Petrography and provenance of the Early Permian Fluvial Warchha Sandstone, Salt Range, Pakistan

Sedimentary Geology 233 (2011) 88–110

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Petrography and provenance of the Early Permian Fluvial Warchha Sandstone, Salt Range, Pakistan Shahid Ghazi a,b, Nigel P. Mountney a,⁎ a b

School of Earth and Environment, the University of Leeds, LS2 9JT, UK Institute of Geology Punjab University, Lahore-54590, Pakistan

a r t i c l e

i n f o

Article history: Received 20 November 2009 Received in revised form 27 October 2010 Accepted 29 October 2010 Available online 6 November 2010 Keywords: Permian Warchha Sandstone Salt Range Provenance Petrography Palaeogeography Gondwana

a b s t r a c t The Warchha Sandstone of the Salt Range of Pakistan is a continental succession that accumulated as part of a meandering, fluvial system during Early Permian times. Several fining-upward depositional cycles are developed, each of which is composed of conglomerate, cross-bedded sandstone and, in their upper parts, bioturbated siltstone and claystone units with distinctive desiccation cracks and carbonate concretions. Clast lithologies are mainly of plutonic and low-grade metamorphic origin, with an additional minor sedimentary component. Textural properties of the sandstone are fine- to coarse-grained, poorly to moderately sorted, sub-angular to sub-rounded, and with generally loose packing. Based on modal analyses, the sandstone is dominantly a feldspathoquartzose (arkose to sub-arkose). Detrital constituents are mainly composed of monocrystalline quartz, feldspars (more K-feldspar than plagioclase) and various types of lithic clasts. XRD and SEM studies indicate that kaolinite is the dominant clay mineral and that it occurs as both allogenic and authigenic forms. However, illite, illite-smectite mixed layer, smectite and chlorite are also recognised in both pores and fractures. Much of the kaolinite was likely derived by the severe chemical weathering of previously deposited basement rocks under the influence of a hot and humid climate. Transported residual clays deposited as part of the matrix of the Warchha Sandstone show coherent links with the sandstone petrofacies, thereby indicating the same likely origin. Illite, smectite and chlorite mainly occur as detrital minerals and as alteration products of weathered acidic igneous and metamorphic rocks. Based primarily on fabric relationship, the sequence of cement formation in the Warchha Sandstone is clay (generally kaolinite), iron oxide, calcareous and siliceous material, before iron-rich illite and occasional mixed layer smectite–illite and rare chlorite. Both petrographic analysis and field characteristics of the sandstone indicate that the source areas were characterised by uplift of a moderate to high relief continental block that was weathered under the influence of hot and humid climatic conditions. The rocks weathered from the source areas included primary granites and gneisses, together with metamorphic basement rocks and minor amounts of sedimentary rocks. Regional palaeogeographic reconstructions indicate that much of the Warchha Sandstone detritus was derived from the Aravalli and Malani ranges and surrounding areas of the Indian Craton to the south and southeast, before being transported to and deposited within the Salt Range region under the influence of a semi-arid to arid climatic regime. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The Salt Range of Pakistan forms part of the Sub-Himalayan Mountains, which stretch for more than 180 km east–west between the Jehlum and Indus rivers, along the southern margin of the Potwar Basin (Fig. 1). Within the Salt Range, a thick sedimentary cover of Precambrian to recent deposits unconformably overlies low-grade metamorphic and igneous rocks (Gee, 1989) and within this cover sequence the Early Permian Nilawahan Group of the Gondwana

⁎ Corresponding author. Tel.: +44 113 3435249. E-mail address: [email protected] (N.P. Mountney). 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.10.013

Realm is subdivided into the Tobra, Dandot, Warchha and Sardhai formations. The Nilawahan Group, which attains a maximum preserved thickness of 350 m, consists mainly of Early Permian deposits of glacio-fluvial, marginal marine and fluvial origin and is separated by a major unconformity from the overlying Upper Permian Zaluch Group, which is itself dominated by carbonate sediments of the Tethyan realm (Gee, 1989). During the Early Permian, the palaeoclimate in the region represented by the Nilawahan Group succession gradually evolved from very cold–cold (the Tobra Formation), to cold–cool (the Dandot Formation), to cool–warm (the Warchha Sandstone), to warm–hot (the Sardhai Formation) (Singh, 1987; Veevers and Tewari, 1995). The Early Permian Warchha Sandstone of the Nilawahan Group forms a fluvial succession of conglomerate, fine- to coarse-grained

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110 O

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Kingriali Peak

Study section

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Fig. 1. Location map showing the extent of outcrop of the Warchha Sandstone in the Salt Range of Pakistan and the position of the eight measured sections from which the data were derived.

sandstone, siltstone and fissile claystone (shale) arranged into repeating fining-upward depositional cycles (Figs. 1, 2; Ghazi and Mountney, 2009). Pebble and cobble clasts of varied lithology and with high textural maturity (well sorted and well rounded) are present within most of the coarser-grained facies and suggest a long transport distance within a large basinal setting (Fig. 2; cf. Pelosi and Fragoso-César, 2003). Although detailed provenance studies have not hitherto been conducted within the Upper Palaeozoic successions of the Salt Range, much of the Gondwana sediment that makes up the various deposits of the Nilawahan Group, including the Warchha Sandstone, is known to have been derived predominantly from an eroded basement of Precambrian granite and granite–gneiss with subordinate amounts of metasedimentary rocks (Pascoe, 1959). Based on detailed analysis of quartz types, Dutta (1983) concluded that there were no changes in the nature of the parent rocks through the time of Gondwana sedimentation. During the roughly 29 m.y. of Early Permian Gondwana sedimentation in the Salt Range area (Fig. 3), both tectonism and climate appear to have influenced the production and deposition of sediment and these factors varied as function of both changes in overall global climate and the movement of the Indian subcontinent towards the equator (Mukhopadhyay et al., 2010; Suttner and Dutta, 1986). Hitherto, the provenance and palaeoclimate of the Warchha Sandstone have not been established and no detailed work has been conducted previously on the analysis of the detrital framework mineralogy of the Warchha Sandstone. Furthermore, the significance of the clay mineral assemblage is little understood. The aims of this paper are: 1) to present for the first time a detailed petrographic overview of the Warchha Sandstone through analysis of textural and mineralogical properties; 2) to establish the provenance of the succession; 3) to establish the palaeogeography and palaeoclimate of the broader region from which the sediment was derived and in which the succession accumulated.

2. Methodology Mineralogical identification of sand- and clay-grade constituents from the Warchha Sandstone was undertaken using thin section petrography, X-ray diffraction (XRD) and scanning electron micrograph (SEM) techniques. A total of 165 representative samples were analysed from eight measured stratigraphic sections (Fig. 1; Table 1). Both the major components (quartz, feldspar, and rock fragments) and minor components (mica, heavy minerals, matrix, organic matter, and authigenic clays) were identified (Table 1). Proportions of various rock components were calculated by counting at least 300 grains per slide, following standard profile traverse methods (Adams et al., 1984; Moorhouse, 1959). Additionally, the relative frequencies of monoand polycrystalline, and undulose and non-undulose quartz grains were also determined (cf. Basu et al., 1975). Generally, ternary diagrams are used by petrographers for compositional analysis of sandstone. However, the interpretation of such diagrams does not have a rigorous confidence level when applied to samples considered to have been derived from complex tectonic settings and variable source areas (Allen and Johnson, 2010; Ingersoll and Eastmond, 2007; Weltje, 2004, 2006). Two research methods are commonly employed: the Gazzi–Dickinson method (data-acquisition) and Weltje method (statistical method) for provenance analyses based on the interpretation of sandstone composition (Allen and Johnson, 2010; Ingersoll and Eastmond, 2007). Due to the absence of any significant temporal and spatial compositional variation within the Warchha Sandstone succession throughout the Salt Range, the Gazzi–Dickinson method is considered justified for the present study (cf. Ingersoll and Eastmond, 2007). To minimize compositional variability due to grain size, sandstone samples (300 points per sample) were point counted following the Gazzi–Dickinson methodology (Zuffa, 1985). Pervasively altered sandstones and those with significant amounts of altered

90

Sanwans

Eastern Salt Range

Western Salt Range

152m

150m SW46 SW44

Claystone/Shale 140m

Sw42

130m

SW40

Siltstone

120m

110m

115m

SW35

110m

100m

110m

SR48

90m SR39

100m

AM37

90m

AM36 AM34

80m

WAM34 AM33

SR26

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60m

AM25

70m

MK36 MK35 MK34 MK33

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NW47

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WAM15

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Nilawahan area

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Amb area

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Sarin area

SW

Sanwans area

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60m

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50m

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Watli

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Saloi

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Karuli

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Nilawahan

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Medium-grained sandstone

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90m

Very coarse-grained sandstone

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

SW31

Medium-grained sandstone with clay lamination

Sarin

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Fine to very finegrained sandstone

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Fig. 2. Vertical logs showing the variation in lithology and thickness in eight measured sections of Warchha Sandstone, Salt Range Pakistan. See Fig. 1 for locations.

