JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

318 HIMADRI BASU AND OTHERS

Study of Provenance Characteristics and Depositional History on theBasis of U, Th and K Abundances in the Gulcheru Formation,

Cuddapah Basin in Tummalapalle-Somalollapalle Areas,Cuddapah-Anantapur Districts, Andhra Pradesh

HIMADRI BASU1, K. MAHENDRA KUMAR

2, S. PANEERSELVAM3 and A. CHAKI

1

Atomic Minerals Directorate for Exploration and Research,1Hyderabad – 500 016, 2Jamshedpur – 831 002, 3Bangalore – 560 072

Email: basuhimadri@rediffmail.com

Abstract: Provenance studies on the basis of U, Th and K reveal that the sediments of the Gulcheru Formation werederived from a minor heterogeneous but dominantly granitoid provenance, which often underwent moderate chemicalweathering. Mixing of reworked and heavy mineral bearing matured sediments with newly generated detritus took placeat different times. Sedimentation took place in three different phases. Initially the sediments were deposited in fluvio-aeolian environment. Later, subsidence of the depo-centre along basinal growth faults led to the transgression of seawaterover earlier sediments. The last phase of the sedimentation history continued in the marginal marine domain. The depo-centre most possibly experienced semi-arid climate.

Keywords: U, Th and K abundances, Provenance characteristics, Depositional history, Gulcheru Formation, CuddapahBasin, Andhra Pradesh.

Changes in concentrations of Th and K relative to eachother in a sedimentary sequence provide a wealth ofinformation about sedimentation cycles. This along withTh/K ratio provides significant idea about provenancehomogeneity and intensity of chemical weathering in theprovenance. Th/K ratios can be used as an indicator ofprovenance stability and provenance relief. Th/K ratio alongwith U helps to decipher marine transgressive events.Th/U ratio can be used to estimate the redox conditions ofthe depositional environment (Doveton, 1991) and tocharacterize sedimentary environments. Th/U ratio alongwith Th/K ratio provides information on transgression andregression in a marginal marine sequence. Increased Uvalues, and in particular low Th/U ratios, may be associatedwith marine condensed sequences (Myers and Wignall,1987). High Th value may be associated with channel sanddeposits or heavy mineral lags along foreshore. Increasedinput of terrigenous clay may also render high Th values(Hassan et al. 1976). However, it will be associated withlow Th/K values compared to the previous incidents. HighK values may be caused by the presence of K-feldsparsand micas.

INTRODUCTION

Several factors such as source rock characteristics(provenance), tectonic setting, extent and intensity ofchemical weathering, sorting during transport andsedimentation and post-depositional diagenetic changesinfluence the chemical character of clastic sedimentary rocks(McLennan, 1989). The geochemical character ofsedimentary rocks can thus be used effectively to determinesource rock character (McLennan et al. 1995; Bhat andGhosh, 2001), to evaluate palaeoclimate and its role inweathering of provenance (Fedo et al. 1995), to reconstructthe tectonic framework of the sedimentary basin (Naqvi etal. 1988) and also to evaluate the composition and evolutionof the continental crust (Condie, 1993; Raza et al. 2002). Ingeochemical studies of sedimentary rocks, trace elementslike REEs, Y, Sc, Th, Zr, Hf, Cr, and Co are commonly usedfor studying the above parameters. Naturally occurringradioactive elements such as U and K, in addition to Th,can also be used to extract valuable information about sourcearea character, palaeoclimate and palaeo-redox conditionsand tectonic processes that are involved in the formation ofthe sedimentary rocks.

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.74, September 2009, pp.318-328

0016-7622/2009-74-3-318/$ 1.00 © GEOL. SOC. INDIA

JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

PROVENANCE CHARACTERISTICS AND DEPOSITIONAL HISTORY OF THE GULCHERU FORMATION, A. P. 319

In the present study we have utilised the abundances ofU, Th and K in the siliciclastics (mainly 0.0625 – 2.0 mmsized grains) of the Gulcheru Formation, in and aroundTummalapalle-Somalollapalle areas, to understand thesource area character and the depositional history.

