PROCEDURE IN PALEONTOLOGICAL EXAMINATION OF SUBSURFACE SECTIONS
Depositional environments of subsurface geologic sections of interest are interpreted primarily from occurrence of species and assemblages of foraminifera. Criteria for interpretation begin with studies of Recent foraminifera obtained from plankton tows and bottom samples collected from sea floor traverses and supplemented by similar studies in the other areas. This method permits correlation of species to environment of deposition, and information on water depths, salinities, temperatures and other significant factors may be obtained. Many Recent foraminifera are either closely related or identical to Tertiary forms; thus, knowledge of depositional environment of modern species may be used to reconstruct ancient environments from occurrences of fossil foraminiferal species. However, associated stratigraphic factors, such as lithology, indicate that some fossil species in the geologic section occurred at shallower water depths than to their modern counterparts in the Recent. This emphasizes the fact that all available stratifgraphic information must be used in making paleoecologic interpretation. By this method, information has been accumulated to show that each environment, from the up-dip transitional to the abyssal zone of the ocean deeps, has characteristic species and assemblages of foraminifera. Only few genera and species are actually restricted to one environment. However, the over-all association or assemblage, together with relative abundance or scarcity of significant genera and species, will usually permit sufficiently accurate interpretation of the representative environment. For paleontological purposes, the species with the greatest tolerance for many different environments and uniform extinction makes the best “time correlation” marker fossil, but it obviously a poor paleontologic indicator. Despite these reservations, in Gulf Coast paleontology studies, collections of Recent species in Gulf Mexico are closely related to fossil species and of most use. As pointed out by Tipsward et al., (1966), excerpted from Crouch (1955), world-wide species distribution data are not applicable to all local stratigraphic problems, and widely scattered ecology data may be confusing.
Paleoecological studies of well sections are usually made simultaneously with paleontological determinations. For paleontological purpose, species of foraminifera and their relative abundance in each sample of drill cuttings are recorded, along with lithologic description and notation of other fossil. After the complete set of samples has been examined from the top to bottom, the detailed record of fossil occurrences and lithology is reviewed and a paleoecological summary is prepared. This summary gives the depositional environment represented according to depth, the approximate location with respect to the ancient shoreline, the cyclic nature of the section (transgressive or regressive), a brief lithologic description, and the geologic age.
The paleoecology may then be plotted on electriclogs and incorporated in cross section and maps to assist exploration as previously described (Tipsward et al., 1966).
Depositional environments of subsurface geologic sections of interest are interpreted primarily from occurrence of species and assemblages of foraminifera. Criteria for interpretation begin with studies of Recent foraminifera obtained from plankton tows and bottom samples collected from sea floor traverses and supplemented by similar studies in the other areas. This method permits correlation of species to environment of deposition, and information on water depths, salinities, temperatures and other significant factors may be obtained. Many Recent foraminifera are either closely related or identical to Tertiary forms; thus, knowledge of depositional environment of modern species may be used to reconstruct ancient environments from occurrences of fossil foraminiferal species. However, associated stratigraphic factors, such as lithology, indicate that some fossil species in the geologic section occurred at shallower water depths than to their modern counterparts in the Recent. This emphasizes the fact that all available stratifgraphic information must be used in making paleoecologic interpretation. By this method, information has been accumulated to show that each environment, from the up-dip transitional to the abyssal zone of the ocean deeps, has characteristic species and assemblages of foraminifera. Only few genera and species are actually restricted to one environment. However, the over-all association or assemblage, together with relative abundance or scarcity of significant genera and species, will usually permit sufficiently accurate interpretation of the representative environment. For paleontological purposes, the species with the greatest tolerance for many different environments and uniform extinction makes the best “time correlation” marker fossil, but it obviously a poor paleontologic indicator. Despite these reservations, in Gulf Coast paleontology studies, collections of Recent species in Gulf Mexico are closely related to fossil species and of most use. As pointed out by Tipsward et al., (1966), excerpted from Crouch (1955), world-wide species distribution data are not applicable to all local stratigraphic problems, and widely scattered ecology data may be confusing.
Paleoecological studies of well sections are usually made simultaneously with paleontological determinations. For paleontological purpose, species of foraminifera and their relative abundance in each sample of drill cuttings are recorded, along with lithologic description and notation of other fossil. After the complete set of samples has been examined from the top to bottom, the detailed record of fossil occurrences and lithology is reviewed and a paleoecological summary is prepared. This summary gives the depositional environment represented according to depth, the approximate location with respect to the ancient shoreline, the cyclic nature of the section (transgressive or regressive), a brief lithologic description, and the geologic age.
The paleoecology may then be plotted on electriclogs and incorporated in cross section and maps to assist exploration as previously described (Tipsward et al., 1966).