Tag Archives: Precambrian

Atlas of stromatolites and cryptalgal laminates

Stromatolites. The Precambrian is replete with them. In many ways they define the Precambrian, that period of earth history, about 90% of it, that set the scene for the world we currently live in – its atmosphere, hydrosphere, lithosphere, and biosphere. It’s the period when life began more than 3.4 billion years ago, taking its time (about 3 billion years) to get over that first rush of DNA replication.

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Stromatolites are the sedimentary record of that really prolonged period of geological time. Some of the oldest known, bona fide cryptalgal structures are found in the 3.4 Ga North Pole deposits. They represent fossil slime – mats of photosynthetic, prokaryotic cyanobacteria. They were responsible for producing the oxygen we, and most other life forms breath.

Stromatolites really came into their own by about 2.5Ga, forming extensive buildups, and reef-like structures, by slow, incremental addition, mat-by-mat, in the ancient shallow seas. Growth habits varied from broad flat domes to intricately branched columns. Stromatolite structure, shape and distribution were primarily controlled by environmental conditions such as water depth, wave and current energy, and substrate (muddy, sandy). Glacially polished rock outcrops on Belcher Islands (where all the following images are from) show these structures in exquisite detail.

Stromatolites in outcrop commonly appear huge, as columns or domes extending vertically several metres. But their sea floor profiles, or synoptic relief during growth was low. We can visualize this when tracing individual laminae or sets of laminae (ie. the original mat surface) from one column to the next. Your average shallow shelf or platform stromatolite extended no more than a few millimeters or centimeters above the sea floor. Some large mounds, or reef-like structures had a few metres relief; but nothing like more recent coral reefs. This also means that the environmental conditions for incremental growth must have been stable for long periods of time. This needs to be kept in mind when looking at cryptalgal structures in outcrop; their apparent size can be misleading.

Check out this post for outcrop descriptions of stromatolite morphological features

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A few publications that have a bearing on the set of images below:

Ricketts, B.D. 1983: The evolution of a Middle Precambrian dolostone sequence – a spectrum of dolomitization regimes; Journal of Sedimentary Petrology, v. 53, p. 565-596.

Ricketts, B.D. and Donaldson, J.A. 1981: Sedimentary history of the Belcher Group of Hudson Bay; Geological Survey of Canada, Paper 81-10, p. 235-254. F.A.H. Campbell, Editor

Ricketts, B.D. and Donaldson, J.A. 1989: Stromatolite reef development on a mud-dominated platform in the Middle Precambrian Belcher Group of Hudson Bay; Canadian Society of Petroleum Geologists, Memoir 13, p. 113-119.

Donaldson, J.A. and Ricketts, B.D. 1979: Beachrock in Proterozoic dolostone of the Belcher Islands, Northwest Territories; Journal of Sedimentary Petrology, v. 49, p. 1287-1294.

The images:

Bulbous, dolomitized stromatolites in the lower part of the outcrop become progressively more branched towards the top. The view is oblique to bedding; the surface polished by Laurentide glacial ice. McLeary Fm. Right: dashes follow the synoptic surface, which approximates the actual growing mat morphology and relief at the sea floor. Whereas the stromatolites in outcrop appear large, at the time of growth (2 billion years ago) the sea floor would have looked vaguely dimpled or domed. Bedding-parallel stylolites have thinned the rock sequence by 10-20%.

The cartoon refers to the synoptic surface outlined in the image above. Even though columns and dome appear in outcrop to be quite large (10s of cm to metres), their actual growth profiles at the sediment-water interface was measured in only millimetres to centimetres.

Bulbous stromatolites similar to those shown above. The original carbonate (calcite-high Mg calcite-aragonite) has been completely replaced by dolomite. Some of the upstanding, resistant edges are subsequent chert replacement. Image on the right shows excellent preservation of original laminae that in some cases can be traced across 2 and 3 branches. Both are oblique to bedding. McLeary Fm. Intercolumn sediment is dolomitized carbonate mud.

