Reef Deposition

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Introduction

Reefs are carbonate sedimentary systems based on the continual buildup and breakdown of carbonate-dependent biotic assemblies in shallow marine environments. Reefs are incredibly fossil-rich on account of being made up by the calcitic skeletons of reef organisms. Being deeply intertwined with the biology of the living reef, individual reefs will vary in terms of grain and fossil content in different areas and across geologic time, making them difficult to classify in ways similar to siliciclastic systems regardless of environment.

Reefs occur today as living assemblages of marine animals with calcareous tests and skeletons secured to the substrate, including corals, sponges, crinoids, anemones, and related organisms that house unattached marine animals like snails, polychaete worms, crustaceans, cephalopods, and fish. Fossil reefs may show similar patterns of attached and unattached organisms, as well as historic algal and microbial mounds.

Sedimentology

The primary rocks that make up reef systems are limestones, which are often described using the Dunham or Folk schemes. Reef limestones, however, are entirely derived from skeletons of organisms and require more specific classifications than what either scheme proposes. A revised Dunham scheme in Embry and Klovan (1971) expanded the Durham scheme’s “boundstone” category:


Where earlier systems distinguish between allochthonous (not organically bound) and autochthonous (organically bound) limestones, Embry and Klovan expanded the categories based on grain size and percent makeup of binding materials.

Allochthonous limestones within reef deposits will be a combination of mechanical breakdown of the reef through biotic activity and environmental events and will be restricted to the area surrounding the reef (James 1983). The sediments get cemented together through the pressure applied by overlying sediment, which increases the likelihood of diagenetic alteration of the original sediments. These sediments will range in size as well as composition of identifiable fossiliferous material and micrite. Sediment from the surrounding environment may be introduced into the reef limestones by environmental factors, as seen in Wallace et al. 2015.

Autochthonous limestones develop as the reef builds on itself through the activity of skeletonizing organisms. The three methods outlined by Embry and Klovan (1971) include: Baffling: the skeleton reduces flow velocity due to its extensive growth, leading to deposition of suspended particles. Binding: the skeleton encrusts and binds sediment to the reef surface, resulting in laminar or tabular fossils that are supported by a non-fossil matrix. Framing: the skeleton building itself up forms the material, usually resulting in large three-dimensional fossils that make up the majority of the limestone. Boundstone as a category is kept for indeterminable autochthonous limestones.

Geomorphology

Reefs are dependent on the organisms that build them, most of which depend on calcareous bioprecipitation to build their skeletons. Most of these organisms are sedentary filter feeders or dependent on sunlight for photosynthesis, which restricts reef development to nearshore environments on continental shelves, or to the tops of mid-ocean carbonate platforms. The organisms that primarily make-up reefs today are corals (phylum Cnidaria) and algae (a catch-all term for several groups of organisms).

Biotic Deposition Controls

Corals are colonial filter-feeding aquatic animals within the phylum Cnidaria. Corals will prefer environments where they can easily filter feed, which restricts their growth to environments with clear waters. Depending on coral growth type, they may be more susceptible to toppling and breaking depending on flow velocity. Finally, because corals rely on carbonate to create their skeletons, and so reef building ones will not be found below the carbonate compensation depth, which is where calcite concentrations in the ocean are not high enough to prevent dissolution of calcite.

Many algae that make up reefs are dependent on sunlight to fuel photosynthesis, and will not be found outside of the eutrophic zone. Calcareous algaes are also dependent on carbonate and will not be found below the carbonate compensation depth. Both corals and algae are dependent on nutrient cycling in order to reproduce, which can further restrict where reefs are found.

Modern Reef Morphology

Coral-algal reefs are typically found on the windward sides of the landforms or submarine platforms they form on. While it’s not precisely known why these reefs favor windward sides, it’s possible that the wind brings currents that carry away sediment made by the reefs, which benefits the filter-feeding organisms that make up the reefs. These reefs will tend to form extensive complexes that can block water flow and create areas of low flow velocity between the reef and landform.

