Thursday, August 23, 2018

Proterozoic Upper Vindhyan Succession: Fossils And Age

I came across a remarkable paper by S. Kumar published in the December 2016 issue of The Palaeontological Society of India. It is an overview of all the published reports of megafossils from the Proterozoic age Vindhyan Basin of Central India. The findings read like a special issue of Retraction Watch. I am not implying any fraud or scientific misconduct (neither does S. Kumar), and the papers in question have not been retracted either. Rather, he concludes, based on a rereading of earlier reviews and new analysis, that there has been a widespread misinterpretation of data. Practically all megascopic features interpreted variously as trace fossils (burrows, trails, scratch marks), impressions and body fossils are not fossils. They are abiotic in origin. Only some carbonaceous megafossils represent the remains of large microbial communities.

The map below shows the outcrop pattern of the Vindhyan Basin.

Source: Candler C. Turner et. al. 2013

..and this figure summarizes our evolving understanding of the age range of the Vindhyan sedimentary succession.

Source: Geoffrey J. Gilleaudeau et. al. 2018

The most sensational and controversial of fossil reports from the Vindhyans are from the lower part of the succession. In the late 1990's and continuing in the early 2000's, Dr. Azmi, a researcher from the Wadia Institute of Himalaya Geology, reported microscopic embryo like globules,  rod like and filamentous microfabrics and oddly shaped mineral fragments. These fossils were found in the Rohtas Limestone and the Tirohan Dolomite of the Semri Group. He interpreted some of the globules to be animal embryos and the odd fragments as 'small shelly fossils', essentially bits and pieces of shells of marine animals. Based on this identification he claimed that the rocks were Late Neoproterozoic to Early Cambrian in age (~ 650 Ma to 520 Ma; Ma = mega annum = million years). This claim though has never gained wider acceptance.

First, since his discovery, improving geochronology of the Lower Vindhyans firmly brackets their age as between 1.7 Ga years to 1. 6 Ga (Ga= giga annum= billion years). And second, no Cambrian age animal fossils have been found from rocks younger than the Rohtas Limestone and Tirohan Dolomite. These shallow marine sediments should have yielded a prolific Cambrian fossil record. The consensus now is that those fossil remains are microbial and eukaryotic algal forms, and the odd shaped mineral fragments are just that; odd shaped inorganic mineral growths. Dr. Azmi's claim does not require a revision of Vindhyan stratigraphy.

An even more subversive fossil claim was made by A. Seilacher, P.K. Bose and F. Pfluger in 1998. They reported to have found burrows of triploblastic animals from the Chorhat Sandstone (dated to about 1.6 Ga) of the Lower Vindhyan Semri Group.  They accepted that the rocks are more than a billion years old and suggested that multicellular animals had evolved more than half a billion years before the appearance of the first animal body fossils in the Latest Neoproterozoic to Early Cambrian times. This claim too has been rejected by most workers. As S. Kumar writes in his review, the burrows have been shown  to be syneresis cracks, resulting from subaqueous shrinkage of sediment. They are not fossils. Molecular phylogeny indicates that multicellular animals originated between 800 Ma and 650 Ma and discernible megascopic fossils, in the form of traces, impressions, and body parts, appear first by around 560- 550 Ma. Any claim of megascopic metazoan 'fossils' in rocks older than 600 Ma needs to be treated with extreme skepticism.

In summary, most paleontologists now accept that there is no credible evidence of fossils of multicellular animals in the Vindhyan sediments. The youngest Vindhyan sediments are thought to be considerably older than the Cambrian (base of the Cambrian is 541 Ma).

When did Vindhyan sedimentation end? What is the age of the youngest Vindhyan rocks? This is one of the conundrums in Indian stratigraphy. Different types of data and methods of dating rocks have come up with conflicting ages.

Only three direct dates for the Upper Vindhyan Bhander Group are available. The Lakheri Limestone in the Rajasthan sub basin shows a date of 1073 Ma +- 210 Ma. The overlying Balwan Limestone has been dated to 866 Ma +- 180 Ma. About 60 m of sediment overlies the Balwan Limestone.  The Bhander Limestone in the Son Valley sub basin has yielded a date of 908 Ma +- 72 Ma. In the Son Valley, there is about 400 m of sediment overlying the Bhander Limestone. There is no direct method available yet to date these youngest rocks.

