A study jointly conducted for the Saurashtra Basin by the Indian Institute of Technology Bombay (IIT Bombay) and National Centre for Earth Science Studies, Thiruvananthapuram is significant to help us piece together the region’s paleogeography—a historical study that tells us what the parts of the earth looked like in the past. The study reveals some secrets of the history of India and the ancient supercontinent configurations, providing insights into how continents were formed and have moved over time.

Saurashtra Basin lies in an area of 2,40,000 sqkm area spanning sea and land across western Gujarat and North of the Mumbai Coastline. Here much of the landscape is buried in the volcanic rocks called Deccan Traps, which was created by the volcanic eruptions at the Western Ghats during the Cretaceous period, 66 million years ago.

Dr. Pawan Kumar Rajak from the Department of Earth Sciences, IIT Bombay, and the lead author of the study said that Saurashtra Basin was formed by the separation of India from Madagascar and Seychelles, 100 million years ago. After the separation, the western margin of India became lowlands and the north and north-eastern parts of the study became highlands. Rivers from these highlands brought sediments and deposited them in the low-lying Saurashtra basin.

“The eruption of Deccan volcanism (which happened later) covered a vast area of the Saurashtra Basin, making it difficult to study the sediments. Today, only the mountains/hills, river sections and road-cuts expose the sediments of that time,” adds Dr. Rajak

The study focuses on the ‘Mesozoic’ era (the age of diansaurs – 66 million years ago) sandstones in the Saurashtra Basin. The study used the cutting edge dating techniques on these sandstones, the study pieces together the sources and routes of the materials that is located in the basin.

The team used two techniques:

• electron probe microanalysis (EPMA)
• laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)

The high precision and low detection limits of LA-ICP-MS allowed to know the minerals’ composition and ages on a small scale. The EPMA determine the composition of minerals as well as the Uranium-Thorium (U-Th) age of a mineral, such as monazite, and helps identify the possible sources, Dr Rajan said.

The team focused on zircon and monazite, minerals (rare earth elements minerals) known for preserving geological data remarkably well. Decaying U or Th into lead (Pb) is used as a geological clock. So studying these minerals helped to gain information about important geological events.

The northeastern part of the Saurashtra Basin is a region with 600 metres of thick sediment, called the Dhrangdara Group, from the late Jurassic-early Cretaceous period.

The new study showed that the sandstones in the Dhrangdara Group primarily originated from two primary ‘Precambrian’ sources. The Precambrian is the earliest part of Earth’s history, covering most of our planet’s timeline. Researchers found that the Neoproterozoic rocks (about 1 billion to 540 million years ago) and Archean rocks (dating back from 4.5 billion to 2.5 billion years ago) are the primary sources of the sediments in the Saurashtra Basin.

The study also hints at revelations about paleo-drainage patterns— the ancient river systems. These findings are crucial for understanding how landscapes evolved and reshaped over time due to geological forces. Future studies could use the knowledge gained from this study to shed light on the paths of ancient river systems. An analysis of minerals, like rutile and tourmaline, in the sandstones indicated a varied derivation from multiple geologic sources such as granites, metapelites (metamorphosed clay-rich sediments), and tourmalinites.

Their chemical characteristics helped trace the rocks back to their origins in older terrains like the Aravalli and Delhi supergroup of rocks—major regional features known for their long-standing geological history. “The analysis of the samples shows multiple sources contributing to sediments. Initially, it was thought that the Aravalli-Delhi Fold Belt (source region) was the only contributor to the sediments in the basin,” says Dr. Rajak.

Geochemical analysis and dating give us a sneak peek into significant global events. The study of the zircon minerals suggested a predominant contribution from formations dating back to 3.5 billion to 539 million years ago. These timelines correlate with ancient supercontinent cycles such as the formation and breakup of Columbia, Rodinia, and Gondwana. These are names given to massive landmasses that once contained most or all of the Earth’s continents but eventually split and drifted apart to form the continents as we see them today.

“During the supercontinents Columbia (1800 Ma) and Rodinia (1200 Ma), all the continents of the current globe were part of a single mass. The geological ages we obtained in our study indicate that the source rocks (mountains) formed during those time cycles,” explains Prof. Santanu Banerjee, a Professor at the Department of Earth Sciences at IIT Bombay. These findings are important not just for academic curiosity but also to offer practical insights into regional geology and resources.

The Saurashtra Basin, along with nearby basins like Cambay, Kutch, and Narmada, forms a part of India’s western margins, which have been identified as potential sites for hydrocarbon resources. Knowing the origin of these sediments can, therefore, assist in exploration efforts and better management of these resources.

Moreover, the study also touches on larger geological phenomena, such as significant orogenies—mountain-building events—and tectonic configurations that shaped the earth’s crust. The sands from the Mesozoic era in Saurashtra capture a historical narrative of the Bhilwara, Aravalli, and South Delhi progenies. These events represent significant periods where the earth’s crust was dynamically altered due to tectonic activities, leading to the formation of mountains and other geological structures.

The team behind the study is looking to explore the minerals of the basin further to improve our understanding of the geological history of the region. “The next plan is to work in the same area to refine our understanding of the source areas and paleogeographic changes of that time. We must check whether the sediments were also sourced from Madagascar and Seychelles. We plan to contact ONGC to get seismic data for the study area to determine the basin configuration and trace sediments across the Arabian Sea,” shares Prof. Banerjee about future plans to continue the research.