Rock magnetic characterization of pedogenesis under high energy depositional conditions: A case study from the Mio–Pliocene Siwalik fluvial sequence near Dehra Dun, NW Himalaya, India

Sedimentary Geology 177 (2005) 229 – 252 www.elsevier.com/locate/sedgeo

Rock magnetic characterization of pedogenesis under high energy depositional conditions: A case study from the Mio–Pliocene Siwalik fluvial sequence near Dehra Dun, NW Himalaya, India V. Kumaravel, S.J. SangodeT, N. Siva Siddaiah, Rohtash Kumar Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehra Dun-248 001, India Received 15 January 2004; received in revised form 3 January 2005; accepted 11 March 2005

Abstract Systematic rock magnetic studies accompanied by field investigations are presented for 51 pedogenic horizons from magnetostratigraphically constrained Middle–Upper Siwalik fluvial sequence (~ 9–4 Ma) near Dehra Dun in the Himalayan Foreland Basin (HFB). Detailed sedimentologic studies infer predominantly proximal to distal alluvial fan setting with considerably high rate of sedimentation (N 55 cm/ka). Paleosol profiles in the Mohand Rao (MR) section are commonly truncated by channel activity. These paleosols display few rootlets, diffused color contrast amongst variegated pedogenic horizons, rare concretions, intense gleying at places, and frequent incursions by sand/silt layers (b 30 cm) characterizing their vicinity to channel. The mean mass specific magnetic susceptibility (v lf) for pedogenic horizons is ~ 0.95  10 8 m3/kg (Max = 1.4  10 8 m3/ kg; Min = 0.5  10 8 m3/kg) and the associated non-pedogenic layers is ~ 0.82  10 8 m3/kg (Max = 1.210 8 m3/kg; Min = 0.4  10 8 m3/kg). The low frequency dependence of the susceptibility (v fd mean = 2%) and high coercivities (B OCR mean = 425 mT) of the pedogenic layers infer the absence of finer pedogenic superparamagnetic (SP) fraction that might have depleted during post depositional burial processes. Acquisition of isothermal remanent magnetization (IRM) shows undersaturation and gentle upward trend even at high applied fields (4000 mT) advocating the predominance of hematite and goethite. Sympathetic variation in the total organic and inorganic carbon content with iron oxides stable under warm humid, oxidative arid and cold restricted conditions confirm the sensitivity of the rock magnetic parameters to ambient climatic condition. Relative variations in goethite, hematite and ferrimagnetic contents have been derived from ratios mainly based upon selective IRM saturations and their coercivities. These studies are validated by sandstone petrography and clay mineralogy for the same section. However, the variation in rock magnetic parameters shows a better amplification and allows relative quantification of the ambient climatic conditions. The assemblage of iron oxides using the rock magnetic ratios of the pedogenic layers in these high energy conditions broadly infer the prevalence of semi-humid to semi-arid climate entirely during the Late Miocene to Early Pliocene period in the

T Corresponding author. Fax: +91 135 2625212. E-mail address: [email protected] (S.J. Sangode). 0037-0738/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.sedgeo.2005.03.006

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HFB with an increased seasonality during early Pliocene. The present approach envisages a great scope for quantitative database under the given setup of high energy conditions that are common in the Mio–Pliocene HFB. D 2005 Published by Elsevier B.V. Keywords: Himalaya; Siwalik group; Paleosol; Rock magnetism; Semi-humid; Semi-arid

1. Introduction The Siwalik fluvial sequence of the HFB represents continuous alternation of the channel sedimentation and flood plain aggradations (Parkash et al., 1980; Tandon, 1991; Willis, 1993; Willis and Berhensmeyer, 1995). HFB is therefore an ideal area to study the interrelation of sedimentation to accretionary processes and their response to climate and tectonism. The channel sedimentation has been traditionally studied in great details in this basin (Kumar, 1993; Willis, 1993; Burbank, 1996). However, the accretionary processes especially pedogenesis that has greatly been influenced by the dynamic channel activity have so far been given little attention due to its complexity in providing ideal climatic records. Frequent interruption of soil forming processes due to channel activity and the post depositional/burial modifications disallow a classical approach of describing the pedogenesis in the Mio–Pliocene Siwalik basin and thus demands a robust yet sensitive approach like rock magnetism. The rock magnetic/ mineral magnetic studies offer quantitative and qualitative estimates of pedogenic changes reflected by iron oxide alterations and precipitations, and proved successful under various depositional conditions (Thompson and Oldfield, 1986; Maher, 1986, 1998; Maher and Thompson, 1999; Retallack, et al., 2003; Evans and Heller, 2003). Paleosols are the soils developed over ancient geologic landforms under the contemporary ecosystem (Kraus, 1999; Wright, 1992). Over alluvial plains in the HFB they are mainly developed during geologic diastems, unconformities and during nondepositional periods when the country rock/bed rock is exposed to the atmosphere for a significant period (Retallack, 1986; Kraus, 1992; Wright, 1992). As a profile is accreted, the parent rock is weathered downwards and there is an upward shift of the soil– atmosphere interface (Wright, 1992; Tardy and

Roquin, 1992). Pedogenesis initiates the upward and downward elemental transfer with re-seggregation of mobile and immobile elements. These processes are controlled by the immediate atmospheric conditions, water table and the type of vegetation (Retallack, 1988). Many studies have been undertaken on paleosol sequences of continental depositional settings (e.g. Retallack, 1988, 1995; Arundorff, 1993; Wright and Platt, 1995; Soreghan et al., 1997; Maher, 1998; Kraus, 1999; Nettleton et al., 2000). These studies describe the significance of paleosols to provide holistic information on ancient climate, vegetation and stability of landforms (Retallack, 1988; Wright, 1992; Kraus, 1999). Iron oxides quickly respond to the ambient climatic conditions and provide the most visible and first hand information on the characterization of soils and paleosols (Kampf and Schwertmann, 1983). Rock magnetism in the recent years has greatly evolved into accurate quantification and grain-size determination without disturbing the natural conditions of the sample (Thompson and Oldfield, 1986; Maher and Thompson, 1999; Evans and Heller, 2003 and references therein). However, the detailed rock magnetic approaches are based upon initial site specific approaches to derive universality of inferences (Maher and Thompson, 1999; Evans and Heller, 2003). And such studies are not available for high energy and dynamic fluvial condition like that observed in the HFB. Hence, we selected an ideal high energy depositional condition during Mio–Pliocene time documented from the Mohand Rao (MR) section near Dehra Dun in the HFB.

2. Study area Dehra Dun is located in the west-central sector of the HFB. The studied section falls on the southern flank of the broad Dehra Dun syncline (Fig. 1).

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Fig. 1. (A) Geological map of the study area showing the location of Mohand Rao section (Valdiya, 1980). The study area is delimited by Yamuna and Ganges tear faults on the west and east, respectively. (B) Map showing distribution of the Siwalik Group sediments in the study area (after Karunakaran and Rao, 1976).

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Sediments of the Middle and Upper Siwalik subgroups are exposed in the Mohand Rao (MR) section. This Mio–Pliocene Middle Siwalik fluviatile succession is characterized by multistorey grey sandstone and grey to brown mudstone intervals incorporating the pedogenic mudstones and profiles. We divide the section into lower 1800 m sandstone mudstone dominant part and the upper 1500 m conglomerate dominating part. Due to monotonous occurrence of conglomerates after 1800 m, the magnetostratigraphic ages are restricted to the lower 1800 m part of the MR section that represents an age in between 10 to 5 Ma on magnetic polarity time scale published by Sangode et al. (1999) and Kumar et al. (2004).

Detailed sedimentologic attempts in the MR section infer a predominance of fine to coarse grained grey multistorey sandstones representing braided streams with frequent avulsion and sheet flood events in a sandy alluvial fan regime (Kumar, 1993; Kumar and Ghosh, 1994; Kumar et al., 2004). The entire sequence represents relative variation of distal to proximal fan setting in an overall proximal setup (Kumar, 1993; Kumar and Ghosh, 1994; Kumar et al., 2004). Reconstruction of magnetic polarity stratigraphy by Sangode et al. (1999) infer an age of 9.73–4.86 Ma for the studied section (Fig. 2) with a prolonged high rate of sedimentation (an average of N 55 cm/ka) in relation to other sections studied by them in the adjoining area (Sangode and Kumar,

Fig. 2. Details of lithofacies association and Magnetostratigraphic ages for the Mohand Rao section (after Sangode et al., 1999). Arrows on the right side of the litho-column represents the paleoflow directions. Lithology, depositional environments and paleoflow for the studied section are summarized.

