LATE QUATERNARY MORPHOTECTONIC EVOLUTION OF THE SONE MEGAFAN, MIDDLE GANGA PLAIN, INDIA

Abstract

The middle Ganga plain is tectonically most active part of the Indo- Gangetic foreland basin. Though extensive study of neotectonics, geomorphology and sedimentology of the Ganga plain has been carried out in last decades, most of those are confined to the north of the Ganga River. However, the region south of the Ganga River has relatively gained lesser attention in the context of geomorphic and tectonic evolution. The southern middle Ganga plain is mainly drained by the Sone River and its distributaries, forming a megafan and ultimately meeting the axial river Ganga. Fluvial geomorphology of the megafan is mainly dominated by the Sone River by different phases of channel migration triggered by block tilting. Tectonically, the Sone megafan rests over two major subsurface basement faults, namely, West Patna Fault (WPF) and East Patna Fault (EPF). In the present work, morphotectonic evolution of the Sone megafan during the Late Quaternary period has been studied, and the tectono-geomorphic characteristics of the bounding faults have been worked out using an integrated approach such as remote sensing and GIS, Optically Stimulated Luminescence (OSL) dating, shallow subsurface stratigraphy and Ground Penetrating Radar (GPR). Reconnaissance of the study area was carried out by remote sensing and GIS techniques using Landsat ETM+ images to identify the different geomorphic domains of the megafan. Old alluvial plains, flood plains, active and paleochannels, and spatial drainage anomalies were marked on the image. Based on spatial drainage anomaly, such as convergent drainage, offsetting streams, generation of new streams, change in channel sinuosity etc. possible fault zones of the EPF and WPF were demarcated. Preferred orientation of the ponds revealed some recently abandoned channels. Comparative distribution and morphology of these abandoned ponds indicate that the western part of the megafan is relatively older than the eastern part. Most of the ponds in the eastern part show elongated morphology and their orientation reveals paleochannel pattern. Given the evidence of channel avulsion of Sone River, it is highly likely that these large ponds have resulted from the quick abandonment of some earlier course of either the Sone or its distributary in ii recent past. Whereas numerous ponds on the western part of the megafan surface indicate a similar hydrological condition but their shrunken size and almost circular shape associates them with relatively old channel-abandonment events. Different geomorphic units identified from remote sensing and GIS study were examined and verified in the fieldwork. The vertical sections exposed in pits were investigated for studying the degree of soil development and samples for optically stimulated luminescence (OSL) dating were collected. Different horizons and sub-horizons of the soil profiles were noted, and sedimentary logs of the sections were prepared. Soils in the western part of the megafan are welldeveloped with 0.5 to 3 m variation of solum thickness as compared to the eastern part where the solum thickness varies from 0.3 to 1.5 m, which is too supported by the micromorphological studies. The surface of the soil profile is blanketed with a silt and clay layer which is characteristic of flooding. OSL dating of different geomorphic units were carried out to decipher the chronological evolution of the megafan. Based on the OSL ages, the Sone megafan has been divided into Oldest Sone plain (OdSP, age varies from 22 ± 1.84 Ka to 14.25 ± 0.78 Ka) in the west, Old Sone plain (OSP, age varies from 8.0 ± 0.7 Ka to 7.31 ± 0.55 Ka) in the east and Young Sone plain (YSP, age varies from 0.89 ± 0.02 Ka to 0.57 ± 0.1 Ka) in the middle of the megafan surface. The distal part of the megafan shows recent overlap of sediments of the Ganga River (1.3 ± 0.16 and 2.4 ± 0.14 Ka). To decipher the geomorphic variation, spot heights and contours from the Survey of India toposheets on a scale of 1:50000 were used to create digital elevation model (DEM) of the megafan surface. A 3-D view of the DEM shows the regional topographic architecture of the Sone megafan. The profiles extracted from DEM across the fault zone indicated the fault trace of the EPF and WPF. However, surficial signatures of West Patna Fault are comparatively low due to erosion, extreme settlement and agricultural activities. Ground Penetrating Radar (GPR) study was carried out across the demarcated fault zone of WPF and EPF. Displacement in the GPR profile iii confirms the effect of these faults in the cover sediments in their respective fault zones. The WPF and EPF are normal faults which dip towards the west and east directions, respectively, forming a horst below the megafan. Soft sediment deformation structures identified as seismites in the present study area were reported for the first time from the south of Ganga River in the middle Ganga plain. Their proximity to the EPF indicates moderatesize earthquakes (or aftershock triggered by major earthquakes) on the fault in the past or mega earthquake in the nearby Himalaya. The integrated approach adopted for the present study shows that the Late Quaternary evolution of the Sone megafan occurred largely under the tectonic controls exhibited by probable uplift along the peripheral bulge region. As the megafan rests on the tectonic block bounded by the EPF and WPF, compression from SW and the Himalaya promotes block tilting which in turn influences the fluvial geomorphology of the megafan in space and time. Similar block tilting and megafan growth has been reported in the northern part of the present area i.e. in the Gandak megafan. By analyzing the present result, it is quite evident that similar geological processes are too acting in the present area of study. The Sone River initially flowing in the western part of the megafan responded to first event of tectonic uplift along northwestern part of the tectonic block formed between the WPF and EPF by eastward shifting of the channel. This is supported by the OSL ages from the western part of the megafan. The river continued to shift eastward until the second event of tectonic tilt uplifted the southwestern part of the block causing the river to reverse the direction of shifting. The river was shifting westward until it attained a rather stable course which is the present course of the river. The river may have attained this stability in the past also, allowing only the downstream part of the river to undergo avulsions in east-west directions as many paleochannels appear in the downstream as compared to the upstream. Based on the inferences drawn from combined study of geomorphic, and pedological features with OSL dating, the Sone megafan was identified with 4 geomorphic units, namely, OdSP lying to the west of the Sone River, OSP covering the eastern part of the plain, YSP spread in triangular shape near downstream part of the river, and AFP belt of iv fresh sediments along present channels of Sone and Ganga rivers. The block tilting and channel migration can be well correlated with the influence of the Himalayan tectonics on the Ganga plain and especially on the fault bounded tectonic blocks. The tectonic blocks tilt and twist due to compression between the peninsula and the Himalaya. Hence the rivers of the alluvial plain shift their course in response to the tectonic tilting which in turn modifies the geomorphology, sedimentology and pedology of the region.