Marking Vindhyan Time

About this time last year I wrote a piece for Nature India on the discovery of the fossil Dickinsonia from the uppermost Vindhyan strata exposed at Bhimbetka caves in Central India. Dickinsonia is considered to be an early animal that lived between 560 to 550 million years ago. This finding seems to confirm an Ediacaran age  (635-541 million years ago) for the youngest Vindhyan strata, termed formally as the Bhander Group. Earlier thinking was that these rocks are 1000 to 900 million years old! 

I have been thinking on and off about this discovery, not as much doubting it as trying to understand its implication from a different angle, that of the ways in which sediments accumulate in a basin and how the passage of time is recorded in sedimentary successions. 

The sedimentary rocks of the Vindhyan Basin have been subdivided by field geologists into four units based on characteristic sediment associations. From the oldest to the youngest, these are, the Semri Group, the Kaimur Group, the Rewa Group, and the Bhander Group.  Deposition of the Semri sediments began around 1700 million years ago and ended by 1600 million or so. Tectonic movements then uplifted, tilted, and eroded these rocks. Between 1200 million and 1100 million years ago the basin floor subsided and new sediment was deposited on the inclined layers of the Semri Group. There is thus a marked  'angular unconformity' between Semri and the overlying Kaimur strata, marking two distinct phases of basin history separated by 400 million years.

The age of Kaimur, Rewa and Bhander Groups, which are often referred to as the Upper Vindhyans, has been more difficult to pin down due to a lack of reliable radiogenic dating and age diagnostic biota. Despite this, a combination of magnetic properties, a few Uranium series dates from limestones, and the age ranges from zircon found in Bhander sandstones,  hinted that the youngest Vindhyan sediments are about 900 million years old.

The fossil Dickinsonia has upended this assumption and appears to have expanded the life of the Vindhyan Basin by a whopping 350-400 million years. This revision has been bolstered by two other age criteria. Microbial fossils typical of the Ediacaran Period have been reported from the Bhander. And secondly, in the year 2020, a detrital zircon dated to 548 million years ago was recovered from the Bhander. Zircons form in magmas or during high temperature metamorphic reactions. The mineral is then eroded away from these source rocks and deposited as a sedimentary particle in adjacent basins. The youngest zircon population in a sedimentary layer sets its maximum age,  since the strata cannot be older than the detritus it is made up of. It could be much younger than the contained zircon, but in this case a lack of Cambrian fossils has constrained the age of the Bhander to be older than 541 million years ago.

Let me now dive in to what has been pricking my geological curiosity. The Kaimur, Rewa and the Bhander are thought to be three distinct episodes of sedimentation. Stratigraphers may recognize them as 'depositional sequences' formed when accommodation space for sediment to accumulate is being created either by the basin floor sinking due to tectonic movements or due to eustatic (global) sea level changes. 

Such depositional sequences are building blocks of sedimentary successions in basins of all ages, each episode lasting at most few tens of million of years. V.S. Kale in a discussion note on Indian Purana basins points out that the physical characters of the sediments are not suggestive of slow sedimentation rates, and given their thickness, estimates that these sequences span 10-20 million years each. Even assigning the upper limit, Kaimur, Rewa and Bhander were deposited in about 60 million years or so. If Upper Vindhyan sedimentation began around 1200 -1100 million years ago and ended by 550-540 million years ago, that leaves about 550 million years of unrecorded elapsed time.

Sedimentation is an inherently episodic process with periods of non-deposition alternating with sediment accumulation. See this close up of an outcrop of a cross bedded sandstone from the Badami area in Karnataka.  

These sediments were deposited in a long lasting river system. Surfaces that result from non-deposition are marked as S1, S2.  In this hierarchical scheme, S1 spans perhaps a few minutes to a few hours and is a break in deposition on the lee side of  migrating ripples due to fluctuations in current energy. The S2 surface has developed over the channel sand body and spans months to years and represents seasonal fluctuations in currents or even a long drought which dries up the channel. 

 Now, cast  your eye on this larger outcrop of the Badami sandstone. 

 
It has formed by the stacking of smaller sandstone units, each containing several surfaces of non deposition. The higher order surface S3 in this thicker pile marks a longer period of non-deposition and indicates a shift in course of the river channel. Hundreds, even thousands of years will pass, before the channel migrates back to its original location and deposition resumes at that locale.

