About This Item
- Full TextFull Text(subscription required)
- Pay-Per-View PurchasePay-Per-View
Purchase Options Explain
Share This Item
A simple, easily obtained ratio of planktonic/benthonic foraminifera is shown to yield a considerable amount of geologic information. The utility of the ratio is demonstrated by application to an interpretation of regional environments in Upper Cretaceous strata in East Texas, southern Arkansas, and northern Louisiana. Planktonic/benthonic ratios plotted against stratigraphic horizon in the form of a well log appear to reveal the developmental history of barriers to oceanic circulation. Contoured ratio values for the Upper Cretaceous rocks studied show relationships to structural and lithofacies patterns. The contoured ratio surfaces indicate the general patterns of oceanic circulation in the study area during the Late Cretaceous and their change with time.
During an environmental study of the Gulfian Cretaceous strata of northeast Texas, southern Arkansas, and northern Louisiana, an attempt was made to obtain and analyze both stratigraphical and paleontological information. Selection of the type of paleontological information to be employed was difficult. Initially, an effort was made to consider quantitatively on the specific and generic levels, areal variations in the foraminiferal population of each of several stratigraphic units. This approach proved enormously time-consuming because of the large area and thick section under study; and although it promised to provide a wealth of information, some simpler method of obtaining paleontological information was required. It was decided to accumulate data on the relative abundances of cert in large groups of foraminifera that were easily identifiable yet characterized by adaptations for particular modes of existence. Abundance counts of planktonic and benthonic foraminifera provided the simplest and most rapidly obtainable paleontological parameter.
Measurements of the ratio of planktonic to benthonic foraminifera have several advantages over counts of specific or generic groupings. First, the two types of organisms are generally readily separable by an operator who has had only a few hours of instruction. Secondly, counts based on these two groups are relatively objective because specific determinations of juvenile and poorly preserved specimens are not required. Thirdly, because only two groups need be distinguished, the counts can be made rapidly. Finally, the use of large groups, each characterized by a pronounced environmental preference, makes it possible to obtain useful information for rocks so old that evolution might have altered in detail the environmental significance of groups at the specific or generic level. The Cr taceous origin of planktonic foraminifera appears to be the only time limit imposed upon the method.
The major disadvantage of the use of the ratio of planktonic to benthonic foraminifera is the loss of resolution to be expected as larger and larger taxonomic units are employed in paleoecological work. In the study reported here, this ratio was used as a means of obtaining information about the large-scale regional environmental changes rather than those of a more local nature. Stratigraphic and structural information of a similarly regional nature was also accumulated to serve as a framework within which paleontological information could be evaluated.
Organisms, sediments, and structural elements exist in the sea as interrelated components in a dynamic depositional environment, not as isolated entities. It is an unfortunate but common practice in geological work to study organisms, sediments, and structures separately. The study
reported here was designed to consider jointly these three classes of information for the Gulfian Cretaceous rocks of northeast Texas, southern Arkansas, and northern Louisiana and to reconstruct the depositional environment as completely as possible. This integrated approach has been followed insofar as possible during the course of the work but problems relative to the preparation of maps and manuscripts have required essentially separate presentation of the three classes of information.
In their initial phases, the paleontological, stratigraphical, and structural approaches to a problem of environmental interpretation are the same. It is essential in pursuing each avenue of approach that a stratigraphic framework be developed within which the data may be appropriately distributed for comparison in both space and time. Fundamental to such a framework is a development of sound and consistent correlations to permit the recognition of synchronous events in distant areas. The correlations used in this study were based on closely spaced electrical logs. More than 4,000 logs were used in the total area covered. Where possible, the type units of the surface sections in southern Arkansas and northeast Texas were projected into the subsurface and carried on the electrical logs throughout the study area.
The electrical log correlations were made and checked within roughly circular traverses involving six to eight closely spaced wells. If the traverse closed, another correlation loop was developed to extend the correlation network. If a traverse failed to close, additional wells were studied until the problem was resolved and the traverse could be closed. Larger loops involving several townships or even several counties were made and checked periodically to further assure internal consistency in the correlations. Once the correlations were considered to be as accurate as possible, sampling for paleontological and stratigraphical study was begun.
In the course of this environmental study, rocks representing the whole Gulfian Cretaceous section were studied. For the sake of simplicity, only the beds overlying the Austin unconformity are considered in this report. This restricted portion of the Gulfian section has been selected because it forms a more or less natural major depositional unit. The Austin and post-Austin Gulfian beds represent a transgressive depositional phase and, in many areas, rest unconformably on older rocks. The unconformity at the base of the Austin is especially pronounced in the area of the Sabine uplift and around the margins of the Upper Cretaceous depositional area. In several areas, however, especially toward the downdip limit of penetration of these beds in Louisiana, there is little evidence of an u conformity and it appears that the magnitude of the erosion surface beneath the Austin decreases markedly downdip.