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S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

10 km

25 km

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Saloi

Watli

Karuli 11 km

5.5 km

27 km

east

Palaeoenvironment & palaeoclimate

Shallow marine

Sardhai Formation N

(Warm to hot)

A

Marginal marine facies

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I

Warchha Sandstone

R

Fluvial

P

E

(Warm)

Glacio-fluvial facies

Tillite facies

Y

Dandot Formation

Nilawahan Group

west

Amb

Sarin

Sanwans

91

L

Fluvio-glacial

Tobra Formation Glacio-marine facies

E

A

(Cold to cool)

R

Shallow water facies

Marginal to shallow marine

(Very cold to cold)

Cambrian

Cambrian Sequence Shallow marine

Fig. 3. Lithostratigraphic relationships of the Early Permian Nilawahan Group, Salt Range, Pakistan. See Fig. 1 for location of named sections.

grains (N10%) were excluded from the data set to avoid potentially misleading results. Detrital modes were obtained after reassigning framework grains to the QFL and QmFLt categories (Table 1; Folk, 1980; Pettijohn, 1975). Provenance discriminations were based on the schemes of Dickinson et al. (1983). Sandstone textural properties were determined from thin sections by means of visual comparison (cf. Compton, 1962; Powers, 1953; Wentworth, 1933). Twenty X-ray diffraction (XRD) analyses were performed to identify type and relative abundance of clay minerals. Three X-ray diffractometer traces were presented as natural, glycolated, and heated; associated diffraction peaks and have been interpreted according to Carroll (1970, 1979). The morphology of clay and sand grains was also observed with a fluid emission scanning electron microscope (FESEM), according to the methods of Gillott (1969) and Welton (1984). Additional images were taken on 16 gold-coated samples using a scanning electron microscope (SEM) for better visualization of authigenic grains coatings and mineral replacements. 3. Lithostratigraphy The Warchha Sandstone is informally divided into several conglomerate, sandstone and claystone units, with roughly equal proportions of sand and clay and less conglomerate (Ghazi, 2009; Ghazi and Mountney, 2010). Clasts lie in a poorly sorted sand-, siltand clay-grade matrix of varied composition. In places, intraformational clasts of claystone (derived from and reworked within the local depositional setting) are abundant. Sandstone units are medium- to thick-bedded, mainly light-brown to pinkish-white in colour, fine- to coarse-grained, poorly- to moderately-sorted, with grains that are sub-angular to sub-rounded. Commonly, the sandstone-dominated units contain 10 to 30 mm-thick, dark-brown, grey and green coloured claystone layers. Additionally, granule and pebble lags composed of clasts of pink granite are abundant. The sandstone units are locally speckled in appearance and, in places, contain carbonaceous material. Argillaceous claystone units are red, maroon,

dark-brown, grey and light-green. They are commonly massivebedded, blocky and splintery, though in places are interlaminated with thin, red, maroon, dark grey and dark green siltstone layers to form shales. Sedimentary structures include different forms of bedding, cross-bedding, ripple marks and stratification, channels, flute casts, load casts, desiccation cracks, rain prints, cone-in-cone structures, a variety of concretions and bioturbation (Ghazi and Mountney, 2009). Seven lithofacies types (Gt, St, Sp, Sr, Sh, Fl and Fm) are recognised in the Warchha Sandstone (Fig. 4a-h; Ghazi and Mountney, 2009) and are here classified using the scheme of Miall (1985, 1996). Stratified gravely sandstone (Gt) facies represents 10% of the total succession and is always present as the lowermost deposits at the base of each complete cycle. This facies is characterised by trough cross-bedded, stratified gravels that commonly infill channel-like erosive basal surfaces (Fig. 4a-b). Clasts are mostly of granite, gneiss or quartzite, though rare claystone and sandstone intraclasts are also present. Geometrically, this facies consists of lens- or ribbon-shaped bodies, commonly interbedded with sandy deposits. Coarse-grained trough cross-bedded sandstone facies (St) represents 15% of the total succession and commonly overlies facies Gt. It consists of medium- to very coarse-grained, moderately- to poorlysorted sandstone arranged into trough cross-bedded sets and cosets (Fig. 4c), some containing claystone and siltstone concretions. The cross-bedded units are mostly large scale (1 to 3 m thick), though some small-scale examples (0.3 to 1 m thick) are also evident. A medium- to coarse-grained cross-bedded sandstone facies (Sp), which represents 13% of the total succession, consists of poorly sorted, arkosic sandstone arranged into lenticular or tabular sets up to 2 m thick, which are characterised internally by planar cross-bedding (Fig. 4d). A ripple cross-laminated sandstone (Sr) facies, which represents 12% of the total succession, usually overlies facies Sp and consists of fine- to coarse-grained sandstone (Fig. 4e), which is generally well sorted and interlaminated with thin siltstone and claystone horizons. It occurs as thin wedge-shaped bodies that pinch-out laterally within few metres

2.6 2.3 2.3 2.4 2.3 2.2 2.2 2.2

32.6 31.4 45.2 47.7 44.4 38.9 44.1 48.5

9.2 9.2 10.6 8.8 10.1 10.1 8.3 7.7

2.1 1.7 2.1 2.2 2.3 2.1 1.8 1.8

2.5 2.5 4.1 4.1 4.1 2.5 1.8 3.8

9.2 10.7 8.5 8.1 8.4 12.1 8.6 9.4

23.1 24.2 25.3 23.1 25.1 27.1 20.4 22.1

9.9 9.2 5.4 2.9 4.6 8.9 8.1 2.5

9.1 10.1 6.5 7.1 7.3 6.9 7.7 7.9

1.1 1.1 1.2 1.7 1.2 1.1 1.1 1.3

6.4 6.3 2.9 2.6 2.6 3.9 4.2 3.3

1.1 1.4 1.8 1.7 1.8 1.5 1.4 1.5

5.1 5.6 3.7 4.1 4.1 3.8 4.4 4.2

1.8 1.7 1.5 2.1 2.1 1.5 1.5 2.1

14.1 15.1 9.4 10.2 10.1 9.7 11.4 10.3

2.5 2.4 1.6 1.4 1.4 2.2 2.1 1.6

0.5 0.5 0.6 0.9 0.7 0.4 0.4 0.6

3.7 3.2 1.9 1.8 1.6 2.8 2.5 1.4

2.9 2.8 3.3 4.7 4.2 2.1 2.8 3.8

2.1 1.1 1.5 1.3 1.4 1.2 1.1 1.4 30.1 29.3 43.2 45.5 42.2 36.8 42.1 46.6 Saloi Watli Karuli Matan Nilawahan Amb Sarin Sanwans

Accessory minerals Cement Feldspar Quartz

Table 1 Summary of the average petrographic composition of eight measured sections of the Warchha Sandstone, Salt Range, Pakistan.

Organic Kaolinite Chlorite / Mica Heavy Calcareous Total Matrix Chert Argillaceous Siliceous Iron Rock Orthoclase Microcline Plagioclase Altered Total Monocrystalline Polycrystalline Total Illite / minerals matters % cement % % % oxides % % % feldspar feldspar fragments (clay) % % % quartz % Smectite% % % % % % % % % %

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Section

92

and which contain abundant ripple marks, flat bedded surfaces, smallscale trough and planar cross-stratification and load casts. Very fine- to medium-grained sandstone with flat bedding (Sh) facies represents 10% of the total succession and consists of horizontally laminated sandstone (Fig. 4f), with micaceous siltstone laminae arranged into thin beds that possess a parting lineation within units with a sheet-like or tabular geometry (Fig. 4f). This facies is discontinuous, generally pinching out laterally within a few tens of metres where it is cut out by low angle erosion surfaces. Parallel laminated siltstone and claystone (Fl) facies represents 15% of the succession and consists of laminated siltstone or massive claystone units interbedded with millimetre-thick siltstone horizons (Fig. 4g). Lower contacts of this facies are gradational with facies Sh or Sr, whereas in almost all cases upper contacts are gradational with facies Fm. Geometrically, this facies is arranged into 2 to 3 m-thick, laterally extensive sheet-like bodies. Massive claystone facies (Fm) is the most abundant type in nearly all cycles, representing 25% of the total succession. It consists of red, dark-brown, purple, green, light-grey and yellow claystone and weakly laminated mudstone, with rare grey to greenish-grey and white siltstone and white to off-white fine-grained sandstone interbeds (Fig. 4h). This facies is commonly massive, though at a few horizons it contains abundant bioturbation, clay balls, iron concretions, desiccation cracks, rain-drop imprints, rootlets and caliche nodules up to 0.1 m in diameter. 4. Conglomerate Petrography Pebble-grade conglomerate with a mixture of intraformational (i.e. locally derived and reworked) and extraformational (i.e. exotic material derived from outside the immediate deposition setting) clast types constitutes 10% of the Warchha Sandstone. Clasts range in diameter from 2 to 160 mm and average 20 mm. Matrix constitutes 20% of the total volume and consists of varying amounts of fine-grained sand, silt and clay. The cement is calcareous, ferruginous and siliceous, comprising about 8% of the total volume. As a result of compaction, lithic grains commonly appear deformed and bent around more resistant quartz grains, making their identification difficult. Poorly sorted intraformational conglomerate clasts are composed of a mix of sub-rounded compacted sandstone pebbles and platy rounded siltstone pebbles. More abundant extraformational conglomerate clasts are composed predominantly of pink granite, granite–gneiss, schist and, more rarely, of massive or layered metaquartzite pebbles. Sub-angular fragments of potassium feldspar up to 30 mm in diameter are locally common. Rare clasts of limestone, dolomite and chert up to 40 mm in diameter, though usually no more than 20 mm, are also present, especially in the eastern Salt Range. Most chert clasts exhibit an outer, light-coloured weathered surface surrounding a darker interior. The Warchha Sandstone in the eastern Salt Range (Saloi and Watli areas; Fig. 1) and western Salt Range (Sarin and Sanwans areas) contains a greater percentage of pebble and granule-grade clasts (Fig. 2). Compositionally, clasts from the eastern part of the succession are dominantly sandstone, claystone, limestone, dolomite, pink granite, quartzite and chert, whereas in the western part of the succession they are dominantly granite, schist, sandstone and claystone. 5. Sandstone Petrography Sandstone constitutes 45% of the Warchha Sandstone. The texture (grain size, sorting, and grain morphology) is very heterogeneous (Fig. 5), varying from coarse-grained sandstone in the channel deposits, to very fine-grained sandstone interbedded with claystone in the floodplain deposits. The Warchha Sandstone generally exhibits a loose-to-normal packing with tangential, concavo-convex and sutured contacts between the grains. Sandstone colour varies from light- to dark-pink, red, reddish-brown, light-grey, dark-grey, greenish-grey to

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

93

Fig. 4. Characteristic example of lithofacies in the Warchha Sandstone in the Salt Range. a) and b) stratified gravely facies Gt. c) coarse-grained trough cross-bedded sandstone facies St. d) medium- to coarse-grained planar cross-bedded sandstone facies Sp. e) ripple cross-laminated sandstone facies Sr. f) horizontal bedded and laminated sandstone facies Sh. g) interlaminated siltstone and claystone facies Fl. h) massive claystone facies Fm.

white and changes as a function of source material, the amount of interstitial haematite in the matrix, and the type of cement (cf. Schluger, 1976). Major detrital framework components of the sandstones – especially quartz, feldspar and rock fragments – have been recalculated as 100% (Table 1) for QFL diagrams, allowing these to occupy one of the three poles. Plots in the QFL diagrams proposed by

Pettijohn (1975) and Folk (1980) show that the sandstone is a feldspathoquartzose, with 95% being classified as arkose and arkosicarenite, and 5% being sub-arkose (Fig. 6). The average modal composition of the sub-arkose is Q70% F22% and L8% and that of arkose and arkosic-arenite is Q60% F30% and L10%. The overall average composition is Q57% F34% and L9%.

