Geology of Aligarh, Uttar Pradesh, India.

Aligarh is one of historical city in western Uttar Pradesh, India. It forms a part of the Central Ganga Basin and is underlain by the alluvium comprising clay, silt, sand and gravels of Quaternary age.The Indo-Gangetic Plain is the extensive alluvial plain of the Ganga, Indus and Brahmputra rivers and their tributaries and separates the Himalayan ranges from Peninsular India. The Ganga Plain occupies the central position in the Indo-Gangetic Plain. Geographically, the Ganga Plain extends from Aravalli-Delhi ridge in the west to the Rajmahal hills in the east; Himalayan foothills (Siwalik Hills) in the north to the Bundelkhand-Vindhayan plateau-Hazaribag plateau in the south (fig 1.). The Ganga basin of northern India contains several kilometers of alluvial strata and constitutes one of the world’s most extensive alluvial plain. The Ganga plains are of great significance as they constitute an important link now and during the earlier Quaternary between the Himalayan Orogen and the Indian Ocean. Indo-Gangetic plain represents a deep (>8 km thick sediments).The length of Ganga Plain is about 1000 km and the width is ranging between 450-200 km; being wider in the western part and narrower n the eastern part (fig.1). It occupies an area about 250,000 km² (Singh, 1996).

There are various shades of opinion regarding to origin of Ganga Basin. According to Dickinson (1974) between fold-thrust belts and the craton, over which the mountan belt is thrust. These are called as Fore-land basin. The Ganga Foreland Basin (Ganga Plain Foreland Basin) occupies the central part of the Indo-Gangetic Foreland System formed in response to the collision of Indian and Asian plates and downflexing of the Indian lithosphere due to loading in Himalayas. According to Singh (1989) the Gangetic plain is part of an active fore-land basin (peripheral type) developed on the thrusting Indian plate in response to the thrust fold belt loading in the Himalayas. The Ganga Plain Foreland Basin exhibits all the important components of a foreland basin, namely orogen (Himalaya), deformed foreland sediments (Siwalik), active foreland (Ganga Plain) and peripheral bulge (Bundelkhand Craton).

The Ganga Plain can be identified into four disdinctive regions (fig. 1.) (Pathak, 1982)

(1) Bhabar Belt- This is a 10-30 km wide belt of gravel-liferous sediment adjacent to Himalaya with steep slopes and ephemeral streams.

(2) Terai Belt- This is a 10-50 km wide low-lying area adjacent to Bhabr Belt with extensive development of swaps, ponds, small sandy rivers.

(3) Central Alluvial Plain- This is the Bhabar-Terai Belt and the axial river. The drainage is mostly aligned in a SE direction.

(4) Marginal Alluvial Plain- This is the north sloping surface, located south of the axial river, and charecterised by NE-flowing gravelly to coarse sandy rivers showing entrenched meandering.

Fig. 1. The Ganga plains in the Himalayan foreland basin. The plains are fed by the rivers originating in the Himalaya to its north as well as the Craton to its south. The Ganga river forms the axial drainage with a total length of 2510 km and catchment area 980,000 km2.

However, it is more practical to identify three broad areas in Ganga Plain.
   1-         Piedmont Zone (it includes both Bhabar and Terai Zone). 

                      2-         Central Alluvial Plain
                      3-         Marginal Alluvial Plain

2.1 Piedmont zone (it includes both Bhabar and Terai Zone) - It is located adjacent to the Siwalik Hills and slopes southward. It is essentially in contractional tectonic regime, showing features like thrusts dipping northward, conjugate system of strike-slip faults (NNE-SSW and NW-SE), linear-ridges, warping and deeply incised river channels.

2.2 Central Alluvial Plain- It is characterized by NW-SE, WNW-ESE and W-E trending lineaments, which have controlled the positioning of most of the river. Entrenchment of river channels within their river valleys is very prominent, often making cliffs along the river channels. In some segments the   lineaments along the river channels have acted as gravity faults, with northern side as downthrown block. This area shows strong distortion of river channel patterns (meanders) and tens of kilometers scale warping of the surface.

2.3 Marginal Alluvial Plain- It makes the southern most part of the Ganga Plains located south of the axial river upto craton margin. It shows rivers flowing in SW-NE direction, which become almost W-E in its northern part. This region shows the development of gravity faults and graben-like structures causing highly variable sediment thickness over a short distance. Main tectonic trend in this area is SW-SE. This region is characterized by extensional tectonics. Vertical uplift of the region is also prominent. An important event of active tectonics has produced tens of kilometer-scale undulations in the Ganga Plain. This tectonic event shaped the fluvial landscape, and many present-day geomorphic features are related to this event.

The study of Siwalik succession shows that inception of this foreland basin started most probably in middle Miocene, which are the oldest known sediments in the Siwalik succession. The northern part of the foreland sediments has been uplifted in several pulses as thrust sheets to make Siwalik ranges. The last major thrusting and uplift in the northern foreland produced Siwalik ranges of Upper Siwalik sediments which attain height upto 1000. Initially the foreland basin was rather narrow, but it increased in its width with time. The foreland basin gradually expanded over the southern craton. The last major southward expansion of the basin is probably related to uplift and thrusting of Upper Siwalik in middle Pleistocene. The southward expansion of the foreland-basin is about 100 km since middle Pleistocene. The southward (cratonward) migration of the basin caused onlap of Himalayan-derived sediments (micaceous litharenite) over the craton-derived sediments (subarkose), which make a thin veneer over the basement rocks.

The Ganga Plain is an active area of sediments from the Himalaya, and some from the Peninsular craton. The Indian lithosphere beneath the Ganga Plain shows much inhomogenety in the form of basement highs and lows.The Himalayan-derived gravel beds are present only in the Bhabar and Terai belt. The gravel horizontal of Central Alluvial Plain are reworked kankar and carbonate-cemented sand. The gravel horizons of the southern part of the Ganga Plain are derived from the Peninsular craton. Kankar development is common in the southern pat of the Central Alluvial Plain and in Marginal Alluvial Plain. The top few meters of succession throughout the Ganga Plain show a distinctive fining upward sequence, mostly terminating in mud rich sediments.

2.4 Sub-surface Geology of the area

In the light of the above discussion and the lithological data of deep wells drilled by O.N.G.C. at Kasganj and Ujjhani and by C.GW.B. at Salempur and Aligarh, the sub-surface geological sequence of the study area are probably as follows-

Table 1. Geological sequence of Aligarh.