The major transgressive episode which began with Austin deposition seems to have proceeded with relatively minor interruptions through the remainder of the Cretaceous. Over a large part of the area no important stratigraphic break separates Cretaceous beds from the overlying Midway formation of Paleocene age. A stratigraphic break of some significance is apparent in the south-central part of the map area where a considerable section of the youngest Upper Cretaceous beds has been removed by pre-Midway erosion.
Gulfian Cretaceous strata below the Austin unconformity have undergone considerable erosion and are now represented by essentially isolated rock bodies, one in Louisiana and the other in Texas on the two sides of the Sabine uplift. Because of this loss of section, principally over the Sabine uplift, the pre-Austin beds are less amenable to a regional environmental study than are those above the Austin unconformity. Most of the beds above this unconformity are widespread throughout the study area and though in places they are missing because of intra-Cretaceous erosion, the volume of lost beds is relatively small; and the erosion does not destroy the essential continuity of strata between major areas. A further advantage of this part of the section accrues from the frequency with which the beds have been penetrated by the drill.
In the area studied, beds overlying the Austin unconformity range in thickness from 1,500 to 5,500 feet. Clearly, environmental interpretations based on a section of such magnitude would be limited to the most persistent features of the region. For more detailed study, as well as for the possible recognition of evolution in the regional environment through time, it was necessary to increase resolution by subdividing the stratigraphic section. Initially, an attempt was made to project from the outcrops in Arkansas and northeast
Texas the classical units of surface stratigraphy and to follow them in the subsurface by means of electrical logs. It was found that some of the surface units could be readily recognized in this way whereas others could not. The final procedure in subdividing the section, therefore, was to carry only those units which could be precisely and consistently followed throughout the area involved. This procedure has resulted in the use of four subunits within the beds under consideration. From the base of the section toward the top, these units are as follows.
1. Base of the Austin to the base of the Buckrange
2. Base of the Buckrange to the base of the Annona
3. Base of the Annona to the top of the Nacatoch
4. Top of the Nacatoch to the top of Upper Cretaceous
These units, though still rather gross, have the advantage of being recognizable over a very broad area and of subdividing the section into a sufficient number of subunits to permit a recognition of changing environmental conditions.
The objective of our paleontological work, as it finally developed, was to obtain quantitative information about the distribution of foraminifera which could be used, in combination with stratigraphic and structural data, to deduce ancient environments.
The foraminifera were subdivided into planktonic and benthonic types (Grimsdale and Morkhoven, 1955). The benthonics were further subdivided into calcareous-shelled benthonics and arenaceous-shelled benthonics. This classification provided three easily recognizable, relatively long-ranging foraminiferal groups. The ostracods were counted as a fourth group. Because both ostracods and arenaceous benthonic foraminifera are rare in the samples studied, the results are reported here in terms of the data for total benthonic foraminiferal population and for the planktonics.
All paleoecological studies heavily depend on knowledge of, or assumptions about, processes, conditions, and responses existing at the present. In short, a model must be used which provides a basis for interpretation of the particular facets of ancient environments which are under study. The model used in this study and the observations and assumptions underlying it are presented in the following paragraphs together with an estimate of the kinds of regional environmental information that could be obtained.
Planktonic foraminifera live primarily in the surficial layers of the open ocean in present-day seas (Schott, 1935). Because they can not significantly control their movements, they move with the water masses they inhabit and are at the mercy of currents. Currents distribute over the continental shelves open-ocean water masses, carrying with them as an exotic faunal element a vast population of planktonic foraminifera. The abundances of planktonic foraminifera in the waters of the continental shelves provide a measure of the transport and mixing of allochthonous and autochthonous water masses (Phleger, 1960).
Upon death, the calcareous tests of planktonic foraminifera fall to the bottom and become incorporated in the accumulating sediment. Where the supply of planktonic tests is large, as it should be along the course of a current carrying open-ocean waters across the shelf, one would expect to find the bottom sediment rich in such tests. If no other factors interfered, the abundance of planktonic foraminiferal tests in bottom sediments could be used to determine the direction and extent of transport of open-ocean waters across the continental shelf. Unfortunately, this situation is complicated by variations in the sedimentation rate of the other clastic materials forming the bulk of the sediment. The effect of variation in the rate of clastic sedimentation is readily seen if the tests of lanktonic foraminifera are visualized as falling from suspension at a more or less constant rate. Assume, as a hypothetical case, that in a particular region the tests of planktonic foraminifera fall to the bottom and accumulate on a sediment surface at a rate of 1,000 per cm2 per year. Assume further that the sedimentation rate for other clastics varies by a factor of ten across the region so that the eastern edge receives, each year, a blanket of sediment one centimeter thick while the western edge receives a 10-centimeter blanket. The result, in the east, will be 1,000 planktonic tests distributed through each cubic centimeter of sediment (1,000 tests per cm3). In the west, however, 1,000 tests
will be found distributed through 10 cm3 (100 tests per cm3). If the 10-fold difference in the clastic sedimentation rate were not known or could not be evaluated, one might conclude the two sample areas vary greatly in their proximity to the open ocean's source of plankton. It thus becomes evident that mere abundance of planktonic tests in bottom sediments can not, in all instances, provide usable information about the superjacent water masses. Information of this kind can be provided only by some parameter which avoids or at least minimizes the effects of variation in the rate of clastic sedimentation.