Stromatolite form here changes from bulbous to more digitate branching, back to bulbous. McLeary Fm.

Large, laterally extensive stromatolite domes more than 8m thick, but having synoptic relief of only a few centimetres. There were very few interruptions in growth; they probably accumulated on a subtidal shelf-platform lacking strong bottom currents. Kasegalik Fm.

Large, closely-spaced, low relief stromatolite domes; synoptic relief was 5-8cm. Look closely at the laminae and in some you will see continuity from one dome to another, and in others discontinuities and overlaps 2-4 laminae thick. Mavor Fm.

Large stromatolite domes like those above, can transform to more digitate columns higher in the bedding unit. This probably represents subtle changes in environment, such as local bottom currents, or growth that was interrupted by storms. Mavor Fm.

Exhumed stromatolite domes on bedding, McLeary Fm. Their internal structure is similar to the domes shown above. The domes are slightly elongated, with long axis parallel to subtidal paleocurrents (determined from other sedimentary structures). Inter-dome sediment is dolomitized carbonate mud. Hammer, centre-right.

Bedding and cross-section views of subtidal platform, domal stromatolites. Synoptic relief here is a bit less than in the image above. McLeary Fm. Stromatolites in the uppermost bed are eroded, overturned, or oversteepened, probably by storm waves.

This distinctive stromatolite unit can be traced 10s of kilometres. Closely spaced vertical, digitate columns grew on a shallow subtidal platform. Columns are relatively uniform width, usually branched, with tangential laminae forming a sturdy wall. Synoptic relief was only a few millimetres. McLeary Fm.

Polished slabs of the digitate stromatolites shown above. The scale on the right is centimetres. Preserved laminae are mm to sub-mm thick. The rock has been completely dolmitized, and yet delicate structure is preserved. McLeary Fm.

Isometric reconstruction of slabbed digitate stromatolites (based on several polished slabs like the one above). McLeary Fm. The (barely visible) scale is in centimetres.

Closely packed columnar stromatolites – bedding view. Raised rims on each column is due to silicification. McLeary Fm.

Several growth stages from domal stromatolites to narrower, closely-spaced, digitate columns, Mavor Fm. Intercolumn sediment is dolomitized mud. Three stylolites (top, centre, bottom) have reduced section thickness by 15-20%. Although completely dolomitized, mm and sub-mm scale laminae are well preserved.

Disruption of stromatolite columns and small domes by erosion. Rip-ups include largish mudstone slabs. McLeary Fm.

Digitate stromatolite columns in cross-section (left) and bedding (right). Dolomitiztion here has produced coarse crystalline textures that have partly obliterated outlines and laminae. Mavor Fm.

Domal, digitate, and coalescing stromatolite columns, growth habits that changed with environmental conditions or interruptions in growth (e.g. storms), McLeary Fm. Image on right has an erosional discordance at the pen tip. branching began during mat regrowth.

Radiating, digitate, branching columns. Left: the radiating cluster is a solitary buildup in surrounding flat, laminated mats. Right: The digitate cluster has been disrupted and partly eroded by crossbedded sandstone, indicating a significant change in local environmental conditions (shallow subtidal to intertidal).

Domal masses with silicified, subsidiary columns growing from the margins. An erosional discordance (just below the coin) terminated growth. Kasegalik Fm.

Wavy mats give way to columnar stromatolites with cone-shaped laminae. This form has historically been called Conophyton. McLeary fm.

Irregularly branched columns with significant silica replacement. The white crystals are coarse, late diagenetic dolomite

Ornately branched stromatolite, a possible example of what historically was called Tungussia. Mavor Fm.

Left: Dolomite pseudomorphs of gypsum in dolarenite. Right: Fine-grained dolarenites interbedded with carbonate mudstone (dolomite) and simple, laminated crpytalgal mats (partly silicified). Gypsum psuedomorphs (spots) are scattered throughout. A layer of algal mat and mud rip-ups is present at the lens cap. McLeary Fm.

