PROCEDURE IN PALEONTOLOGICAL EXAMINATION OF SUBSURFACE SECTIONS

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).

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ECOLOGY OF RECENT ORAMINIFERA

ECOLOGY OF RECENT ORAMINIFERA

The distribution of foraminiferal taxa is influenced by many different factors. Although many authors consider water depth the most significant one, water depth specifically is not the main variable, the controlling factor being the various physical and chemical conditions associated with depth. Typical factors are temperature and temperature variability, light availability, sedimentation rate, bottom characters, energy conditions and pressure.
Studies of recent foraminiferal ecology have provided numerous distinct criteria by which many depositional environments can be characterized and which can be applied to fossil assemblages from sedimentary rocks. Some the main variables can be summarized as follows:
1. The total number of species and of individual increases away from the shoreline, and with increasing depth of water, to maximum values on the outer shelf and in the upper bathyal zone.
2. Porcellaneous forms show their present diversity in shallow, nearshore environment.
3. Arenaceous foraminifera with simple interior wall structure become dominant in shallow waters or in intertidal areas. The percentage occurrence of these arenaceous forms reaches a maximum near the effluence of rivers.
4. Calcareous foraminiferal tests become smaller and thinner near sources of fresh water. In carbonate rich environments, tests may reach a large size and be very robust
5. The percentage occurrence of the most common species in a foraminiferal population relates to the variability of the environment. As marginal marine conditions are approached, environmental parameters become more pronounced resulting in a tendency towards single species dominance in the most unfavorable environment.
6. Planktonic forms occur most abundantly within the outer neritic and deeper waters. Under ideal sedimentation conditions, especially in clastic deposits, planktonic foraminifera can show a more or less regular increase in abundance in depth.
7. Arenaceous taxa with labyrinthic wall structures occur most abundantly in bathyal or deeper waters. In sediments deposited below the calcium carbonate compensation depth (CCD) these forms may become dominant since the calcareous shells of other foraminifera are dissolved.

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APPLICATION TO ANCIENT ENVIRONMENTS

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.

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benthic marine environment

The benthic marine environment compiled from Hedgpeth (1957) and INGLE (1980) :

Non-marine (supralittoral) – top delta, alluvial plain etc.
Lorenz (1863) proposed supra-littoral for the term above extreme high water (Hedgepeth, 1957). This zone include top delta, alluvial plain etc.

Transitional zone (littoral) - brackish water, bay, marsh, lagoon, estuaries, deltas, beaches, tidal flats.
The term littoral has been used to mean the region between high-tide and low-tide level (Forbes and Hanley, 1853). Brakish water, bay, marsh, lagoon, estuaries, deltas (except top delta) are grouped into the transitional zone.

Neritic zone (0 to 100 m)
The region between the low-water line and the edge of the continental shelf, or betweeen shore and extends downwards to about 20 to 50 fathoms (about 36.54 – 91.35 m) is continental shelf. The term for the aquatic environment overlaying this region, the water over the continental shelves, is “neritic”, proposed by Haeckel to complement “oceanic”, the environment of the “blue water”. But paleontologist, working with the remains of the benthic organism of shallow seas, have applied the term “neritic” to the environment of the bottom itself (Hedgpeth, 1957).

Inner neritic - shallowest open marine (inner shelf or shallow inner sub-littoral) environment between 0 to 20m (approx. 0 to 66 feet)
Effective fair weather wave base at about 10m to 15m (Figure 3 of Ingle, 1980).
The lower boundary of this depth zone is roughly equivalent to the maximum effective weather wave base.

Middle neritic - intermediate open marine (middle shelf or deep inner sublittoral) environment - 20 to 100m (approx. 66 to 328 feet).
The region between water and the maximum depth of large attached algae or of reef-coral growth is called as middle neritic (Hedgepeth, 1957). Everage lower boundary of photic zone at tropic marine at about 100m to -120m depth.
The lower boundary of this depth zone is roughly equivalent to the lower boundary of photic zone at tropic marine, or to the maximum depth of large attached algae or of reef-coral growth.
Everage lower boundary of reefal ecologic is 50m. There are the moderate to abundant large foraminiferes associate to the reef. Bellow of this depth, they are very rare (Ingle, 1980). So that, we divided the middle neritic zone into two zones, shallow middle neritic (20m-50m) and deep middle neritic (50m-100m).

