Friday, 1 February 2019

Ediacaran fossils from the Taroudant Group of the Anti-Atlas Mountains, Morocco.

The Anti-Atlas Mountains of southern Morocco have a rich Palaeozoic fossil record, and have long been visited by palaeontologists from outside of the area for decades. The possibility that this fossil record might extend back in time to before the beginning of the Cambrian in the rocks of the Taroudant Group was first suggested in the 1950s, but at that time it was more-or-less universally accepted that Precambrian rocks did not contain fossils beyond the occasional Stromatolite or other microbial structures, and the putative Precambrian fossils of the Anti-Atlas were rejected as being diagenetic in origin (i.e. formed by physical and chemical processes within sediments after those sediments have been buried). Today it is known that the fossil record does not begin in the Cambrian, and Precambrian fossils of the Ediacaran Fauna have been reported from several locations around the world. Despite this the possible Ediacaran-age fossils of the Anti-Atlas have remained largely overlooked by modern researchers.

In a paper published in the journal Precambrian Research+ on 26 November 2019, Dominik Letsch of the Institute of Geochemistry and Petrology at Eidgenössische Technische Hochschule Zürich, and Dr. von Moos AG, Simon Large, also of the Institute of Geochemistry and Petrology at Eidgenössische Technische Hochschule Zürich, and of the Natural History Museum, Stefano Bernasconi of the Geological Institute at Eidgenössische Technische Hochschule Zürich, Christian Klug of the Palaeontological Institute and Museum at the University of Zurich, Thomas Blattmann and Wilfried Winkler, also of the Geological Institute at Eidgenössische Technische Hochschule Zürich, and Albrecht von Quadt, again of the Institute of Geochemistry and Petrology at Eidgenössische Technische Hochschule Zürich, revisit the putative Precambrian fossils of the Anti Atlas, in order to examine their authenticity and age.

The Anti Atlas is a range of arid mountains in southern Morocco, located between the High Atlas, to the north, and the Sahara Desert, to the south, and formed by deformation during the Carboniferous and Permian. Geologically these mountains form the northern margin of the West African Craton, comprised of a Palaeoprotorezoic crystalline basement, overlaid by the Palaeoprotorezoic -Neoproterozoic sedimentary rocks of the Anti-Atlas Supergroup, then the volcano-sedimentary rocks of the Ediacaran Ouarzazate Supergroup, associated with the closure of a Cryogenian ocean, and finally a Palaeozoic Sedimentary succession. The uppermost parts of the Ouarzazate Supergroup, formed as volcanic activity in the area declined and the rocks began to subside, with the Taroudant and Tata groups forming in a volcano-influenced delta system. The Taroudant Group comprises two formations, the Adoudou and the Lie de Vin, both of which vary considerably across their extent, comprising a mixture of siliclastic and carbonate rocks. Above the Lie de Vin Formation the Tata Formation of the Tata Group contains a Cambrian fauna dominated by Trilobites and Archaeocyathids.

(a) Geological overview map of the Anti-Atlas. (b) (inset) schematic stratigraphic column of the Precambrian and early Palaeozoic of the Anti-Atlas area. Sites with fossils described in this paper are indicated by green (discoidal fossils) and red (problematic microfossils) stars in both (a) and (b). Letsch et al. (2018). 

Discoidal structures thought to be fossils have been reported sporadically from the Adoudou Formation near the village of Warmandaz since the 1970s, and have variously been described as fossil Medusae (Jellyfish), Ediacaran soft-bodied organisms, or Aspidella (a form-genus name widely used for discoidal Ediacaran fossils of uncertain origin). These discs are preserved variously as negative-relief impressions on the upper parts of beds with corresponding moulds on the base of the beds above (gravity casts) or as three dimensional structures making impressions on both the beds above and the beds below, and infilled by fine-grained rhombohedrons of cryptocrystalline quartz (endorelief fossils) , and range from about 1 cm in diameter to about 13 cm, though most are in the 2-5 cm range. Larger specimens often have marginal rings, and one specimen was found to comprise a series of stacked bowl-shaped disks separated by thin clay layers, which grow larger moving up the section.

