Tag Archives: Belcher Islands

Mineralogy of carbonates: Stromatolite reefs

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Large, elongate stromatolite mounds - a Precambrian reef complex

Large, elongate stromatolite mounds – a Precambrian reef complex

An example of a Paleoproterozoic stromatolite reef

This is part of the How To…series  on carbonate rocks

Reefs are the largest biological constructions on Earth. They are ecological powerhouses, domicile to a multitude of invertebrates and vertebrates, algae and phytoplankton, all playing their part in acts of communion, competition, and symbiosis.

Reefs are also geological constructions:

  • Modern coral reefs, and most reefs since the beginning of the Cambrian (541 million years ago) have rigid frameworks of corals, stromatoporids or rudists.
  • Reefs fringe islands, platforms and continents.
  • They range in size from small, isolated pinnacles and oceanic atolls, to extraordinarily large barrier systems; Great Barrier Reef extends about 2300km along the eastern coast of Australia.
  • Reefs act as baffles to ocean waves. Waves support reef life by constantly refreshing oxygen and nutrients, but they can interrupt progress during damaging storms.
  • Reefs have a partnership with lagoons.

These are the reefs we are most familiar with. They even evoke romantic notions, the splendours of nature; diving amongst brilliantly coloured corals and fish, home to apex predators, ships dashed upon… etc.

Earth’s earliest reefs looked quite different. For almost 3 billion years, Precambrian reefs consisted of stromatolites, constructed by micro-organisms; primarily cyanobacteria.  They lacked rigid frameworks – most were susceptible to erosion, although sea floor cementation might have rendered them crusty. There were no grazing invertebrates – the ecological web must have been much simpler. Despite these fundamental differences, Precambrian reefs occupied and responded to similar oceanographic conditions as their Phanerozoic cousins:

  • Cyanobacteria are photosynthetic prokaryotes which means the buildups accumulated within the photic zone, responding to changes in filtered light and seawater temperature.
  • Stromatolite reefs defined platform margins, sometimes extending across entire platforms and shelves.
  • Lagoons, tidal flats and sabkhas were partnered with the offshore buildups.
  • Reefs that developed on platforms or shelves were usually associated with deeper water slopes that deposited hemipelagic material plus any coarse-grained shallow water sediment that bypassed the reefs. Slope facies may include mass transport deposits (slumps and slides, turbidites and debris flows).
  • Stromatolite reefs and associated platform facies responded to fluctuating sea levels.
  • Extensive buildups acted as baffles to waves and currents, and in turn were moulded by these forces.
  • They were occasionally battered by storms.
  • They were long-lived constructions. Large reefs probably represent hundreds of thousands and even millions of years accumulation.

 

Stromatolite reefs of the Mavor Formation

Landsat image of Belcher Islands, Hudson Bay.

LANDSAT’s view of Belcher Islands (north at top). The aerial view (above image) is from Tukarak Island. Image credit: NASA, Jesse Allen, University of Maryland.

The Paleoproterozoic succession on Belcher Islands (Hudson Bay, Canada) contains carbonate platforms replete with stromatolites and cryptalgal laminates. One stratigraphic unit, the Mavor Formation, contains spectacular mounded buildups. One of the best exposures is shown below; here one can walk through a continuous succession, from beach to platform margin. The margin itself is delineated by an abrupt change in sedimentology, from large, high synoptic-relief stromatolitic buildups, to thin bedded, carbonate-rich rhythmites and shale deposited on the seaward slope.  Slump packages and turbidites do occur but are not common. A few small, isolated stromatolites domes grew on the upper slope (i.e. outboard of the platform margin) – this probably reflects the maximum limit of the photic zone. The platform margin is a mappable boundary; slope deposits belong to the Costello Formation. The stratigraphic thickness of the reef package in the aerial view is 244m.

