The Deccan Traps, India (Part 4): Quaternary sediments of the Godavari River basin, Maharashtra
Mugdha Chimote (India)
The discovery of Quaternary sediments around Godavari River in Maharashtra (Fig. 1) was something of an accident. Sankalia (1952) first encountered these sediments while excavating the Lower Palaeolithic Industry in the region. Upon discovery, Sankalia brought onboard many geologists, such as Prof S N Rajaguru, Shanti Pappu, Gudrun Corvinus and R V Joshi, to bring an interdisciplinary approach to the study. Based on their geomorphic studies, Sankalia et al. (1952) concluded that the Quaternary palaeoclimates differed from present climates: the bedload stream represented wet climates, while the fine-grained sediments represented dry climates. Following this discovery, the Archaeological Society of India conducted similar such studies in the Narmada, Pravara and Tapi basins.
Quaternary records of upland Maharashtra include colluvial (material transported by the action of gravity) and alluvial (material transported by river) sediments. Along the river channels, alluvial sediments occur as discontinuous outcrops, whereas those in the basins do not exceed a thickness of 50m. Quaternary sediments account for the recent geological strata, which lie on the uppermost layers of earth and have been exposed relatively to the least amounts of erosion.
The Earth underwent dynamic climatic variations in the Quaternary period, from glacial-interglacial events, development of monsoonal wind patterns, the formation of deserts and palaeomagnetic reversals, to mass extinctions. These incidents in turn led to geomorphic processes, such as the rejuvenation of rivers, alteration of those rivers’ courses during each rejuvenation and the occurrence of flash floods. It is thus obvious that these geomorphic processes altered the geometry of the Quaternary sections over the decades. We can therefore safely claim that, the sections studied are not as originally laid down but products of similar such processes (Fig. 2).
In the past 70 years, the Godavari River basin has been thoroughly examined from an archaeological perspective. I conducted a detailed sedimentological study, with the intention of supplementing the already existing vast archaeological data with geological information, to understand the depositional history of major Quaternary units in the region. Part 3 of the Deccan Trap series discussed the Quaternary tectonic activities in the study area and their relationship with the geomorphology. This fourth and the last part of the Deccan Trap series discusses the detailed sedimentology and lithostratigraphy of Quaternary units along Godavari River in Nashik district.
The Deccan Basalts in the Godavari River Basin are characterised by Aa and Pahoehoe lava flows. The lava flows exposed in the area belong to the Ratangad Formation of the Sahayadri Group. The Quaternary deposits have a limited lateral and vertical extent, and are mostly confined to the river channel.
In extensive independent research carried out by Corvinus in 1972, she divided the Godavari alluvium into three Formations:
- Late Holocene beds, with Chalcolithic pottery.
- Older Alluvium comprised of coarser, boulder gravel, overlain by sandy pebbly gravel, which yielded Stone Age tools in the Gangapur area.
- Younger Alluvium – a formation comprising finer yellow-brown sandy silt, silt and dark brown sandy silt, with fragmentary molluscan shells, and pollens of Accasia, Eugenia and Holopteria.
In 1987, Kale and Rajaguru corroborated the formations given by Corvinus and generalised the stratigraphy of the Quaternary Alluvium of Maharashtra as follows:
Field observations and sedimentological analyses
In my research, a total of six localities were studied along the stretch of the river, from the Gangapur Dam to the Gangapur Waterfalls (Fig. 3).
Based on the observations of these sections, the lithostratigraphy of the Gangapur region can be broadly divided into three lithounits:
- Brown to yellow-brown sandy pebbly gravel at the base.
- Overlain by calcretised yellow-brown silty sand and clay.
- Dark brown fine clay forming a terrace, which is currently under cultivation.
Apart from the general stratigraphy, here are few of the features unique to each location:
- Location 1 (L1): A silty sand unit exhibiting laminations and cross stratification. An artefact (core) belonging to the Large Flake Acheulian Industry was retrieved here, analysis of which indicates that the core has undergone substantial weathering and erosion, thus implying that Gangapur is not a site of primary activity.
- Location 2 (L2): Bimodal river activity (inferred from the analysis of bimodal distributions for grain-size frequencies) at this location has caused the silty sand unit to acquire a hummocky appearance.
- Location 3 (L3): The entire section has been calcretised. The base of the section is not visible above ground level.
- Location 4 (L4): The section is fairly calcretised and the lowermost unit is covered with scree.
- Location 5 (L5): The complete absence of sedimentary structures implies an aggradational phase.
- Location 6 (L6): The entire section is fairly calcretised.
Fig. 4 illustrates a relative lithostratigraphic comparison of all the sections studied in the Gangapur region.
All the samples collected from various sections in the Gangapur region were sieved and the data collected was subjected to various statistical analyses. Four statistical parameters, namely, the graphical mean, inclusive graphical standard deviation, inclusive graphic skewness, and graphic kurtosis were calculated, using the formulae given by Folk and Ward (1957).
Mean grain sizes for all samples showed a gradual shift from coarse, to medium, to eventually fine grain size. Standard deviation values indicated that the samples were poorly sorted. Strong positive skewness for samples indicated presence of fines, which implies that these are of riverine origin. In the final parameter, kurtosis (which defines the specified form of peak in a graph of grain-size distribution), samples showed a transition from being platykurtic, to leptokurtic, to mesokurtic (that is, going from a relatively high probability of extreme events to a low probability). All statistical parameters thus imply a shift from turbulent to relatively stable flow conditions, giving a fining upward sequence.
To gain insight on the degree of saltation, traction and suspension load deposited in each section, Visher’s curves (which are graphical representations of grain-size distribution and cumulative probabilities calculated using the Visher method, which is a technique for analysing grain size distributions in terms of sediment transport mechanisms as reflected in cumulative frequency plots on arithmetic probability paper) were plotted based of the calculated statistical values. They were also used to calculate the stream competence. The stream competence values are suggestive of a lateral shift with low energy conditions, thus affirming the interpretations from statistical parameters (Fig. 5).
Implications from sedimentological analyses
The silty sand and clay lithounits show calcretisation because of replacive introduction of vadose carbonates in the soil within a weathering profile. (The vadose zone, also called the unsaturated zone, is the part of the earth between the land surface and the upper level at which groundwater is at atmospheric pressure.) Their absence in the sandy pebbly gravel indicates that the valley terrace was covered with regolith (the blanket of loose, superficial deposits covering solid rock), devoid of calcisols (formerly known as ‘desert soils’).
Based on the nature of deposited sediments and the calcretisation patterns, it can be suggested that the climate during Acheulian activity in the upper reaches of the Godavari was semi-arid yet relatively wetter than during the deposition of overlying strongly calcretised silty sand and clay lithounits. The sandy pebbly gravel units signify moderate aggradation (that is, by a river, stream or current) during the Acheulian phase. The post-Acheulian phase is represented by secondary processes, such as the river incisions of present-day rivers. The Early Holocene witnessed a wet, semi-arid climate, with increased flood frequency and incision, shaping the current geomorphology of the Gangapur region.
The sequence of lithounits in the study area reflects the dominance of traction and saltation load in the upstream region; the proportion of suspension load along with traction and saltation gradually increases downstream. The variations in lithofacies – a consequence of fluctuating stream competence – were possibly controlled by semi-arid/arid and sub-humid climates in the late Quaternary.
My study of the Quaternary sediments around the Godavari River basin has brought out some interesting results and interpretations. We continue to research this topic further upstream, in collaboration with the Deccan College of Archaeology in Pune.
|This is the last 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|
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