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2004). Paleosols in the MR section are characterized by weak mottling, rare and finer root traces and sporadic concretions/calcretes. The profiles are commonly truncated by channels with a sharp contact. These paleosols are thus developed in the vicinity of channels in a relatively high energy sedimentation regime with dynamic channel activities. 2.1. Rock magnetic attempts in soils and paleosols Rock magnetic methods constitute measurement of the response of samples to laboratory induced magnetic fields of known directions and intensity (Thompson and Oldfield, 1986; Dunlop and Ozdemir, 1997). The rock magnetic techniques are being widely used in sediments (marine and lacustrine), windblown dust (loess), soils and paleosols as climate proxy, paleorainfall proxy, pollution monitoring tool etc. (Verosub and Roberts, 1995; Maher and Thompson, 1992; Matzka and Maher, 1999; Evans and Heller, 2001, 2003; Maher et al., 2002, 2003). Iron oxide minerals in the soils/paleosols are of major interest in rock magnetic study (Table 1), as they quickly respond to the pedogenic processes governed by ambient climates (Maher, 1986, 1998; Sangode and Bloemendal, 2004). Fine et al. (1993) and Verosub et al. (1993) established direct relationship between magnetic susceptibility and pedogenesis. During pedogenesis, fine grains of ferrimagnetic minerals (SP magnetite) are produced through microbial activity in well drained soils (Fine et al., 1993; Verosub et al., 1993; Retallack et al., 2003) and thus the pedogenic horizons shows appreciable enhancement of magnetic susceptibilities (Zheng et al., 1991; Banerjee et al., 1993; Maher, 1998). Retallack et al. (2003) differ-

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entiated gleyed (hydric) soils from ungleyed soils by the variation of magnetic susceptibility with iron content, inferring the well drained (ungleyed) soils to have high magnetic susceptibility and water-logged regions or soils affected by groundwater gleization to show low susceptibilities. Maher (1998) reviewed the relationship between soil magnetic properties and ambient climatic conditions. She summarize that the oxidative conditions favor the formation of non-ferrimagnets (hematite) and the intermittent reductive conditions favor the formation of ferrimagnets (magnetite). Maher and Thompson (1991, 1992, 1999) and Evans and Heller (1994) studied the mineral magnetic records of the loess–paleosol sequence and their paleoclimatic significance across loess plateau of China. Maher et al. (2002, 2003) recognized correlation between rainfall and pedogenesis and derived paleorainfall quantifications using soil magnetic properties across Russian steppe. Sangode et al. (2001) and Sangode and Bloemendal (2004) successfully used the rock magnetic methods in the Plio–Pleistocene Siwalik paleosols inferring the pedogenic transformation of magnetic minerals.

3. Field and laboratory methods Each profile was trenched by removing ~ 60 cm of the exposed surface. Color notations were made by Munsell chart in field as well moist colors in laboratory (Munsell color, 2000). Calcareousness (Retallack, 1988) was observed in field using 1 N HCL and the organic and inorganic carbon contents were determined by the method of loss on ignition. About 1000 g of fresh samples was collected from

Table 1 Magnetic minerals in soils and their magnetic status and environmental association (after Maher, 1986) Minerals

Formula

Magnetic status

Colour

Environmental conditions

Magnetite

Fe3O4

Ferrimagnetic

Greyish black

Maghemite Hematite

gFe2O3 aFe2O3

Ferrimagnetic Canted antiferromagnetic

Dark brown Red

Goethite Lepidocrocite Ferrihydrite

aFeOOH gFeOOH 5Fe2O3.9H2O

Canted antiferromagnetic Paramagnetic Paramagnetic

Yellow Reddish-yellow Dark red

Restricted occurrence, primarily preserved in less oxidizing conditions Abundant in highly weathered tropical/sub-tropical soils Relatively dry, highly oxidized soils, usually in areas of elevated temperature Moist soils, abundant in well drained temperate regions Occurs in poorly drained soils Poorly drained and podzolised soils

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each pedogenic level, parent material/C-horizon and corresponding bed rock/R-horizons. Rock magnetic samples were prepared in standard non-magnetic (styrene) cylindrical pots (hard samples are fragmented using agate mortar without affecting

the grain size). Magnetic susceptibility was measured in six orthogonal directions on Bartington MS2B sensor (sensitivity 2  10 6 SI) for low and high frequency (0.465 and 4.65 kHz). An anhysteretic remanent magnetization (ARM) was imparted with a

Table 2 Summary of the rock magnetic and geochemical parameters studied Parameters k and v

Units and Derivation k = 10 v = 10

11 8

3

m; m 3/kg

v fd

10 8 m3/kg; v fd = (v lf v hf) / v lf

B OCR

mT

ARM

10

5

m3/kg

v ARM = ARM/Bias Field v ARM / v lf

–

SIRM

10

SIRM/mlf

103 A/m

HIRM

10 5 Am2/kg (IRM1T + IRM 300mT) / 2 (IRM 300mT / SIRM2.5T)

S-ratio

G (Goethite) H (Hematite) G /H (dhydration indexT) IRMsoft

5

Am2/kg

10 5Am2/kg (IRM2T IRM1T) 10 5Am2/kg (IRM 1T IRM 0.5T) – 10 5Am2/kg (IRM1T IRM (IRM1T + IRM

H / IRMsoft(doxidation indexT) v ARM / H

–

TOC% and CaCO3%

%

Explanation /description Magnetic susceptibility: The ratio of the induced magnetization to applied magnetic field and is indicative of bulk magnetic mineral content in a sample. This is measured in a reversible small magnetic field of the order of 0.1 mT. Measurement on a volume (k) or mass specific basis (v). Susceptibility is also sensitive to change in grain size. Frequency dependence of susceptibility: The variation of susceptibility between two frequencies. Large value of this parameter indicates presence of ferrimagnetic grains lying at the stable single domain/superparamagnetic boundary (~0.02 Am) when measured at room temperature. Remanence coercivity: the reverse field strength required to return a magnetized sample its saturation isothermal remanence to zero. Anhysteretic remanence magnetization: is generally imparted by subjecting a sample to a strong alternating field decaying to zero in the presence of small steady field and it is useful for characterizing magnetic particle. Susceptibility of Anhysteretic remanence (v ARM) is the ratio of ARM to bias field that varies with the quantity of SD grains. v ARM is particularly sensitive to the presence of small grains (single domain and pseudo-single domain(PSD)) whereas v lf is relatively more sensitive to the presence of larger grains (PSD-MD). Hence, v ARM / v lf variation may be used as indicators of grain size variation from SD to MD. SIRM represents the saturation of isothermal remanence. It indicates volume concentration of magnetic minerals in a sample and varies with magnetic mineralogy. The SIRM is the indicator of magnetic mineral concentration and v lf represents the bulk magnetic mineral content. It is also sensitive to grain size, hence the ratio of saturation remanence to susceptibility can be used as a rough estimate of magnetic mineralogy and grain sizes. HIRM varies with the high coercivity (canted antiferromagnetic) component in magnetic mineral assemblage. S-ratio is the ratio obtained by using IRM 300mT and SIRM and indicates the relative proportion of antiferromagnetic to ferrimagnetic minerals. It is negative for low coercivity minerals and positive for high coercivity minerals. G indicates the presence of high coercivity mineral. Especially the larger values varies with goethite. H is sensitive to presence of canted antiferromagnetic mineral hematite and is less sensitive to goethite. The ratio G / H represents the relative variation in goethite over hematite and increases as a function of hydration. IRMsoft can be used for approximating the concentration of MD ferrimagnetic grains.

20mT) / 2 300mT) / 2

The ratio of H / IRMsoft is indicative of relative oxidative conditions. Since v ARM varies with the SD concentration that can be preserved under reducing/ alkaline conditions in soil forming environment. The ratio v ARM / H therefore varies with these conditions. TOC% and CaCO3% are the approximation percentage of total organic carbon and inorganic carbon, respectively derived by the loss on ignition method at 550 and 950 8C.

