the Cretaceous outcrop area best developed units in the southern Indian (Fig. 2). Foraminiferal evidence by Narayanan Cretaceous and has yielded this index fossil con­ (1977), however, presents a different picture as sistently along the strike from near Siruganur (N will be discussed later. 11°01': E 78°47'30") to the south of Paravay (N 11°17': E 79°02'30"). In contrast, the Eu- Remarks on previous zonations at the Ceno­ calycoceras pentagonum Zone is limited to the manian - Turonian contact. northern part of the area. The extension to the southern part of the Cretaceous basin is depen­ Kossmat (1895-98) was the first to propose a dent entirely on inferences based on field biostratigraphic zonation for the Upper Cre­ lithostratigraphic observations as no ammonoids taceous sediments of southern India. He showed are collected beyond Mungilpadi village (N beds with Acanthoceras cf. rhotomagense over­ 11°12': E 79°00'30"). Further discussion of the lain by those yielding Pseudaspidoceras concil- extension of this zone is dealt with subsequently iatum at the contact