Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

P. K. Raha , S. Sinha Roy , C. P. Rajendran

Abstract

A lithostratigraphic classification of the Cenozoic sedimentary sequence of the coastal plains of Kerala has been developed through an analysis of the exposed and subcrop litho-facies and their interrelationship based on sedimentologic parameters. The entire Cenozoic sequence is designated as the Malabar Supergroup comprising the Vembanad Formation (Quaternary-Holocene) and the Warkalli Group (Tertiary) separated by an unconformity marked by laterite. The Warkalli Group comprises the Ambalapuzha, the Quilon and the Mayyanad Formations from top to bottom. The Ambalapuzha and the Mayyanad Formations are basin margin fluvial and deltaic facies comprising sandstones, clays and lignites, and having a wedge of calcareous platform facies of the Quilon Formation. With the thinning out of the Quilon Formation (calcareous facies) in the eastern margin of the basin, the distinction between the constituent formations of the Warkalli Group disappears. Thus the purely terrigenous sediments in the eastern and southern margins of the basin form a single non-classifiable unit, the Warkalli Formation.

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P. K. Raha ¹, C. P. Rajendran ², R. K. Kar ³

Affiliations
1 Geological Survey of India, Calcutta, IN
2 Centre for Earth Science Studies, Trivandrum, IN
3 Birbal Sahni Institute of Palaeobotany, Lucknow, IN

Abstract

Eocene palynofossils are reported for the first time from a bore hole core drilled near Ambalapuzha in Alleppey District, Kerala between the depths of 400-571 m. Characteristic pollen genera recovered are: Palmaepollenites, Couperipollis, Proxapertites, Meliapollis, Striacolporites, Retistephanocolpites etc.

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C. P. Rajendran ¹, Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695031, IN

Abstract

Laterite overlying the Tertiary sediments is a common feature throughout Kerala. The occurrence of lateritic zones underlying the Tertiary sediments are reported from north Kerala. Their nature and mode of occurrence are suggestive of two lateritization spells.

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C. P. Rajendran ¹, P. K. Raha ², M. K. Sen ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum, IN
2 Geological Survey of India, Calcutta, IN

Abstract

The occurrence of Actinosiphon sp. a larger foraminifera, obtained from the subcrop section at Ambalapuzha, at a depth of 454-494 m is reported. This find further confirms the presence of Eocene sediments in the onshore Kerala sedimentary basin.

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C. P. Rajendran ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695031, IN

Abstract

Planktonic foraminifera comparable to Rugoglobigerina rugosa encountered in the sediments occurring at a depth, ranging from 577 metres to 585 metres in a borehole at Ambalapuzha in the coastal district of Alleppey in Kerala is reported. Significance of these foraminifera present in the bottom most part of the onshore sedimentary basin of Kerala is briefly discussed.

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C. P. Rajendran ¹, G. Rajagopalan ², Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrurn 695031, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow 226007, IN

Abstract

An event of transgression (8000-6000 Y.B.P.) and another event of regression (30Q0..5000 Y.B.P.) which gave rise to extensive peat and shell deposits during Holocene in Kcrala is recorded. The initiation of the backwater system in Kerala is linked to these events.

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Kusala Rajendran ¹, C. P. Rajendran ¹

Affiliations
1 Centre For Earth Science Studies, Trivandrum-695 031, IN

Abstract

The occasional incidences of microearthquakes in Kerala have been interpreted to be caused by the reactivation of ancient preexisting faults. While not negating the traditional wisdom, we propose an unconventional approach to analyze the mechanism of microseismicity in Kerala. A moot point considered here is the perceptive increase in the frequency of the microearthquakes in the state over the last decade, quite often coinciding with periods of increased rainfall. Although the occurrence of rain may not be a sufficient or necessary condition for the increase in the level of seismic activity, the correspondence between the two appears to be more than casual. On the basis of this observation. we suggest here that the tremors in Kerala may have been caused by transient increase in the hydrostatic pressure at shallow crustal depths. The mechanism of shallow earthquakes may be similar to what is proposed in the "hydroseismicity" model. Changing land use patterns, deforestation, soil erosion and increased siltation in the drainage basins are the robust parameters in this model.

Keywords

Micro- and Hydroseismicity, Rainfall, Kerala.

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Biju John ¹, C. P. Rajendran ²

Affiliations
1 Geological Survey of India, Lucknow - 226 020, IN
2 Centre for Earth Science Studies, Thiruvananthapuram - 695 031, IN

Abstract

Peninsular India is considered tectonically stable even though a few damaging earthquakes occurred here. Most of the causative faults are detected only after the occurrence of the earthquakes. The present study is an attempt to demarcate such active structures, from the Precambrian crystalline terrain of Peninsular India, through remote sensing, geomorphic analysis and field verification. The study area falls in the vicinity of Palghat Gap, a prominent physiographic break in the Western Ghats, where a low level seismicity is reported since 1989. Our study identified a WNW-ESE regional structure, which has influenced the drainage network of the area. The right angled turn of Bharathapuzha River at Desamangalam near this structure shows the influence of the structure to the drainage system. This structure is also characterized by the presence of a waterfall and a brittle fault dated neotectonic activity. The drainage basins lying south of this structure show anomalous values of asymmetry factors (>65 or <35). The Transverse Topographic symmetry factors also show anomalous pattern on either end of this structure and display a consistent shift in drainages towards southwest in the southern block. It is further observed that the ratio of valley floor width to valley height is very low (<8) in the river segment flowing along the WNW-ESE structure where the river might have been influenced by the on going activity of the structure. The slope induced by the reverse movement of the structure/fault results in the adjustments of drainage network in the form of channel blocks or shifts leaving paleochannels. The present study suggests that the structure, defined by the abandoned river channels, geomorphic anomalies, local seismicity and a brittle fault, is an active fault and capable of producing moderate seismic events.

