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PLATE TECTONICS The Indian Craton was once part of the Supercontinent of Pangaea , attached on what is now its west coast to Madagascar and southern Africa . About 160 million years ago during the Jurassic Period, rifting caused Pangaea to break apart, firstly into two supercontinents (Gondwana to the south and Laurasia to the north). The Indian craton for a time remained attached to Gondwana , but this too began to rift apart about in the early Cretaceous , about 125 million years ago, creating the Indian Plate . The Indian Plate then sped northward toward the Eurasian Plate , at a pace that is the fastest movement of any known plate. About 90 million years ago, it separated from Madagascar. The Indian Plate first collided with the Eurasian Plate about 50 million years ago. This collision, which is ongoing today, closed up an intervening sea, then lifted the Himalaya Mountains and the Tibetan Plateau , and caused parts of the Asian continent to deform westward and eastward on either side of the collision. Just before or simultaneous with this collision, the Indian Plate sutured on to the adjacent Australian Plate , forming a new larger plate, the Indo-Australian Plate . THE DECCAN TRAPS During its journey northward after breaking off from the rest of Gondwana, the Indian Plate passed over a geologic Hotspot , the Réunion Hotspot , which caused extensive melting underneath the Indian craton. The melting broke through the surface of the craton in a massive Flood Basalt event, creating what is known as the Deccan Traps . It is also thought that the Reunion hotspot caused the separation by rifting of Madagascar and India. GEOLOGICAL PHASES The earliest geologic phase was marked by the cooling and solidification of the upper Crust of the earth surface in the Pre-Cambrian era (prior to 600 million years) represented by the exposure of gneisses and granites especially on the Peninsula. These form the core of the Indian craton. Minor igneous activities, subsequent Metamorphism and crumpling along with the folding of the Aravalli Mountains represent the main activities. The levelling of the undulations, crushing and crumpling of the sediments of the Dharwaian group (Bijawars) mark the second phase. The igneous activities and intrusions imparted it the character of the mixed sedimentaries. The calcareous and arenaceous deposits corresponding to humid and semi-arid climatic regimes in the cuddapah and vindhyan basins bordering or lying within the existing landmass and its uplift during the Cambrian (500 million years ago) mark the main sequence of events in this period. These formations have preserved their horizontal stratification. Cuddapahs are certainly ordered as they underlie the Vindhyans, but the controversy regarding the chronology of upheaval of vindhyans is yet to be resolved. The imprints exist only in the form of faults, joints and minor folds at places. The Permo-Carboniferrous glaciation form a southerly source and extensive glacio-fliuvial deposition in the depression and subsequent sag faulting where these deposits, designated as the Gondwanas, are preserved, along with the transgression of the Permian (270 million years ago), exhibit the major activities of this phase. This phase coincided with the fracturing and drift of the continental mass of Gondwana . To this drift probably owes the subsequent uplift of the vindhyans, the circumferencial zone and the northern peripheral sediments in the Himalayan Sea. The close of the Mesozoic witnessed one of the greatest volcanic eruptions, the Deccan Lava Flows , covering more than 500,000 km&2 area followed by the first phase of the Teriary orogeny, the Karakoram phase. MAIN ROCK GROUPS Pre-Cambrian A considerable area of peninsular India consists of Gneisses and Schists , which are the oldest rocks found in India. The Pre-Cambrian rocks of India have been classified into two systems, namely the Dharwar system and the Archaean system. The rocks of the Dharwar system are of mainly sedimentary origin, and occur in narrow elongated synclines resting on the gneiss found in Bellary district, Mysore and the Aravalli s of Rajputana. Manganese and Iron Ore also occur along with them which makes these rock formations very important. The other important Metal associated with these rocks is Gold found in the Kolar Gold Mine s located in Mysore. A few formations in the north and west of India like the Vaikrita system, which occurs in Hundes , Kumaon and Spiti areas, the Dailing series in Sikkim and the Shillong series in Assam are believed to be of the same age as the Dharwar system. The Pre-Cambrian system consists of gneisses which are further classified into the Bengal gneiss, the Bundelkhand gneiss and the Nilgiri gneiss. The Niligiri system comprises the Charnockite s ranging from Granite