Heavy mineral contents and provenance of Late Quaternary sediments of southern Kerala, Southwest India

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1 Indian Journal of Geo-Marine Sciences Vol. 42(6), October 2013, pp Heavy mineral contents and provenance of Late Quaternary sediments of southern Kerala, Southwest India S Anooja, D Padmalal*, K Maya, S Vishnu Mohan & B Baburaj Centre for Earth Science Studies, Thiruvananthapuram , India *[ drdpadmalal@gmail.com] Received 21 March 2012; revised 22 August 2012 Present paper deals with provenance and depositional history of the Late Quaternary sediments including heavy mineral placers in the coastal lands of Kollam district, SW India. Kollam coast is endowed with estuaries, old coastal plains with ridge-runnel systems and barrier beaches. Mineralogical analysis reveals that sediments in the estuarine basins that are seen entrenched over the Neogene sedimentary deposits were derived from dual sources. Heavy mineral residues of the upper estuarine zones are generally garnet bearing and are of alluvial origin. At the same time, sediments in the lower estuary are garnet-free and are derived from the nearby littoral zones during the tidal processes. Heavy mineralogical data together with statistical analysis discloses that the garnet and pyribole-free heavy mineral suite in the beaches and nearby coastal plains of the Kollam coast are primarily derived from denudation of the Neogene sedimentary deposits in the coastal lands. They are evolved during the rising phases of the sea level in the Late Quaternary period. [Keywords: Late Quaternary, Heavy mineral, Holocene, Provenance, Neogene] Introduction Southern Kerala coast, south of the Achankovil Shear Zone (ASZ) is endowed with coast parallel and coast perpendicular water bodies. The former falls under the Partly Closed and Closed category of lagoons whereas the latter under the Estuarine Lagoon category. Estuarine Lagoons are seen entrenched over the Warkalli and Quilon Formations of Neogene age. Holocene sediments in the coastal lowlands, inclusive of the Estuarine Lagoons have a maximum thickness of ~40 m. These sediments are deposited under a complex depositional regime of the Holocene epoch marked by fluctuating sea level positions and climatic conditions 1. Interestingly, the coastal stretch of Kollam and parts of Thiruvananthapuram districts, considered for the present study, is known for many economically viable and strategically significant deposits of heavy mineral placers. A micro-level review of literature reveals that, although many studies have been carried out on the heavy mineral deposits/contents in the coastal sands of southern Kerala 2-7 adequate attention has not been given to unfold the provenance and sedimentation history of these deposits. Therefore, a detailed study has been performed here to characterise the quantity and quality of heavy minerals in the finer sand fraction of sediments in the coastal lands of Kollam and parts of Thiruvananthapuram districts, using a set of borehole cores and surface sediment samples. *Corresponding author: Materials and Methods Area selected for the present study falls mainly within the coastal lands of Kollam district and partly within Thiruvananthapuram district. Major landform features of the coastal lands of the study area include barrier beaches, ridges and swales, tidal flats, lagoons, floodtide islands, bay-head delta, etc. (Fig. 1). The study area is located between North latitudes 8 45'-9 50' and East longitudes 76 25'-76 45'. The region is generally undulating with low altitude hillocks ranging in height between 10 m and 40 m. Study area forms a part of the South Kerala Sedimentary Basin (SKSB) 8,9 with maximum sediment fill of ca. 700 m around Ambalappuzha. Morphotectonic and sea level oscillation studies of the coastal tracts of the southwestern coast of India reveal that a major part of the SKSB is in a submerged block, and areas in the south and north of it are characterised by cliffs and bays indicating emergence and subsequent erosion under the rising sea levels in the Late Quaternary period 10,11. Though the region covers only a small portion of Kerala, geologically it is the most important formation in the entire west coast, south of Narmada rift 12. Estuaries along the southwestern part of peninsula such as the Kayamkulam estuary, Ashtamudi estuary, Paravur estuary and Nadayara estuary are seen successively towards the southern part of the Achankovil Shear Zone. Out of these, the latter three estuaries show antecedent characteristics and are entrenched over the Neogene sedimentary deposits. In addition to this, Sasthamkotta

