Controls of basin margin tectonics on the morphology of alluvial fans in the western Ganga foreland basin's piedmont zone, India

Abstract

Six distinct alluvial fans have developed in the western Ganga basin's piedmont zone between the Ganga (aka Ganges) River in the west and the Sarda River in the east. The basin margin is demarked by the active Himalayan Frontal Thrust along which the Siwalik ranges are uplifted and placed over the alluvia of the Ganga basin. The 6 alluvial fans show marked morphological variances, but their morphometric parameters exhibit proportionality relationships with the “mountain‐front sinuosity index (Smf),” “stream‐gradient index (SL),” and “valley‐floor width to height ratio (Vf)” of the adjoining mountain‐front segments. Correlation and regression analyses reveal that these relationships are statistically significant. The size of the fan is directly proportional to the magnitude of Smf and inversely proportional to the magnitude of SL, whereas the fan gradient is inversely proportional to Smf. The higher the Smf and the lower the SL values, the larger and gentler is the alluvial fan. Given that the studied alluvial fans have developed in the same climatic zone and their drainage basins have similar physiographic, geologic, and climatic conditions, the morphological diversities among the alluvial fans are attributed to the tectonics of the basin margin. Moreover, statistical analyses further rule out any significant relationships of the fan size with area, or drainage density of the drainage basin. The lower values of Smf and Vf and higher values of SL indicate a more active nature and, thus, greater/faster uplift of the mountain‐front and expected deepening of the accommodation space in the adjoining basin, which favoured vertical aggradation and development of smaller and steeper alluvial fans. Conversely, the higher values of Smf and Vf and lower values of SL indicate a less active nature and, thus, relatively lower/slower uplift of the mountain‐front and thus creation of shallower accommodation space in the adjoining basin, favouring the development of larger and gentler alluvial fans.

Fossilized autogenic responses of grain‐size transition to sediment supply and water discharge: Alluvial fan experiments

“…Considering an example field case, a modern channel in the Malin Fan with the dimensions of 100 m (depth), 400 m (width) and 25 km (length), can be entirely filled within 100 years, and thus T Ac = 100 years using channel and fan dimensions reported in Goswami ( 2017) and an estimated Q s = 10 6 m 3 /yr. This Q s was approximated for Malin Fan, Northern India with an opening angle of 60°, length of 25 km, average slope of 0.01 and area of ca 330 km 2 to be built within ca 27 kyr, similar to the reported age of the Malin Fan (Goswami & Mishra, 2013;Goswami, 2017Goswami, , 2018Kralia & Thakur, 2021). In this field example, cyclic sedimentation at 10 2-3 year scales could originate from the autogenic processes without any variations in the external controls, for example, climate and tectonics.…”

“…Zigzag GST trajectories associated with cyclic sedimentation are often observed in natural alluvial fans, such as Malin Fan, Northern India (Powell et al ., 2012; Goswami, 2017, 2018; Kralia & Thakur, 2021), and Poplar Fan, North‐western China (Gao et al ., 2020). This episodic sedimentation is traditionally considered as a result of repeated cut‐and‐fill cycles, and/or explained by varying external forcing such as flash floods in ephemeral streams and tectonics (Dorsey et al ., 1997; Blair & McPherson, 2009; Thomas, 2011; Plink‐Björklund, 2015).…”

Climate as a Stratigraphic Tool for Basin Margin Deposition in Intracontinental Basins

“…The identified variations occur because of both localised autocyclic variations (i.e. lobe switching, vegetation), halokinetic controls (e.g., Venus et al, 2015) and by regional allocyclic control including climate (e.g., Waters et al, 2010;Ventra et al, 2017), tectonics (e.g., Harvey et al, 2005Goswami, 2018), and sediment supply. This paper presents a qualitative assessment of how climate can lead to vertical facies changes.…”