Isolated Tissues or Cells (PI. 11-1, fig. 18) 



Other types of isolated tissues or cells, such as bordered pits and woody tis- 

 sues, can be used for general age determinations. A detailed knowledge of fossil- 

 plant anatomy is necessary to use these fragments in micropaleontological inves- 

 tigations. 



Animal Microfossils 



Foraminifera 



Foraminifera are small, one-celled animals belonging to the phylum Proto- 

 zoa. Most of them have calcareous shells (tests), although some of the early and 

 fresh-water forms have only chitinous tests, whereas those living in areas of 

 turbulence often have arenaceous or agglutinated tests. Because foraminifera dif- 

 fer considerably in size, shape, and ornamentation, they have been used exten- 

 sively to zone various parts of the stratigraphic column. Foraminifera are found 

 in rocks from the early Paleozoic to Recent. 



No other group of microfossils has contributed so much to our knowledge 

 of the subsurface as the foraminifera, which have been used by the oil industry 

 for approximately 40 years. All the major oil companies support microfossil 

 laboratories primarily devoted to the use of the foraminifera for stratigraphic 

 purposes. Besides being used extensively in zoning wells and determining the 

 age of sediments, they are important as paleoecological indicators, and therefore 

 help to interpret favorable or unfavorable environments for oil accumulation. 

 Current research in the Gulf of Mexico and Caribbean areas on Recent foramin- 

 ifera should improve our knowledge of the different habitats that a particular 

 genus, species, or assemblage prefers. This information may be used as an aid 

 for interpreting ancient environments. Because the planktonic (free-floating) 

 forms are facies tolerant and generally widespread, they are especially useful 

 for long-distance correlation. 



This discussion will treat separately three types of foraminifera, subdivided 

 mainly on the basis of size. These include: (1) the microforaminifera; (2) the 

 small foraminifera; and (3) the large foraminifera. 



Microforaminifera (PI. 11-111, figs 1-4) 



These are the minute forms of foraminifera, which for many years have 

 passed through the fine-mesh screen in oil companies' paleontological labora- 

 tories. Only recently they have been noted in residues prepared for the studies 

 of plant spores and pollen. Whereas, foraminiferal studies have been confined by 

 most workers in oil exploration to the small foraminifera whose average size is 

 500 microns (0.5 mm.), the microforaminifera have size ranges from 50 to 150 

 microns. Unlike the small foraminifera, the microforaminifera have to be 



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