A measure independent of variations in the clastic sediment rate might be obtained if the abundance of planktonic foraminifera tests could be compared with the abundance of some other class of objects equally influenced by the sedimentation rate. A suitable class of objects for such a comparison would appear to be the tests of benthonic foraminifera endemic to the shelf. If, as appears likely, the production rate of benthonic tests is relatively constant, their abundance in the sediment would be affected by variation in the clastic sedimentation rate in precisely the same manner as the planktonic tests. If this assumption is correct, a ratio of planktonic to benthonic foraminiferal tests could provide a measurement in which variation in the rate of clastic sedimentation could be negle ted.
If a ratio of planktonic to benthonic tests can be used to avoid the complications produced by variation in sedimentation rate, it must still be demonstrated that meaningful information amenable to interpretation in terms of environmental conditions can be obtained. It is acknowledged, generally, that the abundance of planktonic tests should decrease with increasing distance from their source in the open sea. We have assumed that the distribution of benthonic foraminifera is more or less constant across the shelf. This assumption should be considered both in terms of its reasonableness and in terms of its importance for the use of the ratio measurements. Five distribution patterns of benthonic foraminifera appear possible.
1. Random variations in distribution
2. Increasing abundance away from shore
3. Increasing abundance toward the shore
4. Essentially uniform abundance across the shelf
5. More complex but regular patterns
Regardless of which situation actually exists, useful data could be gathered if any gradients in abundance were small: however, if strong gradients were present, the existence of distribution patterns 1 or 2 would make the method difficult or impossible to apply. Distributions 1 or 5 would substitute various regular or random fluctuations in abundance for fluctuations caused by variation in the clastic sedimentation rate. Distribution 2 would produce a gradient in the same direction as the planktonic gradient. Under such a condition, it would be conceivable that a planktonic to benthonic ratio could remain constant or vary only slightly across the shelf, despite the introduction of large volumes of open-ocean water and associated plankton. The remaining two distributions (3 and 4) wou d permit useful comparisons. Distribution 4 would justify the greatest confidence because the ratio data would bear a direct and simple relationship to the amount of mixing of allochthonous and autochthonous water masses across the shelf.
No comprehensive studies of the continental shelf distribution of foraminifera known to us have considered as entities the large groups employed here and at the same time introduced definitive independent evidence bearing on the rate of clastic sedimentation over any large region. Thus, it is not possible to state as observational fact which of the five suggested distributions prevails. Instead, we can only attempt to deduce the conditions that are most likely to exist. It appears that the availability of energy in the form of food is one of the major factors in controlling the abundance of benthonic foraminifera. Type of bottom sediment, salinity, and many other factors may control distribution at lower taxonomic levels, but the availability of food results in the presence of organis s to consume it and among the benthonic foraminifera are types adapted to many kinds of bottom conditions. The availability of adequate food probably prevails across most of the continental shelf since it is overlain by a prism of water in which photosynthetic organisms can flourish. In addition, the water is sufficiently shallow to insure that much undecayed food reaches the bottom. The possibility still remains that food distribution is heterogeneous; but because a fixed condition is not likely to prevail across the continental shelf for long periods of time, strong distributional gradients seem unlikely. Since it appears that gradients sufficiently
pronounced to interfere with the use of planktonic to benthonic ratios are not likely to exist, it seems probable that such a measure could be usefully applied at least to Recent sediments.(FOOTNOTE *)
APPLICATION OF MODEL
It appears by analogy to supposed present-day conditions that the ratio of planktonic to benthonic foraminifera might provide regional environmental information relative to the mixing of ancient water masses, the position of the open ocean, the direction to the shore line, and the position, nature, and history of barriers to oceanic circulation.
If the model thus outlined is to be applied usefully to ancient rocks, it must be demonstrated or assumed that the present conditions on which the model is based were the same or little changed in the past. One immediate limitation is imposed by the time range of planktonic foraminifera which did not become abundant until the Cretaceous time. Lacking any evidence to the contrary and having the morphology of the tests of ancient planktonic foraminifera to attest to a way of life similar to that of living forms, we are probably justified to assume that they lived in the open ocean and drifted shelfward with the currents. Their distribution should be the same as that of their modern counterparts.
