Field Photos: Fluvial Bedforms

The Deccan Volcanic Province in best known as a thick pile of basalt lava. It is igneous rock country. In Maharashtra, the plateau east of the western ghats is drained by rivers small and big flowing ultimately into the Bay of Bengal. Many of these rivers originate in the very humid mountainous region known as the western ghats. Eastwards in the more arid climatic zone prevailing over the plateau these rivers have deposited a lot of sediment throughout the Quaternary and possibly earlier.

Here is the regional context:

Source: Mishra et al 2003

I was visiting a mineral museum north of the town of Sangamner a couple of weeks ago and came across some superbly preserved fluvial bedforms  along a road cut just south of Sangamner. Fluvial bedforms are sedimentary structures formed as sediment is moved and is deposited by current action in the river bed.  If you zoom and pan southwards along National Highway 50 south of Sangamner in the embedded image below you can see a trace of a small tributary of the river Pravara.


View Larger Map
The sediments in the images below were deposited by that tributary. The river has incised or cut into its own deposits and the active channel today is about 10-15 meters below the section seen in the images. So these deposits form an ancient probably late Pleistocene fluvial terrace a few hundred meters wide.

1) View of fluvial bedforms with pebbly and sandy planar and cross stratification clearly seen. At right center is a pebbly cross bedded wedge, possibly a point bar deposit. So, the sediment you see was being rolled along the bed of the river. The morphology of the sediment surface was like a sheet of sand and pebbles (the planar layers are a cross section of these sheets). Here and there the sediment sheet was wrinkled into large waves (the large cross beds are likely the cross section of these large wave forms). Some sediment was being deposited along the banks along inclined surfaces forming the point bar deposits.

2) Another wide view of planar and cross beds.

3) Shallow cut and fill structures. These are common features in the river bed as pulses of high energy events such as floods may scour the sand in the river bed and then fill up the trough formed by sediment.

4) A small channel filled by gravel and sand overlain by planar and cross bedded sand and pebbly layers.

I couldn't stay long at the outcrop as traffic was zooming perilously close to me and we had to go some distance. These deposits though have been interpreted to preserve a record of Quaternary climate change. The entire section records a phase of mid-late Pleistocene - early Holocene aggradation in which sediment was deposited and the river channel built upwards. This was followed by a phase of incision later in the Holocene when the river cut into its own deposits leaving stranded terraces.

I hope this outcrop survives for long. Apart from aiding our understanding of fluvial geomorphology and climate change it is a superb teaching tool for sedimentary geology and geomorphology classes. Unfortunately intensive farming activity and excavations for construction are slowly degrading these fluvial deposits near Sangamner.