Effect of tectonics and example of Eastern Doon Valley, N. India

The crossings of older and younger fanglomerates or river terrace deposits along the margin of a subsidence area are a sign of tectonic activity. The example of the alluvial aquifer in S Iran discussed in the chapter on Processing of Images, shows uplifted fans on the western margin, whereby the degree of dissection represents the relative age and the degree of uplift. As in the above example of the eastern Zanjan aquifer, important river recharge was also illustrated. Uplifted and dissected older fans are also shown in Figure 7.19 above.

It should be remembered that the older deposits along the margins of a subsidence basin have their continua-tion below the younger ones, whereby knowledge of the facies changes occurring on alluvial fan deposits allows some postulation of what can be expected at depth. Tectonics affecting Quaternary deposits may leave evidences on the surface, not only in the form of lineaments corresponding to faults or flexures, but also in the form of dis-sected deposits, widening and narrowing of floodplains, as the next example demonstrates.

Eastern Doon Valley

Pleistocene and Holocene tectonism affected groundwater in the intramontane Doon Valley in north India (Figure 7.23). In the northeast is a no-flow boundary formed by the Main Boundary Fault, a huge high angle thrust fault and Himalayan rocks bordering the fault are impermeable (slates and phyllite).

In the south the northern flank of the Tertiary Siwalik Formation is seen, with the Upper Siwalik boulder and clay beds dipping NE covered by a small alluvial fan zone. In the NW the Siwalik rocks are folded in an anticline, which is bordered by an inferred fault. Strong outflow of groundwater occurs in the wide braided Song River bed just north of the fault. The Song River further upstream of the fault is dry in the dry season because base flow from the Himalayan catchment is lost by diversion and transmission loss. South of the fault in the NW the emerged groundwater also infiltrates fully in the riverbed during the dry season over a length of a few km and the same is true for the river from the NW. However, at location S2 groundwater emerges again during the dry season and gains much groundwater discharge in a downstream direction (Meijerink, 1974). The image shown was from

4 November 2004, just a month after cessation of the monsoon rainfall. Therefore the watercourse is still con-tiguous.

The fault in the centre of the image shown was first noted by the shift in the anticlinal axis of the Siwalik rocks in the south during air photo interpretation and may continue to the north in the alluvial deposits of the northern rivers. East of the fault the fanglomerates have been tilted and have been dissected by drainage originating on the fans and deeply (several tens of m) incised by the rivers with catchments in the Himalayas.

West of the fault subsidence took place, attracting sedimentation by coarse-grained (sands to boulders) deposits due to high flood discharges in the monsoon and the relatively steep gradients of the rivers.

There are several groundwater flow systems (ignoring the one upstream of the fault in the NW):

1. A system with the infiltration area along the northern part of the subsidence area, fed by important transmis-sion loss and local runoff and rainfall recharge, which infiltrates nearly fully in the younger deposits. Already during November, all base flow from the mountain catchment is infiltrated in the wide riverbed. The exfiltra-tion area coincides with the roughly E-W course of the Song River.

2. An adjoining flow system in the uplifted, tilted and dissected fans. Recharge is less than in the adjoining flow system because soil development took place, reducing infiltration. For that reason local (ephemeral) rivers

Uplifted & tilted

Aster VNIR B3, November 2004, image of the eastern Doon Valley, north India, showing effect of neotectonism on sedimentation, recharge and groundwater outflow (see text). Sub-recent wrench fault in the centre was first detected by shift in anticlinal axis (AA) in the Upper Siwalik Formation and could be traced further north.

The fault borders an area of tilting and uplift in the east and a subsidence area with active sedimentation and transmission losses in the west.

Inferred faults are shown in white. The fault in the north crossing the Song River causes strong (>4 m³s^-1) outflow of groundwater at S1. s = approximate place where groundwater appears during the dry season.

U=upthrow, D=downthrow. MBF = Main Boundary Fault Figure 7-23

could develop and dissection take place. Exfiltration takes place in the riverbeds approximately along the line indicated by (s) and a little further downstream at the junction with the Ganges terrace deposits, where the water infiltrates again to emerge in the Ganges River.

3. A flow system in the local Siwalik fans, fed by transmission loss and rainfall recharge.

7.5 Summary and conclusions

The main types of unconsolidated deposits are briefly introduced as an aid to linking geomorphological features that can be observed on images to hydrogeological properties. Terrain consisting of unconsolidated deposits often has important groundwater occurrences and recharge is influenced by the nature of the surface materials, while in dry climates salinity due to shallow groundwater is associated with type of deposit (e.g. fluvial backswamp clay) or micro-relief (natural levees, dunes, beach ridges). It is difficult to map the Quaternary geology at a regional scale, as a basis for hydrogeological maps, by fieldwork only, but by image interpretation this mapping can be done in an efficient manner, as is illustrated by a few examples.

The soils of Quaternary deposits often support agriculture, but waterlogging and salinization can be a prob-lem in some areas. Images are eminently suitable to take stock of the situation and to study the seasonal variations, as the example from Argentina illustrates.

Although image observations are restricted to surface features, some subsurface conditions may be inferred by considering geomorphological development. This is particularly so in the case of an alluvial fan, which is char-acterized by a groundwater flow system, due to an elevated intake area in the permeable upper part of the fan with phreatic conditions. Lateral groundwater flow is dominant in the middle part of the fan, while a semi-confined condition is common in the lower part because of presence of clay and silt layers. Upward groundwater flow in the lower fan causes appearance of water in riverbeds, seepage zones or even marshy areas, and such phenomena can be observed on images, as given by a few examples. Much of the recharge occurs in the form of transmission losses in the ephemeral riverbeds. The origin of alluvial fans is often linked to tectonic activities, which can also influence depositional patterns and associated surface permeability.

Aquifers in alluvial fans are often used for irrigation in dry climates, but groundwater levels are declining from over-use. Chapter 13 discusses the role of alluvial fans for artificial recharge schemes, which may alleviate ground-water decline.

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