The assumption that the distribution of present-day benthonic foraminifera is rather uniform across the continental shelf is based on the availability of solar energy in the water overlying the shelf and the existence of photosynthetic plants. Neither of these conditions has changed for an immensely long time and certainly not since the beginning of the Cretaceous. The assumption that, in the past, the distribution of benthonic foraminifera was identical or very similar to that deduced for today seems justified.
Since it appeared that the underlying assumptions regarding past distribution of both planktonic and benthonic foraminifera are reasonable, a test of the application of planktonic to benthonic foraminifera ratios on ancient rocks was justified. Areal variations in the ratio within time-rock units should be susceptible to interpretation in terms of ancient current circulation and of barriers to such circulation. The data here presented have resulted from a test of planktonic to benthonic foraminifera ratios in the environmental interpretation of Gulfian Cretaceous rocks. The data offer empirical evidence of the utility of the measurement in environmental studies.
In order to have adequate sample control, it became apparent that cuttings rather than cores would have to be used in this study, and a way was sought in which they could be handled in a minimum amount of time and with a minimum loss of resolution. Because of time limitations, it was not possible to actually log and pick the cuttings from each well to obtain representative samples. Instead, the cuttings were laid out in watch glasses and portions were taken for study where the lithology of the cuttings agreed with the lithology of the interval interpreted from the electrical and sample log. This procedure undoubtedly resulted in the inclusion of some caved material in the samples studied. Wells in which a severe caving problem was recognized were not used.(FOOTNOTE ^dagger)
The samples selected for study were prepared by a standard treatment and examined under a binocular microscope. Tallies were recorded on a laboratory counter. To determine the value to be used for one of the stratigraphic units being studied at a given control point, the value for each sample from the unit was weighted according to the number of feet of section represented and an average value for the entire unit was obtained (see Appendix B). Other than the omission of barren intervals (usually coarse-grained sandstone) from the calculations, the effects of lithology were not considered.
In this study, we counted a total of 640,000 microfossils in 3,719 samples. Of the total of 3,719 samples, we found 455 to be barren or so poor in microfossils as to be worthless for our purposes. In 2,935 samples, we were able to count more than 100 microfossils; and the average count for this group was 211 specimens per sample. In 329 samples, less than 100 microfossils were
FOOTNOTE *. Phleger (1960), p. 188-189, discusses the "standing crop" of foraminifera, indicating that there is considerable areal variability. He indicates that data are not available to fully evaluate the affect. Since he considered living individuals, while we are concerned with dead populations subject to current transport and the averaging effect of both our sampling methods and the long time intervals involved, it is considered that the effects of ephemeral local variability are minimized.
FOOTNOTE ^dagger. For a list of the wells used and their locations, see Appendix A.
APPENDIX A. WELLS PROCESSED FOR PALEONTOLOGICAL DATA
counted; and for this group, the average count was 54 specimens per sample.
RESULTS AND DISCUSSION
DATA IN LOG FORM
Early in this test when data were available for only a few control points, an attempt was made
APPENDIX B. CALCULATION OF WEIGHTED PERCENTAGES
1. Barren intervals are excluded from the calculations.
2. Average percentage of planktonics to total foraminifera are figured by weighting the observed data as follows.
Example: A 300 foot interval has the following observed percentages: 100^prime--50%; 50^prime--40%; 50^prime--60%; 100^prime--30%.
The average percentage for the entire interval is expressed as:
The foregoing is the weighted average of planktonic foraminifera to the entire foraminiferal population for 300-foot interval.
to derive information about depositional environments from the relative abundance of planktonic and benthonic foraminifera presented in the form of a log. The abundance of planktonic foraminifera was expressed as a percentage of the total foraminifera population and plotted against the stratigraphic position of the sample. In this way, variations in relative abundance throughout the entire Gulfian Cretaceous section could be studied for a particular well. It was also possible to compare the planktonic percentage logs of different wells and to search for characteristics indicative of similarities or differences in the depositional environments represented.
Two wells almost 200 miles apart were selected to test the variation in the percentage of planktonic foraminifera observed in areas of markedly different depositional environment (Fig. 1). One well, the Burnett Producing Company, No. A-1, Archie Hale, is located in Little River County, Arkansas, not far from the general position of the depositional edge of the Gulfian Cretaceous. The other well, the American Liberty, No. 1, Cameron Heirs, is located in Polk County, Texas, near the downdip limit of penetration of the Gulfian Cretaceous and close to the probable position of
the edge of the Gulfian continental shelf (see Fig. 4 for location of wells). It was expected that the Polk County well would show planktonic percentages significantly higher than those of the well in Little River County. Examination of the logs in Figure 1 shows that this expectation was fulfilled. The Polk County well shows planktonic percentages for the Gulfian section which fall almost entirely at or above the 80 per cent level and average 85 per cent. The Little River County well shows more variation in the observed values, which range from 42 per cent to 75 per cent, and also a far lower average value of 60 per cent. This first test was encouraging as it suggested that measurements of the relative abundance of planktonic and benthonic foraminifera could differentiate extremes of depositional environment.
