Chapter 6 Sedimentology of the Jutana Formation

6.1 Introduction

6.2 Vertical profile

6.3 Facies and Facies Association

6.4 Facies Interpretation

6.5 Classification of dolomite Texture

6.6 Microfacies analysis of Jutana Formation

6.7 X- Diffraction analysis

6.7.1 Methods of sampling and analysis

6.8 Origin of Dolomite in Jutana Formation

6.9 Sedimentary structures

6.9.1 Beds and Bedding

6.9.2 Bedding Planes

6.9.3 Use of Sedimentary Structures

6.9.4 Cross bedding

6.9.4.1 Tangential cross stratification

6.9.4.2 Trough Cross Stratification

6.9.4.3 Herringbone cross stratification

6.9.4.4 Hummocky Cross Stratification- HCS

6.9.5 Current Ripples

6.9.6 Ball-and-pillow structure

6.9.7 Convolute Lamination

6.9.8 Stromatolites

6.10 Environment of deposition

6.11 Compaction, pressure dissolution and cavity structures

6.11.1 Stylolites

6.11.2 Fenestral Porosity

Sedimentology of Jutana Formation

6.1 INTRODUCTION

The middle Cambrian Jutana Formation is a thick mixed dolomite and

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. 32°38'45.4”;

Long. 73°00’18.4”), where it is 57 meter thick. Section was fully exposed with its

lower contact with Kussak Formation. 24 samples were collected from outcrop,

out of which 16 samples were selected for microfacies analysis. The detailed

investigation of lithology, bedform, sedimentary structures, texture and

microfacies shows varying depositional environment from subtidal to supratidal.

Field work has been done along the escarpment of Khewra Gorge where

excellent outcrop of Jutana Formation occur. The stratigraphic section of Jutana

Formation forming left escarpment face of Khewra Gorge above the PMDC

Tourist Resort was investigated and measured. Section was measured with the

help of Jacob’s Staff. Twenty four samples were collected to represent the

vertical and lateral facies of the Jutana Formation. Fifteen thin sections of

indurated samples were prepared and examined under a polarizing microscope.

Thin sections were stained with Alizarin Red S and K-ferricyanide for carbonate

mineral determination. Mineral types of representative oriented clay samples

were identified by an X-ray diffractometer with Ni-filter and Cu radiation (XRD,

type—Philips 1980).

Figure 6.1 showing location of Jutana Formation section above PMDC Tourist Resort, Khewra.

6.2 VERTICAL PROFILE

The lithofacies established through section measurement were presented in the

form of graphic log. The graphic log comprises of two individual columns. One

column is used to show lithology and is of constant width. The other column

shows sedimentary structures by various symbols and grain size by column

width. Both of the columns show the thickness of each lithologic unit according to

the scale 1 cm = 2 feet.

Vertical profiles provide an essential basis of facies model. Stratigraphic section

of Jutana Formation, measured from Khewra Gorge, comprises our data base

and used for all interpretations.

6.3 FACIES AND FACIES ASSOCIATION

“A facies is a body or packet of sedimentary rock with features that distinguish it

from other facies”. A facies is the product of deposition, and it may be

characteristic of a particular depositional environment, or a particular depositional

process.

Or

Facies are defined on the basis of distinctive combinations of lithology, rock

color, sedimentary structure, trace fossils and body fossils.

The facies identified on the basis of lithologic character are known as

‘Lithofacies’ while facies distinguish by paleontologic characteristic are known as

‘Biofacies’.

The word facies is used as in both a descriptive and interpretive sense.

Descriptive facies include Lithofacies and Biofacies. Both of which are terms

used to refer to certain observable attributes of sedimentary rock bodies that can

be interpreted in terms of depositional or biological processes. An individual

Lithofacies is a rock unit defined on the base of distinct lithological feature

including composition, Grain size, bedding characteristics and sedimentary

structures. Each Lithofacies represents an individual depositional event.

Lithofacies may be grouped in Lithofacies association or assemblages, which are

characteristic of particular depositional environment. These assemblages form

the basis for defining Lithofacies models, they are commonly cyclic. (Andrew

miall)

Facies analysis involves the description and classification of a sedimentary rock

unit by interpreting its depositional processes and environmental settings (Brown,

1943). Facies interpretation is based on the way in which the facies are

associated, and on the geometry and orientation of rock units.

