Regional Geology

An estuary is the wide mouth of river or arm of the sea where the tide meets the river currents or flows and ebbs (from Emery and Stevenson, 1957)
Estuaries have areas of quiet water, were sedimentation of stream constributed material takes place. When these deposits fill up the estuary and become intertidal in position, their top surface is called a tidal flat. If covered with vegetation, it is termed a salt marsh and if not, a sand or mud flat. The tropical equivalent is a mangrove swamp.
Estuaries are hyposaline in character and can be subdivided in upper part, subject to the greatest freshwater influence and a lower part connected with the sea. This difference in salinity is reflected in the faunas of both parts:
Upper estuary : Miliammina sp, Ammobaculites sp.
Lower estuary : Miliammina sp., Ammobaculites sp., Ammonia beccarii, Elphidium spp.

Tidal flats develop alonggently clipping sea coasts, with marked tidal rhytms, where enough sediment is available and strong wave action is not present. This may be the case in estuaries, lagoons, bays or behind barrier islands or other sand bars (Reineck and Singh, 1973)
Sediment mostly consist of sands, muds or mixtures of the two. Characteristic foraminifera inhabiting this environment are Ammoni beccarii, elphidium spp., Quinqueloculina sp.
Rotaliids predominate in tidal flat assemblages, miliolids are rare to absent and arenaceous species not common.

Marshes and mangrove swamps

Tidal marshes and mangrove swamps represent transitional regions between marine/brackish water and terrestrial environments. Tidl marshes occur in temperate areas, whereas the mangrove swamp is characteristic for tropics

Murray (1972) lists the following characteristics of these environments :

1. It is developed in the upper part of the intertidal zone
2. During much of each tidal cycle it is exposed to the atmosphere
3. It spends a smaller part of the tidal cycle submerged beneath sea or estuarine water
4. It is subject to the more dramatic fluctuations in temperature of the atmosphere
5. Fresh water is periodically introduced from rain showers
6. There is usually high organic productivity by the marsh plants and microflora
7. The fauna comprises few permanent indigenous species although migratory terrestrial animals are sometimes common

Tidal marshes can be subdivided in three groups according to salinity;

1. Hyposalina marshes
2. Normal marine marshes
3. Hypersaline marshes

Species diversity is highest in hyposaline marshes, although the general diversity is low.

The hyposaline marshes are characterized by the predominance of arenaceous species (Miliammina sp., Ammotium sp., Trochammina inflate) and rotaliids ( Elphidium spp.) and the absence of miliolids

Normal marine marshes are inhabited by dominantly arenaceous species with minor miliolieds and rotaliids.

In hypersaline marshes the percentage of arenaceous species, miliolids and rotaliids is about equal.

Typical cosmopolitan marsh species are:

-          Ammotium salsum

-          Arenoparrella Mexicana

-          Miliammina fusca

-          Taclammina macrescens

Interpreting an ancient marsh environment may be difficult. Due to reducing conditions calcareous test are easily destroyed after death. After complete solution of calcareous species, it is impossible to distinguish between the various marsh environments. Only the low diversity and an assemblage consisting of small arenaceous species such as Trochammina sp.. Haplophragmoides sp., Ammobaculites sp. points to a marsh origin. If Miliammina sp is present in this assemblage, a hyposaline lagoon could also be indicated.

The rocky substratum of this environment and presence of seaweeds and algae, allow a habitat for attached foraminiferal species. Typical forms can recent coasts are: Elphidium spp., Cibicides lobatulus, Planorbulina sp., Massilina sp.

However, as rocky coast is principally a non depositional environment, the tests of the foraminiferal fauna after death will be either destroyed by abrasion or be transported offshore into a region of deposition

The microfauna inhabiting this environment is little diagnostic. The species diversity is low and planktonic,larger foraminiferal and attached calcareous benthonic forms are absent. As most high energy sandy beaches face open  sea, salinities are generally normal.

Cahracteristic species are: Quinqueloculina sp., Milionella sp., Ammonia beccarii, Elphidium spp.

Fossil sandy beaches can be recognized by poorly preserved, abraded specimens. However, due ti continuous transport ot the tests after death many small or thin shelled specimens may be destructed and foraminifera are sometimes completely absent from an exposed beach sand. A further complication in determining an ancient sandy beach is that many allochthonous forms may have been washed in.

Error in determining a palaeoenvironment can be made due to the following factors:

1. Transport

This will cause a mixing of faunas from different environment

-          Reworking of older sediments

-          Contemporaneous transport

-          As suspended load.The empty shells of dead foraminifera can be transported hundreds of miles offshore. Result:shallow water forms in deep water deposit

-          By currents. This may be reflected in species or size sorted assemblages.

-          By turbidity currents or slides.

-          Vegetation. Attached living species may be transported over vast distances, when the vegetation is uprooted

-          Wind. Empty shells of dead foraminifera may be blow land inwards

2. Bioturbation

The effect of bioturbation is rather minimal. Burrowing organisms may cause the mixing of different assemblages.

3. Diagenesis

Solution of calcareous test or the calcareous cement of arenaceous species can result in the complete absence of a fossil

4. Caving and contamination

Different preservation, colour or the degree of abrasion can be clue in determining transported or reworked faunas.

The distribution of foraminiferal taxa is influenced by many different factors. Although many authors consider water depth the most significant one, water depth in itself is not important, the controlling factors being the various physical and chemical conditions associated with depth. Some of these factor are:

1. Temperature and temperature fluctuations

Some foraminiferal species have a wide tolerance, other have narrow tolerance limits. Most of the shallow water environments on a world wide basis, are controlled by temperature differences.

2. Salinity and salinity variations

Most foraminiferal species tolerate only small changes in salinity. In marginal marine environments, where salinities can be low, due to the mixing with freshwater, characteristics high tolerance assemblages will be found. Especially miliolids are suspectible to low salinities, if less than 32% no miliolids will be present.

3. Calcium carbonate availability

Salinity, temperature and CO2 content of the seawater influence the solubility of calcium carbonate. In tropical marine and hypersaline waters calcium carbonate is most readily available. Cool hyposaline waters contain very little calcium carbonate and consequently faunas will consists mostrly of arenaceous species

4. Oxygen content

Foraminifera do not live in an oxygenic conditions, otherwise oxygen is no limiting factor

5. Energy

6. Pressure

7. Light

Foraminiferal growth will be affected through algae or non algae food supple. The precise effect of light on the distribution of foraminifera is unknow

8. Rate of sedimentation

Krashininnikov (1960): different faunal facies occur at the same depth. Depending on the rate of sedimentation and bottom charecters. For instance, Elphidium prefers mobile water aand rapid sedimentation, while miliolids prefer quiet water and slow sedimentation

9. Bottom Characters

The presence of a sandy, muddy or rocky substratum will influence the distribution of foraminifera.

Biostratigraphic units
In stratigraphy have 3 kinds of unit are used
1. Lithostratigraphic units : defined by lithology of the beds. (fundamental unit = formation)
2. Chronostratigraphic units : defined by absolute age (fundamental unit = chronozone)
3. Biostratigraphic units : defined by paleontologic events (fundamental unit =biozone)
Biostratigraphic units are not mappable in the filds. To use them laboratory work is necessary
Different biozones :
- Range zone
- Partial range zone
- Concurrent range zone
- Assemblage zone