2 750 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER 2013 Sl No. Sub-Environment Length (km) River Table 1 Salient features of various sub-environments of the study area Area (km 2 ) Width (km) Depth (m) Remarks 1 Pallikkal river Coastal plain river 18 2 Kallada river Mountainous river 18 3 Ithikkara river Lowland river 18 4 Ayroor river Coastal plain river 18 Lagoon/Estuaries 5 Kayamkulam Lagoon Coast parallel 6 Ashtamudi estuary Coast perpendicular 7 Paravur estuary Coast perpendicular 8 Nadayara estuary Coast perpendicular Lake, the largest freshwater lake in Kerala, and several other wetland bodies like Chelupola, Chittumalachira, Polachira, and Kotta lakes, are also located in the uplifted block 12. Table 1 summarises the salient features of the lagoons and their feeder channels. A total of 26 borehole cores with a depth range of 6-32 m were collected using rotary drilling. Upper sand dominant litho units ranging in thickness from 1.5 to 12 m were chosen for analysis. A total of 115 subsamples of 10 cm thickness were taken at 1 m interval and were subjected to the present study. In addition to the borehole samples, 51 surface sediment samples were also collected from various sub-environments such as beach, older coastal plain and rivers debouching into the estuary and subjected to heavy mineral examinations. Fig. 1 shows the location of the borehole cores and surface samples. Sand samples were subjected to dry sieving on a Ro-Tap sieve shaker using a standard set of ASTM sieves 13. Fine sand fractions were separated from the sieve sets and subjected to heavy mineral separation, using bromoform with specific gravity 2.89 at 20 C. Stain/iron oxide coating over the grains was removed boiling the sample with SnCl 2 -HCl mixture and the mineral grains were spread over glass slides and mounted using Canada balsm. Heavy fractions in the fine sands, thus obtained, were studied for mineral species following Mange and Maurer 14. Radiocarbon (C 14 ) dates of a few organic rich sediments at specific depths were determined at Birbal Sahni Institute of Palaeobotany, Lucknow for establishing chronological control of various lithounits including the sand layer in the borehole cores. Results Lithologic characteristics Figure 2 depicts the lithological characteristics of the borehole cores selected for the present study. Borehole cores are generally composed of an upper sand dominant layer which is followed downward by silt and clay dominant sediments. Organic rich sediments lying below the sand yeilded an age of 6117 ± 101 yrs BP in Puthenthuruthu borehole and 5350 ± 100 yrs BP in the Pozhikkara borehole. This clearly indicates that the sands in the coastal area have been deposited during Middle to Late Holocene. But the initiation of the deposition was in the end phase of Holocene Climatic Optimum (HCO) when the sea level had risen 3 to 4 m above the present sea level 11,15. Oldest date recorded in the study area is ± 148 yrs BP at 31 m in the Puthenthuruthu borehole. Heavy minerals contents The total heavy mineral content in the fine sand fraction of the Kayamkulam lagoon varies from 6.92% to 13.79% (av ). Heavy mineral residue comprises opaques (av %, range: %) and sillimanite (av %, range: %) as the major minerals (i.e. mineral with number percentage >5%), zircon as minor mineral (with number percentage 1-5%) and rutile and monazite as the trace minerals (Table 2). Pallikkal river is the major drainage system bringing sediments from the landward side. River merges with the lagoon in its southern arm near Vatta kayal. THM content in the fine sand fraction of the river varies from 4.85% to 19.57% (av.11.01%). Heavy mineral residue comprises opaque and sillimanite as the major minerals and garnet as the minor mineral (Table 2). These three minerals together constitute 95.36% of the total heavy mineral species. Rutile, monazite and pyribole are seen only in traces. Based on the energy regime prevailing in the Ashtamudi Estuary 16, the entire system can be divided into three zones: 1) lower estuary 2) central basin and 3) upper estuary. Lower estuary is subjected to constant ebbing and flooding and has a high energy regime in most part of the year. Many flood-tide islands are developed in the landward side of the tidal inlet. Eastern arm of the upper estuary, influenced by the influenced by the Kallada river, also enjoys high