Keywords

Palghat Gap, Seismicity, Geomorphic Indices, Active Fault, Neotectonism, Bharathapuzha River, Kerala.

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Biju John ¹, C. P. Rajendran ²

Affiliations
1 Geological Survey of India, Northern Region, Lucknow - 226 020, IN
2 Centre for Earth Science Studies, Trivandrum - 695 031, IN

Abstract

Surface exposure of a suspected active fault in an intraplate region, in the state of Kerala, south India revealed subtle expressions of brittle deformation in exhumed crystalline bedrock. The deformation zone consists of fracture sets with Small-Scale displacement and slip planes containing gouge; the propagation of displacement at depth being accommodated through fracture planes in the upper levels of the crust. It is spatially coincident with a linear hill scarp, and also the isoseismal elongation of a recent slight earthquake (M=4.3) in this zone. The fault rocks present within the fracture zone indicate cyclic episodes of frictional slips along the fault, which may have led to a progressive increase in the gouge thickness. Major part of the gouge zone along the fault shows alteration at the expense of feldspar and mica. The occurrence of repeated rupture may also imply that the fault zone re-strengthened between each rupture. The structural analysis suggests different generations of fault gouge and the last major episode of frictional sliding may have occurred close to the surface. Available data on the electron spin resonance (ESR) dating of the gouge indicates an age of 430±43 ka, probably representing the last major displacement on this fault Our structural interpretation of the fault rocks suggests that the suitably oriented discrete fault strands, within the Precambrian shear zone can be the source of moderate earthquakes (M≥6.0)bu t slip events are separated by long periods of quiescence. We also suggest possible ways of constraining the nature of activity and the pattern of deformation associated with low-displacement faults that occur within the Precambrian cratons.

Keywords

Craton, Faulting, Gouge, Cataclasite, Earthquake, Paleoseismicity, Quaternary, Kerala.

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C. P. Rajendran ¹

Affiliations
1 Centre for Earth Science Studies Akkulam, Trivandrum - 695031, IN

Abstract

No Abstract.

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Kusala Rajendran ¹, C. P. Rajendran ¹, Sreekumari Kesavan ¹

Affiliations
1 Centre for Earth Science Studies, Akkulam, Thiruvananthapuram - 695 031, IN

Abstract

This note deals with the mild earth tremors that occurred near Thalasseri, Kannur District on November 8,2003. Based on the activity detected by the broadband observatory at Peechi (PCH), we suggest an additional source zone in Kerala.

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C. P. Rajendran ¹, John Biju ², K. Sreekumari ³, Rajendran Kusala ¹

Affiliations
1 Centre for Earth Sciences, Indian Institute of Science, Bangalore - 560 012, IN
2 National Institute of Rock Mechanics, Champion Reefs, Kolar - 563 117, IN
3 Seismic Station, Kerala Forest Research Institute Campus, Thrissur - 680 653, IN

Abstract

Given the lack of proper constraints in understanding earthquake mechanisms in the cratonic interiors and the general absence of good quality database, here we reassess the seismic hazard in the province of Kerala, a part of the 'stable continental interior', based on an improved historical and instrumental database. The temporal pattern of the current seismicity suggests that >60% of the microtremors in Kerala occurs with a time lag after the peak rainfall, indicating that hydroseismicity may be a plausible model to explain the low-level seismicity in this region. Further, an increment in overall seismicity rate in the region in the recent years is explained as due to increased anthropogenic activities, which includes changes in hydrological pathways as a consequence of rapid landscape changes. Our analyses of the historical database eliminate a few events that are ascribed to this region; this exercise has also led to identification of a few events, not previously noted. The improved historical database essentially suggests that the central midland region is more prone to seismic activity compared to other parts of Kerala. This region appears to have generated larger number of significant earthquakes; the most prominent being the multiple events (doublets) of 1856 and 1953, whose magnitudes are comparable to that of the 2000/2001 (central Kerala) events. Occurrences of these historical events and the recent earthquakes, and the local geology indicative of pervasive faulting as shown by widely distributed pseudotachylite veins suggest that the NNW-SSE trending faults in central midland Kerala may host discrete potentially active sources that may be capable of generating light to moderate size earthquakes. The frequency of earthquakes in central Kerala evident from the historical database requires that the seismic codes stipulated for this region are made mandatory.

Keywords

Seismic Hazard, Earthquakes, Stable Continental Interiors, Central Kerala.

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