A late Neoproterozoic to early Paleozoic section (~600-500 million years old) of the Tethyan Sedimentary Sequence is exposed around the villages of Dantu, Boun, and Philum, in the vicinity of the famous Panchachuli Glacier in Kumaon. The lower part, made up of low grade metamorphic rocks is accessible along the many local trails. The higher summits are made up of sandstone and conglomerate. These are harder to reach, but blocks eroded from the summit can be found in streams near Boun and Philum.
Two years ago I had posted a picture of a sandstone block showing convoluted folded layers and asked whether this folding was tectonic in origin or due to synsedimentary deformation of semi lithified sediment. In the latter case, shaking of the sea floor and mass movement of sediment due to an earthquake or a severe storm results in the sediment layers contorting and deforming in various ways.
Sandstone in stream bed near Boun village.
I could not decide between these alternatives although I favored a synsedimentary origin of these features. My reasoning was that such deformation appears local, since there were also examples of undeformed sandstone with delicately preserved primary bedding. The short wavelength folding observed in my example also is very different from the longer wavelength folds present in the lower part of the exposed Tethyan sequence.
Last month on a trip to Boun village I came across another block from those high ridges which I think lends additional weight to the synsedimentary deformation scenario.
You will notice that the block is made up of a conglomerate (pebbly layer) in the lower part, overlain by a beautifully cross bedded sandstone towards the top. The ten rupee coin gives a sense of scale. The lower half of the block is a classic flat pebble conglomerate. You will realize the meaning of the term as you read along. A more detailed examination of the conglomerate hints at an unusual mode of formation. The clues lie in the four boxes I have drawn. I will focus on them one by one to make my case. This is the cross section of the pebbly layer. The bedding plane view is not exposed in the boulder.
Before that, let me put up this picture of another conglomerate. This sample too has rolled down from the high ridges near Dantu village.
Sedimentologists will be confident in interpreting this as deposition in a gravelly stream or in the surf zone of a beach. The smooth and rounded shape of the cobbles is due to long transport from the source to the site of deposition, followed by the particles rubbing against each other in a high energy current and wave environment.
Now take a look at the pebbles in Box 1.
They have straight and jagged sides and pointed edges. This indicates very little transport and attrition before burial. The source rock of these pebbles must have been near by.
In fact, the source can be observed in Box 2.
The dark grey elongated pebbles were derived by the breakage of the bed in the lower part of the block. The dark grey layer has a fragmented fabric. I have outlined in yellow some larger blocks of the remnant bed. They are surrounded by smaller broken pieces. It looks like a layer which hardened quickly on the sea floor broke due to a disturbance and yielded these flat pebbles. These pebbles are called intraclasts, since they are derived from a source from within the depositional environment. The slab like shape of the pebbles suggests breakage along parallel planes of weakness. The breakage is not due to tectonic overprinting since the overlying cross bedded sandstone is not affected.
Complete disarticulation of an early cemented layer will ultimately yield individual centimeter scale pebbles which make up the pebbly layer highlighted in Box 3.
Notice the mostly horizontal disposition of the pebbles suggesting transport in a viscous laminar flow and quick burial. The sandy matrix has prevented pebbles from bumping into each other, thus preserving their sharp faces and edges. In contrast, constant exposure to waves and currents would have resulted in the sand being winnowed out and caused these platy pebbles to be rounded, imbricated and stacked at an angle.
Fine quartz and lime mud sediment was cemented by calcium carbonate on the sea floor within a few tens of centimeters of burial. This semi lithified layer then broke during an earthquake or when the sea floor was pounded during a severe storm.
Box 4 captures this transition from an in-place unbroken layer which show signs of breakage towards the top.
Slope failure and mass movement of such a layer eventually resulted in complete breakage of the rigid bed and the formation of discrete flat chips which then were deposited as a flat pebble conglomerate. Box 1 to Box 4 represent different stages of the deformation and sedimentation process. The sharp contact of the layer in Box 3 with the underlying layer (see pic of the entire block) suggests that it may be material transported from an adjacent area where an equivalent bed was completely disarticulated.
Occurrence of slope failure induced flat pebble conglomerates have been previously observed and reported from the Cambrian age Snowy Range Formation in northern Wyoming and southern Montanan, U.S.A.
I have observed only one example of this during my recent visit and I have proposed only a tentative answer. Without observing and understanding the stratigraphic and sedimentologic context in an outcrop I cannot be certain that it is correct.
The association of undeformed and deformed blocks does suggest that intermittent disturbances resulting in brittle and ductile deformation of semi hardened sediment masses alternated with quieter periods of sedimentation. The overlying cross bedded sandstone is an example of deposition during quieter phases.
Flat pebble conglomerates mostly form in sedimentary carbonate environments. This make sense since rapid cementation of the sea floor by calcium carbonate saturated sea water is common. This example though is from a predominantly siliciclastic setting where quartz rich silt lithified fairly rapidly.
These conglomerates also show a peculiar temporal range. They are common in Proterozoic and Cambrian age sequences, but become exceedingly rare in younger rocks. Paleoecologists suggest that this is due to the diversification of burrowing animals that took place during the Great Ordovician Biodiversification Event about 485 to 460 million years ago.
The churning of sediment by bioturbation kept the sediment loose and granular and prevented frequent cementation of the sea floor and shallow buried layers. Carbonate intraclasts became rarer, forming only in more geographically restricted harsh hypersaline settings. Flat pebble conglomerates give us a glimpse in to the ecology and physical properties of the sea floor before the evolutionary radiation of burrowing macrofauna.
Geological processes and evolution interact and feed off each other. Through earth history, the formation of diverse topography and chemical environments by geological circumstance have been triggers for evolutionary innovation. In this example, the evolution of animals making deep vertical burrows resulted in the disappearance from the geologic record of a distinctive sedimentary rock type. Yet, the churning and resulting oxygenation of the sedimentary profile opened up new ecologic spaces for the colonization and diversification of a more complex web of marine communities.
My quest for a more complete answer to the origin of these deformed sandstone continues.
On my next trip to Boun I will try to find more of these blocks to gather evidence in support of my theory. Or, who knows, try to find an easier route towards those high ridges! Stay tuned.
Further Reading:
1) Rapid Uplift - Field Photos: Folds- Tectonic Or Soft Sediment Deformation?
2) Thematic Posts - Rapid Uplift- Geological Processes and Evolution.