GEOLOGY AND SAMPLING

Tummalapalle-Somalollapalle area (Fig.1) exposes partsof the Gulcheru Formation occurring all along thesouthwestern margin of the Proterozoic Cuddapah Basin. Itnonconformably overlies the basement crystallinescomprising TTGs (Tonalite-Trondhjemite-Granodiorites),Kolar-type greenstone belts, younger granitic plutonsand basic dykes. The Gulcheru Formation, exposed in thearea, has been subdivided as pink massive quartzite(Godu Konda Member), dark-brown ferruginous quartzite(Bolaguntacheruvu Member), grey crossbedded quartzite(Gandi Member) and pitted quartzite (RachakuntapalleMember) (Basu, 2007). It is conformably overlain by purpleshale and dolostone of the Vempalle Formation. Sedimentarystructures, preserved in the Gulcheru Formation, indicatethat it was deposited mainly in beach environment (Reddyet al. 1990). Basu et al. (2007) on the basis of detailed faciesanalysis showed that the Gulcheru Formation is a deposit ofa variety of marginal marine set up. In Tummalapalle-Somalollapalle areas the Gulcheru Formation has been

traversed by a number of E-W trending and a few NW-SEand NE-SW trending faults (Fig.1). Some of the E-W faultsare found to be occupied with dolerite/gabbro dyke.

In Tummalapalle-Somalollapalle areas the GulcheruFormation is mainly represented by fine to medium grainedquartzarenite with subordinate amounts of subarkose. Sub-angular to well rounded clasts of monocrystalline (75-95%of quartz grains) and polycrystalline quartz, minor amountsof K-feldspar, chert and metaquartzite are main frameworkcomponents. Clasts are cemented together mainly withauthigenically grown silica. At a particular stratigraphichorizon, hematite is also found as cementing material. Atplaces, the cement between framework quartz clasts arecorroded and infiltrated with minor amounts of chlorite andrarely with subordinate amounts of sericite (matrix material).Zircon, rutile and anatase are main heavy minerals thatform a minor fraction of framework clasts. Sometimes,zircon, spehene and rarely epidote occur as fine inclusionswithin framework quartz grains.

Atomic Minerals Directorate for Exploration andResearch (AMD) carried out subsurface exploration foruranium mineralization, hosted in the Gulcheru Formation,in the southwestern margin of the Cuddapah Basin. Anumber of boreholes have been drilled into it. However,samples from only two boreholes viz. RAP-1 and RAP-6,drilled in Tummalapalle-Somalollapalle areas (Cuddapahand Ananthapur Districts, A.P.) have been taken for the

Fig.1. Geological Map of Rachakuntapalle-Somalollpalle areas, Cuddapah and Anantapur districts, A.P.

Hyderabad

Bangalore

Tummalapalle

Rachakuntapalle

Ella Konda793

11°12°

11°

10°9°

9°

F F

F

FFF

FF

FF

F FF

Gabbro (Diabase)

Vempalle Formation

Gulcheru Formation

Quartz Reef

Granitoids

Fault/Lineament

Dip & Strike

Radioactive Anomaly

Studied Boreholes11°

F F RKP-1

RAP-1

Index

78° 20'

1 km 0.5 0 1 km

78° 20'78° 15'

78° 15'

20'14°

20'14°

16'14°

16'14°

TMP-1

RAP-1RAP-6

RKP-1

RKP-5

RKP-4

(Toposheet No. 57 J/7)

F

Study Area

Somalollapalle

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JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

320 HIMADRI BASU AND OTHERS

present study. Seventy-three (73) samples have beencollected, at an interval of 1.50 m (except 3 at 2.00minterval), from RAP-1 passing through all the aforesaidMembers of the Gulcheru Formation whereas nine (9)samples have been collected, at 10.00 m intervals, fromborehole RAP-6, which starts just at the contact betweenBolaguntacheruvu (dark-brown ferruginous quartzite) andGodu Konda (pink massive quartzite) members and thereforecuts only through the latter.

Sample Analysis

The samples have been assayed for potassium (K),thorium (Th) and uranium (U) using gamma-ray spectro-meter following the methods given in Rout (2002) andIAEA Technical Report No. 341 (1992). For this, 500 g ofeach sample is taken, crushed and sieved to -100 mesh sizeand then 300 g of the powdered sample is taken for assaypurpose. It is assumed that equilibrium exists between thedaughter products and parent in U and Th decay series.

Discussion on the Sources of Natural Radio-nuclides

Main naturally occurring radioactive elements, whichcommonly found in siliciclastic rocks, are K, Th and U.The sources of these elements in sedimentary rocks, andthe factors that control their concentration are discussedbelow in brief.

