Amazing Inter-trappean beds of Rajmahal Formation, upper Gondwana Fossil park
Inter-trappean beds of the Rajmahal Formation of Upper Gondwana are located in Sahibganj, Jharkhand covering an area of nearly 4300 Sq.km. comprises a series of the volcanic lava flow of basalt and associated intertrappean sedimentary beds. The individual intertrappean beds (Rajmahal formation) vary in thickness from 0.5m to 11m. It has a great significance regarding the preservation of Ptilophyllum flora that is also known as Rajmahal plant beds.
Inter-trappean beds of the Rajmahal Formation is Localized areas. These fossils belong mainly to the fern, Cycads, and conifers. Among these Ptilophyllum, Pterophyllum, Dictyozamites, Taeniopteris, Williamsonia, Brachyllum, Thinnfeldia, and Cladophlebis are the most common plant fossils. These are preserved in the exposed lower three or four intertrappean beds in the northern, central, and southern parts of the Rajmahal hills.
Fossils in the form of either petrification or impression are extremely well preserved- a rarity for remnants of that age. The richest assemblage of fossils is found in the Mandro area. Petrified wood has preserved the original structure of the stem.
The nearest airport from the Rajmahal hills in Ranchi and the nearest railway station is Mirza chowki (14km). Rajmahal hills are 100 km. and Mandro Block is 32 km from Sahibganj. Intertrappean beds of Rajmahal declared a National Geological Monument for its Protection and Maintenance
GEOLOGICAL SETTING
The Gondwana sediments of the Rajmahal Basin are a part of North-South trending Rajmahal-Purnea graben which extends over a large area of Jharkhand and West Bengal, Eastern India.
The Rajmahal Basin has one of the best-developed sequences of the Upper Gondwana sediments which is known for its rich wealth of plant fossils. The basin comprises a series of volcanic lava flows (Rajmahal Traps) and associated intertrappean sedimentary beds. It is located dominantly composed of basalt, occupies a surface area of about 4300 sq. km, and attains a maximum thickness of 600 meters.
The Microfloral assemblage indicating the age of Rajmahal Traps is Jurassic to lower cretaceous. The Intertrappean consists of an alternation of variegated Siltstone, Sandstone, and Conglomeratic.
Inter-trappean beds of Rajmahal Formation are very thin as compared to the thickness of individual lava flow in which thickness of intertrappen beds varies from a few centimeters to about 11m, whereas in subsurface these are up to 100m thick. These sedimentary beds are composed of sandstone, siltstone, arenaceous clay, white and grey color baked shale, carbonaceous shale, tuffite (Volcanic materials), and chert bed with Ptilophyllum flora. The intertrappean beds are well exposed in the southern, central, and northern regions of the Rajmahal areas.
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The extent of the Rajmahal lava flow is not limited to the Rajmahal area only but extends up to the Burdwan, Galsi, Jalangi, and Panagarh areas of West Bengal. Geo-scientist Identified fifteen lava flows with eleven intertrappean beds in the Rajmahal Basin. The Rajmahal intertrappean beds also animal fossils of Clupeiformes fish from the siliceous shales of an intertrappean bed exposed at Sakrighat.
