Marine environments II - carbonate:

In everything that follows, remember three principal issues:

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:

Material from the carbonate factory is constantly redistributed around the platform and off into the abyssal deep. Facies are subdivided into depth-restricted zones including: Among these there are extreme variations in water depth, salinity, organismal contribution, and bioturbation. These carbonate environments resemble equivalent clastic environments that have been discussed previously, and may grade into them.

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.

Supratidal

Supratidal refers to environments that are:

Peritidal

Subtidal

Subtidal carbonate environments are below the tidal range and above storm wave base. The Bahama Banks is an excellent laboratory of a subtidal carbonate factory. Subtidal environments have:

Carbonate can be formed by:

However, globally, biogenic processes predominate. These produce thick carbonate blankets that can be up to 1000s of meters in thickness, depending on the rate of subsidence along the continental margin.

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.

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:
  • Mudmounds aka Biostromes: Piles of loose, unconsolidated, skeletal material and sediment that stand above their surroundings. E.G.: The Muleshoe Mudmound near Alomogordo, NM.

  • Reefs: Piles of intergrown organismal skeletons and sediment that form a rigid wave-resistant interlocking framework. E.G.: MacKay Reef, part of the Great Barrier Reef comples, QLD, Australia. Note: Even though reefs form through the action of framework building organisms, the nature of the framework can vary from bafflestone to bindstone to rudestone. Also, framework tends to represent a minority of actual reef volume, most of which is carbonate sediment infilling pore space.

    Images of contemporary reefs often don't give a sense of a reef's potential scale. For that, see famous ancient reef deposits:

    Reefs, which form at the edges of carbonate banks, can be excellent oil traps. Note, reefs develop at topographic highs on the sea floor, but far enough from the surf zone to have access to clear water. As a result, distinct reef facies form, including:

    The architects of reefs (framework builders) include:

    In the past even microbial mats could built up reefs. However, framework builders are generally only 10% of the total volume of the reef, the remainder is composed of skeletal fragments, micrite, breccia and cements, which fill in the interstitial spaces of the reef framework

  • Reef builders: Reefs have been around since the Cambrian. Major reef builders throughout the Phanerozoic include.


    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.

    Carbonates as paleoclimate indicators - Calcite and aragonite seas:

    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: