Calcitic ooids

Calcitic ooids Nodules of calcite that are less than 2 mm in diameter. They are commonly found in and on the sediments of shallow tropical seas. 

Calcitic oolite A pavement of calcitic ooids formed by continuing precipitation of calcium carbonate between the calcitic ooids. 

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. 

Figure 10.17. Trend in relative percentage of originally calcite and aragonite ooid compositions during the Phanerozoic. (After Mackenzie and Agegian, 1989.)...

High magnesian calcite cements, ooids and whitings all necessitate innovative new approaches to explain their formation and distribution. 

Tucker M.E. (1984) Calcitic, aragonitic and mixed calcitic-aragonitic ooids from the mid-Proterozoic Belt Supergroup, Montana. Sedimentology 31, 627-644. 

Wilkinson B.H., Bucznski C. and Owen R.M. (1984) Chemical control of carbonate phases implications from upper Pennsylvanian calcite-aragonite ooids of southeastern Kansas. J. Sediment. Petrol. 54, 932-947. 

Mass-age distribution and recycling rates Dolomite/calcite ratios Ooids and ironstones Calcareous shelly fossils The carbonate cycle in the ocean... 

Holland and Zimmermann, 2000). The latter authors do accept, however, that there is generally lower dolomite abundance in carbonate sediments deposited during the past 200 Myr. These qualihcations notwithstanding, a number of previously described parameters (Sr/Ca, Mg/ Ca, aragonite/calcite, possibly dolomite/calcite, and frequency of ooids and iron ores) appear to be related in some degree to sea-level stands, the latter at least in part a reflection of plate tectonic activity during the Phanerozoic. 

Oolitic limestones (or ooliths) are made up of tiny spherical grains (known as ooids) of 1 mm or less in diameter, precipitated by algal action in turbulent waters and cemented in calcite. Under a hand lens, they have the appearance of fish roe. 

Ooliths are limestones composed of ooids cemented in calcite. 

A systematic study of compressional and shear wave attenuation in limestones was published by Assefa et al. (1999). Limestones contain a complex porosity system with interparticle macropores and micropores within the ooids, calcite cement, and mud matrix. The authors fotmd that ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed . 

---