These reefs have many shapes, including:

  • Patch Reef: isolated structures ranging in size from 5 to 50 meters wide and up to 10 meters tall. These reefs are also called lagoon reefs, because they grow in calm water zones between an open-water barrier and the coastline. These are typically made of coral rudstones and grainstones in their core, with extensive coral growth
  • Barrier Reefs: These reefs form along the edges of platforms, often forming continuous barriers parallel to a shoreline or a large submarine platform. These reefs have specific ranges within them:
  • * The reef crest is the highest part of the reef at any point in the reef’s growth, and typically receives the most wind and wave energy in shallow waters. Flow velocity and water clearness determines the kinds of corals that grow in this area, and biologic diversity is comparatively low in these areas.
  • * The reef front is the stretch of reef from the crest out to the open ocean down to the carbonate compensation depth, where organisms cannot continue precipitating calcite into their skeletons. Biodiversity in the reef front is comparatively high, with corals dominating to a depth of 30 meters and then growth of calcareous algaes below this depth.
  • * The reef flat is the area directly behind the reef crest towards the platform, and is typically cemented skeletal debris from storm events.
  • * The back reef is behind the reef flat and is dominated by mud-producing organisms like algaes, molluscs, and crustaceans. Only corals that can withstand highly muddy areas tend to grow here
  • Algal cups: cup-shaped algae-based reefs that grow on break-off slopes of island, shelves, and platforms. They can grow up to 10 meters tall and have inward sloping sides that create micro-lagoons a few meters deep. The edges of these algal cups get exposed to the atmosphere at low tides.


Modern reefs are commonly found within the tropics of Earth today. Notable modern reef examples include the Great Barrier Reef in Australia, the Atafu atoll in the Pacific Ocean, and the Florida Keys Reef in the Caribbean Sea.


Ancient Reefs

Reef fossilization and preservation do not follow traditional environment-dependent depositional sequences due to each reef being dependent on location and biotic activity. This means that every instance of a reef recorded in the fossil record is unique to its particular environment and time, most notably in the organisms featured in the reefs.

The oldest studied reefs include the Tonian Little Dal Group (Batten et al. 2004) and the Cryogenian Balcanoona reef complexes (Wallace et al. 2015), both dated to the Neoproterozoic. Both were built up by calcitic stromatolites, or layers of microbial colonies built on top of each other with carbonate cement. The slightly more recent Ediacaran Nama Group features stromatolite content as well as stem-metazoan organisms and evidence of bioturbation (Cribb et al. 2019).

Transitioning into the Phanerozoic, reef composition fluctuates depending on relative organism diversity. During the early Cambrian, the enigmatic archaeocyathids dominated reef structures alongside skeletal algaes, but then disappeared in the middle Cambrian. It wasn’t until the Ordovician that sponges (phylum Porifera), specifically the Stromatoporoidea, and corals (phylum Cnidaria) took over until the end of the Devonian. During this time, framework reefs began building up. After the end-Devonian extinction, reef mounds were commonly built out of bryozoans (phylum Bryozoa), algaes, and smaller corals and sponges until the end-Permian extinction.

In the Mesozoic, the middle Triassic reef mounds start being built with Tubiphytes, an enigmatic group of marine organisms. Corals and sponges by the end-Triassic and build up framework reefs until the Cretaceous, when rudists (phylum Mollusca) built up the most reefs until their disappearance at the end-Cretaceous. Through the Cenozoic most reefs have been made of corals or of skeletal algae.


Facies Models

In the fossil record, reefs will tend to present as limestone bodies constructed by organisms with different parts formed at different times (James 1983). Determining reef migration and activity in the fossil record can be complicated by dolomitization and erosion, which can overwrite important details about reef growth. Terminology used to describe reef deposits has fluctuated over the past century, since reefs have not been consistent through time.

Some common terms used to describe reef deposits in older literature include:

  • Bioherm: a lenticular organic-based body embedded in rocks of differing lithology
  • Biostrome: beds consisting of fossiliferous organisms that do not show swelling into lenses or mounds
  • Carbonate build-ups: laterally restricted carbonate sediment that displays topographic relief
  • Stratigraphic reefs: Carbonate build-ups made of pure or mostly pure carbonate material, may not contain fossils but rather describes the shape of the structure
  • Ecologic reef: Rigid, wave-resistant structure created by reef-building organisms, where all binding is done through biotic activity

Some of these terms may be restrictive of how modern reefs function, particularly in how much biotic activity actually binds a reef to its substrate and resistance to mechanical weathering. Terminology used to describe one fossil reef may not be used to describe another fossil reef, or to describe modern reefs because of variability between reefs.

