Mavor Formation Archives

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.

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.

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.

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.

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.

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 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. 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.

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.

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

A post in the How to… series on carbonate mineralogy – carbonate mud

Micrite – a sensible contraction of microcrystalline calcite

Lime muds are important components of most shallow carbonate-forming environments: platforms, platform margins and slopes, back-reef and fore-reef settings, and lagoons. They also accumulate in shallow alkaline lakes and as oozes on the deep ocean floor. Like their terrigenous cousins, they accumulate as discrete beds and as matrix to coarse-grained carbonate frameworks.

Lime mud, or micrite includes aragonite and calcite (and perhaps dolomite) having crystal dimensions less than 4-5 microns. Thus, any detailed study requires an SEM or similar technology. Their grain size, and hence high unit-volume surface area means they are quite reactive and prone to recrystallization to coarser calcite-dolomite spar mosaics.

Today, the largest volumes of carbonate mud are produced in shallow warm seas at depths usually less than 10 m. All carbonate mud is produced by precipitation, either directly (i.e without biological intervention), or as a by-product of biological activity. Although there is a degree of certainty about this, the relative contribution of each source remains contentious, even after decades of study. Volumetrically less significant but perhaps no less important sources of mud include bioerosion and mechanical breakdown of skeletal material.

Several species of green algae produce micron-sized aragonite needles and plates that are shed when the alga dies. Of these, the codiacean algae Halimeda and Penicillus are the most prolific producers. For example, in Florida Bay and reef measurements of Penicillus’ aragonite productivity (as needles 2-3µm long) indicate that this species alone can account for all the mud deposited there (Stockman et al. 1967). Other codiacean species plus breakdown of skeletal material also contribute mud, but at this location Penicillus is it. The aragonite produced in these shallow waters is also transported to tidal flats and farther offshore.

Across the ditch, Grand Bahama Bank also hosts swaths of aragonitic mud. Calculations like those penned for Florida, demonstrate that most of this mud (on a volumetric basis) could also have come from codiacean algae. However, this is disputed by Milliman and others who contend that a large proportion of the micron-sized aragonite (up to 75%) precipitated directly from sea water because:

The aragonite crystal morphology is more like that produced by direct precipitation, and
The strontium content of the mud is too high – Sr/Mg ratio ≈ 4 compared to average values of ≈ 2 for codiacean algae.

One phenomenon that has aroused interest as a potential source of aragonite mud is the spontaneous development of milky white clouds of suspended carbonate, or whitings. They form over shallow platforms and persist only as long as it takes for the suspension to disperse or fall out of suspension. Two mechanisms are frequently invoked as explanations:

Where bottom muds are stirred up by feeding fish (this has been observed), and
By direct precipitation from seawater. Measurement of whitings over Grand Bahama Bank show that most of the suspension is micron-sized aragonite, but up to 20% high Mg-calcite may be present (Shinn et al 1989).

Some support for the direct precipitation hypothesis is linked to examples of whitings in areas devoid of bottom-dwelling red and green algae (e.g. Trucial Coast). However, this does not preclude the possibility that suspended algae may play a role. J.W. Morse has suggested that en mass precipitation of aragonite in normal seawater can occur if phytoplankton photosynthesis (during algal blooms) reduces dissolved CO₂.

The Bahamian aragonite is also transported to tidal flats and shallow lagoon, and to the platform margin where it accumulates from suspension on the adjacent deep water slope; carbonate mud that accumulates in this way is called hemipelagic mud.

During the Late Paleozoic, the calcareous algae niche was occupied by phylloid algae that were responsible for building mud mounds. Classic examples are found in Pennsylvanian cyclothems of Kansas and Oklahoma. Here they form part of the regressive limestone facies, where bladed and leaf-like phylloids acted as structural frameworks for the buildups. The facies has been well summarized by Phil Heckel (and lots of references therein).

Precambrian carbonate mudrocks (mostly dolomitized) are invariably associated with stromatolites of all shapes and sizes, from simple laminar mats to complex buildup reef-like structures like the example shown at the top of the post . It is generally understood that their construction was promoted by prokaryotic cyanobacteria (Phanerozoic red and green algae are all eukaryotes). Sedimentological evidence indicates that, like their Phanerozoic counterparts, they too inhabited shallow marine shelf, platform and inter-supratidal environments (here’s a link to the Stromatolite Atlas).

However, carbonate mud was deposited on the outer edges of platforms and adjacent slopes. An example from Belcher Islands (Costello Formation) consists of thin bedded dololutites and calcilututes, with the odd calci-turbidite, but no evidence of cryptalgal laminates. These deposits are also thought to have accumulated from mud suspended in the water column (i.e. hemipelagic) that originated in the carbonate mud ‘factory’ on the shallow platform.

Deep sea oozes are composed primarily of coccoliths and planktonic foraminifera that live in uppermost ocean waters (in the photic zone), and when dead gradually sink to the sea floor. Oozes accumulate in oceanic regions that are devoid of terrigenous sediment.

links to other posts in this series

Mineralogy of carbonates

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates: Stromatolite reefs