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constant biasing field of 0.1 mT superimposed over a decaying alternating field from a peak field of 100 mT using Molspin alternating field demagnetizer. IRM was imparted at the intervals of 20/50/100/200 mT up to 2500 mT and back field of 1000 mT on ASC Scientific Impulse Magnetizer (IM-10-30) and the remanence was measured using Minispin Rock Magnetometer of Molspin (sensitivity 10 9Am2). For some high coercivity samples, a forward IRM was measured up to 4000 mT with a backfield of 1000 mT. We used multiple rock magnetic parameters and their ratios based on IRM (Forward and backward fields), magnetic susceptibility (and its frequency dependency) and ARM susceptibility (Summarized in Table 2). We encountered total 9 sites of paleosol occurrences at different stratigraphic levels in the Mohand Rao section (Fig. 3). Although we followed Retallack

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(1988) for describing the individual pedogenic levels, we adopted a simplistic terminology to denote B1, B2, B3 and etc. as subsequent pedogenic horizons after C-horizon for convenience of interpretation of the rock magnetic parameters with incrementally higher pedogenic levels. The dAT, dOT or dET horizons are absent for majority of the profiles due to erosion/ truncation by overlying channel sandstones, that shows sharp contact. At each paleosol site, the systematic sampling includes the pedogenic horizons, incorporated parental material, underlying unaltered sandstone, siltstone and non-pedogenic mudstones in the sequence. This accounts to a total of 51 pedogenic horizons (excluding C-horizon), 11 sandstones, 17 siltstones and non-pedogenic mudstones from the nine sites. Field and laboratory observations and variations in rock magnetic properties for each site is described below (see Figs. 4, 5 and 6 for the

Fig. 3. Litholog showing the occurrences of paleosols profiles in the Mohand Rao section with corresponding stratigraphic levels and Magnetostratigraphic ages.

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Fig. 4. Detailed litholog showing pedogenic features of each paleosol horizons (alpha numeric in the parenthesis indicates the Munsell soil color notations).

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Fig. 5. IRM acquisition and demagnetization curves for some representative samples.

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profile logs and respective variation of rock magnetic parameters).

4. Results The extremely high coercivities of the studied paleosols of the MR section indicate an overall predominance of antiferromagnetic minerals—Goethite ( G) and Hematite (H). These minerals are stable under the climatic conditions of moist temperate regions and dry and highly oxidized soils of the elevated temperature regions (Table 1). The ratio of goethite to hematite has been widely used by soil scientists to infer cold, humid and warm climatic phases as a result of chemical processes controlled

by soil Eh–pH, temperature and humidity (see Kampf and Schwertmann, 1983; Tardy and Roquin, 1992 and references therein). Kampf and Schwertmann (1983) have also shown that the ratio of hematite to goethite increases as a function of mean annual air temperature in Brazilian oxisols. Schwertmann (1988) proposed a line in the northern hemisphere at 40 8N latitude to separate the soils without hematite (northwards) from the soils with hematite and goethite (southwards). In monsoonal tropical soils, the hydrated–dehydrated conditions are quite common at seasonal scale and the intensity varies on a millennial scale. Therefore its vertical variation may be obtained by deducing the relative abundance of the two minerals. Hence, we attempt the G / H ratio (hydration index) based upon IRM acquisition

Fig. 6. Paleosol profiles in the Mohand Rao section showing the variation in the Rock Magnetic and geochemical parameters. Shaded segments represents the pedogenic levels (see Table 2 for description of Rock Magnetic and geochemical parameters and their units).

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Fig. 6 (continued).

profile (adopted from Sangode and Bloemendal, 2004) to provide the variability from semi humid to semi arid climatic conditions. 4.1. Macromorphology and rock magnetic description of the paleosols 4.1.1. Site: MP-0 Multiple paleosols are exposed at MP-0 (See Fig. 3) over fining upward grey sandstone. The entire sequence is ~ 8.1 m thick sharply cut by a thick multistorey sand body on top (Figs. 3 and 4). Variegated paleosols of dark brown, reddish brown, yellowish brown to yellow are present in this interval sandwiched by sand and silt horizon. Values of v lf and B OCR are relatively low and uniform for the parental material compared to that of large fluctuations in respective pedogenic horizons (Table 3). S-ratio is negative for the bed rock (b 0.1) and shows positive values in the pedogenic horizons (N0.19; Table 4) indicating ferrimagnetic oxides in the

bed rock altered/oxidized to antiferromagnetic minerals during pedogenesis. The v lf shows higher significant positive correlation (U N 0.93) with parameter sensitive to antiferromagnetic content (HIRM, G and H; Table 4). The HIRM shows significant positive correlation with G and H (N 0.93) and negative correlation with IRMsoft ( 0.94). The B OCR increases towards higher horizons (Figs. 5 and 6) with significant positive correlation to the antiferromagnetic components particularly G- and S-ratio, but more negative correlation with IRMsoft (Table 4). dGT also shows significant positive (0.94) correlation with TOC% (Table 4). The lower pedogenic horizons display spherical calcareous concretions with scanty yellowish brown mottling, whereas the upper pedogenic levels characterize Fe–Mn nodules, yellowish mottles and abundance of root traces (Fig. 4). Thus the rock magnetic parameters and the macromorphology observations infer that these paleosols developed over ferrimagnetically rich substrate initially transformed into ferric oxides (hematite) in the lower

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Fig. 6 (continued).

pedogenic levels and then to hydrous ferric oxides (goethite) in higher pedogenic levels (Fig. 7). These conditions thus favor interplay of the semi-arid to semi-humid conditions. The G / H (Fig. 6) ratio enables to assess the variation in humid to arid conditions in an entire sequence at MP-0. The v fd% values are higher in non-pedogenic layers within the profiles as compared to the lower values in the pedogenic layers (Fig. 6). As observed in recent analogues, the SP fraction is characteristic of soil forming processes (Dearing et al., 1996; Maher, 1998). However the inverse relation in the present case of the paleosols which are quite older (N 8 Ma) and also bears a cumulative overburden of N3 km suggest that the SP fraction is removed/recrystallized during early stages of lithification/diagenesis in the paleosols. On the other hand, its preservation in the sandstone is probably due to dlock inT because of

characteristic early cementation and compaction of these sandstones (Ghosh and Kumar, 2000). 4.1.2. Site-MP-1 The paleosol profile has a thickness of 4.5 m (Fig. 3) and is developed over a 5 m thick friable pale olive (5Y6/4) sandstone bed rock (MP1/1). Paleosols in this profile shows few brown and many yellowish mottles, Fe–Mn nodules and rootlets at the higher horizons (Fig. 4). Paleosol color varies from brown (7.5YR4/3) to yellowish brown (10YR5/6). B OCR shows higher significant positive correlation with HIRM (0.97) and G (0.8) and higher significant negative correlation with IRMsoft ( 0.9; see Table 4). It also increases gradually from non-pedogenic to pedogenic layers and shows consistency in the pedogenic levels (Fig. 6). Thus the pedogenic enrichment of goethite suggests prevalence of humid condition.

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4.1.3. Site: MP-2 The 1.6 m thick paleosol profile is developed over light brownish grey (2.5Y6/2) sandstone bed rock (MP2/1). It comprises 48 cm C-horizon (MPL2/C) overlain by well pedogenised B-horizon with red mottles, root traces, biotubes and development of slickensides (Figs. 3 and 4). The pedogenic layer shows enrichment of antiferromagnetic oxides with interplay of G and H. The G / H ratio shows a decrease with upper pedogenic levels suggesting more semiarid conditions with increased pedogenesis (Fig. 6). 4.1.4. Site: MP-3/1 This profile is located at a stratigraphic height of 1750 m (Fig. 3) with 1.5 m thick paleosols developed over a bed rock of light brownish grey (2.5Y6/2) sandstone (MP3/1). The lower pedogenic horizon (MP3/1B1) contains many yellowish brown mottles (Fig. 4) and shows peak in B OCR, S-ratio and G / H (Fig. 6). These peaks correspond to low CaCO3 wt.%. Moreover, B OCR shows negative correlation with v ARM (0.79) and higher positive correlation with Sratio and G / H (N 0.93; Table 4). These variations clearly indicate the prevalence of goethite and hence humid conditions. On the other hand, higher pedogenic horizons (MP3/1B2, MP3/1B3, MP3/1B4) showing brown (7.5YR4/2) to dark brown (7.5YR3/2) colors consist of few brown mottles and scarce yellow mottles (Fig. 4). CaCO3 wt.% shows higher negative correlation with G ( 0.76) than H ( 0.69; Table 4). It further indicate an enrichment of goethite in the initial pedogenic levels (MP3/B1; Fig. 7), and predominance of hematite in the higher pedogenic levels (B2, B4 and B5; Fig. 7) suggesting prevalent semiarid conditions. This profile is thus developed under well drained humid conditions later on dominated by arid conditions. 4.1.5. Site: MP-3/2 This sequence of the compound paleosols is developed over a bed rock of pale olive (5Y6/3) sandstone (MP3/2) and in between truncated by the deposition of yellow (2.5Y7/6) color crevasse splay (MP3/2S) that further becomes parent for another overlying profile (Figs. 3 and 4). The v fd% values are higher for non-pedogenic horizons compared to the pedogenic horizons in the profile (Fig. 6; Table 3). As