In siliciclastic rocks, K is contained mainly inK-feldspars and clay minerals (e.g. illite). Survival ofK-feldspars in sedimentary environment is a function ofrelief and climatic factors that prevailed in provenance(Pettijohn, 1957). Under arid or cold climatic conditionsnear quantitative transfer of K-feldspars takes place fromprovenance to the site of deposition. Same is the case fortectonically active (fault controlled) basins. However, intectonically stable set up, where relief is low, if climate isfavourable (warm and humid), effective elimination ofK-feldspars renders sediments more quartz rich. Thepost-depositional factors that can potentially change theconcentration of K in sedimentary rocks are K-meta-somatism and loss of K during burial diagenesis andweathering.

In clastic sedimentary rocks, Th is mainly contained inheavy / resistate minerals such as thorite, monazite, rutileetc. (Boyle, 1982). In addition to this, Th is generallyassumed to be contained by clay minerals in clastic sediments(Taylor and McLennan, 1985; Condie 1991). Th is mainlytransported as solid particulate load (Taylor and McLennan,1985; Condie, 1991) and because of remarkable stability ofheavy minerals in sedimentary environment quantitativetransfer of Th takes place from provenance to basin.

However, concentration of Th may be influenced byhydraulic sorting (Bauluz et al. 2000) and recycling ofheavy minerals in sedimentary environments.

U in sedimentary rocks is present in uraninite (only incase of sediments deposited before oxy-atmosphericconversion), allanite, zircon, monazite etc.; in various rock-forming minerals, particularly feldspars, chlorite, biotite,amphiboles, pyroxenes, epidote, fluorite and apatite; and inassociation with various colloidal and carbonaceoussubstances in sediments (Boyle, 1982). Chemical weatheringin provenance, recycling (McLennan et al. 1993), as well aspH and Eh of depositional environment (Boyle, 1982)influence U concentration in sedimentary rocks. Post-depositional factors that can potentially change theconcentration of U in sedimentary rocks are loss of U as aresult of its mobilization by oxygenous solutions orintroduction of U through epigenetic solutions.

Data Analysis and Interpretation

Seventy-three samples from borehole RAP-1 (drilleddepth: 114.00 m) and nine samples from borehole RAP-6(drilled depth: 105.95 m; the Gulcheru Formation up to90.05m) are assayed by four-channel gamma-rayspectrometer for their K, Th and U concentrations. Theoriginal data of RAP-1 is slightly modified by replacing threeU and Th nuggets each with the respective mean (3.90 and4.99). The data set thus obtained is used, along with theoriginal data of RAP-6, in this study. Brief statisticalinformation about the different radio-elemental parametersof the samples is given in Table 1.

Th/K Ratio and its Implications

Th/K ratio is largely a function of detrital mineral contentin sands, but of clay mineral content in shales (Rider, 1996).Heavy minerals dominate in sandstones with high (>6)Th/K ratios, whereas mainly K-feldspars dominate insandstones with low (<4) Th/K ratio. Thus, for coarsersiliciclastics (quartzarenite and subarkose) of the GulcheruFormation Th/K ratio may be used to discriminate detritalmineralogy. Since, the detrital mineralogy of siliciclasticrocks is controlled by source rock, provenance reliefand its stability (tectonics), intensity of chemical weatheringin the provenance (therefore climate), distance of sedimenttransport and depositional environment; here, attempt hasbeen made to use Th/K ratio as an indicator of the abovefactors. The anomalous Th/K values i.e., >6 or <4 aredefinitely outcome of the factors, which influence survivalof Th and K bearing minerals in sedimentary environment.Th bearing minerals are more stable compared to K bearingminerals in natural sedimentary environment. Thus, Th/K

SAMPLE ANALYSIS

DATA ANALYSIS AND INTERPRETATION

JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

PROVENANCE CHARACTERISTICS AND DEPOSITIONAL HISTORY OF THE GULCHERU FORMATION, A. P. 321

ratios of coarser siliciclastics are mainly dependent uponfactors that are responsible for survival of K bearingminerals (K-feldspar).

