SEDIMENTOLOGY OF THE EARLY MIDDLE CAMBRIAN JUTANA FORMATION OF KHEWRA AREA, EASTERN SALT RANGE,DISTRICT CHAKWAL, PAKISTAN.
Abstract: The sedimentology of the early middle Cambrian, Jutana Formation was investigated from Khewra area, above PMDC Tourist Resort, Eastern Salt Range, District Chakwal, Pakistan to elaborate its microfacies and diagenetic settings. The Jutana Formation is composed of thick bedded to massive sandy dolomite and dolomitic sandstone with interbedded shales and sandstone. The dolomites are dirty white, light green, hard, micaceous, sandy and glauconitic towards the base. The sandstones are whitish grey, fine grained, well sorted, dolomitic, silty and glauconitic. The shales are greenish grey, glauconitic and highly micaceous. A detailed study was conducted after collecting systematically more than 30 rock samples in vertical thickness of aprox.31.45m and studying selecting more than 30 thin sections.
To investigate its sedimentology, microfacies package and diagenetic settings, the petrographic study of unstained and stained thin sections has been executed. Detailed field observations and laboratory investigations revealed that it contains microfacies mainly of five types; lower sandy dolomite facies, silty dolomitic sandstone facies, sandy dolomite facies, silty sandy dolomite facies and highly porous dolomite facies. Research work demonstrates presence of cement and diagenetic process. The Jutana Formation was deposited in the tidal environments.
Key words: Jutana Formation, Cross Bedding, Microfacies, Dolomite, Stylolites
Fleming  named this unit "Magnesian sandstone". The Stratigraphic Committee of Pakistan named the unit as "Jutana Formation". The type locality lies near Jutana Village in the eastern Salt Range [2 and 3]. Thickness at the type locality is about 75 meters which is the maximum recorded thickness. It thins towards west about 60 m near Khewra Gorge and about 45 meters in the Khisor Range, at Saidwali. Thinning towards south in Punjab Plain is observed, where the Jutana formation is very thin .
In the Eastern Salt Range, it is well developed and is exposed at Karangal, Diljabba, Chambal and the Jogi Tilla ridges as well as in Mount Chello area. It is also exposed in the eastern side of Nilawahan while in the south-eastern part of the Khisor Range (Saiyiduwali) . But it is absent in the Western Salt Range. It is present in subsurface in the Potwar area (Adhi, Kallar Kahar and Dhariala), but is absent in the Mahesian and Joya Mair wells .
Jutana Formation shows siliciclastics assemblage and represents the upper middle unit of Jhelum Group. It is very well exposed in the southern part of East Central and Eastern Salt Range and has its subcrop extension in the Potwar also. However, it was measured above the PMDC Tourist resort, Khewra Gorge (Lat. 32deg38'44.6"; Long. 73deg00'16.6"), where it is measured approximately 31.45 meter thick.
The dolomite of the formation form cliffs and it is very difficult to measure a stratigraphic section due to over steeping of the slopes. The lower and upper contact with Kussak and the overlying Baghanwala Formation is conformable. In areas where the Tobra Formation unconformably overlies the Jutana the contact is strictly marked with a change from sandy dolomite to conglomerate. Jutana Formation is mainly characterized by, trough-, herring-bone-, hummocky- cross bedding at upper bedding planes and bedding parallel stylolites. Above the shale interval, there is dolomite which becomes fine grained and then gradually changes to dolomitic sandstone. Dolomites are dirty white, light green, hard, micaceous, sandy and glauconitic towards the base. The sandstones are whitish grey, fine grained, well sorted, dolomitic, silty and glauconitic.
The shales are greenish grey, glauconitic and highly micaceous. Important features of the upper part include fenestral porosity (bird eye about 2-7 cm).
Depositional setting of sedimentary rocks used to assess and predict the fabric of the rocks at that locality Extrapolate their distribution along and perpendicular to the strike, or vertically within that geologic section. Observed sedimentary structures in the Jutana Formation is trough, herringbone, hummocky cross bedding and fenestral porosity .