According to S. Kumar, the Maihar Sandstone, which is the uppermost Vindhyan strata exposed in the Son Valley, are around 650 Ma or slightly younger. The few non-carbonaceous remains that actually are fossils according to S. Kumar are found in these rocks. They are the microbial mat structure Arumberia and the body fossil Beltanelliformis minuta, which recently has been shown to be of cyanobacterial affinity. They are both Ediacaran age (637 Ma - 541 Ma) fossils. Kumar concludes that Vindhyan sedimentation in the Son Valley continued well into the Neoproterozoic, finally terminating in earliest Ediacaran times.

In contrast, an older age range is being hinted at by two very different types of data.  Bijaigarh black shales of the Kaimur Group have been dated to around 1200 Ma. The diamond bearing Majghawan kimberlite intrudes the Baghain Sandstone of the Kaimur Group near the town of Panna in Madhya Pradesh. It has been dated to about 1073 Ma. The Kaimur Group is dated between 1200 Ma and 1100 Ma. The kimberlite does not intrude the overlying Rewa and Bhander strata. They are younger than 1073 Ma, but apparently not much younger. The magnetic signatures frozen in the kimberlite match those preserved in the Rewa and Bhander rocks, suggesting that deposition of the Rewa and Bhander sediments did not persist for too long into the Neoproterozoic. The Bhander Limestone has recently yielded a date of 908 Ma (+- 72 Ma). Vindhyan sedimentation may have ended by 900 Ma.

Support for this scenario comes from the age of detrital zircon found in the Maihar Sandstone. The zircon has yielded a date of 1020 Ma. This date reflects the time when the zircon crystallized out of a magma. Zircons from this igneous rock were then eroded, transported and deposited in the Vindhyan basin. The Maihar Sandstone thus cannot be older than 1020 Ma. But, there is also an absence of zircons younger than 1020 Ma in the Maihar Sandstone. Geologists doing provenance work on the Vindhayans agree than the Aravalli orogenic belt to the west was an important source of sediments into the Vindhyan basin, as was the Central Indian Tectonic Zone (CITZ) to the south. Both of these belts could have yielded the 1020 Ma zircons, since they both contain igneous rocks of that age. The CITZ lacks igenous activity younger than this. But magmatism took place to the west of the Aravalli orogen between 873 Ma to 800 Ma (Erinpura Granite) and between 780 Ma and 750 Ma (Malani rhyolites and associated plutons). The erosion of these two igneous provinces could have provided a source of younger zircons to the Vindhyan basin. Their absence has been taken to imply that the Vindhyan Basin had closed by that time.

I am putting up these contrasting age signals here, but I don't really have a strong position on either of them. For one, geochronology of the Upper Vindhyans is still sparse and the few dates have come with large margins of error. I don't have the expertise to evaluate the magnetic data. The zircon data might seem to make a strong case for an earlier termination of Vindhyan sedimentation. But there is an alternate explanation for the lack of younger zircons in the uppermost Vindhyan sediments. The Erinpura Granite and the Malani igneous rocks lie to the west of the Aravalli belt. Orogenic activity  around 1 Ga, which deformed and uplifted the Delhi Group sediments, would have formed significant topography and a barrier to rivers flowing eastwards into the Vindhyan basin. Thus, a provenance cutoff rather than an end to sedimentation could explain the lack of younger age zircons.

Some workers have suggested that the Vindhyan sub basin to the west in Rajasthan closed earlier by around 900 Ma, but sedimentation continued until early Ediacaran times in the eastern parts of the basin exposed in the Son Valley in Madhya Pradesh.  Geoffrey J. Gilleaudeau and colleagues used variations in carbon isotopes of carbonate rocks to suggest age ranges for Vindhyan rocks in Rajasthan and the Son Valley. They found that carbon isotope values in carbonate formations in Rajasthan did not vary much and fitted the pattern observed for the late Mesoproterozoic (slightly older than 1 Ga) elsewhere around the globe. However, another study by Bivin George and colleagues of  the Balwan Limestone, which is the uppermost carbonate unit in the Rajasthan sub basin, showed substantial shifts in carbon isotope values through the section. The Balwan Limestone has been recently dated to about 866 Ma, but with a margin of error of +- 180 Ma! Bivin George and colleagues suggest that these variations match the 800 Ma globally synchronous Bitter Springs carbon isotope anomaly. Considering the presence of a 60 m shale layer above the Balwan Limestone, they put  the closure of the Vindhyan Basin in Rajasthan at around 770 Ma.

In Son Valley, carbon isotope values in the Bhander Limestone showed oscillations from enriched in the heavier isotope to sharply lighter values. This pattern is seen throughout the Neoproterozoic (younger than 1000 Ma) in sections elsewhere around the world. The sharp variations are thought to be due to large environmental perturbations that punctuated the Neoproterozoic from time to time.