Teepee structures in carbonate mudstone and laminated cryptalgal mats; disruption of the mudstone slabs was probably caused by salt-gypsum expansion. McLeary Fm.

Beachrock is common in the McLeary Fm. Here, a block of dislodged beachrock (preferentially cemented dolomitic sandstone) has been overturned, as evidenced by the small, upside-down stromatolite columns.

Molar tooth structures in dolomitic mud. Their origin has been described variously as shrinkage cracks caused by changes in salinity, CO2 gas expansion (from decaying mats?), wave loading, clathrates, and seismically-induced changes in pore pressures. They are not worm burrows!

Subtidal to outer platform stromatolite mounds that have undergone more intense recrystallization during dolomite replacement of the original carbonate, such that original column-bulb outlines are partly obscured. Remnants of small columns are visible in the upper dome layers (right). There is a hint of coloumn or mat detachment, and possibly pisoliths in the centre. The vugs are secondary diagenetic features from dissolution of (?) sparry calcite and dolomite replacement. Tukarak Fm (immediately overlies the McLeary Fm).

Recrystallized, dolomitic mounds where the original carbonate has been replaced by one or two generations of dolomite spar. The void is lined with late diagenetic dolomite spar, and even later calcite (white crystals). Tukarak Fm.

Microdigitate mats, here associated with grainstone. Left: mats above the dark cherty layer show at least three stages of growth, each following an episode of erosion. Mats below the chert are more simple wavy forms. The grainstone above contains numerous mud and mat rip-ups. Right: Slightly larger, but no less delicate microdigitate mats and columns, again showing evidence of erosion and regrowth. Both examples formed in intertidal to supratidal flats. McLeary Fm.

A coarse grainstone (completely dolomitized) containing abundant mat rip-ups, pisoliths, and a single continuous mat that has regrown over pisoliths. Subtidal to supratidal flat, McLeary Fm.

Wavy and crinkley mats, and faintly preserved microdigitate columns, show the changes in growth habit possible over a scale of millimetres to centimetres. Scale top (bottom left) is 20 mm wide. McLeary Fm.

Left: Small dome, beginning with flat laminae at the base, and successions of microdigitate columns above. Right: Small domes capped by microdigitate columns. Laminated mudstone above are discordant and eroded. The white, silicified masses were probably larger domal structures. McLeary Fm.

Partly silicified microdigitate mats overlying a pavement of edgewise lutite slabs, or beach rosettes. Grainstone above contains mat rip-ups and pisoliths. McLeary Fm.

Dolomitized carbonate mudstone and thin mats, totally disrupted, ripped up, and folded by storm surges into a supratidal flat. McLeary Fm.

Successive microdigitate columns and laminated dololutite-mat interbeds. The resistant ridges are silicified, cherty mats. McLeary Fm.

Both images show wavy mats and microdigitate columns, disrupted by supratidal desiccation, storm-loading pull-aparts, and fragmentation. The interval in the left image is capped by larger domal masses that in turn have been locally overturned. McLeary Fm.

Bulbous to domal masses, partly disrupted and overturned, have stabilized an edgewise conglomerate (beach rosette) pavement. Slabs in the pavement are thin, probably partly lithified-cemented lutite, ripped up during earlier storm events. McLeary Fm.

One of the more spectacular stromatolite buildups, or reefs, in the Proterozoic Mavor Formation, Belcher Group. The aerial view shows a transition from shallow subtidal, flat laminates to simple mounds, to large domes with 3-5m synoptic relief at the platform margin – slope deposits (Costello Fm) extend from the margin on the right. Smaller mounds on the left coalesce into larger mounds. Field of view along mound length is about 800m. Stratigraphic thickness is about 150m along this section of Tukarak Island.

Slightly oblique view across several large mounds and intervening troughs. The relief here is close to synoptic relief. Keep in mind the entire structure was made up of cryptalgal laminates. There were interruptions in growth, at the scale of individual mounds, evidenced by numerous discontinuity surfaces. There is little evidence for wholesale erosion, and the conclusion is that the larger mound structures accumulated below storm wave-base. Mavor Fm.