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S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

a

b

Grain size classes

Sorting of sandstone

c

Roundness classes

25% 35%

40%

Coarse to medium

Medium to fine

25% Moderate to well

45% Poor

25%

30%

Fine to very-fine

Moderate to poor

Angular to sub-rounded sandstone clasts

75% Sub-angular to sub-rounded sandstone clasts

Fig. 5. Textural classification of the sandstone fraction of the Warchha Sandstone according to a) grain size, b) sorting, c) roundness.

6. Mineralogical composition 6.1. Quartz One hundred grains were counted in each thin section of sandstone to determine quartz types (Folk, 1980). Quartz was the most abundant

Q 0

0%

10

a

Quartz arenite Sub-litharenite

LITHIC ARENITE

10

0

0%

ARKOSIC ARENITE

%

75

25

%

%

50

50

%

%

25

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%

Sub-arkose

F

25%

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50%

75%

100%

L

QFL diagram based on the scheme of Pettijohn (1975)

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ITE N RE HA LIT

C FELDSPATHI E LITH ARENIT

LITHIC ARKOSE

E AR

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0%

%

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75

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F

0

detrital constituent, comprising 35 to 65% (average 57%) of the total rock components. Percentage quartz increases gradually lower in the succession (in relation to increasing depth of burial). Grain sizes range from 0.07 mm to 0.84 mm, grain shape is mostly sub-angular to subrounded and sorting is poor (1.5) to moderate (0.75). A few quartz crystals contain irregular inclusions of biotite, zircon, rutile and/or tourmaline. Both monocrystalline (Q m) and polycrystalline (Q p) quartz grains are present (Fig. 7a–f), with monocrystalline quartz grains comprising 92% of the total quartz content and being characterised mostly by non-undulatory (unstrained) and more rarely by slightly undulatory (strained) extinction types with a wide range of extinction angles. Most grains have a simple structure composed of a single quartz unit (Fig. 7a–d). Many grains contain abundant inclusions, either scattered irregularly throughout the grains or as near-continuous straight lines (Fig. 7e). Rutile needles are generally rare, but occur in up to 4% of the quartz grains in some samples. The Polycrystalline quartz (Q p) grains consisting of two or more crystals comprise 8% of the total quartz (Fig. 7f). These grains possess the greatest structural complexity, having either straight or sutured boundaries or, in some cases, a schistose fabric. Rarely, quartz crystals contain irregular inclusions of biotite, zircon and/or tourmaline. A few grains are fractured, crenulated and cloudy in appearance (Fig. 7f).

25%

50%

75%

100%

L

QFL diagram based on the scheme of Folk (1980) Fig. 6. Interpretation of the sandstone composition from the petrography of eight sections of the Warchha Sandstone based on the schemes proposed by a) Pettijohn (1975) and b) Folk (1980). Standard plots: Quartz, Feldspar, Lithic grains (Q, F, L), showing arkosic to subarkosic nature of the sandstone.

6.2. Feldspar Feldspar is the second most abundant detrital constituent of the Warchha Sandstone, averaging from 20 to 34% in various grain sizes and types. Alkali feldspar (orthoclase and microcline) and plagioclase feldspar constitute 30% and 10% of total feldspar, respectively (Fig. 8a–d). Rarely, feldspar grains show perthitic intergrowth. K-feldspar grains are subrounded to rounded, whereas plagioclase grains are angular to subrounded (Fig. 8a). Sixty percent of the feldspar grains are partially altered, and appear cloudy or turbid in thin section, with diffuse outer lines. Commonly, altered feldspar is partially or totally (preserving only its shape) replaced by kaolinite, which in many cases has itself altered to illite. This process appears to have been more intense with increasing burial depth (and temperature), based on its widespread occurrence lower in the succession. In partially decomposed feldspars, alteration products tend to concentrate on the surface where they form microcrystalline aggregates of kaolinite. Microcline is locally abundant, comprising 8 to 15% of the total feldspar in the sandstone (Fig. 8b) and shows distinctive cross-hatched twinning, with clasts commonly broken preferentially along composition planes (cf. Basu, 1976). Orthoclase is untwinned (Fig. 8c–d). The quantitative determination of orthoclase and microcline abundance was somewhat dependant on grain size (cf. Basu, 1976), however, most of the small grains that lack twinning are orthoclase. Plagioclase feldspar occurs in both twinned and untwinned forms (Fig. 8a), with the untwinned variety being more abundant in finer grain sizes (Fig. 8c). Zoning of plagioclase is also observed, particularly in the untwinned

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95

Fig. 7. Thin section photomicrographs of detrital quartz grains in the Warchha Sandstone. a) fine-grained sandstone having monocrystalline quartz grains showing tangential contacts with abundance of matrix. b) fine-grained, sub-angular to sub-rounded quartz grains (1); with haematite as cement (2); intergranular porosity (3). c) loosely packed, monocrystalline, fractured quartz in coarse-grained arkosic facies. d) an authigenic overgrowth of detrital quartz grain, thin layer of clay and iron oxides on its surface. e) polycrystalline quartz (Q p) grains (1); medium- to coarse-grained, sub-angular to sub-rounded monocrystalline quartz (Qm) grains showing inclusions (2); iron oxide cement (3). f) Abundance of polycrystalline quartz (Q p) grains having long and sutured contacts and an abundance of haematite and argillaceous cement . All examples in plane polarized light and depicted at the same scale.

variety and in coarser grains. The ratio of plagioclase to total feldspar varies from 0.11 to 0.18. 6.3. Lithic fragments Rock fragments formed from clusters of multiple mineral grains comprise 3 to 10% of the Warchha Sandstone (Fig. 8e–f) and are mostly medium- or coarse-grained, sub-rounded, rounded or angular. Igneous, metamorphic and sedimentary lithic fragments are all identified. Igneous clasts constitute 5 to 6% of total rock volume and are mainly fragments of acidic plutonic rocks and, more rarely,

volcanic rock. These clasts include granitic and granodioritic rock types with sub-rounded grain shapes. Metamorphic rock fragments, which constitute 4 to 5% of the total rock volume, are sub-angular to sub-rounded in shape and are mainly composed of quartzite, phyllite, schist and slate. Sedimentary rock fragments of sandstone, siltstone, shale, claystone, limestone, dolomite and chert constitute 2 to 3% of the total rock volume, are fine- to medium-grained, and are rounded in shape. Chert grains alone constitute 0.5 to 1% of the total rock volume and are mostly composed of microcrystalline quartz grains with light- to dark-grey interference colours. The chert grains are generally better rounded than associated quartz and feldspar grains

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Fig. 8. Thin section photomicrographs of detrital feldspar grains in the Warchha Sandstone. a) plagioclase feldspar (arrows) showing polysynthetic albite twinning. b) microcline feldspar (arrows) of plutonic origin showing characteristic grid twinning. c) detrital grains of plagioclase (1); feldspar, orthoclase (2); feldspar and alteration of feldspar to haematite (3). d) detrital grains of altered orthoclase (o) feldspar. e) altered rock fragment with corroded edges in coarse-grained sandstone. f) highly deformed metamorphic rock fragment in coarse-grained sandstone. All examples in plane polarized light and depicted at the same scale.

and their frequency tends to increase with decreasing grain size. Although the value of a detailed study of rock fragment types using visual classification techniques has been demonstrated (e.g. Garzanti and Vezzoli, 2003) such a detailed investigation is beyond the scope of this multi-approach study. 6.4. Accessory minerals Accessory minerals, including mica, heavy minerals and organic matter, are present in all samples in minor or trace amounts. Both muscovite and biotite (more common) micas form a minor detrital grain component, accounting for 1 to 2% of the total rock volume (Table 1), and occurring as well defined to very-finely comminute

flakes and shreds. Mica flakes up to 4 mm (average 0.25 mm) in their longest dimension tend to be aligned parallel to bedding. Further, because of their thin, sheet-like shape and resultant lower settling velocity they tend to occur associated with smaller quartz and feldspar grains (cf. Doyle et al., 1983). Twisting of mica reflects the degree of physical compaction. Heavy minerals make up considerably less than 1% of the total rock volume, with only a few grains of garnet, zircon, tourmaline, rutile, epidote, magnetite and haematite present. It is possible that an unspecified component of the original heavy mineral fraction could have been removed through dissolution, although there is no evidence for this. Organic matter is an accessory detrital component of the sandstones and constitutes 1 to 4% of the total rock volume

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(Table 1). In thin sections, organic matter is characterised by dark yellowish-brown to black coloured isolated grains or grain clusters, with translucent edges.