Trough Cross Bedding
In the section boundaries are characterized as being either planar or parallel. Trough cross bedding sets have strongly curved concave-up lower set boundaries as seen in vertical sections transverse to flow (Fig. 2).
Herringbone Cross Bedding
Periodic flow of current is observed in opposite direction.
Opposite dipping of cross beds resembles with herring ribs. These bidirectional cosets are deposited by tidal currents, nearly equal in duration and strength (Fig. 3).
Hummocky Cross Bedding
Hummocky cross bedding is the medium-scale to large-scale cross bedding, generally produced by kind of oscillatory flow. Laminae sets that are both concave upward and convex upward, bounded by sharp bounding surfaces which themselves may be either concave or convex upward. Hummocky cross beds formed at water depth of 5 to 15m, where a strong storm wave produces (Fig. 4).
These are small cavity structures in micritic dolomite-birds eyes which is laminoid (lamina cracks parallel to bedding). It is developed where a gap in the rock framework larger than the normal grain-supported pore spaces are present. It is most commonly associated with tidal flat deposits and other peritidal sediments. This type of porosity is less frequently encountered (Fig. 5, Plate 5a, 5b, 5c).
Classification of Dolomite
Four different dolomite types are identified within the Jutana Formation .
1. Type-1 Dolomite
This type of dolomite is showing early replacive, dirty, non- ferroan, unimodal and nonplanar-anhedral crystalline. It has homogenous dully orange to orange luminescent pattern.
2. Type-2 Dolomite
It is mimetically and/or nonmimetically replaced packstone/ grainstone and its crystals are unimodal, nonplanar-anhedral. They show orange, bright yellow and red luminescence.
3. Type-3 Dolomite
Unimodal, non-ferroan and nonplanar-anhedral post- compaction dolomite crystals associated with quartz grains. It shows dull cores and orange luminescent rims.
4. Type-4 Dolomite
Early to intermediate replacive, non-ferroan polymodal and planar-subhedral crystalline. Crystals of dolomite show orange luminescent cloudy cores and dull or nonluminescent rims.
Microfacies Analysis of Jutana Formation
Jutana formation is divided into five microfacies on the basis of thin sections studies [8 and 9].which are;
1. Lower Sandy Dolomite Facies
2. Silty Dolomitic Sandstone Facies
3. Sandy Dolomite Facies
4. Silty Sandy dolomite Facies
5. Highly Porous Dolomite Facies Lower Sandy Dolomite Facies (MF-1) Field Interpretation
This facie is 4.7m thick .The formation mainly consists of medium to thick bedded dolomite in the lower part .There is cyclic deposition i.e. the alternating beds of sandy dolomite and impure micaceous rich silty sandstone.
Under the microscope, two types of grains are observed i.e. dolomite in the form of rhombs which constitute 80% to 90 % and Quartz which are nearly 20%. Dolomite shows light green color in X-Nicole and gives greenish tint in PPL. Glauconite, muscovite and biotite are also present. Hematite is present as cement (Plate 1a, 1b).
Silty dolomitic Sandstone Facies (MF-2) Field Interpretation
Facie is 4.33m thick, shows characteristic tidal bedding. This facies is mainly yellowish brown which shows cyclic deposition of dolomitic sandstone and shale. The thickness of individual cycle is increased and dolomitic sandstone thickness increases upward. Weavy bedding is present in this facies.
Dominant grains in this facies are Quartz. Very fine grained dolomite is present. Sorting is very poor. Quartz constitutes approx.10%. Other minerals like dolomite, muscovite and biotite are present. Hematite is present as cement (Plate 2a, 2b, 2c, 2d). Sandy dolomite Facies (MF-3) Field Interpretation Facie is 0.90m (90 cm) thick. This is very hard and contains intraclasts, and is light yellow in color.