Another test of the method was made, using samples from two wells about 30 miles apart and presumably representing much the same type of general depositional environment. Both wells are located on the Sabine uplift, a major positive feature of the Gulfian Cretaceous. The test wells were the Stanolind Oil and Gas Company, No. 1, Hattie Cole in Harrison County, Texas, and the Arkansas Fuel Oil Company, No. 1, Jacobs Land Company, Caddo Parish, Louisiana (Fig. 4). The planktonic percentage logs for these two wells are presented in Figure 2. It is evident that the two logs show a great similarity. This excellent correlation attests to the similarity of the depositional environment in the two areas throughout Gulfian time. One can, in fact, go further in the interpretation of these two log with their three synchronous minima separated by more persistent maxima. The minima probably represents times at which it was more difficult than usual for planktonic foraminifera to be carried to the loci of deposition. Because both wells are located on the strongly positive Sabine uplift, it appears likely that the minima reflect times at which the uplift closely approached sea-level and formed an effective barrier to the circulation of water rich in planktonic foraminifera. It seems that these two logs reflect either episodic structural development of the Sabine uplift or brief periods of generally lowered sea-levels which reduced water depth over the uplift.
A choice between the possible interpretations of the two closely spaced planktonic percentage logs required data from control points throughout
Fig. 1. Foraminiferal percentage logs for areas of different environment. Polk County, Texas, well is located close to downdip limit of control and, thus, presumably represents near open-ocean conditions. Little River County, Arkansas, well is close to outcrop and probably represents near-shore conditions. Contrast between high percentages of planktonics downdip, and much lower percentages near shore are evident from logs.
Fig. 2. Foraminiferal percentage logs for areas of similar environment. Logs of these two wells, 30 miles apart and both located on Sabine uplift, show strong similarities and indicate variations in ease with which plankton-rich open-ocean waters reached area.
a larger area. Additional data were therefore gathered; and further tests of the method were made by considering average values for particular stratigraphic intervals as seen regionally in map form. From these maps, it was possible to develop a reasonably comprehensive picture of some of the major features of the depositional environment both in terms of their position and their duration.
DATA IN MAP FORM
The area used in this study was selected because the wealth of control available from drilled wells provided a background of stratigraphic and structural information bearing on regional environments which was independent of the paleontological data and thus available for comparison in the evaluation of the planktonic/benthonic ratios. Areal limits were established on the north and west by the belt of outcrop of the stratigraphic units under consideration. In most areas, the outcrop belt appears to approximate the shore line; but there has, nevertheless, been a significant amount of erosional retreat. The southern limits of the area as studied were established by the downdip limit of well penetration of the Austin unconformity or its equivalent horizon. In general, this downdip limit c incides with the initiation of rapid downdip increases in thickness of the overlying Tertiary units. On the east, the area is arbitrarily bounded by the Mississippi River because studies were not extended into Mississippi.
Within the circumscribed area, there are pronounced structural features (Fig. 3). Three of these features are strongly positive in nature: the Monroe uplift in northeastern Louisiana, and the Sabine uplift lying close to the Texas-Louisiana border, together with the connecting Winn axis. Three important negative areas also are within the region studied: the Tyler basin in East Texas along the western margin of the Sabine uplift, the Interior Salt basin in northwestern Louisiana lying between the north end of the Sabine uplift and the Monroe uplift, and the Desha basin north of the Monroe uplift in southeastern Arkansas.
INTERPRETATION OF DATA
The major positive and negative structural features form a convenient framework for discussion of the ratio data. As many of these features persisted through much or all of the time interval under consideration, it is convenient to discuss the evolution of individual features rather than all features of the area for each stratigraphic interval. An attempt is also to be made to show the interrelations of these structural features as indicated by the ratio data.
Examination of Figure 4, a ratio map for the interval between the base of the Austin and the base of the Buckrange, shows a region of markedly high ratio values along the southern border of the map. In Louisiana, this area of high values is poorly defined because of the sparsity of downdip control, and there are no values as high as those occurring in adjacent East Texas. It is clear, however, that from the southern margin of the map in Louisiana northward, there is a rapid decrease in the importance of the planktonic element in the foraminiferal population. This decrease takes place along an elongate northeast-southwest-trending axis which we have termed the Winn axis. The Winn axis appears to extend from the general area of the Monroe uplift to East Texas. The rapid decrease of rati values as the axis is approached from the south suggests that it may have existed as a positive element connecting the two major uplifts at its ends. Figure 4 suggests that very little water was able to enter the Interior Salt basin across the Winn axis at this time.
