Marine environments II - carbonate:
- Recall the major factors leading to carbonate precipitation:
- Warm temperature
- Low pressure
- Thus, carbonate sediment tends to be autochthonous - forms where it is deposited. Carbonate platforms have often been considered as "factories" due to the rapid buildup of carbonate in appropriate warm and shallow marine environments. From them, allochthonous carbonate material can be exported to adjacent environments.
- Most carbonate production is biogenic. Because carbonate secreting critters tend to be intolerant of muddy waters, carbonate dominated environments tend to be distinct from clastic environments.
Carbonate factories frequently shallow to the surface where they may be exposed due to sea level fluctuations on various time scales, resulting in erosion and karstification. To continue carbonate sedimentation, either:
- accommodation space would have to increase (sea level would have to rise or the platform subside. The development of atolls around eroding volcanic islands is an example.)
- the facies would have to prograde - migrate out toward deeper water.
- Subtidal (including bioherms)
A historical note: Because the Quaternary is an interval of comparatively cool climate and low sea level, shallow epeiric seas and continental shelf are geographicaly limited. The modern world is a poor analogy to past carbonate-rich worlds like that of the Cretaceous.
- Generally above the tidal range and are thus wet with seawater only during storm events, which transport carbonate sediment to form intraclast breccias called tempestites. (E.G. 1992 Hurricane Andrew deposit, FL).
- Otherwise dominated by brackish or fresh water sources.
- A typical environment would be a marsh where it is likely to find high abundances of organic matter and where coal is likely to form.
- The organisms that inhabit this environment must be able to tolerate brackish conditions, and under extreme evaporitic conditions, may be absent altogether.
- Where carbonates are present, brought up from the ocean by storm events, they are often masked by:
- clastic sediments from the continent
- evaporites (in arid environments like Persian gulf sabkhas) that form in situ
- Within the tidal range (E.G.: Three Creeks area of Andros Island, Bahamas)
- Daily exposure to seawater
- Subject to storm events
- Periodic exposure to air
- Often protected by barrier islands, thus tide dominated.
- Experience extremes of salinity
- Hospitable to calcareous algal mats.
- Because of their shoaling upward tendency, carbonate banks can present topographic profiles that may cause waves to break farther from shore.
- During subaerial exposure, carbonate can dissolve then reprecipitate as sparry cement, forming beachrock.
- carbonate (dolomite) mudflats
- evaporites and clastic drapes that form a characteristic chicken-wire texture.
- herringbone crossbeds
- shell debris
- intraclasts (a.k.a. rip-up clasts or plate breccias, mud cracks and algal mats.
- Fenestral (sheet like) porosity or birds-eye structures are caused by the dissolution of evaporites or from voids originally filled with gas.
Peritidal carbonate platforms:(analogous to clastic peritidal environments) commonly form:
Because many carbonate allochems behave hydrodynamically like sand, most of the sedimentary structures we associate with clastic shorelines are found in peritidal carbonate environments, with some odd twists:
Sabkha:This environment is seen in modern and ancient tidal flats similar to those of the Persian Gulf. Due to the daily alternation of the tides peritidal environments often contain:
Peritidal environments are often associated with lagoons, which are, strictly speaking, subtidal. In these, finer sediments and biological allochems are common. These may be heavily bioturbated.
- Normal marine waters with a narrow range of salinity
- Well oxygenated
- Support a wide range of organisms, hence diverse fauna and bioturbation.
Carbonate can be formed by:
- Direct precipitation, as in the ooid shoals of the Bahamas (right)
- Biogenically, as in reefs.
Subtidal environments have a marine fauna which is more diverse than in peritidal environments, and sediments are often bioturbated.
Subtidal environments can have a wide distribution of facies associated with both water depth and wave energy.
- Bioherms in high energy environments
- Ooid and skeletal shoals in high energy environment (right)
- Mud and pellet carbonate sands in protected lagoonal environments
Repetitive changes in sea level cause cyclic deposition on carbonate platforms, which can be readily identified in the field. For subtidal facies, relative water depth may be inferred by the relative abundance of shaley interbeds and the thickness of carbonate layers.
Bioherms:(reefs) are sediment systems built entirely from the organisms that call it a home. We see two varieties in the rock record:
Images of contemporary reefs often don't give a sense of a reef's potential scale. For that, see famous ancient reef deposits:
- Devonian coral-stromatoporoid reefs of the Canning region of West Australia.
- Permian Guadalupe Reef, at Guadalupe Mountains National Park, TX.
- Forereef: Sediment shed off of the reef front (includes reef slope and proximal talus)
- Reef framework and crest: The densest zone of in situ framework growth. The crest receives the most wave energy.
- Backreef: Including reef flat, backreef sands, and lagoonal muds. Sheltered from wave energy by reef front.
The architects of reefs (framework builders) include:
- Algae -(Calcareous algae with and without soft tissue.)
- Early-Middle Cambrian: Archeocyathids (shelled sponges, in essence)
- Silurian - Devonian: Rugose and tabulate corals and stromatoporoids.
- Permian: Sponges.
- Cretaceous: Rudist clams
- Cenozoic: Scleractinian corals.
Thus largely due to mass extinction, the types of framework builders in reefs have changed through time. Indeed, large chunks of the Phanerozoic (Carboniferous, Triassic - Jurassic) were largely reef-free.
The primary deposition of calcite or aragonite is sensitive to the ratio of Mg/Ca, as aragonite can accommodate more Mg through cation substitution. Normal ocean environments are very close to the boundary, such that minor changes result in global shifts in carbonate deposition. Thus, Earth history has seen alternations between periods of aragonite seas and calcite seas. These, reflect rates of sea floor spreading and are indirectly connected to ocean current circulation:
- Calcite seas: During intervals of rapid sea floor spreading:
- hydrothermal activity near spreading zones pumps Ca into the oceans, but tend to withdraw Mg through hydrothermal reactions with ocean bedrock. Thus, oceans are Mg-poor and primary deposition is of low-Mg calcite.
- Because sea floor bedrock is warmer, these also tend to be intervals of higher sea level
- resulting in unrestricted tropical circulation and greenhouse conditions. (E.G. Cretaceous).
- Aragonite seas: During intervals of slow sea floor spreading:
- Oceanic concentrations of Ca are lower and Mg are higher, favoring aragonite and high-Mg calcite deposition.
- Because sea floor bedrock is cooler, these tend to be intervals of lower sea levels.
- resulting in restricted tropical circulation and deviation of ocean currents toward the poles, resulting in ice-house conditions. (E.G. Carboniferous).