62.15: Stromatolite diversity in the Belcher Islands

Figure \(\PageIndex{1}\): South-plunging anticlines along the east side of Belcher Islands, Hudson Bay. The islands are skinny, separated by narrow, frequently stormy inlets. There are no trees. View approximately to the south.

Field work in Hudson Bay (Canada)

A walk through some of the most ancient life forms on Earth

I like the expression “rock record.” For me, these two words encapsulate Earth’s readable history, page by page. Pick up a rock, turn the page, delve into its mysteries, solve the puzzles. There is so much to discover – such a journey.

The journey here will walk you through the world of our most ancient organisms, not the oldest, but pretty old at 2 billion years. This was a world where life was in full swing, where the biomass was huge but occupied solely by bacteria and algae, or more specifically cyanobacteria (sometimes called blue-green algae). These organisms thrived in vast carpet-like communities, now preserved in the rock record as delicately laminated mats (or cryptalgal laminates), molded into simple domes or complexly branched columns (stromatolites), and even vast, reef-like build-ups. The environments in which they grew ranged from geothermal pools to the shallow confines of freshwater lakes, and marine settings that extended from the most landward extent of ocean tides (supratidal) to deeper shelf and platform.

Stromatolites are known from sedimentary rocks as old as 3.4 billion years (North Pole deposits, Western Australia – this 1980 paper was one of the first to fully describe these). They dominated Earth’s biosphere for the next 3 billion years; clearly, life was taking its time to get over that first rush of DNA replication. They relinquished this foremost position during the Cambrian invertebrate explosion because of competition for their niche and from predation – lots of critters like to eat algae. Today, cryptalgal laminates eke out a living in a few specialized environments, Shark Bay on the Indian Ocean coast of Western Australia being one of the better-known examples.

We should be immensely grateful for this Precambrian flourish. Cyanobacteria are photosynthetic prokaryotes, and the general consensus is that they were responsible for the production of nearly all our atmospheric oxygen. Most (but not all) modern cyanobacteria thrive in sunlight which means growth is limited to the photic zone (the part of the water column that sunlight penetrates, at depths less than 200 m). This condition appears to be borne out in the rock record where fossil cryptalgal structures are commonly associated with other sedimentary features also developed in shallow water under the influence of tidal currents, the effects of waves, and storms. We will see some of these sedimentary facies on this trip.

This field excursion will take you through a Paleoproterozoic succession in Belcher Islands, a small group of islands and islets in eastern Hudson Bay. Exposure here is superb – look again at the image at the top of the page. But the weather can be atrocious. I spent 20 weeks in total in 1976-77. For a Kiwi having just arrived in Canada, meeting the local Inuit, and absorbing the scenery and geology were life-affirming experiences.

Figure \(\PageIndex{2}\): A relatively sheltered cove on Tukarak Island, the site of an abandoned Hudson’s Bay Post, was our base camp for the 1976-77 field seasons. We shared this locale with Inuit who spend summers here. The Inuit lodgings are the large. roomy white tents, that have oil heaters and stoves. The two orange blobs behind were our less-salubrious digs.

Figure \(\PageIndex{3}\): Summer is the only time to do field work. We used two zodiacs to move fly camps around the numerous islands. They are open to the elements, and the weather was not always kind. Hence the two boats – if one sank, the other could return to tell the tale. Here, we are negotiating remnants of sea ice en route to our first destination.

We begin this field excursion at the high tide mark, making our way across the intertidal zone, through the shallow subtidal zone where waves break, and thence to deeper offshore waters beyond the immediate influence of anything other than storm waves. You can check your ‘paleo-location’ at each step using the diagram below.

At each location we will compare modern examples with the ancient rock record. We make these comparisons because they are an essential part of how we interpret ancient environments. This way of thinking (sometimes stated as ‘The Present is the Key to the Past’) helps us in two ways: first, it provides us with a warrant to reason inductively, to interpret or to predict; and second, it allows us to use the actual processes and products we observe in modern environments to help us solve the puzzles of the past.

Supratidal zone

We begin our geological transect on a broad tidal flat. Its landward extent may include a zone accessed only by spring tides and storm surges – the supratidal zone. In arid regions evaporite minerals like gypsum and halite may accumulate (the best modern analogues for these are sabkhas along the Persian Gulf). Any cryptalgal mats that grow will be subjected to frequent desiccation; they are susceptible to reworking by strong winds and storm tides where they are mixed with other sedimentary fragments ripped from the tidal flat.

In the next two images, compare the ancient example with some modern supratidal microbial-cryptalgal mats.