Outer neritic zone - deeper open marine (outer shelf or outer sublittoral) environment - 100 to 200m (approx. 328 to 656 feet).
Average maximum depth of continental shelf is 200m depth (Ingle, 1980). The term outer shelf (outer neritic or outer sublittoral) applied to the region between the maximum depth of large attached algae or of reef-coral growth to the average maximum depth of continental shelf.

Upper bathyal zone, -200 to -500m (approx. 656 to 1640 feet).
Bathyal have frequently been applied to the environment of the continental slope down to 2000m, but its usage has not gone much beyond the diagrams in the textbooks.

Middle bathyal zone, -500 to -2000m

Upper middle bathyal zone (500-1500m)
The upper middle bathyal zone is effectively coincides with the oxygen minimum zone (500 to 1500m) (Ingle, 1980), a parcel of water containing only marginal amounts of dissolved oxygen (0.1-0.5 ml/l) due to the oxidation of the organic debris derived from high productivity in the overlying photic zone (Richad, 1957 in Ingle ,1980).

Lower middle bathyal zone (1500-2000m)
Continental slope and bathyal have been applied to the environment of the continental slope down to maximum 2000m (Hedgpath, 1957).

Lower bathyal zone, -2000m to -4000
Bellow 2000m (roughly between 2000m and 3000m, and extends downward to about 6000 m) the temperature is never above 4oC, (Hedgpeth, 1957). The base of lower bathyal zone is delimited by the top of the CCD in mid-latitude oceanic areas today (Ingle,1980)

Abyssal zone, -4000m to 6000m.
Critical deeper boundaries include the lysohaline (LCD) and the calcium carbonate compensation depth (CCD) at 3000 to 4000m (Berger, 1970, 1974); top of the abbysal zone (4000m) is essentially coincide with the top of the CCD in mid-latitude oceanic areas today (Ingle, 1980). Below 4000m, the living and presence of foraminifera was under the influence of CCD. The calcareous foraminiferal tests are dissolved (Berggren, in Haq and Boersma, 1980; Ingle, 1980).
The temperature of the water body at roughly between 2000 and 3000meters and extends downward to about 6000 meters is never above 4oC (Hedgpeth, 1980).

Hadal zone, more than –6000m.
The distinct characteristic of the fauna of the deep trenches suggest that Hedgpeth is dealing with another environment region, for which Bruun has proposed the term “hadal” (from Hades). The term for the region bellow 6000m depth.

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GONDWANAN PALYNOMORPHS FROM THE PALEOGENE SEDIMENTS
OF EAST JAVA: ?THE EVIDENCE OF EARLIER ARRIVAL

BY EKO BUDI LELONO*)

*) R&D Center for Oil and Gas Technology “LEMIGAS”, Jakarta

ABSTRACT
The palynological investigation of the Paleogene sediments is based on cutting samples collected from the exploration wells which are drilled in East Java area. The occurrence of pollen Meyeripollis naharkotensis and spore Cicatricosisporites dorogensis in the upper well sections suggests the pollen zone of Meyeripollis naharkotensis which is equivalent to Oligocene age. Meanwhile, the occurrence of pollen Proxapertites operculatus and spore Cicatricosisporites eocenicus below Meyeripollis naharkotensis zone indicates the appearance of Proxapertites operculatus zone within the lower sections which is equivalent to Eocene. In addition, foraminiferal and nannoplankton analyses confirm the Oligocene-Eocene age by identifying the occurrence of letter stage of Te4-Tb and nanno zone of NP20-NP25. The appearance of the Gondwanan/ Australian elements including Dacrydium and Casuarina with common and regular occurrences throughout the studied sections are controversial as these pollen are recorded in the younger sediments (Early Miocene) of other areas such Java sea, South Sumatra and Natuna sea following the collision of the Australian plate and the Sundaland in the latest Oligocene. Further more, the absence of these palynomorphs within the Paleogene sediments of Central Java and South Sulawesi strengthens the above assumption. Therefore, in regard to East Java, the appearance of Dacrydium and Casuarina may indicate earlier arrival of the Gondwanan/ Australian fragment in this area compared to that in other areas of Indonesia.

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