Wednesday, January 27, 2021

Fossil Dickinsonia Indicates Ediacaran Age For Upper Vindhyans

 It looks like one of the long running debates in Indian stratigraphy has been resolved. 

The Vindhyan Basin is one of several Proterozoic age basins in Peninsular India. The youngest rocks in this basin have been accepted to be of Neoproterozoic age i.e. less than 1 billion years old. But absolute dating of these rocks is tough. Criteria like youngest detrital zircon ages, which tell us the age of the youngest source rocks from which sediments were derived, were hinting at an age of about 850- 900 million years for the uppermost Vindhyan strata. On the other hand, presence of fossils like Arumberia and Beltanelliformis minuta, both considered microbial organisms, were suggesting of an Ediacaran age (637-541 million years) for youngest Vindhyan rock units, although there wasn't a widespread acceptance of these dates. Then, last year, a paper by Lan and colleagues published new detrital zircon dates of 548 million years from the Maihar Sandstone. They indicated that the uppermost Vindhyans are late Ediacaran in age. Detrital zircon age tells us that the sediment cannot be older than the zircon, i.e. it gives a maximum age for the deposition of the sedimentary layer.

Now, a stunning fossil find seems to confirm this chronology. The Ediacaran age fossil Dickinsonia has been dated to about 555 million years from sites in Russia. This fossil was found as impressions on Maihar Sandstone at the site of Bhimbetka in Madhya Pradesh. Bhimbetka caves are famous for pre-historic cave art. Gregory Rettalack and colleagues report Dickinsonia, in their own words.. " hiding in plain sight, on the roof of the Audi-torium Cave (N22.938402° E77.613504°), the first of several Paleolithic and Mesolithic cave painting and petroglyph sites on the path into Bhimbetka Rock Shelters"..

The fossils remain in place at the site as negative relief impressions and has been documented photographically (pics below).

 

            Source: Retallack G. et.al., 2021: Dickinsonia discovered in India and late Ediacaran biogeography.

The biological affinity of Dickinsonia is contested. Some paleobiologists think it is a representative of an extinct group of early animals. Others think it is likely to be a macro algae. No unequivocal indicators of animal life, either in the form of body or trace fossils, has been found in the Vindhyan Basin so far. Such a recent age for the uppermost Vindhyan strata may renew hope that the earliest stages of animal evolution are preserved in these rocks. 

I had written a long post on the age problem of the upper Vindhyans some time back. In that I had mentioned that physical signals like magnetic signatures and chemical criteria like carbon isotope trends were suggestive of the uppermost Vindhyans being not much younger than 850 million years. But similar magnetic signals and carbon isotope trends may be repeatedly produced under similar physical and chemical conditions.

Fossils are a better guide to age. Evolution produces unique life forms. Species exist over a restricted time range. This is the basis for constructing a relative time scale for global strata. Presence of the same species or same group of organisms in physically disconnected rocks indicate contemporaneity of these units. To ascertain their absolute age they need to be dated using one or more of radiogenic dating methods, usually relying on interlayered volcanic ash which contains the  appropriate minerals. Or, as in this case, using detrital zircon, which gives us the maximum age of the unit.  Global surveys of late Neoproterozoic strata show that Dickinsonia is not found in rocks younger than about 550 million years. This brackets the age range of the Dickinsonia containing unit of the Maihar Sandstone at Bhimbetka to between 555 -550 million years. Detrital zircon ages of 548 million years for Maihar Sandstone indicate that Vindhyan sedimentation continued for some time after.

I find myself compelled to ask one question here. What is the source of the 548 million year old zircons reported from the Maihar Sandstone? I haven't read Lan et. al.'s paper, so I am guessing. The age of the youngest zircons from the Aravalli and Central Indian terrains which were the main source of sediments supplied to the Vindhyan Basin are about 800- 900 million years. The Bastar region could be one source for these even younger zircons. High grade gneiss and granitoids from this craton have yielded 500 million year old zircons likely produced during the collision of the Eastern Ghat terrain with the eastern margin of India. Or perhaps they were sourced from terrains which are now far far way in Antarctica.

There are other Proterozoic basins in Peninsular India where age uncertainties of especially the uppermost units continue to be the subject of lively debate. This fossil find might just inspire geologists to start a fresh search for hitherto elusive age indicators.

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