During warmer climes, in healthy ecosystems, photosynthesizing marine organisms preferentially suck up the lighter carbon (C12) isotope. Sea water gets enriched in the heavier isotope (C13) which makes its way into carbonate minerals, resulting in 'heavier' or positive C isotope values of carbonate rock.  Environmental crises may cause a population crash of photosynthesizers. More of the lighter isotope is now available to enter carbonate minerals, resulting in  'lighter' or negative C isotope values of carbonate rock.

One problem in having a high confidence in this isotope curve matching is the inadequate geochronology of the Upper Vindhyans. Limestone ages in the Rajasthan sub basin have large margins of error (up to 200 Ma). Just one direct date of 908 Ma +- 72 Ma is available for the Bhander Limestone in the Son Valley sub basin. This means it is difficult to assign a particular pattern of variation to a specific Neoproterozoic interval. The technique has real potential but currently also underscores the need for sharper geochronology data.

However, the fossils Arumberia and Beltanelliformis found in the Maihar Sandstone support this scenario of a diachronous end to Vindhyan sedimentation. But there are niggling doubts too. The Sturtian (717 -643 Ma) and Marinoan glaciations (650 - 635 Ma) took place during the Neoproterozoic, covering much of the earth in ice. These glaciations ended with ice transported tillite deposits overlain by distinctive limestone or dolomite layers known as the 'cap-carbonates'. But there are no reports of unequivocal glacial deposits in the Rewa and Bhander sediments. Why their absence? Is that a hint that they are older than the Cryogenian Period i.e. older than 720 Ma, or is it because India was located at lower latitudes which were not glaciated? It is noteworthy that the latest Neoproterozoic (~ 590 Ma) Blaini Formation exposed in the Lesser Himalaya near Nainital and Mussoorie does contain glacially transported deposits.

But if the Upper Vindhyans are older than the Cryogenian, then what about the Ediacaran age fossils from the Maihar sediments?...

watch this space.

Saturday, August 18, 2018

Technology: Carbonate Sedimentology

Just a quick note to point out the range of instrumentation now available to sedimentologists for analysis of samples. This is from a study of Ordovician age sediments by Yihang Fang and Huifang Xu, published in the June 2018 issue of the Journal of Sedimentary Research.

An excerpt:

 A micro-laminated carbonate with alternating dolomite–calcite layers from the mid-Lower Ordovician St. Paul Group from the Central Appalachians in southern Pennsylvania was examined using optical microscopes, X-ray diffraction (XRD), scanning electron microscopy (SEM) with X-ray energy-dispersive spectroscopy, electron microprobe analysis (EPMA), scanning transmission electron microscopy (STEM), laser-induced fluorescence (LIF) imaging, short-wave infrared (SWIR) imaging, and X-ray fluorescence (XRF) imaging. The sample is composed mainly of two types of layers. Dolomite-dominated layers are darker in color, generally thinner, and contain detrital minerals such as quartz and feldspar. In contrast, calcite-dominated layers are lighter in color, thicker, and contain less detrital minerals supported by microcrystalline calcite matrix. In situ XRD, LIF, XRF, and SWIR results show that organic remnants are enriched in the dolomite layers. The coincided spatial distribution confirmed a positive correlation between dolomite and organic matter, and hence provide evidence for microbial-EPS-catalyzed formation of sedimentary dolomite.

That is a lot of heavy weight toys!!

The researchers were trying to come up with an answer to one of the long lasting problems in sedimentary geology; that of the origin of the mineral dolomite.

I am not going into this paper in detail. I have not read it.  Let me just say that it has been suspected that microbial communities living on the ocean floor and in the sediment column may catalyze the precipitation of dolomite and this study confirms that dolomite rich layers contain organic matter. The dolomite may form at the sediment sea water interface by precipitation directly out of sea water or more commonly by replacement of the aragonite or calcite sediments during shallow burial.

On a broader time scale, dolomite abundance through the Phanerozoic does seem to correlate well with episodes of lower atmospheric oxygen concentrations and consequently less oxygenated sea water. See this figure spanning the Phanerozoic Era ( McKenzie J.A. and Vasconcelos, C.  2009). This relationship between ocean anoxia and dolomite abundance may well hold for the Precambrian too.

Anaerobic bacteria can thrive under such anoxic conditions.  Sulfate ions in sea water are thought to be a hindrance for dolomite formation. Anaerobic bacteria  remove these ions from sea water and pore fluids by sulfate reducing respiration, thus creating conditions favorable for precipitation of the mineral. They may also present specific types of organic substrates which enable easy nucleation of dolomite crystals.

For more details on the origin of sedimentary dolomite, do see my post The Dolomite Problem- Peeking Under the Hood