One of the better aerial views of the stromatolite reefs and platform margin (dashed line), east Tukarak Island. The horizontal distance across the buildups (left to right) is about 800m; beds dip right (east) 10o-15o. Buildup stratigraphy is 244m thick. Small subtidal mounds (left) coalesce towards the outer margin into larger buildups. Locations (a) through (d) refer to the 3D reconstructions shown below.

One of the better aerial views of the stromatolite reefs and platform margin (dashed line), east Tukarak Island. The horizontal distance across the buildups (left to right) is about 800m; beds dip right (east) 10o-15o. Buildup stratigraphy is 244m thick. Small subtidal mounds (left) coalesce towards the outer margin into larger buildups. Locations (a) through (d) refer to the 3D reconstructions shown below.

Shallow water facies

Depending on location, the landward part of the platform changes from sandy beach to intertidal-shallow subtidal flats. Facies indicative of beach settings include crossbedded grainstone, edgewise conglomerate pavements, and beachrock. Typical tidal flat structures are lenticular and flaser cross bedding, plus indications of prolonged exposure with mud cracks and gypsum crystals (replaced by dolomite). Local clusters of branched stromatolites and laminates show frequent scouring and reworking by storms.

Cross-section of tabular beds and lenses of edgewise conglomerate, interbedded with lenticular crossbedded grainstone. Conglomerate slabs consist of early cemented carbonate mud and cryptalgal laminates, probably ripped up during storm surges. Bedding views of the conglomerate show them to have formed extensive pavements.

Cross-section of tabular beds and lenses of edgewise conglomerate, interbedded with lenticular crossbedded grainstone. Conglomerate slabs consist of early cemented carbonate mud and cryptalgal laminates, probably ripped up during storm surges. Bedding views of the conglomerate show them to have formed extensive pavements.

Reef transitions across the platform

Three-dimensional reconstruction of Mavor platform mounds, from shallow subtidal laminates at the base, to high relief mounds at the platform margin. The stratigraphic section corresponds to that shown in the aerial view. Total stratigraphic thickness here is 244m.

Three-dimensional reconstruction of Mavor platform mounds, from shallow subtidal laminates at the base, to high relief mounds at the platform margin. The stratigraphic section corresponds to that shown in the aerial view. Total stratigraphic thickness here is 244m.

In the image above, the four panels reconstructed from field sketches and photos, show the stratigraphic changes in buildup organization. Beyond the beach, shallow subtidal mounds are relatively small, have synoptic reliefs of a few centimetres, and consist of laterally linked hemispheroidal domes and columns. Mounds are elongate, parallel to the overall trend seen in the aerial view; elongation direction is normal to the paleoshoreline, the strike of which was established from bedform azimuths.

Shallow subtidal, relatively simple, laterally linked and bridged mounds with synoptic relief of a few centimetres.

Shallow subtidal, relatively simple, laterally linked and bridged mounds with synoptic relief of a few centimetres.

Moving across the platform, corresponding to higher stratigraphic levels, mounds coalesce into fewer but larger buildups, with concomitant increases in synoptic relief. Synoptic relief is a maximum 2m to 3m at the platform margin. Here, buildups consist almost entirely of wavy, undulating and crinkly laminates; digitate branching is far less common. In addition, smaller scale elongate mounds are superposed on the larger structures. Thin beds of mud and laminate rip-ups, floored by shallow scours, indicate the passage of storms. Disruption by storms is seen in all the buildups including those near the platform margin. This implies that the depth of the margin itself was close to storm wave-base.

The exposure in places allows one to walk out individual beds from mound crest through the adjacent trough to the opposite mound; the only thing that’s missing is the water over your head.

High synoptic relief reef mounds viewed towards the platform margin, slightly oblique to elongation axes. In places, individual beds can be traced across mounds. These large structures were constructed entirely of laminated cryptalgal laminates. Eastern Tukarak Island.

High synoptic relief reef mounds viewed towards the platform margin, slightly oblique to elongation axes. In places, individual beds can be traced across mounds. These large structures were constructed entirely of laminated cryptalgal laminates. Eastern Tukarak Island.