Left: View approximately along strike. Second-order mounds are nicely exposed here (by the hammer). Right: View is slightly oblique to depositional dip. Here too we can see smaller mounds superposed on the larger structure. Mavor Fm.

View down dip across smaller mounds that are superposed on the larger structures. Mavor Fm.

Cross-section through the smaller mounds (hammer right centre) showing the distinctive geometry and regularity of the laminae. There are numerous stylolites (thin dark bands) that tend to mimic the mound outline. Synoptic relief is 20-40 cm. Mavor Fm.

Detail of the wavy and crinkley cryptalgal laminae, through a (2nd order) mound crest (left) and trough (right). The synoptic relief on any lamination is rarely more than a centimetre.

Flat to wavy cryptalgal laminae in a 2nd order mound, with prominent stylolites. At least 5 seams here account for about 20% loss of stratigraphic thickness. Below the upper stylolite seam is a thin layer of mat rip-ups, evidence for briefly interrupted growth. Mavor Fm.

Reconstruction of the progressive changes in mound amplitude and spacing, from shallow subtidal platform at the base (corresponds with the left side of the aerial image above), through coalescing mounds at the platform margin, to the slope deposits beyond (Costello Fm). For completeness, an example of the slope rocks is shown below.

Regular bedded (that can be traced laterally for 100s to 1000s of metres) calci-dololutite and red marls in slope deposits, outboard of the Mavor Formation platform-wide buildups-reefs. There are a few slumps and the occasional small channel filled with eroded lutite and shale. There are a few thin, graded beds, likely deposited as calci-turbidites.

In the field: from one extreme to another

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Have you ever looked at some locale on a map or photograph and thought “that looks like an intriguing place to work”, only to find, sometime later that you are smack in the middle of that same spot? Time-warp? Some god’s lap?

I was preparing to travel to Canada. The plan was to do a PhD, and because I had not long completed a Masters thesis on geologically very young sedimentary deposits, had entertained the idea that research on very old rocks would add a kind of symmetry to my geological outlook – from one end of the geological time-scale to the other. In preparation, I borrowed The Geology of Canada, a weighty tome, and homed in on the Precambrian system (basically everything older than 540 million years). What caught my attention were some squiggly-shaped islands about 150km off the southeast coast of Hudson Bay; the Belcher Islands. Their shape belied some interesting geological structures, and the strata a mix of sedimentary and volcanic rocks about 2 billion years old. What a neat place to work, although I envisioned the islands to be treed.

I arrived in Ottawa (early January, 1976) to minus 25^oC and snow; I had never seen so much snow. I thought it quite beautiful, which elicited wry comments from the Ottawans I was meeting who were sick of shovelling driveways and digging vehicles out of snow drifts. Destination – Carleton University. My supervisor was to be Alan Donaldson, well-known in Precambrian geology circles. Following the introductions, he announced that my project, unless I had some objection, was to focus on the Belcher Islands. LOL. I was to spend 5 months there in total over the summers of 1976-77. The social environment, the weather, and the geology were remarkable.

Getting to the islands was a milk run: a drive to Montreal airport, a flight to Moosonee near the southern shores of James Bay (northern Ontario), a very noisy DC3 leg to Umiujaq (Quebec) where we picked up field equipment (kindly loaned to us by the Geological Survey of Canada), then Twin Otter across the 150 km to Sanikiluaq, the sole village on Belcher Islands. We were able to stay in a small house owned by the Hudson’s Bay Company for the two days needed to sort gear, buy food, and make sure the two inflatable Zodiacs and outboards worked. My assistants (John McEwan in 1976, Mike Ware in 1877) and I always used two boats as a safety measure (and for visitors).

