Sandstone Composition and Provenance of the Nari Formation, Central Kirthar Fold belt, Pakistan

Abstract The Oligocene-Early Miocene Nari Formation is widely distributed in the Kirthar thrust-fold belt. The formation in the study area is mainly consist of sandstone and shale. Field observations and detailed petrographic study reveal that these sandstones are mostly fine to medium grained, subangular to subrounded and poorly to moderately sorted. Detrital grains are dominantly quartz ranging in proportion 36-76%, feldspar 7-17% and lithic grains 1-13%, reflecting that these sandstones are compositionally submature. Quartz is mostly monocrystalline with some polycrystalline grains. Feldspar is dominantly plagioclase (albite) with some alkali feldspar (orthoclase and microcline). Lithic fragments are siltstone, mudstone and chert. Biotite and muscovite are present as accessory minerals. Heavy minerals such as apatite, tourmaline, and zircon are present in trace amount. The QFL diagrams show that the sandstones of the Nari formation are subarkose and lithic subarkose. The QtFL, QmFLt ternary diagrams and paleocurrent direction suggest that the sediments were transported from the Indian shield exposed to the northeast of the Nari Basin.


AGE GROUP FORMATION LITHOLOGY Holocene
Recent-Subrecent Mixture of clay, sand and gravel

Materials and Methods
Six stratigraphic sections of the Nari Formation were measured in the study area, and were described in detail, noting their sedimentary structures, texture and other features. 25 samples were collected from different horizons of Nari Formation for detailed petrography (Table 2). Petrographic analysis of 20 representative samples were carried out using the Leica petrographic microscope equipped with digital camera and automatic point counter. Minimum 300 points in each thin section were counted for the comparative proportion of mineral composition using Gazzi-Dickinson method [10][11][12]. Nomenclature of was used to determine the framework grains for siliciclastic sandstones (Table 3) [7]. For classification of sandstone ternary diagrams were used ( Figure  3) and for provenance and tectonic settings of sandstone of the Nari Formation, ternary diagrams and were used ( Figure 4) [10][11][12][13][14].

Texture
Field observations and petrography of the Nari Formation reveal that the formation in the study area is consist of dominantly sandstone and shale. Limestone beds are present in the southern part of the Basin [15]. The sandstone is fine to medium grained, in places coarse grained, thin to thick bedded, in places massive, moderately sorted, framework grains are mostly subangular to subrounded ( Figure 2A) with low to medium sphericity. Common sedimentary structures are parallel lamination, cross lamination, normal grading, flutes, load casts and grooves. It is mainly grain supported, with calcite cement 2-35% and ferruginous cement 1-9%.

Sandstone Composition
The sandstones of the Nari Formation are mainly composed of quartz, feldspar and lithic grains. Biotite and muscovite present as accessory minerals and heavy minerals such as apatite, epidote, tourmaline and zircon are present in trace amount ( Table 2).

Quartz
Quartz is found as the most common detrital mineral in sandstones of the Nari Formation, ranging from 36% to 76%. Quartz is mostly monocrystalline and shows uniform extinction ( Figure 2B). Some of the quartz grains show undulose extinction. Polycrystalline Quartz are also present in minor amount ( Figure  2B). The Quartz grains are commonly subangular to subrounded in shape.

Feldspar
Feldspar is the next abundant mineral in these sandstones ranging from 7 to 17%. It is mostly plagioclase with minor amount of alkali feldspar (orthoclase and microcline). Plagioclase feldspar is characterised by perfect albite twining ( Figure 2C), low relief and some show zoning whereas orthoclase and microcline feldspar show cross hatch and Carlsbad twining ( Figure 2C). The orthoclase grains are usually cloudy and dirty. Some grains with perthitic and myrmekitic intergrowth texture were also observed in some thin sections ( Figure 2D). The shape of feldspar grains are mostly subangular to subrounded, partially to completely altered to sericite and calcite.

Lithic grains
Lithic grains are less common, ranging from 1 to 13% with an average of 6% (Table 2). Among the lithic fragments mudstone is the most common with minor amount of sandstone, siltstone, mudstone, limestone, volcanic igneous rocks and bioclasts. Bioclasts of foraminifer are also present most probably derived from older formations.

Mica
Mica in the form of biotite and muscovite are present up to 4%. Biotite is more common than muscovite. They are present in the form of elongate and bended flakes. (Figure 2E, F). Some grains of muscovite are strained and altered to chlorite.