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previously suggested (in the description of site MP 0), it indicates that the SP fraction was removed/recrystallized during early stages of lithification/diagenesis whereas it get locked into the sandstone showing early cementation and compaction. B OCR, SIRM / v lf and Sratios are higher for pedogenic layers than nonpedogenic units (Fig. 6) indicating enrichment of the antiferromagnetic oxides ( G and H) during pedogenesis over a ferrimagnetically rich substrate (Fig. 7). S-ratio shows significant positive correlation with B OCR (0.98) and H / IRMsoft (0.8) and significant negative correlation with v ARM / H ( 0.8; Table 4). The lower pedogenic horizons (MP3/2B2L) shows noticeable peaks for v lf, v ARM, and CaCO3 wt.% and corresponds to low values for G / H (Fig. 6). There is a decreasing trend of G / H ratio for higher paleosol horizons. All these parameters suggest that the hematite is more abundant amongst antiferromagnetic minerals in the pedogenic horizon (Fig. 7). TOC% and CaCO3 wt.% although significantly increased in the pedogenic levels, do not show a trend upwards (Fig. 6). However, TOC shows a positive correlation with B OCR (0.6), G (0.7) and H (0.6; Table 4). HIRM shows significant positive correlation with both G (0.96) and H (0.99; Table 4). Moreover, pedogenic horizons preserve many yellowish and brown mottles (Fig. 4). This indicates that the entire profile have experienced both semi-arid and semihumid climatic conditions. 4.1.6. Site: MP-3/3 Sandstone (MP3/3) of strong brown (7.5YR5/6) color is the bed rock for this profile (Fig. 3). Pedogenic horizons are mainly brown colored and consist of few pale yellowish brown and green mottles. The topmost horizon preserved abundant grass root traces (Fig. 4). Values of v lf, v ARM and v ARM / H are relatively higher in contrast to decreasing trend for B OCR towards higher horizons (Fig. 6). The v lf shows higher positive correlation with v ARM (+ 1). Mean B OCR values for non-pedogenic parental layers are higher than the pedogenic layers, indicating abundance of antiferromagnetic minerals in the parental horizon (Fig. 7). TOC% increases towards higher pedogenic horizons complimenting the abundance of root traces. The rock magnetic parameters in this profile infer enrichment of SD ferrimagnetic oxides during pedogenesis over an antiferromagnetically rich parental material. Such

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Table 3 Rock magnetic and geochemical results of the studied paleosols Height v lf (m)

v hf

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

0.25 0.75 1.13 1.38 1.55 1.87 2.08 2.40 2.98 3.60 4.45 5.45 6.33 7.65 2.50 5.25 5.90 6.68 7.30 7.80 8.30 8.80 9.30 0.25 0.63 0.83 1.15 1.45 0.25 0.70 1.05 1.43 1.88 2.28 0.25 0.68 0.95 1.35

0.48 0.51 0.86 0.93 0.66 2.41 1.08 1.05 0.85 1.01 0.88 0.85 0.96 1.18 1.06 0.46 1.15 0.86 0.57 0.69 0.50 0.61 0.69 0.83 0.81 0.86 1.02 0.93 1.03 1.12 1.11 1.20 1.09 1.10 0.98 1.01 1.10 0.39

MPO-1 MPO-2 MPO-1C MPO-1B1 MPO-1B2 MPO-1B3 L MPO-1B3U MPO-1B4 MPO-3 MPO-4 MPO-5L MPO-5U MPO-6 MPO-2C MP1-1 MP1-2 MP1-1C MP1-1B1 MP1-1B2 MP1-1B3 MP1-1B4 MP1-1B5 MP1-1B6 MP2-1 MP2-1C L MP2-1CU MP2-B1 L MP2-B1 U MP3-1 MP3-1C1 MP3-1B1 MP3-1B2 MP3-1B3 MP3-1B4 MP3-2 MP3-2C MP3-2B1 MP3-2S

0.53 0.51 0.82 0.98 0.69 0.97 1.11 1.04 0.87 0.99 0.91 0.85 0.97 1.20 1.09 0.49 1.15 0.78 0.55 0.63 0.50 0.65 0.69 0.84 0.86 0.86 1.08 0.96 0.96 1.06 1.13 1.18 1.11 1.07 0.98 1.06 1.10 0.43

v fd%

IRM1

T

8.89 51.52 0.00 47.35 0.00 67.48 5.45 76.69 5.00 19.66 0.00 61.17 2.53 90.38 0.00 53.37 1.69 32.46 0.00 44.77 3.03 39.51 0.00 17.07 1.39 31.11 1.19 45.35 2.53 23.86 6.67 39.42 0.00 119.15 0.00 87.95 0.00 67.62 0.00 77.82 0.00 24.20 5.41 30.68 0.00 31.65 1.45 15.89 5.26 43.19 0.00 20.42 5.41 80.07 3.08 93.51 0.00 15.66 0.00 35.99 1.41 94.21 0.00 114.67 2.44 162.15 0.00 43.58 0.00 8.32 4.76 22.50 0.00 65.73 8.57 19.73

IRM2.5 60.06 55.73 80.67 89.19 23.29 74.39 110.99 69.89 39.94 56.14 50.55 22.33 40.00 57.87 26.95 46.54 134.98 100.57 80.34 91.98 29.69 37.43 39.52 17.91 48.64 23.61 98.93 111.35 18.74 39.52 111.99 124.11 176.24 47.99 10.25 24.03 79.78 21.73

T

IRM

300 mT

7.58 5.68 15.62 24.41 6.46 31.25 58.13 32.44 7.45 14.86 20.84 6.20 15.98 25.45 5.98 6.73 21.31 33.66 18.37 21.03 3.75 6.18 5.95 0.17 7.97 0.49 40.34 44.52 3.43 4.57 42.84 8.54 9.69 2.26 0.60 8.82 25.02 6.70

B OCR v ARM SIRM / lf v ARM / lf HIRM 230 250 410 440 470 535 600 610 455 500 560 475 550 575 150 420 425 475 450 450 420 430 425 300 420 325 500 490 430 255 490 350 325 350 375 160 470 160

2.63 3.70 4.32 7.52 2.08 3.96 3.79 11.42 8.80 12.93 5.84 3.18 4.66 5.21 3.71 6.23 20.18 9.70 6.76 7.95 4.84 4.46 7.47 5.24 8.65 5.47 9.71 9.18 3.10 5.99 7.96 20.28 18.06 11.99 4.89 3.37 7.56 2.83

113.41 108.71 98.26 90.81 33.71 76.97 99.75 67.50 46.12 56.98 55.83 26.27 41.17 48.28 24.82 95.13 117.25 128.71 146.00 146.39 58.99 57.78 57.51 21.30 56.54 27.37 92.01 115.71 19.44 37.30 99.13 105.16 158.08 44.88 10.49 22.69 72.60 50.53

4.97 7.23 5.26 7.66 3.02 4.10 3.41 11.03 10.16 13.13 6.45 3.74 4.79 4.34 3.42 12.74 17.53 12.41 12.28 12.66 9.62 6.89 10.87 6.24 10.05 6.34 9.04 9.54 3.22 5.65 7.05 17.18 16.20 11.21 5.00 3.18 6.88 6.59

21.97 20.83 41.55 50.55 13.06 46.21 74.25 42.90 19.96 29.81 30.17 11.63 23.54 35.40 8.94 23.08 70.23 60.81 43.00 49.42 13.97 18.43 18.80 7.857 25.58 10.45 60.21 69.02 9.55 15.71 68.53 61.61 85.92 22.92 4.46 6.84 45.37 6.51

IRMSoft G 1.79 0.26 1.15 2.70 0.09 4.21 7.57 4.05 0.34 0.93 3.95 1.51 2.96 4.09 6.04 0.02 2.29 2.54 2.42 2.04 0.29 0.38 0.90 1.05 0.24 0.09 6.41 6.82 0.40 1.47 6.34 2.06 1.64 0.10 0.11 1.51 3.37 2.23

8.55 8.38 13.19 12.51 3.63 13.22 20.61 16.52 7.47 11.36 11.04 5.26 8.89 12.52 3.09 7.11 15.83 12.61 12.72 14.16 5.50 6.76 7.87 2.02 5.45 3.20 18.86 17.84 3.08 3.53 17.78 9.44 14.09 4.42 1.94 1.53 14.05 2.01

H

G /H

10.78 11.17 21.28 22.25 5.06 21.87 29.76 20.85 9.60 14.14 14.95 5.17 11.09 16.90 3.99 10.80 31.82 30.85 18.91 22.04 6.40 8.25 8.46 3.51 10.47 4.67 28.54 32.64 3.78 6.70 32.13 26.06 38.23 10.08 1.84 2.68 20.11 2.74

0.79 0.75 0.62 0.56 0.72 0.60 0.69 0.79 0.78 0.80 0.74 1.02 0.80 0.74 0.77 0.66 0.50 0.41 0.67 0.64 0.86 0.82 0.93 0.58 0.52 0.68 0.66 0.55 0.81 0.53 0.55 0.36 0.37 0.44 1.05 0.57 0.70 0.73

