Abstract:
The Khewra Formation of Cambrian age, exposed in the Karuli area Central Salt
Range (Upper Indus Basin) was selected for the present petrographic study. Formations
exposed in the study area ranges from Precambrian to Eocene. Khewra Sandstone overlies
Salt Range Formation without any apparent disconformity and underlies Kussak Formation
disconformably. Measured section is 85 m thick and mainly consists of sandstone with some
interclations of shale. Thirty samples were collected for general study, out of these sixteen
were selected for detailed analysis. On basis of grain size, exposed section is divided into
three parts. The variation of the grain size in the studied samples shows a more or less regular
pattern from the base of Formation upwards, that is, there is a gradual increase in the grain
size. Lower part is mostly very fine with some fine units in its middle portion. Middle part is
generally fine and upper part is medium grain. Some silt size units also occur in the lower
portion. The grain contacts are long and concavo-convex. Samples have grain supported
fabric. Lower and middle part is moderately to moderately well-sorted while upper part is
poorly-sorted. Formation is texturally immature. Framework grains in the studied samples
essentially consist of variable amounts of quartz (69-87%), feldspar (0.9-2.1%), and rock
fragments (0.1-0.5%). Accessory minerals include muscovite, biotite. Brown tourmaline,
zircon and monazite are in trace amounts. The percentage volume of opaque minerals is high
in lower and middle parts relative to upper part in the studied section. The cement in the
studied samples consists of two types, i.e. calcite and silica. Khewra Sandstone is
mineralogically mature. The model composition suggested that on the basis of relative modal
proportions of three essential framework constituents (quartz, feldspar, rock fragments) and
matrix contents all the samples can be classified as quartz arenites except four samples from
the lower part of the Formation which fall in the class of quartz wackes. Diagenetic changes
can be observed as physical and chemical compaction, cementation, decementation,
replacement of quartz, and reddish color of the formation during petrographic study.