 

Small mounds commonly adorned the crests and flanks of the high relief structures. Their elongation directions parallel the trend of the larger mounds. Jacob’s Staff (centre) is 1.5m long. Eastern Tukarak Island.

Small mounds commonly adorned the crests and flanks of the high relief structures. Their elongation directions parallel the trend of the larger mounds. Jacob’s Staff (centre) is 1.5m long. Eastern Tukarak Island.

 

Smaller-scale domes superposed on the large reef structures, are simple, laterally linked laminates, having synoptic relief in the range 20-30 cm. (hammer centre right). Eastern Tukarak

Smaller-scale domes superposed on the large reef structures, are simple, laterally linked laminates, having synoptic relief in the range 20-30 cm. (hammer centre right). Eastern Tukarak Island.

 

Wavy cryptalgal laminates from an outer platform buildup, disrupted by a shallow scour surface and overlain by rip-up clasts. Disruption by storms was common.

Wavy cryptalgal laminates from an outer platform buildup, disrupted by a shallow scour surface and overlain by rip-up clasts. Disruption by storms was common. Also some nice stylolites.

 

Wavy and crinkly laminates make up the bulk of the high-relief buildups that grew on the middle and outer platform.

Wavy and crinkly laminates make up the bulk of the high-relief buildups that grew on the middle and outer platform.

The slope

The transition from reef buildups to slope shale and carbonate rhythmites is abrupt, although a few small, isolated stromatolite domes occur immediately outboard of the platform margin, within slope mudrocks; these occurrences are inferred to represent the photic zone depth limit of stromatolite growth . The rhythmites are particularly striking, with alternating intervals of red and white dolomitized carbonate mudstones and calcilutites. There are a few graded beds containing more sandy sediment (mudstone fragments, ooids) and climbing ripples, deposited by turbidity currents. Clearly, some coarse-grained, shallow water sediment was moved onto the platform, perhaps through troughs between the mounds, eventually bypassing the margin.  Slump structures also occur but like the turbidites, are not common.

Slope rhythmites composed of calcilutite and dololutite. The ‘lumpy’ appearance is mainly due to incomplete replacement of calcitic mud to dolomite; this is one of the few units in the Belcher stratigraphy that still contains some calcite. Left: A relatively continuous succession of rhythmites, cut by numerous small extension faults. Top right: Bouma c (climbing ripples) and thin d (laminated mud) intervals of a turbidity current. Sandy material in the c interval was derived from the inshore deposits. Bottom right: Dololutite with some terrigenous mud (red hues), interbedded with white calcilutite rhythmites.

Slope rhythmites composed of calcilutite and dololutite. The ‘lumpy’ appearance is mainly due to incomplete replacement of calcitic mud to dolomite; this is one of the few units in the Belcher stratigraphy that still contains some calcite. Left: A relatively continuous succession of rhythmites, cut by numerous small extension faults. Top right: Bouma c (climbing ripples) and thin d (laminated mud) intervals of a turbidity current. Sandy material in the c interval was derived from the inshore deposits. Bottom right: Dololutite with some terrigenous mud (red hues), interbedded with white calcilutite rhythmites.

The Mavor platform was dominated by muddy carbonate sediment. It was a large carbonate factory where the production line consisted entirely of cyanobacteria.  Reef and slope stratigraphy indicate long-term transgression, conditions conducive to the viability of factory production. There are no chronostratigraphic controls on this part of the succession, but I speculate the cycle duration is roughly equivalent to a 3rd-order Sequence.

 

Links to other posts in this series:

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates; Karst

Mineralogy of carbonates; Burial diagenesis

Mineralogy of carbonates; Neomorphism

Mineralogy of carbonates; Pressure solution

Stromatolites in outcrop

 

References

Bosak, A.H. Knoll, and A.P. Petroff, 2013. The Meaning of Stromatolites. Annual Reviews of Earth & Planetary Science, v. 41, p. 21-44. Free Access. Lots of great references.

