Formation of Ooids

Experimental studies of ooid formation During the past 20 years relatively few experimental studies have been carried out on the formation of ooids. Most of these studies have emphasized the role of organic matter. The primary reason for this emphasis is the observation of abundant protein matrices in obids (e.g.,... 

Their studies are remarkable in indicating that organic matter may be important for the formation of magnesian calcite radial ooids, but not aragonitic tangential ooids. This observation is contrary to general concensus on the formation of these two types of ooids. It may also offer a major clue to the formation of aragonitic and calcitic carbonate cements. 

Ferguson J., Bubela B. and Davies P.J. (1978) Synthesis and possible mechanism of formation of radial carbonate ooids. Chem. Geol. 22, 285-308. 

Hypotheses for the processes responsible for the existence of ooids are based on bacterial-mechanical, algal, and chemical mechanisms for ooid formation. Some investigators have suggested models that involve more than one of these mechanisms. An extensive review and discussion of the hypotheses for ooid formation can be found in Morse and Mackenzie (1990). 

The amount of carbonate which is deposited in the oceans today is almost exclusively of biogenous origin. The long controversy whether chemical precipitation of lime occurs directly in the shallow waters of the tropical seas, such as the banks of the Bahamas and in the Persian Gulf (Fig. 1.5), during the formation of calcareous ooids and oozes of acicular aragonite, has been settled in preference of the concept of biomineralization (Fabricius 1977). The tiny aragonite... 

Often iron stones have an oolithic texture, i.e. they consist of perfectly rounded bodies, the so-called ooids. Iron oxide ooids are mm to cm in size, hardened and showing concentric Fe accumulations (Plate 15.1), which suggest cyclic precipitation of the Fe oxide. Ghemical point analysis by the electron microprobe indicates enrichment of Ti, P and V in the Fe oxides, whereas Si and Al concentrations are very low (Fig. 15.5). The mode of Fe ooid formation and in particular the need for some mechanical action to produce the perfect rounding has been a matter for discussion for a long time. In principle, both terrestrial and marine formation is possible and it... 

Before continuing, it is important to note that the solubility behavior of aragonite in seawater has proven rather strange, in that for equilibration periods of less than about a month, different solubility values can be obtained in an apparently reversible manner. This result points to some sort of short-term, solubility controlling surface phase and may have implications for processes such as ooid formation which is discussed in Chapter 5. 

We will not attempt to review the occurrence and general characteristics of odlites here, because this has been done by several authors (e.g., Milliman, 1974 Bathurst, 1975 Fabricius, 1977 Simone, 1981) however, to quote Cayeux (as translated by Bathurst, 1975), "it is remarkable that so large a body of writing should contain so small a measure of achievement." Instead, we will concentrate on the major hypotheses for ooid formation and summarize the chemical characteristics of odids and experimental studies aimed at determining their processes of formation. 

Bathurst has presented a model that solves some of the problems associated with laminated aragonitic ooid formation. His suggestion involves a means for growing inorganic precipitates of tangentially oriented rods. His arguments also offer plausible explanations for limitations on ooid size and other characteristics. 

One of the difficulties in understanding typical aragonitic ooid formation is the fact that the aragonite needles, or battens, are arranged with their c crystallographic axes parallel to the solid surface. Bathurst (1974) observed that the... 

Shallow marine environments include coral and algal reefs as well as other bioherms and many favour calcification by benthic fauna. Stromatolites and stromatolitic environments are also typical shallow marine formations. The shallow marine carbonate environment may be subdivided into more or less agitated waters with dominantly benthic fauna, calm shallow areas with carbonate muds (e.g. Bahama Banks) with ooids as typical forms of deposits and reef areas with their complicated patterns of calcification and deposition (Bathurst, 1975 Kinsey and Davies, Chapter 2.5). 

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