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Young Scientist Journal

Exploring the Use of Gravel Occurrence Data as an Indicator of Large-Scale Fluvial Events in the Bengal Basin

ABSTRACT

Flooding is one of the most pressing challenges posed by climate change. The Bengal Basin is known for its up to 90-meter-deep deltaic sediment sequences, preserving a detailed record of fluvial events across the climatic variations of the Holocene (last 11,000 years). Over the past 11 years, the Goodbred lab has worked with University of Dhaka geology students to collect 440 soil cores from across the Bengal Basin, as well as a detailed library of driller’s logs, which contain hitherto unexplored observational data on gravel occurrences in the delta. Gravels are commonly used as auxiliary descriptors in facies analysis, although their use in identifying cores that contain evidence of past fluvial events is unexplored. The driller logs were compiled into a single usable Masterfile, and multiple delta wide visualizations. These visualizations displayed a distinct cluster of gravel occurrences in the lower basin. Facies analysis on two of the gravel containing cores unveiled potential evidence for a large, Early Holocene flashflood. Previous research has shown that in the Early Holocene, a hot and humid climate induced violent flash floods in rivers stemming from the Chota Nagpur plateau which deposited similar facies to those discovered in the lower Bengal Basin.

INTRODUCTION.

The most common form of natural disaster is flooding, accounting for 43% of world-wide natural disasters from 1998-2017, affecting some 2 billion people in that period [1]. Moreover, due to climate change and demographic movement, 758 million people are expected to live in flood prone areas in 2030, from just 580 million in 2010 [2]. Further, the 100-year flood, as reported in 2000 CE, could have a return period of just 2-25 years in some areas by 2100 CE [3]. In order to mitigate the worldwide risks of flooding, a detailed understanding of the effects of various environmental stimuli from the past is necessary to better evaluate the risks they might pose in the future.

The Bengal Basin stretches across 100,000 km2 in the northeastern section of the South Asian Continent. Sediment transported by the Brahmaputra and Ganges from the Himalayan Mountains to the basin rapidly constructed largest fluvio-deltaic system and a sediment sequence that is 90 meters deep in certain areas.

Moreover, because of the relationship between the Bengal Basin, the Himalayan Mountains, and these great rivers, this region has a long history of flooding. Research enabled by its uniquely deep Holocene  sedimentary sequence has unveiled detailed information about the history of high-energy fluvial events in the basin [4,5].

Facies description and interpretation have long been a primary means for studying this unique region and its floods. Gravel occurrences are frequently used as an auxiliary to grainsize descriptors when conducting facies analyses but have a hitherto unexplored use in identification of high-energy patterns in the Bengal Basin. Concentrations of gravel that are not constant across depth could indicate a major fluvial event. Thus, a basin wide analysis of gravel occurrences could enable the discovery of previously unknown flood sediments or river patterns enabling future research on the effects and causes of those floods.

In this paper, we draw from existing gravel occurrence data to visualize basin-wide gravel occurrence patterns, enabling the identification and further analysis of surprising clusters.

MATERIALS AND METHODS.

Data Collection.

From April 2011 to November 2022, University of Dhaka Geology students collected 440 soil cores from across the Bengal Basin. The average core depth was ∼60 meters. The drillers collected visual observational data of 1.5-meter chunks, referred to below as Driller’s Logs. The Driller’s Logs were compiled into a single Masterfile, which had categorical yes-no data on all occurrences observed in the Driller’s Logs.

Sediment Provenance.

Strontium concentrations are used to determine sediment provenance in the Bengal Basin. Sr concentration of ≤110 ppm indicates Ganges sediment, and Sr ≥140 ppm indicates Brahmaputra sediment [6]. Sr concentration has been collected by the Goodbred lab for many of the soil samples in the Bengal Basin using an X-ray fluorescence spectrometer (XRF) machine.

Geologic Epoch.

The Pleistocene boundary is visually determinable by the presence of a red clay paleosol, a highly weathered layer formed due to prolonged air exposure during the Last Glacial Maximum, at the end of the Pleistocene [7,8]. The depth to the paleosol for each core was determined by the Goodbred Lab prior to this research [9].

Analysis of Basin Gravels.

Data from the Masterfile was used to plot gravel occurrences across depth and latitude in Seaborn and map the core containing gravel in ArcGIS (Figure 1b). An unusual cluster of gravels was observed in the Lower Delta (Figure 1a). Sr Concentrations were mapped onto these Lower Delta cores where it was available (Figure 1c). Two cores, G13 and G19, were found with Sr concentrations consistent with Ganges River sediments. Due to the huge distance the Ganges River travels after leaving its Himalayan headwaters, it has not been previously thought that the Ganges would ever have the energy required to transport gravel to the Lower Bengal Basin in the quantities in which it was observed.  To better understand these surprising occurrences, the depth-segments containing gravel and the surrounding depth-segments were photographed (Figure 2) to enable facies identification.

