10.5: Carbonate Environments
Distribution in Space and Time
Most of the world’s coastlines today are dominated by clastic sediments, with only a few regions exhibiting significant carbonate production. These carbonate-producing areas are often in shallow, tropical areas that are removed from major rivers and other sources of clastic sediment. In environments where clastic sediments are present, carbonate production is often inhibited. Clastic sediments reduce light penetration, dilute carbonate material, and generally create unfavorable conditions for carbonate-producing organisms. While mixed clastic-carbonate systems do exist, it is often more practical to consider carbonate and clastic depositional environments as distinct end-members due to their differing sedimentary processes.
Carbonate deposition is most prevalent during warm climate phases and in shallow seas where sea levels are high. Although abiotic carbonate production can be locally important (oolitic limestones, whiting events, etc.), a large percentage of modern carbonate production is biological, derived from shelly organisms, calcareous algae, and calcareous plankton. Since most of these organisms thrive in the photic zone, carbonate deposition is generally restricted to shallow marine environments where light penetration is sufficient to support biological activity.
Carbonate rocks are significant in the geological record, but the organisms associated with their formation have changed dramatically over time. Unlike clastic sediments, carbonate sediments are intrabasinal, meaning they originate within the basin where they are deposited. The maximum grain size in carbonate sediments is controlled by the skeletal material of the organisms rather than by hydrodynamic processes. Additionally, the proportion of carbonate mud present in a deposit serves as a reliable proxy for assessing energy levels within the depositional environment.
A stunningly vague, complex, and sometimes contradictory collection of terms has arisen to describe the spectrum of carbonate environments. In this section we adopt the following terminology based on our experiences and reading of the literature:
Basin - A deep marine environment well below storm wave base. Dominated by suspension deposition. Basinal deposits can be nearly identical to outer ramp or outer shelf deposits.
Lagoon - A relatively narrow body of water that sits between the mainland and some barrier with the open ocean (in this context, typically a shoal or reef). Communication with fully marine waters is restricted and oxygen and/or salinity stress are typical. Generally very low energy; some have peritidal areas around their margins.
Peritidal - Areas that are tidally influenced or immediately adjacent to them. Subenvironments include subtidal (below avg. low tide), intertidal (variously exposed and inundated by normal tides), and supratidal (above avg. high tide).
Platform - A nearly horizontal, shallow marine environment that is surrounded by marine waters. Flooded areas of large platforms can be considered shelves. Can be in direct communication with the open ocean (unrimmed) or can be bounded by a reef or shoal (rimmed).
Ramp - A moderately dipping, relatively shallow marine environment. Comparable to a shelf but a somewhat steeper slope. Can be in direct communication with the open ocean (unrimmed) or can be bounded by a reef or shoal (rimmed). Outer ramp deposits can be nearly identical of basinal facies.
Reef - An in situ accumulation of wave-resistant organisms (cf. shoal). Can form on the margins of platforms, ramps, or shelves.
Shelf - a very low gradient, relatively shallow marine environment in shallow epieric seas, continental margins, or atop platforms. Comparable to a ramp, but a shallower gradient. Can be in direct communication with the open ocean (unrimmed) or can be bounded by a reef or shoal (rimmed). Outer shelf deposits can be nearly identical of basinal facies.
Shoal - an accumulation of sand-sized carbonate grains that that builds up in higher energy, wave-dominated environments (cf. reef). Can form on the margins of platforms, ramps, or shelves.
Slope - A steeply dipping area that is transitional between the relatively shallow waters of a shoal, shelf, ramp, or reef and deep marine waters of the basin.
Major Depositional Environments
Carbonate sediments primarily form in relatively shallow water environments (meters to tens of meters) within the subtidal carbonate factory, where sediment is transported both basinward and shoreward. In addition to these regions, carbonate accumulation occurs in relatively shallow water reef and peritidal environments, while deepwater carbonate deposition can occur where calcareous skeletal material accumulates.
Peritidal Areas
Peritidal carbonates form in environments influenced by tides and the areas immediately adjacent to them. These include supratidal environments (only inundated during storm events), intertidal environments (regularly exposed and submerged by the tides), and shallow subtidal environments (just below the low tide level).
The most proximal (updip) areas experience extended periods of subaerial exposure and may consist of pebble- to boulder-sized carbonate breccias with a matrix that varies from (lime) mudstone to siltstone or sandstone. Supratidal to upper intertidal areas are more frequently inundated and may be composed of lime mudstone with variable amounts of interbedded skeletal wackstone to packstone. Lime mudstones may be planar laminated or have crinkly algal laminae; mudcracks and intraclasts can be present locally.