Under microscope, it shows two types of Dolomite. One is in the form of subhedral to euhedral crystals and second is in the form of intraclasts of dolomite. The intraclasts contain very fine grained dolomite. Quartz is also present in intraclasts as well as in the form of grains and it constitutes nearly 25% of the facies (Plate 3a, 3b).
Silty Sandy Dolomite Facies (MF-4) Field Interpretation
This facie contains a 12.29m thick sequence which shows cyclic repetition and some beds were very hard and some were soft. Mainly color is changing from yellow to light green and somewhat dark green (weathered color). Microscopic Interpretation Under microscope it shows mixing of silt and sand contents as at different levels increasing vice versa. In the thin section some dolomite rhomb show zoning, filling showing dedolomitization in upper part. Dolomite and quartz are in various proportions. Other minerals are muscovite and biotite. Cement is mainly of hematite (Plate 4a, 4b, 4c, 4d,4e, f4, 4g).
Highly Porous Dolomite Facies (MF-5) Field Interpretation
This facie is composed of massive bedded yellow dolomite. It is 9.23m thick. The lower upper part is thickly massive bedded and the middle upper part 2-4m thick zone is highly porous having fenestral pores. The macrostylolites are present throughout this facies. Upper part is brecciated and cataclasite is present. Absence of glauconite, bioturbation and sand grains but the presence of herringbone cross stratification in dolomite suggests intratidal to supratidal environment of deposition.
Dolomite is the most commonly occurring mineral in this facies and their percentage ranges from 80 to 90. Other minerals are quartz, muscovite, biotite and hematite. Thin section shows stylolites in this facies. At the lower part dolomite crystals are very fine grained while in the middle part crystals size increases and planar subhedral crystals with xenotopic texture are present. At the upper part, very fine grained dolomite crystals are dominant. This part shows laminations due to difference in grain size and color. The percentage of hematite cement increases upwards (Plate 5- a, b, c).
Origin of Dolomite in Jutana Formation
Dolomites are mostly medium to fine grained and best preserve the original features, form in semi-arid regions on high intratidal to supratidal flats . On the basis of very fine to medium grain size of dolomite crystals and syndepositional sedimentary structures like herringbone-, trough-, and the hummocky cross beds, dolomite of the Jutana Formation is considered to be primary in origin  Stylolites Stylolites formed by pressure solution, the study section also depict presence of stylolites in between different grains of dolomite and clearly identified by the minor traces of clay along the contact between dolomite grains (Plate 2a, 3a, 5b), .
On the basis of very fine grained size of dolomite crystals and syndepositional sedimentary structures like herringbone cross bedding, trough cross bedding, hummocky cross stratification, we consider dolomite in the Jutana Formation to be primary in origin.
The lithology of the formation in lower part suggests that the area comes under both clastic and non-clastic influx cyclically. The cyclicity in deposition is because of many reasons: the presence of alternating beds of sandy dolomite, dolomitic sandstone, pure dolomite, mica rich silty sandstone shows a transition from sub-tidal to intra-tidal environment.
In the middle part of the formation, percentage of glauconite decreases and bioturbation is not observed, the thickness of individual cycle is increased and dolomitic bed up to 2-3 meters thick in which trough and herring bone cross bedding is clearly observed. Herring bone cross bedding shows bidirectional flow of water which is typical feature of tidal flat environment.
The upper part of formation is massive bedded yellow dolomite. The lower upper part is thickly massive bedded and the middle upper part 2-3 m thick zone is highly porous having elongated fenestral pores. Absence of glauconite, bioturbation and sand grains but the presence of herringbone cross bedded dolomite in the upper part of formation suggests intratidal to supratidal depositional environment. REFERENCES
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|Author:||Ahmad, Nazir; Ahsan, Naveed; Sameeni, Shahid Jameel; Mirag, Muhammad Armaghan Faisal; Khan, Babar|
|Date:||Sep 30, 2013|
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