On the basis of Th/K values (in 10-4), the samples canthus be grouped into three classes viz., Gr-1 with Th/K values> 6, Gr-2 with 6 � Th/K � 4 and Gr-3 with Th/K <4 (Fig.2).Th/K ratio (>6) of first group of samples may indicate either(a) reworking of sediments (e.g., in beaches), during whichunstable minerals like feldspars are destroyed and stableand resistant minerals like quartz and heavy minerals areselectively concentrated; or (b) intense chemical weathering(warm and humid climate) over a stable provenance of lowrelief, during which feldspars and other unstable mineralsare destroyed while resistant minerals, capable ofwithstanding chemical weathering, are concentrated; or(c) mixing of highly matured and heavy mineral (especiallymonazite) rich sediments with sediments of moderatemineralogical (and textural) maturity; or (d) differentcombinations of above factors. Moreover, elimination offeldspars through mechanical destruction (Wright, 2004) inaeolian environment and concentration of heavy mineralsin fluvial channels or beaches also enhance Th/K ratio.

Th/K ratio of second group of samples may indicate(a) moderate relief and/or (b) moderate chemical weathering(semi-arid climate) in provenance, during which partialdestruction of feldspars takes place and near quantitativetransfer of Th and semi-quantitative transfer of K takes placefrom provenance to the site of deposition. Th/K ratio (<4)of third group of samples may indicate either (a) high relief,thus faster erosion of source rocks giving little scope forelimination of unstable minerals; or (b) cold and/or aridclimatic conditions in provenance. These lead to quantitativetransfer of K-feldspars and heavy minerals from provenanceto site of deposition.

Statistical analysis of Th/K ratios of the samples (n =82) show that they are almost lognormally (x̄: 4.32, G.M.:3.18, Median: 3.08) distributed (Table 1). Th/K values arenow plotted against depth along boreholes to know aboutprovenance characteristics, its relief and stability during thegeneration of sediments corresponding to a particular depth(along the borehole) (Figs.3 and 4).

Th/U Ratio and its Implications

Based on laboratory analysis of many samples of varyinglithologies Adams and Weaver (1958) concluded that Th/Uratio is often strongly linked with depositional environment.They suggested that when the ratio is <2 (U rich), thedepositional environment promoted U fixation underprobable reducing conditions and is most commonly marine.By contrast, Th/U ratio values >7 (U poor) imply Umobilization through weathering and/or leaching andtherefore indicate an oxidizing, possibly terrestrial,environment.

Contrary to suggestions made by Adams and Weaver(1958) on depositional environment it is found that low Th/U ratio (<2), in addition to marine environment, may alsobe an indicator of aeolian environment. Leaching out of Uin sedimentary environment is primarily dependent uponthree factors viz., availability of oxygen, presence of waterand presence of organic (humic) materials. In case of

Table 1. Summary statistics of radio-elements’ concentration and elemental ratios

Borehole RAP-1 [n = 73] RAP-6 [n = 9] All Samples [n = 82] Proterozoic Cratonic(Pitted, grey, ferruginous and (Pink massive quartzite) Sandstone

pink massive quartzites) (Condie, 1993)

Elements Range Mean S.D. Range Mean S.D. Range Mean S.D. Mean

K [%] 0.10 – 4.20 1.67 1.23 0.15 – 1.70 0.66 0.49 0.10 – 4.20 1.56 1.22 0.88

Th [ppm] 1.00 – 12.00 3.87 2.53 1.00 – 11.00 4.69 3.94 1.00 – 12.00 3.96 2.70 4.20

U [ppm] 1.00 – 12.00 3.15 2.74 2.00 – 17.00 7.11 4.83 1.00 – 17.00 3.58 3.25 1.20

Th/K 0.43 – 10.00 3.82 2.83 2.14 – 20.00 8.43 6.45 0.43 – 20.00 4.32 3.65 4.77

Th/U 0.08 – 7.00 2.13 1.93 0.30 – 1.43 0.63 0.37 0.08 – 7.00 1.96 1.88 3.50

0.0

1.5

3.0

4.5

0 3 6 9 12

Th (ppm)

K (

%)

RAP-1

RAP-6Th/K < 4.00

6.00 ≥ Th/K ≥ 4.00

Th/K > 6.00

Fig.2. Th-K plots of the samples from boreholes RAP-1 and 6.

JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

322 HIMADRI BASU AND OTHERS

Fig.3. Variation of Th, K, U, Th/K and Th/U vs depth (m) in borehole RAP-1.

Fig.4. Variation of Th, K, U, Th/K and Th/U vs depth (m) in borehole RAP-6.

JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

PROVENANCE CHARACTERISTICS AND DEPOSITIONAL HISTORY OF THE GULCHERU FORMATION, A. P. 323

sediments of the Proterozoic Gulcheru Formation, wherelittle or no organic matter is found, leaching out of U hadbeen mainly controlled by other two factors. Thus, sedimentsdeposited in an aeolian environment are also likely to showlow Th/U values, even if oxygenic atmospheric conditionsprevailed. Presence of thick (2-5 m) beds of hematite richsediments (Basu et al. 2007) within the Gulcheru Formationindicates that atmosphere had sufficient oxygen during itsdeposition. Since, sediments of the Gulcheru Formation aremainly deposited in marginal marine (Basu et al. 2007;Reddy et al. 1990) set up; the Th/U ratio could be used asan indicator of availability and non-availability of water.Thus, high Th/U ratios may indicate fluvial environment orshallow marine transgressions during which both oxygenand water are available to leach out U from sediments.Moreover, concentration of heavy minerals (especiallymonazite) in beaches and fluvial channels may renderhigh Th/U ratio to the sediments. Low Th/U values on theother hand indicate deeper water, marine regression oraeolian environment during which non-availability ofeither oxygen or water prohibits leaching out of U fromsediments. Moreover, concentration or addition of heavymineral (especially zircon) rich sediments may render lowTh/U ratios to the sediments. Zelt (1985) showed that Th/Uratio of Upper Cretaceous sediments in Colorado, Utah andNew Mexico are correlatable with transgression andregression cycles. Doveton (1991) used Th/U ratios toestimate palaeo-redox conditions at the time of deposition,which he in turn used to identify transgressive andregressive intervals.

Th-Depth (Th-D) and K-Depth (K-D) Curves and theirImplications

Th-D and K-D curves are obtained by plotting Th andK concentrations against depth along the boreholes (Figs.3and 4). Separate consideration of the curves does not implyanything other than depth-wise variation in concentrationof the concerned elements; but simultaneous considerationof Th-D and K-D curves is found to give useful informationon a number of provenance characteristics such as reliefand stability of source area, nature of source-area weathering(climatic condition of provenance) and source-areahomogeneity. A number of combinations that may arisedepending upon relative concentrations of Th and K, andtheir implications are outlined below:

1. Concentration of both Th and K increases steadily: Thisimplies generation of more and more fresh materialsand their quick dumping into the site of depositionwithout any major changes in mineralogical

composition of the sediments. A period of instabilityand high relief in provenance (beginning of a tectono-sedimentary event).

2. Concentration of both Th and K decreases steadily:This implies generation of less and less fresh materialdue to lowering of relief. A period of ensuing stabilityin provenance (end of a tectono-sedimentary event).

3. Concentration of both Th and K remains fairlyconstant: This implies a steady state of weathering inprovenance during which the rates of production ofresidues through weathering is balanced by the ratesof removal of residues by erosion. Near constanttectonic and climatic conditions over long periods.

4. Concentration of Th increases and K decreases:This implies efficient mixing of heavy-mineral(especially monazite) rich matured sediments withnewly generated detritus from a chemically weatheredprovenance or chemical weathering of a provenancein which pockets of Th-rich granites are present.

5. Concentration of Th decreases and K increases: Thisis an abnormal situation in case of a granitoidprovenance as Th bearing minerals are both chemicallyand mechanically much more resistant than K-bearingminerals. Thus, this may indicate source areaheterogeneity and the sediments are probably derivedfrom a source rock having less Th and relatively moreK concentration viz. granodiorite, syenite and basicrocks (possibly from schist belts).

6. Concentration of Th is near constant and K decreases:This implies loss of K-feldspar during bed-loadtransport or by mechanical destruction in aeolianenvironment. This may also imply increasing intensityof chemical weathering in the provenance.

7. Concentration of Th is near constant and K varies: Thisimplies variable chemical weathering of the provenancerock. Thus, Th contribution remains constant but Kconcentration varies depending upon the extent ofchemical weathering.

Interpretation of Data and Discussion

The siliciclastics, deposited in any sedimentary basin,are derived from the surrounding highlands (provenance)and their chemical character depends on source-areacomposition, mineral fractionation (hydraulic sorting),climate, tectonic setting (Bhatia, 1983; McLennan, 1989)and depositional environment. There could not be anyambiguity that the vast granitoid terrain with small enclavesof schistose belts surrounding the Cuddapah Basin providedthe detritus for the Gulcheru Formation. From the radio-elemental signature, registered in these detrital sediments,

INTERPRETATION OF DATA AND DISCUSSION

JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

324 HIMADRI BASU AND OTHERS

we will try to decipher their provenance characteristics anddepositional history.