T.E. Playton, X. Janson and C. Kerans. 2010. Carbonate slopes and margins. In N.P.James and R Dalrmple (Editors), Geological Association of Canada, Facies Models 4, Chapter 18. P. 449-476.  Available for download

E.P. Suosaari, R.P. Reid, and M.S. Andres, 2019. Stromatolites, so what?! A tribute to Robert N Ginsberg. Depositional Record, v. 5, p. 486-497. Open Access. Evaluates some of the main controversies with stromatolites and microbialites. Lots of great references.

B.D. Ricketts and J.A. Donaldson, 1989. Stromatolite reef development on a mud-dominated platform in the Middle Precambrian Belcher Group of Hudson Bay. In, H. Geldsetzer, N.P. James and G. Tebbutt (Eds.), Reefs, Canada and adjacent areas. Canadian Society of Petroleum Geologists, Memoir 13.

H.D. Williams and others, 2011. Investigating carbonate platform types: Multiple controls and a continuum of geometries. Journal of Sedimentary Research, v. 81, p.18-37. Good account of factors influencing platform and ramp geometries using 2D modelling. Available for download

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Atlas of stromatolites and cryptalgal laminates

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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.

The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite, and professional thing to do).

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

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

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.

 

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In the field: Windows into two billion year-old rocks

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My early geological education was very much New Zealand centered; the gamut of sedimentary, igneous and metamorphic rocks (there are no Precambrian rocks in New Zealand), in the context of a landmass (and attached submerged bits) still rent by active faults and erupting volcanoes. The timing was fortuitous. We were taught at the cusp of the ‘new tectonics’, sea-floor spreading, and the morphing of continental drift into plate tectonics.  The fixists were a disappearing breed; now everything was on the move, attached in some way to one tectonic plate or another, rifted, drifted, and eventually subducted. Now, the rock formations, faults (particularly the Alpine Fault), and the volcanoes, were all connected in one, all-encompassing global, plate tectonic system.  Geologically active New Zealand had a place in this grand scheme.

Admittedly, not all our professors found it easy to teach these revolutionary ideas. We would be exhorted to go and read the latest journal papers, and come back with questions – I guess this gave the teachers time to read the articles themselves. But it was an exciting time, reading the claims and counterclaims. It really was a (Thomas Kuhn) paradigm shift.

Landing on the shores of Belcher Islands (Hudson Bay) was also something of a mind warp; from a country that straddles a plate boundary, has a volcanic rift zone in central North Island, and faces a subduction zone within a stone’s throw of the east coast, to a part of the Canadian Shield where not much has happened over the last two billion years.  Perhaps that’s a bit of an exaggeration, but this prolonged period of stasis had its advantages.  The rocks, despite being about 2000 million years old, are loaded with beautifully preserved structures and fossils.  They were not cooked by metamorphism during the time they spent being buried, nor fractured beyond recognition by tectonic forces. Basically, everything was intact. Stunning.

For someone interested in deciphering ancient sedimentary environments, being parachuted into the Belchers and being told to take the rocks apart, layer by layer, sequence by sequence, was initially a tad scary; an emotional response that quickly dissipated once the measuring, observation, and interpretations had begun. On finishing the work on one set of exposures, we couldn’t wait to get to the next, and the next.

Ancient and modern stromatolite domes

If you were to stand all the Belcher strata in a single pile, it would be almost 9 km thick. But this pile was subsequently tipped on its side. Over the eons, the rocks were eroded by rivers and scraped by ice, fortuitous levellers that provided windows into each layer. Geologists are enticed to enter these portals, at least in their mind’s eye; the rewards are huge.  We can envisage times when there were broad platforms of limestone (now all converted to the mineral dolomite), that harboured a massive biomass of primitive algae, stromatolites of all shapes and sizes; layers as thin as a fingernail, and reefs 10s of metres high. The platforms were covered by warm, seas that shoaled into tidal flats and (deserted) beaches. Some areas infrequently inundated by high tides, became desiccated; there are remnants of minerals like gypsum and halite (common salt) that attest to salty seas. Walking over rocks like these kindles the imagination; a beach stroll, waves rolling in like they have done for billions of years, or parched landscapes exposed to the full effects of sunlight uninhibited by oxygen and the UV dampening effects of ozone (the incidence of UV light must have been intense). The experience is humbling.