H / IRMsoft v ARM / S-ratio TOC% CaCO3% H 6.04 42.47 18.52 8.25 53.92 5.20 3.93 5.14 28.10 15.16 3.78 3.42 3.74 4.14 0.66 668.85 13.90 12.17 7.81 10.78 22.21 21.62 9.36 3.33 43.88 53.80 4.45 4.78 9.34 4.55 5.07 12.62 23.30 103.63 15.98 1.78 5.97 1.23

0.24 0.33 0.20 0.34 0.41 0.18 0.13 0.55 0.92 0.91 0.39 0.62 0.42 0.31 0.93 0.58 0.63 0.31 0.36 0.36 0.76 0.54 0.88 1.49 0.83 1.17 0.34 0.28 0.82 0.89 0.25 0.78 0.47 1.19 2.66 1.26 0.38 1.03

0.13 0.10 0.19 0.27 0.28 0.42 0.52 0.46 0.19 0.26 0.41 0.28 0.40 0.44 0.22 0.14 0.16 0.33 0.23 0.23 0.13 0.17 0.15 0.01 0.16 0.02 0.41 0.40 0.18 0.12 0.38 0.07 0.05 0.05 0.06 0.37 0.31 0.31

1.65 1.55 3.02 3.97 2.72 3.62 4.22 4.00 2.87 3.23 2.63 3.28 2.84 3.15 1.94 1.61 4.23 3.52 2.40 2.63 2.08 2.44 2.91 1.91 2.71 5.28 3.54 3.37 2.14 2.70 3.69 3.58 3.22 3.78 2.69 3.50 3.79 1.95

8.05 1.01 3.84 2.37 18.62 3.11 2.99 2.97 1.94 2.61 2.16 2.18 2.12 2.67 1.29 0.91 2.13 2.08 1.38 1.59 1.54 1.62 1.97 1.46 2.57 2.47 2.42 2.46 1.85 2.38 2.32 2.52 2.29 2.46 1.63 1.78 2.25 1.53

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

Serial Sample No. No.

MP3-2B2 L MP3-2B2 U MP3-2B3 L MP3-2B3 U MP3-3 MP3-3C MP3-3B1 MP3-3B2 MP3-4 MP3-4C MP3-4B1 MP3-4B2 MP3-4B3 MP3-4B4 MP3-4B5A MP3-4B5B MP3-4B5C MP3-4B5D MP3-4B5E MP3-4B5F MP3-4B5G MP3-4B6 MP3-4B7 MP3-4B8 MP3-5 MP3-5C MP3-5B1 MP3-5B2 MP3-5B3 MP3-6 MP3-6C MP3-6B1 MP3-6B2 MP3-6B3 MP3-6B4 MP3-6B5 MP3-6B6 MP3-6B7 MP3-6B8 MP3-6B9 MP3-6B10

1.85 2.40 2.88 3.28 0.25 0.78 1.40 2.08 0.25 0.75 1.30 1.90 2.50 3.23 3.90 4.53 5.03 5.35 5.73 6.13 6.50 6.85 7.18 7.55 0.25 0.88 1.63 2.10 2.48 0.25 0.73 1.13 1.55 1.80 2.10 2.40 2.70 3.05 3.35 3.63 3.93

0.93 0.93 1.00 1.11 0.53 0.60 0.82 1.36 0.79 0.83 0.90 1.07 0.78 0.90 1.15 0.96 1.01 1.01 1.02 1.05 1.00 0.68 1.00 0.96 0.59 0.89 0.79 0.93 1.01 0.61 0.59 1.07 1.10 1.03 0.97 0.91 0.92 0.82 0.87 1.01 1.02

0.90 3.03 130.68 0.94 0.00 98.07 0.97 2.78 36.93 1.10 1.30 115.83 0.50 5.71 57.98 0.59 2.17 59.79 0.83 0.00 79.77 1.24 8.33 138.04 0.72 8.70 16.73 0.81 2.13 26.10 0.88 1.82 51.42 1.05 1.89 64.75 0.76 2.22 50.18 0.85 5.26 27.50 1.09 5.41 155.25 0.96 0.00 172.10 1.03 0.00 121.86 0.97 4.05 114.18 1.00 1.47 109.29 1.05 0.00 141.73 0.94 6.15 131.97 0.71 0.00 112.51 1.01 0.00 119.64 0.92 3.28 218.53 0.59 0.00 38.28 0.85 4.84 36.65 0.82 0.00 78.05 0.90 3.51 64.62 0.99 1.28 205.42 0.57 6.52 77.54 0.66 0.00 35.83 1.04 3.08 196.97 1.06 3.80 191.27 1.02 1.41 144.88 0.86 11.11 92.58 0.95 0.00 83.87 0.93 0.00 103.37 0.85 0.00 72.12 0.90 0.00 47.68 0.99 1.89 125.13 1.04 0.00 72.02

149.07 115.60 42.50 134.18 67.25 67.78 98.69 148.75 18.94 33.45 60.87 76.86 62.55 34.63 184.14 198.33 143.33 132.92 126.98 169.88 155.69 132.69 154.70 296.74 46.74 40.76 95.72 74.74 255.96 91.51 41.49 228.11 220.38 170.84 116.58 100.45 122.63 88.34 62.88 141.61 86.57

24.90 34.80 4.00 40.79 13.43 6.08 6.34 27.95 1.40 5.26 12.12 24.38 16.34 7.24 73.53 67.30 50.41 39.94 37.97 53.67 41.33 41.61 73.94 150.81 10.40 6.44 26.76 10.55 142.99 25.09 2.99 73.89 73.19 62.84 42.55 29.36 32.93 23.16 13.14 37.78 22.26

375 450 350 445 400 350 340 200 265 400 400 450 450 440 480 450 465 450 440 450 425 450 575 600 425 225 450 375 600 450 260 450 455 475 475 450 445 440 440 420 420

13.34 6.79 6.07 10.62 7.90 10.49 22.62 80.87 4.38 6.08 7.72 6.04 6.97 5.58 12.27 13.41 11.03 12.08 11.04 12.08 13.15 9.28 4.70 9.91 6.30 8.01 8.93 10.48 8.04 8.09 8.51 16.50 14.73 10.02 9.64 9.34 10.80 10.10 9.00 12.36 8.53

161.05 124.38 42.61 120.43 127.48 113.02 121.02 109.60 23.96 40.34 67.61 71.88 80.30 38.42 160.00 206.21 141.82 131.17 124.51 161.81 154.97 193.99 154.54 310.39 79.57 45.78 120.67 80.16 254.00 150.77 70.68 212.93 199.69 165.33 120.59 110.72 133.86 107.57 72.32 139.65 84.89

14.41 7.31 6.08 9.53 14.97 17.50 27.74 59.59 5.54 7.33 8.58 5.65 8.94 6.19 10.66 13.94 10.92 11.92 10.83 11.51 13.09 13.57 4.70 10.37 10.72 9.00 11.25 11.24 7.98 13.32 14.50 15.40 13.35 9.69 9.97 10.29 11.79 12.30 10.35 12.19 8.37

77.79 66.44 20.47 78.31 35.70 32.93 43.06 55.04 7.66 15.68 31.77 44.56 33.26 17.37 114.39 119.70 86.13 77.06 73.63 97.70 86.65 77.06 96.79 184.67 24.34 15.11 52.40 37.58 174.21 51.31 16.42 135.43 132.23 103.86 67.56 56.62 68.15 47.64 30.41 81.46 47.14

4.83 5.18 0.62 5.94 1.03 0.41 0.45 27.74 0.76 0.40 2.42 4.25 2.63 1.18 11.25 10.00 7.92 6.64 5.47 8.65 4.54 4.99 13.28 33.11 1.05 2.96 5.04 0.55 22.08 3.09 2.54 11.88 11.05 10.25 8.69 5.25 4.98 3.85 3.59 5.21 4.87

18.39 17.54 5.57 18.35 9.27 7.99 18.91 10.71 2.21 7.35 9.45 12.11 12.37 7.13 28.90 26.23 21.47 18.73 17.70 28.15 23.72 20.17 35.06 78.21 8.46 4.11 17.67 10.12 50.54 13.97 5.66 31.14 29.10 25.96 24.00 16.58 19.27 16.22 15.20 16.47 14.55

32.18 30.36 9.07 35.15 16.20 14.97 18.91 25.41 3.42 7.34 14.61 21.50 15.16 8.47 52.95 54.08 39.66 35.29 33.74 44.41 39.91 34.52 46.83 87.38 11.41 6.26 24.11 17.10 88.54 23.80 6.88 61.64 60.05 47.83 28.95 26.17 30.84 22.19 13.78 36.00 22.15