Facies association is a collection of commonly associated sedimentary attributes.

Lithofacies group together into assemblages because they represent various

types of depositional events that frequently occur together in the same overall

environment (Miall, A. D., 2000).

6.4 Facies Interpretation

The key to facies interpretation is that the facies occurring in a confirmable

vertical sequence were formed in laterally adjacent environment. This was the

basis of Walther’s law, which states “Facies that occur in conformable vertical

succession of strata also occurred in laterally adjacent environments”. (Boggs, S.

1987)

Facies exhibiting specific environmental information were rare. A better approach

is to study the vertical sequence and lateral relationships, which may contribute

more information about the environment than the individual facies themselves.

Sedimentary sequences contain assemblages of facies with a restricted range of

interrelationships between them. These assemblages are called “Facies

Association”, were commonly cyclic and characteristics of particular depositional

environments.

6.5 Classification of dolomite Texture

An early attempt at classifying dolomites based on texture was made by

Friedman (1965). Friedman’s classification system was later modified by Sibley

and Gregg (1987), who classify dolomite texture on the basis of crystal boundary

relationship and crystal size distribution (Figure 6.2). Crystal boundary shape is

classified as either planar or nonplanar. Planar dolomite is characterized by

straight compromise boundaries with many crystal-face junctions. Nonplanar

dolomite is characterized by boundaries between crystals that tend to be curved,

lobate, serrated, or otherwise irregular, with few preserved crystal-face junctions.

Planar dolomite texture develops when crystals undergo faceted growth and is

characteristic of dolomite crystals formed during early diagenesis and, under

certain conditions, at an elevated temperature in the burial environment.

Nonplanar boundaries develop when crystals undergo nonfaceted growth. This

growth occurs most commonly at an elevated temperature (>50oC) in the burial

environment and /or less commonly at high supersaturation. Both planar and

nonplanar dolomite can form as cement, by replacement of calcium carbonate, or

by neomorphic recrystallization of precursor dolomite (Gregg and Sibley, 1984;

Sibley and Gregg, 1987).

Figure 6.2 Classification of dolomite textures (From Sibley and Gregg, 1987).

6.6 Microfacies analysis of Jutana Formation

Jutana formation is divided into five microfacies, these are;

1- Lower Sandy Dolomite Facies

2-Thick Bedded Dolomite Facies

3- Silty Dolomitic Sandstone Facies

4- Dark Green Shale Facies

5- Porous Dolomite Facies

These are described below with field and microscopic interpretation.

6.6.1 Lower Sandy Dolomite Facies (MF 1)

Field Interpretation:

This facies is 1.2 m 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.

Microscopic Interpretation:

Under the light of microscope, two types of grains are observed i.e. dolomite in

the form of rhombs which constitute 80% to 90 % and Quartz which constitutes

nearly 10%. Dolomite shows light green color in X-Nicole and gives greenish tint

in PPl. muscovite and biotite is also present. Hematite is present as cement.

Plate 6.1: (1a, 1b, 1c)

Table 7.3 (see Appendix):

6.6.2 Cyclic thick bedded dolomite facies (MF 2)

Field Interpretation:

This facies is 11.6 m thick. This facies is mainly yellowish brown and pure

dolomite which shows cyclic deposition. The thickness of individual cycle is

increased and dolomitic bed up to 3.6 meters thick in which trough and herring

bone cross bedding is clearly observed. It shows intertidal environment of

deposition

Microscopic Interpretation:

Dominant grains in this facies are dolomite which is present in the form of

anhedral to subhedral crystals. Sorting is moderate to well. Xenotopic to idiotopic

texture is present. Dolomite constitutes more than 90%.other grains which are

present in minor amount are quartz, muscovite, biotite and.

Plate 6.2: (2a, 2b, 2c, 2d)

Table 7.3 (see Appendix):

6.6.3 Silty dolomitic Sandstone Facies (MF 3)

Field Interpretation

This facies is 4.1 m 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. Ripple marks, wavy bedding, flaser bedding and bioturbation

are present in this facies.