3 ANOOJA et al.: HEAVY MINERAL CONTENTS & PROVENANCE OF LATE QUATERNARY SEDIMENTS 751 Fig. 1 Study area showing locations of surface and boreholecore sediments energy regime due to river influx. Central basin is not considered for the present study as the region is floored essentially by silt and clay. Borehole core retrieved from the Puthenthuruthu island a flood tide island begins with 12 m thick sand which is subjected to detailed heavy mineral study. THM content in the borehole core varies from 8.27% to 20.50% (av.14.29%). Heavy mineral species include opaque (av %) and sillimanite (av. 8.71%) as the major member and zircon (av. 3.96%) as the minor member. Rutile and monazite are recorded in trace quantities (Table 2). Interestingly, the heavy mineral suite in the upper estuarine zone is quite different from that of the lower estuary. THM content varies from 4.23% to 46.67% (av %). Major minerals noticed in the upper estuary include opaque, sillimanite and garnet (almandine variety). Pyriboles (pyroxene amphibole); zircon and biotite occur as minor minerals and, monazite and rutile occur as trace minerals. THM content in the Kallada river is almost similar to that of the upper estuary. Like the case of the Ashtamudi Estuary, the energy regime prevailing in the lower estuary, central basin and upper estuary are almost similar in the case of

4 752 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER 2013 Fig. 2 Lithologs of the borehole cores chosen for the present study. Note the C 14 dates at specific levels of the borehole cores. Depth is given in metres Paravur estuary as well. THM content of the lowe estuary varies from 2.61% to 43.78% (14.87%). Heavy mineral residue comprises opaque and sillimanite as the major minerals and zircon the minor mineral. Rutile and monazite occur in trace amounts. Upper estuarine region comprises opaque and sillimanite as major mineral, zircon as the minor and, monazite and garnet as trace minerals. Ithikkara river sediments exhibit almost similar content of heavy minerals as that of the upper estuary. THM content in the river sediment varies from 8.27% to 13.95% (av %). Opaques and sillimanite occur as major minerals and, garnet and zircon as the minor minerals. Minerals like rutile and pyribole are seen in trace quantities. Nadayara Estuary is fed by the Ayroor river which is a small coastal plain river (Table 1). In the Nadayara Estuary, the heavy mineralogical study is confined only to the upper estuary. THM content of the Nadayara Estuary varies from 3.44 to 14.74% (av. 7.32%). Heavy mineral residue comprises opaques and sillimanite as the major minerals, zircon and garnet as minor mineral, and monazite and pyribole as trace minerals. Ayroor river exhibits an almost similar variation in the heavy mineral content as that of the upper estuary. Coastal sand comprises two categories - beach sand and strand plain/old coastal sand. THM content in the beach sand between Parvur and Alappad varies from

5 ANOOJA et al.: HEAVY MINERAL CONTENTS & PROVENANCE OF LATE QUATERNARY SEDIMENTS 753 Table 2 Mean and standard deviation of total heavy minerals (Wt %) and various heavy mineral species (number %) in the sediment samples of the study area Location/sample THM Opaques Sillimanite Zircon Rutile Monazite Garnet Pyriboles Biotite Sphene Borehole cores Puthenthuruthu ( ) Pangod Sasthamkotta (main) ( ) West Kallada ( ) ( ) ( ) ( ) ( ) Munrothuruthu 8.31 Ashtamudi ( ) ( ) ( ) ( ) Kotapuram Kidappuram ( ) ( ) ( ) ( ) Pozhikkara Nedungolam ( ) ( ) ( ) ( ) Adichanalloor ( ) Nadayara 7.32 ( ) Tevalakkara 9.78 ( ) Chavara Maruthadi ( ) ( ) ( ) ( ) ( ) ( ) ( ) Kollam ( ) Eravipuram ( ) Ayiramthengu Surface sediments ( ) ( ) ( ) ( ) ( ) 8.71 ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 5.67 ( ) ( ) ( ) ( ) ( ) 3.96 ( ) 4.21 ( ) 1.96 ( ) 2.22 (0-4.44) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 1.29 ( ) ( ) ( ) 2.13 ( ) 2.68 ( ) 4.08 (0-2.22) ( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2.44 ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 1.11 ( ) 0.88 ( ) 2.49 ( ) 1.85 ( ) 5.19 ( ) 1.10 ( ) 3.86 ( ) ( ) ( ) ( ) ( ) ( ) 1.88 ( ) 6.04 ( ) ( ) 2.80 ( ) ( ) ( ) ( ) ( ) 1.04 ( ) ( ) ( ) ( ) ( ) ( ) ( ) Beach (PVR - ALPD) ( ) ( ) ( ) ( ) ( ) ( ) ( ) Kollam (ASRM- PLMK) ( ) ( ) ( ) 1.64 ( ) 1.29 ( ) 0.45 Kayamkulam ( ) ( ) ( ) 2.78 ( ) ( ) ( ) 0.79 ( ) 0.5 Pallikkal thodu ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) Red sand ( ) ( ) ( ) 3.77 ( ) 3.42 ( ) 0.69 Warkalli ( ) ( ) ( ) ( ) PVR-Paravur, ALPD-Alappad, ASRM-Asramam, PLMK-Pallimukku, THM-Total heavy minerals