Intertidal environments represent low-energy, sheltered settings that occasionally experience high-energy storm events. Common facies include laminated mudstones with variable amounts of intraclastic, skeletal and/or peloidal wackestone to grainstone. Mudcracks, fenestrae, crinkly algal laminae, pisoids, peloids, and intraformational conglomerates may be abundant.
Shallow subtidal areas, which transition into lagoonal environments, can contain mudstone to packstone facies with rip-up clasts, peloids, and fossils present locally. Herringbone cross-beds and variably oriented ripple cross-laminae may be present and record reversing tidal currents. Thin quartz sandstones or arenaceous carbonates are present in some successions.
In all of these settings, dolomitization can be pervasive and may destroy or overprint primary textures.
Lagoons
Lagoons are low-energy environments located on the landward side of reefs or shoals. Although entirely subaqueous, their limited connection to the open ocean makes them susceptible to oxygen and salinity stress. Deposition is primarily by suspension and organic-rich lime mudstones are very abundant, but peloidal grainstones can be abundant locally and intraclast and/or skeletal grainstones record washover events during storms.
Both trace and body fossil abundance and composition are highly variable, though low-diversity assemblages are most common. While scattered marine fossils may be present, organisms tolerant of stressed conditions (ostracods, calcispheres, etc.) tend to dominate. Algal laminae, stromatolites, and/or stromatoporoids may be present locally.
Overall, lagoons are typically preserved as organic-rich lime mudstones to wackestones, which are internally massive to faintly laminated. Bioturbation ranges from scattered to abundant. Dolomitization can be pervasive in lagoons that become chemically evolved.
Shelf/Platform Interior
Shelves and platforms are both very low-gradient shallow marine environments; the margin of shelves and platforms can be in direct communication with the open ocean (unrimmed) or can be bounded by a reef or shoal (rimmed). Shelves can be adjacent to continental crust and represent a transition to the open ocean or form the interior of platforms. Platforms are isolated and surrounded by marine waters.
The spectrum of environments and processes on shelves can be extremely variable depending on local conditions. Carbonate deposition can occur seaward of, and pass laterally into, clastic-dominated areas on the margins of continental shelves. Common facies include lime mudstones, wackstones and packstones with largely in situ skeletal material, storm-generated wackestone to packstone beds with abraded skeletal material, as well as fossiliferous sandstones, siltstones, and shales.
Higher energy shelves have been described from epicontinental seas and the interior portions of isolated platforms. Common facies include grainstones that may contain a variety of transported grains, including skeletal fragments, ooids, pisoids, peloids, and/or intraclasts. Grainstones are commonly cross-bedded and may contain isolated larger grains transported during storms. In shallow, protected areas near the shore or behind shoals/reefs, fenestral-pisoid laminated facies and tepee complexes may be present. Scattered sandstones and/or siltstones may be present.
Shoals
Carbonate shoals are high-energy environments where transported skeletal fragments and/or ooids accumulate above fair-weather wave base. While shoals and reefs occupy similar bathymetric positions, reefs consist of in situ accumulations of wave-resistant organisms, whereas shoals are composed of transported sediment. Both can form along broad shelf margins or isolated platforms, provided clastic input is minimal and fully marine conditions with abundant oxygen, energy, and warm temperatures are present. Although modern examples exist (e.g., the Bahama Banks), they were far more common in the geologic past when epeiric seas flooded continental crust, and global climates were warmer.
Shoals and reefs act as barriers, separating calm lagoons or shelf environments from more energetic open-marine conditions. Shoal development follows processes similar to those of a clastic shoreface. The upper shoal is well above fair-weather wave base, consists of shifting oolitic and/or skeletal sand (grainstone) with sedimentary structures such as cross-beds, ripple cross-laminae, low-angle laminae, cross-cutting erosion surfaces, and rip-up clasts. More distal foreshoal deposits, also above fair-weather wave base, contain more interbedded skeletal to oolitic packstones and wackestones. Bioturbation increases in abundance and diversity compared to the upper shoal. While sedimentary structures are similar, cross-beds become less common and may be replaced by hummocky cross-strata.
Reefs
Reefs are wave-resistant, in situ accumulations of benthic organisms that thrive in high-energy, shallow-water environments. While they can endure crashing waves and grow rapidly enough to keep pace with sea-level rise, they generally require clear, warm, fully marine waters. The morphology of reef-building organisms is shaped by energy levels within the reef environment, with different organisms contributing in various ways—some form rigid, wave-resistant frameworks, others generate sediment, and some bind and stabilize the seafloor. Reef-building communities have varied over time, including stromatolites, archaeocyathids, corals, stromatoporoids, algae, sponges, and bivalves, among others. Major subenvironments of the reef are described below.