Cement
Calcite, quartz and hematite cement ( Figure 2) is present ranging from 5-14%, Calcite cement is more abundant, while one sample (ZA-33) contains 37% cement. Quartz overgrowth has also been observed in several thin section. Calcite replacing quartz and feldspar grains is common ( Figure 2G).

Heavy minerals
Heavy minerals such as Zircon ( Figure 2H), tourmaline, apatite and epidote are present as accessary minerals. Brown leached material may indicate the presence of iron oxide.

Sandstone Classification of Nari Formation
The values of recalculated detrital mineral composition of the studied thin sections (Table 3) were plotted into the ternary diagrams for classification of sandstones ( Figure 3) fall in the field of subarkose and few samples fall in the field of lithic subarkose, indicating that the sandstones of the Nari Formation are mostly subarkose and with minor lithic suarkose, reflecting that these sandstones are compositionally submature [13,14].

Provenance and Tectonic Settings
To interpret sandstone provinance and tectonic setting of the ancient sedimentary basins with the help of the quantitative detrital modes, calculated from point counts of thin section is well established [10,11,16]. Sandstone composition is greatly affected by parent rock type, tectonic setting, climate, topography of the source area, mechanism of transport, depositional environament and diagenesis [10][11][12][13][14][15][16][17]. Prolong weathering produces sandstone depleted with unstable minerals such as feldpar and mafic minerals and enriched with stable minerals such as quartz. Sandstone composition is also modified during transportation from source to depositional site by eliminating the unstable minerals. Mixing of sediment from multiple source and addition of sediment like bioclast, glauconite within basin followed by diagenesis further modify final composition of sandstone [17][18][19][20]. Major categories of provinance include Craton Interior, Transitional Continental, Basement Uplift, Magmatic Arc (Dissected Arc and Undissected Arc) and recycled orogen [10,11,16].
Using the QtFL plot, the detrital mineral composition of sandstone of the Nari formation fall in craton interior and recycaled orogen field ( Figure 4A) and the data plotted in the QmFLt ternary diagrams, fall in the field of craton interior and transitional continetal bolck provenance ( Figure 4B) indicating that the sandstones of the Nari formation were derived mostly from shield areas and some from uplifted basement and recycle orogens provenance [16]. The sheild areas, commonly composed of granitic and gneissic rocks with low topography, usually provide quartz rich and subarkose sandstones. In the LmLvLs plot, the samples fall in suture belt field (Fugure 4C) indicating that some sediments have also been derived from the uplited older sedimentary strata exposed in the kirthar fold belt [11]. In the QmPK plot it falls near to Qm end ( Figure 4D) indicating derivation of the detritus from continental block [11]. The abundance of monocrystalline quartz with uniform extinction indicates that these sandstones were derived from granitic source [17,18].
The presence of some of polycrystalline quartz consisting of more than 3 grains and undulose quartz grains suggest dervation from plutonic igneous and metamorphic rocks [14]. The presence of zircon inclusion in the quartz grains indicates derivation from plutonic felsic igneous rocks. The presence of albite plagioclase and alkali feldspar (orthoclase) suggest derivation of sediments from plutonic felsic igneous rock. The presence of microcline indicates that derivation of the sediments from felsic plutonic igneous and metamorphic rocks. The presence of mica (biotite and muscovite) indicates derivation from metamorphic anf igneous plutonic felsic rocks [21][22][23][24][25]. The presence of some lithic fragments such as siltstones, limestones chert indicate derivation of some sediment from older sedimentary rocks. The paleocurrent direction recorded from the flutes show that the flow was from southwest. This indicates that source area was to the northeast of the Nari Basin. It is envisaged that the Indian Craton to the NE have provided sediments for the Nari Formation with minor contribution from nearby exposed sedimentary rocks [26,27].

Conclusions
Following conclusions can be drawn from the petrographic analysis of sandstones of the Nari Formation: 1. The sandstones are texturally and compositionally submature and moderately sorted indicating high relief, rapid erosion in the source area and short transport distance from source area.
2. The sandstones are subarkose and lithic subarkose. These sandstones are rich in quartz (36% to 76%) with subordinate feldspar (7-17%) and minor amount of lithic fragment (1-13%). The absence of distinctive variation in the composition of these sandstones indicate homogenous source.
3. The detrital mineral components indicate that these sandstones mostly were derived from acidic igneous granitic source with partial contribution from metamorphic and sedimentary rocks.
4. The Ternary Tectonic diagrams and paleocurrent direction reveal that the Indian Craton present to the northeast of the study area is believed to be the major source area with minor contribution from locally exposed sedimentary rocks.