6 754 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER 2013 Table 3 Heavy mineral contents in the Pozhikkara and Pangod borehole cores Sample location and Depth (m) THM Opaques Sillimanite Zircon Rutile Monazite Garnet Pyriboles Biotite Sphene Pozhikkara Pangod to 99.37% with an average 74.57%. Among the two categories of coastal sands, the beach sand exhibits the highest content of THM. Heavy mineral assemblage consists of opaque, sillimanite and zircon as the major minerals, rutile and monazite as the minor and garnet as the trace minerals. Contrary to beach sand, the strand plain sand exhibits marked difference in the content of heavy minerals. Opaques and sillimanite are present as the major minerals, zircon as the minor and, monazite and garnet as the trace minerals. Content of total heavy minerals (THM) in the fine sand fraction of the Tertiary sediments exposed in the boundary of the lagoons vary from 7.82% to 24.40% with an average of 14.50%. On an average, the heavy mineral residue is composed of 62.88% of opaque and 37.12% of non-opaque minerals.non-opaque comprised of sillimanite (av %, range: %) as the major mineral. Traces of garnet, monazite and some other unidentified heavies are also noticed in the sample. Discussion Study of detrital minerals received considerable importance as it is one of the reliable tools to unfold depositional regimes and source rock characteristics of sedimentary environments 17. In the present paper, the results of the detailed analysis of heavy minerals in the fine sand fractions of the coastal lands of southern Kerala comprising the Kayamkulam, Ashtamudi, Paravur and Nadayara estuaries as well as the adjoining landforms are discussed to decode the provenance and depositional history of sediments. Downcore variations of heavy minerals revealed marked changes in the quantity and quality of minerals in certain borehole cores retrieved from the nearshore regions as well as estuarine heads. In the Pozhikkara borehole core which is located in the littoral zone of the Paravur basin, the THM content exhibited an increasing trend towards the top of the borehole core (Table 3). This has been resulted from the constant winnowing of finer and lighter detrital grains seaward attributing a preferential enrichment of heavies rich in high dense minerals like opaque and monazite. Contrary to the Pozhikkara borehole core, the core collected near the estuarine head at Pangod in the Ashtamudi estuary revealed marked variation in the distribution of heavy mineral species (Table 3). In general, sediments in the river influenced upper estuarine regions show garnet as the major heavy mineral and are derived from the khondalitic suite of rocks in the hinterlands. However, the upper yellowish layer in the Pangod borehole core is devoid of garnet compared to the lower unaltered sand layer. This observed variation may be attributed to post depositional oxidation of garnets and other iron containing less stable minerals like pyriboles (pyroxene represented by hypersthene and amphibole represented by hornblende) under exposed conditions. Spatial distribution of heavy minerals in the surface sediments of the Ashtamudi estuary reveals that the content of minerals in the estuarine mouth is substantially high due to the input of heavy mineral rich sands from the adjacent beach/near shore areas 16. Spatial distribution of garnet indicated in Fig. 3 reveals the extent of riverine processes taking place in the upper estuary and adjoining regions. In short, the estuarine head is dominated by river input whereas the estuarine mouth by tidal processes, leaving the central part of the estuary a calm environment favourable for the deposition of mud dominated sediments. Such a process could be seen in the Paravur and Nadayara estuaries as well.