Although I may be amiss in my specific interpretation of these surfaces of non-deposition, what I want to convey is that sediment deposition in all types of environments is interrupted by periods of non-deposition ranging from a few minutes, to thousands, even millions of years. The vast majority of elapsed time a sedimentary succession represents may be accounted for by periods of non-deposition and erosion. 

The Kaimur, Rewa and Bhander collectively account for a 60 million year time-span. If the revised age range for the Upper Vindhyans is correct then there were gaps of hundreds of millions of  years between the deposition of these Groups.

But these very long breaks invariably result due to tectonic uplift and cause pronounced erosion and chemical alteration of the exposed sedimentary surface. In outcrop it is recognizable as an undulating surface with the debris of weathering consolidated into hard soils, and in the formation of solution pits in limestone terrains. Such basin wide surfaces of marked erosion have not been found separating the Kaimur and the Rewa, and between the Rewa and the Bhander strata. 

Smaller breaks though do occur. Chandan Chakraborty in a study of sedimentary cycles of the Vindhyan Basin finds an angular discordance between the Kaimur and the Rewa, and the Rewa and the Bhander. However, this is restricted only to the south of the Vindhyan Basin. The discordance between the older and younger sequences dies out towards the north. This stratigraphic relationship  suggests that the tectonic movements were contemporaneous with sedimentation. The basin floor was tilted up on one side interrupting sedimentation and eroding the exposed strata there, while at the other end (north), subsidence and deposition continued. The periods of non-deposition and erosion between these sequences was a more localized event, lasting few millions of years at most. The kind of tectonic upheavals that may halt deposition for hundreds of millions of years across the entire basin are simply not attested to in the stratigraphic record of the Upper Vindhyans. 

It seems to me, and I say this mischievously, that the earlier estimate of 1000-900 million year age of the uppermost Bhander strata and for the end of Vindhyan sedimentation sounds quite reasonable! 

How secure is the finding of Dickinsonia and the assigned Ediacaran age of the Bhander Group? I am in no way trying to rebut this finding. The microbial fossil Arumberia found in the Bhander is considered by experts to be a reliable Late Ediacaran age indicator. However, the identification of Dickinsonia is based on a 3D digital reconstruction of photographs taken at the site of discovery. Due to the protected status of the Bhimbetka Caves, the scientists were unable to excavate the impression and subject it to a physical examination or a chemical test for organic residue. This is the only report of Dickinsonia from the entire Vindhyan Basin.

The young 548 million year date of the detrital zircon is based on one grain. The researchers are confident of their analysis and see no reason to reject this data point, though they admit that further corroboration of this date will be necessary given that previous studies did not report such young zircons from the Bhander strata. Curiously, no source terrain of this young zircon is identified in the paper.  

The news about Dickinsonia received wide coverage in India. That was expected given the long standing uncertainty of the age of the Vindhyans and the importance of the fossil for understanding early animal evolution and Ediacaran ecology and paleogeography.  But these new proposed timelines have thrown up a puzzle about rates of accumulation of sedimentary sequences and magnitudes of intervening breaks. We are quick to appreciate changes in sedimentary layer color, their structures, and their geometry, and interpret these changes in the context of fluctuating depositional conditions. But what about surfaces of non-deposition? Do they have anything valuable to contribute to our understanding of geological processes. 

The paleontologists Niles Eldredge and Stephen Jay Gould once perceptively observed that 'stasis is data'. The unchanging morphology of many fossil species lineages tell us something about the mode and tempo of evolution. Geological surfaces of stasis, when sedimentation has come to a standstill, are richly informative too in their own way. Surfaces representing very long time-spans may develop during convulsions in the earth's crust and periods of mountain building. The soils that often mantle such surfaces archive information about past climate and terrestrial life. Breaks of smaller and smaller time-spans like wise have a variety of drivers such as orbital control on climate and sea level, migration of depositional environments, and even tidal cycles.  

How many more hiatuses of unknown duration lie hidden within the Vindhyan sediment pile?  They may mark phases when no sediment accumulated but they are as much an integral part of basin history as are the iconic sandstone faces of Bhimbetka.