Figure \(\PageIndex{4}\): Compare the profile view of structures in this image with the modern example from Texas. The laminated carbonate mudstones and thin cryptalgal mats accumulated on a two billion year-old supratidal to intertidal flat. The muds periodically dried, resulting in the upturned edges of desiccation polygons. Subsequent erosion by the occasional storm produced mud-pebble conglomerates, or rip-ups.

Figure \(\PageIndex{5}\): These algal mats are forming today in a tidal marsh south of Galveston, Texas. They are only covered during spring, or king tides. Here they have partly dried. The desiccated edges tend to curl upwards. The dried fragments are susceptible to reworking by the next tide. Compare this bedding view with fossil examples opposite.

Intertidal zone

Working seawards we pass through the intertidal zone, a region washed twice daily by tides. Desiccation of mats is less intense here, but they are more susceptible to erosion by waves, particularly storm waves. The example here shows many erosional discordances where mats were regularly disrupted and regrown.

Again, we can compare modern analogues with the 2 billion year-old examples. The modern structures below are from an Abu Dhabi sabkha.

Figure \(\PageIndex{6}\): Detail of pustular algal (microbial) mats on an Abu Dhabi tidal flat. Sediment between the pustules is mainly carbonate mud. Coin (circled) for scale, gives a sense of the synoptic, or growth relief of each microbial structure. Compare the scale of these structures with the images of fossil mats. (Photo by Stephen Lokier, University of Bangor, Wales.)

Figure \(\PageIndex{7}\): This photo of a modern tidal flat in Abu Dhabi shows how extensive these environments can be. The tidal flats are covered by algal, or microbial mats, all showing signs of desiccation. They are covered more frequently by shallow tides. Evaporation here is intense. (Photo by Stephen Lokier, University of Bangor, Wales.)

Figure \(\PageIndex{8}\): Laminated intertidal mats overlain by some very nice but small, finger-like (digitate) columns. The digitate mats were subsequently eroded, possibly during a storm surge and covered by coarse sand. Coin is 20 mm diameters.

In the images that follow, fan-shaped clusters mid-image are edgewise conglomerates that consist of semilithified slabs of carbonate mud and cryptalgal laminates, ripped up and stacked one against the other (note this is a profile view). This kind of structure commonly forms on beaches – the modern example (right image) consists of shale slabs that when stacked, present a crude radial pattern (this is a bedding view).

Figure \(\PageIndex{9}\): These intertidal cryptalgal (microbial) laminates show many stages of growth, disruption of mats by erosion during storms or desiccation, and regrowth. Zoom in on different parts of the image to see the detail. Chert replacement of carbonate in some mats causes them to be more resistant to erosion at the outcrop. Oncoids represent mats that grow as they are rolled around the sea floor – a bit like oversized ooids. A modern example of edgewise conglomerate accompanies this image.

Figure \(\PageIndex{10}\): A modern edgewise conglomerate pavement (sometimes called stone rosettes) formed by wave action on a beach. The slabs are stacked end-on in crude radial patterns. In the ancient example shown above, slab composition is different (cemented carbonate mud), but the stacking arrangement is identical.

Ripples are extremely common on sandy tidal flats. In the rock record, layers containing ripple crossbeds may alternate with layers containing mats. The examples below are interference ripples where two sets of ripples almost at right angles, appear to interfere. These structures are excellent indicators of ancient tides where one ripple set formed on the incoming (flood) tide, the other on the outgoing (ebb) tide. If we can determine the orientation of the ancient shoreline, then we can hazard a good guess as to which set records the tidal flux.

Figure \(\PageIndex{11}\): Compare these two images of interference ripples that formed during alternating flood and ebb tidal currents across a tidal flat.

Shallow subtidal

The subtidal zone below low tide is always submerged. This is the region of constant water movement; it includes the surf zone. Here, cryptalgal buildups tend to be larger and more complex; they may form as simple domes or complex branched columns. In outcrop they can be spectacular, and quite beautiful. Finding modern analogues for this kind of stromatolite is more difficult – there are very few locations where they form today. One notable exception is Shark Bay in Western Australia, an iconic location where modern, actively growing domes bear an uncanny resemblance to ancient structures, as illustrated below.

Even more spectacular are stromatolites that developed branched columns of many shapes and sizes. An excellent example is shown below.

Figure \(\PageIndex{12}\): Bedding exposure of simple stromatolite domes, remarkably similar in outward appearance to the modern structures at Shark Bay (W. Australia). One could imagine stepping gingerly between the domes on the 2 billion year-old beach. The domes stand out because rock between each dome has been removed by erosion. Hammer for scale (circled).