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whether some of the material that is apparently matrix might actually be pseudomatrix. 6.6. Cement

6.5. Matrix Matrix constitutes 5 to 10% of the total rock components (Table 1) and is mostly composed of quartz, feldspar, clay minerals and other argillaceous materials. In particular, soft aggregates of clay minerals, which underwent physical disintegration during transport and which readily deformed during compaction, likely form much of the matrix. Although it is difficult to separate matrix from cement in thin section, XRD and SEM analyses show that kaolinite is the dominant clay mineral in the matrix. Given the form of preservation, it is unclear

Argillaceous, ferruginous (haematite) and calcareous (calcite) cements are the most abundant types in the sandstone, each constituting 6 to 8% of the total rock volume. Additionally, siliceous cement is also present as a secondary overgrowth and constitutes 3% of the total rock volume (Fig. 9a–f). Ferruginous cement is dominantly haematite (Figs. 8c; 9a) and rarely siderite and constitutes 3 to 5% of the total rock components. Haematite occurs as both discrete disseminated grains and masses along fractures, whereas siderite occurs as small individual or clusters of rhombohedra. The presence of siderite indicates the operation of reducing agents, like organic matter

Fig. 9. Scanning electron micrographs of different types of cement in the Warchha Sandstone. a) fine crystalline, authigenic haematite cement, occurs as platy material which is aggregated in to rosette like clusters. b) authigenic calcite causing displacement of feldspar fragments by displacive crystallization. c) quartz overgrowth cementation. Numerous very small overgrowths give appearance of pitted surface on detrital quartz grains. d) authigenic kaolinite cement formed by the alteration of feldspar. e) and f) pore spaces were created by dissolution of feldspar and filled with kaolinite cement.

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(Bjørlykke, 1983). The occurrence of authigenic calcareous cement is variable but averages 2% of the total rock components (Table 1) and is present as a mosaic of interlocking crystals that passively fill pore spaces, fracture planes or occur along the corroded edges of detrital framework grains, and additionally as a form that has partially replaced detrital grains (Fig. 9b). In thin section, calcareous cement is colourless and of variable relief under plane polarized light. Calcite is recognised by its rhombohedral cleavage, and by its higher-order pink and green birefringence colours under crossed nicols. Siliceous cement occurs either in optical continuity with detrital grains or as cementing materials filling the pore spaces (Fig. 9c) and constitutes 1 to 3% of the total rock components (Table 1). Quartz overgrowths, although common within the Warchha Sandstone (Fig. 9c), yield a low volumetric percentage (b1%). Kaolinite is the most abundant authigenic clay and is present as either pseudohexagonal-stacked plates (books), in a vermicular form, or as irregular and dispersed aggregates within pore spaces (Fig. 9d–f). It is more abundant in feldspathic sandstones than in claystone. Banfield and Eggleton (1990) and Bjørlykke (1998) have suggested that dissolution and decomposition of feldspar is likely to be the major source of

kaolinite precipitation (Fig. 9d–f). It constitutes about 2 to 4% of the total rock components, with an average of just over 2% (Table 1). Optically, kaolinite is present in pore spaces as clusters of granules. The boundaries of kaolinite clusters are commonly attached to adjacent K-feldspar grains (Fig. 9d–f) in a manner indicative of authigenic growth (cf. Bucke and Mankin, 1971). 7. Clay mineralogy Finer-grained rock types, including claystone (exclusively of clay grade), siltstone (exclusively of silt grade), mudstone (of a mix of clay and silt) and shale (laminated mud), constitute 45% of the measured thickness of the Warchha Sandstone. Of these types, mudstone constitutes 50%, claystone 30% and siltstone 20%. Semi-quantitative analysis was performed following the method of Moore and Reynolds (1989) (Figs. 10, 11). Finer-grained components show important compositional mixing with clay-sized quartz and feldspar grains and they account for 40 to 45% of the fine-grained lithologies in each sample. This could be explained by the mixing of mud- and silt-sized fractions produced by the transport agent within a floodplain setting.

K

a

K

a

Natural

K

Natural

K

Glycolated Glycolated Heated

I/S Heated Ch K

0

10

20

30

Q

40

0

10

20

2 Theta (°)

b

b

K

30

40

2 Theta (°)

K

Sm Natural

Q

Sm

Natural

K

Glycolated

Q

K

Glycolated

Heated

Heated Sm Q

K

Q Q

Q Q

0 0

10

30

20

10

20

40

40

30

2 Theta (°)

2 Theta (°)

Natural

c

Glycolated Q

c

Natural

Ch

Glycolated K

Heated Ch

Ch

Heated K

I

I

Q Q

I

0

10

20

30

40

0

10

20

30

40

2 Theta (°)

2 Theta (°)

Fig. 10. a) XRD analysis of the Warchha Sandstone, Salt Range, Pakistan. a) kaolinite (K) and chlorite (Ch). b) kaolinite (K), smectite — montmorillonite (Sm) and quartz (Q). c) illite (I), kaolinite (K) and quartz (Q).

Fig. 11. XRD analysis of the Warchha Sandstone, Salt Range, Pakistan. a) illite/smectite — montmorillonite mixed layer (I/S), kaolinite (K) and quartz (Q). b) Smectite — montmorillonite (Sm), kaolinite (K) and quartz (Q). c) Chlorite (Ch).

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To enable recognition of the clay mineral association, XRD techniques were used and for this 20 samples were treated (normal, glycolated and calcinated). The amount of clay slightly decreases with depth. Both allogenic and authigenic clays occur in the Warchha Sandstone. Allogenic clays, emplaced during deposition of the Warchha Sandstone, occur as both clay-sized particles and silt- to gravel-sized clay aggregates whereby physical and biogenic processes are likely to have controlled their size, shape and distribution. Individual clay particles are dispersed in the sandstone as matrix, thin laminae and mud clasts. Kaolinite is the dominant type of clay, with illite, smectite, chlorite and mixed layers of illite-smectite also occurring in varying amounts and in several forms (Figs. 10, 11; Table 2).

99

The weathering of acidic plutonic rocks was likely responsible for generating the kaolinite-rich clay in the Warchha Sandstone (Iniguez et al., 1989; Islam et al., 2002; Manassero et al., 1991; Potter et al., 1980). This weathering likely involved very intense kaolinization under the influence of a climate characterised by high temperatures and heavy rainfall (Nakagawa et al., 2006; van de Kamp, 2010) and it is therefore suggested that the kaolinite present in the Warchha Sandstone was derived through the intense chemical weathering of crystalline source rocks containing abundant of feldspar in the presence of meteoric water drainage (cf. Bertier et al., 2008; Bjørlykke, 1998; Islam et al., 2002; van de Kamp, 2010). 7.2. Illite

7.1. Kaolinite Kaolinite, which occurs as both allogenic and authigenic forms, is the dominant clay mineral and constitutes 60% of the total clay volume (Table 2). On X-ray powder diffractograms of oriented slides, kaolinite is characterised by 7.17 Å, 7.16 Å, 3.58 Å and 3.57 Å reflections (Fig. 10). Profiles do not show any change upon glycol saturation. However, upon heating at 500 C, samples become amorphous, which confirms the presence of kaolinite (Carroll, 1970, 1979). Scanning electron micrographs of kaolinite show good crystals of pseudo-hexagonal plates, commonly with face-to-face stacking (Fig. 12a–f). Mostly, kaolinite fills only scattered pores in the sandstone, and only rarely are all pores in a specimen plugged. These clay associations consistently occur both within and between sandstone detrital grains, suggesting the same sources of origin. Kaolinite occurs in varying amounts and as several forms in the Warchha Sandstone: a) kaolin pellets enclosing silt-sized quartz grains in channel and bar facies associations and forming a component of the matrix rather than the framework of the rock; b) a mixture of kaolinite with illite and smectite occurring in floodplain facies associations; c) authigenic kaolinite occurring as books in channel and bar facies associations (Fig. 12a–f); d) very poorly crystalline kaolinite with minor amounts of illite and quartz and with carbonaceous material at the top of the depositional cycles. The sharp X-ray diffraction pattern and resistance to heat treatment (Fig. 9) confirms a crystalline form for the kaolinite. The following factors are likely to have played major role for generation of the authigenic kaolinite within the Warchha Sandstone (cf. David et al., 1971): a) porosity and permeability, by allowing migration of interstitial water and by providing growth space; b) the presence of K-feldspar, which acted as a likely source of Al and Si; c) the presence of partly degraded illite, which acted as a K+ acceptor; d) the presence of organic material to maintain a low pH.

Illite constitutes 10% of the total clays by volume in the Warchha Sandstone (Table 2) and it occurs as a detrital mineral, as an authigenic phase, and as an alteration product of kaolinite, micas and feldspars. Illite is characterised on X-ray powder diffractograms of oriented slides, by 10.06, 5.00, and 3.35 Å reflections (Fig. 10). The profile at 10.06 Å does not show any changes upon glycol saturation. SEM images display irregular flakes with lath-like projections (Fig. 13 a–b). The growth of illite varies in appearance depending on the style of development of these laths; principally depending on whether they are attached to the surface of sand grains, or whether they develop as sheets that curl from the point of attachment (Fig. 13 a–b). Illite is stable under alkaline conditions and is stable in the presence of kaolinite with increasing temperature. Illite in the Warchha Sandstone may also be formed by the degradation of smectite at temperatures around 100 C (cf. Hower et al., 1976). During diagenesis, potassium feldspar was likely the main source of the potassium utilized in the illitization process in the Warchha Sandstone (cf. Bertier et al., 2008; Mosser-Ruck et al., 2001; Weaver and Beck, 1971; Whitney, 1990). Meshri (1986) postulated that illite can be formed by the conversion of microcline at 25 C and 1 bar pressure in H2 CO3. 7.3. Mixed layers Illitte–Smectite (I–S) mixed layers constitute 10–15% of the clay volume (Table 1). A mixed layer mineral tends to show a broad a diffraction peak between the basal spacing normally shown by its pure components (Fig. 11). The X-ray diffraction patterns of this I–S mixed layer are similar to those calculated by Reynolds and Hower (1970) and Reynolds (1983) for an order I–S interstratifications containing about 20% smectite. SEM images of the mixed-layer have characteristics similar to illite and smectite host clay minerals (Fig. 13c–d). Illite–smectite mixed layer clays in the Warchha

Table 2 Summary of the properties of various identified clay minerals in the Warchha Sandstone, Salt Range, Pakistan. Clay type

% XRD analyses Volume

Kaolinite

60%

Illite

10%

I/S mixed layer 10–15%

Smectite

10%

Chlorite

5–10%

SEM analyses

Pseudo-hexagonal plates (books); 7.17–7.16 Å and 3.58–3.57 Å in air-dried commonly faceto-face stacking. state; no change upon glycol saturation; upon heating at 500 °C becomes amorphous.