Letsch et al. note that fossils assigned to Aspidella from Newfoundland, Australia, and Argentina also have cryptocrystalline quartz infills and similar size ranges, suggesting a common origin, though Aspidella fossils from other locations do not follow this pattern. They suggest that this may be indicative of an animal with high infant mortality, with most individuals dying when they are 2-5 cm in diameter, but a few growing above this size and then being able to survive light burial with sediment, and subsequently surviving a series of such events and growing to in excess of 10 cm, whilst developing a structure if concentric rings. They suggest that these discoidal structures may have been the holdfasts of frondose organisms, but were not able to find any trace of such structures.

Discoidal fossils from the Warmandaz site. (a) Fossil preserved as negative epirelief with two well-developed marginal rings (hand lens, 3 cm, for scale; note that the protruding central part of the fossil is an adhering piece of the upper bed). (b) Two close-by discoidal fossils preserved as negative epireliefs (above hammer handle). (c) Bedding plane with four discoidal fossils (outlined by rectangular boxes) preserved as negative epireliefs. (d) Discoidal fossil with marginal rings preserved as negative epirelief (cutting edge of chisel, 3 cm, for scale). (e) discoidal fossil preserved either as an endorelief or as an epirelief filled by multiple sediment laminae. (f) Discoidal fossil preserved as intrastratal endorelief. Hammer (total maximum length 32 cm) for scale. Letsch et al. (2018). 

It was not possible to directly date any of the strata at the Warmandaz site. However, the beds which contain the discoidal fossils overlay a layer of rhyolite (volcanic rock) from which uranium-lead zircon dates were obtained from the village of Tafendra, 4.5 km away. Zircon is a volcanic mineral that forms as liquid magma slowly cools to form solid rock. As zircon forms it can incorporate a variety of different elements into its crystal matrix, including uranium but not lead. This is useful as over time uranium decays to form lead, so any lead in a zircon mineral must be the result of the decay of uranium. Since the decay of uranium to lead occurs at a steady rate, it is possible to determine the age of zircons by measuring the ratio of uranium to lead within them.

Seven zircons were obtained from the Tafendra village site. Five of these produced palaeoprotorezoic dates, ranging from 2.1 to 1.9 billion years in age. This is clearly much older than the rock layer from which they were obtained, though this is not in itself surprising, as zircons are extremely resilient once formed, and are known to be capable of surviving subduction at plate margins and then re-eruption into plate-margin igneous strata, such as those of the Ouarzazate Supergroup. The remaining two zircon grains yielded ages of ~561 million years, consistent with that obtained from the uppermost layers of volcanic rock within the Ouarzazate Supergroup at other locations, suggesting that the Warmandaz discoidal fossils can be assumed to be less than 561 million years old.

The discoidal fossils of Warmandaz are located within a violet sandstone that forms part of the Tabia Member of the Adoudou Formation, for which no dating information is available. The Tabia Member is overlain by the Tifnout Member, comprised largely of stromatolitic dolomites (high magnesium-content limestones, laid down by calcifying microbial mats), the base of which has, elsewhere, been dated to between 544 and 542 million years old, based upon regional chemostratigraphic data (comparing isotope ratios to other strata in the region for which more direct date information is available). This leads Letsch et al. to conclude that the discoidal fossils must be older than 542 million years and younger than 561 million years old, making them Late Ediacaran in age.

Warmandaz composite section: Schematic stratigraphic section of the Warmandaz locality with the level with discoidal fossils indicated. Letsch et al. (2018).

Letsch et al. also looked at microfossils in the Adoudou and Lie de Vin formations. Relatively little evidence for microbial life has been reported from these formations before; Stromatolites are known from both formations at a number of locations, with some skeleton-forming macro Algae assigned to the genus Kundatia reported from exposures of the Adoudou Formation south of the Zenaga Inlier, and calcified Cyanobacteria assigned to the genera Renalcis, Tarthinia, and Epiphyton from the Lie de Vin Formation. An unconfirmed claim for Tommotids (Earliest Cambrian Small Shelly Fossils) from the upper part of the Lie de Vin Formation has also been made.

Hemispheroid Stromatolites with onlapping micrite filling microbial microtopography (lower Tifnout Member). Hammer (maximum length 32 cm) for scale. Letsch et al. (2018).

