IAPRS & SIS, Vol.34, Part 7, "Resource and Environmental Monitoring", Hyderabad, India, 2002
Jaintia
Hills
PA - eShillong
9 ^e fherrapunji
FF
BANGLADESH
Fig.!. Tectonic framework of Shillong plateau.
Prolonged erosion since Miocene period has resulted in the
engraving of the present geomorphology in the southern parts
of the plateau.
In the terrain east of Kynsiang river, the region of the present
study, most of Tertiary cover has been removed as sheet wash,
exposing the dip surface of the Mahadek sandstone. The
remnant uneroded Tertiary sediments stand out as buttes and
mesas depending upon the level of erosion they have
undergone. The buttes like Lum Kohkhlum, Lum Kuttraw, etc.
in this area confirm that the entire terrain was covered by the
complete sequence of sediments and that except for Mahadeks
almost all the overlying formations have been eroded away by
fluvial processes.
As the sediments were having a gentle southwesterly dip slope
(5-10°), a characteristic dendritic drainage developed on the
sediment surface with a general southwesterly flow, which was
subsequently superimposed over the basement. Besides,
reactivation of basement fractures during the upliftment of the
plateau played a role in the southwesterly course of major rivers
like Kynshiang, Wah Podhra, Um Sophew, etc., in the area.
South of Kynshiang river, the complete succession of
Cretaceous-Tertiary sediments are preserved. Here the
Mahadeks sandstones are exposed in the deeper portion of the
Kynshiang river. For all practical purposes the sediments
exposed south of Kynshiang river are in simple physical
continuity with the sediments in the north, excepting that in the
north all the sedimentary formations other than Mahadeks have
been eroded away.
The channel-filled sandstone facies is characterized by cross-
bedding, medium to coarse grain size, frequent variation in
grain size both vertically as well as laterally and poor sorting.
It incorporates abundant coaly matter as disseminations and
stringers along with pyrite. Geomorphologically, this facies
expresses itself as cuesta with a gentle southwesterly slope. It
occurs as linear patches of upto 10 km long flanked on either
side by floodplain sediments and supports a dense vegetation,
imparting a dark tone in aerial photographs (fig.1). In valley
section it has a vertical scarp up to the contact with
Precambrian basement rocks. The gentle cuesta slope of this
unit is crowded with randomly distributed and smoothly
rounded mounds of about 60 m diameter and 10-15 m height,
together imparting a custard-apple-skin-like appearance. This
534
geomorphological expression, a characteristic feature of this
unit, is the photointerpretation key which has been used to
discriminate channel-filled facies from other sedimentary units.
The floodplain sandstone facies is massive in nature,
characterized by fine to medium grain size, incorporation of up
to 30% pyrite nodules, presence of coaly matter as chunks and
well-developed joints. Geomorphologically, this unit also
expresses itself as cuesta but does not support good vegetation,
with the result a light grey tone and mottled texture is observed
in aerial photographs, besides flat, smooth and well-jointed
surface. Because of the presence of well-developed joints, a
subdentritic to subparallel drainage pattern has been developed
on these sediments. The intimate relation of pyrite with this
sandstone facies indicates that it was formed in a marshy
environment. Such a marshy floodplain favours the thrival of
sulphur-reducing bacteria.
The following three linear patches of channel-filled sediments
were identified between Kynshiang river in the west and Um
Rilang river in the east:
1. A north-south patch of 1-1.5 km in width exposed on
the eastern bank of Kynshiang river, north of its confluence
with Wah Blei river: This unit continues further south of the
confluence and except for a small patch of 1.5 x 0.5 km,
occurring as a river bench, it is covered by thick sequence of
Cretaceous-Tertiary sediments in the south. This channel-filled
sandstone is flanked on the west and east by Lum Kuttraw and
Nongmalong flood plain sediments, respectively.
2. A north-south patch exposed between Wah Podhra
and Um Sophew rivers and south of Phlangdiloin village: The
width of this unit is nearly 1 km in the north, which broadens to
about 4 km in the south. The Domiasiat uranium deposit is
situated on this patch only. This patch of channel-filled
sandstone is flanked on the west and east by sediments of
Nongmalong and Sindumdum floodplain, respectively.
3. A small patch of 1 x 1.5 km exposed north of
Phlangmawphraw and west of Um Rilang valley: Most part of
this sedimentary unit appears to have been eroded away, giving
way for the formation of Um Rilang valley.
Sandstone-type uranium deposits are epigenetic concentrations
of uranium in unmetamorphosed sandstones of fluvial,
lacustrine or marginal marine origin. The minimum
requirements for the formation of this type uranium deposits are
: (1) a provenance rock containing labile uranium, e.g. granite ;
(2) a transporting medium to carry the oxidized uranium, e.g.
ground water ; (3) a favourable host rock having the capacity to
reduce and retain uranium.
By virtue of its higher porosity and permeability incorporation
of disseminated reductants like carbonaceous matter and pyrite,
presence of permeability barriers such as siltstones and a score
of other factors, the channel-filled sandstone of Mahadek
formation is considered to be a good host for uranium
mineralization. This is evidenced by a number of uranium
occurrences like Domiasiat, Phlangdiloin, Tyrnai, etc. The aim
of the present study was. to reduce target areas for survey and
exploration, which has been successfully achieved by utilizing
effectively the geomorphology characteristics to discriminate
the channel-filled facies from the others. By the study of aerial
photographs and subsequent field checks, a photogeological
map has been prepared delineating the three aforementioned
channel-filled sedimentary units. Of these, Domiasiat has
already been proved to be having a low-grade
