UNIT 7 GONDWANA SUPERGROUP AND DECCAN TRAPS Structure_______________________________________________ 7.1 Introduction 7.4 Activity Expected Learning Outcomes 7.5 Summary 7.2 Gondwana Supergroup 7.6 Terminal Questions Distribution 7.7 References Classification 7.8 Further/Suggested Readings Description of Formations 7.9 Answers Economic Significance 7.3 Deccan Traps Distribution Classification Age and Duration Economic Significance 7.1 INTRODUCTION The Precambrian (Archaean and Proterozoic) rocks are well developed in the peninsular India. You have already studied some of the rock supergroups that belong to Precambrian age such as Dharwar, Cuddaphah, Vindhyan and Delhi in Unit 4. In peninsular India, Lower to Middle Palaeozoic rocks record is almost absent. However, Palaeozoic rocks are well developed in the extra-peninsular or Himalayan region which you have read in Unit 5. From the Upper Carboniferous to Lower Cretaceous, peninsular India witnessed the huge deposition of freshwater sediments in numerous …………………………………………………………………….…………………………………………………Block 2 Stratigraphy of India interconnected inland basins, which are collectively known as Gondwana Supergroup. This supergroup is famous for its coal deposits and plant fossils wealth. At the end of Cretaceous, western part of the peninsular India underwent numerous volcanic eruptions that resulted in the formation of the Deccan Traps. It has been proposed that these volcanic eruptions might have been responsible for the mass extinction at the Cretaceous/Palaeogene boundary. In this unit, we will discuss the geographic distribution, classification, climate and lithology of the Gondwana Supergroup. We will also discuss the Deccan traps and associated sediments. Expected Learning Outcomes__________________ After studying this unit, you should be able to: define and classify Gondwana Supergroup; discuss the lithology of various formations of the supergroup; outline the economic significance of the supergroup; describe the distribution and classification of the Deccan traps; and explain the age and duration of the Deccan traps. 7.2 GONDWANA SUPERGROUP Gondwana Supergroup comprises a thick sequence of fluviatile and lacustrine sediments having a cumulative thickness of about 6 to 7 km with glacial sediments at the base. It covers a vast tract of India, particularly prominent in peninsular region, occupying about 50,000 km2 areas. The deposition of the sequence began in the Upper Carboniferous and continued up to the Lower Cretaceous. It may be noted that the lower and upper boundaries of the supergroup are determined by marine fossil records and do not coincide with standard chronostratigraphic system. The Gondwana sequence is largely considered as a continental sedimentary sequence with occasional marine incursions and deposited either in the river valley grabensor in the down-faulted grabens (Kumar, 1988; Vaidyanadhan and Ramakrishnan, 2010). It is dominantly composed of sandstones and shales with rich coal seams as well as fossil remains of plants and animals. The name“Gondwana” was given by H. B. Medlicott in 1872. It is derived from the Gond Kingdom of the Narmada river, Madhya Pradesh, where the supergroup was first studied by him. Later, investigations carried out in other parts of the world such as in South America, South Africa, Australia, Antarctica and Madagascar, which shows that these rocks are also present in these southern continents. The Gondwanan rocks of these areas bear spectacular similarities in terms of lithology and fossil contents. Keeping these facts in view, Edward Suess in 1885 coined the term “Gondwanaland” to referall these southern continents into a supercontinent, which was separated from its northern counterpart known as “Laurasia” by then present equatorial sea called “Tethys”. 150 Gondwana Supergroup and Deccan Trap …………………………………………………………………….………………………………………………….Unit 7 7.2.1 Distribution The rocks of the Gondwana Supergroup in peninsular India mainly occur in the four isolated patches represented by linear tracts. These tracts are Koel- Damodar basin of West Bengal and Jharkhand, Son-Mahanadi basin of Chhatisgarh and Madhya Pradesh, Satpura basin of Madhya Pradesh and Pranhita-Godavari basin of Telangana and Andhra Pradesh (Fig. 7.1). In addition, a few Gondwana outcrops also present in the eastern coastal area of India such as Athgarh basin of Odisha, Krishna trough of Andhra Pradesh and Palar-Cauvery troughs of Tamil Nadu. In the Himalayan region, a linear belt of the Lower Gondwana rocks occurs along the Himalayan foot-hills Assam and Arunachal Pradesh and also in Kashmir valley. Fig. 7.1: Map of India showing the distribution of the Gondwana basins. (Source: simplified after Vaidyanadhan and Ramakrishnan, 2010) 7.2.2 Classification Classification of the Gondwana Supergroup is always a topic of debate in Geology of India. On the basis of lithology and plant fossils, two schemes of classification have been proposed for the rocks of the supergroup. These schemes are: Two-fold classification Three-fold classification 151 …………………………………………………………………….