Microscopic Interpretation:

Dominant grains in this facies are Quartz. Very fine grained dolomite is present.

Sorting is very poor. Quartz constitutes approx.50%.other grains present are

dolomite, muscovite, biotite and.

Plate 6.3: (3a, 3b, 3c, 3d)

Table 7.3 (see Appendix):

6.6.4 Sandy dolomite Facies (MF 4)

Field Interpretation:

This facies is 50 cm thick. This is very hard and contains intraclasts, and is light

yellow in color.

Microscopic Interpretation:

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

20% of the facies.

Plate 6.4: (4a, 4b, 4c, 4d)

Table 7.3 (see Appendix):

6.6.5 Dark green shale facies (MF 5)

Field Interpretation:

This facies contains a 3 meter thick horizon of light green splintery and

variegated shale.

XRD Analysis of shale:

XRD of shale shows presence of chlorite clay mineral in high amount. The

percentage of chlorite in the shale was more than 40% of the clay present. The

percentage of Illite is more than 20%. Both Illite and chlorite make 70-80% of the

clay in the shale. Other minerals presents are quartz, feldspar and traces of

Montmorillonite (Figure No. 6.2). Detailed analysis is described at the end of

microfacies part.

6.6.6 Highly porous Dolomite facies (MF 6)

Field Interpretation:

This facies is composed of massive bedded yellow dolomite. It is 28.8 meter

thick. The lower upper part is thickly massive bedded and the middle upper part

4-6m thick zone is highly porous having fenestral pores. Macrostylolites are

present throughout this facies. Upper part is brecciated and cataclasite is

present. Honeycomb and chopboard weathering are clearly seen in this facies.

Absence of glauconite, bioturbation and sand grains but the presence of

herringbone cross stratification in dolomite suggests interatidal to supratidal

environment of deposition,

Microscopic Interpretation:

Dolomite is the most commonly occurring mineral in this facies and their

percentage ranges from 80 to 90. Other minerals are quartz, mica (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 again very fine grained dolomite crystal are dominant. This part

shows laminations due to difference in grain size and color. Upward the

percentage of hematite cement increases

Plate 6.5: (5a, 5b, 5c, 5d)

Table 7.3 (see Appendix):

6.6.7 Flaggy bedded silty dolomitic Sandstone facies (MF 7)

Field Interpretation:

This facies is 7.8 meter thick. It consists of flaggy bedded dolomitic sandstone

which gradually changes into overlying baghanwala formation i.e. again the

presence of oscillatory ripple marks on the top of the Jutana formation suggest a

stagnant water condition showing rise in water level or transgression or shifting

from intra-tidal to sub-tidal environment.

Microscopic Interpretation:

In this facie the percentage of quartz grain is increased as this formation was

called Magnesian Sandstone earlier. Quartz may be up to 40% and dolomite

percentages nearly 30%. Other minerals present are micas (muscovite, biotite).

Ankerite is the dominant cement in the facies with hematite. It shows porosity in

under microscope which are filled with mounting material.

Plate6.6: (6a, 6b, 6c, 6d)

Table 7.3 (see Appendix):

6.6.7 X-ray Diffraction analysis of Shale Facies:

X-ray diffraction is a valuable tool in determining the mineralogy of sedimentary

rocks. By this semi-quantitative determination of mineral substance can be made,

(Tucker, 1988).

6.6.7.1 Methods of sampling and Analysis

Sample of shale present in the Jutana Formation was collected from the field and

one sample was selected for analytical work, comprising whole rock and clay

mineralogy by XRD.

For whole-rock mineral identification by the XRD technique, the sample was

dried at 60oC for 10-12 hours. About 20-30gram of dried sample was gently

crushed in an agate mortar. The material passing through 325mesh sieve was

analyzed using a Siemens 500D Diffractometer operated at 40 kV/30 mA using

Ni filter and CuKα radiation, and an on-line computer control. The sample was

scanned from 2o to 30o2 θ, at a running speed of 1o2 θ per minute.

For clay mineralogy, <2 micro meter fractions of samples in accordance to

Stoke’s Law were separated after dispersion in de-ionized water. Sodium

polyhaxametaphosphate was used as deflocculant agent. Oriented mounts were

prepared by smearing the clay suspension onto glass slides. All samples were

scanned from 2o to 30o2 θ, at a running speed of 1o2 θ per minute.