During this study, it is assumed that the sediments didnot suffer any significant loss of K during diagenesis andthey did not undergo K-metasomatism. Further, the datafrom top 5.00m of both the boreholes have been excludedfrom this study as physical and chemical weatheringinfluences the elemental signature of the near surface rocks.In addition to this, the data up to 14.00m from boreholeRAP-1 is not considered, as a small mineralized band isintercepted in this zone.

Provenance Characteristics

K-D and Th-D curves pertaining to borehole RAP-1(Figs. 3 and 5) show that between depths 105.00-103.50 m,96.25-94.35 m, 93.15-91.30 m, 88.40-87.20 m, 85.20-81.20m, 75.10-73.60 m, 52.95-51.50 m, 41.20-38.60 m, 27.50-25.85 m, 23.80-22.50 m and 21.00-19.70 m concentrationof Th increases in spite of decreasing K content. Combinedeffects of moderate chemical weathering of heterogeneousprovenance and/or mixing of heavy mineral bearingsediments with chemically weathered detritus may producesediments of such character. Moderate (2 - 7) Th/U ratioscorresponding to these depths, except for 96.25-94.35 m,88.40-87.20 m, 52.95-51.50 m and 23.80-22.50 m, implyincreased contribution of sediments from pockets of Th-rich rocks in a moderately chemically weathered provenance(e.g., a TTG terrain with pockets of granites). Contrary toit, sediments corresponding to depths 96.25-94.35 m, 88.40-87.20 m, 52.95-51.50 m and 23.80-22.50 m show low Th/U ratio (<2) and thus possibly imply addition of zirconbearing sediments. Again, between 94.35-93.15m, 54.00-52.80m and 30.00-27.90m along borehole RAP-1 (Fig.3 and5) and between 19.75-9.90m along borehole RAP-6 (Fig.4)Th concentration remains constant but K concentrationincreases. At these depths sediments show low Th/U (<2)and low to moderate Th/K (<6) ratio. Decrease in Uabundance against that of K at depths between 94.35-93.15 m and 30.00-27.90 m (RAP-1, Fig.3) implies partialcontribution of sediments from pockets of probably alkalicultramafic rocks (e.g., kimberlite) in the source area.Presence of K-rich granites in the provenance, which aregenerally known to be enriched in U, is indicated in othercases. Similarly, the presence of pockets of some other rocktypes with low Th/K ratio [felsic volcanics (Th/K = 0.86-2.16), basalt (Th/K = 2.32), data from Condie, 1993; gabbro,diabase (Th/K = 2.57), data from Kobranova, 1989] in theprovenance is indicated by decrease in Th with increasingK content at depths between 81.20-78.90m, 70.30-68.50m,51.40-49.25m, 47.50-45.60m, 43.00-41.90m, 38.60-37.00m

and 18.75-16.50m along borehole RAP-1; and between50.90-40.30m along borehole RAP-6 (Fig.4). DecreasingU abundances in conjunction with low Th/U (<2) ratiobetween 51.40-49.25m and 47.50-45.60m implies derivationof sediments from basic rocks; while in other cases felsicvolcanic source-rock is implied. Possibly parts of thesediments deposited at these depths were derived from theschist belts where bimodal suits of volcanics are known toexist. An idea of either chemical weathering in the sourcearea or loss of K due to mechanical breakdown of feldsparsin aeolian environment can be conceived from the fact thatat depths between 37.00-35.30 m along borehole RAP-1(Figs.3 and 5); and between 30.00-22.75 m and 9.90-6.00 m along borehole RAP-6 (Fig.4) Th concentrationremains nearly constant but K decreases. ModerateTh/U ratios (2-7) of the sediments corresponding to depth37.00-35.30 m (RAP-1) possibly imply chemical weatheringbecause this will cause loss of both K and U from thesediments. On the other hand low (<2) Th/U ratios at allother depths imply loss of K possibly due to mechanicaldestruction in aeolian environment as this prevents leachingout of U from sediments.

