15 May WESTERN GHATS, NEOTECTONICS AND KERALA BLACKSAND PLACERS: AN APPRAISAL
WESTERN GHATS, NEOTECTONICS AND KERALA BLACKSAND PLACERS: AN APPRAISAL
Dr. Thrivikramji.K.P. (Retd.)
Professor of Geology, University of Kerala
Karuiavattom Campus 695 581, India
Western ghats (WG), a.k.a Shyadri, roughly coinciding with west coast (WC) of peninsular India and facing the Laccadive sea, is a 1300 km long and nearly contiguous (but for the Palghat gap in Kerala) wall-like mountain, characterized by rather steep-western-slopes interrupted only by a step-like-aspect (hence the name ghats) in the cross profiles. Stepped-cross-profiles are indicative of multiple planation surfaces (i.e., due to repeated rejuvenations or uplifts) of varying geological ages. The WC is marked by a relatively straight and long shoreline flanking the WG escarpment, which is roughly 100 km away from the modern shoreline
The near-straightness of WC is a consequence of its rifting process and passivity of a trailing edge, which liberated the peninsula from the Gondwanaland to propel itself toward the Asian plate (a continent-to-continent collision) in the north, to create the lofty fold mountain belt of Himalayas. Rifting of Indian peninsula from the Madagascar-Seychelles marks the beginning of creation and further evolution of the WC, with features like inland plateau, steep escarpment and low-lying coastal land. Based on evidences available from the WC, Subrahmanya (2006) states that doming occurred around 93.0 ma and rifting started by about 88.0 ma, marking the origin of the WC of India and Shyadris. Subsequently, around 67.0 ma, Sahyadris and northern part of WC witnessed exceptional levels of basaltic magmatism connected to the Re-union hotspot.
Unlike the younger-fold-mountains, the WG is geologically heterogeneous, in that its northern part is built on flow basalts of Deccan volcanism, while older Dharwar rocks of the Indian shield underlie the central part and further to the south occurs the typical lithologies of southern granulite terrain. Younger sedimentary fills (Age: Cenozoic) of both marine and non-marine affinities have been located partly in the modern onshore and largely in the offshore and truly the onshore occurrences are only extensions of the larger offshore Kerala- and Konkan-basins
In fact, the trinity of relief, climate and lithology (including structure) plays a major role in the development of landforms by process of weathering and erosion Relief aspects of the WG are as old as late Mesozoic coinciding with separation of peninsula from Madagascar-Seychelles land mass and since then, this region underwent sub-aerial processes of weathering and erosion. Moreover, relief has been demonstrated by geomorphologists as a critical factor in controlling depth of weathering, steepness of surface and stream gradients, and hence in transfer of detritus down stream channels. In terrains of relatively higher relief and set in moist tropical climate with clear wet and dry spells (like in tropical monsoon regime), the amplitude of fluctuation of water table will be quite wide promoting good drainage in a thick zone of rock cover leading to the formation of an exceptionally thick layer of gruss with a capping of laterite. Presence of multiple planation surfaces in the WG itself needs to be taken as evidences for rejuvenation of the relief of the landmass.
According Radhakrishna (1993) the monsoon climate had dawned in the peninsula and WG in particular from Eocene period (~46.0 ma B.P.) onwards. However, later investigators fine tuned the monsoon circulation further. For example, Qiang et al (2001) reported evidences for a stronger monsoon system at 3.5 ma and 2.6 ma, i.e., in Pliocene But Shackleton et al (1984) based on continental and marine paleo-climatic records proposed that since the onset of glaciation around 2.5 ma, the strength of the Asian monsoon showed variability both on a short-term and long-term perspective. Data records from China, Indian and N.Pacific oceans denoted that Indian and east Asian monsoon systems were established about 8.0 ma ago (An et al., 2001) or during Miocene. The foregoing data thus reiterates the fact that monsoon system had perhaps undergone some degree of variation in respect of intensity during various periods of earth’s history.
Chemical weathering or Lateritisation
An extensive tract (~60%), closely spread over most of the midland region (Elv.=7.5 – 75.0 m) of Kerala, is literally clad with an ubiquitous cover of laterite (originally defined by Bucchanan, a French traveler, in 1807 from Angadipuram in Malappuram Dist. Kerala) manifesting as low-standing–nearly-flat-topped ridges and/or hills (or mesas) overlying the Pre-Cambrian crystalline basement. Laterite capping is also noticed over the Tertiary sedimentary fill, based on which An et al., 2001tended to identify a high-level laterite (residual on primary rocks) and a low-level laterite (secondary capping the sedimentary fill).. Following the line of King (1883), Narayanaswamy (2001) classified laterite into two, viz. primary laterite forming in situ and secondary/detrital laterite forming by partial or complete consolidation of transported “lateritic material”.