The XRD patterns of whole-rock samples obtained following Brown and Brindley

(1984), show that in general, the shale of Jutana Formation is rich in clay

minerals comprising chlorite, Illite along with appreciable amount of quartz and

some samples contained minor quantity of feldspar and Montmorillonite.

6.6.7.2 Chlorite:

Chlorite is a 2:1:1 mineral characterized by a series of basal reflections at 14.07

Ao (002), 7.15 Ao (004) and 3.5 Ao (008). The percentage of chlorite in the shale

was more than 40% of the clay present in shale. Both Illite and chlorite make 70-

80% of the clay in the shale. Most of the chlorite was produced by the weathering

of granite in which Biotite was changed into chlorite (Velde, 1992). Chlorites are

usually derived from weathering profiles, where in early Diagenesis smectite are

transformed into chlorite. During burial diagenesis chlorites are more stable and

regular. It is suggested that reducing pore waters cause the reduction of ferric

iron to ferrous iron involving a raise in pH. In these alkaline conditions chlorites

forms instead of kaolinite (Curtis, 1977, Kantorowicz, 1984). It may be related to

kaolinite and smectite as they provide Mg, and Fe (Weaver, 1989). So Chlorite is

formed from kaolinite by reaction with Fe+2, Fe+3 or Mg+2 present in the sediments

as temperature increases during burial causing kaolinite instability. The Fe is

believed to come from Siderite (Weaver and Bradley, 1984). Hower et al., (1976)

suggested that chlorite is most likely to form from iron and magnesium released

from smectite during diagenesis. Diagenetic chlorite is produced at temperatures

70oC and at the depth of 2450 meters depth (Hower’s et al. 1976).

Chlorite can be produced as a bi product during the illitization process. Low

temperature chlorites are Fe-rich and Fe/Mg ratio generally decreases with

increasing temperature of formation, i.e. chlorite at higher depth contain more Mg

than Fe. Mg rich chlorite starts to form at temperatures of 130oC to 165oC

(Steiner, 1967), Chlorite starts forming from montmorillonite at temperatures of

less than 100oC.

6.6.7.3 Illite:

Illite is a 2: 1 clay mineral characterized by a reflections at 10 A o. The percentage

of Illite in shale was more than 20%. Illite occurs as a detrital mineral, as an

authigenic phase and as an alteration product of Koalinite, micas and feldspars.

Illite is stable under alkaline conditions and is stable in presence to Koalinite with

increasing temperature. Authigenic illite is probably forms by the illitization of

Koalinite in a closed system at intermediate burial depth (3-4 km) at

temperatures of 130 o to 150 o C with K-feldspar as a source for the potassium

(Bjorkum and Gjelsvik, 1988). It may also form by the degradation of smectite at

temperatures around 100oC (Hower et al., 1976). It is suggested that illite /

smectite phases undergo progressive illitization during the burial of sediments

(Imam and Shaw, 1985). The illitization is probably commences at about 60 o C

and produces a significance reduction in the proportion of smectite layers at

burial temperatures of 100 to 110 o C. Shaw (1980) postulated that in the

presence of alkaline pore water the smectite are progressively transformed to

illite through dehydration, absorption of alkaline cations and lattice

rearrangements via illite smectite mixed layers (or the promotion of chlorite

formation via chlorite / smectite mixed layer phase depending on whether the

pore waters are potassium or magnesium rich).

Illitization is probably involves in three fundamental processes, dissolution, solute

mass transfer and precipitation or crystallization (Whitney, 1990). In a fluid

deficient smectite system, as the primary smectite dissolves and more stable Illite

begins to form, the rate and extent of illitization is affected by the abundance of

water in the system (reducing the water contents retards illitization). Weaver and

Beck, (1971) suggested that potassium feldspar is the main source of the

potassium for the illitization during Diagenesis. Meshri (1986) postulated that Illite

can be formed by the conversion of microcline at 25 o C and 1 bar pressure in H2

CO3 such that:

3KAlSiO3O8 + 2H2CO3 + 12 H2O → KAl3Si3O2 (OH) 2 + 2K+ + 6H2SiO2 + 2HCO3

(Microcline)+ (Carbonic Acid) = (Illite)

6.8 Origin of Dolomite in Jutana Formation:

Despite intensive research over more than 200 years, the origin of dolomite, the

mineral and the rock, remains subject to considerable controversy. This is partly

because some of the chemical and/or hydrological conditions of dolomite

formation are poorly understood, and because petrographic and geochemical

data commonly permit more than one genetic interpretation.

