The Deccan Traps, India (Part 2): Its geomorphology and stratigraphy
Mugdha Chimote (India)
The Deccan Traps occupy approximately 25% of the total of peninsular India, that is, the triangular shaped landscape of southern India. They traverse the states of Maharashtra, Karnataka, Madhya Pradesh, and Gujarat. The Deccan Traps are currently believed to occupy about 500,000km2 of northwest peninsular India. It is estimated that the total exposure prior to erosion (including the region beneath the Arabian Sea) is of the order of 15 million square kilometres (Krishnan, 1956) or even up to 18 million square kilometres (Todal and Eldham, 1999). The differences in estimations of the total area of the Deccan Traps resulted from the fact that, an unknown area of the Deccan Volcanic Province (DVP) was rifted away as the Cambay rift system moved south and the Seychelles-Mascarene Plateau, along with part of the DVP, migrated to the west.
The earliest basaltic eruptions took place along the north-western margins of the Indian continent, that is, in the Nashik-Narmada region. Later lava successions were emplaced on the southern flank of the evolving volcanic edifice as India migrated northwards over the plume head. The last of the flows were erupted in the southern DVP near Belgaum in Karnataka. As a result, the thickness of the flows gradually reduces from north-western to southern region of Indian subcontinent. Given the massive extent and volumes of Deccan Basalts, extensive studies have been carried out over the years to better understand the petrography, geochemistry, stratigraphy and volcanic features of the terrain.
The first part of this article on the Deccan Traps (see The Deccan Traps (Part 1): The story of its genesis) discussed the various theories of the genesis of the Deccan Traps, their inadequacies and possible alternative models. Now that we have established how DVP (probably) formed, let us dive into the details of its geomorphology, stratigraphy and volcanogenic structures.
Geomorphology of the Deccan Traps
Geomorphological studies of the Deccan Traps initiated in the late 1860s. There were numerous speculations over the type of rock underlying the DVPs and whether this rock was responsible for the structure of the terrain. Earlier, the traps were perceived as mountains rather than basalt flows. As a result, geomorphic studies were conducted with that context in mind. The Gazetteer’s reports from early 1900s reported water divides in the peninsula, upheavals in the latitudes and the flatness of scenery in Upland Maharashtra, all suggesting a sedimentary succession beneath. In 1950, Pascoe proposed that differential erosion has played a vital role in mountain formation rather than any tectonic activity, as the successive lava flows are practically undisturbed.
The breakthrough came when Radhakrishna, in 1965, pointed out an important observation that the Western Ghats are not “true mountains”, but represent the steep edges of an elevated plateau, a combined effect of uplift and erosion. (Geologically, the term, ghat, is used in the Indian subcontinent to refer to a range of stepped hills with valleys – ghati in Hindi – such as the Eastern Ghats and Western Ghats.)
He reasoned that sediments older than the Miocene are not recorded, further substantiating that land must have existed prior to this date and the retreat of the ghats must have been accelerated by westerly flowing rivers causing headword erosion evident by well-established drainage patterns in the plateau. The steep slopes of Western Ghats, and their high humidity and heavy rainfalls have promoted intense chemical weathering and erosion over the years, resulting in the current topography.
In 1993, Radhakrishna proposed a sequence of events starting from Cretaceous to Present (Table 1):
|Recent||Reduction of old plateau to new plateau surface, the carving of the present-day landscape|
|Pleistocene||Migration and nick point formation and river capture, excavation of deep gorges, retreat of scarps, waterfalls|
|Pliocene||Formation of Western Ghat scarp and retreat, rejuvenation of rivers|
|Miocene||Stability and plantation, lateritization of parts of the surface|
|Eocene||Rifting of uplifted segment, creation of the Western Ghat and its scarp facing sea|
|Cretaceous||DVP episode, extensive trap cover over eroded sediments of an earlier surface|
Geomorphometric studies carried out by Kale et al (2008) for 30 rivers in the Deccan Trap region show that the geomorphometric tools available now provide compelling evidence for the widespread belief that the western margin of India has undergone prolonged uplift and tectonic deformation since Tertiary times. Subtle imprints on the present-day landscape by any recent significant uplift failed to be detected by commonly used geomorphic indices of active tectonics, at least on a spatial scale. Kale thus labels the “Indian great escarpment” as a “creeping divide”, rather than a “leaping divide”.
Lithological variations in the DVP
Lithological variations within the Deccan basaltic flows can be seen without the need of a microscope. The rock types in Deccan show similar petrographic composition. However, they vary in their appearance, that is, their size, shape, texture, colour and shape of vesicles. It is interesting to realise that occurrence of various basaltic rock types is a function of magma viscosity, vesicle zonation, direction of flow of magma and the pre-eruptive topography of the region.
Some of the commonly found rock types in Deccan traps are discussed below:
- Giant Porphyritic Basalts (GPB): These are unusual porphyritic basalts (that is, containing distinct crystals or crystalline particles embedded in a compact groundmass) containing large phenocrysts (which are early forming, relatively large and conspicuous crystals that are larger than the grains of the igneous rock that contain them) of plagioclase (that is, silicate minerals within the feldspar group). GPB is a variety of compact aphanitic basalts. The size of plagioclase laths may range from few millimetres to 10cm. Geologists are now trying to date the plagioclase in GPB belonging to the older Deccan flows in an effort to obtain their exact ages.
- Compact Aphanitic Basalts (CAB): The word “aphanitic” describes a very fine-grained rock whose constituents are not visible to the naked eye. CAB is a compact rock devoid of gas vesicles. The rock shows various textures under a microscope, such as glomeroporphyritic (that is, grouping of phenocrysts, not necessarily of the same mineral, into distinct clusters within porphyritic igneous rocks), ophitic (in which crystals of feldspar are interposed between plates of augite) and inequigranular (that is, material composed chiefly of crystals with marked differences in their orders of magnitude to one another).
- Amygdaloidal Basalts: The term “amygdale” comes from the word almond. Amygdaloidal basalts are thus rocks that have almond shaped cavities filled with cavity minerals, such as zeolites (crystalline structures made of silicon, aluminium, and oxygen). If the cavities remain unfilled, the rock is termed as vesicular basalt.
- Volcanic Breccia: This rock is formed by the agglomeration of angular fragments caught up in a lava matrix.
- Red Bole Horizons: These are red/brown/green-coloured interfaces found between two successive lava flows. There is still debate over the origin of these horizons, which have been attributed to either a chilling effect of the overlying lava flow or the erosion of the previous flow.
Flow morphology of Deccan basaltic flows
Why are volcanoes in Iceland cone shaped while those in Hawaii are flat? Well, that’s because Icelandic eruptions yield cinder cones and shield volcanoes. Hawaiian eruptions are fissure eruptions, which means, each basaltic flow is emplaced like a layer of cake, one on top of the other. The flow morphology of the DVP and Hawaii is remarkably similar. As such, to understand DVP mechanism in real-time, comparative studies between DVP and Hawaiian volcanism are often carried out.
Under the Hawaiian Nomenclature System, basaltic flows have been traditionally classified into two categories: Pahoehoe and Aa, based on their surface morphology, initial viscosity and style of emplacement/eruption:
1. Pahoehoe lava flow (pronounced paw-hoy-hoy)
A pahoehoe lava flow is made up of numerous smaller flow units called toes. Each toe is generally 30cm thick, around 2m long and 40cm wide. These flows have low viscosity. As a result, the skin maintains a smooth, well-insulated and unbroken surface. The advancing flow front is made of an active toe. Each toe stops flowing after some time as it gets inflated with still erupting lava. Eventually, the flow front cools down and develops fractures. The toe breaks apart and gets curled up beneath the still hot lava flow.
Numerous such flow actions lead to formation of a layer of volcanic breccia beneath the lava flow. Pahoehoe lava flows can be identified with the help of field structures, such as lava lobes, ropy lavas and inflation clefts. A typical lava lobe consists of three zones: the lowermost made of inverted Y shaped pipe vesicles, the middle zone is a compact core with very few vesicles and the uppermost zone is made of vesicular basalt.
2. Aa lava flow (pronounced ah-ah)
Aa and Pahoehoe are, strictly speaking, identical in composition. However, the striking differences in appearance are because Aa flows have higher viscosity than Pahoehoe flows. Aa flow is characterised by rough rubbly crust, dense interiors and clinkery bottom surfaces. These clinkers – the broken lava blocks – are formed as lava is pulled apart by frictional forces during the flow. The process of Aa lava emplacement is very similar to the mechanism of a conveyor belt. The cooled fragments at the front-end tumble down and are engulfed by the advancing lava.
These clinkers cover the top and bottom of the flow, thus acquiring the name, “top” and “basal” clinkers, respectively. Aa flows travel through open channels, thus forming lava tubes and lava channels. With distance, Aa flows become thicker. As such, they are devoid of vesicles. The Aa flows can be identified with the help of field structures, such as colonnade and entablature, pillow lavas and tongues.