0.57 0.58 0.61 0.52 0.57 0.53 1.00 0.42 0.64 1.00 0.65 0.56 0.82 0.84 0.55 0.49 0.54 0.53 0.52 0.63 0.59 0.58 0.75 0.90 0.74 0.66 0.73 0.59 0.57 0.59 0.82 0.51 0.48 0.54 0.83 0.63 0.62 0.73 1.10 0.46 0.66

6.67 5.87 14.61 5.92 15.68 36.28 41.65 0.92 4.48 18.41 6.03 5.06 5.76 7.19 4.71 5.41 5.01 5.31 6.17 5.13 8.79 6.91 3.53 2.64 10.89 2.12 4.79 31.04 4.01 7.70 2.71 5.19 5.44 4.66 3.33 4.99 6.19 5.76 3.83 6.90 4.55

0.41 0.22 0.67 0.30 0.49 0.70 1.20 3.18 1.28 0.83 0.53 0.28 0.46 0.66 0.23 0.25 0.28 0.34 0.33 0.27 0.33 0.27 0.10 0.11 0.55 1.28 0.37 0.61 0.09 0.34 1.24 0.27 0.25 0.21 0.33 0.36 0.35 0.46 0.65 0.34 0.39

0.17 0.30 0.09 0.30 0.20 0.09 0.06 0.19 0.07 0.16 0.20 0.32 0.26 0.21 0.40 0.34 0.35 0.30 0.30 0.32 0.27 0.31 0.48 0.51 0.22 0.16 0.28 0.14 0.56 0.27 0.07 0.32 0.33 0.37 0.36 0.29 0.27 0.26 0.21 0.27 0.26

3.72 3.51 2.97 3.40 1.59 2.08 2.79 3.22 3.02 2.76 3.44 3.91 2.84 3.99 4.32 5.04 4.10 4.89 4.08 3.87 4.03 2.66 4.74 6.18 3.21 4.80 3.97 4.86 6.47 3.12 3.26 5.75 3.68 3.96 3.39 3.50 3.52 3.46 3.85 4.20 4.46

2.48 2.30 2.50 2.53 1.19 1.62 2.28 2.13 1.98 1.80 2.31 2.88 1.68 2.36 2.71 7.16 4.97 4.58 2.61 2.31 3.53 3.47 2.98 3.56 1.37 2.01 1.81 1.74 3.00 1.40 1.37 2.50 2.31 2.26 1.94 2.16 2.26 2.03 2.06 2.32 2.49

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

243

244

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

Table 4 Correlation matrix for the rock magnetic parameters of representative paleosol profiles MP0

v lf

v lf v fd% v ARM v ARM / v lf IRM 2.5T (B o)cr SIRM2.5T / v lf HIRM IRMSoft G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

1.000 0.482 0.479 0.369 0.935 0.638 0.898 0.934 0.914 0.983 0.975 0.050 0.918 0.343 0.752 0.983 0.919 0.929 G

G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3% MP1 v lf v fd% v ARM v ARM / v lf IRM 2.5T (B o)cr SIRM2.5T / v lf HIRM IRMSoft G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

1.000 0.952 0.082 0.834 0.364 0.848 0.943 0.839 0.850 v lf 1.000 0.073 0.701 0.187 0.433 0.583 0.117 0.487 0.361 0.312 0.551 0.504 0.304 0.231 0.637 0.948 0.821 0.884

v fd%

v ARM

v ARM / mlf

IRM

–

– –

– – –

– – – –

1.000 0.350 0.289 0.210 0.789 0.154 0.264 0.498 0.518 0.392 0.356 0.476 0.030 0.750 0.356 0.508 0.481

1.000 0.992 0.241 0.325 0.222 0.170 0.136 0.406 0.304 0.292 0.564 0.631 0.186 0.556 0.560 0.558

1.000 0.132 0.232 0.122 0.054 0.011 0.289 0.191 0.288 0.479 0.709 0.074 0.456 0.474 0.471

2.5T

1.000 0.401 0.990 0.988 0.889 0.904 0.982 0.315 0.861 0.564 0.592 0.937 0.854 0.871

(B o)cr

SIRM2.5T / v lf

HIRM

IRMSoft

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.284 0.498 0.736 0.745 0.529 0.642 0.400 0.060 0.958 0.525 0.424 0.420

1.000 0.961 0.827 0.845 0.962 0.441 0.874 0.585 0.490 0.912 0.865 0.881

1.000 0.943 0.931 0.980 0.225 0.810 0.609 0.686 0.913 0.807 0.824

1.000 0.959 0.931 0.025 0.728 0.581 0.885 0.841 0.737 0.747

H

G /H

H / IRMSoft

v ARM/H

S-ratio

TOC%

LOI%

CaCO3%

–

– –

– – –

– – – –

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.227 0.906 0.529 0.700 0.954 0.906 0.917

1.000 0.303 0.558 0.419 0.105 0.286 0.288

1.000 0.278 0.520 0.928 0.999 1.000

1.000 0.332 0.261 0.281 0.287

v fd%

v ARM

v ARM / v lf

IRM

–

– –

– – –

– – – –

1.000 0.597 0.856 0.406 0.277 0.446 0.392 0.500 0.426 0.381 0.252 0.592 0.011 0.262 0.218 0.088 0.138

1.000 0.829 0.815 0.807 0.636 0.832 0.811 0.756 0.852 0.727 0.735 0.449 0.820 0.867 0.390 0.629

1.000 0.797 0.657 0.809 0.772 0.856 0.828 0.741 0.594 0.822 0.455 0.630 0.448 0.121 0.151

2.5T

1.000 0.946 0.939 0.994 0.961 0.963 0.973 0.912 0.639 0.869 0.921 0.590 0.103 0.158

1.000 0.633 0.542 0.541

1.000 0.923 0.936

1.000 0.999

1.000

(B o)cr

SIRM2.5T / mlf

HIRM

IRMSoft

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.812 0.973 0.906 0.835 0.988 0.979 0.505 0.856 0.996 0.721 0.092 0.323

1.000 0.904 0.939 0.967 0.849 0.780 0.660 0.866 0.762 0.298 0.417 0.169

1.000 0.949 0.928 0.992 0.947 0.595 0.864 0.956 0.644 0.033 0.227

1.000 0.949 0.922 0.838 0.775 0.798 0.879 0.559 0.127 0.116

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

245

Table 4 (continued) G G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3% MP3-1

1.000 0.877 0.771 0.753 0.818 0.795 0.460 0.226 0.017 v lf

v lf v fd% v ARM v ARM / v lf IRM 2.5T (B o)cr SIRM2.5T / v lf HIRM IRMSoft G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

1.000 0.077 0.508 0.418 0.498 0.105 0.436 0.529 0.393 0.357 0.481 0.246 0.760 0.410 0.145 0.206 0.014 0.258 G

G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3% MP3-2 v lf v fd% v ARM v ARM / v lf IRM 2.5T (B o)cr SIRM2.5T / v lf HIRM IRMSoft G

1.000 0.871 0.451 0.775 0.998 0.744 0.389 0.064 0.769 v lf 1.000 0.337 0.100 0.342 0.192 0.495 0.403 0.100 0.031 0.054

H

G /H

H / IRMSoft

v ARM / H

S-ratio

TOC%

LOI%

CaCO3%

–

– –

– – –

– – – –

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.969 0.546 0.836 0.979 0.708 0.060 0.318

1.000 0.342 0.872 0.974 0.643 0.027 0.266

1.000 0.311 0.484 0.470 0.031 0.183

1.000 0.820 0.308 0.342 0.155

v fd%

v ARM

v ARM /mlf

IRM

–

– –

– – –

– – – –

1.000 0.040 0.014 0.771 0.096 0.809 0.804 0.345 0.748 0.836 0.068 0.590 0.746 0.242 0.772 0.603 0.945

1.000 0.995 0.515 0.799 0.486 0.309 0.523 0.265 0.213 0.959 0.388 0.200 0.750 0.601 0.692 0.364

1.000 0.500 0.853 0.477 0.280 0.591 0.307 0.185 0.981 0.332 0.244 0.804 0.625 0.739 0.336

2.5T

1.000 0.165 0.998 0.966 0.245 0.651 0.940 0.378 0.907 0.693 0.014 0.920 0.741 0.543

1.000 0.768 0.170 0.394

1.000 0.740 0.875

1.000 0.952

1.000

(B o)cr

SIRM2.5T / v lf

HIRM

IRMSoft

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.171 0.096 0.910 0.630 0.176 0.937 0.145 0.592 0.987 0.468 0.723 0.336

