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Marine Index

Paleoenvironment curve has been developed by assigning to each sample a minimum and maximum value between zero and six to express the degree of marine incursion. A similar "marine index" (MI) was first utilized in Lower Cretaceous of the Western Canada Sedimentary Basin by Banerjee and Davies (1988) and is based on the abundances and diversities of key dinoflagellate species which are paleoenvironmental indicators. This has been adapted for the Neocomian of the Sable Basin as illustrated in following figure:

 

DISTRIBUTIONS OF PALYNOMORPHS

Each group of fossils has its own utility range within time and environmental space. The group of organic-walled microfossils, which are studied under the discipline called Palynology, are called palynomorphs in a general sense. This group consists of a broad microfossil spectrum of palynomorphs encompasses the three living kingdoms: animals, plants and protists.

The aquatic forms comprise a diverse spectrum of fossils, such as testate amoebians (protozoans) and scolecodonts (worm teeth), but more commonly as zygospores of chlorophytic algae and dinoflagellates (algal resting cysts) and those cysts of unknown affinity called acritarchs.

The aquatic elements will have distributions that closely represent their living environments. However, these forms can also be transported by currents to another depositional site. Generally the movement will be from onshore to offshore. The more nearshore forms can be deposited in offshore sites. Thus freshwater forms such as the chlorophytic Zygnemataceae and Pediastrum may occur in all marine enviroments and may indicate a level of freshwater influx into the basin. The most common forms of aquatic algae in palynological assemblages are the dinoflagellates. These are generally one-celled photosynthetic forms which must live in the photic zone with enough depth for their diurnal movements. Thus their distribution does not directly reflect water depth, but rather the surface conditions of the water body. These factors include turbidity, salinity, temperature and nutrient concentration.

The great marity of palynomorphs consist of terrestrial debris from terrestrial land plants from which the reproductive elements such as pollen from gymnosperms (conifers) and angiosperms (flowering plants), and the spores from bryophytes (mosses) and pteridophytes (ferns) can be recognized. The productive elements of upland to shoreline plants can be washed out by currents and wind into far reaching enviroments but generally become lower in diversity and more predominantly saccate pollen offshore.


DEFINITIONS OF PALEOENVIRONMENTS

Because the Marine Index model is a simple two dimentional it cannot reflect the wide diversity of paleenvironments available in a depositional system. The Marine Index reflects the tendancy for farther offshore, marine enviroments. In the model four broad paleoenvironments are discerned and are defined below:

Non-marine - coastal plain or lacustrine with no marine influence at the depositional site.

Paralic - environments close to shore with an interaction of freshwater influx and marine influence including lagoonal, littoral, estuarine and innermost neritic. It is marked by low diversity with forms highly tolerant to large swings in the aquatic conditions.

Restricted marine - environments that are not fully marine with high salinity or freshwater or turbidity factors where opportunistic species are prevalent, typical of inner neritic to middle neritic environments, including embayments, outer estuaries, margins of plume jets, etc.

Open Marine - fully marine environment with minimum freshwater influence, usually typical of middle to outer neritic and deeper realms.


APPLICATION OF THE MARINE INDEX

The application of the MI should reflect fluctuating paleoenvironments and help determine the maximum flooding surfaces (i.e. the most marine influence) within a studied interval.

An MI value equal to 0.0 reflects a barren assemblage and is considered non-marine. There are, however, occasional marine paleoenvironments which produce barren assemblages.

An MI value between 0.0 and 1.0 reflects aquatic non-marine paleoenvironments which produce assemblages predominated by terrigenous spores and pollen, and freshwater algae with occasional brackish water forms near the upper limit. The presence of rare zygnemataceous chlorophytic cysts, which grow in stagnant water with unrestricted sunshine (no canopy) indicates a marsh environment.

An MI value between 1.0 and 2.0 reflects brackish to marginally marine, paralic paleoenvironments which produce assemblages usually predominated by terrigenous miospores and brackish opportunistic dinoflagellates. A MI value between 2.0 and 3.0 reflects a transitional interval between paralic to restricted marine paleoenvironments such as lagoonal, tidal marsh and high estuarian settings.

An MI value between 3.0 and 4.0 reflects restricted marine conditions where opportunistic marine dinoflagellates predominate and occasional neritic species occur near the upper limit. Enclosed marine embayments, harbours, estuarine mouths and marine shore face would be examples. An MI value between 4.0 and 5.0 reflects a transition to the fully marine setting, such as inner to middle neritic paleoenvironments where dinoflagellate are more abundant and diverse.

An MI value between 5.0 and 6.0 reflects fully marine setting middle to outer neritic paleoenvironments which contain highly diverse assemblages and morphologically complex dinoflagellate species in high abundances.

By assigning a minimum and maximum MI value for each sample a Marine index curve is developed. Thus the curve reflects the fluctuating marine flooding and freshwater influx.

 

 

© Branta Biostratigraphy Ltd.
Edward H. Davies Ph.D., P.Geol.