Origin

Occurs both as allogenic and authogenic forms as pellets, mixtures and crystalline states. Mostly fills and plugs pores. Mainly generated by weathering of acidic plutonic rocks in hot temperature and high rainfall conditions. Mainly occurs in detrital and rarely in authigenic forms; 10.06, 5.00, and 3.35 Å in air-dried state; no Irregular flakes with lath-like change upon heating and glycol saturation. projection; developed as sheets that attached on the surface of sand grains. K-feldspar is the main source of illization. curl from the point of attachment. Sheet with short lath like digitate Originated by the mechanical rearrangement of layers in 11.88 Å in air dried state; upon glycol wetting and drying conditions in an open system. Also saturation give rise to reflections at 13.37 Å edges. developed from K-feldspar as temperature, burial depth and and 10.08 Å. time increased. Mainly associated with palaeosol development. Originates via 14.6 Å in air dried state; expand upon glycol A crinkly coating and cellular weathering in wet and dry conditions. Transported due to structure similar to honeycomb saturation (17.18 Å) and collapse to10.6 Å erosion of continental soil. chlorite. upon heating at 500 °C. 14.72 and 3.5 Å in air-dried state; no change Plattes, rosettes, honeycombs, or Occurs mainly as a pore-lining material. Mainly formed by the upon glycol saturation and heating at 500 °C. cabbage-head growths. weathering of granitic-rich rocks in which biotite was changed to chlorite.

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Fig. 12. Scanning electron micrographs of kaolinite in the Warchha Sandstone. a) and b) kaolinite formed due to alteration of feldspars and filling pore spaces. c), d), e) and f) stacked plates of kaolinite showing face-to-face arrangement and pseudo-hexagonal (books) outline of individual plates.

Sandstone were probably produced by the mechanical rearrangement of layers facilitated by wetting and drying in an open system that fixed the K+ between the high charge smectite layers (cf. Whitney, 1990). As burial depth increased, smectite was likely converted into illite to make the mixed layer (Weaver, 1989). The conversion of smectite into illite was probably controlled by a combination of burial depth, temperature, time and percentage of K-feldspar in the succession (cf. Pollard, 1971; Ylagan et al., 2000).

montmorillonitic clay was mainly recorded from samples taken from the uppermost parts of depositional cycles where palaeosols are developed in the Warchha Sandstone. Such floodplain soils, which typically have high shrink-swell potential, were probably developed in a wet–dry seasonal climate. Smectite was most likely liberated for transport by erosion of soil and was deposited mostly in floodplain areas during the deposition of the Warchha Sandstone. The smectite likely originated mostly from the alteration of these continental soils (cf. Chamely et al., 1990; Deconinck et al., 1991).

7.4. Smectite 7.5. Chlorite Montmorillonite is the most abundant smectite mineral and constitutes 10% of the total clay volume (Table 1). Montmorillonite is characterised on X-ray powder diffractograms of oriented slides by 14.6 Å reflections (Fig. 11). SEM images display two basic growth habits: a crinkly coating on detrital sand grains (Fig. 13e), and a cellular structure similar to honeycomb chlorite. Smectite-rich

Chlorite constitutes 5 to 10% of the total clay volume (Table 1) and is recorded from the top and bottom parts of the succession. Chlorite is characterised by a series of basal reflections at 14, 7.26 and 3.5 Å (Fig. 11) and shows no change upon glycol saturation and heating at 550 C. It occurs as pore linings, which are extremely variable in the

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101

Fig. 13. Scanning electron micrographs of different types of clay minerals in the Warchha Sandstone. a) and b) Illite with short lath-like projections. c) and d) Illite/smectite mixed layers. Mixed layer minerals appear to take on the characteristics of the host minerals. e) smectite (montmorillonite); highly crenulated and interlocking crystals are typical of montmorillonite. f) chlorite plates attached on edges to the detrital grains.

sandstones and may take the form of plates attached to detrital sand grains (Hayes, 1970; Pittman and Lumsden, 1968). SEM analyses (Fig. 13f) show plates, rosettes, honeycombs, and cabbage-head-like growths (cf. Wilson and Pittman, 1977). Most of the chlorite in the Warchha Sandstone was likely produced by the weathering of granite, whereby biotite was altered to chlorite (cf. Velde, 1985), which itself underwent further degradation during transportation. Chlorite in the Warchha Sandstone may also be derived from weathering profiles whereby smectite transformed to chlorite (cf. Velde, 1985). 8. Provenance Eighty representative samples of the feldspathoquartzose Warchha Sandstone from each of the eight measured sections (640 in total) were selected for interpretation of provenance. Provenance discriminations are based on the schemes of Dickinson et al. (1983) and

consider the hierarchy of different depositional environments for provenance interpretation defined by Ingersoll et al. (1993). Plots on QFL and Q mFLt triangular diagrams indicate that the Warchha Sandstone was likely derived mostly from cratonic interiors and transitional continental blocks (Fig. 14; Dickinson, 1985). This simple classification is potentially confusing since the plotted samples are concentrated along the Q-F/Q m-F lines within the stable cratonic, continental transitional and recycled orogenic areas in the continental block provenances (Fig. 14; Dickinson, 1985; Dickinson et al., 1983). Provenance studies of sediments derived from continental blocks have been conducted in modern settings subject to a diverse range of climatic conditions, ranging from the arid/hyperarid Red Sea and Gulf of Aden (Garzanti et al., 2001), to the temperate setting of the Alps (Garzanti et al., 2006a), to monsoonal subtropical settings in Ethiopia and Sudan (Garzanti et al., 2006b). Thus, a variety of possible source area palaeoclimatic configurations are considered possible for the site

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Q TOTAL QUARTZ

a

3

CRATON INTERIOR

18

TRANSITIONAL CONTINENTAL RECYCLED OROGENIC

45 UP LIF T

37 E

ECT

ME NT

DISS

25

ARC NAL

TIO

NSI

BA SE

13

C D AR

TRA

L

50

LITHIC/ROCK FRAGMENT

Qm MONOCRYSTALLINE

b

QUARTZ

CRATON INTERIOR 20 TRANSITIONAL CONTINENTAL

11

E OS TZ D AR LE QU ECYC R

43

T LIF UP NT ME

CTED DISSE C AR

32 29 18 13

L ARC

ITIONA

BA

TRANS

ED SECT UNDIS RC A

23

47

HIC D LIT CLE CY RE

SE

42

L NA IO SIT D AN LE TR CYC RE

MIXED

FELDSPAR

E ISS

D

15 FELDSPAR

F

A

CT

UN

F

18

ED

RC

Lt

TOTAL LITHIC GRAINS

Fig. 14. Interpretation of provenance from the petrography of the Warchha Sandstone in the Salt Range of Pakistan based on schemes proposed by Dickinson et al. (1983). a) quartz, feldspar, lithic fragments (Q, F, L). b) monocrystalline quartz, feldspar, total lithic grains (Qm, F, Lt).

of weathering of the sediments of the Warchha Sandstone. For this reason, several other petrographic and sedimentologic characteristics were additionally used to determine the distribution of the likely source areas and the type of source terrane of the Warchha Sandstone. Palaeocurrent directions (principally from planar and trough crossbedding — Ghazi and Mountney, 2009) indicate transport to the north and northwest and suggest a source area to the south and southeast. The clast and sandstone composition does not suggest any change in the source area over time. Analysis of sandstone composition in the Warchha Sandstone has been undertaken to reconstruct the sedimentary history of Salt Range basin during Early Permian times in terms of its palaeoclimate and palaeophysiography, and in relation to the probable tectonic controls on sediment production, supply and dispersal (cf. Ingersoll and Eastmond, 2007; Weltje et al., 1998). Based on the detrital grain composition of the sandstones, several conclusions can be made regarding the likely provenance of the sediments (Fig. 15a–f). The quartz grains in the Warchha Sandstone were likely derived from the erosion and recycling of crystalline igneous or metamorphic rocks (Dutta, 2007; Pascoe, 1959; Pettijohn et al., 1987). Detrital quartz grains showing evidence of plastic deformation (Fig. 15a–f), as demonstrated in thin section by grains possessing an undulatory extinction, are common in the Warchha Sandstone and these most