The seas around the islands are mostly ice-free during the summer months, but the water is still only a few degrees above freezing, and the air close to the water cold. Even in the summer, we had to bundle up with wet-gear, fleeces, and life jackets (I was told the life jackets were necessary for insurance purposes – so they could retrieve the bodies). The islands and intervening channels are also elongated north, so that wave set-up could change drastically any time there was a wind shift. We were caught out a few times with unfavourable seas, but there was always somewhere to shelter.

Belcher Islands are mostly held together by a thick volcanic unit that creates more or less linear coastlines. The strata were folded, like a series of waves, into simple anticlines and synclines, such that the package of sedimentary rocks is exposed in the anticlines, while the synclines are drowned by major channels and inlets. The terrain is subdued with low relief – the islands were scraped clean by the Laurentide Ice Sheet during the Last Glaciation.

Our base camp was to be in a small, relatively sheltered inlet along the western shore of Tukurak Island (one of the largest and easternmost island). It was a 3-4 hour journey, depending on weather. This is the site of an abandoned Hudson’s Bay post. It was also the favoured summer holiday spot for local Inuit, primarily because it is close to their source of soapstone. Belcher Island soapstone has an enviable reputation amongst northern communities, because of its uniform, deep green colour, and general lack of fractures that would render carving difficult. Whenever we were in base camp, we would watch the elders carving, and teaching their younger folk the same skills. They would also bring us bannock and Arctic Char. And there was never a shortage of Inuit kids around, checking in, telling stories, or simply hanging out. We would spend 4-5 days away from base camp, returning to stock up and cache samples. Time in base camp was always a delight.

Belcher Islands sit well below the Arctic Circle at 56^oN (latitude), and yet the landscape is typically Arctic. The northern Canada tree-line is located south of Hudson Bay, such that the Islands have a typical Arctic flora (especially wild flowers), and no trees – so much for my earlier, wistful image of the place.

The weather alternated between gorgeous, with light winds and clear skies, and abysmal. On more than one occasion we returned to camp from a day’s work to find tents down and sleeping bags soaked. High winds also prevented longer excursions with the boats, unless we were riding through sheltered channels and inlets. With the boats, there was always one eye kept on the weather.

During the first couple of weeks in 1976, Bill Morris (Geological Survey of Canada) had joined us to sample rocks for geophysical measurements (looking for ancient magnetic poles). The day he was to leave base camp (and fly out of Sanikiluaq) was particularly inclement. He insisted on attempting the trip, but instead of using our inflatable boats, I decided to rent one of the larger, sturdier, Inuit canoes with twin outboard motors (I was the only one with boating experience). We ventured out of the sheltered inlet, into the maelstrom – at least that’s what it looked like from the perspective of our small craft. I doubt we got any further than 50m from the inlet entrance; a lull in the waves, a quick decision to about-face, a beeline back to calmer waters, and the colour returned to the faces of my two passengers.

“Guess I’m going to miss my flight”. We all new he probably would have missed it, even if we had continued. Back to base camp to drain what was left of a bottle of scotch, and cogitate on an earlier field season on a warm New Zealand Pacific coast.

This is the first blog on my Belcher Islands episode

The Ancient Earth 7. The Art of the Stromatolite

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Algae, Fossil Slime and Organic Precambrian Art

Stromatolites are the earliest physical life forms on earth; they were the precursors to pretty well everything you see living today. There may be indications of earlier life forms preserved as chemical signatures, but as fossils go, something you can see and touch, stromatolites are it. The oldest stromatolites known are from Western Australia – about 3400 million years old. These ancient structures were built by primitive algae and bacteria, aka cyanobacteria, sometimes referred to as blue-green algae. Clearly life had already evolved to something quite complex by 3400 million years ago.