7 ANOOJA et al.: HEAVY MINERAL CONTENTS & PROVENANCE OF LATE QUATERNARY SEDIMENTS 755 Table 4 gives the summary of the heavy mineral investigations in various sub-environments of the coastal lands of Kollam district. The interrelationship existing Fig. 3 Variation of Garnet and Sillimanite along a transect from West Kallada (WK) to Estuarine Mouth (EM); RC River confluence with the Ashtamudi estuary among the various sources worked out from the heavy mineralogical data is depicted in Fig. 4. It is revealed from Fig. 4 that the beach environment is a lone component in the cluster-tree, indicating its unique depositional behaviour under the high energy, wavedominated regime. This peculiar energy regime is favourable for the deposition of high dense heavy minerals and transportation of low dense (lighter) components by the long shore current. Comparative low content of heavy mineral residue in the older coastal plain may be attributed to the role of aeolian activity in enriching the lighter minerals compared to heavier, denser minerals. Most of the garnet rich upper estuarine samples are closely knitted and deviation if any noticed is attributed to the contribution of garnet poor heavies from the Tertiary/ Neogene formations. One of the striking observations noticed in the examination of Table 4 Summary table of the heavy mineralogical studies carried out in the study area Sl. No. Environment/ sub-environment Reach Major heavies (>5%) Minor heavies (1-5%) Traces (<1%) Remarks/ observations 1 Kayamkulam Lagoon 2 Ashtamudi Estuary Lower estuary Opaques, sillimanite Opaques, sillimanite Upper estuary Opaques, sillimanite, garnet Zircon Rutile, Monazite Kayamkulam Lagoon is a coast parallel, partly closed lagoon separated from the Arabian Sea by a barrier beach. Zircon Rutile, monazite Pyribole, zircon, biotite Monazite, rutile. Ashtamudi Lagoon is a coast perpendicular 'Estuarine Lagoon' seen entrenched over the Neogene sedimentary formations. 3 Paravur Estuary 4 Nadayara Estuary Lower estuary Opaques, sillimanite Upper estuary Opaques, sillimanite. Upper estuary Opaques, sillimanite 5 Rivers Opaques, sillimanite, garnet 6 Coastal sand Beach (littoral zone) Ridges and runnels/strand plains Opaque, sillimanite, zircon Zircon Zircon Zircon, garnet Opaque, sillimanite Zircon, Rutile Rutile, monazite. Monazite, garnet. Monazite, pyribole. Zircon, pyribole Garnet in some samples, rutile. Rutile, monazite Garnet in some samples. Monazite, garnet in some samples Lagoon character and setting are similar to the above; the lower part of the borehole core ( m) collected from the estuarine mouth is composed of high content (11.11%) of garnet. Lagoon character and setting are similar to that of the earlier cases. Garnet is not a major member in the upper estuary; instead the mineral occurs as minor or even in trace amounts. The opaque minerals contain substantial percentage of magnetite. The thickness of this sand is <10m in certain borehole cores, the bottom sand is riverine and with high contents of garnet. 7 Red sand Opaque, sillimanite Zircon, Rutile Monazite, garnet. Occurrence of red sand is noticed as pockets, especially in Karunagappalli and Edava. 8 Warkalli sand stone (Neogene sediments) Opaque, sillimanite Zircon Monazite, rutile. Warkalli sand stones of Kundara, Shsthamkotta and Varkala are examined under this category.

8 756 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER 2013 features of high energy hydrodynamic regime. The present study highlights the fact that although the Kerala Khondalite Belt is the primary source of the heavy minerals in the study area, the garnet-free sediments might have evolved from chemical weathering and leaching of the minerals under exposed conditions. Therefore, the probable source of the garnetfree suite of minerals in the study area is the Neogene formations which was subjected to erosion during the rising spells of sea in the Late Quaternary period. On the other hand, the garnet and pyribole suite of minerals in the river mouth areas might have derived from the Kerala Khondalite Belt in the more recent erosive phases of Holocene. Fig. 4 Dendrogram showing linkages existing among various sample/sample groups. Note the deviation of beach-borne heavy minerals from the rest of the samples heavy mineral species is that the monazite grains in the beach and coastal plains resemble to that the Tertiary sediments. The study highlights that although the Kerala Khondalite Belt is the prime source of heavy minerals in the study area, the garnet-free suite in the beach and coastal plains might have evolved due to chemical weathering and dissolution of Fe- rich minerals under exposed conditions prior to deposition in the beach and coastal plains. Further, from the geomorphic view point, the coastal areas of southern Kerala host well developed cliffs. A large part of the Tertiary cliff would have been eroded by wave activity (i.e., cliff retreat) during the rising spells of sea in the Quaternary period. Therefore, the heavy minerals derived from the Tertiary sediments especially that of the Warkalli Formation, may be the probable source of heavy minerals in the beach, coastal plain and the nearshore environments. Conclusions The content of heavy mineral residue of the sand dominant sediments ranges from 4.23% to 46.67% in the Ashtamudi estuary, 2.61% to 43.78% in the Paravur estuary and 3.44% to 14.74% in the Nadayara estuary. Respective heavy mineral contents in the beach and inland coastal sands are 27.3%-99.37% and 5.03% %, respectively. Minerals include opaques, sillimanite and garnet as the major minerals and zircon, rutile, monazite and pyriboles as the minor/trace minerals in the river influenced areas. Marine influenced areas, on the other hand, are generally devoid of garnets and pyriboles. Further, the minerals in the seaward side are polycyclic and contain surface textural Acknowledgements Authors are thankful to the Director, Centre for Earth Science Studies (CESS), Thiruvananthapuram and Dr K. P. N. Kumaran, AGI, Pune for encouragements and support. Dr. C. M. Nautiyal, Senior Scientist, BSIP, Luknow for C 14 dates. One of the authors (SVM) acknowledges CSIR, New Delhi for SRF [09/909(0005)/2012-EMR-I]. References 1 D, Silting up of a Holocene mega lagoon along Kerala Padmalal D, Kumaran K P N, Nair K M, Baijulal B, Limaye R B & Vishnu Mohan S, Evolution of the coastal wetland systems of SW India during the Holocene: Evidence from marine and terrestrial archives of Kollam coast, Kerala, Quat. Int., 237 (2011) Rao G P, Sediments of the nearshore region of Neendakara coast and Ashtamudi and Vembanad estuaries, Kerala, India, Bull. Natl. Inst. Sci., 30 (1968) Sajan K, Studies on Mineralogy, geochemistry and Origin of modern sediments of the Ashtamudi lake, Kerala, PhD Thesis submitted to Cochin University of Science and Technology, Kochi, (1998) 199pp. 4 Babu D S S & Thrivikramji K P, Palaeogeographic interpretation of Kerala beach placers, southwest coast of India, Ind. J. Mar. Sci., 22 (1993) Prakash T N, Sediment distribution and placer mineral enrichment in the inner shelf of Quilon, SW coast of India, Ind. J. Mar. Sci., 29 (2000) Vinodkumar N, Sedimentology of the placer sands of Kerala coast, PhD Thesis submitted to the university of Kerala, Thiruvananthapuram, (2003) 117pp. 7 Ravindrakumar G R & Sreejith C, Relationship between heavy mineral placer deposits and hinterland rocks of southern Kerala: A new approach for source to sink link from the chemistry of garnets, Ind. J. Geo-Mar. Sci., 39 (2010), Nair K M, Sajikumar S & Padmalal coast, In National seminar on coastal evolution, process and products, KUSAT, Kochi, (1998) 12pp. 9 Nair K M, Padmalal D & Kumaran K P N, Quaternary Geology of South Kerala Sedimentary Basin an outline, J. Geol. Soc. Ind., 67 (2006) Bruckner H., Late Quaternary shorelines. In: Scott DB et al., (eds.), Late quaternary sea level correlations and applications,

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