likely indicate substantial tectonic uplift of crystalline basement rocks in the source region. Such undulose extinction did not develop postdepositionally, since the Warchha Sandstone was not buried sufficiently deeply or heated to a sufficiently high temperature to induce plastic deformation of the quartz grains (Ghazi, 2009). Monocrystalline grains of detrital quartz of medium- to coarsegrain size (Fig. 15a–b) were likely derived from the weathering of granites (Basu et al., 1975; Datta, 2005; Dutta, 2007; Pettijohn et al., 1987), whereas the fine-grained monocrystalline grains are most likely the product of breakage and chipping of larger quartz grains derived from an igneous provenance (Table 3; Dickinson, 1970). By contrast, polycrystalline elongated and stretched quartz grains were most likely derived from a metamorphic source (Blatt, 1967), such as granite–gneiss or schist (Fig. 15c–d). Additionally, the exceptionally rounded to well-rounded nature of some quartz grains is indicative of a recycled sedimentary source (Basu et al., 1975; Pettijohn et al., 1987). Detrital K-feldspar grains were mainly derived from either acidic igneous rocks (Ghose and Kumar, 2000) or from granite and gneisses (Datta, 2005), whereas plagioclase feldspar grains were mainly derived from low-grade mica-schist (Table 3). The ratio of plagioclase to total feldspar varies from 0.11 to 0.18 (Table 4), values that are lower than those expected for sand derived exclusively from volcanic terranes, but typical of sand derived from mixed plutonic and metamorphic terranes of granitic composition (Dickinson, 1970; Dickinson and Rich, 1972; Ingersoll, 1979). The detrital feldspar grains therefore likely indicate both an igneous and a metamorphic source (Fig. 15e–f). Detrital micas were most likely derived from low-grade metamorphic rocks like quartzite, schist, and gneiss (Fig. 15e–f), or from plutonic igneous rocks of granitic composition (Khan, 1991). Petrographic data (Table 4) reveal a ratio of total feldspar to total lithic fragments (F/L) of 4.7, which is indicative of a plutonic provenance (Dickinson, 1970). Plagioclase to total feldspar ratio (P/F) varies from 0.11 to 0.18, probably as a function of source rock type and degree of weathering in the provenance region (Dickinson, 1970). Given that the percentage of lithic fragments is less than 25%, and commonly less than 10% (average 9%), and that the succession has an average composition of 57% quartz with a low ratio of polycrystalline to total quartz (Q p / Q), it is likely that the main source for the sediment was via the erosion of plutonic, granitic material (cf. Dickinson, 1970). The average values of the Q p / F + R and Q m + Q p / F + R ratios are 0.049 and 1.37 (Table 4), which are indicative of a mineralogically submature sandstone (Suttner and Dutta, 1986). The mineralogically submature composition combined with a general textural maturity suggests a derivation of detrital materials under conditions of predominantly mechanical disintegration and supports the interpretation of a hot, arid climate during transportation and deposition (cf. Datta, 2005; Dutta, 2007; Suttner and Dutta, 1986). Indeed, the average QFL ratio values of 57:34:9 are very similar to the calculations of Suttner and Dutta (1986), which suggests accumulation under the influence of a semi-arid to arid climate. Further, an invariant log/log plot between Qp / F + R and Qm + Qp / F + R (Suttner and Dutta, 1986) is also indicative of a semi-arid to arid climate (Table 4; Fig. 16). The ratio of Q / (Q + F) is 0.6 and the ratio of P/F is 0.11 to 0.18, which suggests accumulation under the influence of a temperate, subtropical climate (van de Kamp, 2010). The predominant association of kaolinite with silts and muds and comparatively lower abundance of chlorite, smectite and illite is indicative of severe weathering and temperature (van de Kamp, 2010). Overall, the characteristic clast populations indicate a provenance that consisted of uplifted crystalline basement terranes of granitic to granodioritic composition, as well as low- to high-grade metasedimentary terranes (Ingersoll, 1979). Measured QFL and Q mFLt compositions of the Warchha Sandstone (Fig. 14) indicate a dominant ‘transitional continental’ provenance representative of a source region within a relatively stable shield or uplifted continental block (Dickinson, 1985; Dickinson et al., 1983). Gondwana sediments in

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

103

Fig. 15. Thin section photomicrographs of detrital grains in the Warchha Sandstone indicating their source rock type. a) monocrystalline quartz grains (Qm) of igneous origin. b) quartz grains of plutonic origin having microlites showing typical granitic source. c) coarse-grained, poorly sorted sandstone with angular to sub-angular quartz grains with convex– concave contacts (highlighted). Feldspar is dominantly plagioclase. d) Polycrystalline, stretched, elongated quartz grains (Q p) of metamorphic origin having sutured contacts (highlighted). e) Deformed rock fragments of metamorphic origin. f) Partially fractured quartzite (1) with few quartz grains; sheared quartz (2); microcline (3). All examples in plane polarized light and depicted at the same scale.

the region have long been considered to have been mostly derived from uplifted horst blocks of Precambrian granite and granite–gneiss, with subordinate amounts of metasedimentary rocks (Pascoe, 1959). The Warchha Sandstone of the Salt Range is therefore here considered to have been mainly derived from a stable part of the Indian craton within an interior basin. This, together with the presence of an overall northerly sediment transport direction, indicates the provenance of the study unit to have been a combination of the plutonic and metamorphic complexes that lay to the south and southeast of the Salt Range. The presence of minor amounts of sedimentary rock fragments, including shale, siltstone, sandstone, limestone, dolomite, chert fragments and altered feldspar grains, indicates an additional sedimentary

provenance. QFL and Q mFLt plots for the eastern Salt Range (Saloi and Watli areas) suggest that a recycled orogenic basement may have been uplifted locally prior to and during Permian time in this area, as indicated by the greater abundance of sedimentary clasts in the eastern Salt Range sections. The thin nature or complete absence of the some parts of the Cambrian sedimentary sequence in the central and western Salt Range also supports this interpretation. The recycled trend in the QFL and Q mFLt plots of the Sarin Section (western Salt Range) is also an indicator of possible local uplift or at least of the supply of sediments from more than one source. The clay mineralogy also supports the hypothesis that the provenance was mainly an area of plutonic and metamorphic rocks, with kaolinite likely having originated via the weathering of the granitic rocks in a hot climate (Millot, 1970). Illite and

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Table 3 Summary of the sandstone detritus compositional grain type and the possible source of origin in the Warchha Sandstone. Detrital Average % Description grain volume

Main source

Qm

40%

Plutonic rocks; mainly granitic rocks

Qp O M

2% 9% 2%

P

3%

Mi

2%

C

1%

92% of the total quartz; non-undulatory (unstrained) to slightly undulatory (strained) extinction. Single quartz units contain straight lines of inclusions with rare rutile. 8% of the total quartz, strained or unstrained, two or more quartz units. Dominant feldspar type, angular to sub-rounded, usually untwinned. Locally abundant feldspar type, angular to sub-rounded, commonly shows cross-hatched twinning. Twinned grains often break along composition planes. Locally abundant feldspar type; angular grains, mostly altered, both untwinned and twinned; untwinned is more abundant in fine grains; zoning is also present. More biotite than muscovite; average b 0.2 mm long flakes; larger than adjacent quartz and feldspar grains. Light-grey to dark-grey chert grains are generally better rounded than associated quartz and feldspar grains; composed of microcrystalline quartz grains.

Plutonic and low grade metamorphic rocks Plutonic rocks; mainly granite and gneiss Plutonic rocks; mainly granite and gneiss Low grade metamorphic mainly mica-schist Low-grade metamorphic rocks indicating quartzite, schist, and gneiss; igneous rocks like granite. Sedimentary rocks of Precambrian to Cambrian age.

Q m monocrystalline quartz; Q p polycrystalline quartz; O orthoclase; M microcline; P plagioclase; Mi mica; C chert.

8.1. Source rock The mineral assemblage recorded in the Warchha Sandstone indicates that a granitic or gneissic terrane ultimately contributed much of the straight extinction quartz and the K-feldspar. The phyllite, schist and slate clasts are derived from low-grade metamorphic rocks. Additionally, sub-rounded to rounded clasts of sedimentary rocks were possibly derived from the reworking of texturally mature sedimentary rocks of Cambrian or Precambrian age (Table 3). Heterogeneous weathering of feldspar grains is indicative of active tectonism, moderate relief, rapid erosion and hot climatic conditions, all of which indicates that the most probable geographic locations of the source areas was: 1) the Aravalli System, 2) the Malani Range, 3) an unrecognized but more local source of sedimentary rocks (see below). 8.2. Aravalli System The Precambrian Aravalli Supergroup is exposed in a range of hills in the region around Udaipur in the Rajasthan area of India, more than 600 km to the southeast of the Salt Range (Fig. 17), and is composed of a complex suite of granitic, sedimentary (feldspar-rich sandstone, polymictic conglomerate, orthoquartzite, limestone, dolomite, stromatolitic phosphorite, carbonaceous-rich sandstone) and metamorphic (phyllite, phyllitic slate) rocks (Sisodia and Chauhan, 1998), which themselves rest on an earlier banded gneiss complex (Heron, 1953). There is some controversy over the age of the Aravalli Supergroup. Geochronological studies (Crawford, 1970; Sarkar, 1972) and structural Table 4 Average detrital compositional (QFL) and (Qm Flt) mode from eight sections of the Warchha Sandstone. Quantitative modes are recalculated. Recalculated QFL compositional modes

Bivariant logs/log plot values

Section

Q%

F%

L%

Q m%

F%

Lt %

Q p/F + R

Q p + Q m/F + R

Saloi Watli Kaurli Matan Nilawahan Amb Sarin Sanwans Mean

50 48 59 62 58 51 60 66 57

35 37 34 33 36 38 28 30 34

15 15 7 5 6 11 12 4 9

48 47 58.5 61 57 49 59 65 55.5

36 38 34.3 34 37 39 29 31 35

16 15 7.2 5 6 12 12 4 9.5

2.6 / 50 = 0.052 2.0 / 52 = 0.038 1.8 / 41 = 0.044 1.8 / 38 = 0.047 2.1 / 42 = 0.050 2.4 / 50 = 0.048 2.3 / 39 = 0.058 1.8 / 33 = 0.055 0.049

50 / 50 = 1.00 48 / 52 = 0.92 59 / 41 = 1.44 62 / 38 = 1.63 58 / 42 = 1.38 50 / 50 = 1.00 61 / 39 = 1.56 67 / 33 = 2.03 1.37

Q m monocrystalline quartz; Q p polycrystalline quartz; Q total quartz; F total feldspar; R (Lt) total rock fragments.

and stratigraphic correlations (Roy et al., 1988) indicate an Early Proterozoic age of 2000–2500 Ma. By contrast, biostratigraphic studies of stromatolites (Banerjee, 1971; Chauhan, 1973) indicate an age of 1600–1900 Ma, though a more recent biostratigraphic reinterpretation (Raaben, 1981) suggests an ‘Early Proterozoic’ age. Regardless of their true age, the provenance analysis presented herein demonstrates that rocks of the Aravalli Supergroup formed an uplifted, positive-relief feature by Permian times, during which they were actively shedding detritus to the northwest. 8.3. Malani Range The Malani Suite forms a series of volcanic porphyritic rhyolite lavas, intrusive granites and ash beds that cover an area of 51,000 km2 in the Barmer District of Rajasthan, India and which extend into the Sind Province of Pakistan, more than 500 km to the south of the Salt 10.0

Polycrystalline Quartz / Feldspar + Rock Fragments

chlorite, by contrast, likely originated via the weathering of metamorphic and acid igneous rocks (Weaver and Pollard, 1973). Smectite was generated most likely in response to erosion of a continental area (Deconinck et al., 1991).