s to Gabbros . Palaeozoic The earliest rocks of the Cambrian period are found in the Salt range in Punjab and the Spiti are in central Himalaya s and consist of fossiliferous sediments. In the Salt range there are various zones found. The Salt Pseudomorph zone has a thickness of 450 ft consists of Dolomite s and Sandstone s. The Magnesian Sandstones have a thickenss of 250 ft and are near to the Dolomites in characteristic. They have very few fossils. The Neobolus Shale composed of dark Shales have a thickness of 100 ft. Finally there is a zone consisting of red or purple sandstones having a thickness of 250 to 400 ft called the Purple Sandstone. These are unfossiliferous and show sun-cracks and worm burrows. The deposists in Spiti are known as Haimanta system and they consist of Slate s, micaceous Quartzite and dolomitic Limestone s. The Ordovician rocks comprise flaggy shales, limestones, red quartzites, quartzites, sandstones and Conglomerates . The siliceous limestones belonging to the Silurian succed the Ordovician rocks. These limestones are covered by white quartzite and this is known as Muth quartzite. Silurian rocks which contain typical Silurian Fauna are also found in the Vihi district of Kashmir . Typical Devonian fossils and Coral s are found in grey limestone in the central himalayas and in black limestone in the Chitral area. The Carboniferous comprises two series namely Po series, also known as upper carboniferous, and Lipak series also known as lower carboniferous. The fossils of Brachiopods and some Trilobites are found in the calcareous and sandy rocks of the lipak series. The po series overlays the lipak series, and the Fenestella shales of the upper carboniferous age are found here in the qaurtzites and dark shales. The Syringothyris limestone in Kashmir also belongs to the lipak series. The carboniferous strata is overlaid by Agglomeratic slates, believed to be of volacanic origin, which are grey in color. Mesozoic Various Genus of Productus are found in the limestones of the Triassic, which has led to the deposits being referred to as productus limestone. This limestone is of marine origin and is divided into three stratas. The Chideru stage is formed by the upper productus and contains ammonites, the middle productus represents the Virgal stage and the Amb stage formed by the lower productus. Ceratite beds, named after the genus Ceratite , consisting of arenaceous limestones, calcerous sandstones and marls also belong to the Triassic. The Jurassic consists of three stratas. Kioto limestones comprise the lower and middle Jurassic and are 2000 to 3000 ft thick. The upper Jurassic comprises Spiti shales, which are black, and extend from the Karakoram to Sikkim. Cretaceous rocks are found in south India and are divided into Niniyur stage, Ariyalur stage, Trichonopoly stage and Utatur stage. The utatur stage consists of phosphatic nodules which constitutes an important source of Phosphate s in the country. The well developed beds of Lameta in the central provinces contain fossil records which are helpful in estimating the age of the Deccan Trap. This trap was formed near the end of the Cretaceous period due to volcanic activity. This volcanic area occupies about 200,000 square miles of area. These traps give rise to really good building stone and also yields very fertile clayey Loam , particularly suited to Cotton cultivation. Cenozoic
In this period occurred the uplift of the Himalayas and the volcanic activity of the Deccan Traps. The rocks of this era have valuable deposits of Petroleum and Coal . Sandstones of Eocene are found in Punjab, which pass into chalky limestones with oil seepages. Further north the rocks found in the Simla area are divided into various series, the Sabathu series consisting of grey and red shales, the Dagshai series comprising bright red clays and the Kasauli series comprising sandstones. Towards the east in Assam Nummulitic limestone is found in the Khasi hills. Oil is associated with these rocks of the Oligo-Miocene age. The Siwalik system also has rocks with thickness of 16000 to 20000 ft which belong to the Cenozoic age. These rocks mainly comprise clays and sandstones and contain remains of Vertebrate animals.
The Alluvium which is found in the Indo-Gangetic Plain belongs to this era, and was brought down from the Himalayas by the rivers. These alluvial deposits consist of clay, loam, silt etc. and are divided into the older alluvium and the newer alluvium. The older alluvium is called Bhangar and is present in the ground above the flood level of the rivers. Khaddar or newer alluvium is confined to the river channels and their Flood Plain s. This area has some of the most fertile soil found in the country as new silt is continually laid down by the rivers every year. NOTES |
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