1.000 0.962 0.231 0.654 0.940 0.364 0.882 0.693 0.017 0.936 0.763 0.589

1.000 0.486 0.823 0.995 0.135 0.953 0.855 0.245 0.806 0.559 0.637

1.000 0.867 0.548 0.734 0.541 0.851 0.955 0.106 0.434 0.474

H

G /H

H / IRMSoft

v ARM / H

S-ratio

TOC%

LOI%

CaCO3%

–

– –

– – –

– – – –

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.041 0.935 0.897 0.325 0.771 0.509 0.698

1.000 0.158 0.395 0.903 0.571 0.744 0.370

1.000 0.817 0.272 0.671 0.406 0.407

1.000 0.709 0.427 0.102 0.745

v fd%

v ARM

v ARM / v lf

IRM

–

– –

– – –

– – – –

1.000 0.452 0.516 0.013 0.933 0.082 0.108 0.349 0.315

1.000 0.968 0.828 0.140 0.802 0.760 0.585 0.656

1.000 0.811 0.262 0.842 0.720 0.520 0.616

2.5T

1.000 0.241 0.974 0.989 0.917 0.943

1.000 0.322 0.605 0.456

1.000 0.941 0.524

1.000 0.344

1.000

(B o)cr

SIRM2.5T / v lf

HIRM

IRMSoft

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.122 0.361 0.573 0.542

1.000 0.941 0.843 0.895

1.000 0.965 0.969

1.000 0.977

(continued on next page)

246

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

Table 4 (continued) MP3-2

v lf

H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3% G G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3% MP3-4

1.000 0.972 0.453 0.932 0.882 0.655 0.726 0.150 0.104 v lf

v lf v fd% v ARM v ARM / v lf IRM 2.5T (B o)cr SIRM2.5T / v lf HIRM IRMSoft G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

v ARM

0.192 0.325 0.584 0.698 0.886 0.417 0.466 0.812

v ARM / v lf

0.688 0.485 0.453 0.225 0.024 0.476 0.274 0.474

IRM

0.639 0.465 0.386 0.180 0.148 0.461 0.104 0.427

2.5T

(B o)cr

0.967 0.633 0.766 0.697 0.380 0.581 0.014 0.190

SIRM2.5T / v lf

0.431 0.184 0.780 0.807 0.987 0.583 0.273 0.658

HIRM

0.908 0.600 0.696 0.640 0.257 0.570 0.135 0.150

IRMSoft

0.994 0.636 0.820 0.779 0.498 0.579 0.028 0.138

0.986 0.567 0.895 0.903 0.695 0.590 0.022 0.024

H

G /H

H / IRMSoft

v ARM / H

S-ratio

TOC%

LOI%

CaCO3%

–

– –

– – –

– – – –

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.647 0.837 0.828 0.567 0.552 0.029 0.102

1.000 0.141 0.286 0.030 0.257 0.489 0.699

1.000 0.926 0.843 0.853 0.292 0.402

1.000 0.880 0.636 0.326 0.417

1.000 0.594 0.217 0.562

1.000 0.498 0.541

1.000 0.833

1.000

v fd%

v ARM

v ARM / v lf

IRM

1.000 0.244 0.328 0.128 0.080 0.140 0.035 0.306 0.205 0.165 0.314 0.352 0.307 0.333 0.313 0.522 0.436 0.102

–

– –

– – –

– – – –

H

G /H

H / IRMSoft

v ARM / H

S-ratio

TOC%

LOI%

CaCO3%

1.000 0.930 0.377 0.655 0.712 0.840 0.744 0.272 0.184

–

– –

– – –

– – – –

– – – – – 1.000 0.667 0.272 0.231

– – – – – – 1.000 0.706 0.466

– – – – – – – 1.000 0.896

– – – – – – – – 1.000

G G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

v fd%

0.058 0.199 0.140 0.044 0.454 0.026 0.550 0.005

1.000 0.121 0.028 0.353 0.104 0.156 0.069 0.024 0.001 0.059 0.202 0.340 0.345 0.200 0.163 0.016 0.241

1.000 0.042 0.597 0.824 0.852 0.759 0.481 0.416

1.000 0.890 0.497 0.212 0.482 0.509 0.152 0.204 0.474 0.598 0.163 0.268 0.050 0.256 0.557 0.533

1.000 0.256 0.171 0.168 0.130 0.424 0.492

1.000 0.577 0.272 0.518 0.410 0.072 0.145 0.369 0.504 0.302 0.182 0.059 0.014 0.381 0.529

1.000 0.625 0.802 0.602 0.192 0.092

2.5T

1.000 0.233 0.606 0.519 0.311 0.372 0.503 0.242 0.011 0.491 0.337 0.331 0.516 0.691

1.000 0.899 0.513 0.301 0.345

(B o)cr

SIRM2.5T / v lf

HIRM

IRMSoft

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.649 0.702 0.868 0.847 0.731 0.505 0.789 0.707 0.926 0.663 0.113 0.052

1.000 0.956 0.852 0.890 0.950 0.045 0.463 0.778 0.758 0.603 0.402 0.449

1.000 0.898 0.916 0.999 0.012 0.576 0.819 0.834 0.744 0.494 0.437

1.000 0.986 0.914 0.370 0.746 0.699 0.860 0.790 0.322 0.209

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

247

Table 4 (continued) MP3-6

v lf

v lf v fd% v ARM v ARM / v lf IRM 2.5T (B o)cr SIRM2.5T / v lf HIRM IRMSoft G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

1.000 0.318 0.637 0.276 0.294 0.140 0.740 0.825 0.798 0.700 0.822 0.670 0.070 0.741 0.573 0.542 0.638 0.699 G

G H G /H H / IRMSoft v ARM / H S-ratio TOC% LOI% CaCO3%

v fd%

v ARM

v ARM / v lf

IRM

–

– –

– – –

– – – –

1.000 0.182 0.055 0.294 0.577 0.266 0.294 0.520 0.505 0.256 0.043 0.451 0.324 0.667 0.082 0.346 0.328 H

1.000 0.909 0.450 0.096 0.706 0.787 0.395 0.073 0.270

– 1.000 0.737 0.252 0.803 0.666 0.500 0.315 0.535

1.000 0.916 0.181 0.059 0.896 0.876 0.697 0.755 0.871 0.604 0.408 0.506 0.326 0.600 0.211 0.508

1.000 0.369 0.008 0.739 0.668 0.452 0.575 0.664 0.429 0.501 0.264 0.115 0.463 0.060 0.278

2.5T

1.000 0.573 0.150 0.215 0.380 0.314 0.220 0.103 0.330 0.414 0.577 0.480 0.020 0.183

(B o)cr

SIRM2.5T / v lf

HIRM

IRMSoft

– – – – –

– – – – – –

– – – – – – –

– – – – – – – –

1.000 0.380 0.372 0.625 0.647 0.363 0.046 0.501 0.428 0.805 0.240 0.440 0.405

1.000 0.989 0.882 0.907 0.988 0.732 0.315 0.789 0.655 0.465 0.201 0.469

1.000 0.916 0.915 0.999 0.731 0.247 0.803 0.677 0.490 0.295 0.516

1.000 0.975 0.912 0.507 0.146 0.771 0.841 0.433 0.206 0.343

G /H

H / IRMSoft

v ARM / H

S-ratio

TOC%

LOI%

CaCO3%

– –

– – –

– – – –

– – – – –

– – – – – – 1.000 0.669 0.762

– – – – – – – 1.000 0.861

– – – – – – – – 1.000

1.000 0.660 0.856 0.458 0.345 0.422 0.659

1.000 0.246 0.227 0.089 0.151 0.379

conditions are favored in restricted reducing, relatively cold environments. 4.1.7. Site: MP-3/4 This site is placed at a stratigraphic level of 1760 m showing a thickness of 7.8 m (Fig. 3). The paleosol sequence is developed over a light yellowish brown (2.5Y6/3) silty bed rock (MP3/4). This profile comprises massive pedogenic layers with incursions of silty layers in between. This suggests multi-phase soil forming processes and forming the composite paleosols (Fig. 4). B OCR, SIRM and S-ratio increases towards higher horizons (Fig. 6). B OCR shows significant positive correlation with S-ratio and G (N0.84) and negative correlation with IRMsoft ( 0.8; Table 4). SIRM / v lf shows positive correlation with HIRM (0.9) and negative correlation with IRMsoft ( 0.85; Table 4). This indicates abundance of antiferromagnetic minerals in the pedogenic levels. The pedogenic horizons