Later workers described it as a product of tropical weathering of rocks by desilication as well as enrichment of iron and/or aluminum (King, 1962). In general, estimating rate of weathering has all along been a difficult proposition. Yet, some new approaches have been designed and perfected by some workers. Table 1 shows the various estimates aggregated from several field studies.
Table 1 Rate of Formation of Laterite
|Freyssinet & Farrah (2000)||French Guyana||3.0|
|Boeglin & Probst (1998)||Up.Niger basin||1.3- 3.7|
|Thomas (1994)||Tropical areas||22.0|
In Kerala, two periods of lateritisation have been recognized, viz., one younger phase of post-Warkalli times (vouched by a thick and ubiquitous Laterite capping on Warkalli sediments) and an older phase that prepared and enhanced erodibility of source rocks of older Vaikom beds. Menon (1966), in yet another compelling piece of evidence from Karichal Cliffs, reported the unconformable relation between altered Pc-basement basement rock (Khondalite) and overlying Warkalli sediments, demarcated by an oligomictic conglomerate, in which clasts are bounded by a crudely laminated sandy-bauxitic-matrix. Later Goplakrishnan and Nair (1976) re-interpreted this conglomerate as an evidence lateritisation of Pre-Warkalli age and perhaps equating with the Pre-Vaikom phase of lateritisation. Ubiquitous laterite caps occur at varying elevations capping the Warkalli sediments at several sectors in the coastal land, like at Karichal, Varkala beach, and Kannur.
Being a trailing edge, with a complex lithological assemblage of variable geological ages, i.e., ranging from Proterozoic to Tertiary and modern, the WC including the Shaydris contains a host of evidences to underscore the active tectonism of this land mass. Yet, evidences of neotectonism, both in the Konkan and Malabar coasts, are much more compelling in comparison with paleotectonism.
Fig.1 Major and minor lineaments, Kerala (Vardarajan and Nair, 1978)
Signatures and proxies of active tectonism of the geologic past in peninsular India need to be deciphered from landform features, nature of drainage net, sedimentary stratigraphy or sedimentation history, presence of passive/dormant/active faults or fault scarps and seismicity. The single most important evidence for active tectonism in the Peninsula is the fact that it is perhaps the only land mass on planet earth that had propelled from the southern hemisphere to its present position in the northern hemisphere. Multiple erosion or planation surfaces in the WG is yet another piece of data in support of the episodic change in relief and hence rejuvenation of landmass that occurred in tandem with northerly and easterly movement of Peninsula. Regarding neo-tectonism (including Quaternary), evidences are immense. River terraces, knick points or cascades, sharp turns in channel geometry, ponding of water in the channels with or with out accumulation of fine sediment like silt and clay are some pieces of data in support of neotectonic adjustments or disturbances in other wise quiescent river flow.
Indeed (in the southern granulite terrain of which Kerala is part of), satellite imageries show a large number of small, medium and large lineaments belonging to major (e.g., NW-SE, NNW-SSE and N-S) and minor (e.g., NE-SW, NNE-SSW, and NW-SE) types, which primarily cut across lithological boundaries and even align themselves with the gross structure of the basement rocks (Fig.1). Differential movement along some of these lineament sets, co-promoted the formation of the proto-Vembanad Kayal basin. Similarly, down (gravity) faulting along some sets of lineaments toward the end of Tertiary era and the emergence of Warkalli sediment fill to subaerial conditions enabling lateritisation, created kayal basins of proto-Sasthamlkotta and proto-Ashtamudi (Thrivikramji et al. 2007).
The remarkable presence of the Tertiary sedimentary rocks below the ground, but at varying depths, between Neendakara in the south and Ponnani in the north, itself vouches for the continued subsidence of the Cenozoic sedimentary basin guided by the activation of lineaments related faulting. Fault control of this sedimentary basin and continuation of displacement of sediment fill (Fig.2) are other compelling evidences for the neotectonic activity in Kerala. Like wise, central and northern sectors of the WG also hold overwhelming evidences of neotectonism that manifest as raised beaches, raised shell beds etc.