The ratio map for the interval from the base of the Buckrange to the base of the Annona (Fig. 5) indicates the continued existence of the Winn
Fig. 3. Major tectonic features of study area. Structural features appear to have strongly influenced circulation patterns in study area during Upper Cretaceous. Most of them show considerable persistence through time interval considered.
axis as a positive feature. As a barrier, it appears to have been somewhat more effective during this interval than during the preceding one. Figure 6, showing the ratio map for the interval from the base of the Annona to the top of the Nacatoch, indicates the persistence of the Winn axis into this time interval. The northeastern end, close to the Monroe uplift, appears to have been very positive; and, over most of the course of the axis, ratio values are less than one.
The ratio map for the interval from the top of the Nacatoch to the top of the Cretaceous (Fig. 7) suggests a rather abrupt alteration of the pattern typifying the older intervals. For this latest part of Late Cretaceous time, it appears that the Winn axis was distinctly less positive and no longer constituted an effective barrier to oceanic circulation.
To summarize the environmental conditions along the southern border of the map area in Louisiana, it may be said that high ratio values were confined on the southern side of a barrier (Winn axis) extending between the southern end of the Sabine uplift and the Monroe uplift. This feature persisted as a barrier to oceanic circulation from the beginning of Austin deposition until the end of the deposition of the Nacatoch. During the last part of Late Cretaceous time, however, the Winn axis no longer constituted an effective barrier to circulation. Open-ocean waters rich in planktonic foraminifera apparently spilled across it into the Interior Salt basin of
Fig. 4. Foraminiferal ratio map for interval from base of Austin to base of Buckrange. Relationships between circulation patterns shown here and structural framework of Figure 3 are evident.
Fig. 5. Foraminiferal ratio map for interval from base of Buckrange to base of Annona.
Fig. 6. Foraminiferal ratio map for interval from base of Annona to top of Nacatoch.
Fig. 7. Foraminiferal ratio map for interval from top of Nacatoch to top of Cretaceous (Arkadelphia).
Louisiana for the first time. This circumstance suggests either that sea-level rose so markedly that the Winn axis no longer constituted an effective barrier, or that this axis, despite its prolonged positive tendency, foundered late in Late Cretaceous time.
The southern edge of the map area as seen in East Texas during the Austin-Buckrange interval (Fig. 4) shows the highest ratio values encoun-countered. Here the values for some wells exceed eight, and a large region is characterized by values greater than four. The situation differs considerably from that in Louisiana where high values terminated along an axis. In East Texas, the high values extend northward for a considerable distance along the western edge of the Sabine uplift, essentially following the structural axis of the Tyler basin (Fig. 8). This tongue of plankton-rich sediments along the axis of the Tyler basin is interpreted as marking the position of a current flowing northward across the shelf. The eastern edge of this current is clearly related to the edge of the Sabine u lift, but the western margin is not clearly related to any well known structural high. Instead, the western margin appears to be roughly defined by the position of the Luling-Mexia-Talco fault system. Concurrent stratigraphic studies clearly indicate that this fault
Fig. 8. Structure contour map on base of Buckrange in East Texas. Note the structural sag in the crest of the Sabine uplift in Panola County, Texas, and DeSoto Parish, Louisiana, which may be related to an apparent current crossing uplift during interval from base of Austin to base of Buckrange (Fig. 4).
system experienced repeated movements during Late Cretaceous time. A zone of locally shallower water may have coincided with its position during the Late Cretaceous; but the available data suggest that still farther west, in an area not now containing Upper Cretaceous outcrops, there was another source of open-ocean planktonic-rich waters.
North of northern Cherokee County, Texas, the iso-ratio contours do not clearly outline any specific current path, though high values continue toward the north and appear to be swinging eastward into southwestern Arkansas around the northern end of the structural uplift. Figure 8 indicates the existence of a structural sag in approximately this position. Despite the inconclusive nature of the evidence for a northward extension of the current, there appears to be rather good evidence for an eastward flow of current across the Sabine uplift and into the Interior Salt basin of Louisiana. Eastward flow over the top of the Sabine uplift appears to have been taking place through Panola County, Texas, and adjacent DeSoto Parish, Louisiana. Figure 8 indicates the existence in this area of a s ructural sag which may have coincided with the course of this current.
Examination of Figure 5, representing the interval between the base of the Buckrange and the base of the Annona, shows a more pronounced current pattern than was previously evident. High ratio values extend north along the entire length of the Sabine uplift. Again, there appears to be evidence for a current crossing the uplift; but this time the crossing seems to have been farther north than during the previous interval. A rather pronounced current appears also to have followed the structural sag previously noted around the north end of the Sabine uplift. Once again, the western margin of this supposed current system shows lower ratio values in a region which appears to coincide roughly with the Luling-Mexia-Talco fault zone. Scattered evidence also exists for this interval which sugg sts the distribution of plankton-rich waters farther west across an area now devoid of Upper Cretaceous rocks.