Gypsum (replaced by dolomite) in a Paleoproterozoic tidal-supratidal flat

However, idylls have a tendency to dissipate in the fog of time or, as was the case here, a smothering by erupting ash columns and lava flows. Now we get to walk across the tops of really ancient lava flows, around piles of pillow lavas, or along catastrophic pyroclastic flows of ash and pumice.  The earlier tropical paradise had been obliterated, but even in this volcanic brutality there is wonder.

Mud cracks in a 2 billion year river deposit

Other strata tell of deep seas fed by turbulent mud flows cascading down an ancient submarine slope, and of sandy rivers turned red by iron oxidized by the gradually increasing levels of oxygen in the ancient atmosphere (deposits like this are commonly referred to as red beds). In every layer, every rock we looked at, there were mysteries waiting to be unravelled. A geologist cannot hope to solve all such questions, but finding a solution to even one of them is incredibly satisfying.

Lots of turbidites in a deep, Paleoproterozoic basin

I spent a total of 5 months in the field during the 1976-77 summers. This was not the kind of location where, if I’d forgotten to do something, I could whip back for a couple of days to sort things out. Several of my student colleagues were doing similar kinds of research in remote parts of the country – field seasons were long. Once you had arrived, you were there for the duration. And despite the sense of excitement and discovery, it was always good to get back home.

Some details of turbidite beds showing complete and partial Bouma intervals

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In the field: from one extreme to another

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Aerial photo of Belcher Islands, Hudson Bay

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 25oC 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.

Plunging anticline-syncline folds, Mavor Island

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).

Sanikiluaq in 1976

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.

Negotiating sea ice, 1977

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.

Base camp on Tukarak Island - the local Inuit holiday spot

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.

Camp visitors, 1976

 

Davidee Kavik, soapstone carver on holiday on Tukarak Island

Belcher Islands sit well below the Arctic Circle at 56oN (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

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Ropes, pillows and tubes; modern analogues for ancient volcanic structures

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Analogies are the stuff of science. In geology, we frequently employ modern analogies of physical, chemical, or biological processes to help us interpret events that took place in the distant past. We cannot observe directly geological events beyond our own collective memory. Instead, we must infer what might have taken place based on evidence that is recorded in rocks, fossils, chemical compounds, and the various signals that the earth transmits (such as acoustic or electrical signals).  Analogies are not exact replicas of things or events, although they may come quite close. Their primary function is to guide us in our attempts to interpret the past.  As such, they are part of our rational discourse with deep time. Analogies are at the heart of the concept of Uniformity espoused by our 18th and 19th century geological heroes, James Hutton and Charles Lyell; they are the foundation for the common dictum “the present is the key to the past”, coined by Archibald Geikie, an early 20th century Scottish geologist.

Even though lots of people have written about this, I figure one more example that illustrates the methodology won’t hurt. Forty years ago, I worked on some very old rocks on Belcher Islands, Hudson Bay, that included volcanic deposits. Looking at the photos (35mm slides), I still marvel at the geology, the fact that something almost 2 billion years old is so well preserved, makes it look like the volcano just erupted.

Here are three ancient structures that were constructed by flowing basalt lava. Each can be compared with modern volcanic structures and processes that we can observe directly.  We can interpret the ancient structures according to the similarities and differences between the modern analogues and the ancient versions. The examples are from strata known as the Flaherty Formation, a succession of volcanic rocks exposed on Belcher Islands, Hudson Bay. Continue reading

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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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