Was the entire DVP emplaced in one place or even all at one time? Stratigraphy provides the answer to these questions. Stratigraphy is the branch of geology concerned with the order and relative positioning of strata in relation to the geological timescale.
Stratigraphic classification of the Deccan basalts was attempted way back in the nineteenth century by Blandford (1867), who classified the traps into three divisions. Krishnan (1960) also classified the Deccan basalts into three stratigraphic groups: Lower, Middle and Upper traps. The groups have since been further divided into sub-groups and then subsequently into formations. Formations are the smallest mappable or traceable units and must have internal lithological consistency and mappability.
The lower traps cover Madhya Pradesh and eastern margin, the Middle traps cover Malwa and the Central Deccan, and the Upper traps cover the western areas around Bombay and Saurashtra. The Deccan Traps unconformably lie over the Proterozoic basins of the Vindhyan, Sausar, Sakoli, Mahakoshal and Bhima Supergroups. The Gondwana supergroup is exposed in the eastern and central Deccan province. The Gondwanas and Deccan Traps are separated by intertrappean horizons of Bagh beds of the Lameta Formation. These are sedimentary successions of lacustrine and fluvial origin assigned to Turonian age, with rich records of faunal and floral remains. These stratigraphic relations establish a Palaeogene period for the Deccan volcanic eruption.
The Western Deccan Province of the Main Deccan Plateau is the best studied area in the DVP. The lithostratigraphy has been established by the Geological Survey of India (Godbole et al, 1996), based on extensive field studies with emphasis on lithological characters, such as presence of interflow horizons and physical characters of the flows.
Most of the lithostratigraphic work has now been authenticated with geochemical studies and consequently new formations have been added. Cox and Hawkeshworth, in 1985, proposed a detailed chemostratigraphic succession for the DVP. Later, modifications were made by Subbarao and Hooper in 1988. The Deccan Traps have been divided into formations based on geochemical criteria, reliably the Sr/Sr ratio, and have been shown to be traceable around the Mahabaleshwar plateau, where they are proposed to have originally formed.
Table 2 shows a relative comparison between the lithostratigraphy and chemostratigraphy for Deccan Volcanic Province.
Fresh data from real-time eruption studies in Hawaii and Iceland have raised numerous questions regarding the decade old concepts of DVP emplacement, DVP features and their correct terminological usage. Until concrete evidence is provided by these studies, we will continue to explore the DVP in India and try to understand it using the chemical, petrographic and magnetic techniques available.
|Natural arches are bridge-like structures, cut in basaltic lava flows within a stream valley (Fig. 1). These are commonly found at many places in India, New Zealand and Hawaii, among others. Natural arches in sedimentary terrains have been extensively studied. However, those in igneous terrain remain a mystery.|
Although no direct evidence is available, the overall appearance of these features suggests that they were formed by the collapse of a cavity within the lava. The existence of lava channel systems in the nearby regions of Ahmednagar provide compelling evidence for this theory. It is also proposed that the collapse is usually facilitated by the joint system existing in the region. However, further work needs to be carried out to substantiate these propositions.
|This is the second of four articles on the Deccan Traps of India. The others consist of:|
|The Deccan Traps, India (Part 1): The story of its genesis|
|The Deccan Traps (Part 2): Stratigraphy and geomorphology|
|The Deccan Traps, India (Part 3): Evidence of recent tectonic activities in Deccan basalts|
|The Deccan Traps, India (Part 4): Quaternary sediments of the Godavari River basin, Maharashtra|
Heming. R, (1979). Natural bridges in basalt lavas, Northland, New Zealand. N.Z. Journal of Geology and Geophysics, vol. 22, no. 2, pp. 239-243.
Jay, A, and Widdowson, M. (2007). Stratigraphy, structure and volcanology of the SE Deccan continental flood basalt province: implications for eruptive extent and volumes. Journal of the Geological Society, London, vol. 164, pp. 1-12.
Jay, Anne E. (2005). Volcanic architecture of the Deccan Traps, western Maharashtra, India: an integrated chemostratigraphic and paleomagnetic study, PhD thesis The Open University.
Sengupta, P., and Ray, A. (2006). Primary volcanic structures from a type section of Deccan Trap flows around Narsingpur–Harrai–Amarwara, central India: Implications for cooling history. Journal of Earth Sciences, 115, no. 6, pp. 631- 642.
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