Data from the interval between the base of the Annona and the top of the Nacatoch (Fig. 6) suggest once more a distinctive current system with a possible crossing of the Sabine uplift in the Panola County area and evidence for a passage of current around the northern end of the uplift into Arkansas and Louisiana. The ratio values at the southern margin of the map appear to be unusually high, possibly indicating the development of deeper water in this region. Low ratio values continue to coincide with the fault zone, along the western margin of the current system, and somewhat higher ratio values again come into the west.
Figure 7, representing the data for the interval between the top of the Nacatoch and the top of the Cretaceous, still shows the suggestion of a current system, though there is much change evident between this interval and those which preceded it. It seems evident that whatever events caused the disappearance of the Winn axis as an effective barrier in Louisiana simultaneously affected the southern end of the Sabine uplift. Very high ratio values now occur in the position of the southern end of the uplift, which was formerly characterized by abnormally low values. The erosion of a considerable thickness of beds in this south-central part of the map region in post-Cretaceous time has considerably obscured the record. West of the major current system, the positive feature associated with the Luling-Mexia-Talco fault system is again evident.
In summary, it may be said of the East Texas region of the map that extremely high ratio values characterize its southern margin and extend northward along the axis of the Tyler basin for a considerable distance. The distribution of the values suggests that a current system flowed northward along the western margin of the Sabine uplift. That the distribution pattern observed is not to be explained by the existence of deeper waters along the axis of the Tyler basin, is suggested by the shale color map presented in Figure 9. This map indicates a gradation from predominantly black shale at the southern margin of the map where planktonic values are high to gray shale over the bulk of the map area. There is no extension of the black shales into the Tyler basin. It would seem, therefore, th t different environmental features control the distribution of planktonic forams and the color of shales. The difference is here interpreted to indicate that deeper waters in which black shale predominated were prevalent along the southern margin of the map area but did not extend into the Tyler basin which, while structurally negative, was an area of relatively shallow water. The sediments, rich in planktonic foraminifera, are believed to have resulted from the presence of a current system and
not from the existence of deep water in this area.(FOOTNOTE *) Through the course of the time interval examined, there appears to have been a strong tendency for the major current system to extend itself farther northward. This tendency can be seen both in the increasingly strong suggestion of current flowing east around the north end of the Sabine uplift and in the northward shift of the eastward-flowing current across the top of the Sabine uplift. The planktonic ratio data indicate rather clearly that the bulk of the open-ocean water reaching the Interior Salt basin of Louisiana came from the west around and across the Sabine uplift, rather than from the south where the Winn axis constituted an effective barrier until most recent Late Cretaceous time. Both the apparent northward ext nsion of the current system and the latest Cretaceous disappearance of the southern end of the Sabine uplift as a barrier to current flow could be interpreted as indicating progressive deepening of the waters over the shelf. That at least some structural forces were active, however, is indicated by the local removal of latest Cretaceous beds over the south-central portion of the map region prior to the deposition of the Midway formation.
Fig. 9. Shale color map of East Texas for interval from base of Austin to top of Cretaceous. Shale color map does not seem to bear close relation to foraminiferal ratio maps, suggesting control of two parameters by different environmental conditions.
FOOTNOTE *. Bandy (1956) has suggested a method based on the number of species of planktonics as a function of depth which might have been brought to bear on this problem but was not tested because species were not identified in this study.
NORTHEAST LOUISIANA AND SOUTHEAST ARKANSAS
The strongly positive Monroe uplift in the northeastern portion of the map area appears to have retained its positive character through the entire Late Cretaceous. Information for the lower two intervals is somewhat difficult to interpret because of severe loss of beds at the unconformity underlying the Monroe gas-rock. Nevertheless, the available data indicate ratio values dropping below 0.25 for the interval between the base of the Austin and the base of the Buckrange. Still lower values are encountered for the interval between the base of the Buckrange and the base of the Annona, and values as low as 0.25 again characterize the area for the interval between the base of the Annona and the top of the Nacatoch. For this latter map (Fig. 6), the information is more complete than for th older intervals. It suggests that the Monroe uplift was an elongate extension of the Winn axis and that, as a positive feature on the sea floor, it may have differed considerably in form from the structural uplift with which the name is generally associated. For the same interval, we see for the first time a suggestion of plankton-rich water entering the region from the northeast. The control here is admittedly very poor, but there is a suggestion of a current which must have flowed up the Mississippi Embayment and then swung westward through the Desha basin around the north side of the Monroe uplift. Whether or not this situation may have occurred earlier can not be determined due to the loss of beds beneath the Monroe gas-rock. The general disappearance of positive structural features as effective current barriers, which characterizes latest Cretaceous time in the southern part of the map area, appears to have affected the Monroe uplift to a less serious degree than either the Winn axis or the Sabine uplift. Our control again is poor; but the suggestion is that the Monroe uplift, although still positive, was more deeply submerged than previously.