1.0

0.1 Humid

Semi - Humid

0.01 Semi - Arid

Arid

0.001

0

1

10

100

Total Quartz / Feldspar + Rock Fragments Fig. 16. Bivariant log/log plot of the ratio of various framework constituents of eight sections of the Warchha Sandstone, Salt Range, Pakistan. Inferences regarding palaeoclimate are indicated by the arrow and values of ratio of polycrystalline quartz to feldspar plus rock fragments against the ratio of total quartz to feldspar plus rock fragments are shown by circles. The trend of the framework composition in the Warchha Sandstone is consistent with a semi-arid to arid climate setting during the deposition of formation (based on Suttner and Dutta, 1986).

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

S

N

Chitral

N

Gigit

Salt Range

Skardu

A

t

y

T

T

e

h

a

e

s

105

S

Pakistan e

Malani Range

ng

I

Ra

0

km

200

Peshawar

Islamabad

A

Ar

N

lli

a av

H

I n d i a

G

Gondwanaland

F

KH

A

K

Sargodha Zira

P A K I S T A N Kp Sulaiman Lobe

International boundary

Jacobabad

Dehli

KLWI BLW1

iR

an Mount Abu

Salt Range

Jodhpur

Malani

Karachi

all

Jaislmer

Settlement

Palaeotransport direction Aravalli Super Group

Tosham

Sanga Nagar

Thrust HFT, MBT Drill holes

Malani-Kirana Group

Dehradun

Ferozepur

Malani-Kirana Basin

Strike line in Axial belt Thrust MCT, MMT

Marwar Super Group Salt Range, Lesser Himalayan, Hazara.

Ambala

Quetta

Jaipur I N D I A

av

Mesozoic in Rajastan

600

Ar

Tertiary deposits

km

ge

0

Nagar Parkar

Fig. 17. Geological maps of Pakistan and northwest India depicting the location of the Salt Range together with the likely source areas for the Warchha Sandstone, the Aravalli and Malani Ranges (modified after Virdi, 1998). Analysis of provenance indicates that these regions of the stable Indian craton formed uplifted and actively eroding continental blocks of igneous and metamorphic basement rocks during Permian times. The Warchha Sandstone represents the preserved remnant of a major river system that drained northwards into the Tethys Ocean. The Aravalli Range was located more than 600 km to the southeast, and the Malani Range was located more than 500 km to the south of the Salt Range region. In addition, clasts of sedimentary origin within the Warchha Sandstone were mainly derived more locally from the Eocambrian to Cambrian succession of the Salt Range.

Range (Fig. 17; Bhushan, 1984; Pascoe, 1959). The rhyolites, which form the bulk of this suite, are mostly reddish-brown, with abundant pink orthoclase and some sanidine, feldspars, oligoclase, quartz and rare hornblende and magnetite. The pinkish, medium- to coarsegrained Malani Granite, which intrudes the rhyolites, is composed predominantly of orthoclase and quartz, with varying amounts of acidic plagioclase, hornblende, biotite and less common occurrences of muscovite. Accessory minerals include tourmaline and fluorite. The Tobra Formation of Early Permian age in the Salt Range contains pebbles and boulders of Malani origin as an important constituent. All these grain types are also common within the Warchha Sandstone and the Malani region is therefore likely to have formed another source area that shed detritus northwards during Permian times. 8.4. Local Sedimentary Source An additional but relatively minor source of sedimentary clasts within the Warchha Sandstone has been the local recycling of underlying Permian, Cambrian and Eocambrian strata in the Salt Range region itself. Extraformational sandstone, limestone and dolomite clasts derived from the Cambrian and Eocambrian formations have all been observed. For example, well rounded pebbles of pink granite of a similar type to but larger in size than those in the

underlying Tobra Formation, are found in lower and middle part of the Warchha Sandstone. The varied detrital grains of the Warchha Sandstone suggest heterogeneous source terranes and transported sediments were mainly supplied from areas to the south and southeast of the Salt Range. 9. Palaeogeographic implications The Salt Range region occupied a palaeogeographic position that formed part of East Gondwanaland at the margin of Tethys during the Permian (Mukhopadhyay et al., 2010; Valdiya, 1997) and was located in a geographic region conducive to the development of semi-arid to arid climate conditions during deposition of the Warchha Sandstone (Teichert, 1967). The Gondwana basin in which the Warchha Sandstone accumulated, along with other formations of the Nilawahan Group, was initiated as a north–northwest to south–southeast trending, elongated depression in Precambrian times (Sengupta, 1970). The basement rocks beneath the Salt Range are composed of Precambrian metamorphic and plutonic constituents of the northern extension of the Indian Shield. In the Salt Range, the Eocambrian Salt Range Formation directly overlies crystalline basement rocks and is followed by a shallow marine sequence of Cambrian age known as the Jehlum Group. After the Cambrian sequence there was a pronounced interruption in basin filling

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in the Tethyan realm in the northern margin of the Indian Peninsula (cf. Gee, 1989; Mukhopadhyay et al., 2010; Valdiya, 1997). The entire of Gondwanian India was uplifted at this time in response to the PanAfrican orogeny in Gondwanaland (Valdiya, 1997). Marine depositional conditions eventually returned to the greater Salt Range region in Early Permian times along a narrow elongated depression that was formed due to subsidence (possibly rift related) of the crust along what is today the Outer (Southern) Lesser Himalaya (Valdiya, 1980a,b, 1989, 1997; see also Garzanti and Sciunnach, 1997 and references therein). This basinal feature stretched from the Salt Range in northern Pakistan through Pir-Panjal in Kashmir to Central India (Valdiya, 1997). As this basin developed it commenced filling with Gondwanian sediments of the Nilawahan Group, initially with glacio-fluvial deposits of the Tobra Formation (the Talchir) in a cold climate (cf. Ghazi, 2009; Singh, 1987; Valdiya, 1997). At this time, glaciers covered the Aravalli Range at least 600 km to the south and these extended northward to the Salt Range (Fig. 18). The glaciers retreated as the climate ameliorated and sea levels rose, resulting in transgression across the basin, as recorded by the Dandot Formation (cf. Singh, 1987; Veevers and Tewari, 1995). The basement rocks were uplifted in the east of the Salt Range and a marked relative sea-level fall occurred, eventually culminating in sub-aerial exposure of the entire Salt Range area. By Early Permian times, the basin was occupied by a major Gondwanian fluvial system that flowed to the north or north–northwest (cf. Sengupta, 1970; Valdiya, 1997). Mineralogical analyses and the north or north–northwest-directed palaeoflow inferred from palaeocurrent analysis (Ghazi and Mountney, 2009) confirm that the detritus carried by this fluvial system was dominantly derived from granitic and low grade metamorphic sources located south–southeast of the Salt Range (the Aravalli System and the Malani Range). Seasonally heavy rainfall in a relatively hot and humid climate to the south of the Salt Range helped to establish this major river system, which discharged into a marine embayment that lay to the north–northwest of the Salt Range at the north margin of Gondwana. An additional, more localised, source of detritus could have been from the erosion and denudation of Neotethyan rift shoulders or related rejuvenated basement uplifts that were actively developing during Early Permian times in areas adjacent to the Salt Range region. The meandering Warchha River repeatedly shifted its position with time in its floodplain. Each time it abandoned a meandering channel reach, a sinuous strip of coarse-grained channel-fill deposits became cut off from the main channel and these were gradually buried under slowly accumulating, very fine-grained floodplain deposits (Ghazi, 2009; cf. Sengupta, 1970). These in turn were cut by subsequent younger channels as they migrated or avulsed across the fluvial plain. The resultant pattern of cyclic sedimentation records the repeated avulsion and cut-and-fill channel sedimentation events during accumulation of the Warchha Sandstone (Ghazi and Mountney, 2009; cf. Shukla et al., 2006). The Precambrian, north–south trending uplands of the Aravalli Range mainly formed a drainage divide between the western and eastern Indian Peninsula (Ghazi, 2009; Mehdiratta, 1954). The Salt Range being part of western Indian Peninsula had a north–northwest-directed drainage system during the Early Permian, and the Warchha River flowed from the Aravalli Range northwards to the Tethys Sea (Fig. 18), marine deposits of which are now exposed in the foothills of the Himalayas (Gansser, 1981; Valdiya, 1997). The maintenance of the north–westerly-dipping palaeoslope during the sedimentation of the Warchha Sandstone was probably influenced by the continued downwarp in the basinal area of the Salt Range. 10. Discussion Detrital grain compositions of the Warchha Sandstone were influenced by several factors including tectonic setting and composition of the source rock, weathering processes, climate during transportation to the depositional basin, and diagenesis (cf. Korsch,