Dolomites that originally form very close to the surface and from evaporitic brines

tends to be recrystallize with time and during burial. Those dolomites that

originally form at several hundred to a few thousand meters depth commonly

show little or no evidence of recrystallization.

Penecontemporaneous dolomites form almost syndepositionally as a normal

consequence of the geochemical conditions prevailing in the environment of

deposition. There are many such settings, and most commonly they form only a

few per cent of microcrystalline dolomite(s). Many, if not most,

penecontemporaneous dolomites appear to have formed through the mediation

of microbes. (Colin et al., 2004)

Penecontemporaneous dolomites are mostly fine grained and best preserve the

original features. They form in semi-arid regions on high intratidal-supratidal flats,

preserving dessication cracks, evaporates and their pseudomorphs, microbial

lamination. On the basis of very fine grained size of dolomite crystals and

syndepositional sedimentary structures like herringbone cross bedding, trough

cross bedding, ripple marks, hummocky cross stratification, stromatolites which

form in intratidal to supratidal environment, dolomite in the Jutana Formation is

considered to be primary in origin.

Plate 6.1

1a 1b

1c

1a. Shows anhedral to subhedral dolomite crystals in cross-nicols, 40X unstained.

1b. Shows coarse euhedral in fine anhedral to subhedral crystals, hematite is present around the large crystals in plane polarized light, unstained, 40X.

1c. Shows alignment of platy minerals (Muscovite and biotite) in sandy dolomite in cross nicols, unstained, 40X.

Plate 6.2

2a 2b

2c 2d

2a. Shows larger crystal of dolomite in smaller subhedral crystals in cross-nicols, 40X unstained.

2b. Shows plane polarized view of larger dolomite crystal, 40X unstained.

2c. Planar subhedral to euhedral crystals forming xenotopic texture in cross-nicols, 40X unstained.

2d. Shows planar subhedral crystals of dolomite with some quartz grains in cross-nicols,

40X unstained.

Plate 6.3

3a 3b

3c 3d

3a. Very fine dolomite is seen in micaceous sandstone in cross-nicols, 40X unstained.

3b. Shows orientation of platy minerals in dolomitic sandstone, plane polarized light 40X

unstained.

3c. Alignment of platy minerals and dolomite crystals are seen in cross-nicols, 40X

unstained

3d. Shows very fine dolomite in sandstone, cross-nicols, 40X unstained.

Plate 6.4

4a 4b

4c

4a. Large rounded intraclasts of very fine sandy dolomite in subhedral crystals shows two

phases of dolomitization, cross-nicols, 40X unstained.

4b. Tooth like dolomite crystals are seen in between larger intraclasts, plane polarized

light 40X unstained.

4c. Close view of intraclasts of sandy dolomite, cross-nicol, 40X unstained.

Plate 6.5

5a 5b

5c 5d

5a. Large pores filled with hematite cement are present in fine dolomite crystals, cross-

nicols, 100X unstained.

5b. Shows larger subhedral dolomite crystals in fine dolomite, plane polarized light , 40X

stained.

5c. Shows laminations in very fine grained dolomite due to difference in grain size and

color, cross-nicols, 40X stained.

5d. Shows stylolite in planar subhedral dolomite crystals partially filled with hematite

cement, cross-nicols, 40X stained.

Plate 6.6

6a 6b

6c 6d

6a. Shows alignment of platy minerals (muscovite and biotite), cross-nicols, 100 X

Stained.

6b. Bedding parallel pores filled with hematite cement with orange color Ankerite

cement in dolomitic sandstone, plane polarized light 100X unstained.

6c. Bedding parallel pores and platy minerals in micaceous sandstone, plane polarized

light 100X unstained.

6d. Porosity is seen in the centre of thin section along a line filled with mounting

material.