APPLICATION TO ANCIENT ENVIRONMENTS
The study of paleoenvironments is based on the concept of uniformitarianism i.e. “The present is the key to the past”.
Care must be taken. Within young Tertiary rocks there is very close correspondence between fossil and recent foraminiferal assemblages. But in older sediments, of Early Tertiary to Mesozoic age, gross differences may occur as in these periods certain groups of foraminifera common in modern environments had not appeared and other group existed which have no modern counterparts. Examples are the rotaliids, which are essentially a tertiary development, and Cretaceous Globotruncana which are thought to have had similar, but not identical, requirements to modern planktonics. The ecology of extinct groups can be determined by carefull study of sedimentology and analysis of large numbers of fossil assemblages.
Additional complications may be added since many foraminifeal tests may be transported vast distances before actually becoming incorporated into a sediment, thus two assemblages : biocoenosis (living assemblage) and thanatocoenosis (dead assemblage). For example of transportation which may result in mixtures of faunas are as follows :
1. Reworking of foraminifera into younger
rocks.
2. Contemporaneous transport :
· as suspended load; the empty shells of dead foraminifera can be transported hundreds of kilometers offshore, resulting in the presence shallow water forms in deep water deposits.
· by currents; this may be reflected by the presence of size sorted or species sorted assemblages.
· by turbidity currents or slides; resulting again in the presence of shallow water forms in deep water environments.
· wind; empty shells of dead foraminifera may blown land wards.
Diagenesis may also seriously effect to fossil assemblages; solution of the calcareous tests or calcareous cement of arenaceous forms may result in the complete absence of a fossil fauna.
When material from well sections is studied and cuttings are examined, contamination may occur from higher in a well section in the form of caving. Caving may be recognized by differences in preservation, color of the degree of abrasion of foraminifera. Often, however, it may not be at all clear whether foraminifera are in situ, or caved. Analysis of carefully selected core material or sidewall cores from a sequence in question would provide an indication of the true in situ assemblage.
The study of paleoenvironments is based on the concept of uniformitarianism i.e. “The present is the key to the past”.
Care must be taken. Within young Tertiary rocks there is very close correspondence between fossil and recent foraminiferal assemblages. But in older sediments, of Early Tertiary to Mesozoic age, gross differences may occur as in these periods certain groups of foraminifera common in modern environments had not appeared and other group existed which have no modern counterparts. Examples are the rotaliids, which are essentially a tertiary development, and Cretaceous Globotruncana which are thought to have had similar, but not identical, requirements to modern planktonics. The ecology of extinct groups can be determined by carefull study of sedimentology and analysis of large numbers of fossil assemblages.
Additional complications may be added since many foraminifeal tests may be transported vast distances before actually becoming incorporated into a sediment, thus two assemblages : biocoenosis (living assemblage) and thanatocoenosis (dead assemblage). For example of transportation which may result in mixtures of faunas are as follows :
1. Reworking of foraminifera into younger
rocks.
2. Contemporaneous transport :
· as suspended load; the empty shells of dead foraminifera can be transported hundreds of kilometers offshore, resulting in the presence shallow water forms in deep water deposits.
· by currents; this may be reflected by the presence of size sorted or species sorted assemblages.
· by turbidity currents or slides; resulting again in the presence of shallow water forms in deep water environments.
· wind; empty shells of dead foraminifera may blown land wards.
Diagenesis may also seriously effect to fossil assemblages; solution of the calcareous tests or calcareous cement of arenaceous forms may result in the complete absence of a fossil fauna.
When material from well sections is studied and cuttings are examined, contamination may occur from higher in a well section in the form of caving. Caving may be recognized by differences in preservation, color of the degree of abrasion of foraminifera. Often, however, it may not be at all clear whether foraminifera are in situ, or caved. Analysis of carefully selected core material or sidewall cores from a sequence in question would provide an indication of the true in situ assemblage.