1984). Data collected for this study are here used to infer the nature of the depositional basin and climatic and tectonic character of the source area, or at least to place constraints on the tectonic setting of ancient orogens (Dickinson and Suczek, 1979; Ingersoll and Suczek, 1979). Vertical profiles through the Warchha Sandstone succession demonstrate a regular alteration between coarse- and fine-grained units on a scale of metres to tens of metres. Coarse-grained units are composed of conglomerates and sandstones, which are generally moderately to poorly sorted and texturally immature. The presence of a wide range of sedimentary structures, indicative of unidirectional traction currents of fluctuating strength implies that sediment transport within the channels took place via the accumulation of a series of downstream migrating bedforms and laterally accreting point bars (Ghazi and Mountney, 2009). Floodplain sediments indicative of deposition via overbank flooding dominate the finegrained units. Thin siltstone and sandstone beds within the otherwise fine-grained units were likely deposited via crevasse-splay events due to the localised breaching of channel banks during flood episodes (Ghazi and Mountney, 2009). The textural elements of channel and point bar elements (see Ghazi and Mountney, 2009) consist largely of grains with minor amounts of silt- and clay-size material as matrix. Although the clay component makes up 15 to 36% of the rock, it occurs mainly as matrix and only rarely as grains either in the form of pellets of clay, which enclose minor amounts of silt-size quartz, or as clay worn material from those grains. All the channel and bar elements contain a significant percentage of kaolin. The presence of hard kaolin grains may be detrital or intraformational in origin. Of these two possibilities, a detrital origin is the most plausible. It seems likely that the kaolin was eroded and transported as pellets in the bed load of the river rather than being derived from reworked clay sized floodplain material. Kaolin present in the floodplain sediments was also likely derived from attrition of the kaolin pellets. The integrated mineralogical, petrological and field data suggest that the streams that carried the detritus from the source areas descended onto a broad, low relief fluvial plain. The sluggish, generally northward flowing, streams could not continue to transport their load. The coarsest material was consequently deposited in the river channels and structures therein indicate deposition in a lowflow regime (Ghazi and Mountney, 2009). Deposition of somewhat finer sands occurred as bars in the meandering setting. The sandstones were deposited both in-channel and on bars and deposits are characterized by differing grain size, mineralogical composition and sedimentary structures depending on their setting within the fluvial channel system. When floodwater overflowed the channel banks, fine silt and clay carried in suspension was deposited. The angular blocks of floodplain clay embedded in the channel units records the undercutting of the floodplain deposits at the eroding outer banks of laterally accreting channels (Bridge, 2006). Petrographic study demonstrates that weathering played a major role in determining the detrital grain composition of the Warchha Sandstone. The weathering of individual quartz and feldspar grains commenced at grain boundaries and proceeded along fracture planes and other planes of weakness (Figs. 7d; 9c), forming cloudy or amorphous material that eventually altered to secondary minerals, predominantly kaolinite. Plagioclase seems to have been more readily altered than K-feldspar by weathering processes. The majority of the kaolinite clay was likely derived by the chemical weathering of the older basement rocks at the site of weathering (cf. Iniguez et al., 1989; Manassero et al., 1991; Potter et al., 1980) in the Aravalli and Malani ranges (Fig. 17). This weathering seems to have involved very intense kaolinization at high temperature and in the presence of heavy rainfall (cf. Nakagawa et al., 2006). Vegetation in the source area might have provided significant quantities of organic acids, which could have been responsible for intensifying the chemical weathering process

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

a Early Permian (Asselian) 0

km

b Early Permian (Sakmarian) 0

500

km

N

s

N

s

Se

hy

t Te

Pr Salt esentRan day ge a rea

hy

t Te

500 a

a

Se

107

Pr Salt esentRan day ge a rea

Transgression direction

Tobra Formation do Dan

ra

Aravalli Range

d Early Permian (Kungurian) 0

500 a

Se

Pr Salt esentRan day ge a rea

km

500

N

h

t Te

rm

Fo

Malani Range

c Early Permian (Artinskian)

ys

n

b To

Aravalli Range Malani Range

km

at

t Fo

Sediment transport direction

0

io

n

tio rma

Sediment transport direction

N ys

a Se

th Te

Pr Salt esentRan day ge a rea

Transgression direction

n

io

at

Warchha Sandstone

m or F i

ha

rd Sa

Malani Range

Aravalli Range

Malani Range

Aravalli Range

Fig. 18. Tectono-stratigraphic evolution of the present-day Salt Range area through the Early Permian. a) Asselian. b) Sakmarian. c) Artinskian. d) Kungurian. These maps illustrate the change in palaeoenvironment from fluvioglacial (Tobra Formation), to shallow to marginal marine (Dandot Formation), to meandering fluvial (Warchha Sandstone), and to shallow marine (Sardhai Formation).

(Nakagawa et al., 2006). These weathered clay-rich materials were transported from the source area by streams and rivers in which numerous ponds and areas of slack water developed on the interfluves and such areas acted as sediment traps for kaolinite clay, along with minor amounts of silt and fine sand. Clay associations show coherent links with the sandstone petrofacies, which themselves indicate the same source.

Ongoing subsidence of the basinal area in the Salt Range region allowed continued sedimentation of the Warchha Sandstone and the accumulation of a series of stacked fluvial cycles. Sedimentation of the Warchha Sandstone was terminated in response to a regional marine transgression across the area in which fluvial sedimentation was replaced by shallow marine sedimentation represented by the overlying Sardhai Formation.

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The Warchha Sandstone is of varied composition (Q:F:R = 57:34:9), which is consistent with its accumulation in a warm, arid climate. A log/log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz total feldspar plus rock fragments also supports the interpretation of a semi-arid to arid climate at the site of accumulation. Further, the abundance of kaolinite, together with the presence of haematite and calcite cement, thick clays, desiccation cracks, rain prints and caliche within the fine-grained units also supports the interpretation of a relatively arid climate at the time of the deposition of the Warchha Sandstone. Provenance analysis indicates that sediments were mainly derived from cratonic interiors and transitional continental blocks of plutonic origin. Igneous and, to a lesser extent, metamorphic sources provided most of the constituent grains for these sandstones. Based on analysis of composition and textural maturity, the Warchha Sandstone is composed predominately of detritus that was derived from coarsegrained, crystalline parent rocks and transported relatively long distances before accumulating in a fluvial plain setting. None of the sandstones were buried deeply enough, nor exposed to sufficiently intense thermal activity, to experience significant late-stage diagenetic destruction of framework minerals. A lack of significant variation in the framework composition indicates the same sediment source throughout deposition of the succession. The presence of kaolinite in almost all the samples is indicative of intense chemical weathering of the source rock under the influence of a hot and probably humid climate. A granitic or gneissic terrane ultimately contributed much of the straight extinction quartz and the K-feldspar grains and such rocks were probably the main sources. The source areas were characterised by regions of uplift subject to active chemical and mechanical weathering under the influence of a hot and humid climate, in marked contrast to the area of ultimate deposition in what is now the Salt Range which was significantly more arid. The most probable geographic locations of the source areas were: 1) the Aravalli Range to the south–southeast, 2) the Malani Range to the south and 3) Precambrian to Cambrian age sedimentary rocks. Of these, the Aravalli Range probably supplied the majority of the detritus.

The climate of both the source and depositional area influenced the detrital composition of the Warchha Sandstone (Fig. 19). The formation of the kaolinite through the alteration of feldspar in the source area indicates a warm and humid climate developed under tropical and subtropical conditions (van de Kamp, 2010; Fig. 19). Poorly crystalline, possibly reworked kaolinite (Fig. 9) was generated in this acidic, reducing environment. By contrast, the dominance of monocrystalline quartz and the varied composition (Q:F:R = 57:34:9) support the interpretation of a warm, semi-arid climate during the deposition of the Warchha Sandstone (Suttner and Dutta, 1986), as does the red colour of the formation (Turner, 1980) and the occurrence of abundant sedimentary structures such as desiccation cracks, rain pits and caliche nodules. Calcite and haematite cementation occurred within the vadose zone of this arid environment (Bjørlykke, 1983). 11. Conclusions Petrographically, the Warchha Sandstone is mainly a sub-arkose (5%) to arkose (95%). Kaolinite is the dominant, widespread clay; it constitutes 60% of the total clay volumes and occurs in both allogenic and authigenic forms. Minor compaction and alteration of feldspar by dissolution and replacement with kaolinite and haematite are the important diagenetic changes that occurred in the Warchha Sandstone during shallow burial. Authigenic minerals in the Warchha Sandstone were formed either by direct precipitation from formation water or through reactions between precursor materials and contained waters. Based primarily on fabric relationship, the sequence of cement was clay (mostly kaolinite), iron oxide, calcareous and siliceous, before iron-rich illite and occasional mixed layer smectite–illite and rare chlorite. Facies analysis and petrographic data indicate that the Warchha Sandstone accumulated in a meandering fluvial plain setting, in a palaeogeographic position close to the Tethys Ocean margin. Vertical profiles reveal a regular alteration between coarse- and fine-grained units on a scale of metres to a few tens of metres. This is indicative of repeated channel avulsion and lateral accretion over a broad, low relief floodplain.

Warchha Sandstone Mainly composed of arkosic to sub-arkosic sandstone (high % age of quartz) QUARTZ ARENITE

ite n re b Su

-lit

ha

e os

% 60 40

%

50

%

1.5

20

%

Depositional site northeast to northwest of the Aravalli and Malani ranges

LITHIC ARENITE

ARKOSIC ARENITE

)] +L /(F 0.25 [Q ex ind .67 ity 1 0

tur

Ma

Su

80

4

bar k

%

95

%

Q

Aravalli and Malani ranges mainly composed of granite - gneiss of basement uplift and low grade metamorphic rocks (high % of feldspar)

Salt Range 0

F0

0

(semi-arid to arid climatic conditions throughout the deposition of the Early Permian Warchha Sandstone)

25%

50%

75%

100%

L

Sediment composition source south to southeast of the Salt Range

Transport distance (~ 600 km)

Source area (hot and humid climate with severe chemical weathering conditions)

Fig. 19. Schematic model to explain the change in composition between the source areas (Aravalli and Malani ranges) and the site of deposition (Salt Range) for the Warchha Sandstone, modified after Diekmann and Wopfner (1996).

S. Ghazi, N.P. Mountney / Sedimentary Geology 233 (2011) 88–110

Acknowledgements This research was supported by a scholarship to SG from the University of Punjab and by an award from the Steve Farrell Fund of the British Sedimentological Research Group. We are grateful to Gilbert Kelling (Keele University, UK) for providing valuable discussions regarding the interpretation of parts of this sedimentary succession. This manuscript has been much improved by the valuable comments of reviewers Salvatore Critelli and Eduardo Garzanti, and by editor Gert Jan Weltje, to whom we are grateful.

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