The back reef is a low-energy zone transitional between the reef and protected areas (ex: lagoon) behind it. It typically has a muddy substrate, sometimes with domal or robust branching organisms. A low-gradient reef flat , composed of storm-transported clasts cemented into a pavement, may separate it from the reef crest. The reef crest extends from the high tide mark to several meters below, this zone varies with energy levels. In extreme wave conditions, it consists of sheet-like encrusting organisms adapted to exposure and intense wave action; in calmer settings, robust branching forms dominate. The reef front contains a diverse collection of organisms that display a systematic progression of forms including massive corals near the reef crest, branching forms near wave base, and platy forms extending to the lower limit of the photic zone.
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Figure \(\PageIndex{16}\): Carbonate deposits associated with reefs. A) Modern reef ( USFWS - Pacific Region via Flickr ; CC BY-NC 2.0 ), B) Sponge boundstone block transported downslope from the Permian Reef Complex. The reef complex was entombed in a thick succession of evaporites when the basin became restricted and modern erosion exposes the amount of original relief on the platform. The Capitan Limestone is the core of this sponge-dominated reef and it prograded out over the slope and deep basinal deposits of the Cherry Canyon through Bone Spring Formations. The amount of topographic relief present today approximates the amount of bathymetric relief when the reef was active. C) Fossil brain coral ( Diploria ) at the Windley Key Fossil Reef Geological State Park . US Quarter near top for scale ( Jstuby via Wikimedia Commons ; public domain ), D) Stromatoporoid reef from the Devonian Cairn Formation ( Georgialh via Wikimedia Commons ; CC BY-SA 4.0 ). E) Fenestral limestone and F) carbonate breccia deposited in a backreef setting, Permian Reef Complex, New Mexico ( Michael Rygel via Wikimedia Commons ; CC BY-SA 4.0 )
Middle Ramp/Slope
Depending on geomorphic and plate tectonic settings, shoals and reefs transition seaward into either a middle ramp on gently dipping shelves or steeply dipping slopes along the margins of rimmed shelves or platforms.
The middle ramp extends from fair-weather wave base to storm wave base, comparable to the offshore transition (inner shelf) zone of clastic coasts. Common lithologies include lime mudstone interbedded with skeletal wackestones to packstones and thin grainstones. Skeletal material accumulates in situ in wackestones to packstones, while grainstones likely form from downslope sediment movement during storms. Fully marine faunas are present, and bioturbation is generally more intense and diverse than in shoals.
Steeply dipping slopes lie seaward of reefs or shoals on rimmed shelves or platforms, extending from the photic zone to depths of tens to hundreds of meters before transitioning to the low-gradient basin floor. These carbonate slopes contain fine-grained carbonate sediment that settled from suspension and coarser debris transported downslope from reefs or shoals. Common facies include interbedded lime mudstones, wackestones/packstones, and rudstones deposited by debris flows or turbidity currents, with occasional clastic sandstones or siltstones. Much of the skeletal material is transported downslope, and bioturbation varies from absent to intense. At the distal toe of slope , deep basin deposits alternate between lime mudstones formed by suspension deposition and rudstones from turbidity currents and debris flows.
Outer Ramp/Basin
As per the discussion in the abyssal plain and carbonate compensation depth sections in 10.4: Clastic Marine Environments , carbonate accumulation in deep marine outer ramp or basin environments primarily results from the suspension deposition of calcareous plankton skeletons (calcareous ooze), with occasional gravity-driven transport from the slope. Deposition of carbonate only occurs above the carbonate compensation depth. Common facies include laminated lime mudstones and/or chalk, often interbedded with fine-grained clastics and bedded cherts. Lime mudstones may contain microfossils, sparse bioturbation (typically Zoophyco s or Nereites Ichnofacies), and peloids. Deep marine carbonates are often organic-rich and can serve as significant hydrocarbon source rocks.
Readings and Resources
- Carbonate World website - https://carbonateworld.com/
- James, N.P. and Jones, B., 2015, Origin of Carbonate Sedimentary Rocks , Wiley, 464 p.
- Laugié, M., Michel, J., Pohl, A. et al. Global distribution of modern shallow-water marine carbonate factories: a spatial model based on environmental parameters. Sci Rep 9, 16432 (2019). https://doi.org/10.1038/s41598-019-52821-2 ; CC BY 4.0 .