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IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring", Hyderabad, India, 2002
and (iii) Broad Structural / Intermontane Valley and (iv)
Narrow Structural / Intermontane Valley. Under the second
category, the following geomorphic units / landforms have been
mapped — (i) Alluvial Fan, (ii) Upper Piedmont Alluvial Plain,
(iii) Lower Piedmont Alluvial Plain, (iv) Alluvial Plain, (v)
Flood Plain, (vi) Older Flood Plain of Sarda River, (vii) Infilled
Valley, (viii) Palaeochannel, (ix) abandoned channel, (x)
meander scar, (xi) ox-bow lake, (xii) Point Bar and (ix)
Channel / Braid Bar.
The denudo-structural mountains of Pre-Tertiary rocks are
characterized by higher elevation, moderate to deep dissection
and fine to medium drainage texture, sharp crested ridges with
narrow intermontane valleys. The denudo-structural hills of
Siwaliks and Pre-Siwalik Tertiaries are relatively low and
exhibit less dissection, fine drainage texture and rounded to
sharp crests. The bedding traces and flat-irons are clearly seen.
The broad valleys having their longitudinal extent parallel to
the general strike of the formations and occurring within the
Siwaliks / Pre-Siwalik Tertiaries have been mapped as broad
structural / intermontane valleys. The narrow valleys occurring
within the mountains / hills have been mapped as narrow
structural / intermontane valleys. Though, there are number of
such valleys, but only the mappable ones have been depicted in
the geomorphological map.
Few discrete alluvial fans having typical conical to sub-conical
shape are developed at the foot of Siwalik hill ranges, where the
major streams emerge on to the plains from hills depositing
their sediment load due to sudden change in their gradient. A
mega-alluvial fan is developed north of Lalkuan, where the
Gola River emerges from the Siwalik hills. There are few more
discrete alluvial fans developed on either side of this Kosi Fan.
These alluvial fans display characteristic distributary type of
drainage pattern, while the main stream shows braided pattern.
These fans consist of highly porous and permeable coarse
clastics or fanglomerates. The highly porous and permeable
nature of the material is clearly evident on the satellite imagery
as the discharge of rivers/streams reduces considerably as they
flow across the alluvial fans.
The coalescence of several alluvial fans has given rise to
formation of piedmont alluvial plain in front of the Siwalik
hills. Considering the objective of the study, this piedmont
alluvial plain has been divided into two parts — (i) Piedmont
alluvial plain — upper and (ii) Piedmont alluvial plain — lower.
The upper part is dominated by coarse clastics and the lower
part by the relatively finer facies. The upper part has a good
recharge potential with reference to deeper aquifers, whereas
lower part acts as recharge zone for shallow aquifers only and
also as discharge zone. Many streams emerge within this unit
due to favorable disposition of aquifers. Artesian aquifers are
also common within this unit. Though, the piedmont alluvial
plain extends much south in the subsurface as far as
Shahjahanpur (CGWB, 1998), but the lower part of this unit has
been significantly modified surficially by the present rivers and
streams. Therefore, on the geomorphological map, ‘we have
depicted the geomorphic units and landforms visible on the
surface based on the interpretation of satellite imagery.
7. SUBSURFACE GEOLOGY
Extensive geological, hydrogeological and geophysical surveys,
and exploratory drilling carried out by various organizations,
such as ONGC, OIL, GSI, GGWB, etc have provided excellent
database for understanding the subsurface geology and
tectonics of the study area. Tectonically, the GB lying south of
531
the HFF has been divided into a number of depressions
separated by subsurface ridges/spurs (Raiverman et al., 1983).
In the study area, the western part forms part of the Ramganga
Depression and the eastern part forms part of the Sarda
Depression. These two depressions are separated by the
Kasganj-Tanakpur spur, the eastern end of which coincides
with the subsurface extension of the Great Boundary Fault of
Rajasthan separating Aravallis from the Vindhyans. Within
these two major depressions, there.are minor digitations/ridges.
The courses of few of the streams/rivers emerging from the
Siwalik hill ranges are controlled by these subsurface
structures.
The lithostratigraphic contacts and the underlying structures in
the GB have been brought out by the seismic surveys carried
out by the ONGC and OIL, deep boreholes of ONGC. The
conclusions drawn from the interpretations of these data sets
are dealt in detail in the report of CGWB (1998), and the
highlights are given here. The seismogeological sections and
borehole data reveal that in general Siwalik rocks dip towards
NE, however, reversal of dip has been inferred in the Alluvium
and Upper Siwaliks near the foothills. The two-way isochron
map of OIL showing depth to the bottom of Upper Siwaliks
depicts sag in the Upper Siwaliks near Sitarganj, Lalkuan and
Khatima. Similarly, the isochron map showing depth to the
bottom of Middle Siwaliks also shows sag in the Middle
Siwaliks near Sitarganj and Khatima. These sags have also been
inferred from the seismogeological sections and deep resistivity
soundings, which could be the important tectonic features for
hosting the potential aquifers under high hydrostatic pressure.
The deep resistivity surveys indicate the presence of highly
resistive layer, mainly consisting of boulder and gravel, north
of Lalkuan, which does not seem to continue towards south.
The seismic surveys have indicated two thick channels near
Bilaspur and Bazpur, which are about 700 m and 1400 m deep
respectively. The deep borehole data of ONGC at Puranpur,
Shahjahanpur and Tilhar indicates that the thickness of
alluvium is maximum at Shahjahanpur (590 m) followed by at
Tilhar (520 m) followed by at Puranpur (400 m). In addition,
the interface between the fresh/brackish ground water occurs in
the Middle Siwaliks, which is deepest at Puranpur (2668 m),
followed by at Shahjahanpur (1249 m) followed by at Tilhar
(1178 m) (CGWB, 1998).
8. ARTESIAN CONDITIONS IN AQUIFERS
The artesian (auto-flow) conditions exist in the shallow aquifers
(up to about 400 m) in the lower part of the piedmont alluvial
plain, also known as Terai belt. The development of artesian
conditions is essentially due to extremely favourable
hydrogeological set-up in the form of inter-layered sand and
clay sequences, lateral variation in facies, hydraulic head, and
good recharge potential. The favourable geological set-up can
be attributed to the (i) fluvial-cum-debris flow type of
sedimentary environment typically existing in front of the rising
mountain ranges, and (ii) climatic variations. Considering the
similar kind of environ existing during the time of deposition of
Siwalik sediments, it is highly likely that the Siwalik rocks
present below the Ganga Alluvium also have favourable
geological set-up to host excellent aquifers under artesian
conditions, provided adequate and perennial recharge is
available. The tectonic sag in the Upper and Middle Siwalik, as
mentioned in the previous section, could play a significant role
in generating the artesian conditions in the deep aquifers. The
deep borehole data of ONGC at Puranpur, Shahjahanpur and
Tilhar in the study area also suggest that the Formation Pore
Water Pressure in Siwaliks above the fresh/brackish water