In summary, it may be said of the conditions in the northeastern part of the map area that the Monroe uplift persisted throughout this part of Late Cretaceous time as a strongly positive element. It appears to have been most strongly positive in the interval between the base of the Buckrange and the base of the Annona and to have been least strongly positive near the end of Late Cretaceous time, though it was not affected by latest Cretaceous events in nearly so pronounced a fashion as the other positive elements farther south. Sparse control suggests the existence of a current flowing up the Mississippi Embayment, northward around the Monroe uplift and thence westward through the Desha basin and into the Interior Salt basin of Louisiana.
SUMMARY OF RESULTS
The results of this attempt to apply the ratio of planktonic to benthonic foraminifera to a regional interpretation of depositional environments may be summarized in two parts. First, we may consider the results as they affect this particular test area; secondly, as they bear on the utility of the method.
In the test area the planktonic to benthonic ratio maps suggest that deposition in the southern margin of the area took place close to the open ocean and in an environment which communicated freely with the open ocean. A barrier to the circulation of open-ocean waters existed along the southern part of the map area in Louisiana where the Winn axis, extending between the Monroe uplift and the southern end of the Sabine uplift, evidently formed a remarkably effective deterrent to the northward passage of open-ocean waters.(FOOTNOTE *) The existence of this barrier resulted in the entrance of open-ocean waters into the Interior Salt basin not from the south but primarily from the west. Open-ocean waters flowed northward along the western side of the Tyler basin. A rather pronounced curre t system appears to have carried waters north into southwestern Arkansas and eastward at various positions across the crest of the Sabine uplift and thus into the Interior Salt basin of Louisiana. The western margin of this current system appears to have closely coincided with the position of the Luling-Mexia-Talco fault zone which evidently constituted a moderately effective barrier to current flow. West of this fault zone barrier is scattered evidence of a second current system entering the region from the southwest across an area now devoid of rocks belonging to the part of the Upper Cretaceous under study. This information suggests that Upper Cretaceous rocks once extended considerably farther west in that region. In the northeast part of the map area the Monroe uplift was a persiste t positive feature
FOOTNOTE *. It now appears that the Winn axis may approximately coincide in position with a Lower Cretaceous "rudistid reef" trend which may have remained topographically somewhat positive into Late Cretaceous time.
A minor current may have flowed northward up the Mississippi Embayment and westward through the Desha basin to supply some open ocean waters to the Interior Salt basin from the east, toward the end of the Late Cretaceous.
The use of the relative abundances of planktonic and benthonic foraminifera appears to provide information about some parameters of ancient environments of deposition. The method appears to be effective in revealing:
1. Paleocurrent systems
2. Ancient structural or topographic features of the sea floor
3. The former existence of strata across areas now laid bare by erosion
4. The evolution of tectonic and topographic features and of current systems.
Maps based on the areal distribution of foraminiferal ratio values for particular stratigraphic intervals reveal patterns which can be meaningfully interpreted in terms of regional structural patterns which are deduced independently from stratigraphic evidence. It also suggests that our underlying assumptions about the distribution of planktonic and benthonic foraminifera may be reasonably close to the actual distribution. Further, it appears that the necessarily crude sampling techniques forced upon us by the use of cuttings and the lack of time necessary to run detailed sample logs did not badly prejudice the data obtained.
Bandy, O. L., 1956, Ecology of Foraminifera in northeastern Gulf of Mexico: U.S. Geol. Survey Prof. Paper 274-G, p. 179-204.
Grimsdale, T. F., and Van Morkhoven, F. P. C. M., 1955, The ratio between pelagic and benthonic foraminifera as a means of estimating depth of deposition of sedimentary rocks: Proc. 4th World Petroleum Cong., Sec. 1/D, paper 4, p. 473-491.
Phleger, F. B, 1960, Ecology and distribution of Recent Foraminifera: Johns Hopkins Press, Baltimore 18, Md.
Schott, W., 1935, Die Foraminiferen in den Aquatorialen Teil des Atlantischen Ozeans: Deutsche Atlantische Exped. 11, Heft 6, p. 411-616.
End_of_Article - Last_Page 1827------------
(2) Department of Geology, Western Reserve University, Cleveland 6, Ohio.
(3) Pan American Petroleum Corporation, Research Department, Cleveland, Tulsa 2, Oklahoma.
The writers are indebted to many persons for help in the conduct of this study. Particularly, it is desired to acknowledge the assistance of E. E. Hoffman, who made many of the counts, and of J. M. Forgotson, Jr., who assisted us in setting up the stratigraphic framework and in making various statistical tests of the data. J. R. Dodd and P. O. Banks critically read the manuscript and made valuable suggestions for its improvement.
Pay-Per-View Purchase Options
The article is available through a document delivery service. Explain these Purchase Options.
|Protected Document: $10|
|Internal PDF Document: $14|
|Open PDF Document: $24|
Members of AAPG receive access to the full AAPG Bulletin Archives as part of their membership. For more information, contact the AAPG Membership Department at [email protected].