Depositional History

Th-D and K-D curves, Th/K-D and Th/U-D plots,pertaining to borehole RAP-1, show three major phases oftectono-sedimentary events. The boundaries are fixed at108.00m and 102.00m depths along the borehole (Fig.3) onthe basis of three parameters viz.

1. Sharp fall in Th/K ratios [rapid change in provenancerelief – faulting or rifting].

2. Simultaneous steady increase in Th and K from verylow concentrations [generation of fresh material].

3. Sharp fall in Th/K ratio and initial low concentrationsof U followed by higher concentrations and low Th/Uratio [transgression of marine water].

The boundary between first and second phases matcheswell (minor mismatch of the order of 50cm is due to samplinginterval) with the lithological boundary between the GoduKonda and Bolaguntacheruvu Members (shown in thegraphic litho-log; Fig.3). This boundary is characterized bya sharp fall in Th/K ratio [8.33 to 3.33] and is followed byhigher concentrations of U (Fig.3). Van Buchem et al. (1992)showed that the early-Pliensbachian transgression in thecostal section of Redcar and Robin Hood’s Bay ischaracterized by a sharp fall in Th/K ratio and is followedby high concentrations of U. Thus, this boundary possiblymarks a transgressive event. On the basis of sedimentologicalstudies also, Basu et al. (2007) showed that theBolaguntacheruvu Member (dark-brown ferruginous

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quartzite) was deposited over the Godu Konda Member(pink massive quartzite) during marine transgression. Asharp fall in Th/K ratio also indicates a sharp change(upliftment) in provenance relief. Thus, it is most likely thatsubsidence of the basin along basinal growth faults (Kale,1995) paved the way for marine transgression over earlierdeposited sediments.

Sediments below this boundary (i.e., sediments of firstphase - the Godu Konda Member), are characterized by highTh/K ratios (>6; RAP-1, mean 8.08; RAP-6, mean 8.43)and low Th/U ratios (<2.00; RAP-1, mean 0.87; RAP-6,mean 0.63) (Figs.3,4 and 5). High Th/K ratios indicate thatthe sediments might have been derived from an intenselychemically weathered, stable granitoid provenance of low

Fig.5. Summary of provenance history.

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relief and/or underwent sufficient reworking. Alternatively,high Th/K values are due to elimination of feldsparsthrough mechanical destruction (Wright, 2004) in aeolianenvironment. Low Th/U values support aeolian environmentof deposition (see implications of Th/K and Th/U ratios).Again, sediments deposited in beach environment may alsoattain very high Th/K values due to reworking. But it willalso have high Th/U ratios due to leaching out of U, providedthe atmosphere is oxygenous. Presence of hematite richsediments (Bolaguntacheruvu Member) above the GoduKonda Member indicates that the atmosphere had sufficientoxygen. Since, the presence of water is must for leachingout of U in sedimentary environment, even if oxygenicatmospheric conditions prevailed; here, the low Th/U valuesare thought to be caused by deposition of sedimentspredominantly in aeolian environment. Here, it could bementioned that though, low Th/U values (<2) are suggestedto be indicator of reducing environment (deep marine) ofdeposition by Adams and Weaver (1958), it is observed thatmixing of zircon (richer in U content compared to Th) richmatured sediments may also render low Th/U ratio to thesediments giving a false impression of reducing environment.Petrographic studies have revealed the presence ofzircon as detrital grains and also as fine inclusions in detritalquartz grains in the Gulcheru Formation. The fluctuationsin Th/U values (Fig.4) may be attributed to availability ofwater from ephemeral streams, generated during occasionalraining (semi-arid climate), which helped in leaching out ofU from sediments. Otherwise, selective concentration ofheavy minerals (explicably zircon), particularly in thesestream sands, at different stratigraphic levels may haveresulted fluctuations in Th/U values. Thus, the pink massivequartzite (Godu Konda Member) was deposited in fluvio-aeolian environment and the climate of the depo-centre wasmost probably semi-arid.

