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The Notion Of Nari Formation

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In Karachi and surrounding areas only the middle and Upper Tertiary Rocks are exposed. The lowest of these exposures are of Nari formation of Oligocene age. Above Nari formation lies the Gaj formation of lower Miocene age. This is followed by Manchar formation of upper Miocene to Pliocene and partly of Pleistocene age. Above this a cover of conglomerate if found which is considered to be of Pleistocene times. Quaternary deposits are represented by extensive conglomerate which over lies unconformable over Manchar formation and at place over laps and Gaj formation. Alluvial deposits windblown material and shore line deposits of recent origin also occur frequently in this region (Shah, 2009). Geologically the study area comprises of Aluvium, Manchar formation and Gaj formation including Mundro, Mol and Gulistan e Jauhar members of Recnet, Plistocene/Pliocene and Miocene age respectively which overlies the Nari formation of Oligocene.

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Nari Formation consists of sandstone, shale and subordinate limestone. The lower part of the formation, the Nari Member is predominantly composed of crystalline limestone which is white to grey, brown or yellow shelly, nodular or rough surfaced, thin to thick bedded and even massive. Thin stringers of dark shale and of thin bedded and fine-grained brown sandstone are also present. The upper part of the Nari Formation consists predominantly of sandstone, which is greenish grey, grey brown and white colored, fine to coarse grained often gritty and calcareous with subordinate shale, sandstone and varicolored claystone. Upper contact of Nari Formation is conformable with Gaj Formation (Iqbal et al., 1980). According to Iqbal et al., (1980), typical fauna found in this formation include Forams, Corals, and Mollusks, Echinoids, Algae and other fossil groups. Foraminifera of wide variety (Alveolinia, Lepidocyclina, Nummulites, Discocyclina) are present in Nari formation.

LITHOSTRATIGRAPHIC DIVISIONS OF NARI FORMATION

Litho stratigraphically Nari formation is further divided into:

  • TOBO MEMBER The Tobo member is composed of sandstone, shale and breccia. The breccia is composed of up to boulder and even larger size angular fragments (Niamatullah et al., 2011).
  • PIR MANGHO LIMESTONE The Pir Mangho overlies the Tobo member which is also grayish green shale with subordinate siltstone. It also contains a few beds of highly fossiliferous limestone (Niamatullah et al., 2011).
  • HALKANI SANDSTONE Halkani is composed of shale interbedded with siltstone. Shale is of green and grayish green color and mainly constitutes lower part of the unit while ratio of siltstone increases upward (Niamatullah et al., 2011).
  • GHORA LAKI LIMESTONE Ghora Laki is composed of shale interbedded with a few fossiliferous beds. The shale is of greenish color, soft and friable. Fossiliferous beds are of yellowish color and full of foraminiferas (Lepidocyclina) (Niamatullah et al., 2011).
  • ORANGI SANDSTONE Orangi sandstone, the top most member of the Nari Formation. This is quartz arenite and is multi-colored. At places its weathered color give rusty appearance (Niamatullah et al., 2011).

GAJ FORMATION

Gaj Formation is composed of shales with subordinate sandstone and limestone. Shales are mostly gypsiferous. At some places, minor conglomerate beds containing pebbles, separately derived from the Nari Formation are also present. Its lower contact with Nari Formation is transitional and conformable (Iqbal et al., 1980). The paleontological collections from the Gaj Formation were assisting as Miocene. According to Shah et al., (1980), the formation includes abundant forams, Bivalves, Gastropods and Echinoids. These including Foraminifera (Lepidocyclina) and Echinoids (Clypeaster). Gaj/Nari contact is exposed in north of Karachi.

LITHOSTRATIGRAPHIC DIVISIONS OF GAJ FORMATION

Lithostratigraphically Gaj Formation is divided into:

  • Mol Member The lower member (Mol) compises of clay in the lower part and limestone in the upper part.
  • Metan Clay The Metan clay is composed of yellowish green colour soft clay. This member is overlain by Jhil limestone (Niamatullah et al., 2011).
  • Jhill Limestone It is mainly composed of white colour nodular limestone interbedded with subordinate light yellow color limestone. White limestone is mainly a precipitate deposit and it is hard compact and of coralline nature at places. Subordinate beds of allochemical limestone are also present. Jhill limestone is overlain by Talawa Limestone (Niamatullah et al., 2011).
  • Talawa Limestone Talawa Limestone which is limestone interbedded with shale. Limestone is of golden yellow to brown colour and is allochemical (Niamatullah et al., 2011).
  • Sherji Member This member is dominated by clay interlayered with limestone and sandstone. It includes
  • Drig Clay Clay is brown, yellow, grey, chocolate and red colored with soapy touch. Limestone is brown with black specks, hard nodular, thin to thick bedded, sandy with derived fossils fragments, sandstone is grey, greenish grey to brownish grey in color and is thick bedded, medium to coarse grained, soft and friable lateral lithological variation in clay, sandstone and limestone in common.

MANCHAR FORMATION

Blandford (1876) proposed the name Manchar Formation for rocks exposed near Manchar Lake. The Manchar Formation is widely distributed in the Kirthar province (HSC, 1960). A part of Manchar formation represented by sandstone is exposed in southeastern part of the study area extending from Goth Ibrahim Hyderi to GothRehri along the coast (topographic-sheet 35, p/l). The sandstone is thick bedded and massive grey, greenish- grey, soft, friable, coarse grained, micaceous, intercalated with sandy clay. No fossils could be found in Manchar formation. Sratigraphically the study area comprises of coastal geological deposits like Alluvium deposits, stream bed deposits and sand bar deposits. Stream Bed Deposits: Streambed deposits consist of unconsolidated gravel, sand, silt and clay. These are located all along the stream and rivers in the surveyed area.

01 April 2020

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