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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 50 (1966)

Issue: 3. (March)

First Page: 612

Last Page: 613

Title: Diagenetic Phases: ABSTRACT

Author(s): Rhodes W. Fairbridge

Article Type: Meeting abstract

Abstract:

Diagenesis, interpreted as all those changes that may occur in or to a sediment after deposition--short of dynamic or high-temperature metamorphism--is a major process of the geocycle, potentially leading to sedimentary lithogenesis. However, this process may be subdivided into distinctive geochemical phases, each of which tends toward an equilibrium condition, only to be upset by the introduction of a new set of environmental parameters. An intermediate phase may be bypassed or reinstated repeatedly.

From the moment of deposition of a sediment grain to the eventual exposure to weathering and erosion, there are ideally three principal phases.

(a) Syndiagenesis (term proposed by Bissell, 1959) is the "bacterial phase," during which the sediment's organic matter provides the nutrient for vigorous bacterial metabolism and various "in-fauna." In an oxygenated basin there is a secondary subdivision into the following: (1) an upper oxygenated layer, where CO2 is the principal organic waste product and the pH will be 7 or less; carbonate shells tend to dissolve unless present in overwhelming numbers; and (2) a lower layer beneath a boundary marked by zero redox potential (Eh = 0); here there is no free O2 and the principal bacterial flora utilize CO4-- ions of the connate sea water, leading to sulfite and sulfide production, and commonly the formation of pyrite nodules. In barred basins the Eh = 0 boundary is above the sediment-water interface and the upper layer is eliminated. Other modifications occur in freshwater and supersaline basins.

(b) Anadiagenesis (writer's term) is the "compaction and cementing phase," during which the progressive new sediment accumulation and loading of the buried sediment lead to closer packing of grains and the slow expulsion of connate water. Organic geochemistry is replaced by inorganic reactions. By molecular filtration, clay adsorption, base exchange, etc., the connate residual solutions become progressively stronger, commonly until brines evolve. Important authigenic minerals are formed. Mg-rich brines favor dolomite metasomatism. Complete cementation leads to connate-water entrapment, but diaclastic revival during further subsidence or tectonics may remobilize the circulation. In some basins igneous activity leads to introduction of juvenile water and elements, including metallic ion . It is postulated that at times in the past these have joined the ascending connate fluids, emanating in submarine springs to enrich bottom waters, with which "raw materials" it has been possible for syndiagenetic bacteria

End_Page 612------------------------------

to stabilize vast layers of metal sulfides, the feasibility of which has been experimentally demonstrated by Baas Becking.

(c) Epidiagenesis (writer's term) is the "meteoric phase," during which tectonic emergence of the basin occurs. The ascending waters (of high pH) are replaced by or mixed with descending CO2- and oxygen-rich waters of meteoric origin (pH7, or even less when they drain from some lakes and streams). Pyrites commonly are oxidized, and the liberated iron forms Liesegang diffusion rings throughout porous rocks like sandstone, or along the joint planes of impervious types. Limestone develops karst features. Calcitic fossils that escaped syndiagenetic destruction may now be reduced to hollow casts.

Continental-shelf sediments under eustatic oscillations may pass through several epidiagenetic interludes before anadiagenesis, this leading to early lithification of carbonate layers. In a thick rock sequence containing several unconformities, multiple incidences of anadiagenesis and epidiagenesis are expected.

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Copyright 1997 American Association of Petroleum Geologists