In addition to these earlier records, Letsch et al. report possible microfossils from the Adoudou Formation at Tazgla on the northwestern edge of the Zenaga Inlier, and close to the village of Aït Saoun at the southwest end of the Saghro inlier.

The Zenaga Inlier material comes from a pebble of apparent Stromatolitic origin within the upper part of the Tabia Member, and two thin dolostone layers at the base of the Tifnout Member. They comprise a large number of molds filled with sparite or microsparite, with an outer rim that is opaque and dark brown or red in colour. These molds range from 50 to 1000 μm in size, though the majority are between 200 and 300 μm, and can be dog-bone or dumbbell in shape, with some longer spindles, circles or ellipses.

Problematic microfossils from the Tazgla site. All microphotographs were taken under plane polarized light. Letsch et al. (2018).

Dating the Tazgla material proved difficult. as no datable material was found in the Tabia or lower Tifnout Members. The Ouarzazate Supergroup here overlies an ignimbrite (volcanic rock) layer, from which a uranium-lead zircon age of 574.9 million years was obtained, though this was some way beneath the layer which produced the microfossil-producing pebble, with an unconformity (layer at which deposition has stopped and erosion occurred) in between. Zircons were also obtained from a sandstone horizon in the upper part of the Tifnout Member, though again these were some way from the microfossil-producing layers, which produced a date of 553 million years, though there would have been some time between the volcanic formation of the zircons and their deposition in the sandstone. Based upon this and correlations with the Tabia and Tifnout members elsewhere, Letsche et  al. conclude that these microfossils are Late Ediacaran or Early Cambrian in age.

The Aït Saoun microfossils were obtained from a layer of detrital calcarenite (sedimentary rock made up of detached limestone fragments) with ripple patterning, close to the top of the Tifnout Member, which here is a much reduced layer, predominantly made up of laminated microbial dolostones,s sandwiched between the Tabia Member and Lie de Vie Formation. These microfossils are less distinct than at Tazgla, and comprise a mixture of dog-bone shapes and irregular shapes with radial protrusions.

Problematic microfossils from the Aït Saoun site. Some microfossils are indicated by white arrows. All microphotographs are taken under plane polarized light. Letsch et al. (2018).

Letsche et al. were able to obtain dates from derived zircons within the sedimentary sequence at Aït Saoun. A layer of detrital silt and volcanic material within the Tifnout Member and above the microfossil producing horizon produced a minimum uranium-lead zircon age of 529 million years (Middle Cambrian), while a layer below the microfossils produced an age of 540 million years (Early Cambrian). A layer in the Lie de Vin produced an age of 523 million years.

Lithostratigraphic logs and chemostratigraphic data from the Tazgla and Aït Saoun section). Fossil-bearing levels, geochronology samples (asterisks, with maximum depositional or volcanic ages, legend in upper-right corner of figure). Letsch et al. (2018).

The nature of the Tazgla and Aït Saoun microfossils is unclear. There is nothing about their mineralogy or structure that could not be produced abiotically (by non-biological means), but the fact that several shapes are repeated numerous times pushes Letsche et al. towards the conclusion that these are of organic origin. They note that these structures show a number of similarities to the Ediacaran macrofossil Namacalathus, including outlines similar to cross sections of the macrofossil (though while a fossil would only ever be preserved in cross section in one direction is unclear), as well as a thin wall-structure with signs for flexible behaviour during compaction. The possibility that these might be Algal cells is thought less likely, due to the lack of a distinct cell-wall.

See also...

http://sciencythoughts.blogspot.com/2019/01/putative-discoidal-fossils-from.htmlhttp://sciencythoughts.blogspot.com/2019/01/using-ordovician-tafilalt-lagerstatte.html
http://sciencythoughts.blogspot.com/2019/01/reconstructing-ediacaran-nilpena.htmlhttp://sciencythoughts.blogspot.com/2019/01/ediacaran-trace-fossils-suggest-that.html
http://sciencythoughts.blogspot.com/2019/01/understanding-deep-marine-origin-of.htmlhttp://sciencythoughts.blogspot.com/2019/01/hylaecullulus-fordi-new-species-of.html
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