The rock formations of Cenozoic, are chiefly confined to the coastal land (<7.5 m) and in parts of the midland. Table 2, a classification proposed by Najeeb (1999) is very instructive in that here the Vembanad formation of Quaternary and intrusives of Mesozoic find a place. Further laterite formation by implication initiated itself only by Quaternary. The cross section in Fig. 2, complied using well logs and data from outcrops do show how the lineaments manifest as a series of normal faults of variable displacement, resulting in the creation of the larger Kerala Basin most of which is now in the offshore of Kerala. A closer look at the stratigraphy reveals that during Tertiary, clastic Vaikom beds formed prior to the deposition of the shallow water carbonate (viz., Quilon limestone. Age: Mid. Miocene) and then it was followed by deposition of clastic sediments of younger Warkalli beds, which Raha et al (1983) interpreted as typifying an oscillation of the shoreline oceanward, to begin with, then landward and finally to oceanward again – an example of a regression-transgression-regression sequence.
Table 2 Stratigraphic sequence, Kerala (Najeeb, 1999)
|Vembanad formation||Sands, clays, molluscan shell beds, riverine alluviam and flood plains deposits, Latetite capping crystallines and Tertiary sediments|
1.6 to 66.4 ma
|Sandstone and clay with lignite seams
Limestone, marl, clays/calcareous clays with marine and lagoonal fossils.
Sandstones with pebble and gravel beds, clays and lignite and carbonaceous clay.
|Intrusives with veins of quartz, pegmatite, granites, granophyres, dolerite & gabbro
Garnet-sillimanite gneiss, hornblende biotite gneiss, garnet biotite gneiss, quartzo felspathic gneiss, charnockites, charnockite gneiss.
Fig. 2 Geological cross-section, Kerala Coastal land (Varadarajan and Nair, 1978)
Black Sand Placers (BSP)
In the WC, though heavy mineral placers are known to occur in several sectors of the coastal land, viz., in Ratnagiri coast in Maharashtra, Chavara-Kayamkulam (CK) coast in Kerala and Manavalakurichi (MK) coast TamilNadu, the Kerala occurrence of Black sand placers earned all the name and fame. It is immensely rich in black opaque mineral ilmenite (FeO.TiO2), rutile (TiO2), magnetite, sillimanite, garnet, zircon and monazite. Interestingly, unlike other mineral deposits, the BSP of Kerala as well as of Tamil Nadu are special categories of placer deposits in that these are renewed annually during the SW monsoon season, like a water table is recharged during every snowmelt or rainy seasons. It is a mineral deposit with “life”.
During every SW monsoon season, the beaches of this sector goes through a cycle of erosion – seaward transport – shoreward re-transport – deposition on eroded beach face – repair of erosional damage. During the beach rebuilding phase fantastic volumes of black sand are deposited on the shore face of the eroded beach and while the backwash further winnows away to the inshore the light fraction of quartz and shell fragments. This annual-beach-cycle of erosion and rebuilding creates a large reserve of blacksand in the
sector between Chavara and Kayamkulam. The mining lease holders use bulldozers and tipper trucks to cart away the bounty to run their mineral separation operations through one year. The ancient coastal land to the east has proved occurrence of ancient BSP, which is recovered using a dredging. The story of the MK BSP deposit is no different from that of the CK deposit, where gathering black sand makes use of dragline excavators.
Logically, in the past several workers had closely examined the ultimate and penultimate sources of placer minerals, transport to the depositional sink, sorting and separation from lighter minerals, processes of formation, reserve and economic worth of the deposit etc. Though the recent Asian Tsunami of Dec. 26, 2004, devastated life and property worth a few hundred million rupees in Kerala, it alongside brought forth an impressive piece of evidence on the extent of blacksand reserve in the seabed off Chavara -Kayamkulam sector of the coast. The tsunami wave caused a huge seawater surge, carrying along phenomenal volumes of blacksand rich seabed sediment to be deposited as washover fans and sheets (on the backshore tract to heights of 2.0-3.0 m and occasionally reaching 4.0 m), blocking the traffic of 4 wheeled emergency and relief vehicles in the paved streets of the affected area. Indeed, JCBs had to be deployed to remove the sediment (blacksand) cover of the pavement to make them motorable. This phenomenon underscored the point that the seabed off this region is essentially a very large and extensive reservoir of blacksand rich sediment.