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The Ancient Earth 6. Life and all that… Part 2

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The Origin of Life? – Science persists in asking…

Galileo Before the Holy Office Joseph Nicolas Robert-Fleury

When our Renaissance heroes Galileo, Copernicus, and Tycho Brahe cogitated the celestial sphere and removed humanity from the centre of the physical universe, they must have developed cricks in their necks – regularly looking over their shoulders for the ever-watchful ecclesiastical authorities who brutally insisted on maintaining the status quo. Six hundred years later we still want to understand the unknown and persist in our curiosity, although the consequences of our enquiry are not usually as dire as those faced by Copernicus. We continue to ask questions and one of these, for which there are still few answers, is ‘how did life begin?’. Embedded in a question like this are metaphysical quandaries that consider consciousness and our humanity; one wonders whether such attributes will ever enter the realm of the empirical. However, science can attempt to deal with explicitly empirical parts of the question, such as ‘how might organic molecules, that are critical to functioning cells, have formed on the ancient earth?’.

The first article on this thorny topic dealt with an iconic set of chemical experiments conducted in the early 1950s. This post takes these experiments incrementally further.

In 1953 Stanley Miller, then a young postgraduate student under the tutelage of Harold Urey, conducted experiments that produced several organic compounds, including amino acids. The chemical reactions were all abiotic. They demonstrated that some of the important chemical ingredients of living cells could be produced without the interaction of life-forms. This was a huge step forward in scientists’ understanding of how life arose on earth.

However…

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The Ancient Earth 5. Life and all that… Where, How, When? Part 1

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The Origin of Life?

This is a vexing question; perhaps the ultimate puzzle! It invokes wonder and intrigue for many, but for others it’s a question that invites derision and disbelief. Of course, we may never know the complete answer, or answers to the question “How Did Life Come About?“, but there is also no reason why science shouldn’t persist in asking it, in looking for the geological evidence, either here or in some other solar system, or devising experiments and models to help explain it. For all we know, it could be as simple an answer as it was to Richard Adams’ ultimate question in Restaurant At The End Of The Universe. The next two posts look at some scientific experiments that help us imagine how it might all have begun.

The oldest fossils

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The Ancient Earth: 4. Water – Oceans of the stuff

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The Ancient Earth 4. Where did all that water come from?

The oceans cover 71% of our planet, and account for 97% of its total surface water. The greatest ocean depth is 10,994m in the Mariana Trench (near the Island of Guam), and the shallowest …, well most of you have been to the beach. They harbour a massive biomass (micro and macro) that comprises beautiful and complex ecosystems; they help feed our burgeoning population. They provide opportunities for explorers and metaphors for poets. But where did all that water come from? Continue reading →

Ancient earth. 3 The air we breath; how our atmosphere evolved

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The really ancient earth: How our atmosphere evolved

Take a deep breath. Savour it. One of the few absolutes of our physical world (that we probably haven’t looked after as well as we might have). This post continues the theme “The Really Ancient Earth” by looking at what we know about the origin of our atmosphere; some of the evidence and some of the hypotheses. What was it like on day 1 (about 4600 million years ago) and how did it evolved into our breath-taking world today? Continue reading →

Ancient earth. 1 A time-line for the first 4 billion years

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A time-line for the first 4 billion years of Earth history

The Cambrian, that relatively brief period in geological history (40 odd million years) was witness to one of the most amazing series of biological events in the entire history of the Earth; the rapid, almost explosive appearance of marine critters with preservable shells and skeletons – a real first. Trilobites are probably the best known fossils from that period, but there are also some pretty weird and wonderful looking soft-bodied creatures (one famous fossil locality is the Burgess Shale near the town of Field, British Columbia). Most animal life today can track its origin to those early life forms. These events began about 540 million years ago (how easy these numbers roll off the tongue, or pen). But we also know that our Earth is pretty close to 4600 million years old (4.6 billion – How old is Earth); in other words there is almost 4 billion years, a humongous period of time in which, seemingly, not much happened. 4000 million years worth of boredom! This period is know as the “Pre” Cambrian, or Precambrian. Most Precambrian events did take place pretty slowly, but these events also determined the kind of world we now live in: the air we breath, the oceans and rivers, the biosphere and indeed life itself, all originated and evolved over this, the deepest of geological time. Continue reading →

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