Sediments of the second phase initially show asimultaneous steady rise in Th and K concentrations (Fig.3),which indicates generation of more fresh material inprovenance and their quicker deposition. The Th/K valuescorresponding to these depths indicate a relatively high reliefin provenance. Later, sediments show a steady decline bothin its Th and K concentrations, implying generation of littleor no fresh material, low relief and a period of ensuingstability in provenance. Interestingly, initial sediments ofthis phase that were deposited during marine transgressionshow low Th/U values in spite of availability of both oxygenin the atmosphere and water (interpretation given above).This may be due to deepening of water column (implyingreducing environment) or by addition of zircon richsediments. However, here low Th/U values have been

attributed to adsorption of U on hematite, which occurs inabundance both as matrix and cementing material in thedark-brown ferruginous quartzite (BolaguntacheruvuMember). From graphic litho-log (Fig.3) it is evident thatthe boundary between second and third phases ofsedimentation corresponds to the lower part of the greycrossbedded quartzite (Gandi Member). Thus, during secondphase of sedimentation, the Bolaguntacheruvu Member wasdeposited near shore and the lower part of the Gandi Memberwas deposited in relatively deeper water, where water depthswere sufficient enough to prohibit the precipitation ofhematite. The second phase ended with short-term marineregression and lowering of provenance relief, which is wellreflected in corresponding low Th/U (1.00) and high Th/K(10.00) values.

Third phase of sedimentation continued for longperiod. Within this phase two sub-phases, 3A and 3B, withboundary at 56.00m are recognized (Fig.3). Beginning ofthe 3A sub-phase experienced drastic change in provenancerelief and transgression of marine water (subsidence of thebasin) as reflected by simultaneous steady rise in Th and Kconcentrations, sharp fall in Th/K ratio (10.00 to 2.11), lowTh/U value (1.00) and it is followed by higher concentrationsof U. Subsidence of the basin along growth faults, like secondphase, possibly led to transgression of marine water.Beginning of the 3B sub-phase is also characterized bysimultaneous rise in Th and K concentrations, low Th/Uvalues and minor fall in Th/K ratio. But it shows very highU concentration. This condition prevailed up to a depth of50.00 m (Fig.3). This indicates deepening of water-columnso much so that it led to the development of reducingenvironment, which prohibited mobility of U from thesediments. The Th/K values (<4) corresponding to thesedepths, indicating upheaval of provenance (or reciprocalsubsidence of the basin), also corroborate it. Th/U – D curveshows zigzag pattern between 102.00 – 56.00m and 45.00– 18.00 m (Fig.3) indicating fluctuation in availability ofthe conditions (i.e., oxygen or water-depth) that helped inleaching out of U. These fluctuations in a marginal marineset up may be attributed to minor transgression andregression episodes. A major regression and lowering ofprovenance relief is reflected in simultaneous steady declineof Th and K, low Th/U (0.50) and high Th/K (10.00) valuescorresponding to 14.00m depth (Figs.3 and 5).

CONCLUSION

Study of provenance characteristics and depositionalhistory, on the basis of U, Th and K abundances, shows thatthe sediments of the Gulcheru Formation were derived from

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a moderately chemically weathered granitoid terrain.Pockets of felsic volcanics, basalts and alkalic rocks werealso present in the provenance. Mixing of reworked andheavy mineral bearing matured sediments with newlygenerated detritus took place at different times. Three majorphases of sedimentations led to the deposition of theGulcheru Formation in Tummalapalle-Somalollapalle areas.Sediments of the first phase (pink massive quartzite: GoduKonda Member) were deposited in fluvio-aeolian set up insemi-arid climatic condition. Second phase of sedimentationstarted with marine transgression over pink massivequartzite. Seawater transgressed as the depo-centre sankalong basinal growth faults. In other words, this enhancedprovenance relief. During this phase dark-brown ferruginousquartzite (Bolaguntacheruvu Member) and lower part of thegrey crossbedded quartzite (Gandi Member) were deposited.This phase was short-lived and ended with short-term marine

regression and lowering of provenance relief. Third phasestarted with marine transgression as the basin sank furtherdown. Minor marine regressions and transgressionscharacterized the rest of sedimentation history of the thirdphase. At the middle of this phase, the basin perhaps sankfurther down and the site of deposition became quite deep.Towards the end of this phase the seawater regressed and alow relief in the provenance was maintained.

Acknowledgement:The authors are thankful to thereviewer for his critical comments and suggestions, whichhave helped immensely to improve the quality of thepaper. Authors are also thankful to all geoscientists of thedepartment who gave their valuable suggestions during thecourse of geological work and preparation of this paper.The spontaneous help of M. Rengarajan, SO/F duringstatistical analysis of data is duly acknowledged.

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(Received: 23 September 2008; Revised form accepted: 20 March 2009)