1.000 0.834 0.228 0.247 0.451

1.000 0.044 0.071 0.015

MP3/4B4 and MP3/4B7 characteristically showing yellow mottles display (Fig. 4) higher G / H ratio and lower v ARM, SIRM / v lf and CaCO3 wt.% (Fig. 6; Table 3). This indicates abundance of goethite suggesting predominantly well drained semi-humid conditions. There is abundance of calcareous nodules (N 5 mm diameter) in pedogenic horizons MP3/4B2 and MP3/4B5B coinciding with noticeable peaks in CaCO3 wt.% and lower values in G / H ratio (Fig. 6). Alternate enrichment of goethite and hematite with presence of calcareous nodules infer semi-humid to semi-arid seasonality of the climate. 4.1.8. Site: MP-3/5 Brown (7.5YR5/4) to reddish brown (5YR4/4) pedogenic horizons are developed over a brownish yellow siltstone at MP3/5 (Fig. 3). Paleosols show well rounded peds, few brownish and yellowish mottles, calcareous as well as iron nodules and root traces (Fig. 4). SIRM / v lf, S-ratio and B OCR shows

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V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

Fig. 7. Cartoon diagram illustrates the pedogenic transformation of iron oxides from parent to the subsequent pedogenic horizons (refer to Fig. 4 for the true thickness of the paleosol profiles). Fm—Ferrimagnetics; Af—Antiferromagnetics; H—Hematite; G—Goethite.

positive trend towards top of the profiles, and v ARM shows corresponding negative trend (Fig. 6). B OCR shows higher negative correlation with v ARM ( 0.93) and IRMsoft ( 0.99) and positive correlation with HIRM (0.97) and S-ratio (0.99). This indicates predominance of ferrimagnetic minerals in the parent material that transformed to majority of antiferromagnetic minerals during pedogenesis (Fig. 7). dHT has higher values than dGT (mean H = 43, G = 26) and shows decreasing trend of G / H ratio towards higher horizons (Table 3). Moreover HIRM shows ideal positive correlation (+ 1) with dHT. This suggests predominance of hematite in this profile. The presence of iron nodules in pedogenic layer (MP3/ 5B2) coincide with low G / H ratio and higher CaCO3 wt.% (Figs. 4 and 6). Thus the increasing value of H and CaCO3 at higher pedogenic levels from parental towards pedogenic layers indicate seasonality from humid to arid condition. 4.1.9. Site: MP-3/6 Paleosol sequence is developed over a bed rock of strong brown (7.5YR5/6) fine grained sandstone (Fig. 3). v lf shows higher positive correlations with HIRM (0.8) and H (0.8; Table 4). HIRM shows high negative

correlation with IRMsoft ( 0.9) and higher positive correlation with G and H (N0.9). S-ratio shows high negative correlation with IRMsoft ( 0.84) and high positive correlation with G (0.79; Table 4). The horizon MP3/6B8 which contains many yellow mottles (Fig. 4) shows peak in G / H ratio with corresponding low values for v ARM, SIRM / v lf and CaCO3 (Fig. 6). The lower and uppermost pedogenic horizons (MP3/6B1 and MP3/6B9) indicate low G / H ratio, high SIRM / v lf and CaCO3 wt.% (Fig. 6). Better variegation of the color of the paleosols from reddish brown, strong brown to yellowish brown and respective peaks/drops in G / H ratio suggest an increased seasonality. This profile is developed under initial brief ponding conditions later shows an interplay of semi-arid and humid conditions.

5. Mio–Pliocene sedimentation and climate: inferences from rock magnetic variations of MR paleosols Paleosols in the MR section are developed in a braided river system over sandy alluvial fan set up (Kumar, 1993; Kumar and Ghosh, 1994; Sangode et

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

al., 1999; Kumar et al., 2003). The MR section exhibits predominance of fine to coarse grained grey multistorey sandstones with frequent avulsion and sheet flood events. The entire succession is represented by relative variation from distal to proximal fan setting and a significantly high rate of sedimentation (Kumar, 1993; Kumar and Ghosh, 1994; Sangode et al., 1999; Kumar et al., 2003). As a result there is very little preservation of the overbank material that must have been cannibalized by the dynamic channel activity (mud balls are observed at several places). The paleosol sequences preserved at nine sites in the 1800 m thick stratigraphic section of MR are thus largely affected by the high energy depositional condition. The Miocene–Pliocene period (~ 10–3 Ma) in the HFB is characterized by the widespread deposition of the grey sandy interval of Nagri formation followed by mudstone-paleosol rich facies of Dhok Pathan Formation after 8 Ma. The period around 7.5 Ma records a major shift in vegetation from C3 to C4 plants that has been related to the initiation of monsoon over the Indian Sub-continent (Quade et al. 1989, 1995; Cerling et al. 1997). Hence the Mio– Pliocene provides an important period to study the foundations of monsoon system over the Indian sub-

249

continent and its fluvial response. In this scenario we attempted the present rock magnetic study on pedogenic horizons initially to characterize their climatic signatures and to test its suitability for more detailed attempts in the HFB. We tried to assess the rock magnetic signatures with some the pedogenic characteristics. The present study thus advocates the pedogenic control of the iron oxides in all the studied horizons e.g., a) the ferrimagnetically rich substrate shows canted antiferromagnetic enrichment in its pedogenic layers and vice versa; b) enrichment of the yellow mottles in the pedogenic levels coincides with the peak in G (i.e., goethite); c) similarly red mottling, iron concretions and their association with calcareous concretions coincides with peak in H (hematite); d) intense gleying shows richness of ferrimagnetic minerals; e) horizons with calcareous nodules show an interplay of H and G indicating the seasonal dry and wet conditions. These conditions witness the climogenic control of the rock magnetic signatures and the parameters allow relative quantification of the ambient climate. Fig. 7 summarize the pedogenic transformations at each profile and Fig. 8 depict the climatic inferences drawn from the present study.

Fig. 8. Variation of the G / H ratio of the pedogenic layers that can be used to infer the relative variation in semi-humid to semi-arid conditions.

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V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

Fig. 9. Synthesis of climatic inferences derived from Rock Magnetic parameters (this study), clay mineralogy and petrography.

Further we summarise the rock magnetic inferences with the previously available information based on independent approaches of petrography (Ghosh and Kumar, 2000) and clay mineralogy (Fig. 9; Bagati and Kumar, 1994).

6. Conclusions The rock magnetic studies in the MR section infer an over all predominance of the antiferromagnetic minerals; goethite and hematite. The pedogenic processes are thus mainly reflected by the interplay of these two minerals. We used the ratio G / H based upon differential acquisition of IRM (Sangode and Bloemendal, 2004) to infer the relative variations between these two oxides that are indicative of humid to arid conditions in an over all semi-humid climate (Fig. 8). Further a combination of these oxides with other

pedogenic indicators e.g. carbonate nodules and lithogenic (non-pedogenic) observations of Bagati and Kumar (1994) and Ghosh and Kumar (2000) in the studied section allows us to infer the relative intensities of seasonality, humidity and aridity (Fig. 9). The present study confirms the absence of SP fraction in the pedogenic horizons of the Siwaliks as previously reported by Sangode et al. (2001) and Sangode and Bloemendal (2004). It restricts the use of climo-functions as applicable in Chinese loess–paleosol sequence (Maher and Thompson, 1999). Interestingly it is observed that the SP fraction is well preserved in some of the parental horizons (sandstones). The detailed petrographic studies (Ghosh and Kumar, 2000) indicate that parental and bedrock sandstones in the MR section show an early cementation (prior to compaction). Hence, these processes might have preserved the SP fraction escaping the post depositional changes that are prevalent in paleosols.

V. Kumaravel et al. / Sedimentary Geology 177 (2005) 229–252

Thus the combination of rock magnetic parameters with other lithogenic records, infer predominantly semi-humid conditions. The studied rock magnetic parameters are highly sensitive to the minor variations (as recorded by lithogenic signatures) towards semiarid conditions and rare cold events (Fig. 8). Thus the relative intensity of these conditions has been shown more precisely by the rock magnetic parameter dG / HT (proportional to the variation in goethite and hematite content). Spells of semi-arid and cold conditions can be noticed around 4.75 and 4.72 Ma, respectively. Thus, the rock magnetic approach appears to be more suitable compared to other methods to characterize such complex paleosol sequences developed under high energy sediment depositional conditions due to higher sensitivity of rock magnetic parameters to record minor changes in pedogenically altered iron oxides.

Acknowledgements The authors are grateful to the Director, Wadia Institute of Himalayan Geology, Dehra Dun for encouragement and for providing facilities to carryout this work. We acknowledge Prof. G. J. Retallack, Dr. France Lagroix and an anonymous referee for their critical review and suggestions greatly improving the manuscript. We also acknowledge Dr. T. N. Bagati for his generous help in providing the facilities and Mr. Rakesh Kumar for assisting in analytical work. This work was carried out under the research grant ESS/23/ VES/146/2001 funded by Ministry of Science and Technology, New Delhi, India.

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