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Abstract

Synsedimentary diagenesis in the marine realm is relatively uncomplicated (by comparison with meteoric and burial diagenesis) because it generally operates over short time spans (only years to thousands of years, in most cases) and involves a restricted range of pore fluid chemistries. Nevertheless, through a combination of physical, chemical and biological processes, coupled with access to a nearly unlimited supply of dissolved materials in seawater, marine diagenesis can often bring about remarkable change in carbonate sediments and produce some very complex fabrics. Furthermore, the subsequent overlay of meteoric or burial diagenetic alterations can greatly complicate the recognition of marine diagenetic fabrics in ancient carbonate rocks. That is especially true because the aragonitic or Mg-calcitic cements that result from marine diagenesis are essentially just as unstable in meteoric or burial-stage pore fluids as primary grains of those compositions.

The intensity or extent of marine cementation is a function of the supply of solutes from seawater. Solute supply, in turn, depends on sedimentation rates and the effectiveness of water transport from the surface into the interior of a sediment pile. Mechanisms of water movement include, among others, wave forcing, tidal pumping, thermal convection, and diffusive transport. Areas of very slow sedimentation (e.g., hiatus surfaces, low-sedimentation-rate platform interiors, or low-productivity deep sea settings) can have substantial marine cementation (including hardgrounds) because they all have long times of contact between seawater and a thin package of sediment, even with no special mechanism for water pumping. In high-sedimentation rate areas, on the other hand, substantial marine cementation occurs mainly in reef front or coastal settings where wave or tidal action can force seawater through the sediments to a considerable depth. Likewise, atoll margins and steep carbonate platform flanks are sites of extensive marine cementation because of convective water input coupled, in some cases, with low sediment accumulation rates. Hot or cold seeps on the sea floor also represent sites of exceptional water throughput and extensive cementation.

Grain and matrix dissolution are widespread in certain marine environments, particularly in cold- and deep-water areas. Modern oceanic waters have an aragonite compensation depth or ACD at roughly 1,500 m (the ACD is the depth below which aragonite does not accumulate because the rate of dissolution exceeds the rate of aragonite supply). Aragonite also is extensively dissolved in cool and cold-water shelf areas. The modern calcite compensation depth (CCD) lies at roughly 4,500 m (but that depth, as well as that of the ACD, varies with latitude, productivity, and other factors, and undoubtedly has varied significantly with geologic time).

Bored (biodegraded) grains with cement infill of borings and generation of micrite envelopes (also discussed in the sections on pellets/peloids and sedimentary structures-borings).

Isopachous crusts of fibrous to bladed, peloidal, or aphanocrystalline high-Mg calcite cement. The aphanocrystalline crusts consist of equant, less than 4 μm-sized rhombs that look much like micrite.

Isopachous crusts of fibrous aragonite cement within grain cavities and as intergranular cements (predominantly found in warm-water, slightly hypersaline settings and tropical beachrock deposits).

Marine-cemented hardground formation in selected areas (see above) — associated, in many cases, with phosphate and glauconite cementation, boring and faunal encrustation, and intraclast formation.

Large botryoids of cavity-filling aragonite and high-Mg calcite cement.

Internal sediment fills of primary cavities or neptunian dikes in framework-supported sediments.

Coastal beachrock and spray-zone cements.

Microbe/cement associations in marine methane and thermal seeps.

Modern marine cements in warm-water settings consist mainly of high-Mg calcite (~12-18 mol% Mg), but with extensive aragonite as well. In colder-water areas (temperate, polar and deep marine), high-Mg calcite cements predominate, but become scarcer and less Mg-rich at higher latitudes. Many ancient carbonate deposits certainly had aragonite and high-Mg calcite cements, perhaps with secular variations in their abundance (e.g., Wilkinson and Given, 1986), but low-Mg calcite marine cements may also have formed at some times. In older limestones, original aragonite and high-Mg calcite cements generally have been converted diagenetically to low-Mg calcite and must be recognized by micro-inclusions, geochemical analysis (especially Mg and Sr contents), relict morphologies or crystal outlines, or, as a last resort, characteristic patterns of preservation or alteration (former aragonitic cements, for example, typically have poor primary fabric preservation.)

Characteristic morphologies of marine cements

A diagrammatic depiction of some common types of modern marine high-Mg calcite and aragonite cements. Most of these morphologies will be illustrated in this section. Adapted from James and Choquette (1983).

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