Though blacksand laden sediment accumulation perhaps dates back to the origin of the WC, the changes in climate and relief would have modulated the rate of erosion, transport and accumulation of sediment in the ocean. The geometry of the modern coastal land of Kerala is as recent as early Holocene, and prior to that time sediment west flowing Kerala rivers (and especially those between N.Lat 9 and 10 degrees) would have delivered the alluvial load to the modern innershelf or even beyond, due to the lower sealevel stands of glacial periods. A cursory estimate of shelf are of Kerala suggests that it is nearly thrice as large as the land area. As lateritisation or chemical weathering either predates or contemporaneous with the formation of sediments of Vaikom beds, it is argued that the ancient landmass of Kerala had been exposed to a monsoon climate or a proto-monsoon of some sort enabling the chemical decay of rocks of the basement. But for the suggestion of a semiarid to arid and Humid conditions during the late Pleistocene and early Holocene by Rao and Thamban (1994), during the Cenozoic, we have scarcely any strong evidence warranting a climate-shift from wet and moist type.
As a consequence, the process of lateritisation of basement rocks (like Khondalites and Charnockites) unabatedly continued and the Kallada R delivered huge volumes of detritus carrying black sand minerals to the ancient coastal plain now under the modern shelf and presently to the shelf itself. The Kallada Rr is very special in this context in that unlike like other rivers rising from the WG of Kerala, Kallada R cuts through the rocks of Tertiary Warkalla basin declaring its antecedent nature. Recently Vinodkumar (2005) demonstrated that primarily Khondalite and to a lesser degree the associated charnockite have been the source of BSP minerals and cumulatively over the last several million years a vast reservoir of BS minerals have been deposited in the ancient and modern shelf off the CK sector. The dominant longshore current, like now, have been heading northerly in the past – Holocene to be precise – as indicated by the extensive patch of Chertala glass sand deposit (reserve=42.0 million tons) by a process of hydrauklic sorting. The heavier BSP minerals any way lagged behind the lights and accumulated as ancient placer in the ancient coastal land.
- Though the WC of India is a passive trailing margin, it had been tectonically active in the geologic past as well as later (as indicated by several intra-plate earthquakes of varying magnitudes in ancient and current times), right from the time of its parting by rifting from Antartica and later from the Seychelles-Madagascar landmass.
- The WG, bordering the WC of Indian peninsula, is a very lofty mountain range of 1300 km. extending from north to south practically uninterruptedly and is constituted by ancient crystallines rocks of Dharwar craton in the central part, the Deccan flood basalts in the northern part and rocks like khondalite and charnockite of the southern granulite terrain in the south.
- Dawn of monsoon or a proto-monsoon climate at the early Tertiary is responsible for the extensive chemical weathering or lateritisation that the basement crystalline rocks went through and the great deal of erosion of the uplands of WG. At least, 3 planation surfaces exist in this tract and are caused by the rejuvenation of the WG.
- Several sets of large and small lineaments crossing the terrain, at varying orientations did play a critical role in dissipating the stress accumulating from the northerly movement of Peninsula by triggering seismic episodes, and in creating the Kerala basin in early Cenozoic time enabling the accumulation of marine or terrestrial sediments from time to time. Some of the proto-basins of Kayals of Kerala coastal land owe their origin to the lineament-controlled-neotectonics.
- Laterite (first described by Bucchanan from Angadipuram in Malappuram Dist., Kerala) and the mechanism of lateritisation is an example of near-intense chemical weathering in a tropical monsoon climate, is primarily controlled by climate and relief of a terrain. Though difficult to quantify, the rate of weathering (ranging between 1.3 to 22.0 m/ma) has been estimated by some workers. It is no surprise to find deeply weathered zones of primary rock with a laterite cap in many parts in the midland of Kerala.
- Dawn of the Proto-monsoon climate and episodic rejuvenation of the WG led to the etching of slopes and terrain by several west-flowing antecedent streams in the early Cenozoic. With progress of time, monsoon also evolved and refined to its contemporary style. Since that time, the fluvial processes erosion, transport and deposition, delivered several billion tons of detrital sediment to the ancient Laccadive sea, along with the associated heavy mineral particles.
- In any continent, coastal geologic scenery is mostly contributed by the fluctuating sea levels of the Cenozoic. The WC and Kerala in particular, is no exception. Huge sediment accumulations in the proto- or modern-shelf of the Laccadive Sea, was controlled by fluctuating sea levels during the Pleistocene glaciations and tectonism of earlier periods of the Cenozoic.
- The shelf sediment of the modern Laccadive Sea, off CK sector, is a huge reservoir of blacksand minerals and the recent Asian tsunami (Dec.26, 2004) proved its potential beyond doubt.
I sincerely thank Dr. D.S.S. Babu (the convener) and Dr. M. Baba (Director, CESS) for offering me a berth in the workshop of IGCP-514 Project. Ms. Frincy, R.M., rendered invaluable (Dept. of Geology, Univ. of Kerala, Kariavattom Campus) rendered invaluable assistance in completing the manuscript.
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