Figure 1. a) Density plot of the discovered gravels across depth and Latitude. Density function derived using Kernal Density Estimation (KDE) with the Seaborn visualization library in Python. The cluster of Lower Basin gravels is circled. b) Map of the quantity of gravel found in cores across the delta. Background is a Digital Elevation Map of the Basin. c) Map of the 5 Lower Delta cores with Sr concentrations available for the gravel containing depth-segments. The average Sr concentration of each core is displayed on the map, with higher Sr concentrations displayed as a dark blue, and lower concentrations as a light white.
Figure 2. Photographs of the discovered gravel facies and the sur-rounding sediment deposits. The depth at which the cores were discovered is noted. Facies A was observed by the researchers in G13 from 67.5 to 69 m, and in Core G19 at 73.5 and 76.5 m. Facies B was observed in Core G13 from 64.5 to 66 m, and G19 from 70.5 to 72 m, and at 75 and 78 m. Facies C was observed in G13 from 70.5 to 72 m.

RESULTS.

Lower Delta Gravels.

An unusual cluster of Lower Delta Gravels in both the plot of the gravel occurrences across depth and latitude and in the map of gravel occurrences across the basin was observed. (Figure 1).

Strontium Concentration and Provenance Analysis.

Strontium concentrations (ppm) were available for 9 of the Lower Delta gravels. The Sr concentrations observed in the western cores was consistent with Ganges River sediments (∼80-110 ppm) while the eastern cores were more consistent with sediment deposited by the Brahmaputra River (∼140-180 ppm) (Figure 1c). Cores G13 and G19 had Sr concentrations consistent with observed values of the Ganges at depths where there were gravels reported, (96.5 ppm and 85.7 ppm respectively).

Facies Analysis of G13 and G19.

Three facies were observed in the analyzed depth segments of G13 and G19 (Figure 2). The gravel containing facies (Facies A) of G13 and G19 is defined as coarse sands and gravels with a yellowish tinge. Facies B is defined as grey fine sands with large amounts of mica. Facies A was discovered to greater and lesser extents interspersed in Facies B layers. Facies C is defined as a grey fine mud. Facies C was only found in G13, where Facies A roughly overlies it.

DISCUSSION.

Facies Analysis.

Facies A is defined by its yellowish coarse sand gravel mixture. Facies A is unique, anomalous within the rest of the core, and has not been described in large scale facies analyses in the region, or in the Ganges Braidbelt. In contrast, both facies B and C have been previously described [10]. Facies B’s fine grey sands and abundant mica is particularly associated with Himalayan fluvial channel deposits [11].

Provenance of Facies A.

Gravel is difficult to move across long distances, and the Ganges travels over 2,500 km across mostly flat plains after leaving the Himalayas. It is difficult to describe an event in which these gravels could have been sourced from the Himalayas. The lack of mica in Facies A also serves to indicate that Facies A is not of Himalayan origin.

If we discount a Himalayan origin, what else could be the provenance of Facies A? It is unlikely that it was tidily transported, as the nearest tidal sediments are those of the upper Bengal Submarine Fan, which are mostly made up of Himalayan sediments [12,13]. It is also improbable that this was a debris-flow event based on the generally low elevation of the lower delta (Figure 3). Therefore, fluvial transport remains the most likely form of movement for Facies A. The nearest point of high elevation is the Chota Nagpur plateau of West Bengal, which is drained by multiple major rivers. One of those rivers, the Damodar River, is well known for its intense and frequent flash floods, possessing one of the largest flash flood magnitudes in the Bengal Basin [14].

The Damodar currently flows along a southeastern channel, before rapidly turning south to become a tributary of the Hooghly River (Figure 3). However, the Damodar has multiple paleochannels, suggesting that it could be a mobile river [15].

Figure 3. Digital Elevation Map of the Bengal Basin with the modern track of the Damodar River. I widened the Damodar River to make it more visually obvious. The Chota Nagpur Plateau is highlighted. The lower Basin cores containing gravel are displayed and cores G13 and G19 are circled.

Furthermore, yellowish coarse sands and pebbles found in cores from the Damodar River are associated with the high flash-flood regime from the Early Holocene. [16]. We therefore propose that a large Early Holocene flash flood in the Damodar, or a kin river, caused a sudden avulsion in the Damodar, where it maintained its South-Eastern course, depositing the flood sediments at G13 and G19. As to the cause of this flashflood, we suggest that future research begin by looking at the summer monsoon and pre monsoon cyclones. The Early Holocene was characterized by high humidity and heat, leading to a strong summer monsoon [17]. These conditions are also beneficial to the formation and intensification of pre-monsoon cyclones [18].

CONCLUSION.

The Damodar is well known for its powerful flash floods. However, no similar event to the one we propose has occurred in the human record of the Damodar. Such a flood in modernity would be devastating. Flood events across the globe are becoming more frequent and powerful, as are cyclones, especially in Bangladesh [19]. Modern mathematics and science have given paleoclimate researchers more tools than before to estimate the size and strength of paleo flood events. This data is crucial to build resilience against our rapidly changing climate.

ACKNOWLEDGMENTS.

I would like to extend my gratitude to Dr. Goodbred, Farzana Rahmin, and Dr. Stanton for supporting me through this project with so much kindness. DEM data was obtained from the NASA Langley Research Center Atmospheric Science Data Center.

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Posted by on Friday, May 15, 2026 in May 2026.

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