Figure \(\PageIndex{13}\): The modern microbial domes growing in Shark Bay, Western Australia are iconic analogues for ancient stromatolites. Comparison with the ancient domes shown above is uncanny. Sediment between the domes is fine-grained carbonate, plus fragments of dome broken off during storms. Penknife for scale bottom centre. (Image credit: Alicejmichel 8 June 2007, https://commons.wikimedia.org/wiki/F...omatolites.jpg)

Figure \(\PageIndex{14}\): Coarse-grained, crossbedded sandstone that was probably deposited on subtidal sand bars, in areas where shifting sand prevented growth of microbial mats and domes.

Figure \(\PageIndex{15}\): This example (one of my favourites) show some broad simple domes in the lower half of the image, changing to branched, digitate (finger-like) columns – the transition occurs just below the hammer. Each millimetre-thick curved line represents a former cryptalgal mat, now converted to dolomite. Zoom in and see how far you can trace a single lamination from one column or dome to another. This will give you some insight into the relief of the growing mat on the seafloor. These structures formed in shallow, agitated, subtidal water.

Figure \(\PageIndex{16}\): Fine sediment adheres to sticky algal filaments, or the microbes themselves promote precipitation of calcite or aragonite crystals

Each of the skinny layers, commonly less than a mm thick, represents a once-thriving mat of cyanobacteria, growing on the seafloor. The laminates consist of very fine carbonate mud that adhered to the algal filaments; the filaments may also have promoted precipitation of fine carbonate minerals, such as aragonite. Most of the original organic matter was removed by chemical processes as the sediments were buried.

Figure \(\PageIndex{17}\): This 3D reconstruction of branching stromatolites was made from a large block, collected in the field. The block was slabbed (with a diamond saw), each slab was polished, the fine details traced onto paper, and the 3D representation of branches and laminae drawn as an isometric image. (Note, this was done before the advent of computer graphics).

The stromatolites shown above consist of columns that are about 50 cm high – in fact the columns in some stromatolites can reach several metres. However, the column height is a bit deceiving. Each lamination represents a period of growth on the seafloor (days, months). If you trace a lamination, or perhaps a set of laminae from one column to its nearest neighbours, you will see how much relief the stromatolite had during growth. We call this synoptic relief. In many stromatolites synoptic relief was only a few mm or cm. In other words, if you were walking across the surface you would only see low mounds and ridges rather than tall columns. Large columns and domes therefore represent relatively continuous growth over long periods (perhaps hundreds of years) where columns gradually build, keeping pace with sediment accumulation on the seafloor. Sometimes growth was interrupted by storms or long periods of desiccation during exposure; when growth re-established, a new branch or dome would develop over the earlier-formed structures – hence the branched stromatolite columns we now see in cross-section views in outcrop.

Figure \(\PageIndex{18}\): A nicely polished outcrop containing cross-sections of bulbous stromatolites. We can get a sense of how much these stromatolites were exposed at the sea floor by tracing individual laminae from one column to the next, as in the cartoon below. The traced line represents the sea floor at a point in time when the mats were growing.

Deeper water platform, but within the photic zone

Our transect takes us to deeper water where normal or fairweather waves no longer interact with the seafloor. Here, the cryptalgal laminates grow in quieter waters within the photic zone (perhaps a few tens of metres deep), washed by tidal currents. There is much less disruption of growth by storms (and no desiccation), such that domes extend much farther across the sea floor than in shallower waters. The surrounding sediments are also finer grained, perhaps with ripples migrating across the seabed. However, the synoptic relief is greater; in the example shown, the domes are 2 m to 4 m wide with synoptic relief of 30-60 cm. Branching is less common because there were fewer disruptions to mat growth.

Figure \(\PageIndex{19}\): A cross-section view of cryptalgal domes, some up to 4m wide, interpreted to have formed on a broad carbonate platform at depths below the influence of normal, fair-weather waves. Most are simple structures lacking the complex branching more common in shallower, agitated waters. The dashed black line in front of this keen geologist traces laminae across two domes. indicating synoptic relief up to 60 cm. The carbonate rocks here are 100% dolomite. Bedding dips to the right.

The field excursion ends beneath the waves offshore, figuratively in our interpretations of the two billion year-old rock record, and practically as we made our way across the modern sea floor. Surfacing will allow us to breathe, and reflect on what we have observed.

Unless indicated otherwise, all the images used in this chapter are from geological-digressions.com