Framework Reefs

When viewing a Phanerozoic sessile reef in cross-section, there are four different reef growth stages that may be recognizable:

  • Pioneer stage: The initializing stage that starts the core of the reef. This usually consists of accumulations of sediments colonized by algae, plants, and “rooting” animals that bind and stabilize the substrate the reef builds on.
  • Colonization stage: Typically the thinnest unit within a reef structure. Initial colonization by a small number of branching reef-building organisms with limited biodiversity. This branching may create smaller sub-environments for other organisms to live in.
  • Diversification stage: Typically the thickest unit within a reef structure and grows upwards towards sea level through typical carbonate accommodation-based growth. This stage of a reef has the most biodiversity, as many organisms begin to thrive within the reef structure and compete with each other.
  • Domination stage: A typically sudden transition from increased biodiversity to sudden restriction to encrusting or laminating types of organisms. It’s unclear whether this shift from diversity to restriction comes from changes in the environment or due to biotic activity within the reef itself.

Superimposition of reefs on top of other reefs is incredibly common, and many stratigraphic reefs may be composed of multiple reefs that have grown on top of each other. These stacked reefs may not demonstrate new pioneer stages since the underlying reef provides a hard surface for reefs to grow on.

Mounds

Reefs that form with non-sessile skeletal organisms typically get called reef mounds. These reefs have lower amounts of biodiversity. They tend to form in more calm-water environments with high amounts of micrite in their rocks. Mounds tend to show three stages in their growth: Stage 1: base lime mudstone or wackestone pile with lots of bioclastic debris, but no sessile organisms. Stage 2: mudstone or bafflestone core that makes up the bulk of the mound with delicate or dendritic forms and upright growth patterns in a mudstone matrix. Stage 3: a cap consisting of encrusting or lamellar organisms.

Further Reading

James (1983) “Reef” in AAPG Memoir 33 is the source for many of the diagrams and much of the facies information within this Wiki-article, and works as an incredibly good reference for understanding reef systems.

References

[1] [2] [3] [4] [5] [6] [7] [8] [9]

Image Sources

[10]

  1. Batten, K.L., Narbonne, G.M., and James, N.P., 2004, Paleoenvironments and growth of early Neoproterozoic calcimicrobial reefs: platformal Little Dal Group, northwestern Canada: Precambrian Research, v. 133, p. 249–269, doi:10.1016/j.precamres.2004.05.003.
  2. Burton, E.A., 1998, Carbonate compensation depthcompensation depth, in Geochemistry, Dordrecht, Kluwer Academic Publishers, p. 73–73, doi:10.1007/1-4020-4496-8_46.
  3. Embry III, A.F., and Klovan, J.E., 1971, A Late Devonian Reef Tract on Northeastern Banks Island, N.W.T: Bulletin of Canadian Petroleum Geology, v. 19, p. 730–781.
  4. John M. Kennard and Noel P. James, 1986, Thrombolites and Stromatolites: Two Distinct Types of Microbial Structures: Palaios, v. 1, p. 492–503, https://www.jstor.org/stable/3514631.
  5. Johnson, C.C., 2002, The Rise and Fall of Rudist Reefs: Reefs of the dinosaur era were dominated not by corals but by odd mollusks, which died off at the end of the Cretaceous from causes yet to be discovered: American Scientist, v. 90, p. 148–153, https://www.jstor.org/stable/27857629 (accessed December 2021).
  6. Noel P. James, 1983, Reefs, in Carbonate Depositional Environments, The American Association of Petroleum Geologists, AAPG Memoirs, v. 33, p. 345–440. Robert Riding and Li Guo, 1992, Affinity of Tubiphytes: Paleontology, v. 35, p. 37–49, http://go.palass.org/4iz.
  7. Rowland, S.M., 2001, Archaeocyaths—a history of phylogenetic interpretation: Journal of Paleontology, v. 75, p. 1065–1078, doi:10.1666/0022-3360(2001)075<1065:AAHOPI>2.0.CO;2.
  8. Wallace, M.W., Hood, A. v.S., Woon, E.M.S., Giddings, J.A., and Fromhold, T.A., 2015, The Cryogenian Balcanoona reef complexes of the Northern Flinders Ranges: Implications for Neoproterozoic ocean chemistry: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 417, p. 320–336, doi:10.1016/j.palaeo.2014.09.028.
  9. What are corals? | ICRI, https://www.icriforum.org/about-coral-reefs/what-are-corals/ (accessed December 2021).
  10. Noel P. James, 1983, Reefs, in Carbonate Depositional Environments, The American Association of Petroleum Geologists, AAPG Memoirs, v. 33, p. 345–440.