Geology of the area around Jaipur
The beautiful city of Jaipur is situated in the northern part of the Aravalli Mountain ranges (also known as the Aravalli-Delhi Orogenic Belt) which feature a horst-like structure, consisting of a series of Proterozoic rocks that are intensely deformed and metamorphosed. The Aravalli Mountains consist of the Aravalli and Delhi fold belts, and are collectively known as the Aravalli-Delhi orogenic belt. The Aravalli-Delhi Orogenic Belt trends in a northeast-southwest direction and has a length of 700 km. Width varies from about 150 km in the south to about 40 km in the north. Beginning in Palanpur near Ahmedabad in Gujarat, the Belt extends through Rajasthan, Haryana and Delhi. The northern end of the Aravalli Range is the famous Delhi Ridge which traverses through south Delhi and terminates in the Raisina Hill in Central Delhi. Three main subdivisions of rocks constitute the stratigraphy of the mountain range:
The lowermost unit comprising the Aravallis is the Bhilwara Gneissic Complex of Archaean age, over which lie the Aravalli Supergroup followed by the Delhi Supergroup. The northern part of the mountain range, which is exposed in and around Jaipur, only consists of the Delhi Supergroup, also known as the North Delhi Fold Belt. Rocks of the Delhi Supergroup overlie the Aravalli Supergroup with a distinct unconformity. On the southern side, however, both the Aravalli and Delhi supergroups are present. The mountain range is bounded by the Eastern and Western marginal faults, where the former is also termed as the Great Boundary Fault.
Whereas the precise evolutionary processes culminating in the uplift of the Aravalli Mountain Range remains poorly understood, it is generally agreed that primary mechanism for development of the mountain range is the collision between the Bundelkhand and Marwar cratons.
The site of the Aravalli Mountains bears evidence of repeated phases of crustal rifting, development of rift basins, generation of oceanic troughs, deposition of various sedimentary facies in different tectonic settings and emplacement of various kinds of acid and basic igneous rocks. During the Paleoproterozoic Era (~2500 Ma), the opening of Aravalli oceanic basin separated the eastern Bundelkhand craton and the western Marwar craton. Sedimentation of the Aravalli Supergroup took place in this basin simultaneously with subsidence of the Aravalli Basin and basic magmatism. The sedimentation was followed by an end of rifting and onset of a compressional phase during which the eastern Bundelkhand Craton subducted beneath the western Marwar Craton. Continued compression resulted in the development of an island arc between the two cratons. Further compression led to a steepening of the subduction zone and collision of the two cratons, leading to uplift of the Aravalli Supergroup at around 1800 Ma. In the last stage of convergence, the thrust fault further steepened and the colliding blocks eventually become sutured. The suture zone is marked by the Great Boundary Fault.
During the Mesoproterozoic Era (~1600 Ma), another rifting phase began. This rifting separated the Bundelkhand-Aravalli craton on the east from the Marwar craton on the west. This basin received the Delhi Supergroup sediments. The Delhi Supergroup of rocks thus occur on the western side of the Aravalli-Delhi Orogenic Belt.
The compressional phase that followed the Delhi Supergroup sedimentation led to eastward subduction of the western Marwar craton. Continuous subduction of the western block might have created another island arc, and similar to the Aravalli orogeny, further collision between the two blocks with island arc in between gave rise to the development of the Delhi orogeny around 1100 Ma. The suture zone between the two cratons is marked by the Western Marginal Fault and the emplacement of the Phulad Ophiolite Suite in the region.
Rocks exposed in and around Jaipur belong to the Delhi Supergroup.
The Delhi Supergroup:
Rock units comprising the Delhi Supergroup lie over those of the Aravalli Supergroup with a distinct unconformity. This Delhi Supergroup consists of two main types of rocks: a thick sequence of volcanic rocks that is of continental affinity; and sedimentary rocks that represent fluvial and shallow marine to deep marine depositional environments. The depositional age of these sequences is approximately from 1.7 to 1.5 Ga.
In and around Jaipur, the Delhi Supergroup is classified into three groups:
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)