…………………………………………………Block 2 Stratigraphy of India Two-fold Classification Two-fold classification was proposed by W. T. Blanford. He divided the Gondwana sequence into two subdivisions namely, Lower Gondwana Sequence and Upper Gondwana Sequence. These two sequences are separated by a slight unconformity, which lies at the top surface of the Panchet Formation of the Lower Triassic age. Lower Gondwana Sequence is characterised by the dominance of Glossopteris flora whereas the Upper Gondwana Sequence is characterised by appearance of Ptilophyllumflora. The two-fold classification was strongly supported by C. S. Fox, R. D. Oldham, G. Cotter, M. S. Krishnan and other workers. Three-fold Classification Three-fold classification was proposed by O. Feistmantal and further it was supported by E. Vredenburg and D. N. Wadia. This classification is primarily based on plant fossils and the prevailing characteristic climatic conditions of the Gondwana Supergroup. According to this classification, Gondwana Supergroup is divided into three sequences such as Lower, Middle and Upper Gondwana, corresponding roughly to the Permo-Carboniferous, Triassic and Jurassic, rock systems of Europe, respectively. The Lower Gondwana Sequence is characterised by presence of Glossopteris flora, a warm and humid climate with numerous coal seams. The Middle Gondwana Sequence is characterised by appearance of Dicroidium flora, a warm and dry climate and by the presence of amphibian and repltilian fossils. The Upper Gondwana Sequence is marked by the appearance of Ptilophyllumflora as well as a warm and humid climate (Fig.7.2). Fig. 7.2: Three-fold classification of the Gondwana Supergroup based on floral relationships. 152 Gondwana Supergroup and Deccan Trap …………………………………………………………………….………………………………………………….Unit 7 Over the centuries, most of the workers have been following two-fold classification of the Gondwana Supergroup and hence, the same has been discussed here. A generalised stratigraphic classification of the Gondwana Supergroup is given in Table 7.1. Table 7.1: Generalised stratigraphic classification of the Gondwana Supergroup. Super Sequence Group Formation Age group Jabalpur Lower Jabalpur Formation Group Cretaceous Rajmahal Lower Rajmahal Formation Upper Group Cretaceous Gondwana Kota Formation Middle Jurassic Sequence Mahadeva Maleri Formation Upper Triassic Group Pachmarhi Lower Triassic Formation ~~~~~~~~~~~~~~~~~~~~~~ Unconformity ~~~~~~~~~~~~~~~~~~~ Panchet Panchet Formation Lower Triassic Group Gondwana Supergroup Raniganj Formation Upper Permian Lower Barren Measures Damuda Middle Permian Gondwana Formation Group Sequence Barakar Formation Lower Permian Karharbari Formation Lower Permian Talchir Upper Talchir Formation Group Carboniferous 7.2.3 Description of Formations The Permo-Carboniferous sedimentary successions of the Gondwana Supergroup is largely known as Lower Gondwana Sequence and the Mesozoic Gondwana succession forms the Upper Gondwana Sequence (Fig. 7.3). However, the Panchet Formation of the Lower Triassic age comes under the Lower Gondwana Sequence. The Lower Gondwana groups are characterised by the presence of Gangomopteris-Glossopteris flora and the Mesozoic Upper Gondwana groups by containing Dicroidium-Lepidopteris flora. It has been inferred that the Upper Carboniferous sediments of the Lower Gondwana sequence are deposited under a glacial/cold climate, whereas coal-bearing Permian sediments were deposited under warm and humid climate. 153 …………………………………………………………………….…………………………………………………Block 2 Stratigraphy of India Fig. 7.3: Distribution of Lower and Upper Gondwana sequences in the major Gondwana basins of India. During the beginning of the Upper Gondwana Sequence, sudden changes in climate took place in the Triassic. As a result, warm and humid climate of the Permian was replaced by warm and dry climatic conditions during the Triassic. This warm and dry climate was considered responsible for the disappearance of the Glossopteris flora of the Lower Gondwana. The warm and humid climatic conditions again appeared during the Late Triassic and most of the Jurassic, which resulted in the appearance of Ptilophyllum flora. A generalised stratigraphic classification of the Gondwana Supergroup of India is presented in Table 7.1. All formations mentioned in the table may not occur in any single Gondwana basin of India. For example, Karharbari, Barakar, Barren Measures and Raniganj formations are well